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Development of metallic ion beams using ECRIS
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 252 (2006) 354–360 www.elsevier.com/locate/nimb
Development of metallic ion beams using ECRIS P. Kumar a
a,*
, G. Rodrigues a, P.S. Lakshmy a, D. Kanjilal a, Beer Pal Singh b, R. Kumar b
Inter University Accelerator Centre (IUAC), UGC, Aruna Asaf Ali Marg, Post Box No. 10502, New Delhi 110067, India b Department of Physics, Ch. Charan Singh University, Meerut 250004, UP, India Received 17 May 2006; received in revised form 14 July 2006 Available online 14 September 2006
Abstract The low energy metallic ion beams find wide applications in various research fields of the materials science. Several metallic ion beams have been developed successfully using the electron cyclotron resonance (ECR) ion source based low energy ion beam facility (LEIBF) at Inter University Accelerator Centre (IUAC), New Delhi. These metallic ion beams were developed by different techniques and utilized for the synthesis of the metal nanoparticles inside various host matrices. The special emphasis was put on the development of the nickel (Ni) and iron (Fe) ion beams using volatile compounds. The hydrocarbon cluster beams were also observed in the charge state distribution (CSD) of the ECR plasma produced by the dissociation of the vapors from the volatile compound of iron. Ni and Fe ion beams were utilized to make a dilute magnetic semiconductor phase (nickel in silicon and iron in silicon) by implantation method. The ion beams extracted from the metallic ECR plasma have been analyzed in energy and momentum using a high mass resolution dipole magnet. Studies of the CSD of the output metallic ion beams and the co-relations among various source parameters are presented. 2006 Elsevier B.V. All rights reserved. PACS: 52.50.Sw; 52.50.Dg Keywords: Electron cyclotron resonance ion source; Plasma; Ion beams; Metallic ions; Charge states distribution; Vapor pressure
1. Introduction The development of the low energy (keV) metallic ion beams is essential for different research areas of materials science like nanotechnology, synthesis of the compound and dilute magnetic semiconductors, interface mixing, rare earth doping of the semiconductors, etc. The metal nanoparticles inside various host matrices exhibit different optical, electrical, magnetic and structural properties which can be utilized to develop new technologies. For example, gold nanoparticles doped SiO2–TiO2 sol–gel films show nonlinear optical absorption [1] and are used for switching pur-
*
Corresponding author. Tel.: +91 11 26893955; fax: +91 11 26893666. E-mail address: [email protected] (P. Kumar).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.07.026
pose in the control circuits. The nanoparticles consisting of a metallic iron core and an amorphous silica shell present a much stronger magnetic response than any composite material [2]. The dilute magnetic semiconductors which are widely used in making spintronic devices [3,4] are formed by the doping of the magnetic impurities like nickel and iron inside the semiconductors. The doping of such magnetic impurities by implantation method has a great advantage in controlling the fluence and the range/position of the projectile. The range of projectile inside host matrices and the fluence can be optimized using the stopping and range of ions in matter (SRIM) simulation code [5]. The deposition of the carbon and hydrocarbon clusters is used to grow several types of films from polymeric to diamond-like carbon [6]. Such films could be used as protective, lubricating coatings for computer hard disk, coating for medical
P. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 354–360
implants or as dielectrics. In case of metal cluster deposition, the incident energy plays a crucial role in determining the structure and properties of the films. For example, when carbon cluster beams are deposited on metallic and polymeric surfaces at low velocities, nanostructured films form because the carbon clusters tend to retain their structure. These films are found to have good adhesion to the substrate and may be used in electrochemical application.
2. Description of the facility 2.1. Facility layout The LEIBF [7] was set up at IUAC to provide research facilities in the field of materials sciences and atomic/ molecular physics. For this purpose, fully permanent magnet based ECR ion source [8] known as NANOGAN [9] operating at 10 GHz frequency was placed on a high voltage (HV) platform and extraction, acceleration, and beam lines were designed and developed accordingly. The details of the source and the facility have been reported in earlier publications [10–12]. The ECR ion source along with all its peripheral electronics (10 GHz (200 W) ultrahigh frequency transmitter, power supplies, etc.) and vacuum components are placed on a 200 kV HV platform. The system provides multiply charged positive ions in a widely varying energy range from a few tens of keV to a few MeV. The schematic of the facility (LEIBF) is shown in Fig. 1. The
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LEIBF consists of two beam lines for experiments. The 90 beam line is mainly dedicated for the materials science related experiments specially implantation studies at room temperature as well as at elevated temperatures while 15 beam line is used to study the interaction of the ion beam with liquid/gas targets [13]. 2.2. The plasma generation and confinement The plasma inside the source is produced by the electron impact process and confined using the frozen min-B magnetic structure. If the gyration frequency of the electrons (xe = eB/m) in a magnetic field matches with input microwave frequency (xrf), the ECR condition is satisfied and the electrons gain energy from the input microwave causing intense ionization of the medium in the source and producing a hot ECR plasma. The source utilizes the permanent magnets for the radial and axial confinement of the plasma. The hexapoles placed around the plasma chamber (cylindrical) produce a nonlinear radial magnetic field, which is zero at the centre of the plasma chamber and maximum at the walls. In the design of NANOGAN source, the maximum radial magnetic field is 0.8 T. The axial magnetic field is achieved by placing two permanent magnet solenoids at the two ends of the plasma chamber. The peak axial fields for the injection and the extraction sides are 5.5 kG and 3.5 kG respectively. As both the fields (axial and radial) are due to the permanent magnets, so magnetic field structure of the NANOGAN source is a frozen one for the confinement of the charge particles. The superposition of the two fields (axial and radial) results in min-B configuration of the magnetic fields. The min-B configuration acts as a strong trap for the electrons and ions, and stabilizes the plasma against magneto hydrodynamic (MHD) instability. This feature, plasma stabilization in a min-B field configuration, produces an enormous improvement in the life time of the ions in the source (102 s) and thereby also in CSD. Although min-B structure is most suitable for the high intensities of high charge states, yet the high currents of lower charge states can be obtained by increasing the throughput/vapor or operating the source at higher pressures. 2.3. Extraction system
Fig. 1. Schematic of low energy ion beam facility.
The extraction system consists of a extraction aperture (plasma electrode), a puller electrode in the conical shape (at ground potential), a pumping tank which houses an Einzel lens [14] and a large acceptance accelerating tube. The puller electrode is kept close to the extraction aperture (15 mm). After polarizing the source (plasma chamber) at a positive potential (maximum of 30 kV) with respect to the puller electrode (ground potential), an electric field is set up to extract the parallel positive ion beam from the ECR plasma. Accelerating tube together with the Einzel lens focuses the ion beam at the object plane of the analyzing magnet. The extracted beam intensities of different charge
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states are found to be strongly dependent on the size of the extraction aperture. The extraction aperture of bigger diameter is suitable for the extraction of ion beams of lower charge states. The optimization of the beams of different charge states needs a variation of the diameter of the extraction aperture between 3 and 7 mm. 2.4. Analyzing system The extracted ion beam is analyzed (E(energy) /p(momentum)) using a high resolution dipole magnet which has two bending ports at 15 and 90. The mass resolution of dipole magnet is 1 in 300 (4 mm slit width) for 90 bend. The beams having same m/q values cannot be resolved using the dipole magnet. Such beams are termed as contaminated beams and are avoided from the delivery at target in the regular operation of the source. The bending radius of the magnet is 600 mm for 90 bend and 2000 mm for 15 bend. The maximum magnetic field of the dipole magnet is 1.4 T for the analysis of the ion beams. A log amplifier module interfaced with computer is used to record the CSD. With the variation of the magnetic field (magnetic field ramping), the analyzed currents collected by the Faraday cup are taken as the input by log amplifier and the corresponding output voltage is generated to record the on-line spectrum. 3. Metallic ion beam production techniques To produce metallic ECR plasma, followings techniques are widely used: (a) (b) (c) (d)
use of micro-oven, sputtering, insertion, metal ion from volatile compound (MIVOC).
Each technique has its own advantages and limitations. The use of the micro-oven [15] is beneficial if the melting point of the material used for the production of the ion beam is around 1000 C. Since higher temperatures are detrimental to the properties of the permanent magnet used for the radial as well as axial confinement of the ECR plasma, micro-oven has to be used with caution. So, for the materials of very high melting points (>1000 C), sputtering/insertion [16] method is most suitable. In the sputtering method, metallic vapors are introduced in the ECR source by inserting the metallic wire in the source close to the background plasma. The output beam current in this method is limited by the sputtering yield. The optimization of the ion beams of various charge states is strongly dependent on the sputtering yield/throughput. For the optimization of the ion beams of lower charge states, the sputtering method is found to be inadequate. In the insertion method, metallic wire is injected step by step into the background plasma to get the higher yield/throughput, which is essential for high intensities of lower charge state ion beams.
Fig. 2. MIVOC system coupled to injection side of ECR ion source at IUAC.
The continuous operation of the ECR source to get the stable output beam by the insertion method is interrupted because of higher material consumption and the local cooling of the background ECR plasma. Most metals have low vapor pressure even at quite high temperature and the production of high intensity metallic ion beams of lower charge states by conventional methods like micro-oven, sputter gun, insertion method is quite difficult. Since the volatile compounds have vapor pressure of greater than 103 mbar at room temperature, an elegant solution of this problem is to switch over to metal ions using volatile compounds (MIVOC) method [17], in which vapors of the volatile compounds having metal atoms in their molecular structure are used to release metallic elements. This facility consists of a small chamber and a vapor flow control valve with the control unit and is shown in Fig. 2. The MIVOC chamber containing the volatile compound is connected to the injection side of ECR ion source from where the vapors of the compound are allowed to diffuse into the source. Decomposition and ionization of the compound then take place in the plasma. 4. Beam requirement for the implantation studies In the implantation studies of the materials, it is desirable to implant a specific fluence (particles/cm2) inside the host matrices/targets to study the changes in the various properties like optical, structural and electrical of the target materials. The implantation time taken to complete a desired fluence is strongly dependent on the beam parameters and is given by t ðsÞ ¼ ðDqeAÞ=I; where D is the fluence (particles/cm2), q is the charge state of the ion beam, e is the electronic charge in Coulomb, A is the area of the implantation region in cm2 and I is the beam current is ampere. A high intensity/current beam of low charge states is essential to complete the implantation at lower energy within acceptable time period.
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5. ECR operation with metallic ion beams and results For the production of the metallic ion beams of different charge states, the required throughput of vapor varies a lot. The tuning of higher charge states ion beams needs a very little amount of throughput and it can be easily achieved by the sputtering of the material or using the material inside the micro-oven. The oven temperature and the sputter rod position can be optimized for the required throughput. However, the throughput required for the optimization of the ion beams of lower charge states and high intensities (implantation purpose) is relatively quite high (10 times). It is very difficult to get that throughput by sputtering of the material. Since the metals have very high melting point, the use of micro-oven also cannot be implemented to develop such beams. The MIVOC method was tested to develop a few metallic ion beams for the implantation purpose. This method is used only to extract the ion beams of the materials for which synthesis of the volatile compounds is possible. The iron and nickel ion beams were developed for materials science research in synthesis of the dilute magnetic semiconductor (nickel in silicon) as well as to study the magnetic properties of the polymer films. Since both nickel and iron have their volatile compounds available, the MIVOC method is used to develop the ion beams from these materials. For the optimization of the gaseous ion beams of low charge states, the observed change in the injection side pressure is 2 · 106 mbar and effective pumping speed of the pumping system (installed at the extraction side) at the injection side is 20 l/s. Thus, the total throughput of the gas at the injection side is 4 · 105 mbar l/s and it allows us to design the MIVOC chamber with the minimum conductance of 0.04 l/s. With this conductance, volatile compounds (vapor pressure 103 mbar) are expected to provide sufficient throughput for the optimization of the low charge states metallic ion beams. In principle, the MIVOC method can also be used to develop ion beams of higher charge states using a needle valve (placed between source and MIVOC chamber) to control throughput. The nickel and iron ion beams were developed using nickelocene [(C5H5)2Ni] and ferrocene [(C5H5)2Fe] respectively. The MIVOC chamber and couplings were designed to get the required conductance and throughput for optimization of ion beams of lower charge states. For Fe+ and Ni2+ ion beams, the calculated implantation time for the fluence 1017 particles/s and beam current of tens of nA is quite large for a single sample (1 cm2 area). Since there are numbers of samples/targets to be implanted with a variation in fluence (1015–1017 particles/s) as well as with constant fluence in most experiments, the high beam current is essential for such types of implantation studies. Furthermore, the implantation of many samples with constant fluence helps in characterization of the samples by different techniques. To enhance the beam currents, the pellets (6 mm in diameters and 3 mm thick) of the compounds (nickelocene [(C5H5)2Ni] and ferrocene [(C5H5)2Fe]) were made and placed into
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the source inside the bias tube (a negatively polarized copper tube placed axially into the source from the injection side to reflect the electrons back into the ECR plasma to maintain the enough electron densities for ionization) for the development of the ion beams. A mesh of metal wires was used at the open end (towards the plasma) of the bias tube to avoid falling of the volatile compounds from the tube during initially pumping of the source for vacuum. With this technique, we got enough intensities (of the order of lA) of the beams and the implantation of iron and nickel beams were carried out. For the nickel beam, the source parameters have been optimized for Ni2+ with total potential differential (E/q) of 100 kV. The total potential difference seen by the extracted ion is the sum of the platform and the extraction voltage. The recorded CSD of nickel is shown in Fig. 3 and analyzed beam currents for different charge states are listed in Table 1. The nickel has five isotopes corresponding to the mass numbers 58, 60, 61, 62 and 64 and the mass 58 has been chosen because of its highest natural abundance (68.08%). The HV platform and the source potential (extraction voltage) were set at 85 kV and 15 kV resulting in
Fig. 3. Charge state distribution of nickel. The MIVOC method was used and the ion source parameters were optimized for Ni2+. The dipole magnetic field was set at 0.387 T to analyze the beam.
Table 1 Analyzed beam currents of
58
Ni ions having E/q as 100 kV
Charge state (q)
M/q (M = 58)
Beam current (e lA)
1 2 3 4 5 6 7 8 9 10
58 29 19.33 14.5 11.6 9.67 8.28 7.25 6.44 5.8
1.63 2.14 2.36 3.61 4.47 3.28 2.34 1.02 0.45 0.10
The highest intensity/current was achieved for q = 5 (m/q = 11.6).
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P. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 354–360
a total potential difference of 100 kV. The typical optimized source parameters are listed in Table 2. The optimized source parameters for the iron beam (56Fe+) are listed in Table 3 and the CSD is shown in Fig. 4. The hydrocarbon cluster beams (singly ionized) are seen in the CSD and are marked as CnHn. The 56Fe2+ ion beam has same m/q (28) as singly ionized C2H4 beam and is contaminated in nature. In fact, we tried to develop the iron beam using the sputtering system but we could not get sufficient intensities. The iron wire mounted on the sputter rod was found to be bent in the radial direction (off axis position) after opening the source. Since along the axis of the source, B (magnetic field) radial is zero and the B axial is parallel to the velocities of the charge particles, the loss of the charge particles from the plasma is maximum along the axis. If the wire is
Table 2 The optimized source parameters for the extraction of Ni2+ ion beam from the ECR plasma Source parameters
Values
E/q Beam Einzel lens voltage Source pressure Microwave power Bias voltage Beam current
100 kV Ni2+ 7.0 kV 5.0 · 106 mbar 20 W 130 V 2.14 lA 58
Table 3 The optimized source parameters for the extraction of Fe+ ion beam from the ECR plasma Source parameters
Values
E/q Beam Einzel lens voltage Source pressure Microwave power Bias voltage Beam current
150 kV Fe+ 7.5 kV 6.5 · 106 mbar 8W 100 V 12 lA 56
Fig. 4. Charge state distribution of iron. The MIVOC method was used and the ion source parameters were optimized for Fe+. The dipole magnetic field was set at 0.66 T to analyze the beam.
Table 4 The metallic ion beams developed for various experiments with LEIBF Beam
E/q (kV)
q (selected)
Technique
Current (e lA)
58
100 100 93.75 150 150 100
2 2 4 1 10 1
MIVOC Sputtering Insertion/sputtering MIVOC MIVOC Sputtering
2.14 0.10 0.05 12.00 0.10 0.10
Ni Zn 153 Eu 56 Fe 63 Cu 107 Au 66
not exactly positioned on the axis of the source, the sputtering of the wire by charge particles will not occur and the beams of higher intensities cannot be developed. The bending of the iron wire may be due to the magnetic force on the wire exerted by the permanent magnets of the source. The metallic ion beams which have been developed successfully and delivered for experiments are Ni, Cu, Eu, Zn, Au and Fe. Table 4 shows the output currents of these ion beams. The different techniques to develop these ion beams are also mentioned in Table 4. Since the maximum platform and the extraction voltages are 200 kV and 30 kV respectively, the charge state was selected based on the requirement of energy of the beam for implantation during various experiments. 6. Discussion The very first interest of ECRIS technology is to get enough intensities of high charge state beams. Mostly, we use the ECRIS based LEIBF for implantation studies. For this kind of studies, high particle current is essential. To achieve high particle current, the electrical currents of higher charge state beams should be high. We have optimized the source parameters for lower values of charge states. To produce high intensity beam at lower charge state, we need high throughput/vapor pressure. Due to lower throughput (case of sputtering method), the intensity of ion beams at lower charge state is low as shown in Table 4. Furthermore, the efficiency of sputtering of heavy element (target metal) by lighter projectile (support gas) is also lower. The background plasma of the elements heavier than the metallic element of interest will also not help to get the higher sputtering yield. The sputtering is mainly due to the ions lost from the ECR plasma. Therefore, heavy ions of background plasma will be confined strongly by magnetic mirror configuration of ECRIS due to their relatively higher average charge states and ion cooling effect of internal gas mixing [18]. In case of MIVOC method, the beam currents of high charge states (63Cu, 153Eu: Table 4) are lower because of the limitation on input microwave power (<200 W) in our system. The efforts were made to get stable beams with enough intensities. Fluctuations and a constant decay in metallic beams were noticed in earlier studies [19]. We have successfully extracted nickel and iron beams for the implantation studies. There were small fluctuations in beam currents but no decay in beam currents has been
P. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 354–360
noticed during the implantation period of one week. This was made possible by changing the old extraction system with new one having puller electrode of smaller length and placed closer to extraction aperture (15 mm). This configuration reduces the probability of beam hitting to HV insulator between the plasma chamber and the puller electrode flange and stops the formation of a conducting coating on inner surface of HV insulator, which has been observed in case of extraction of other metallic beams. The high leakage current through this conducting layer causes high voltage fluctuations resulting in instabilities in extracted and analyzed beam currents. In fact, these instabilities are mainly caused by plasma oscillations, different modes of rf coupling to the system, loading of power supplies, etc. and are more pronounced in case of metallic ECR plasma as has been observed during the source operation. A typical beam stability graph for 58Ni4+ is shown in Fig. 5. The pallets of the volatile compounds placed inside the bias tube (on the source axis and inside the source) undergo the additional sputtering process and the expected throughput in this configuration is more. Furthermore, the vapors released from the volatile compounds are forced to diffuse direct into the plasma through the copper bias tube. Since the bias tube is close to the plasma and is hot, the residence time of the vapors on the wall of the tube is fairly small. In the configuration where the material is placed in the MIVOC chamber (outside the source), the vapors have the high probability to sit on the walls of the couplings/ source body before diffusing into the interaction region (plasma surface). In such case, the vacuum guage shows higher pressure indicating the desired throughput but the ionization efficiency is very poor. For the higher ionization efficiency, the vapors should not escape before ionization. The configuration where the material is placed inside the bias tube is more efficient for ionization. As there is no control on the throughput in this configuration, the tuning of
Fig. 5. Beam (Ni2+, E/q = 100 kV) fluctuations at Faraday cup.
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high charge states ion beams was found to be difficult. Furthermore, the high power levels which are essential for the high intensities of highly charged ions lead to the plasma instabilities and loading of the power supplies due to high operating pressure of the source. The hydrocarbon clusters (CnHn) observed in the CSD of the iron are due to the low microwave power (8 W). At this power level, ferrocene undergoes incomplete fragmentation. At higher power, intensity of these clusters decreases steadily. To optimize the intensity of Fe+ (singly ionized), we need to operate the ECRIS at lower level of microwave power. 7. Conclusion Various metallic ion beams for implantation have been developed using all permanent magnet ECR ion source on a high voltage platform. Micro-oven, MIVOC method and sputtering set up have been used to produce these beams. Ni and Fe ion beams have been used regularly for various experiments at typical fluence of about 1 · 1017 particles/cm2. In MIVOC method, the volatile compounds placed inside the source resulted in high intensities of low charge state ion beams required for ion implantation. The cluster beams (hydrocarbon) produced by dissociation of volatile compounds have also been extracted and analyzed. New experiments will be carried out using these cluster beams. References [1] N. Venkatram, R. Sai Santosh Kumar, D. Narayana Rao, S.K. Medda, Sucheta De, Goutam De, Nanosci. Technol. 1–5 (2006) 6. [2] Rodrigo Ferna´ndez-Pacheco, Manuel Arruebo, Clara Marquina, Ricardo Ibarra, Jordi Arbiol, Jesu´s Santamaria, Nanotechnology 1188–1192 (2006) 17. [3] V.A. Ivanov, T.G. Aminov, V.M. Novotortsev, V.T. Kalinnikov, Russ. Chem. Bull. 2357–2405 (2004) 53. [4] Xiangyang Huang, Adi Makmal, James R. Chelikowsky, Phys. Rec. Lett. 236801 (2005) 94. [5] J.F. Zeigler, The Stopping and Range of Ions in Matter, Pergamon Press, 1977–1985. [6] L. Diederich, E. Barborini, P. Piseri, A. Podesta, P. Milani, A. Schneuwly, R. Gallay, Appl. Phys. Lett. 2662 (1999) 75. [7] D. Kanjilal, T. Madhu, G.O. Rodrigues, U.K. Rao, C.P. Safvan, A. Roy, Ind. J. Pure Appl. Phys. 25 (2001) 39. [8] R. Geller, Electron Cyclotron Resonance Ion Sources and ECR Plasmas, IOP, Bristol, 1996. [9] P. Sortais, C. Bieth, P. Foury, N. Lecesne, R. Leroy, J. Mandin, C. Marry, J.Y. Pacquet, E. Robert, A.C.C. Viliari, in: Proceedings of the 12th International Workshop on ECR Ion Sources, RIKEN, Japan, 1995, p. 44. [10] D. Kanjilal, G. Rodrigues, U.K. Rao, C.P. Safvan, P. Kumar, A. Roy, G.K. Mehta, in: Proceedings of Second Asian Particle Accelerator Conference, Beijing, China, 2001, p. 878. [11] G. Rodrigues, P. Kumar, U.K. Rao, C.P. Safvan, D. Kanjilal, A. Roy, in: Proceedings of the 15th International Workshop on ECR ion sources, Jyvaskyla, Finland, JYFL Research Report 4, 2002, p. 210. [12] P. Kumar, G. Rodrigues, U.K. Rao, C.P. Safvan, D. Kanjilal, A. Roy, Pramana-Ind. J. Phys. 805 (2002) 59. [13] G.K. Padmashree, A. Roy, D. Kanjilal, G. Rodrigues, R. Ahuja, R. Somashekar, C.P. Safvan, Rev. Sci. Instr. 5094 (2004) 75. [14] F.H. Read, J. Phys. E: Sci. Instr. 679 (1969) 2.
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[15] P. Lehe´rissier, C. Barue´, C. Canet, M. Dubois, M. Dupois, J.L. Flambard, G. Gaubert, P. Jardin, N. Lecesne, F. Legmanen, R. Leroy, J.Y. Pacquet, F. pellemoine-Landre´, Rev. Sci. Instr. 1488 (2004) 75. [16] P. Kumar, G. Rodrigues, D. Kanjilal, A. Roy, Beer Pal Singh, R. Kumar, Nucl. Instr. and Meth. B 440 (2006) 246. [17] J. Arje, H. Koivisto, M. Nurmia, in: A.G. Drentje (Ed.), Proceedings of 11th International Workshop on ECRIS, Groningen, 1993, p. 27.
[18] A.G. Drentje, A. Girard, D. Hitz, G. Melin, Rev. Sci. Instr. 953 (1996) 67. [19] C. Barue´, T. Been, M. Dupuis, J.L. Flambard, G. Gaubert, Y. Huguet, P. Jardin, S. Kandri-Rody, F. Landre´, N. Lecesne, D. Lecler, P. Lehe´rissier, F. Lemagnen, R. Leroy, J.Y. Pacquet, J.P. Rataud, A.C.C. Villari, in: Proceedings of 14th International Workshop on ECR Sources, ECRIS99, 1999, p. 111.
Nuclear Instruments and Methods in Physics Research B 252 (2006) 354–360 www.elsevier.com/locate/nimb
Development of metallic ion beams using ECRIS P. Kumar a
a,*
, G. Rodrigues a, P.S. Lakshmy a, D. Kanjilal a, Beer Pal Singh b, R. Kumar b
Inter University Accelerator Centre (IUAC), UGC, Aruna Asaf Ali Marg, Post Box No. 10502, New Delhi 110067, India b Department of Physics, Ch. Charan Singh University, Meerut 250004, UP, India Received 17 May 2006; received in revised form 14 July 2006 Available online 14 September 2006
Abstract The low energy metallic ion beams find wide applications in various research fields of the materials science. Several metallic ion beams have been developed successfully using the electron cyclotron resonance (ECR) ion source based low energy ion beam facility (LEIBF) at Inter University Accelerator Centre (IUAC), New Delhi. These metallic ion beams were developed by different techniques and utilized for the synthesis of the metal nanoparticles inside various host matrices. The special emphasis was put on the development of the nickel (Ni) and iron (Fe) ion beams using volatile compounds. The hydrocarbon cluster beams were also observed in the charge state distribution (CSD) of the ECR plasma produced by the dissociation of the vapors from the volatile compound of iron. Ni and Fe ion beams were utilized to make a dilute magnetic semiconductor phase (nickel in silicon and iron in silicon) by implantation method. The ion beams extracted from the metallic ECR plasma have been analyzed in energy and momentum using a high mass resolution dipole magnet. Studies of the CSD of the output metallic ion beams and the co-relations among various source parameters are presented. 2006 Elsevier B.V. All rights reserved. PACS: 52.50.Sw; 52.50.Dg Keywords: Electron cyclotron resonance ion source; Plasma; Ion beams; Metallic ions; Charge states distribution; Vapor pressure
1. Introduction The development of the low energy (keV) metallic ion beams is essential for different research areas of materials science like nanotechnology, synthesis of the compound and dilute magnetic semiconductors, interface mixing, rare earth doping of the semiconductors, etc. The metal nanoparticles inside various host matrices exhibit different optical, electrical, magnetic and structural properties which can be utilized to develop new technologies. For example, gold nanoparticles doped SiO2–TiO2 sol–gel films show nonlinear optical absorption [1] and are used for switching pur-
*
Corresponding author. Tel.: +91 11 26893955; fax: +91 11 26893666. E-mail address: [email protected] (P. Kumar).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.07.026
pose in the control circuits. The nanoparticles consisting of a metallic iron core and an amorphous silica shell present a much stronger magnetic response than any composite material [2]. The dilute magnetic semiconductors which are widely used in making spintronic devices [3,4] are formed by the doping of the magnetic impurities like nickel and iron inside the semiconductors. The doping of such magnetic impurities by implantation method has a great advantage in controlling the fluence and the range/position of the projectile. The range of projectile inside host matrices and the fluence can be optimized using the stopping and range of ions in matter (SRIM) simulation code [5]. The deposition of the carbon and hydrocarbon clusters is used to grow several types of films from polymeric to diamond-like carbon [6]. Such films could be used as protective, lubricating coatings for computer hard disk, coating for medical
P. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 354–360
implants or as dielectrics. In case of metal cluster deposition, the incident energy plays a crucial role in determining the structure and properties of the films. For example, when carbon cluster beams are deposited on metallic and polymeric surfaces at low velocities, nanostructured films form because the carbon clusters tend to retain their structure. These films are found to have good adhesion to the substrate and may be used in electrochemical application.
2. Description of the facility 2.1. Facility layout The LEIBF [7] was set up at IUAC to provide research facilities in the field of materials sciences and atomic/ molecular physics. For this purpose, fully permanent magnet based ECR ion source [8] known as NANOGAN [9] operating at 10 GHz frequency was placed on a high voltage (HV) platform and extraction, acceleration, and beam lines were designed and developed accordingly. The details of the source and the facility have been reported in earlier publications [10–12]. The ECR ion source along with all its peripheral electronics (10 GHz (200 W) ultrahigh frequency transmitter, power supplies, etc.) and vacuum components are placed on a 200 kV HV platform. The system provides multiply charged positive ions in a widely varying energy range from a few tens of keV to a few MeV. The schematic of the facility (LEIBF) is shown in Fig. 1. The
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LEIBF consists of two beam lines for experiments. The 90 beam line is mainly dedicated for the materials science related experiments specially implantation studies at room temperature as well as at elevated temperatures while 15 beam line is used to study the interaction of the ion beam with liquid/gas targets [13]. 2.2. The plasma generation and confinement The plasma inside the source is produced by the electron impact process and confined using the frozen min-B magnetic structure. If the gyration frequency of the electrons (xe = eB/m) in a magnetic field matches with input microwave frequency (xrf), the ECR condition is satisfied and the electrons gain energy from the input microwave causing intense ionization of the medium in the source and producing a hot ECR plasma. The source utilizes the permanent magnets for the radial and axial confinement of the plasma. The hexapoles placed around the plasma chamber (cylindrical) produce a nonlinear radial magnetic field, which is zero at the centre of the plasma chamber and maximum at the walls. In the design of NANOGAN source, the maximum radial magnetic field is 0.8 T. The axial magnetic field is achieved by placing two permanent magnet solenoids at the two ends of the plasma chamber. The peak axial fields for the injection and the extraction sides are 5.5 kG and 3.5 kG respectively. As both the fields (axial and radial) are due to the permanent magnets, so magnetic field structure of the NANOGAN source is a frozen one for the confinement of the charge particles. The superposition of the two fields (axial and radial) results in min-B configuration of the magnetic fields. The min-B configuration acts as a strong trap for the electrons and ions, and stabilizes the plasma against magneto hydrodynamic (MHD) instability. This feature, plasma stabilization in a min-B field configuration, produces an enormous improvement in the life time of the ions in the source (102 s) and thereby also in CSD. Although min-B structure is most suitable for the high intensities of high charge states, yet the high currents of lower charge states can be obtained by increasing the throughput/vapor or operating the source at higher pressures. 2.3. Extraction system
Fig. 1. Schematic of low energy ion beam facility.
The extraction system consists of a extraction aperture (plasma electrode), a puller electrode in the conical shape (at ground potential), a pumping tank which houses an Einzel lens [14] and a large acceptance accelerating tube. The puller electrode is kept close to the extraction aperture (15 mm). After polarizing the source (plasma chamber) at a positive potential (maximum of 30 kV) with respect to the puller electrode (ground potential), an electric field is set up to extract the parallel positive ion beam from the ECR plasma. Accelerating tube together with the Einzel lens focuses the ion beam at the object plane of the analyzing magnet. The extracted beam intensities of different charge
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states are found to be strongly dependent on the size of the extraction aperture. The extraction aperture of bigger diameter is suitable for the extraction of ion beams of lower charge states. The optimization of the beams of different charge states needs a variation of the diameter of the extraction aperture between 3 and 7 mm. 2.4. Analyzing system The extracted ion beam is analyzed (E(energy) /p(momentum)) using a high resolution dipole magnet which has two bending ports at 15 and 90. The mass resolution of dipole magnet is 1 in 300 (4 mm slit width) for 90 bend. The beams having same m/q values cannot be resolved using the dipole magnet. Such beams are termed as contaminated beams and are avoided from the delivery at target in the regular operation of the source. The bending radius of the magnet is 600 mm for 90 bend and 2000 mm for 15 bend. The maximum magnetic field of the dipole magnet is 1.4 T for the analysis of the ion beams. A log amplifier module interfaced with computer is used to record the CSD. With the variation of the magnetic field (magnetic field ramping), the analyzed currents collected by the Faraday cup are taken as the input by log amplifier and the corresponding output voltage is generated to record the on-line spectrum. 3. Metallic ion beam production techniques To produce metallic ECR plasma, followings techniques are widely used: (a) (b) (c) (d)
use of micro-oven, sputtering, insertion, metal ion from volatile compound (MIVOC).
Each technique has its own advantages and limitations. The use of the micro-oven [15] is beneficial if the melting point of the material used for the production of the ion beam is around 1000 C. Since higher temperatures are detrimental to the properties of the permanent magnet used for the radial as well as axial confinement of the ECR plasma, micro-oven has to be used with caution. So, for the materials of very high melting points (>1000 C), sputtering/insertion [16] method is most suitable. In the sputtering method, metallic vapors are introduced in the ECR source by inserting the metallic wire in the source close to the background plasma. The output beam current in this method is limited by the sputtering yield. The optimization of the ion beams of various charge states is strongly dependent on the sputtering yield/throughput. For the optimization of the ion beams of lower charge states, the sputtering method is found to be inadequate. In the insertion method, metallic wire is injected step by step into the background plasma to get the higher yield/throughput, which is essential for high intensities of lower charge state ion beams.
Fig. 2. MIVOC system coupled to injection side of ECR ion source at IUAC.
The continuous operation of the ECR source to get the stable output beam by the insertion method is interrupted because of higher material consumption and the local cooling of the background ECR plasma. Most metals have low vapor pressure even at quite high temperature and the production of high intensity metallic ion beams of lower charge states by conventional methods like micro-oven, sputter gun, insertion method is quite difficult. Since the volatile compounds have vapor pressure of greater than 103 mbar at room temperature, an elegant solution of this problem is to switch over to metal ions using volatile compounds (MIVOC) method [17], in which vapors of the volatile compounds having metal atoms in their molecular structure are used to release metallic elements. This facility consists of a small chamber and a vapor flow control valve with the control unit and is shown in Fig. 2. The MIVOC chamber containing the volatile compound is connected to the injection side of ECR ion source from where the vapors of the compound are allowed to diffuse into the source. Decomposition and ionization of the compound then take place in the plasma. 4. Beam requirement for the implantation studies In the implantation studies of the materials, it is desirable to implant a specific fluence (particles/cm2) inside the host matrices/targets to study the changes in the various properties like optical, structural and electrical of the target materials. The implantation time taken to complete a desired fluence is strongly dependent on the beam parameters and is given by t ðsÞ ¼ ðDqeAÞ=I; where D is the fluence (particles/cm2), q is the charge state of the ion beam, e is the electronic charge in Coulomb, A is the area of the implantation region in cm2 and I is the beam current is ampere. A high intensity/current beam of low charge states is essential to complete the implantation at lower energy within acceptable time period.
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5. ECR operation with metallic ion beams and results For the production of the metallic ion beams of different charge states, the required throughput of vapor varies a lot. The tuning of higher charge states ion beams needs a very little amount of throughput and it can be easily achieved by the sputtering of the material or using the material inside the micro-oven. The oven temperature and the sputter rod position can be optimized for the required throughput. However, the throughput required for the optimization of the ion beams of lower charge states and high intensities (implantation purpose) is relatively quite high (10 times). It is very difficult to get that throughput by sputtering of the material. Since the metals have very high melting point, the use of micro-oven also cannot be implemented to develop such beams. The MIVOC method was tested to develop a few metallic ion beams for the implantation purpose. This method is used only to extract the ion beams of the materials for which synthesis of the volatile compounds is possible. The iron and nickel ion beams were developed for materials science research in synthesis of the dilute magnetic semiconductor (nickel in silicon) as well as to study the magnetic properties of the polymer films. Since both nickel and iron have their volatile compounds available, the MIVOC method is used to develop the ion beams from these materials. For the optimization of the gaseous ion beams of low charge states, the observed change in the injection side pressure is 2 · 106 mbar and effective pumping speed of the pumping system (installed at the extraction side) at the injection side is 20 l/s. Thus, the total throughput of the gas at the injection side is 4 · 105 mbar l/s and it allows us to design the MIVOC chamber with the minimum conductance of 0.04 l/s. With this conductance, volatile compounds (vapor pressure 103 mbar) are expected to provide sufficient throughput for the optimization of the low charge states metallic ion beams. In principle, the MIVOC method can also be used to develop ion beams of higher charge states using a needle valve (placed between source and MIVOC chamber) to control throughput. The nickel and iron ion beams were developed using nickelocene [(C5H5)2Ni] and ferrocene [(C5H5)2Fe] respectively. The MIVOC chamber and couplings were designed to get the required conductance and throughput for optimization of ion beams of lower charge states. For Fe+ and Ni2+ ion beams, the calculated implantation time for the fluence 1017 particles/s and beam current of tens of nA is quite large for a single sample (1 cm2 area). Since there are numbers of samples/targets to be implanted with a variation in fluence (1015–1017 particles/s) as well as with constant fluence in most experiments, the high beam current is essential for such types of implantation studies. Furthermore, the implantation of many samples with constant fluence helps in characterization of the samples by different techniques. To enhance the beam currents, the pellets (6 mm in diameters and 3 mm thick) of the compounds (nickelocene [(C5H5)2Ni] and ferrocene [(C5H5)2Fe]) were made and placed into
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the source inside the bias tube (a negatively polarized copper tube placed axially into the source from the injection side to reflect the electrons back into the ECR plasma to maintain the enough electron densities for ionization) for the development of the ion beams. A mesh of metal wires was used at the open end (towards the plasma) of the bias tube to avoid falling of the volatile compounds from the tube during initially pumping of the source for vacuum. With this technique, we got enough intensities (of the order of lA) of the beams and the implantation of iron and nickel beams were carried out. For the nickel beam, the source parameters have been optimized for Ni2+ with total potential differential (E/q) of 100 kV. The total potential difference seen by the extracted ion is the sum of the platform and the extraction voltage. The recorded CSD of nickel is shown in Fig. 3 and analyzed beam currents for different charge states are listed in Table 1. The nickel has five isotopes corresponding to the mass numbers 58, 60, 61, 62 and 64 and the mass 58 has been chosen because of its highest natural abundance (68.08%). The HV platform and the source potential (extraction voltage) were set at 85 kV and 15 kV resulting in
Fig. 3. Charge state distribution of nickel. The MIVOC method was used and the ion source parameters were optimized for Ni2+. The dipole magnetic field was set at 0.387 T to analyze the beam.
Table 1 Analyzed beam currents of
58
Ni ions having E/q as 100 kV
Charge state (q)
M/q (M = 58)
Beam current (e lA)
1 2 3 4 5 6 7 8 9 10
58 29 19.33 14.5 11.6 9.67 8.28 7.25 6.44 5.8
1.63 2.14 2.36 3.61 4.47 3.28 2.34 1.02 0.45 0.10
The highest intensity/current was achieved for q = 5 (m/q = 11.6).
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a total potential difference of 100 kV. The typical optimized source parameters are listed in Table 2. The optimized source parameters for the iron beam (56Fe+) are listed in Table 3 and the CSD is shown in Fig. 4. The hydrocarbon cluster beams (singly ionized) are seen in the CSD and are marked as CnHn. The 56Fe2+ ion beam has same m/q (28) as singly ionized C2H4 beam and is contaminated in nature. In fact, we tried to develop the iron beam using the sputtering system but we could not get sufficient intensities. The iron wire mounted on the sputter rod was found to be bent in the radial direction (off axis position) after opening the source. Since along the axis of the source, B (magnetic field) radial is zero and the B axial is parallel to the velocities of the charge particles, the loss of the charge particles from the plasma is maximum along the axis. If the wire is
Table 2 The optimized source parameters for the extraction of Ni2+ ion beam from the ECR plasma Source parameters
Values
E/q Beam Einzel lens voltage Source pressure Microwave power Bias voltage Beam current
100 kV Ni2+ 7.0 kV 5.0 · 106 mbar 20 W 130 V 2.14 lA 58
Table 3 The optimized source parameters for the extraction of Fe+ ion beam from the ECR plasma Source parameters
Values
E/q Beam Einzel lens voltage Source pressure Microwave power Bias voltage Beam current
150 kV Fe+ 7.5 kV 6.5 · 106 mbar 8W 100 V 12 lA 56
Fig. 4. Charge state distribution of iron. The MIVOC method was used and the ion source parameters were optimized for Fe+. The dipole magnetic field was set at 0.66 T to analyze the beam.
Table 4 The metallic ion beams developed for various experiments with LEIBF Beam
E/q (kV)
q (selected)
Technique
Current (e lA)
58
100 100 93.75 150 150 100
2 2 4 1 10 1
MIVOC Sputtering Insertion/sputtering MIVOC MIVOC Sputtering
2.14 0.10 0.05 12.00 0.10 0.10
Ni Zn 153 Eu 56 Fe 63 Cu 107 Au 66
not exactly positioned on the axis of the source, the sputtering of the wire by charge particles will not occur and the beams of higher intensities cannot be developed. The bending of the iron wire may be due to the magnetic force on the wire exerted by the permanent magnets of the source. The metallic ion beams which have been developed successfully and delivered for experiments are Ni, Cu, Eu, Zn, Au and Fe. Table 4 shows the output currents of these ion beams. The different techniques to develop these ion beams are also mentioned in Table 4. Since the maximum platform and the extraction voltages are 200 kV and 30 kV respectively, the charge state was selected based on the requirement of energy of the beam for implantation during various experiments. 6. Discussion The very first interest of ECRIS technology is to get enough intensities of high charge state beams. Mostly, we use the ECRIS based LEIBF for implantation studies. For this kind of studies, high particle current is essential. To achieve high particle current, the electrical currents of higher charge state beams should be high. We have optimized the source parameters for lower values of charge states. To produce high intensity beam at lower charge state, we need high throughput/vapor pressure. Due to lower throughput (case of sputtering method), the intensity of ion beams at lower charge state is low as shown in Table 4. Furthermore, the efficiency of sputtering of heavy element (target metal) by lighter projectile (support gas) is also lower. The background plasma of the elements heavier than the metallic element of interest will also not help to get the higher sputtering yield. The sputtering is mainly due to the ions lost from the ECR plasma. Therefore, heavy ions of background plasma will be confined strongly by magnetic mirror configuration of ECRIS due to their relatively higher average charge states and ion cooling effect of internal gas mixing [18]. In case of MIVOC method, the beam currents of high charge states (63Cu, 153Eu: Table 4) are lower because of the limitation on input microwave power (<200 W) in our system. The efforts were made to get stable beams with enough intensities. Fluctuations and a constant decay in metallic beams were noticed in earlier studies [19]. We have successfully extracted nickel and iron beams for the implantation studies. There were small fluctuations in beam currents but no decay in beam currents has been
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noticed during the implantation period of one week. This was made possible by changing the old extraction system with new one having puller electrode of smaller length and placed closer to extraction aperture (15 mm). This configuration reduces the probability of beam hitting to HV insulator between the plasma chamber and the puller electrode flange and stops the formation of a conducting coating on inner surface of HV insulator, which has been observed in case of extraction of other metallic beams. The high leakage current through this conducting layer causes high voltage fluctuations resulting in instabilities in extracted and analyzed beam currents. In fact, these instabilities are mainly caused by plasma oscillations, different modes of rf coupling to the system, loading of power supplies, etc. and are more pronounced in case of metallic ECR plasma as has been observed during the source operation. A typical beam stability graph for 58Ni4+ is shown in Fig. 5. The pallets of the volatile compounds placed inside the bias tube (on the source axis and inside the source) undergo the additional sputtering process and the expected throughput in this configuration is more. Furthermore, the vapors released from the volatile compounds are forced to diffuse direct into the plasma through the copper bias tube. Since the bias tube is close to the plasma and is hot, the residence time of the vapors on the wall of the tube is fairly small. In the configuration where the material is placed in the MIVOC chamber (outside the source), the vapors have the high probability to sit on the walls of the couplings/ source body before diffusing into the interaction region (plasma surface). In such case, the vacuum guage shows higher pressure indicating the desired throughput but the ionization efficiency is very poor. For the higher ionization efficiency, the vapors should not escape before ionization. The configuration where the material is placed inside the bias tube is more efficient for ionization. As there is no control on the throughput in this configuration, the tuning of
Fig. 5. Beam (Ni2+, E/q = 100 kV) fluctuations at Faraday cup.
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high charge states ion beams was found to be difficult. Furthermore, the high power levels which are essential for the high intensities of highly charged ions lead to the plasma instabilities and loading of the power supplies due to high operating pressure of the source. The hydrocarbon clusters (CnHn) observed in the CSD of the iron are due to the low microwave power (8 W). At this power level, ferrocene undergoes incomplete fragmentation. At higher power, intensity of these clusters decreases steadily. To optimize the intensity of Fe+ (singly ionized), we need to operate the ECRIS at lower level of microwave power. 7. Conclusion Various metallic ion beams for implantation have been developed using all permanent magnet ECR ion source on a high voltage platform. Micro-oven, MIVOC method and sputtering set up have been used to produce these beams. Ni and Fe ion beams have been used regularly for various experiments at typical fluence of about 1 · 1017 particles/cm2. In MIVOC method, the volatile compounds placed inside the source resulted in high intensities of low charge state ion beams required for ion implantation. The cluster beams (hydrocarbon) produced by dissociation of volatile compounds have also been extracted and analyzed. New experiments will be carried out using these cluster beams. References [1] N. Venkatram, R. Sai Santosh Kumar, D. Narayana Rao, S.K. Medda, Sucheta De, Goutam De, Nanosci. Technol. 1–5 (2006) 6. [2] Rodrigo Ferna´ndez-Pacheco, Manuel Arruebo, Clara Marquina, Ricardo Ibarra, Jordi Arbiol, Jesu´s Santamaria, Nanotechnology 1188–1192 (2006) 17. [3] V.A. Ivanov, T.G. Aminov, V.M. Novotortsev, V.T. Kalinnikov, Russ. Chem. Bull. 2357–2405 (2004) 53. [4] Xiangyang Huang, Adi Makmal, James R. Chelikowsky, Phys. Rec. Lett. 236801 (2005) 94. [5] J.F. Zeigler, The Stopping and Range of Ions in Matter, Pergamon Press, 1977–1985. [6] L. Diederich, E. Barborini, P. Piseri, A. Podesta, P. Milani, A. Schneuwly, R. Gallay, Appl. Phys. Lett. 2662 (1999) 75. [7] D. Kanjilal, T. Madhu, G.O. Rodrigues, U.K. Rao, C.P. Safvan, A. Roy, Ind. J. Pure Appl. Phys. 25 (2001) 39. [8] R. Geller, Electron Cyclotron Resonance Ion Sources and ECR Plasmas, IOP, Bristol, 1996. [9] P. Sortais, C. Bieth, P. Foury, N. Lecesne, R. Leroy, J. Mandin, C. Marry, J.Y. Pacquet, E. Robert, A.C.C. Viliari, in: Proceedings of the 12th International Workshop on ECR Ion Sources, RIKEN, Japan, 1995, p. 44. [10] D. Kanjilal, G. Rodrigues, U.K. Rao, C.P. Safvan, P. Kumar, A. Roy, G.K. Mehta, in: Proceedings of Second Asian Particle Accelerator Conference, Beijing, China, 2001, p. 878. [11] G. Rodrigues, P. Kumar, U.K. Rao, C.P. Safvan, D. Kanjilal, A. Roy, in: Proceedings of the 15th International Workshop on ECR ion sources, Jyvaskyla, Finland, JYFL Research Report 4, 2002, p. 210. [12] P. Kumar, G. Rodrigues, U.K. Rao, C.P. Safvan, D. Kanjilal, A. Roy, Pramana-Ind. J. Phys. 805 (2002) 59. [13] G.K. Padmashree, A. Roy, D. Kanjilal, G. Rodrigues, R. Ahuja, R. Somashekar, C.P. Safvan, Rev. Sci. Instr. 5094 (2004) 75. [14] F.H. Read, J. Phys. E: Sci. Instr. 679 (1969) 2.
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P. Kumar et al. / Nucl. Instr. and Meth. in Phys. Res. B 252 (2006) 354–360
[15] P. Lehe´rissier, C. Barue´, C. Canet, M. Dubois, M. Dupois, J.L. Flambard, G. Gaubert, P. Jardin, N. Lecesne, F. Legmanen, R. Leroy, J.Y. Pacquet, F. pellemoine-Landre´, Rev. Sci. Instr. 1488 (2004) 75. [16] P. Kumar, G. Rodrigues, D. Kanjilal, A. Roy, Beer Pal Singh, R. Kumar, Nucl. Instr. and Meth. B 440 (2006) 246. [17] J. Arje, H. Koivisto, M. Nurmia, in: A.G. Drentje (Ed.), Proceedings of 11th International Workshop on ECRIS, Groningen, 1993, p. 27.
[18] A.G. Drentje, A. Girard, D. Hitz, G. Melin, Rev. Sci. Instr. 953 (1996) 67. [19] C. Barue´, T. Been, M. Dupuis, J.L. Flambard, G. Gaubert, Y. Huguet, P. Jardin, S. Kandri-Rody, F. Landre´, N. Lecesne, D. Lecler, P. Lehe´rissier, F. Lemagnen, R. Leroy, J.Y. Pacquet, J.P. Rataud, A.C.C. Villari, in: Proceedings of 14th International Workshop on ECR Sources, ECRIS99, 1999, p. 111.