Etching and hydrogen diffusion mechanisms during a hydrogen plasma treatment of silicon thin films

We present here an in situ spectroscopic ellipsometry (SE) study of hydrogen diffusion through hydrogenated amorphous silicon, when exposed to a hydrogen plasma. The spectroscopic data were analyzed by a combination of the tetrahedron model and the Bruggemann effective medium approximation. Our results indicate that etching of amorphous silicon is not just a surface process. Indeed, the formation of a rather thick sub-surface layer appears as a necessary condition for etching. The hydrogen content and thickness of the sub-surface layer were also confirmed by secondary ion mass spectrometry (SIMS) measurements. These results are discussed with respect to the nucleation process of microcrystalline silicon films.