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Lifetime enhancement of a multicusp ion source for lithography
MICROELECTRONIC ENGINEERING ELSEVIER
Microelectronic Engineering 41/42 (1998) 241-244
LIFETIME ENHANCEMENT
OF A MULTICUSP ION SOURCE FOR LITHOGRAPHY
Y. Lee, a~R.A. Gough, W.B. Kunkel, K.N. Leung, J. Vujic, a~M.D. Williams, D. Wutte, b~and F.L. Yang Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA The acceleration system designed by Ion Microfabrication System (IMS) for ion projection lithography (IPL) generates a divergent beam of which only a small portion is employed in the process. Computer simulation shows that by optimizing the accelerator design, the source can be operated with lower discharge power, resulting in longer lifetime and lower energy spread. RF induction discharge operation provides long ion source lifetime with energy spread comparable to that of the filament discharge multicusp source when RF modulation in the extraction voltage is eliminated through proper shielding. New antenna designs have been developed which can further extend the lifetime of the RF-driven ion source.
1.
INTRODUCTION
Multicusp ion sources have been used in man), applications, such as neutral beam injectors for fusion devices, particle accelerators, ion implantation systems, neutron tubes for oil well logging and proton therapy machines. Currently, large number of applications of this kind of source has been extended to include ion projection lithography because of its low axial energy spread and long source lifetime. In the ion projection lithography (IPL) approach developed by the Advanced Lithography Group (ALG), positive ion beams are accelerated with the help of a three-electrode extraction system, designed by IMS (Ion Microfabdcation Systems, Vienna, Austria), that will project patterns from a stencil mask onto a wafer substrate. The axial energy spread of the ions is an important factor to reduce chromatic aberration, which is a variation of focal length due to the energy of the particles being focused) It has been demonstrated that filamentdischarge multicusp ion sources can deliver beams with axial energy- spreads as low as 2 eV. 2.3
In the IMS accelerator design, a total beam current of approximately 200 ~tA is extracted from a 0.3-mm-radius aperture. In order to meet this operational requirement, high discharge power is needed which is demanding for the ion source in CW operation, and can significantly reduce the lifetime of the source. It is demonstrated that further optimization of the accelerator design will allow the ion source to be operated at lower power. Furthermore, the radio frequency (RF)-driven source has been chosen as an alternative to a filament cathode which has a limited lifetime. RF induction discharges have long operational lifetimc with axial energy spread comparable to that of filament discharges. In this article, the approaches for enhancing the lifetime of the multicusp source for IPL applications will be presented. 2.
ACCELERATOR SYSTEM DESIGN In the present IPL system designed by IMS. ion beams are formed by the use of a three-electrode extraction system. Ions are accelerated through a hole of 0.6-mm-diameter with an energy of 15 kV. The second electrode is electrically biased at -lkV for electron suppression, and the last electrode is
*
]his work was supported by the Advanced Lithography Group (ALG) under CRADA BG-94-212(00) with file Lawrence Berkeley National Laboratory and the Division of Nuclear Physics. Office of Energy Re,arch. IJ.S. Department of Energy under Contract No. DE-AC03-76SF00098.
~
Also with the Department of Nuclear Engineering, University of California at Berkeley.
b,
Also with the Institut Allgemeine Physik, Technical University, Vienna. Austria.
0167-9317/98/$19.00 © ElsevierScience B.XZ All rights reserved, PII: S0167-9317(98)00055-0
242
Y. Lee et al./Microelectronic Engineering 41/42 (1998) 241-244
connected to ground. The acceleration system is designed for an expanding beam and only a small portion (~10%) of the accelerated beam is employed in the process. The machine operates with a useful beam current of 20 rtA within a 3° cone half angle (55 nn'ad).
l JjM)l)JJljji)) J I 15 n|lll l
Figure 1 IGUNE simulation of the original design at 80 mA/cm 2. The beam has a focal point about 4mm from the first electrode.
Figure 2 IGUNE simulation of the design at 20 mA/cm 2. The beam has a focal point about 4.15 mm from the first electrode.
Figure 3 IGUNE simulation of the design at 10 mA/cm 2. The extraction hole was increased from 0.3 mm to 0.4 mm. The beam has a focal point about 4.26 mm from the first electrode. In normal operation, the ion source must provide a total current of approximately 200 p.A (80 mA/cm=). The beam has a focal point outside the
source between the first and second electrodes. At higher ion beam currents space charge (Boerch effect) can have a detrimental effect on the ion energy spread intrinsic to the ion source. Figure 1 shows the IGUNE simulation of the accelerator design. The beam divergence is approximately 180 mrad (half angle), and there is only a small clearance between the ground electrode and the edge of the beam. In order to meet the beam current requirement (-20 ~tA within a 6 ° cone), a discharge power as high as 3 kW is employed. As a result the tungsten filament cathodes can be easily worn out, resulting in a short source lifetime. Evaporation of tungsten material from the filament can also modify the shape of the small exit aperture, and therefore, the uniformity of the extracted beam current. Computer simulations using the ion trajectory' code, IGUNE, have been performed with the goal of increasing the useful beam fraction. If the required ion beam current density is reduced, it will allow the ion source to be operated at a lower discharge power. For this purpose, the divergence of the beam is decreased by increasing the distance between the first and second electrodes. Figure 2 shows the simulation of the system using a current density of only 20 mA/cm 2, a onequarter of the original design value. The total divergence is approximately 90 mrad, the focal point is approximately 4.15 mm from the first electrode, and the current within the 3° ( - 57 mrad) cone is approximately 24 gA. This adjustment does not increase the sensitivity of the design to changes in density. At 80 mA/cm :, the beam divergence is approximately 100 mrad, and the current within the cone is approximately 104 ~A. Additionally the extraction hole size can be increased from 0.3 mm to 0.4 mm to operate the source at even lower power, as shown in figure 3, where the current density is reduced to 10 mA/cm-'. The focal point in this case is at 4.26 nun from reference, the current within the cone is approximately 24 i.tA, and the beani divergence is approximately 80 mrad. This will allow the ion source to operate at low discharge power and provide ions with low energy spread. The summary of these different simulations is shown in Table 1
Y Lee et at/Microelectronic Engineering 41/42 (I 998) 241-244
Changes
hole diameter
Current density (mAlcm2)
Beam divergence (mrad)
Original
0.6
80
180
Increase diameter and distance
0.6
20
90
hl¢l'ease distance belween frist and second electrodes
0.8
t0
80
Table 1 Summary of beam trajectory simulations. 3.
RF-DRIVEN MULTICUSP SOURCE
Tungsten filament driven multicusp sources have been shown to produce low axial energy" spread ion beams. 2 However, the main disadvantage of the filament cathode is that it can be easily worn out, resulting in a short source lifetime. Evaporation of tungsten material from the filament can also modify the shape of small exit apertures, and therefore, the uniformity of the extracted beam current. On the other hand, radio-frequency (RF) induction discharge sources have a long operational lifetime, and have a clean plasma discharge. 1 Continuous rf source operation for over 200 hours has been demonstrated. Thc only concern with the RF-driven source has been its large energy spread values. Energy spreads as large as 100 eV have been reported by Zackhary 5 and Olthoff et.al. 6 In recent measurements, the energy spread of the if-driven multicusp ion source was found to be comparable to that of the filament-discharge source. 7 The ion axial energy spread was measured in a 10-cm-diameter bv 10-cm long rf-driven multicusp ion source, with 20 columns of samarium-cobalt permanent magnets surrounding the source chamber which arc arranged with alternating polarity to generate longitudinal line cusp magnetic fields. The rf supply is a 13.56 MHz generator with a maximum output power of 2.5 kW. The if-power is coupled into the plasma through a matching network which is connected to the leads of the if antenna. The antenna is made of copper tubing and eoatcd with a thin layer of porcelain which
243
enhances the efficiency and cleanliness of the discharge and prolongs the source lifetime. A magnetic filter is installed in the front part which provides a limited region of transverse magnetic field for plasma potential adjustments and species enhancements.2 The ion energy distribution of the accelerated beam is analyzed by using a retarding field energy analyzer. 7 The axial ion energy spread for the if source has been found to be greater than 100 eV when the if power supply is operated at ground potential. The energy spread is reduced to approximately 47 eV when the rf power supply is installed at the same high voltage platform as the ion source (Figure 4). This large energy spread is due to the if coupling to the high voltage which causes fluctuation in the beam energy. The axial energy spread is dramatically reduced to 3.2 eV when (i) the leads between the antenna leads or induction coil and the matching network is shielded properly, and (ii) capacitors are used to reduce the if coupling to the extraction voltage.7 The capacitors function essentially as a lowpass filter, largely eliminating the modulation of the dc acceleration voltage with if interference. The energy spread of the if-driven source is comparable to that of the filament discharge source having the same input power. The estimated useful current per unit power for the rf discharge is about the same as for the filament discharge. The if-driven source. however, provides a much longer lifetime than the filament driven source, and a cleaner discharge which preserves the uniformity of the extracted beam. 4. R F S O U R C E - L I F E T I M E I M P R O V E M E N T
RF-antennas have a longer lifetime than the filament cathodes. The induction coil can be made of different materials. For example, metals such as copper, aluminum, and titanium have been used, and the source can be operated with the antenna bare or with an insulation coating. The performance of the source is much improved when an insulation layer is applied to the external surface of the conductor. The antenna is usually made out of copper with a thin layer of porcelain enamel on the
244
Y. Lee et al. / Microelectronic Engineering 41/42 (1998) 241-244
surface. The insulation prevents the sputtering of the antenna nmterial due to ion bombardment from the plasma. The coating arrangement is also energy efficient in plasma production since it eliminates the short-circuit between the plasma and the metallic coil. It has been demonstrated that a porcelaincoated antenna-coil can be operated with various types of plasmas. Porcelain-coated antennas have been tested in hydrogen plasmas under CW operation for up to 20 hours at an rf input power of 10 kW, and tens of minutes at 20 kW without any sign of degradation. At lower power levels (-5 kW), antenna lifetime in excess of 260 hours have been reported. 4 For low duty factor pulsed operation, the lifetime of this kind of antenna can easily exceed 500 hours. With the increasing RF power requirements and different plasma conditions, even the porcelaincoated antenna can sometimes fail to perform satisfactorily. Due to the high temperature gradient, an uneven thermal expansion can occur between the porcelain-coating and the copper surface. As a result, the porcelain coating can peel off. A new antenna design using a better insulation, quartz tubing, has been investigated. This quartz antenna can provide a much longer lifetime and cleaner operation of the ion source. Similar to the porcelain-coated copper antenna, the quartz tubing is wound in the shape of a multiturn induction coil. Silver coated copper wire strands are threaded through the tubing to serve as a conductor. Connection to the braid is made outside the vacuum. The antenna can be gas or watercooled for high duty-factor or CW operation. Quartz antennas can be operated in pulsed or CW nmde. For low duty factor (<1%) operation, at a frequency of 2 MHz, this antenna can be operated at very high pulse power (70 kW) and by far outlast the porcelain-coated antenna. In CW operation, at a frequency of 13.56 MHz, the quartz antenna was tested for different gases, for example hydrogen, argon and reactive gases such as oxygen. Under conditions where porcelain-coated antennas are not able to withstand even twenty hours of operation, quartz antennas can be operated for over
85 hours without any sign of physical damage. 8 Porcelain antennas' failure can be readily seen on the surface of the antenna where parts of the coating is missing and black pits are observed. The performance comparison of both antenna types in the pulsed mode shows no discrepancies in terms of minimum starting pressure and peak extractable current density. However in the CW mode, the performance of the quartz antenna surpassed that of the porcelain coated antenna. 8 With the new quartz antenna together with an optimized accelerator design, the RF driven multicusp source should be operated at lower discharge power, resulting in long lifetime and low energy spread. REFERENCE [1] J. Orloff, Sci. Am. Oct. 96 (1991) 12] Y. Lee, L.T. Perkins, R.A. Gough, M. Hoffmann, W.B. Kunkel, K.N. Leung, M. Sarstedt, J. Vujic, M. Weber, and M.D. Williams, Nucl. Instrum Methods Phys. Res. A 374, 1 (1996) [3] Y. Lee, R A . Gough, W.B. Kunkel, K.N. Leung, L.T. Perkins, D.S. Pickard, L. Sun, J. Vujic, M.D. Williams, and D. Wutte, Nucl. Instrum. Methods Phys. Res. B [4] S.T. Melnychuk, T.W. Debiak, and J. Sredniakwski, Proceedings of the 6 th International Conference on Ion Sources, Whistler, B.C., 10-16 Sept. Canada, 1995. [51 S. G. Zakhary, Rev. Sci. Instrum. 66, 5419 (1995). [6] J.K. Olthoff, R.J. Van Brant, and S.B. Radovanov, Rev. Sci. Instrum. 67, 473 (1995). [7] Y. Lee, R.A. Gough, W.B. Kunkel, K.N. Leung, L.T. Perkins, D.S. Pickard, L. Sun, J. Vujic, M.D. Williams, and D. Wutte, Rev. Sci. Instrum. 68 (3), March 1997, 1398-1402 [8] Y. Lee, R.A. Gough, K.N. Leung, L.T. Perkins, D.S. Pickard, J. Vujic, L.K. Wu, and M. Olivo, Proceedings of the 7th International Conference on Ion Sources, Taormina, Italy, Sept. 7-13, 1997.
Microelectronic Engineering 41/42 (1998) 241-244
LIFETIME ENHANCEMENT
OF A MULTICUSP ION SOURCE FOR LITHOGRAPHY
Y. Lee, a~R.A. Gough, W.B. Kunkel, K.N. Leung, J. Vujic, a~M.D. Williams, D. Wutte, b~and F.L. Yang Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA The acceleration system designed by Ion Microfabrication System (IMS) for ion projection lithography (IPL) generates a divergent beam of which only a small portion is employed in the process. Computer simulation shows that by optimizing the accelerator design, the source can be operated with lower discharge power, resulting in longer lifetime and lower energy spread. RF induction discharge operation provides long ion source lifetime with energy spread comparable to that of the filament discharge multicusp source when RF modulation in the extraction voltage is eliminated through proper shielding. New antenna designs have been developed which can further extend the lifetime of the RF-driven ion source.
1.
INTRODUCTION
Multicusp ion sources have been used in man), applications, such as neutral beam injectors for fusion devices, particle accelerators, ion implantation systems, neutron tubes for oil well logging and proton therapy machines. Currently, large number of applications of this kind of source has been extended to include ion projection lithography because of its low axial energy spread and long source lifetime. In the ion projection lithography (IPL) approach developed by the Advanced Lithography Group (ALG), positive ion beams are accelerated with the help of a three-electrode extraction system, designed by IMS (Ion Microfabdcation Systems, Vienna, Austria), that will project patterns from a stencil mask onto a wafer substrate. The axial energy spread of the ions is an important factor to reduce chromatic aberration, which is a variation of focal length due to the energy of the particles being focused) It has been demonstrated that filamentdischarge multicusp ion sources can deliver beams with axial energy- spreads as low as 2 eV. 2.3
In the IMS accelerator design, a total beam current of approximately 200 ~tA is extracted from a 0.3-mm-radius aperture. In order to meet this operational requirement, high discharge power is needed which is demanding for the ion source in CW operation, and can significantly reduce the lifetime of the source. It is demonstrated that further optimization of the accelerator design will allow the ion source to be operated at lower power. Furthermore, the radio frequency (RF)-driven source has been chosen as an alternative to a filament cathode which has a limited lifetime. RF induction discharges have long operational lifetimc with axial energy spread comparable to that of filament discharges. In this article, the approaches for enhancing the lifetime of the multicusp source for IPL applications will be presented. 2.
ACCELERATOR SYSTEM DESIGN In the present IPL system designed by IMS. ion beams are formed by the use of a three-electrode extraction system. Ions are accelerated through a hole of 0.6-mm-diameter with an energy of 15 kV. The second electrode is electrically biased at -lkV for electron suppression, and the last electrode is
*
]his work was supported by the Advanced Lithography Group (ALG) under CRADA BG-94-212(00) with file Lawrence Berkeley National Laboratory and the Division of Nuclear Physics. Office of Energy Re,arch. IJ.S. Department of Energy under Contract No. DE-AC03-76SF00098.
~
Also with the Department of Nuclear Engineering, University of California at Berkeley.
b,
Also with the Institut Allgemeine Physik, Technical University, Vienna. Austria.
0167-9317/98/$19.00 © ElsevierScience B.XZ All rights reserved, PII: S0167-9317(98)00055-0
242
Y. Lee et al./Microelectronic Engineering 41/42 (1998) 241-244
connected to ground. The acceleration system is designed for an expanding beam and only a small portion (~10%) of the accelerated beam is employed in the process. The machine operates with a useful beam current of 20 rtA within a 3° cone half angle (55 nn'ad).
l JjM)l)JJljji)) J I 15 n|lll l
Figure 1 IGUNE simulation of the original design at 80 mA/cm 2. The beam has a focal point about 4mm from the first electrode.
Figure 2 IGUNE simulation of the design at 20 mA/cm 2. The beam has a focal point about 4.15 mm from the first electrode.
Figure 3 IGUNE simulation of the design at 10 mA/cm 2. The extraction hole was increased from 0.3 mm to 0.4 mm. The beam has a focal point about 4.26 mm from the first electrode. In normal operation, the ion source must provide a total current of approximately 200 p.A (80 mA/cm=). The beam has a focal point outside the
source between the first and second electrodes. At higher ion beam currents space charge (Boerch effect) can have a detrimental effect on the ion energy spread intrinsic to the ion source. Figure 1 shows the IGUNE simulation of the accelerator design. The beam divergence is approximately 180 mrad (half angle), and there is only a small clearance between the ground electrode and the edge of the beam. In order to meet the beam current requirement (-20 ~tA within a 6 ° cone), a discharge power as high as 3 kW is employed. As a result the tungsten filament cathodes can be easily worn out, resulting in a short source lifetime. Evaporation of tungsten material from the filament can also modify the shape of the small exit aperture, and therefore, the uniformity of the extracted beam current. Computer simulations using the ion trajectory' code, IGUNE, have been performed with the goal of increasing the useful beam fraction. If the required ion beam current density is reduced, it will allow the ion source to be operated at a lower discharge power. For this purpose, the divergence of the beam is decreased by increasing the distance between the first and second electrodes. Figure 2 shows the simulation of the system using a current density of only 20 mA/cm 2, a onequarter of the original design value. The total divergence is approximately 90 mrad, the focal point is approximately 4.15 mm from the first electrode, and the current within the 3° ( - 57 mrad) cone is approximately 24 gA. This adjustment does not increase the sensitivity of the design to changes in density. At 80 mA/cm :, the beam divergence is approximately 100 mrad, and the current within the cone is approximately 104 ~A. Additionally the extraction hole size can be increased from 0.3 mm to 0.4 mm to operate the source at even lower power, as shown in figure 3, where the current density is reduced to 10 mA/cm-'. The focal point in this case is at 4.26 nun from reference, the current within the cone is approximately 24 i.tA, and the beani divergence is approximately 80 mrad. This will allow the ion source to operate at low discharge power and provide ions with low energy spread. The summary of these different simulations is shown in Table 1
Y Lee et at/Microelectronic Engineering 41/42 (I 998) 241-244
Changes
hole diameter
Current density (mAlcm2)
Beam divergence (mrad)
Original
0.6
80
180
Increase diameter and distance
0.6
20
90
hl¢l'ease distance belween frist and second electrodes
0.8
t0
80
Table 1 Summary of beam trajectory simulations. 3.
RF-DRIVEN MULTICUSP SOURCE
Tungsten filament driven multicusp sources have been shown to produce low axial energy" spread ion beams. 2 However, the main disadvantage of the filament cathode is that it can be easily worn out, resulting in a short source lifetime. Evaporation of tungsten material from the filament can also modify the shape of small exit apertures, and therefore, the uniformity of the extracted beam current. On the other hand, radio-frequency (RF) induction discharge sources have a long operational lifetime, and have a clean plasma discharge. 1 Continuous rf source operation for over 200 hours has been demonstrated. Thc only concern with the RF-driven source has been its large energy spread values. Energy spreads as large as 100 eV have been reported by Zackhary 5 and Olthoff et.al. 6 In recent measurements, the energy spread of the if-driven multicusp ion source was found to be comparable to that of the filament-discharge source. 7 The ion axial energy spread was measured in a 10-cm-diameter bv 10-cm long rf-driven multicusp ion source, with 20 columns of samarium-cobalt permanent magnets surrounding the source chamber which arc arranged with alternating polarity to generate longitudinal line cusp magnetic fields. The rf supply is a 13.56 MHz generator with a maximum output power of 2.5 kW. The if-power is coupled into the plasma through a matching network which is connected to the leads of the if antenna. The antenna is made of copper tubing and eoatcd with a thin layer of porcelain which
243
enhances the efficiency and cleanliness of the discharge and prolongs the source lifetime. A magnetic filter is installed in the front part which provides a limited region of transverse magnetic field for plasma potential adjustments and species enhancements.2 The ion energy distribution of the accelerated beam is analyzed by using a retarding field energy analyzer. 7 The axial ion energy spread for the if source has been found to be greater than 100 eV when the if power supply is operated at ground potential. The energy spread is reduced to approximately 47 eV when the rf power supply is installed at the same high voltage platform as the ion source (Figure 4). This large energy spread is due to the if coupling to the high voltage which causes fluctuation in the beam energy. The axial energy spread is dramatically reduced to 3.2 eV when (i) the leads between the antenna leads or induction coil and the matching network is shielded properly, and (ii) capacitors are used to reduce the if coupling to the extraction voltage.7 The capacitors function essentially as a lowpass filter, largely eliminating the modulation of the dc acceleration voltage with if interference. The energy spread of the if-driven source is comparable to that of the filament discharge source having the same input power. The estimated useful current per unit power for the rf discharge is about the same as for the filament discharge. The if-driven source. however, provides a much longer lifetime than the filament driven source, and a cleaner discharge which preserves the uniformity of the extracted beam. 4. R F S O U R C E - L I F E T I M E I M P R O V E M E N T
RF-antennas have a longer lifetime than the filament cathodes. The induction coil can be made of different materials. For example, metals such as copper, aluminum, and titanium have been used, and the source can be operated with the antenna bare or with an insulation coating. The performance of the source is much improved when an insulation layer is applied to the external surface of the conductor. The antenna is usually made out of copper with a thin layer of porcelain enamel on the
244
Y. Lee et al. / Microelectronic Engineering 41/42 (1998) 241-244
surface. The insulation prevents the sputtering of the antenna nmterial due to ion bombardment from the plasma. The coating arrangement is also energy efficient in plasma production since it eliminates the short-circuit between the plasma and the metallic coil. It has been demonstrated that a porcelaincoated antenna-coil can be operated with various types of plasmas. Porcelain-coated antennas have been tested in hydrogen plasmas under CW operation for up to 20 hours at an rf input power of 10 kW, and tens of minutes at 20 kW without any sign of degradation. At lower power levels (-5 kW), antenna lifetime in excess of 260 hours have been reported. 4 For low duty factor pulsed operation, the lifetime of this kind of antenna can easily exceed 500 hours. With the increasing RF power requirements and different plasma conditions, even the porcelaincoated antenna can sometimes fail to perform satisfactorily. Due to the high temperature gradient, an uneven thermal expansion can occur between the porcelain-coating and the copper surface. As a result, the porcelain coating can peel off. A new antenna design using a better insulation, quartz tubing, has been investigated. This quartz antenna can provide a much longer lifetime and cleaner operation of the ion source. Similar to the porcelain-coated copper antenna, the quartz tubing is wound in the shape of a multiturn induction coil. Silver coated copper wire strands are threaded through the tubing to serve as a conductor. Connection to the braid is made outside the vacuum. The antenna can be gas or watercooled for high duty-factor or CW operation. Quartz antennas can be operated in pulsed or CW nmde. For low duty factor (<1%) operation, at a frequency of 2 MHz, this antenna can be operated at very high pulse power (70 kW) and by far outlast the porcelain-coated antenna. In CW operation, at a frequency of 13.56 MHz, the quartz antenna was tested for different gases, for example hydrogen, argon and reactive gases such as oxygen. Under conditions where porcelain-coated antennas are not able to withstand even twenty hours of operation, quartz antennas can be operated for over
85 hours without any sign of physical damage. 8 Porcelain antennas' failure can be readily seen on the surface of the antenna where parts of the coating is missing and black pits are observed. The performance comparison of both antenna types in the pulsed mode shows no discrepancies in terms of minimum starting pressure and peak extractable current density. However in the CW mode, the performance of the quartz antenna surpassed that of the porcelain coated antenna. 8 With the new quartz antenna together with an optimized accelerator design, the RF driven multicusp source should be operated at lower discharge power, resulting in long lifetime and low energy spread. REFERENCE [1] J. Orloff, Sci. Am. Oct. 96 (1991) 12] Y. Lee, L.T. Perkins, R.A. Gough, M. Hoffmann, W.B. Kunkel, K.N. Leung, M. Sarstedt, J. Vujic, M. Weber, and M.D. Williams, Nucl. Instrum Methods Phys. Res. A 374, 1 (1996) [3] Y. Lee, R A . Gough, W.B. Kunkel, K.N. Leung, L.T. Perkins, D.S. Pickard, L. Sun, J. Vujic, M.D. Williams, and D. Wutte, Nucl. Instrum. Methods Phys. Res. B [4] S.T. Melnychuk, T.W. Debiak, and J. Sredniakwski, Proceedings of the 6 th International Conference on Ion Sources, Whistler, B.C., 10-16 Sept. Canada, 1995. [51 S. G. Zakhary, Rev. Sci. Instrum. 66, 5419 (1995). [6] J.K. Olthoff, R.J. Van Brant, and S.B. Radovanov, Rev. Sci. Instrum. 67, 473 (1995). [7] Y. Lee, R.A. Gough, W.B. Kunkel, K.N. Leung, L.T. Perkins, D.S. Pickard, L. Sun, J. Vujic, M.D. Williams, and D. Wutte, Rev. Sci. Instrum. 68 (3), March 1997, 1398-1402 [8] Y. Lee, R.A. Gough, K.N. Leung, L.T. Perkins, D.S. Pickard, J. Vujic, L.K. Wu, and M. Olivo, Proceedings of the 7th International Conference on Ion Sources, Taormina, Italy, Sept. 7-13, 1997.