WHOI Workshop on Biological and Chemical Sensors in the Ocean Trends in Microfabrication Technology Toward High-Performance, Low-Cost Sensor Systems Prof. Tsu-Jae King Electrical Engineering and Computer Sciences Department University of California, Berkeley, CA USA July 14, 2003

2 T.-J. King, UC-Berkeley Outline Introduction – Micro-ElectroMechanical Devices (MEMS) Integration of MEMS with Electronics Large-Area Electronics Technology Summary

3 T.-J. King, UC-Berkeley sacrificial layer MEMS Technology Surface Micromachining (cross-sectional view) Si wafer substrate structural film Structures are freed by selective removal of sacrificial layer(s) Mechanical structures are made using conventional microfabrication techniques Polycrystalline silicon is the preferred structural material as strong as steel resists fatigue -- requires high annealing temperatures (  900 o C)

4 T.-J. King, UC-Berkeley Examples of MEMS Products Chemical and pressure sensors Inertial sensors —accelerometers and gyroscopes Optical modulators —micro-mirrors for communications, projection displays

5 T.-J. King, UC-Berkeley Texas Instruments Inc. DMD TM Projection Display Chip SEM image of pixel array Schematic of 2 pixels Mirrors are made using metal layers (Al, alloys) - sacrificial material is photoresist Each mirror corresponds to a single pixel, programmed by an underlying memory cell to deflect light either into a projection lens or a light absorber.

6 T.-J. King, UC-Berkeley Outline Introduction Integration of MEMS with Electronics Large-Area Electronics Technology Summary

7 T.-J. King, UC-Berkeley Integrated Microsystems Monolithic integration of MEMS with electronics is desirable for improving system performance and reliability, and for lowering cost Modular, electronics-first approach is attractive Allows for separate development and optimization of electronics & MEMS fabrication processes MEMS can be stacked directly on top of electronics …but the metal wiring in electronic circuitry cannot withstand very high temperatures  A low-temperature micromachining process is desirable

8 T.-J. King, UC-Berkeley The Ideal MEMS Technology For high performance & low cost, we want: Low thermal process budget  can use semiconductor foundry for electronics Capabilities similar to poly-Si MEMS technology  can leverage existing MEMS foundry processes  can leverage MEMS industry design experience

9 T.-J. King, UC-Berkeley Enter Silicon-Germanium… SiGe can be processed at significantly lower process temperatures than Si (  450 o C) - Conventional process tools are used for deposition and patterning Properties are similar to those of Si, and can be tailored by adjusting Ge content IC industry has significant experience with SiGe CB IV N SiAl VIII P GaGeAs

10 T.-J. King, UC-Berkeley Schematic cross-sectional view of modularly integrated devices SiGe iMEMS Technology A. E. Franke et al., Solid-State Sensor and Actuator Workshop Technical Digest, pp , June 2000 Conventional CMOS process (Al wiring) Structural layer: ~65% Ge, 2.5  m thick Sacrificial layer: 100% Ge, 2  m thick

11 T.-J. King, UC-Berkeley SiGe iMEMS Demonstration Resonator next to Amplifier conventional layout Resonator on top of Amplifier smaller area --> lower cost reduced interconnect parasitics --> improved performance A. E. Franke et al., Solid-State Sensor and Actuator Workshop Technical Digest, pp , June 2000

12 T.-J. King, UC-Berkeley SiGe iMEMS Resonator Response S. A. Bhave et al., Solid-State Sensor and Actuator Workshop Technical Digest, pp , 2002

13 T.-J. King, UC-Berkeley Outline Introduction Integration of MEMS with Electronics Large-Area Electronics Technology Summary

14 T.-J. King, UC-Berkeley “Macroelectronics” Low-density integration of thin-film transistors (TFTs) distributed over a large area (~1 m 2 ) Applications:  large-area flat-panel displays  sampling or control of the properties and environment over a large surface

15 T.-J. King, UC-Berkeley Technology Targets 100 MHz circuit operation  semiconductor material must be poly-Si (TFT minimum feature size  1  m) Manufacturing cost < $100 per ft 2  roll-to-roll processing (plastic substrates) Courtesy of FlexICs, Inc.

16 T.-J. King, UC-Berkeley Challenges for TFTs on Plastic Substrate cannot be exposed to temperatures above ~150 o C for long periods of time  It is difficult to achieve the high-quality thin films necessary for good TFT performance Substrate shrinkage and swelling  Large misalignment tolerances are needed, which result in degraded TFT performance  Thin-film stress can be an issue  A new DARPA program in Macroelectronics aims to address these challenges…

17 T.-J. King, UC-Berkeley Excimer Laser Annealing (ELA) Typical fluences: mJ/cm 2 (500Å Si) mJ/cm 2 (1000Å Si) raster-scan process A short-pulse (~20-40 ns) excimer laser can be used to form high-quality polycrystalline thin films on coated plastic substrates homogeneous laser beam (XeCl, =308 nm)

18 T.-J. King, UC-Berkeley Thermal Simulation of ELA 5000 A 1000 A Small thermal budget  no damage to plastic

19 T.-J. King, UC-Berkeley Integrated Macrosystems? MEMS technology for large-area substrates has yet to be developed High-performance sensors generally require high fabrication temperatures, which are incompatible with plastic Self-assembly (to embed pre-fabricated sensors into the substrate) may be a viable approach

20 T.-J. King, UC-Berkeley Parallel Microassembly Concept K. Böhringer et al, ICRA, Leuven, Belgium, May 1998 Process “dis- integration” Heterogeneous integration –electronics –photonics –MEMS –…

21 T.-J. King, UC-Berkeley Stochastic Assembly 3-D shape matching:  Binding sites are etched wells  Assembly occurs spontaneously in solution, due to gravitational potential energy minimization J. Stephen Smith, UC Berkeley Product application: RF ID tags Alien Technology Corp., Morgan Hill, CA

22 T.-J. King, UC-Berkeley Chemical Binding Sites Complementary hydrophobic shapes are patterned onto parts and substrates, using monolayer coatings no shape constraints on parts no micromachining of substrate submicron, orientational alignment Uthara Srinivasan, Ph.D. thesis, Chemical Engr., UC-Berkeley (May 2001)

23 T.-J. King, UC-Berkeley Directed Self-Assembly Orientation of part to binding site occurs within one second after capture 0:00 s 0:31 s 0:32 s 0:33 s 0:47 s 0:36 s 0:41 s 0:56 s

24 T.-J. King, UC-Berkeley Outline Introduction Integration of MEMS with Electronics Large-Area Electronics Technology Summary

25 T.-J. King, UC-Berkeley Summary Monolithic integration of sensors with electronics is necessary for high performance and reliability —Low-temperature (  450 o C) surface-micromachining processes have been successfully developed to leverage both semiconductor and MEMS industry infrastructures Technologies to enable large-area sensing systems require significant additional R&D investment —macroelectronics —large-area MEMS —packaging…

26 T.-J. King, UC-Berkeley Acknowledgements Prof. Roger T. Howe Former Ph.D. students: –Dr. Andrea E. Franke, Dr. Yeh-Jiun Tung Former Visiting Researchers/Scholars: –Dr. Y.-C. Jeon, Prof. Y.-S. Kim Funding: –NSF and DARPA –UC-SMART program –Analog Devices Inc. and the Robert Bosch Corporation