Performance and reliability of MEMS coatings in liquids

Reliability of MEMS in liquids is an important concern for MEMS operating in harsh environments in petrochemical, biomedical, and defense applications. Our previous studies have demonstrated reliability concerns for silicon microcantilevers in liquid environments such as saline. In this paper, the effect of coatings on reliability of MEMS in liquids, such as DI water and saline, is compared to that in air. Silicon microcantilevers coated with titanium (Ti) and SU-8 were vibrated in liquids for 10⁸-10⁹ cycles. The resonant frequency of the microcantilevers was periodically monitored to evaluate performance. Reliability of microcantilevers coated with Ti and SU-8 was compromised as indicated by a gradual decrease in resonant frequency in liquids. These changes in resonant frequency were attributed to factors such as stress-corrosion cracking, water absorption, and intrinsic stresses.