An integrated digital silicon micro-accelerometer with MOSFET-type sensing elements

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Abstract
A fully digital integrated accelerometer having a new sensing element is designed and fabricated based on CMOS processes and micromachining. The sensing elements of this accelerometer are constructed on the bulk silicon proof mass with metal air-gap MOSFETs (MAMOS) whose drain current is dependent on the applied acceleration. A current-controlled oscillator converts the change of this drain current to a digital pulse train. A 20-bit synchronous binary counter is monolithically integrated to digitize the output pulse of the oscillator. A bulk micromachined silicon proof mass provides perfect electrical isolation of the MOSFET sensing elements from a peripheral CMOS readout circuit. The suspension springs of this accelerometer are formed from thick MEMS (microelectromechanical systems) polysilicon. A CMOS compatible doping and annealing process for the MEMS polysilicon is developed to optimize the trade-off between the mechanical properties and the electrical requirements. The shift in the overall device characteristics of CMOS circuitry integrated with MEMS polysilicon is well below 5% of those fabricated by a standard CMOS process. Using the slightly modified 1.5 µm CMOS circuit process followed by an anisotropic silicon etch, an integrated digital silicon accelerometer is fabricated. The measured sensitivity of the fabricated MAMOS accelerometer is 63 kHz G-1.
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