Physical Principles of in situ Sidewall Profile Control in Cryogenic Plasma Etching of HAR Silicon Microstructures

The results of experimental studies and mathematical modeling of cryogenic plasma etching of high aspect ratio (HAR) microstructures in silicon are presented. Observed morphological defects on the surface at the sidewalls of formed microstructure are explained. It is shown that all of these defects are associated with changes in the conditions of sidewall passivation by the SiOXFY layer as the aspect ratio increases. Based on analysis of the physical phenomena leading to the formation of morphological defects, the solutions are proposed for stabilizing the passivation layer at various depths within the formed HAR structure. It based on in situ control of the plasma species composition during etching. This approach demonstrated the advantages of continuous cryogenic deep silicon etching (up to 120 um) compared to the widely used Bosch process.