- Email: [email protected]
CH4 jet
Accepted Manuscript Characterization of diamond-like carbon thin film synthesized by RF atmospheric pressure plasma Ar/CH4 Jet Farshad Sohbatzadeh, Reza Safari, G.Reza Etaati, Eskandar Asadi, Saeed Mirzanejhad, Mohammad Taghi Hosseinnejad, Omid Samadi, Hanieh Bagheri PII:
S0749-6036(15)30281-0
DOI:
10.1016/j.spmi.2015.11.016
Reference:
YSPMI 4069
To appear in:
Superlattices and Microstructures
Received Date: 25 July 2015 Revised Date:
7 November 2015
Accepted Date: 16 November 2015
Please cite this article as: F. Sohbatzadeh, R. Safari, G.R. Etaati, E. Asadi, S. Mirzanejhad, M.T. Hosseinnejad, O. Samadi, H. Bagheri, Characterization of diamond-like carbon thin film synthesized by RF atmospheric pressure plasma Ar/CH4 Jet, Superlattices and Microstructures (2015), doi: 10.1016/ j.spmi.2015.11.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Characterization of Diamond-Like Carbon Thin Film
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Synthesized by RF Atmospheric Pressure Plasma Ar/CH4 Jet
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Farshad Sohbatzadeh1,†, Reza Safari1 , G.Reza Etaati2, Eskandar Asadi3,
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Saeed Mirzanejhad1, Mohammad Taghi Hosseinnejad4, Omid Samadi1, Hanieh Bagheri1
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Department of Atomic and Molecular Physics, Faculty of Basic Sciences, University of Mazandaran, Babolsar, Iran.
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Nuclear Engineering and Physics Department, Amir Kabir University of Technology, Tehran, Iran.
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Laser-Plasma Research Institute, Shahid Beheshti University, Evin, 1983963113, Tehran, Iran.
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Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran.
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Email: [email protected]
Abstract
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Keywords: Atmospheric pressure deposition, Diamond like carbon coating, Thin film, RF plasma jet
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The growth of diamond like carbon (DLC) on a Pyrex glass was investigated by a radio frequency (RF) atmospheric pressure plasma jet (APPJ). The plasma jet with capacitive configuration ran by a radio frequency power supply at 13.56 MHz. Alumina ceramic was used as dielectric barrier. Ar and CH4 were used in atmospheric pressure as carrier and precursor gases, respectively. Diamond like carbon thin films were deposited on Pyrex glass at substrate temperature and applied power of 130 o C and 250 Watts, respectively. Performing field emission scanning electron microscope (FE-SEM) and laser Raman spectroscopy analysis resulted in deposition rate and the ID/IG ratio of 21.31 nm/min and 0.47, respectively. The ID/IG ratio indicated that the coating possesses relative high sp3 content. The optical emission spectroscopy (OES) diagnostic was applied to diagnose plasma jet species. Estimating electron temperature and density of the RF-APPJ resulted in 1.36 eV and 2.75×1014 cm-3 at the jet exit, respectively.
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ACCEPTED MANUSCRIPT 1. Introduction
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Atmospheric-pressure plasma processing has attracted great interest for industrial
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applications such as surface cleaning, deposition, etching, and sterilization due to its low cost,
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high processing speeds and simple system, which uses no vacuum equipment. Much attention
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has been focused on deposition processing as one of the surface modification methods.
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Deposition of diamond like carbon (DLC) coating has attracted considerable attention due to
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its excellent properties including outstanding abrasion, wear resistance, chemical inertness,
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exceptional hardness, low coefficient to friction and high dielectric strength [1-3].
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DLC is considered to be an amorphous material, containing a mixture of sp3, sp3sp2 and
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sp3 bonded carbon. Based on the percentage and fraction of these bonds and the hydrogen
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content, four various types of DLC coatings have been divided: amorphous carbon (a-c),
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tetrahedral amorphous carbon (tα-C), hydrogenated amorphous carbon (α-C:H) and
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hydrogenated tetrahedral amorphous carbon (ta-C:H) [4-6]. DLC is a metastable material that
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has high degree of disorder owing to distortion in bond and bond length [7].
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The properties of this coating such as excellent mechanical, electrical and optical properties depend
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on sp3/sp2 ratio that is controlled by different cases such as ion energy, self-bias, gas pressure,
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deposition temperature and gas flow rates [7]. The mechanical property is controlled by sp3 and the
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electrical and optical properties are controlled by sp2 [8].
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DLC films have residual stresses in its structure so they show poor adhesion to substrate [7, 9] that for
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solving this problem, various foreign elements have been incorporated to DLC such as copper, Si and
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nitrogen [7]. In the recent years DLC coatings were deposited at low pressure by various
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methods such as plasma-enhanced chemical vapor deposition [10], cathodic arc spray [11],
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ion beam deposition [12] and pulsed laser deposition [13, 14]. The properties of electrical,
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mechanical and optical properties have been investigated in Refs [7-9,15-17]. Deposition of DLC in
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vacuum has disadvantages due to high cost and restriction on shape and size of surfaces at
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deposition processing. Recently, deposition of DLC by atmospheric pressure plasma jet
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ACCEPTED MANUSCRIPT (APPJ) has demonstrated by several researchers [18-22]. The results showed that DLC was
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successfully deposited on substrate by APPJ and prepared sp3 content is comparable with
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low pressure condition [15, 21, 22] in addition surface roughness and hardness of the film is
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less than that of low pressure [18,19,23-28].
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In the present work, for the first time, we deposited DLC thin film on Pyrex glass substrate,
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using a self-design RF atmospheric pressure Ar/CH4 plasma jet in open air. We used several
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methods to investigate RF plasma jet characteristics and DLC deposited samples.
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This paper is organized as follows: In section 2, we represent experimental results, in section
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3, results and discussion will be given. Finally, conclusion will be given in section 4.
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2. Experimental setup
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Fig. 1 shows a schematic view of a self-designed RF-APPJ that was used in our experiment.
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The RF-APPJ consisted of a tube of alumina ceramic with a length of 100mm, internal
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diameter of 10 mm and external diameter of 14 mm. As shown in Fig. 1, powered electrode,
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made of copper with diameter of 5 mm, was set inside the alumina ceramic (acting as
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dielectric barrier). Grounded electrode, made of copper, was attached to the external surface
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of the alumina ceramic. The RF-APPJ was driven by a radio frequency (RF) power supply at
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13.56 MHz. The mixture of Ar (99.999%) and CH4 (99.99%) as carrier and precursor gases
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were flourished through the annular space between the alumina tube and the concentric
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copper electrode. The gap between powered electrode and alumina ceramic was 2.5 mm.
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Fig. 1
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The RF-APPJ was generated at the gas gap between two copper electrodes and exited into the
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surrounding air outside the nozzle (see Fig. 1). The flow rates of argon and methane were
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controlled by digital mass flow meters. In order to cool the plasma head, cool system using
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water has been used. Diamond like carbon coating was deposited on the glass substrate that
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was cleared ultrasonically with alcohol and later with acetone, each for a period of 10 min.
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The operating parameters for deposition of DLC thin film are presented in table1. Table 1
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The voltage and discharge current were measured by a high voltage probe (Tektronix
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P6015A) and a current monitor (Pearson 4100) with a digital oscilloscope (Tektronix
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DPO2012). Typical I-V characteristics of the argon plasma jet in the absence and presence of
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methane gas are shown in Fig. 2, respectively.
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As can be seen in figure 2a, there is a phase difference between discharge voltage and current
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in Ar plasma. The current phase lag is due to capacitive behavior of the APPJ. It actually
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comprises two electrodes forming a capacitor. On the other hand, the voltage and current
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peaks increase in Ar/CH4 plasma, as can be seen in figure 2b. These changes can be attributed
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to the increasing breakdown voltage of the Ar/CH4 mixture with respect to the Ar. It is well
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known that the breakdown voltage of the methane is higher than that of argon. Therefore,
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when we use argon/methane mixture, the discharge voltage increases. On the other hand,
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more power consumption requires to sustain such a plasma that leads to current enhancement
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versus argon plasma.
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Fig.2
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The components and characteristics of the DLC thin film were analyzed by laser Raman
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spectroscopy (LABRAM (Laser532nm-Nd:YAG)) and field emission scanning electron
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microscope (HITACHI S4160). Meanwhile, optical emission spectroscopy IR-UV (SOLAR
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ACCEPTED MANUSCRIPT LASER SYSTEM, S100) with scanning range from 190 nm to 1100 nm was used to evaluate
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RF plasma jet species such as variations of the electron density and excitation temperature.
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3. Results and discussion
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The RF atmospheric pressure argon plasma jet in the absence of methane gas is shown in Fig.
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3(a). It is seen that the jet length is about 30 mm. In the presence of methane gas, as shown in
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Fig. 3(b), the length of the plasma jet reduces to 10 mm. It is worth to note that the length of
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plasma jet of a noble gas such as Ar, is longer than that of molecular gases such as N2, O2 and
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CH4. Ar plasma has metastable species that could retain the energy in their excited states.
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This energy can ionize neutral atoms by Penning ionization [29] that eventually sustains the
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plasma jet. When a molecular gas is used to produce plasma jet, the corresponding length is
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shorter. This can be attributed to electronegativity effect, vibrational and rotational excited
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states of the molecules that consume the energy and lower the ionization process [30]. Fig. 3
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(c) and (d) show coating process and DLC thin film deposited on the glass substrate,
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respectively. The focused points for OES measurements are indicated in Fig. 3(b).
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Fig.4 shows the Raman spectra of DLC thin film coated on glass substrate by RF plasma jet.
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Raman spectroscopy is a non-destructive technique to characterize the structural properties of
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carbon coatings and their quality [21, 22, 31]. Raman spectra of carbon materials such as
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DLC structures show common features that so-called the G and D peaks. The G peak related
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to stretching vibration mode of all pairs of sp2 sites in aromatic rings or C=C chains and D
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peak assigned to breathing mode of sp2 sites only in six-foldrings [32]. The intensity ratio of
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the D and G peaks, ID/IG and the position of the G peak, Position (G), have been widely used
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for qualitative estimation of sp3content in DLC [32]. S. Zhang et al. showed that the sp3
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fraction is inversely proportional to the band ratio ID/IG [33, 34].
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ACCEPTED MANUSCRIPT If ID/IG ratio becomes smaller, the sp2 phase organizes rather in chains whereas a higher ID/IG
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ratio indicates an increase in sp2 phase in aromatic rings [35] but also a higher overall sp3
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content. Both D and G peaks originate from the sp2 bonded part in amorphous carbon films
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and the integrated intensity ID/IG ratio of these two peaks is closely related to the graphite
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crystallite size formed during the film growth [36]. Ferrari and Robertson [37] proposed an
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important rule stating that the lower ID /IG ratio would be straightly related to easier formation
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of sp3 phase in the DLC films that leads to higher quality of DLC structure.
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Fig. 4
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In Fig.4, It is seen the peaks at 1354 and 1604 cm-1are related to D and G bands, respectively
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and integrated intensity, ID /IG ratio of DLC coating is 0.47. This result indicates that DLC
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thin film deposited with this method possesses a relatively high sp3 content and high quality
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of DLC structure.
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Fig.5 shows the top and cross sectional view FE-SEM images of DLC deposited thin film. In
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Fig. 5(a), the top view image of DLC deposited sample indicates appearance of voids spaces
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between the DLC cauliflower micro-structures. Regarding to the results of several works [18,
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22, 38], one can conclude that the voids and pits structure of deposited DLC coating is not a
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direct consequence of high frequency plasma reactor. It may be attributed to gas composition
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and impurities therein in atmospheric pressure plasmas. The cross sectional FE-SEM image
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of the DLC sample (which is deposited at 35 min) is shown in Fig. 5(b). As it can be seen
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from Fig. 5(b), the average thickness of deposited DLC thin film is 746 nm. Therefore,
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deposition rate in these conditions is about 21.31 nm in minute.
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Fig. 5
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We know that the plasma density changes with distance from the nozzle. Therefore, we
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deposited DLC thin films on the substrate in different distances from the nozzle. The 6
ACCEPTED MANUSCRIPT thickness of the DLC was 746 nm and 4 µ m with distance 10 and 5 mm from the nozzle,
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respectively. The plasma jet emission spectrum was measured at different positions with
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respect to the jet exit. Fig. 6 shows the emission spectra of argon/methane plasma jet, at three
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points (0 cm, 0.5 cm and 1 cm from the jet exit) as shown in Fig. 3(b). The growth of
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diamond like carbon is attributed to the C2, CH, CH2 and CH3. The emission band of CH is
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located at 430-435 nm and that of C2 are at 465, 516 and 558 nm.
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Fig.6
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It is worth noting that the CH, CH2 and CH3 radicals are active species for DLC growth in
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methane plasma [21, 39]. As shown in Fig. 6, CH3 and CH2 radical lines have not been
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detected in our RF Ar/CH4 plasma jet. The CH3 radical has a long lifetime (about several
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milliseconds) in plasma [21] so it is supposed that the absence of CH3 radical line in emission
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spectra of plasma is due to CH3 radical does not emit light in our spectrograph range. Also
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the CH2 radical is immediately converted into CH radical through the following pathway
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[39]:
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Therefore, in Ar/CH4 plasma, CH is formed directly from the H abstraction of the CHx
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fragments by electron impact dissociation, and C2 species are formed mainly by
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recombination of CHx species [21].
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Relation between active species of CH and C2 with plasma jet length has been shown in Fig.
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7. The position of CH and C2 bands are at 430-435 nm and 516 nm, respectively. As can be
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seen in this figure, the concentration of CH radical and C2 species reduce with increasing the
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distance from jet exit. This result shows that for high deposition rate, the distance between
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the nozzle exit and sample must be low.
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In non-thermal plasma (also called non-equilibrium plasmas) the species such as neutral, ions,
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radicals and electrons have different energy states. The electron temperature is much higher
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than the kinetic temperature of heavy particles (gas temperature) in non-equilibrium plasma.
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Electron temperature (Te) is one of the basic parameters in gas discharge physics because of
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its role in the excitation, dissociation, and ionization of atoms and molecules. For
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measurement of the kinetic temperature of free electrons, the electronic excitation
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temperature (Texc) must be evaluated [40]. This temperature is widely utilized in high
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pressure plasma and Te measurement is based on Texc determination because of their close
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relationship. If a system obeys the local thermodynamic equilibrium (LTE), Texc is close to
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Te[40, 41]. A number of different methods based on optical emission spectroscopy for
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evaluating electron excitation temperature Texe, have been described. In the presence of local
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thermodynamic equilibrium for plasma, the Boltzmann technique is effective. For
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atmospheric pressure plasmas where the collision frequency is dominant over radiative decay
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probability of the excited states, the upper energy levels of the selected atomic transitions are
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in local thermodynamic equilibrium, so the density of these levels were obtained with the
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Boltzmann law [42]. Based on above assumptions, the Te has been estimated by conventional
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Boltzmann technique, given by [43, 44]:
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I ij λij ln Ag ij i
− Ei = k BTe
(2)
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Where Iij is the relative intensity of the emitted line for transition level i to j, k B is the
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Boltzmann constant, λij is wavelength, gij is statistical weight, Aij is transition probability and
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Ei is energy of the upper level. The atomic lines of Ar (738, 763, 794, 801, 811, 826 and
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912nm) have been used to estimate the Te. Spectroscopic data of ArI emission lines 8
ACCEPTED MANUSCRIPT associated with wavelengths of the emission peaks are given in Table 2, for estimating the Te
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[45]. Plot of the ln ( I ij λij Aij gi ) versus the energy of the upper level, Ei (ev) gives a straight
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line slope of 1 k B Te that Figure 8 represents the electron temperature at the nozzle exit and 1
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cm far from the nozzle that were obtained by Boltzmann plot assuming local thermodynamic
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equilibrium.. As shown in this figure, Te decreases from 1.36 eV to 1.16 eV and then 0.87 eV
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with the increase in distance from the jet exit.
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Fig.8
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The Saha-Boltzmann equation was used for electron density evolution [46]:
ne =
− E + Ei , z − X z −3 I z* 32 6.04 ×10 21 (Te ) × exp i , z +1 cm * I z +1 k T B e (3)
Where I z* = ( I z λij , z ) / ( gij , z Aij , z ) and X z is the ionization energy of the species in the ionization
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stage z. As shown in Fig. 9, to evaluate at three points, it is better to average the electron
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density obtained using intensity ratio of ArI lines to ArII lines (696 nm ArI to 648 nm ArII
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and 763 ArI to 648 nm ArII). Figure 9 indicates that the temperature and density of the
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plasma electrons diminish after exiting the nozzle. The electron temperature is 1.35 and 0.85
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eV at the nozzle exit and 1 cm far from the nozzle respectively. On the other hand, the
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electron density reduces from 2.7 × 1014 to 0.3 × 1014 cm-3. The higher electron temperature
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and density, the closer distance to the plasma jet exit.
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Fig.9
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ACCEPTED MANUSCRIPT 4. Conclusion
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Deposition of diamond like carbon by RF atmospheric pressure plasma jet with Ar as
3
dilution gas and methane as precursor gas was reported successfully. The structure and
4
morphologies of DLC thin film were analyzed by laser Raman spectroscopy and field
5
emission scanning electron microscope (SEM). The result obtained from laser Raman
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spectroscopy demonstrated that integrated intensity ID/IG is 0.47, indicating high quality of
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DLC film and relative high sp3 content. Deposition rate of 21.31nm/min was obtained at
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applied RF power of 250 Watt shaving substrate temperature of 130 oC. OES was used to
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identify active species of plasma jet for deposition of DLC and to evaluate the electron
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temperature and density. The electron temperature and density of the atmospheric pressure
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RF argon/methane plasma jet were evaluated to be 0.87 eV and 1×1010 (1/cm3) respectively,
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at the DLC deposition place. It should be noticed that the CH, CH2 and CH3 radicals are
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active species for DLC growth and C2 species are formed mainly by recombination of CHx
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species.
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Carbon Films Synthesized by DC-Plasma Enhanced Chemical Vapor Deposition, Journal of
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fusion energy 30 (2011) 447-452.
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[33] S. Zhang, X. T. Zeng, H. Xie, P. Hing, A phenomenological approach for the ID/IG ratio
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and sp3 fraction of magnetron sputtered a-C films, Surface and Coatings Technology 123
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(2000) 256-260.
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[34] A. Zeng, E. Liu, P. Hing, S. Zhang, S. N. Tan, I. F. Annergren, J. Gao, Microstructure
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and Electrochemical Behavior of Sputtered Diamond -Like Carbon Films, International
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Journal of Modern Physics B 16 (2002)1024-1030.
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[35] M. Kahn, M. Cekada, T. Schoberl, R. Berghauser, C. Mitterer, C. Bauer, et al.,
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Structural and mechanical properties of diamond-like carbon films deposited by an anode
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layer source, Thin Solid Films 517 (2009) 6502-6507.
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[36] J. Robertson, Properties of diamond-like carbon, Surface and Coatings Technology 50,
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(1992) 185-203.
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[37] A. C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and
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amorphous carbon, Physical review B 61 (2000) 14095-14107.
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[38] L. Marcinauskas, A. Grigonis, V. Valincius, P. Valatkevicius,. Surface and structural
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analysis of carbon coatings produced by plasma jet CVD, Materials science 13 (2007) 269-
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272.
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Plasma Generated in an Atmospheric Pressure DBD Micro-Jet by Optical Emission
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Spectroscopy, Chinese Physics Letters 26 (2009) 035203.
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[40] H. Park, W. Choe, Parametric study on excitation temperature and electron temperature
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in low pressure plasmas, Current Applied Physics, 10(6) (2010) 1456-1460.
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[41] H. R. Griem, Validity of local thermal equilibrium in plasma spectroscopy, Physical
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review, 131(3) (1963) 1170.
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[42] M. Bashir, J. M. Rees, S. Bashir, W. B. Zimmerman, Characterization of atmospheric
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pressure micro plasma produced from argon and a mixture of argon-ethylenediamine, Physics
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Letters A 378 (2014) 2395-2405.
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[43] S. Forster, C. Mohr, W. Viol, Investigations of an atmospheric pressure plasma jet by
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optical emission spectroscopy, Surface and Coatings Technology 200 (2005) 827-830.
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[44] J. L. Walsh, F. Iza, M. G. Kong, Atmospheric glow discharges from the high-frequency
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to very high-frequency bands, Applied Physics Letters, 93(25) (2008) 251502.
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[45] http://physics.nist.gov/PhysRefData/ASD/lines_form.html
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[46] J. Gomba, C. D'Angelo, D. Bertuccelli, G. Bertuccelli, Spectroscopic characterization of
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laser induced breakdown in aluminium–lithium alloy samples for quantitative determination
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of traces, Spectrochimica Acta Part B: Atomic Spectroscopy 56 (2001) 695-705.
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Figure Captions Fig. 1: Experimental setup for diamond like carbon deposition process1: Grounded electrode 2: Powered electrode 3: Alumina dielectric 4: Substrate
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5: Digital mass flow meter
Fig. 2: Time dependence of discharge voltage and current a) In the presence of argon gas b) In the presence of argon/methane gases
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Fig. 3: a) Image of argon plasma jet with length of 3.5 cm b) Plasma jet in the presence of argon/methane gases with length of 1 cm c) Image of plasma jet
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during deposition process d) DLC film prepared by the RF argon/methane plasma jet
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Fig. 4: The Raman spectra of DLC film, prepared by RF plasma jet
Fig. 5: SEM morphology of DLC film a) The top view b) Cross sectional
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Fig.6: The emission spectra of Ar/methane plasma formed in atmospheric
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pressure RF plasma jet a) 0 cm b) 0.5 cm c) 1 cm from the jet exit.
Fig. 7: Relationship between active species with plasma jet length a) C2 b) CH
Fig.8: The electron temperature of Ar/methane plasma formed in atmospheric pressure RF plasma jet a) 0 cm b) 0.5 cm c) 1 cm from jet exit
Fig.9: Relationship between electron temperature and density with plasma jet length
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Table Captions Table 1: The parameters for DLC film deposition
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Table 2: Spectroscopic data of ArI emission lines selected for estimating electron
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temperature [45]
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Table 1 Deposition time(min)
Power (Watt)
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0.1
3 5
250
Substrate temperature (.c) 130
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Distance from jet exit to substrate (mm) 10
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Methane (Lit/min)
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Ar (Lit/min)
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Table 2
Ei (ev)
nm 738 763 794 801 811 826 912
0.08 47 0. 245 0.1 86 0.09 28 0.3 31 0.1 53 0.1 89
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13.30 13.17 13.28 13.09 13.07 13.32 12.90
Aij (108 / s)
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ACCEPTED MANUSCRIPT Highlights The growth of DLC using atmospheric pressure plasma jet. Investigation of structural and morphological properties of DLC deposited thin film.
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Estimation of plasma jet electron temperature and density using OES analysis.
S0749-6036(15)30281-0
DOI:
10.1016/j.spmi.2015.11.016
Reference:
YSPMI 4069
To appear in:
Superlattices and Microstructures
Received Date: 25 July 2015 Revised Date:
7 November 2015
Accepted Date: 16 November 2015
Please cite this article as: F. Sohbatzadeh, R. Safari, G.R. Etaati, E. Asadi, S. Mirzanejhad, M.T. Hosseinnejad, O. Samadi, H. Bagheri, Characterization of diamond-like carbon thin film synthesized by RF atmospheric pressure plasma Ar/CH4 Jet, Superlattices and Microstructures (2015), doi: 10.1016/ j.spmi.2015.11.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Characterization of Diamond-Like Carbon Thin Film
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Synthesized by RF Atmospheric Pressure Plasma Ar/CH4 Jet
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Farshad Sohbatzadeh1,†, Reza Safari1 , G.Reza Etaati2, Eskandar Asadi3,
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Saeed Mirzanejhad1, Mohammad Taghi Hosseinnejad4, Omid Samadi1, Hanieh Bagheri1
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Department of Atomic and Molecular Physics, Faculty of Basic Sciences, University of Mazandaran, Babolsar, Iran.
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Nuclear Engineering and Physics Department, Amir Kabir University of Technology, Tehran, Iran.
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Laser-Plasma Research Institute, Shahid Beheshti University, Evin, 1983963113, Tehran, Iran.
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Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran.
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Email: [email protected]
Abstract
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Keywords: Atmospheric pressure deposition, Diamond like carbon coating, Thin film, RF plasma jet
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The growth of diamond like carbon (DLC) on a Pyrex glass was investigated by a radio frequency (RF) atmospheric pressure plasma jet (APPJ). The plasma jet with capacitive configuration ran by a radio frequency power supply at 13.56 MHz. Alumina ceramic was used as dielectric barrier. Ar and CH4 were used in atmospheric pressure as carrier and precursor gases, respectively. Diamond like carbon thin films were deposited on Pyrex glass at substrate temperature and applied power of 130 o C and 250 Watts, respectively. Performing field emission scanning electron microscope (FE-SEM) and laser Raman spectroscopy analysis resulted in deposition rate and the ID/IG ratio of 21.31 nm/min and 0.47, respectively. The ID/IG ratio indicated that the coating possesses relative high sp3 content. The optical emission spectroscopy (OES) diagnostic was applied to diagnose plasma jet species. Estimating electron temperature and density of the RF-APPJ resulted in 1.36 eV and 2.75×1014 cm-3 at the jet exit, respectively.
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ACCEPTED MANUSCRIPT 1. Introduction
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Atmospheric-pressure plasma processing has attracted great interest for industrial
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applications such as surface cleaning, deposition, etching, and sterilization due to its low cost,
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high processing speeds and simple system, which uses no vacuum equipment. Much attention
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has been focused on deposition processing as one of the surface modification methods.
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Deposition of diamond like carbon (DLC) coating has attracted considerable attention due to
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its excellent properties including outstanding abrasion, wear resistance, chemical inertness,
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exceptional hardness, low coefficient to friction and high dielectric strength [1-3].
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DLC is considered to be an amorphous material, containing a mixture of sp3, sp3sp2 and
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sp3 bonded carbon. Based on the percentage and fraction of these bonds and the hydrogen
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content, four various types of DLC coatings have been divided: amorphous carbon (a-c),
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tetrahedral amorphous carbon (tα-C), hydrogenated amorphous carbon (α-C:H) and
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hydrogenated tetrahedral amorphous carbon (ta-C:H) [4-6]. DLC is a metastable material that
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has high degree of disorder owing to distortion in bond and bond length [7].
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The properties of this coating such as excellent mechanical, electrical and optical properties depend
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on sp3/sp2 ratio that is controlled by different cases such as ion energy, self-bias, gas pressure,
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deposition temperature and gas flow rates [7]. The mechanical property is controlled by sp3 and the
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electrical and optical properties are controlled by sp2 [8].
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DLC films have residual stresses in its structure so they show poor adhesion to substrate [7, 9] that for
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solving this problem, various foreign elements have been incorporated to DLC such as copper, Si and
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nitrogen [7]. In the recent years DLC coatings were deposited at low pressure by various
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methods such as plasma-enhanced chemical vapor deposition [10], cathodic arc spray [11],
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ion beam deposition [12] and pulsed laser deposition [13, 14]. The properties of electrical,
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mechanical and optical properties have been investigated in Refs [7-9,15-17]. Deposition of DLC in
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vacuum has disadvantages due to high cost and restriction on shape and size of surfaces at
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deposition processing. Recently, deposition of DLC by atmospheric pressure plasma jet
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successfully deposited on substrate by APPJ and prepared sp3 content is comparable with
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low pressure condition [15, 21, 22] in addition surface roughness and hardness of the film is
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less than that of low pressure [18,19,23-28].
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In the present work, for the first time, we deposited DLC thin film on Pyrex glass substrate,
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using a self-design RF atmospheric pressure Ar/CH4 plasma jet in open air. We used several
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methods to investigate RF plasma jet characteristics and DLC deposited samples.
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This paper is organized as follows: In section 2, we represent experimental results, in section
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3, results and discussion will be given. Finally, conclusion will be given in section 4.
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2. Experimental setup
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Fig. 1 shows a schematic view of a self-designed RF-APPJ that was used in our experiment.
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The RF-APPJ consisted of a tube of alumina ceramic with a length of 100mm, internal
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diameter of 10 mm and external diameter of 14 mm. As shown in Fig. 1, powered electrode,
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made of copper with diameter of 5 mm, was set inside the alumina ceramic (acting as
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dielectric barrier). Grounded electrode, made of copper, was attached to the external surface
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of the alumina ceramic. The RF-APPJ was driven by a radio frequency (RF) power supply at
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13.56 MHz. The mixture of Ar (99.999%) and CH4 (99.99%) as carrier and precursor gases
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were flourished through the annular space between the alumina tube and the concentric
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copper electrode. The gap between powered electrode and alumina ceramic was 2.5 mm.
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Fig. 1
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The RF-APPJ was generated at the gas gap between two copper electrodes and exited into the
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surrounding air outside the nozzle (see Fig. 1). The flow rates of argon and methane were
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controlled by digital mass flow meters. In order to cool the plasma head, cool system using
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water has been used. Diamond like carbon coating was deposited on the glass substrate that
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was cleared ultrasonically with alcohol and later with acetone, each for a period of 10 min.
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The operating parameters for deposition of DLC thin film are presented in table1. Table 1
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The voltage and discharge current were measured by a high voltage probe (Tektronix
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P6015A) and a current monitor (Pearson 4100) with a digital oscilloscope (Tektronix
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DPO2012). Typical I-V characteristics of the argon plasma jet in the absence and presence of
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methane gas are shown in Fig. 2, respectively.
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As can be seen in figure 2a, there is a phase difference between discharge voltage and current
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in Ar plasma. The current phase lag is due to capacitive behavior of the APPJ. It actually
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comprises two electrodes forming a capacitor. On the other hand, the voltage and current
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peaks increase in Ar/CH4 plasma, as can be seen in figure 2b. These changes can be attributed
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to the increasing breakdown voltage of the Ar/CH4 mixture with respect to the Ar. It is well
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known that the breakdown voltage of the methane is higher than that of argon. Therefore,
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when we use argon/methane mixture, the discharge voltage increases. On the other hand,
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more power consumption requires to sustain such a plasma that leads to current enhancement
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versus argon plasma.
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Fig.2
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The components and characteristics of the DLC thin film were analyzed by laser Raman
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spectroscopy (LABRAM (Laser532nm-Nd:YAG)) and field emission scanning electron
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microscope (HITACHI S4160). Meanwhile, optical emission spectroscopy IR-UV (SOLAR
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RF plasma jet species such as variations of the electron density and excitation temperature.
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3. Results and discussion
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The RF atmospheric pressure argon plasma jet in the absence of methane gas is shown in Fig.
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3(a). It is seen that the jet length is about 30 mm. In the presence of methane gas, as shown in
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Fig. 3(b), the length of the plasma jet reduces to 10 mm. It is worth to note that the length of
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plasma jet of a noble gas such as Ar, is longer than that of molecular gases such as N2, O2 and
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CH4. Ar plasma has metastable species that could retain the energy in their excited states.
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This energy can ionize neutral atoms by Penning ionization [29] that eventually sustains the
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plasma jet. When a molecular gas is used to produce plasma jet, the corresponding length is
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shorter. This can be attributed to electronegativity effect, vibrational and rotational excited
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states of the molecules that consume the energy and lower the ionization process [30]. Fig. 3
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(c) and (d) show coating process and DLC thin film deposited on the glass substrate,
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respectively. The focused points for OES measurements are indicated in Fig. 3(b).
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Fig.4 shows the Raman spectra of DLC thin film coated on glass substrate by RF plasma jet.
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Raman spectroscopy is a non-destructive technique to characterize the structural properties of
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carbon coatings and their quality [21, 22, 31]. Raman spectra of carbon materials such as
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DLC structures show common features that so-called the G and D peaks. The G peak related
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to stretching vibration mode of all pairs of sp2 sites in aromatic rings or C=C chains and D
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peak assigned to breathing mode of sp2 sites only in six-foldrings [32]. The intensity ratio of
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the D and G peaks, ID/IG and the position of the G peak, Position (G), have been widely used
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for qualitative estimation of sp3content in DLC [32]. S. Zhang et al. showed that the sp3
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fraction is inversely proportional to the band ratio ID/IG [33, 34].
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ratio indicates an increase in sp2 phase in aromatic rings [35] but also a higher overall sp3
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content. Both D and G peaks originate from the sp2 bonded part in amorphous carbon films
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and the integrated intensity ID/IG ratio of these two peaks is closely related to the graphite
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crystallite size formed during the film growth [36]. Ferrari and Robertson [37] proposed an
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important rule stating that the lower ID /IG ratio would be straightly related to easier formation
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of sp3 phase in the DLC films that leads to higher quality of DLC structure.
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Fig. 4
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In Fig.4, It is seen the peaks at 1354 and 1604 cm-1are related to D and G bands, respectively
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and integrated intensity, ID /IG ratio of DLC coating is 0.47. This result indicates that DLC
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thin film deposited with this method possesses a relatively high sp3 content and high quality
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of DLC structure.
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Fig.5 shows the top and cross sectional view FE-SEM images of DLC deposited thin film. In
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Fig. 5(a), the top view image of DLC deposited sample indicates appearance of voids spaces
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between the DLC cauliflower micro-structures. Regarding to the results of several works [18,
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22, 38], one can conclude that the voids and pits structure of deposited DLC coating is not a
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direct consequence of high frequency plasma reactor. It may be attributed to gas composition
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and impurities therein in atmospheric pressure plasmas. The cross sectional FE-SEM image
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of the DLC sample (which is deposited at 35 min) is shown in Fig. 5(b). As it can be seen
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from Fig. 5(b), the average thickness of deposited DLC thin film is 746 nm. Therefore,
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deposition rate in these conditions is about 21.31 nm in minute.
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Fig. 5
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We know that the plasma density changes with distance from the nozzle. Therefore, we
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deposited DLC thin films on the substrate in different distances from the nozzle. The 6
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respectively. The plasma jet emission spectrum was measured at different positions with
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respect to the jet exit. Fig. 6 shows the emission spectra of argon/methane plasma jet, at three
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points (0 cm, 0.5 cm and 1 cm from the jet exit) as shown in Fig. 3(b). The growth of
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diamond like carbon is attributed to the C2, CH, CH2 and CH3. The emission band of CH is
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located at 430-435 nm and that of C2 are at 465, 516 and 558 nm.
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Fig.6
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It is worth noting that the CH, CH2 and CH3 radicals are active species for DLC growth in
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methane plasma [21, 39]. As shown in Fig. 6, CH3 and CH2 radical lines have not been
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detected in our RF Ar/CH4 plasma jet. The CH3 radical has a long lifetime (about several
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milliseconds) in plasma [21] so it is supposed that the absence of CH3 radical line in emission
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spectra of plasma is due to CH3 radical does not emit light in our spectrograph range. Also
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the CH2 radical is immediately converted into CH radical through the following pathway
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[39]:
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Therefore, in Ar/CH4 plasma, CH is formed directly from the H abstraction of the CHx
17
fragments by electron impact dissociation, and C2 species are formed mainly by
18
recombination of CHx species [21].
19
Relation between active species of CH and C2 with plasma jet length has been shown in Fig.
20
7. The position of CH and C2 bands are at 430-435 nm and 516 nm, respectively. As can be
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seen in this figure, the concentration of CH radical and C2 species reduce with increasing the
22
distance from jet exit. This result shows that for high deposition rate, the distance between
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the nozzle exit and sample must be low.
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In non-thermal plasma (also called non-equilibrium plasmas) the species such as neutral, ions,
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radicals and electrons have different energy states. The electron temperature is much higher
4
than the kinetic temperature of heavy particles (gas temperature) in non-equilibrium plasma.
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Electron temperature (Te) is one of the basic parameters in gas discharge physics because of
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its role in the excitation, dissociation, and ionization of atoms and molecules. For
7
measurement of the kinetic temperature of free electrons, the electronic excitation
8
temperature (Texc) must be evaluated [40]. This temperature is widely utilized in high
9
pressure plasma and Te measurement is based on Texc determination because of their close
10
relationship. If a system obeys the local thermodynamic equilibrium (LTE), Texc is close to
11
Te[40, 41]. A number of different methods based on optical emission spectroscopy for
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evaluating electron excitation temperature Texe, have been described. In the presence of local
13
thermodynamic equilibrium for plasma, the Boltzmann technique is effective. For
14
atmospheric pressure plasmas where the collision frequency is dominant over radiative decay
15
probability of the excited states, the upper energy levels of the selected atomic transitions are
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in local thermodynamic equilibrium, so the density of these levels were obtained with the
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Boltzmann law [42]. Based on above assumptions, the Te has been estimated by conventional
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Boltzmann technique, given by [43, 44]:
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I ij λij ln Ag ij i
− Ei = k BTe
(2)
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Where Iij is the relative intensity of the emitted line for transition level i to j, k B is the
21
Boltzmann constant, λij is wavelength, gij is statistical weight, Aij is transition probability and
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Ei is energy of the upper level. The atomic lines of Ar (738, 763, 794, 801, 811, 826 and
23
912nm) have been used to estimate the Te. Spectroscopic data of ArI emission lines 8
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[45]. Plot of the ln ( I ij λij Aij gi ) versus the energy of the upper level, Ei (ev) gives a straight
3
line slope of 1 k B Te that Figure 8 represents the electron temperature at the nozzle exit and 1
4
cm far from the nozzle that were obtained by Boltzmann plot assuming local thermodynamic
5
equilibrium.. As shown in this figure, Te decreases from 1.36 eV to 1.16 eV and then 0.87 eV
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with the increase in distance from the jet exit.
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Fig.8
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The Saha-Boltzmann equation was used for electron density evolution [46]:
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Where I z* = ( I z λij , z ) / ( gij , z Aij , z ) and X z is the ionization energy of the species in the ionization
12
stage z. As shown in Fig. 9, to evaluate at three points, it is better to average the electron
13
density obtained using intensity ratio of ArI lines to ArII lines (696 nm ArI to 648 nm ArII
14
and 763 ArI to 648 nm ArII). Figure 9 indicates that the temperature and density of the
15
plasma electrons diminish after exiting the nozzle. The electron temperature is 1.35 and 0.85
16
eV at the nozzle exit and 1 cm far from the nozzle respectively. On the other hand, the
17
electron density reduces from 2.7 × 1014 to 0.3 × 1014 cm-3. The higher electron temperature
18
and density, the closer distance to the plasma jet exit.
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Deposition of diamond like carbon by RF atmospheric pressure plasma jet with Ar as
3
dilution gas and methane as precursor gas was reported successfully. The structure and
4
morphologies of DLC thin film were analyzed by laser Raman spectroscopy and field
5
emission scanning electron microscope (SEM). The result obtained from laser Raman
6
spectroscopy demonstrated that integrated intensity ID/IG is 0.47, indicating high quality of
7
DLC film and relative high sp3 content. Deposition rate of 21.31nm/min was obtained at
8
applied RF power of 250 Watt shaving substrate temperature of 130 oC. OES was used to
9
identify active species of plasma jet for deposition of DLC and to evaluate the electron
10
temperature and density. The electron temperature and density of the atmospheric pressure
11
RF argon/methane plasma jet were evaluated to be 0.87 eV and 1×1010 (1/cm3) respectively,
12
at the DLC deposition place. It should be noticed that the CH, CH2 and CH3 radicals are
13
active species for DLC growth and C2 species are formed mainly by recombination of CHx
14
species.
15
Reference
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Figure Captions Fig. 1: Experimental setup for diamond like carbon deposition process1: Grounded electrode 2: Powered electrode 3: Alumina dielectric 4: Substrate
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5: Digital mass flow meter
Fig. 2: Time dependence of discharge voltage and current a) In the presence of argon gas b) In the presence of argon/methane gases
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Fig. 3: a) Image of argon plasma jet with length of 3.5 cm b) Plasma jet in the presence of argon/methane gases with length of 1 cm c) Image of plasma jet
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during deposition process d) DLC film prepared by the RF argon/methane plasma jet
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Fig. 4: The Raman spectra of DLC film, prepared by RF plasma jet
Fig. 5: SEM morphology of DLC film a) The top view b) Cross sectional
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Fig.6: The emission spectra of Ar/methane plasma formed in atmospheric
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pressure RF plasma jet a) 0 cm b) 0.5 cm c) 1 cm from the jet exit.
Fig. 7: Relationship between active species with plasma jet length a) C2 b) CH
Fig.8: The electron temperature of Ar/methane plasma formed in atmospheric pressure RF plasma jet a) 0 cm b) 0.5 cm c) 1 cm from jet exit
Fig.9: Relationship between electron temperature and density with plasma jet length
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Fig. 1
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Table Captions Table 1: The parameters for DLC film deposition
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Table 2: Spectroscopic data of ArI emission lines selected for estimating electron
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temperature [45]
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Table 1 Deposition time(min)
Power (Watt)
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0.1
3 5
250
Substrate temperature (.c) 130
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Distance from jet exit to substrate (mm) 10
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Methane (Lit/min)
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Ar (Lit/min)
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Table 2
Ei (ev)
nm 738 763 794 801 811 826 912
0.08 47 0. 245 0.1 86 0.09 28 0.3 31 0.1 53 0.1 89
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13.30 13.17 13.28 13.09 13.07 13.32 12.90
Aij (108 / s)
gi 5 5 3 5 7 3 3
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Wavelength (λij) nm
ACCEPTED MANUSCRIPT Highlights The growth of DLC using atmospheric pressure plasma jet. Investigation of structural and morphological properties of DLC deposited thin film.
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Estimation of plasma jet electron temperature and density using OES analysis.