Meetings & Events

1998 MRS Fall Meeting & Exhibit

November 30 - December 4, 1998 | Boston
Meeting Chairs:
 Clyde L. Briant, Eric H. Chason, Howard E. Katz, Yuh Shiohara

Symposium AA—Materials Science of Microelectromechanical System (MEMS) Devices

-MRS-

Chairs


        

Arthur H. Heuer, Case Western Reserve Univ 
S. Joshua Jacobs, Texas Instruments

* Invited paper

SESSION AA1: MECHANICAL AND PHYSICAL PROPERTIES
Chair: Arthur H. Heuer
Tuesday Morning, December 1, 1998
St. George B/C/D (W)
8:50 AM CHAIRMEN'S REMARKS

9:00 AM *AA1.1
ADVANCES IN MECHANICAL TESTING OF SILICON FOR MICROELECTROMECHANICAL SYSTEMS. Roberto Ballarini , Department of Civil Engineering, Case Western Reserve University, Cleveland, OH.

The mechanical properties of materials used to fabricate microelectromechanical systems (MEMS) is a topic that has received significant attention in the past few years. Polycrystalline silicon is the most widely used structural material in surface micromachined MEMS devices. The first part of this talk will first present a critical review of recently developed experimental techniques to measure, using specimens with characteristic dimensions comparable to MEMS devices, its Young's modulus, Poisson's ratio, tensile strength, fracture toughness, and fatigue crack growth rates. The second part of the talk will describe, in detail, the author's development of a strength/fracture/fatigue specimen that is fully integrated with a simultaneously microfabricated electrostatic actuator, and thus allows on-chip testing without the need of an external loading device, and without any possible influences from external sources.

9:30 AM AA1.2
ENVIRONMENTAL EFFECTS ON CRACK GROWTH IN MICROMACHINED SINGLE CRYSTAL SILICON STRUCTURES. A.M. Fitzgerald , Stanford Univ, Dept of Aeronautics and Astronautics, Stanford, CA; R. Suryanarayanan Iyer, R.H. Dauskardt, Dept of Materials Science; T.W. Kenny, Dept of Mechanical Engineering.

Long-term reliability and lifetime prediction of MEMS devices fabricated from micromachined single crystal silicon requires understanding of the fracture and crack growth processes in this material. Recent studies in the literature have shown that micro-scale silicon structures may be susceptible to slow crack growth by stress-corrosion processes; however, the effects of stress and environment on crack growth rate are unclear. In this study, we present crack growth data for micromachined single crystal silicon structures as a function of applied stress intensity and environmental variables (humidity and temperature). We use a special double cantilever beam geometry to obtain stable crack propagation in our samples. The samples are fabricated using standard lithographic techniques and released from the wafer in a High Density Low Pressure (HDLP) plasma etcher built by STS Ltd. Tests were conducted in controlled environments and loading conditions. Direct measurements of crack growth were made via optical and electrical resistance techniques. Results will be discussed with respect to operative mechanisms and their implications for MEMS device reliability.

9:45 AM AA1.3
DEPENDENCE OF SILICON FRACTURE STRENGTH AND SURFACE MORPHOLOGY ON DEEP REACTIVE ION ETCHING PARAMETERS. Kuo-Shen Chen , Arturo A. Ayon, Kevin A. Lohner, Mark A. Kepets, Terran K. Melconian, S. Mark Spearing, Massachusetts Institute of Technology, Cambridge, MA.

The development of a high power-density micro-gas turbine engine is currently underway at MIT. The initial goal is to produce the components by deep reactive ion etching (DRIE) single crystal silicon. The capability of the silicon structures to withstand the very high stress levels within the engine limits the performance of the device. This capability is determined by the material strength and by the achievable fillet radii at the root of turbine blades and other etched features rotating at high speeds. These factors are strongly dependent on the DRIE parameters. Etching conditions that yield large fillet radii and good surface quality are desirable from a mechanical standpoint. In order to identify optimal dry-processing conditions, a mechanical testing program has been developed. The designed experiment involves a matrix of 55 silicon wafers with radiused hub flexure specimens etched under different DRIE conditions. The resulting fracture strengths are determined through mechanical testing, while SEM analysis is being used to characterize the corresponding fillet radii. The test results will provide the basis for process optimization of micro-turbomachinery fabrication and play an important role in the overall engine redesign.

10:30 AM AA1.4
INFLUENCE OF SPECIMEN SIZE AND SUB-MICRON NOTCH ON THE FRACTURE BEHAVIOR OF SINGLE CRYSTAL SILICON MICROELEMENTS AND NANOSCOPIC AFM DAMAGE EVALUATION. Kohji Minoshima , Kyoto Univ, Dept of Mechanical Engineering, Kyoto, JAPAN; Shigemichi Inoue, Hitachi Ltd., Hitachi, JAPAN; Tomota Terada, Kenjiro Komai, Kyoto Univ, Dept of Mechanical Engineering, Kyoto, JAPAN.

Simple bending tests of single-crystal silicon microelements fabricated by photoetching were performed. For this purpose, the authors have developed a specially designed testing machine (load range: 0.1 mN - 5N, accuracy: 0.02 mN), which enables mechanical testing including fatigue of microelements. Force was applied to a specimen by means of an electromagnetic actuator, and the displacement was measured with a differential transformer. Mechanical testing including fatigue of microelements could be performed with sufficient precision. Single-crystal silicon microelements deformed elastically until final catastrophic failure, showing a brittle nature. Although the elastic modulus of single-crystal Si was independent of specimen size, the fracture strength increased with a decrease in specimen size, and the maximum strength became over 8 GPa. Some fatigue tests were conducted in laboratory air and in deionized water: water reduced the strength of microelement under fatigue loading. A Focused ion beam was also used to machine a sub-micron scale notch at a root of a miscocantilever. Such a small sub-micron size notch decreased the fracture strength of a microelement. Fracture surface and sample surface were closely examined with a scanning electron microscope and an atomic force microscope, and the fracture mechanisms were discussed from the nanoscopic points of view.

10:45 AM AA1.5
MECHANICAL PROPERTIES OF THIN POLYSILICON FILMS BY MEANS OF PROBE MICROSCOPY. Wolfgang G. Knauss , Ioannis Chasiotis, Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA.

A new method for tensile testing of thin films is being developed. The method employs Atomic Force Microscope (AFM) or Scanning Tunneling Microscope (STM) acquired surface topologies of deforming specimens to determine (fields of) strains. An electrostatic grip apparatus was designed and implemented to measure the elastic and failure properties (Young's modulus, Poisson's ratio, tensile strength) of surface micromachined polysilicon specimens. The tensile specimens are 'dog-bone' shaped ending in a large 'paddle' for electrostatic gripping. The test section of the specimens is 400 microns long and measures 2 microns x 50 microns in cross section. The deformation of a part of the test section is monitored by an AFM. The acquisition of surface topographies before and after deformation allows the deduction of strains by means of the method of Digital Image Correlation (DIC). Thus this method uses naturally occurring marks on the surface for defining the gage for the deformation process. No additional markings need to be added to the surface. The effect of other parameters on property measurements, such as surface roughness, has been examined computationally. Results of the tensile tests on small specimens derived from this effort will be presented.

11:00 AM AA1.6
RAPID THERMAL ANNEALING FOR RESIDUAL-STRESS RELAXATION IN PHOSPHORUS OR BORON DOPED POLYSILICON THIN FILMS. Xin Zhang, Tong-Yi Zhang and Yitshak Zohar, Dept of Mechanical Engineering, Hong Kong Univ of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, CHINA.

Rapid thermal annealing (RTA) reduces or eliminates residual stresses in doped polysilicon thin films in a few seconds. In this study, residual stresses, materials characterization and surface analysis in either phosphorus or boron doped polysilicon films at various RTA stages are fully investigated. The as-deposited 0.5 um-thick doped polysilicon films have compressive stress in the range of 100 350 MPa depending on the doping element and the doping level. The residual stresses are relaxed quickly after a few cycles of RTA at high temperatures. Further rapid thermal annealing at these temperatures changes even the state of the residual stress from compression to tension. Using micro Raman spectroscopy (MRS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), we have studied changes in the microstructure of the thin films and variations in the composition of the surface layer during the stress relaxation. The experimental results suggest that the residual stress evolution during rapid thermal annealing is induced by two main factors, partial grain growth and nitridation at the top surface of the doped film, and the surface nitridation dominates the final stage of residual stress during rapid thermal annealing.

11:15 AM AA1.7
STRESS RELAXATION AND CYCLIC SOFTENING OF ALUMINUM MICROBEAMS. G. Cornella , J.C. Bravman, Stanford University, Department of Materials Science and Engineering, Stanford, CA; R.P. Vinci, Lehigh University, Department of Materials Science and Engineering, Bethlehem, PA.

Knowledge of mechanical properties is essential for successful modeling and design of MicroElectro Mechanical Systems. Fatigue behavior, for instance, may ultimately limit product lifetime in certain applications. Bulk aluminum is known to exhibit a lack of an endurance limit, and is therefore subject to fatigue even at extremely small strains. Despite this, released thin film aluminum torsion hinges have been reported to suffer no obvious sign of fatigue damage. Plasticity of metal films on stiff substrates has been studied extensively, but thin film fatigue processes and plasticity of released metal films are largely unknown.
Uniaxial tension-tension tests were performed in order to investigate effects of cyclic loading on mechanical behavior. The samples consisted of aluminum beams 2 $\mu$m thick, 50 $\mu$m wide and 600 $\mu$m long patterned over a window in a silicon substrate. Cyclic load-displacement behavior was recorded during constant amplitude strain-controlled testing. The peak load relaxation behavior was modeled using a phenomenological relaxation law. Static stress relaxation tests were performed for comparison. In both cyclic and static tests, significant stress relaxation was observed below the nominal yield stress. Deformation kinetics models were used to evaluate the mechanisms acting in order to determine the effect of cycling on relaxation behavior.

11:30 AM AA1.8
MICRO-TENSILE AND FATIGUE TESTING OF COPPER THIN FILMS ON SUBSTRATES. Martina Hommel , Oliver Kraft, Eduard Arzt, Max-Planck-Institut fuer Metallforschung, Stuttgart, GERMANY; Shefford P. Baker, Department of Materials Science & Engineering, Cornell University, Ithaca, NY.

For technical applications, the reliability of microelectromechanical devices has become an important issue, in particular, the mechanical properties of the materials used may determine the lifetime of devices. It is well known that in small dimensions the mechanical properties of materials differ from their bulk values. The design of reliable MEMS structures requires the knowledge of the mechanical properties, such as yield strength, fracture toughness or fatigue resistance, of the thin film materials used.
In this paper, we present results obtained by using a specially developed micro-tensile tester, which can be mounted in an x-ray diffractometer. This provides the unique opportunity to study the development of stresses in thin metal films in-situ during tensile loading. Copper thin films were magnetron-sputtered onto polyimide substrates. Due to the compliant nature of the polyimide, the film/substrate composite can be subjected to large strains where only the film deforms plastically. Using this technique, we studied the yield behavior of the copper films as a function of their thickness, grain size and texture. It was found that the film strength increases with decreasing thickness and grain size. Finally, the method was extended to investigate the plastic behavior of the films under cyclic loading where the film is subjected to a compressive stress state when the substrate is unloaded. As a result, the film material undergoes plastic deformation under tension and compression in each cycle. It was found that the yield strength increases within the first load cycles (``cyclic hardening'').

11:45 AM AA1.9
TORSION TESTING OF DIFFUSION BONDED LIGA FORMED NICKEL. T.R. Christenson , Electromechanical Engineering Dept., T.E. Buchheit, Materials Performance and Reliability Dept., D.T. Schmale, Direct Fabrication Dept., Sandia National Laboratories, Albuquerque, NM.

Diffusion bonding has recently been applied to the batch fabrication of multi-level mechanical LIGA structures. The process accommodates precision aligned substrates decorated with LIGA patterned geometry in a hot press to allow diffusion bonding between two separate layers. The mechanical strength of LIGA deposited materials, however, degrades when subjected to higher temperatures due to grain growth, restricting the temperature that can be applied during the bonding process. To characterize the bond stregth versus temperature of diffusion bonded LIGA structure a method to measure a distributed array of test structures across a substrate has been developed. The resulting test method uses an arrangement consisting of a motorized micrometer driven into a load cell, and LVDT displacement transducer, and the end of a torsion bar which provides a torsion load to a 3mm diameter circular array of six precisely located gauge pins. These gauge pins are then aligned and mated to a LIGA formed scalloped circular ring that is the diffusion bonded test specimen. This scheme is amenable to the testing of a large array of specimens over a given substrate to accumulate a significant amount of data and yield a measure of the variation in bond strength versus temperature and bond geometry. A detailed description of the torsion testing machine and generated data will be presented.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC-04-94AL85000.

SESSION AA2: MECHANICAL AND PHYSICAL PROPERTIES (continued)
Chair: S. Mark Spearing
Tuesday Afternoon, December 1, 1998
St. George B/C/D (W)
1:30 PM AA2.1
TAILORING THE MECHANICAL PROPERTIES OF LIGA SYNTHESIZED MATERIALS AND COMPONENTS. T.E. Buchheit , Materials Performance and Reliability Dept., T.R. Christenson, Electromechanical Engineering Dept., D.T. Schmale, Direct Fabrication Dept., Sandia National Laboratories, Albuquerque, NM.

The LIGA fabrication process in an additive process in which structural material is electrodeposited into a precision mold of PMMA realized through deep x-ray lithography. Representative component dimensions range from 5 microns to about 10 millimeters with thickness usually between 100 microns - 500 microns and tolerances less than 1 micron. LIGA fabricated materials exhibit several processing issues affecting their metallurgical and mechanical properties currently limiting their usefulness for MEMS applications. For example, LIGA processing is known to be very sensitive to deposition conditions which results in processing lot variations of mechanical and metallurgical properties. Furthermore, the electroforming fabrication process produces a highly textured lenticular microstructural morphology suggesting an anisotropic material response. Understanding and controlling out-of-plane anisotropy is desirable for LIGA component designs with out-of-plane flexures. Previous work (Spring '98 MRS Meeting - Symposium N) by the current authors which focused on results from a miniature servo-hydraulic mechanical test frame built for testing LIGA materials demonstrated microstructural and mechanical properties dependencies with plating bath current density in LIGA fabricated nickel. This presentation builds on that work to address these issues and foster a methodology for controlling the properties of LIGA fabricated materials through processing. New results include measurement of out-of-plane and localized mechanical properties using nanoindentation and compression testing, and exploitation of the magnetic properties of LIGA fabricated nickel to impart changes on its microstructure during processing.
Sandia is a multiprogram laboarotary operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy under contract DE-AC-04-94AL85000.

1:45 PM AA2.2
EXTRACTION OF THE COEFFICIENT OF THERMAL EXPANSION OF THIN FILMS FROM BUCKLED MEMBRANES. Volker Ziebart , Henry Baltes, ETH Zurich, Physical Electronics Lab, Zurich, SWITZERLAND; Oliver Paul, Univ of Freiburg, Inst of Microsystem Technology, Freiburg, GERMANY.

We report a novel method to measure the temperature-dependent (TD) coefficient of thermal expansion $\alpha(T)$ of thin films. The method exploits the TD buckling of clamped square thin film plates. It was used to extract $\alpha(T)$ of PECVD silicon nitride in the temperature range of 298 K to 420 K.
The deflection profile of a buckled thin film membrane changes with the temperature in response to the thermal expansion of the thin film and the supporting substrate, characterized by respective coefficients $\alpha_{tf}(T)$ and $\alpha_{sub}(T)$. If Poisson's ratio and the dimensions of the membrane are known, $\alpha_{tf}(T)$ can be extracted from the TD buckling deflection.
This extraction requires an accurate model of square membrane buckling far beyond the limits of simple critical buckling. Such a model were developed using finite element simulations and an energy minimization method. Both approaches show excellent agreement, even for large initial strains where complex buckling shapes were observed. By analyzing the measured TD membrane deflections using this model, the TD strain of the thin film on its substrate is extracted. From this, $\alpha_{tf}(T)$ is finally deduced.
We applied the method to 3.55 $\mu$m thick PECVD silicon nitride films on silicon wafers. Square membranes with side length 2.62 mm were released using silicon bulk micromachining. Center deflections ranged from 0 $\mu$m to 18 $\mu$m between 298 K and 420 K. From these results, with the independently measured Poisson's ratio and published values for $\alpha_{sub}(T)$ we obtained

\begin{displaymath}
        \alpha_{tf}(T)=[1.9+4\times10^{-3}(T-298 {\rm
        K})]\times10^{-6} {\rm K}^{-1} \end{displaymath}

for the PECVD silicon nitride.



2:00 PM AA2.3
DETERMINING THE HIGH-TEMPERATURE PROPERTIES OF THIN FILMS USING BILAYERED CANTILEVERS. Haruna Tada , Ioannis N. Miaoulis, and Peter Y. Wong, Thermal Analysis of Materials Processing Laboratory, Mechanical Engineering Department, Tufts University, Medford, MA; Patricia Nieva and Paul Zavracky, Microfabrication Laboratory, Department of Electrical and Computer Engineering, Northeastern University, Boston, MA.

Designing and fabricating MEMS for high-temperature applications (above 200$^{\circ}$C) require accurate data of thermo-mechanical properties of different MEMS materials for proper modeling of the structure behavior. Properties of thin films vary from bulk properties, and must be studied for MEMS materials such as silicon, silicon nitride, silicon dioxide, and other thin films. The variables in thermo-mechanical properties of thin films include the stoichiometry of the film, crystallinity, and quality of the films as fabricated with standard and advanced VLSI technologies. Thermo-mechanical properties (e.g., coefficient of thermal expansion, elastic modulus, and yield strength) of thin films are determined through a combination of indirect experimental measurements, analytical expressions, and numerical analysis. Bilayered cantilever beams made of thin-films were designed to simultaneously determine the properties of two thin-film materials. The beams are fabricated using SOI wafer with 1 $\mu$m silicon on 1 $\mu$m silicon dioxide layer. The two layers are patterned to define the beam shape, then the beam is released from the substrate by anisotropic wet etching with TMAH. The resulting beams are suspended over silicon substrate, and the difference in thermal expansion coefficients between the top and bottom layers cause the beam to deflect down and touch the substrate at some temperature. The temperature at which contact is made depends on the thermal expansion coefficients and the elastic moduli of the film materials, as well as the length of the beam and the relative widths of the layers. During testing, the specimen are heated to a known temperature, then brought back to room temperature for inspection. Adhesion between the beam and substrate is required to permanently record the contacts made at high temperature. The material properties at the temperature can be calculated by comparing the experimental results to the analytical model of curvature of bilayered cantilever beams.

2:15 PM AA2.4
THIN FILM STRESS MEASUREMENT WITH A TUNNELING SENSOR. Ping Zhang , Richard P. Vinci, John C. Bravman, Stanford Univ, Dept of Materials Science and Engineering, Stanford, CA; Thomas W. Kenny, Stanford Univ, Mechanical Engineering Dept, Stanford, CA.

In thin films, especially bi-layer or multi-layer thin films used for MEMS applications, film stress is a key issue to consider. High levels of stress through a film, and stress gradients across a film, may lead to device failure, while stress variations across a wafer may limit the uniformity and/or yield of MEMS devices. While many ex-situ film stress measurement techniques have been developed, in situ stress monitoring during film deposition remains largely unexplored, especially for films deposited in production-grade equipment. In-situ monitoring, it is hoped, will provide valuable information on the evolution and distribution of stress within a film. This should enable deposition conditions to be optimized in order to minimize stress and thus improve device performance.
The device we are developing for in-situ stress monitoring is a tunneling sensor based on an IR detector previously developed at Stanford. The device consists of a deflection membrane (approximately 2x2mm), capacitive deflection electrodes, and a tunneling tip, together with feedback circuitry to achieve force-balanced working conditions for the membrane. A small bias voltage is applied between the tip and the membrane, and a deflection voltage is applied between the membrane and the bottom deflection electrodes. As the membrane is pulled towards the tip by the deflection voltage, significant electron tunneling occurs between the tip and the membrane at a distance of about one nanometer. At this point, the electrostatic force generated by the deflection voltage force balances the force exerted by the thin film (due to stress) deposited on top of the supporting deflection membrane. Therefore, changes in stress are detected by changes in deflection voltage, with the aid of the feedback circuitry. In this paper we will describe the fabrication and functioning of this in-situ monitoring device, and address issues such as the range of stress accessible to measurement, thermal noise, and compatibility with standard deposition equipment.

2:30 PM AA2.5
MECHANICAL PROPERTIES OF SU-8. Andrew McAleavey, George Coles, Johns Hopkins University, Department of Mechanical Engineering, Baltimore, MD; Richard L. Edwards, Johns Hopkins University Applied Physics Laboratory, Laurel, MD; William N. Sharpe, Jr. , Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD.

Most current MEMS devices employ chemically vapor deposited polycrystalline silicon as their major structural material. Polysilicon is appropriate for some MEMS applications, like accelerometers and pressure transducers, but often, materials which can transmit higher forces and torques are needed. Thicker MEMS materials which meet this criterion can currently be manufactured by the LIGA process, which can produce thick metallic components, on the order of 200 mm, with a high aspect ratio.
The negative-tone UV photoresist SU-8, invented by IBM and marketed by MicroChem Corp. under the NANOô XP SU-8 brand name, is a promising material for low-cost thicker MEMS components. Micromechanical structures can be manufactured in SU-8 by conventional photolithographic techniques. Reports in the literature have focused on SU-8's high-aspect-ratio (18:1), thickness and chemical resistance, but its mechanical properties remain largely undetermined.
Efforts are underway at Hopkins to manufacture microtensile test specimens with SU-8 and perform tensile tests to determine the elastic modulus, yield and ultimate strengths of this material. The specimens are 3 millimeters in overall length with wedge-shaped grip ends, and have a gage cross-section of 200 micrometers wide by 100 micrometers thick. The overall specimen shape is that of a dog biscuit.
The SU-8 specimen preparation process will be described, procedures for microtensile testing will be reviewed and explained, and tensile test results will be presented.

2:45 PM AA2.6
A THERMOMECHANICAL MATERIALS DATA BANK FOR MEMS MATERIALS. H.J. Fecht , B. Kocdemir and M. Mahlich, University of Ulm, Ulm, GERMANY.

The technology of microsystems allows the integration of electronic, acoustic, mechanical, thermal, optical and magnetic functions within one product. Examples are actuators, sensors, electronic packaging, metallization of integrated circuits, optoelectronic devices and barriers for atomic and heat transport resulting in multiple stacking of materials. Due to the multifunctional demands a wide variety of materials is being used uncluding semiconductors, insulators, metals and alloys, polymers as well as special functional materials. The tendency to a continuous miniaturization and the corresponding increase in the density of integration is a further challenge to the materials in use. The mechanical properties governing elasticity, strength, creep, fatigue and fracture toughness are additionally controlled by defects, such as grain and interphase boundaries, pores etc. typically in the submicrometer range. Therefore, the thermomechanical properties and also atomic or thermal transport properties can be completely different for single crystalline (epitaxial), polycrystalline, nanocrystalline or amorphous thin films. These issues of the reliability of microsystems become even more important at elevated temperatures. An short review will be given on critical materials issues of MEMS devices used for sensors and high power applications. Furthermore, the importance of a special database for such devices has been realized recently by a research consortium including several industrial and University institutions. As such, an overview will be given on the type of materials and type of data forming the data bank (Simiko) as well as the types and results of thermomechanical measurements leading to an improvement of lifetime and reliability of MEMS. The financial support by the German Ministry for Research and Technology (BMBF, Simiko) is gratefully acknowledged.

SESSION AA3: ADHESIONS AND COATINGS
Chair: S. Mark Spearing
Tuesday Afternoon, December 1, 1998
St. George B/C/D (W)
3:30 PM *AA3.1
MICROMECHANICS AND SURFACE MODIFICATION METHODS FOR REDUCING ADHESION IN MICROELECTROMECHANICAL SYSTEMS. K. Komvopoulos , Department of Mechanical Engineering, University of California, Berkeley, CA.

The rapidly evolving field of microelectromechanical systems (MEMS) is expected to lead to new technologies with impacting effects on science and engineering. The interdisciplinary nature of MEMS devices has generated a high demand for integrating basic knowledge of mechanical, electrical, chemical, and thermal phenomena encountered at the microscale, and the identification of novel methods for the development of versatile micromachines. As the growth of MEMS continues to increase at a high rate, micromachine reliability and long-term durability are expected to assume even greater importance. Due to the low stiffness of most micromachine devices, the development of high attractive forces at MEMS interfaces often leads to permanent surface adhesion, a phenomenon known as stiction. The importance of various stiction mechanisms, such as solid bridging due to residue precipitation at small interfacial gaps, van der Waals force, liquid meniscus (capillary), and electrostatic charging, is discussed in light of results obtained from a new stiction theory of MEMS devices based on the characterization of approaching surfaces by fractal geometry and the elastic-plastic material response at asperity microcontacts. The efficacy of various surface modification methods to reduce the magnitude of the total stiction force at MEMS interfaces is analyzed, and simulation results for the critical micromachine stiffness required to overcome stiction are presented for different material systems possessing different surface roughness. It is demonstrated that the magnitudes of adhesive forces can be reduced significantly by reducing the real area of contact through surface texturing and fabrication of micro-bumps, passivation of the surfaces with hydrophobic thin solid films (e.g., diamond-like carbon), and alteration of the surface chemical state by the adsorption of low surface energy substances (e.g., self-assembled monolayers).

4:00 PM AA3.2
ADHESION HYSTERESIS OF COATED POLYSILICON BEAMS IN CONTROLLED HUMIDITY AMBIENTS. M.P. de Boer 1, J.A. Knapp1, R. Maboudian2, U. Srinivasan2, T.M. Mayer1, and T.A. Michalske1. 1Sandia National Laboratories, Albuquerque, NM; 2Dept. of Chemical Engineering, Univ. of CA, Berkeley, CA.

An important reliability unknown in MEMS is the adhesion of structures to each other in various ambients. If initially non-contacting structures come into contact while in operation, interfacial forces may cause them to adhere. If they remain adhered, these forces may increase over time, giving rise to the phenomenon of adhesion hysteresis. It is important to quantify and develop chemical models for adhesion hysteresis so that the reliability of MEMS structures is characterized and understood. Here we develop mechanics for, and report on measurements of, adhesion hysteresis in surface micromachined polycrystalline silicon beams subject to dry and wet ambients. The measurements reflect in-situ values taken with our Environmental Inteferometric Microprobe Station on test structures activated near the support post. Beams are treated with hydrophobic molecular coatings such as octadecyltrichlorosilane (ODTS, C18H37SiCl3) or perfluorodecyltrichorosiliane (FDTS, C10H4F17SiCl3). Results indicate that apparent adhesion values increase after an incubation period at relative humidities (RH) of 90$\%$ and above. By measuring apparent energies during crack healing and repropagation, we determine that the higher energy values are observed only upon repropagation. Hence the forces involved are very short range. Furthermore, upon repropagation, we observe apparent non-uniformities in the adhesion energy, suggesting that the high humidity environment induces localized breakdown of the molecular film coating.

4:15 PM AA3.3
VACUUM DEPOSITED FLUORINATED ALKYL SILOXANE FILMS FOR ADHESION CONTROL IN MEMS DEVICES. T.M. Mayer , M.P. de Boer, N.D. Shinn, P.J. Clews, T.A. Michalske, Sandia National Labs, Albuquerque, NM.

Monolayer films of polymerized alkyl siloxanes have been employed for surface passivation and adhesion control in MEMS devices. However, reproducible film formation and properties have been difficult to achieve due to process sensitivity to substrate preparation conditions, presence of small quantities of adsorbed water, and high aspect ratio structures typical of MEMS devices. In contrast to the normal solution coating process using alkyl trichlorosilane precursors, we have developed a vacuum-based film deposition process, using volatile fluorinated alkyl trichloro silane precursors. Reproducible substrate conditions are obtained by plasma oxidation followed by sequential or simultaneous exposure to the chlorosilane precursor and water vapor. Efficient transport of reactants into high aspect ratio structures is accomplished by maintaining Knudsen flow conditions at low pressures. We measure kinetics of film growth by in-situ ellipsometric and quartz-crystal microbalance techniques, and evaluate film composition and structure by XPS and IR spectroscopies. We also measure the work of adhesion and surface energy of coated cantilever beams under equilibrium fracture mechanics conditions. We compare results to uncoated structures, and to structures coated from solution with alkyl and fluoro-alkyl siloxane films.
Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the U. S. Dept. of Energy under contract DE-AC04-94AL85000.

4:30 PM AA3.4
PARTIALLY FLUORINATED ORGANIC THIN FILMS FOR COATINGS APPLICATIONS. Michael Graupe, Thomas Koini, Ramon Colorado, Jr., Hyun I. Kim, Oloba Olugbenga, Nupur Garg, Yasuhiro F. Miura, Mitsuru Takenaga, Scott S. Perry and T. Randall Lee , Department of Chemistry, University of Houston, Houston, TX.

Our research explores the molecular basis for wetting, adhesion, and friction in fluorinated thin films. We construct well-defined thin films via the self assembly of alkanethiols and partially fluorinated alkanethiols on the surface of gold. Films generated from simple alkanethiols, CH3(CH2)nSH (where n = 9-15) are fully characterized, and the effects of systematically introducing fluorine into the films are then examined. We characterize the films using a variety of techniques including contact angle goniometry, atomic force microscopy (AFM), ellipsometry, and infrared spectroscopy (FT-IRRAS). Films generated from CF3(CF2)m(CH2)n-mSH (where n = 9-15; m = 0-9) will be compared to those generated from the normal alkanethiols. The properties of both single-component and mixed-monolayer films will be discussed.

SESSION AA4: POSTER SESSION
Tuesday Evening, December 1, 1998
8:00 P.M.
America Ballroom (W)
AA4.1
PARTICULARY ASPECTS OF RESIDUAL STRESS IN THICK MEMBRANES BY BORON DIFFUSION PROCESSES. Elena Manea , Ileana Cernica, Ralu Divan, National Institute for Research and Development of Microtechnologies, Bucharest, ROMANIA.

Micromachining techniques have an increasing importance in the development of microsensors applications. We obtained thick membrane by thermal diffusion from Boron+ solid source by a performed program (concentration of 1x1020cm-3). This value is enough for stop-etch layer in anisotropy etching of silicon (100) in KOH solution at 80$^{\circ}$C. Spreading resistance has measured the Boron profile. We define square geometries for membranes, with 1.5 and 2.5 mm2 areas. One drawback of the heavily doped boron layer is that the structure made by etch stop techniques usually has residual stress, which can deteriorate the device performances. The origin of the residual stress in p+ layers is due to the different size of boron and silicon atoms. An other cause of the stress induced in the heavily doped layer is the presence of dislocations. The crystal defects are configured due to the selective removed rate (higher rate on field than in the region bounding the defect). The increase of these differences of the etching rates in the two regions (the defect place and the surrounding area) is achieved by decreasing the solution etching rate. This is particularly allowed for the silicon surface. A new solution of revealing defects on silicon, not necessitating ultrasound agitation (like Secco solution) was used: 2HF:1MCrO3:2H2O. We present in detail the chemical mechanism and the causes of the revealed defects.
Experimentally was found that the residual stress can be changed, from tensile to compressive, by post diffusion processes as oxidation and/or annealing. In this way the crystalline defects decrease and we could correlate what kind of defects have most stress effect.
The performance could be noticed from the thickness of the membranes of 10­12$\mu$m having both good uniformity and minimal stress (membrane curvature is 5 $\mu{\mu}%%\mu{$m).{\newline\newline
        \noindent{\textbf{ AA4.2 }}\newline\noinden...
        ...MEMBRANES FOR HIGH TEMPERATURE SENSORS APPLICATIONS.
        Christophe Gourbeyre $^1$, Patrick Aboughe-nze2, Christophe Malhaire1, Martine Le Berre1 ,Yves Monteil2, Daniel Barbier1; 1Laboratoire de Physique de la Matiere, INSA de Lyon, Villeurbanne, FRANCE. 2Laboratoire des Multimateriaux et interfaces UMR CNRS, Universite Claude Bernard Lyon1, Villeurbanne, FRANCE.

During a work relating to the high temperatures pressure sensors , a study was undertaken on the thermopneumatic behaviour of the membranes as well as the effect of the built-in stress on the structure. 3C-SiC films were grown on (100) Si substrate in a vertical reactor by an atmospheric-pressure chemical vapour deposition (APCVD). Silane and propane were used as precursor gas and hydrogen as carrier gas. Prior to the growth, Si surfaces were annealed at 1000ƒC for 5 minutes and carbonized with C3H8 at 1150ƒC during 10 minutes. At 1350ƒC, SiH4, C3H8 and H2 were introduced with a flow rate of 8 sccm, 9 sccm and 10 slm respectively for the SiC deposition. The atomic ratio of Si/C in the gas phase was 0.3 and the growth rate was 3 µm/h. Only the process time is varying to obtain different SiC layers thickness varying from 3 to 9 µm. Stress measurements of thin SiC layers deposited on thick Si substrates were performed at room temperature using the bending plate method and the Stoney's equation. Curvature measurements were studied by optical profilometry before and after deposition. The stress for a 3 µm thick SiC layer is evaluated to be about 200MPa in tension. The SiC layer thickness was measured by FT-IR spectrometry, ellipsometry, and SIMS crater. The surface layer roughness was also determined by optical profilometry. SiC/Si, and SiC/SiO2/Si membranes were obtained by KOH etching and studied as a function of the temperature (up to 300ƒC) and the pressure in the [0-1 bar] range. Measured deflections were in a 10% agreement with 3D finite element simulations using the Ansys 5.3 software.

AA4.4
A DIRECT MOLDING TECHNIQUE TO FABRICATE SILICA MICRO-OPTICAL COMPONENTS. Choon-Keat Terence Lee, Michael J. Vasile , Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA; Jost Goettert, Forschungzentrum Karlsruhe, Karlsruhe, GERMANY.

A technology for the direct molding and release of silica micro-optical components in polymethylmethacrylate (PMMA) molds is developed. The objectives of this work are to by-pass some of the usual steps in deep X-ray lithography (DXRL) and to determine the suitability of GR650 resin (polysilsequioxane) for molding thick, high aspect ratio structures.The process is initiated by DXRL exposure of PMMA , followed by GR650 application, a cure at 85 deg. C to set the shape in the resin, and then demolding. The cure is below the glass transition temperature of PMMA and the demolding process allows re-use of the mold.There is no need for electroforming in the replication process. PMMA molds with thickness of 250, 350 and 500 um were fabricated by DXRL and standard development. A novel metal sacrificial layer technique is used to release the molded components with about 50% yield. Micro-optical elements that were fabricated include lenses, prisms, grating structures and transmission test structures. These components are cured at 400 deg. C after release, to convert the organosilicon polymer to a silica-like material. Weight loss from the 400 deg. cure was in the range of 15 to 25% while linear dimensions of the components shrunk by less than 1%. Composition of the molded, cured structures was measured by XPS.

AA4.5
BI-LAYER PZT FILMS FOR MEMS APPLICATIONS. D.F.L. Jenkins and W.W. Clegg, Centre for Research Information Storage Technology, University of Plymouth, Plymouth, UNITED KINGDOM; G Velu, E Cattan and D. Remiens, Laboratoire des Materiaux Advances Ceramique, CRITT Ceramique Fines, Universite de Valenciennes et du Hainaut Cambresis, Maubeuge, FRANCE.

A potential application for ferroelectric thin layers is micro-positioning and actuation, with recent developments for micro-motors and micro-pumps. Today there is an increasing requirement for very precise positional control over a broad range of frequencies. For example, in scanning probe microscopy, and in particular magnetic force microscopy, there is a requirement to dynamically control the position of the cantilever used to sense the magnetic force gradients of the sample. For using PZT films as micro-actuators it is desirable to have film thicknesses of comparable size to the underlying structure. The amount of actuation possible is determined by the electric field across the film. A great advantage of ferroelectric materials (PZT) in thin layer form is the enhanced breakdown strengths (on the order of 10-7 - 10-8 V/m); as a consequence higher energy densities can be achieved, which holds promise for micro-positioning. The amount of actuation should therfore be increased by using a bi or multi-layer. PZT films of thickness 0.5 micron have been deposited as a bi layer and 1.0 micron as mono layer. Micro-actuators have been fabricated using these structures and their performance characterised and evaluated using optical beam deflection. The results of this work will be presented and the implications discussed.

AA4.6
STUDY OF SOL-GEL TECHNOLOGIES APPLIED TO MICROFLUIDIC STRUCTURES. C.M. Chan and G.Z. Cao, University of Washington, Department of Materials Science and Engineering, Seattle, WA.

Microfluidic applications provide tough materials challenges for current MEMS fabrication technologies. Silicon based technologies have proven adequate for sensors; however, to create true microfluidic systems a larger materials base is required. Furthermore, in biologically active fluids, corrosion and contamination presents other materials challenges. We report a study of sol-gel technologies applied to the fabrication of microfluidic structures. Organic/inorganic hybrid materials, particularly silica based hybrids, were developed by sol-gel processing. A weak silica network was made possible by using a deficient amount of water and acid catalyst in the sol preparation. Since the weak network has a high tendency to collapse, the result is a denser material. Incorporation of organic ligands modifies the surface chemistry of gel network so that a greater drying stress was developed and a denser hybrid structure was achieved without high temperature treatment. In addition, the incorporation of organic ligands prevented formation of cracks. Nanoscale oxide particles were dispersed and incorporated into the gel network by surface condensation. The incorporation of solid nanoparticles into sols greatly reduced the shrinkage of gels and enhanced the mechanical strength of the components. The relationships between the processing conditions, low temperature densification, and microstructure are discussed.

AA4.7
DRY ETCHING OF SOL-GEL PZT. Robert Zeto , Bernard Rod, Madan Dubey, Matthew Ervin, Richard Piekarz, Army Research Laboratory, Adelphi, MD; Susan Trolier-McKinstry, Tao Su, Joseph Shepard, Pennsylvania State University, Intercollege Materials Research Laboratory, University Park, PA.

Dry etching of sol-gel lead zirconate titanate (PZT 52/48) thin films was investigated by reactive ion etching and argon ion milling. Etched profiles were characterized by scanning electron microscopy. Reactive ion etching was conducted in a Plasma Therm 720 system using HC2ClF4, an eviornmentally safe etch gas as described by S. Desu and coworkers. Etch rates were measured and compared as a function of electrode shield (ardel, graphite, alumina) and RF power (150 to 550 watts). Etch rates varied from 10 to 100 nm/min. RIE sidewall angles 12 degrees off normal were consistently produced over a wide range of RF powers and etch times, but overetching was required to produce clean sidewalls. Argon ion milling was conducted in a Commonwealth Scientific system using 72 mPa argon chamber pressure and a 300 mA/500 V beam with an incidence angle of 80 degrees to the substrate. These initial conditions produced an etch rate of 25 nm/min with sidewall angles 55 degrees off normal. The ion milling rate for sol-gel PZT was significantly faster than reported for bulk PZT. Experiments to reduce the sidewall angle will be described. The PZT films used in this study, 500 nm thick, were prepared by the sol-gel process using methoxyethanol solvent, spin coating of Pt/Ti/SiO2/Si substrates, and rapid thermal annealing for 30 sec. at 650 $^{\cir}$C for crystallization of the perovskite phase.

AA4.8
MESO-SCALE PRESSURE TRANSDUCERS UTILIZING LOW TEMPERATURE CO-FIRED CERAMIC TAPES. Jaiyoung Park, Patricio Espinoza-Vallejos , Heather Lynch, Jorge J. Santiago-Aviles, University of Pennsylvania, Philadelphia, PA; Luis Sola-Laguna, DuPont Experimental Station, Wilmington, DE.

Pressure transducers with promising characteristics at high pressure and temperatures have been developed using low temperature co-fired ceramic (LTCC) tape technology. All parts for the transducer were machined from DuPont 951 series LTCC tapes utilizing either a numerically controlled milling machine, or an isotropic etching technique involving the removal of the glassy binder of a partially sintered LTCC tape. Device dimensions are in the meso (intermediate) scale range, with the smallest device size of 8mm in diameter, and the cavity of 2 mm. Utilizing the anisotropy induced during the casting process a chemical exfoliation technique was developed. This technique allowed us to separate the original tape in three layers, the middle one being highly elastic, isotropic and homogeneous. The middle layer can be chemically thinned to achieve membrane like behavior, with thickness of the order of 50 mm. The pressure is measured as a function of the membrane deformation where two piezo-resistors are screen printed. Two piezo-resistors were used to achieve temperature compensation. Using shrinkage matched paste, nominal thick film technology was used in the screen printing of the piezo-resistors. The rest of the transducer was fabricated using several layers of LTCC tapes, which were laminated and fired. Devices of different sizes were fabricated and compared. Computer simulations of membrane deflection as a function of the vacuum load where obtained and utilized for the design and scaling of the piezo-resistors.

AA4.9
MICROSTRUCTURAL EVOLUTION OF CO-FIRED CERAMIC TAPE FOR MEMS APPLICATIONS. Suebpong Charoenmechaikul and David E. Luzzi, Univ of Pennsylvania, Dept of Materials Science and Engineering, Philadelphia, PA.

A detailed study of the structure and thermodynamics of the densification process in a model alumina-based ceramic tape system during thermal processing is presented. Low temperature co-fired ceramic tape is a model material system for near net­shaped production of meso-scale MEMS devices. In addition to its inherent ease of patterning in the green state, the material is compatible with existing silicon technology. The primary drawback to this device fabrication technique is the contraction that occurs in the plane of the tape upon firing. One possible route to avoid this complication is through the use of post-anneal HF-based etching which also enables batch processing of multiple devices. The etching behavior should be closely linked to the microstructure of the as-fired material. In the present study, DuPont's Green Tape(TM) 951, which is primarily composed of frit ($\gt 50\%$ silica), alumina and an organic binder, is used as the model system. The microstructures of specimens annealed at temperatures between 650 C and 890 C for various times are characterized by scanning electron microscopy. The volume fraction, distribution and shape of pre-existent pores and frit particles are found to be strongly dependent on the processing conditions. Samples held at temperatures of 730 C and above showed significant interconnection of the frit. With the processing temperature significantly below conventional sintering temperatures of alumina, the behavior of the frit is the critical determinant of the final microstructure and of the etching behavior. During HF etching, in addition to local etching of frit, separation of the tape into surface layers and an interior layer occurs in directions perpendicular to the tape casting direction. The origin of this preferential etch process within the tape processing and the resultant microstructure will be discussed.

AA4.10
Abstract Withdrawn.

AA4.11
THE SYNTHESIS OF NOVEL POLYNUCLEAR ORGANOGOLD COMPLEXES. J. Thomson1, A.H. Fzea 1 and J. Lobban1, J.A. Cairns2, A.G. Fitzgerald2, G.J. Berry2, M.R. Davidson2, D. Rodley2, and Drapacz2. 1Department of Chemistry, 2Department of Applied Physics and Mechanical and Electrical Engineering, University of Dundee, Dundee, UNITED KINGDOM.

The importance and potential of nanotechnology has been recognised by the scientific community since the late 1980s. Hence a number of individual studies in areas of chemistry, physics and engineering are linked to move the established microtechnology fabrication methods towards nanoscale dimensions. One important route which has been established [1, 2] for achieving such miniaturisation is the use of novel organometallic materials with controlled molecular structures and specific physical and chemical properties. Hence, many new organometallic complexes have been synthesised and employed as precursors for the production of micron and sub-micron metallic features [3, 4]. Using carefully designed preparative methods, we have recently shown that various noble metal organometallic complexes can be prepared with increased metal content and well controlled molecular functionalisation [5, 6, 7]. These novel materials have been successfully tested in our laboratories for the purpose of producing metallic features with dimensions less than 300nm. In the present study, we report a new strategy towards obtaining novel polynuclear organogold complexes which contain high metal density where the Au-ligand units are linked in a network by $\mu$-fluorine bridges. This new type of organogold-fluoride complex undergoes direct conversion to pure metal under the influence of electron beam bombardment. The presence of the $\mu$-fluoro bridges enhance the adhesion of the material to the substrate. We have found that the edge definition of the metal features obtained from these organogold precursor materials is also enhanced. The preparation and chemical characterisation used to identify these organometallic complexes will be described in this paper.
References:
[1]. K.W. Beeson and N.S. Clements, Appl. Phys. Lett., 53 (1988), 7.
[2]. W. Gopel and Ch. Ziegler, Eds., Nanostru. based on mole. mater., VCH, Weinheim, 1992.
[3]. T.J. Stark, T.M. Mayer, D.P. Griffiths and P.E. Russel, J. Vacuum Sci. Technol., B10 (1992), 6.
[4]. W. Gopel, Microelectronic Engineering, 32 (1996), 75.
[5]. G.J. Berry, J.A. Cairns and J. Thomson, J. Mat. Scie. Lett., 14 (1995), 844.
[6]. J.A. Cairns and J. Thomson, Eur. Pat. 0, 670, 055 (1997).
[7]. M.R. Davidson, G.J. Berry, J.A. Cairns, A.G. Fitzgerald, B. Lawrenson, J. Thomson, I.C.E. Turcu, W. Shaikh, N. Spencer and Microelectronic Engineering, 41/42 (1998), 279.

AA4.12
MICROMETER-SCALE MACHINING OF METALS AND POLYMERS ENABLED BY FOCUSED ION BEAM TECHNIQUES. David P. Adams and Gilbert Benavides, Sandia National Laboratories, Albuquerque, NM; Michael J. Vasile, Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA.

We demonstrate a technique that is capable of fabricating complex, micron-size features in a diverse set of materials. This includes producing three-dimensional structures and geometries inaccessible by conventional materials removal processes, in metals, metal alloys and plastics. In this work focused ion beam (FIB) micromachining is used as an enabling technology for fabrication at the micron level. Microtools, including micro-end mills, are made using focused ion beam sputtering. A beam of 20keV Ga+ ions define the cutting edges in microtools typically made of Cobalt M42 high speed steel. Facets are patterned into an initially cylindrical stock with an ion beam impinging normal to the cylinder axis, but tangential to the circumference. This produces sharp cutting edges having submicrometer radii of curvature and avoids rounding due to the Gaussian intensity distribution of the ion beam. The portion of the tool used for cutting is < 25 microns wide and 75 microns in length. Numerous tool designs are investigated, and we demonstrate their effectiveness on patterning different materials. Multifaceted end mills having 2,4,5 and 6 cutting edges successfully micromachine a number of polymers and metal alloys, such as 6061 Al. Using feed rates of 2 mm/minute we find evidence of cutting (not burnishing) in metal alloys. Ultraprecision machining of 6061 Al demonstrates dimensional control on the micron length scale. Using 25 micron-wide tools, we fabricate trenches $\sim$25 microns deep and wide with submicrometer tolerances over several millimeters. A discussion of tool durability and wear is also included.

AA4.13
LASER MACHINING WITH ULTRASHORT PULSES: EFFECTS OF PULSE-WIDTH, FREQUENCY AND ENERGY. David E. Bliss , David P. Adams and Stewart M. Cameron, Sandia National Laboratories, Albuquerque, NM.

The benefits of short pulse laser machining over conventional ns lasers have been well demonstrated: higher aspect ratio features with smaller remelt zones and less redeposited material. However, shock induced defects can degrade the mechanical, electrical and optical properties of the remaining material, especially in brittle materials. Our focus is to reduce the amount of defects created while maintaining the machining advantages of using ultra-short pulses. Utilizing a chirped pulse amplified Ti:Sapphire laser with a pulse energy of 1 mJ and a pulsewidth of 100 fs we investigated the effects of pulsewidth, laser frequency and pulse energy on the controlled micro-ablation of Au, Si and Quartz films. Pulsewidths were varied from 100 fs - 100 ps, fundamental, second and third harmonics of the Ti:Sapphire laser were used. Samples were characterized using optical microscopy and the microstructure was evaluated by electron microscopy.
*Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

AA4.14
CIRCUIT PATTERM Cu DEPOSITION ON POLYIMIDE SURFACE USING ArF EXCIMER LASER. Masaaki Tomita Masataka Murahara Department of Electrical Engineering, Tokai University, Kanagawa, JAPAN.

The polyimide is far superior in heat resistance,mechanical strength, and chemical stability, electric insulation and low dielectric constant; permittivity is widely used as aerospace and flexible print circuit board. These excellent properties, on the other hand, make surface modification difficult. We have reported on photochemical modification of polyimide surface: the C-H bonds on the photo-excited surface are easily dehydrated by hydrogen atoms dangling bond of the dehydrated carbon atoms are bonded with hydrogen atoms. The Cu thin films were grown on the Cu nuclei of the polyimide surface. The CuSO4 water solution was dropped on the polyimide surface and covered with a fused silica glass plate to make a thin liquid layer with capillary action. Then, the mask patterned ArF laser image was projected on the polyimide surface through the grass plate. The experimentalconditions were as follow; in the ArF laser fluence was 60mJ/cm2, The concentration of CuSO4 water was 1% and the thickness of the layer solution about 50$\mu$m. While polyimide and CuSO4 solution has the absorption band in the wavelength of ArF excimer laser. Therefor, CuSO4 solution was photodissociated with ArF excimer laser to generate the Cu radical. The laser shot number dependence of the Cu nucleus density was measured. With increasing laser shot number, the Cu nucleus density increases at the shot number 10, the density become 90%. Chemical composition of the laser-irradiated surface was analyzed by x-ray photoelectron spectroscopy (XPS). It is consider that O-Cu bonds were produced on the polyimide surface. The Cu thickness 2000$\approx$ and the line and space100mum.

AA4.15
Abstract Withdrawn.

AA4.16 FABRICATION OF HONEYCOMB MICROCOMPOSITES USING SOFT LITHOGRAPHY. Bing Xu , Francisco Arias, Anthony G. Evans$\dagger$, George M. Whitesides, Department of Chemistry and Chemical Biology, $\dagger$Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA.

This work uses soft lithography to fabricate honeycomb microcomposites that consist of honeycomb core sandwiched between laminate layers. We made polymeric honeycomb cores with small features (20$\sim$150 $\mu$) by micromolding prepolymers with polydimethylsiloxane (PDMS) masters. We also prepared metallic honeycombs microstructures ($\sim$100 $\mu$) by combing electroplating with rapid prototyping. The mechanical properties of these structures (Young's modulus and Poisson's ratio) are presented.

 

 

 

SESSION AA5: NEW MATERIALS
Chair: S. Joshua Jacobs
Wednesday Morning, December 2, 1998
St. George B/C/D (W)
8:50 AM CHAIRMEN'S REMARKS



9:00 AM *AA5.1
SHAPE MEMORY AND GIANT MAGNETOSTRICTIVE MATERIALS FOR MEMS. Manfred Wuttig , Dept. of Matls Sci. & Eng., Univ. of MD, College Park, MD.

The presentation will concentrate on the actuation properties of Shape Memory (SMAs) and Giant Magnetostrictive (GMA) Alloys under a substrate constraint. For SMAs the equilibrium microstructure develops irreversibly with changing temperature, i.e. the microstructure evolution paths for the direct and reverse transformations are different. During the reverse transformation incompatibly stressed austensite forms from the martensite phase. Therefore, a considerable shift of the temperature interval of transformation must be expected. Experimental studies of the stress evolution with changing temperature in NiTi and NiTiPd polycrystalline films on Si substrates support the principle thermodynamic conclusions. The magneto-mechanical properties of Terfenol-D thin films and (approx. 10nm FeTb)/(approx. 10nm (Co)Fe) multilayers also show evidence of substrate constraint: a pronounced damping maximum at a magnetic field of about 1 kOe oriented perpendicular to the plane of the film is the result of a magneto mechanical instability in the Terfenol film. The magneto-mechanical response (of rare earth amorphous/high permeability) nanosized multilayers behaves similarly.

9:30 AM AA5.2
MICROFABRICATED SILICON CARBIDE MICROENGINE STRUCTURES. Kevin A. Lohner , Kuo-Shen Chen, Arturo A. Ayon, S. Mark Spearing, Massachusetts Institute of Technology, Cambridge, MA.

A research and development program is underway to develop technology for a MEMS-based micro-gas turbine engine. The thermodynamic requirements of power-generating turbomachinery drive the design towards high rotational speeds and high temperatures. To achieve the specified performance requires materials with high specific strength and creep resistance at elevated temperatures. The thermal and mechanical properties of silicon carbide make it an attractive candidate for such an application. Silicon carbide as well as silicon-silicon carbide hybrid structures are being designed and fabricated utilizing chemical vapor deposition of relatively thick silicon carbide layers (10-200 microns) over time multiplexed deep etched silicon molds. The silicon can be selectively dissolved away to yield high aspect ratio silicon carbide structures with features that are hundreds of microns tall. Research has been performed to characterize the capabilities of this process. Conformality, microstructure, bonding and etching techniques, as well as mechanical properties of the resultant structures will be discussed.

9:45 AM AA5.3
MECHANICAL AND THERMOPHYSICAL PROPERTIES OF SILICON NITRIDE THIN FILMS AT HIGH TEMPERATURES. P. Nieva , P.M. Zavracky, G. Adams, Northeastern Univ, Dept of Computer and Electrical Engineering, Boston, MA; H. Tada, I. Miaoulis, P. Wong, Tufts Univ, Mechanical Engineering Dept, Medford, MA.

The optimization of MEMS technology depends in many cases on the availability of the mechanical and thermophysical properties of thin film materials. The thickness, stoichiometry, structure and thermal history can all affect the properties of thin films causing their mechanical and thermophysical properties to diverge from bulk values. Moreover, it is known that the mechanical and thermophysical properties of thin films vary considerably at different temperatures. Bulk properties of semiconductors have been characterized over a wide range of temperatures; however there is limitted information on thin film properties of silicon-based compounds such as silicon nitride and silicon dioxide, specially at high temperatures. In our work, MEMS devices have been designed to determine the material properties of silicon nitride at high temperatures. Mechanical properties (Young's modulus and yield strength) and thermophysical properties (coefficient of thermal expansion) of Silicon Nitride thin films are determined through a combination of indirect experimental measurements, analytical expressions, and numerical analysis.
The devices are in the form of a thin film bridge, designed to concentrate the stress in the film at the neck of the bridge. They are fabricated using a low-stress silicon nitride (Si3N4) and suspended over a substrate. The behavior of the devices is a function of the shape of the thin film bridge, its Young's modulus, its yield strength, as well as the thermal coefficients of expansion of both the Si3N4 film and the substrate. During testing, the devices are thermally loaded in tension by heating the sample. The thermally induced stress at the neck will cause the device to break at a certain high temperature. Devices have been fabricated and tested at a variety of temperatures. A numerical model was generated and is compared with experimental data. Some of the material properties of the film can be calculated from the breaking temperatures of various structures, if the experiments are conducted using at least two different substrates of known temperature-dependent coefficients of thermal expansion. The two candidate materials for the substrate are silicon and sapphire.

10:30 AM AA5.4
PERFORMANCE OF ULTRA HARD CARBON WEAR COATINGS ON MICROGEARS FABRICATED BY LIGA. Joel W. Ager III , Othon R. Monteiro, Ian G Brown, Lawrence Berkeley National Laboratory, Berkeley CA; David M. Follstaedt, James A. Knapp, Michael T. Dugger, and Todd R. Christenson, Sandia National Laboratory, Albuquerque, NM.

Stiction and friction are of concern for the reliable, long-term application of micromachines. We have found that the application of a 30 - 70 nm hard carbon coating produces a significant reduction in the friction coefficient and wear rate of electroformed Ni substrates in reciprocating sliding contact under simulated MEMS operating conditions. To evaluate the performance of coated components, a series of microgears ranging in diameter from 300 microns to 2.2 mm were fabricated from electroformed Ni via standard LIGA processes and fixtured on posts in preparation for the coating procedure. A pulsed vacuum-arc deposition process was used to deposit a carbon coating with 50 nm nominal thickness. A sample bias of 2 keV was used in order to produce a coating with relatively low stress and good adhesion while maintaining high hardness (27 GPa). Cross-section TEM indicates that a thin ($\sim$7 nm) ion-mixed zone forms just below the carbon/Ni interface, which is expected to promote layer adherence. The coating conformity to the surface of the component is varied by modifying the bias conditions. The coating uniformity, particularly in the high-aspect-ratio areas between the gear teeth, will be evaluated with micro-Raman and analytical SEM analyses. The microgears are components of a speed-reduction gear-train. The results of a comparison of the performance of treated and untreated components in extended operation, if available, will be presented.

10:45 AM AA5.5
AMORPHOUS DIAMOND MICROMECHANICAL STRUCTURES. J.P. Sullivan , T.A. Friedmann, A.J. Magerkurth, M.P. de Boer, M.M. Bridges, and C.I.H. Ashby, Sandia National Laboratories, Albuquerque, NM.

Micromechanical structures synthesized from stress-free amorphous diamond (a-D) films are described. Stress-free amorphous diamond is an extremely hard (90 GPa hardness), wear-resistant, atomically-smooth, and transparent material, which is deposited at room temperature using pulsed-laser deposition and then stress-relieved at 600$^{\circ}$C. The material is particularly well-suited for the formation of microelectromechanical systems (MEMS) which require low thermal budget, extreme wear resistance, chemical inertness, integrated optics capability, and chemical compatibility with Si microelectronics processing. Singly- and doubly-clamped cantilever beams, tensile test rings, comb-drive actuators, and resonant fatigue micromechanical structures were designed to test the mechanical properties of a-D and to evaluate compatibility with conventional MEMS fabrication processes. These micromechanical structures were fabricated by depositing 1 $\mu$m thick stress-free a-D films on to 2 $\mu$m thick sacrificial layers of SiO2 on Si. The a-D films were photolithographically patterned to produce critical features as fine as 2 $\mu$m and then etched in an O2 electron cyclotron resonance plasma. The structures were released by wet chemical etching of the SiO2 sacrificial layer in order to undercut the a-D film. An environmentally-controlled interferometric microprobing station was used to measure stiction, residual strain gradients in the a-D film, and thin film mechanical properties, including elastic modulus. This material appears promising for unique integrated MEMS applications, and the prospects for future novel MEMS devices will be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.

11:00 AM AA5.6
CONFORMAL ELECTROLESS COPPER AND NICKEL DEPOSITION ON MEMS STRUCTURES. Hercules P. Neves , Thomas D. Kudrle, Jia-Ming Chen, Noel C. MacDonald, School of Electrical Engineering, Cornell University, Ithaca, NY; Scott G. Adams, Kionix Inc., Ithaca, NY; Sergey Lopatin, AMD Co., Sunnyvale, CA.

We propose electroless metallization as a method for conformal metal deposition on high aspect ratio MEMS. We have successfully deposited electroless copper and nickel on silicon structures built using the SCREAM process with excellent conformality and film quality.
Electroless deposition consists of a redox reaction. A catalytic (seeding) surface is needed to start it, after which the process becomes autocatalytic. We have used two different seeding techniques: sputtering deposition of a seed layer and activation of the silicon surface using a palladium displacement reaction. Seed layer deposition can be accomplished even in the presence of high aspect ratio structures, since a few monolayers are enough to start the process; in fact, it has been observed that the initial seed does not need to be a continuous film. Palladium activation allows deposition directly on silicon and makes it selective with respect to adjacent insulating surfaces.
For copper deposition, the seed layer consisted of three sputtered layers: an adhesion layer (20 nm of Ta or Ti), the seed layer itself (100 nm of Cu) and a protective layer (20 nm of Al) to prevent copper oxidation; for nickel, a single 50 nm Al layer was used. Samples processed through palladium activation were immersed in a PdCl + HF solution. The plating solution consisted of a sulfide as the source of metal ions, a reducing agent, a hydroxide for pH adjust, and additives (for controlling stability, conformality, reaction rate, microsctructure, and even elastic properties). In order to ensure a stable deposition, the pH was kept above 12 for copper and around 6 for nickel. The bath temperature was adjusted between 60$^{\circ}$C and 70$^{\circ}$C. Deposition rates between 20 nm/minute and 50 nm/minute were obtained. Due to the process selectivity, no subsequent metal patterning was required.

11:15 AM AA5.7
DC MAGNETRON REACTIVE SPUTTERING OF LOW STRESS AlN PIEZOELECTRIC THIN FILMS FOR MEMS APPLICATION. Peter Hsieh , MIT, Dept of Materials Science and Engineering, Cambridge, MA; Rafael Reif, MIT, Dept of Electrical Engineering and Computer Science, Cambridge, MA; Brian Cunningham, Draper Laboratory, Cambridge, MA.

Many MEMS devices require piezoelectric excitation and readout to actuate and sense motion of mechanical structures. Aluminum nitride is advantageous for MEMS fabrication because it is compatible with silicon integrated circuit foundry impurity contamination requirements, can be deposited at low temperatures, provides a high piezoelectric coefficient, and is easily patterned using conventional photolithographic techniques. In this work, AlN thin films were deposited on silicon substrates for use in MEMS silicon membrane ultrasonic resonator. The ultrasonic resonator is configured as a gravimetric sensing device for chemical detection. Issues of concern with regard to device performance and yield include the maximization of the electromechanical coupling constant (K2), film stress control, and film uniformity; these issues were addressed through a central composite design set of experiments to resolve the film property responses as a function of the deposition parameters. Film characterization was conducted with x-ray diffraction, spectroscopic ellipsometry, and surface profilometry. Optimization of film deposition parameters improved sensor performance and enabled further device miniaturization with the use of thinner films.

11:30 AM AA5.8
RELIABILITY AND PROPERTIES OF PZT THIN FILMS FOR MEMS APPLICATIONS. D.F. Bahr , J.C. Merlino, M.R. Diaz, A. Bandyopadhyay, Mechanical and Materials Engineering, Washington State University, Pullman, WA.

Piezoelectric lead zirconate titanate (PZT) thin films are an attractive material for use in MEMS devices for both sensing and actuating applications. The ability to act as efficient high load actuators should enable new MEMS device designs. However, before they can consistently be integrated into a MEMS processing stream, the basic properties as well as the mechanical and electromechanical reliability of these films must be determined. Solution based sol-gel processing methods are used to deposit PZT films onto platinized Si substrates with thicknesses varying from 100 to 2000 nm. The microstructure and the electromechanical properties of the films are determined as a function of heat treatment conditions and film chemistry. Additionally, the micromechanical properties of these films are tested to determine the mechanical strength, which demonstrates a hardness around 4 GPa. The adhesion of these films to the underlying Pt electrode is measured using indentation techniques, and is shown to improve with respect to sintering temperature and time. In many potential applications of PZT thin films, such as for driving the motion of fluidic pumps and valves, both ferroelectric and mechanical fatigue may be critical to cause materials failure. Therefore, the reliability of these films has also been investigated using both electrical and thermal cycling.

11:45 AM AA5.9 EFFECT OF MICROSTRUCTURE ON ELECTROMECHANICAL PROPERTIES OF MULTILAYER ACTUATORS. Manu M. Srivastava , Robert F. Speyer, School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA; Christopher S. Lynch, The G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA.

In generating mechanical action from electric signals, multilayer PLZT ferroelectric actuators (MLA) have lesser response time and higher volumetric efficiency and require a lower drive voltage than conventional single-layer actuators. The two major issues concerning performance are linear response over a wide range of applied electric fields and resistance to damage during cycling. The performance of MLA's is deteriorated by the presence of micro-cracks and residual stresses, which are exacerbated by extended exposure to high electric fields ($\sim$75% of the coercive field ). PLZT multilayer specimens were sintered to form average grain sizes between 1 and 10 micrometers. Reported are 1) the effect on electromechanical linearity with grain size, 2) decay of microstructure and electromechanical response with electric field cycling, and 3) the fracture toughness variation with grain size using indentation methods.

 


SESSION AA6: NEW MATERIALS (continued)
Chair: Terry J. Garino
Wednesday Afternoon, December 2, 1998
St. George B/C/D (W)
1:30 PM AA6.1
ALTERNATIVE MATERIAL FOR LIGA-LIKE MICROELECTROMACHINING. Peter Mardilovich, Dmitri Routkevitch , Carrie Wyse, Tapesh Yadav, Nanomaterials Research Corp., Longmont, CO.

Approach alternative to LIGA for high aspect ratio micromachining based on inorganic resist with anisotropic morphology will be presented. This material has unique combination of high etching anisotropy, superior adhesion to the substrate, thermal, mechanical, and chemical stability, and can be used in severe environments. This is also a multifunctional material, which can be used as a resist, as a mold, as structural, and functional MEMS components. Basic and multilevel 2- and 3-D MEMS structures from proposed material were demonstrated. The approach improves the manufacturability by streamlining or eliminating altogether some of the typical processing steps required with organic polymer resists. Use of the conventional exposure sources instead of synchrotron X-ray significantly reduces the time and the cost involved in development and production of MEMS devices. The application potential for the proposed technology will be evaluated. This work was in part supported by DARPA.

1:45 PM AA6.2
FULLERENE DERIVATIVES AS NOVEL RESIST MATERIALS FOR FABRICATION OF MEMS DEVICES BY ELECTRON BEAM LITHOGRAPHY. A.P.G. Robinson , R.E. Palmer, Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham, UK; T. Tada, T. Kanayama, Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research, Tsukuba, JAPAN; E.J. Shelley, J.A. Preece, School of Chemistry, University of Birmingham, Birmingham, UK.

The fabrication of MEMS devices relies heavily on lithography. Recent experiments [1] have shown that C60 is fragmented by electron beam irradiation, and furthermore that it has several desirable features as an electron beam resist, not least its very high dry etch durability. However, there are two major problems with the use of C60 as an electron beam resist; it has a very low sensitivity, and it must be deposited onto the substrate by high vacuum evaporation as it cannot be spin coated. We have studied several chemical derivatives of C60 including ${\it mono}$, ${\it tris}$, and ${\it tetra}$ adducts and have found that they also behave as effective high resolution negative electron beam resists [2, 3], allowing sub 20 nm resolution patterning of silicon which compares favourably with other commercially available negative resists. Films of the derivatives, ranging in thickness from 20 nm to 200 nm, can be prepared easily by spin coating. Various organic solvents such as monochlorobenzene and chloroform can be used to develop the exposed films. The films have sensitivities of 1 x 10-2 to 8 x 10-4 C/cm2 for 20 keV electrons, more than an order of magnitude higher than the sensitivity of C60 itself. The dry etch durabilities of these compounds which increase marginally as the sensitivity to electron dose decreases, range from $\sim$6 to $\sim$8 for electron cyclotron resonance microwave palmsa etching with SF6 on silicon, considerably higher than that of conventional resists.
[1] T. Tada and T. Kanayama, Jpn J. Appl. Phys., 35, L63 (1996)
[2] A.P.G. Robinson, R.E. Palmer, T. Tada, T. Kanayama and J.A. Preece, Appl. Phys. Lett., 72, 1302 (1998)
[3] A.P.G. Robinson, R.E. Palmer, T. Tada, T. Kanayama, J.A. Preece, D. Philp, U. Jonas and F. Deiderich, Chem. Phys. Lett., 289, 586 (1998)

2:00 PM AA6.3
A NOVEL RANGE OF NOBLE METAL ORGANOMETALLIC FLUORIDES FOR USE IN THE FABRICATION OF SUBMICRON METAL FEATURES BY E-BEAM OR UV IRRATION. J. Thomson1, J. Lobban 1 and A.H. Fzea1, J.A. Cairns2, A.G. Fitzgerald2, G.J. Berry2, M.R. Davidson2, D. Rodley2, and Drapacz2. 1Department of Chemistry, 2Department of Applied Physics and Mechanical and Electrical Engineering, University of Dundee, Dundee, UNITED KINGDOM.

In recent years the mass production of submicron metal features has been an important goal for the microelectronics industry. Submicron metallic features can have a range of applications in electronics such as i) interconnects for chip on chip technology, ii) bump solder contacts, iii) interconnect circuitry deposited directly onto glass substrate for use in digital lens technology or flat panel displays, iv) nanoscale metal features and v) X-ray photomasks. We have recently synthesised a range of novel noble metal organometallic fluorides containing two noble metal atoms in their molecular structure [1]. Dependent upon the synthetic route, these organometallic compounds can contain the same metal to give $\mu$-bridged fluoro compounds of organoplatinum, palladium or gold respectively. Alternatively, the constituent metal components of the bimetallic compounds can be different, and hence produce alloy precursor compounds of organogold-palladium, organoplatinum-palladium or organogold-platinum respectively. These materials are readily reduced to the metal by electron bream or uv irradiation. Utilising these novel materials in conjunction with photomask technology, we have prepared submicron metal features comprising i) fine lines in gold, palladium and platinum on a range of semiconductor and insulator substrates, ii) arrays of submicron noble metal dots in palladium and platinum, iii) gold circuits directly onto glass substrate. In this paper, the results of chemical characterisation of these materials will be presented, together with some examples of their applications.
References: [1]. J. A. Cairns and J. Thomson. Eur. Pat. 0, 670, 055 (1997).

SESSION AA7: PROCESSING
Chair: Terry J. Garino
Wednesday Afternoon, December 2, 1998
St. George B/C/D (W)


2:15 PM *AA7.1
DEEP REACTIVE ION ETCHING OF SILICON. Arturo A. Ayon , Martin A. Schmidt, Department of Electrical Engineering and Computer Science, Kuo-Shen Chen, Kevin A. Lohner, S. Mark Spearing, Department of Aeronautics and Astronautics, Herb H. Sawin, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA.

The ability to etch deep trenches in silicon while controlling not only the profile of etched features but also the etching rate, uniformity and selectivity enable us to expand the number and scope of MEMS devices. In fact, the increase of MEMS applications in different and varied fields requiring deep silicon etching or high aspect ratio structures (HARS) has even been extended to include microturbomachinery which was recently introduced as a feasible source of power generation. Many projects also place additional demands on surface morphology. Thus, the scalloping observed on vertical walls during time multiplexed deep etching (TMDE), the roughness of horizontal surfaces exposed to the glow discharge and the radius at the bottom of etched features are also relevant. Therefore, it is important to understand not only the plasma processes involved but also the dependence of response variables on operating conditions. For this purpose we have designed, performed and analyzed sets of experiments adequate to fit a quadratic model. The data was collected using interferometry, atomic force microscopy (AFM), profilometry and scanning electron microscopy (SEM). The exercise involved eight variables and it was conducted in an inductively coupled reactive ion etcher. The mapping of the dependence of response variables on dry processing conditions produced by this systematic approach provide additional insight in the plasma phenomena involved and supply practical tools to locate and optimize robust operating conditions. The measured performance allows the tailoring of silicon etching rates in excess of 4 mm/min with anisotropic profiles, nonuniformities of less than 3% across the wafer, and reactive ion etching lag control with a depth variation of less than one micron for trenches of dissimilar width. Furthermore it is feasible to prescribe the slope of etched trenches from positive to reentrant.

2:45 PM AA7.2
ETCH SELECTIVITY OF NOVEL EPITAXIAL LAYERS FOR BULK MICROMACHINING. J.T. Borenstein , N.D. Gerrish, Charles Stark Draper Laboratory, Cambridge, MA; M.T. Currie and E.A. Fitzgerald, Dept of Materials Science and Engineering, MIT, Cambridge, MA.

The present work demonstrates vew high etch selectivity for a novel epitaxial layer structure in several standard etchants used for bulk silicon micromachining. Historically, high selectivities for etch-stop layers are achieved by high-concentration boron diffusions into low-doped silicon substrates. While this process has been used successfully to build a wide array of high performance micromechanical devices, several drawbacks associated with the boron diffusion remain. Doping gradients, boron precipitates and dislocation arrays generated during the diffusion process can have deleterious effects on device performance, causing curvature of suspended structures and undesirable surface topography. Epitaxial layers utilizing heavy boron doping can avoid some, but not all of these undesirable phenomena. We have recently developed a novel epitaxial layer structure which employs a silicon-germanium alloy without boron doping to provide an etch-stop for bulk micromachining. The mechanism for providing an etch-stop appears to be related to the energy band structure of the SiGe alloy, mimicking the hole injection and oxide passivation phenomenon ascribed to boron-diffused etch-stops. In this work, the selectivity of this layer and of boron-diffused layers are determined for a variety of etching conditions. High selectivity is demonstrated for several conditions typically utilized to fabricate bulk-micromachined silicon sensors, using standard etchants such as ethylene-diamine-pyrocatechol (EDP) and potassium hydroxide (KOH.) Resulting structures exhibit smooth surfaces and precise build dimensions.

3:30 PM AA7.3
THE BENEFITS OF PROCESS PARAMETER RAMPING DURING THE PLASMA ETCHING OF HIGH ASPECT RATIO SILICON STRUCTURES. Huma Ashraf , Jyoti Kiron Bhardwaj, Janet Hopkins, Alan Michael Hynes, Ian Johnston, John Nicholas Shepherd, Surface Technology Systems Ltd., Newport, UNITED KINGDOM.

The plasma etching of Silicon with highly anisotropic profiles, high aspect ratios and fine critical dimension control, is required for many MEMS (Micro Electro-Mechanical systems) applications. The STS Advanced Silicon Etch (ASE) process satisfies the current requirements of the industry. However, as applications move to a production environment, an increased etch-rate is highly desirable. A >100% etch-rate improvement can often be achieved, but this is accompanied by degradation of profile/critical dimension control. Process parameters changes which increase etch-rate, also cause loss of critical dimension control. The key parameters for etch-rate improvement are identified in this paper. Their effect on profile control/ critical dimension control is detailed. A parameter ramping scheme has been designed to overcome the conflicting requirements of etch-rate and critical dimension/profile control. The results are presented in this paper.
Parameter ramping processes have also been developed for the etching of deep (>100mm), high aspect ratio openings where the optimum process conditions to etch the top of a feature are very different to the conditions required to etch the bottom. The main causes of the types of profile degradation typically observed, are discussed, and parameter ramping schemes to overcome some of the limitations are presented.

3:45 PM AA7.4
SILICON ELECTROMACHINING PROCESSES IN AQUEOUS SOLUTIONS. Mark Kovler , David Starosvetsky, Joseph Yahalom, Technion, Dept. of Materials Engineering, Haifa, ISRAEL; Yael Nemirovsky, Technion, Dept. of Electrical Engineering, Haifa, ISRAEL.

Three-dimensional structures formed from silicon and other semiconductor substrates are generally produced by anisotropic etching processes. At present, silicon is usually removed by simple chemical etching, that is, by immersing silicon specimens in a liquid bath of the chemical etchants. These are usually hydrofluoric solutions, strong alkalis such as KOH, or ethylene diamine-pirocathecol (EDP).
There are several inherent limitations in the etching techniques presently known and used in the art. The principal problem is a lack of control over the rate of silicon dissolution during the etching process. Furthermore, at present, there is only one electrochemical technique which is reasonably effective in increasing the rate of silicon etching. This technique combines anodic biasing in aggressive HF solutions with illumination. However, the use of etchants like hydrofluoric acid, which are highly corrosive and hazardous, gives rise to serious problems in production and waste disposal.
The purpose of the present research was to study the effect of electrochemical polarization on the etch rate of silicon. The reason behind this was to find ways to accelerate and control the etching rate in solutions other than HF.
The work focused on silicon etching under cathodic biasing. The electrochemical behavior of both <100> p- and n-type low doped silicon in aqueous KOH solutions was studied. The current-voltage characteristics of silicon in the temperature range between 25 and 90$^{\circ}$C were examined by using potentiodynamic, potentiostatic, and galvanostatic polarization modes. The etch rate of silicon with both doping types increased at potentials more negative than -5V when combing polarization with illumination. Silicon surfaces etched under such conditions were polished. Whereas the etch rate of p-Si in dark does not depend on the bias over a wide range of potentials, the etching rate of n-Si accelerates with potential both in dark and under illumination. Acceleration in the etch rate of silicon could also be shown for neutral aqueous electrolytes. Possible mechanisms of these phenomena will be discussed.

4:00 PM AA7.5
FABRACATION OF MEMS DEVICES BY POWDER-FILLING INTO LIGA-FORMED MOLDS. Terry Garino , Todd Christenson and Eugene Venturini, Sandia National Laboratories, Albuquerque, NM.

Techniques for fabricating MEMS devices of materials not readily formable using standard processes such as silicon micromachining and electroplating into LIGA formed-molds have been developed utlizing powder filling into LIGA-formed molds. Three techniques have been developed, all of which involve filling a LIGA formed mold with a powder of the desired material by either pressing or calendering. In the first technique, a binder such as epoxy is added to the powder, for example NdFeB permanent magnet powder, to create a bonded part after lapping and removal from the mold. While this allows easy fabrication of precisely dimensioned parts, it sacrifices properties such as magnetic energy product and strength. The second technique involves forming the parts with a powder that is then sintered to form dense parts with better properties. Due to the shrinkage variability during sintering, the dimensional precision is somewhat compromised in this technique. The final technique involves uniaxial hot pressing o

SESSION AA8: THEORY AND SIMULATION
Chair: Terry J. Garino
Wednesday Afternoon, December 2, 1998
St. George B/C/D (W)
4:15 PM AA8.1
DIAMOND AND POLYCRYSTALLINE DIAMOND FOR MEMS APPLICATIONS: SIMULATIONS AND EXPERIMENTS ON FRICTION AND WEAR PROCESSES. Jianwei Che, Tahir Çagin, California Institute of Technology, Materials Simulation Center, Pasadena, CA; Michael N. Gardos, Raytheon, Los Angeles, CA, and William A. Goddard, III, California Institute of Technology, Materials Simulation Center, Pasadena, CA.

To date most of the MEMS devices are based on Silicon. This is due to the technological knowhow on manipulation and micromachining of Silicon. However, due to rapid wear arising from high friction, there are very few Silicon MEMS devices involve moving parts. Recent tribometric experiments carried out by Gardos show that this rapid wear is caused by a variety of factors, related both to surface chemistry and cohesive energy density of Silicon and diamond. In particular, the 1.8-times strength of the C-C bond in diamond as opposed to the Si-Si bond in the bulk translates into more than 104-times difference in wear rates, even though the difference in flexural strength is only 20-times, in hardness 10-times and the fracture toughness 5-times. It has been shown that the wear rates of Silicon and PCD are controlled by high friction induced surface cracking and the friction is controlled by the number of dangling, reconstructed or adsorbate-passivated surface bonds.
Theory and simulation could play an important role in the rational design of MEMS. Predicting materials properties such as friction and wear is of critical importance for the components of MEMS. Here we present theoretical studies of frictional process on diamond surfaces using a steady state Molecular Dynamics Method. We simulated the atomic level friction on diamond 100 surface using an extended bond-order dependent potential for hydrocarbon systems.[1] Unlike the traditional empirical potentials, bond order dependent reactive potentials can mimic the bond breaking and formation processes. Therefore, it is a natural choice to study surface dynamics under friction and wear.
[1] J. Che, T. Cagin, W. A. Goddard, III, Extension of Bond Order Dependent Potentials to include Long Range Interactions, preprint.

4:30 PM AA8.2
MICROACTUATORS MADE OF ACTIVE MATERIALS: A FEW SUGGESTIONS FROM THEORY. Kaushik Bhattacharya , Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA; Richard D. James, Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN; Yi-Chung Shu, Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA.

A common problem facing MEMS devices in many areas is that the actuators proposed for use in these systems lack adequate work output and displacement magnitude. Substantial improvements in both these quantitities can be achieved by modifications of both material and design of actuators. On the material side, active materials show great promise because the underlying mechanism is a first order phase transformation; further they allow designs which combine the structural and actuator elements. In particular, shape-memory alloys are particularly attractive since they arguably have the largest work output per unit volume. Further, at small sizes the frequency of cycling is not limited because of fast heat transfer. However, the microstructure can be very different in thin films compared to the bulk and thus the shape-memory characteristics of a material can change. The talk will discuss a recently developed theory which captures this change in the microstructure; moreover the theory shows the need to use the films in membrane rather than bending mode for maximum work output. Based on this we have proposedmaterials systems and designs which are currently being experimentally evaluated. We also extend the theory to heterogeneous films - polycrystalline and multilayer - and show that sputtering texture is unfavorable for common shape memory alloys and propose other textures which yield large recoverable strains.