AIP ADVANCES 5, 097137 (2015)

Atmospheric pressure plasma polymerization using doublegrounded electrodes with He/Ar mixture

Dong Ha Kim,1 Hyun-Jin Kim,1 Choon-Sang Park,1 Bhum Jae Shin,2Jeong Hyun Seo,3 and Heung-Sik Tae1,a1School of Electronics Engineering, College of IT Engineering, Kyungpook NationalUniversity, Daegu, 702-701, South Korea2Department of Electronics Engineering, Sejong University, Seoul 143-747, South Korea3Department of Electronics Engineering, Incheon National University,Incheon 406-772, South Korea

(Received 26 June 2015; accepted 1 September 2015; published online 11 September 2015)

In this study, we have proposed the double grounded atmospheric pressure plasmajet (2G-APPJ) device to individually control the plasmas in both fragmentation (oractive) and recombination (or passive) regions with a mixture of He and Ar gases todeposit organic thin films on glass or Si substrates. Plasma polymerization of acetonehas been successfully deposited using a highly energetic and high-density 2G-APPJand confirmed by scanning electron microscopy (SEM). Plasma composition wasmeasured by optical emission spectroscopy (OES). In addition to a large numberof Ar and He spectra lines, we observed some spectra of C2 and CH species forfragmentation and N2 (second positive band) species for recombination. The exper-imental results confirm that the Ar gas is identified as a key factor for facilitatingfragmentation of acetone, whereas the He gas helps the plume of plasma reach thesubstrate on the 2nd grounded electrode during the plasma polymerization process.The high quality plasma polymerized thin films and nanoparticles can be obtained bythe proposed 2G-APPJ device using dual gases. C 2015 Author(s). All article content,except where otherwise noted, is licensed under a Creative Commons Attribution 3.0Unported License. [http://dx.doi.org/10.1063/1.4931036]

I. INTRODUCTION

Polymer thin films can be prepared by various techniques such as a chemical synthesis, elec-trochemical polymerization, and plasma enhanced chemical vapor deposition (PECVD). In general,most plasma polymerization has been performed at low pressure (less than a few Torr) in orderto form uniform films and obtain high molecular weight. Vacuum plasmas1,2 and radio frequency(RF) atmospheric plasmas are the most common methods for depositing plasma-derived thin filmsand nanoparticles.3,4 However, the necessary equipment is difficult to operate and maintain, as wellas being large and expensive because of the vacuum process and matching, respectively. Recently,many groups developed the atmospheric pressure glow plasma system to reduce the cost of theapparatus and to simplify the experimental operations.5–9 In spite of intensive studies, only a fewcases have reported a successful polymerization for electronic and optics based applications.7–9 Thisis mainly because organic films often show poor thermal and chemical stabilities, poor mechan-ical toughness, and low molecular weight due to use of plasma with low density and electrontemperature in monomer activation (or fragmentation) region. Therefore, it is very important toincrease the density and electron temperature of plasma during plasma polymerization processes toobtain the high quality and high molecular weight for a variety of industrial applications. Recentresearch confirms the usefulness of one or more grounded electrode on plasma jet in specific appli-cations, in which a plasma with high density and electron temperature between the applied voltage

aAuthor to whom correspondence should be addressed; electronic mail: hstae@ee.knu.ac.kr

2158-3226/2015/5(9)/097137/8 5, 097137-1 ©Author(s) 2015

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097137-2 Kim et al. AIP Advances 5, 097137 (2015)

and grounded electrodes during the plasma polymerization process occurs.3,4 However, the plasmabehavior and polymerized characteristics in both fragmentation (or active) and recombination (orpassive) regions were not investigated in detail.

In this study, we have proposed the double grounded atmospheric pressure plasma jet (2G-APPJ) device to individually control the plasmas in both fragmentation and recombination regions.Unlike the conventional APP jet with only one grounded electrode, the newly proposed 2G-APPJdevice having the auxiliary grounded electrode, twined around the side tube, can separately controlthe fragmentation and recombination regions in combination with gas-flow configuration, whichresults in generating the plasma with high plasma particle energy in both regions. Furthermore, byemploying the dual gas mixture of Ar and He, the proposed 2G-APPJ enables the Ar plasma tofacilitate the fragmentation of the acetone monomer within the tube, and simultaneously enables Heto enhance the propagation of plasma plume toward the substrate on the 2nd grounded electrode.10,11

Accordingly, plasma polymerization of acetone monomer has been successfully deposited using anovel atmospheric pressure plasma jet, featuring the double grounded electrodes with dual gases ata sinusoidal wave with low frequency. The experimental results confirm that the discharge inten-sities in both fragmentation and recombination regions can be significantly improved thanks to theuse of the two grounded electrodes, and therefore, the high quality plasma polymerized thin filmsand nanoparticles can be obtained.

II. EXPERIMENTAL

Figure 1 shows a schematic diagram of experimental setup for proposed 2G-APPJ devicewith four different gas-flow configurations (cases I-IV). The 2G-APPJ device consists of one highvoltage electrode and two grounded electrodes. The high voltage (H.V.) electrode (copper ring-type)and the 1st ground electrode (copper ring-type) were twined around the main tube and side tube,respectively. Whereas the 2nd ground electrode was adjoined to the indium tin oxide (ITO) side. Thesinusoidal power supply was connected to the high voltage electrode and two grounded electrodeswith a peak value of 10 kV and a frequency of 25 kHz. The inner and outer diameters were 12.5 and14.0 mm for the main tube, and 2.5 and 4.0 mm for the downstream tube, respectively. The dimen-sion of the side tube was equal to that of the downstream tube. For each four case, we changed twogases flowing through the main and side tubes. In cases I and II, only Ar and He gases were used,respectively. In case III, the Ar gas flowed through the main tube and He gas through the side tube,while the He gas flowed through the main tube and Ar gas through the side tube in case IV. A smallquantity of acetone vapor was added to the gas, which flowed through the main tube by bubblingmethod of liquid acetone monomer. The main gas and bubbling gas flow rates were fixed at 500and 150 sccm, respectively. The gas flow rate flowing through the side tube was fixed at 650 sccm.The striking feature of 2G-APPJs can generate two distinct plasma jets in the fragmentation andrecombination regions, respectively. In the fragmentation region where the high voltage of 10 kVwas applied between the H.V. electrode and the 1st grounded electrodes, the acetone monomer wasmixed with gases from both tubes and the discharge was initiated near the HV electrode, whichwas on the downstream tube. Consequently, the plasma jet was initiated in mostly pre-dissociatedacetone in fragmentation region. Whereas, in the recombination region where the high voltage of10 kV was simultaneously applied between the H.V. electrode and the 2nd electrode, the plasma jetwas produced by the flow of the gas mixtures (He and Ar) with the cracked acetone fragments fromthe main and side glass tube, thus inducing the plasma polymerization.

The voltage and discharge current were measured by the high voltage probe (TektronixP6015A) and current monitor (Pearson 4110), respectively. The photo sensor amplifiers (Hama-matsu C6386-01) were used to observe plasma emissions in both fragmentation and recombinationregions, respectively. The measurement positions of the two optical fibers from the photo sensoramplifier were 2 and 5 cm from the ITO glass (2nd grounded electrode) for vertical location, respec-tively, and 1 cm from the glass tube for horizontal location. An optical emission spectrometer (OES,Ocean Optics USB-4000UV-VIS) was employed to verify the excited N2, Ar, He, and carbonaceousspecies. The optical emission spectra were collected from the OES behind the ITO glass (2nd

grounded electrode). All photographs of the devices and plasma plumes were taken with a DSLR

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FIG. 1. Schematic diagram of experimental setup employed in this paper and various gas-flow configurations (cases I-IV).

camera (Nikon D56300) with a Macro 1:1 lens (Tamron SP AF 90 mm F2.8 Di). The substratesused for the plasma polymerization were Si wafers and glasses. Before plasma polymerization, theslide glass was ultrasonically cleaned in 99.99% acetone, isopropanol, and distilled (DI)-water for20 min, respectively, so as to remove contamination on the surface of the substrates. The cleanedsubstrates located at a distance of 2 cm below the end of the main tube were polymerized for60 min. for each four case. The field emission scanning electron microscopy (FE-SEM, HITACHISU8220) was employed to analyze the surface morphology.

III. RESULTS AND DISCUSSION

Figure 2 shows the plasma images of fragmentation and recombination regions of the 2G-APPJunder various gas-flow configurations (cases I- IV). In case I (Main tube: Ar and Side tube: Ar), thestrong visible emissions were observed in fragmentation region. However, weak visible emissionswere monitored in recombination region, meaning that the propagation distance of the Ar-acetoneplasma plume was too short to reach the 2nd grounded electrode. This absence of the outer dischargein case I may have more to do with the lower conductivity of Ar versus He.10,11 On the contrary,in case II (Main tube: He and Side tube: He), the propagation distance of the He-acetone plasma

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FIG. 2. Images of plasmas produced in both fragmentation and recombination regions of 2G-APPJ under various gas-flowconfigurations (case I: only Ar, case II; only He, case III: He gas flowing through 1st grounded electrode in Ar/He gasmixtures, and case IV: Ar gas flowing through 1st grounded electrode in He/Ar gas mixtures).

plume was long enough to reach the 2nd grounded electrode in recombination region, however, theemission intensity in fragmentation region was decreased in comparison with the case I. In case III(Main tube: Ar and Side tube: He), the visible emissions in both fragmentation and recombinationregions were weakened in comparison with the cases I and II. The observation of intense visibleemissions from the discharge region confirmed that a strong plasma could be produced in both frag-mentation and recombination regions in case IV (Main tube: He and Side tube: Ar). In particular,we observed that the strong green visible light was produced in fragmentation region; it was mainlydue to more creation of emission of species such as CH [431.3 nm A2∆ → X2] and C2[516.4 nmA3 → X3] peak.7–9,12 These optical emission spectra will be shown in Fig. 4. The observationof the emission of green light in fragmentation region implied that the fragmentation process waseffectively carried out in the gas-flow configuration of the case IV. In addition, we also observed thatthe plasma plume could reach the 2nd grounded electrode.10,11

Figure 3 shows the applied voltage, discharge current, and optical intensity measured in bothfragmentation and recombination regions relative to various gas-flow configurations (cases I-IV).The optical peaks of Fig. 3 were mainly emitted from the Ar excited species and not He excitedspecies because the measurement scale of the used photo sensor amplifiers was just fixed at Aremission intensity due to the large difference between the Ar and He emission intensities.13 Asshown in Fig. 3, the high optical emissions were observed in fragmentation region in the caseof flowing the Ar gas through the first active discharge path between the HV and 1st groundedelectrode (cases I and IV), whereas the high optical emissions were observed in recombinationregion in the case of flowing the Ar and He mixture gases through the second active discharge pathbetween the HV and 2nd grounded electrode (cases III and IV). By flowing the Ar instead of Hegas through the first active discharge path between the HV and 1st grounded electrode, the emissionintensity of the fragmentation region in cases I and IV was increased, which appeared presum-ably due to the better energy transfer efficiency between Ar-acetone mixtures in He-Ar-acetonedischarge.7–9 On the other hand, no optical emission peak of case I was observed in recombinationregion, meaning that it was not easy for the plasma plume without He flow to escape the maintube toward downstream tube. Unlike the case I, high optical emission peaks were observed in thecase of flowing the Ar and He mixture gases through the second active discharge path betweenthe HV and 2nd grounded electrode, as shown in cases III and IV, implying that the propagationof the plasma plume was improved mainly due to the increase in the conductivity by flowing theHe gas in the Ar-acetone mixtures. The discharge currents of Fig. 3 were a sum of two differentdischarge currents flowing through the parallel-connected electrical path made by the H.V. electrode

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FIG. 3. Applied voltages, discharge currents and optical intensities measured in both fragmentation and recombinationregions of 2G-APPJ under various gas-flow configurations (cases I-IV).

and the two grounded electrodes. For the cases of I, III, and IV, the ignition time and magnitudeof the total discharge currents were exactly matched to those of the optical emissions measured inboth fragmentation and recombination regions. The optical emission measurement result of Fig. 3confirms that the best experimental gas configuration of the proposed 2G-APPJ is the case IV,implying that the flow of He gas through the main tube and the flow of Ar gas through the side tubewith the 1st grounded electrode can be suitable for a sufficient fragmentation of acetone monomerin fragmentation region and an efficient generation of abundant radical species in recombinationregion.

Figure 4 shows the emission spectra from 200 to 900 nm relative to various gas-flow configu-rations with magnifying from 300 to 600 nm to verify the excited N2, Ar, He, and carbonaceousspecies. Some spectra from carbonaceous species emitted during fragmentation process of theacetone monomer, such as CH [431.3 nm A2∆ → X2] and C2[516.4 nm A3 → X3],wereonly monitored in cases I and IV. It means that Ar plasma can facilitate an efficient fragmentationof the acetone monomer in fragmentation region. On the contrary, He plasma do not contributethe fragmentation of the acetone monomer in fragmentation region. Interestingly in our

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FIG. 4. Optical emission spectrum of full spectrum plots and magnified region from 300-600 nm of 2G-APPJ under variousgas-flow configurations (cases I-IV).

experiment, when He gas was mixed with Ar gas in cases III and IV, He peaks were not de-tected, as shown in Fig. 4. However, the peaks of excited N2 such as 337.1, 353.7, 357.7, 375.5,380.5 nm and strong Ar peaks were observed. Metastable species of both He and Ar can excitestrong N2 emission in open-air plasma jets. These results imply that the use of He gas withAr gas can increase a conductivity of the plasma plume in the He-Ar-acetone discharge. There-fore, thanks to the presence of He, the plasma can propagate further toward the substrate andthe corresponding interaction with ambient air is improved, thereby resulting in generating moreexcited nitrogen species in the vicinity of the substrate especially in case IV. This result showsthat the gas-flow configuration of case IV is expected to be suitable for both fragmentation andrecombination processes to obtain the high quality polymer thin film in the proposed 2G-APPJdevice.

Figure 5 shows the SEM images of the polymerized acetone thin film grown for 60 min onGlass and Si wafer relative to four different gas-flow configurations (cases I-IV). The distance be-tween the jet end and the 2nd grounded electrode was fixed at 2 cm in cases I-IV, as shown in Fig. 1.In addition, it was located at 5 and 6 cm in case IV. In case I, the thin films were observed to becracked, meaning that the polymer layer was very weak because the weak plasma was produced inrecombination region. In cases II and III, the thin films were not observed, meaning that the crack ofthe acetone monomer was not produced during the plasma polymerization process due to the weakplasma especially in fragmentation region. However, high quality acetone polymer was observed incase IV. As shown in Fig. 5, many spherical particles were gathered into clumps which tended toform an irregular cross-linked network in case IV, meaning that the high quality plasma polymer-ized thin films and nanoparticles could be obtained in the proposed 2G-APPJ with proper gas-flowconfiguration. In addition, as shown in Fig. 5, case IV (5 cm-distance) and case IV (6 cm-distance)also showed the SEM images of the plasma polymerized thin films. In these cases, the outer plasmaplume was unstable because the plasma plume was not able to reach the 2nd grounded electrode [notshown here], thereby resulting in inducing the rod type polymerized layer. The detailed reason isnot clear and need further study. Consequently, these experimental results confirm that the proposed2G-APPJ device driven at 25 kHz by the sinusoidal high voltage waveform can obtain the high

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FIG. 5. SEM images of plasma polymerized thin film grown on Glass and Si wafer under various gas-flow configurationswhere distance between jet end and 2nd grounded electrode is fixed at 2 cm in cases I-IV: extra two cases (5 cm-distance caseand 6-cm distance case) are additionally studied.

quality plasma polymerized thin films and nanoparticles by individually controlling the Ar and Heplasmas in both fragmentation and recombination regions.

IV. CONCLUSIONS

This paper investigates the double grounded atmospheric pressure plasma jet (2G-APPJ) devicefor plasma polymerization. By individually feeding gases under various gas-flow configurations, theelectrical and optical emission characteristics of plasmas were compared. When feeding the He gasto the main tube and Ar gas to the side tube, the strong plasma was generated between the H.V.electrode and the auxiliary 1st grounded electrode on the side tube, and also it can easily interactwith ambient air by using the 2nd grounded electrode. In this case, high quality acetone polymeron the glass and Si wafer were obtained, respectively. These results may be caused by its sufficientfragmentation inside the tube and the supply of abundant radicals by comparing other cases. In addi-tion, this newly proposed 2G-APPJ device can control plasmas in the active and passive regions,respectively, and is expected to be used for plasma polymerization with various monomers.

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ACKNOWLEDGMENTS

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