A polymer trench filling based silicon isolation technique and its application to two-axis scanning comb-drive micromirrors

Electrostatic micromirrors can be widely used in LiDAR, displays, AR/VR and medical imaging. Electrically insulated silicon enabled by trenches filled with thermal oxide and polysilicon is commonly used for 2-axis electrostatic comb-drive scanning mirrors made on SOI (silicon-on-insulator) wafers. These trenches provide both electrical isolation and mechanical stability. However, this method has drawbacks such as high deposition temperature, voids, and high residual stress. In this work, a polymer filling method is proposed to form electrical insulation trenches in the silicon device layer of SOI wafers. Only low-temperature processes are used in this method and the silicon trenches can be fully filled up reliably. Furthermore, the filling polymer not only relieves residual stresses but also significantly increases the shock resistance of the micromirrors made with this method. In particular, Benzocyclobutene (BCB) is employed and a process has been developed to effectively fill deep silicon trenches. Using this method, two-axis electrostatic micromirrors have been successfully fabricated and tested. Experimental results show that the BCB-filled trenches demonstrate excellent electrical insulation performance and mechanical stability and the fabricated 2-axis mirrors can achieve a large field of view of 60°×40° and stand high impact over 2500 g.