An all-silicon single-wafer micro-g accelerometer with a combined surface and bulk micromachining process

This paper reports an all-silicon fully symmetrical z-axis micro-g accelerometer that is fabricated on a single-silicon wafer using a combined surface and bulk fabrication process. The microaccelerometer has high device sensitivity, low noise, and low/controllable damping that are the key factors for attaining micro g and sub-micro g resolution in capacitive accelerometers. The microfabrication process produces a large proof mass by using the whole wafer thickness and a large sense capacitance by utilizing a thin sacrificial layer. The sense/feedback electrodes are formed by a deposited 2-3 microns polysilicon film with embedded 25-35 microns-thick vertical stiffeners. These electrodes, while thin, are made very stiff by the thick embedded stiffeners so that force rebalancing of the proof mass becomes possible. The polysilicon electrodes are patterned to create damping holes. The microaccelerometers are batch-fabricated, packaged, and tested successfully. A device with a 2-mm x 1-mm proof mass and a full bridge support has a measured sensitivity of 2 pF/g. The measured sensitivity of a 4-mm x 1-mm accelerometer with a cantilever support is 19.4 pF/g. The calculated noise floor of these devices at atmosphere are 0.23 micro g/sqrt(Hz) and 0.16 micro g/sqrt(Hz), respectively.