Reliable piezoelectric FEM-simulations of MEMS microphones: Basis for intrinsic stress reduction

We derived a 3D FEM-simulation model for a piezoelectric MEMS-microphone, which includes - in contrast to commonly used models - intrinsic stress effects. The stress values included in the model are determined by measurements of cantilever test structures. The challenging task in the simulation is to combine a piezoelectric analysis with intrinsic stress in a harmonic response analysis, as to this, several restrictions exist in commercially available tools. The arising difficulties could be overcome by employing a coupled thermo-electro-mechanical simulation in order to get consistent static and harmonic response results. In that way, we were able to demonstrate successfully that intrinsic stress dramatically reduces the microphone sensitivity and necessarily has to be integrated in the FE-model for reliable and predictive device simulations. Additionally, the derived model is fully 3D and, thus, allows for design studies and optimization as well as for the evaluation of new design concepts since also non-axisymmetric geometries can be assembled.