Numerical study of aperture shape effects in deep cryogenic etching of silicon

Cryogenic etching of silicon in SF6/O2 plasma is the process of choice for fabrication of deep high aspect ratio structures with strict requirements for sidewall angle, surface roughness and contamination, including MEMS, X-ray optics, and supercapacitors. The manufacturing of such structures, containing features of various size and shape, poses new challenges for the etching process optimization, since the key parameters of the resulting structures depend on the local aspect ratio and the shape of the mask aperture. As an aid in process optimization, we propose a three-dimensional Monte-Carlo simulator for the cryogenic etching of complex structures. It employs a surface kinetics model tuned to SF6/O2 process in 2 1⁄2D geometry and cubic voxel representations of simulation domain. Systematic study on cryogenic etching of test structures of different mask shape and aspect ratio is performed and principal mechanisms affecting etching results are identified.