Hydrogenated amorphous silicon films are formed in a large-volume (63 liters) silane plasma by a low-pressure (<0.1 Pa) DC discharge under a toroidal magnetic field. In order to elucidate the film formation process, we have developed two novel techniques which can control the composition of particle influxes to a substrate. One is an electrostatic technique to deposit only neutral flux and to exclude ion flux by a positive bias to the substrate; the other is a magnetic technique to exclude the hydrogen ion flux by means of magnetic deflection. By means of these techniques, the contributions of neutral and ion species are discriminated, as well as the surface reactions associated with hydrogen influx to the substrate. As a result, it turned out that hydrogen atoms break the Si-Si bond and terminate in a dangling bond to increase SiH2 bonds in the films.