Real-time, in situ infrared study of etching of Si(100) and (111) surfaces in dilute hydrofluoric acid solution

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The chemical nature of Si(100) and (111) surfaces during immersion in dilute hydrofluoric acid (HF) solution was investigated ‘‘in situ’’ and in real time using infrared absorption spectroscopy in the multiple internal reflection geometry. In dilute HF solution, the Si surface is not completely terminated with hydrogen, but may be covered in part with hydrogen-associated Si fluorides, such as SiH2(SiF) and SiH2F2. It is found that the hydrogen coverage of the surface depends on the HF concentration of the solution. At HF concentrations above 1%, the surface concentration of Si hydrides is reduced while that of Si fluorides is enhanced. We confirm that rinsing in water following HF immersion leads to complete hydrogen termination of the surface. Based on the present experimental results, we suggest that in dilute HF the Si surface is in chemical equilibrium with the solution to allow the coexistence of Si hydrides and Si fluorides on the surface.