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Current-Voltage Characteristics of DC Plasma Discharges Employed In IRAQI JOURNAL OF APPLIED PHYSICS Current-Voltage Mohammed K. Khalaf 1 Characteristics of DC Plasma Oday A. Hammadi 2 Firas J. Kadhim 3 Discharges Employed in Sputtering Techniques In this work, the current-voltage characteristics of dc plasma discharges were 1 Ministry of Science studied. The current-voltage characteristics of argon gas discharge at different and Technology, inter-electrode distances and working pressure of 0.7mbar without magnetrons Baghdad, IRAQ 2 were introduced. Then the variation of discharge current with inter-electrode Department of Physics, distance at certain discharge voltages without using magnetron was determined. College of Education, The current-voltage characteristics of discharge plasma at inter-electrode Al-Iraqia University, Baghdad, IRAQ distance of 4cm without magnetron, with only one magnetron and with dual 3 Department of Physics, magnetrons were also determined. The variation of discharge current with inter- College of Science, electrode distance at certain discharge voltage (400V) for the cases without University of Baghdad, magnetron, using one magnetron and dual magnetrons were studied. Finally, the Baghdad, IRAQ discharge current-voltage characteristics for different argon/nitrogen mixtures at total gas pressure of 0.7mbar and inter-electrode distance of 4cm were presented. Keywords: Plasma discharge; Magnetron; Glow discharge; Sputtering 1. Introduction emission effects, which lead to increase the flowing Glow discharge is low-temperature neutral current gradually approaching the breakdown point plasma where the number of electrons is equal to the beyond which the avalanche occurs and the current number of ions despite that local but negligible increases drastically in the Townsend discharge imbalances may exist at walls [1]. Glow discharge is region [3]. described as self-sustaining plasma, i.e., the The breakdown voltage is directly related to the avalanche effect of electrons keeps the continuous pressure of the gas sample and the mean free path of production of ion species. The avalanche condition secondary electrons. This relation between from the initial applied voltage in a typical low- breakdown voltage (VB) and gas pressure (p) is pressure discharge is shown in figure (1). known as “Paschen’s law” and given by [4] 푝푑퐵 푉 = (1) 퐵 1 푙표푔(푝푑퐴)−푙표푔(푙표푔(1+ )) 훾푒 where A and B are constants and their values are determined by the properties of the used gas, as shown in Table (1), d is the distance between discharge electrodes and e is the emission coefficient of the secondary electrons The breakdown voltage (VB) depends on the product (p.d), and weakly depends on the cathode material that defines the emission coefficient of secondary electrons. As well, the breakdown voltage is proportional to the product p.d at large values of this product and the electric field (E=V/d) is scaled Figure (1) Different regions of plasma discharge on the I-V characteristics [10] linearly with the pressure [5]. In case of small values of the product p.d, only If an initial voltage is applied on a gas sample few collisions occur and higher voltage is applied to between two electrodes at sufficient separation, little increase the probability of breakdown per collision. current will flow through this sample due to the Hence, the minimum voltage required to ignite the ionization effects in the gas. As the applied voltage discharge of a gas sample of pressure p over a is increased to reach the breakdown voltage, the distance d is defined at the minimum of Paschen’s energy given to ions is increased too that increases curve, as [6] 1 1 their collisions with atoms and electrodes [2]. 푝푑|푉 = 푙표 (1 + ) (2) Accordingly, more ions and electrons are produced 푚푖푛 퐴 훾푒 If the pressure and/or separation distance is too due to the ionization and secondary electron large, ions generated in the gas are slowed by All Rights Reserved ISSN (printed) 1813-2065, (online) 2309-1673 Printed in IRAQ 11 IJAP, vol. (12), no. (3), July-September 2016, pp. 11-16 inelastic collisions, so that they strike the cathode 퐴+ + 푒− → 퐴 + ℎ푣 (4b) with insufficient energy to produce secondary here A* is an excited atom and e- is a high-energy electrons. In most sputtering glow discharges, the electron discharge starting voltage is relatively high [7]. Table (2) shows the values of secondary electron Figure (2) shows the Paschen’s curves of different emission coefficients for three different metals when gases. The electron mean free path (λe) is then different ions are bombarding these targets. related to the pressure by Table (2) The secondary electron emission coefficients for 휆푒 ≅ 푝. 푑 (3) To initiate the discharge, the following condition three different metals when different ions are bombarding these targets [8] must be satisfied [8] 휆푒 Target Ion Energy (eV) 푝 ≥ Incident Ion 푑 Material 200 600 1000 According to the self-sustaining feature of glow He+ 0.524 0.24 0.258 discharge, the number of the produced electrons is Ne+ 0.258 0.25 0.25 + just sufficient to produce the same number of ions to W Ar 0.1 0.104 0.108 Kr+ 0.05 0.054 0.108 generate again the same number of electrons. These Xe+ 0.016 0.016 0.016 ions liberate electrons from the electrodes He+ 0.215 0.225 0.245 Mo (secondary), atom-ion and ion-ion collisions [9]. He++ 0.715 0.77 0.78 When this condition is satisfied, the voltage He+ 0.6 0.84 Ni Ne+ 0.53 decreases and the current increases. This is said to + be a “normal discharge”. Ar 0.09 0.156 2. Experimental Part The magnetron sputtering system used in this work was designed to include vacuum chamber, discharge electrodes and magnetron assembly, vacuum unit, dc power supplies, gas supply system, cooling system and measuring instruments. The system is schematically shown in figure (3). Figure (2) Paschen’s curves of different common gases used in electric discharge applications [11] Table (1) Typical values of A and B constants, E/p and ionization energy (Vi) for various gases [8] Gas A B E/p V (V) i Figure (3) Schematic diagram of the system used in the H2 5 130 150~600 15.4 present work N2 12 342 100~600 15.5 O2 - - - 12.2 The vacuum chamber was constructed from CO2 20 466 500~1000 13.7 stainless steel. It is a cylinder with internal diameter air 15 365 100~800 - of 35cm, outer diameter of 45cm, and height of H O 13 290 150~1000 12.6 2 37.5cm. There were four side holes of 10.8cm in HCl 25 380 200~1000 - He 3 34 (25) 20~150 (3~10) 24.5 diameter on the circumference of the cylinder; one Ne 4 100 100~400 21.5 of them was closed with a quartz window, while the Ar 14 180 100~600 15.7 other three were closed with glass windows. These Kr 17 240 100~1000 14 windows were mounted by stainless steel flanges Xe 26 350 200~800 12.1 each of 21cm in diameter and four screws. They Hg 20 370 200~600 10.4 were used for monitoring the discharge and events A in (Ion/pairs)/(cm.torr), B in V/(cm.torr), E/p in V/(cm.torr) inside the chamber. Each window was far from the discharge region by a neck of 7.5cm diameter to Due to the recombination effects and excited avoid the effects of heat developed by the glow atoms returning to ground state, the plasma begins discharge. to glow, as [10] The vacuum chamber was sealed from lower end 퐴∗ → 퐴 + ℎ푣 (4a) with a stainless steel flange of 40cm in diameter 12 © Iraqi Society for Alternative and Renewable Energy Sources and Techniques (I.S.A.R.E.S.T.) IRAQI JOURNAL OF APPLIED PHYSICS containing a feedthrough for electrical connections tube. required for experiment. The upper end was sealed A single-stage rotary pump (Leybold-Heraeus) with a similar flange but containing two of 9 m3/hr pumping speed was used to get pressure feedthroughs; one for gas inlet and Pirani gauge and down to about 10-3 mbar inside the vacuum the other for the Penning gauge. Both flanges chamber. A water-cooled diffusion pump was include a central hole for the electrode hollow available for use in this work for lower vacuum holder. Rubber O-rings and silicon vacuum grease pressure and connected to the vacuum chamber via a were used in all sealing points. trap. However, all results presented were obtained Both discharge electrodes were constructed from using the rotary pump only as vacuum pressure of stainless steel (St. St. 304) hollow disks of 80mm in 10-2 mbar was easily reached. The minima of diameter and 8.5mm in thickness. The electrode was Paschen’s curves for the inter-electrode distances 2- joined to holder of 295mm in length and outer and 6 cm were achieved at pressures higher than 10-3 inner diameters of 16.2mm and 11.6mm, mbar. Pirani gauge (down to 10-3 mbar) was used. respectively, to include a stainless steel channel of The electrical power required for generating 78.5mm in length and 5.6mm in diameter through discharge inside vacuum chamber was provided by a which the cooling water was flowed to the inside 5 kV dc power supply (Edwards 2A) through high- volume of the electrode. The holder tube includes a tension cables. A current-limiting resistance (3.25 1mm-step screw thread of 25.88mm in length to k, 1 kW) was connected between the negative connect the cooling channel tightly. terminal of the dc power supply and the cathode Two permanent ring magnets were placed at the inside the vacuum chamber, while the positive back side of each electrode to form the magnetron terminal of the dc power supply was connected with a separating distance of 1cm.
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