Thermal stress analysis for a CMOS-MEMS microphone with various metallization and materials

Many micro-electromechanical systems (MEMS) microphone structures are developing along with the growing market of portable and smart devices. Therein, the impact of thermal loading is one of the inevitable concerns that need to be considered from the viewpoint of microphone's reliability. This study focused on the thermal stress analysis for a complementary metal oxide semiconductor (CMOS)-MEMS microphone with various metallization and materials in diaphragm and back-plate under thermal loading; meanwhile, the weak points in the microphone structure under thermal loading were identified through simulation and experiment. In simulation, three-dimensional finite element (FE) models of the MEMS microphone with different metallization and materials in the diaphragm, including aluminum and copper, were built by commercial software ANSYS. The FE models were subjected to the thermal loading from −40 °C to 125 °C. In experiment, the failure mode of a CMOS-MEMS microphone chip by TSMC (Taiwan Semiconductor Manufacturing Company Limited) 0.18 μm CMOS process after 500 cycles of thermal loading from −40 °C to 125 °C was analyzed using the images obtained from scanning electron microscope (SEM). The results from simulation are consistent with those from experiment. The stress at the interface of the metal located at the bottom of diaphragm is higher than that at its top according to FE analysis and the stress concentration was found at the interface between the diaphragm and Si substrate in the microphone. The SEM images showed the delamination and crack locations of the microphone after thermal cycling test agreed with those from FE analysis. Moreover, diverse metallization and materials were studied by FE analysis for structural enhancement of the microphone. The results indicated that adopting Cu layer instead of Al layer in the diaphragm could decrease the thermal stress around 33% when under thermal loading. The weak spots of the CMOS MEMS microphone chip were found in this study and an effective enhanced approach was proposed for this microphone structure with alternative metallization and material.