Modeling of corrugated diaphragms for condenser microphones

The working principle of a condenser microphone is based on the variations of capacitance due to the movements of the diaphragm corresponding to the effect of sound pressure. A well-designed condenser microphone should be equipped with a diaphragm with very large compliance. Because of micromachining processes, significant residual stresses will appear on the diaphragm. Residual stresses could result in reducing the diaphragm compliance, frequency response and the sensitivity of the microphones. To lessen these undesired characteristics, the design of the diaphragm is aimed to lower the residual stress effects. A feasible approach to reduce the residual stress is to incorporate a corrugated diaphragm in the microphone structure. This appears to be a more convenient way to increase the mechanical sensitivity of the condenser microphones than the approach of changing the deposition process during the fabrication. The purpose of this research is to find the design to optimize diaphragm compliance, frequency response and damping effects in terms of the mechanical sensitivity of the microphones. Effects of varying the designed geometries of corrugated diaphragm such as depth, thickness, configurations (square or round), and applying different residual stresses on the diaphragm are investigated by the use of a commercial MEMS software. Simulation results show that a corrugated diaphragm is more flexible than a planar diaphragm for both square and round configurations. Reduction of residual stress plays an important role in improving the mechanical sensitivity of the condenser microphone. The increase of corrugated depth can greatly enhance the sensitivity due to the reduction of diaphragm stiffness. Furthermore, a thinner corrugated diaphragm with circular shape can increase the sensitivity significantly.