Influence of nitrogen–argon gas mixtures on reactive magnetron sputtering of hard Si–C–N films

Si–C–N films were deposited on p-type Si(100) substrates by dc magnetron co-sputtering of silicon and carbon using a single sputter target with variable Si/C area ratios in nitrogen–argon mixtures. The film characteristics were primarily controlled by the argon concentration (0–75%) in the gas mixture at a fixed 40% silicon fraction in the magnetron erosion track area. The total pressure and the discharge current on the magnetron target were held constant at P=0.5 Pa and Im=1 A, the substrate temperature was adjusted at Ts=600 °C by an ohmic heater and the rf-induced negative substrate bias voltage, Ub was −500 V. Mass spectroscopy was used to explain differences between sputtering of carbon and silicon in nitrogen–argon discharges. The films, typically 1.0–1.5 μm thick, were found to be amorphous with a very smooth surface (Ra≤0.8 nm). It was shown that the nitrogen–argon gas mixture composition is an important process parameter in a reproducible production of Si–C–N compounds with controlled properties by dc magnetron co-sputtering using a composed C–Si target with variable Si/C area ratios. With a rising argon concentration in the gas mixture, the Si content in the films rapidly increases (from 19 to 34 at.% for a 40% Si fraction in the erosion target area), while the C content decreases (from 34 to 19 at.%) at an almost constant (39–43 at.%) N concentration. An intensified bombardment of growing films by argon leads to its subplantation into the films (up to 5 at.%) and to a decrease in a volume concentration of hydrogen (from 5 to 1 at.%). As a result, the NSi and SiN bonds dominate over the respective NC and SiO bonds, preferred in a pure nitrogen discharge, and the film hardness increases up to 40 GPa. The effect of the negative substrate bias voltage (from a floating potential of −18 to −500 V) on characteristics of the films prepared in a 50% N2+50% Ar gas mixture is not too marked at Ts=600 °C. A decrease in the Ts values to 135–210 °C leads to a higher incorporation of hydrogen into the films (up to 6 at. %) and to a stronger influence of the negative substrate bias voltage.