THE APPLICATION OF FINE-GRAINED, TENSILE POLYSILICON TO MECHANICALLY RESONANT TRANSDUCERS

Resonant force sensors are devices which convert axially applied forces to changes in resonant frequency. These structures are fundamentally wires or beams or more complicated structures which are in a vacuum envelope. They become interesting and useful if they can be miniaturized, can be fabricated from a single material in a cost effective manner and can be excited and read via simple techniques. The devices which are reported here satisfy most of the above criteria. The construction material involves a silicon substrate, tensile strain polysilicon films and strain-compensated silicon nitride deposits. Clamped-clamped beams of polysilicon, typically 400,μm long, 40,μm wide and 2μm thick are fabricated with an isoplanar process over an oxide filled tub. Low-pressure chemical-vapor-deposited (LPCVD) nitride is used as a second sacrificial layer which also serves to support a second polysilicon layer which is part of the vacuum envelope. Internal surface adhesion problems are avoided by freeze-sublimation procedures which remove surface tension-induced beam deflections. Passivation and sealing is accomplished via LPCVD nitride and reactive sealing. Excitation and sensing is accomplished via ion implanted resistors. Experimental results always produce quality factors, Q, above 35 000. Resonant frequencies to 750 kHz have been achieved. It is estimated that these devices can measure axially applied forces below 0.1 dyne with standard electronic interfaces. © 1990.