Study of DC Cylindrical Magnetron by Langmuir Probe

WDS'2 Proceedings of Contributed Papers, Part II, 76 8, 22. ISBN 978-737825 MATFYZPRESS Study of DC Cylindrical Magnetron by Langmuir Probe A. Kolpaková, P. Kudrna, and M. Tichý Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. We present results of the Langmuir probe measurements in the cylindrical magnetron apparatus in this paper. Measurements were performed in low-temperature and low-pressure neon plasma. This work aims to study the impact of different values of macroparameters (e.g. discharge current, the strength of the magnetic field, pressure) on plasma microparameters (floating potential, plasma potential, electron density). The radial profiles of plasma parameters were obtained from Langmuir probe characteristics. w results of neon plasma are mutually complementary and well comparable with previous measurements in argon. Introduction Cylindrical magnetron is often used for deposition of thin films with interesting physical or chemical properties or materials with special characteristics (e.g. magnetic or optical). The configuration of cylindrical magnetron is relatively simple and the cylindrical symmetry is suitable for computer simulations of measurements [Golubovskii et al., 26]. Experimental data can be compared and verified by results obtained from model. The basic physics and various configuration of magnetron sputtering sources are reviewed in [Thorton, 977]. The principle of the cylindrical magnetron is based on the radial electric field and magnetic field applied parallel to the axis. This configuration of crossed electric and magnetic fields causes ExB drift of charged particles. Electron drift velocity is studied in work [Borah et al., 2]. High-energetic electrons are confined by applied magnetic field and they are responsible for the ionization of the neutral noble gas atoms. Formed positive ions are accelerated towards the negatively biased cathode target and their impact on cathode cause secondary electron emission and sputtering of cathode material. Secondary electrons are also by the influence of Lorentz force confined in E B drift loops in vicinity of cathode and they help to sustain the discharge. The sputtered atoms deposit thin film on the substrate situated near the anode. Understanding the behavior of discharge requires making systematic experimental studies of the spatial and temporal plasma parameters. Langmuir probes are used as suitable experimental tools for determining the plasma parameters during deposition of thin films when the discharge burns. This plasma diagnostic is comparatively cheap and easily technically practicable. Several methods are used for interpretation of probe data [Pfau et al., 2]. Simple explanation of probe characteristic interpretation is described in work [Merlino, 27]. In the previous experiments [Holík et al., 22, 24] the Langmuir probe method was used to study axial and radial dependences of plasma parameters in the argon plasma. In this paper we present radial profiles of floating potential, plasma potential and electron density for neon discharge. Our results of measurements in neon plasma are also used as input parameters for numerical model. This model is being developed at Department of Physical Electronics of Faculty of Science on Masaryk University in Brno. Experimental apparatus and measurements Experiments are performed in low temperature weakly magnetized DC plasma using the cylindrical magnetron apparatus. The scheme of this experimental system is shown in Figure. The system of cylindrical magnetron consists of two cylindrical electrodes placed in the stainless steel vacuum chamber. The inner electrode with diameter of 8 mm is negatively biased and coaxially placed in the chamber and is cooled by water to prevent its overheating. The grounded outer electrode is 58 mm in diameter. The discharge length is 3 mm. Constant and homogeneous magnetic field parallel to the axis is created by six coils mounted round the vacuum chamber. Magnetic field can vary up to 4 mt. The vacuum chamber is equipped by six vacuum ports for Langmuir or emissive probes and two ports with glass windows for optical 76

emission spectroscopy situated between coils. The apparatus is pumped by tandem of piston and turbomolecular pump to the ultimate pressure in the order of 3 Pa. The working gas is usually argon or neon. Typical working pressure range is of Pa and working gas flow rate range is of.5 2 sccm. The discharge current can be set up in the range of 25 ma. The apparatus is detailed described in [Golubovskii et al., 26]. Measurements are performed by cylindrical Langmuir probe, which is shown in Figure 2. Langmuir probe consists of tungsten wire with a diameter of 35 μm fixed in a copper tube. The probe holder is made up of two degussit tubes placed one into other. This construction aims to minimize impact of the sputtered cathode material on the probe (to prevent conductive connection between deposited layer on the ceramic holder and the wire of the probe). The probe is radially movable and placed at the axial position of 6 mm from the center of magnetron. The orientation of probe wire is perpendicular to the magnetic field lines to minimize the effect of the magnetic field on the probe characteristics, see [Pfau et al., 2, Kudrna, 997]. The probe characteristics are obtained by means of computer controlled data acquisition system. The measuring probe circuit consists of DC power supply Siemens calibrator B35, which is used as probe bias voltage generator and Siemens multimeter B322, which measures the probe current. Battery powered circuit with high-voltage operational amplifier Apex PA24 is used to double the voltage sweep of the Siemens calibrator. The exact value of generated voltage is calibrated using the control software. The entire probe electronics is driven by GPIB interface and it is controlled by software, which is created in graphical programming language Agilent VEE. One probe characteristic consists of about 2 points and it takes approximately one minute to record it. For displaying and analyzing measured probe characteristics the software START [Kudrna, 993] is used. The program estimates the floating potential V fl from the measured probe characteristic. The plasma potential V p is estimated from the zero-cross of the second derivative. The electron density N e is evaluated from the slope of the square of the probe current in the electron accelerating regime versus probe voltage dependence. All measurements presented in this paper were performed in neon plasma. The work aims to study the impact of macro-parameters (e.g. discharge current, the strength of the magnetic field, pressure) on plasma micro-parameters. Probe characteristics were measured for different values of macroparameters: discharge current was changed in the range of 5 25 ma, magnetic field was adjusted Figure. Schematic diagram of the cylindrical magnetron apparatus. Figure 2. The scheme of the cylindrical Langmuir probe construction. 77

in the range of 5, and also the gas pressure was altered from 4 to. According to [Pfau et al., 2] the influence of the magnetic field to the probe electron current in the collision-free case depends on the parameter β = r p /r L (r p is the probe radius; r L is the radius of the cyclotron motion of a charged particle). In our case for magnetic field strengths 5 mt, 2 mt and parameter β was calculated: β 5mT =.5, β 2mT =.6, β 22.5mT =.7. For parameter β << the influence of magnetic field is small and can be neglected. Magnetic field was therefore not considered in the evaluation of plasma parameters from Langmuir probe characteristics. Results and discussion Presented figures show dependences of plasma parameters on relative radial distance between cathode and anode, r a is the inner radius of the anode and r denotes the probe distance from the magnetron axis. Cathode surface corresponds to position x=.3 and anode surface is at position x=. The typical radial profiles of floating potential V fl are shown in Figures 3, 6 and 9. Radial dependences of plasma potential V p are depicted in Figures 4, 7 and. From these figures, we can see that values of plasma potential are higher than the values of floating potential and this difference is higher near the cathode where the higher electron temperature is expected. This kind of potential profile is typical for glow discharge. It can be observed the cathode fall in the cathode region and also anode fall in the anode region. Between these two regions the plasma potential has only very slow, approximately linear increase. This area of glow discharge is associated with positive column. The plasma potentials slightly go beyond V in the vicinity of anode. We assume that reason for this effect could have been the absence of vessel wall and the presence of the vacuum port for probe in that place. The influence of pressure and discharge current on plasma and floating potential is visible in Figures 3, 4, 6 and 7. Potentials increase with higher value of pressure and discharge current. Radial distributions of electron densities N e in the magnetron for different discharge conditions are shown in Figures 5, 8 and. Electron concentration slightly varying is in order of 6 m 3 for all measurements. The Figures 6 and 9 show that electron concentration increases for higher values of working pressure and discharge current. We observed that its maximum is located approximately at position x=.5 of relative distance and with increasing distance from the cathode N e decreases gradually. The influence of magnetic field strength on floating and plasma potentials is depicted in Figure 9 and. We observed that for higher magnetic fields the potential decreases. The reason is that at higher magnetic fields charged particles are confined closer to the cathode. For the same pre-set current flow the higher radial electric field is necessary to move charged particles across magnetic field lines. This fact causes increasing of radial electric field in the positive column. From Figure, we can see decreasing electron density for higher magnetic filed. We observed that its maximum moves towards the cathode, because electrons are stronger confined near the cathode by stronger magnetic field. It was found that for the magnetic field values greater than 2 mt, the amplitude of potential fluctuations increases [Kudrna et al., 997]. The probe current is much noisier for higher magnetic field and it can not be correctly interpreted. Therefore we limited our measurements to values of magnetic field around 2 mt. Conclusion Radial profiles of plasma parameters were determined from probe characteristics obtained for different values of gas pressure, discharge current and magnetic field strength. Measurements were performed in the neon weakly magnetized DC plasma. All obtained results agree with previous measurements in argon plasma published in works [Rusz, 23, Holík et al., 24]. It can be concluded that behavior of plasma parameters for both neon and argon are very well comparable. Obtained experimental results will be compared with numerical model of neon plasma, which is being developed at Masaryk University in Brno. Acknowledgments. This research has been supported by the Charles University Grant Agency, grant No.25 and No. 6462, by the CEEPUS CII-AT3 and by EURATOM. The financial support by the Czech Science Foundation, grant 4/9/H8 and by project P25//386 is also gratefully acknowledged. 78

5 6 Pa 25 ma 5 Pa 4 Pa 5.4.5.6.7.8.9. Figure 3: The radial dependences of floating potential for different neon pressures obtained using discharge current of 25 ma and strength of magnetic field of. 6 Pa -7 25 ma 5 Pa 4 Pa Figure 4: The radial profiles of the plasma potential for different value of gas pressure obtained using discharge current of 25 ma and strength of magnetic field of. Electron density [ 6 x m ] 3,5 3, 2,5 2,,5,,5 25 ma 6 Pa 5 Pa 4 Pa, Figure 5: The radial distributions of the electron density for different neon pressures obtained using discharge current of 25 ma and strength of magnetic field of. 5 25 ma 5 Pa 2 ma 5 ma 5.4.5.6.7.8.9. Figure 6: The radial profiles of the floating potential for different discharge currents obtained magnetic field of. 25 ma 5 Pa 2 ma -7 5 ma Figure 7: The radial dependences of the plasma potential for different discharge currents obtained magnetic field of. Electron density [ 6 x m ] 3,2 2,4,6,8 5 Pa 25 ma 2 ma 5 ma, Figure 8: The radial distributions of the electron density for different discharge currents obtained magnetic field of. 79

2 4 6 5 ma 5 mt 8 2 mt Figure 9: The radial profiles of the floating potentials for different strength of magnetic field obtained using neon pressure of and discharge current of 5 ma. 5 ma 5 mt 2 mt Figure : The radial dependences of the plasma potential for different strength of magnetic field obtained using neon pressure of and discharge current of 5 ma. Electron density [ 6 x m ] 3,5 3, 2,5 2,,5,,5 5 ma 5 mt 2 mt, Figure : The radial distributions of the electron densities determined from square of the probe current in the electron accelerating regime versus probe voltage. The parameter is magnetic field strength. References Borah, S. M., A. R. Pal, H. Bailung, and J. Chutia, Effect of E B electron drift and plasma discharge in dc magnetron sputtering plasma, Chin. Phys. B, 2, 47, 9, 2. Golubovskii, Yu. B., I. A. Porokhova, V. P. Sushkov, M. Holík, P. Kudrna, and M. Tichý, Electron kinetics in cylindrical discharges of magnetron configurations, Plasma Sources Sci. Technol., 5, 228 236, 26. Holík, M., P. Kudrna, O. Bilyk, J. Rusz, M. Tichý, J. F. Behnke, I. A. Porokhova, Yu. B. Golubovskii, 2-D experimental study of plasma parameters in the cylindrical magnetron dc discharge, Czechoslovak Journal of physics, 52, D673 D68, 22. Holík, M., O. Bilyk, A. Marek, P. Kudrna, J. F. Behnke, and M. Tichý, 2-D Experimental study of the plasma parameter variations of the magnetically sustained DC discharge in cylindrical symmetry in argon, Contrib. Plasma Phys., 44, 63 68, 24. Kudrna, P., PhD. Dissertation, Charles University in Prague, Faculty of Mathematics and Physics, Prague, 997. Kudrna, P., E. Passoth, Langmuir probe diagnostics of a low temperature non-isothermal plasma in a week magnetic field, Contributions to Plasma Physics, 5, 47 429, 997. Merlino, R. L., Understanding Langmuir probe current-voltage characteristics, Am. J. Phys., 75, 78 85, 27. Pfau, S., M. Tichý, Langmuir probe diagnostics of low-temperature plasma, in Hippler R., Pfau, S., Schmidt M., Schoenbach K. H., Low temperature plasma physics, WILEY-VCH, Verlag, Berlin, 3 72, 2. Rusz, J., PhD. Dissertation, Charles University in Prague, Faculty of Mathematics and Physics, Prague, 23. Thorton, J. A., Magnetron sputtering: basic physics and application to cylindrical magnetrons, J. Vac. Sci. Technol., 5, 7 77, 977. 8