Applying Macro Design Tools to the Design of MEMS Accelerometers

This paper describes the design of two different surface micromachined (MEMS) accelerometers and the use of design and analysis tools intended for macro sized devices. This work leverages a process for integrating both the micromechanical structures and microelectronics circuitry of a MEMS accelerometer on the same chip. In this process, the mechanical components of the sensor are first fabricated at the bottom of a trench etched into the wafer substrate. The trench is then filled with oxide and sealed to protect the mechanical components during subsequent microelectronics processing. The wafer surface is then planarized in preparation for CMOS processing. Next, the CMOS electronics are fabricated and the mechanical structures are released. The mechanical structure of each sensor consists of two polysilicon plate masses suspended by multiple springs (cantilevered beam structures) over corresponding polysilicon plates fixed to the substrate to form two parallel plate capacitors. One polysilicon plate mass is suspended using compliant springs forming a variable capacitor. The other polysilicon plate mass is suspended using very stiff springs acting as a fixed capacitor. Acceleration is measured by comparing the variable capacitance with the fixed capacitance during acceleration.