Advances in High-Aspect-Ratio Deep Reactive Ion Etching of 4H-Silicon Carbide Wafers

This study presents recent advances in high-aspect-ratio Deep Reactive Ion Etching (DRIE) of bulk 4H-SiC and thick 4H-SiC on Insulator (SiCOI) substrates at the wafer level. Utilizing an electroplated nickel mask, we successfully achieved high aspect ratios ranging from 10:1 to 18:1 in deep trenches with critical dimensions in the range of 1- $10~\mu $ m on the wafer. Trenches having an opening of $\sim ~4~\mu $ m were etched to greater than the target depth of $45~\mu $ m, with a tapering angle of 88.5° and smooth sidewalls (roughness <200nm), achieving trench depth uniformity of ~2% ( $\pm 0.85~\mu $ m) across the wafer. These results facilitated batch fabrication of capacitive 4H-SiC bulk acoustic wave disk resonators with high quality factor (Q) approaching 5 million at 3MHz. These achievements in high-aspect-ratio DRIE of 4H-SiC at the wafer level mark a significant stride towards enabling volume manufacturing of ultra-high Q SiC microresonators.[2024-0119]

I. Introduction

4H-Silicon Carbide (4H-SiC) is a monocrystalline semiconductor that offers exceptional material properties such as a wide bandgap [1], high thermal conductivity [2], [3], and low intrinsic phononic dissipation [4]. These properties enable the deployment of SiC in harsh environment microsystems [5], including high temperatures, high voltage [6], high radiation [7], and high vibration environments [8], holding promise for 4H-SiC Micro-Electro-Mechanical Systems (MEMS) devices to demonstrate exceptional performance. While SiC boasts numerous advantages, research in this area remains relatively nascent, primarily due to the high cost of commercial 4H-SiC substrates and the challenges encountered during wafer-level fabrication, especially in the precision high-aspect-ratio (HAR) deep reactive ion etching (DRIE) process, which is essential to improve the performance of MEMS resonators and other devices such as accelerometers [9] and low-thrust propulsion systems [10].