In situ ATR-FTIR analysis of surfactant adsorption onto silicon from buffered hydrofluoric acid solutions

Buffered hydrofluoric acid (BHF) solutions containing HF and NH₄F are widely used in the manufacturing of silicon-based integrated circuits. The adsorption/desorption characteristics of a commercially available, high purity, polyglycidol type surfactant (OHS) onto/from silicon from buffered hydrofluoric acid (BHF) solutions was studied by in situ attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The challenge in these measurements was to resolve the C-H peaks, in the 2800-3000 cm^-1 region of the surfactant spectrum, that were masked by the strong absorbance due to N-H caused by a large amount of NH₄⁺ ions in the solution. A technique has been developed to overcome this limitation. The principle of this technique is to carry out the surfactant adsorption in BHF solutions followed by the replacement of NH₄⁺ ions by alkali-metal cations, such as K⁺ and Cs⁺, to allow better resolution of the C-H peaks from the baseline. Extrapolation of the adsorption density to time zero yields the adsorption density in the presence of NH₄⁺. Using this technique, the adsorption density of OHS surfactant in a buffered HF solution containing 7 parts of NH₄F (40%) and 1 part of HF (49%) was found to be approximately 20% higher than that in dilute HF solutions.