Thin Film Residual Stress Assessment of MEMS Microphone Using Process Modeling Technology

The characteristic of MEMS component is affected by different process procedures such as depositing and etching thin films on substrate. These processes might induce residual stresses or deformations in MEMS component, which will reduce its efficiency and quality, and it is very unfavorable in MEMS development. This study develops a methodology that uses Finite Element Analysis (FEA) along with process modeling technology to analyze the residual stress in a MEMS microphone structure. The residual stress of thin film is composed of thermal and intrinsic stress. The thermal stress can be obtained directly in FEA but not for the intrinsic stress, which is a process-dependent material property. The intrinsic stress in multilayered structure is obtained from Stoney’s experiment which measures the curvature on blanket wafer after each process. A comparison of the experimental and simulated results showed that dislocation induced intrinsic stress in aluminum can be rearrangement after first annealing. The lattice mismatch induced intrinsic stress, however, will influence the residual stress of the aluminum film when its thickness is under 1μm. The residual stresses and the deformations in two electrode plates are presented for the process simulation in the MEMS microphone. The polysilicon would buckle and warp downward if it is subjected to compressive stress. However, the polysilicon diaphragm would be flat in the positive intrinsic stress.