Structural Enhancement for a CMOS-MEMS Microphone Under Thermal Loading by Taguchi Method

Structural optimization is a necessary procedure to make progress toward mass production for a new device. Both of structural robustness and superior performance are targets for structural optimization. In this study the structural weakness of a complementary metal oxide semiconductor (CMOS) - microelectromechanical systems (MEMS) microphone chip with 4 by 3 microphone cells by TSMC 0.18 µm CMOS process during thermal loading was identified first by thermal cycling test and thermal stress analysis; then, the optimal structures of the microphone were discussed from viewpoints of thermal stress and sensitivity by Taguchi method. Therein, the finite element (FE) method was adopted for thermal stress analysis and capacitive sensitivity of the microphone was obtained from the equation of sensing capacitance. Moreover, the weakness spots at bottom of the diaphragm in the microphone chip from simulation were verified by the images of scanning electron microscope (SEM) for the chip after 500 cycles of thermal loading in experiment. The results of structural optimization by Taguchi method showed that the microphone with thicker metal and thinner SiO2, wider anchor, and larger diaphragm could reduce the thermal stress in the diaphragm up to 68% than that of the original design. However, for the capacitive sensitivity of microphone chip, the results indicated that the microphone with thicker metal and SiO2, narrower anchor, and larger diaphragm had 5.8 times increase of microphone capacitive sensitivity than that of the original design. This study could provide helpful suggestions for the design and structural robustness of MEMS microphone.