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

摘要

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 mu m 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). [References: 17]