Investigation of Niobium oxynitride thin films deposited by reactive magnetron sputtering

Niobium oxynitride films were deposited using reactive magnetron sputtering of a niobium target in an Ar/O2/N2 atmosphere with fixed nitrogen flux in direct current (DC) and pulsed modes. For the DC sputtering mode the deposition rate was found to be twice as high as for the pulsed mode at lower oxygen to nitrogen ratios (O/N). Morphology investigation by scanning electron microscopy and atomic force microscopy showed that the coatings are getting very smooth with increasing oxygen content (average roughness Ra < 0.4 nm at oxygen contents > 40 at.%). X-ray diffraction measurements revealed that the niobium oxynitride films are X-ray amorphous for oxygen contents > 40 at.%. The electrical conductivity of the coatings was studied by the 4 point-probe method and was found to decrease with increasing oxygen content. Optical properties of Nb–O–N films were analysed by spectroscopic ellipsometry and transmission spectroscopy. The refractive index of transparent and semi-transparent films was found to be in the range of 2.3 and 2.6 (at 633 nm). The experimental results will be discussed with respect to the O/N ratios (range 1.2 < O/N < ∞) or the oxygen content (range 33.7 at.% < O < 67.3 at.%) in the films as measured by Rutherford backscattering spectroscopy and particle induced X-ray emission.