An Ellipsometer Investigation of Vapor Adsorption on Etched Silicon

Ellipsometer measurements have been made of vapor adsorption on single crystal silicon etched in mixtures. Approximate equations, derived originally by Drude from the exact theory of reflection, have been used to evaluate film thicknesses. Adsorption isotherms have been plotted for water, some lower monohydric alcohols, acetone, benzene, and hexane. The results confirm the feasibility of using an ellipsometer for this type of measurement. In the absence of further surface treatment the amount of adsorption is small, even at saturation. However, when the etched surface is either treated with caustic soda solution or boiled successively in nitric acid and deionized water, adsorption is enhanced and smooth type II isotherms result. This behavior is attributed to the removal of chemically bonded fluoride ions from the surface of the adsorbent. Monolayer thicknesses calculated from the experimental results agree reasonably well with those to be expected from the dimensions of the adsorbed molecules assuming the latter to have a particular orientation on the surface. Anomalous results obtained on some samples are thought to be due mainly to the existence of localized areas of the surface on which adsorption is inhibited.