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 = 푙표𝑔 (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

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

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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. The inner magnet directly to the anode. The output voltage of the was 12.5mm in height and 31.5mm in diameter with power supply could be varied precisely over 0-5 kV a central hole of 17.5mm in diameter, while the to control the current flowing between discharge outer magnet was 15.2mm in height and 80mm in electrodes. However, the maximum supply voltage diameter with a central hole of 40mm in diameter. did not exceed 800V. Another dc power supply (0- Therefore, the two magnets were separated by a 250 V PHYWE-7532) was used to provide bias distance of 10mm around their opposing surfaces. A potential for Langmuir probe measurements. In metallic disk of 69.7mm in diameter and 2.5mm in addition, a third dc power supply (DHF-1502DD, thickness was used to choke the outer magnet to 1.5-15V, 0.6-2A) was used for electrical prevent the magnetic field lines from extending to measurements performed on the samples prepared the backside of the electrode. as photodetectors. The design of dual magnetrons proposed in this Gas supply unit consists of argon and nitrogen work provides two concentric regions of high cylinders, flowmeters, gas flow regulators, needle magnetic field intensity on electrode surface. At valves and connections and joints. Argon gas of these regions, the charged particles are totally 96% purity and nitrogen gas of 90% purity were confined because the particles escaping from the used. inner region towards the walls of vacuum chamber A compact unit was used to cool and circulate will lose some energy and then be trapped by the the room-temperature water through a channel in outer region. In conventional configurations, a discharge electrodes. This unit can cool more than fraction of charged particles can escape from the 51 liters of distilled water down to about 4°C and confinement region and hence decrease the circulate it with maximum flow rate of 30 L/min. ionization rate near the cathode. The temperature inside the chamber was measured The advantage of magnetron on the anode is by a thermometer located near the wall of the observed in the film deposition of ferromagnetic chamber while the temperatures of both electrodes materials because the magnetic field intensity forces were measured by thermocouples connected to the deposited particles to distribute on the substrate digital instruments. The maximum surface according to its distribution. This makes possible to temperature of the substrate placed on the anode produce films with selective optical densities to was 40-45°C with uncooled circulating water and serve multipurpose devices as in the optical data reasonably reduced with cooled circulating water. storage applications [103]. The operating conditions of the system were In order to prevent any variation in the classified into two groups; constant and variable. arrangement of internal components, a teflon host of The constant operating conditions include inter- 100mm diameter and 44.5mm height with a central electrode distance, vacuum pressure, current groove of 84.75mm diameter and 26.3mm depth limiting resistance, discharge voltage, discharge was used to maintain the magnetron from the current, cooling temperature, cooling water flow backside of the electrode. This piece was locked rate and deposition time. The variable operating from movement by cylindrical teflon piece of conditions include gas pressure and gas flow rate. 37.5mm in diameter and 35.25mm in height Varying discharge voltage was almost possible containing an M5 screw driven towards the holder during the operation. In addition, turning the cooling

IJAP, vol. (12), no. (3), July-September 2016, pp. 11-16 system off would raise the temperature of either the discharge current increases with increasing electrode to 40-45°C with circulating water, while discharge voltage. This behavior is the same at all stopping the circulation of water would raise distances. At high voltages (>350V), the discharge electrode temperature more (up to 150°C). current is reasonably increased with increasing the distance, while the variation in the discharge current 3. Results and Discussion with distance is very small at lower voltages Electrical characteristics of the discharge plasma (<300V). The electrodes are fully covered by the were determined by measuring the breakdown discharge and any increase in discharge current voltage and discharge current as functions of inter- leads to an increase in the cathode fall. So, the electrode distance, gas pressure, gas mixing ratio voltage across the electrodes rises sharply. This and supply voltage. High-impedance voltmeters and behavior is attributed to the mobility-limited version DC high-precision ammeters were used to perform of Child-Langmuir equation, where the discharge these measurements. current is proportional to V3/2/d2 [110]. As shown in The electrical characteristics of discharge these figures, at inter-electrode distance of 5cm, plasma, such as dependencies of discharge current maximum discharge current was measured. on the applied voltage and gas pressure inside the vacuum chamber as well as Paschen’s curve, are of 100 importance to introduce the homogeneity of the 250V generated plasma. In the following, these 80 300V characteristics are presented. 350V In figure (4), the discharge current was measured 60 as a function of discharge voltage at different inter- electrode distances (2-6cm) without magnetron. It is 40 clear that no current flows when the voltage is 20 below 200V as the gas between the electrodes not (mA) current Discharge reached the breakdown. At voltages 200-250, 0 discharge current initiates as the breakdown voltage 1 3 5 7 Inter-electrode distance (cm) is reached. Obviously, discharge current flows at Figure (5) Variation of discharge current with inter- smaller distance between the electrodes as the electrode distance at certain discharge voltages without using charge carriers (electrons) have to pass shorter magnetron distance to transfer from cathode to anode. However, this was not exactly satisfied here due to In case of using a magnetron, the same relation some experimental imperfections. between discharge current and discharge voltage was recorded at certain inter-electrode distance of 100 4cm, as shown in figure (6). Despite the discharge 2cm current measured at this distance was lower than 80 3cm 4cm that measured at 5cm, this distance was chosen for 5cm this relation to match the optimum distance of 60 magnetron configuration, as shown before. 40 It is clear that higher discharge current is resulted at each discharge voltage compared with 20 the case when using only one magnetron (at the

Discharge current (mA) current Discharge cathode), which in turn shows higher current when 0 compared to no magnetron case. This effect is 150 250 350 450 550 attributed to the role of magnetron in confining Discharge voltage (V) charged particles (electrons and ions) near the Figure (4) Current-voltage characteristics of argon gas discharge at different inter-electrode distances (2-6cm) and electrodes, increasing number of collisions with working pressure of 0.7mbar without magnetrons neutral gas atoms and hence producing much more particles those carry much more charges in time At certain discharge voltages, the discharge (higher current). current was measured as a function of inter- Using only one magnetron at the cathode results electrode distance as shown in figure (5). Initially, in an increase of about 13% in the discharge current this current slightly increases at distance from 2 to 3 compared with the case without magnetron, while cm, slightly decreases at distance of 4 cm, using dual magnetrons at both electrodes results in reasonably increases at distance of 5cm, and an increase of about 24%. decreases at distance of 6 cm. This nonlinear On the other side, in case of using a magnetron, behavior between discharge current and discharge the variation of discharge current with inter- voltage is a characteristic of abnormal region in electrode distance was measured at a discharge glow discharge. voltage of 400V, as shown in figure (7). As in figure At certain inter-electrode distance (e.g., 2 cm), (6), similar behaviors were obtained when using

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IRAQI JOURNAL OF APPLIED PHYSICS only one magnetron (at the cathode) and using dual electrons under the effect of applied electric field magnetrons with the values of discharge current and/or by the secondary electrons. The difference is shifted upward. Maximum currents were obtained at very small at low discharge voltages (200-300V) inter-electrode distance of 5cm. because the major contribution of nitrogen molecules is due to the ionization by primary 100 electrons. This difference is clearly observed at high Without magnetron discharge voltages (>300V) because the secondary One magnetron electrons are accelerated by higher electric field 80 Dual magnetrons (V/d) and then the ionization of nitrogen molecules by collisions with these electrons is increased. Therefore, the contributions of ionized molecules 60 are added.

100 40 Argon only Ar 1:1 N2

Discharge current (mA) current Discharge 80 Ar 3:2 N2 20 Ar 7:3 N2 Ar 4:1 N2 d=4cm Ar 9:1 N2 0 60 150 250 350 450 550 Discharge voltage (V) 40 Figure (6) Current-voltage characteristics of discharge

plasma at inter-electrode distance of 4cm without magnetron, (mA) current Discharge with only one magnetron and with dual magnetrons 20 120 Without magnetron One magnetron 0 100 Dual magnetrons 150 250 350 450 550 Discharge voltage (V) Figure (8) Discharge current-voltage characteristics for 80 different argon/nitrogen mixtures at total gas pressure of 0.7mbar and inter-electrode distance of 4cm 60 References [1] F.F. Chen, Introduction to Plasma Physics and 40 Controlled Fusion, 2nd edition, Plenum Press Discharge current (mA) current Discharge (NY), p. 22 (1974). 20 [2] T.J. Boyd and J. Sanderson, Physics of Plasmas, Cambridge University Press (2003). 0 [3] F. Ghaleb and A. Belasri, Numerical and 1 3 5 7 theoretical calculation of breakdown voltage in Inter-electrode distance (cm) the electrical discharge, Radiation Effects and

Figure (7) Variation of discharge current with inter-electrode Defects in Solids, 1, 1-7 (2012). distance at certain discharge voltage (400V) for the cases [4] M.A. Lieberman and A.J. Lichtenberg, without magnetron, using one magnetron and dual Principles of Plasma Physics and Materials magnetrons Processing, Second edition, Wiley, p. 547

(2005). Since one of the main goals of this work is to [5] M.M. Mansour, N.M. El-Sayed, O.F. Farag and produce silicon nitride nanostructures, pure nitrogen M.H. Elghazaly, Effect of He and Ar Addition gas (N ) is mixed with argon gas (Ar) to form the 2 on N Glow Discharge Characteristics and mixture required for this purpose. Different mixing 2 Plasma Diagnostics, Arab J. of Nuclear Sci. ratios of Ar:N are characterized to determine the 2 and Applications, 46(1), 116-125 (2013). optimum ratio at which the required silicon nitride [6] A.A. Solov’ev, Investigation of Plasma will be prepared, as shown in figure (8). Characteristics in an Unbalanced Magnetron If nitrogen gas mixed with argon at mixing ratio Sputtering System, Plasma Phys. Rep., 35(5), Ar:N of 4:1, an increase in the discharge current 2 399–408 (2009). was observed, as shown in figure (8), which may be [7] A. Bojarov, M. Radmilovic-Radjenovic and Z. attributed to the contribution to total current from Petrovic, Effect of the ion induced secondary the ionized nitrogen molecules. Ionization of these electron emission on the characteristics of RF molecules can occur by collisions with primary

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