The relationship between the characteristics of silane-hydrogen plasmas and the growth of polymorphous silicon films has been studied by means of three simultaneous diagnostics: spectral analysis of the harmonics of the RF current, laser light scattering from silicon powders in the plasma, and in situ ellipsometry. The deposition rate of the films has been measured as a function of the plasma pressure in the range of 50 up to 400 Pa. In this way we could identify the polymorphous silicon deposition regime. Using a high hydrogen dilution allows to avoid a sharp transition and thus define a wide range of pressure (from 160 up to 270 Pa) in which pm-Si:H films are obtained. The spectral analysis of the RF current has been identified as the most sensitive technique to characterize the plasma allowing to determine the optimal conditions of cluster and nanoparticle formation while avoiding powders.