A nanoscopic kinetic model of controlled plasma-assisted microcrystallite formation ͑PACF͒ of Si in pre deposited a-Si:H films at low temperatures is proposed. The model suggests mechanisms for enhancement of the Si crystallization in a-Si:H films at low temperatures by treatment of the films in plasma. The model reveals certain kinetic advantages of hydrogen plasmas for the formation of Si crystalline nuclei in a-Si:H compared to other plasmas ͑Ar plasma, etc.͒. These advantages make the hydrogen plasma substantially more efficient in the PACF of Si in a-Si:H films. The proposed mechanism for PACF of Si is associated with the formation on the surface of the a-Si:H film and in the adjacent nanometer material layer of nanoscale ͑picosecond͒ short-lived hot spots of high energy density ͑or effective temperature͒. The hot spots are generated in the material by energetic plasma ions of energy is ϭ20-100 eV accelerated by the electrical field in the thin plasma layer near the solid surface. The hot spots promote Si crystallization in a-Si:H. It is shown how the plasma composition, energy, mass, and fluxes of the plasma ions impinging on the surface of the a-Si:H film determine the Si nucleation rate and density of Si microcrystallization.