Thin films prepared by sputtering MgF2 in an rf planar magnetron

Magnesium fluoride granules have been sputtered in an rf planar magnetron at 20 MHz. Films deposited in Ar developed a strong optical absorption at high power levels arising from fluorine depletion and magnesium oxidation. The latter was attributed to a reaction between residual water vapour and the dissociated compound. Films prepared at a low power input with a growth rate of about 2 nm min^?1 had the lowest optical absorption and a refractive index near to that of bulk MgF₂ but their growth was too slow for most practical uses. The magnetron discharge probably resulted in greater target dissociation than a non-magnetron discharge operated at the same power input because of the ion impact localisation at the magnetron target. Film analysis showed that biasing the film substrate positively with respect to ground to enhance impact of fluorine negative ions raised the fluorine content of the film. Sputtering in a CF₄+Ar+O₂ mixture to enhance fluorine sorption by the film and reduce carbon deposition by oxidation was ineffective. The resultant deposit was a fluorocarbon polymer with a low Mg/F content.