We use in situ spectroscopic ellipsometry to analyze the microstructure of the μc‐Si:H/a‐Si:H interface deposited by reactive magnetron sputtering of a Si target in (Ar+H2). We increase the hydrogen pressure to promote μc‐Si:H formation and observe several effects. Initially, H penetrates ∼45 Å into the a‐Si:H substrate and increases its hydrogen content. Then ∼55 Å of hydrogen‐rich a‐Si:H deposits. Finally, μc‐Si:H nucleates on top of this ∼100 Å thick, high H‐content a‐Si:H interface layer. As μc‐Si:H grows, the thickness of the amorphous interface layer decreases by ∼40 Å; the void fraction in the μc‐Si:H layer is always ≤15 vol %, ruling out the possibility that the a‐Si:H is etched away. These results suggest that a‐Si:H can be transformed into μc‐Si:H in a subsurface region under appropriate conditions. © 1994 American Institue of Physics.