Meetings & Events

fall 1997 logo1997 MRS Fall Meeting & Exhibit

December 1 - 5, 1997 | Boston
Meeting Chairs:
 Harry A. Atwater, Peter F. Green, Dean W. Face, A. Lindsay Greer 

Symposium NN—Thin Films-Stresses and Mechanical Properties



Esteban Busso Robert Cammarata, Imperial College Johns Hopkins University
Michael Nastasi Warren Oliver, Los Alamos National Laboratory Nano Instruments Inc.

Symposium Support 

  • Los Alamos National Laboratory

Proceedings published as Volume 505 
of the Materials Research Society 
Symposium Proceedings Series.

* Invited paper

Chairs: Robert C. Cammarata and C. A. MacDonald 
Monday Morning, December 1, 1997 
Essex South (W)

8:30 AM *NN1.1 
POLYCAPILLARY X-RAY OPTICS FOR THIN FILM STRAIN ANALYSIS. F.A. Hofmann, W.M. Gibson, S.M. Lee, C.A. MacDonald, University at Albany Center for X-Ray Optics, Albany, NY; Q.F. Xiao, X-Ray Optical Systems, Inc., Albany, NY; B.R. York, IBM, Dept. of Thin Films and Metallurgy, San Jose, CA.

Polycapillary optics, shaped arrays consisting of hundreds of thousands of hollow glass capillary tubes, can be used to redirect, collimate, or focus x-ray beams from conventional, laboratory-based sources. A multifiber polycapillary collimating optic was placed without system optimization into a flat crystal Bragg-Bretano diffractometer. The lens is 108 mm long and 35 mm in output diameter, and has a transmission of 30% and output beam divergence of 3.8 mrad FWHM at 8 keV. Measurements on thin magnetic CoPtCr films with and without the lens yielded a gain of 8. The collimation also resulted in peak symmetrization, which simplified peak shape analysis. Measured gains would be much higher in comparison with systems employing pinhole rather than one dimensional slit collimation. Further, the measured gain would be approximately a factor of two larger if the detector and soller slit area diameters were increased to match that of the lens. Polycapillary optic alignment is also faster and more convenient than the usual parafocusing geometry. Focused beam optics provide even larger gains. Gains of 100 have been demonstrated using polycapillary optics with spot sizes as small 20 m. This is of particular significance for texture and structure determination for low signal applications such as thin films. The inverse dependence of the critical angle for total external reflection on photon energy also results in suppression of high energy photons. This suppression of Brehmsstrahlung can allow use of higher tube potentials to increase characteristic line emission. Background suppression from a polycapillary optic "soller slit" is also enhanced due to the two dimensional collimation and much smaller acceptance angle. Preliminary measurements yield nearly complete scatter rejection. The combination of background suppression, intensity gain and increased tube emission by employing polycapillary optics would greatly increase the signal to noise ratio for thin film stress analysis.

9:00 AM NN1.2 
FULL FIELD MEASUREMENTS OF CURVATURE USING COHERENT GRADIENT SENSING: APPLICATION TO THIN FILM CHARACTER4IZATION. A.J.Rosakis 1, R. P. Singh 1, Y. Tsuji 2, E. Kolawa 2,3, and N.R. Moore, Jr. 3, 1Dept of Aeronautics, 2Dept of Applied Physics, California Institute of Technology, Pasadena, CA; 3Jet Propulsion Laboratory, Pasadena, CA.

Coherent gradient sensing (CGS) is presented as an optical, full-field, real-time, non-intrusive and non-contact technique for measurement of curvature and curvature changes in thin film and micro-mechanical structures. The technique is applied to determine components of the curvature tensor field in multilayered thin films deposited on silicon wafers. Curvature field measurements using CGS are compared with average curvatures obtained using high-resolution x-ray diffraction. Finally, examples are presented to demonstrate the capability of CGS in measuring curvature in a variety of thin film and micro-mechanical structures.

9:15 AM NN1.3 
CALIBRATION OF MICRO INSTRUMENTS FOR SUB MICRON MATERIAL CHARACTERIZATION. M. T. A. Saif, University of Illinois at Urbana-Champaign, Mechanical and Industrial Engineering, Urbana, IL; Noel C. MacDonald, Cornell University, School of Electrical Engineering, Ithaca, NY.

A well known challenge with micro instruments for material characterization is the calibration of the force that they generate, and their linear and higher order spring constants. Such non-linearity of the springs are unavoidable when the instruments undergo large displacements, e.g., during material characterization involving large deformations and/or strains. We present a methodology of calibrating micro instruments consisting of micro electro mechanical systems (MEMS) actuators. The method is based on post buckling (post bifurcation) deformation of a long slender single crystal silicon (SCS) beam. We demonstrate the method by fabricating micro instruments and calibrating them. The same instruments are then employed to fracture a SCS bar subjected to torsion. Calibration: A long slender beam, clamped at both ends, buckles under an axial compressive force, , where E, I and L are the modulus of elasticity, minimum moment of inertia, and the length of the beam. After buckling, the maximum transverse deformation, D, is related to the axial force P and the end displacement, , along the axial direction by 
The force generated by a micro instrument consisting of comb actuators is given by , where is a constant. If the actuator buckles a clamped-clamped beam, then the force equilibrium requires: 
where K0 is the linear and Ki, i=1,3,... are the higher order spring constants of the actuator springs. From the buckling experiment, the transverse displacement, D, and the applied voltage, V, can be measured, and using Eq , V2 can be plotted against . The data can be least square fitted to 
Comparing and 3, which gives the calibration parameters , K0, and Ki. Demonstration: We designed and fabricated two micro actuators to twist a vertical SCS pillar attached to the substrate. The pillar is also connected to a lever arm at the top. The pillar is fractured by torsion. The micro actuators are then calibrated. The calibration parameters are , K0 = .75 N/m, K1=.16N/m3. The spring restoring force is thus, R=.75x + .16x3 + .... Failure of the torsion bar occurs at 36.5 V. The applied torque prior to failure is 375 . A high resolution SEM fracture surface analysis distinctly demonstrates the shear mode of failure of the bar.

9:30 AM NN1.4 
MEASUREMENT OF RESIDUAL STRESSES IN MEMS THIN FILMS USING ROTATING MICRO STRUCTURES. Xin Zhang, Tong-Yi Zhang and Yitshak Zohar, Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, HONG KONG.

Rotating micro structures for local measurements of residual stresses in thin films were simulated using the finite element method (FEM) in the present work. Effects of every structural parameter on the rotating deflection were systematically studied. According to the FEM results, a variety of optimized microstructures were designed, fabricated and characterized. Residual stresses in both silicon nitride and polysilicon thin films, deposited by LPCVD on silicon wafers, were fully investigated before and after rapid thermal annealing (RTA). Residual-stress measurements by the micro structures were further confirmed by the wafer curvature method. The residual stresses in the polysilicon films at different RTA stages were also characterized by the micro Raman spectroscopy (MRS). The experimental results verified the sensitivities of the structures analyzed by FEM. The rotating microstructures exhibited indeed the ability to spatially and locally measure the residual stresses in MEMS thin films with appropriate sensitivities.

10:15 AM NN1.5 
THE EFFECTS OF SURFACE STEPS ON LOCAL MECHANICAL PROPERTIES OF Au(111). J.D. Kiely, R.Q. Hwang, J.E. Houston, Sandia National Laboratories.

As the scale of technology continues to shrink, local materials properties are becoming increasingly important in defining the operation of devices and their longevity. In the area of mechanical properties, the behavior of defects will be crucial in fields such as microelectronics, MEMS and thin films. To begin to develop a fundamental understanding of mechanical properties on this length scale, we have applied the interfacial force microscope (IFM) to quantitatively investigate the influence of surface steps on the elastic and plastic responses of an Au(111) surface. Well characterized Au(111) samples were prepared that exhibited large defect free terraces separated by steps ranging from one atomic height (3‰) to about 50‰. These steps were found to have a pronounced asymmetric influence both on the locally measured elastic modulus and the threshold of plastic deformation. Across a 40‰ high step, the measured elastic moduli varied by 15%, being larger on the low side of the step. In the plastic regime, the distance from the step had a significant effect on the threshold shear stress, that is, the onset of plasticity. 570 ‰ from the step, the threshold shear stress decreased by 16% from that of the planar (111) surface, while directly on the step, the threshold decreased by 38%. Furthermore, imaging the resulting pits after deformation clearly shows the influence of the step crystallographic orientation. These effects will be discussed in terms of the step structure and the intrinsic stresses around steps.

10:30 AM NN1.6 
A NOVEL METHOD TO MEASURE POISSON'S RATIO OF THIN FILMS. Volker Ziebart, Oliver Paul, Henry Baltes, ETH Zurich, Physical Electronics Laboratory, Zurich, SWITZERLAND.

We report a novel method to determine Poisson's ratio of compressive thin films with high accuracy. The method subtly exploits a mechanical instability of compressive membranes made of the material under investigation. It is demonstrated with the extraction of a value of of PECVD silicon nitride films. The method is based on the load-deflection of long rectangular membranes under differential pressures. The membranes are so long that their smaller sides have negligible influence on the mechanical response of an extended middle section. Due to the strongly compressive residual stress of the constituent material, such a membrane exhibit a complex buckling pattern in the absence of a differential pressure. In contrast, sufficiently high differential pressures straighten out the buckling topography. In its middle section, then, the membrane has a plane-strain deflection profile independent of the longitudinal coordinate. As the pressure is lowered, the buckling reappears via a mechanical instability transition at a critical pressure Pcr: the translational symmetry of the membrane deflection profile is broken and a ripple with characteristic wavelength appears. The unknown is uniquely and accurately determined from Pcr and , and from the dimensions and the independently measured bending stiffness of the membrane, using an analytical model developed by us. We applied the method to m thick PECVD silicon nitride films on silicon wafers. Membranes with lengths between 2.4 and 8òmm and side length ratios up to 1:15 were released using silicon bulk micromachining. Critical pressures ranged from 18 to 27 kPa, and was between 129 and 184 m. From these results we extracted . The extension of the new method to tensile films is possible.

10:45 AM NN1.7 
A METHOD FOR INDEPENDENT MEASUREMENT OF ELASTIC MODULUS AND POISSON'S RATIO OF DIAMOND-LIKE CARBON FILMS. Seong-Jin Cho, Kwang-Ryeol Lee, Kwang Yong Eun, Thin Film Research Center, KIST, Seoul, , KOREA; Dea-Hong Ko, Department of Ceramics, Yonsei University, Seoul, KOREA.

Unusual combination of physical and chemical properties of diamond-like carbon (DLC) films has stimulated studies for various applications. For example, DLC coated Ti tweeters and DLC overcoat for surface acoustic wave (SAW) devices have been developed by utilizing the high elastic modulus and the low mass density of the films. However, only a few results were reported on the elastic modulus and Poisson's ratio of DLC films. The present work demonstrates a noble technique to determine the elastic modulus and Poisson's ratio of DLC films. DLC films has high residual stress up to 10GPa. Although the high residual stress is the main reason for poor adhesion, the films are deformed when being removed from the substrate. The quantity of the deformation is dependent on the residual stress, elastic modulus (E) and Poisson's ratio () of DLC films. can be thus determined by the simple elastic theory, if the resiudal stress and the deformation were independently measured. On the other hands, can be obtained from the unloading curve of load-displacement data of nano-indentation. By combining these two techniques, the elastic modulus and Poisson's ratio could be independently measured. DLC films were depostied on Si (100) wafer by RF glow discharge of benzene at a self bias voltage of -400V and a deposition pressure 10mTorr. In order to measure the deformation when the film was seperated from the substrate, part of the Si wafer was removed by a conventional MEMS technology. The deformation of the film was then observed by optical microscope and SEM and was analyzed to obtain . By comparing the value with that of obtained by nano-indentation, E and were estimated to be 13530 GPa and 0.340.05, respectively.

11:00 AM NN1.8 
FATIGUE STRENGTH OF THIN FILMS BY MEANS OF IMPACT TESTER. E. Lugscheider, O. Knotek, Christian Wolff, Materials Science Institute, RWTH-Aachen, Aachen, GERMANY.

Machine parts like rolling bearings or gears are stressed during operation in a changing mechanical strain. This causes a wear by impacts and a wear by rolling which is marked by the so called surface ruin. This corporate identity of the surface fatigue is based upon structural transformation, cracking processes and cracking growth processes and ends with the separation of debris particles caused by the above mentioned permanent changing strain. The final stages, which is equivalent with the failure of component, is the so called `pitting' on the technical surface, which is named characteristically `surface fatigue' The impact tester is used for closer researches of the failure mechanism in the field of thin films. Statements about the adherence of the deposited hard material films under dynamic pressure load can be made with this test method. This impact tester is used to simulate a part of the load that breaks or fatigue with a frequency up to 50 Hz into the hard films. The altitude stress can be varied to differentiate the analysis of the fatigue strength under reversal strain. Selected film systems of an impact force of 300 N, 500 N and 700 N have been analysed in the described test. Different MSIP-films on titanium- and chromium basis were analysed. The fatigues defects of the thin films will be discussed depending on structure and morphology of the thin films.

11:15 AM NN1.9 
FRACTURE TESTS OF POLYSILICON FILM. W. N. Sharpe, Jr. and Bin Yuan, Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD; R. L. Edwards, Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD.

Other attempts have been made to measure the fracture toughness of polysilicon thin film by forcing open a double-cantilever fracture specimen with a mechanical probe and measuring the position of the probe when the specimen breaks. The obtained information is used with a finite element model of the specimen to extract a value of fracture toughness. This paper presents a more direct measure of fracture toughness in which the specimen is a center-cracked panel - the first fracture geometry that was analyzed in the early 1900s. The polysilicon specimen is 3 millimeters wide, 6 millimeters long, and 3.5 microns thick. A center slot perpendicular to the loading axis is patterned into the specimen. The slot is 10 microns wide, 100 microns long, and tapered at each end to a tip with a radius of approximately 1/2 micron. This is the classic center-cracked-panel geometry of fracture mechanics, and the relation between applied load and crack opening displacement at the center of the crack is well-known. Two gold pads are deposited astride the center of the slot and serve as the reflective markers for a laser interferometry system that measures the crack opening displacement. Excellent agreement is found between the measured and predicted crack opening displacements. The maximum load at failure of this brittle material can be used to compute its fracture toughness. An initial value of 1.6 MPa-m1/2 is found; more tests are in progress.

11:30 AM NN1.10 
TENSILE BEHAVIOR OF FREE-STANDING GOLD THIN FILMS AT ELEVATED TEMPERATURE. R.D. Emery, B. Mazin, C. Simons, and G.L. Povirk, Yale University, Dept. of Mechanical Engineering, New Haven, CT.

An experimental method for tensile testing free-standing thin films at elevated temperatures is presented. The tensile specimens were prepared by evaporating gold onto a patterned oxidized silicon wafer. Using fabrication techniques common to the microelectronics industry, free-standing films were produced that span rectangular holes etched through the wafer. Prior to testing, the silicon frame was cleaved at its mid-section so that the load is carried by the metal film. The specimen was then encased within a small furnace capable of temperatures up to 650 C. The tensile behavior of sub-micron thin films is reported as a function of temperature and the results compared to that of bulk gold. Details regarding the design and construction of the experimental apparatus are also discussed.

11:45 AM NN1.11 
STUDY OF THE DYNAMIC BEHAVIOUR OF CARBON THIN FILMS DURING INDENTION USING ACOUSTIC EMISSION METHOD. Nikolay Novikov, Oleg Lysenko, Svetlana Maletskaya, Inst for Superhard Materials of the National Academy of Science, Kiev, UKRAINE.

Fracture of carbon films was investigated by experimental and theoretical analyses of acoustic emission (AE) signals recorded during Vickers hardness tests at loads from 0.1 N to 2.0 N. A special AE transducer based on PKR piezoceramic was developed. AE signals were detected in the frequency range from 10 kHz to 10 MHz. Two different types of AE signals were observed and indentifield to deformation and fracture, respectively. The application of the AE method is discussed in the development of technique for testing thin films adhesion.

Chairs: Sean G. Corcoran and Joost J. Vlassak 
Monday Afternoon, December 1, 1997 
Essex South (W)

1:30 PM *NN2.1 
MICROINDENTATION MEASUREMENTS OF DOPED Si AND GaAs-AlGaAs THIN FILMS. J.S. Williams, A. Kerr, J. Wong-Leung, Y. Chen and M.V. Swain*, Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, AUSTRALIA; *CSIRO Division of Telecommunications and Industrial Physics, Linfield, AUSTRALIA.

Doped Si wafers and GaAs/AlGaAs thin films have been subjected to microindentation experiments, to extract mechanical properties such as hardness and elastic modulus, using both Berkovich and spherical indentors with a UMIS-2000 apparatus. In the case of Si, boron bulk doped and uniformly boron doped, ion implanted surface layers of varying thickness were used to examine the depth-dependent elastic/plastic behaviour and hardness. Heavily boron doped Si is considerably harder than lightly doped Si and the bulk and ion implanted layers were found to be ideal for separating stress/strain contributions from the films and substrate for different indentors and loads. In the case of GaAs-AlGaAs structures, again bulk sample measurements were compared with thin films (grown by MOCVD) of softer AlGaAs on GaAs of harder GaAs on AlAs of varying thicknesses. Results of loading/partial unloading experiments are quite revealing of interesting structural modifications which take place. Comparisons are also made between results for different indentors and loads.

2:00 PM NN2.2 
CRITICAL ISSUES IN MEASURING THE MECHANICAL PROPERTIES OF HARD FILMS ON SOFT SUBSTRATES BY NANOINDENTATION TECHNIQUES. Jack C. Hay, Oak Ridge National Laboratory, Oak Ridge, TN; George M. Pharr, Rice University, Dept. of Materials Science, Houston, TX.

This work examines the difficulties associated with using conventional nanoindentation analysis techniques to measure the properties of hard films on soft substrates. The effective indentation modulus, evaluated by these standard methods, depends on the Young's moduli and hardness of both the film and substrate. Here, a 'model' system consisting of NiP on annealed Cu is used to reduce the effect of the Young's moduli mismatch as a variable. In contrast, the hardness of the NiP is approximately 8 GPa, and that of the annealed copper is less than 1 GPa, representing a factor of 10 difference. Preliminary findings indicate that standard analysis methods do not work well for this case of a hard film on a soft substrate. At shallow contact depths, the indentation modulus represents that of the thin film, but at increasing depths a large sink-in phenomenon leads to a measured indentation modulus which is less than both the film and substrate properties. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide critical details of the physical processes which result in the erroneous measurements.

2:15 PM NN2.3 
A NEW TECHNIQUE FOR VISUALIZING THE DISPLACEMENT FIELD OF INDENTATIONS: THE CASE OF A SOFT FILM ON A HARD SUBSTRATE. Joost J. Vlassak, T. Tsui*, and W. D. Nix; Department of Materials Science and Engineering, Stanford University, Stanford, CA; *Materials Technology Development, Advanced Micro Devices, Sunnyvale, CA.

We have developed a new technique for visualizing displacement fields of indentations in thin films. In this technique, the indented film consists of alternating layers of two different materials. One of the materials serves as a marker for visualizing the plastic flow induced by the indentation. A Focused Ion Beam (FIB) is used to cross-section the indentation, revealing the deformed layers. This technique can be used to study how the presence of the substrate affects the plastic displacement field around the indentation. The technique is applied to a multilayered film of aluminum and titanium nitride on a silicon substrate. The titanium nitride layers are much thinner than the aluminum layers and serve the function of marker. Indentations are made to various depths in the film. Pile-up of the film material around the indenter and the effect of the hard substrate are easily revealed and a mechanism for pile-up is suggested. The technique also shows that the grain structure in the deformed zone around the indentation is altered profoundly.

2:30 PM NN2.4 
A NOVEL ENERGY-BASED APPROACH TO THE HARDNESS OF COATINGS. Alexander Korsunsky, Martin McGurk, Steve Bull, Trevor Page, University of Newcastle, Department of Mechanical, Materials and Manufacturing Engineering, Newcastle upon Tyne, UNITED KINGDOM.

Hardness is a principal parameter which determines how effectively any particular protective coating system operates. Yet because the coatings in many such systems are very thin, an extension of the conventional hardness concept is required, both in terms of the definition, and the measurement techniques appropriate for the task. Depth-sensing nanoindentation techniques, together with the interpretation methods developed for them, have largely taken the measurement and interpretation limits to sub-micron coating thicknesses. In order to improve the coating design methodology based on these results, predictive quantitative models are required which would show agreement with the experimental data over a broad range of systems and conditions. The models existing to date rely either on systematic or mechanistic concepts, i.e. concentrate either on the partitioning the deformation between the substrate and the coating, or on the deformation modes in each. These clearly don't have sufficient generality to describe the full variety of systems and responses observed. An alternative approach is to calculate the expenditure of energy in an indentation experiment, and to relate it to the composite hardness value. The energy analysis highlights the influence the coating response mode has on the variation of composite hardness with the indentation depth, but also allows similarities to be identified and a unified approach to be developed. The model uses dimensionless quantities which are equally applicable to plasticity- or fracture-dominated coating response, and contains a single fitting parameter. Excellent agreement with experimental results obtained using a variety of techniques (macro-Vickers to nanoindentation) is demonstrated for a number of coated systems. Furthermore, it is shown how, by interpreting the dependence of the model parameter on coating thickness, dominant deformation mechanisms can be identified and design recommendations be made.

3:15 PM *NN2.5 
NANOINDENTATION OF ATOMICALLY MODIFIED SURFACES. Sean G. Corcoran, Hysitron, Inc., Nanomechanics Research Laboratory, Minneapolis, MN; Stanko R. Brankovic, Nikolay Dimitrov, Karl Sieradzki, Arizona State University, Department of Mechanical and Aerospace Engineering, Tempe, AZ.

Nanoindentation studies on metal and semiconducting surfaces often display instabilities in the load-displacement curves which have been attributed to phase transitions, oxide breakthrough, surface contamination effects, and dislocation nucleation under the indenter tip. We have shown recently that displacement excursions were present for nanoindentation on single crystal Au (111), (110) and (100), and were attributed to dislocation nucleation since all other phenomena were ruled out. We present our recent results which have been aimed at understanding the effects of surface modification at the nanoscale on dislocation nucleation. The effects of modifying the Au surface with electrochemically deposited metal monolayers (Pb and Ag), with an electrochemically deposited oxide monolayer and an electrochemically reconstructed surface will be presented. Hardness differences as great as a factor of 3 have been observed for these surfaces. These experiments are unique in that they were carried out under electrochemical control where strict control of the surface cleanliness can be maintained.

3:45 PM NN2.6 
ANALYSIS OF INDENTATION CURVES OBTAINED USING CONICAL AND PYRAMIDAL INDENTERS. Yang-Tse Cheng, General Motors Research and Development Center, Warren, MI; Che-Min Cheng, Institute of Mechanics, Chinese Academy of Sciences, Beijing, CHINA.

We apply a similarity approach to conical and pyramidal indentation in elastic and plastic solids. New equations are derived which describe indentation loading and unloading curves. These equations are compared with the existing theories of indentation, including the slip-line theory for rigid-plastic solids, Sneddonís analysis for elastic solids, and Johnsonís spherical cavity model for elastic-perfectly plastic solids. In the limit of small ratio of yield strength (Y) over Youngí s modulus (E), both the new results and Johnsonís model approach that predicted by slip-line theory for rigid-plastic solids. In the limit of large Y/E, the new results agree with that for elastic solids. For a wide range of Y/E, a difference is seen between Johnsonís model and the present results. The effects of indenter tip rounding on the shape of loading and unloading curves are also shown. We further demonstrate the possibilities and limitations of using indentation curves to extract fundamental mechanical properties of solids.

4:00 PM NN2.7 
THE EFFECTS OF RESIDUAL STRESS ON MODULUS MEASUREMENTS BY INDENTATION. D.F. Bahr, D.A. Crowson, W.W. Gerberich, Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN.

Nanoindentation has been used to determine the modulus of a variety of thin metal films under various amounts of residual stress. Platinum deposited by both sputter deposition and evaporation onto silicon wafers exhibits a higher measured modulus than bulk platinum when large tensile residual stresses are present in the film. These were measured by both x-ray diffraction and wafer bowing techniques. Residual stresses between 500 and 1200 MPa correspond to a range of apparent modulus between 200 and 250 GPa, an increase of up to 50% above the measured modulus of bulk platinum. These values are significantly higher than the modulus of a bare silicon substrate. Using the identical test conditions, thin films with low residual stresses, such as titanium, do not exhibit this apparent increased modulus. These effects have been observed with both research and commercial indentation instruments. Evaporated chromium films have been used to verify this behavior on a variety of substrates, including silicon, glass, and copper. Residual stress levels in the film have been altered by controlling the substrate temperature during deposition. Proposed explanations regarding this apparent increased modulus as measured by indentation will be presented.

4:15 PM NN2.8 

A method that is becoming increasingly common for measuring the mechanical behavior of thin films is low-load indentation testing. However, there can be complications in interpreting the results. For instance we have observed enhanced hardness and elastic moduli in thin (50-110 nm) films of AlN on sapphire substrates grown at 700ƒC. These enhanced elastic moduli and hardness effects diminish as thicker films of AlN are grown. Since many factors can affect hardness and modulus measurements using nanoindentation techniques such as surface roughness and pile up surrounding an indentation, the properties were also evaluated using a scanning force microscopy (SFM) based pico-indentation device to allow imaging of the surface and indents.

4:30 PM NN2.9 
MECHANICAL CHARACTERIZATION OF SUB-MICRON POLYTETRAFLUOROETHYLENE (PTFE) THIN FILMS. B. N. Lucas, Nano Instruments, Inc., Oak Ridge, TN; C. T. Rosenmayer, W. L. Gore and Associates, Inc., Eau Claire, WI; W. C. Oliver, Nano Instruments, Inc., Oak Ridge, TN.

Polytetrafluoroethylene (PTFE) thin films have been demonstrated as potential candidates for low dielectric constant materials in Ultra Large Scale Integration (ULSI) applications. Previous studies have indicated excellent electrical, thermal and chemical properties for sub-micron PTFE films. However, very little is known about the mechanical properties of these and other polymer films in this thickness regime due to the limitations of conventional techniques used for characterization of these types of materials. Currently, reliability modeling techniques for integrated circuit interconnects use mechanical properties data determined from either bulk material or from films with a thickness of several microns. The in-situ mechanical properties of sub-micron polymer films must be known in order to better understand the implications of such modeling. This study reports the results of an investigation of the mechanical properties of PTFE thin films of a variety of thicknesses in the sub-micron regime using frequency specific depth-sensing indentation. These results are compared to a number of other mechanical characterization techniques including dynamic mechanical analysis (DMA), wafer curvature measurements for film stress, and film adhesion measurements.

4:45 PM NN2.10 
NANO-SCALE VISCOELASTIC PROPERTIES OF POLYMER MATERIALS.. S.A. Syed Asif and J.B.Pethica, Dept. of Materials, University of Oxford, Parks Road, Oxford, UNITED KINGDOM.

The AC force modulation technique in nanoindentation can be used to study the dynamic viscoelastic properties of polymer materials and thin films. Very careful calibration of the apparatus dynamic properties is required to enable accurate results from thin films. We show the frequency and temperature dependent loss and storage moduli of polyisoprene and of sub-micron thicknesses of PMMA. The results are in good agreement with the reported literature results obtained from conventional dynamic testing. We also show that time-temperature superposition, the mechanical relaxation and the activation energy for the relaxation process can all be directly measured from nano scale contact. The technique is likely to be of particular value for polymer thin films.

Chair: Warren C. Oliver 
Tuesday Morning, December 2, 1997 
Essex South (W)

8:30 AM *NN3.1 
NANOINDENTATION OF SOFT FILMS ON HARD SUBSTRATES: EXPERIMENTS AND FINITE ELEMENT SIMULATIONS*. George M. Pharr, Rice University, Dept. of Materials Science, Houston, TX; Alexei Bolshakov, Baker Hughes Inteq, Houston, TX; Ting Y. Tsui, Advanced Micro Devices, Sunnyvale, CA.

Experiments have been conducted on a model material system to examine a how hard substrate influences the measurement of mechanical properties of a soft film by load and displacement sensing indentation techniques (nanoindentation). The experimental system consisted of aluminum films with thicknesses in the micron range sputter deposited onto soda-lime-silicate glass substrates. Since the hardness of aluminum is about a factor of 10 smaller than that of glass but the elastic moduli of the two materials are very similar, substrate influences are manifested in the deformation behavior primarily by differences in the plastic flow characteristics of the two materials. When standard nanoindentation methods are use to analyze the load-displacement data to estimate the hardness and elastic modulus of the film, several unusual behaviors are observed, including an apparent increase in the film elastic modulus with increasing indentation depth to a value which is approximately 50% greater than the true value. Finite element simulations show that the unusual behavior is caused by changes in pile-up behavior as the indenter approaches the substrate and plastic deformation in the film is severely constrained by the hard material below it. The geometry of the pile-up and its influence on the measurement of mechanical properties depends in important ways on the work hardening characteristics of the film.

9:00 AM NN3.2 
FINITE-ELEMENT MODELING OF NANOINDENTATION FOR EVALUATING MECHANICAL PROPERTIES OF THIN DIAMOND-LIKE CARBON LAYERS. J. A. Knapp, D. M. Follstaedt, T.A. Friedmann, A. J. Magerkurth, S. W. Clarke, Sandia National Laboratories, Albuquerque, NM; O. R. Monteiro, J. W. Ager, I. G. Brown, Lawrence Berkeley National Laboratory, Berkeley, CA; B. N. Lucas, W. C. Oliver, Nano Instruments, Inc., Oak Ridge, TN.

Amorphous tetrahedral diamond-like carbon (DLC) has many potential applications as a very hard, thin coating. One proven technique for characterizing the mechanical properties of these films is depth-sensing indentation. However, as the thickness of these films continues to decrease, obtaining a substrate-independent measure of the film properties becomes increasingly complicated due to the influence of the underlying material. For DLC films which may approach the hardness of bulk diamond, the deformation and possible yielding of the indenter tip is an additional complication. We have developed procedures based on finite-element modeling of depth-sensing indentation data to reliably extract mechanical properties for thin, hard films on softer substrates. The method accurately deduces the yield stress, Young's modulus, and hardness from indentations as deep as 50% of the layer thickness and for films less than 50 nm thick. Simulations are performed using ABAQUS, a commercial finite-element code. Deformation of the indenter tip, friction between tip and surface, and pre-existing stress in the layers are all modeled. The yield stress and Young's modulus of the layer are extracted from the fit of simulation to experiment, and the intrinsic hardness of the layer material is then deduced by an additional simulation using the yield stress and Young's modulus of the layer for a hypothetical bulk ''sample''. We have applied these methods to two types of DLC layers; one formed by vacuum arc deposition and the second by pulsed laser deposition. Hardness as high as 88 GPa has been measured.

9:15 AM NN3.3 

Thin film mechanical properties including elastic modulus and hardness can be derived from measurements of force as a function of depth when a probe tip is pressed into a surface, as in IFM, AFM, and nanoindentation experiments. The strain produced by elastic compression of the surface under a load can be partially relieved by the formation of dislocations and defects underneath the indenter. We have modeled this behavior using the embedded atom method for a gold surface and a nearly-hard-sphere repulsive potential to represent a spherical indenter tip with an 80 Angstrom radius. This new technique simulates a passivated surface by preventing adhesion between the tip and the surface. Force vs. depth profiles for the passivated (111) and (001) Au surfaces were calculated during indentation and retraction of the indenter, via minimum energy calculations. Detailed images of these dislocation structures were obtained by a new method which calculates the deviation from a centrosymmetric environment around each atom. The first yield point, corresponding to the onset of plastic deformation, produced multiple stacking faults and dislocation loops in the thin film for both surfaces. We will also discuss the plastic deformation of the two surfaces past the first yield point as well as the full or partial recovery of the thin films during retraction of the tip from various stages of the indentation process.

Chairs: Gary L. Povirk and John E. Sanchez 
Tuesday Morning, December 2, 1997 
Essex South (W)

10:00 AM *NN4.1 
EFFECTS OF MECHANICAL PROERTIES IN THIN FILMS AND MICROELECTRONIC INTERCONNECTS. John E. Sanchez, Jr., Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI.

Deposited thin films and metallization interconnects are often subjected to contraints or conditions which produce significant intrinsic and extrinsic (or applied) strains. For example, the contraint due to Al film adhesion and thermal expansion mismatch to nearly rigid Si substrates results in large applied strains in the film during processing. In another example electromigration induced mass fluxes produce large stresses and stress gradients in passivated and confined metallization interconnects. In both cases the material response to such applied strains often determines critical parameters such as processing yield, manufacturing costs or integrated circuit reliability. Additonally the film or interconnect microstructure may play a crucial role in controlling the material response. Specific examples will be described in which microstrucure may control stress-induced hillock and void formation in polycrystalline films. Several other effects of induced or applied strains will be described. Recent work has shown that Al films subjected to extremely large hydrostatic pressures can be induced to diffusively flow to fill interlevel via structures in integrated circuit metallization structures. The role of diffusion creep in this process will be described. Finally, the effects of hydrostatic stresses induced by electromigration forces in confined interconnects will be described. The modeling of effects such as early resistance changes and resistance saturation due to Blech effects will be presented.

10:30 AM NN4.2 
MICROSTRUCTURAL EVOLUTION IN THERMALLY CYCLED PASSIVATED ALUMINUM THIN FILMS. M. Legros, K. J. Hemker, Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD; A. Gouldstone and S. Suresh, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA.

The mechanical performance of an Al thin film on a Si substrate, as measured in wafer curvature experiments, has been shown to depend on the presence and thickness of the passivation layer. Films with Si02 passivation layers have been found to require a large number of cycles to reach steady-state behavior, and the evolution that occurs prior to this steady-state has been related to the microstructural changes that are caused by the biaxial stresses and temperatures associated with thermal cycling. In this study, in-situ cross-sectional TEM observations of passivated Al thin films have been used to characterize the microstructural evolution that occurs within each thermal cycle. In-situ observations of grain growth, dislocation recovery, and subsequent dislocation activity have been recorded and will be use to explain and model the transient mechanical behavior that has been measured in these films.

10:45 AM NN4.3 
MILLION-LINE FAILURE DISTRIBUTIONS FOR NARROW INTERCONNECTS. M.C. Bartelt, N.C. Bartelt, J.J. Hoyt, W.G. Wolfer, Sandia National Laboratories, Computational Materials Science Department, Livermore, CA.

A central goal of materials degradation and reliability testing is the understanding of the key factors or constraints which determine details of the distribution of early failure events. These may be fundamentally different from the mechanisms controlling the overall distribution of median times to failure. Test sample sizes are in practice limited, often forcing questionable extrapolations of failure behavior to service conditions of very large scale production devices. Here, we examine the distribution of stress- and electromigration-induced failure times in a simple but fairly authentic model of interconnect reliability that allows consideration of statistically significant samples. The model includes an effective description of the interconnect grain texture, a treatment of stress evolution associated with mass transport along grain boundaries, and relaxation of stresses due to void formation. Stress evolution is based on a discrete variation [1] on Korhonen et al.'s model [2], which connects relative changes in local atomic density to relative increments in local stress, via an appropriate elastic modulus function. Spatial maps of this function, and data for stresses around voids are specified from finite- and boundary-element method calculations of the materialís elastic response [1]. Failure time distributions for populations of idealized structures are also analyzed to aid in interpretation of general model behavior.

11:00 AM NN4.4 
SIMULATIONS OF ELECTROMIGRATION AT A TUNGSTEN/ALUMINUM JUNCTION. A. Enver, Digital Semiconductor, Hudson, MA; and G. L. Povirk, Department of Mechanical Engineering, Yale University, New Haven, CT.

Two-dimensional, plane strain simulations on the evolution of electromigration-induced stresses at the junction between a tungsten via and an aluminum interconnect are presented. Rather than accounting for diffusion along individual grain boundaries, the model is simplified by using the governing equations that describe bulk diffusion. The effects of grain boundary diffusion are incorporated into the analysis by assuming an effective diffusity constant, and the resultant boundary value problem is solved via the finite element method. The predicted stress fields are compared with experimental observations of void initiation and growth near a tungsten/aluminum junction. The results suggest that modeling interconnect structures in this manner may provide a relatively simple yet effective tool for evaluating the potential reliability of different interconnect/via configurations.

11:15 AM NN4.5 
AND MECHANICAL PROPERTIES OF ELECTROPLATED Cu FILMS FOR DAMASCENE ULSI METALLIZATION. Valery M. Dubin, Guarionex Morales, Advanced Micro Devices, Sunnyvale, CA; Changsup Ryu and S. Simon Wong, Center for Integrated Systems, Stanford University, Stanford, CA.

Copper is considered as alternative materials to Al in ULSI metallization because of low resistivity, high electromigration resistance and low stress induced voiding. In this paper we study microstructure and mechanical properties of Cu films deposited by electroplating on sputtered Cu seed layer with Ta diffusion/adhesion layer. The Cu film texture was measured using x-ray diffraction fiber pole plot technique. Electroplated Cu films exhibit a strong (111) texture ( random component and <5 degree tilting angle ). After annealing, the (111) texture becomes stronger with narrower tilting distribution ( random component with less than 3 tilt range). Electroplated Cu lines formed in sub-micron trenches also exhibit (111) texture (for 0.5 m wide Cu lines, the random component is 48% with less than 10 degree tilting angle _95)%%) which becomes stronger after annealing ( random component with less than 4 tilt range). The grain size distribution of copper were obtained from the plan-view TEM micrographs (for blanket Cu films) and dual beam microscopy (for submicron Cu lines). For 1.5 m thick as-plated Cu film, the median grain size is 0.26 m and the lognormal standard deviation, , is 0.69. After annealing, the median grain sizes increases to 1.04 m and is decreases to 0.44 m. Large grains occupying the entire sub-micron trenches have been found in damascene electroplated Cu lines after annealing. Stress in electroplated Cu films was measured by using FLEXUSTM in the 10 -8 dyne/cm2 range (tensile). Elastic modules was found to be about 150 GPa. Microhardness decreases from 1.5 GPa to 1 GPa when the grain size increases. Adhesion of plated Cu films to substrate was measured by pull test in the range of 200 Kg/mm2. Strong <111> texture, large grains, small grain size distribution, low stress, large microhardness and good adhesion of electroplated Cu films will improve the reliability related attributes in Cu metallization over Al metallization.

11:30 AM NN4.6 
UNDER BUMP METALLIZATION DEVELOPMENT FOR HIGH SN SOLDERS. T.M. Korhonen, S.J. Hong, P. Su, C. Zhou, M.A. Korhonen, and C.-Y. Li, Dept of Materials Science and Engineering, Cornell University, Ithaca, NY.

The current flip chip technology introduced by IBM uses high Pb solder (97Pb-3Sn or 95Pb-5Sn), which necessitates the use of reflow temperatures in the range of 330-350 C to attach the chip to a ceramic substrate. In order to use flip chip bonding on organic substrates, lower melting solders (such as eutectic Pb-Sn) with reflow around 200 C must be used. It turns out that the conventional Cr/Cr-Cu/Cu/Au under bump metallization (UBM) is not adequate for solders containing large amounts of Sn. The Sn-Cu reaction during reflow is more rapid than in Pb based solders and can consume all the copper in the UBM, causing dewetting and loss of adhesion between the solder and the substrate. Thus an improved metallization must be found to make flip chip interconnect technology compatible with high Sn solders. In this research, several UBM schemes based on Ni and Ni-Cu alloys were investigated, since the intermetallic reactions in the Ni-Sn system are slower than the Cu-Sn reaction. Cr and Ti were used as the adhesion layer. Solder bumps were reflowed on different UBMs and the resulting interfacial microstructures were studied as a function of reflow time. The joints were also mechanically tested in fatigue and shear. Relative merits of the metallization schemes are discussed.

11:45 AM NN4.7 

Thin films may sustain a much higher stress than corresponding bulk materials. This 3thin film effect2 is especially strong in aluminum thin films at high temperature. It has been assumed that interface dislocations are to some extend responsible for the thin flim effect. Knowledge about the dislocation structure and the dislocation behaviour thus is crucial for the understanding of mechanical properties of thin films. We have repeatedly and reproducably observed that the contrast of interface dislocations disappears within the electron beam of a transmission electron microscope (TEM). We assume that the contrast dissolution is due to the spreading of the dislocation core at the crystalline/amorphous interface which is driven by the reduction of the elastic energy. We discuss the dislocation core spreading hypothesis on the basis of a very simple dislocation core model. Another possible explanation for contrast dissolution is that the dislocations may be able to move through the oxide with the aid of diffusion. Based on the present results, we suggest that the interface disloctions may spread along the interface at high temperature. As a consequence, the near field stresse and partly also the far field stresses of the dislocations relaxe. This relaxation reduces the interaction force between dislocations and may thus significantly affect the mechanical properties of thin films.

Chairs: Daniel Josell and Harriet Kung 
Tuesday Afternoon, December 2, 1997 
Essex South (W)

1:30 PM *NN5.1 
CREEP AND THE STABILITY OF MULTILAYERS. D. Josell, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD.

Because of the high density of interfaces that they contain, multilayer materials may not be stable even when the layered materials are nonreactive and immiscible. Thus one may not take for granted that superior properties possessed by a multilayer will remain so when the material is placed in use. A relevant example is a multilayer thermal barrier coating in a high temperature land or air based engine application where, as a result of the increased kinetics at the high operating temperatures, the metastable or unstable layered structure that was frozen in during deposition (e.g. sputtering or evaporation) begins to creep. Generally creep occurs even in the absence of external stresses applied to the coating! Any equilibrium, stable structure toward which the coating might evolve is ultimately determined by the free energies of the interfaces between, as well as within layers and external applied stresses. Theory that relates interfacial free energies and creep behavior for several types of multilayer based creep experiments, combined with results obtained from different multilayer systems, demonstrates the richness of this field.

2:00 PM NN5.2 
AND MICROSTRAIN RELAXATION DURING AGING AT ROOM TEMPERATURE IN ANNEALED Ag FILMS. R.C. Currie, R. Delhez, E.J. Mittemeijer, Laboratory of Materials Science, Delft University of Technology, Delft, NETHERLANDS.

XRD and SEM are applied to study the relaxation of thermally induced strain in polycrystalline Ag layers during aging at room temperature. The 500 nm thick Ag layers are deposited by e-beam evaporation at 325K onto {111}-oriented Si wafers covered with natural oxide. The films exhibit a dominant fiber texture with a majority of a matrix of {111}-oriented crystallites and a minority of {511}-oriented twin crystallites. After annealing at 523K, cooling down to room temperature causes plastic deformation of the layer, leading to dislocation production associated with XRD line profile broadening. Part of the thermal misfit strain is elastically accommodated as is revealed by the occurrence of a large macrostrain associated with XRD line-profile shift. The macrostrain in the plane of the layer is determined from the slope of the interplanar spacing d^{HKL}%%; d^{HKL}HKLHKL} of both texture fractions. For each {%%HKL} a distinct relation is observed between the microstrain and the macrostrain. This relationship and the significant difference in microstrain and macrostrain behaviour between the matrix and twin crystallites suggests an orientation-dependent reduction of the dislocation density in the crystallites during the aging process.

2:15 PM NN5.3 
AGING EFFECTS ON THE DURABILITY OF THIN TANTALUM NITRIDE FILMS ON ALUMINUM NITRIDE SUBSTRATES. N.R. Moody, D. Medlin, Sandia National Laboratories, Livermore, CA; D.P. Norwood, Sandia National Laboratories, Albuquerque, NM.

Structure and properties of thin films can vary markedly with age in service. This is of particular interest for tantalum nitride films on aluminum nitride substrates where high heat transfer permits use of very thin films in high power density applications. However, no data exists on how age affects the durability of these films. We therefore employed nanoindentation and nanoscratch testing to determine aging effects on the structure, properties, and fracture resistance of this thin film system. The films were sputter-deposited, air annealed, then aged to duplicate long term service. These tests showed that the fracture energies were similar to the energy for chemical bonding. Of particular note was the strong role that the high compressive residual stresses had on driving susceptibility to film failure where an apparent decrease in residual stress level with annealing and aging improved durability. These results will be used to show that film fracture is driven by compressive residual stresses with adhesion controlled by bonding across the interface plane.

3:00 PM *NN5.4 
STRUCTURAL STABILITY OF MULTILAYERS. H. Kung, M. Nastasi, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM.

The development of thin film multilayers has been the focus of extensive study due to enhanced properties and the broad range of applications. The stability of the multilayered structure can directly impact the performance and reliability of thin film multilayered devices and applications. For example, in metallic multilayers, certain magnetic properties are destroyed by the interdiffusion between the sublayers. Significant degradation in mechanical properties has also been reported accompanying the layer structure breakdown. The de-stabilization of multilayered structure can be induced by various processes, including thermal treatment, application of external stress field, or ion beam irradiation. The stability of the multilayered structure is often controlled by the length scale, mutual solubility, interdiffusivity, interface characteristics, and the local stress/composition gradient. A review of the different approaches in stabilizing multilayered structure will be presented and discussed. Another important aspect of the stability in multilayers is the stabilization of non-equilibrium crystal structure in multilayers due to the structural constraints. The different mechanisms and the potential impact in exploiting new properties and applications will be reviewed and discussed.

3:30 PM NN5.5 
COHERENCY STRESSES IN MULTILAYERS. P.M. Hazzledine, Materials Directorate, Wright Laboratory, Wright-Patterson Air Force Base, OH; M.A. Grinfeld, ETS Inc, Princeton, NJ; and B. Shoykhet, Reliance Electric Co., Cleveland, OH.

A multilayer consisting of parallel sided, elastically isotropic layers with thicknesses much smaller than their dimensions in the plane of the layers may develop large coherency stresses when the layers are thin enough. General formulae are given for the stress tensors in a flat periodic fully coherent multilayer which may contain any number of layers, each with different lattice parameters, elastic constants and thicknesses. As the thicknesses of the layers increase there is an increasing tendency for the interfaces to become incoherent thereby relaxing the elastic stresses, at the expense of creating higher energy interfaces between the layers. This tendency is particularly marked for unusually thick or unusually rigid layers, or for layers with very large or small lattice parameters. Unlike the case of thin films on massive substrates, there is no single thickness at which coherency is lost. In this paper, the loss of coherency is explored by searching for energy minima in a generalised multilayer.

3:45 PM NN5.6 
MODIFICATION OF MISFIT DISLOCATION PROPAGATION VELOCITIES BY POINT, SURFACE AND VOLUME DEFECTS IN THE (SI) SIGE(C)/SI(001) SYSTEMS. E.A. Stach, J.C. Bean, R. Hull, Univ. of Virginia, Charlottesville, VA; R.M. Tromp, F.M. Ross, M.C. Reuter, M. Copel. F.K. LeGoues, IBM T.J. Watson Research Center, Yorktown Heights, NY; L. Lanzerotti, J.C. Sturm, Princeton Univ., Princeton, NJ; A. Nejim, Univ. of Surrey, Surrey, UNITED KINGDOM; K.S. Jones, Univ. of Florida, Gainesville, FL.

We report investigations of the role of point, surface and volume defects on the process of misfit dislocation propagation in (Si) / SiGe / Si (001) and Si / SiGeC / Si (001) strained layer heterostructures using in-situ transmission electron microscopy (TEM), both during post growth annealing of metastable structures, and during UHV-CVD deposition. In situ TEM anneals of post-growth structures implanted with Si+, B+, F- and BF2 at different energies and doses have shown that the presence of excess point defects within the epilayer may greatly alter the magnitude of the observed misfit dislocation velocities. Additionally, the incorporation of carbon into the SiGe epilayer may result in the formation of SiC precipitates, whose presence again affect dislocation motion. Observation of the heteroepitaxial SiGe / Si (001) growth process using the unique capabilities of a specially constructed UHV-TEM equipped with in-situ UHV-CVD growth facilities has revealed that the modification of the material surface resulting from the formation of a native oxide yields a three times increase in observed velocities. The mechanisms which cause these observed differences in velocity will be discussed as functions of defect type and concentration, and the effect of these differences on the evolution of strain relaxation during material processing will be addressed.

4:00 PM NN5.7 
MECHANISMS FOR ASYMMETRIC RELAXATION IN LATTICE-MISMATCHED ZINCBLENDE SEMICONDUCTORS. Karen L. Kavanagh, Dept. of Electrical and Computer Engineering and Materials Science Program, University of California, San Diego, La Jolla, CA and Rachel Goldman, Dept. of Materials Science and Engineering, University of Michigan, Ann Arbor, MI.

Partially relaxed epilayers in the (001) III-V lattice-mismatched systems often show asymmetric relaxation, in which residual strain or misfit dislocation densities along [110] are not identical to the [10] direction. For many years this has been attributed to differences in core structure of the dislocations in the two directions because one set, dislocations, are associated with the Ga sublattice while the other set, dislocations, with the As sublattice (glide set dislocations assumed). Recently, we and a number of other groups have shown that the degree and direction of asymmetric relaxation can also be influenced by the offcut axis of the substrate. In other words, for (001) GaAs substrates offcut by a small angle, the strain relaxation or the density of and dislocations can be controlled simply by the choice of offcut direction. This talk will discuss new and old mechanisms for this asymmetry in the compressively-stressed InGaAs/GaAs system. If misfit formation is nucleation limited then there is strong evidence from stress calculations and TEM that this nucleation is dependent on the trailing 90 partial dislocation. If so, the asymmetries in relaxation are due to differences in the rate of nucleation of and 90 partials. Alternatively, if the nucleation process depends on the rate of formation of the leading 30 partial, as is commonly assumed, then asymmetries in nucleation rates are not expected from stress calculations and glide processes must be the determining factor. In this case, threading dislocations may act to form both and misfits and the asymmetries are then related to variations in the rate of backglide versus forward glide in the strained epilayer.

4:15 PM NN5.8 
STRAIN RELAXATION IN IV-VI SEMICONDUCTOR LAYERS GROWN ON SILICON (100) SUBSTRATES. Harpreet K. Sachar, Patrick J. McCann and Xiao-Ming Fang, School of Electrical and Computer Engineering and Laboratory for Electronic Properties of Materials, University of Oklahoma, Norman, OK.

The large thermal expansion coefficient mismatch between IV-VI semiconductors and silicon results in significant tensile strain when structures are cooled following growth at high temperatures. Molecular beam epitaxy (MBE) growth of PbSe on Si(100) at 280C on a BaF2/CaF2 buffer layer results in high PbSe crack density because of this strain. Interestingly, crack-free layers of PbSe can be grown on Si (100) by performing liquid phase epitaxy (LPE) growth on MBE-grown PbSe/BaF2CaF2/Si substrates. Although the primary glide system for the IV-VI materials is along the (100) planes in the [110] direction, it is believed that the presence of selenium vacancies during LPE growth can allow slip along higher order glide planes such that the Schmid factor becomes non-zero and strain relaxation can occur by plastic deformation. Quaternary Pb1-xSnxSe1-yTey alloys have smaller bandgaps than PbSe, so it can be used as the active layer in heterostructure lasers. The addition of tin reduces the bandgap while the addition of tellurium increases the lattice parameter such that lattice-matched heterostructures can be grown. Recently, we have been interested in growing these quaternary layers on similar MBE-grown buffer layer structures. However, the layers obtained were not crack-free. It is believed that addition of tellurium, which is known to increase IV-VI material hardness, restricts dislocation glide along the higher order slip planes and that crack formation becomes the thermal strain relief mechanism. Recent LPE growth of lattice mismatched Pb1-xSnxSe layers have resulted in nearly crack-free layers and this supports the conclusion that tellurium induced solid solution hardening occurs in this materials system.

4:30 PM NN5.9 
INTERFACIAL STABILITY AND MISFIT DISLOCATION FORMATION IN InAs/GaAS (110) HETEROEPITAXY. Luis A. Zepeda-Ruiz, Dimitrios Maroudas, and W. Henry Weinberg, Dept. of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA.

Outstanding questions in semiconductor heteroepitaxy involve mechanisms of strain relaxation, such as the formation of misfit dislocations at the interface between a semiconductor substrate and a semiconductor epitaxial film. In this presentation the mechanical behavior of the epitaxial film is addressed, as well as the structural stability of the coherently strained film/substrate system as a function of the thickness of the grown film. The InAs/GaAs (110) system is chosen as a prototypical one because of the significant lattice mismatch, of about 7%. Our theoretical approach, however, is applicable to any case of semiconductor heteroepitaxial growth characterized by a two-dimensional or layer-by-layer growth mode. This approach combines continuum elasticity and dislocation theory with atomic-scale structural relaxation simulations within a valence force field interatomic description. Our analysis of structural stability is based on comparison of the energies of the heteroepitaxial system under different states of strain. The analysis predicts that the fully coherent biaxially strained system becomes unstable for film thicknesses greater than three monolayers. At such film thicknesses, a partly relaxed uniaxially strained structure characterized by a semi-coherent interface with a regular array of edge misfit dislocations becomes energetically favorable. This structure also becomes unstable at higher coverages (greater than ten monolayers), for which a fully relaxed structure characterized by a semi-coherent interface with a network of perpendicularly intersecting edge and 60 misfit dislocations becomes energetically favorable. Our theoretical results for the strain in the coherently strained system, the structural stability, and the stable interface structure as a function of the film thickness are in excellent agreement with recent experimental measurements in InAs/GaAs (110) by RHEED and plain-view TEM. In addition, our analysis elucidates the role of the substrate or buffer layer thickness in strain relaxation during heteroepitaxy.

4:45 PM NN5.10 
STRESS RELAXATION AND MISFIT DISLOCATION NUCLEATION IN HETEROEPITAXIAL FILM GROWTH: A MOLECULAR DYNAMICS SIMULATION STUDY. Liang Dong, Jurgen Schnitker, Richard W. Smith, and David J. Srolovitz, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI.

The growth and relaxation of heteroepitaxial films is simulated using molecular dynamics in two dimensions with Lennard-Jones potentials. The goal of this study is to identify the mechanisms for misfit dislocation nucleation and stress relaxation in heteroepitaxial films with surface morphologies that develop during film growth. Pseudomorphic film growth is observed up to a critical thickness. In some cases, the formation of voids within the film relaxes some of the stress. At the critical thickness, dislocations nucleate and relax most of the misfit. The critical thickness increases with decreasing lattice mismatch and depends on the sign of the misfit. The critical thickness of compressively strained films is smaller than that for films with the same magnitude of misfit, but in tension. The mechanism of dislocation nucleation is different in tension and compression and, in all cases, is associated with the roughness of the film surface. In the compressive misfit case, dislocations nucleate by squeezing-out an atom at the base of surface depressions. In the tensile misfit case, however, the nucleation of misfit dislocations involves the concerted motion of a relatively large number of atoms, leading to insertion of an extra lattice (plane) row into an already continuous film. These results show that the critical thickness depends intimately on the film morphology which, in turn, is determined as an integral part of the film growth process.

Chairs: Esteban P. Busso, Robert C. Cammarata, 
Michael Nastasi and Warren C. Oliver 
Tuesday Evening, December 2, 1997 
8:00 P.M. 
America Ballroom (W)

INDENTATION OF A HARD FILM ON A SOFT SUBSTRATE: VISUALIZATION OF THE DISPLACEMENT FIELD. Ting Tsui*, Joost J. Vlassak, and W. D. Nix; Department of Materials Science and Engineering, Stanford University, Stanford, CA; *Materials Technology development, Advanced Micro Devices, Sunnyvale, CA.

We have developed a new technique for visualizing displacement fields of indentations in thin films. In this technique, a composite thin film that consists of alternating layers of two different materials is indented with a sharp indenter. One of the materials serves as a marker for visualizing the plastic flow induced by the indentation. Focused Ion Beam (FIB) milling is used to cross-section the indentation, revealing the plastically deformed layers. This technique can be used to investigate the plastic displacement field near the indentation. The technique is applied to a tungsten film on top of a multilayered film consisting of alternating layers of aluminum and titanium nitride. The titanium nitride layers are much thinner than the aluminum layers and serve the function of marker. Indentations are made to various depths of the tungsten film. Plastic deformation in the aluminum / titanium nitride film surrounding the indentation is easily revealed. Nanoindentation experiments allow one to detect the effect of the compliant aluminum on the indentation modulus of the tungsten film.

DYNAMIC HARDNESS OF THIN FILMS AND ITS THICKNESS DEPENDENCE. Mikio Iwasa, Koji Tanaka, Osaka National Research Institute, Dept of Materials Physics, Osaka, JAPAN; John A. Barnard, Richard C. Bradt, The University of Alabama, Dept of Metallurgical and Materials Engineering, Tuscaloosa, AL.

The hardness is the important factor to control wear or abrasion resistance of surfaces. The indentation methods are usually used to measure hardness. For the case of thin films, the hardness is complexly dependent on film thickness, indentation depth and also hardness of substrate. The dynamic hardness, which is derived from the relation between indentation load and depth, is a effective method to analyze the hardness properties of thin films. Thin films of Indium Tin Oxide (ITO), Silica and Chromium metal were deposited on the soda-lime-silica glass substrates with the thickness ranging from 50 to 2000 nano-meter. Their dynamic hardness were measured with the indentation depth from 50 to 200 nano-meter. The hardness of ITO film increased and that of silica decreased with increasing film thickness. The hardness of chromium was almost constant with thickness. Their dependence on film thickness and inden- tation depth will be discussed in more details.

INDENTATION CREEP ANALYSIS OF AMORPHOUS NITROGEN-CONTAINING CARBON FILMS. Daisuke Tanaka, Shigeo Ohshio, Hidetoshi Saitoh, Nagaoka Univ Tech, Dept. Chemistry, Nagaoka, Niigata, JAPAN.

Indentation creep behavior has been investigated for amorphous nitrogen-containing carbon films with various nitrogen contents using a nano-indentation apparatus. This technique measured indentation depth under a constant load. The time-dependent indentation depth of samples can be characterized by a stress()-strain rate() relationship. In general, stress is proportional to the m exponent of strain rate. That is, =Km where m is the strain rate sensitivity exponent and K is equivalent to viscosity. The m value is found to be in a range between 0.005 and 0.015. This value increased with increasing the nitrogen content, suggesting that the nanostructure of amorphous nitrogen-containing carbon films changed with nitrogen content.

INDENTATION HARDNESS AND WEAR OF ELECTROCHEMICALLY GROWN TANTALUM OXIDE. A.W. Mulivor, A.B. Mann, P.C. Searson & T.P. Weihs, Materials Science and Engineering Dept., The Johns Hopkins University, Baltimore, MD.

The surface hardness of many metals can be enhanced by the growth of an oxide film. The presence of an oxide can also lead to a dramatic improvement in the metal's wear behavior. For several metallic systems, including tantalum. barrier (non-porous) oxide layers up to 200 nm thick can be grown quickly and efficiently using electrochemical techniques. The thickness of the oxide layer has been found to be linearly proportional to the voltage applied during the electrochemical growth process. Micro and Nano indentation testing of the ex-situ Ta/Ta2O5 system has clearly shown that the indentation hardness increases with oxide thickness, even for indentation depths much greater than the oxide thickness. AFM imaging of indentations and analysis of nanoindentation curves suggests this increased hardness is due to the oxide impeding the pile-up of material at the edges of the indentations which changes the geometry of the contact. Tribometer testing of various oxide thicknesses has shown that the oxide plays a similar role in preventing wear, specifically by preventing the transfer of material between the two contacting bodies and spreading the load over a greater area. Thus, the electrochemical growth of Ta2O5 provides a convenient method for providing protective hard coatings.

NANOINDENTATION AND SCRATCH HARDNESS PROPERTIES OF CARBON NITRIDE THIN FILMS. Ashok V. Kulkarni, Hysitron Inc., Minneapolis, MN; Zhenghong Qian, Jack H. Judy, Department of Electrical Engineering, University of Minnesota, Minneapolis, MN.

In this paper we present results of the nanomechanical properties of amorphous C:N films deposited on Si(100). Amorphous C:N films of various thickness were deposited by Facing Target Sputtering (FTS) technique. The elemental composition and chemical bond characteristics of the C:N films have been determined by X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). The C:N films exhibited high sp3/sp2 bond ratio as evidenced by AES. The surface morphology was studied by Atomic Force microscopy. The film replicated the topography of the substrate with r.m.s roughness less than 1.0 nm. Nanoindentation experiments were performed using Hysitron Triboscope in the load range of 10-500 uN, using a 90 degree 3-sided pyramidal diamond tip. Hardness and Youngís modulus of elasticity for these films were determined using the load-displacement curves. These studies revealed the hardness of C:N films to be 23-30 GPa. Hardness was also measured as a function of N/C ratio. The residual stress of the film in each case was determined. Scratch test were performed on these coatings and critical load at which the film debonds the surface was used to determine the adhesion and yield strength of the film. Nanowear measurement suggest that C:N film have excellent resistance as compared to Si(100). The improved performance of the C:N film is attributed to the high hardness and high yield strength of the coating.


Curves of material elasto-plastic deformations are usually obtained in uniaxial tensile or compression tests. It is impossible to test in such a way thin coatings, brittle materials, small-size samples,. In the present paper, a new method to obtain elasto-plastic deformation curves of materials during depth-sensing indentation tests is proposed. The method implies that an indenter is loaded twice. By the first loading and unloading, (for example, up to 30 mN), the initial indent is formed. Then without shifting, the indenter is loaded for the second time. At the load below 30 mN, an elastic deformation of the initial indent takes place. At higher loads, a plastic deformation of initial indent begins. Thus, a reloading curve contains portions of both elastic and plastic deformations. Using the reloading curve, it is possible to define the dependence of average contact pressure under the indenter on a relative indenter displacement (a relative indenter displacement is (h-ho)/ho, where ho is the initial i ndent depth and h is the current indent depth). The tests performed on Nano Indenter II nanohardness tester by Berkovich indenter show that such a curve depend not on the initial indent depth but on mechanical properties of the material only. In the reloading experiments, the elasto-plasic deformation curves of amorphous silicon and germanium thin coatings, brittle materials, and individual phases in multiphase materials have been obtained.

ADVANCED BULGE TEST SYSTEM. Henning Keiner, Mehrdad N G Nejhad, Univ of Hawaii at Manoa, Dept of Mechanical Engineering, Honolulu, HI; Fred J von Preissig, Hong Zeng, Eun Sok Kim, Univ of Hawaii at Manoa, Dept of Electrical Engineering, Honolulu, HI.

In the thin-film characterization method known as the bulge test, the measured deflection of a membrane as a function of applied differential pressure is used to determine the membrane's residual stress and elastic stiffness. We present a new bulge test system incorporating several innovations that produce probably the best speed, accuracy, and flexibility of any existing system. A primary innovation is the use of a laser-Doppler displacement meter (LDDM) with focused beam to measure deflection. The output is continuous, with unlimited range and sub-0.1-micron resolution. The LDDM measurements do not restrict sample size or displacement rate, and high reflectivity is not required. Another key innovation is the use of an actuator/piston/air-cylinder mechanism to produce smooth and rapid changes in pressure. With the pressure transducer the limiting factor, differential pressures from -40 kPa to +33 kPa can be produced and measured accurately at rates of up to 10 kPa/sec. A computerized environment utilizing commercial software makes pressure control and data acquisition simple, quick, and flexible. A data analysis program, written in a MATLAB environment, allows data to be manipulated, fitted, displayed, and filed easily. The entire required process for a typical test, including mounting, aligning, programming, measuring, and analyzing, takes one person about 20 minutes with this system, as compared with hours for less automated ones. Repeated tests (including remounting) performed on a low-stress silicon nitride membrane show that repeatability is extremely high, with a measured residual stress of 6.5 +- 0.4 MPa (std. dev.) and a Young's modulus (with Poisson's ratio = 0.3 assumed) of 135.0 GPa +- 0.1 GPa. Accuracy must also be very high, provided that sample geometry measurements are. We believe this system will be very useful for characterizing many kinds of thin-film materials.

STRESS-STRAIN CURVES BY TENSILE TESTING OF THIN METALLIC FILMS ON THIN POLYIMIDE FOILS: Al, AlCu, CuNi(Mn). F. Macionczyk, W. Brueckner, and G. Reiss, Institute of Solid State and Materials Research Dresden, Dresden, GERMANY.

For better understanding the mechanical properties of thin films it is helpful to use the same experimental methods as for bulk material, like tensile tests, thereby being able to directly compare the results. However, tensile tests of free standing metallic thin films are often difficult to perform for reasons of preparation, handling, and stresses in the films. By leaving the metallic film on an elastic substrate tensile tests were performed in a new, rather simple, and precise manner, using a commercial tensile testing machine. Stress-strain curves were determined by separating the force working on the substrate from that working on the film-substrate compound. Those measurements were done at room temperature for Al, AlCu(0.5 wt ) and Cu0.57Ni0.42Mn0.01 thin (200-2000 nm) films prepared by magnetron sputtering on 8 m and 13 m thick polyimide (Kapton) foils. The film microstructure was characterized by scanning and transmission electron microscopy and x ray diffraction. The tensile strength of the (fine grained) films was found to be up to one order of magnitude higher than for the corresponding (coarse grained) bulk material. Al and AlCu films showed little, CuNi(Mn) films showed no plastically behavior. Crack formation started between 0.3 and 2 elongation depending on the material, the thermal history and the grain size.

ELASTIC MODULUS MEASUREMENT OF THIN FILM USING A DYNAMIC METHOD. Youngman Kim, Chonnam National Univ., Dept. of Metallurgical Engineering, Kwangju, KOREA.

A two layer composite model was developed using a beam vibration theory and the model was applied for measuring the Young's modulus of thin film using a dynamic method (the sonic resonance method). The metal coated Si wafer composites were produced by RF magnetron sputtering and used to test the developed model. The measured film moduli using the combination of the developed two layer composite model and the sonic resonance method were checked with those using the static method utilizing the bending of cantilever layer composite beam. The values of film moduli measure in both methods were in good agreements.

THE USE OF X-RAY TOPOGRAPHY TO MAP MECHANICAL, THERMOMECHANICAL AND WIRE BOND STRAIN FIELDS WITHIN PACKAGED INTEGRATED CIRCUITS. Patrick J. McNally & John W. Curley, Microelectronics Research Laboratory, School of EE, Dublin City University, Dublin, IRELAND; T. Tuomi & R. RantamAki, Optoelectronics Research Laboratory, Helsinki University of Technology, Espoo, FINLAND; A.N. Danilewsky & J. Meinhardt, Kristallographisches Institut, Universitaet Freiburg, GERMANY.

A packaged integrated circuit (IC) is a complex composite of different materials, each possessing a distinct thermal expansion coefficient. Thermal processing of these can lead to generation of thermomechanical strains. If the strain fields are not properly controlled, IC failure may ensue via e.g. crack propagation, moisture leakage or corrosion. Additionally, the bonding of wires to die contact pads can produce additional strain fields, and improper adhesion (monitored via a change in these strain fields) can result in a worst case scenario of de-bonding of the leads. Current monitoring techniques include the combination of theoretical strain field modelling together with techniques such as scanning acoustic microscopy (resolution dependant on penetration depths), SEM (destructive) or the use of strain gauges (cannot observe entire package strain fields) [1-3]. Synchrotron X-Ray Topography (SXRT) has been used to visualise the strain fields within the Si semiconducting substrate in a packaged IC. A 2764 8Kx8 EEPROM was analysed utilising the Grazing Incidence Diffraction (GID) geometry. Only the cover of the package was removed chemically, the remainder of the IC being unperturbed. Varying the angle of incidence from 15 to 0.6 allows the user to visualise strain fields at varying depth in the silicon, e.g. from 170m to <1m for the ICs examined in this study. As a result, the strain fields due to the adhesive die bonding, the wire bonding, the circuit metallisation and the dielectric protective coating were all observable in the same experiment. This open up possibilities of the use of SXRT for back-end-of-line analysis.

MODELING OF STRESS EVOLUTION DURING ELECTROMIGRATION IN INTERCONNECTS. J. J. Hoyt, R. J. Gleixner*, M. C. Bartelt, N. C. Bartelt, W. G. Wolfer, Computational Materials Science Dept., Sandia National Labs, Livermore, CA; * Department of Materials Science and Engineering, Stanford University, Stanford, CA.

A rate equation for the evolution of the stress during electromigration in thin line interconnects is derived utilizing an elastic response function. The stress depends on two terms, the first is a discrete version of the Korhonen model [1] which relates the normal stress at any point on a grain boundary to the second derivative of the normal stress with respect to distance measured along the length of the boundary. The second contribution is a non-local term which relates the stress at any point to the entire stress field in the line. We demonstrate how the elastic response function can be generated with finite element modeling and how the stress evolution equation can be solved numerically using a boundary element technique. We illustrate the numerical procedure on simulated lines with realistic grain structures, including bamboo patterns.

MICROVOID FORMATION BEHAVIOR OF CAPPING Al LINES ON GLASS SUBSTRATES. Satoshi Tsuji, Katsuhiro Tsujimoto, Hiroshi Takatsuji, IBM Japan, Ltd., Kanagawa, JAPAN; Kotaro Kuroda, Hiroyasu Saka, Nagoya Univ., Dept. of Quantum Eng., Nagoya, JAPAN.

Thin-film transistors (TFTs) are now widely used as elements in active-matrix liquid crystal displays. Aluminum (Al) and Al-based materials are of great technological interest due to their low electrical resistivity. Because of the fabrication on large-sized glass substrates, the stress migration of Al such as hillock or whisker formation is one of the major concern affecting the yield loss in TFT fabrication. An approach to suppress stress migration is to cap Al line with molybdenum (Mo) film. However, the microvoid formation in the encapsulated Al line would become another problem during subsequent thermal processes. Despite its technological importance, little is know about the microvoid formation behavior of Al line fabricated on the glass substrate. Here we describe the investigation of microvoid formation and its behavior in capping Al lines using a vacuum furnace. We observed the microvoid formation (400 nm - 800 nm, starfish-shape). In addition, these microvoids have been restored in another thermal cycling. This result suggests that even under capped conditions Al diffusion behaves like a reversible change. Cross-sectional transmission electron microscopy (TEM) studies support this behavior. It revealed incomplete structure of Al. It also revealed that Al diffusion paths would be along the Al grain boundary plane. We also demonstrate the novel TEM sample preparation method for these microvoids.

DAMAGE INTEGRAL MODELLING AND TESTING FOR FATIGUE LIFE OF FLIP CHIP SOLDER JOINTS. P. Su, S. Rzepka, T.M. Korhonen, S.J. Hong, M.A. Korhonen, and C.-Y. Li, Department of Materials Science and Engineering, Cornell University, Ithaca, NY.

Thermal fatigue of flip chip joints between the chip and organic or ceramic substrates is a serious reliability concern. Differences in the temperature and/or in the coefficients of thermal expansion of the chip and substrate lead to significant stresses which lead to fatigue damage and eventual failure in interconnects. Conventionally, the solder interconnect lives have been estimated by Coffin-Manson type relations. However, these largely empirical approaches become often inadequate when comparing thermal histories that a widely different, as they are in the cases of accelerated thermal cycling and power cycling, for example. In this study, we use a damage integral model, where the fatigue damage rate is calculated based on the specified momentary loading and solder material constitutive equations, and the damage is then integrated over the entire loading history. To verify the model predictions we have devised a simple test vehicle where all joints experience similar stress-strain history. We shall compare the model predictions againsts the observed results, both for thermal and isothermal, cyclic loading cases.

THEORETICAL ANALYSIS OF ELECTROMIGRATION-INDUCED FAILURE OF METALLIC THIN FILMS. Dimitrios Maroudas and Matthew N. Enmark, Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA.

Electromigration-induced failure of metallic thin films used for device interconnections in integrated circuits is among the most serious materials reliability problems in microelectronics. One of the most common failure mechanisms is the propagation and growth of microvoids in the metallic films under electromigration conditions. In this presentation, the mechanisms of electromigration-induced void morphological evolution are examined in detail based on theoretical nonlinear analysis. In our analytical theory, void morphological evolution is governed solely by surface mass transport. The two dominant transport mechanisms are curvature-driven surface diffusion and electromigration-induced drift. Special emphasis is placed on the electric field distribution around the void surface and the resulting current crowding as a function of the void morphology. Specifically, it is shown using potential theory that the component of the electric field tangent to the void surface, which drives the electromigration fluxes, becomes singular in wedge-shaped voids; such void shapes evolve frequently due to anisotropies in the surface atomic mobility. Furthermore, if the void takes the form of a slit-like crack, the singularity resembles the well known singularity of the elastic stress field around the crack tip under applied mechanical loading. In addition, the analysis focuses on the morphological stability of the void surface as determined by the competition between the two dominant surface transport mechanisms. For wedge-shaped voids, linearized stability theory determines the morphological stability of the planar wedge surfaces, while asymptotic bifurcation theory predicts the morphologies that evolve due to the surface instabilities. The validity of our analytical theory is demonstrated through fully self-consistent numerical simulations of void morphological evolution based on finite-element and boundary-element methods. In addition, our theoretical predictions are in agreement with recent experimental data from accelerated electromigration tests in aluminum thin films.

NUCLEATION OF VOIDS IN THIN-FILM INTERCONNECTS THROUGH CRYSTALLOGRAPHIC SLIP. Yu-Lin Shen, University of New Mexico, Dept of Mechanical Engineering, Albuquerque, NM.

Metal interconnects in modern integrated circuits suffer from the voiding damage induced by thermal mismatch straining. Experiments have shown that voids tend to form at the side-wall interface between the aluminum line and the surrounding dielectric, and the pre-existing interfacial debond areas resulting from contamination such as plasma etch residue serve as sites for void formation. While void growth due to the atom diffusive processes have been studied extensively, void nucleation is probably still the least understood aspect of stress-voiding. In this work a two-dimensional void nucleation model based on dislocation slip along crystallographic systems is proposed. Plastic deformation initiates at the edge of local debond, where a concentration of the resolved shear stress on the slip system occurs. A pattern of dislocation motion, which is fully compatible with the overall deformation field, is constructed. The appearance of slip steps at the debond site facilitates void opening, with the net effect being that the metal atoms are displaced, through crystallographic slip, to both sides of the void along the line direction. Although it is difficult to validate the model by direct experimental observations, the nucleation mechanism is supported by noting the fact that, within the time scale normally experienced for void nucleation, the diffusive transport contribution is negligible and dislocation glide is the dominant mechanism in thin-film aluminum. More importantly, numerical modeling using the finite element method shows that the proposed slip morphology is in accord with the evolving plastic yielding and shearing mode resolved on the slip system during cooling from the processing temperature.

MICROTRIBOLOGY OF IRON-OXIDE SURFACE FILMS. Samuel Campbell1, Jian-Mei Pan2, Suzi Steinberg1, Jacob Israelachvili1, 1Department of Chemical Engineering, University of California, Santa Barbara, CA; 2Rockwell Corporation, Thousand Oaks, CA.

Thin films of iron-oxide have been deposited on molecularly smooth sheets of mica using Plasma Enhanced Chemical Vapor Deposition. The chemical composition of the films has been characterized using ESCA and Auger Electron Spectroscopy. Additional characterization has been performed using ellipsometry, alternating gradient field magnetometry, and atomic force microscopy. These novel iron-oxide surfaces were utilized in the surface forces apparatus to measure both normal (adhesion) forces and lateral or shear forces between them. The forces are measured under conditions including inert dry atmosphere, in the presence of volatile organic vapor, in the presence of confined (to a few molecular diameters) organic liquid, in water saturated vapor, and with bulk water confined between the surfaces. Results presented include the adhesion energies of the surfaces under various conditions, and the friction for the surfaces both in contact and separated by a few molecular layers of various liquids. The surfaces are found to be adhesive under all conditions. The friction varies from smooth to stick-slip sliding and possesses a coefficient of friction comparable with macroscopic values ( = 0.2-0.3). These studies provide molecular level results for a surface of engineering importance.


Abstract not available.

8:30 AM *NN7.1 
ELECTROCHEMICAL CONTROL OF MECHANICAL AND TRIBOLOGICAL PROPERTIES IN METALLIC SYSTEMS. A.B. Mann, P.C. Searson, and T.P. Weihs, Materials Science and Engineering Department, The Johns Hopkins University, Baltimore, MD.

In metallic systems the presence of thin oxide films and adsorbate layers, as well as variations in surface forces, can cause dramatic changes in the mechanical response of the surface. A similar, related, variation in tribological properties has also been observed. Though the importance of these surface effects is well known and widely documented, the exact physical and chemical mechanisms that are operating remain poorly understood. Point probe techniques (nanoindentation and APM) can be used to examine both mechanical and tribological properties on the same length scale as the surface films, but achieving a high level of control over the surface chemistry and forces is difficult in a normal laboratory environment. To overcome these difficulties we have used nanoindentation and tribometer testing to examine the mechanical and tribological properties of metals while using electrochemical methods to control the surface chemistry. Our results have shown that surface oxides play a number of different roles in the deformation of metals. These include: impeding the generation of defects at the point of contact; preventing defects generated at the point of contact from propagating into the bulk metal; modifying the stress field under a contact. Additionally, it appears that surface forces and adsorbate layers play an important role in determining the geometry of a contact and the ease of defect generation at the surface. These defects have been found lo significantly impact the observed mechanical and tribological properties of metallic surfaces.

9:00 AM NN7.2 
MICROSTRUCTURE-TRIBOLOGY RELATIONSHIPS IN WO3 THIN FILMS. Oliver D. Greenwood, Robert J. Lad, Univ. of Maine, LASST, Orono, ME; Peter J. Blau, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN.

We have investigated the friction and wear properties of well characterized WO3 thin films which are potentially important tribological coatings for high temperature oxidizing environments. 500 nm thick WO3 films were grown by electron cyclotron resonance (ECR) plasma-assisted e-beam evaporation of WO3, and analyzed in-situ using RHEED and XPS. Stoichiometric WO3 films grown on R-sapphire exhibited amorphous, polycrystalline, or epitaxial microstructure depending on the substrate temperature in the range 50C to 600C. Pin-on-disk tests were performed on films having these different microstructures using sapphire and stainless steel ball bearing sliders, a 1 N load, and a sliding speed of 0.1 m/s. Friction microprobe linear single pass tests for the same slider-film combinations were also carried out (0.1 N load, 10 /s sliding speed) to study intial sliding behavior. The mean pin-on-disk friction coefficient for sapphire sliders on amorphous WO3 films ( 0.6) was twice that of epitaxial WO3 films ( 0.3). In addition, the friction coefficients were higher for steel sliders than for sapphire sliders on all the films. Microprobe friction coefficients, which emphasize the initial stages of sliding contact, were comparable on amorphous and epitaxial films ( 0.2) and higher on polycrystalline WO3 films ( 0.3 to 0.5). Optical micrographs and profilometry of wear tracks indicate that greater film wear occurs for the steel/WO3 system than for sapphire/WO3. In all cases, the lowest wear rate occurs for epitaxial films. AFM analyses of wear tracks help to elucidate crystallographic features involved during the wear process.

9:15 AM NN7.3 
MECHANICAL CHARACTERIZATION OF ULTRA-THIN, HARD-DISK OVERCOATS USING A RECIPROCATING WEAR TEST AND DEPTH-SENSING INDENTATION. J. L. Hay*, R. L. White**, B. N. Lucas*, W. C. Oliver*; * Nano Instruments, Inc., 1001 Larson Drive, Oak Ridge, TN; ** IBM Corporation, San Jose, CA.

Tribological behavior of hard-disk overcoats with varying hydrogen content was investigated using a reciprocating wear test. Two series of diamond-like carbon (DLC) coatings were sputtered under nominally identical conditions, but to different film thicknesses of 200‰ and 1100‰. The wear test consisted of moving the sample back and forth repeatedly under a Berkovich diamond, while subject to a constant load. Tribological performance was assessed by measuring the change in surface profile as a result of the wear test. This performance is compared with the hardness and elastic modulus, as determined by depth-sensing indentation. Between each wear test, indents were made on a fused-silica standard to check the indenter tip for wear. This technique offers an alternative for characterizing ultra-thin films, when indentation alone yields measurements that are significantly affected by the substrate.

9:30 AM NN7.4 
MICROSTRUCTURE AND WEAR RESISTANCE OF DOPED DIAMOND-LIKE CARBON FILMS PREPARED BY PULSED LASER DEPOSITION. Q. Wei, A.K. Sharma, S. Oktyabrsky, K. Jagannadham, J. Sankara and J. Narayan, Dept of Materials Science and Engineering, NC State Univeristy, Raleigh, NC; aDept of Mechanical Engineering, NC AT State University, Greensboro, NC.

We have investigated the microstructure and tribological properties(wear resistance) of diamond-like carbon(DLC), DLC doped with Cu and DLC doped with Ti deposited by a sequential pulsed laser ablation of two targets onto Si(100) substrates. The composition of these films was determined by Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and detailed analysis of the electron diffraction intensity of the films showed typical features of DLC with a structure dictated by sp3 bonded carbon. Wear resistance measurements made on the samples by means of the crater-grinding method showed that DLC + 2.75 Ti has the highest wear resistance, while that of the undoped DLC has the lowest among the samples. The improvement of wear bahavior of the doped films was attributed to the reduction of internal compressive stress due to the presence of more compliant atoms, as indicated by the G-peak position shift to smaller Raman shift. The XPS studies showed the evidence for the formation of Ti-C bonding in the Ti doped films. Thus we expect metal-doped DLC coatings can have better tribological properties than the undoped, highly stressed DLC coatings.

10:15 AM NN7.5 
ULTRA LOW FRICTION IN BOUNDARY LUBRICATION BY SUBNANOMETER NORMAL VIBRATION. Carlos Drummond, Manfred Heuberger and Jacob Israelachvili, Chemical Engineering and Materials Depts, University of California, Santa Barbara, CA.

A Surface Forces Apparatus, modified for friction measurements, was used to investigate boundary lubrication of a double layer of (tridecafluoro-1,1,2,2-tetrahydro-octyl)-1-Triclorosilane self-assembled from vapor on molecularly smooth mica surfaces. A fast subnanometer vertical vibration of the surfaces was introduced by applying a high frequency sinusoidal voltage to a piezo crystal attached to one of the surfaces. The distance between the surfaces, the area and shape of the contact region and the friction force were monitored using multiple beam interferometry, while the surfaces were slid laterally with respect to each other at constant rate. We observed a very reach friction force versus piezo frequency spectrum. Considerable reduction in the friction force between the surfaces (ultra low friction) was achieved by driving the system far from equilibrium in order to reduce the energy dissipation. It was also observed that previous vibrations have a lasting influence on the friction force measured between the surfaces (``memory effects''). Effect of pressure and temperature as well as amplitude and frequency of the vibration on friction forces were also investigated. The implications of these results for the fundamental nature and origin of friction forces in boundary lubricated systems are discussed, and methods for reducing friction in practical engineering situations suggested.

10:30 AM NN7.6 
MICROSTRUCTURAL EFFECTS ON THE FRICTION AND WEAR OF ZRO2. Scott C. Moulzolf, Robert J. Lad, LASST, Univ of Maine, Orono, ME; Peter J. Blau, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN.

Stoichiometric ZrO2 films of varying microstructure characterized by in situ RHEED were deposited on r-cut sapphire via electron beam evaporation of Zr in the presence of an electron cyclotron resonance (ECR) oxygen plasma. Polycrystalline films with nanometer-sized grains are formed at deposition temperatures below 300C. Above 475C, highly textured ZrO2 films are produced which contain coexisting phases of cubic and monoclinic ZrO2. A pin-on-disk tribometer was used to measure friction coefficients and wear rates using sapphire and stainless steel pins in air at 25C. The polycrystalline films exhibited low friction coefficients (0.2 - 0.3) but appear to be unsuitable for unlubricated tribological applications due to high wear rates (> 2.5 mm3 N-1 m-1). The wear rates on the highly textured films were extremely small; the wear tracks were too shallow to be measured by profilometry. The textured films also had dramatically different pin-on-disk friction characteristics depending on the relative percentage of cubic vs. monoclinic phase. For the films containing primarily the monoclinic phase, the pin-on-disk traces exhibited abrupt transitions to high friction coefficients (> 0.8) due to the generation of third body particulates. However, highly textured films containing primarily the cubic phase had low, relatively constant friction coefficients ( 0.3) and exhibited anisotropic microcracking during a scratch test with a conical diamond at a 10 N load. The initial friction characteristics for the same pin/film combinations were studied using a friction microprobe, a specialized micro-contact tribometer developed at Oak Ridge National Laboratory. Friction microprobe traces from the films reveal qualitatively different initial friction coefficients and features related to stick-slip events.

10:45 AM NN7.7 
MECHANICAL PROPERTIES OF FERRO-ELECTRIC COMPOSITE THIN FILMS. C.G. Fountzoulas and S. Sengupta, U.S. Army Research Laboratory, Materials Division, APG, MD. Thin films of novel barium strontium titanate (BSTO) composites, deposited by the pulsed laser deposition (PLD) technique exhibit excellent electronic properties including tunable dielectric constants, and low electronic loss1. The mechanical properties of the films (internal stresses and adhesion) are important factors affecting the mechanical integrity and reliability of a device made of these thin films. This paper presents the initial results of a study of these mechanical properties. Films of 1 m nominal thickness were deposited on single crystals of sapphire, magnesium oxide, and lanthanum aluminate. The substrate temperature was varied from 30C to 700C. FT-Raman spectroscopy and x-ray diffraction analysis were utilized to characterize the thin films. Preliminary results on wear properties of BSTO thin films showed that the friction coefficient, , (at 0.1 N) of the coatings, measured by a pin-on-disk tribometer ranged from 0.14 to 0.18 whereas the friction coefficient of the bare sapphire substrate, measured under the same load and number of cycles, ranged from 0.20 to 0.60. X-ray analysis showed that the films were amorphous or partially crystalline up to 500C, while at 700C were fully crystalline. The film microhardness increased with increasing substrate temperature. While the cohesion failure load of the films remained fairly constant, the adhesion failure load increased with increasing substrate temperature. The mechanical and physical properties and an initial growth-microstructure model of these films will be discussed as a function of the substrate temperature. The performance of the coplanar devices fabricated from these thin films will also be presented.

11:00 AM NN7.8 
SLIDING WEAR BEHAVIOUR OF CERAMIC COATINGS ON TOOLS. M.van der Meer, M.D.Tran, W.P.Vellinga, J.H.Dautzenberg, Eindhoven University of Technology, Dept of Mechanical Engineering, Materials Technology, Eindhoven, THE NETHERLANDS.

In many instances in manufacturing technology, for example in deep drawing, coatings are required to withstand sliding loads. Knowledge of the resistance to sliding wear, and understanding of its relation to other properties of the coatings such as its hardness, elastic modulus, thickness and internal stress is therefore important. Here we report on the sliding wear properties of hard TiN and TiC coatings on tool steel. Thin layers of TiN and TiC were deposited on tool steel, using unbalanced magnetron reactive sputtering. Coatings with different thickness, microstructure and residual stress were deposited, and characterised prior to testing. Characterisation included determination of the strain to failure under different stress states, such as in bulging and pure bending. One of the specific aspects of these applications is the continuous contact of coated toolpieces with new undeformed workpiece material. In laboratory experiments this situation is usually not obtained, and results can be influenced, for example by abrasive particles formed during the test. A testing device developed in our laboratory us does not have such problems. In the apparatus long strips (up to 1(km)) of eg. stainless steel are drawn over coated surfaces. It can therefore be used to simulate continuous sliding, at loads up to 500(N) and speeds up to 100(mm/s), until failure of the coating occurs. Trends in time to failure as well as surface deformation for several test conditions were related to the mechanical properties and residual stress of the coatings.

11:15 AM NN7.9 
COMPARATIVE INVESTIGATION OF MULTILAYER TiB2-C COATINGS FOR LOW-FRICTION APPLICATIONS. R. Gilmore, W. Gissler, M.A. Baker and P.N. Gibson, Institute for Advanced Materials, Joint Research Centre, Commission of the European Union, Ispra, VA.

The feasibility of producing relatively hard and low-friction TiB2 based coatings by the incorporation of carbon has been investigated for both multilayer and co-sputtered coatings. Multilayer coatings were prepared by sequential sputtering, and homogeneous coatings by co sputtering from TiB2 and C targets. Coatings were characterized by glancing angle X-ray diffractometry and photoelectron spectroscopy. Friction coefficients were measured using a pin-on-disk tribometer, and hardness was determined by nanoindentation. Co-sputtered coatings were found to consist of two phases: diamond-like carbon (DLC) and a hexagonal TiB2-type structure into which carbon is incorporated. In the case of the co-sputtered coatings, overall C concentrations as high as 50 at.% were required to produce a friction-reducing effect. This can be explained by the preferential incorporation of the carbon atoms into the TiB2 lattice, the lubricating DLC phase only starting to form once saturation is reached. In the case of multilayers, it was found that there was an increase in the overall carbon content required to obtain a friction reducing effect from about 10 to 50 at.% as TiB2 sublayer thickness was decreased Mom 100 to 1 nm. This was attributed to an increase in the relative proportion of carbon bonded with TiB2 in Me interface regions. It was shown that coatings with a hardness of the order of 20 GPa and displaying friction coefficients as low as0.1 for temperatures not exceeding 200C could be obtained by choosing a suitable composition.

11:30 AM NN7.10 
IMPROVED TRIBOLOGICAL BEHAVIOR OF BORON IMPLANTED Ti-6Al-4V. N.P. Baker, K.C. Walter, and M. Nastasi, Los Alamos National Laboratory, Los Alamos, NM.

The surface of Ti-6Al-4V alloy has been modified using beamline implantation of boron at energies between 30 and 75 keV and fluences between 2 and 8x1017 at/cm2. By tailoring the combinations of ion energy and dose, difrerent B depth profiles in the Ti64 surface have been obtained. A direct comparison between B-implanted (through a beamline process) and N implanted (through a plasma source ion implantation process) can be made. It has been previously shown that while B-implanted Ti64 has a 30% higher surface hardness than N-implanted Ti64, the N-implantation reduced the wear coefficient of Ti64 by 25-120x, while B-implantation reduces the wear coefficient by 6.5x or less. The different depth profiles resulting from the different ion implantation technologies is believed to result in the different tribological behavior. In the case of N-implantation, the surface layer is converted to TiN. In the case of B-implantation, the surface is not completely converted to TiB2, because of insufficicnt B concentrations. In this work, the boron depth profile has been tailored to increase the B concentration nearer the surface. The boron depth profile, retained boron dose, hardness, elastic modulus, and tribological properties of the modified surfaces will be reported. The results nvill be directy compared to those for N-implanted Ti64 surfaces so the two surface modification techniques can be ranked.

11:45 AM NN7.11 
MECHANICAL AND TRIBOLOGICAL PROPERTIES OF a-GeCx FILMS DEPOSITED BY DC-MAGNETRON SPUTTERING. Luiz G. Jacobsohn, Denise C. Reigada, Fernando L. Freire Jr., PUC-Rio, Depto de Fisica, Rio de Janeiro, BRAZIL; Rodrigo Prioli, Susana I. Zanette, Anibal O. Caride, CBPF, Rio de Janeiro, BRAZIL; Carlos M. Lepienski, Fabiana C. Nascimento, UFPR, Depto de Fisica, Curitiba, BRAZIL.

Amorphous carbon-germanium (a-GeCx) films were grown by dc-magnetron sputtering with a 1.7 W/cm2 argon plasma. Different plasma pressures were employed: 0.17, 0.36 and 0.7 Pa. The Si <100> substrates were mounted onto a grounded water-cooled anode, while the 99.99 pure graphite cathode target was kept at -510 V. Ultra-pure Ge strips were put over the target in order to promote the incorporation of Ge in the films. The structure and the chemical bonds were investigated by Raman and Infrared spectroscopies, while the composition and the atomic density was determined by Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis. The films present a Ge/C ratio ranging from 0 to 2. The carbon and germanium atoms tend to segregate into distinct nanometer domains in a amorphous network, as revealed by Raman spectroscopy. The tribological investigation was carried out by means of atomic force microscopy techniques, providing the friction coefficient and the surface roughness. The internal stress was determined by measuring the film-induced bending of the substrates, and the hardness was measured by using a nanoindenter with a Berkovich pyramidal tip and 50, 150 and 450 N loads. While the incorporation of Ge leads to softer (8 GPa, while a-C film hardness is 17 GPa) and more relaxed films (1.0 to 0.1 GPa for Ge/C = 0 to 1.9, respectively), nor the Ge content, nor the plasma pressure seem to have any effect on the friction coefficient. On the other hand, there is a tendency to increase the roughness within plasma pressure. The higher deposition rate and lower internal stress, when compared with a-C films, together with its hardness and low friction coefficient suggest that a-GeCx films are potential candidates for protective coatings.

Chair: Donald S. Stone 
Wednesday Afternoon, December 3, 1997 
Essex South (W)

1:30 PM *NN8.1 
MEASUREMENT OF IN-SITU INTERFACE STRENGTH AT ELEVATED TEMPERATURES. Vijay Gupta, Alexander Pronin, University of California, Los Angeles, Dept of Mechanical and Aerospace Engineering, CA.

A previously developed strategy of measuring the tensile strength of thin film interfaces by using laser-generated stress pulses under ambient conditions is extended for carrying out similar in situ measurements at elevated temperatures (up to 1200 C). The extension of the technique to these temperature regimes was non-trivial as it required development of environmental cells for housing the sample and its communication with both the stress pulse generating YAG laser and the probing interferometeric Argon laser beams. Additionally, to generate sufficiently high stress levels, a new multilayer coating system for the absorption of the laser energy was designed. The strategy at elevated temperature is used to measure the tensile strength of Nb/sapphire interfaces at 1000 C. The results show an average strength of 400 MPa which is much lower than that of 1.9 GPa measured under ambient condition. A plausible mechanism for this strength degradation is proposed by making contact with high resolution TEM micrographs of the Nb/sapphire interfaces which show distribution of stable 250 to 300 nm-dia hemispherical interfacial voids. This research is now focused on thermal barrier coatings to see if their life prediction strategy would require input of bond strength data at the temperature of performance, especially if their bond strength turns out to be lower than that measured under ambient conditions.

2:00 PM NN8.2 
ADHESION MEASUREMENTS OF DUCTILE COPPER THIN-FILMS BY NANOINDENTATION. Michael D. Kriese, William W. Gerberich, Dept of Chemical Engineering and Materials Science, Univ of Minnesota, Minneapolis, MN; Neville R. Moody, Sandia National Laboratories, Livermore, CA.

Nanoindentation has been used to measure the adhesion of 100 - 1000 nm thick sputtered copper films on Si/SiO2 substrates. This method allows rapid testing of as-processed films with a minimum of specimen preparation, and is based on linear elastic fracture mechanics analyses. The indenter tip produces compressive stresses which combine with residual stresses to initiate a crack. As this grows, the buckling condition is reached which then reduces elastic strain energy. However, it has been shown to be problematic for ductile films, for which failure occurs primarily by large-scale yielding rather than by interfacial crack propagation. A modification utilizing overlayers of a stiff, highly stressed film of sputtered tungsten is used to promote interfacial failure. Results are presented on the use of this method with copper processed by annealing the as-sputtered films. This demonstrates the applicability of the method for adhesion testing of ductile films.

2:15 PM NN8.3 
ADHESION AND TIME-DEPENDENT DELAMINATION OF INTERFACES IN MULTILAYER THIN-FILM STRUCTURES. Michael Lane and Reiner H. Dauskardt, Stanford University, Stanford, CA; Qing Ma and Harry Fujimoto, Intel, Santa Clara, CA.

Adhesion and progressive debonding in multilayer thin film structures is of major concern for the reliability and lifetimes of microelectronic devices and interconnects. These fractures are driven by residual stresses, thermo-mechanical cycling and mechanical loading. In addition, subcritical debonding enhanced by temperature (creep), moisture or corrosive species (stress corrosion), or even cyclic loading induced crack extension similar to classic metal fatigue may occur. While quantitative adhesion data for device level interconnect structures is limited, subcritical debonding behavior at interfaces is almost non-existent. In this presentation, we present unique quantitative adhesion and time-dependent delamination data for debonding of interfaces in such thin film structures. Behavior is rationalized in terms of the salient loading and environmental interactions. Implications for the reliability of multilayer structures is discussed.

2:30 PM NN8.4 
EFFECT OF ION IMPLANTATION ON INTERFACIAL BONDING BETWEEN A THIN FILM AND A SUBSTRATE STUDIED BY PICOSECOND ULTRASONICS. G.Tas, J.J. Loomis and H.J. Maris, Department of Physics, Brown University, Providence, RI; A.A. Bailes III and L.E. Seiberling, Department of Physics, University of Florida,

Gainesville, FL. We present a picosecond ultrasonics method for studying the effects of ion irradiation on interfacial bonding between a thin film and a substrate. The measurements were performed on metal films (Au) deposited on a silicon substrate, and subsequently irradiated with 2.5 MeV helium ions with doses between and ions/cm2. Acoustic vibrations are excited in the metal film when a picosecond light pulse is absorbed. The rate at which these vibrations damp out via sound transmission into the substrate was found to be significantly greater for the ion implanted regions compared to the ``as deposited'' regions on the samples. The increase in the damping rate induced by ion irradiation indicates an improved interfacial bonding.

2:45 PM NN8.5 
THE MECHANICAL RESPONSE OF THIN FILM SUBSTATES SUBJECT TO ULTRASONIC JOINING. J.E. Krzanowski, Mechanical Engineering Dept, University of New Hampshire, Durham, NH; and E. Razon and A. F. Hmiel, Kulicke and Soffa Industries, Willow Grove, PA.

Ultrasonic joining processes are commonly used to make interconnections between IC chips and package lead frames. The ability to connect to the chip requires bonding to a thin film on a hard substrate. The uItrasonic wire bonding process applies cyclic stresses to the thin film, and the film can respond by elastical/plastic deformation, shear flow or microcracking. In this study, we have conducted experiments to understand the types of mechanical response in thin film substrates and how thin film composition and structure effects this response. Atomic force microscopy (AFM) has been used to examine the morphology of the thin film surface before and after bonding. Al wires were bonded to thin films of Cu, Cr and Ag, after which the aluminum wire was removed by etching. AFM examination of the bonded surface revealed a wide range of morphological changes, from almost no effect to a complete restructuring of the surface. Cross sections were also examined using TEM and SEM, and included aluminum thin-film substrates subjected to gold ball bonding as well as Al wedge bonding. In the case of the gold wire ball bonded to an Al film, a high density of microcracks and voids were generated in the film, suggesting limited plastic deformation. This result is compared to previous studies of bonding to bulk Al foil substrates, in which extensive plastic deformation and recystallization were observed.

Chair: James E. Krzanowski 
Wednesday Afternoon, December 3, 1997 
Essex South (W)

3:30 PM *NN9.1 
INDENTATION CREEP EXPERIMENTS FOR EXAMINING LOW-TEMPERATURE DEFORMATION MECHANISMS IN THIN FILMS AND NANOLAYER COMPOSITES. D.S. Stone, Materials Science and Engineering; K.B. Yoder, and M.F. Tambwe, Materials Science Program, University of Wisconsin-Madison.

Thin films and nanostructured materials with grain sizes in the range of 1-100 nm are of interest to the mechanical properties community in large part because of the opportunities they provide to test the limits of existing mechanical theory. A number of important questions remain that concern the relative importance of bulk-like dislocation mechanisms in comparison to grain boundary and interface mechanisms. To address such questions as these, we note that thermal activation almost always plays at least a minor role in allowing deformation to take place. This fact has motivated us to use nanoindentation as a tool for characterizing the derivatives of hardness with respect to temperature and deformation rate. The systematic variations in these derivatives observed in correlation with changes in microstructure can be used to distinguish between mechanisms and to identify how the mechanisms in nanoscale materials differ from those in bulk, large-grained materials. In this talk we describe our efforts to develop and apply indentation creep techniques to study deformation mechanisms in thin films, nanolayer composites, and surface layers. A number of examples are provided.

4:00 PM NN9.2 
TENSILE CREEP OF FREE-STANDING POLYCRYSTALLINE TEXTURED THIN FILMS. Haibo Huang and Frans Spaepen, Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA.

A creep apparatus was built to study the tensile properties of free-standing thin films at temperatures up to . Strain rates as low as can be measured. Copper films 3 thick were tested between room temperature and . At constant load between 80 MPa to 300 MPa, a steady state creep is reached after about ten hours. The creep rate can be described empirically as , where T is the temperature in Kelvin, is the stress in MPa, and the coefficients are , B0=0.031 K-1, B1=-0.0036 MPa-1 and MPa-1. B2 increases with temperature, contrary to the decrease predicted for singly activated events. Possible microscopic mechanism will be discussed.

4:15 PM NN9.3 
FRACTURE TOUGHNESS OF DLC COATINGS. M. Nastasi, P. Kodali, K.C. Walter and J.D. Embury, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM; and R. Raj, Dept of Mechanical Engineering, University of Colorado, Boulder, CO.

Diamond-like-carbon (DLC) coatings have been shown to have excellent tribological properties. Recent wear test, at very high loads, have shown DLC to possess very low wear rates. In an attempt to determine the underlying reason for the low wear rates, we have evaluated the fracture toughness of the DLC coatings on single crystal Si substrates by performing microindentation fracture test. A basic bond breaking model, assuming elastic fracture in DLC, predicts a fracture toughness of approximately 1.3 MPa(m)1/2. An analysis of the indentation cracks formed during microindentation in the DLC/Si fracture system, subtracting substrate effects, gives an effective DLC fracture toughness of 9.9 MPa(m)1/2. The influence of intrinsic stress and plastic deformation on the effective DLC fracture toughness will be discusses as will the analytical approach that was used to determine the fracture toughness of a coating on a substrate.

4:30 PM NN9.4 
DEFORMATION AND FRACTURE MECHANISM DURING THERMAL CYCLING IN Al ALLOY THIN FILMS ON Si. J. Koike, S. Utsunomiya, Y. Shimoyama, and K. Maruyama, Dept. of Materials Science, Graduate Shool of Engineering, Tohoku University, Sendai, JAPAN.

Thermal cycling was performed in Al-1mol%Si thin films deposited on Si wafers. After a given number of cycling between room temperature and 723 K, residual stress was measured at room temperature. Residual stress was found to increase with increasing the cycling number up to the 4th cycle, followed by a continuous decrease by further cycling. The initial increase was found to be related to the increase of lattice dislocations and their tangling. The following decrease was caused by crack formation along grain boundaries or by film delamination in some cases. Stress relaxation experiments were also performed during isothermal annealing at various temperatures. Analysis of the relaxation curves revealed three temperature regions that were characterized with different relaxation times. Each temperature region was found to correspond to various deformation mechanisms, using a calculated deformation mechanism map for an Al thin film. Based on the obtained knowledge of the deformation mechanisms, the origin of the microstructure changes and the fatigure failure during thermal cycling is discussed.

4:45 PM NN9.5 
TEM STUDY OF YIELDING IN POLYCRYSTALLINE GOLD THIN FILM. Kwame Owusu-Boahen and Alexander H. King, Dept. of Materials Science & Engineering, State University of New York, Stony Brook, NY.

We have studied the microstructure of thin gold films which were grown on <100> rock salt, using transmission electron microscopy (TEM). The samples were annealed on the rock salt substrate or supported on a gold TEM specimen grid. Films annealed on rock salt had relatively larger grains than films annealed on TEM grids. All of the annealed films have a <111> preferred orientation. Several cracks are observed in the film annealed on rock salt. Plastic yielding of the film was identified by the presence of dislocations, and is caused by tensile stress derived from grain growth. In spite of the uniform texture of the films, the observed dislocations were concentrated only on some individual grains, while their surrounding grains remained relatively dislocation-free. Yielded grains showed no difference of orientation that would lead to higher Schmid factors, so other predictors of yielding must be considered in these thin-film specimens.

Chairs: Esteban P. Busso, Robert C. Cammarata, 
Michael Nastasi and Warren C. Oliver 
Wednesday Evening, December 3, 1997 
8:00 P.M. 
America Ballroom (W)

RESIDUAL STRESS AND STRENGTH OF SOLID OXIDE ELECTROLYTE BI-LAYERS. A. SelÁuk and A. Atkinson, Department of Materials, Imperial College of Science Technology and Medicine, Prince Consort Road, London, UNITED KINGDOM.

Ceramic solid electrolytes consisting of a gadolinia-doped ceria (CGO) substrate with a thin layer of yttria-stabilised zirconia (YSZ) are being considered for application in solid oxide fuel cells. Specimens of this type were produced by co-firing ceramic/polymer tapes at 1500C to give a final structure in which the thickness of the CGO substrate was 180 microns and the YSZ layer 5 microns. The residual stresses at room temperature were estimated from the curvature of the laminates and the elastic constants of the individual layers. These stresses can be interpreted quantitatively by considering the thermal expansion difference between the materials and creep relaxation at temperatures above 1200C. The fracture of the laminates was studied in biaxial flexure using ring-on-ring loading at 25 and 800C. The maximum tensile stress in the laminates at failure was calculated using finite element modelling taking into account both the residual and the applied stresses. The failure initiated close to the CGO/YSZ interface at stresses much higher than would be required to break the individual free-standing materials. Possible reasons for this are discussed.

RESIDUAL STRESS MEASUREMENTS IN DIAMOND FILMS. Arthur J. McGinnis, K. Jagannadham, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC; and T.R. Watkins, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN.

Residual stress analysis is carried out in different types of diamond coatings deposited on molybdenum substrate by hot filament chemical vapor deposition. The diamond coatings include discontinuous diamond, continuous single layer diamond, continuous multilayer diamond with aluminum nitride embedding layer, continuous thick diamond, and thick layer of diamond that peeled. Stresses are analyzed by X-ray method and Raman spectroscopy. The development of stresses with deposition of multilayer structure is investigated in order to understand the contribution to adhesion of diamond to the molybdenum substrate. The results from both these techniques are compared and conclusions are made on the mechanisms of adhesion of diamond to molybdenum.

DEVELOPMENT OF MECHANICAL STRESS IN CuNi(Mn) FILMS DURING TEMPERATURE RAMPING: RELATED MECHANISMS. W. Brueckner, W. Pitschke, St. Baunack, J. Thomas, and G. Reiss, Institute of Solid State and Materials Research, Dresden, GERMANY.

CuNi(Mn) is a material for resistive and thermoelectrical applications. The mechanical stress and the mechanical properties are important for the stability and reliability. The objective of this work was to study the development of the mechanical stress and the related processes during the first and repeated temperature ramps with Cu0.53Ni0.46Mn0.01 films sputtered at RT onto silicon wafers. Beside the biaxial stress by wafer curvature measurement the evaluation of both the electrical resistivity and the film structure (grain size, texture, oxide layer formation, if necessary) was investigated. Concerning athermal stress contributions between 120 C and 300 C, excess-vacancy annihilation, grain-boundary relaxation, and shrinkage of grain-boundary voids are discussed. Grain growth in the fcc solid solution does not occur in this temperature range. Grain-boundary relaxation is found to be the controlling mechanism. With Ar atmosphere, Ni-oxide layer formation was observed by reaction with the remaining oxygen above 300 C. The grain boundary diffusion of Ni out of the film results in a strong tensile stress component. With both a N2 / 5 vol H2 gas mixture and vacuum, stress relaxation was found above 300 C. Flow by grain-boundary diffusion (Coble creep) is discussed to be the responsible mechanism. This mechanism is also considered for understanding the athermal stress-temperature curves during repeated thermal ramping.

INTRINSIC STRESS CHARACTERIZATION OF SPUTTERED PZT FILMS. Mark J. Mescher, Michael L. Reed, T.E. Schlesinger, Integrated Microsystems Laboratory, Carnegie Mellon University, Pittsburgh, PA.

Control of intrinsic stress in piezoelectric thin films is important in micromachined devices. These films are often released from the substrate to achieve some desired mechanical actuation, thus uncontrolled intrinsic stress can cause undesirable buckling, out-of-plane bending, and/or stiction of a released micromechanical structure. In this work we show that stress in sputter deposited lead zirconate titanate (PZT) films can be controlled by variation of both deposition and annealing temperatures. These films were deposited via reactive rf magnetron sputtering using a Pb1.25Zr0.48O3 composite target and O2 as a reactive gas in an Ar ambient. Variation of stress as a function of deposition and annealing temperature was characterized. The deposited film composition was determined from x-ray flourescence measurements. There is a strong correlation between film stress, composition, and crystallographic orientation.Stress was determined from the deflection of released SiO2/Pt cantilever beams. We show that films with a wide range of intrinsic stress can be deposited which still exhibit good piezoelectric properties, making the fabrication of reliable thin film piezoelectric actuators possible.

ANISOTROPIC STRESSES AND STRUCTURES IN SPUTTERED Mo FILMS. Z.B. Zhao, S.M. Yalisove and J. C. Bilello, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI.

It has been observed that both in-plane texture and out-of-plane texture can develop for Mo films deposited under the conditions of rotating substrate and oblique deposition. Curvature measurements by laser scanning technique [LST] and double crystal diffraction topography [DCDT] indicate that for some of these Mo films, the stress induced curvature is not uniform in different orientations. This suggests the occurrence of in-plane stress anisotropy. In this study, we have developed the new relationships between the principal stresses and the principal curvatures by considering the following two cases: (a) anisotropically stressed film bonded on an elastically isotropic substrate and, (b) anisotropically stressed films bonded on a non-elastically isotropic substrate Si(100) wafer. This approach allows the determination of the in-plane principal stresses and stress distribution in the films. A number of Mo films (100 nm 3000 nm) deposited (100) Si wafers mounted on a rotating plate are characterized by DCDT and LST, and the anisotropic stresses are determined by the above approach. Their microstructures and surface morphologies have been also observed by scanning electron microscopy. The results indicate that the stress anisotropy only occurs for the Mo films with the development of in-plane texture and elongated grain structures. Moreover, the degree of stress anisotropy varies as a function of the film thickness, and reaches the maximum for the 1.2 m thick film which shows the most elongated grain structure. Such a thickness dependent variation of stress anisotropy is consistent with the evolving characteristics of Mo films as their thickness increases.

SURFACE AND INTERFACE STRAINS STUDIED BY X-RAY DIFFRACTION. Koichi Akimoto, Takashi Emoto and Ayahiko Ichimiya, Nagoya Univ, Dept of Quantum Engineering, Nagoya, JAPAN.

We have developed a technique of X-ray diffraction in order to measure strain fields near semiconductor surface and interface. The diffraction geometry is using the extremely asymmetric Bragg-case bulk reflection of a small incident angle to the surface and a large angle exiting from the surface. The incident angle of the X-rays is set near critical angle of total reflection by tuning X-ray energy of synchrotron radiation at the Photon Factory, Japan. For thermally grown-silicon oxide/Si(100) interface, the X- ray intensity of the silicon substrate 311 reflection has been measured. From comparison of the full width at half maxima (FWHM) of X-ray rocking curves of various thickness of silicon oxides, it has been revealed that silicon substrate lattice is highly strained in the thin ( less than about 5 nm) silicon oxide/silicon system. In order to know the original silicon surface strain, we have also performed the same kind measurements in the ultra-high vacuum chamber. A clean Si(111) 7x7 surface gives sharper X- ray diffraction peak than that of the native oxide/Si(111) system. From these measurements, it is concluded that the thin silicon oxide film itself gives strong strain fields to the silicon substrates, which may be the reason of the existence of the structural transition layer at the silicon oxide/Si interface.

STRESS MEASUREMENT TECHNIQUES BY X-RAY DIFFRACTION. C. Sarioglu, J.R. Blachere, F.S. Pettit, and G.M. Meier, Department of Materials Science & Engineering, University of Pittsburgh, Pittsburgh, PA.

Stresses play a major role in many applications of thin films deposited on substrates for the electronic industry and in the formation of thermally grown protective oxide scales on high temperature alloys such as Fe-base and Ni-base superalloys. The major sources of stress developed in thin films and oxide scales are growth stresses (developed during film deposition or oxide formation) and thermal stresses (developed due to differences in thermal expansion coefficients of the film or oxide scale and substrate). The quantitative measurement of these stresses is important for the estimation of the adherence performance of films. The measurement of stresses in films or scales by x-ray diffraction (XRD) is discussed. Four XRD techniques using the ``Sin2'' method have been used and compared. One of the goals of this comparison was to develop a technique which can measure stresses in oxide scales at elevated temperature. Excellent agreement is demonstrated between the results of the four techniques in the measurement of residual stresses at room temperature in -Al2O3 thermally grown on FeCrAl alloys and Cr2O3 thermally grown on a NiCr alloy. The residual stresses measured for the alumina scale were confirmed by XRD and laser ruby fluorescence measurements performed by two different laboratories on the same samples. The new multiplane technique established by this comparison is used to measure the growth stresses of oxide scales (-Al2O3) and to measure the stresses in-situ at high temperatures. In-situ measurements at 1000 and 1100C indicated significant growth stresses in an alumina scale formed on FeCrAl alloys during isothermal oxidation.

STRUCTURE DETERMINATION OF B4C AND SiC THIN FILMS VIA SYNCHROTRON HIGH-RESOLUTION DIFFRACTION. J. Hershberger(a), F. Kustas(b), Z.U. Rek(c), S.M. Yalisove(a), and J.C. Bilello(a); (a)Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, (b)Technology Assessment and Transfer, Annapolis, MD; (c)Stanford Synchrotron Radiation Laboratory, Menlo Park, CA.

Thin films of B4C and SiC deposited by magnetron sputtering as components of multilayers have the potential to provide significant property improvements over current wear resistant coating technology. B4C and SiC have previously been found to be amorphous and possibly nanocrystalline under the deposition conditions used. This study reports results of synchrotron x-ray scattering experiments providing information on the degree of crystallinity, strain, average density, and coordination number in 200 nm films of these compounds on Si substrates. Radial distribution functions from B4C and SiC thin films were obtained and used to model the structure. Strain results are compared with stress measurements from double crystal x-ray diffraction topography as a means for establishing a strain-free standard state.

CONTROL OF GRAIN-BOUNDARY INDUCED INTRINSIC STRESSES DURING CHEMICAL VAPOR DEPOSITION. Sumit Nukhawan, Janet Rankin, and Brian W. Sheldon, Division of Engineering, Brown University, Providence, RI; Barbara L. Walden, Physics Department, Trinity College, Hartford, CT.

Intrinsic stress generation during the chemical vapor deposition (CVD) of diamond was investigated both experimentally and theoretically This work considers films which consist of randomly oriented grains, as well as films where local heteroeptixial nucleation produces an oriented microstructure. Diamond is a good model system for studying intrinsic stress generation during CVD for several reasons: (l) observed intrinsic stresses are relatively large ( > 2 GPa), (2) growth mechanisms in CVD diamond have been studied in detail, (3) control of intrinsic stresses is technologically important. Previously, we have reduced intrinsic stresses by more than 70% with a processing sequence which includes an intermediate annealing step prior to complete film coalescence (annealing after longer growth times has little or no effect).* This anneal does not reduce stress directly, rather it subtley alters the film structure in a way which reduces subsequent stress generation. Current research focuses on understanding the origin of these intrinsic stresses, and on explaining how the intermediate annealing step leads to stress reduction These investigations utilize Raman spectroscopy, wafer curvature measurements, X-ray diffraction, high resolution TEM with concurrent EELS, and finite element analysis. The experiments demonstrate that intrinsic tensile stresses are closely associated with grain boundary formation. Based on these results, we have developed a model of grain boundary formation and stress evolution. The observed stress reductions can also be assessed with this model.


The influence of ion bombardment on the mechanical stress and microstructure of sputtered silicon nitride (SiNx) films has been systematically investigated. Applied substrate bias voltage was used to control the bombardment energy in a radio frequency (rf) reactive magnetron sputtering system. The resultant films were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), Rutherford backscattering spectrometry (RBS), stress and chemical etch rate measurements. As the bias voltage was increased, the internal stress in SiNx films became increasingly compressive and reached a value of about 1.8 * 1010 dyne/cm2 at higher bias voltages. These correlated well with the transition of the film microstructure from a porous microcolumnar structure containing large void to the more densely packed one. The obtained results can be explained in terms of atomic peening by energetic particles, leading to densification of the microstructure. It was also found that the amount of argon incorporated in the film is increased with increasing the bias voltage, whereas the oxygen content is decreased. The lowest etch rate in buffered HF solution, approximately 1.2 /s, was observed with the application of a substrate bias of - 50 V.

THERMAL STRESSES IN TBGA SUBSTRATES. V. Balasubramanian, F. Ye, W. O. Soboyejo, The Ohio State University, Columbus, OH; A. O. Ibidunni, Sheldahl Inc, Longmont, CO.

The reliability of tape ball grid array (TBGA) packages could be affected by the thermal expansion mismatch of the various materials used in the packaging. The stresses arising from Joule heating effects under operating conditions may initiate fatigue failures. Stress analysis has been performed on a two-metal layer TBGA polyimide substrate with multilayer metallization. The vias that connect the two metal layers were the focus of this analysis. The stress distributions in these vias were estimated using the finite element method. The principal tensile stresses/strains, and the maximum shear stresses and strains were determined for vias subjected to temperature cycling between -65 and 150 C. The effects of via shape and metallization thickness on the stresses were determined; and the regions of high stress concentrations, in the via, were identified by careful analysis of the finite element results. The implications of the results on the fatigue behavior of vias in TBGA substrates will be discussed.

RESIDUAL STRESS MEASUREMENTS OF PERFLUOROCYCLOBUTANE AROMATIC ETHER POLYMER THIN-FILM ON SILICON. C. K. Chiang, A. S. DeReggi and G. T Davis, Polymer Division, NIST, Gaithersburg, MD; D. A. Babb, The Dow Chemical Company, Freeport, TX.

Residual stress of thin-films of perfluorocyclobutane aromatic ether polymers* on silicon were studied as a function of temperature. The polymer thin-film was prepared by spin-coating precursor solution on silicon wafer and curing at high temperature. The curvature of the silicon wafer was measured by an optical method during the complete processing cycle of the polymer. The observed stress developed as the system cooled from the processing temperature to room temperature. The Stoney equation was used to deduce the residual stress between the thin film and the silicon wafer. Mechanical properties of the polymer thin-film deduced from these measurements are discussed.

INFLUENCE OF TARGET STRUCTURE ON FILM STRESS IN WTI SPUTTERING. Chi-Fung Lo, Hong Wang and Paul Gilman, Materials Research Corporation, Orangeburg, NY.

In this report, the effect of sputtering, target microstructure on the deposited film stress was investigated. By controlling the metallurgical process, two types of W-l0wt%Ti target constitutes, namely multiple phase and single-phase WTi, were prepared. The former one was composed of W, W-rich (Ti,W) and Ti-rich (Ti,W) phases, the latter one was with W-rich (Ti,W) phase only. The stress of the films deposited on 5 inch silicon oxide wafers from the single-phase target tends to be more compressive than that from the multiple-phase target sputterings. By increasing water temperature, the compressive stress was linearly decreased. In addition the level of film stress was also affected by the film thickness and other sputtering parameters. To understand the cause of difference in film stress between the multiple-phase and single phase sputterings. an evaluation on the film structure using a field emission scanning electron microscope (FE-SEM) and mechanical testing using a micro-indentation instrument were performed. Finer grains with denser laminar structure were observed on the films deposited from the single-phase target. This kind of film also showed a higher Young's modulus. It is envisioned that the more uniform solid solution between the W and Ti in the films obtained by sputtering the single-phase targets forms a stiffer film when deposited on the silicon oxide wafers resulting in a higher compressive stress.

MEASUREMENT OF THIN FILM STRESSES BY NEUTRON REFLECTIVITY. Xiao-Lin Zhou, Massachusetts Institute of Technology, Dept of Nuclear Engineering, Cambridge, MA.

A method is developed to measure the stress in the surface layer of a thin film by using the neutron reflectivity technique. At grazing incidence such as at an incident angle of less than a fraction of a degree, the neutron wave penetrates less than a couple of thousand angstroms into the film. A measurement of the reflectivity from a film under investigation would produce a curve showing the reflectivity as a function of the wave vector transfer Q. Such a reflectivity curve would contain a total reflection plateau at small Q and a critical edge at which the reflectivity drops from unity to less than unity. The location of the critical edge is directly related to the strain in the film. By measuring the actual location of the critical edge, the strain in the surface film can be determined. This paper presents the principle of this technique and discusses the instrumentaional requirements for obtaining useful data.

STRESS EVOLUTION AND NOTCH FORMATION DURING POLYSILICON GATE ELECTRODE ETCHING. Michael A. Vyvoda, David B. Graves, University of California at Berkeley, Department of Chemical Engineering, Berkeley, CA.

It is well known that during the over-etch portion of a transistor gate electrode etch process, anomalous notches may form in which the inside of the outermost gate develops a deep undercut near the electrode / gate oxide interface [1]. It has been proposed by several authors [2,3] that this notching is due to deflected positive ion trajectories caused by local potential gradients within the microfeature. However, it has also been suggested [4] that in addition to surface charge-related ion transport effects, stress-related phenomena may be important during notch formation. We have calculated the elastostatic stress field within an over-etched polysilicon gate electrode due to thermal contraction from deposition temperatures down to room temperature using a boundary element method. We find that once a small undercut near the electrode / gate oxide interface is initiated by ions deflected by local electric fields, large gradients in strain energy density develop in the region near the interface, precisely where notching is experimentally observed to occur. This suggests that once a notch is initiated, stress-related phenomena may play a role in notch propagation. We have developed a self-consistent stress field / feature profile evolution code to show that under the influence of these strain energy density fields, notches with shapes similar to those observed experimentally appear to form and propagate.

INTERFACIAL STRAIN RELIEFS IN EPITAXIAL THIN FILM YBa2Cu3O SUPERCONDUCTORS GROWN ON SrTiO3 BUFFERED MgO SUBSTRATES. Xingtian Cui, Q.Y. Chen, and W.K. Chu, Texas Center for Superconductivity and Department of Physics, University of Houston, Houston, TX; Yongxiang Guo, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, NM.

High quality YBa2Cu30 epitaxial thin films grown on MgO substrate with a strain-relieved SrTiO3 (STO) buffer layer have been investigated. The in-situ growth of STO buffer layer of several hundred angstroms along with the YBCO films was carried out by laser ablation. TEM studies showed that the STO layers were strain-relieved by an array of periodic edge dislocations. The YBCO films on STO buffer, as in those grown directly on a STO substrate, evolved from a strained layer to a largely dislocation free area. To further study the degree of perfection of these films, we have studied the ion channeling effects in comparison with the signals from other atoms. We will discuss the growth condition in relation to the interfacial properties obtained from the channeling studies.

CORRELATIONS OF STRESS WITH HYDROGEN MICRO-STRUCTURE IN THIN FILM HYDROGENATED AMORPHOUS SILICON. Daxing Han, Dept of Physics & Astronomy, Univ of North Carolina, Chapel Hill, NC; Tamihiro Gotoh, Motoi Nishio, Shuichi Nonomura, Shoji Nitta, Dept of Electronic and Computer Engineering, Gifu Univ, Gifu, JAPAN; Qi Wang, Eugene Iwaniczko, National Renewable Energy Laboratory, Golden, CO.

Hydrogenated amorphous silicon (a-Si:H) has been used in large area semiconductor devices such as thin film transistors and solar cells that normally deposited on glass substrates. A bending effect [1] is used for studying stress of thin film a-Si:H deposited on thin quartz by glow-discharge (GD) and hot-wire (HW) chemical vapor deposition (CVD) techniques. The hydrogen concentration in the a-Si:H films varies from 1 to 30 at. %, and the hydrogen micro-structure is different in HW and GD films.[2] The film/substrate system acts as bimorph and therefore the sample bends due to the difference in thermal expansion coefficient. The bending angle of the bimorph sample is detected by a laser beam deflection.

8:30 AM *NN11.1 
DEFECT GENERATION AND RESIDUAL STRESSES IN TiN FILMS. J-E. Sundgren, L. Hultman, H. Ljungcrantz and L. Karlsson, Thin Film Division, Department of Physics, Linköping University, Linköping, SWEDEN.

TiN films grown from the vapor phase in the presence of energetic particle bombardment typically exhibit compressive residual stresses s that can be varied over a large range and s-values even exceeding -10 GPa are reported. Films stressed to these levels show high hardness values H and linear relations between H and s have been demonstrated. In this paper, defect and stress generation mechanisms during ion assisted growth of TiN are reviewed and discussed. Nanoindentation results on single crystal (001) and (111) TiN films demonstrate that the intrinsic hardnesses for these two orientations are 20 and 21 GPa, respectively while compressively stressed polycrystalline TiN films can exhibit hardness values up to 35 GPa. Nanoindentation loading curves of single crystal TiN films exhibited clear "pop-in" effects and transmission electron microscopy (TEM) of the indents showed that the deformation occurred through dislocation cascades and not through crack networks. By investigating polycrystalline TiN films grown by both magnetron sputtering and arc evaporation, the variation in defect content and s have been studied as a function of process parameters. The results demonstrates that the residual defect density depends on the competition between ion induced defect generation and annihilation rates. For growth temperatures < 550C, the majority of the defects generated are found to be point defect clusters. The results also show that the defect annihilation depends critically on the detailed structure of the defects and Ar- as well as C-incorporation increases the stability of the generated defects. The results also show a relation between s and H and we demonstrate that the obstruction effect from defects on dislocation motion is the dominating effect that causes H-values of polycrystalline TiN to often be significantly higher than the single crystal values.

9:00 AM NN11.2 
STRESS EFFECTS IN SPUTTERED POLYCRYSTALLINE SPINEL OXIDE FILMS. Rand D. Dannenberg (1,2), Alexander H. King (1), Richard J. Gambino (1) and Alan P. Doctor (2); (1) Department of Materials Science & Engineering, State University of New York, Stony Brook, NY; (2) Servo Corp. of America, Westbury, NY.

Sputtering of oxides under various conditions produces films of varying quality. Delamination from the substrate is a persistent problem, the control of which is a major issue in the development of devices based upon these materials. We have studied a Mn-Ni-Co spinel that has potential for use in infrared detectors compatible with standard CMOS processing. Delamination of this material from its substrate is usually characterized by curling of the film, indicating a gradient of in-plane tensile (or compressive) residual stress, from top to bottom of the film. Post-deposition annealing of delaminated specimens leads to increased curling which indicates an increase of the stress gradient as a function of heat treatment, rather than the expected decrease. We demonstrate that the curling of the film is a function of temperature and time, and derives from the microstructural development of the film. TEM reveals regions of well-separated grains near the film-substrate interface, that densify in a sintering process during annealing. The increasing density of the bottom surface of the film generates the tensile stresses that cause the observed curling.

9:15 AM NN11.3 
MICRO-RAMAN STUDY OF MECHANICAL STRESS IN POLYCRYSTALLINE SILICON BRIDGES. H. Talaat, S. Negm, Faculty of Science, Ain Shams Univ., Cairo, EGYPT; H. Schaffer, F. Adar, Instruments SA Edison, NJ; A.G. Nassipoulos, Institute of Microelectronics, Athens, GREECE.

Micro-Raman spectroscopy is used to study mechanical stress distribution in polycrystalline silicon bridges on cavities michromachined in crystalline Si wafers . These bridges are of relatively large dimensions (550x230 m2) the cavities are as deep as 120m and are fabricated by sacrificial porous-Si layer.The influence of the processing parameters such as the thickness of polycrystalline membrane, the annealing conditions (before or after the removal of porous Si), are studied. The results indicate that the annealing reduces the residual stress by an order of magnitude and that the membrane thickness of 2.5 m has the least residual stress. These results are explained in terms of combined tensile stress acting on the lower surface of the bridge and a compressive stress acting on the upper surface . The tensile stress is much larger and dominants the stress picture over the tunnel.

9:30 AM NN11.4 
TUNING AND MEASURING EPITAXIAL STRESS DURING GROWTH. J.-P. Locquet1, J. Perret2, J. Fompeyrine3 and E. Machler3, 1IBM Research Division, Zurich Research Laboratory, Rueschlikon, SWITZERLAND; 2Institut de Physique, Université de Neuchatel, Neuchatel, SWITERLAND; 3Institute for Inorganic Chemistry, Universitat Bern, Bern, SWITZERLAND.

In the field of high temperature superconductivity there has been no increase in the critical temperature Tc of bulk compounds since 1993. However, since the stress in thin films can drastically change the properties of these materials, a stress induced modulation of Tc should be possible. For instance, in the case of the La2-xSrxCuO4 compound, it has been shown that the critical temperature Tc can be dramatically shifted downwards when the films is under tensile strain and a little upward when the film is under compressive strain. In these materials three components contribute to the final strain: i) the epitaxial mismatch; ii) the thermal mismatch and iii) the oxygen loading during cooling. Using a careful lattice parameter determination based on a RHEED analysis during growth and cooling, each of these components can be analyzed in detail. Finally, under optimized conditions, a large compressive strain together with a drastic increase of the critical temperature could be obtained.

Chair: Clive Hayzelden 
Thursday Morning, December 4, 1997 
Essex South (W)

10:15 AM *NN12.1 
EVALUATION OF RESIDUAL STRESSES IN THIN FILMS BY MEANS OF MICRO-RAMAN SPECTROSCOPY. Kazuyuki Mizuhara, Mecahnical Engineering Laboratory, Tsukuba, JAPAN; Shinichi Takahashi, Nikon Corp., Tsukuba Research Lab., Tsukuba, JAPAN; Jyunichi Kurokawa, Noboru Morita, Yoshitaro Yoshida, Chiba Univ., Chiba, JAPAN.

Evaluation of the residual stresses on surfaces is very important in manufacturing semi conductor devices and micro mechanisms. In most of these applications, a local stress in micro meter scale is crucial. Micro-Raman spectroscopy is capable of analyzing the local stress in micrometer scale, and extensively used for the local stress evaluation. Although the Raman technique inherits a serious problem that is extremely small cross sections for the Raman scattering. Because of this, intense laser illumination is required to obtain a sufficient signal to noise ratio, which is essential for stress analysis, that in turn results in the temperature increase at the point of analysis. Since the spectral peak shift is more sensitive to temperatures than stresses, the increase in temperature shields the effect of stresses, especially for thin films that can easily be heated. In this study the effect of the temperatures on stress evaluation of boron doped silicon in solid and film forms are investigated. Several techniques such as fluid cooling to eliminate the temperature raise and/or simultaneous observation of Stokes and anti Stokes peaks to compensate the temperatures effects are applied. Advantage and disadvantage of each method and ability and limits of these techniques are discussed.

10:45 AM NN12.2 
MEASUREMENT OF NONUNIFORM STRESSES IN SEMICONDUCTOR FILMS BY OPTICAL METHODS. K. Pinardi, S.C. Jain, and H.E. Maes, R. Van Overstraeten, IMEC, Leuven, BELGIUM; M. Willander, Chalmers Univ of Technology, Dept of Physics, Göteborg, SWEDEN; and A. Atkinson, Dept of Materials, Imperial College of Science Technology and Medicine, London, UNITED KINGDOM.

Optical methods (micro-Raman and photoluminescence) have been the techniques of choice for measurement of stresses in semiconductor films [1,2]. The stress distribution in the films becomes nonuniform if the films are laterally small (e.g. stripes, quantum wires, quantum dots) or if there are pin-holes, nanotubes, clusters of dislocations, or cracks in the films. The laser beam used in the optical measurements has a diameter of m. The depth of penetration of the beam varies with the absorption coefficient. The beam samples a volume in which stress variation can be large. In this paper we describe a method to synthesise theoretically the optical spectra in such cases. The stress components in the film are first calculated using the Finite Element method. Using these stress components, the secular equation is solved [2] to obtain the peak positions of the optical spectra for each point (shifted from the unstrained value of the peak depending on the strain at that point) in the volume sampled by the laser beam. Spectra for each of these peaks are calculated and superposed to synthesise the final spectrum. We synthesise the Raman spectra for several stripe films and for GeSi quantum wires grown on Si and compare them with the observed spectra. In all cases we obtain excellent agreement between the synthesised and the observed spectra. We find that considerable modelling is required to determine nonuniform stresses using the shape and peak of the observed optical spectra.

11:00 AM NN12.3 
MEASUREMENT OF RESIDUAL STRESS IN THIN FILMS USING THE OPTICAL MICROPROBE. A. Atkinson1 , D.R. Clarke2, S.C. Jain3, and K. Pinardi3. 1Department of Materials, Imperial College of Science Technology and Medicine, London, UNITED KINGDOM; 2Materials Department, University of California at Santa Barbara, Santa Barbara, CA; 3IMEC, Leuven, BELGIUM.

The laser optical microprobe is a powerful tool for studying the spatial variation of residual stress exploiting the sensitivity of Raman and fluorescence spectra to the local stress. Here, using two different examples, we consider some issues determining the depth and lateral resolution of these techniques and their use in stress mapping. The first example involves a significant variation in stress within the material volume sampled by the optical probe. In such a case the stress distribution must be convoluted with the response function of the probe. We have applied this to interpret Raman micoprobe studies of a microelectronic stripe structure (3 microns wide and 240 nm high) comprising a silicon nitride layer and a polysilicon buffer layer on a silicon substrate[1]. The Raman spectrum calculated from the stress distribution obtained by finite element modelling is in good agreement with the experimentally observed spectrum. The second example involves stress mapping when the stress varies on a scale that is large compared with the sampled volume. The stress sensitivity of chromium ion fluorescence in alumina [2] has been used to map the residual compressive stress in alumina films grown by high temperature thermal oxidation on NiAl single crystals. The factors determining the resolution of the technique in terms of space and stress, and the speed of mapping are discussed. The observed stress variations are correlated with microstructural features generated during the thermal oxidation process and subsequent cooling to room temperature.

11:15 AM NN12.4 
ON THE MEASUREMENT OF RESIDUAL STRESS IN THIN FILMS. Z.B. Zhao, J. Hershberger, S.M. Yalisove and J.C. Bilello, Department of Materials Science & Engineering, University of Michigan, Ann Arbor, MI.

The residual stress in thin films has been determined by double crystal diffraction topography [DCDT] and laser scanning technique [LST]. The usual procedure is to actually measure the strain via lattice or physical curvature analysis and then to calculate the stress using standard mechanical analysis. DCDT and laser scanning techniques measure the different physical quantities. In the former case the curvature of crystal planes is assessed while in the latter situation the curvature of the physical surface of the sample is measured. In principle, these two methods should yield the same results for the residual stress assuming that no strong stress gradient exits near film-substrate interface. However, reports on analysis by both methods on the same samples under identical conditions are not readily available. In this study, a laser scanning device incorporated with a DCDT apparatus has been developed to determine the quantities of interest for a variety of films on Si (100) wafers. The system allows the measurements by the two techniques to be performed on same sample at same time under the exactly identical conditions. For specimens where the residual stresses produce large curvatures (>.05 m-1)of both types (lattice curvature and surface curvature), the results for DCDT and LST agree within the usual experimental error. When small to moderate curvatures are present, the two methods deviate to varying degrees. This deviation is of special significance in determining residual stress in nano-thin films. Nevertheless, DCDT and LST generally yield similar results on differential curvatures, i.e., the stress induced curvature differentials. When proper consideration is taken for the inherent limits of each technique, both DCDT and LST can be used as valid procedure for stress measurement in thin film-substrate systems. The unique features of each curvature measuring technique and other related issues have been also addressed.

11:30 AM NN12.5 
DETERMINATION OF TEMPERATURE DEPENDENT UNSTRESSED LATTICE SPACINGS IN CRYSTALLINE THIN FILMS ON SUBSTRATES. Guido Cornella, Seok-Hee Lee, Oliver Kraft, William D. Nix, John C. Bravman, Stanford Univ, Dept of Materials Science and Engineering, Stanford, CA.

Knowledge of unstressed lattice spacings is essential for the calibration of x-ray stress measurements in crystalline thin films. Unstressed lattice spacings in thin films may differ from tabulated values for bulk materials due to thin film anomalies such as variations in stoichiometry, impurities, and interface effects. A common way to resolve this problem involves calculation of the unstressed lattice parameters using known elastic constants for the film. A limitation of this approach is that Poisson's ratio or the compliances for the chosen material (which are needed for these calculations) may not be available for the film in question at the chosen test temperature. In a novel technique which has been devised by the authors, unstressed lattice spacings for 0.5 m thick (111) textured polycrystalline thin films of aluminum and gold on silicon substrates have been measured directly using the Generalized Focusing Diffractometer (GFD) without any knowledge of the elastic properties of the films. The results were obtained from the intersection of the plots of the interplanar spacing versus sin2 of the angle between the surface normal and the normal of the diffracting planes for different stress states. The interplanar spacing was measured by asymmetric x-ray diffraction of the {422} planes at 2 values of 137.44 (Al) and 135.402 (Au). The different stress states were produced by annealing the thin film/substrate samples at elevated temperatures in air (to produce a tensile stress) and by cooling them in liquid nitrogen (to produce a compressive stress). d spacings and sin2 values corresponding to d were extracted. An extension of this technique also allows the measurement of thermal expansion without removing the film from the substrate and without any knowledge of the elastic properties of the film. Implications of this novel technique for new and poorly characterized thin film materials will be discussed.

11:45 AM NN12.6 
CURVATURE OF RECTANGULAR COATED FOILS WITH DIFFERENT WIDTH-TO-LENGTH RATIOS. M.D. Tran, M. van der Meer, W.P. Vellinga, J.H. Dautzenberg, Eindhoven University of Technology, Dept. of Mechanical Engineering, Materials Technology, Eindhoven, NETHERLANDS.

Deflection methods offer an easy experimental way of determining the mean residual stress in ceramic coatings on metallic substrates. In relating the curvature to the residual stress, equations based on linear elasticity, valid only for small deflections, are widely used. The applicability of the methods may be extended to larger deflections, if geometrically non-linear relations between deflection and stress are used. Moreover, using data for several geometries involving the same coating, the value of the biaxial modulus of the coating may be obtained. In this investigation we have studied the shape of stainless steel foils with thin PVD TiN coatings. Coated rectangular strips with different width-to-length (w/l) ratios, that are easily cut from a single coated foil, were used. The curvature of these strips was measured, and compared to non-linear calculations. In this case the description predicts a spherical shape with an increasing radius of curvature for increasing w/l. Moreover it predicts that the spherical shape becomes unstable above a certain w/l ratio and gives way to ellipsoidal shapes. Both trends were also found experimentally. Finite element modelling is used to check for which shapes the non-linear description is quantitatively correct, and can be used to determine the biaxial modulus of the coating.

Chair: Peter M. Anderson 
Thursday Afternoon, December 4, 1997 
Essex South (W)

1:30 PM NN13.1 
3D DISCRETE DISLOCATION MODELS OF THIN-FILM PLASTICITY. A. Hartmaier2, M. C. Fivel1, G. R. Canova1, and P. Gumbsch2, 1GP2M/ENSPG, Domaine Universitaire BP 46, St. Martin d'Hères, FRANCE; 2MPI für Metallforschung, Stuttgart, GERMANY.

Three-dimensional simulation schemes for discrete dislocation dynamics (DDD) have been used successfully to investigate plasticity of bulk materials /1/. The adaptation of these DDD schemes to a description of thin-film plasticity requires detailed modelling of the interfaces and surfaces of the film. While the geometrical restrictions can easily be dealt with in the simulation scheme, the handling of the image forces needs thorough elaboration. On the other hand, the model cannot be too expensive in terms of computing time in order permit simulations of the evolution of dislocation populations over micro-seconds time scales. One possible method is to calculate the normal stresses that a dislocation distribution exerts on a plane inside an infinite body. Then, the free surface is introduced by compensating this stress distribution by appropriate point loads /2/. This method is extended to a free standing film (two opposing surfaces). A direct application of this is, for example, to model thin TEM samples. The generalisation to a thin film on a substrate is discussed. Another, very general, method to describe plasticity in restricted geometries is the coupling of the DDD and finite element methods (FEM). The results of both models will be compared, and their advantages and shortcomings will be critically discussed.

1:45 PM NN13.2 
DISLOCATION INTERACTIONS IN CU-NI MULTILAYERED THIN FILMS. Sriram Swaminarayan and Arthur F. Voter, Los Alamos National Laboratory, Los Alamos, NM.

A major factor in determining the mechanical properties of nanoscale multilayered thin films is the interaction between the different dislocations within the system. This includes not only the interactions between the dislocations present in the interior of the individual layers, but also the interactions between these interior dislocations and the misfit dislocations located at the interface. We present the results of a series of atomistic simulations of Cu-Ni multilayered thin films investigating these interactions and their effect on the strength of the multilayered thin film.

2:00 PM NN13.3 
MODELLING OF STRESSES GENERATED DURING CONSTRAINED SINTERING OF LAYERED CERAMIC THIN FILM SYSTEMS. L.Chandra, E.P.Busso, R.P.Travis, G.A.Webster, Dept of Mechanical Engineering, Imperial College of Science, Technology and Medicine, London,UNITED KINGDOM.

Sintering of ceramic thin films is a common and cost-effective method for the manufacture of microelectronics circuit packaging and ceramic based fuel cells. Often a thin ceramic layer is screen printed on a rigid substrate and then fired to achieve a desired high porosity. During sintering, significant residual stresses are generated in the film due to the constraint exerted on the film by the substrate which can lead to its bending, cracking or delamination. In some other cases, e.g. during manufacture of microelectronics circuit packaging where multilayered ceramic systems are co-fired, the mismatched densification kinetics of different materials can result in one material layer undergoing rapid densification ahead of the other. Here, although the sintering layers are not constrained by a rigid substrate, the layer with faster sintering kinetics is effectively constrained by the one which has not yet begun to densify. The processing parameters under such complex sintering conditions are normally optimised by a trial and error approach. An analytical plane stress formulation based on an existing constitutive model has been proposed to predict the microstructural and relative density changes and in-plane stresses during constrained sintering of a thin ceramic layer. The model has been calibrated using sintering data for two different ceramic powders on a rigid substrate. The model accurately predicts the relative density and the average grain size. The predicted sample curvature (due to sintering and thermal mismatch stresses) at the end of sintering is compared with the experimentally measured values. There is reasonable agreement between the two. The analytical formulation has been further extended to predict stresses during co-sintering of two layer systems. The study relies on non-linear FE analysis to assess the accuracy of the above plane stress formulation.

2:15 PM NN13.4 
COMPUTER SIMULATION OF STRESS DISTRIBUTION IN AMORPHOUS SiO2 THIN FILMS. Yoshiaki Kogure, Masao Doyama, Teikyo University of Science & Technology, Uenohara, Yamanashi, JAPAN.

A molecular dynamics computer simulation of amorphous SiO2 thin films has been made. A central force potential developed by Tsuneyuki et al. has been used to express the interaction between Si and O atoms. Thickness of the films are 20- 30 . As an initial condition Si and O atoms are arranged in cristbalite structure. The atoms on the lower surface are always fixed to the substrate. Then a particle velocity with random distribution is given to each atom, and the system is quenched to form an amorphous state. The distribution of internal stress is estimated from the atomistic structure through the change of Si-O bond length and the volume of SiO4 tetrahedron. An external stress is applied to the film by expanding and compressing the substrate. Changes of the atmistic configuration and the stress distribution by the external stress are investigated.

Chair: Esteban P. Busso 
Thursday Afternoon, December 4, 1997 
Essex South (W)

3:00 PM *NN14.1 
MECHANICAL PROPERTIES OF MULTILAYERS OF SILVER AND NICKEL. K.O. Schweitz, H. Geisler, J. Chevalier, J. Bøttiger, Institute of Physics and Astronomy, Aarhus University, Aarhus C., DENMARK; R. Feidenans'l, Risø National Laboratory, Roskilde, DENMARK.

By use of magnetron sputtering, <111>-textured Ag/Ni multilayered thin films were produced with bilayer repeat lengths ranging from 1 nm to 50 nm. Bulk and interface stresses were obtained from x-ray diffraction and measurements of substrate curvatures. Both in-plane and out-of-plane expansions were observed in the Ni layers, and the interface stress was found to be tensile, in agreement with the findings of Ruud et al1. The hardness and the elastic modulus were measured by nanoindentations and compared with current theories.

3:30 PM NN14.2 
DISLOCATION-BASED MODELS OF STRESS-STRAIN BEHAVIOR IN MULTILAYERED THIN FILMS. Peter M. Anderson and Eric R. Kreidler, Jr., Dept. of MSE, Ohio State University, Columbus, OH.

This talk will discuss dislocation-based modeling of deformation in multilayered thin films. Two critical events are the propagation of dislocation loops within embedded layers and the initiation of slip in adjoining layers. Features of stress-strain curves will be presented as a function of layer thickness, deformation mode, and mismatch in stress-free lattice parameter, assuming that deformation is controlled by the propagation of loops within embedded layers. Regimes are identified in which deformation is either relatively uniform versus localized. The transition from isolated to co-deformation of layers is expected to occur when the stress is sufficient to initiate dislocation transmission or activation of a source in an adjoining layer. Under such conditions, the stress-strain curve is dramatically changed from the isolated layer regime. The model results are applied to experimental data from multilayered metallic systems.

3:45 PM NN14.3 
ENHANCED MECHANICAL HARDNESS IN EPITAXIAL Mo/NbN AND W/NbN SUPERLATTICES. Anita Madan and Scott A. Barnett, Department of Materials Science and Engineering, Northwestern University, Evanston, IL; Carolina Engström, Henric Ljungcrantz and Lars Hultman, Thin Film Physics Division, Department of Physics, Linköping University, SWEDEN; Marcos Grimsditch, Argonne National Laboratory, IL; Michael Nastasi, Los Alamos National Laboratory, NM.

Mo/NbN and W/NbN superlattices represent a class of non-isostructural superlattices in which a BCC metal is combined with a NaCl-structure nitride. Nanoindentation was used to measure the hardness of epitaxial Mo/NbN and W/NbN superlattices, grown on MgO(001), as a function of superlattice period . Hardness values as high as 32 GPa were observed at superlattice periods down to 1.3 nm for metal fractions of 50, compared to the rule-of mixtures values of 10 GPa. The hardness decreased with increasing , to 14 GPa at = 120 nm. The hardness for 5 nm was 20 - 25 GPa and relatively independent of metal fraction from 15 to 90. Brillouin scattering results showed that there was little variation in the elastic modulus c44 for > 4 nm suggesting that elastic anomalies are not responsible for the observed large hardness enhancements. The decrease in hardness with increasing can be quantitatively explained for > 20 nm by dislocation glide within individual layers. For < 20 nm, available models predict hardness greater than observed. We propose that the maximum observed hardness is due to the metal layers reaching their theoretical yield strength where they deform plastically without dislocation motion. The effects of annealing at temperatures (> 900C) on the stability and hardness of these superlattices will also be presented.

4:00 PM NN14.4 
PREPARATION AND MECHANICAL PROPERTIES OF CU/NI NANOLAYER COMPOSITES. Y.C. Lu, H. Kung, M. Nastasi, N. Baker and T.E. Mitchell, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM.

A systematic study has been conducted on the synthesis, structure and mechanical properties of e-beam evaporated Cu/Ni nanolayers of various compositional wavelengths. TEM investigation of these nanolayer composite thin films shows that each metal layer is single crystal except for the presence of twins, misfit dislocations at Cu/Ni interfaces, and dislocation loops. Microstructures of these Cu/Ni nanolayer composites before and after deformation are compared via TEM to understand the deformation processes in such periodically confined structures. The hardness and elastic modulus of the Cu/Ni nanolayer composites were measured by nanoindentation as a function of compositional wavelength. Results are compared to those measured from polycrystalline Cu/Ni nanolayer composites reported before.

4:15 PM NN14.5 
NANOINDENTATION HARDNESS OF COMPOSITIONALLY MODULATED Ti TiN MULTILAYERED FILMS. Eiji Kusano, Masaru Kitagawa, Hidehito Nanto, Akira Kinbara, Kanazawa Institute of Technology, AMS R&D Center, Matsutou, Ishikawa, JAPAN.

Hardness of multilayered coatings has been studied by many researchers in relation to their mechanical and tribological use. Recently we have prepared compositionally modulated Ti-TiN multilayered thin films by a reactive-gas-flow-rate modulated reactive magnetron sputtering method using a combination of a Ti target and an Ar-N2 gas mixture. By this deposition technique, a film with a compositionally modulated layers but without any interrupted interfaces has been obtained. The detailed study of microhardness of the multilayered films by a nanoindentation method was strongly needed to reveal effects of film layer structures and deposition conditions to film hardness. In this respect we have deposited a several types of Ti-TiN multilayered films and estimated their hardness by a nanoindentation method. The desired compositional modulation was obtained by changing the flow rate of N2 gas accurately using a computer system. The number of layers has been varied up to 80 by changing a flow rate control method. The total thickness of the film ranged between 400nm and 800nm including undercoatings of TiO2(50nm) and Ti(50nm) layers. Substrates used in the experiment were borosilicate glass and not heated during film deposition. The compositional distribution toward to the film depth orientation was estimated by Auger electron spectroscopy. The hardness of the films has been estimated by a nanoindenter as a function of the number of layers. It was found that there existed an optimum number of the layers ranging between 20 and 40 for the maximum hardness. The values of microhardness ranged between 40 and 80GPa. In addition the effects of indenting load on film hardness results are discussed for the multileyered films with different thickness.

4:30 PM NN14.6 
MICROSTRUCTURES AND MECHANICAL PROPERTIES OF SPUTTERED Cu/Cr MULTILAYERS. A. Misra, H. Kung, T.E. Mitchell, T.R. Jervis and M. Nastasi, Los Alamos National Laboratory, Los Alamos, NM.

Cu/Cr multilayers with layer thicknesses ranging from 1 nm lo 200 nm were prepared by sputtering onto {100} Si substrates at room temperature. The microstructures were characterized by transmission electron microscopy and the hardness of the multilayers evaluated by nanoindentation. The films exhibited columnar grain microstructures with nanoscale grain sizes. The interfaces were planar and abrupt with no intermixing as expected from the phase diagram. The multilayers tended to adopt a Kurdjumov-Sachs (KS) orientation relationship: {110} Cr//{111}Cu, <111>Cr//<110>Cu. For layer thickness greater than 25 nm, the hardness of the multilayers appeared to follow a Hall-Petch relationship, whereas for lower thicknesses the hardness of the multilayers was interpreted in terms of an Orowan-type mechanism involving the motion of a single dislocation rather than a pile-up. The possible effects of factors such as density and composition of films as evaluated by ion-backscattering spectroscopy, residual stress in the multilayers as measured by the curvature of the films, and misfit dislocation structure at the interfaces on the hardness of these multilayers are discussed. Comparions are made to the mechanical properties of sputtered Cu/Nb multilayers which, like Cu/Cr, exhibit sharp fcc/bcc interfaces with no intermixing and KS orientation relationship, but have a very small elastic modulus mismatch and much larger lattice mismatch for {110}bcc and {I I I}fcc planes as compared to Cu/Cr.

4:45 PM NN14.7 
STRESS EVOLUTION DURING GROWTH IN SPUTTERED Cu/Pd MULTILAYERS. Vidya Ramaswamy, William D. Nix, and Bruce M. Clemens, Stanford University, Stanford, CA.

It has been previously observed that during growth of Cu/Pd multilayers the apparent stress in the Cu layers changes from compressive to tensile and the apparent stress in the Pd layers is initially tensile or compressive depending on the thickness of the underlying Cu layer [1]. It has been speculated that the stress behavior exhibited by the Cu/Pd system may be a result of island growth of Cu on Pd or alloying at the Cu/Pd interface. This behavior is further explored by studying the effects of deposition parameters such as sputtering pressure, carrier gas species and deposition rates. Stress evolution during growth is obtained from in-situ substrate curvature measurements using a multiple parallel laser beam technique. Superlattice spectra for the multilayers are obtained from symmetric x-ray diffraction scans. Values for interfacial roughness and interdiffusion are obtained by fitting a numerical model to the superlattice diffraction patterns. The effects of growth mode-induced stress are also modeled. The relative contributions of interfacial alloying and island growth to the apparent film stress will be discussed by correlating the model predictions to the in-situ stress measurements.

Chairs: Shefford P. Baker and Wen J. Meng 
Friday Morning, December 5, 1997 
Essex South (W)

8:30 AM *NN15.1 
STRESS CONTROLLED MAGNETO-MECHANICAL INSTABILITY IN TERFENOL-D THIN FILMS. Quanmin Su, Y. Wen, Cecile Bailly and Manfred Wuttig, Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD.

The magneto-mechanical properties of Terfenol-D thin films deposited on Si substrates were studied by magnetic and mechanical measurements of film/substrate composite cantilevers. The , Delta-E effect and mechanical damping of the film were measured simultaneously. The stress in the film was controlled by annealing and deposition at different temperatures as well by the selection of the substrate material below the recrystallization temperature and determined to vary from -500 MPa, compression, in as deposited films to +480 MPa, tension, in annealed films. This presentation highlights the magneto-mechanical response of tensioned 1 µm nanocrystalline Terfenol-D films on 50 µm Si substrates which display a pronounced damping maximum at a magnetic field of about 1.5kOe oriented perpendicular to the plane of the film. The phenomenon is critically dependent on the orientation of the magnetic field and is the result of a magneto-mechanical instability in the Terfenol film.

9:00 AM NN15.2 
ENERGY STORAGE AND RECOVERY IN THIN METAL FILMS ON SUBSTRATES. Shefford P. Baker, Rose-Marie Keller, and Eduard Arzt, Max-Planck-Institut fuer Metallforschung, and Institut fuer Metallkunde, University of Stuttgart, Stuttgart, GERMANY.

Dislocation segments which extend through the thickness of a film can move through the film only if dislocation line length is deposited at the film/substrate and film/passivation (if any) interfaces. Thus the dislocation density and, therefore, the energy stored in the film increase during plastic deformation. This effect has been used to explain the high strength of thin films in comparison with bulk as well as an increase in film strength with decreasing film thickness which is frequently observed. The reverse process, that is, the reduction of strain energy in the film by the reduction of dislocation line length, is here suggested to be the origin of a number of unexplained features of experimentally obtained stress-temperature curves; including very low (or even negative) yield stresses in compression, tensile-compressive flow stress asymmetries, and a very strong Bauschinger-like effect which has been seen in thin Cu films. The results of stress-temperature measurements of Cu thin films on silicon substrates will be presented. Stress levels are shown to be very sensitive to thermomechanical history, in that behavior induced under one set of thermal cycling conditions may persist in several further cycles under different conditions. Interpretation of these results in terms of dislocation configurations will be described along with results of model calculations of stress temperature behavior.

9:15 AM NN15.3 
MICROSTRUCTURAL EFFECTS ON THE HARDNESS, ELASTIC MODULAS AND FRACTURE TOUGHNESS OF CVD DIAMOND FILMS. A. Kant, J.W. Ager III, W.J. Moberly Chan, R.O. Ritchie, Materials Sciences Division, Lawrence Berkely National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkley, CA; N.R. Drory, Crystallume Corporation, Santa Clara, CA; N.R. Moody, Sandia National Laboratories, Livermore CA.

Hardness, elastic modulus and fracture toughness properties of polycrystalline chemical vapor deposited (CVD) free-standing diamond thin (100 m) films have been characterized with respect to different measurement techniques, microstructure and failure mechanisms. Vickers and Knoop indentation techniques of hardness measurement, combined with various modulus measurement techniques such as dynamic resonance and nano-indentation, were used to estimate the indentation fracture toughness. It is proposed that, like many ceramics, the toughness of CVD diamond can be enhanced by weakening the grain boundaries in order to promote intergranular fracture and grain bridging. Several films with slightly different non-diamond carbon impurities were grown and studied for their mechanical properties. A significant variation in the fracture toughness was observed in accordance with the proposed theory. Correlation of mechanical properties with changes in non diamond carbon content and microstructure was established by studying the crack path, fracture surfaces and the location of non-diamond carbon content using Raman spectroscopy, atomic force microscopy, secondary electron microscopy and transmission electron microscopy.

9:30 AM NN15.4 
MECHANICAL TESTING AND MICROSTRUCTURAL CHARACTERISATION OF TITANIUM NITRIDE THIN FILMS. O. R. Shojaei, A. Karimi, J.L. Martin, Institut de Genie Atomique, Departement de Physique, Ecole Polytechnique Federale de Lausanne, Lausanne, SWITZERLAND.

Mechanical properties of titanium nitride thin films have been investigated using the bulge test experiments and the depth sensing nanoindentation measurements. The bulge tests were performed on square free standing membranes made by means of standard micromachining of Si(100) wafers containing a thin layer of low stress silicon nitride. The thin films of titanium nitride (TiNx) with a thickness varying between .5 - 1 m were deposited using the radio frequency magnetron sputtering method. The bulge test was first conducted on the silicon nitride films to determine its proper residual stress and Young's modulus. Then, the composite membrane made of TiNx together with underlying SiNy was bulged and the related load-displacement variation was measured. Finally, using a simple rule of mixture formula, the elastic mechanical properties of TiNx coatings were determined. Both the Young's modulus and residual stress showed increasing values with bias voltage, nitrogen to titanium ratio, and coating density. In contrast, the effect of substrate temperature below 600ƒ C was found less significant compared to other parameters. Nanoindentation data extracted from dynamically loading-unloading of TiN films adhered to supporting substrate roughly confirmed the results of biaxial tensile testing of free standing membranes. However, attempts are made to correlate the local values of hardness and Young's modulus obtained during nanoindentation to the global values of films measured during the bulge test experiments. Scanning electron microscopy of cross sectioned samples showed that coating growth occurs by formation of equiaxial nanocrystallites of size 10 - 30 nm, which leads to columnar morphology beyond a thickness of 100-150 nm. The columns are nearly perpendicular to the film surface. The bulge test and indentation results are discussed in terms of coating microstructure and chemical composition determined by means of electron probe microscopy.

9:45 AM NN15.5 
RELATING MICROSTRUCTURE WITH THE MECHANICAL PROPERTIES OF POLYSILICON THIN FILMS. S. Jayaraman, K. J. Hemker, Dept. of Materials Science and Engineering, and R. L. Edwards, Applied Physics Laboratory; Johns Hopkins University, Baltimore MD.

The microstructural characteristics and mechanical properties of LPCVD deposited 3.5 micron polysilicon thin films have been measured and correlated in the present study. Square and rectangular windows have been etched in the supporting silicon substrates, and pressure-displacement curves of the polysilicon films have been used to determine Youngís modulus (E) and Poissonís ratio (nu). TEM observations evidenced a columnar grain structure with a <011> out-of-plane texture and a random in-plane grain orientation. The bounds for the elastic modulus of the thin films have been calculated using a probabilistic model of the texture. The results of the bulge testing experiments (E = 162+/-4 GPa and nu = 0.2) were found to be in good agreement with the predictions of the texture model (E = 168-171 GPa) and literature values of the isotropic elastic constants of bulk polycrystalline silicon.

10:30 AM NN15.6 
NANOSCALE STRUCTURAL, MECHANICAL AND CHEMICAL CHARACTERIZATION OF III-V THERMAL OXIDES. D.T. Mathes, R. Hull, Department of Materials Science and Engineering, University of Virginia, K.D. Choquette, H.Q. Hou, Sandia National Labs, Albuquerque, NM; R.D. Dupuis, B.P. Tinkham, M.R. Islam, Microelectronics Research Center, The University of Texas at Austin, Austin, TX.

III-V thermal oxides have broad potential applications in optoelectronic and microelectronic III-V compound semiconductor devices and circuits. We describe a detailed Transmission Electron Microscopy (TEM) and Focused Ion Beam study of the nanoscale structural, mechanical and chemical properties of Al203 layers formed by wet oxidation of AlGaAs and InAIP. It is observed that AlGaAs based oxides formed by lateral oxidation techniques exhibit weak interfacial bonding and high internal stresses, as indicated by the frequent delamination of the oxide from the surrounding crystal either during mechanical sample preparation or during electron beam irradiation. The mechanisms inducing these local stresses, and their ramifications for device processing will be discussed. Chemical analysis, using electron energy loss imaging, has shown that As diffuses out of an oxidized AlGaAs layer and builds up at the oxide/crystal interface, with potential ramifications for interfacial state densities and metal-oxide-semiconductor devices. TEM imaging has also revealed that chemical composition variations existing in pre-oxidized layers remain after the oxidation process. This suggests that either Al fails to reach compositional uniformity throughout the oxide layer, and/or that at least some of the other species are not being removed from the oxide layer. It is well established that the oxidation rate in AlCaAs layers increases rapidly with the Al content of those layers. We have investigated multilayer oxide structures in which a thin, high Al concentration layer is sandwiched between two thicker, low Al concentration oxides. TEM imaging has shown that these layers together form a wedge-shape at the oxide front with the inner oxide extending to the point of the wedge. The outer layers oxidize faster than expected and form a sloped front, presumably due to Al or O diffusion from the inner layer. The inner layer oxidizes slower than expected. Implications for the oxidation mechanism will be discussed.

10:45 AM NN15.7 
MECHANICAL PROPERTIES OF POLYCRYSTALLINE Y2O3/ZrO2 SUPERLATTICES DEPOSITED BY HIGH-RATE REACTIVE SPUTTERING. P. Yashar and S.A. Barnett, Dept. of Materials Science and Engineering, Northwestern University, Evanston, IL 60208; W. D. Sproul, Advanced Coatings Technology Group, Northwestern University, Evanston, IL (Current address: Sputtered Films, Inc., Santa Barbara, CA).

Hardness enhancements have been widely reported for metal and nitride superlattices, but little work has been done on oxide/oxide superlattices. In this paper, we report on hardness enhancements in polycrystalline Y2O3/ZrO2 superlattice thin films deposited on Si (100) in an opposed-cathode unbalanced magnetron sputtering system, in Ar + O2 mixtures. Pulsed d.c. power was used to eliminate arcing on the metallic targets, and r.f. power applied to the substrates was used to achieve ion bombardment of the growing film. Simultaneous reactive sputtering of oxide materials with different hysteresis loops makes controlling the O2 gas a difficult problem, so indirect control of the O2 partial pressure via the target voltage was used. A series of superlattices with periods () from 2.9 - 22 nm were deposited with a constant Y2O3/ZrO2 layer thickness ratio of 0.75. The structure was characterized by X-ray diffraction and high resolution cross-sectional transmission electron microscopy (HREM), which both showed well-defined layered structures. At all studied, HREM showed that the ZrO2 layers exhibited the high termperature (> 2400C) cubic-fluorite structure, which was epitaxially stabilized by the fluorite-structure Y2O3. The hardnesses of the films were measured using nanoindentation, and showed an enhancement for < 10 nm of up to 25 relative to rule-of-mixtures. A maximum hardness of 13.5 GPa was observed at = 2.9 nm.

11:00 AM NN15.8 
DEGRADATION OF MECHANICAL PROPERTIES IN SiC-COATED CARBON FIBERS DUE TO THERMAL STRESSES GENERATED DURING CVD PROCESSING. Horacio Nassini, Comision Nacional de Energia Atomica, Centro Atomico Bariloche, Bariloche, Rio Negro, ARGENTINA; Hector Zolotucho, Carlos Gonzalez Oliver, Consejo Nacional de Investigaciones Cientificas y Tecnicas, Centro Atomico Bariloche, Bariloche, Rio Negro, ARGENTINA.

Silicon carbide (SiC) thin films deposited by CVD on carbon fibers are being frequently proposed as a diffusion barrier to prevent the chemical reactions at high temperatures between reinforcing fibers and metallic matrix in aluminum-based composites. It is well-known that SiC coating and carbon fibers present a big mismatch in their respective thermal expansion coefficients and, then, large thermal stresses may develop during cooling from processing temperatures, typically above 1273 K, to room temperature. As axial residual stresses in coating are predicted to be tensile, they could lead to an early cracking of film when coated fibers are axially loaded, and these circunferential cracks (notches) could then propagate easily into the fiber core, reducing strongly its mechanical strength respect to pristine fibers, as it was observed by several researchers. To investigate the degradation of tensile mechanical properties in carbon fibers by applying on them a thin SiC coating, e.g. 0.2-0.3 micrometers thick, deposited by CVD, single-filament tensile tests were performed on uncoated and coated specimens of a commercially available pitch-based carbon fiber (Thornel P-25). Tensile tests were carried out on over 40 specimens of 10 mm gauge length for each fiber type, following the standard ASTM D3379. The results of tensile tests were analyzed in terms of Weibull statistics and they showed that although a decrease both in average tensile strength (about 16 %) and Young's modulus (around 10 %) was observed in SiC-coated fibers, the degradation of mechanical properties was quite less pronounced than expected, according to previous experiments. The unforeseen behavior of SiC-coated carbon fibers could be explained by the higher compliance of low temperature CVD-deposited SiC coating, which was porous in nature, as concluded from SEM examinations and a detailed thermal stress analysis based on a microcomposite coaxial cylinder model complemented by concepts of linear-elastic fracture mechanics.

11:15 AM NN15.9 
GRAZING INCIDENCE STRESS PROFILING OF ZrO2 GROWN ON Zr-2.5%Nb PRESSURE TUBES. M.G. Glavicic1, J.A. Szpunar1, Y.P. Lin2; 1McGill University, Montreal, CANADA; 2Ontario Hydro Technologies, Toronto, CANADA.

Zr-2.5%Nb pressure tubes form an integral part in the heat transport system of the CANDU reactor design. During the final manufacturing stage of the pressure tubes a protective oxide is grown on the surfaces of the tubes in a 400 C steam environment. This oxide is a two phase material consisting of a room temperature and pressure stable monoclinic phase and a metastable tetragonal phase. Several factors are believed to be responsible for the existence of the metastable tetragonal phase; crystallite size, chemical impurities and namely stress. In order to determine whether stress was the main stabilizing factor of the metastable tetragonal phase a grazing incidence method of stress measurement was developed. This method is capable of measuring stress in the oxides as function of depth by varying the incident angle of the incoming radiation which thereby limits the penetration depth of the x-rays. This method was then used to examine the stresses present in oxides of varying thickness. The stresses present in the samples were then compared to the volume fraction of the tetragonal phase present in the specimens as determined by the pole figure integration method1. Based upon these results and previous texture2,3 and TEM3,4 results a model of the oxide microstructure is proposed.

11:30 AM NN15.10 
STRUCTURE AND MECHANICAL PROPERTIES OF DIAMOND FILMS DEPOSITED ON TI-6AL-4V ALLOY AT LOW TEMPERATURES USING CO/O2/H2 AND CH4/O2/H2 GAS MIXTURES.*. Shane A.Catledge and Yogesh K.Vohra, University of Alabama at Birmingham (UAB), Dept. of Physics, Birmingham, AL.

Low temperature diamond deposition on metal substrates is motivated by the need to reduce thermal stresses so that the film adhesion is satisfactory. Although diamond deposition on Ti-6Al-4V has been performed using the standard CH4/H2 chemistry, mechanical property data is still lacking. This study is motivated by the need to lower the processing temperature and to improve adhesion through interfacial chemistry control. To this end, microwave plasma-enhanced chemical vapor deposition was used to grow diamond films on Ti-6Al-4V alloy at low temperatures (500-700 C) in CO/O2/H2 and CH4/O2/H2 gas mixtures. The gas-phase excited species were determined by optical emission spectroscopy which was useful for optimizing the film quality. Micro-Raman spectroscopy was used to determine the quality of the films and their degree of stress. The crystalline structure of the films and film/substrate interface was characterized by grazing angle x-ray diffraction. Film hardness and modulus data were obtained using nano-indentation. This study represents the first attempt at using O2 and CO-based gas mixtures to deposit diamond at low temperature on Ti-6Al-4V substrates and to measure mechanical properties of the film.

11:45 AM NN15.11 
PLASMA ASSISTED DEPOSITION AND CHARACTERIZATION OF Ti CONTAINING DIAMOND-LIKE CARBON COATINGS. W. J. Meng, T. J. Curtis, General Motors R&D Center, Warren, MI; L. E. Rehn, P. M. Baldo, Argonne National Laboratory, Argonne, IL.

We report on synthesis of Ti-containing diamond-like carbon (Ti-DLC) coatings by plasma-assisted physical vapor deposition and characterization of coating microstructure and mechanical properties. We find that Ti-DLC coatings with Ti compositions higher than 5.5 atomic percent are nanocomposites consisting of nanocrystalline TiC in an amorphous carbon matrix. Mechanical properties of the Ti-DLC coatings depend sensitively on Ti composition. The observed mechanical properties variation should stimulate future modeling work to understand the mechanical behavior of this class of nanocomposite materials.