Characteristics of the Broad Beam Glow Discharge Ion Source

Transcription

1 Characteristics of the Broad Beam Glow Discharge Ion Source M.M.Abdel Rahman, N.I. Basal and F.W. Abdelsalam Accelerators & Ion Sources Department, Nuclear Research Center. Atomic Energy P.O.Box:13759, Cairo, Atomic Energy Athority, Egypt Received: 25/5/2012 Accepted: 1/7/2012 ABSTRACT The design, construction, characteristics and also applications of a self extraction glow-discharge ion source,namely broad beam ion source are discussed. A broad beam ion source is achieved to deliver currents of more than 100 µa with applied voltage between anode and cathode of 500 V 4 kv. Discharge and ion beam characteristics of this compact multipurpose ion source were studied with different operating conditions using nitrogen gas. Experimental characteristics of this source and the output ion beam currents are given for a range of operating parameters, e.g. the gas pressure, the applied voltage and the gap distance between the electrodes, to find out an optimal operating condition. The influence of varying distance between the anode and the cathode on the discharge and output ion beam characteristics was studied at different gas pressures. It was found that the optimum distance between the anode and the cathode equals to 3 mm, where maximum output ion beam current and beam energy could be obtained. At the optimum parameters specimens of Al was etched and the surface morphology of it was investigated. This specimen was examined by using SEM technique and EDEX spectra which show the precipitation of Cu on Al specimens. Key words: Glow discharge ion source, plasma sputtering, etching processes. INTRODUCTION Broad beam ion sources with uniform current distributions which are needed for many scientific applications and industrial technologies for surface materials. It is found that surface modification of materials is difficult without highly efficient, simple, and dependable ion sources (1-4). Broad beam ion sources can be used in a wide field of thin film technologies (5, 6). A broad ion beam is typically several centimeters or more in diameter which is equal to the ion source diameter. The beam diameter is also much larger than the Debye length, which is the typical distance that the electric field can penetrate into plasma. In this work, a compact broad beam ion source is designed and constructed in our laboratory. The discharge characteristics are measured at different pressures for various distances between the anode and the cathode (3, 5, 7 and 9mm). From the discharge characteristic curves, we can determine the relation between the discharge voltage versus the gap distance between the anode and the cathode for different discharge currents at various gas pressures. Also, characteristics are measured at different distance between the anode and the Faraday cup (1, 3, 5, 7, and 9 cm) at different gas pressures and constant distance between anode and cathode (3mm). Finally, sputtering characteristics of this ion source were observed using SEM and EDEX techniques. EXPERIMENTAL SETUP A schematic diagram of the broad beam glow discharge ion source and its associated electrical circuit is shown in Figure (1). This ion source consists of non-magnetic stainless steel anode, carrying high voltage, with an internal diameter is 20 mm and its height is 25 mm connected to a grid from 114

2 bottom which is taking the same potential. On the top there is a grounded plain copper cathode of 5mm thickness and 20mm in diameter having an aperture of 1.5mm for gas inlet. Cathode is isolated from anode by Perspex cylinder flanges of the same diameter and different heights 3, 5, 7 and 9mm, to change the distance between anode and cathode. This grid is made from aluminium of diameter 20 diameter. Both the anode cylinder and the plane cathode are immersed in a Perspex cylinder. The collector (Faraday cup F.C.) is situated 30mm away from the grid and used to collect and measure the output ion beam current. The working gas is admitted to the ion source through a hose fixed in a Perspex flange at the upper side of the cathode. A 10 KV power supply is used for initiating the glow discharge between the anode and the cathode. A vacuum system is used to evacuate the ion source chamber up to 10-5 mbar. A liquid nitrogen trap is fixed between the ion source chamber and the diffusion pump in order to prevent the any vapour from entering the ion source chamber. The working gas is transmitted to the ion source from a gas cylinder through a needle valve to regulate the rate of gas flow. This ion source has high efficiency, low gas consumption and free from any troubles, so that, it can operate in a high vacuum (less than 10-3 ) with a stable mode for a long time (2h) without any break down problems. EXPERIMENTAL RESULTS AND DISCUSSION The operating principle of this ion source (7) is based on the ionization mechanism produced by primary electrons colliding with the gas molecules due to a potential difference between the anode and the cathode. Therefore high output ion beam current can be extracted axially in a direction normal to the discharge region without using extraction system. The ion source was cleaned before introducing inside the vacuum chamber. It was polished, and washed by acetone. The polishing of the electrode parts should remove the irregular parts from their surfaces and the contamination due to the erode materials of the discharge. The assembled ion source was placed inside the vacuum chamber. Fig.1: Schematic diagram of the ion source and its associated electrical circuit. Figure (2), is the Paschen, s curve (8) which describes the relation between the discharge voltage and the variation of distances between the anode and the cathode at constant gas pressures and constant discharge current. It shows that, the discharge voltage decreases with increasing the distance between the anode and the cathode until it reaches minimum value after which the discharge voltage 115

3 increases with increasing the distance between the anode and the cathode. This phenomenon is known as the abnormal glow discharge (9). It is clear that the discharge voltage not only depends on the discharge current, but also on the gas pressure. Fig (2):The Paschen, s curve x10 mmbar Discharge voltage (kv) x10 mmbar 1.1x10 mmbar 1.5x10 mmbar Distance (mm) between the anode and the cathode Fig. (3): Variation of the discharge voltage and the distance between the anode and the cathode Figure (3) describes the influence of changing the distances between the anode and the cathode with the applied discharge voltage at constant gas pressure and constant discharge current. It shows that, the discharge voltage decreases with increasing the distances between the anode and the cathode at constant gas pressure and discharge current. 116

4 Distance between the anode and the cathode = 3, 5, 7 and 9 mm. Discharge voltage (kv) Nitrogen gas pressure (mbar) Fig. 4: Variation of the discharge voltage and the gas pressures. Figure (4), describes the influence of changing the gas pressure with the discharge voltage at constant distance between the anode and the cathode and constant discharge current. It shows that the increase of the gas pressure was accompanied with decreasing the discharge voltage at constant distance between the anode and the cathode and constant discharge current. From figures (3 and 4), it shows that the behaviour of the curves is similar to the behaviour of the Paschen, s curve in the region of AB (Fig.2), which means that the broad beam ion source acts in the region of the abnormal glow discharge AB. Figure(5), describes the relation between the output ion beam current, I b, and the gap distance between the anode and the cathode (3, 5, 7 and 9mm) at various gas pressures with a constant discharge current, I d = 1 ma. From this figure, it is clear that, an increase of the gap distance was accompanied by a decrease of the ion beam current and reaches its maximum value at a gap distance equal to 3 mm for a pressure of 7 x 10-4 mbar. So, it is clear that this distance is the optimum value, where maximum ion beam current can be obtained with maximum ion energy. Since the ion energy is the summation of the charge state times the applied voltage between the electrodes (anode and the cathode) and in our ion source (q=1, singly charged ions) times voltage applied to the electrodes (anode and the cathode). 117

5 60 7x10-4 mbar Output ion beam current ( A) x10-4 mbar 1.1x10-3 mbar 1,5x10-3 mbar Distance between the anode and the cathode (mm) Fig. 5: The output ion beam current versus the gap distance between the anode and Cathode at Id = 1 ma. Fig. (6: a, b) describes the relation between the ion beam current versus the distance between the anode and the Faraday cup (1, 3, 5, 7 and 9cm) at I d = 1and I d= 2 ma, respectively at different gas pressures. It is clear from this figure that, an increase of the distance was accompanied with a decrease of the ion beam current and may be due to the large diameter of the anode (20 mm) where the beam contains the entire anode diameter and produced through a grid of different apertures. Each aperture can acts as a focusing lens which focuses the ion beam current and increases its magnitude. This means that the current doesn t increase according to the 3 /2 power law. This result implies that the ion current is somewhat limited by ion diffusion from the discharge and is possibly limited by ion production rather than being entirely by the acceleration process (10). From the curves it must be concluded that the ion beam current increases with decreasing the gas pressure and the rate of etching increases with decreasing the gas pressure (11). Also curves illustrates that the optimum gas pressure is 7x10-4 mbar where the etching rate is maximum due to the stable ion beam current. 118

6 pressuret, pr = 1.5x10-3 mbar, id= 1 ma pressure, pr = 1.1x10-4 mbar, id= 1 ma pressure, pr = 9x10-4 mbar, id = 1 ma pressure, pr = 7x10-4 mbar, id 1 ma ion current ( A) distance, daf (cm) (A) pressure, pr = 1.5x10-3 mbar, id = 2 ma pressure, pr = 1.1x10-3 mbar, id = 2 ma pressure, pr = 9x10-4 mbar, id = 2 ma pressure, pr = 7x10-4 mbar, id = 2 ma ion current ( A) distance, daf (cm) (B) Fig. (6): The ion beam current versus the distance between the anode and The Faraday cup at discharge current I d = 1and I d= 2 ma, respectively. Sputtering characteristics of the ion source Broad ion beam technology can provide a link between a large area manufacturing processes and surface analytical techniques using ion beams. This ion beam is useful for etching a number of small specimens. After measuring and optimizing the ion source characteristics, some atoms from the copper cathode were precipitated on the Al grid and on the Faraday cup, but on the grid it was much dense than that on the Faraday cup due to the difference in the distances between the cathode, grid and the Faraday cup. SEM and EDEX techniques examined the surfaces of the grid and the Faraday cup, Figure (7) shows the surface morphology of the Faraday cup after the precipitation of copper ions on it. Figure (8: a and b) is the EDEX spectra of the Faraday cup before (8-a) and after (8-b) the copper ions precipitation. Spectrum (8-b) detected some impurities as shown in table (1) such as silicon, iron, 119

7 sulphur and nickel. This may be from the ion source materials which has insulators and connections beside its main components. Table (1) shows the elemental analysis of the Al surface of the Faraday cup before and after precipitation of Copper on it. Fig. (7): The surface morphology of the Faraday cup after precipitation of Cu ions Al 14.0 Counts[x1.E+3] Cu Ni Fe Fe Zn Cu Zn Cu kev (A) 4-y Al 20.0 Counts[x1.E+3] S Si Cu S Zn Fe Cu Zn Fe Cu kev 120

8 (B) Fig.(8): EDEX spectra showing the Cu ions on the Al Faraday cup. Table1(1): Elemental analysis of the Faraday cupbefore and after precipitation Element ms% (before preciptation) ms% (after preciptation) Al Si S Fe Cu Zn Ni ACKNOWLEDGMENTS Authors are grateful to the Accelerators and Ion Sources Dept., Nuclear Research Center, Egyptian Atomic Energy Authority, Egypt for financial support and for Dr. B. Ali for his contribution during the investigation of this work. Thanks are due to Mr. I. Abdelbaki and M. Madboli for the skillful construction of parts of the broad beam glow discharge ion source. REFERENCES (1) D. Tang, S. Pu, Q. Huang, H. Tong, X. Cui, Paul K. Chu, Nuclear Instruments and Methods in Physics Research B 257, 801 (2007). (2) B. Wolf, Handbook of ion sources, Boca Raton, New York press, USA (1995). (3) I. N. Martev, G. I. Grigorov, M. V. Stoyanova, Bulgarian Joumal of Physics 25 Nos 5/6 (1998). (4) Barakat A. Soliman, Moustafa M. Abdelrahman, Fatama W. Abdelsalam, Kamal A. Aly, Journal of Nuclear Materials 432 (2013). (5) I.G. Brown, "The Physics and Technology of Ion Sources", J. Wiley & Sons, New York (2004). (6) J.J. Cuomo, S. M. Rossnagel, H. Kaufman, "Handbook of Ion Beam Processing Technology", Noyes Publ., Park Ridge (1989). (7) N. Miyamoto, K. Miyabayashi, T. Yamashita, H. Fujisawa, Rev. Sci. Instrum, 73 (2002). (8) R.K. Marcus, J.A.C. Broekaert, Glow Discharge Plasmas in Analytical Spectroscopy, John Wiley & Sons, (9) M.M. Abdel Brahman, A. Helal, O.A. Moustafa and F.W. Abdel Salam, Journal of Nuclear and Radiation Physics, Vol. 3, 1(2008). (10) G. G. Sikharulidze, Instrum. And Exp. Techniques, 52, 2 (2009). (11) R.G.Wilson and G.R.Brewer, Ion Beam with Applications to Ion Implantation, Willy Interscience Publication (1973). 121