Bottom-inlet-type micro-electro-mechanical system acoustic sensors based on two polyimide/amorphous-Si sacrificial layers

A bottom-inlet-type micro-electro-mechanical system acoustic sensor based on two polyimide/a-Si sacrificial layers is presented. A diaphragm was adapted to be on the top side of the sacrificial layers, showing the bottom-inlet structure for the package, which has the sensitivity of more than 3 dB compared with that of the top-inlet type. Also, the fundamental CMOS process implemented with Al electrodes was applied to have simple releasing steps by O₂ ashing and XeF₂ isotropic etching because of their material etching selectivity. The sensor module had a sensitivity of -38.9 dBV/Pa at 1 kHz with a bias of 9.2 V in the sweep range from 100 Hz to 16 kHz. In addition, to evaluate the open-circuit sensitivity, structure-based equivalent circuit modelling was performed with lumped parameters. The modelled sensitivity was in good agreement with the measured sensitivity in the error rate of 4.3% under 8 kHz, demonstrating the validity of the modelling. The modelled open-circuit sensitivity was determined to be -37.2 dBV/Pa at 1 kHz using the proposed lumped model.