The adhesion mechanisms of plasma-deposited silicon alloys on polymer substrates (polycarbonate) were studied by a new double-modulated IR ellipsometer. This ellipsometer combines the low frequencies typical of Fourier transform spectroscopy with the high frequency (37 kHz) of the phase modulation provided by a photoelastic device. The film adhesion is characterized by the evolution of the vibrational properties of the polymer surface during the early stages of the film preparation. The influence of preliminary plasma treatments on the polymer surface is emphasized. Successive exposure of polycarbonate to Ar and (NH3, Ar) plasmas induces a substrate activation characterized by the formation of CN bonds at the surface. Then it is shown that the SiH4, plasma treatment leads to the formation of a very thin SiOx layer at the substrate surface. The enhancement of the film adhesion appears to be correlated to the presence of a vibration located at 1030 cm(-1).