Carbon nitride deposition by magnetron sputtering: structural, mechanical, electrical and optical properties

Carbon nitride films were deposited using a magnetron sputtering technique based on the Penning type geometry. For deposition, graphite targets were used in a nitrogen/argon gas mixture. The technique employed consists of two opposing cathodes, with two very strong magnets placed behind them. The magnetic field created with this configuration, in conjunction with the electric field, provides a high ion flux at the substrate, which results in high deposition rates (up to 3fim/h) and increased nitrogen incorporation in the films (up to 45 at.% N). The effect of deposition parameters, such as nitrogen partial pressure (Npp), substrate bias, and deposition temperature on the film properties has been investigated. The effect of nitrogen addition on the structural properties of carbon nitride films has been characterised in terms of its composition, infrared and Raman spectra and X-ray photoelectron spectroscopy (XPS). Nitrogen content in the films was measured by Rutherford Backscattering (RBS) and film thickness by a surface profilometer. Infrared spectra revealed that the films have an amorphous structure, in which nitrogen is mainly incorporated in C=N sp2-type bonding configurations, with some proportion of O N sp-type bonding, which increases with nitrogen incorporation. Above 20 at.% N, a structural rearrangement occurs in which N preferentially bonds to itself. Core level XPS peaks due to carbon and nitrogen Is electrons were assigned to different types of bonds in accordance with Raman and IR spectra and by comparison with other assignments found in literature. A Nanoindenter instrument was employed to investigate the mechanical properties of the films. Nanoindentation load-displacement curves provide a 'mechanical fingerprint' of the material’s response to contact deformation. Hardness and elastic modulus values obtained for carbon nitride films using the Oliver and Pharr technique ranged from 8-12 GPa and 90-120 GPa respectively. Hardness was shown to decrease with Npp but was not dependent on total nitrogen content. Hardness was better correlated to the ON bond concentration; the increased amount of ON bonds causes a weakened structure as they serve as network terminators in the carbon backbone structure. The hardness of the films correlates with the presence of a graphite-like structure dominated by sp2 bonding structure. The effect of substrate bias, annealing and deposition temperature on the film’s hardness and structure is also described. Higher coefficients of friction were found for harder films deposited at different substrate bias. The stress is shown to be concentrated at the film-substrate interface whereas the bulk of the film is stress-free and increases at higher substrate bias as ion bombardment increases. The XPS and ultraviolet photoelectron spectroscopy (UPS) valence band (VB) spectra have been reported. The evolution of bands corresponding to jt and a bonding with Npp reveals that there is a greater degree of sp2 bonding as the nitrogen partial pressure (Npp) is initially increased compared to the pure carbon samples. However, with further increase in Npp (reaching 100%), the bonding becomes more sp3-like. The films’ electronic states were further investigated as a function of Npp using techniques such as electron energy loss spectroscopy (EELS) and electron spin resonance spectroscopy (ESR). The observations reported point to the reasons why crystalline (5-C3N4 material has been found only with high Npp despite the fact that nitrogen content is not significantly enhanced in this situation. Various electrical analysis techniques were employed to study the electrical properties of carbon nitride films. The four-point probe and van der Pauw methods were used for resistivity measurements. From these analyses, the resistivity was found to increase with Npp, and to be controlled by the amount of C=N bonding, which causes a decrease in the number density of electrons available for conduction. N is incorporated in the films in a non-doping configuration. From temperature-dependent resistivity measurements, it was proposed a transition from metallic to semiconducting behaviour as nitrogen is incorporated in the films. A conduction mechanism typical of low mobility amorphous semiconductors was suggested. An increase in both negative substrate bias and deposition temperatue produce films with lower resistivity. The optical gap was estimated from absorption coefficient measurements in the ultraviolet-visible region. Refractive index and extinction coefficient measurements are also reported. An electron emission configuration revealed that it is possible to obtain moderate emission current from a number of carbon nitride films. Despite the fact that the analysis techniques considered here give results consistent with an amorphous carbon nitride solid, there is some evidence of crystalline P-C3N4 areas co-existing with the mainly amorphous structure for films deposited at 100% Npp. There is room for further optimisation of the quality of carbon nitride films deposited by reactive Penning-type magnetron sputtering with respect to composition, structure and application-relevant properties