Growth Feature of Diamond-Like Carbon Films by Various Vacuum Plasma Methods

Diamond-like carbon (DLC) films were prepared on silicon substrate using four different vacuum plasma methods: magnetron sputtering (MS), pulse laser ablation (PLA), ion beam deposition (IBD) and pulse cathode vacuum arc evaporation (CVAE). The microstructure, surface morphology and optical properties of the DLC films were investigated by Raman spectroscopy, atomic force microscope (AFM) and UV-visible spectrophotometer. The results showed that the films prepared by different vacuum plasma methods varied in the microstructure. The film prepared by CVAE possessed the smallest grain size of graphite with the highest disordering of Csp2 clusters, while more disordered Csp2 clusters presented in the DLC film due to the high ions energy density during PLA. Meanwhile, the deposition methods mainly affected the size of particles and roughness of the DLC films under the same thickness. The film by PLA shows a quite smooth surface structure with uniform particle size. In terms of the optical properties, the films prepared by magnetron sputtering expressed the best optical performance among the four vacuum plasma methods. Introduction Diamond-like carbon (DLC) is a metastable form of amorphous carbon with a mixture of graphitelike sp and diamond-like sp bonding. DLC films are widely applied as protective coatings, such as mechanical, acoustic, electronic and optical devices and magnetic media because of their excellent properties, including high mechanical hardness, optical transparency, good chemical stability and thermal conductivity [1–3]. In 1971, Aisenberg and Chabot [4] successfully prepared the Diamondlike carbon (DLC) films for the first time. From then on, more and more deposition techniques and methods have been created to improve the comprehensive properties of the DLC films during the past fourty years. Currently, the DLC films are mainly fabricated by the following methods, such as magnetron sputtering (MS) [5,6], pulsed laser ablation (PLA) [7,8], ion beam deposition (IBD) [9,10], and cathode vacuum arc evaporation (CVAE) [11,12]. And the composition, structure and performance are varied with different deposition methods. RF-magnetron (RF-MS) technology is one of the most popular methods. It displays some additional advantages over other methods, including the higher sputtering rate of graphite and the more content of strong sp-hybrized carbon bonding when compared to IBD. When it comes to PLA, it is an effective technique for producing DLC films that possess hard and smooth surfaces with good adhesion to substrates. However, there are many disadvantages existing in the process of the thin film deposition when using laser as a sputtering source, such as the high energy consumption, the small deposition area and nonuniform thickness of the films [13]. Compared with other methods, IBD is a low temperature deposition method with a better ion ratio and higher ion energy. Although the process of IBD can be controlled flexibly, it is hard to deposit the thin films on the substrate material with a large area [14]. When referred to CVAE technique, it can export high valence and strong energy plasmas with high ionization rate. In addition to strong adhesion between the substrate and film, CVAE is an effective technique to deposit large area of wearresistance carbon films. However, the graphite target easily produces some large size of graphitic particles due to the effect of high temperature electric arc, which will form the rough surface and greatly reduce the performance of the film. Some other techniques are applied to CVAE to 552 Advances in Engineering Research (AER), volume 105 3rd Annual International Conference on Mechanics and Mechanical Engineering (MME 2016) Copyright © 2017, the Authors. Published by Atlantis Press. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). eliminate these negative effects, including installing filter pipe, shield plate or supplying pulse source. Somehow, these methods will significantly reduce the deposition rate and the uniform deposition areas. In this work, we fabricated the DLC films with various vacuum plasma techniques, including magnetron sputtering, pulse laser ablation, ion beam deposition and pulse cathode vacuum arc evaporation. The dependence of microstructure, surface morphology and optical properties of the films was investigated on the preparation method. Experimental DLC films were synthesized on single-crystal silicon (100) and quartz glass substrate using RFmagnetron sputtering (RF-MS) [6], pulse laser ablation (PLA) [7,8], ion beam deposition (IBD) [10] and pulsed cathode vacuum arc evaporation (CVAE) [12]. Graphite targets (99.5 % purity) with different size were used as source materials in the deposition process of four techniques. The deposition devices of different techniques have been illustrated in our previous works. The main deposition parameters of different methods were summarized in table 1. The substrate was ultrasonically cleaned for 20 min in acetone, ethanol and deionized water to remove other surface contaminants. At a base pressure of 6×10-4 Pa, the substrate was etched by ion sputtering source for 15 min to clean the oxide layer on the surface. Ar+ energy and ion current density were about 4 KeV and ~ 25 A/m2, respectively. The bias voltage with different deposition methods is about 300V. The substrate was cooled to room temperature after sputtering clean. During films deposition the rotation velocity of the sample holder was kept at 2 rev/min. The film thickness was controlled by deposition time and was measured by a step device (Ambios Technology XP-2). The thickness of DLC films in present work was 150~180 nm. Table 1. Deposition parameters of DLC films by different techniques Preparation methods Graphite target diameter, mm Process pressure, Pa Discharge voltage, V RF-MS 50 0.2 (+Ar) 1250 PLA 20 2×10-3 Nd: YAG laser, 355 nm, 10 Hz and pulse duration of 7 ns IBD 80 5×10-2 (+Ar) 5000 CVAE 30 2×10-3 350 V, pulse frequency 3 Hz The microstructure of the films was analyzed using a Renishaw inVia Raman spectrometer. Raman spectra were excited with the 514.5 nm emitting lines from an Ar-ion laser at an incident power of 20 mW. Surface morphology of the films was observed by atomic force microscope (AFM) using a multimode scanning microscope Solver-PRO P47 (NT-MDT) in tapping mode, with a scanning scope of 1×1 μm and a scan rate of 1.0 μm/s. Optical transmittance in the range of 200 ~ 1100 nm were carried out bya UV-1201 UV-Visible spectrophotometer using a blank substrate as reference. Results and Discussion Raman spectra of DLC films prepared by different plasma techniques are shown in Fig. 1. All spectra display a typical spectra characteristic of amorphous carbon structure: a broad Raman peak in the range of 1100~1800 cm-1 with a shoulder peak at around 1360 cm-1. It can be seen from Fig. 1 a and c that the DLC films prepared by RF-MS and IBD have a stronger shoulder peak, implying a higher content of sp2-hybridized carbon in the films . Raman spectra of DLC films can be fitted with two Gaussian peaks located at about 1360 and 1570 cm-1, which are assigned to the D peak and G peak, respectively [15]. The G peak is dominated by stretching vibration of all pairs of sp– bonded carbon atoms in both aromatic rings and carbon chains. The D peak is attributed to disorder553 Advances in Engineering Research (AER), volume 105