CN x films were deposited on Si(100) substrates at a substrate temperature of 600 °C using dc magnetron sputtering of a high-purity graphite target in pure nitrogen. The film characteristics were primarily controlled by the pressure, p, (0.05–5 Pa), the discharge current on the magnetron target, Im, (0.5–3 A), and the rf induced negative substrate bias voltage, Ub, (−300 to −1200 V). The films, typically 1–2 μm thick, were found to be amorphous, and they possessed the N/C atomic concentration ratio up to 0.35, the hardness up to 40 GPa, the elastic recovery up to 85%, and good adhesion and promising tribological properties. The energy and flux of ions bombarding the target and the growing films were evaluated on the basis of the discharge characteristics measured for both the dc magnetron discharge and the rf discharge in the deposition zone. Optical emission spectroscopy was used to study the behavior of significant atomic and molecular species, such as N, N2, N+, N2+, CN, C, and C2, near the substrate and in the plasma bulk. The measurements indicated that both dc and rf discharges are mutually coupled. We confirmed that CN radicals and N atoms are the principal precursors for reactive magnetron sputtering of the CNx films. Good correlation between the N/CN concentration ratio in the deposition zone and the N/C atomic ratio in the films was found over a wide range of the investigated process parameters. We have shown that the CNx films have a high hardness only when the energy and flux of the nitrogen ions are sufficiently high for effective incorporation of nitrogen in the layers; such conditions require the pressures lower than 0.5 Pa and the Ub values between −500 and −700 V. It has been proved that the frequently used deposition parameters such as Im, Ub, p, and Ts, do not generally suffice to satisfactorily characterize the film growth process.

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Plasma analysis using emission spectroscopy has been carried out in the cathode fall and the negative glow of N2-H2 abnormal glow discharges for steel surface nitriding. The spatial distributions of the selected excited states of N2, N, N2+, N+, NH, H2, H and Fe have been measured in terms of their dependence on the main process parameters, such as the cathode temperature (400-550 degrees C), H2 percentage in the gas mixture (10-70%) and total pressure in the nitriding reactor (400 and 800 Pa), corresponding to the given values of the discharge current (115-260 mA) and applied voltage (465-635 V). The experimental data presented yield reliable information on the occurrence of excited states in the cathode region and on the effect of the process parameters on the cathode fall thickness, which determines the energy distributions of ions bombarding the cathode and is an important input parameter in plasma process modelling.

In the present work, we systematically study the effect of radio frequency induced negative substrate bias voltage, Ub, on the characteristics of CNx films fabricated by direct current (dc) magnetron sputtering of a graphite target in a nitrogen plasma. CNx films, typically 1–2 μm thick, were deposited onto Si(100) substrates at a temperature of 600 °C and at a pressure of 0.5 or 5 Pa. The films’ microstructure and composition were evaluated using elastic recoil detection analysis, Fourier transform infrared spectroscopy, Raman scattering, x-ray diffraction, and atomic force microscopy. The films were found to be amorphous and partially conductive. The nitrogen concentration in the films varied between 12 and 24 at. %. Increasing the |Ub| values from 300 to 700 V resulted in increasing the N/C ratio, while the hydrogen content in the bulk decreased from 5 to 1 at. %. The latter effect clearly demonstrates the sputter-cleaning process during the ion-assisted deposition. Increasing the |Ub| values also resulted in an increase in film density from 1.5 to 2.7 g/cm3, and in a decrease in surface roughness from 13 to 2 nm. This densification was accompanied by an increased hardness (from 2 to 23 GPa), and elastic recovery (from 35% to 74%), and by higher adhesion to the silicon substrate. |Ub| values higher than 700 V lead to the degradation of the films mechanical properties. Even at its low concentration, hydrogen was found to be directly related to the film quality. We suggest that the presence of hydrogen inhibits interconnection of graphite-like planes containing carbon and nitrogen, which may hamper formation of possible fulerene-like microstructure described in recent literature.

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