Synthesis of microcrystalline silicon (μc-Si) film at an ultrafast deposition rate over 100nm/s is achieved from SiH 4 +He by using a high density microwave plasma source even without employing H 2 dilution and substrate heating techniques. Systematic deposition studies show that high SiH 4 flow rate and working pressure increase film deposition rate while high He flow rate decreases the rate. On the other hand, crystallinity of deposited Si film decreases with increasing SiH 4 or He flow rate and working pressure. Enhancements of gas phase and surface reactions during film deposition process are responsible for the achievement of high deposition rate and high film crystallinity.