Reliability of microcantilevers in liquid environments

Reliable operation of MEMS in liquid environments is an important design requirement for numerous MEMS devices in chemical, pharmaceutical, biomedical, consumer product, and defense industries. In this paper, reliability and long-term performance of microcantilevers in liquid environments is investigated. Single crystal silicon microcantilevers are subjected to long-term cyclic actuation (≈10⁸-10⁹ cycles) in enclosures filled with two different liquids- de-ionized water and saline solution. Additionally, silicon microcantilevers are actuated in air to enable comparison of experimental data in air and liquids. The microcantilevers have an electroplated Permalloy layer and are magnetically actuated. The resonance frequency of the microcantilevers is periodically monitored to track changes in stiffness and mechanical performance. The microcantilevers are subjected to peak stresses ranging from 0-10 MPa, which are typical for MEMS applications like AFM tips, and resonating sensors. Since the peak stresses are small compared to the tensile strength of silicon (1-3 GPa), complete structural fatigue failure is neither expected nor observed. However, operational failures characterized by a gradual decrease in resonance frequency of the microcantilevers are observed in saline solution. Changes in resonance frequency of microcantilevers actuated in air and water were negligible to within the limits of experimental accuracy. These results demonstrate that an understanding of MEMS reliability in air cannot necessarily be extended to explain and predict device reliability in liquid environments.