Effect of sacrificial layer compliance on thermal stresses of composite MEMS beams

We present a detailed analysis of thermal mismatch stress (TMS) [1] developed in composite beams built on compliant sacrificial layer (SAC) [2]. Strikingly, the stress-balance of beams is strongly affected by the SAC compliance, which is substantiated by comparison of analytic models, finite element analysis (FEA), and measurements. With this analysis we concluded that the highly compliant organic SAC was a root cause for high and unequal biaxial curvatures in early devices. By switching to a rigid SAC we verified that the beam curvatures were significantly reduced.