A downstream-cluster-collection method of high sensitivity has been developed to obtain information on size distribution, density, shape, and structure of clusters formed in reactive plasmas. The method have been applied together with a cluster-suppressed plasma chemical vapor deposition method in order to study a correlation between a volume fraction V-f of clusters incorporated into a-Si:H films and a hydrogen content C-H(SiH2) associated with Si-H-2 bonds in the films. The C-H(SiH2) value decreases almost linearly from 0.6 at. % to 0.05 at. % with decreasing V-f by about one order of magnitude. The result indicates that the incorporation of clusters, above about 1 nm in size, of amorphous structure into a-Si:H films is an important origin of Si-H-2 bonds in the films. (C) 2004 American Vacuum Society.