Study of Ion Beam Sputtering using Different Materials

Transcription

1 ab Journal of Nuclear Science and Applications, 5(), Study of Ion Beam Sputtering using Different Materials H. El-Khabeary Accelerators & Ion Sources Department, Basic Nuclear Science Division, Nuclear Research Center, Atomic Energy Authority, P.No. 759, Egypt. ABSTRACT In this paper, the electrical discharge and output ion beam characteristics of cold conical cathode ion source were me asured at the optimum operating conditions and different pressures using argon gas. The effect of negative extraction voltage applied on the ion collector plate on the output ion beam current was measured at electrical discharge current equal to ma and different pressures using argon gas. It was found that the output ion beam current increases about 75 % its initial values at extraction voltage equal to - volt. The efficiency of the ion source was determined at electrical discharge current equal to. ma and different argon gas pressures without and with effect of negative extraction voltage applied on the ion collector plate. It was found that at pressure equal to x - mmhg, the efficiency of the ion source reaches.5 % without extraction voltage and 7.5 % at extraction voltage equal to - volt. The sputtering rate of copper and aluminum targets using argon ion beam was calculated without and with effect of negative extraction voltage. A comparison was made between the sputtering rate values of copper and aluminum targets without extraction voltage and with applied extraction voltage on each target equal to - volt. It was found that at pressure equal to x - mmhg and extraction voltage equal to - volt, the sputtering rate values of coppe r and aluminum targets increases about 5 % than that the sputtering rate values without extraction voltage. Key Words: Conical Cathode Ion Source / Sputtering Rate / Extraction Voltage. INTRODUCTION Sputtering (-) is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles (). When an ion beam strikes a surface, several processes will start. The ions might be partly reflected, they might initiate photons and secondary electrons, or they might be slowed down and captured by the surface. When the energy of the ions exceeds the so-called threshold energy, the surface emits uncharged surface atoms. This process is called sputtering. The kinetic energy of the sputtered particles is about electron volt and thus it is much higher than the energy of evaporated particles (. electron volt). When the sputtered particles hit a substrate, their sticking coefficient is much higher because of their higher energy; the use of a second ion beam for direct bombardment of the substrate might even improve the quality of the film. These properties are caused by processes in the growing layer which usually start at higher temperatures. It is commonly used for thin film deposition (5), etching and analytical techniques. The average number of atoms ejected from the target per incident ion is called the sputtering yield and depends on the angle of the incident ion, the energy of the ion, the masses of the ion and target atoms and the surface binding energy of atoms in the target.

2 ab Journal of Nuclear Science and Applications, 5(), Ion beam sputtering, as a potentially (6) useful roughening technique, has recently been used in attempts to modify the surface topography of biocompatible materials, such as metals, alloys, polymers and ceramics. The focused ion beam (FIB) microscope (7-9) is a tool that has a widespread use in the field of materials science because it is able to micromachining with high resolution imaging thus therefore enhancing a broad range of both fundamental and technological applications () in materials science. EXPERIMENTAL ARRANGEMENT A schematic drawing of cold conical cathode ion source () and its associated electrical circuit is shown in Fig.(). It consists of copper anode disc, A, of 7 mm diameter. The copper conical cathode, C, of mm inner diameter, 7 mm outer diameter and aperture in the center of diameter equal to mm. Two confinement rings, I, made from perspex insulator of 7 mm inner diameter, 7 mm outer diameter and thickness equal to mm are fixed, one on the anode inner surface and the other on the cathode inner surface to confine the discharge in the central zone between them. The anode and the cathode are placed inside an insulating cylinder made from pure perspex material of cm inner diameter, 5 cm outer diameter and length equal to 5 cm. The working gas is admitted to the ion source through a hole of mm diameter in the outer surface of the perspex insulator cylinder. The copper collector plate, CP, is placed at a distance equal to 5 cm from the ion exit aperture of the cathode to collect the output ion beam from the ion source. Fig.(): Cold conical cathode ion source and power its associated electrical circuit. Fig.(): Shows the connection of negative supply to the ion collector plate. Figure () shows the connection of negative power supply to the ion collector plate. The anode is connected to kv positive power supply, P.S., for initiating the glow discharge between the anode and the cathode. The collector plate is connected to 5 kv negative power supply, N.P.S., for extraction of ions from the ion source. A milli-ampere meter is used to measure the electrical discharge current, I d, between the anode and the cathode, while the kilo voltmeter is used to measure the electrical discharge voltage, V d, between them. The cathode is connected to earth, while the ion collector plate is connected to earth through micro-ampere meter which is used to measure the output ion beam current, I b, from central aperture of the cathode. A vacuum system consists of stainless steel mercury diffusion pump of speed 7 L/s provided with electrical heater and backed by a 5 L/min. rotary pump was used to evacuate the ion source chamber. The rotary pump is used to get a pressure from - mmhg to - mmhg, while the mercury diffusion pump is used to yield a low pressure from - mmhg to -6 mmhg in the ion source

3 ab Journal of Nuclear Science and Applications, 5(), chamber. The working gas is admitted into the ion source from a gas cylinder through a finely controlled needle valve. EXPERIMENTAL RESULTS In this work, all the experimental investigations were measured using argon gas and at optimum anode cathode distance, d A-C, inner diameter of two confinement rings, D ring, and cathode ion exit aperture ion collector plate distance, d C-CP, which were obtained before (,) for stable electrical discharge current and a high output ion beam current. The input electrical discharge and output ion beam characteristics from the ion source were measured at different pressures. The effect of applied negative extraction voltage on the ion collector plate, V CP, was determined at different pressures. The efficiency of the ion source was determined at discharge current equals. ma and different argon gas pressures without and with extraction voltage equals - volt. The sputtering rate of copper and aluminum targets which are placed at the optimum distance from the ion exit aperture of the cathode equals 5 cm was calculated without and with extraction voltage equals - volt. - Effect of the Discharge Parameters on Output Ion Beam Current Figure () shows the electrical discharge current, I d, versus the electrical discharge voltage, V d, at different pressures. It is clear that an increase of the discharge voltage was accompanied by an increase of the discharge current and the discharge voltage starts at higher value in case of low pressure, P = x - mmhg, than that at high pressure, P = 7 x - mmhg. Figure () shows the output ion beam current, I b, versus the electrical discharge current, I d, at different pressures. It is obvious that the output ion beam current incresases by increasing the discharge current and at P = x - mmhg and I d =. ma, the value of the output ion beam current reaches 55? A. I d (ma) P = x - mmhg P = 5 x - mmhg P = 6 x - mmhg P = 7 x - mmhg D ring = 7 mm d A-C = mm V d (kv) I b (ma) P = x - mmhg P = 5 x - mmhg P = 6 x - mmhg P = 7 x - mmhg D ring = 7 mm d A-C = mm I d (ma) Fig.(): Discharge current versus discharge versus voltage at different pressures using gas. Fig.(): Output ion beam current discharge current at different argon pressures using argon gas.

4 ab Journal of Nuclear Science and Applications, 5(), - Effect of Applied Negative Extraction Voltage on the Ion Collector Plate In this experiment, the performance of the ion source was modified by applying a negative extraction voltage on the ion collector plate. Also, the effect of applied negative extraction voltage on the ion collector plate on the output ion beam current was measured. Figure (5) shows the output ion beam current, I b, versus the negative extraction voltage applied on the ion collector plate, V CP, at different pressures, optimum cathode ion exit aperture - ion collector plate distance, d C-CP, which equals 5 cm and I d = ma. It is clear from the curves that the output ion beam current increases by increasing the negative voltage applied on the ion collector plate and reaches about 75 % its initial values at V CP = - volt. Also a maximum output ion beam current, I b = 86 µa, can be obtained. Figure (6) shows the output ion beam current, I b, versus the electrical discharge current, I d, at different pressures and V CP = - volt using argon gas. It is obvious that the output ion beam current increases by increasing the discharge current and at P = x - mmhg and I d =. ma, a maximum output ion beam current, I b = 55 µa, can be obtained. I b (ma) P = x - mmhg P = 5 x - mmhg P = 6 x - mmhg P = 7 x - mmhg V CP (volt) I d = ma I b (ma) P = x - mmhg P = 5 x - mmhg P = 6 x - mmhg P = 7 x - mmhg V CP = - volt I d (ma) Fig.(5): Output ion beam current versus V CP Fig.(6): Output ion beam current at versus different pressures using argon gas. discharge current at V CP equals - volt using argon gas. - Ion Source Efficiency Without and With Negative Extraction Voltage The efficiency of the ion source was calculated without and with effect of applied the negative extraction voltage on the ion collector plate at different pressures using the experimental results of the electrical discharge current and the output ion beam current. Figure (7) shows the ion source efficiency, (I b / I d ), versus the gas pressure, P, at I d =. ma without and with applied the negative extraction voltage on the ion collector plate. It is clear from the figure that the ion source efficiency increases by decreasing the gas pressure. At P = x - mmhg and without extraction voltage, the efficiency of the ion source reaches.5 % and at P = 7 x - mmhg reaches.5 %, while in case of V CP = - volt, the efficiency of the ion source at P = x - mmhg reaches 7.5 % and at P = 7 x - mmhg reaches. %.

5 ab Journal of Nuclear Science and Applications, 5(), Ion source efficiency ( Ib / Id) % V CP = - volt without extraction voltage I d =. ma P x - (mmhg) Fig.(7): Ion source efficiency versus the pressure at I d =. ma without and with extraction voltage using argon gas. - Determination of the Sputtering Rate Without and With Negative Extraction Voltage In this experiment, the copper and aluminum targets of 5 mm length, 5 mm width and.5 mm thickness were used. The surface of each target was finely polished to a mirror finish and cleaned by ultrasonic bath of acetone, alcohol and distilled water to remove any contaminations left attached to the surface. Each target was placed at a distance equals 5 cm from the ion exit aperture of the cathode. The argon ions emerging from the cathode aperture of the ion source were used for sputtering of copper and aluminum targets respectively. The rate of the sputtering process for copper and aluminum targets placed at a distance equal to 5 cm from the ion exit aperture of the cathode was calculated using the experimental results without and with the effect of applied negative extraction voltage on each target. The sputtering rate, q, can be calculated as the thickness of the target material surface layer removed per second by ion beam sputterting using the equation () : ( V V ) N q CQ. I b. d - = () o where q is the sputtering rate in mass / sec, C Q is a sputtering rate constant, I b is the output ion beam current, V d is the electrical discharge voltage, V o is a turn-on voltage typically about volt in d.c. operation and N is a constant equal to.7. The sputtering rate constant depends principally on two factors: the transfer of energy from the incident argon ion to the target atoms (this will vary with their relative masses) and break atomic bonds in the target to free sputtered atoms (given by the sublimation energy). The sputtering rate constant of a pure material, expressed as a ratio to some reference material, e.g. pure iron, is given approximately by the relation () :

6 ab Journal of Nuclear Science and Applications, 5(), C Q a b Ø M ø Ø M ref + M ø Ø V S( ref ) ø = Œ œ Œ œ Œ œ () º M ref ß º M + M ß º V S ß c where M is the atomic mass of the material (target), M ref is the reference material, M is the atomic mass of argon gas (the sputtering gas), V S is the sublimation energy, V S(ref) is the sublimation energy of the reference material. The constants a, b and c are equal to.,.8 and.5 respectively. Figure (8) shows the rate of sputtering, q, versus the discharge voltage, V d, for copper and aluminum targets at P = - mmhg without negative extraction voltage. It is clear that the rate of sputtering for copper and aluminum targets increases by increasing the discharge voltage and at V d = kv, the sputtering rate values of copper and aluminum targets reaches.7 µm / min and. µm / min respectively using argon gas. Figure (9) shows the rate of sputtering, q, versus the discharge voltage, V d, for copper and aluminum targets at P = - mmhg with applied extraction voltage on each target, V target, equal to - volt. In this case the extracted ions are due to sum of the discharge voltage and the extraction voltage. Therefore at V d = kv and V target = - volt, the sputtering rate values of copper and aluminum targets reaches.8 µm / min and.9 µm / min respectively using argon gas. q (mm / min.)..8 Cu without extraction voltage.6 P = x - mmhg Al V d (kv) q (mm / min.).5..9 V target = - volt.6 P = x - mmhg Cu Al V d (kv) Fig.(8): The sputtering rate versus discharge Fig.(9): The sputtering rate versus discharge voltage for copper and aluminum targets voltage for copper and aluminum targets at P = - mmhg without extraction at P = - mmhg and V target = - voltage using argon gas. volt using argon gas.

7 ab Journal of Nuclear Science and Applications, 5(), CONCLUSION The electrical discharge current, electrical discharge voltage and output ion beam current characteristics of a cold conical cathode ion source were measured at the optimum operating conditions using argon gas. It was found that at P = x - mmhg and I d =. ma, a maximum output ion beam current, I b = 55 µa, can be obtained. The effect of applied negative extraction voltage on the ion collector plate on the output ion beam current was measured at different pressures. It was found that, at V CP = - volt, I d = ma and, the output ion beam current increases about 75 % its initial values using argon gas. While at P = x - mmhg, V CP = - volt, I d =. ma and, a maximum output ion beam current, I b = 55? A, can be obtained. The efficiency of the ion source was determined without and with the negative extraction voltage. It can be concluded that, at P = x - mmhg and I d =. ma, the efficiency of the ion source reaches.5 % and 7.5 % without and with V CP = - volt respectively. The sputtering rate of copper and aluminum targets which are placed at optimum distance equals 5 cm from the ion exit aperture of the cathode was calculated using the experimental results without and with negative extraction voltage applied on each target. A comparison was made between the sputtering rate values of copper and aluminum targets without extraction voltage and with applied extraction voltage on each target equal to - volt. It can be concluded that, at P = x - mmhg and V target = - volt, the sputtering rate values of copper and aluminum targets increases about 5 % than that the sputtering rate values without extraction voltage. Therefore this ion source can be used for different applications such as sputtering, etching, cleaning (removal the surface contiminations), surface modification and thin film deposition. REFERENCES () K.Shimizu, R.Payling, H.Habazaki, P.Skeldon and G.E.Thompson; J. Anal. At. Spectrom.; 9, 69 (). () M.M.Abdelrahman; Brazilian J. Phys.; (), 6 (). () R.A.Baragiola; Phil. Trans. R. Soc. Lond. A.; 6, 9 (). () R.Behrisch and W.Eckstein; "Sputtering by Particle Bombardment: Experiments and Computer Calculations from Threshold to Mev Energies", Springer, Berlin; (7). (5) M.Stepanova and S.K.Dew; J. Phys.: Condens. Matter; (), (9). (6) F.Aumayr and HP.Winter; Phil. Trans R. Soc. Lond. A.; 6, 77 (). (7) L.A.Giannuzzi and F.A.Stevens ; "Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques and Practice", Springer Press, New York; (). (8) C.A.Volkert and A.M.Minor; MRS Bulletin;, 89 (7). (9) V.N.Tondare; J. Vac. Sci. Technol. A.; (6), 98 (5). () L.Repetto, G.Firpo and U.Valbusa; Materials and Technology; (), (8). () A.G.Helal, S.A.Nouh, H.El-Khabeary and S.M.Mahmoud; ab J. Nucl. Sci. Appl.; () () H.El-Khabeary; Brazilian J. Phys.; (), 7 (). () R.Payling; Surf. Interface Anal.;, 79 (99). () B.V.King; " Sputtering: Basic Principles", In: "Glow Discharge Optical Emission Spectrometry", R.Payling, D.G.Jones and A.Bengtson (eds.), John Wiley & Sons LTd., Chichester; (997).