Effects of adding hydrocarbon gas to a high-power impulse magnetron sputtering system on the properties of diamond-like carbon films
Introduction
Diamond-like carbon (DLC) coatings are widely used for many applications owing to their unique properties. These include considerable high hardness, low friction, high wear resistance and chemical inertness. The mechanical properties of DLC films strongly depend on the ratio of sp3 C bonds and sp2 C bonds and the hydrogen content in the films, and DLC films with specific properties have been synthesized for a variety of applications using chemical vapor deposition (CVD) or physical vapor deposition (PVD) techniques [1], [2], [3], [4]. A variety of hydrocarbons are usually used as source gases for the CVD. Discharge plasmas for CVD include direct current and radio frequency discharge plasmas, electron cyclotron resonance plasma, and surface-wave excited plasma [5], [6], [7], [8], [9], [10], [11], [12], [13]. The inclusion of hydrogen in DLC films can reduce internal stress and improve adhesion to various substrates.
Cathodic arc deposition [14], [15], [16], [17] is a PVD technique for preparation of tetrahedral amorphous carbon (ta-C) films. Pulsed laser deposition [18,19] and magnetron sputtering [20], [21], [22], [23] are PVD methods to fabricate amorphous carbon films. It is well known that carbon ion bombardment at an energy of ~100 eV can significantly influence the ratio of sp2 C bonds and sp3 C bonds in non-hydrogenated DLC films. Magnetron sputtering has been widely employed in industrial processes, because uniform films can be deposited over large areas. However, carbon ion bombardment may be insufficient in a conventional magnetron sputtering system. High power impulse magnetron sputtering (HiPIMS) has attracted interest as a way to enhance carbon ion bombardment for manufacture of DLC coatings. Many researchers [24], [25], [26], [27], [28], [29], [30] have used HiPIMS to prepare DLC films. The sp3 C content is increased by applying a negative pulsed voltage to the substrate [25,26], but only a small percentage of the sputtered carbon atoms are ionized [24,25]. The relationship between the applied HiPIMS voltage and the energy distribution functions of ionized argon and carbon atoms has recently been investigated [31]. Ne and Ne/Ar mixture gas are also used as ambient gases to enhance the ionization of sputtered carbon atoms [28], because this raises the electron temperature, which increases the ionization collision frequency between the electrons and the carbon atoms. Tetrahedral amorphous carbon films with sp3 C contents of 13–82% have been prepared using mixed-mode HiPIMS [30]. In mixed-mode HiPIMS, the arc is generated at the end of the pulsed sputtering process, and a large number of carbon ions are produced. In general, the ionization degree of sputtered carbon atoms can be increased by increasing the target current, although the formation of droplets on the film may induce the increase in the surface roughness. On the other hand, the increase in the proportion of sp2 C ring clusters with the increase in the discharge power has been observed. Depending on the operating conditions, the graphite-like carbon films could be prepared [29]. The hardness of films prepared via PVD is higher than that of films prepared using CVD techniques. However, the films are relatively brittle and their adhesion properties tend to be degraded. Therefore, CVD and PVD conditions must be adjusted to prepare films with suitable properties for practical use.
Reactive HiPIMS can be described as the generation of a high-density pulsed plasma that contains a large number of radical ions and neutral radicals. The properties of DLC films prepared via reactive HiPIMS with hydrocarbon gas have recently been reported [32], [33], [34], [35], [36]. The results indicate that the growth rate increases markedly with an increase in the hydrocarbon gas fraction. Hydrocarbon radicals and ions produced in the gas phase can be regarded as an additional carbon source. In previous reports [34], [35], [36], the synthesis of the hydrogenated DLC films with a high hardness required the bombardment of a large number of ions, which had optimum kinetic energies between 200 and 500 eV. Large numbers of hydrocarbon radicals and hydrocarbon ions, which were predicted from the marked increase in the growth rate, can be expected to play a significant role in controlling the film properties. Adding a small amount of hydrocarbon gas (<5%) is expected to improve the quality of coatings, even when HiPIMS is used for their preparation [34]. When hydrogen is introduced, C–C bonds in DLC films are converted to C–H bonds. However, introducing a small amount of hydrogen atoms into the DLC films by adding a small quantity of hydrocarbon gas to Ar HiPIMS can be expected to suppress the formation of graphite-like carbon film. Moreover, the number of C–H bonds formed by the introduction of hydrogen may be reduced by using hydrocarbon ions with the optimum energy for bombardment.
In this study, the properties of DLC films prepared via nonreactive Ar HiPIMS were compared with those of films prepared using a reactive HiPIMS system that contained less than 5% C2H4 or CH4. The influence of the small quantities of hydrocarbon gas (≦5%) on the properties of the films was investigated by changing the HiPIMS target current. The low ionization potential of C2H4 (10.5 eV) should influence on the properties of films formed through the ion process in reactive HiPIMS. Therefore, the properties of the films prepared using C2H4 and CH4 were compared. The mechanical properties of the films prepared on Si substrates were investigated using a nanoindentation method. The microstructure of the films was investigated by Raman spectroscopy. The composition of the films and their bonding states were examined via X-ray photoelectron spectroscopy (XPS).
Section snippets
Experimental setup
A schematic diagram of the experimental apparatus is shown in Fig. 1. The system was equipped with a high-pulse voltage source for the HiPIMS, a negative pulse voltage source for the substrate, a vacuum pumping system, and a cylindrical vacuum chamber. A graphite target with a diameter of 80 mm and a thickness of 2 mm was set on a 10 mm thick glass on the top of the vacuum chamber. The cylindrical vacuum chamber had an inner diameter of 195 mm and a height of 150 mm. A magnetic field was
Results and discussion
The relationship between the properties of the films and the hydrocarbon gas fraction was firstly investigated. The peak current and power densities were approximately 1.1 A/cm2 and 0.7 kW/cm2, respectively. Fig. 4 shows the Raman spectra of the films prepared using different C2H4 gas fractions at p = 0.3 Pa. The Raman spectrum of a DLC film was mainly fitted with two Gaussian peaks after removing the photoluminescence background [1]. The Raman spectrum consisted of D (disorder) and G
Concluding remarks
DLC films were prepared via reactive high power impulse magnetron sputtering (HiPIMS) containing small amount of hydrocarbon gases using small amounts of either C2H4 or CH4 gas. The properties of the films were compared with those of films prepared via nonreactive Ar HiPIMS. The relationships between the properties of the films and the hydrocarbon gas fraction (δHC) were investigated at a target current density of ~1.1 A/cm2 and a power density of ~0.7 kW/cm2. The influence of the peak height
CRediT authorship contribution statement
Takashi Kimura: Supervision, Writing - review & editing, Investigation. Kento Sakai: Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thanked to Mr. K. Nakajima of Nagoya Institute of Technology for fabrication of target holder.
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