Implementation of crack arrest properties for reliability enhancement on MEMS cantilever

Microelectromechanical systems (MEMS) technology is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through various microfabrication processes. While this technology has demonstrated significant utility in numerous applications, current reliance on silicon-based technology creates reliability issues due to the mechanical properties of the substrate. Until now, reliability enhancing designs have been primarily limited to the packaging process, thus creating numerous design limitations and increase in production costs. The current work details efforts to address this limitation through the introduction of a composite design into MEMS structures. The intrinsic brittleness of silicon creates extreme sensitivity to stress concentration and non-deterministic strength values, thus reducing reliability of silicon MEMS devices and forcing use of composites. Composite architectures consisting of sub-surface layers with compressive residual stresses have been developed. To address this need, a suite of macroscopic brittle composite designs have been studied including a phenomenon created by buried compressive residual stress laminar layer called crack arrest. A particular cantilever beam design is fabricated to showcase this phenomenon and to validate the positive impact on the MEMS structural reliability. The fabricated device is a bulk micromachined cantilever beam composed of crystalline silicon with a laminar silicon oxide layer buried underneath the surface. The performance of this cantilever is characterized and revisions are made to improve its structural strength and stress variability. A model is explored to explain the observed trends on the fracture strength with the characteristics of an initial crack. This model, which accounts for the residual compressive stress, is further used to determine the threshold strength