Trends in Microfabrication Technology Toward HighPerformance LowCost Sensor

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 94720 USA July 14, 2003 WHOI Workshop on Biological and Chemical Sensors in the Ocean

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

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

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

TM DMD Projection Display Chip Texas Instruments Inc. • Mirrors are made using metal layers (Al, alloys) - sacrificial material is photoresist SEM image of pixel array Schematic of 2 pixels 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. T. -J. King, UC-Berkeley 5

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

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 T. -J. King, UC-Berkeley 7

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 T. -J. King, UC-Berkeley 8

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

Si. Ge i. MEMS Technology A. E. Franke et al. , Solid-State Sensor and Actuator Workshop Technical Digest, pp. 18 -21, June 2000 Schematic cross-sectional view of modularly integrated devices • Conventional CMOS process (Al wiring) • Structural layer: ~65% Ge, 2. 5 mm thick • Sacrificial layer: 100% Ge, 2 mm thick T. -J. King, UC-Berkeley 10

Si. Ge i. MEMS Demonstration A. E. Franke et al. , Solid-State Sensor and Actuator Workshop Technical Digest, pp. 18 -21, June 2000 Resonator next to Amplifier • conventional layout T. -J. King, UC-Berkeley Resonator on top of Amplifier smaller area --> lower cost reduced interconnect parasitics --> improved performance 11

Si. Ge i. MEMS Resonator Response S. A. Bhave et al. , Solid-State Sensor and Actuator Workshop Technical Digest, pp. 3437, 2002 T. -J. King, UC-Berkeley 12

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

“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 T. -J. King, UC-Berkeley 14

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

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 Ø A new DARPA program in Macroelectronics aims to address these challenges… T. -J. King, UC-Berkeley 16

Excimer Laser Annealing (ELA) • A short-pulse (~20 -40 ns) excimer laser can be used to form high-quality polycrystalline thin films on coated plastic substrates • Typical fluences: m. J/cm 2 260 -285 (500Å Si) 440 -450 m. J/cm 2 (1000Å Si) T. -J. King, UC-Berkeley homogeneous laser beam (Xe. Cl, l=308 nm) raster-scan process 17

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

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 T. -J. King, UC-Berkeley 19

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

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

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

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

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

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… T. -J. King, UC-Berkeley 25

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 T. -J. King, UC-Berkeley 26