Study of sidewall passivation and microscopic silicon roughness phenomena in chlorine-based reactive ion etching of silicon trenches

Abstract

Sidewall passivation layers produced in the formation of Si trenches by HCl/O2/BCl3 reactive ion etching have been characterized by angle-resolved x-ray photoemission measurements and secondary electron microscopy. Electron-shading effects observed at grazing electron emission angles and electrostatic charging of insulating portions have been used to differentiate photoemission contribution from (i) the trench sidewall film (moderately insulating), (ii) the oxide mask (highly insulating), and (iii) the Si trench bottom (conductive). The sidewall passivation film for this process is found to be a silicon oxide. Only minor amounts of chlorine are incorporated in this film. The thickness of the sidewall passivation layer depends on the width of the Si trench and ranges from ≂0.1 μm for narrow trenches to ≂0.7 μm for wide Si trenches for our conditions. The Si taper formed in this trench etching process is produced by the simultaneous formation of the sidewall film which protects the underlying Si from etching. It is also shown that microscopic silicon pillars (‘‘black silicon’’ or ‘‘grass’’) form by micromasking under process conditions where large areas of unmasked silicon are exposed. The microscopic Si pillars are protected by the same sidewall passivation film as the trench sidewalls. Islands of black silicon are observed only in large open silicon areas and separated from oxide masked areas by a characteristic distance which is free of silicon pillars. This roughness-free distance is of the order of the mask thickness employed. Ion scattering from the sloped mask sidewalls and sputtering of involatile micromasking material can explain this observation. Our results are consistent with a mechanism in which the processes that cause the formation of the beneficial sidewall film also initiate and control the production of undesirable silicon roughness in large unmasked Si regions.