Post-release stress-engineering of surface-micromachined MEMS structures using evaporated Chromium and in-situ fabricated reconfigurable shadow masks

We present a novel post-release stress-engineering process that can provide out-of-plane curvature to initially planar MEMS structures. This process uses a layer of evaporated Chromium (Cr) with intrinsic compressive stress to provide the required curvature. The stressor layer is applied post-release, and is patterned using shadow masks fabricated in-situ as part of the surface micromachining fabrication process. The masks can be reconfigured to provide variable stressor layer coverage of the underlying structures without the use of photolithography. A model is presented that encompasses the increase in deflection, and thus non-uniform chromium coverage, during the stress-engineering of released structures.