A Single-crystal Silicon 3-axis CMOS-MEMS Accelerometer

The paper presents a single-crystal silicon (SCS)-based, integrated 3-axis accelerometer fabricated using a post-CMOS micromachining process. This new CMOS-MEMS process provides monolithic integration of electronics and SCS microstructures, and electrical isolation of silicon. By employing a unique vertical sensing mechanism, 3-axis acceleration sensing is achieved with a single proof mass. The symmetric structures and fully differential configuration of all the sensing electrodes can greatly reduce the cross coupling among the 3 axes. By sacrificing one interconnect metal layer, the silicon undercut of the sensing comb fingers is minimized, resulting in much higher sensitivity with the same device footprint. A wet Al etching process was also developed to remove the top Al layer without attacking the Al layers exposed from the sidewalls of the multilayer Al/oxide stacks. A two-stage, open-loop, continuous time chopper stabilized amplifier is integrated on the chip.