Flexible, dry-released process for aluminum electrostatic ... - IEEE Xplore

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 3, NO. 3, SEPTEMBER 1994

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Flexible, Dry-Released Process for Aluminum Electrostatic Actuators Christopher W . Storment, David A. Borkholder, Student Member, IEEE, Victor Westerlind, John W. Suh, Nadim I. Maluf, Member, IEEE, and Gregory T. A. Kovacs, Member, IEEE

Absfrucf- We present an all-aluminum MEMS process (AlMEMS) for the fabrication of large-gap electrostatic actuators with process steps that are compatible with the future use of underlying, pre-fabricated CMOS control circuitry. The process is purely additive above the substrate as opposed to processes that depend on etching pits into the silicon, and thereby permits a high degree of design freedom. Multilayer aluminum metallization is used with organic sacrificial layers to build up the actuator structures. Oxygen-based dry etching is used to remove the sacrificial layers. While this approach has been previously used by other investigators to fabricate optical modulators and displays, the specific process presented herein has been optimized for driving mechanical actuators with relatively large travels. The process is also intended to provide flexibilityfor design and future enhancements.For example, the gap height between the actuator and the underlying electrode(s) can be set using an adjustable polyimide sacrificial layer and aluminum “post” deposition step. Several AI-MEMS electrostatic structures designed for use as mechanical actuators are presented as well as some measured actuation characteristics. [lo21

I. INTRODUCTION

T

HERE are two major approaches to the ultimate integration of MEMS devices and active control circuitry: developing a custom monolithic MEMS/active circuit process or developing a MEMS process that is compatible with an existing (“standard”) active circuit process. With the latter approach, the MEMS process is more clearly decoupled from the underlying circuit technology, allowing the best available circuit process to be used at a given time. Researchers at, for example, U.C. Berkeley [ 1J and Analog Devices [ 2 ] have previously opted for the custom-process option. Hornbeck et ul. [3]-[8] at Texas Instruments have, on the other hand, demonstrated MEMS actuators for digital displays that are fully compatible with an underlying, standard active circuit process. More recently, Berkeley researchers have also demonstrated a decoupled process approach [9]. The groundwork for multi-layer aluminum MEMS processes with organic sacrificial layers has been done by the Texas Instruments group. They have demonstrated torsionally-actuated optical modulators (and complete displays built from them) and Manuscript received January 11, 1994; revised March 22, 1994. Subject Editor, R. S. Muller. This work was supported by the Advanced Research Projects Agency Grant Number N0014-92-J- 1940-pooO01, administered by the Office of Naval Research. C. W. Storment, D. A. Borkholder, V. Westerlind, N. I. Maluf, and G. T. A. Kovacs are with the Department of Electrical Engineering, Stanford University, Stanford, CA 94305-4070 USA. J. W. Suh is with the Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-4070 USA. IEEE Log Number 9401753.

other devices using such a technology. The Texas Instruments process makes use of aluminum metallization and organic sacrificial layers to post-process pre-fabricated CMOS wafers, thus adding mechanical optical modulators above the circuits. While beyond the scope of this paper, the devices described herein are intended to be used as drivers for more complex, out-of-plane actuators based on the addition of vertical extensions (by electroplating or other methods). For these applications, as well as large angular motion optical modulators, we require a larger gap for deflection and few constraints on geometry, but have less stringent requirements for the surface reflectivities or fill factors achievable. Nonetheless, we have adopted a process philosophy similar to that used for presently available aluminum MEMS actuators for displays: additive and CMOS process compatible MEMS structures. As in the Texas Instruments process, thin metal hinges and thicker plate regions are used to form the mechanical elements, suspended above the substrate by aluminum “posts” and released by the dry etch removal of an organic sacrificial layer (shown in Fig. 1). The AI-MEMS process is described below, as well as some test results on the resulting actuators. The primary focus of this paper is the AI-MEMS process itself. We present test results for cantilever-type actuators. Clearly, the process can be used to implement more complex actuator designs, examples of which are shown in Fig. 2-4. 11. MATERIALS AND METHODS

As stated above, the overall process is a multi-level aluminum, organic sacrificial layer sequence, illustrated in Fig. 5. The present process, while somewhat complex, is designed to provide flexibility in modifying the material properties and design constraints upon each layer. All photoresist steps are carried out using standard positive photoresist, normally applied to a 1.O-pm thickness, baked and developed using automated, cassette-feed equipment. Photoresist thickness is increased to 1.6 pm where stated below via spin speed reduction. All lithographic patterning is carried out with automatic alignment using an Ultratech 1: 1 projection mask aligner (General Signal, Santa Clara, CA). Wherever critical, thin film stress levels are determined for a given step and batch of wafers using a SMSI 3800 stress measurement tool (Scientific Measurement Systems, Inc., San Jose, CA). All A1 films are wet etched using “Aluminum etch 11” (a pre-mixed solution from Ashland Chemical, Columbus, OH) at 40°C. On 100 mm diameter, p-silicon wafers, a wet thermal silicon dioxide is grown at 1100°C for 45 min for a final thickness of

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Fig. 3. SEM view of a four-plate, individually-actuated torsional actuator array. Fig. 1. Scanning electron micrograph (SEM) view illustrating the basic structure of an AI-MEMS cantilever actuator, showing the interconnects, deflection and landing electrodes, post, hinge, plate and etch access holes through the plate.

Fig. 4. SEM view of a section of a large array of torsional actuators

wet etching an AlISi alloy a fine residue of Si, often referred to as “freckles,” are left behind. This residue is removed by plasma etching using SFs and C2ClFS reactant gases. The plasma-toughened photoresist in each step is removed with 0.5 pm. The first and second metallization layers (99% Al, 1% a combination of wet stripping solutions and oxygen plasma Si) for interconnects, electrodes and bond pads are deposited etching. Between the first two A1 layers, an insulating low temin a dc magnetron sputtering system, with film thicknesses for each layer of 0.25 pm. The metal pattems are defined perature oxide (LTO) is deposited in an LPCVD fumace. To by coating with photoresist, exposure and wet etching. After minimize built-in stress, two different LTO layers are deposited Fig 2

__ SEM view of a torsional A1-MEMS actuator

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from Amoco, and for thicker films two layers can be applied with a bake cycle between applications. The baked UltradelTM 4212 can be wet etch patterned using photoresist as a mask, but resolution finer than approximately 20 pm is not easily obtained for the thicker films. An oxygen RIE process is therefore used, as described below. For the devices described in this paper, the polyimide was spun on in a single step for 10-pm gaps. The polyimide is cured in a nitrogen purged oven using multistep ramp heating cycles that cycle the wafers from 100°C to 350°C in 2 h. The Polyimide Pos( Sacrificial layer Hole wafers are then held at 350°C for 1 h and then allowed to cool \ to 100°C with nitrogen purging over the period of 30-45 min. B*%P cpc..'". Since the subsequent layers used to form the moving parts of .,,,,,, the actuators are opaque, physical alignment marks must be (b) formed in the top surface of the polyimide. The polyimide is coated with thick photoresist, patterned and oxygen plasma Aluminum Silimn Hinge Aluminum Plate etched to form 1-pm-deep alignment marks in the exposed polyimide. The masking photoresist that remalns is stripped off in acetone and isopropyl alcohol, rather than dry etching, to maintain a smooth surface on the polyimide (critical for the deposition of suitable A1 films above it). After a 30 min 150°C bake to remove any adsorbed moisture . .,.,. ,.,., ,.,.,.,.,.,~ , . . : . , . , c . , I J . y and solvents, a Si/A]/Si trilayer (0.25 pd0.15 pdO.25 pm) is dc magnetron sputter deposited as a masking layer for the oxygen FUE to form the post holes. The top silicon layer is patterned over the post holes using photoresist and plasma etched using SF6. The A1 is wet etched as above, followed by a Si plasma etch using SF6 and C2ClF5 reactant gases. The post holes (approximately 10 pm in diameter) are then formed by oxygen RIE for 1-3 h (depending on the thickness of the polyimide). At this point, the Si protecting the second layer of A1 and the top Si in the masking trilayer is removed using an SF6 and C2ClF5 plasma. This stage of the processing is (d) illustrated in Fig. 5(b). After etching the post holes into the polyimide, the posts are Fig. 5. Diagram illustrating AI-MEMS process flow (not to scale). At stage A, the interconnects and deflection electrodes have been fabricated on the formed using a 2.5-p m-thick A1 layer that is deposited using silicon. At stage E, the sacrificial polyimide layer has been deposited and patterned using a silicon etch mask layer. Stage C shows a completed actuator. high-bias dc magnetron sputtering. Measurements showed that the stress in the post layer was approximately 170 MPa Stage D shows a released actuator. (compressive). The post A1 is then patterned and wet etched as above, removing the trilayer A1 as well. At this point, the to a combined thickness of 0.5 pm, the first layer being lower Si layer in the trilayer is exposed. The hinge layer is phosphorous doped glass (PSG) deposited at 300°C and the then formed by depositing a 0.15-pm A1 layer, followed by second layer consisting of undoped glass deposited at 350°C. a 0.25-pm Si protective layer by dc magnetron sputtering. A To make electrical contact between the two layers, via holes 0.75-pm A1 layer is then deposited in the same manner to form are etched through the oxide to expose the first layer metal the moveable upper electrodes. The stress within any sputtered using RIE etching with 0 2 and CHF3 reactant gases. After layer can be controlled by choosing the appropriate operating resist removal, the second metal layer is deposited, followed parameters (such as sputtering gas pressure, dc power, target by a 0.1 pm protective layer of sputtered Si. The top silicon and substrate bias voltages) as discussed by Thornton [lo]. It and metal layers are then patterned with a photoresist mask was verified that the stress in the A1 hinge and plate layers and etched using an SF6 and C2ClF3 plasma, followed by could be kept below 75 MPa (compressive), corresponding wet etching of the A1 as described above. At this point, the to films of A1 that were extremely smooth to optical and processing has progressed to the stage illustrated in Fig. 5(a). electron microscopic examination. A thick layer of photoresist In order to define the deflection gap at a desired value is applied, patterned and used to pattern the thick upper A1 between 1 and 20 pm, UltradelTM4212 spin-on polyimide plate layer using wet etching down to the Si hinge protection (Amoco Chemical Co., Chicago, IL) is used as the organic layer. The photoresist is stripped by oxygen plasma etching sacrificial spacer layer. Using the unmodified product, it is followed by another application and patterning of photoresist possible to control thickness from 5-15 pm. For smaller gaps and plasma patterning of the silicon protective layer (SF6 and the polymer can be diluted 1:l with the appropriate thinner C~CIFE, reactant gases) into the final hinge shapes. The hinges Aluminum Electmda and Silicon p l o t d i v e Layer

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are then wet etched as above, using the patterned Si layer as a mask. This stage of the processing is illustrated in Fig. 5(c). Having thus completed the fabrication sequence, the wafers are coated with photoresist to protect against particulates and damage from the water jet during dicing. Following dicing and removal of particulates, the individual dice are oxygen plasma etched to remove the protective photoresist, SF6 plasma etched to remove the protective Si layer on the hinges and then oxygen plasma etched to remove the sacrificial polyimide layer. It is important to note that the Si plasma etch is designed to undercut the narrow hinge regions, removing any Si from beneath them. Etch times for the polyimide vary between 35 h depending on its thickness. The released structures are illustrated in Fig. 5(d). 111. TESTING METHODS

The released devices were examined using both optical and scanning electron microscopy. In both cases, they can be actuated and imaged simultaneously, but there are significant differences in the capabilities of each. The optical methods required very little set-up effort but presently can only be used to resolve gross effects such as full deflection. Therefore, optical methods are used where large numbers of devices are to be tested and where measurements of the threshold voltages are sufficient. Electron microscopy, on the other hand, provides sub-micron resolution but requires more extensive sample preparation and a great deal more time due to the delays of loading and unloading samples in the vacuum system. Electron microscopic methods are therefore reserved for precision measurements of limited numbers of devices. In both cases, the dice were attached to deep-well hybrid packages (Spectrum Semiconductor, San Jose, CA) using cyanoacrylate adhesive and electrical connections were made using a conventional gold ball bonder. For optical microscopy, the hybrid packages were either left open for the experiments or covered with a glass coverslip and the desired leads were connected to an adjustable dc power supply. The devices were observed using a long-working distance optical microscope with video camera, connected to video tape recorders and directly into a video digitizer within a Macintosh IIci computer. The combination of equipment allowed high-resolution images to be stored while the voltage applied to the structures under test was varied. For electron microscopy, the hybrid packages were sawed in half prior to die attachment to allow imaging from arbitrary angles. Electrical connections from the pins of the package were routed through high vacuum connectors on the scanning electron microscope. For each applied voltage, an image was recorded on PolaroidTM film. The images were then scanned at 300 dots per in on a standard gray-scale scanner and stored for later image processing and analysis using Image 1.47 (National Institutes of Health, Bethesda, MD) on a Macintosh IIci computer. In order to study actuation performance differences that might be caused by slight process variations and to evaluate the effects of varying the deflection electrode geometries, the threshold voltages of a total of 224 cantilevered structures

were measured using optical methods. The threshold voltage was defined as the voltage at which there was a significant change in contrast (from bright to very dark), indicating full deflection of the cantilever. This corresponds to a change in angle of approximately 10” which is easily observed under the microscope. The estimated error in applied voltage was approximately fl V. To obtain detailed gap- and angle-versus-voltage information, a few randomly selected actuators were tested using SEM methods. The voltage was varied in decreasing steps as the threshold voltage, Vth, was approached and increased until full deflection was reached. The cycles were repeated several times, often applying “manual closed-loop control” to attempt to stabilize the angles just below Vth. This was followed by the acquisition of a series of still images at each of the applied voltages, as described above. The estimated error in applied voltage was again approximately k1 V.

IV. RESULTS As mentioned above, the use of a thin hinge layer and a thick plate layer essentially allows the overall mechanical properties of the finished structures to be controlled by the geometries of the hinges. Several different actuator structures were designed and fabricated to serve as test vehicles for the process, including both cantilever and torsional actuators. Examples of both types are shown in Fig. 1 4 . After the actuators were fabricated, they were evaluated to verify basic functionality and to look at the effects of slight process variations across arrays of actuators. For the devices illustrated and tested herein, the gap heights were 10 pm and hinge thicknesses were 150 nm. Initial fabrication runs showed that, as expected, slight variations in built-in stresses in the hinge and plate layers could cause appreciable, or even destructive, distortion of the released actuators. Such effects are highly design-dependent, with simple cantilevers being the most sensitive to them, and torsional structures showing the least sensitivity. Variation of stresses in these layers between runs, and even across wafers, will require further study, but functional devices could be fabricated even on the first run through the process. Cantilever actuators with three types of deflection electrodes (but with identical cantilever configurations and no etch access holes in the plates) were tested using optical methods. The “standard’ configuration (Fig. 6(a)) is comprised of a 41 pm deflection electrode, a 3 pm space and a 6 pm landing electrode (kept at the same potential as the moving plate). A configuration with a 12 pm, nonplanar landing electrode and the same 3 pm space and 41 pm deflection electrode was designated the “ridged” type (Fig. 6(b)). A third configuration, without landing electrode and with a 52 pm deflection electrode was designated the “nl” type (Fig. 6(c)). In all cases, the top plate, landing electrode and actuating electrode were 50-pm-wide. The top plate length ( L p ) was also 50 pm in all cases. The hinge lengths ( L H )were 5, 10 or 20 pm. The hinge widths ( W Hwere ) 4, 8, 16 or 32 pm. All fifteen possible combinations were tested (80 “standard” designs, 64 “ridged” designs, and 80 “nl” designs).

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Fig. 6. Diagram illustrating the three types of cantilever actuators tested optically. (Not to scale.)

Plots of I& versus W;” for the “nl” design (with no electrostatic field bending due to a landing electrode) for each of the three LH values are shown in Fig. 7(a)-(c). They showed considerable individual variation for each design. Plots of b&, versus Wk” for the other two designs showed comparable variations. Plots of &h versus W;” for the “standard” and “ridged” devices were also generated and showed comparable variability and higher average Vth values (to be expected with less effective deflection electrode areas). As expected, all “nl” devices welded down at full deflection. The bulk of the designs with landing electrodes survived multiple test cycles without sticking in the fully-deflected state. It was noted that, once released, the cantilevers were quite vulnerable to destruction by rapid air currents. Fig. 8 shows a device that was examined in the SEM and shown to be flat, removed and replaced in a Petri dish, examined the following day in the SEM and found to be bent up perpendicular to the substrate (damaged during SEM venting or pumping). Further tests, without removing the samples from the SEM and continuing venting and pumping cycles, gave comparable results. For simple electrostatic cantilever actuators (with thin hinge), we previously presented [ 1 11 the following equation for Vth. based on parallel motion of the plate,

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do where, Wp = width of plate, WH = width of hinge, L p = length of plate, L H = length of hinge, E = elastic modulus ( P a ) , I = bending moment of inertia,

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= dielectric permittivity of free space (F/m), and = constant Petersen [ 121 obtained a similar relationship for electrostatic cantilever actuators without compliant hinges. Assuming that L H = do (reasonable for the structures tested), our previous nonparallel model [ l l ] reduces to an E,

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STORMENT et al.: FLEXIBLE, DRY-RELEASED PROCESS FOR ALUMINUM ELECTROSTATIC ACTUATORS

Fig. 8. SEM view of a torsional actuator apparently damaged by air currents ( L H = 10 pm, W H = 4 LLm, L p = 50 p m and ffrp = 50 pm).

approximate relationship of the form,

where K2 = constant. Electron microscopic tests were carried out on a limited number of torsional structures. It proved possible to obtain high quality images despite a strong voltage-contrast effect (caused by the applied voltages) and quite accurate measurements of angle and gap versus voltage from the digitized images. Fig. 9 is a composite of ten images taken of a torsional actuator (as shown in Fig. 2 but with etch access holes and landing electrodes) that was tested with applied voltages ranging from 0 to 33.0 V. The resulting gap versus voltage characteristics for this device are shown as an inset to Fig. 9. While relatively few Vth measurements were made for torsional structures, it was seen that they too had relatively large variations in this parameter. Overall, the large measured variations in bik, for a given design make extensive analysis somewhat futile. It is possible that future reductions in device-to-device variability, for example in patterning of the hinges or microdefects therein, could be achieved through process modifications. However, we feel that new device designs, rather than new processes, should reduce the sensitivity of Vti, to process variations in a much more useful manner. V. CONCLUSION We have presented an aluminudorganic sacrificial layer MEMS process (Al-MEMS) for the fabrication of mechanical actuators. The process is designed to be compatible with the

Fig, 9. Composite of ten SEM images taken of a torsional actuator (as shown in Fig. 2 but with etch access holes and landing electrodes) with applied voltages ranging from 0 to 33.0 V. The resulting gap versus voltage characteristics for this device are shown as an inset.

future use of underlying, pre-fabricated CMOS or BiCMOS circuits by relying on only process steps known to be compatible with such circuits. In addition, the process developed does retain the capability to fabricate various electro-optic modulators, which can be used for optical data output from future MEMS and non-mechanical transducer systems. As stated in the discussion section, the relatively large variation in threshold voltages was evident in all cantilever designs tested. Our future investigations of electrostatic actuators will be oriented more toward structural design than process development, with potential to greatly improve performance. In addition, new testing methods, such as optical angle measurement, are under development and should improve our ability to test large numbers of devices.

REFERENCES [ I ] C . H. Mastrangelo and R. S. Muller, “Fabrication and performance of a fully integrated p-Pirani pressure gauge with digital readout,” in Trunsducers’91, Proc. 1991 Int. Conf on Solid-State Sensors und Actuurors, San Francisco, CA, June 24-27, 1991, pp. 245-248.

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[2] T. A. Core, W. K. Tsang and S. J. Sherman, “Fabrication technology for an integrated surface-micromachined sensor,” Solid State Technol., vol. 36, no. 10, Oct. 1993, pp. 39-47. [3] L. J. Hombeck, “Deformable-mirror spatial light modulators,” in Spatial Light Modulators and Applications III, in Proc. SPIE-the Int. Soc. for Optical Eng., 1990, vol. 1150, San Diego, CA, Aug. 1989, pp. 86-102. [4] L. J. Hornbeck, “Spatial light modulator and method,” U.S. Patent 5,061,149, Oct. 29, 1991. [5] L. J. Hornbeck, “Spatial light modulator system,” U.S. Patent 5,028,939, July 2, 1991. [6] L. J. Hombeck, “Spatial light modulator,” U.S. Patent 4,956,619, Sept. 11, 1990. [7] L. J. Hornbeck, “Spatial light modulator and method,” U.S. Patent 4,710,732, Dec. 1, 1987. [8] J. B. Sampsell, “Spatial light modulator,” U.S. Patent 4,954,789, Sept. 4, 1990. [9] W. Yun, R. T. Howe and P. R. Gray, “Surface micromachined, digitally force-balanced accelerometer with integrated CMOS detection circuitry,” in Proc. IEEE Solid-state Sensor and Actuator Workshop, Hilton Head, SC, June 22-25, 1992, pp. 126-131. [lo] J. A. Thornton and D. W. Hoffman, “Internal stresses in titanium, nickel, molybdenum, and tantalum films deposited by cylindrical magnetron sputtering,”J. Vacuum Sei. and Technol., vol. 14, no. 1, Jan./Feb. 1977, pp. 164-168. [ I l l K. A. Honer and G. T. A. Kovacs, “Static modeling of thin-hinged electrostatically-deflected micromachined cantilever actuators,” submitted to J. Microelectromech. Syst., July 1993. [I21 K. E. Petersen, “Micromechanical light modulator array fabricated on silicon,” Appl. Physics Lett., vol. 31, no. 8, Oct. 1977, pp. 521-523.

Christopher W. Storment received the B.S. degree in chemistry from the University of California, Riverside, CA, in 1980, and the M.S. degree in material sciences from the University of Southern California, Los Angeles, CA, in 1987. From 1980 to 1987, he was employed at the Hughes Aircraft Torrance Research Center and the TRW Advanced Microwave Circuits Department, working on process development for GaAs digital and analog microwave circuits. From 1987-1993, he worked at Stanford University, Stanford,CA, for the Palo Alto Veterans Administration Rehabilitation Research and Development Center developingfabrication techniques for use in the neural interfaceproject. Since then he has joined the staff at Stanford’s Center for Integrated Systems as a Research and Development Engineer. He is active in co-advising students on several solid-statetransducerprojects in Dr. Kovacs’ laboratoryat Stanford.

David A. Borkholder received the B.S. degree in microelectronic engineering from the Rochester Institute of Technology, Rochester, NY, in 1992. He is currently pursuing the Ph.D. degree in the department of Electrical Engineering at Stanford University under the United States Air Force Laboratory Fellowship Program. From 1989 to 1992 he was employed at Eastman Kodak Company’s Microelectronics Technology Division, working on CCD imager fabrication and characterization.His research activities involve process development and fabncation of MEMS, high-aspect-ratioelectroplated structures, microelectrode arrays for neurophysiological studies, and neural microsensors. Mr. Borkholder is a member of Tau Beta Pi, Eta Kappa Nu, Phi Kappa Phi, and a student member of SPIE.

Victor A. Westerlind received the B.S degree in electrical engineering from Come11 University, Ithaca, NY in 1991 and the M.S. degree in electrical engineering from Stanford University, Stanford, CA in 1993. He was awarded a Hughes Master’s Fellowship in 1991. He is currently researching the design and fabrication of micromachined electrostatic actuators.

John W. Suh received the B.S degree in electrical engineeringfrom the GMI Engineering & Management Institute, Flint, MI, in 1990 and the M.S. degree in Mechanical Engineering from Stanford University, Stanford, CA in 1993. He is currently a Ph.D. candidate in mechanical engineering at Stanford University. His research is on the design and fabricationof electrostaticand thermal microactuators.

Nadim I. Mduf (S’85-M’91) received the B.E. degree in Electrical Engineering from the American University of Beirut, Lebanon, in 1984, the M.S.EE from the California Institute of Technology, Pasadena, CA, in 1985, and the Ph.D. degree in Electrical Engineering from Stanford University, Stanford, CA, in 1991. From 1985 to 1988, he was employed as a design engineer with Siliconix Inc., Santa Clara, CA, working on the development of high voltage integrated circuits. He is currently a Research Associate and a Lecturer in the Department of Electrical Engineering at Stanford University. His research interests are in micro/nanofabrication techniques and their applications in the development of sensors and instrumentation, and in the development of microfabricated neural interfaces.

Gregory T. A. Kovacs (S’82-M’91) received the B.A.Sc. degree in Electrical Engineering from the University of British Columbia,Vancouver, B.C., in 1984, the M.S. degree in Bioengineering from the University of California,Berkeley, CA, in 1985, the Ph.D. degree in Electrical Engineering and the M.D. degree from Stanford University, Stanford, CA, in 1990 and 1992, respectively. His industry experience includes the design of a large number of electronic circuits for commercial and industrial applications, patent law consulting, and the co-founding of three electronics companies. In 1991, he joined Stanford University as Assistant Professor of Electrical Engineering. His present research areas include neurallelectronic interfaces, solid-state sensors and actuators, micromachining, integrated circuit fabrication,medical instruments, and biotechnology. He holds the Robert N. Noyce Family Faculty Scholar Chair and received an NSF Young Investigator Award in 1993.