Structural and mechanical properties of chromium nitride, molybdenum nitride, and tungsten nitride thin films

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Abstract
Cr–N, Mo–N, and W–N thin films are deposited on silicon by rf reactive magnetron sputtering. The crystallographic phase and residual stress are determined by x-ray diffraction analysis. In each of the three material systems, a hexagonal and a face-centred cubic (fcc) phase are observed. Plasma diagnostics using energy-resolved mass spectroscopy reveal that a significant fraction of the Cr+ ions exhibits a high flux and kinetic energy if the nitrogen partial pressure pN2 is low. These high-energy ions effectively bombard the growing film and a densely packed morphology results. In contrast, in absence of a significant amount of high-energy ions at higher pN2 , a columnar crystal morphology is observed by scanning electron microscopy. The grain size strongly depends on the presence of a second phase and on the nitrogen content. The hardness, measured by nanoindentation, increases in every material system if the content of the hexagonal phase increases. Under overstoichiometric conditions, the hardness of fcc compounds decreases. The observed hardness differences are explained by morphological changes and by differences in the electronic structure of the compounds.