Novel interconnection technologies for integrated microfluidic systems1

Sensors and Actuators 77 Ž1999. 57–65 www.elsevier.nlrlocatersna

Novel interconnection technologies for integrated microfluidic systems B.L. Gray

a,2

, D. Jaeggi

1

a,b

a , N.J. Mourlas b, B.P. van Drieenhuizen , K.R. Williams a , ¨ N.I. Maluf a,b,) , G.T.A. Kovacs b

a

b

Lucas NoÕaSensor, 1055 Mission Court, Fremont, CA 94539, USA Stanford UniÕersity, Center for Integrated Systems, CISX 202, Stanford, CA 94305-4075, USA

Abstract A new approach to realize silicon based integrated microfluidic systems is presented. By using a combination of silicon fusion bonding ŽSFB. and deep reactive ion etching ŽDRIE., multi-level fluidic ‘circuit boards’ are fabricated and used to integrate microfluidic components into hybrid systems. A multi-level laminating mixer and a manifold with multiple pressure sensors are presented as application examples. To interface the microfluidic system to the macroscopic world, three types of DRIE-fabricated, tight-fitting fluidic couplers for standard capillary tubes are described. One type of coupler is designed for minimal dead space, while another type reduces the risk of blocking capillaries with adhesive. A third design demonstrates for the first time a siliconrplastic coupler combining DRIE coupler technology with injection-molded press fittings. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Microfluidic systems; Deep reactive ion etching; Fluidic interconnects; Injection molding; Mixing; Silicon bonding

1. Introduction Integrated microfluidic systems have gained interest in recent years for many applications including chemical, medical, automotive, and industrial. A major reason is the need for accurate, reliable, and cost-effective liquid and gas handling systems with increasing complexity and reduced size. While a wide range of miniature fluidic devices such as valves, pumps, mixers, and flow sensors have been demonstrated w1x, efficient interconnections between these devices and coupling to external fluidic structures are not yet available. However, it is primarily these and other packaging issues that will determine the commercial success of microfluidic devices and systems. There remains a need for a microfluidic interconnection technology that mimics the electronic printed circuit board in ease of use, reliability, and versatility, and can accommodate active components as well as coupling to other microfluidic circuits and macroscopic equipment. A miniature hybrid fluidic circuit board based on bonding of plastic was demonstrated by Verlee et al. w2x. A )

Corresponding author Paper presented as part of the SSAW-98 Workshop. 2 Currently at the Micro Instruments and Systems Laboratory, Department of Electrical Engineering, EUII, University of California, Davis, CA 95616, USA. 1

reduction in size and increased integration level was achieved using epoxy printed circuit board technology w3x or gasketed stacked modules w4x. Further size reduction can be obtained with fully monolithic systems. However, the integration complexity of these systems is usually limited by wet silicon etch processes for the definition of channels and vias. The new approach presented in this paper uses deep reactive ion etching ŽDRIE. technology w5,6x to fabricate complex fluidic ‘circuit boards’. DRIE offers many advantages over wet silicon etch processes, in particular, higher density of fluidic interconnects, precise via holes with uniform cross-sections through the wafer thickness, and the ability to fabricate vias and channels of nearly arbitrary size and shape. The fabrication process includes a combination of DRIE, silicon fusion bonding ŽSFB., and anodic bonding to obtain multi-level fluidic substrates. These fluidic circuit boards can be used to build complex multi-level structures or can be combined with surfacemounted microfluidic devices. Two application examples are presented. The first is a mixer structure that relies on multiple levels of flow for operation. The second is an integrated hybrid system for measuring pressure in a microchannel using surfacemounted micromachined pressure sensors. A major issue in integrated microfluidic systems is the coupling of the fluidic circuits to the macroscopic world. Fluidic couplers consisting of a capillary tube glued into

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 1 8 5 - 5

58

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

Fig. 3. Illustration of a press-fit type siliconrplastic coupler. Heat-staked pegs are used to hold the injection-molded plastic part against the silicon.

Fig. 1. Illustration of the fabrication process for silicon, multi-level fluidic circuit boards: Ža. etch first channels using DRIE, then etch through-wafer vias and chambers from the other side using DRIE; Žb. SFB, grind back, and polish second wafer and use DRIE to etch second channels; Žc. SFB third wafer, define metal interconnects, and etch couplers as well as vias to surface-mount components using DRIE; Žd. anodically bond Pyrexe and attach capillaries and pressure sensors.

an insertion channel which is iso- or anisotropically etched into a silicon substrate have been previously demonstrated

w7,8x. These couplers do not have accurately fitted insertion channels resulting in difficult handling and increased dead space. Our approach is to use successive DRIE steps to fabricate accurately sized cross-sections for the connecting capillaries. To circumvent the necessity of gluing the capillaries, a new coupler combining DRIE with injectionmolded press fittings has been developed, allowing the capillaries to be exchanged.

2. Fabrication Fig. 1 illustrates the fabrication process for multi-level fluidic circuit boards. The first deep etch, approximately 100 mm deep, is performed on a double-side polished silicon wafer to define the first channels. This is followed

Fig. 2. SEM photographs of the first level of channels and through-wafer via holes: Ža. 100-mm diameter vias and 100-mm square microchannels; Žb. 100-mm diameter via hole in the center of a spiral structure.

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

by a second DRIE step from the opposite side of the wafer to form interconnection vias, 50–100 mm in diameter, all the way through the 390-mm thick wafer. Photoresist is

59

used to protect the wafer front with the first channels during the through-wafer etch and all subsequent DRIE steps. All silicon etches were performed using time-multi-

Fig. 4. Illustrations of a multi-level laminating mixer. Mixing occurs by successive separation and reuniting of fluid streams to increase the contact area between the two fluids, resulting in faster mixing by diffusion: Ža. SEM photograph of first channel level and vias of the first two Žof six. stages; Žb. CFD simulation result; Žc. photograph of an actual mixer operating at 60 mlrmin using two water streams dyed with different colors.

60

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

plexed SF6 etching and C 4 F8 passivation cycles in a tool from Silicon Technology Systems ŽAbercarn, UK. w9x. Etch rates were 2–3.2 mmrmin. Fig. 2 shows scanning electron microscope ŽSEM. photographs of first level channels and via holes. A second wafer is fusion bonded to the first, and ground and polished to 100 mm in thickness. Second level channels are then etched through the second wafer. In a third SFB step, a third 390-mm thick silicon wafer with 0.3–0.6 mm of thermally grown silicon oxide on top is used to cap the second level of channels. Using three wafers to obtain two channel levels avoids an aligned bonding step that would be necessary if only two wafers, both containing one level of channels and vias, were used. Aluminum metallization Ž0.8-mm thick. is patterned on top of the three-wafer stack to provide electrical interconnects if needed for the surface-mounted components. A final DRIE step through the top wafer defines couplers to capillary tubing and access holes for surface-mounted fluidic components. A transparent glass substrate Ž880 mm Pyrexe. is anodically bonded to the bottom of the stack to aid in flow visualization, but may be replaced by a silicon wafer to result in an all silicon stack. Piezoresistive pressure sensors, commercially available from Lucas NovaSensor ŽFremont, CA., were aligned over 100-mm diameter pressure ports, attached using a silicone rubber sealant, and wire-bonded to bondpads on the microfluidic circuit board. Capillaries were either inserted and fixed in place with adhesive plumbing marine epoxy from ACE ŽOak Brook, IL., or inserted into a siliconrplastic coupler with press fitting which requires no gluing of capillaries ŽFig. 3.. The plastic press fitting was fabricated by H. Weidmann AG, Plastic-Technologies ŽRapperswil, Switzerland. using injection molding of polyoxymethylene ŽPOM. plastic w10x. To align and hold the plastic part in place, the silicon stack and the Pyrexe have additional through-holes for pegs fabricated by DRIE and ultrasonic drilling, respectively. Once the press fitting is in place, the plastic pegs are melted with a heat-staking tool, an adapted soldering iron with a copper block attached, at temperatures reaching 2508C. The result of the overall process is a four-wafer stack with two interconnected channel levels that are connected to surface-mount components and capillaries linked to external devices through vias etched in the top wafer. Pressure sensors were the only surface-mounted components tested, but other components, such as microvalves or pumps, could also be used.

fluids is fundamental to the creation of many ‘on-chip’ microfluidic processing systems. However, the channel sizes and flow rates associated with such processing systems usually imply low Reynolds numbers, precluding turbulence as a mixing mechanism. Mixing by diffusion can be very slow unless the contact area between the two fluids is increased. One well-known method to increase contact area is through the formation of multiple laminations. LIGA-fabricated and silicon wet-etch processed mixers based on this principle have been previously demonstrated by other researchers w11,12x. Fig. 4a shows an SEM photograph of one level of channels and vias of the first two Žof six. stages of a laminating mixer. The channels are 100 mm deep. Two fluids entering the inlet ports laminate at the first horizontal junction, producing two side-by-side fluid streams. The fluid flow is split vertically, then brought back together horizontally, resulting in four streams alternating the two fluids in stage 2. Successive vertical separation and horizontal reuniting of fluid streams increases the number of laminates with each stage and, thus, the contact area between the two fluids w11x. After the third stage, eight

3. Application examples 3.1. Multi-leÕel laminating mixer The process shown in Fig. 1 was used to fabricate a multi-level laminating mixer. Time-efficient mixing of two

Fig. 5. Simulated and measured concentration profiles of fluid 1 across the channel in: Ža. stage 1; and Žb. stage 2. The measured data points have an uncertainty of "15%.

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

61

Fig. 6. Photograph of a fluidic circuit board demonstration system with five pressure sensors mounted on a substrate with an embedded microchannel and two capillary connections. The five pressure sensors are mounted 4 mm apart over 100-mm diameter access holes to the buried 100-mm square channel.

laminates exist, with each following stage doubling the number of laminates. The contact area between the two fluids is thus greatly increased, resulting in faster mixing by diffusion.

Fig. 7. Measured and simulated data for Ža. water at 100–500 mlrmin and Žb. air at 3 mlrmin showing pressure drop along the 100-mm square microchannel for the system shown in Fig. 6. The simulation results were obtained using CFD simulations.

Fig. 8. SEM photograph of a minimum dead space fluidic coupler for capillaries with 150 and 50 mm outer and inner diameters, respectively.

62

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

Fig. 9. Ža. SEM photograph of a fluidic coupler with a 110-mm thick sleeve around the bore to prevent blocking of capillaries with adhesive. The silicon is partially cleaved to show the sleeve; Žb. SEM photograph of capillaries inserted into the sleeve couplers.

Computational fluid dynamics ŽCFD. simulations were performed using the tools from CFD Research Corporation ŽHuntsville, AL. w13x. Fig. 4b shows a simulation result for the first two stages of the mixer, where the differently dyed water streams at each inlet port have flow rates of 30 mlrmin. A diffusion constant, D, of 8.3 = 10y1 0 m2rs was used. The diffusion constant was measured by flowing two side-by-side streams of differently dyed water in a 5-mm long channel, and adjusting the flow rate such that full mixing occurred at the end of the channel. Fig. 4c shows a close-up of the first vertical split and second

reunification in a fabricated mixer with the same input conditions as the simulations. The expected two and four laminates in the first and second stage can be clearly observed. Simulated and measured concentration profiles across the channel for each fluid were compared. For measured data, profiles were obtained through color analysis of captured video images of each stage by separating them into red, green, and blue components. Fig. 5a shows the simulated and measured concentration profiles for liquid 1 in stage 1. Fig. 5b shows the simulated and measured

Fig. 10. Photograph of two arrays of siliconrplastic couplers with inserted capillaries accessing a 50-mm wide, 100-mm deep microchannel.

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

63

concentration profiles for liquid 1 in stage 2. A good qualitative agreement is observed between simulation and measurement. A concentration profile that is not completely symmetric is seen for the second stage, resulting from asymmetries in mixer geometry. 3.2. Hybrid system As a demonstration of an integrated hybrid microfluidic system, five pressure sensors were directly mounted on a silicon substrate with an embedded 100-mm square microchannel and pressure ports leading from the channel to the sensors ŽFig. 6.. The pressure sensors have a pressure range of 0–30 psi Ž0–207 kPa.. Sensitivities, calibrated after surface mounting using a uniform load dead-weight tester, were found to be 4.7–5 mVrpsi Ž0.68–0.73 mVrPa.. A syringe pump was used to provide constant flow rates. Pressure inside the channel was measured for water flowing between 100 and 500 mlrmin ŽFig. 7a. and for air flowing at 3 mlrmin ŽFig. 7b.. The pressures were measured in steady-state and averaged for at least 30 s at approximately 18 samplesrs. Worst case divergence between measured data and simulation results obtained using CFD modeling was 10% for water and 30% for air. The kinematic viscosities used in the models were 1 = 10y6 m2rs and 1.51 = 10y5 m2rs for water and air, respectively.

4. Fluidic couplers Fluidic couplers for standard fused silica capillary tubing obtained from Polymicro Technologies Inc., ŽPhoenix, AZ. were fabricated using the process sequence shown in Fig. 1. The left coupler in Fig. 1d illustrates the first design. DRIE was used to define circular holes matching the inside and outside diameter of the capillaries, thus eliminating any dead space. Fig. 8 shows an SEM photograph of the minimum dead space coupler. The capillary was inserted into the opening and fixed in place with adhesive. It was observed that improperly cleaved capillaries with a cusp of glass and their outer polyimide coating at the end did not fit well into the insertion channel and allowed seeping of adhesive into the capillaryrsilicon interface. Although this problem can be solved with careful cleaving techniques, as an alternative we developed a second type of coupler with a silicon ‘sleeve’ around the etched bore, as illustrated in the right coupler in Fig. 1d. Fig. 9a shows an SEM photograph of a coupler with a 110-mm thick sleeve. The sleeve acts as barrier to adhesive and enhances the mechanical integrity of the coupling, but also introduces some dead volume. Fig. 9b shows fused silica capillaries with 250 and 360 mm inner and outer diameters, respectively, inserted into the sleeve couplers. To

Fig. 11. Measured water pressure loss in a blocked test channel indicating a sealed connection for a siliconrplastic coupler with a 500-mm thick silicone gasket.

quantify the dead volume introduced, CFD simulations were performed. A dead volume on the order of 2 nl at a flow rate of 100 mlrmin was estimated for the structure shown in Fig. 9a. The couplers were used at pressures up to 500 psi Ž3.4 MPa., limited by the test set-up. Adhesiveheld capillary couplers are known to hold pressures up to approximately 2000 psi Ž13.8 MPa. w8x. Fig. 10 shows a photograph of two arrays of the third type of coupler, a siliconrplastic coupler with injectionmolded press fittings. The two capillaries were connected through a microchannel in the substrate. Leakage tests of the new couplers were performed by pressurizing a blocked microchannel with water and measuring the pressure loss vs. time. Fig. 11 shows that the new siliconrplastic coupler alone is not leak proof at a pressure of 60 psi Ž414 kPa.. By introducing a silicone gasket, cut from a 500-mm thick sheet, between the plastic and the silicon, a sealed fluidic coupler system was obtained for pressures up to a maximum of 60 psi Ž414 kPa.. To quantify the mechanical integrity of the plastic press fittings, capillary pull tests with a load cell were performed. Measured pull-out forces ranged from 1 to 2 N.

5. Conclusions A novel technology for the fabrication of fluidic circuit boards including couplers to standard capillary tubes has been presented. A multi-level laminating mixer was used to demonstrate the technology for making multi-level structures. Good agreement was found between simulated and measured mixing of differently dyed water streams at 60 mlrmin. To demonstrate the technology for making fluidic circuit boards, the measurement of pressure in a microchannel was demonstrated with good agreement between measurement and CFD simulation. Novel couplers were fabricated using DRIE and either held in place with adhesive or a siliconrplastic press fitting. The siliconrplastic coupler is easily reusable and is leak-proof

64

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65

for pressures below 60 psi Ž414 kPa.. For higher pressures, adhesive-held capillaries may be necessary.

Acknowledgements Funding for this work was provided by the DARPA MicroFlumes Program ŽContract Number: N66001-96-C8631.. The authors would like to thank R. Scimeca of Lucas NovaSensor, Fremont, CA, M.G. Giridharan of CFD Research Corporation, Huntsville, AL, and T. Callenbach of H. Weidmann AG, Plastic-Technologies, Rapperswil, Switzerland for their help and technical discussion with processing, CFD modeling, and heat-staking, respectively. Load cell measurements by G. Cornella, Department of Materials Science and Engineering, Stanford University, CA are gratefully acknowledged.

References w1x P. Gravesen, J. Branebjerg, O.S. Jensen, Microfluidics—a review, J. Micromech. Microeng. 3 Ž1993. 168–182. w2x D. Verlee, A. Alcock, G. Clark, T.M. Huang, S. Kantor, T. Nemcek, J. Norlie, J. Pan, F. Walsworth, S.T. Wong, Fluid circuit technology: integrated interconnect technology for miniature fluidic devices, Proc. of the Solid-State Sensor and Actuator Workshop ’96, Hilton Head, June 3–6, 1996, pp. 9–14. w3x T.S.J. Lammerink, V.L. Spiering, M. Elwenspoek, J.H.J. Fluitman, A. van den Berg, Modular concept for fluid handling systems: a demonstrator micro analysis system, IEEE Proc. 9th Int. Workshop on Micro Electro Mechanical Syst. ŽMEMS ’96., San Diego, February 11–15, 1996, pp. 389–394. w4x W.K. Schomburg, B. Bustgens, J. Fahrenberg, D. Maas, Compo¨ nents for microfluidic handling modules, in: A. van den Berg, P. Bergveld ŽEds.., Micro Total Analysis Systems, Twente, Netherlands, November 21–22, 1994, Kluwer Academic Publishers, Dordrecht, 1995, pp. 255–258. w5x E.H. Klaassen, K. Petersen, J.M. Noworolski, J. Logan, N.I. Maluf, J. Brown, C. Storment, W. McCulley, G.T.A. Kovacs, Silicon fusion bonding and deep reactive ion etching; a new technology for microstructures, 8th Int. Conf. Solid-State Sensors and Actuators ŽTransducers ’95., Vol. 1, Stockholm, June 25–29, 1995, pp. 556– 559. w6x B.P. van Drieenhuizen, N.I. Maluf, I.E. Opris, G.T.A. Kovacs, ¨ Force-balanced accelerometer with mG resolution, fabricated using silicon fusion bonding and deep reactive ion etching, 9th Int. Conf. Solid-State Sensors and Actuators ŽTransducers ’97., Vol. 2, Chicago, June 16–19, 1997, pp. 1229–1230. w7x R.J. Reay, R. Dadoo, C.W. Storment, R.N. Zare, G.T.A. Kovacs, Microfabricated electrochemical detector for capillary electrophoresis, Proc. of the Solid-State Sensor and Actuator Workshop ’94, Hilton Head, June 13–16, 1994, pp. 61–64. w8x V.L. Spiering, J.N. van der Moolen, G.-J. Burger, A. van den Berg, Novel microstructures and technologies applied in chemical analysis techniques, 9th Int. Conf. Solid-State Sensors and Actuators ŽTransducers ’97., Vol. 1, Chicago, June 16–19, 1997, pp. 511–514. w9x J. Bhardwaj, H. Ashraf, Advanced silicon etching using high density plasmas, SPIE Micromachining and Microfabrication Process Technology Symposium, Vol. 2639, Austin, October 23–24, 1995, pp. 224–233.

w10x J.I. Kroschwitz ŽEd.., Concise Encyclopedia of Polymer Science and Engineering, Wiley, New York, 1990, pp. 469–472. w11x H. Mobius, W. Ehrfeld, V. Hessel, T. Richter, Sensor controlled ¨ processes in chemical microreactors, 8th Int. Conf. Solid-State Sensors and Actuators ŽTransducers ’95., Vol. 1, Stockholm, June 25–29, 1995, pp. 775–778. w12x J. Branebjerg, P. Graveson, J. Krog, C. Nielsen, Fast mixing by lamination, IEEE Proc. 9th Int. Workshop on Micro Electro Mechanical Syst. ŽMEMS ’96., San Diego, February 11–15, 1996, pp. 441–446. w13x CFD-GEOM, CFD-ACE, CFD-VIEW Manuals, CFD Research Corporation, Huntsville, 1997. Bonnie L. Gray received the BS in Electrical Engineering from Rensselaer Polytechnic Institute in Troy, NY, in 1992. Also in 1992, she was awarded an NSF Graduate Student Fellowship. She received the MS in Electrical Engineering from the University of California at Berkeley in 1995, with a thesis on the design and operation of a surface-micromachined tactile sensor array for endoscopic manipulators. Since 1995, she has been an Electrical Engineering doctoral student at the University of California at Davis. She is currently interested in interconnection technologies for MEMS, including mechanical, optical, and especially fluidic interconnection for mTAS. Dominik Jaeggi received the MSc and PhD degrees in Electrical Engineering from ETH Zurich ŽSwiss Federal Institute of Technology., Zurich, Switzerland, in 1991 and 1996, respectively. At ETH Zurich, his research focused on the development of thermal microsensors using industrial IC processes in combination with postprocessing micromachining steps. Most of his PhD research was conducted at the CMOS EM Microelectronic Marin, Marin, Switzerland, where he continued his work on thermal sensor technology as a ETH Zurich Research Associate in 1996–1997. Since 1997, he has been working as a Post-doctoral Fellow at Stanford University, Stanford, CA, and Research Staff Scientist at Lucas NovaSensor, Fremont, CA. His current research interests include design, fabrication, and modeling of microfluidic devices. Nicholas J. Mourlas received the AB and BE degrees from Dartmouth College, Hanover, NH, in 1992. From 1992 to 1994, he was a Design Engineer at Reliable Power Products, Franklin Park, IL. He received the MS degree in Electrical Engineering from Case Western Reserve University, Cleveland, OH in 1996. In 1996, he worked as a research intern and consulted for Lucas NovaSensor. He is currently pursuing the PhD degree in the Department of Electrical Engineering at Stanford University. Since 1992, he has investigated MEMS devices for ink-jet printing, data storage, signal filtering, and chemical processing. His current research activities include interconnection and channel technologies for microfluidic systems. Bert van Drieenhuizen received the MSc with honors in 1991 and the ¨ PhD degree in 1996, both in Electrical Engineering from the University of Technology, Delft, The Netherlands. His doctoral research focused on the development of a surface micromachined RMS to DC converter. Since October 1996, he has been working for Lucas NovaSensor, Fremont, CA as a Research Staff Scientist. Kirt R. Williams received the BS degree with high honors with a double major in Electrical Engineering and Computer Sciences ŽEECS. and Materials Science and Engineering from the University of California at Berkeley in 1987. While pursuing this degree, he was employed at Eastman Kodak and Altera. After receiving the BS, he did digital and analog circuit design at Western Digital. Working in the Berkeley Sensor and Actuator Center at UC Berkeley, he earned the MS and PhD degrees in EECS in 1993 and 1997. His dissertation focused on the design, fabrication, and testing of micromachined hot-filament vacuum devices. He taught and revised the laboratory for the department’s IC-fabrication class, receiving the EE Outstanding Graduate Student Instructor award in 1996. He is presently a Research Staff Scientist at Lucas NovaSensor, Fremont, CA.

B.L. Gray et al.r Sensors and Actuators 77 (1999) 57–65 Nadim I. Maluf received the BE degree from the American University of Beirut, Lebanon, in 1984, the MS degree from the California Institute of Technology, Pasadena, CA, in 1985 and the PhD degree from Stanford University, Stanford, CA, in 1991, all in Electrical Engineering. From 1985 to 1988, he was employed at Siliconix, Santa Clara, CA, where he was responsible for the development of high-voltage integrated circuits. From 1991 to 1994, he was a Research Associate and Lecturer at Stanford University where he worked on micromachined transducers for medical applications and taught graduate courses on solid state physics and semiconductor devices. He is presently Manager of Advanced Technologies at Lucas NovaSensor, Fremont, CA, where his group is responsible for the development of advanced technologies and products for the automotive, medical, consumer, industrial and aerospace markets. He is also a Consulting Assistant Professor of Electrical Engineering at Stanford University and serves on the advisory boards of a number of organizations. He has authored or co-authored over 60 articles and patents in the areas of sensors and actuators, semiconductor technology and microinstrumentation.

65

Gregory T.A. Kovacs received the BASc degree in Electrical Engineering from the University of British Columbia, Vancouver, BC, in 1984, the MS degree in Bioengineering from the University of California, Berkeley, in 1985, the PhD degree in Electrical Engineering from Stanford University, in 1990 and the MD degree from Stanford University in 1992. His industry experience includes the design of high-speed data acquisition systems, the design of instruments for GaAs device fabrication, commercial and consumer product design, extensive patent law consulting, and the co-founding of several companies, most recently Cepheid, in Sunnyvale, CA. In 1991, he joined Stanford University and is currently an Associate Professor of Electrical Engineering. He teaches courses in electronic circuits and micromachined transducers. He held the Robert N. Noyce Family Faculty Scholar Chair in 1992–1994, received an NSF Young Investigator Award in 1993, was appointed a Terman Fellow in 1994, was appointed to the Defense Sciences Research Council in 1995 and was appointed a University Fellow in 1996. His present research areas include solid-state sensors and actuators, micromachining, neuralrelectronic interfaces, integrated circuit fabrication, medical instruments, and biotechnology, all with emphasis on solving practical problems.