Effects of adding hydrocarbon gas to a high-power impulse magnetron sputtering system on the properties of diamond-like carbon films

Highlight

• DLC films are synthesized by nonreactive Ar HiPIMS or reactive HiPIMS with hydrocarbon gas.

• Reactive HiPIMS system contains the small quantities of C₂H₄ or CH₄ gas.

• Properties of the films prepared by nonreactive and reactive HiPIMS are compared.

• Adding hydrocarbon gas less than 5% to Ar HiPIMS is crucial to improve mechanical properties.

• Properties of the films prepared by reactive HiPIMS do not strongly depend on peak target current.

Abstract

Diamond-like carbon (DLC) films were prepared via reactive high-power impulse magnetron sputtering (HiPIMS) using a mixture of argon and a small amount of C₂H₄ or CH₄ gas. Synthesis of the DLC films was based on both plasma-enhanced chemical vapor deposition and physical vapor deposition. At a peak target current of 30 A corresponding to 1.1 A/cm², the hardness of films prepared with hydrocarbon gas fractions of less than 5% was somewhat higher than the hardness of films prepared via nonreactive Ar HiPIMS. The absence of background due to photoluminescence in the Raman spectra indicated that the hydrogen contents in the films were less than 20%. X-ray photoelectron spectroscopy (XPS) was performed, and the peaks related to the sp³ C in the C 1s core level spectra were investigated. The sp³ C peaks in the C 1s spectra of the films prepared at hydrocarbon gas fractions below 5% were more intense than sp³ C related peaks in the spectra of the films prepared via nonreactive HiPIMS. The sp³ C related peaks accounted for more than 30% of the total at a peak target current of 30 A. The influence of the target current on film properties was also investigated. At peak values of target current exceeding 40 A, the hardness of the films prepared via the nonreactive HiPIMS was somewhat higher than that of the films prepared via reactive HiPIMS. This was predicted from the relationship between the target current and the sp³ C content estimated from the XPS spectra.

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).