Configurable Post-Release Stress-Engineering of Surface Micro-Machined MEMS Structures

Stress-engineering is the method for inducing curvature in initially planar micromachined structures by the deposition of a stressor layer. Post-release stress engineering occurs when the stressor layer is deposited and patterned after the release of the underlying structures. In this paper, we present a novel post-release stress-engineering process, which patterns the stressor layer without the use of photolithography using a set of in-situ fabricated configurable shadow masks. The configuration of these masks enables variable coverage of the evaporated stressor layer, and is automated with the help of surface-micromachined scratch drive actuators, which maneuver the shadow masks to their desired configuration. We present both theoretical and experimental results showing the viability of this new stress-engineering method. In particular, theoretical models validated by experimental results are presented, describing the gradual increase in deflection of the stress-engineered structures. A new set of stress-resisting microelectromechanical systems I-beams is also presented, allowing the development of support structures that resist to curving during blanket stressor layer deposition.