APPLIED PHYSICS LETTERS 89, 161504 共2006兲

Discharge characteristics of atmospheric-pressure radio-frequency glow discharges with argon/nitrogen Hua-Bo Wang, Wen-Ting Sun, He-Ping Li,a兲 and Cheng-Yu Bao Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China

Xing Gao School of Public Health and Family Medicine, Capital University of Medical Sciences, Beijing 100069, People’s Republic of China

Hui-Ying Luo Beijing Center for Diseases Control and Prevention, Beijing 100013, People’s Republic of China

共Received 5 July 2006; accepted 29 August 2006; published online 19 October 2006兲 In this letter, atmospheric-pressure glow discharges in ␥ mode with argon/nitrogen as the plasma-forming gas using water-cooled, bare copper electrodes driven by radio-frequency power supply at 13.56 MHz are achieved. The preliminary studies on the discharge characteristics show that, induced by the ␣-␥ coexisting mode or ␥ mode discharge of argon, argon-nitrogen mixture with any mixing ratios, even pure nitrogen, can be employed to generate the stable ␥ mode radio-frequency, atmospheric-pressure glow discharges and the discharge voltage rises with increasing the fraction of nitrogen in the argon-nitrogen mixture for a constant total gas flow rate. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2362631兴 Glow discharges at atmospheric pressure exhibit tremendous advantages over traditional low-pressure glow discharges.1 As one of the atmospheric-pressure glow discharge 共APGD兲 plasma sources, the uniform APGD plasmas using bare metal electrodes driven by radio-frequency 共rf兲 power supply can be generated with much lower breakdown voltage compared with atmospheric-pressure dielectric barrier discharges,1 and thus, possess promising prospects in different fields, such as plasma-aided etching,2 deposition,3 decontamination of chemical and/or biological warfare agents,4 etc. However, up to now, only helium or argon can be employed as the primary plasma-forming gas for generating the rf APGD plasmas using bare metal electrodes, and the amount of the admixed gas 共e.g., O2, N2, CF4, etc.兲 used for producing a flux of chemically active species is also very small.1–8 In actual applications, the chemically active species and their concentrations in plasmas can influence the qualities of the treated materials obviously. Nitrogen-containing plasmas have been employed widely for improving surface hardness, wear and corrosion resistance, fatigue life of different materials, as well as for changing the wettability and promoting adhesion of the surfaces of organic polymers.9,10 It is indicated that a high atomic nitrogen content produced by plasmas is necessary for accomplishing these processes.9 In this letter, uniform rf APGD plasmas operated in a ␥ mode for argon-nitrogen mixture with any mixing ratio ␹关=QN2 / 共QN2 + QAr兲兴 are generated using water-cooled, bare copper electrodes. A schematic diagram of the experimental setup is shown in Fig. 1. The plasma generator is composed of two 5 ⫻ 8 cm2 planar, bare, water-cooled copper electrodes, which are connected to the rf 共13.56 MHz兲 power supply through a manually adjustable matching network and the ground, respectively. The electrode surfaces are polished before experi-

ments. Teflon spacers are used to seal the plasma generator on both sides and adjust the distance between the electrodes. In this study, the gap spacing between electrodes is 1.2 mm. The plasma-forming gas 共argon with purity of 99.999% or better and/or nitrogen with purity of 99.999% or better兲 is admitted into the plasma generator from the left side, flows through a cross-section-variable channel 共gradually converged in the direction normal to the electrode surface and enlarged in the direction parallel to the electrode surface兲 for improving the uniformity of the flow field before the gas enters the discharge region, ionized between electrodes under the applied rf electric field, and flows out of the generator through a rectangular outlet 共1.2⫻ 80 mm2兲 on the right side. In this experiment, a current probe 共Tektronix TCP202兲 and a high voltage probe 共Tektronix P5100兲 are used to measure the current 共including the electron current and the displacement current11兲 passing through the plasma generator and the voltage between the electrodes. The signals detected by these two probes are input into a digital oscilloscope 共Tektronix TDS3054B兲, which can give the root-mean-square values and wave forms of the current and voltage, as well as the current-voltage phase difference 共␪兲. The discharge pictures

a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

FIG. 1. Schematic diagram of the experimental setup.

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FIG. 2. 共Color online兲 V-I characteristics for argon/nitrogen discharge, d = 1.2 mm; A–F: pure argon discharge, QAr = 5.0 SLPM; G-H: argon-nitrogen mixture discharge, QN2 = QAr = 2.5 SLPM; I-J: pure nitrogen discharge, QN2 = 5.0 SLPM.

are taken by a digital camera 共Fujifilm S5500兲. Similar to discharges at intermediate pressure, rf APGD plasmas can operate in two distinctively different, but stable modes, i.e., ␣ mode and ␥ mode. Externally, these two modes differ in the intensity and luminosity along the discharge length. Essentially, they differ in the ionization processes, i.e., the ␣ mode discharge is sustained by volumetric ionization process, while ionization by secondary electrons from the electrode surfaces is important for the ␥ mode discharge.11,12 It is found in this study that the fraction of nitrogen added into a uniform, ␣ mode, rf APGD plasma with argon is very limited, but it is much easier to add any amount of nitrogen into an argon rf APGD plasma operated in an ␣-␥ coexisting mode or ␥ mode, and even a uniform, ␥ mode, rf APGD plasma with pure nitrogen using watercooled, bare copper electrodes can be achieved. With this method, the expensive helium is avoided for obtaining nitrogen rf APGD plasmas, as that presented in Ref. 13. The voltage-current characteristic 共V-I curve兲, representing the whole discharge process from the ignition with pure argon to a stable and uniform rf APGD plasma in the ␥ mode with pure nitrogen, for the case with initial argon flow rate QAr = 5.0 SLPM 共SLPM denotes standard liters per minute兲, is shown in Fig. 2. Before breakdown 共A-B兲, the voltage increases linearly with current due to the purely capacitive character of the discharge circuit; at point B, breakdown occurs accompanied by a large voltage drop and a uniform glow discharge in a ␣ mode appears covering the whole area of the water-cooled bare copper electrodes; then, the discharge voltage goes up with increasing the discharge current when the argon discharge lies in the ␣ mode 共C-D兲; at point D, an abrupt restructuring of the discharge occurs accompanied by a breakdown of the ␣-discharge sheaths,8,11 i.e., several filamentlike contracted positive columns, which is the so-called ␥ discharge, appear and are surrounded by the ␣ mode discharge region, resulting in the ␣-␥ coexisting discharge mode, where the discharge voltage varies a little with the increase of the rf current 共E-F兲 by adjusting the rf power input, corresponding to an average voltage of 153.0± 0.8 V. Under such condition, nitrogen can be added into argon and a uniform rf APGD plasma in the pure ␥-discharge mode is obtained for argon-nitrogen mixture 共QN2 = QAr = 2.5 SLPM兲 corresponding to the line G-H, where with decreasing the discharge current, the discharge voltage also varies a little

Appl. Phys. Lett. 89, 161504 共2006兲

FIG. 3. 共Color online兲 Photographs of the rf APGD plasmas with gap spacing d = 1.2 mm, where the white solid lines indicate the positions of the electrodes. 共a兲 ␣-␥ coexisting discharge mode with pure argon, QAr = 5.0 SLPM, Pin = 297 W; 共b兲 ␥ mode discharge with argon-nitrogen mixture, QN2 = QAr = 2.5 SLPM, Pin = 368 W; and 共c兲 ␥ mode discharge with pure nitrogen, QN2 = 5.0 SLPM, Pin = 384 W.

with an average voltage of 218.5± 3.5 V; decreasing the argon flow rate, and simultaneously, increasing the flow rate of nitrogen. Finally, a stable rf APGD plasma with pure nitrogen 共QN2 = 5.0 SLPM兲 operated in the ␥ mode is achieved, which corresponds to the V-I curve I-J, where the discharge voltage varies a little, too, when decreasing the discharge current with an average voltage of 267.2± 1.3 V. The experiments in this study show that 共1兲 once a ␥ mode discharge with argon appears, the preceding method can be employed to generate a stable glow discharge with argon-nitrogen mixture or pure nitrogen, also working in a ␥ mode; 共2兲 keeping the total plasma working gas flow rate QN2 + QAr = 5.0 SLPM, the discharge voltage rises with the increase of the mixing ratio ␹ from 0 to 1.0, and the corresponding discharge voltages for pure argon and pure nitrogen are 155.2± 0.8 and 273.7± 3.7 V, respectively; 共3兲 in the discharge process corresponding to the V-I curve E-F in Fig. 2, the coverage of the electrodes with the ␥ mode discharge and the number of the filamentlike columns increases with increasing the discharge current; and 共4兲 while for the discharge processes indicated by the lines G-H and I-J in Fig. 2, the ␥-discharge region only covers a small part of the electrodes, and initially the area of the ␥ mode discharge region increases with increasing the discharge current, and then, no obvious enlargement of the ␥ mode discharge region on the electrodes is observed with the further increase of the discharge current. The discharge pictures are taken by a digital camera and presented in Figs. 3共a兲–3共c兲 with the shutter speeds of 1 / 315, 1 / 416, and 1 / 630 s, respectively. The white solid lines in Fig. 3 indicate the positions of the electrodes. Figure 3共a兲 shows the discharge picture of a glow discharge in the ␣-␥ coexisting mode for pure argon 共QAr = 5.0 SLPM兲 with the power input Pin = 297 W. In experiment, the positions of the ␥ mode discharge region are random, and in Fig. 3, the ␥ mode discharge stays in the left part of the discharge region with many brighter filamentlike columns, while the uniform ␣ mode discharge covers the other part between the electrodes. Keeping the total plasma-forming gas flow rate constant, decreasing the argon flow rate, and simultaneously, increasing the flow rate of nitrogen, stable ␥ mode dis-

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FIG. 4. Voltage 共solid兲 and current 共dashed兲 wave forms of rf APGD plasma with pure nitrogen in ␥ mode; d = 1.2 mm, QN2 = 5.0 SLPM, Pin = 384 W, and ␪ = 38°.

charges for argon-nitrogen mixture 共QN2 = QAr = 2.5 SLPM兲 and for pure nitrogen 共QN2 = 5.0 SLPM兲 are achieved with Pin = 368 and 384 W, as shown in Figs. 3共b兲 and 3共c兲, respectively. It can be seen from Fig. 3 that under the same gap spacing between electrodes and the constant total gas flow rate, with increasing the gas mixing ratio 共␹兲, the ␣ mode discharge region coexisting with the ␥ discharge for pure argon disappears for the case using argon-nitrogen mixture or pure nitrogen; the luminous structure of the ␥ discharge for argon is significantly different from those for argonnitrogen mixture or pure nitrogen. There exist very bright filamentlike contracted columns in the argon ␥ mode discharge region, which is consistent with experimental observations presented by Laimer et al.7 while for the case with addition of nitrogen in argon or with pure nitrogen, the ␥ mode discharge region is columnar and uniform. In this study, no high intensity afterglow for the ␥ discharge is observed with the naked eyes and no transitions from glowlike discharge to arc occur. The experimental results show that the current wave forms always lead the voltage wave forms by ⬃40° for the cases studied in this letter, which indicates the capacitive nature of the discharge. The wave forms of the discharge voltage and current are shown in Fig. 4 for a ␥ mode discharge of pure nitrogen with ␪ = 38°. By analyzing the wave forms using fast Fourier transform, it is found that the amplitudes of the third and fifth harmonics of the voltage14 wave form are 13.0% and 8.9% of the fundamental wave, while in case of the current, the amplitudes of the third, fifth, and seventh harmonics reach the values of 4.6%,

4.6%, and 6.3% of the fundamental wave, respectively. In summary, the rf APGD plasmas operated in a ␥ mode using bare copper electrodes with different argon-nitrogen gas mixing ratios 共␹兲 varying from 0 to 1.0 共where ␹ = 0 and 1.0 correspond to the cases of pure argon and pure nitrogen discharges, respectively兲 are obtained. The observed discharge images and the corresponding V-I characteristics are different for different plasma-forming gases 共i.e., different gas mixing ratios兲 with the same gap spacing between electrodes and the same total gas flow rate. Further studies concerning the influences of the plasma-working gas flow rate, the gas mixing ratio, etc., on the concentrations and distributions of the chemically active species, the emission intensity profiles of the discharge region, and so on, are needed in future work. This work has been supported by the project sponsored by SRF for ROCS, SEM. 1

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