A study of residual stress effects on CMOS-MEMS microphone technology

In this study, a process modeling methodology applied in finite element (FE) analysis has been developed to investigate the evolution of residual stress during and after the CMOS-MEMS process. The MEMS (micro-electromechanical systems) capacitive microphone structure which contains a large membrane for sound sensing is selected to be studied, and it is fabricated using a standard foundry CMOS process. From the FE analysis, the thermal and intrinsic stresses are induced and continuously monitored during the process steps. It is found that the buckling of membrane happens after the post-CMOS micromachining, and it leads to the degradation of microphone performance. The membrane deflection becomes larger as higher compressive stress exists after thin film deposition, and the residual normal stress is the major reason of membrane buckling. Besides, the residual gradient stress effect becomes influential as the constraint of the diaphragm is released. To improve the effective sensing area of membrane, the designed slots near the circumference of membrane are applied. On the other hand, the curling-type deformation due to the residual gradient stress can be controlled by the patterned adding layer. The possible approaches to mitigate the residual stress effects are investigated, and thus the qualified CMOS-MEMS capacitive microphone design can be achieved.