Bonding structure of carbon nitride films by infrared ellipsometry

Carbon nitride (CNx) films were deposited by reactive sputtering to study the effect of the ion bombardment during deposition (IBD) on their bonding structure. Fourier-transform infrared (IR) ellipsometry (FTIRE) was used to identify and distinguish the characteristic bands of the sp(3) C-N, sp(2) C=N, sp(1) (-C=N, -N=C) and the IR-inactive C=C bonds. The results are compared and discussed in view of the films' electronic behavior through the dielectric function epsilon(omega) in NIR-visible-UV region and with those obtained by nanoindentation measurements. The low-energy IBD is suggested to promote the homogeneous N distribution in the films, resulting in films with low hardness (similar to6 GPa) and stress. On the contrary, the high-energy IBD results in high-N concentration in localized regions of the films, where possibly the formation of fullerenelike and C3N4 structures is favored. Indeed, hardness values up to 45 GPa were measured at some regions of these films, along with the high stress and hardness that they exhibit. Their absorption due to pi-->pi* electronic transitions is higher and exhibit strong absorption similar to1.6 eV where the low-energy IBD films are transparent. Furthermore, the effect of postdeposition thermal annealing to 900 degreesC on the bonding structure of the films was investigated. It was found that the structural modifications induced by the N removal from the carbon-nitrogen bonds depend on the bonding structure of the films, as determined by the IBD energy. The N evolution from spa C-N bonds is more intense in low-energy IBD films and more pronounced around 450 degreesC, while the C-N bonds of pentagons and C3N4 structures, contained mainly in high-energy IBD films, are more stable and break at higher temperatures. Above 600 degreesC, N is evolved from the sp(2) C=N bonds, while the most stable structures (i.e., sp(1) -N=C and -C=N groups) break above 700 degrees C. The thermal treatment differentially affects the electronic transitions; the pi-->pi* are almost stable, while the strength and energy position of the sigma-->sigma* decrease. This reduction is dramatic in low-energy IBD films, suggesting an intensive N evolution from the sp(3)- and sp(2)-bonded nitrogen. The slight modification of epsilon(omega) and the unaffected film thickness at 900 degreesC reveal the high-thermal stability of high-energy IBD films. The above results indicate how IBD) affects the bonding and electronic structure and the thermal stability of the CNx films, contribute to the understanding of the bonding mechanisms during N incorporation in the a-C network, and provide insight towards the production of CNx films with desired properties.