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Simple concepts for ion source improvement
Nuclear Instruments and Methods in Physics Research B 190 (2002) 402–404 www.elsevier.com/locate/nimb
Simple concepts for ion source improvement P.A. Hausladen 1, D.C. Weisser *, N.R. Lobanov, L.K. Fifield, H.J. Wallace Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia
Abstract We report on improvements in the overall intensity of a sputter ion source that evolved originally from an NEC MCSNICS. Beam output increases benefit both AMS measurements and nuclear physics experiments using low natural abundance beams. In particular, minor changes in source geometry suggested by a combination of electrostatic calculations and simple design principles have yielded increases in extracted negative ion intensity of nearly a factor of 4. Ó 2002 Published by Elsevier Science B.V.
1. Introduction A sputter ion source operates by delivering neutral Cs to a hot surface where it is ionized, then accelerated in an electric field towards a sample that is sputtered by the energetic Csþ ions. Some of the sample material will be ejected as negative ions that are then extracted for injection into a tandem accelerator. The effects of primary Csþ beam intensity, extraction optics, source vacuum, and sample chemistry can confound diagnosing the origin of low extracted negative ion intensity. Following the success of ion source work at the LLNL CAMS facility, we took a closer look at the constraints imposed on the magnitude of the Csþ current by the applied electric field [1]. Ready estimation of the Csþ current can be made because the bulk of the electron current
*
Corresponding author. E-mail address: [email protected] (D.C. Weisser). 1 Present address: Physics Division, ORNL, Oak Ridge, TN, USA.
leaving the cathode is collected and measured on the extractor. The electron contribution can then be subtracted from the cathode current to yield a good estimate of the Csþ current. The negative ion current can essentially be neglected for a sample material such as Al metal. Measurements using the original geometry of our source indicated that the primary difference between it and higher intensity sources was the magnitude of the sputtering Csþ current. Furthermore, the maximum Csþ current was not limited by the flow of neutral Cs to the ionizer, but fundamentally constrained by Child’s law [2], a quantitative statement of the fact that an ion current is limited at such a value that the electric field from the ions cancels the applied electric field extracting those ions. This condition of space charge limited current was observed by increasing the supply of neutral Cs to the ionizer until the V3=2 dependence of the Csþ current on the cathode voltage occurred. Our approach was therefore to investigate the electrostatics of the original source to see whether modified electric fields, which could support larger Cs currents while maintaining efficient focus onto
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 3 0 7 - 6
P.A. Hausladen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 402–404
the sample, were possible with minimal changes to electrode geometry and use of existing power supplies. Calculations of electric fields and ion trajectories for the present work were performed using the code SIMION-6 [3].
2. The original ANU geometry The major components of our ion source at the outset of the present work are shown schematically in Fig. 1(a). Because many of the components have been modified from the original NEC supplied ones and because the design of the NEC source has evolved since delivery of this source 6.5 years ago, performance of this source should not be taken as indicative of NEC MCSNICS sources [4]. In particular, beam transport subsequent to the ion source effectively limits the cathode voltage to 5 kV. Nonetheless, the understanding of physical processes applies to sputter sources generally and informs their most productive use. The region of interest for calculations of potentials and ion trajectories for the original geometry is shown in Fig. 1(b). Notice that while good Cs focus is achieved, the majority of ions initially have directions inconsistent with striking the sample (Fig. 1(b)). The observed focus is made possible by the nearly spherical bulges in the equipotentials between the Cs focus electrode and the ionizer, a condition that can only exist when the field modification due to the Cs focus electrode reduces the electric field near the surface of the ionizer. For this reason, the Csþ current cannot be both intense and well focused. In this incarnation, the source consistently produced 15–20 lA 12 C .
403
3. The improved geometry In the improved geometry, the ionizer surface was changed from conical to spherical. Because the initial Csþ directions for this geometry are then consistent with converging to a sharp focus, the need to have a large change in electric field across the Cs focus electrode is eliminated, and a higher field on the ionizer surface is achieved. The shape of the Cs focus electrode is then chosen to follow a natural equipotential that would exist in its absence, thereby leaving the focus unchanged for the ‘‘natural’’ voltage setting, and moving the focus closer to the ionizer for voltages nearer to that of the ionizer. The selected equipotential was chosen to be consistent with the full-scale voltage of the existing cathode and Cs focus power supplies. Fig. 2(b) shows the calculated trajectories and equipotentials of the new geometry, and Fig. 2(a), by comparison to Fig. 1(a), shows that the changes are possible with minimal reshaping of the ionizer and Cs focus electrode. A photograph of the new ionizer assembly can be seen in Fig. 2(c). The improved geometry was first evaluated to determine the magnitude of the Csþ current it could support. Fig. 3 shows the Csþ current from the spherical ionizer as a function of cathode voltage for three Cs reservoir temperatures, effectively different Cs neutral supply rates. Also shown is Csþ current vs cathode voltage for the conical ionizer. For each cathode voltage, the Cs focus voltage was chosen to be the natural value from the equipotential calculations. The approach to the V3=2 dependence of space charge limited current over the range of potentials measured is evident. The space charge limited Csþ current is an
Fig. 1. The original ANU geometry: (a) schematic diagram of the major components, (b) equipotentials and Csþ trajectories, and (c) photograph of the ionizer assembly.
404
P.A. Hausladen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 402–404
Fig. 2. The improved geometry: (a) schematic diagram of the major components, (b) equipotentials and Csþ trajectories and (c) photograph of new ionizer assembly mounted in the ion source.
output is 60–70 lA, consistent with the usable increase in Csþ current.
4. Future efforts
Fig. 3. Estimated Csþ current vs cathode voltage for three reservoir temperatures and fixed ionizer power of 130 W.
order of magnitude greater than for the original geometry. Unfortunately, we cannot capture all of this improvement in the present incarnation. The reason is that the size of the Cs beam spot increases rapidly with increasing Csþ current as a result of the mutual repulsion among Csþ ions. In practice, the actual space charge limited Csþ current; at the smaller Cs focus to ionizer potential difference necessary to maximize the Csþ current striking the sample, is about a factor of 4 higher than for the previous geometry. Correspondingly, the 12 C
While the changes implemented in the present work were successful, directions for further improvement are clear. Calculations of ion trajectories that include the effects of space charge are anticipated to suggest geometries capable of adequate focusing of even larger Cs currents. Perhaps more importantly, improvements in the ion source and subsequent beam transport to make them compatible with cathode voltages over the present 5 kV will yield gains in intensity in proportion to V3=2 .
References [1] T.A. Brown, M.L. Roberts, J.R. Southon, Nucl. Instr. and Meth. B 172 (2000) 344. [2] C.D. Child, Phys. Rev. 32 (1) (1911) 492. [3] D.A. Dahl, SIMION 3D Version 6.0, 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, Georgia, May 21–26, 1995, p. 717. [4] G.A. Norton, in: J. Benson, L. Rowton, J. Tesmer, R. Darling (Eds.), Proceedings of the 25th Symposium of North Eastern Accelerator Personnel, Santa Fe, New Mexico, USA, 1991, World Scientific, Singapore, 1992, p. 295.
Simple concepts for ion source improvement P.A. Hausladen 1, D.C. Weisser *, N.R. Lobanov, L.K. Fifield, H.J. Wallace Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia
Abstract We report on improvements in the overall intensity of a sputter ion source that evolved originally from an NEC MCSNICS. Beam output increases benefit both AMS measurements and nuclear physics experiments using low natural abundance beams. In particular, minor changes in source geometry suggested by a combination of electrostatic calculations and simple design principles have yielded increases in extracted negative ion intensity of nearly a factor of 4. Ó 2002 Published by Elsevier Science B.V.
1. Introduction A sputter ion source operates by delivering neutral Cs to a hot surface where it is ionized, then accelerated in an electric field towards a sample that is sputtered by the energetic Csþ ions. Some of the sample material will be ejected as negative ions that are then extracted for injection into a tandem accelerator. The effects of primary Csþ beam intensity, extraction optics, source vacuum, and sample chemistry can confound diagnosing the origin of low extracted negative ion intensity. Following the success of ion source work at the LLNL CAMS facility, we took a closer look at the constraints imposed on the magnitude of the Csþ current by the applied electric field [1]. Ready estimation of the Csþ current can be made because the bulk of the electron current
*
Corresponding author. E-mail address: [email protected] (D.C. Weisser). 1 Present address: Physics Division, ORNL, Oak Ridge, TN, USA.
leaving the cathode is collected and measured on the extractor. The electron contribution can then be subtracted from the cathode current to yield a good estimate of the Csþ current. The negative ion current can essentially be neglected for a sample material such as Al metal. Measurements using the original geometry of our source indicated that the primary difference between it and higher intensity sources was the magnitude of the sputtering Csþ current. Furthermore, the maximum Csþ current was not limited by the flow of neutral Cs to the ionizer, but fundamentally constrained by Child’s law [2], a quantitative statement of the fact that an ion current is limited at such a value that the electric field from the ions cancels the applied electric field extracting those ions. This condition of space charge limited current was observed by increasing the supply of neutral Cs to the ionizer until the V3=2 dependence of the Csþ current on the cathode voltage occurred. Our approach was therefore to investigate the electrostatics of the original source to see whether modified electric fields, which could support larger Cs currents while maintaining efficient focus onto
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 3 0 7 - 6
P.A. Hausladen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 402–404
the sample, were possible with minimal changes to electrode geometry and use of existing power supplies. Calculations of electric fields and ion trajectories for the present work were performed using the code SIMION-6 [3].
2. The original ANU geometry The major components of our ion source at the outset of the present work are shown schematically in Fig. 1(a). Because many of the components have been modified from the original NEC supplied ones and because the design of the NEC source has evolved since delivery of this source 6.5 years ago, performance of this source should not be taken as indicative of NEC MCSNICS sources [4]. In particular, beam transport subsequent to the ion source effectively limits the cathode voltage to 5 kV. Nonetheless, the understanding of physical processes applies to sputter sources generally and informs their most productive use. The region of interest for calculations of potentials and ion trajectories for the original geometry is shown in Fig. 1(b). Notice that while good Cs focus is achieved, the majority of ions initially have directions inconsistent with striking the sample (Fig. 1(b)). The observed focus is made possible by the nearly spherical bulges in the equipotentials between the Cs focus electrode and the ionizer, a condition that can only exist when the field modification due to the Cs focus electrode reduces the electric field near the surface of the ionizer. For this reason, the Csþ current cannot be both intense and well focused. In this incarnation, the source consistently produced 15–20 lA 12 C .
403
3. The improved geometry In the improved geometry, the ionizer surface was changed from conical to spherical. Because the initial Csþ directions for this geometry are then consistent with converging to a sharp focus, the need to have a large change in electric field across the Cs focus electrode is eliminated, and a higher field on the ionizer surface is achieved. The shape of the Cs focus electrode is then chosen to follow a natural equipotential that would exist in its absence, thereby leaving the focus unchanged for the ‘‘natural’’ voltage setting, and moving the focus closer to the ionizer for voltages nearer to that of the ionizer. The selected equipotential was chosen to be consistent with the full-scale voltage of the existing cathode and Cs focus power supplies. Fig. 2(b) shows the calculated trajectories and equipotentials of the new geometry, and Fig. 2(a), by comparison to Fig. 1(a), shows that the changes are possible with minimal reshaping of the ionizer and Cs focus electrode. A photograph of the new ionizer assembly can be seen in Fig. 2(c). The improved geometry was first evaluated to determine the magnitude of the Csþ current it could support. Fig. 3 shows the Csþ current from the spherical ionizer as a function of cathode voltage for three Cs reservoir temperatures, effectively different Cs neutral supply rates. Also shown is Csþ current vs cathode voltage for the conical ionizer. For each cathode voltage, the Cs focus voltage was chosen to be the natural value from the equipotential calculations. The approach to the V3=2 dependence of space charge limited current over the range of potentials measured is evident. The space charge limited Csþ current is an
Fig. 1. The original ANU geometry: (a) schematic diagram of the major components, (b) equipotentials and Csþ trajectories, and (c) photograph of the ionizer assembly.
404
P.A. Hausladen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 402–404
Fig. 2. The improved geometry: (a) schematic diagram of the major components, (b) equipotentials and Csþ trajectories and (c) photograph of new ionizer assembly mounted in the ion source.
output is 60–70 lA, consistent with the usable increase in Csþ current.
4. Future efforts
Fig. 3. Estimated Csþ current vs cathode voltage for three reservoir temperatures and fixed ionizer power of 130 W.
order of magnitude greater than for the original geometry. Unfortunately, we cannot capture all of this improvement in the present incarnation. The reason is that the size of the Cs beam spot increases rapidly with increasing Csþ current as a result of the mutual repulsion among Csþ ions. In practice, the actual space charge limited Csþ current; at the smaller Cs focus to ionizer potential difference necessary to maximize the Csþ current striking the sample, is about a factor of 4 higher than for the previous geometry. Correspondingly, the 12 C
While the changes implemented in the present work were successful, directions for further improvement are clear. Calculations of ion trajectories that include the effects of space charge are anticipated to suggest geometries capable of adequate focusing of even larger Cs currents. Perhaps more importantly, improvements in the ion source and subsequent beam transport to make them compatible with cathode voltages over the present 5 kV will yield gains in intensity in proportion to V3=2 .
References [1] T.A. Brown, M.L. Roberts, J.R. Southon, Nucl. Instr. and Meth. B 172 (2000) 344. [2] C.D. Child, Phys. Rev. 32 (1) (1911) 492. [3] D.A. Dahl, SIMION 3D Version 6.0, 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, Georgia, May 21–26, 1995, p. 717. [4] G.A. Norton, in: J. Benson, L. Rowton, J. Tesmer, R. Darling (Eds.), Proceedings of the 25th Symposium of North Eastern Accelerator Personnel, Santa Fe, New Mexico, USA, 1991, World Scientific, Singapore, 1992, p. 295.