Using plasma growth sources with concurrent particle bombardment, silicon thin films can be deposited with various phases and microstructures. DC Reactive Magnetron Sputtering (RMS), in particular, can produce amorphous, mixed-phase, nanocrystalline, polycrystalline, porous columnar, and epitaxial Si films. In particular, a large flux of low energy, heavy ions strongly affects the phase and microstructure, and therefore the resulting film qualities. Lowpressure (1.6 mTorr) RMS is particularly suited for this type of plasma manipulation: we bias the substrate to produce the ion energy of choice, and use an external magnetic field to control the ion/neutral flux ratio, therefore decoupling the parameters of bombardment energy and flux. In this work, we study the influence of slow (<40eV), heavy (Ar+), ions in RMS deposition on the formation kinetics and microstructures of microcrystalline, mixed-phase, and poly crystalline Si films. The analytical methods are ellipsometry, Raman scattering, and AFM. We will show how ion bombardment influences the direct nucleation of extremely smooth polycrystalline Si on glass at temperatures below 400°C, enhances the crystallinity of this polycrystalline Si in the bulk, and produces nanocrystalline Si with tailored grain sizes at temperatures below 200°C.