Anchor-induced localized stress evolution and deformation prediction in SOI MEMS structures

Suspended structures in silicon-on-insulator (SOI) microelectromechanical systems (MEMS) are critical components for sensing and actuation. However, undesired deformation in these structures significantly degrades device performance. Although such deformation is commonly attributed to compressive stress, the underlying mechanisms governing stress evolution and deformation remain unclear, limiting predictive capability and effective suppression strategies. Here, we investigate the evolution of localized stress and reveal its role in driving structural deformation. We demonstrate that the localized stress is not intrinsic to the device layer but is mechanically transferred from the buried oxide (BOX) anchor. Specifically, the deformation is an extrinsic response governed by a three-stage mechanism: lateral anchor expansion, interfacial stress transfer, and the formation of a localized bending moment. To enable deformation prediction, a theoretical model is established by introducing an equivalent thickness and a characteristic length that defines the effective stress-transfer region. The resulting models reveal deformation behaviors fundamentally distinct from traditional theories, including linear deflection scaling in cantilevers and pre-buckling nonlinear deflection in double-clamped beams. To validate the proposed mechanism, micro-Raman stress mapping provides direct experimental evidence of a transition from tensile stress in the anchor region to compressive stress in the suspended region. The proposed theoretical models are further validated through FEM simulations and experiments, showing good agreement with average relative errors consistently below 10%. Furthermore, a mechanism-guided stress-isolation beam design is demonstrated, reducing the deflection of a double-clamped beam by ~93%. This work proposes a unified, mechanism-based framework for understanding, predicting, and suppressing anchor-induced deformation in MEMS devices.