Investigation of the Etching of Silicon under Subcritical Water Conditions

Abstract

Herein, we present a study investigating the reaction conditions to wet etch monocrystalline silicon in the subcritical region of water, specifically from 200 to 300 °C. In the subcritical region there is a departure of thermodynamic, transport, and chemical properties from normal conditions. This has the potential to affect the reaction paths for the dissolution of silicon, by modifying the local concentration of reactants and products, by altering the relative activation energy of different reactions, or by changing the polarity of the water. Furthermore, our methodology uses concentrations of etching agent markedly lower than previous works (our reactions were conducted at [KOH] = 0.03–0.7%/wt). The results of the study suggest that dilute etchant solutions at high temperatures can produce results comparable to those reported at atmospheric conditions using higher concentrations of etchants. Based on the results obtained, the main key factors in the etch rates were crystallographic orientation of the wafer, composition of the etching solution, and temperature.

Degradation of silicon photonic biosensors in cell culture media: analysis and prevention

“…In particular, there is a high concentration of sodium bicarbonate in DMEM (3.7 g/L), and this has been reported to etch crystalline silicon on its own [20]. In basic solutions, hydroxide ions (OH − ) initiate the hydrolysis reaction: $\mathit{Si}+2{\mathit{OH}}^{-}+2H_{2}O\to {\mathit{SiO}}_2{\left(\mathrm{italicOH}\right)}_2^{2-}+2H_2$ [21]. The significant reduction in etch rate observed when using the HEPES-buffered CCM may be due to the superior control of the pH of the solution over extended periods of time, where it is prevented from becoming too alkaline.…”

An assessment of the viability of hydrogen generation from the reaction of silicon powder and sodium hydroxide solution for portable applications

“…The ratio of the etch rates for the different planes in potassium hydroxide was found to be 160:100:1 for (110)/(100)/(111) crystal planes at room temperature [38]. It is this variation in the etching rates that leads to the anisotropic etching observed with bulk silicon powders and silicon wafers [41].…”

“…It has been reported that the surface atoms on silicon remain hydride terminated during the course of etching by hydroxide solutions [30]. Hydroxide ions and water are substituted for the surface hydrides, thus weakening the Si-Si back bonds and enabling the removal of surface silicon atoms in the form of silicates [41]. A postulated stepwise reaction scheme is shown in Scheme 1 [39], although various others also exist in the literature.…”