Synthesis of microcrystalline silicon (μc-Si) film at an ultrafast deposition rate over 100nm/s is achieved from SiH4+He by using a high density microwave plasma source even without employing H2 dilution and substrate heating techniques. Systematic deposition studies show that high SiH4 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 SiH4 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.