Tetrahedral Amorphous Carbon Films Research Articles

Preparation of ultra-thin carbon overcoat for magnetic recording medium by filtered cathodic vacuum arc technology

The deposition process of carbon coating by filtered cathodic vacuum arc (FCVA) system was studied for the development of a high-density magnetic disk with an ultra-thin overcoat of about 2 nm thickness. A FCVA system was installed on disk production equipment for the deposition of carbon thin coating on the disk sample. The FCVA method, processed at a low pressure of 10 −4 Pa, using no reactant gas, had some advantages over the plasma CVD method in deposition of the hard carbon thin coating on an electrically floated substrate. The carbon coating of less than 3 nm in thickness was examined for the various properties with respect to the coating requirements for magnetic recording media. The properties of this ultra-thin coating were the same as those of a relatively thick tetrahedral amorphous carbon (ta-C) film. Moreover, the injection of the energetic ions from the arc source formed a mixture layer on the surface of the magnetic film. Several effective measures were proposed to avoid the deterioration of the magnetic properties.

Space charge limited conduction and electron paramagnetic resonance studies of as grown and nitrogen incorporated tetrahedral amorphous carbon films deposited by pulsed unfiltered cathodic vacuum arc process

This paper reports the space charge limited conduction (SCLC) and electron paramagnetic resonance (EPR) studies of as grown and nitrogen incorporated tetrahedral amorphous carbon (ta-C) films deposited by pulsed unfiltered cathodic vacuum arc process. The effect of varying substrate bias and nitrogen content on the properties of as grown and nitrogen incorporated ta-C films have been studied. The values of density of states ( N( E F)) evaluated from SCLC measurements and spin density ( N s) evaluated from EPR studies of as grown ta-C films deposited at 5 V substrate bias are found to be 1.6×10 19 cm −3 eV −1 and 6.7×10 19 cm −3, respectively, which decrease to 8.7×10 18 cm −3 eV −1 and 4.6×10 19 cm −3, respectively, with the increase of substrate bias up to 80 V and beyond 80 V substrate bias these values are found to increase. A small amount of nitrogen incorporation up to 3.6 at.% nitrogen content in nitrogen incorporated ta-C films reduces the values of N( E F) and N s to 1.4×10 19 cm −3 eV −1 and 4.3×10 19 cm −3, respectively. Beyond 3.6 at.% nitrogen content, the values of N( E F) and N s are found to increase monotonically to 2.6×10 19 cm −3 eV −1 and 1.0×10 20 cm −3, respectively, in nitrogen incorporated ta-C films with further increase of nitrogen content up to 15.6 at.%. The local minimum in the values of N( E F) and N s in as grown ta-C films deposited at 80 V substrate bias arises due to the ability of sp 2 sites to pair up. Nitrogen incorporation up to 3.6 at.% in nitrogen incorporated ta-C films seems to compensate the p-type nature and beyond 3.6 at.% nitrogen content the donor electron increases the values of N( E F) and N s in the films.

Deposition of ta-C:N:H as Function of Experimental Parameters

The incorporation of nitrogen into tetrahedral amorphous carbon films (ta-C:H) was investigated as a function of different gas sources, ion energies and substrate temperatures. Samples were deposited using a highly energetic plasma method: electron cyclotron wave resonance. Composition was measured by nuclear techniques or by XPS. The optical gap was determined by reflection and transmission measurements of samples deposited on quartz substrates and the bonding state was investigated by Raman and infrared (ir) spectroscopy. All the results suggest that tetrahedral bonding is lost, as the nitrogen content increases independently of the deposition parameters used. Structural changes indicated a more polymeric structure with the formation of >C=N, -C≡N and >NH groups.

Bonding configurations in DBOP-FCVA nitrogenated tetrahedral amorphous carbon films studied by Raman and X-ray photoelectron spectroscopies

Nitrogenated tetrahedral amorphous carbon (ta-C:N) films having nitrogen content from 0 to 10.3 at.%, have been produced by a double bend off-plane filtered cathodic vacuum arc system. X-ray photoelectron and Raman spectroscopies have been applied to study the effect of nitrogen in the bonding structure in the films. Deconvolution of the XPS spectra revealed a decrease in the amount of sp 3-bonded carbon in the ta-C:N films. A comparative study between the Raman parameters at 514 and at 633 nm excitation wavelength was presented. Nitrogen (N) incorporation led to a lower G peak position and higher I D/ I G ratios, indicating the development of larger sp 2 domains. Q (−ve) parameter (which measures the skewness of G line) increased sharply with the increased of N content in the films. The hardness of the films was evaluated by nanoindentation.

Microstructure effect on field emission from tetrahedral amorphous carbon films annealed in nitrogen and acetylene ambient

Tetrahedral amorphous carbon (ta-C) films have been deposited by filtered cathodic vacuum arc technique. The samples were then annealed at various temperatures in nitrogen and acetylene ambient. The surface morphologies and microstructure of the films were characterized using atomic force microscopy, scanning electron microscopy, visible and ultraviolet Raman spectroscopy. A thin layer of amorphous carbon was deposited on the surface of the ta-C films after annealed at 700 and 800 °C while submicro crystalline pyrolytic graphite was formed on the surface of the ta-C film annealed at 900 °C. The surface layer was found to enhance the sp 2 clustering of the underlying ta-C layer. Field emission results reveal that the sp 2 cluster size plays an important role in electron field emission properties. The threshold field decreases as the sp 2 cluster size increases. For the film annealed at 800 °C, the lowest threshold field and the largest cluster size concurred.

The microstructure and properties of tetrahedral amorphous carbon films deposited by filtered arc with accelerating at different energetic grades

Tetrahedral amorphous carbon (ta-C) films were deposited on single crystalline silicon by accelerating the species with different impinging energetic grades produced by static-electricity pulse substrate bias from 0 to -2000 V. The microstructure of the ta-C films consist of amorphous sp3 hybridization skeleton enchased with sp2 clusters with sizes less than 1 nm by visible Raman measurement. At low energetic grade, sp3-rich energetic window, and sub-high energetic grade, the more the content of sp3 in the film, the smoother the surface of the film. The relationship between the impinging energy of the species and the surface morphology can be illustrated perfectly in the light of subimplantation growth mechanism. Nevertheless, at high energetic grade, the impinging ions with appropriate energy and angle can sputter and smoothen the surface, even the roughness is lower than the surface of the films with the most sp3. The hardness and Young's modulus of the films deposited at high energetic grade (-2000 V) are higher than those of the films prepared on the floating conditions. At the same time, the critical scratching load of the films deposited with the substrate bias of -2000 V is even larger than the one of the sp3-rich films.

Deposition of carbon films by direct ion beam

In this study diamond-like carbon (DLC) films have been grown onto the Si(111) and steel substrates by direct hydrocarbon (hexane, acetylene) ion beam deposition. Raman spectra of the room temperature deposited carbon films were typical of the hard tetrahedral amorphous carbon films. At 500 and 750 K temperatures, polymer-like carbon films have been synthesized. Hydrocarbon gas dilution by nitrogen resulted in growth of the more graphitic films. Increase of the deposition temperature resulted in increased internal stress in carbon films deposited from hexane–hydrogen gas mixture. Thin film stress decreased as a result of the hexane–hydrogen mixture dilution by nitrogen. Stress decreased with increase of the nitrogen concentration. It has been revealed that DLC films can be deposited by direct ion beam onto the AISI 316 steel at room temperature. Ion beam nitridation was the best method for increase of adhesion between the DLC and steel substrate.

A study of ta-C, a-C:H and Si-a:C:H thin films on polymer substrates as a gas barrier

Two types of diamond-like carbon (DLC) films were grown on Poly (ethylene terepthalate) (PET) substrates have been investigated for density, internal stress, gas permeability and structural properties by an X-ray reflectometry, surface profilometer, Raman spectroscopy and water vapour permeation analysis system, respectively. The high density tetrahedral amorphous carbon (ta-C) films (3.27 g/cm 3) prepared by filtered vacuum cathodic arc (FCVA) showed unexpected high water vapour transmission rate (WVTR) (1.3 g/m 2 day) and a surface covered by a network of deep micro-cracks, which is due to intrinsic stress inside the ta-C films (up to 12 GPa). The soft Si doped hydrogenated amorphous carbon (Si-a:C:H) films prepared by plasma enhanced chemical vapour deposition (PECVD) exhibited low transmission rate (0.03 g/m 2 day) with a water vapour reduction factor up to 98% and a surface almost completely free of micro-cracks. Si incorporation in the a-C:H films reduced both the film density from 2.3 g/cm 3 to 1.85 g/cm 3 and the compressive stress to <0.5 GPa. This could be understood by two possibilities. Firstly, the increasing in the hydrogen content within the films (as indicated by increasing the Raman background slope) developed more polymer–like bonds, which weakens the microstructure. Second, replacing the stronger CC bonds (3.7 eV) by CSi (3.21 eV) bonds where the relaxation of residual stress would occur with large strains in the CSi bonds.

Ultra-thin tetrahedral amorphous carbon films with strong adhesion, as measured by nanoscratch testing

Amorphous carbon can form hard and dense films, which find applications for tooling components, magnetic recording, etc. The challenge is to prepare hard layers that have good adhesion to the substrate. Moreover, mechanical characterisation of sub-20 nm layer is still difficult and semi-quantitative. The paper is addressing these issues. We prepared 10-nm and 50-nm thick tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H) films, respectively, by filtered cathodic arc deposition (FCVA) and plasma enhanced chemical vapour deposition (PECVD). The films were characterised by nanoindentation and nano-scratching. With such hard films, blunting of the diamond tip can affect the measurements and we used a hardness slope ratio protocol, which gave more accurate results. The ta-C films were the hardest and more wear resistant, i.e. for the 50-nm thick ta-C film on Al 2O 3–TiC; H (25 nm)=51 GPa and residual depth=4 nm for a 5 mN scratch load. This is typical of the FCVA conditions, i.e. a relatively high energy, mono-energetic (approx. 25 eV) non-hydrogenated ion flux, resulting in formation of a dense sp 3 network. Ramping the scratch load to observe delamination events, we measured the highest critical loads (approx. 6 mN) again for the ta-C films. We used SEM/EDX thickness measurements to confirm these delaminations. Again, the ta-C films show the best adhesion. We believe that, the FCVA beam is sufficiently energetic to provide a dense and rigid network structure and a good ion beam mixing in the substrate but, appropriately, not enough to produce the large internal stress observed at −100 V substrate bias.

Evaluating the fracture properties and fatigue wear of tetrahedral amorphous carbon films on silicon by nano-impact testing

A repetitive contact technique, nano-impact testing, has been used to investigate the fracture properties of tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The impact test has shown clear differences in the resistance to impact wear of ta-C films with their thickness, for film thicknesses between 5 and 80 nm. The resistance to impact-induced fracture decreases as the film thickness increases. This may be due to the maximum shear stress being closer to the film–substrate interface, or reflect reduced toughness in the thicker films that are less able to deform as the substrate deforms plastically during the repetitive contact test. The mechanism of impact-induced failure on these thin films is (i) the initial impact stage where plastic deformation causes cracks to nucleate sub-surface, (ii) fatigue—further nucleation and growth of sub-surface cracks (with little or no change in probe depth) and (iii) crack coalescence and film fracture leading to a rapid change in probe depth as the film fails. The influence of impact load on fracture probability has been investigated. Fracture probability increases sharply as the impact load is increased from 100 to 300 μN. The greater load provides the stresses necessary to nucleate and propagate the sub-surface cracks; at low load the driving force for the cracks to coalesce and spall the coating is much reduced, so failure is less likely to occur within the test duration.

Dynamic Roughening of Tetrahedral Amorphous Carbon

The roughness of tetrahedral amorphous carbon (ta-C) films grown at room temperature is measured as a function of film thickness by atomic force microscopy, to extract roughness and growth exponents of alpha approximately 0.39 and beta approximately 0-0.1, respectively. This extremely small growth exponent shows that some form of surface diffusion and relaxation operates at a homologous temperature of 0.07, much lower than in any other material. This is accounted for by a Monte Carlo simulation, which assumes a smoothening during a thermal spike, following energetic ion deposition. The low roughness allows ta-C to be used as an ultrathin protective coating on magnetic disk storage systems with approximately 1 Tbit/in.(2) storage density.

Nanoscale manipulation of tetrahedral amorphous carbon films

The outstanding mechanical, chemical and tribological properties of tetrahedral amorphous carbon (ta-C) films have attracted much attention. For a wide variety of applications, a great effort should be focused on the nanoscale structure control. In the present work, we have adopted a novel technique for nanoscale manipulation of ta-C films; incorporating nano Ni dots at the interface between the ta-C film and the substrate. For Ni dot pretreatment, the Ni thin film was deposited and annealed prior to the ta-C films deposition. TEM and Raman spectrum analysis shows that the nano Ni dots at the interface between the film and the substrate results in nanoscale graphitic phase embedded in hard ta-C matrix. Mechanical and electrical properties were strongly dependent on the changes of the Ni-induced second phase. The reduction of mechanical properties and electrical resistivity with increasing the size of Ni dots could be understood in view of the local increase of sp 2 bonds in hard ta-C matrix.

Nanocomposite ta-C films prepared by the filtered vacuum arc process using nanosized Ni dots on a Si substrate

Recently, structural manipulation of tetrahedral amorphous carbon (ta-C) film at the nanometer scale has attracted much attention. We demonstrate a novel method to obtain a nanocomposite film where nanoscale columns of graphitic phase are embedded in a tetrahedral amorphous carbon matrix. When using a Si substrate with nanosized Ni dots on the surface, graphitic columns grew selectively on the Ni dots, while a dense ta-C film was deposited on the bare Si surface. The growth of the graphitic columns is closely related to the nanosized Ni dots that catalyze the graphitic-carbon formation in a filtered vacuum arc deposition condition.

Nano-tip emission of tetrahedral amorphous carbon

Various forms of tetrahedral amorphous carbon were deposited on n-type silicon (100) substrate. Tetrahedral amorphous carbon is an excellent electron emitter for field emission array (FEA) applications. The negative electron affinity (NEA) of diamond surface was believed to facilitate the electron emission for diamond-like carbon. However, in this research, we have demonstrated that the geometric enhancement factor may be even more effective to promote electron emission. Moreover, we have succeeded in depositing high-density (4×10 10 emitters/cm 2) of nano-sized emitters. The electrons are emitted under both the effect of enhanced field due to the sharp tips, and the optimized distortion of tetrahedral bonding of carbon atoms. The sp 3/(sp 3+sp 2) ratio of C–C bonded tetrahedral amorphous carbon films was measured by ESCA and Raman; and the geometry of emitters by AFM. The lowest turn on applied field strength of 4.6 V/μm was found at the current density of 10 μA/cm 2. High reproducibility of field emission was also observed in this study.

Correlation of surface, mechanical and microproperties of tetrahedral amorphous carbon films deposited under different magnetic confinement conditions

The effect of using a magnetic field to confine and focus the carbon plasma in a filtered cathodic vacuum arc (FCVA) deposition system was investigated in the preparation of tetrahedrally bonded amorphous carbon (ta-C) thin films. The design of the magnetic field was such that the plasma can be confined into a high-density focussed spot of ∼2 cm diameter or de-focussed into a wide beam of ∼10 cm diameter. The microstructural, optical, tribological and surface properties of the ta-C films grown in the high-density magnetic field were subsequently studied in detail. Under a high-density magnetic field, ta-C thin films were deposited on Si and quartz substrates. Laser-induced surface acoustic wave (L-SAW) measurements confirmed an increase in the Young’s Modulus and the bulk density of these films as compared to ta-C films deposited under no or low magnetic field [Phys. Rev. B 48 (1993) 4777; J. Appl. Phys. 79 (1996) 7239]. XPS results showed a high >85% sp 3 content on the surface while combined EELS and Raman measurements showed high and constant sp 3 content of >85% in bulk of all deposited films. An increase in optical bandgap was observed, from 3.6 to 3.9 eV as the density of the plasma (and hence, the films) was increased. In addition, the overall surface free energy was also observed to decrease from 44 to 40 dyn/cm. This confirms that under a high external magnetic field, the carbon plasma is confined and focussed, and is thus able to deposit highly densified ta-C thin films with high optical bandgap and low sp 2 defect density. Tribological measurements showed that by comparing the high-density ta-C films with lower density ta-C films, the former have a better adhesion to the substrate, as well as a better coefficient of friction and wear rate.

XPS and XAES studies of as grown and nitrogen incorporated tetrahedral amorphous carbon films deposited by pulsed unfiltered cathodic vacuum arc process

This paper reports the X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) studies of as grown and nitrogen incorporated tetrahedral amorphous carbon (ta-C) films deposited by pulsed unfiltered cathodic vacuum arc process. The effect of varying substrate bias and nitrogen content on the properties of the as grown and the nitrogen incorporated ta-C films has been studied. The XPS spectra indicate that in the as grown ta-C films the C 1s peak occurs at 285.15±0.08 eV and the binding energy (BE) marginally decreases with the increase of substrate bias. The nitrogen incorporated ta-C films show N 1s peak at 399.46±0.10 eV in addition to the C 1s peak. The peak position of C 1s remains unchanged with nitrogen incorporation, whereas the N 1s peak seems to marginally decrease in BE, with increasing nitrogen content. From the values of the parameter D, the energy width between the highest maximum and the lowest minimum value of the derivative of C (KLL) spectra, sp 2, sp 3 and sp 3/sp 2 ratio were evaluated. The values of D, sp 2, sp 3 and sp 3/sp 2 ratio for the as grown ta-C films are found to be 17.16 eV, 35.7%, 64.3% and 1.80, respectively. The values of D and sp 2 fraction are found to decrease to 16.19 eV and 23.9% and those of sp 3 and sp 3/sp 2 ratio are found to increase to 76.1% and 3.18 in the case of ta-C films deposited at an applied substrate bias of 40 V. In the case of applied substrate bias beyond 40 V, the values of these parameters show a reversal in the trend. The nitrogen incorporated ta-C films with 3.6 at.% nitrogen content do not show any change in the values of D, sp 2, sp 3 and sp 3/sp 2 ratio. With further increase in the nitrogen content up to 15.6 at.%, the values of D and sp 2 fraction decrease continuously to 15.87 eV and 20.1% and the values of sp 3 and sp 3/sp 2 ratio increase to 79.9% and 3.98, respectively. The study suggests that, at certain conditions of nitrogen incorporation in the case of ta-C films, there is an increase in sp 3 fraction or the diamond like nature of the ta-C films.

Correlations between substrate bias, microstructure and surface morphology of tetrahedral amorphous carbon films

The microstructure and surface morphology of ta-C films deposited on p-type (1 0 0) single crystal silicon with the substrate negative bias varying from 0 to 2000 V by the filtered cathodic vacuum arc technology have been investigated by means of Raman spectroscopy and atomic force microscope. The optimal deposition process of sp 3-rich ta-C films can be confirmed in light of the relations between the coupling coefficients or full-width at half-maximum and the substrate negative bias. The surfaces of these films are uniform and smooth and RMS surface roughness is less than 0.4 nm. At the lower energetic grades, the more the content of sp 3 is in the film, the smoother the surface of the film is. The dependence of the surface morphology and the impinging energy of the species can be illustrated according to the subimplantation growth mechanism. Nevertheless at the high energetic grade, the impinging ions with appropriate energy sputter and smoothen the surface so that the roughness might be even lower than the one of the films with the richest sp 3 component.

Rapid thermal annealing study on the metal containing amorphous carbon films

The tetrahedral amorphous carbon (ta-C) films deposited by filtered cathodic vacuum arc (FCVA) technique contain very high sp 3 fraction. However, the high compressive stress of the ta-C films causes film delamination and limits its potential applications. Stress reduction by incorporating a small percentage of a metallic element such as Al has been reported. In this work, we investigate the rapid thermal annealing (RTA) of Al containing amorphous carbon (a-C:Al) films. The results are compared with that of ta-C films. The microstructure and surface morphology of the films subjected to different annealing temperature were studied by Raman spectrometer and atomic force microscope (AFM), respectively. Some mechanisms were suggested to explain the stress reduction of the films that were subjected to different annealing temperatures. Complete stress relief was observed when the a-C:Al (5 at.%) films were RTA at 800 °C for duration of 2 min.

Ultrathin amorphous carbon overcoats by filtered cathodic arc deposition

We studied properties of 2- to 25-nm-thick tetrahedral amorphous carbon films produced by filtered cathodic arc deposition. We found that nitrogen doping was effective in controlling film properties. Both the amount of sp/sup 3/ bonding in the film and the hardness decreased with nitrogen incorporation. The adhesion of lubricant to the carbon film was improved by nitrogen doping. Tests of wear and corrosion on a nanometer scale showed the superior tribological and anticorrosion performance of tetrahedral amorphous carbon films over hydrogenated amorphous carbon films.

Deposition of permalloy films by filtered cathodic vacuum arc

The filtered cathodic vacuum arc technique was employed to deposit magnetic films such as FeNi on silicon substrate. Using the normal cathode design, tetrahedral amorphous carbon and metals films can be deposited. If a magnetic target is used, the arc spot always preferably moves to the edge of the target and then extinguishes. Therefore, the arc is not stable and no film can be deposited. A cathode was designed to obtain stable and continuous arc and uniform erosion pattern. The atomic force microscopy, grazing X-ray diffraction and vibrating sample magnetometry were used to characterize the morphological, structural and magnetic properties of the FeNi film. When the film deposited at substrate bias of −100 V, it exhibits a square magnetization–hysteresis loop, the lowest coercivity and the highest saturated magnetization among all the investigated samples, which may be related to its good surface roughness and crystallinity.