Characterization of sputter-deposited chromium oxide thin films

Abstract

Cr2O3 thin films exhibit high hardness values and low friction coefficients. These properties make chromium oxide a serious candidate to replace transition metal nitrides or Al2O3 in special applications. The CrOx films were deposited on silicon, glass and HSS substrates by r.f. reactive magnetron sputtering at different oxygen partial pressures and substrate temperatures, TS, between 360 and 590 K. The crystallographic phase was determined by X-ray diffraction analysis. Polycrystalline films with a single α-Cr2O3 phase are obtained in the range between 15 and 25% oxygen in the sputtering gas at a TS value exceeding 500 K. The surface morphology was examined by atomic force microscopy and revealed a very low roughness on amorphous films. The chemical composition was measured by electron probe microanalysis. The grain size varied significantly with the oxygen content and the substrate temperature. The optical constants and the optical band gap of crystalline films were determined by spectroscopic ellipsometry in the photon energy range of 1.5–5.0 eV. The optical constants appeared to be extremely sensitive to the chemical composition of the Cr2O3 films. Hardness values up to 32 GPa were obtained by nanoindentation at compressive residual stress levels below 1 GPa. Mixed phase CrN/Cr2O3 thin films combine the high hardness of Cr2O3 and the toughness of CrN.