David La Van Yale University
Mark Spearing University of Southampton
Srikar Vengallatore McGill University
Mark da Silva Exponent Inc.
DD1: Micromechanics I
Monday PM, November 26, 2007
Room 313 (Hynes)
9:30 AM - **DD1.1
A Review of Tension Test Methods for Thin Films.
William Sharpe 1 Show Abstract
1 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States
The tensile test is the accepted standard for determining mechanical properties of materials used in large structures and components. It, along with the compression test, generates uniform states of stress and strain thereby permitting their direct measurement. However, tension test methods applicable to freestanding thin films were virtually non-existent only 15 years ago. While several researchers using different techniques and procedures now generate comparable results, there is as yet no accepted standard for tension testing of thin films. Neugebauer conducted tensile tests of gold films ranging in thickness from 50 nanometers to 1.5 microns in 1960! It is remarkable that no similar tests followed until the new test method of Read and Dally in 1992 – a gap of 30 years. Neugebauer evaporated gold films with a tensile specimen pattern onto rock salt crystals, glued the crystal ends to a unique test machine, and then dissolved the salt away from the tensile region. Read and Dally’s method is quite similar except the substrate is silicon crystal and removed by selective etching. An easier test method in which one end of the patterned tensile specimen remains attached to the silicon substrate was introduced by Tsuchiya in 1997. The free end of the specimen can be gripped by electrostatic attraction to a probe or by various gluing methods. Unique test machines are developed by each researcher.Strain can be measured by overall elongation of the specimen, which in many cases is sufficient since the specimen can be relatively long compared to its end conditions. Direct strain measurement is of course preferred, and various techniques are used. Sharpe introduced an interferometric technique in 1997. Long used optical digital image correlation (DIC) for gold films in 2001, and Chasiotis has used AFM images in the same manner. Currently, various forms of DIC are being used at this size scale, and it certainly appears to be the preferred technique. These DIC methods have advanced rapidly in the past few years and promise to be even more powerful with computer improvements.Finally, MEMS structures can be used to produce tiny test machines incorporating thin tensile specimens in the processing. Saif’s work of 2004 is an excellent example.This paper is a review of the development of thin-film test methods over the past 15 years. It will describe and discuss the merits of each and include a complete bibliography.
10:00 AM - DD1.2
Reliability of MEMS Materials: Mechanical Characterization of Thin-Films using the Wafer Scale Bulge Test and Improved Microtensile Techniques.
Joao Gaspar 1 , Marek Schmidt 1 , Jochen Held 1 , Oliver Paul 1 Show Abstract
1 Department of Microsystems Engineering (IMTEK), University of Freiburg, Freiburg, BW, Germany
The mechanical characterization of thin films is of great importance for their use as structural layers in MEMS or ICs since device functionality and reliability depend strongly on mechanical parameters. Towards a consistent extraction of mechanical properties, this paper reports on key improvements of both bulge and microtensile techniques applied to an extensive set of materials, brittle and ductile, typically used in MEMS applications.Membranes and microtensile specimens made of silicon nitride, silicon oxide, polycrystalline silicon and aluminum have been fabricated and characterized. Deposition methods such as LPCVD, PECVD, thermal oxidation and PVD have been used. Properties extracted from these materials included pre-stress σ0, elastic modulus E or plane-strain modulus Eps, Poisson’s ratio ν, Weibull parameters (strength μ and modulus m) and strain hardening coefficients n. Their dependence on deposition conditions will be emphasized.The bulge test uses a setup where up to 80 membranes on a silicon wafer can be characterized in a single series of sequential measurements. This enables the efficient acquisition of large data sets necessary, e.g., for the determination of reliable fracture data. The membrane profiles resulting from pressure loads are measured with an auto-focus sensor integrated with automated position tables for scanning purposes. A peculiarity of these bulge experiments is the use of diaphragms with large side length aspect ratios. This makes it possible to formulate and apply an accurate, analytical model of the diaphragm mechanics including multilayers with individual prestress and mechanical constants, and the support stiffness.The microtensile test exploits a wafer-detachable test structure design where the specimens are produced on a c-Si frame with coplanar fixed and mobile components defined by DRIE. Delicate handling of the tensile samples is thus avoided. The specimens bridge the gap between the frames experiencing truly uniaxial loads by geometrical constraint when displacements are imposed to the moving component. Such test structures allow the characterization of several material samples in a single experiment and thus contribute to the efficient acquisition of statistically relevant data. Force data are read from a load-cell while local strains are obtained from 2D-interferometric measurements, stroboscopic image analysis and calibrated actuators movement. Current work is developing a wafer scale tensile setup complementing the capability of the bulge test setup.More than 500 membranes of some films have been measured: from the brittle materials analyzed, LPCVD nitride films have the highest fracture strength and are highly predictable (m up to 60). In contrast, oxides are weak and have the lowest Weibull moduli (∼5). Elastic, fracture and ductile properties obtained from both techniques agree with each other and literature, however with narrower uncertainties due to the high throughputs of data.
10:15 AM - DD1.3
Multipurpose on-chip Nanomechanical Laboratory for Testing Thin Films.
Thomas Pardoen 1 2 , Raskin Jean-Pierre 1 3 , Coulombier Michael 1 2 , Sebastien Gravier 2 , Nicolas Andre 1 3 , Asmahan Safi 2 , Thomas Gets 2 , Vincent Delongueville 3 , Stanislas Sobieski 3 , Samer Houri 3 Show Abstract
1 Research Center in Micro and Nanoscopic Materials and Electronic, Université catholique de Louvain, Louvain-la-Neuve Belgium, 2 Department of Materials Science and Processes, Université catholique de Louvain, Louvain-la-Neuve Belgium, 3 Department of Electricity, Université catholique de Louvain, Louvain-la-Neuve Belgium
The measurement of the mechanical properties of submicron sized specimens is extremely challenging due to difficulties for manipulating samples, for applying small load and for extracting accurate stresses and strains. We present a novel, versatile concept of micromachines to measure the mechanical response, up to large strains and fracture, of thin films allowing multiple loading configurations and geometries. This new concept relies on MEMS microfabrication techniques. The first idea is to use internal stresses to actuate and impose the load to the tested sample which is directly deposited on a patterned substrate. The second idea is to multiply elementary machines rather than to build on single complex multi-purpose micromachine. The applied stress and strain are directly inferred from a local displacement measurement. The versatility of the concept allows other loading configurations than simple tension such as shear, biaxial, compression, three-point bending and double cantilever opening. About 5000 elementary micromachines have been processed on top of a single silicon wafer, delivering statistically representative information. The mechanical response of pure Al, Al/Si alloys, Ti and silicon nitride films, with thicknesses ranging between 80 and 500 nm, has been addressed. For instance, stress strain curves have been measured for the Al films, with the strength increasing linearly with 1/thickness to reach values larger than 1 GPa for the thinnest samples while keeping more than 8% of uniform elongation. The strength of Al/Si alloys is two times larger than for Al films. The strength of the Ti films reaches more than 2 GPa. The fracture stress of silicon nitride films attains about 4 to 5 GPa, approaching the theoretical cleavage stress. This method based on MEMS fabrication techniques constitute a combinatorial experimental platform for addressing several critical MEMS reliability issues.
10:30 AM - DD1.4
Fatigue of Nanostructured Ni MEMS Thin Films.
Yong Yang 1 , Guojiang Fan 2 , Benjamin Imasogie 3 , Brad Boyce 4 , Peter Liaw 2 , Winston Soboyejo 1 Show Abstract
1 Princeton Institute for the Science and Technology of Materials (PRISM) and The Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 2 Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee, United States, 3 Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife Nigeria, 4 Materials Science and Engineering Center, Sandia National Laboratories, Albuquerque, New Mexico, United States
This paper presents the results of an experimental study of fatigue in nanostructured Ni thin films produced using electro-deposition techniques. The underlying mechanisms of fatigue are elucidated in stress-life and short/long crack fatigue crack growth experiments. The studies reveal the potential role of pre-existing material defects in crack nucleation. They also show new experimental evidence of the role of microvoid formation and grain growth during short and long fatigue crack growth of nanostructured Ni MEMS thin films. The implications of the observed phenomena are discussed for potential applications of Ni MEMS structures with nanostructured grain sizes.
10:45 AM - DD1.5
Fatigue of Al Thin Films at ultra-high Frequencies: Discrete Dislocations and Damage Behavior.
Diana Courty 1 , Chris Eberl 1 2 , Werner Ruile 3 , Oliver Kraft 1 2 Show Abstract
1 izbs, University of Karlsruhe, Karlsruhe Germany, 2 Institute for Materials Research II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 3 , EPCOS AG, München Germany
Micro devices are widely used in applications for radio frequency communications. In many of these applications, they are exposed to cyclic mechanical loading at typical operating frequencies in the GHz regime. In our work, we have used patterned Al thin films on a piezoelectric substrate to study ultra-high-cycle fatigue. The working principal is based on frequency filtering by the to and fro transduction of an electrical signal into an acoustic wave. An alternating applied electric field generates acoustic deformations on the surface of the piezoelectric substrate by the transduction and can thus be used to stress metallizations at extremely high frequencies. At the resonance frequency, large deflection amplitudes and, hence, high stresses may occur. The devices are tested in the GHz-regime, which allows studying cycle numbers up to 10^14. Fatigue effects, such as the formation of voids and extrusions that result in a frequency shift or even in short circuits are observed.In our work, we have concentrated on the question how cyclic stresses at extremely high frequencies activate defect mechanisms, in particular if they are able to drive dislocations in thin metal films leading to the observed irreversible damage.For this, we have conducted discrete dislocation dynamics simulations showing that dislocations are still able to move in the GHz-regime. It is observed in these simulations that even small asymmetries in the loading conditions lead very quickly to an accumulated plastic strain due to the extremely high frequencies. As a result, the role of the microstructure of the metal films such as grain size or texture is discussed.In order to explore further possible failure mechanisms, tested Al thin film devices were analyzed by Focused Ion Beam microscopy (FIB) and Scanning Electron Microscopy (SEM) evaluating quantitatively the observed extrusions and voids. Furthermore FIB micrographs were used to determine the mean grain size, to investigate whether or not there is a significant influence of the grain size on the extrusion and void formation. Our attempt to correlate details of the dislocation behavior with the damage formation at ultra high cycle fatigue in thin metal films is of general importance for the reliability of micro devices operated at radio frequencies. It becomes obvious that even very moderate loading conditions may become detrimental due to the high frequency connected to an extremely fast accumulation of strain.
11:30 AM - **DD1.6
Strategies for Testing of Material and Geometrical Properties of MEMS.
Joerg Bagdahn 1 , Matthias Ebert 1 , Ronny Gerbach 1 , Falk Naumann 1 , Ralf Schaefer 1 , Jan Schischka 1 Show Abstract
1 Microsystems, Fraunhofer Inst for Mechanics of Materials, Halle/Saale Germany
Testing of materials properties of micro electromechanical systems (MEMS) is required during the development stage of devices, during fabrication and for analysis of failed components. The testing approaches can be classified in techniques for testing on wafer-level, on die-level and on lift-out samples. Testing on wafer-level is a key element in in-line quality management during fabrication. A fast (few seconds) and non destructive test procedure is required. The testing on wafer-level or die-level is used to detect faulty devices or divide devices on a wafer into different quality classes. The possibility of the detection of bad / or good devices on the wafer-level is an essential cost factor, since unnecessary packing and assembly cost can be avoided and the yield during inte-gration of several dies, e.g. in stacking applications, is dramatically increased. Furthermore ma-terials and also geometric parameters can be monitored during fabrication. Due to the complex-ity of MEMS components a simple electrical measurement is not sufficient. Rather mechanical, chemical, optical, electrical excitations are required and either optical or electrical measurement methods are needed. In the papers the focus is given on techniques for mechanical stimulation and evaluation. Techniques for excitation of MEMS structures on the wafer level, methods for detection of static deformation as well as dynamic vibrations and numerical methods for data evaluation will be presented. Tests on die-level are used to fabricate dedicated test structures, which are tested off-line. The off-line test allows also the destructive-testing of devices. An example of such an test struc-ture are micro-chevron-samples, which can be used for strength characterization of wafer-bonded interfaces. In the paper the current status of this test approach will be shown. Lift-off techniques are required if materials properties of parts of devices has to be measured, e.g. in failed components. An new approach for testing of parts of MEMS structures was devel-oped. The test includes the preparation of test samples by focused ion beam technique milling, handling of test samples and subsequent testing with dedicated test equipment. The approach allows the measurement of mechanical properties like tensile or bend strength or fatigue prop-erties of already fabricated devices. Results of strength investigations will be presented.
12:00 PM - **DD1.7
Adhesion of Microstructures in Dry and Humid Ambients.
Maarten deBoer 1 , Frank DelRio 2 , Martin Dunn 3 Show Abstract
1 MEMS Core Technologies, Sandia National Labs, Albuquerque, New Mexico, United States, 2 Chemical Engineering, UC Berkeley, Berkeley, California, United States, 3 Mechanical Engineering, Univ. of Colorado at Boulder, Boulder, Colorado, United States
The miniaturization of functional structures to the micron and nanometer scales is of widespread technological and scientific interest. Examples include nanotubes, nanowires, mechanical switches, robotic tools, and non-volatile memory elements. As dimensions decrease, surface-to-volume ratios increase, and the effects of properties such as adhesion, friction and damping on system performance become increasingly important. Here, we address the interaction between adhesion and surface topography in dry and humid ambients. To measure adhesion, we employ microcantilever structures using a fracture-mechanics based methodology. In dry ambients, adhesion is as low as several microjoules per square meter. Analysis of the surface topography shows that the highest surface asperities control the average surface separation, but that the main contribution to adhesion is from non-contacting areas. In humid ambients, we observe a strong interaction between relative humidity and surface roughness with millijoule per square meter adhesion values. To analyze the results, a constitutive model for an asperity bridged by a liquid condensate to a flat surface is needed. Under equilibrium conditions, it can be shown that the work of adhesion is less than the surface energy created. This apparent paradox is resolved by a thermodynamic control-volume analysis, and is due to heat-induced evaporation of the liquid during the pull-off process. A more general Griffith criterion results. Experimentally, topographic correlations between the upper and lower surfaces, asperity plasticity and disjoining pressure of the ambient vapor are also important factors. When these effects are all taken into account, good agreement between the experimental and calculated results are found. This work will help enable more reliable micro and nanostructures.
12:30 PM - DD1.8
Vapor Phase Lubrication of Silicon MEMS.
Michael Dugger 1 , David Asay 2 , James Ohlhausen 1 , Seong Kim 2 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Pennsylvania State University, University Park, Pennsylvania, United States
MicroElectroMechanical Systems (MEMS) that have contacting surfaces necessitate surface modification to mitigate adhesion, friction and wear. Most MEMS applications avoid rubbing surfaces because of the challenges associated with reliability in such devices. Surface treatment for non-contacting MEMS is focused mainly on obtaining structures that survive handling associated with production and assembly. A lubrication approach to create reliable sliding contacts would enable a significant expansion of the design space.Chemisorbed monolayers have been used to modify the surface energy of oxidized silicon after etching the sacrificial oxide layers, in order to resist water adsorption and capillary adhesion. A variety of other surface treatments for silicon microsystems have also been investigated, including carbides and oxides to improve wear resistance. Recent emphasis in MEMS lubrication has been on systems that allow for damaged areas to be healed by lubricant diffusion over time. This approach offers some promise, although surface diffusion may limit the effectiveness of this approach for applications involving long duration operation.Interfaces that are deeply buried in complex structures present particular challenges for lubrication. Line-of-sight surface treatment approaches are not applicable to these contacts, and chemisorbed monolayers have exhibited limited mechanical durability in repetitive contact. To address these problems, vapor phase lubrication has been developed for silicon MEMS to enable extremely long duration operation with little or no wear. Molecules in the vapor phase adsorb on silicon surfaces and react to form a friction and wear reducing film. Therefore, as long as the molecule is available in the environment, the lubricant film can be replenished. Micromachined silicon tribometers have been used to demonstrate the vapor phase lubrication concept on MEMS devices, and to compare friction and adhesion behavior with and without the wear-reducing film. Operational lifetime is increased from the order of 1E4 cycles with a chemisorbed monolayer alone, to in excess of 1E8 cycles with vapor phase lubrication. Macro-scale pin-on-disk measurements have been used to produce wear areas large enough for surface analysis by ToF-SIMS. These measurements indicate the formation of a high molecular weight product in the wear scar that is hypothesized to be responsible for the dramatic improvement in tribological behavior. Probable reaction pathways leading to the reactant formation will be discussed.
12:45 PM - DD1.9
MEMS Lubrication: An Atomistic Perspective of a Bound + Mobile Lubricant.
Douglas Irving 1 , Donald Brenner 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Chemically bound octadecyltrichlorosilane (ODTS) self-assembled monolayers have been successful in protecting microelectricalmechanical systems (MEMS) from stiction related failure in both the processing stage and the initial stage of use (i.e. “single shot” applications). This lubricant, however, is not successful for MEMS devices expected to cycle repetitively for extended time due, primarily, to the inability to replenish the ODTS in situ. Successful lubrication by ODTS is also limited to a narrow range of temperatures and pressures. A new “Bound + Mobile” lubrication scheme, which combines the bound ODTS with a replenishable mobile tricresyl phosphate (TCP) phase, has been proposed as a solution to these deficiencies. This talk will focus on recent atomistic simulations that characterize both the self-diffusion and the incorporation into defects of the mobile phase as well as the adhesive properties and interfacial structure of the newly formed ODTS/TCP/ODTS interface. This work has been supported by the Air Force Office of Scientific Research, MURI grant # FA9550-04-1-0381.
DD2: Micromechanics II
Monday PM, November 26, 2007
Room 313 (Hynes)
2:30 PM - **DD2.1
Fracture of Brittle Polycrystalline and Amorphous Materials for MEMS.
Ioannis Chasiotis 1 , Krishna Jonnalagadda 1 Show Abstract
1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Fracture of brittle materials fabricated for surface micromachined devices hinges upon understanding the stochastic nature of material failure in conjunction with its microstructure and the material volume under stress. Experiments and numerical analyses were conducted to understand the deterministic and probabilistic parameters entering the problem of fracture in two material systems: Polycrystalline silicon and tetrahedral amorphous diamond-like carbon. Fracture investigations were carried out to determine the effect of material inhomogeneity and stress gradients in brittle films with prefabricated mode-I and mixed-mode critical cracks. It was shown that the location of the crack tip determines the macroscopic failure stress and, thus, the effective critical stress intensity factor for both mode-I and mixed-mode cracks. As a result, the stochastic nature of polysilicon strength is strongly affected by variations in the local critical stress intensity factor. This finding limits extrapolations of statistical probability of failure functions to small material domains that are comparable to grain size, or to specimen sizes that do not guarantee statistical homogeneity. Furthermore, it was shown that through-the-thickness stress gradients are critical in the determination of the material fracture toughness, especially for films that are thicker than one micron. These data were corroborated with an investigation of the accuracy of a single two-parameter Weibull cumulative probability density function (material scale parameter and Weibull modulus) used to describe the failure strength of brittle MEMS components with non-uniform geometries and stress distributions. It was concluded that failure of self-similar MEMS geometries is not always described by the same Weibull constants due to competing flaw populations whose severity is controlled by the stress distribution and the material volume subjected to large stresses.
3:00 PM - **DD2.2
On-chip Microscale Mechanical Properties Measurements.
H Kahn 1 , R. Ballarini 2 , A. Heuer 1 , S. Eppell 3 Show Abstract
1 Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio, United States, 2 Civil Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 3 Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Because of the actuation schemes and processing techniques developed over the past years and their inherent micrometer size scales, MEMS are excellent platforms for performing materials properties investigations. For example, chemical vapor deposition can produce films with wide ranges of microstructures and residual stresses, and electrostatic actuation can generate well controlled forces for monotonic or resonant loading. Submicron displacement accuracy adds to the capabilities for highly precise mechanical measurements. This talk will demonstrate this capacity by describing results for on-chip tests, including: the stresses generated during oxidation of silicon, the effects of microstructure on fracture toughness and fracture strength of polysilicon, fatigue of polysilicon, and stress corrosion of silicon dioxide. In addition, MEMS devices can be used to test non-MEMS materials. The example of tensile testing of collagen fibrils will be presented.
3:30 PM - DD2.3
Effect of Surface Oxide Layer on Mechanical Properties of Single Crystalline Silicon.
Kenji Miyamoto 1 , Koji Sugano 1 , Toshiyuki Tsuchiya 1 , Osamu Tabata 1 Show Abstract
1 Microengineering, Kyoto University, Kyoto-shi, Kyoto, Japan
Single crystal silicon (SCS) is widely used for MEMS devices as structural material, because it has excellent mechanical properties and is suitable for microfabrication processes. Since silicon is a brittle material, the mechanical reliability, especially the fatigue fractures under long term cyclic loading, is concerning and the fatigue properties and mechanisms are being investigated widely. The previous works have revealed that the fatigue fracture of single crystalline silicon is sensitive to the humidity of the test environment and the native oxide layer on the surface affects the fatigue properties (1), (2). In this research, we focus on the effect of surface oxidation on the mechanical properties, such as the Young’s modulus, fracture strength and fatigue. The tensile testing of the surface oxidized SCS specimens was performed. The SCS specimens were fabricated from SOI (silicon-on-insulator) wafer using standard SOI surface micromachining, which contains deep reactive ion etchings (DRIEs) for specimen patterning and substrate removal and sacrificial etching. The thickness, width, and length of the specimens were 4 μm, 3 μm, and 120 or 600 μm, respectively. The specimen surface orientation was (100), and tensile axis was aligned to the <110> direction. Then about 200-nm-thick oxide layer was grown by dry thermal oxidization at 1100 °C for 90 min. The tensile testing was performed using the tensile tester for thin films that can control the humidity of test environment by covering the specimen with a small humidity-controlled chamber. The electrostatic chuck was used for specimen fixing(1). The tests were performed in two conditions, in lab air (humidity: 50 %RH) and in the humidity chamber (humidity: 90 %RH) both at room temperature (26 °C). The quasi static tensile tests showed that the averaged fracture strengths of surface oxidized SCS specimens were 2.3 GPa in the lab air and 1.8 GPa in the high humidity, respectively, which is lower than that without oxide layer, which was 2.8 GPa. The fracture surface of the surface oxidized SCS specimen indicates that the fracture was initiated from inside silicon near the corner, not from surface oxide layer. The oxidization of silicon caused the tensile strain in silicon near the oxide layer interface due to the large compressive strain in oxide layer, which may relate to the fracture behavior. Now the thickness effect on the tensile and fatigue strength of the silicon is being investigated and will be reported.(1)Y. Yamaji, et al. Proc MEMS 2007, pp.267-270.(2)C.L. Muhlstein, et al., J. Microelectromechanical Syst. 10(4), 2001, 593-600.
3:45 PM - DD2.4
A Comparison of the Strength of Poly- and Single-Crystalline Silicon Subject to Galvanic Corrosion in Hydrofluoric Acid.
David Miller 1 , Collin Becker 1 , Brad Boyce 2 , Conrad Stoldt 1 Show Abstract
1 Mechanical Engineering, University of Colorado, Boulder, Colorado, United States, 2 Microsystem Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States
Immersion of silicon-based microelectromechanical systems (MEMS) in hydrofluoric acid (HF) solutions is a standard post-fabrication procedure used to render mechanically moveable structures. Recent studies have demonstrated that HF etching can result in autonomous corrosion of silicon, when an added metallic layer is present. We utilized microfabricated tensile specimens to examine the reduction in mechanical strength when silicon is galvanically coupled to a gold metallic layer. The resulting corrosion damage is compared between polycrystalline silicon (polySi) and single crystal silicon (SCS) tensile specimens, manufactured using the standard commercial Multi-User MEMS Process (MUMPs) technologies. The etching solutions used include undiluted 48% HF (UDHF), as well as HF mixed with either water (an oxidizing agent) or Triton-X-100 (a surfactant). Micro-tensile testing was performed using a custom electromechanical probe station where the applied load could be measured to 0.01 mN. The specimens’ fractured surface was examined in a field emission scanning electron microscope (FESEM), with representative specimens being further characterized in a transmission electron microscope (TEM). For polySi, strength decreased precipitously with etch time, either due to preferential grain-boundary attack (leading to intergranular fracture) to generalized material removal (leading to transgranular fracture), depending on the etch solution. Alternately for SCS, strength could increase or decrease with etch time, depending on the effect of etch chemistry in blunting or deepening sidewall crevices. For both types of silicon, the severity of damage and corresponding change in strength depended on etch duration and etch chemistry. Our results uniquely indicate that galvanic corrosion is dominated by the surface wetting characteristics of the HF-based chemistry utilized. For example, etching in UDHF resulted in cracks and voided regions. Alternately, etching in the presence of a surfactant resulted in a very different rounded etch-front. In all cases, the corroded surface layer primarily composed of oxidized silicon as observed using energy dispersive X-ray spectroscopy (EDX). We will discuss the results of mechanical and morphological examination, which compare the different types of silicon and etchant chemistries. Additional nanoindentation, mathematical modeling, electrical resistance, and electrochemical based studies support the mechanical test data and the related conclusions. Our work provides an understanding of the drastic degradation in mechanical properties for the commonly used silicon and gold material couple, with implications concerning the design, performance, and reliability of MEMS structures.
4:30 PM - DD2.5
MEMS Stress Camera for High-Throughput Materials Characterization.
Noble Woo 1 , Jonathan Petrie 2 , R. van Dover 2 Show Abstract
1 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
4:45 PM - DD2.6
Indentation Characterization of Fracture Toughness and Interfacial Strength of PECVD Nitrides after Rapid Thermal Annealing.
Hong-Yi Yan 1 , Kuang-Shun Ou 1 , Kuo-Shen Chen 1 Show Abstract
1 Mechanical Engineering, National Cheng-Kung University, Tainan Taiwan
Plasma enhanced chemical vapour deposited (PECVD) silicon nitrides have been widely used in microelectromechanical systems (MEMS) and integrated circuits (IC) devices as mask or structural materials and their mechanical properties are traditionally important information for the structural design to enhance the device integrity. In particular, the mechanical properties of PECVD nitride significantly depend on the associate thermal processing history. Recently, rapid thermal annealing (RTA) has been widely applied in IC and MEMS processes for reducing thermal budget during fabrication. However, the influence of RTA on the mechanical properties of PECVD nitride, especially on the fracture toughness and interfacial strength, is still not clear. Since the information is vital for device longevity concern, a detail investigation is therefore required.This work presents the result of mechanical characterization for the fracture toughness and interfacial strength of PECVD silicon nitride films deposited on silicon subjected to rapid thermal annealing (RTA) processing between 200 and 800 oC. Both micro- and nano-indentation techniques are employed to perform the experiments. In conjunction with the model proposed by Marshall and Lawn for data reduction, the fracture toughness of un-heat treated nitride is obtained as 1.33 MPa.sqrt(m) based on a series of Vickers micro-indentation tests and this value is essentially unchanged if the RTA temperature is below 400 oC. Further increase in RTA temperature would significantly enhance the fracture toughness. On the other hand, using nanoindentation testing in conjunction with the model proposed by Marshall and Evans, the interfacial strength between the nitride and silicon is determined as 17.2 J/m2 for un-heat treatment nitrides and it could be significantly enhanced by RTA processes with temperature exceeding 400 oC. These results should be useful for related MEMS or IC structure fabrication for the concerns of maintaining the structural integrity and improve fabrication performance in related applications.
5:00 PM - DD2.7
Incomplete Martensitic Transformations of NiTi Shape Memory Alloy Thin Films.
Xu Huang 1 , Hoo-Jeong Lee 2 , Ainissa Ramirez 1 3 Show Abstract
1 Mechanical Engineering, Yale University, New Haven, Connecticut, United States, 2 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Kyongki-Do Korea (the Republic of), 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Nickel titanium thin films were deposited onto silicon wafers and subjected to thermal cycles to create incomplete martensitic transformations. With wafer curvature methods, it was found that the film stress plateaus briefly when forward (austenite-to-martensite) transformations follow incomplete transformations. No stress plateau was found in reverse transformations. X-ray diffraction measurements show the phases remained constant over this plateau when austenite converts to martensite, but changed immediately in the opposite direction. These observations are consistent with the thermodynamic model that suggests the elastic strain energy is stored in forward transformations and impedes them, yet assists them when released in reverse ones.
5:15 PM - DD2.8
A Novel Micro Tensile Testing Instrument with Replaceable Testing Specimen by Parylene Passivation Technique.
Yung-Dong Lau 1 , Tso-Chi Chang 1 , Hong Hocheng 1 , Rongshun Chen 1 2 , Weileun Fang 1 2 Show Abstract
1 Power Mechanical Engineering, National Tsing Hua University, Hsinchu Taiwan, 2 Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu Taiwan
5:30 PM - DD2.9
Atomistic+Continuum Multi-scale Modeling of Single Asperity Gold-Gold Contact in an RF MEMS Device.
Wes Crill 1 , D. Irving 1 , C. Padgett 3 , O. Rezvanian 2 , M. Zikry 2 , D. Brenner 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Department of Chemistry and Physics, Armstrong-Atlantic State University, Savannah, Georgia, United States, 2 Mechanical and Aerospace Engineering , North Carolina State University, Raleigh, North Carolina, United States
The gold contacts of failed RF MEMS devices often show what appear to be melted and re-solidified asperity tips. The cause of these features is not yet understood, but it is thought that the formation of nanowires during contact pull-out could potentially be a precursor to this behavior. To understand the details of nanowire formation in gold-gold RF MEMS contacts, we have carried out simulations of the pull-apart of a single asperity contact using large-scale molecular dynamics simulations coupled to a continuum treatment of Joule heating and heat transfer. The initial asperity geometry, which was derived from a finite element fractal model of a contact blunted by plasticity, contained a 550 nm2 asperity on a 7500 nm2 substrate that is brought into contact with another flat substrate of the same size. As the contact is pulled apart, the simulations show dislocation emission in the substrate surrounding the asperity contact as well as a disordering of the region between the surfaces. Without Joule heating, the severed contact retains a non-cylindrical geometry that reflects that of the initial contact shape. With an applied current, Joule heating that starts at the edges of this contact drives the structure towards a more cylindrical geometry. The influence of this local heating on the formation of long nanowires will be discussed and compared to experimental structures observed for hot switching of gold MEMS.This work was supported by the Office of Naval Research and the Extreme Friction Multi-University Research Initiative sponsored by the Air Force Office of Scientific Research.
5:45 PM - DD2.10
Microscopic Motion and Deflection Quantifying Chip for Lab-on-chip Application.
Xiaoyu Zheng 1 , Xin Zhang 1 Show Abstract
1 , Boston University, Brookline, Massachusetts, United States
This paper presents a double layered polymer structure for quantifying local distortions on polymer micro/nano structures. Recently, researchers have been engineering polymer based periodic substratum for cell migration and cellular force measurement. The substratum ranges from thin elastomer films to micro beads or micro vertical pillar arrays which quantifies the force evolution on cells upon their substratum. However, the determination of beads or pillar motion was only based on measurement of direct observation on each individual measuring unit via optical microscopy, which has to rely on algorithms to determine the centroid for each unit and track their motions individually. Such method has several drawbacks. Firstly, it has to rely on the algorithm on determining the motion of unit individually without knowing their initial state, which, on the other hand, could make the measurement problematic especially after fabrication, elastomer materials such as poly dimethylsiloxane(PDMS), is distorted by stress, due to the contraction on curing of the elastomers and contact with a substrate. Such distortion could cause each individual sensors distribute non-uniformly which violate the assumption that the sensor were distributed evenly on the substratum hence affect the accuracy of such measurement. Secondly, conventional tracking of each individual unit requires visibility of not only each individual unit but also their distortions. As a result, this method confines the sensors within micro meter domains due to the capacity determined by the microscope and camera, therefore requires the sensors must have very low stiffness to make the distortion distinguishable, which in turn, violates the linear load-displacement assumption. In this paper, we fabricated double layered chip by soft lithography technique. The top layer is the substratum consisting of each individual unit sensor such as pillars or micro beads. The bottom layer consists of a pattern layer made of the same material but with a much higher rigidity which has slightly different scales than the top one and tilted by a small angle. A He-Ne 633nm laser was used as an imaging source that transmitted vertically through the plane of the substratum. A 10X to 50 X objective and CCD camera was followed by the placement of the substratum. A circular aperture 3mm diameter was mounted to increase the depth of focus. Upon saturation of laser, diffracted moiré pattern from the double layer was formed and visualized on monitor. The angle of the double layer was chosen such that the flexible fringes of moiré pattern and the spatial period satisfy measurement needs which map the distortion of pillars or motion of beads in two planar directions. Therefore, the information of initial state of periodic substratum can be visualized and the motion of every unit can be directly mapped in real time via diffracted moiré patterns.
DD3: Poster Session I
Tuesday AM, November 27, 2007
Exhibition Hall D (Hynes)
9:00 PM - DD3.1
Mechanical Stress Sensors for Copper Damascene Interconnects.
Romain Delamare 1 , Christian Rivero 2 , Sylvain Blayac 1 , Moustafa Kasbari 1 , Karim Inal 1 Show Abstract
1 PS2, Centre de Microelectronique de Provence, Gardanne France, 2 , STMicroelectronics, Rousset France
We propose embedded microsensors to investigate the mechanical stress in copper damascene lines. Stress induced voids and cracks in interconnect structures is known to be responsible in reduced yield at all levels of device processing: the local mechanical stress measurement during process steps is of major interest. In this study, we describe sensors based on silicon piezoresistive effect where strain in the active silicon is induced by orientated copper lines. The challenge is to correlate the electrical sensors signal directly to stress variation in lines. Experimental characterizations have been realized versus copper stress variation induced by both temperature and uniaxial external stress application. The relation between the carrier mobility and the silicon strain variation was extracted via four points bending measurements. In addition MEMS were integrated in interconnects levels to estimate the stress. A good agreement between Finite Element Analysis and measurements permit to propose an electro-mechanical model of our structures. Moreover, these microsensors are process compatible, thus it permits to show the relevance in the monitoring of stress induced by interconnects during the fabrication step process.
9:00 PM - DD3.10
Modification of Conductivity and Stiffness of Electroactive Polymer (EAP) Thin Films by Metal Ion Implantation.
Muhamed Niklaus 1 , Samuel Rosset 1 , Philippe Dubois 1 , Massoud Dadras 2 , Herbert Shea 1 Show Abstract
1 LMTS - Microsystems for Space Technologies Laboratory, EPFL - Ecole Polytechnic Federal de Lausanne, Neuchatel Switzerland, 2 Institute of Microtechnology, University of Neuchâtel, Neuchatel Switzerland
Compliant electrodes for micron-scale dielectric electroactive polymer actuators can be fabricated by implantation into polydimethylsiloxane (PDMS) of metal ions with energies from 2 to 35 keV , creating a thin conductive layer a few nm below the surface without forming a continuous metallic film. Electrical conductivity of a few kΩ/sq can be reached with less than 1 MPa increase in Young’s modulus for doses of order 1016 ions/cm2, allowing efficient downscaling of artificial muscle actuators.We present the impact of implantation method, ion species, energy, and dose on the mechanical and electrical properties of PDMS membranes. Dose (D) and energy are chosen to be in the range that produces useful electrodes for EAP actuators. Two implantation techniques were used: Filtered Cathodic Vacuum Arc (FCVA) and Low Energy broad beam Implanter (LEI). Ti and Au were implanted at 5 keV by FCVA, and Ti at 10 keV and 35 keV by LEI. Doses varied from 1016 to 6x1016 ions/cm2.Stress, Young’s modulus (E) and surface resistivity (R) were measured. Initially E was 0.85 MPa and R was >2000 MΩ/sq. All data sets show an increase in E and decrease of R with increasing dose. Different conditions however give up to a factor of 100 difference in E for a given dose.Best performance for actuators was obtained for FCVA implantations of Gold at 5 keV. For D=1.5x1016 ions/cm2, E increased by 100% (to 2 MPa) while R decreased to less than 1kΩ/sq. R vs. time shows two regimes: for R<1kΩ, R decreases slightly then stabilizes, while for R>1kΩ, R rapidly increases. We explain these two behaviors by Au diffusion which leads to annealing for thicker films, but can bring barely continuous films to below the percolation limit.For Ti implanted by LEI at 10 keV and 35 keV, to get comparable R a dose 50% larger than for FCVA Au 5 keV is required, consistent with simulations. However E is 2 orders of magnitude larger, primarily due to the nature of the implantation process: FCVA is pulsed (600 µs pulses at 1 Hz, 4x1013 ions/cm2/pulse) and generates large heating at every pulse. LEI rasters a 3 µA beam over the surface for several hours. Charging plays a large role in the final microstructure and the two techniques have orders of magnitude difference in beam current.Implantation changes surface roughness (SR) of polymers by sputtering, stress modification, charging and heating. The PDMS films had initial SR of 3 nm rms. SR increases with implant dose, up to 100 nm for FCVA, and over 1 µm for LEI. This is an indication of the dramatic difference in microstructure obtained at comparable doses with different implanters. TEM samples have been prepared.This study enabled the definition the implant conditions where stable low resistivity is reached with only limited stiffening of the elastomer membrane. To create compliant electrodes, FCVA type implanters are much better suited than conventional implanters.  P. Dubois et al, Sensors and Actuators A: Physical, 130-131, p.147 (2006)
9:00 PM - DD3.11
C-Axis Oriented ZnO Film by RF Sputtering and its Integration with MEMS Processing.
Sudhir Chandra 1 , Ravindra Singh 1 Show Abstract
1 Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, New Delhi, Delhi, India
During recent years, extensive research has been carried out on zinc oxide (ZnO) thin films because of their applications in surface acoustic wave (SAW) devices, bulk acoustic wave (BAW) resonators, optical waveguides, transparent conducting coatings, light emitting diodes (LED), photodetectors and electroluminescence devices. These films can also be used in micro-electro-mechanical-systems (MEMS) as a sensing material because of its good piezoelectric properties, and as a sacrificial layer in surface micromachining. More recently, piezoelectric based MEMS have shown great potential for bio-sensor applications. For applications of ZnO films based on its piezoelectric properties, it is a requirement that the films should be c-axis oriented and have high resistivity. Furthermore, ZnO films deposition at comparatively low temperature is of great interest for realization of MEMS in post-CMOS integration process. Among various deposition techniques, sputtering is the preferred method as oriented and uniform films of ZnO can be obtained even at relatively low substrate temperatures. In the present work, we report the preparation of high resistivity c-axis oriented ZnO thin films on different substrates by RF magnetron sputtering. The self-heating of the substrate during sputtering process has been advantageously exploited to prepare c-axis oriented ZnO thin films.The ZnO films are very sensitive to most of the chemicals used in MEMS fabrication process such as H2SO4, hydrofluoric acid (HF), buffered HF (BHF), silicon anisotropic etchants (e.g. KOH, EPW), metal (e.g. chromium, gold, aluminum) etchants etc. In another words, the subsequent fabrication processes after ZnO deposition could degrade or damage ZnO film if it is not protected properly. Therefore, the integration of ZnO films with MEMS processing is a very challenging task. The present work also focuses on how to protect the ZnO film against degradation in subsequent micromachining processes. A modified fabrication process has been demonstrated to integrate the ZnO films with bulk-micromachined diaphragms. A Si3N4 layer was incorporated in the diaphragm structures to protect the ZnO film from the etchants of chromium and gold used for patterning the top electrodes. The square diaphragms of different dimensions were fabricated using standard IC fabrication techniques and bulk micromachining process. A mechanical jig has been used for protecting the front side (ZnO film side) of the wafer from ethylenediamine pyrocatechol water (EPW) during the anisotropic etching of silicon. The proposed technique can be advantageously applied for the fabrication of various MEMS devices such as micro-sensors, micro-actuators, accelerometers, SAW devices, bulk acoustic resonators, RF switches and bio-sensors etc.
9:00 PM - DD3.12
Optimization of the Geometry of the MEMS Electrothermal Actuator to Maximize In-Plane Tip Deflection.
Edward Kolesar 2 , Jeffrey Tippey 2 , Brandon Least 2 , Thiri Htun 2 Show Abstract
2 Department of Engineering, Texas Christian University, Fort Worth, Texas, United States
Several microactuator technologies have been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion represent the most common modes of microactuator operation. This investigation optimized the geometry of the asymmetrical electrothermal actuator to maximize its in-plane deflection characteristics. The motivation was to present a unified description of the behavior of the electrothermal actuator so that it can be adapted to a variety of microsensor and microactuator applications. The MEMS polysilicon surface micromachined electrothermal actuator uses resistive (Joule) heating to generate differential thermal expansion and movement. In the traditional asymmetrical electrothermal actuator design, the single-hot arm is narrower than the cold arm, and thus, the electrical resistance of the hot arm is greater. When an electrical current passes through the device (both the hot and cold arms), the hot arm is heated to a higher temperature than the cold arm. This temperature differential causes the hot arm to expand along its length, thus forcing the tip of the device to rotate about its flexure. In this investigation, a 3D model of the electrothermal actuator was been designed, and its geometry was optimized using the finite-element analysis (FEA) capabilities of the ANSYS computer program. The electrothermal actuator's geometry was systematically varied to establish optimum values of several critical geometrical ratios that maximize tip deflection. The value of the ratio of the length of the flexure component relative to the length of the hot arm was discovered to be the most sensitive geometrical parameter ratio that maximizes tip deflection.
9:00 PM - DD3.13
Polymer Gel-gate Field Effect Transistor for Quantitative Detection of Sugar Molecules.
Akira Matsumoto 1 , Naoko Sato 1 , Toshiya Sakata 1 , Chiho Kataoka 2 , Kazunori Kataoka 1 , Yuji Miyahara 1 2 Show Abstract
1 Center for NanoBio Integration, The University of Tokyo, Tokyo Japan, 2 Biomaterials Center, National Institute for Materials Science , Tsukuba Japan
Development of a Field Effect Transistor (FET) device capable of detecting cell membrane dynamics in a noninvasive manner is being attempted. Phenylborate derivative is used as a functional molecule exhibiting specific bindings with carbohydrate chains, particularly targeting to sialic acid, which generally exists with the largest population of all other sugar moieties on the cell membrane. Modification of the transistor gate with phenylborate moiety may therefore lead to a promoted cell adhesion onto the gate surface. Further, due to negative charges on the sialic acid molecules, construction of “cell transistor” may be possible through which cell membrane dynamics can be continuously and quantitatively analyzed. The introduction of phenylborate moiety onto the gate surface was accomplished by forming a copolymer gel: poly(N-isopropylacrylamide-co-3-acrylamidophenylboronic acid) (9:1 in molar ratio = NB10 gel) covalently attached onto the gate. The gel can undergo a reversible volume changes in response to the change in sugar concentrations. For the check of sugar recognizing ability of the transistor, it was tested for its glucose sensitivity. Change in threshold voltage (VT) of the gel-modified transistor was monitored with varying glucose concentrations using an FET realtime analyzer. It was clearly seen that the VT value increases with increase of the glucose concentration in linear or non-linear manners depending on the temperatures. Reversibility was also confirmed. Based on systematic evaluation of the glucose-sensitivity and comparison to previously reported phase diagrams of the NB10 gel, it was suggested that the glucose-dependent volume change causes a change in permittivity at the vicinity of the gate surface resulting in a change in VT. In ordinary types of FET devices, change in charge density is detectable only within the Debye length, which can reach in length no greater than 10 nm. Noteworthy is that, in the present gel-based device, very drastic and overall volume change is induced and, consequently, despite the large gel thickness, distinctive signal with high S/N ratio can be achieved. Under the physiological pH (7.4), binding constant between phenylborate group and sialic acid is about 10 times higher than that between phenyborate and glucose. Thus detection of sialic acid can be expected with an even better S/N ratio using the herein-proposed FET device. Further, some derivatives of sialic acid moieties are known, whose composition and density on the cell membrane vary depending on type of cells and between those of normal and diseased. With ability to distinguish such differences of cell membranes on molecular level, and to do so continuously in a noninvasive manner, the device may represent a candidate for a new type of cytology.
9:00 PM - DD3.14
Electroosmotic Micro-pump for Local Control of Droplets.
Amit Gupta 1 , Heather Denver 2 , Amir Hirsa 2 1 , Julie Stenken 3 , Diana-Andra Borca-Tasciuc 2 Show Abstract
1 Chemical Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Chemistry and Chemical Biology Department, Rensselaer Polytechnic Institute, Troy, New York, United States
This work demonstrates the principle of localized, low-voltage DC electroosmotic pumping across thin membranes- a novel approach to transport liquid droplets onto and off a surface. Localization of the pumping effect is obtained by employing ring electrodes, microfabricated on each side of a nanoporous alumina membrane and coating the membrane surface with a non-wetting agent, except for the designated pumping site. Low voltage actuation is achieved by having the separation of the two electrodes in the microscale range- the thickness of the membrane. The actuation voltage is more than an order of magnitude lower than typically used in electroosmotic pumping applications. The measured electroosmotic velocity increases linearly from 40 µm/s at 5V to 200 µm/s at 20V. Experimental results compare well with predictions of an electroosmotic flow model considering the nonuniform distribution of the electric field between the electrodes. These results establish the potential for microfabricated devices employing local, cross-membrane electroosmotic pumping. Possible applications include addressable liquid microlens arrays, reconfigurable, two-dimensional photonic crystals and highly localized sampling of extracellular fluid for brain studies.
9:00 PM - DD3.15
Aligned Low Temperature Wafer Bonding for MEMS Manufacturing: Challenges and Promises.
Viorel Dragoi 1 , Thorsten Matthias 1 , Gerald Mittendorfer 1 , Paul Lindner 1 Show Abstract
1 Technology Development, EV Group, Sankt Florian Austria
Wafer bonding became during last decade a very attractive technology for applications in various fields. Due to the specific 3D architecture of Micro-Electro-Mechanical Systems (MEMS), wafer bonding was relatively easy adopted as an important component of MEMS manufacturing tool box.The “early MEMS” applications (e.g. pressure sensors) were using mainly the advantage offered by this technique to join two substrates (typically silicon-silicon or silicon-glass), eventually with possibility to encapsulate certain vacuum levels. The complexity of current MEMS devices impose new requirements for wafer bonding. Among these can be mentioned low process temperature (<400°C), precise optical alignment of substrates, ability to bond a large variety of substrates and the possibility to bond with defined intermediate layers.Important efforts were focused during last years on developing various wafer bonding processes to fulfill the above requirements. The most used wafer bonding processes are: anodic bonding (known also as electrostatic assisted bonding) of silicon and glass, direct (fusion) bonding (based on molecular bonds between the two surfaces in contact), adhesive bonding using polymer intermediate layers, eutectic bonding (based on formation of an eutectic alloy as bonding layer), metal diffusion bonding (based on formation of metal bonds) or glass frit bonding (using low melting temperature glass as bonding layer).An important aspect in aligned wafer bonding is that alignment accuracy needs to be correlated to the type of bond process. Especially in the case of processes using intermediate layers the post-bond alignment accuracy will be given by the behavior of the bonding layers.If for mainstream IC industry standardization solves numerous technical problems, in MEMS and particularly in wafer bonding the lack of standards imposes additional challenges to MEMS manufacturing from the design stage and going through the entire pr