FT-IR ATR ANALYSIS OF THE SILICON/AQUEOUS SOLUTION INTERFACE USING SPUTTERED SILICON THIN-FILMS TO ACCESS THE 1550-1100 CM(-1) SPECTRAL REGION/

An FT-IR attenuated total reflection (ATR) method is described for qua ntitative in situ analysis of the adsorption and rinsing-removal of su rfactants from silicon surfaces. Spectral bands at wavenumbers below 1 550 cm(-1) are nearly inaccessible when single-crystal silicon ATR int ernal reflection elements (IREs) are used. A new ATR technique was att empted in order to overcome this limitation. The silicon was sputtered as a thin film onto a thin Al2O3 buffer layer, which had been previou sly sputtered onto a ZnSe IRE to improve adhesion of the silicon layer . The method allowed observation of species at the silicon/aqueous sol ution interface below 1550 cm(-1), to 1100 cm(-1). Absorption bands du e to adsorbed octylphenol polyethylene oxide (OPEO) and dodecyl trimet hyl 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 str etching vibrations for DTAB. A mathematical method to calculate adsorp tion density for stratified ATR IRE systems having more than three pha ses (i.e., ZnSe/Al2O3/Si/aqueous solution) was developed and applied t o the determination of the adsorption density of DTAB and OPEO surfact ants on silicon, in situ. The method was confirmed through spectra obt ained with a single-crystal Si IRE and the previous three-phase calcul ation method. The agreement indicates that the two surfaces have very similar physisorption chemistry. In addition, this method allows direc t, in situ observations of the oxidation-induced growth of a Si-O-Si b and near 1150 cm(-1) and its removal by dilute HF solutions.