Pure hydrogenated microcrystalline silicon (muc-Si/H) films were deposited by SiF4-H-2-He plasmas at temperatures as low as 120 degreesC and frequencies of 13.56 and 75 MHz. The plasma diagnostics by optical emission spectroscopy has allowed us to control the growth of muc-Si/H films. The silicon structure can be changed from pure amorphous to pure microcrystalline under the controlled variation of the densities of F atoms, H atoms and SiFn radicals in the plasma phase.