Galvanic corrosion of silicon-based thin films: A case study of a MEMS microphone

MEMS microphones are widely utilized in commercial applications such as mobile phones and laptops. Microphones cannot be sealed completely from the atmosphere since sensing requires propagation of sound waves to the sensing element. Therefore, the sensing element is vulnerable to airborne impurities and humidity. The prevalent material of choice for MEMS microphones is silicon, which is generally considered reliable and inert material in normal atmospheric conditions, although micron-scale silicon is known to be susceptible to delayed fracture failures under cyclic stresses. Especially humidity has been shown to affect its fatigue properties. Furthermore, the fabrication history of the device can affect the mechanical performance of the devices significantly. Hence, there exist large discrepancy for the fatigue limit and fracture strength of micron-scale silicon in the literature. In addition, there is little information on how harsh environments can affect the reliability of silicon-based thin films in MEMS applications. Therefore, harsh environmental tests were conducted on MEMS microphones. The silicon-based sensing elements of the microphones were observed to degrade during the testing. Failure analysis was conducted employing microscopy and chemical analysis techniques. The degree of degradation was evaluated qualitatively with nanoindentation, and finite element method was employed in explaining the influence of the observed degradation.