Kinetic ellipsometry study of the hydrogen plasma etching of hydrogenated amorphous silicon films

Abstract

rf hydrogen plasmas have been used to etch the surface of hydrogenated amorphous silicon (a‐Si:H) films, at different deposition and etching temperatures. The capability of spectroscopic phase‐modulated ellipsometry (SPME) to characterize in situ the etching processes is illustrated. The modifications of the pseudodielectric function of a‐Si:H films under H2 plasma exposure are measured in the range 2.5–5.0 eV. They are described by the formation of a rough surface which is parametrized by a density deficient overlayer beneath a dense ultrathin layer. At the ambient, it is shown that the density deficient overlayer thickness increases as the square root of the etching time. At a higher temperature (240 °C) the surface roughness increase is slower. In order to control such a model, a real‐time SPME characterization of the whole etching process is performed on a very thin film (160 Å). By using experimental conditions corresponding to a very low etching rate (0.04 Å s−1), and combining both experimental procedures, two steps are evidenced in the etching process of a‐Si:H: (a) first at the scale of 25 s, the formation of a dense film–plasma interface layer and (b) the increase of the surface roughness at the scale of 2000 s. This last process is described accurately by the creation and development of hemispherical craters at the film surface.