In-situ differential reflectance study of the etching of SiO2 films

A differential reflectance technique has been applied to study the etching of silicon dioxide films on silicon substrates in-situ in the liquid hydrogen fluoride/Water environment as well as ex-situ. Essentially, the technique scans an incident optical beam in the spectral range of 200 to 800 nm across two adjacent samples at near normal incidence and the reflected beams are detected using lock-in amplification so as to measure only the difference in the reflectance spectra. This study demonstrates the great sensitivity of the technique to follow the etching experiment in-situ, and in conjunction with simulated spectra and ex-situ results, a new spectral feature is seen which may be evidence for an electric double layer at the Si surface in the etch solution. In microelectronics processing, the cleaning of silicon substrates prior to processing and the etching of SiO2 films on Si for various lithographic procedures often involves the use of aqueous HF solutions. Previous studies have shown that the use of HF alters the Si surface and the oxidation kinetics. More recent studies have confirmed these results and indicate that the major effect is at the Si surface.