AVS 45th International Symposium
    The Science of Micro-Electro-Mechanical Systems Topical Conference Monday Sessions
       Session MM+VT-MoA

Invited Paper MM+VT-MoA1
Polysilicon Sealed Vacuum Cavities for MEMS

Monday, November 2, 1998, 2:00 pm, Room 324/325

Session: Vacuum MEMS and Microanalysis
Presenter: J.D. Zook, Honeywell
Authors: J.D. Zook, Honeywell
W.R. Herb, Honeywell
Y.C. Ahn, University of Wisconsin
H. Guckel, University of Wisconsin
Correspondent: Click to Email
Sealed vacuum cavities are highly useful in silicon-based micro-electrical-mechanical structures (MEMS). They serve as the reference chambers for absolute pressure sensors and provide enclosures for high-Q mechanical resonators. A process for fabricating sealed vacuum cavities in polysilicon was developed and described by Burns and Guckel in 1988.@footnote1@ The cavities are produced by the sacrificial etching of SiO@sub 2@. The vacuum is generated by the out-diffusion of hydrogen following the polysilicon sealing step. As an additional precaution the devices are coated with silicon nitride. The process was first applied to the fabrication of piezoresistive pressure transducers with a polysilicon diaphragm and a vacuum cavity used as a pressure reference. In 1989 a multi-level polysilicon process was used to fabricate resonant microbeams and to demonstrate that high mechanical Q values require a hard vacuum inside the cavity.@footnote 2@ The micromachined polysilicon resonant microbeams are sensitive strain transducers that provide the basis for temperature, pressure, strain, acceleration and vibration sensors. The polysilicon microbeams are fabricated monolithically on single crystal silicon microstructures, are sealed high vacuum shell enclosures and are characterized by high mechanical Q, typically between 20,000 and 100,000, with recent values as high as 220,000. Two devices have been running continuously for 7 years with no observable change in Q, i.e., no change in the vacuum level. The most recent use of the vacuum encapsulation process has been for fiber optic sensors which combine the advantages of silicon microfabrication with those of optical fiber communication.@footnote 3@ The microbeams are optically excited into resonance by either an optothermal mechanism or a photovoltaic mechanism. They can be driven by modulated light or can be self-resonant. The vibration of the beam modulates the light reflected back into the fiber, which is then detected using a photodetector. Fiber optic sensors also have advantages for aerospace because of their light weight and EMI immunity. A network of 16 optically resonant microbeam temperature sensors driven and read by the same laser was recently demonstrated. Optically driven self-resonant microbeams have been operating continuously for 4 years without measureable change in Q. The most recent demonstration of the vacuum integrity of the polysilicon cavities has been the high temperature operation of the microbeams. Operation up to 510 C for several hours resulted in no loss of vacuum as evidenced by the Q of the resonators after they were returned to room temperature. Thus polysilicon-based vacuum-encapsulated devices are potentially suitable for fiber-optic-based sensors that withstand harsh environments, including high temperature. The value of Q is determined not only by residual gas in the cavity but also by the end losses and by electrical losses induced by the vibrating polysilicon capacitor composed of the microbeam and the bias electrode. By measuring Q as a function of dc bias, the electrical contributions to Q can be subtracted, providing an upper limit on the partial pressure of residual gas in the vacuum cavity. @FootnoteText@ @footnote 1@D. W. Burns, Ph. D. Thesis, Dept. Mat. Sci., UW, Madison, WI (1988). @footnote 2@J. J. Sniegowski, Ph. D. Thesis, Dept. Nuc. Eng. and Eng. Phys., UW, Madiso n,WI (1989). @footnote 3@J. D. Zook, D. W. Burns, W. R. Herb, H. Guckel, J. W. Kang and Y. C. Ahn, S ensors and Actuators A52 (1996) pp. 92-98.