Effect of Si incorporation on the properties of niobium nitride films deposited by DC reactive magnetron sputtering

Thin films of NbSiyNx have been deposited by reactive magnetron sputtering from confocal Nb and Si targets in mixed Ar/N2 atmosphere, at a substrate temperature of 250 °C. The total pressure, the nitrogen partial pressure and the current on the Nb target were kept constant, while the current on the Si target was varied in order to obtain Si concentrations between 1 and 34 at.%. For Si contents below 11 at.%, X-ray diffraction (XRD) reveals that the films are crystalline and have a fcc δ-NbN structure. For higher Si concentrations, the films exhibit a multiphase structure consisting of δ-NbN nanocrystallites and an amorphous matrix. The texture and the crystallite size depend on the Si content. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements allowed to identify the amorphous phase as Si3N4. Transmission electron microscopy (TEM) observations reveal that the microstructure of films is columnar, and that each column is formed by an agglomerate of crystallites. Amorphous regions were observed on the high-resolution transmission electron microscopy (HRTEM) images of the films with CSi=11 and 20 at.%. dentation measurements show that the hardness increases with increasing Si content up to 5 at.%, and reaches a maximum of 34 GPa. Above 11 at.% of Si, it decreases to the value comparable to that reported for amorphous Si3N4 (22 GPa). The hardness variation of the NbSiyNx films appears to be related to the formation of the amorphous Si3N4 phase and its volume fraction within the film. This behavior is similar to that observed in nc-MeN/a-Si3N4 nanocomposites (Me=Ti, W, V).