Relationship between CH4/Ar ratio, stoichiometry, and mechanical properties of TaCx coatings produced by reactive magnetron sputtering

Abstract

This study focused on the influence of the CH4/Ar flow ratio on the microstructure, chemical composition, morphology, and mechanical properties of TaCx coatings deposited on (1 1 1) silicon substrates using magnetron sputtering. To characterize the coatings, X-ray diffraction for the microstructural analysis, auger electron spectroscopy (AES) for the stoichiometry, a mechanical profilometer for roughness and thickness, and a nanoindenter for mechanical measurements were used. Two experiments were designed: the first consisted of producing TaCx coatings at room temperature (RT) using various CH4/Ar ratios, from 0.1 to 0.3, to identify the phases, stoichiometry, and hardness. These samples did not present crystalline phases and showed a transition from sub-stoichiometric to over-stoichiometric materials. The over-stoichiometric coatings produced at CH4/Ar ratios greater than 0.2 exhibited a decrease in their hardness. For the second experiment, the substrate temperature was set at 500 °C to reach crystalline phases; the CH4/Ar ratio values were set to 0, 0.1, 0.125, 0.15, 0.175, and 0.2 to avoid over-stoichiometric coatings that exhibited lower hardness. At low CH4/Ar ratios, TaCx coatings were produced in an amorphous phase (α-Ta) and underwent a mixture of phases (α-Ta and cubic TaC). The materials tended to crystallize at CH4/Ar ratios greater than 0.175 (cubic TaC). All of the coatings were sub-stoichiometric, showing values from Ta to TaC0.56. Most of the samples exhibited a hardness of ~30 GPa and a high Young’s modulus, except for the coatings with the highest carbon concentration (TaC0.56). The Ta coatings exhibited the lowest roughness, although they had the highest thickness. This behavior was attributed to Ta exhibiting a higher deposition rate than TaC.In conclusion, the CH4/Ar ratio strongly influenced the crystallinity and stoichiometry of the coatings. Moreover, the materials exhibited a high hardness caused by their tendency to be amorphous.