In-situ FTIR studies of reactions at the silicon/liquid interface: Wet chemical etching of ultrathin SiO2 on Si(100)

we have developed an experimental setup to facilitate study of the surface reactions of single-crystal silicon with Fourier transform infrared (FTIR) spectroscopy in a variety of aqueous environments. Employing a short optica l path length through the silicon sample allows access to the critical low- frequency region of the spectrum (similar to 850-1500 cm(-1)) that cannot b e probed with traditional multiple internal reflection (MIR) techniques. Th e utility of this technique is demonstrated in a study of the etching of ul trathin SiO2 on Si(100) in dilute hydrofluoric acid. This approach provides in-situ access to the SiO2/Si(100) interface that is revealed as the overl ying oxide is stripped away. We find that this layer is, indeed, structural ly distinct from the rest of the SiO2 film, consistent with a marked change in reactivity as etching nears the Si(100) substrate.