Fully microcrystalline silicon, μc-Si, thin films (<100 nm) have been deposited at low temperature (60 °C) on Corning glass and plastic flexible polyimide substrates by plasma enhanced chemical vapor deposition (PECVD) using SiF4-H2-He. The effect of deposition temperature on the structure, i.e., crystallinity and density, of μc-Si films is investigated by spectroscopic ellipsometry in the 1.5−5.5 eV energy range. Modeling of spectroscopic ellipsometry data is used for highlighting crystallinity of the substrate/film interface, i.e., the absence of any amorphous incubation layer. It is found that film crystallinity does not depend on film thickness, and it increases with the decrease of deposition temperature. The temperature dependence is explained on the basis of a like-Arrhenius kinetic analysis of the etching process by atomic fluorine and hydrogen of both μc-Si and a-Si phases.