Amorphous Diamond-like Carbon Films Research Articles

Solid-State NMR Studies of the Bonding Structure of Diamondlike Amorphous Carbon Films

ABSTRACTAmorphous carbon films are (a-C:H) of interest because of their useful physical properties. They are extremely hard and chemically inert, resisting degradation by both acids and alkalis. They are insoluble and can be conformably coated onto virtually any substrate. These properties make the films ideal protective coatings on magnetic disks and tools. We have studied several thin (one to two micron) films prepared by plasma enhanced chemical vapor deposition with varying radiofrequency fields strengths to determine structural differences at the atomic level. Several properties of the films, such as hardness and wear rate, are dependent on deposition power. We have found that the sp2/sp3 ratio increases with increasing deposition power. Thus, films that are harder are more “graphitic” and less “diamondlike”. The films studied here contain 11–16 atomic percent hydrogen, most of which is associated with sp3 carbon sites. At least two distinct phases of hydrogens exist. Variable temperature studies reveal that, in contrast to amorphous hydrogenated silicon, proton linewidths in carbon films are temperature dependent, suggesting some molecular motion is present at room temperature.

Pulsed-Laser Deposition and Characterization of Amorphous Diamondlike Carbon Films

ABSTRACTUnhydrogenated amorphous carbon films were deposited by KrF pulsed laser ablation of graphite under a laser power density of 8×108 W/cm2. The films were characterized using x-ray reflectivity, optical transmittance, spectroscopic ellipsometry and Raman spectroscopy. The physical, optical and bonding properties of the films have been investigated in detail as functions of substrate temperatures Ts (22-300°C) and post-annealing temperatures. Films deposited at Ts, <200°C possessed diamond-like properties with mass densities p=2.8-3.1 g/cm3 and optical bandgaps Eg,=1.5-2.2 eV. Above Ts,=200°C, the films showed typical graphite characteristics. The diamond-like films annealed at temperatures up to 750°C show excellent thermal stability.

Surface Structure of Tetrahedral-Coordinated Amorphous Diamond-Like Carbon Films Grown by Pulsed Laser Deposition

ABSTRACTThe structure and composition of tetrahedral-coordinated amorphous diamondlike carbon films (a-tC) grown by pulsed laser deposition (PLD) of graphite has been studied with atomic force microscopy (AFM). The nanometer-scale surface structure has been studied as a function of growth parameters (e.g., laser energy density and film thickness) using contact-mode and tapping-mode AFM. Although the surfaces were found to be generally smooth, they exhibited reproducible structural features on several size scales which correlate with the variation of laser energy and the effects of excited ion etching.

Structure and properties of amorphous diamond-like carbon films produced by ion beam assisted plasma deposition

The microstructure and tribological properties of carbon film produced by ion beam assisted plasma deposition in a plasma source ion implantation (PSII) chamber with energies varied from 0 to 30 keV are examined. The process is illustrated schematically, and Raman spectra as well as TEM images and corre-sponding diffraction patterns of carbon films are shown.

DLC films by plasma assisted chemical vapor deposition near room temperature

This work concentrates on the preparation of diamond-like amorphous carbon films using a d.c. glow discharge with and without coupling to a microwave source. Films were deposited on single crystal silicon, glass and several plastics at temperatures between room temperature and 150°C. A variety of carbon containing feed gases have been used at pressures ranging from few milliTorrs to 1 Torr with an applied electric field up to 3300 V in −1. The films obtained range from transparent to semi-transparent and can be very hard as observed by an ASTM scratch tester. The visible, IR and Raman spectra of the films have been obtained, and these are related to their structure and hardness. The DLC films grown on plastics were found to suffer from deformations due to the flexibility of the underlying material, and hence their performance was judged to be less than ideal despite the hardness of the overlying material. The results are discussed in terms of chemical composition and growth kinetics of the films.

Pulsed laser deposition of amorphous diamond-like carbon films with ArF (193 nm) excimer laser

We have deposited hydrogen-free diamond-like amorphous carbon films by ArF (193 nm) pulsed laser ablation of graphite. The deposition process is performed with a laser power density of only 5 × 108 W/cm2 at room temperature without any auxiliary energy source incorporation. The resulting films possess remarkable physical, optical, and mechanical properties that are close to those of diamond and distinct from the graphite target used. The films have a mechanical hardness up to 38 GPa, an optical energy band gap of 2.6 eV, and excellent thermal stability. Analysis of electron energy loss spectroscopy reveals the domination of diamond-type tetrahedral bonding structure in the films with the sp3 bond fraction over 95%. Compared with other reported results of pulsed-laser-deposited diamond-like carbon films, our experimental results confirm that the laser wavelength or photon energy plays a crucial role in controlling the properties of the pulsed-laser-deposited diamond-like carbon films.

Structure, dynamics, and electronic properties of diamondlike amorphous carbon.

An amorphous carbon network generated by quenching high-density high-temperature liquid carbon using tight-binding molecular-dynamics simulation is found to contain a large fraction of tetrahedral bonding sites. Investigations of the structural, vibrational, and electronic properties of the theoretical model provide fresh insights into the microscopic structure of diamondlike amorphous carbon films produced in ion-beam deposition experiments.

Optical and electronic properties of amorphous diamond

The optical and electronic properties of highly tetrahedral amorphous diamond-like carbon (amorphous diamond, a-D) films were investigated. The structure of the films grown on silicon and glass substrates, under similar deposition conditions using a compact filtered cathodic vacuum arc system, are compared using electron energy loss spectroscopy (EELS). Results from hydrogenation of the films are also reported. The hydrogenated films show two prominent IR absorption peaks centred at 2920 and 2840 cm −1, which are assigned to the stretch mode of the CH bond in the sp 3 configuration on the CH 3 and CH sites respectively. The high loss EELS spectra show no reduction in the high sp 3 content in the hydrogenated films. UV and visible transmission spectra of a-D thin films are also presented. The optical band gap of 2.0–2.2 eV for the a-D films is found to be consistent with the electronic bandgap. The relationship between the intrinsic compressive stress in the films and the refractive index is also presented. The space charge limited current flow is analysed and coupled with the optical absorption data to give an estimate of 10 18 cm −3 eV −1 for the valence band edge density of states.

Microhardness and Young’s modulus of diamond and diamondlike carbon films

The microhardness, H, and Young’s modulus, E, of a polycrystalline diamond film and several amorphous diamondlike carbon (DLC) films were determined from force-displacement curves obtained using an ultralow-load microhardness instrument (UMIS-2000). Measurements were made at a constant loading rate of 3 mN/s, to a maximum applied force of 67 and 100 mN with contact force of 0.06 and 1.07 mN, respectively. The diamond film had a surface morphology typical of microwave plasma chemical vapor deposition films (crystallite size 0.5–3 μm), and the force-displacement curves showed nearly complete elastic behavior. The average values of hardness (80–100 GPa) and modulus (500–533 GPa) are comparable to those of natural (001) diamond reference standards (H=56–102 GPa, E=1050 GPa). The DLC films were prepared by low-energy ion-assisted unbalanced magnetron sputtering. By varying the bombarding ion energy, five films were prepared having different sp3/sp2 bonding ratios (3–6), optical gaps (1.2–1.6 eV), and hydrogen concentrations (4–20 at %). The force-displacement measurements are characterized by substantial elastic recovery, and individual films show a very narrow range of hardness and modulus values. It is found that high hardness and improved modulus in DLC films correlate with increasing ion energy, sp3/sp2 bonding ratio, and energy gap. Individual films have mean values of hardness and elastic modulus in the range 12–30 GPa and 62–213 GPa, respectively.

Diamond-like amorphous carbon films prepared by r.f. sputtering in argon: electron-assisted investigations

We reliably verified by various electron-assisted methods (electron probe microanalysis, scanning electron microscopy, cross-sectional transmission electron microscopy (XTEM), high resolution XTEM, TEM, radial distribution function, electron energy loss spectroscopy) the impurities, morphology and atomic structure which are important for the macroscopic film behaviour of diamond-like carbon films r.f. sputtered in argon at different target voltages, cooled and uncooled. The relatively smooth surface morphology of our low nitrogen-1 and argon-contaminated films depends on target voltage and substrate temperature. Until now, the influence of up to 4.2 at.% oxygen impurities on our resulting solid state has not been clear. We find in comparison with earlier investigations of the physical film properties for our low hydrogen, hard, IR-transparent amorphous carbon films a dominating film structure of irregular cross-linked regions of sp 2-hybridized carbon predominantly arranged in six-membered rings.

Diamond-like amorphous carbon films prepared by r.f. sputtering in argon

In depositing thin carbon films by the r.f. sputtering of a graphite target in argon, an optimal parameter window was found in which it is possible to prepare adherent, IR-transparent, insulating, hard layers. Approximations for the sputtering process (total sputtering yield, energy of the sputtered carbon particles on the substrate) are demonstrated. A characteristic dependence of the film properties on the target voltage, an essential sputtering parameter, exists. In all cases, the target voltage influences the bombardment of the growing film by charged energetic particles. An anisotropic layer behaviour was established for more strongly absorbing films by three different methods of investigation. Finally, preliminary considerations concerning the film structure were made.

The hydrogen concentration of diamondlike amorphous carbon films by elastic recoil detection analysis

By using elastic recoil detection analysis, the hydrogen concentration of amorphous carbon films prepared by ion beam deposition was determined. The dependence of the hydrogen concentration on the deposition parameters (energy of ion beam, beam current density, and ratio of hydrogen to methane in gas mixture) was observed for these films. It seems clear that the diamond cubic structure in a-C:H films increases monotonically with decreasing hydrogen concentration.

Hardness and Young's modulus of diamond-like carbon films prepared by ion beam methods

Amorphous diamond-like carbon films were prepared in a dual ion beam facility equipped with two Kaufmann sources of 3 cm diameter and of a maximum beam voltage of 1500 eV. Films were prepared: (i) by sputtering carbon atoms from a graphite target and assisting the deposition process with an Ar + and H + ion beam of variable mixture and energy; (ii) by direct deposition using methane as the working gas without the assistance of a second ion beam. Hardness and Young's modulus of the films were determined with a depth-sensing ultralow-load nanoindentation method. The atomic hydrogen content was determined by FTIR spectroscopy from the integrated absorbance of the CH stretching modes. The values were absolutely calibrated by ERD measurements. The sp 3 sp 2 bond ratio of hydrogen atoms was determined from an analysis of the individual components of the CH stretching modes. The chemical composition of the films was determined by RBS. Films of atomic hydrogen content, x < 5% displayed values for hardness up to 57 GPa and Young's modulus up to 335 GPa. It was observed that hydrogen content is the most significant parameter for hardness, Young's modulus, density and and the sp 3 sp 2 ratio, whereas the energy, E A, supplied to a condensing C-atom plays a major role only for x < 5%.

Synthesis and characterization of diamond-like carbon films for optical and mechanical applications

Diamond-like carbon (DLC) films were directly deposited onto several substrates utilizing the ion beam deposition technique. By impacting high kinetic energy (1000 eV) of high molecular weight hydrocarbon ions onto the substrate, adherent, amorphous, high density and transparent DLC films were produced. Cross-sectional transmission electron microscopy demonstrates that the DLC films are homogeneous, free of pinholes and produce no damage to the silicon substrate. The hydrogen content of the DLC films is 30% and is free from other impurity contamination. These films are hard (Knoop hardness 1548 ± 200 kg mm -2), and resistant to organic solvents, mineral acids, extreme temperature cycling and moisture attack. The friction coefficient of the DLC films in the beginning of the experiment is 0.2, and then rapidly decreases to 0.1 with increasing sliding distance. An increase in humidity results in a slight increase in friction coefficient, but a significant decrease in wear factor. The optical band gap is 1.0 ± 0.1 eV and films are transparent in the IR region. The refractive index at 10.6 μm is 1.91 ± 0.02, and the absorption coefficient is 189 ± 10 cm -1. Raman spectra demonstrate that amorphous DLC films change to graphitic carbons upon rapid thermal annealing above 600 °C and also upon high ion energy with excess fluence and high power density laser irradiation by losing hydrogen in the films.

Characterization of chemical bonding and physical characteristics of diamond-like amorphous carbon and diamond films

Diamond-like (amorphous) carbon (DLC) films were prepared by dc magnetron sputtering and plasma enhanced chemical vapor deposition (PECVD) and diamond films were prepared by microwave plasma enhanced chemical vapor deposition (MPECVD). For the first time, chemical and mechanical characterization of the films from each category are carried out systematically and a comparison of the chemical and physical properties is provided. We find that DLC coatings produced by PECVD are superior in microhardness and modulus of elasticity to those produced by sputtering. PECVD films contain a larger fraction of sp3-bonding than the sputtered hydrogenated carbon films. Chemical and physical properties of the diamond films appear to be close to those of bulk diamond.

Physical and tribological properties of rapid thermal annealed diamond-like carbon films

This paper reports the physical characterization and tribological properties of diamond-like carbon (DLC) and rapid thermal annealed (RTA) DLC films. Amorphous DLC films were deposited by the direct ion beam deposition technique using methane ions. Rapid thermal annealing (RTA) of the films was performed at several fixed temperatures (350, 400, 500, 600, 700, 800, 900 and 1000°C) in a nitrogen atmosphere for 2 min. The films were then characterized by proton recoil detection analysis for hydrogen content, Rutherford backscattering for carbon content and Raman spectroscopy for structural change. The hydrogen content of the DLC films remains constant at 30 at.% in the RTA temperature range 25–500°C, decreases rapidly from 25% to 9% in the range 600–800°C and finally remains at 5% in the range 900–1000°C. The Raman spectra of the DLC films do not change in the RTA temperature range 25–400°C and begin to show the appearance of crystalline graphitic carbon above 500°C. The coefficients of friction of the DLC films do not change significantly with the RTA process; however, the wear factors are strongly dependent on the annealing temperature and the moisture in the environment.

Deposition of diamond-like carbon films by low energy ion beam and d.c. magnetron sputtering

Amorphous diamond-like carbon films were prepared using low energy (1.0–1.25 keV) ion beam plasma of hydrocarbons (methane and n-butane) and by d.c. magnetron sputtering of a graphite target in argon plasma. Optical measurements of the refractive indices n and k in the photon energy 1.0–4.0 eV are reported. The optical properties of sputtered films shows a dependence on sputtering power. The optical gap E opt for sputtered films was found to be 0.78 eV and the gap increased to 1.27 eV for ion-beam-deposited films. Knoop hardness was determined for different loads, and it varied from 1000–2500 kgf mm -2. Internal stress measurements were also performed. Films prepared by magnetron sputtering were found to have high compressive stress which depended on the partial pressure of the sputtering gas. A typical value of the internal stress in ion beam films prepared from butane plasma was 2.0 × 10 9 N m -2. The effective number of valency electrons per carbon atom n eff, was calculated and the ratio of fourfold sp 3 to threefold sp 2 coordination of carbon atoms was found to be 8.08. The application of these films as a corrosion resistant coating and a single layer antireflection coating on germanium for 10.6 μm wavelength was demonstrated.

Brillouin light scattering from the Rayleigh surface phonon of thermally annealed a-C:H films

Brillouin light scattering experiments are performed on amorphous diamond-like carbon films (a-C:H). Inelastic scattering from the Rayleigh surface phonon is found. Young's modulus, the shear modulus, bulk modulus, and microhardness of a-C:H films annealed at various temperatures are evaluated combining light scattering and indentation techniques.

Vapor deposition processes for amorphous carbon films with sp3 fractions approaching diamond

Trends in recently reported data on high sp3 fraction (up to 85%), nonhydrogenated amorphous diamond-like carbon films deposited by ion beam sputtering and laser vaporization are examined. The degree of diamondlike film character is found to depend upon the deposition technique as well as the substrate temperature and thermal diffusivity. The data suggest that the combination of incident particle kinetic energy and surface accommodation determine the physical properties of the resultant film. A model is proposed for the condensation of energetic carbon atoms into diamondlike films in which a quench-type surface accommodation mechanism is operative.

Hard diamond-like carbon films prepared by various physical vapour deposition techniques

Amorphous diamond-like carbon (DLC) films were prepared both by dual-ion-beam sputtering (DIBS) and by ion-beam-assisted d.c. magnetron sputtering (IBAM) of a graphite target under an argon atmosphere. The growing film was bombarded with 50–300 eV inert gas ions. Homogeneous, adherent films up to 0.8 μm thick were deposited on glass, steel and silicon substrates at a highest deposition rate of 5 nm min −1 observed with IBAM. This technique, as compared with DIBS, allowed the ion energy per deposited carbon atom to vary in a larger range and achieved a reduced stress level. The film properties were carefully examined and related to the process parameters in order to determine the growth conditions favouring DLC films with respect to poor quality carbon ones. The hardness reached 40 000 MPa and the morphology was very compact. The inert gas concentration rose from 1 to 2.5 at.% with film bombardment. The low hydrogen content (0.5–1 at.%) measured for DIBS films was increased to 10 at.% with IBAM. The density was strongly dependent on the ion bombardment parameters (ion energy, nature and flux). It ranged from 1.6 to 2.7 g cm −3 for the DIBS films and from 1.4 to 2.3 g cm −3 for the IBAM layers. Ball-on-disc tests performed in a laboratory environment pointed out a low friction coefficient varying from 0.08 to 0.1. Furthermore, the ion bombardment raised the optical indices.