The nonliner in MEMS touch mode capacitive devices

Capacitive touch MEMS devices have better performance and higher sensitivity than conventional capacitive MEMS device. Because the diaphragm of these capacitive MEMS device touches the substrate in the operation, it also can withstand large over distortion, thus it is very suitable for industrial applications. However, hysteresis has been observed on some devices, especially for high sensitivity devices, when the load is cycled. In fabrication experiments, it is found that the P+ diffused diaphragm is deformed after the Si-Si bonding and subsequent high temperature annealing. The fabricated P+ diaphragm of some Si-Si sensor buckles downward toward the substrate, with no applied distortion. From an analysis of fabrication processes and a finite element modeling study, it is suggested that the P+ diaphragm was deformed when no load is applied. When a positive load is applied, the diaphragm touches the substrate creating a “compression” stress in the touched section of the diaphragm and making this part of the diaphragm raised. The hysteresis observed is the result of this unstable diaphragm. This hypothesis can be used to calculate the size of hysteresis and explain some of the observed experimental results.