Hydrogenation using plasma deposited silicon nitride (SiN : H) overlayers has been inves-tigated for improvement of the electrical properties of polycrystalline silicon thin film transistors (poly-Si TFT). In order to evaluate the poly-Si films prepared for the channel layer of TFT's by low pressure chemical vapor deposition, grain boundary defects and redistribution of ionimplanted arsenic after thermal annealing were evaluated by Raman scattering and Rutherford backscattering, respectively. It was found that the best ON/OFF ratio was achieved at a temperature of 450-500°C in the case of annealing in the atmosphere. By annealing in a vacuum chamber evacuated to 100 Pa which is subsequent to the SiN deposition, the properties of TFT's were improved in comparison with those obtained by annealing at atmospheric pressure. This process can reduce the hydrogenation process steps and minimizes impurity contamination durting the hydrogenation process.