Synthesis of carbon nitride thin film by magnetron sputtering technique: its structural characterization and application
Chowdhury, A. K. M. Shaestagir (1998) Synthesis of carbon nitride thin film by magnetron sputtering technique: its structural characterization and application. PhD thesis, Dublin City University.
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The purpose of this investigation was to establish a technique to deposit crystalline carbon nitride material and study its structural properties with the view to its use a s hard coating. For the first time, carbon nitride thin films have been deposited, using a Penning-type opposed-target DC reactive sputtering source, that contain large continuous nanocrystalline areas ( > 10|j,m2) of crystallography consistent with the P-C3N4 structure. In addition the creation of these P-C3N4 regions has been achieved with low substrate temperatures (<270°C) and high deposition rates (2.5 - 3 |j,m.hr"1).
The IR absorption due to carbon-nitrogen bonding was observed to be independent of actual nitrogen content above —25 at.% N. It has been shown that over the range of 25-44% N/(N+C) there is no systematic variation of absorption coefficient. It was predicted and shown that films with >25 at.% nitrogen content, the nitrogen is mostly bonded to carbon either a s C=N or C^N bonds and a significant amount of nitrogen bonded with itself in IRinvisible structures. It was also seen that the C=N bond (absorbance at 2200 cm"1) concentration which controls the hardness of the film, can be eliminated at 600°C. The physical explanation of the weakness of the polymeric CN network is probably due to the formation of this C=N bonding which terminates the carbon backbone leading to less tightly bound C atoms. This feature was indicated by AES in the C KLL Auger spectrum and defined a s a defect related 71 state in the structure. It was also seen that nitrogen incorporation in the film not only increases the nitrogen-nitrogen bonding but also stabilizes the C-C sp3 type bonding. The breaking of C-C sp3 bonds results from the input thermal energy a s annealing progresses and leads to graphitisation of the film.
It is also seen that between the Raman D and G peaks there exists a third peak at —1455 cm'1, designated the "N" peak, which has been assigned to the N=N stretching vibration. As nitrogen incorporation in the film increases, the N=N, C=N and C-C bonding intensities increase.
The presence of different bonding structures in CN network was also determined by XPS. The core level XPS peaks were assigned to different types of bond by correlating their behaviour a s annealing takes place at different temperatures with changes in the bond structure as detected by vibrational spectroscopy. The valence band XPS spectra show the interlinked carbon backbone nature of the carbon nitride solid and thus identify the structural nature of this solid which is significantly different from diamond-like and graphitic features.
It was seen that the hardness decreases a s the C^N bond concentration increases in the film. The intrinsic film stress was found to be lower for the nitrogenated films than for the pure carbon films. Unlike the film hardness it was found to be independent of the nitrogen content for films with >25 at. % N. It was also found to be independent of the film thickness indicating that the stress was introduced at the film-substrate interface during the initial growth process rather than in the bulk of the film.
This is the first time that structural modification of carbon nitride solid with negative bias was observed by valence band XPS spectra. Valence band XPS spectra show a significant change in structure, i.e., sp2 to sp3, in carbon nitride solid when the substrate negative bias w as increased from -75 to -150V.
Carbon nitride in thin film form is a good candidate for hard coating but it has poor adhesion on tool steel due to diffusion of nitrogen or carbon atoms into the substrate at the deposition temperature (typically ~325°C). Carbon nitride thin film has been deposited successfully for the first time directly on tool steel. Ca se hardened surfaces act a s a diffusion barrier for nitrogen or carbon atoms from the film. The adhesion properties of the film was considerably improved on the nitrided samples compared to untreated substrates on which the films do not adhere.
The technique discussed here opens a new era in the production of crystalline carbon nitride solids. Successful fabrication of this C-N solid enlightens a new possibility in the field of super hard material.
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