Reliability of Cyclically Actuated Silicon Microcantilevers in Liquid Environments

Microstructures are exposed to a variety of liquid environments in applications such as biosensors, BioMEMS, and microfluidic devices. Environmental interactions between the liquids and microstructures can undermine the reliability of MEMS in liquid environments. In this paper, reliability of silicon microcantilevers is investigated in two common liquids, de-ionized water and saline solution, and contrasted with results in air. These single crystal silicon microcantilevers have a magnetic Permalloy tip and are magnetically actuated. The microcantilevers are subjected to long-term cyclic actuation (10⁸-10⁹ cycles) in a liquid filled enclosure. Reliability of the microcantilevers is evaluated by periodically monitoring changes in resonance frequency. The microcantilevers are subjected to peak stresses of ~5 MPa, which is typical for MEMS applications in liquids, e.g. AFM tips and biosensors. Since the tensile strength of silicon (1-3 GPa) is much higher than the applied stresses, structural fatigue failures were neither expected nor observed in air, water or saline. Changes in resonance frequencies of microcantilevers tested in air and water were insignificant to within the limits of experimental accuracy. However, environmental interactions influenced the reliability of microcantilevers tested in saline as indicated by a gradual decrease in resonance frequency. This decrease in resonance frequency is attributed to gradual mass adsorption of ionic species on the microcantilevers from the saline solution as indicated by scanning electron microscope imaging and electron dispersive spectroscopy.