MAMOS-a novel displacement sensitive transducer for fully digital integrated ac accelerometer

A fully digital integrated accelerometer was designed and fabricated based on CMOS and micromachining technologies. The sensing elements of this accelerometer are floating gated n-type MOSFETs with the variable airgap proportional to applied acceleration as gate insulator. The change of this air-gap distance alters the drain current of the sensing MOSFET (metal air-gap MOSFET: MAMOS). Current controlled oscillator converts the change of the drain current of MAMOS to frequency output. Twenty-bit synchronous binary counter is integrated to digitize the frequency output. CMOS compatible doping and annealing process for 2-μm-thick polysilicon used as suspension springs is developed to optimize the trade-off between the mechanical properties and the electrical requirements. Through the slightly modified 1.5 μm CMOS integrated circuit process followed by anisotropic silicon etch, integrated silicon digital accelerometer was fabricated