Physical and chemical properties of sputter-deposited TaCxNy films

The structural, electronic, optical, and mechanical properties of stoichiometric TaCxNy=1-x were simulated using an ab initio calculation based on density functional theory (DFT) within the generalized gradient approximation. The calculations revealed the theoretical lattice parameter, density of states, refractive index, and elastic constants as a function of carbon and nitrogen content. TaCxNy films were subsequently produced on Si wafers using unbalanced magnetron sputtering. The structural, optical, and mechanical properties were measured using x-ray diffraction/transmission electron microscopy, vacuum ultraviolet spectroscopic ellipsometry, and nanoindentation, respectively. The computational and experimental properties were compared. The lattice parameter, the energy of the 2p bands in the density of states, and the energy of the interband transitions were found to decrease with increasing C content. No significant changes in the elastic constants were observed as a result of substituting N atoms with C atoms. The hardness and the elastic modulus were in the 40 and 380 GPa range, respectively. The experimental Young's modulus was much smaller than the computational one and this discrepancy was attributed to the nanocrystalline nature of the films. Also, the elastic constants were found to decrease dramatically for overstoichiometric films.