Radio frequency sputter deposition and properties of calcium fluoride thin films

Abstract

Calcium fluoride thin films have been deposited on various substrates by radio frequency sputtering of a hot-pressed CaF2 target in a pure argon plasma using load-locked sputtering equipment. The sputtering power was less than 100 W to avoid crack formation and fracture of the target. As a result, the maximum deposition rate of films was only 2.1 nm/min. The films analyzed by Rutherford backscattering spectroscopy (RBS) were found to be fluorine deficient; depending on the deposition conditions, the content of oxygen atoms incorporated in the films can be as low as the RBS detection limit of 2 at. %. The crystallographic structure of the deposited material was identified to be a CaF2 fluorite phase by x-ray diffraction technique; this phase was preferentially oriented in the (111) direction for films of 0.4–0.5 μm in thickness. This preferred orientation vanished with increasing film thickness. Films of 2.5 μm in thickness were slightly (110) oriented. The sputter-deposited CaFx films (with x≤1.85) exhibited compressive residual stresses. The apparent Knoop hardness of the films deposited on silicon substrates (harder than CaF2) and measured under a load of 10 and 20 g was about 3500 MPa, i.e., twice the hardness of the bulk CaF2 compound. The surface and cross-section morphology of CaFx films was examined by scanning electron microscopy. These films were resistant to oxidation in air at 500 °C. Tribological characteristics of CaFx films sputter-deposited on cobalt-based alloy disks were determined by pin-on-disk tests. The friction coefficient and wear resistance of these films sliding against Cr–C coated pin specimens in air at 500 °C were found to be encouraging. These films are potential candidates to serve as solid lubricant films for precision components (ball bearings) operating at high temperatures in oxidizing ambient.