Electron-cyclotron-resonance (ECR) plasma are used extensively to deposit amorphous and microcrystalline Si and (Si,Ge) films, and for depositing oxides and nitrides of Si. In this paper, we discuss the results of optical emission spectroscopy, Langmuir probe measurements and mass-spectrometery measurements of the ECR plasma when used to deposit a-Si films. We study He and H diluted plasmas. We find that the addition of hydrogen to the gas mixture immediately reduces the flux of the active (e.g. SiH, SiH2,SiH3) radicals and ions that are responsible for the growth of films. Thus, introducing hydrogen in the mixture reduces the growth rate significantly, not only because it etches the film during growth, but primarily because it reduces the production of the needed growth radicals. We find an excellent correlation between growth rates and SiH intensity signal. A surprising result is that the densities of all three ions, SiH, SiH2 and SiH3, arriving at the substrate are comparable in magnitude in this low pressure reactor, with the SiH ion density becoming larger than the SiH3 density at higher powers. This observation raises some doubt about the standard model for growth that states that SiH3 is the dominant radical responsible for growth during plasma CVD processes. We also find that decreasing the pressure increases the ratio of H/H2 arriving at the substrate, which in turn means that more H ions and atoms are available to do etching of the growing film and change its properties at lower pressures.