This project was designed to investigate the subcritical etching of silicon using alkaline etchants.
"This optical image demonstrates the anisotropic etching of the silicon in the presence of alkaline water at high temperature and pressure. Surface of silicon that has been etched in sub-critical water: this work represents the transition of techniques developed for the preservation of historic metal applied to microelectronic materials. The goal of this project was to investigate the anisotropic etching of silicon in subcritical water conditions. Specifically, we investigated the etch rate and resulting morphology as function of time, temperature, pH, and other reaction conditions of silicon wafers in subcritical water. Subcritical is the region where water remains in the liquid phase and that extends from the normal boiling point of water (100ºC, 1 bar) to its critical point (373.9ºC, 220 bar). While there has been significant work on the etching of silicon conducted below the boiling point of water, there have been very few studies into the etching behavior in the subcritical region. The understanding of the reaction mechanism for subcritical silicon etching could assist in the advancement of the silicon-etching and other related fields of surface modification of materials. The image demonstrates the highly anisotropic nature of the subcritical reaction. That is, the etching of silicon in these reaction conditions (250ºC and a pH of 11) favors certain crystalline planes over others. Therefore, by changing the reaction conditions, the surface morphology can be altered. This control makes the process extremely favorable to the formation of microelectronic devices and photovoltaic devices. An understanding of wet etching on monocrystalline silicon surfaces with low concentrations of etchants and high pressure/temperature reactions could improve the renewable energy field by providing an effective method of producing silicon photovoltaic cells with higher absorbance properties than are currently attained. Not only would the process be cost effective due to low material and process costs, but also environmentally friendly due to the highly diluted concentration of etchants in solution.