Spectroscopic study on the evolution of graphite ablation plume in ECR argon plasma during the deposition of diamond-like carbon films

A spectroscopic study of carbon plume produced by pulsed laser ablation of a graphite target in argon plasma generated by electron cyclotron resonance (ECR) microwave discharge has been carried out in an attempt to clarify the processes involved in ECR argon plasma assisted pulsed laser deposition of diamond-like carbon (DLC) films. Optical emission spectroscopy measurements show that the plume exhibits different behaviors in its space distribution and time evolution when produced and expanding in argon plasma compared with the plume in vacuum or low-pressure argon ambient. In vacuum or low-pressure argon ambient, lines from carbon atoms and ions dominate the plume emission. In argon plasma, the emission evolves from consisting of lines from carbon atoms and ions to being dominated by bands from dimer C₂, with the strong C₂ bands governing the plume emission at the later stages of the plume expansion. The differences in the plume emission reveal the different gaseous species composing the plumes expanding in different environments, suggesting the different gaseous precursors for film deposition by PLD with or without the assistance of argon plasma. It was also found that the emissions of C₂ Swan bands and C₂ Deslandres D'Azembuja bands exhibit different space distribution.     

Spectroscopic study on pulsed laser ablation of graphite target in ECR nitrogen plasma for carbon nitride film deposition

The influence of nitrogen plasma generated by electron cyclotron resonance (ECR) microwave discharge on deposition of carbon nitride films by means of ECR plasma assisted reactive pulsed laser deposition was investigated by a comparative study on the optical emission of the plume produced by pulsed laser ablation of a graphite target in ECR nitrogen plasma, in vacuum and in low-pressure nitrogen gas ambient. Spatial and temporal spectroscopic measurements show that in vacuum optical emission lines originating from carbon atoms and ions dominate the plume emission, while in low-pressure N₂ ambient weak CN emissions appear. In nitrogen plasma, the plume emission exhibits itself very differently. It evolves from consisting of emissions almost from mono-atomic carbon atoms and ions to being dominated by emissions from CN molecules. The presence of the reactive nitrogen plasma greatly enhanced the CN emissions and eliminated the emission lines from carbon atoms and ions. The appearance of the CN emissions occurs after the emissions from carbon atoms and ions have decayed considerably, indicating that the dominant mechanism for the formation of CN molecules is gaseous phase reaction. Carbon nitride films with nitrogen content of about 40 at.% were obtained. Possible processes for CN molecule formation in gaseous phase and mechanisms responsible for efficient nitrogen incorporation and carbon nitride film deposition were suggested.     

Deposition of amorphous carbon films from C60 fullerene sublimated in electron beam excited plasma

C₆₀ fullerene clusters are used as a carbon source for amorphous carbon films deposition in an electron beam excited plasma. C₆₀ clusters are sublimated by heating a ceramic crucible containing the C₆₀ powders up to 850 °C, which is located in a highly vacuumed process chamber. The sublimated fullerene powders are injected to the electron beam excited argon plasma and dissociated to be active species that are propelled toward the substrates. Consequently, the carbon species condense as a thin film onto the negatively biased substrates that are immersed in the plasma. Deposition rates of approximately 1.0 μm/h and the average surface roughness of 0.2 nm over an area of 400 μm² are achieved. Decomposition of the C₆₀ fullerene after injecting into the plasma is confirmed by optical emission spectroscopy that shows existence of small carbon species such as C₂ in the plasma. X-ray diffraction pattern reveals that the microstructure of the film is amorphous, while fullerene films deposited without the plasma show crystalline structure. Raman spectroscopic analysis shows that the films deposited in the plasma are one of the types of diamond-like carbon films. Different negative bias voltages have been applied to the substrate holder to examine the effect of the bias voltage to the properties of the films. The nano-indentation technique is used for hardness measurement of the films and results in hardness up to about 28 GPa. In addition, the films are droplet-free and show superior lubricity.     

Etching process of hydrogenated amorphous carbon (a-C:H) thin films in a dual ECR–r.f. nitrogen plasma

Hydrogenated amorphous carbon (a-C:H) films deposited from CH₄ in a dual electron cyclotron resonance (ECR)–r.f. plasma were treated in N₂ plasma at different r.f. substrate bias voltages after deposition. The etching process of a-C:H films in N₂ plasma was observed by in situ kinetic ellipsometry, mass spectroscopy (MS), and optical emission spectroscopy (OES). Ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the etched film surface. XPS analysis proves that the nitrogen treatment on the a-C:H film, induced by r.f. substrate bias, causes a direct nitrogen incorporation in the film surface up to 15–17 at.% to a depth of about 20–40 Å depending on the r.f. bias. Various bonding states between carbon and nitrogen, such as tetrahedral sp³ C–N, and trigonal sp² C–N were confirmed by the deconvolution analysis of C 1s and N 1s core level spectra. The evolution of etching rate and the surface roughness in the film measured by AFM exhibit a clear dependence on the applied r.f. bias. MS and OES show the various neutral species in the N₂ plasma such as HCN, CN, and C₂N₂, which may be considered as the chemical etching products during the N₂ plasma treatment of a-C:H film.     

On the effect of treating poly(acrylic acid) with argon and tetrafluoromethane plasmas: Kinetics and degradation mechanism

Poly(acrylic acid) (PAAc) films were treated with either an argon or a tetrafluoromethane (CF₄) plasma and subsequently analyzed with X-ray photoelectron spectroscopy (XPS). PAAc films were decarboxylated during both types of plasma treatments. In addition, during the CF₄ plasma treatment, the PAAc films became fluorinated. The plasma phase during the argon plasma treatment of PAAc films was investigated with optical emission spectroscopy. It was shown that during this plasma treatment carbon dioxide, water, and possibly hydrogen were liberated from the PAAc surface. By covering the surface of PAAc films with different materials (lithium fluoride, UV fused silica, and glass) during the plasma treatment, it was possible to differentiate between photochemically induced and particle-induced changes of the surface. This method was used to show that decarboxylation during the argon plasma treatment was caused by vacuum UV radiation (wavelength < 150 nm) and the decarboxylation/fluorination during the CF₄ plasma treatment was induced by reactive fluorine-containing species from the plasma phase. Furthermore, during both processes, etching of the PAAc surface occurred. Based on these mechanisms, kinetic models were derived that could be used to describe the measured kinetic data adequately. © 1994 John Wiley & Sons, Inc.     

The effect of argon on the electron field emission properties of α-C:N thin films

Amorphous carbon nitride (α-C:N) thin films were synthesized on silicon as electron emitters by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system in which a negative dc bias was applied to the graphite substrate holder and a mixture of C₂H₂ and N₂ was used as precursors. The addition of Ar combined with the application of a negative dc bias can increase nitrogen content (N/C) measured by X-ray photoelectron spectroscopy (XPS), eliminate the dangling bonds in the film determined by Fourier transform infrared (FTIR) spectroscopy, decrease the film thickness measured by field emission scanning electron microscope (FE-SEM), increase the film roughness measured by atomic force microscope (AFM) and raise the graphitic content examined by Raman spectroscopy. The result shows that the onset emission field of α-C:N with Ar addition to the precursors can be as low as 4.5 V μm⁻¹ compared with 9.5 V μm⁻¹ of the film without the addition of Ar.     

Electrochemical characteristics of amophous carbon coated silicon electrodes

The properties of carbon films deposited by the radio frequency plasma sputtering of a fullerene C₆₀ target were investigated to elucidate the dependence on the plasma power. A radio frequency argon plasma power ranging from 50 to 300W at a pressure of 1.3 Pa was applied for sputtering. This corresponds to a self-bias potential on the target ranging from −95 to −250 V and a maximum argon ion energy ranging from 240 to 575 eV. The analysis of the G and D peaks in the Raman spectra shows that the films are similar to tetragonal hydrogenated amorphous carbon annealed at 600–1,000 °C. The electron band structure of the carbon films deposited by the sputtering of C₆₀ depends on the plasma power. The coating effect of these carbon films on the capacity performance of the silicon film electrode of lithium secondary batteries was significant in our experimental range. An electrochemical test revealed that such carbon thin film on the silicon electrode plays an important role in mitigating the capacity fading during the charge and discharge processes. The test revealed that the film formed at a plasma power of 300 W is the most effective.     

Duplex DLC coatings fabricated on the inner surface of a tube using plasma immersion ion implantation and deposition

A duplex plasma immersion ion implantation and deposition (PIIID) process, involving carbon ion implantation and diamond-like carbon (DLC) deposition, is proposed to modify the inner surface of a tube. In the research, samples of GCr15 bearing steel were placed inside a tube in the vacuum chamber. After the vacuum chamber was evacuated to a base pressure of 6 × 10^− 3 Pa, C₂H₂ gas was introduced into the chamber, and the tube was biased by a negative pulsed bias. Since a pulsed glow discharge (PGD) plasma can be formed by the bias, carbon ion implantation and DLC film deposition process can be obtained by biasing the tube with a high and low bias, respectively. To synthesize different DLC films, single PIIID processes employing a low voltage (several kV) PGD method and duplex PIIID processes combining the high (several tens kV) and low voltage PGD techniques were carried out. The as-synthesized films were characterized by Raman spectrum, nano-indentation, scratch, tribological and electrochemical tests. Raman results show that duplex DLC films were synthesized by this duplex PIIID process. In addition, compared with the single DLC film synthesized by the low voltage PGD process, the duplex DLC films can obtain a high wear and corrosion resistances. Furthermore, using this duplex PIIID method, batch treatment of outer-rings of the bearing was realized.     

Surface morphology and evolution of amorphous carbon thin films

The surface morphology of disordered carbon films grown by nanosecond pulsed laser ablation of graphite is reviewed. It is shown that the presence of a background gas can have a profound effect on the plume of material ejected during ablation. At low pressures smooth films are produced but at higher pressures rough films with an evolution from a nodular morphology to a large area cluster-assembled morphology occurs. The surface morphology changes with increasing background pressure as a result of collisions, which reduce the kinetic energy of the ejected material and allow for cluster formation within the plume. It is shown that the energy of some of the carbon ablated species in vacuum can exceed 100 eV. The nature of the species present in the plume is discussed in terms of electron–ion recombination and impact ionisation/excitation. The cluster-assembled films are shown to be useful as a scaffold for supporting metal nanoparticles to produce substrates for surface enhanced Raman spectroscopy.     

Effect of ambient gas ionisation on the morphology of a pulsed laser deposited carbon film

The paper reports a study of the mechanism of ambient gas ionisation and its effects on a growing carbon film. Such an ionisation occurs during the expansion through an inert gas of an ablation plume generated by laser irradiation of a target. Charge transfer reactions between ablated ions and inert gas atoms lead to the formation of a charged layer in contact with plume front. The energy lost by fast ablated ions associated with plume slowing down is calculated. In the case of carbon ablated in helium and argon atmospheres, where ionisation plays a very different role, we discuss the microstructure changes observed in the deposited films in terms of dramatic differences of plume dynamics.     

ArF (193nm) laser ablation of poly(methyl methacrylate)

We report results from a range of complementary studies of pulsed ArF (193 nm) laser ablation of poly(methyl-methacrylate), PMMA, in vacuo, and in the presence of low background pressures of Ar and He, designed to identify correlations between the properties of the plume and those of the a-C:H films that result when the plume is incident on a NaCl substrate. The plume itself has been investigated by wavelength, spatially and/or temporally resolved measurements of the emission from electronically excited H¹, C¹, O¹, CH¹ and C₂¹ fragments, and by Langmuir probe (time-of-flight) measurements of the positively and negatively charged ablated particles, as a function of laser fluence and of ambient gas pressure. Infrared absorption spectroscopy suggests that a-C:H films deposited following pulsed laser ablation of PMMA under low pressures of Ar contain similar C:H:O ratios to the parent polymer, but also confirms previous reports that films deposited in vacuo have a reduced H content.     

Tribological properties of DLC films with different hydrogen contents in water environment

Diamond-like carbon (DLC) films were deposited on silicon wafers by thermal electron excited chemical vapor deposition (CVD). To change the hydrogen content in film, we used three types of carbon source gas (C₇H₈, CH₄, and a CH₄+H₂) and two substrate bias voltages. The hydrogen content in DLC films was analyzed using elastic recoil detection analysis (ERDA). Tribological tests were conducted using a ball-on-plate reciprocating friction tester. The friction surface morphology of DLC films and mating balls was observed using optical microscopy and laser Raman spectroscopy.Hydrogen content in DLC films ranged from 25 to 45 at.%. In a water environment, the friction coefficient and specific wear rate of DLC films were 0.07 and in the range of 10⁻⁸–10⁻⁹ mm³/Nm, respectively. The friction coefficient and specific wear rate of DLC film in water were hardly affected by hydrogen content. The specific wear rate of DLC film with higher hardness was lower than that of film with low hardness. Mating ball wear was negligible and the friction surface features on the mating ball differed clearly between water and air environments, i.e., the friction surface on mating balls in water was covered with more transferred material than that in air.     

Study on metal-doped diamond-like carbon films synthesized by cathodic arc evaporation

In this study, we developed a novel method of synthesizing metal-doped diamond-like carbon films (DLC) using the cathodic arc evaporation (CAE) process. Intense Cr plasma energy activated the decomposition of hydrocarbon source gas C₂H₂ to form a metal-doped amorphous carbon film on steel substrates. We deposited a Cr interlayer to prevent interdiffusion between DLC and the steel substrates. When the C₂H₂ partial pressure is higher than 1.3 Pa, the deposition reaction switched from Cr₃C₂ to DLC formation. The result is a hydrogenated DLC thin film possessing excellent microhardness as high as 3824 Hv(25g), and for which the incorporation of a Cr interface and Cr doping in the DLC matrix ensure film ductility and sufficient film adhesion. We employed Raman spectroscopy to evaluate the influences of reactive gas flow and substrate bias on the DLC composition; we carried out the microstructure and mechanical property measurements by scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical spectroscopy (GDS) and wear tests.     

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.     

Diamond film growth in an oxygen atmosphere

We propose a new method for the synthesis of diamond films via a hydrogen-free vapor-phase route of pulsed laser ablation of a graphite target in a low-pressure pure oxygen atmosphere. The evidence from microscopic, diffraction and spectroscopic techniques indicates that high-quality diamond crystals can be nucleated and grown epitaxially on sapphire (single-crystal aluminum oxide) substrates without diamond-powder treatment at temperatures lower than 600°C under the optimized growth conditions of oxygen pressure and pulsed KrF-excimer laser ablation. The spectroscopic property of a laser plasma plume produced during pulsed laser ablation of graphite in an oxygen atmosphere was examined by using time-resolved optical emission measurements in order to discuss the vapor-phase reaction and film growth mechanism.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

Effects of vacuum ultraviolet on the structure and optical properties of poly(tetrafluoroethylene) films

The effects of vacuum ultraviolet (VUV) at wavelengths of 5–200 nm on the microscopic structure and optical properties of poly(tetrafluoroethylene) (PTFE) films were investigated. X‐ray photoelectron spectroscopy analysis showed that the C_1s spectra changed from a single peak at 292.8 eV to multiplex peaks with binding energies of 284.6, 286.6, 288.6, 290.5, and 293.0 eV after VUV irradiation at 680 esh. With an increasing irradiation dose, the C_1s peaks at 290.5 and 293.0 eV disappeared. After the PTFE film specimens irradiated at 1600 esh were sputtered with argon ions for 3 min, the C_1s peaks at 290.5 and 293.0 eV appeared again, and the height of the peaks at 286.6 and 288.6 eV increased. The content of fluorine decreased after VUV irradiation. The content of fluorine in the film surface layer decreased significantly with the increase in the VUV intensity, but it did not change with the irradiation dose. Fourier transform infrared (FTIR) analysis results indicated that some conjugated bonds, such as ? FC?CF? , were formed during VUV irradiation, but no CH absorption bands were observed in the FTIR spectra; this indicated that the increase in the height of the C_1s peak at 284.6 eV arose mainly from the carbon–carbon bonds, that is, from carbonification. The spectral transmittance of the PTFE film decreased gradually with an increasing VUV irradiation dose, and at a given dose, the lower the intensity was of the VUV irradiation, the greater the change was in the spectral transmittance. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 115–121, 2003     

Structural,mechanical and biological comparison of TiC and TiCN nanocomposites films

The vacuum deposition provides great flexibility for manipulating material's chemistry and structure. A combination of metallic (Ti) and carbon phase can enhance certain physical properties of nanocomposite thin films.In this work, the comparison of nanocomposite films composed of TiC or TiCN grains embedded in amorphous carbon matrix is reported. The films were prepared by dc magnetron sputtering at 200 °C in argon and nitrogen. In the case of argon deposition, 4–5 nm TiC grains in carbon matrix were observed. The nitrogen deposition combined with low content of Ti (～1.2 at%) proved to be insufficient for the development of larger crystals. The carbon had carbide character in TiC film, whereas in TiCN film all the carbon had graphite type environment. TiC film deposited in argon exhibited better mechanical properties than TiCN films deposited in nitrogen. In both cases, the good biocompatibility was observed after 7 days osteoblast cells seeding.     

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.     

Nanodiamond synthesis by pulsed laser ablation in liquids

Pulsed laser ablation in liquids (PLAL) has become an attractive method for the synthesis of nanodiamond. This work deals with the growth kinetics study of structures of nano-diamonds embedded in sp² carbon synthetized by this method. The plasma created by the laser pulse has been monitored by time resolved spectroscopy to analyze the evolution of the plume and therefore the transient species created. Typical C₂ vibrational bands appear, as well as a continuous spectrum due to various phenomena. The study of both the background and the vibrational features gives information on the reaction kinetics and on the plasma density. The presence of nanodiamonds has been confirmed by Raman spectroscopy as well as TEM analysis.