Macro-electronic devices such as photovoltaic cells and active matrix displays are based on the deposition of thin semiconductor films onto large area substrates at low temperatures. Silicon presents an interesting case because the microstructure can range from amorphous to nanocrystalline to polycrystalline. These different microstructures can be produced by manipulating the concurrent particle bombardment during PVD growth by dc reactive magnetron sputtering of a Si target in Ar + H2. Three types of particles impinge on the film: (i) sputtered Si atoms of a few eV; (ii) H atoms with ~ 100 eV, generated by the acceleration and reflection of H2+ ions at the target; and (iii) bulk plasma Ar+ and H2+ ions with ~ 25 eV, whose flux is controlled using an externally-generated magnetic field to unbalance the magnetron. We analyze the growth process using real-time mass spectroscopy, spectroscopic ellipsometry, and reflection IR absorption, including isotopic H2/D2 exchange experiments. We combine these data with binary collision simulations in the gas-phase and substrate to show how each flux modifies the microstructure. The essential results are: (i) Few-eV sputtered Si atoms produce a dense microstructure, as predicted by the Thornton zone diagram, but also lead to the random formation of nanocrystalline Si particles in an amorphous Si matrix. These particles can serve as nuclei for solid-phase crystallization processes. (ii) 100 eV H atoms penetrate ~ 50 A into the growing film, where they drive crystalline nucleation and subsurface transformation through bond-insertion and momentum-transfer events. Fully nanocrystalline films can be deposited on glass substrates using large fluxes of fast H or D atoms. (iii) 25 eV Ar+ ions modify the competitive growth of polycrystalline grains at the film surface, which leads to a coarsening of the grain structure at modest substrate temperatures.