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 accelero meter that is fabricated on a single-silicon wafer using a combined surface and bulk fabrication process. The microaccelerometer has high device sensi tivity, low noise, and low/controllable damping that are the key factors fo r attaining mug and sub-mug resolution in capacitive accelerometers. The mi crofabrication process produces a large proof mass by using the whole wafer thicknes and a large sense capacitance by utilizing a thin sacrificial lay er. The sense/feedback electrodes are formed by a deposited 2-3 mum polysil icon him with embedded 25-35 mum-thick vertical stiffeners. These electrode s, while thin, are made very stiff by the thick embedded stiffeners so that force rebalancing of the proof mass becomes possible. The polysilicon elec trodes are patterned to create damping holes. The microaccelerometers are b atch-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 p F/g. The measured sensitivity of a 4-mm x 1-mm accelerometer with a cantile ver support is 19.4 pF/g. The calculated noise floor of these devices at at mosphere are 0.23 mug/root Hz and 0.16 mug/root Hz, respectively.