In situ infrared observation of etching and oxidation processes on Si surfaces in NH₄F solution

We use infrared absorption spectroscopy in the multiple internal reflection geometry to investigate in situ and in real-time the chemical process at Si(100) surfaces that are immersed in a dilute solution of ammonium fluoride, NH₄F. We have followed spectral changes in the Si-H stretch vibration region of the surface during immersion in the solution. Infrared spectroscopic data show that the native oxide on the surface is initially removed by NH₄F solution and subsequently, H-terminated Si(111) microfacets are increasingly populated. Long-term immersion in dilute NH₄F leads to oxide formation on the surface. The rate of oxide formation strongly depends on the concentration of NH₄F; oxide formation is enhanced at concentrations below 10% and it does not occur at concentrations above 20%. We suggest that oxide formation and subsequent oxide removal proceed on Si surfaces in dilute NH₄F solution and that at low NH₄F concentration, the rate of oxide formation is higher than that of oxide removal.