Growth of TaC thin films by reactive direct current magnetron sputtering: Composition and structure

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Abstract
Single-phase TaC films were grown by reactive dc magnetron sputtering onto high speed steel substrates kept at 650 °C (0.22 Tm ). The films were grown in both Ar/CH4 and Xe/CH4 mixtures using target-to-substrate distances, dts, of 6 and 15 cm. For all growth conditions, plasma-probe measurements were carried out. As-deposited films were analyzed using Auger electron spectroscopy (AES), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The C content of the films, as measured with AES, was found to increase linearly with increasing CH4 partial pressure in all cases. However, both the slope of these curves and the CH4 pressure needed in order to obtain a certain C content in the films were different depending on both dts and the sputtering gas. These results are explained with arguments based on cracking of the CH4 molecules into more reactive CHn radicals in the plasma and a different gas scattering of these radicals compared to the sputtered Ta atoms. Also, the structure of the deposited films was depending on the sputtering gas and all films grown in Ar/CH4 showed a considerably higher strain level compared to films grown in Xe/CH4 discharges. This higher strain level is due to the generation of defects in the films induced by a large flux of backscattered energetic neutral Ar atoms that imping on the growing film. In the Xe case, the energy of the backscattered neutrals is not sufficient to generate defects in the films. The structure of the films was also found to depend on the C content. For over-stoichiometric films, TEM analyses showed that the grain size was only a few nanometers and that there are low density regions surrounding the grains suggesting a precipitation of the excess C. When this precipitation occurred, the preferred orientation of the films changed from (200) to (111).

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