S. Muhl

A review of the preparation of carbon nitride films

Abstract Using a semiempirical rule based on a range of existing hard materials, Liu and Cohen predicted that some of the crystalline forms of C3N4 might have values of bulk modulus close to, or even greater than, that of diamond. Subsequent theoretical work has supported this result, although modifying which of the crystal structures had the most extreme properties. Based on the assumption that hardness is primarily determined by the bulk modulus, a large number of groups initiated research to try to make this ‘harder-than-diamond’ material. Although today there are doubts as to the validity of this assumption, the ability to synthesise materials with such a large bulk modulus is of great interest because such substances can be expected to have extreme properties, such as thermal conductivity, compression strength, acoustic velocity, etc. A review is presented of the methods used, and the results obtained, by a variety of groups in their attempts to prepare carbon nitride films. The preparation techniques have been somewhat arbitrarily divided into the following sections: (1) atmospheric-pressure chemical processes, including pyrolysis and explosive shock; (2) ion-beam deposition, including cathode arc; (3) laser techniques; (4) chemical vapour deposition processes, including direct-current (DC) arc, hot-filament chemical vapour deposition (HFCVD) and plasma-enhanced chemical vapour deposition (PECVD); and (5) reactive sputtering, with and without deliberate substrate biasing. We have also included a section describing some of the problems associated with the analysis of carbon nitride films. A brief description of the techniques is provided and we discuss some of the problems associated with the characterisation methods that have been used in the analysis of the deposited material. A somewhat speculative set of conclusions is presented which we hope will be of assistance to colleagues working in this field.

Properties of carbon films and their biocompatibility using in-vitro tests

In this paper we report the results of a comparative study of the biological response of amorphous carbon coated stainless steel. Films of amorphous carbon (a-C), amorphous carbon nitride (a-CN) and hydrogenated amorphous carbon (a-C:H) were deposited on stainless steel substrates (AISI 316L) using a dc magnetron sputtering system. In-vitro studies were carried out on the coated samples using human osteoblasts cell culture lines and fibroblasts. Preliminary biocompatibility was assessed by cell adhesion and proliferation, as determined by a spectroscopic technique. Comparison of the optical absorbance results between control uncoated disks and the test cultures provided a semi-quantitative analysis of the cytotoxic effect of the different carbon compounds. Osteoblasts cells were grown on uncoated steel, a-C, a-CN and Ti coated steel samples. The degree of fibroblast adhesion measured at 24 h is very similar for all the test samples, however, osteoblasts adhesion was higher for a-C films. Similarly, cellular proliferation at 7 days showed an outstanding increase of osteoblasts cells for a-C and Ti in contrast with uncoated steel. The physical film properties, such as, roughness measured by atomic force microscopy, surface composition determined by both Rutherford Backscattering and Auger Spectroscopy and the electro-optic properties of the films were also determined. The relation between film properties and cellular response is discussed.

Infrared spectra of carbon nitride films

The assignment of the vibrational modes in amorphous carbon nitride (CN) films is discussed by considering CN films deposited using a variety of methods. The infrared (IR) spectra of CN show three main absorption bands. In hydrogenated CN samples, CHx and NHx groups give rise to stretching vibrations at 3000 and 3400 cm−1, respectively. A weaker sharp band is observed approximately 2200 cm−1 due to CN–sp1 bonds. Finally, there is a broad band between 1000 and 2000 cm−1. It is usually stated that the effect of nitrogen into carbon films is to break the symmetry of the sp2 carbon bonds making the Raman ‘G’ (graphitic) and ‘D’ (disorder) modes IR active, so the broad band between 1000 and 2000 cm−1 is similar in both IR and Raman spectra. However, it is shown that nitrogen is not necessary to have significant IR activity in the 1000–2000 cm−1 region. Also, Raman spectroscopy in carbon is always a resonant process, so that the spectra depend on the excitation energy. Therefore, the similarity of the visible Raman and IR spectra of some CNs is generally a coincidence. We show that the IR broad band in the 1000–2000 cm−1 region is an electronic effect and is not due to activation of IR forbidden modes due to symmetry breaking. This explains the IR spectra not only of CN films but also of N-free amorphous carbon films and is related to the presence of the system of delocalized π bonds with increasing conjugation.

Optical and surface analysis of DC-reactive sputtered AlN films

A set of aluminium nitride (AlN), thin films were prepared by DC-reactive magnetron sputtering.The deposition parameters, such as substrate temperature, sputtering gas composition and plasma current were varied.Spectroscopic ellipsometry, XPS, RBS, XRD, SEM, AFM and FTIR techniques were utilized to study the relationship between film properties and preparation conditions. We observed that the optical and surface properties have a strong dependence on the deposition rate.All prepared films present a composition close to AlN stoichiometry, even for nitrogen to argon gas concentrations below 1:2.The near surface of AlN films exposed to atmosphere was primarily composed of Al O while the bulk was AlN with some minor contamination of 23 oxygen and carbon.The thickness of the oxide layer was reduced when higher plasma current and lower nitrogen concentration were used.Deposits prepared at 400 8C presented the best refractive index and deposition rate, for both utilized plasma currents. 2003 Elsevier Science B.V. All rights reserved.

Optical gap in carbon nitride films

Abstract In this paper we study the effect of introducing nitrogen into different carbon networks. Two kinds of carbon nitride films were deposited: (a) Using a DC-magnetron sputtering system sp 2 bonded carbon nitride (a-CN) films were deposited and (b) Using a combination of filtered cathodic vacuum arc and a low-pressure N 2 plasma source, N was introduced into sp 3 carbon networks (ta-C), leading to the formation of a more dense CN film named ta-CN. For ta-CN films we found that the optical gap initially decreases as the N content and the sp 2 fraction rises, but above a certain N quantity there is a level-off of the value, and the gap then remains constant despite further increases in the fraction and clustering of the sp 2 phase. However, for a-CN films the optical gap increases with the nitrogen content. These two different trends are not easily explained using the same framework as that for carbon films, in which any decrease in the band gap is associated to an increase in the sp 2 fraction or its clustering. Here we discuss the conditions that lead to high optical gap in sp 2 -bonded carbon nitride samples, which are clearly not associated to the presence of any crystalline super-hard phase. We also compared other differences in properties observed between the two films, such as deposition rate, infrared and Raman spectra.

The Preparation, Characterization and Tribological Properties of TA-C:H Deposited Using an Electron Cyclotron Wave Resonance Plasma Beam Source

A compact electron cyclotron wave resonance (ECWR) source has been developed for the high rate deposition of hydrogenated tetrahedral amorphous carbon (ta-C:H). The ECWR provides growth rates of up to 900 A/min over a 4 diameter and an independent control of the deposition rate and ion energy. The ta-C : H was deposited using acetylene as the source gas and was characterized in terms of its sp 3 content, mass density, intrinsic stress, hydrogen content, C-H bonding, Raman spectra, optical gap, surface roughness and friction coefficient. The results obtained indicated that the film properties were maximized at an ion energy of approximately 167 eV, corresponding to an energy per daughter carbon ion of 76 eV. The relationship between the incident ion energy and film densification was also explained in terms of the subsurface implantation of carbon ions into the growing film.

Corrosion of a Zn–Al–Cu alloy coated with TiN/Ti films

Abstract TiN and Ti coatings were deposited by d.c. magnetron sputtering on a Zn–Al–Cu alloy. The coatings were deposited at a relative low temperature of 115–130°C. X-ray diffraction showed that the films were crystalline. A pure titanium interlayer was used to improve the adhesion and corrosion resistance of the titanium nitride films. The corrosion behavior of samples in NaCl solutions was studied using polarization curves. Scanning electron microscopy was used to study the corroded samples. It was found that corrosion resistance of coated alloy was higher than the substrate resistance. Furthermore the corrosion resistance is mainly controlled by the presence of defects in the TiN and Ti films.

The relation between the plasma characteristic and the corrosion properties of TiN/Ti multilayers deposited by unbalanced magnetron sputtering

The TiN/Ti multilayers were prepared by balanced magnetron sputtering with a concentric electromagnet coil around the magnetron in order to produce a variable unbalance of the magnetron, and in this way provide changes in the ion bombardment of the substrate. It was observed that the additional magnetic field increased the plasma temperature as well as altering the other plasma characteristics. The field induced changes in the ion bombardment of the substrate and was found to modify the film microstructure. In this work multiple TiN/Ti layers on H13 steel substrates were prepared by reactive magnetron sputtering, with the aim of improving the corrosion resistance. The ion current density incident on the substrate and the plasma parameters, such as, the electron temperature, plasma density, floating potential and plasma potential were studied as a function of the additional magnetic field. The corrosion resistance of these layers was studied by means of potentiodynamic polarisations in a 0.5 M NaCl solution. The composition of the films was determined by ion beam techniques. We report the results of studies of the influence of magnetic field on the plasma parameters and on the improvement of the corrosion resistance of TiN/Ti multilayers.

Vibrational and optical properties of carbon nitride films prepared by reactive magnetron sputtering

Carbon nitride films were prepared by reactive magnetron sputtering of a graphite target in a nitrogen atmosphere at different pressures and temperatures. The films were characterized by infrared, Raman and ultraviolet-visible spectroscopies, as well as by ellipsometry. We obtained hydrogen-free dark films when a negative bias was applied to the substrates; the samples prepared at room temperature without bias were soft and hydrogen could be detected in their infrared spectra. The most prominent features, between 1000 and 1700 cm-1, in both the Raman and infrared spectra were modelled using two Gaussian curves. As the nitrogen content was varied, the ratio between their areas followed opposite trends in the Raman and the infrared spectra. The presence of hydrogen caused the main feature to absorb more strongly at lower frequencies but did not alter the trend. The presence of six and five membered heteroaromatic rings is suggested to explain the increase of the infrared vibrations associated with double conjugated double bonds. The index of refraction can be correlated with features in the infrared spectra, suggesting that its value increases as the number of carbon and nitrogen double bonds increases. Ion bombardment reduces nitrogen incorporation; it is responsible for the densification of the films and the introduction of several defects that make films prepared under bias more absorbing in the low energy region.

Aluminium nitride films prepared by reactive magnetron sputtering

The exceptional properties of aluminium nitride make this material a very promising candidate for a variety of technological applications. In this paper we report our work on the preparation of thin films of aluminium nitride by reactive DC and RF magnetron sputtering. The physical and electrical properties of the films were studied as a function of the preparation conditions: concentration of nitrogen in the reactive gas mixture, the substrate temperature, the plasma power and the horizontal distance from the centre of the target. X-ray diffraction data indicated that highly oriented polycrystalline films could be fabricated. Rutherford backscattering and nuclear reaction analysis showed that nearly stoichiometric films could be prepared using nitrogen concentrations greater than 50%. For the DC plasma process, bombardment of the growing film plays a very significant role. The film growth mechanism was found to be very different for the RF plasma. The nuclear analysis of the films prepared at different lateral distances, together with measurements of the substrate potential, permitted modelling of the film formation process for both the DC and RF plasmas.