Atmospheric pressure plasma polymerization using double grounded electrodes with He / Ar mixture

Atmospheric pressure plasma polymerization using double grounded electrodes with He/Ar mixture Dong Ha Kim,1 Hyun-Jin Kim,1 Choon-Sang Park,1 Bhum Jae Shin,2 Jeong Hyun Seo,3 and Heung-Sik Tae1,a 1School of Electronics Engineering, College of IT Engineering, Kyungpook National University, Daegu, 702-701, South Korea 2Department of Electronics Engineering, Sejong University, Seoul 143-747, South Korea 3Department of Electronics Engineering, Incheon National University, Incheon 406-772, South Korea


I. INTRODUCTION
Polymer thin films can be prepared by various techniques such as a chemical synthesis, electrochemical 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 order to form uniform films and obtain high molecular weight.Vacuum plasmas 1,2 and radio frequency (RF) atmospheric plasmas are the most common methods for depositing plasma-derived thin films and nanoparticles. 3,4However, the necessary equipment is difficult to operate and maintain, as well as being large and expensive because of the vacuum process and matching, respectively.8][9] This is mainly because organic films often show poor thermal and chemical stabilities, poor mechanical toughness, and low molecular weight due to use of plasma with low density and electron temperature in monomer activation (or fragmentation) region.Therefore, it is very important to increase the density and electron temperature of plasma during plasma polymerization processes to obtain the high quality and high molecular weight for a variety of industrial applications.Recent research confirms the usefulness of one or more grounded electrode on plasma jet in specific applications, in which a plasma with high density and electron temperature between the applied voltage and grounded electrodes during the plasma polymerization process occurs. 3,4However, the plasma behavior and polymerized characteristics in both fragmentation (or active) and recombination (or passive) 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-APPJ device having the auxiliary grounded electrode, twined around the side tube, can separately control the fragmentation and recombination regions in combination with gas-flow configuration, which results in generating the plasma with high plasma particle energy in both regions.Furthermore, by employing the dual gas mixture of Ar and He, the proposed 2G-APPJ enables the Ar plasma to facilitate the fragmentation of the acetone monomer within the tube, and simultaneously enables He to enhance the propagation of plasma plume toward the substrate on the 2 nd grounded electrode. 10,11ccordingly, plasma polymerization of acetone monomer has been successfully deposited using a novel atmospheric pressure plasma jet, featuring the double grounded electrodes with dual gases at a sinusoidal wave with low frequency.The experimental results confirm that the discharge intensities in both fragmentation and recombination regions can be significantly improved thanks to the use of the two grounded electrodes, and therefore, the high quality plasma polymerized thin films and nanoparticles can be obtained.

II. EXPERIMENTAL
Figure 1 shows a schematic diagram of experimental setup for proposed 2G-APPJ device with four different gas-flow configurations (cases I-IV).The 2G-APPJ device consists of one high voltage electrode and two grounded electrodes.The high voltage (H.V.) electrode (copper ring-type) and the 1 st ground electrode (copper ring-type) were twined around the main tube and side tube, respectively.Whereas the 2 nd ground electrode was adjoined to the indium tin oxide (ITO) side.The sinusoidal power supply was connected to the high voltage electrode and two grounded electrodes with a peak value of 10 kV and a frequency of 25 kHz.The inner and outer diameters were 12.5 and 14.0 mm for the main tube, and 2.5 and 4.0 mm for the downstream tube, respectively.The dimension of the side tube was equal to that of the downstream tube.For each four case, we changed two gases 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 small quantity of acetone vapor was added to the gas, which flowed through the main tube by bubbling method of liquid acetone monomer.The main gas and bubbling gas flow rates were fixed at 500 and 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 and recombination regions, respectively.In the fragmentation region where the high voltage of 10 kV was applied between the H.V. electrode and the 1 st grounded electrodes, the acetone monomer was mixed with gases from both tubes and the discharge was initiated near the HV electrode, which was on the downstream tube.Consequently, the plasma jet was initiated in mostly pre-dissociated acetone in fragmentation region.Whereas, in the recombination region where the high voltage of 10 kV was simultaneously applied between the H.V. electrode and the 2 nd electrode, the plasma jet was produced by the flow of the gas mixtures (He and Ar) with the cracked acetone fragments from the main and side glass tube, thus inducing the plasma polymerization.
The voltage and discharge current were measured by the high voltage probe (Tektronix P6015A) and current monitor (Pearson 4110), respectively.The photo sensor amplifiers (Hamamatsu C6386-01) were used to observe plasma emissions in both fragmentation and recombination regions, respectively.The measurement positions of the two optical fibers from the photo sensor amplifier were 2 and 5 cm from the ITO glass (2 nd grounded electrode) for vertical location, respectively, 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 N 2 , Ar, He, and carbonaceous species.The optical emission spectra were collected from the OES behind the ITO glass (2 nd grounded electrode).All photographs of the devices and plasma plumes were taken with a DSLR camera (Nikon D56300) with a Macro 1:1 lens (Tamron SP AF 90 mm F2.8 Di).The substrates used for the plasma polymerization were Si wafers and glasses.Before plasma polymerization, the slide glass was ultrasonically cleaned in 99.99% acetone, isopropanol, and distilled (DI)-water for 20 min, respectively, so as to remove contamination on the surface of the substrates.The cleaned substrates located at a distance of 2 cm below the end of the main tube were polymerized for 60 min.for each four case.The field emission scanning electron microscopy (FE-SEM, HITACHI SU8220) 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-APPJ under various gas-flow configurations (cases I-IV).In case I (Main tube: Ar and Side tube: Ar), the strong visible emissions were observed in fragmentation region.However, weak visible emissions were monitored in recombination region, meaning that the propagation distance of the Ar-acetone plasma plume was too short to reach the 2 nd grounded electrode.This absence of the outer discharge in 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 FIG. 2. Images of plasmas produced in both fragmentation and recombination regions of 2G-APPJ under various gas-flow configurations (case I: only Ar, case II; only He, case III: He gas flowing through 1 st grounded electrode in Ar/He gas mixtures, and case IV: Ar gas flowing through 1 st grounded electrode in He/Ar gas mixtures).
plume was long enough to reach the 2 nd grounded electrode in recombination region, however, the emission 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 recombination regions were weakened in comparison with the cases I and II.The observation of intense visible emissions from the discharge region confirmed that a strong plasma could be produced in both fragmentation 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 mainly due to more creation of emission of species such as CH [431.3 nm A 2 ∆ → X 2  ] and C2[516.49]12 These optical emission spectra will be shown in Fig. 4. The observation of the emission of green light in fragmentation region implied that the fragmentation process was effectively carried out in the gas-flow configuration of the case IV.In addition, we also observed that the plasma plume could reach the 2 nd grounded electrode. 10,11igure 3 shows the applied voltage, discharge current, and optical intensity measured in both fragmentation 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 excited species because the measurement scale of the used photo sensor amplifiers was just fixed at Ar emission intensity due to the large difference between the Ar and He emission intensities. 13As shown in Fig. 3, the high optical emissions were observed in fragmentation region in the case of flowing the Ar gas through the first active discharge path between the HV and 1 st grounded electrode (cases I and IV), whereas the high optical emissions were observed in recombination region in the case of flowing the Ar and He mixture gases through the second active discharge path between the HV and 2 nd grounded electrode (cases III and IV).8][9] On the other hand, no optical emission peak of case I was observed in recombination region, meaning that it was not easy for the plasma plume without He flow to escape the main tube toward downstream tube.Unlike the case I, high optical emission peaks were observed in the case of flowing the Ar and He mixture gases through the second active discharge path between the HV and 2 nd grounded electrode, as shown in cases III and IV, implying that the propagation of the plasma plume was improved mainly due to the increase in the conductivity by flowing the He gas in the Ar-acetone mixtures.The discharge currents of Fig. 3 were a sum of two different discharge currents flowing through the parallel-connected electrical path made by the H.V. electrode and the two grounded electrodes.For the cases of I, III, and IV, the ignition time and magnitude of the total discharge currents were exactly matched to those of the optical emissions measured in both fragmentation and recombination regions.The optical emission measurement result of Fig. 3 confirms 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 tube with the 1 st grounded electrode can be suitable for a sufficient fragmentation of acetone monomer in fragmentation region and an efficient generation of abundant radical species in recombination region.
Figure 4 shows the emission spectra from 200 to 900 nm relative to various gas-flow configurations with magnifying from 300 to 600 nm to verify the excited N 2 , Ar, He, and carbonaceous species.Some spectra from carbonaceous species emitted during fragmentation process of the acetone monomer, such as CH [431.experiment, when He gas was mixed with Ar gas in cases III and IV, He peaks were not detected, as shown in Fig. 4.However, the peaks of excited N 2 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 excite strong N 2 emission in open-air plasma jets.These results imply that the use of He gas with Ar gas can increase a conductivity of the plasma plume in the He-Ar-acetone discharge.Therefore, thanks to the presence of He, the plasma can propagate further toward the substrate and the corresponding interaction with ambient air is improved, thereby resulting in generating more excited nitrogen species in the vicinity of the substrate especially in case IV.This result shows that the gas-flow configuration of case IV is expected to be suitable for both fragmentation and recombination processes to obtain the high quality polymer thin film in the proposed 2G-APPJ device.
Figure 5 shows the SEM images of the polymerized acetone thin film grown for 60 min on Glass and Si wafer relative to four different gas-flow configurations (cases I-IV).The distance between the jet end and the 2 nd 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 be cracked, meaning that the polymer layer was very weak because the weak plasma was produced in recombination region.In cases II and III, the thin films were not observed, meaning that the crack of the acetone monomer was not produced during the plasma polymerization process due to the weak plasma especially in fragmentation region.However, high quality acetone polymer was observed in case IV.As shown in Fig. 5, many spherical particles were gathered into clumps which tended to form an irregular cross-linked network in case IV, meaning that the high quality plasma polymerized thin films and nanoparticles could be obtained in the proposed 2G-APPJ with proper gas-flow configuration.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 plasma plume was unstable because the plasma plume was not able to reach the 2 nd grounded electrode [not shown here], thereby resulting in inducing the rod type polymerized layer.The detailed reason is not clear and need further study.Consequently, these experimental results confirm that the proposed 2G-APPJ device driven at 25 kHz by the sinusoidal high voltage waveform can obtain the high

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

FIG. 3 .
FIG. 3. Applied voltages, discharge currents and optical intensities measured in both fragmentation and recombination regions of 2G-APPJ under various gas-flow configurations (cases I-IV).
FIG.4.Optical emission spectrum of full spectrum plots and magnified region from 300-600 nm of 2G-APPJ under various gas-flow configurations (cases I-IV).

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FIG. 5. SEM images of plasma polymerized thin film grown on Glass and Si wafer under various gas-flow configurations where distance between jet end and 2nd grounded electrode is fixed at 2 cm in cases I-IV: extra two cases (5 cm-distance case and 6-cm distance case) are additionally studied.quality plasma polymerized thin films and nanoparticles by individually controlling the Ar and He plasmas in both fragmentation and recombination regions.