Stress Characterization in Si/SiO₂ Spherical Shells Used in Micro-robotics

Abstract

An in-depth parametric and stress analysis study was accomplished using CoventorWare™finite element method (FEM) modeling to evaluate 1–3 μm thick Si/SiO₂ layers which are being used to create spherical shells of 1.0 mm in diameter to be used as a baseline for realizing cubic millimeter micro-robotics. FEM was performed on spherical shells to optimize both the level of curvature of the petal designs which, once released, deflect upward to create one component of the fully self assembled spherical shell. In addition, FEM modeling was used to determine petal spacing in an effort to maximize the total functional surface area of the self-assembled spheres for future circuit integration and electrostatic actuation which will enhance shell movement. As expected, the radius of curvature of the petal is primarily based on the design of the petal, material thicknesses, and residual stresses in the structural layers. Four different petal designs were analyzed, modeled and fabricated to determine which petal design would likely satisfy the self assembly requirement and desired spherical shape. To fabricate the shells, both bulk and thin-film micromachining processes were performed on the silicon-on-insulator (SOI) wafer. To release the spherical shells from the substrate, a multistage wet and dry etching process was used. Once the petals are released, the petals curl up, self-assembling into spherical shells.