In this contribution, the results of ellipsometry measurements on the growth of hydrogenated amorphous silicon (a-Si:H) films using an expanding thermal plasma in Ar/H-2/SiH4 are presented. The measurements are simulated with an optical model in which roughness and optical parameters of the bulk are time dependent during deposition. From this simulation it can be concluded that in the first 2-13 s of the deposition, depending on the substrate temperature, a top layer containing Voids is created. At substrate temperatures above 400 degrees C and at a silane flow of 10 sees, corresponding to conditions at which high quality a-Si:H is grown, the smallest roughness of 3 nm is measured.