Polysilicon sealed vacuum cavities for microelectromechanical systems

Sealed, vacuum cavities are highly useful in silicon-based microelectromech anical systems. Sealed polysilicon cavities serve as reference chambers for commercially available pressure sensors and provide enclosures for high-e mechanical resonators. They also provide isolation of resonant strain gauge s from ambient gases and liquids for use in precision pressure transmitters . The residual pressure in such cavities is less than 10(-5) Ton as evidenc ed by the polysilicon resonator Q: values which can be as high as 300000. M easured values of Q can be dominated by parasitic electrical resistance los ses in the electrical drive circuit. After these losses are accounted for, the dominant loss is due to mechanical radiation of acoustic waves from the supports to the rest of the die,and package. The vibration of the beams ca n be excited and sensed in-several different Bays for testing purposes. Fib eroptically driven and sensed microbeams operating at up to 510 degrees C: for several hours resulted in no loss of vacuum as evidenced by the restora tion of resonator Q upon its return to room temperature. Thus optically res onant microbeams are-potentially suitable for fiberoptic-based sensors that withstand harsh environments, including high temperature. (C) 1999 America n Vacuum Society. [S0734-2101(99)16404-2].