Design, Simulation and Novel Fabrication Method for High Sensitivity and Low Cross-axis Sensitivity Out-of-Plane Micro Accelerometer

In the present work, MEMS based acceleration sensor with high sensitivity and out-of-plane sensing is designed. An important highlight of this work is the location of the beams on the proof mass and the novel fabrication process to achieve it. The sensor has an operational range of 15g with differential capacitive sensing configuration. It is designed considering silicon as the structural material and has proof mass of 50 micrometer thickness and beams of 25 micrometer thickness, located in the central plane of the proof mass. Analytical design has been carried out to arrive at the preliminary dimensions. FEM simulations have been carried out using MEMS Design and analysis software, CoventorWare®. Modal analysis shows the first resonance mode occurring in the out-of-plane direction at a frequency of 1.389 KHz. The displacement response to acceleration is found to have non-linearity less than 0.1 percentage full scale output (FSO) and the variation of capacitance with acceleration has a non-linearity of less than 1 percentage FSO. Base capacitance for the design is 4.5 pF and sensitivity is 118.2 fF/g. Harmonic analysis shows linear response characteristic of the sensor over the operational bandwidth of 100 Hz. A significant advantage of the reported design is its negligible cross-axis sensitivity which is found to be less than 0.01 percentage. Fabrication process with novel etching method to realize the beams in the central plane of the proof mass, is presented. This fabrication process provides designers the freedom to design the beams independent of the proof mass thickness.