Abstract

An FT-IR attenuated total reflection (ATR) method is described for quantitative <i>in situ</i> analysis of the adsorption and rinsing-removal of surfactants from silicon surfaces. Spectral bands at wavenumbers below 1550 cm^-1 are nearly inaccessible when single-crystal silicon ATR internal reflection elements (IREs) are used. A new ATR technique was attempted in order to overcome this limitation. The silicon was sputtered as a thin film onto a thin Al₂O₃ buffer layer, which had been previously sputtered onto a ZnSe IRE to improve adhesion of the silicon layer. The method allowed observation of species at the silicon/aqueous solution interface below 1550 cm^-1, to 1100 cm^-1. Absorption bands due to adsorbed octylphenol polyethylene oxide (OPEO) and dodecyl trimethyl ammonium bromide (DTAB) surfactants were observed in the 1550-1100 cm^-1 spectral region, which were assigned to benzene-ring modes and the aliphatic stretching vibrations for OPEO and to the aliphatic stretching vibrations for DTAB. A mathematical method to calculate adsorption density for stratified ATR IRE systems having more than three phases (i.e., ZnSe/Al₂O₃/Si/aqueous solution) was developed and applied to the determination of the adsorption density of DTAB and OPEO surfactants on silicon, <i>in situ.</i> The method was confirmed through spectra obtained with a single-crystal Si IRE and the previous three-phase calculation method. The agreement indicates that the two surfaces have very similar physisorption chemistry. In addition, this method allows direct, <i>in situ</i> observations of the oxidation-induced growth of a SiO-Si band near 1150 cm^-1 and its removal by dilute HF solutions.

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