Design And Simulation Of Micromachined Silicon Condenser Microphone With Free Floating Diaphragm

Micromachined silicon condenser microphone is used in many applications ranging from consumer products to professional audio systems. It offers advantages in higher sensitivity, better stability, lower power consumption, batch-production and better compatibility with conventional silicon micro-fabrication technology. While microphone working, expect the sensing diaphragm has lower rigidity and the backplate has upper rigidity, to provide adequate response linearity, suitably wide dynamic range and sufficiently high pull-in voltage. At the same time, expect the simple technics facture to meet with the requirements of industrialization batch production. During fabrication process of microphone, the tensile stress is always produced in the diaphragm. The excessively large residual tensile stress often results in low performances of microphone.A micromachined condenser microphone with a free floating sensing diaphragm and a perforated thick copper backplate is presented. The diaphragm consists of a heavily doped layer of polycrystalline silicon and two layers of low stress silicon nitride. One end of rectangled sensing diaphragm sticks on silicon substrate while the other parts float from substrate, so that the intrinsic stress of diaphragm can be released. The backplate is fabricated using a photoresist-molded electroplating technology and perforated with circular vent holes. Perforated circular holes on backplate are laid out as hexagon to adjust air squeeze-film damping between diaphragm and backplate to critical damping.A new approach to design micromachined condenser microphone based on Multi-objective Genetic Algorithm is proposed. In this method, the parameter of the sandwich diaphragm and the backplate as well as the space between them as the design variables, and the sensitivity and the pull-in voltage and the frequency bandwidth are optimized. A Pareto-optimal set is obtained by the multi-objective genetic algorithms. The set, in accordance with demand of design mostly is chosen as the parameter of microphone from the Pareto-optimal set.We simulate the microphone structure that has been optimized, use ANSYS11.0 to simulate the mode of sandwich diaphragm, use micro-electronics mechanism system designed Intellisuite to simulate the open-circuit sensitivity and the pull-in voltage with, use the moving grid of FLUENT to simulated the damping and calculate the resonance frequency accordingly educe the frequency bandwidth.Based on the analogy that exists between electric and mechanical systems, we build the equivalent circuit Lump model of silicon condenser micromachined microphone. According as this macro-model, we use Multisim to simulate the curve of the open-circuit sensitivity with mutative audio frequency. According to this curve, we can hold the open-circuit sensitivity and frequency bandwidth of microphone. At last were compared the simulation results with FEA numerical results, and the validity of this equivalent circuit was proved.