In situ process diagnostics using coherent anti-Stokes Raman spectroscopy (CARS) have been performed under state-of-the-art a-Si:H film deposition conditions in a reactor designed for a-Si:H solar cell fabrication. The silane plasma of device-quality a-Si:H film depositions was monitored by measuring the silane ν 1 band CARS spectrum to determine the depletion induced by the RF glow discharge. The silane depletion is linearly dependent on the RF power in the region of 4 to 12 W with a slope of O.5%/mWcm−2. The depletion is also dependent on the SiH4 flow rate starting with a 50% depletion at a low flow rate of 5.6 sccm and asymptotically approaching an 8% depletion at a high flow rate of 80 sccm. The a-Si:H film deposition rate is systematically measured as a function of the flow rate and the RF power. Linear correlations between the silane depletion and the film deposition rate are observed. The results are discussed in terms of primary electron impact dissociations of silane and the residence time of SiH4 molecule in the glow discharge region.