High-Depth FIB Etching on Silicon Sidewall: Coupling Incidence Angle and Initial Multi-Groove Surface Topography

Focused-ion-beam (FIB) etching on the silicon sidewall surface is significant for improving the performance of micro-electro-mechanical system (MEMS) device. However, the high-depth etching on the functional surface requires the ion beam to be inclined to the surface with an initial topography, posing a notable challenge in revealing the ion beam-silicon solid interaction mechanism. In this article, we propose a molecular dynamic model for the ion-solid interaction of FIB etching on the silicon sidewall by introducing the coupling of incidence angle and initial multi-groove surface. The model is validated by conducting FIB tilted etching on a multi-groove silicon sidewall surface fabricated by inductively coupled plasma (ICP). By tilting the ion beam with an incidence angle, the target surface roughness Ra can be reduced from 126 nm to 4 nm on a $125~\mu $ m-depth silicon sidewall. We also found the inheritable effect of initial multi-groove topography on the processed sidewall surface. Results show that the inheritable effect can be reduced by decreasing the incidence angle of the ion beam. Furthermore, FIB tilted etching is applied to the sidewall micro-mirror surface of an ICP etched MEMS optical switch. By using an incidence angle of 20°, the sidewall surface with an area of $177~\mu $ m $\times 125~\mu $ m is processed. The optical transmission efficiency of the MEMS switch increases from 6.9% to 34.6%. We believe this work is significant for expanding the application range of FIB etching on MEMS devices. [2024-0212]