Argon Plasma Sustained in Different Discharge Configurations

Transcription

1 WDS'1 Proceedings of Contributed Papers, Part II, 76 8, 21. ISBN MATFYZPRESS Argon Plasma Sustained in Different Discharge Configurations L. Schmiedt, V. Hrachová Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. A. Nikiforov, C. Leys Universiteit Gent, Faculty of Engineering, Gent, Belgium. Abstract. Two different types of discharges positive column of the DC glow discharge and DBD discharge are studied in pure argon by means of optical emission spectroscopy. The pressure 1 Pa was kept constant for all our measurements. We have focused on the qualitative differences in emission spectra of both types of discharges. Similar dependences of intensities of studied spectral lines on fed power have been observed. Introduction The DC glow discharges in various gases and its mixtures can be utilized in miscellaneous applications such as plasma etching or plasma treatment of surfaces. The most of these applications usually requires low operating pressure and also high level of system purity. This fact makes experimental set-up more complicated both technically and financially, because some kind of vacuum apparatus is needed. Dielectric-barrier discharges (DBD) became nowadays very popular surface modification technique used for example as a tool for plasma treatment of textiles [Kang and Sarmadi, 4]. The discharge is formed through streamer breakdown, featuring a large number of short-living current filaments on macroscopic scale [Kogelschatz et al., 1997]. One of the crucial discharge parameters influencing the efficiency of the surface treatment is the pressure [De Geyter et al., 8] although DBD discharge itself can be sustained for wide range of pressures. Study of processes in both types of discharges is an important topic. Particularly, possibility of finding the same operating pressure for utilization of both types of discharges give us a unique chance to study physical properties of two different phenomena under the same pressure conditions. Optical emission spectroscopy has already been employed for study of parameters of both types of discharges for various pressures [e.g., Garamoon et al., 7; Shi and Kong, 7; Zhang et al., ; Zhang et al., 2; Yu et al., 8]. Nevertheless, the study comparing spectral properties of both discharges under same pressures is still missing. Series of measurements have been therefore started in our laboratories in order to compare properties of both discharges utilized under so-called medium pressures, i.e. pressures of hundreds of Pascals. The DC glow discharge and DBD discharge were both sustained in argon, which is commonly used as a working gas in various applications. The main aim of our study is quantitative analysis of spectra emitted by particular discharges. The example of the results for the pressure of 1 Pa is shown in presented contribution. Measurements in DC Glow Discharge Experimental The DC glow discharge was sustained in Silica U-shaped discharge tube with inner diameter 22 mm. The central part of the discharge tube was 34 mm long. It was equipped with head-on planar windows on both sides and also with two pairs of cylindrical platinum probes (5 mm long,.1 mm in diameter) used for measurements of electric field strength. Argon of Linde production with declared purity better than 1 ppm was used as an active medium. Spectra of emitted radiation were detected in axial direction by means of monochromator Jobin Yvon-Spex Triax (focal length 55 mm) using plane grating (1 grooves/mm). The spectral resolution of this set-up given by manufacturer was.24 nm. The monochromator was equipped with MTE CCD detector (thermo-electrically cooled) linked to the CCD 3 controller which was connected to the PC. This arrangement allowed us to detect emission spectra in the range 15 nm. The experimental set-up of our measurements is schematically shown in Figure 1. 76

2 Figure 1. Experimental set-up for measurements in DC glow discharge. Discharge tube was heated up to 42 C before each measurement and it was pumped for several hours by diaphragm pump in series with turbomolecular pump. The pressure in vacuum system checked after this procedure by full range gauge was better than Pa. Results Double-probe method [Raizer, 1991] has been employed for determination of axial electric field strength. As can be seen in Figure 2, the values of electric field strength decrease with the discharge current, which is standard behaviour in DC glow discharges. The neutral argon lines were observed in emission spectra of positive column of the discharge within the range of 7 9 nm. We have focused on study of several lines summarized in Table 1. The most intense line was found to be the line 763 nm (transition 2p 6 1s 5 ) which is in good agreement with previous results [Garamoon et al., 7]. The curves of dependences of intensities of particular lines on power fed to the discharge are shown in Figure 3. It can be seen that intensities of all particular lines increase with power fed to the discharge as a result of more efficient excitation by electron impact. This increase seems to be linear. Table 1. Atomic data of studied argon lines [Ralchenko et al., 21]. Line Wavelength [nm] Transition p 3 1s 5 2p 6 1s p 4 1s p 2 1s 2 7 E [V/m] I [ma] Figure 2. Dependence of electric field strength on discharge current (DC glow discharge). 77

3 5 Intensity [cps] Power [W] Figure 3. Dependences of intensities of particular lines on power fed into the DC glow discharge. Measurements in Dielectric-Barrier Discharge Experimental A schematic diagram of the plasma configuration is depicted in Figure 4. Two rectangular copper electrodes (145 mm 8 mm) are placed within a cylindrical enclosure. Upper electrode is covered with a ceramic (Al 2 O 3 ) plate with a thickness of.5 mm and an area of 165 mm 12 mm. The gas gap between the lower electrode and the ceramic plate can be set within a range 1 2 mm. The used working gas was Argon of Air Liquide Alphagaz production. The upper electrode is connected to the pulse generator Velonex 36 with maximal peak amplitude of 25 V and frequency of Hz, while the lower electrode is grounded through a resistor of 1 Ω. The voltage applied to the electrodes was measured using a high voltage probe Tektronix P615A. The discharge current was obtained by measuring the voltage over the resistor of 1 Ω, connected in series to ground. Both parameters were recorded using a digital oscilloscope Tektronix TDS21 6 MHz. Spectra of emitted radiation were analysed by means of USB Fiber Optic Spectrometer produced by Ocean Optics with best possible spectral resolution.3 nm. The experimental system was equipped with oil rotary pump, which enabled us to pump down the system before each set of measurements to a pressure of 13 Pa. Figure 4. Experimental set-up for measurements in DBD discharge. 78

4 Results Emission spectra of DBD discharge were affected by the worse vacuum purity of the system. Clear traces of background molecular nitrogen were found between and 5 nm with relative intensity about 1 % of intensity of 763 nm line, which is more than twice higher compared to the DC glow discharge case. Moreover, broad spectral lines around 3 nm were observed. These are probably of an organic origin, caused as a result of evaporation of organic materials inside the chamber (rubber sealing, glue, plastic electrode holders, etc.). The band at 31 nm also contains probably the band of OH radical. The experimental set-ups in the most already published studies used constant distance between the both electrodes. First we have therefore focused on possible dependence of intensities of particular studied lines on the distance between electrodes. Results can be seen in the left part (a) of the Figure 5. Increase of the gas gap leads to need of higher voltage and charge cumulated on the surface of the ceramics in order to obtain local breakdown. Keeping the amplitude of voltage pulses constant during our measurements lead to worse ionization process, less count of free charges and therefore less count of local breakdowns and micro-discharges. This resulted in decrease of intensities with increasing distance of electrodes, which is in Figure 5 (a) well documented. In order to compare qualitatively spectral properties of both discharges, we also studied the dependence of intensities on power fed to the discharge which is to be seen in the right part (b) of Figure 5. The most intensive line was found to be again argon line nm which is in agreement with previous results for DBD in various operating pressures [e.g. Shi and Kong, 7]. Moreover, it can be seen that intensities of all studied lines increase with increasing power. The nonlinearity of the course will be subject of further studies. Conclusion Optical emission spectroscopy has been employed for study of DC glow discharge and DBD discharge sustained in 1 Pa of pure Argon. Partial quantitative analysis has been performed. Argon line at nm was found to be the most intensive in both DC glow and DBD discharge. Moreover, increase of intensities of studied spectral lines with increasing power fed to the discharge has been observed in both types of discharges. Since corresponding states are excited directly by electron impact, the observed courses can be explained by increasing excitation probability due to increasing number of electrons as a result of increasing power [Raizer, 1991]. In DBD discharge, decrease of intensities of studied spectral lines with increasing distance of electrodes was observed. This fact can be explained by less count of local breakdowns and consequently less count of micro-discharges. In order to explain different ratios of studied spectral lines for both types of discharges, detailed study will follow a b Inrtensity [cps] 1 8 Intensity [cps] d [mm] Power [W] Figure 5. Dependences of intensities of particular lines on the gas gap (a) and power fed into the DBD discharge (b). 79

5 Acknowledgments. This research has been supported by the research plan MSM that is financed by the Ministry of Education of the Czech Republic. The first author would also like to thank to financial support of the Grant Agency of Charles University Prague, Project GAUK-4631/21. References Garamoon A.A. et al., IEEE Transactions on Plasma Science 35 (7), 1. Geyter De, N., R. Morent and Ch. Leys, IEEE Transactions on Plasma Science 36 (8), 138. Kang, J.Y. and M. Sarmadi, AATCC Rev. 1 (4), 28. Kogelschatz, U., B. Eliasson and W. Egli, J.Phys. 7 (1997), 47. Raizer, Y.P., Gas Discharge Physics (1991), Springer-Verlag Berlin Heidelberg, 122. Ralchenko, Y, A.E. Kramida, J. Reader and NIST ASD Team (21). NIST Atomic Spectra Database (ver. 4..), [Online]. Available: [21, November 7]. National Institute of Standards and Technology, Gaithersburg, MD. Shi, J. J. and M. G. Kong, Applied Physics Letters 9 (7), Yu Y., Z. Du, M. Chen and J. Wang, Angew. Chem. Int. Ed. 47 (8), 799. Zhang, J.L., X.L. Deng, P.S. Wang and T.C. Ma, Vacuum 59 (), 8. Zhang, J.L., S.J. Yu and T.C. Ma, Vacuum 65 (2),