[en] Nucleation and subsequent growth processes of clusters, i.e. particles below a few nm in size, have been studied in capacitively coupled high-frequency SiH₄ discharges. Time evolution of size and density of clusters has clearly shown the existence of bottleneck in the size distribution at around Si₄H_x's. After the nucleation, the clusters of the large size group, Si_nH_x's (n>4), grow mainly due to influxes of those of the small size group, Si_nH_x's (2≤n≤4) and other molecular species SiH_x's (0≤x≤3). These results have led to the growth model of clusters, which can reasonably explain the results obtained until now. Furthermore, effects of gas flow, discharge modulation, gas temperature gradient and hydrogen dilution on growth and behaviour of clusters have been studied to control their growth. The reactor, which suppresses the growth of clusters both by thermophoretic force and by gas flow and evacuation without stagnation, has been newly developed to make a-Si:H films of high qualities. The qualities of films deposited using the reactor are much better than those of conventional high-quality films