Silicon dioxide thick films (1-18 mum) have been deposited at very reasonable deposition rates (20-80 nm/min) with no intentional heating of the substrate (T approximately 200-degrees-C) using SiH4/O2 plasmas coupled in a new type of plasma reactor : The radio frequency plasma excitation used in the helicon diffusion reactor induces the formation of high-density plasmas (approximately 10(12) cm-3) with low plasma potentials. Three main parameters have been investigated; the total gas flow, the oxygen/silane gas flow ratio, and the magnetic confinement in the diffusion chamber. An in situ control of the refractive index and deposition rate has been obtained and correlated to an ex situ analysis of the deposited films (infrared transmission spectroscopy and chemical etch rate measurements) and the effects of the deposition kinetics on the film properties such as the hydrogen content and the voids fraction have been analyzed.