Amorphous hydrocarbon film deposition in a DECR Multipolar Microwave Plasma takes place from two deposition mechanisms: Hydrocarbon species sticking and film erosion by hydrogen formed in the discharge. This last mechanism leads to the formation of volatile species such as CxHy that can be dissociated and modify the film deposition process. For example, in CH4 plasma, it appears that the discharge is modified during the first minutes of the process. Indeed, excited volatile species can be observed by optical emission spectroscopy such as C2. As the probability of recombination in plasma volume is low, these species can not be created from the reactor walls. To simulate this competition between deposition and erosion on the reactor walls, the process on uncooled substrate was analyzed by in-situ spectroscopic ellipsometry. From the evolution of ellipsometric angles, it appears that erosion is highly dependant on the substrate temperature. Indeed, above 100°C, erosion is more important than deposition process – leading to the decrease of the film thickness – although the sticking coefficients are not affected in this range of temperature. Moreover, from the analysis of ellipsometric spectra, the evolution of the film structure will be shown. Then, during the first deposition times, the discharge is highly modified (volatile species formation and dissociation) inducing an evolution of the film structure for controlled processes (i.e. at substrate temperature fixed). This evolution will then be analyzed by spectroscopic ellipsometry and compared to those of the discharge from optical emission spectroscopy.