Epitaxial encapsulation of fully differential electrodes and large transduction gaps for MEMS resonant structures

This work demonstrates, for the first time, a wafer-scale encapsulation process for hermetic sealing of MEMS resonant structures incorporating fully differential electrodes, in and out of the device plane, and large lateral transduction gaps. The polysilicon electrode and device layers were deposited using an epitaxial silicon reactor with Tetra-Ethyl-Ortho-Silicate (TEOS) as a spacer material. Large transduction gaps were fabricated using vapor-phase xenon difluoride (XeF ₂ ) and hydrofluoric acid (vHF) etches, with silicon and silicon dioxide as sacrificial materials, respectively. Transduction gaps of 0.7μm up to 50μm were fabricated and the polysilicon devices were driven into a strongly nonlinear regime using the bottom, top, and in-plane capacitive electrodes.