A novel process for the preparation of thick porous silicon layers with very high porosity

Abstract

A novel process for the preparation of thick porous silicon layers having very high porosities is proposed. Starting with thick layers of intermediate porosity the porosity is further increased in an additional step. This is done by means of controlled partial oxidation of the silicon skeleton followed by selective removal of the oxide film in an HF vapor phase etching process. Due to this, porosities beyond 90% are accessible since any further liquid phase contact is avoided and therefore no additional drying steps are necessary.

Porous Silicon Formation by HNO3/HF Vapor Etching

“…To avoid liquid condensation, one efficient way consists of heating the wafer during vapor HF etching (Lee et al 1997). HF vapor etching was also used to enlarge the porosity of porous silicon layers by attacking SiO 2 just after an oxidation step; no drying is needed and therefore no cracking or other damages can occur (Feyh et al 2005). HF vapor etching was also used to etch SiO 2 and to make freestanding silicon microstructures of SOI wafers (Fukuta et al 2003).…”