The surface roughness of HWCVD deposited a-Si:H films has been monitored as a function of the film thickness for substrate temperatures between 70 and 450 degrees C using real-time spectroscopic ellipsometry. Information on the microstructural evolution of the a-Si:H films has been deduced from the data and different growth phases in this microstructural evolution are discussed in terms of the underlying surface processes such as nucleation and initial growth, surface smoothening and roughening processes, and surface diffusion. From the data, it is concluded that, for the specific conditions studied, the best material properties are obtained at similar to 250-350 degrees C. (c) 2005 Elsevier B.V. All rights reserved.