Because of the importance of the influence of the plasma-substrate interaction on the characteristics of films deposited by sputtering, a method of measurement of the striking force of Ar ions on the growing film surface is proposed. It is based on conversion of the force by a small piezoelectrical stick used as a pseudo-substrate. We applied this method to measure the force of Ar ions impinging on the substrate during sputter deposition of a-Si:H thin films. The influence of deposition parameters, such as the r.f. power and gas pressure, is emphasized. Increases in the r.f. power and gas pressure are shown to enhance the striking strength of the ions, and thus to promote densification of the film network. This is consistent with the microstructural characterization of films grown under comparable conditions of Ar ion force.