Density functional study of the stability and electronic properties of TaxNy compounds used as copper diffusion barriers

Considerable efforts have been dedicated to the growth of the cubic TaN films for copper barrier application but deposition of various phases from TaN1.1 to Ta3N5 has mainly been reported. In order to solve this point, we have undertaken a systematic study on the stability of each compound and especially on the determination of the influence of point defects in the TaN cubic structure. We perform a first-principles calculation within the local density approximation for the TaxNy compounds. Our calculations show that the compounds in the Ta–N system with the lowest energy are Ta4N5 and Ta5N6 for which the Ta atoms are enclosed in octahedrons of N, while the N atoms are coordinated by five atoms of Ta. The cubic TaN structure for which the N atoms are enclosed in an octahedron of Ta is found to be a metastable phase. Our main result stands in the fact that a vacancy of Ta changes the coordination number of the six N atoms surrounding the vacancy from six to five, mimicking the geometry encountered in the most stable Ta4N5 and Ta5N6 compounds. As a result, the cubic symmetry is energetically favourable. Moreover, proof is added that the vacancy of Ta modifies only in a negligible manner the desired electronic properties of TaN.