Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coatings

Tetrahedral amorphous carbon (ta-C) film was coated on aligned carbon nanotube (CNT) films via filtered cathodic vacuum arc (FCVA) technique. Field electron emission properties of the CNT films and the ta-C/CNT films were measured in an ultra high vacuum system. The I–V measurements show that, with a thin ta-C film coating, the threshold electric field (E_thr) of CNTs can be significantly decreased from 5.74 V/μm to 2.94 V/μm, while thick ta-C film coating increased the E_thr of CNTs to around 8.20 V/μm. In addition, the field emission current density of CNT films reached 14.9 mA/cm² at 6 V/μm, while for CNTs film coated with thin ta-C film only 3.1 V/μm of applied electric field is required to reach equal amount of current density. It is suggested that different field emission mechanisms should be responsible for the distinction in field emission features of CNT films with different thickness of ta-C coating.     

Diamond-like carbon overcoat for TFMH using filtered cathodic arc system with Ar-assisted arc discharge

The deposition system described for sub-30 Å and thicker carbon (ta-C) overcoat that includes two RF ion beam guns and Filtered Cathodic Arc (FCA) module mounted on a single vacuum chamber. The system is capable of flattening the Thin Film Magnetic Heads (TFMH) surface by ion beam etching; smoothing scratches, trenches, steps on boundaries of different materials, and enhancing the adhesion by ion assisted ion beam sputtering. It provides the highly controllable deposition of carbon using an FCA module with Ar-assisted arc discharge. Low-level particulates are achieved on the deposited film surface (< 5/cm² ). It was shown that crucial impact on filtering the particles with size < 1 μm has the electrostatic field distribution across the plasma guide that can be controlled by duct bias. Mechanical and electrical properties, optical and Raman spectra of ta-C films were investigated as a function of Ar flow in the arc discharge area. At Ar flow rates 0–12 sccm, stress of the films was varied in a range 2.9–7.5 GPa while hardness and Young's Modulus stayed in ranges of 45–60 GPa, and 230–300 GPa, respectively. Density of the obtained films was greater than 2.8 g/cm³. Optical absorption and electrical conductivity of ta-C films showed a significant rise while stress came down with Ar flow. Raman G-peak was higher for ta-C films with lower stress and shifted to lower energy. The low stress films versus high stress films showed a few orders reduced electrical resistance and anisotropy of specific resistance with respect to substrate plane: ρ_⊥ ≫ ρ_∥. In situ ellipsometric control of growing film thickness was implemented on the system. Run-to-run standard deviation was less than 1 Å for 20–25 Å thick films. High corrosion resistance of FCA coatings was exhibited. The impact of Ar gas–carbon plasma interaction on the deposition conditions and microstructure of ta-C films was discussed.     

Evolution of compressive stress and electrical conductivity of tetrahedral amorphous carbon films with phosphorus incorporation

Successful modification of stress and conductivity for tetrahedral amorphous carbon (ta-C) films is realized by phosphorus incorporation via filtered cathodic vacuum arc technique with PH₃ as the impurity source. By establishing the structure as a function of phosphorus content, it is found that phosphorus fraction in phosphorus incorporated ta-C (ta-C:P) films increases with varying levels of PH₃ from 3 to 30 sccm, and that all samples retain their amorphous structures without remarkable changes, just exhibiting the clustering of sp² sites and the evolution of structural ordering. Furthermore, the addition of phosphorus causes the compressive stress relaxation in terms of the rearrangement in atomic bonding structures. The increased number of localized electronic π and π⁎ states as hopping sites after phosphorus incorporation results in several orders of magnitude increase in the conductivity, and the films represent the hopping conduction in band tail states in the temperature range of 293–463 K. However, more H induced by excessive PH₃ may saturate some defects and compensate the hopping sites, leading to a slight drop in the conductivity. The nature of ta-C:P films as n-type semiconductors is proved from the features of rectifying current–voltage cures.     

Mechanical and electrical properties of micron-thick nitrogen-doped tetrahedral amorphous carbon coatings

The effect of nitrogen doping on the mechanical and electrical performance of single-layer tetrahedral amorphous carbon (ta-C:N) coatings of up to 1 μm in thickness was investigated using a custom-made filtered cathode vacuum arc (FCVA). The results obtained revealed that the hardness and electrical resistance of the coatings decreased from 65 ± 4.8 GPa (3 kΩ/square) to 25 ± 2.4 GPa (10 Ω/square) with increasing nitrogen gas ratio, which indicates that nitrogen doping occurs through substitution in the sp² phase. Subsequent AES analysis showed that the N/C ratio in the ta-C:N thick-film coatings ranged from 0.03 to 0.29 and increased with the nitrogen flow rate. Variation in the G-peak positions and I(D)/I(G) ratio exhibit a similar trend. It is concluded from these results that micron-thick ta-C:N films have the potential to be used in a wide range of functional coating applications in electronics.     

Properties and tool performance of ta-C films deposited by double-bend filtered cathodic vacuum arc for micro drilling applications

This paper describes the results of the application of ta-C films to micro drilling operation for deep and small machining boreholes. Tetrahedral amorphous carbon (ta-C) films were successfully deposited on WC-Co substrates by a double-bend filtered cathodic vacuum arc (FCVA) system. The structure, mechanical and tribological properties of both pure ta-C and ta-C incorporated argon gas (ta-C:Ar-flow) films were systematically investigated. And then, high-speed through-hole drilling tests were performed on the PCB (printed circuit board) workpiece to investigate the machining performance of ta-C coated micro drills. The experimental results show that the ta-C:Ar-flow (2 sccm) coated micro drill has excellent microstructure, microhardness, and friction coefficient properties and represents the optimal coatings for micro drilling applications.     

The microstructure and properties of energetically deposited carbon nitride films

The intrinsic stress, film density and nitrogen content of carbon nitride (CN_x) films deposited from a filtered cathodic vacuum arc were determined as a function of substrate bias, substrate temperature and nitrogen process pressure. Contour plots of the measurements show the deposition conditions required to produce the main structural forms of CN_x including N-doped tetrahedral amorphous carbon (ta-C:N) and a variety of nitrogen containing graphitic carbons. The film with maximum nitrogen content (~ 30%) was deposited at room temperature with 1.0 mTorr N₂ pressure and using an intermediate bias of − 400 V. Higher nitrogen pressure, higher bias and/or higher temperature promoted layering with substitutional nitrogen bonded into graphite-like sheets. As the deposition temperature exceeded 500 °C, the nitrogen content diminished regardless of nitrogen pressure, showing the meta-stability of the carbon–nitrogen bonding in the films. Hardness and ductility measurements revealed a diverse range of mechanical properties in the films, varying from hard ta-C:N (~ 50 GPa) to softer and highly ductile CN_x which contained tangled graphite-like sheets. Through-film current–voltage characteristics showed that the conductance of the carbon nitride films increased with nitrogen content and substrate bias, consistent with the transition to more graphite-like films.     

Heat treatment of tetrahedral amorphous carbon films grown by filtered cathodic vacuum-arc technique

Tetrahedral amorphous carbon (ta-C) is a potential low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. The thermal stability of tetrahedral amorphous carbon (ta-C) films has been investigated by heat treatment of the films at temperatures from room temperature to 450°C in high vacuum, low vacuum and oxygen ambient. It was found that heat treatment in oxygen ambient leads to a much more prominent variation in film thickness, stress and hardness than in both low and high vacuum. Raman studies also show an increase of the G-band frequency to higher values, an increase of the integrated intensity ratio and a narrowing of the G bands for films annealed in oxygen ambient with increasing temperature. By contrast, ta-C films exhibit a high resistance to degradation during treatment in low and high vacuum. They sustain their structure, thickness, stress and hardness for temperatures up to 400°C.     

Chemical analysis and vibrational properties of boronated tetrahedral amorphous carbon films

Boronated tetrahedral amorphous carbon (ta-C:B) films were prepared by filtered cathodic vacuum arc technique using boron mixed graphite targets. The effect of boron content on the chemical bonding and vibrational properties of these films has been investigated by X-ray photoelectron spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. It has been found that boron atoms are predominantly configured in a graphitic network, while the carbon atoms in the ta-C:B films are mainly in sp³ hybridization which tend to decrease as boron content increases. The Raman and infrared spectra of ta-C:B films both show prominent features in the regions of 1100–1900 cm^− 1 and 900–1600 cm^− 1 respectively. It was identified that the Raman parameters are strongly correlated with the boron content which is due to the clustering of sp² domains induced by B introduction. The activation of infrared spectrum of ta-C:B film is a consequence of heteroatomic (C–B) vibration combined with changes in the sp² carbon configuration. And the enhanced infrared absorption of ta-C:B with increased boron incorporation results from the increased effective charges in the delocalized sp² carbon phase.     

Effect of increasing hardness on Si-containing diamond-like carbon film during tribo-test

In this study, silicon-DLC film has been especially treated by plasma-enhanced chemical vapor deposition (PECVD) process at 500 °C in the same chamber without compound-layer for enhancement of hardness and adhesion. The effects of different levels of silicon content on the silicon-containing DLC films were tested in air condition at room temperature with relative humidity using a ball-on-disk tribometer. After the wear test, Raman spectrum analysis on the tested surface of silicon DLC showed the changed structure on the surface. Especially, it has shown the increasing hardness value in proportional to increase TMS gas rate after wear test. At the same time, it was shown that I_D/I_G values increased higher G-peak values and positions on wear track of silicon-containing DLC surfaces. Therefore, the structure of the coated DLC surface changed between the wear-tested surface and the original surface. High silicon content DLC showed increased I_G value with suddenly increased I_D/I_G value after the wear test.     

Formation and characteristics of undoped and doped tetrahedral amorphous carbon films

Synthesis of undoped and doped tetrahedral amorphous carbon (ta-C) films has been achieved using magnetic field filtered plasma stream system in an ambient gas of pure Ar and Ar with N₂, respectively. The optical and electrical properties of these films as a function of the substrate bias voltages (V_b) or nitrogen partial pressures (P_N) have been studied using UV-visible optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and measurements of electrical conductivity. The results show that ta-C films with a high sp³ fraction were formed when the V_b was in the range of −10 to −50 V. The optical band gap of such ta-C films was found to be larger than 3 eV. The incorporation of nitrogen into the ta-C films deposited at low P_N (P_N<25%), results in a slight drop in activation energy, which indicates that there is evidently some doping effect of nitrogen. The configurations of N atoms in ta-C network are identified and discussed.     

The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance

Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R_a and R_z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the “Skewness” (R_sk) and “Kurtosis” (R_ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R_a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R_a = 10 nm. This phenomenon is explained by analyzing the R_sk and R_ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.     

Comparison of the wear particle size distribution of different a-C coatings deposited by vacuum arc

For biomedical application in the field of artificial hip joints diamond-like carbon (DLC) coatings have been widely studied due to their tribological properties. The wear particles as the main factor limiting the life expectancy of hip joints have attracted more and more interest, not only the number of them, but also the distribution of their size. In this study we have deposited DLC coatings on stainless steel (P2000) by a vacuum arc adjustable from anodic to cathodic operation mode, with the anode–cathode diameter ratio of d_a/d_c = 3/1 at a DC bias of − 250 V to − 1000 V. To improve the adhesion of the DLC coating on P2000, titanium as a metallic interlayer was deposited by cathodic vacuum arc evaporation. The internal structure of the coating was investigated by the visible Raman spectroscopy with the four-Gaussian curve fitting method. Comparing the results with the previous work (coatings deposited with d_a/d_c = 1/1), it was found that the anode–cathode diameter ratio has an effect on the structure (e.g. I_D/I_G) as well as the wear particle size distribution. It was shown that the maximum of the frequency distribution e.g. at − 1000 V bias can be shifted to below 1 μm with increasing d_a/d_c.     

Preparation of tetrahedral amorphous carbon films by filtered cathodic vacuum arc deposition

Tetrahedrally bonded amorphous carbon (ta-C) and nitrogen doped (ta-C:N) films were obtained at room temperature in a filtered cathodic vacuum arc (FCVA) system incorporating an off-plane double bend (S-bend) magnetic filter. The influence of the negative bias voltage applied to substrates (from −20 to −350 V) and the nitrogen background pressure (up to 10⁻³ Torr) on film properties was studied by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, X-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and X-ray reflectivity (XRR). The ta-C films showed sp³ fractions between 84% and 88%, and mass densities around 3.2 g/cm³ in the wide range of bias voltage studied. In contrast, the compressive stress showed a maximum value of 11 GPa for bias voltages around −90 V, whereas for lower and higher bias voltages the stress decreased to 6 GPa. As for the ta-C:N films grown at bias voltages below −200 V and with N contents up to 7%, it has been found that the N atoms were preferentially sp³ bonded to the carbon network with a reduction in stress below 8 GPa. Further increase in bias voltage or N content increased the sp² fraction, leading to a reduction in film density to 2.7 g/cm³.     

Improved wear imbalance with multilayered nanocomposite nanocrystalline Cu and tetrahedral amorphous carbon coating

Tetrahedral amorphous carbon (ta-C) has a relatively high hardness, and it can be used to enhance film properties such as wear resistance. However, the high hardness of ta-C can adversely affect a counterpart and accelerate its wear, and the resulting wear imbalance between the film and its counterpart can cause vibrations. This issue may be resolved by improving the wear of the counterpart. This study aimed to reduce the hardness and improve the fracture toughness of ta-C films to enhance the durability of a tribosystem, which was achieved by toughening a composite and ductile phase. A multilayered nanocrystalline (nc)-Cu/ta-C nanocomposite film was fabricated that allowed for reductions in the wear of the film and its counterpart of more than 88% and 99%, respectively.     

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm³) and sp³ content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Young's modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Young's modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm², producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.     

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 sp² clustering of the underlying ta-C layer. Field emission results reveal that the sp² cluster size plays an important role in electron field emission properties. The threshold field decreases as the sp² cluster size increases. For the film annealed at 800 °C, the lowest threshold field and the largest cluster size concurred.     

Diamond like carbon used as antireflective coating on crystalline silicon solar cells

Amorphous carbon with different structures was used as antireflective coating on crystalline silicon solar cells. Polymeric-like carbon (PLC) and diamond-like carbon (DLC) were deposited by the PECVD technique on the anode and cathode electrode, respectively. Tetrahedral-like carbon (ta-C) was deposited by the filtered cathodic vacuum arc (FCVA). An increase in the short circuit current comparable to that obtained by conventional antireflective coating (SnO₂) was obtained using PLC antireflective coating. The effect on the short circuit current of the other structures (DLC and ta-C) is reduced mainly due to the band gap and/or a mismatch on the index of refraction of the film and the crystalline silicon substrate.     

Structure and optical properties of Cu-DLC composite films deposited by cathode arc with double-excitation source

Copper-doped diamond-like carbon films (Cu-DLC) were fabricated on silicon and quartz substrates by cathode arc technique with direct-current and pulse double-excitation source. The microstructure, composition, morphology, hardness and optical properties of the films were studied by Raman, XPS, AFM and SEM, UV-Vis, laser ellipsometer and Vickers sclerometer. The results showed that Cu doping increases the size, ordering and amount of sp²-C clusters in the Cu-DLC composite films. The microstructure parameters enhance with increasing the Cu content to 22.4 wt%. All the films show specific nano-structural surface, however, the lower Cu content induces finer particle formation in the Cu-DLC films. When the arc current is higher than 60 A, the roughness and particle size of Cu-DLC composite films increase with increasing the Cu content. The average transmittance of the Cu-DLC films in Vis-NIR region is smaller than 40% when the Cu content exceeds 12.6 wt%. With increasing the Cu content, the optical band gap (E_g) of the films decreases from 3.54 eV to 0.25 eV. The relationships among the E_g, refractive index and extinction coefficient for the Cu-DLC films were found and indirectly revealed the correlation of microstructure and optical properties of the films with the Cu content variation.     

Effect of nitrogen pressure on the microstructure,mechanical and electrochemical properties of CrAlN coatings deposited by filter cathode vacuum arc

CrAlN coatings were prepared on Al–Si alloys using filter cathode vacuum arc deposition technique with nitrogen as the reactive gas and Cr₂₅Al₇₅ alloy target as the arc source. The effect of nitrogen pressure on the microstructure, mechanical properties and electrochemical properties of the coatings had been systematically studied. The results showed that the composition, structure and performance of the CrAlN coating depended on the nitrogen pressure. As the nitrogen pressure increased, the Al and Cr content decreased while the N content increased slowly in the coating. Meanwhile, the phase structure gradually changed from AlN phase to CrN phase. The hardness of the CrAlN coating increased significantly with the increase of nitrogen pressure from 0.04 to 0.06 Pa due to the formation of CrN phase and grain refinement. However, further increasing the nitrogen pressure to 0.07 Pa, the hardness was reduced owing to the deterioration of the surface quality caused by target poisoning. Moreover, the adhesion strength of the coating gradually decreases, and the corrosion resistance of the CrAlN coating first increased and then decreased with increasing the nitrogen pressure. The CrAlN coating deposited at a nitrogen pressure of 0.05 Pa had the best corrosion resistance, with the highest polarization resistance, charge transfer resistance and pore resistance, which was related to the combined effect of great compactness and AlN-dominant phase structure in the coating.     

Techniques for filtering graphite macroparticles in the cathodic vacuum arc deposition of tetrahedral amorphous carbon films

Filtered cathodic vacuum arc (FCVA) deposition has been found to be a reliable technique for the production of high quality tetrahedral amorphous carbon films (ta-C). These coatings can be used as protective coatings for different applications ranging from cutting tools to human hip joint prosthesis. The FCVA technique is widely used in different laboratories around the world with somewhat different technical implementations. A serious disadvantage in the FCVA technique is the graphite particles that are emitted from the solid graphite cathode during the arc-discharge. A variety of different techniques exist to diminish their production and transport. However, some of the magnetic filtering designs that are used to reduce the macroparticle transport into the substrate do not work well with high melting point cathode materials such as graphite. Although the influence of graphite particles for the ta-C coating performance in some applications is controversial, many applications demand that the produced ta-C film is practically particle-free. This is especially important in corrosion resistance, electrical and optical applications. In this paper an introduction to different FCVA devices is presented. Different magnetic filtering designs together with control techniques for macroparticle generation have been reviewed and their advantages and disadvantages in the plasma transport and particle filtering efficiency have been discussed.