HF Under-Etching Prevention for Advanced THz Micromachined Waveguide Devices

This letter presents an under-etching prevention method for an aqueous hydrofluoric acid (HF) releasing process by adding a single-side low-pressure chemical vapor deposition (LPCVD) silicon protection layer. The proposed method enables advanced silicon-on-insulator (SOI) based millimeter-wave/terahertz (mmW/THz) MEMS waveguide devices, which require for RF performance a complete metallization film over the SOI buried-oxide (BOX) layer, and simultaneously need a locally under-etched BOX layer for implementing MEMS actuators reconfiguring the devices. A comparison between the WR-3.4 waveguide (220-330 GHz) using the proposed under-etching prevention method and the one without using it proves the effectiveness and feasibility during HF releasing processes. A MEMS tunable phase shifter driven by comb-drive actuators has been successfully implemented by applying this method in a micromachined waveguide. [2021-0036]