Integrated silicon accelerometer with MOSFET-type sensing element

A fully digital integrated accelerometer having new sensing element is desi gned and fabricated based on CMOS process and micromachining. The sensing e lements of this accelerometer are constructed on the bulk silicon proof mas s with metal air-gap MOSFETs (MAMOS) whose drain current is proportional to the applied acceleration. Current controlled oscillator converts the chang e of this drain current to digital pulse train. Twenty-bit synchronous bina ry counter is monolithically integrated to digitize the output pulse of the oscillator. Bulk micromachined silicon proof mass provides perfect electri cal isolation of MOSFET sensing elements from CMOS readout circuit. A CMOS compatible doping and annealing process for this MEMS polysilicon is develo ped to optimize the trade-off between the mechanical properties and electri cal requirements. Through the slightly modified 1.5 mu m CMOS circuit proce ss followed by anisotropic silicon etch, integrated digital silicon acceler ometer is fabricated. Measured sensitivity of the fabricated accelerometer is 125 kHz/g.