Boron ion beam generation using a self-sputtering planar magnetrona) Aleksey Vizir, Aleksey Nikolaev, Efim Oks, Konstantin Savkin, Maxim Shandrikov, and Georgy Yushkov Citation: Review of Scientific Instruments 85, 02C302 (2014); doi: 10.1063/1.4824643 View online: http://dx.doi.org/10.1063/1.4824643 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ion Source of Pure Single Charged Boron Based on Planar Magnetron Discharge in Self‐Sputtering Mode AIP Conf. Proc. 1321, 472 (2011); 10.1063/1.3548454 A self-sputtering ion source: A new approach to quiescent metal ion beamsa) Rev. Sci. Instrum. 81, 02B306 (2010); 10.1063/1.3272797 Boron ion source based on planar magnetron discharge in self-sputtering modea) Rev. Sci. Instrum. 81, 02B303 (2010); 10.1063/1.3258029 Broad, intense, quiescent beam of singly charged metal ions obtained by extraction from self-sputtering plasma far above the runaway threshold J. Appl. Phys. 106, 023306 (2009); 10.1063/1.3177336 Linear ion source with magnetron hollow cathode discharge Rev. Sci. Instrum. 76, 113502 (2005); 10.1063/1.2130933

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 02C302 (2014)

Boron ion beam generation using a self-sputtering planar magnetrona) Aleksey Vizir,b) Aleksey Nikolaev, Efim Oks, Konstantin Savkin, Maxim Shandrikov, and Georgy Yushkov High Current Electronics Institute, Russian Academy of Sciences, Tomsk 634055, Russia

(Presented 10 September 2013; received 28 August 2013; accepted 10 September 2013; published online 15 October 2013) A boron ion source based on planar magnetron discharge with solid boron target has been developed. To obtain a sufficient conductivity of the boron target for high current discharge ignition, the target was heated to the temperature more than 350 ◦ C. To reach this temperature, thermally isolated target was heated by low-current high-voltage magnetron DC discharge. Applying a high-current pulse (100 μs range) provides a self-sputtering mode of the discharge, which generates the boron plasma. Boron ion beam with current more than 150 mA was extracted from the plasma by applying an accelerating voltage of 20 kV. The boron ion fraction in the beam reached 95%, averaged over the pulse length, and the rest ions were working gas (Kr+ ). It was shown that “keeping alive” DC discharge completely eliminates a time delay of pulsed discharge current onset, and reduces the pulsed discharge minimal working pressure. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824643] I. INTRODUCTION

Boron ion beam generation is important in semiconductor industry, for implantation into silicon as a single-charged acceptor. Also boron ion beams are used in materials science and metal surface hardening. Freeman ion sources1 (the similar type are Bernas, Calutron ion sources) are widely used for boron ion beam generation. These sources operate with gases containing boron, such as BF3 or solid boron-hydrogen compounds.2 A disadvantage of these methods is the high fraction of impurity ions (other than boron) in the beam. Vacuum arc boron ion sources3 require special filtering from cathode debris produced by the arc, and the filtering is less than perfect. Magnetron discharge in planar geometry of electrode configuration is commonly used for a thin film deposition. A self-sputtering mode of a magnetron sputtering device is attracting a wide interest due to its promising applications.4–9 In this mode, the plasma is self-supported by ions generated from the sputtering target (cathode) material. A metal ion density in the plasma can substantially exceed the ion density of a working gas, even when the gas pressure is sufficiently high for gaseous discharge operation. The high fraction of metal ions in self-sputtering mode of the magnetron discharge makes it attractive for application in metal ion sources. This mode can be reached for target materials having high sputtering coefficient, for example, copper and silver, at a high discharge current of several tens of amperes or more. The selfsputtering mode can occur only if the ion sputtering coefficient for target material ions bombarding the target is greater than unity. For other target materials, this mode is unachievable. Being a semiconductor, boron has a high electrical resistance at room temperature (∼106  cm), decreasing with temperature to about 30  cm at 400 ◦ C. Having this resistance, boron can be used as a cathode (target) in a planar a) Contributed paper, published as part of the Proceedings of the 15th Interna-

tional Conference on Ion Sources, Chiba, Japan, September 2013. b) Author to whom correspondence should be addressed. Electronic mail:

[email protected] 0034-6748/2014/85(2)/02C302/3/$30.00

magnetron discharge system. Thus, a key concern in realizing a magnetron discharge with boron target is operation with hot target and cold target holder, to protect permanent magnets from overheating. Another goal is realizing the self-sputtering mode with boron target. These goals have been successfully accomplished using argon as a working gas earlier.10 Heavier gases generally have a higher sputtering coefficient. New results for the boron ion source based on planar magnetron in self-sputtering mode with krypton as a working gas are presented in this article.

II. EXPERIMENTAL SETUP

A schematic view of the ion source and its electrical connections are presented in Fig. 1. The design of the ion source is axially-symmetric. An arc-shape magnetic field over the target surface is generated by water-cooled pair of permanent magnets. Working gas (Kr) is fed into the plasma expander, having the anode potential. Single, or two-electrode, accelerating system11 is used for ion extraction and beam formation. A mass-to charge state composition of the ion beam was measured by the time-of-flight (TOF) method.12 The discharge power supply consists of two units connected in parallel: DC and pulsed. The parameters of the power supplies are shown in Fig. 1. The vacuum system was pumped by a turbo pump with pumping speed of 900 l/s, down to a base pressure of 1 × 10−5 Torr.

III. RESULTS AND DISCUSSIONS

During first several minutes of low current DC operation, the weakly conductive boron target was heated up to 350 ◦ C. That was reached due to both ion bombardment and ohmic heating. After that, high-current discharge pulses were applied between the cathode and the anode of the discharge. In low current DC mode, the ion fraction of boron was less than

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FIG. 3. Particle fractions of different ion beam species. Pressure is 5 × 10−4 Torr. DC discharge current is 50 mA, TOF gate pulse delay is 40 μs, repetition rate is 5 pps, working gas is krypton.

FIG. 1. Experimental arrangement and power supply scheme.

5%, whereas in high current pulsed mode, the ion fraction of boron reached 99% for optimal conditions (Fig. 2). The two major peaks on the spectrum (Fig. 2) correspond to boron isotopes 10 B+ and 11 B+ correspondently. Fractions of working gas (Kr+ and Kr++ ) are almost 2 orders of magnitude lower. All results presented were obtained for diffuse mode of the discharge. Transition to cathodic arc with cathode spot form of the discharge was observed at high currents of the pulsed discharge, but it was identified as an arc both visually and by discharge voltage drop. Fig. 3 demonstrates that the boron ion particle fraction in total ion beam current increases with the discharge current from 70% at 2 A to 95% at 24 A (integrated over the pulse

FIG. 2. TOF gate pulse voltage and TOF Faraday current response to the gate pulse. Pressure is 7 × 10−4 Torr, pulsed discharge current is 15 A, repetition rate is 2 pps, DC accelerating voltage is 18.5 kV, discharge pulse duration is 100 μs, delay of the TOF gate pulse regarding to beginning of the discharge pulse is 40 μs, DC discharge current is 50 mA, working gas is krypton.

length). No stepwise transition between regular and selfsputtering mode of the discharge was observed. The massto-charge composition of the ion beam also changes in time through the pulse. The maximum boron ion fraction is detected at the beginning of the pulse (99% at first 10 μs). At 120 μs the boron fraction reduces to 88%. This happens due to electrode surface outgassing during the pulse. The influence of working pressure on the boron ion fraction in the beam has a maximum. The optimal pressure (providing highest boron ion fraction) was 10−3 Torr for krypton as an operating gas, and 2.5 × 10−4 Torr for argon.10 Before the maximum, gaseous ion fraction growth is can be simply explained by pressure rise. After the maximum, working gas ion fraction decrease requires explanation. Estimations, taking into account the energy dependence of ionization probability for working gas and for boron, and the energy distribution of ionizing electrons in the discharge plasma were performed. Lower ionization energy threshold for boron than for krypton (8.29 and 14 eV, respectively) and “cooling” of electrons in the plasma with pressure growth explain this unexpected fact. Compared to argon used as a working gas,10 for krypton, the highest fraction of boron ions turns out to be approximately the same (99%). This is because krypton, having a higher sputtering coefficient, as a heavier ion, also has a lower ionization potential than argon (15.76 eV). It is well known that pulsed magnetron discharge is characterized by a certain current growth delay after applying the voltage to a discharge gap.13 “Keeping alive” DC discharge, even with minimal current (1 mA and lower) completely eliminates this delay in a full range of discharge operating parameters. Moreover, DC discharge decreases minimal working pressure for the pulsed one (Fig. 4). This reduction is important in terms of beam transport and minimizing beam losses because of ion charge exchange and scattering in the ion beam drift space. As mentioned above, the boron ion fraction during DC discharge is below 5%, and the highest boron ion fraction is reached only during high-current pulses. To increase

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on discharge stable initiation, and decrease of minimal working pressure were retained. Fig. 5 shows the current-voltage characteristics of the boron ion source. A saturation of the ion beam current was observed at voltages about 4 kV. But at higher voltages the current starts to rise again. This happens due to a secondary ion-electron emission increase from the suppressor mesh with ion energy. Thus, the actual ion current is in the range of 150– 200 mA. Measurements of an ion collector current give the same estimations of ion beam current. IV. SUMMARY

FIG. 4. Dependence of the lowest possible working pressure on the DC discharge current. The target is anticipatory heated to 400 ◦ C for each point of dependencies. Pulsed discharge current is 10 A.

time-averaged boron ion fraction, DC discharge current was reduced to 3 mA, and repetition rate of the pulses was increased to 25 pps. The power of the pulses was enough to keep the target hot, and, at the same time, the positive effect

The boron ion source has been designed, manufactured, and tested with krypton as a working gas. The plasma from which ions are extracted is generated by planar magnetron discharge with boron target. The special design of plasma generation system, as well as special features of power supply of the discharge, provide operation of the discharge in selfsputtering mode. This mode is characterized by extremely high fraction of boron ions compared to working gas ions in the extracted ion beam. Maximal boron ion fraction reached 99%, and time-averaged fraction was 95%. ACKNOWLEDGMENTS

The work was supported by Russian Foundation for Basic Research under Grant No. RFBR 13-08-00185a. 1 J.

H. Freeman, Nucl. Instrum. Methods 22, 306 (1963).

2 A. S. Perel, W. K. Loizides, and W. E. Reynolds, Rev. Sci. Instrum. 73, 877

(2002). M. Williams, C. C. Klepper, D. J. Chivers, R. C. Hazelton, and J. J. Moschella, J. Vac. Sci. Technol. B 26, 368 (2008). 4 A. Anders, J. Andersson, and A. Ehiasarian, J. Appl. Phys. 102, 113303 (2007). 5 A. Anders, J. Andersson, and A. Ehiasarian, J. Appl. Phys. 103, 039901 (2008). 6 J. Andersson and A. Anders, Appl. Phys. Lett. 92, 221503 (2008). 7 J. Andersson and A. Anders, Phys. Rev. Lett. 102, 045003 (2009). 8 A. Anders and E. M. Oks, J. Appl. Phys. 106, 023306 (2009). 9 E. M. Oks and A. Anders, J. Appl. Phys. 105, 093304 (2009). 10 V. I. Gushenets, A. Hershcovitch, T. V. Kulevoy, E. M. Oks, K. P. Savkin, A. V. Vizir, and G. Yu. Yushkov, Rev. Sci. Instrum. 81, 02B303, (2010). 11 A. V. Vizir, E. M. Oks, and I. G. Brown, IEEE Trans. Plasma Sci. 26(4), 1353–1356 (1998). 12 I. G. Brown and J. C. Kelly, J. Appl. Phys. 63(1), 254 (1988). 13 G. Yu. Yushkov and A. Anders, IEEE Trans. Plasma Sci. 38(11), 3028– 3034 (2010). 3 J.

FIG. 5. Dependence of the ion emission current on accelerating voltage. Curves (1) pressure is 10−3 Torr, (2) 8 × 10−4 Torr, (3) 5 × 10−4 Torr. Pulsed discharge current is 10 A, pulse duration is 100 μs, prepetition rate is 5 pps, suppressor voltage is −1 kV, working gas is krypton.

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Boron ion beam generation using a self-sputtering planar magnetron.

A boron ion source based on planar magnetron discharge with solid boron target has been developed. To obtain a sufficient conductivity of the boron ta...
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