Plasma polymerized methylsilane (PPMS) films are promising photosensitive layers for 248 and 193 nm lithography applications. These a-Si1-xCx:H films are deposited in a low power plasma of methylsilane gas using a commercial tool. The effect of substrate temperature, pressure in the chamber and rf power injected in the source on optical properties and chemical composition of the PPMS films have been studied using spectroscopic ellipsometry and Fourier transform infrared (FTIR) spectroscopy. FTIR experiments show that the PPMS structure is organized around a SI:H network with intact methyl groups. The correlation between deposition parameters and PPMS films properties is discussed.