Design Optimization of MEMS Comb Accelerometer

MEMS (Microelectromechanical Systems) refers to the technology integrating electrical and mechanical components with feature size of 1~1000 microns. MEMS comb accelerometers have been successfully applied for air-bag deployment systems in automobiles. In this paper, the design optimization of a polysilicon surface-micromachined MEMS comb accelerometer is discussed. The device uses folded-beam structure to enhance the sensitivity. The movable mass is connected to two anchors through folded-beams. There are movable fingers extruding from both sides of movable mass. Each movable finger has left and right fixed comb fingers surrounding it, so that a differential capacitance pair is formed. Any acceleration along the sensitive direction will induce inertial force on movable mass and deflect the beams. Hence the differential capacitance gap will change. By measuring this differential capacitance change, the experienced acceleration can be measured. ANSYS FEM simulation is used to extract the device sensitivity and resonant frequency of the device. By gradually varying the design parameters in ANSYS simulation, the relationship between the device sensitivity and various design parameters is derived. The curves of device sensitivity versus beam width, beam length and mass width are derived and they are in good agreement with theoretical prediction. From the analysis it is concluded that the device behavior strongly depends upon various design parameters. By adjusting design parameters, desired sensitivity can be obtained. Based on the simulation results, a set of optimized design parameters for the comb accelerometer is decided. The ANSYS simulation results show that the device has displacement sensitivity of 3nm/g. The above-proposed MEMS comb accelerometer may be used for many applications, such as automobile airbag deployment and navigations, fabrication sequence of the comb accelerometer is also proposed. The device is to be fabricated using surface-micro machining process with sacrificial layer technique.