Three-axis SOI capacitive accelerometer with PLL C-V converter

A three-axis capacitive accelerometer has been developed using silicon-direct-bonding SOI wafer. Z-axis accelerometer has 1 mm×1 mm plane mass and spiral shape beams of few millimeters in length. The mass and beams were formed using 6 μm thick single-crystal silicon. A gap of 1 μm has been maintained between the mass and one-side plate. X- and Y-axis accelerometers have comb structure of 3 mm×1 mm size and folded beams of few millimeters in length. Length of comb electrodes is 200 μm, and a gap of 4 μm is maintained between them. Initial sensor capacitance was about 5 pF for Z-axis accelerometers and 2 pF for X- and Y-axis accelerometers. Sacrificial etching process was performed using 73% HF solution without attacking aluminum metallization. `After-rinse stiction' was prevented using photoresist buried plug and side stopper with on yield of better than 90%. `In-use stiction' was prevented using fluorocarbon film formed by plasma polymerization equipment. The sensor characteristics were measured with PLL capacitance to voltage converter IC that was custom designed to reduce parasitic capacitance error and electric static error with offset and sensitivity calibration functions.