Synthesis and properties of tantalum nitride films formed by ion beam assisted deposition

Tantalum nitride films TaNx were deposited onto 316L austenitic stainless steel and silicon wafer substrates by evaporation of tantalum under simultaneous nitrogen ion irradiation, applying an acceleration voltage of 2 kV. In order to study the influence of the nitrogen content, the atom-to-ion transport ratio ([Ta]/[N]) was varied. The compositional and structural characterization of the films was carried out using Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction. It was observed that the surface morphology depends on the deposition conditions. X-ray diffraction patterns show the crystal structure of the films changing from Ta3N5, TaN, Ta2N and Ta-N solid solution when the transport ratio is increased from 0.7 to 10. The crystal structure of the film deposited at [Ta]/[N]=6 has the feature of a b.c.c. phase plus an amorphous phase containing 30 at.% nitrogen. The chemical bonding state of tantalum atoms observed by X-ray photoelectron spectroscopy changes with increasing transport ratio [Ta]/[N], whereas the bonding state of nitrogen atoms is independent of the transport ratio. The sheet resistivity and the temperature coefficient of resistance (TCR) of the films were measured by the four-point probe method. The TCR values of all observed TaNx films were negative, showing a minimum value at the lowest observed value of [Ta]/[N]=0.7. The corrosion behavior was evaluated in an oxygen saturated aqueous solution of sulfuric acid, using multisweep cyclic voltammetry measurements. The best corrosion protection was observed for the Ta-N solid solution film prepared at [Ta]/[N]=6. All other films, existing in the compound state, show less resistivity.