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Improvement of the emittance of a Middleton type sputter source
NUCLEAR
INSTRUMENTS
IMPROVEMENT
AND
METHODS
[3 8 ([976)
OF THE EMITTANCE
4o7-4IO;
©
NORTH-HOLLAND
OF A MIDDLETON
PUBLISHING
TYPE SPUTTER
CO.
SOURCE
G. IHMELS, E. JAESCHKE and R. REPNOW
Max-Planck-lnstitutfiir Kernphysik, Heidelberg, 14I.Germany Received 16 August 1976 The emittance of a Middleton type sputter source was investigated. Considerable improvement was achieved by a modification of the target and extraction geometry. With this source particle transmission rates of about 50% for various heavy ion species were obtained at the Heidelberg MP tandem accelerator. 1. Introduction
Since the d e v e l o p m e n t o f the M i d d l e t o n type sputter source 1) for negative heavy ions this source has been used in n u m e r o u s t a n d e m laboratories. F i r s t emittance m e a s u r e m e n t s 3' 4) showed t h a t 70% o f the total beam has an emittance smaller t h a n the acceptance o f an EN t a n d e m Van de G r a a f f accelerator. However, at M P t a n d e m accelerators particle transmission rates o f a b o u t 15-20% only were r e p o r t e d S), implying a m u c h higher emittance. Systematic emittance measurements have been performed2), therefore, in o r d e r to investigate this descrepancy. F o r all m e a s u r e m e n t s the c o m m e r c i a l sputter source M a r k I1 m a n u f a c t u r e d by Extrion Corp. was used. A detailed description o f this source can be found in ref. 1. 2. Apparatus and emittance measurements
I n o r d e r to measure the emittance, the mass separated negative ion b e a m was split by h o r i z o n t a l slits. Beyond a drift space the intensity distribution was registered with a F a r a d a y cup with a slotted a p e r t u r e moving in vertical direction. F r o m p e a k width a n d p e a k distance o f the intensity d i s t r i b u t i o n the a r e a A
A = fdr
is evaluated. The emittance E is then o b t a i n e d by multiplication o f A with (W+)/~ ( W = particle energy):
E = (W)÷/Tr f dr
dr'.
E m i t t a n c e m e a s u r e m e n t s were p e r f o r m e d in the horizontal direction only because o f r o t a t i o n a l s y m m e t r y o f the extraction geometry. A detailed description o f the emittance m e a s u r e m e n t m e t h o d a n d a p p a r a t u s can be f o u n d in refs. 6 a n d 7. First emittance m e a s u r e m e n t s o f the original version o f the source were m a d e with a A u - - b e a m . F r o m the a s y m m e t r y in the intensity distributions (fig. 1) it was evident that two b e a m c o m p o n e n t s o f different origin were emitted by the source. Besides the b e a m c o m i n g from the target surface, a second b e a m comp o n e n t was emitted from the s o - c a l l e d b e a m f o r m i n g electrode (L in fig. 2a). This can be u n d e r s t o o d as follows: neutral m a t e r i a l sputtered from the target A
K
C
D L H
M
A
[3
C
D
E J
F
G
H
i
.
dr'
1 I J (arbitrary
unit)
a)
i, -16
-12
-8
-4
0
4
8
12
16
x (rnrn)
Fig. 1. Intensity profile of a Au--beam at the end of a drift space, obtained with the original target and extraction geometry of the source.
Fig. 2. Target and extraction geometry of the source: (a) The original geometry; (b) target and extraction geometry for improved emittance. (A) primary beam, (B) channel target, (C) target wheel, (D) front plate, (E) modified gas feed supply, (F) modified and insulated extraction electrode, (G) isolator, (H) secondary negative ion beam, (J) primary beam spot, (K) cone target, (L) beam forming electrode, (M) original extraction electrode, (V~) extraction potential, (VR) reflector potential.
408
G. IHMELS et al.
TABLE ! Measured emittances for various source conditions: (a) as a function of ion species (UE = 10 kV, lcs =0.6 mA); (b) as a function of Cs-current and extraction voltage (C--beam). (a) Ion species mass number E [mm"mrad" (MeV)~]
C12 6.3
O16 6.9
C~" 24 6.6
Cu63/65 6.7
Ag107/109 6.9
(b) Ics (mA) UE (kV) E [mm"mrad" (MeV)~]
0.6 10 7.7
0.6 15 8.4
0.6 20 9.1
0.5 10 8.0
0.6 10 7.7
surface is deposited on this electrode and again sputtered by that part of the primary beam which escapes through the cone aperture and is then reflected towards this electrode. A fraction of this material is extracted as a negative ion beam. This mechanism can explain experimental results obtained by Hyder a' 4) who reported a steep increase of emittance with larger beam fraction including more than 50% of the total beam. This steep increase is caused by the second beam component born on the electrode at a rather high distance from the beam axis. Because of space charge expansion of the primary beam with high Cs-current, fewer ions are escaping through the target aperture. Therefore with high Cs-current the intensity of the secondary ion beam sputtered from the electrode is relatively small and the steep increase of emittance begins only at 70% of the total negative ion beam. In order to avoid the extraction of negative ions from the beam forming electrode as well as from the flat back surface the beam forming electrode was eliminated and the negative ion extraction electrode was replaced by a conically shaped electrode with a diameter of only 4.5 mm (see fig. 2b). With this extraction geometry the asymmetry in the lines of fig. ! disappeared, and it was now possible to measure the emittance of the secondary negative ion beam sputtered from the inner target surface only. After these changes the dependence of the emittance on the three following parameters was investigated: a) ion species, b) extraction voltage, c) primary beam current. The results of these measurements are summarized in table 1. The table hows that the emittance of the negative ion beam is widely independent of ion mass, and that
Au197 7.4 0.75 10 7.4
increasing the primary beam current and lowering the extraction voltage tend to lower the emittance. Space charge effects from the primary Cs beam can account for all three observations. The space charge of the primary beam in the target canal is only partially compensated by the secondary electrons and negative ions and produces a radial potential accelerating the negative ions towards the beam axis. This process enhances the radial momentum of the sputtered ions. Because space charge forces are not Hamiltonian, the emittance of the negative ion beam becomes worse. An increase of the extraction voltage reduces the space charge expansion of the primary beam. Thus higher space charge density near the beam axis creates a stronger radial potential deteriorating the emittance of the secondary beam. A more intense primary beam at constant extraction voltage will enhance the absolute sputtering yield of negative ions by the more efficient surface activation. Therefore the positive space charged density is better compensated by the sputtered negative ions and electrons and the radial potential is somewhat less pronounced resulting in a lower emittance with high Cs-current. 3. Improvement of source emittance
The results of the emittance measurements suggested two possiblities for emittance improvement: 1) the distance from the negative ion creation area to the beam axis should be minimized; 2) the space charge influence of the primary beam should be avoided. Maintaining high negative ion currents with both requirements can be fulfilled by varying the target and extraction geometry in the following way (fig. 2c): The primary beam - typically UE -----25 kV, Ics= I mA is reduced by the hollow target cylinder of 3.5 m m diameter to 0.3 mA. Only the outer ring of the clipped
409
EMITTANCE OF A MIDDLETON S P U T T E R SOURCE TABLE 2
r (mrad)
Negative ion currents measured with the modified target and extraction geometry (fig. 2b).
Ion species
C-
C~-
O-
AIO-
Cu-
Ni-
Current (/tA)
26
22
14
4
10
2
~ -16 -12 beam is striking the inner target surface; the beam fraction escaping through the cylinder is reflected by the extraction electrode to the front surface of the target. With a slightly positive potential at the extraction electrode the reflected primary beam can be focussed towards a small ring on the front surface. The origin of the sputtered ions is now located at a distance less than 2.5 mm from the axis and in a high extraction field of about 4 kV/mm ensuring a high extraction efficiency and avoiding space charge effects of the primary beam. With this target and extraction geometry high currents of negative ion beams were obtained as listed in table 2. For the last ten months the modified source has been in regular use and is now the standard source for all types of ions with A > 7 , even for production of negative ion beams from enriched gaseous isotopes. To minimize the consumption of these gases a modified conically shaped gas feed supply (fig. 2c) was mounted at the end of the target channel in order to spray the gas directly onto the primary beam spot. During an 180 beam time of 36h 66 standard cm 3 1802 were consumed, the negative ionization efficiency (negative ion/gaseous particle) was estimated to be 10 - 3 . A negative ionization efficiency of 1% was measured during a 32S beam time using powderous PbS as target material pressed into a copper insert (fig. 4). This high efficiency will allow the preparation of special targets of enriched stable isotopes available as solid compounds in order to produce beams of expensive materials economically. Eminance measurements (table 3) obtained under various source conditions show that the emittance has been improved by a factor of two.
-8
2520 15 10 5
-4
o5~'~ 4 I ~ '12 116 rir~m) -10 -20 -25
Fig. 3. Phase space diagrams of a C - - b e a m using (a) original conically shaped target, (b) channel target.
a
b
t/
18
__~ J_
LIo
(mm)
'
Fig. 4. Size of a channel target: (a) copper insert, (b) powderous target material.
These results are seen to be nearly independent of extraction voltage and Cs-current, and the emittance is therefore independent of space charge influence. Phase space diagrams of C-beams emitted from the original cone and a channel target under identical source conditions are compared in fig. 3. From these phase space diagrams the emittance is calculated to be 6.3 and 3.1 m m . mrad. (MeV) ~ respectively. 4. Transmission tests at the M P tandem accelerator
The modified sputter source was tested at the MP tandem with a terminal voltage of 11 MV and a
TABLE 3
Emittances measured with a C - beam using the modified target and extraction geometry (fig. 2c).
lcs (mA) UE (kV) E [mm" mrad" (MeV)~]
0.3 15 3.1
0.5 15 3.0
0.65 15 3.0
a )
0.5 10 3.1
0.5 15 3.0
0.5 20 3.3
410
G. IHMELS et al.
TABLE 4 Results of transmission tests at the MP tandem. Ion species
C O F AIO S Ni
Target material
C Ni+Oz Ni q- S F 6 AI+Oz Pbs Ni
1njected current (hA)
Analysed current (#A)
Charge state
0.76 1.7 1.8 0.27 0.2 0.l
1.07 2.35 2.7 0.03 0.23 0.025
4 6 6 7 6 9
p r e a c c e l e r a t i o n v o l t a g e o f 200 kV. T a b l e 4 s h o w s currents and t r a n s m i s s i o n rates for v a r i o u s ion species o f a single c h a r g e state. T h e t r a n s m i s s i o n rates were c a l c u l a t e d using the t h e o r e t i c a l c h a r g e state distrib u t i o n 8) o f the ion b e a m passing a C-foil resp. a gas stripper. T h e t r a n s m i s s i o n rates for the m o d i f i e d s p u t t e r s o u r c e are e q u i v a l e n t to t h o s e o f the c h a r g e e x c h a n g e d u o p l a s m a t r o n o r the P e n n i n g s o u r c e 9) at the H e i d e l berg M P t a n d e m . T h e a u t h o r s are i n d e b t e d to the late G. H o r t i g w h o initiated this w o r k . T h e successful o p e r a t i o n o f the source owes m u c h to K . H . H e m b e r g e r . T h e e m i t t a n c e m e a s u r e m e n t s w o u l d n o t h a v e been p o s s i b l e w i t h o u t the c o l l a b o r a t i o n o f H. B a u m a n n . T h e s u p p o r t
Transmission (%)
48 5l 53 46 47 43
and a d v i c e given by the a c c e l e r a t o r staff is g r a t e f u l l y appreciated.
References t) R. Middletron and C.T. Adams, Nucl. Instr. and Metb. 118 (1974) 329. 2) G. Ihmels, Thesis (MPI Heidelberg, 1975). 3) G. Doucas and H. R. Mck. Hyder, Nucl. Instr. and Meth. 119 (1974) 413. 4) G. Doucas and H. R. Mck. Hyder, Nucl. Instr. and Meth. 124 (1975) 11. s) H. Wegener, Brookhaven National Laboratory, private communication. 6) A. van Steenbergen, Nucl. Instr. and Meth. 51 (1967) 245. 7) H. Bauman and G. Klein, Annual Report MPI Heidelberg (1974). 8) A.B. Wittkower and H. D. Betz, At. Data5 (1973) 113. 9) R. Repnow et al., Nucl. Instr. and Meth. 122 (I974) 179
INSTRUMENTS
IMPROVEMENT
AND
METHODS
[3 8 ([976)
OF THE EMITTANCE
4o7-4IO;
©
NORTH-HOLLAND
OF A MIDDLETON
PUBLISHING
TYPE SPUTTER
CO.
SOURCE
G. IHMELS, E. JAESCHKE and R. REPNOW
Max-Planck-lnstitutfiir Kernphysik, Heidelberg, 14I.Germany Received 16 August 1976 The emittance of a Middleton type sputter source was investigated. Considerable improvement was achieved by a modification of the target and extraction geometry. With this source particle transmission rates of about 50% for various heavy ion species were obtained at the Heidelberg MP tandem accelerator. 1. Introduction
Since the d e v e l o p m e n t o f the M i d d l e t o n type sputter source 1) for negative heavy ions this source has been used in n u m e r o u s t a n d e m laboratories. F i r s t emittance m e a s u r e m e n t s 3' 4) showed t h a t 70% o f the total beam has an emittance smaller t h a n the acceptance o f an EN t a n d e m Van de G r a a f f accelerator. However, at M P t a n d e m accelerators particle transmission rates o f a b o u t 15-20% only were r e p o r t e d S), implying a m u c h higher emittance. Systematic emittance measurements have been performed2), therefore, in o r d e r to investigate this descrepancy. F o r all m e a s u r e m e n t s the c o m m e r c i a l sputter source M a r k I1 m a n u f a c t u r e d by Extrion Corp. was used. A detailed description o f this source can be found in ref. 1. 2. Apparatus and emittance measurements
I n o r d e r to measure the emittance, the mass separated negative ion b e a m was split by h o r i z o n t a l slits. Beyond a drift space the intensity distribution was registered with a F a r a d a y cup with a slotted a p e r t u r e moving in vertical direction. F r o m p e a k width a n d p e a k distance o f the intensity d i s t r i b u t i o n the a r e a A
A = fdr
is evaluated. The emittance E is then o b t a i n e d by multiplication o f A with (W+)/~ ( W = particle energy):
E = (W)÷/Tr f dr
dr'.
E m i t t a n c e m e a s u r e m e n t s were p e r f o r m e d in the horizontal direction only because o f r o t a t i o n a l s y m m e t r y o f the extraction geometry. A detailed description o f the emittance m e a s u r e m e n t m e t h o d a n d a p p a r a t u s can be f o u n d in refs. 6 a n d 7. First emittance m e a s u r e m e n t s o f the original version o f the source were m a d e with a A u - - b e a m . F r o m the a s y m m e t r y in the intensity distributions (fig. 1) it was evident that two b e a m c o m p o n e n t s o f different origin were emitted by the source. Besides the b e a m c o m i n g from the target surface, a second b e a m comp o n e n t was emitted from the s o - c a l l e d b e a m f o r m i n g electrode (L in fig. 2a). This can be u n d e r s t o o d as follows: neutral m a t e r i a l sputtered from the target A
K
C
D L H
M
A
[3
C
D
E J
F
G
H
i
.
dr'
1 I J (arbitrary
unit)
a)
i, -16
-12
-8
-4
0
4
8
12
16
x (rnrn)
Fig. 1. Intensity profile of a Au--beam at the end of a drift space, obtained with the original target and extraction geometry of the source.
Fig. 2. Target and extraction geometry of the source: (a) The original geometry; (b) target and extraction geometry for improved emittance. (A) primary beam, (B) channel target, (C) target wheel, (D) front plate, (E) modified gas feed supply, (F) modified and insulated extraction electrode, (G) isolator, (H) secondary negative ion beam, (J) primary beam spot, (K) cone target, (L) beam forming electrode, (M) original extraction electrode, (V~) extraction potential, (VR) reflector potential.
408
G. IHMELS et al.
TABLE ! Measured emittances for various source conditions: (a) as a function of ion species (UE = 10 kV, lcs =0.6 mA); (b) as a function of Cs-current and extraction voltage (C--beam). (a) Ion species mass number E [mm"mrad" (MeV)~]
C12 6.3
O16 6.9
C~" 24 6.6
Cu63/65 6.7
Ag107/109 6.9
(b) Ics (mA) UE (kV) E [mm"mrad" (MeV)~]
0.6 10 7.7
0.6 15 8.4
0.6 20 9.1
0.5 10 8.0
0.6 10 7.7
surface is deposited on this electrode and again sputtered by that part of the primary beam which escapes through the cone aperture and is then reflected towards this electrode. A fraction of this material is extracted as a negative ion beam. This mechanism can explain experimental results obtained by Hyder a' 4) who reported a steep increase of emittance with larger beam fraction including more than 50% of the total beam. This steep increase is caused by the second beam component born on the electrode at a rather high distance from the beam axis. Because of space charge expansion of the primary beam with high Cs-current, fewer ions are escaping through the target aperture. Therefore with high Cs-current the intensity of the secondary ion beam sputtered from the electrode is relatively small and the steep increase of emittance begins only at 70% of the total negative ion beam. In order to avoid the extraction of negative ions from the beam forming electrode as well as from the flat back surface the beam forming electrode was eliminated and the negative ion extraction electrode was replaced by a conically shaped electrode with a diameter of only 4.5 mm (see fig. 2b). With this extraction geometry the asymmetry in the lines of fig. ! disappeared, and it was now possible to measure the emittance of the secondary negative ion beam sputtered from the inner target surface only. After these changes the dependence of the emittance on the three following parameters was investigated: a) ion species, b) extraction voltage, c) primary beam current. The results of these measurements are summarized in table 1. The table hows that the emittance of the negative ion beam is widely independent of ion mass, and that
Au197 7.4 0.75 10 7.4
increasing the primary beam current and lowering the extraction voltage tend to lower the emittance. Space charge effects from the primary Cs beam can account for all three observations. The space charge of the primary beam in the target canal is only partially compensated by the secondary electrons and negative ions and produces a radial potential accelerating the negative ions towards the beam axis. This process enhances the radial momentum of the sputtered ions. Because space charge forces are not Hamiltonian, the emittance of the negative ion beam becomes worse. An increase of the extraction voltage reduces the space charge expansion of the primary beam. Thus higher space charge density near the beam axis creates a stronger radial potential deteriorating the emittance of the secondary beam. A more intense primary beam at constant extraction voltage will enhance the absolute sputtering yield of negative ions by the more efficient surface activation. Therefore the positive space charged density is better compensated by the sputtered negative ions and electrons and the radial potential is somewhat less pronounced resulting in a lower emittance with high Cs-current. 3. Improvement of source emittance
The results of the emittance measurements suggested two possiblities for emittance improvement: 1) the distance from the negative ion creation area to the beam axis should be minimized; 2) the space charge influence of the primary beam should be avoided. Maintaining high negative ion currents with both requirements can be fulfilled by varying the target and extraction geometry in the following way (fig. 2c): The primary beam - typically UE -----25 kV, Ics= I mA is reduced by the hollow target cylinder of 3.5 m m diameter to 0.3 mA. Only the outer ring of the clipped
409
EMITTANCE OF A MIDDLETON S P U T T E R SOURCE TABLE 2
r (mrad)
Negative ion currents measured with the modified target and extraction geometry (fig. 2b).
Ion species
C-
C~-
O-
AIO-
Cu-
Ni-
Current (/tA)
26
22
14
4
10
2
~ -16 -12 beam is striking the inner target surface; the beam fraction escaping through the cylinder is reflected by the extraction electrode to the front surface of the target. With a slightly positive potential at the extraction electrode the reflected primary beam can be focussed towards a small ring on the front surface. The origin of the sputtered ions is now located at a distance less than 2.5 mm from the axis and in a high extraction field of about 4 kV/mm ensuring a high extraction efficiency and avoiding space charge effects of the primary beam. With this target and extraction geometry high currents of negative ion beams were obtained as listed in table 2. For the last ten months the modified source has been in regular use and is now the standard source for all types of ions with A > 7 , even for production of negative ion beams from enriched gaseous isotopes. To minimize the consumption of these gases a modified conically shaped gas feed supply (fig. 2c) was mounted at the end of the target channel in order to spray the gas directly onto the primary beam spot. During an 180 beam time of 36h 66 standard cm 3 1802 were consumed, the negative ionization efficiency (negative ion/gaseous particle) was estimated to be 10 - 3 . A negative ionization efficiency of 1% was measured during a 32S beam time using powderous PbS as target material pressed into a copper insert (fig. 4). This high efficiency will allow the preparation of special targets of enriched stable isotopes available as solid compounds in order to produce beams of expensive materials economically. Eminance measurements (table 3) obtained under various source conditions show that the emittance has been improved by a factor of two.
-8
2520 15 10 5
-4
o5~'~ 4 I ~ '12 116 rir~m) -10 -20 -25
Fig. 3. Phase space diagrams of a C - - b e a m using (a) original conically shaped target, (b) channel target.
a
b
t/
18
__~ J_
LIo
(mm)
'
Fig. 4. Size of a channel target: (a) copper insert, (b) powderous target material.
These results are seen to be nearly independent of extraction voltage and Cs-current, and the emittance is therefore independent of space charge influence. Phase space diagrams of C-beams emitted from the original cone and a channel target under identical source conditions are compared in fig. 3. From these phase space diagrams the emittance is calculated to be 6.3 and 3.1 m m . mrad. (MeV) ~ respectively. 4. Transmission tests at the M P tandem accelerator
The modified sputter source was tested at the MP tandem with a terminal voltage of 11 MV and a
TABLE 3
Emittances measured with a C - beam using the modified target and extraction geometry (fig. 2c).
lcs (mA) UE (kV) E [mm" mrad" (MeV)~]
0.3 15 3.1
0.5 15 3.0
0.65 15 3.0
a )
0.5 10 3.1
0.5 15 3.0
0.5 20 3.3
410
G. IHMELS et al.
TABLE 4 Results of transmission tests at the MP tandem. Ion species
C O F AIO S Ni
Target material
C Ni+Oz Ni q- S F 6 AI+Oz Pbs Ni
1njected current (hA)
Analysed current (#A)
Charge state
0.76 1.7 1.8 0.27 0.2 0.l
1.07 2.35 2.7 0.03 0.23 0.025
4 6 6 7 6 9
p r e a c c e l e r a t i o n v o l t a g e o f 200 kV. T a b l e 4 s h o w s currents and t r a n s m i s s i o n rates for v a r i o u s ion species o f a single c h a r g e state. T h e t r a n s m i s s i o n rates were c a l c u l a t e d using the t h e o r e t i c a l c h a r g e state distrib u t i o n 8) o f the ion b e a m passing a C-foil resp. a gas stripper. T h e t r a n s m i s s i o n rates for the m o d i f i e d s p u t t e r s o u r c e are e q u i v a l e n t to t h o s e o f the c h a r g e e x c h a n g e d u o p l a s m a t r o n o r the P e n n i n g s o u r c e 9) at the H e i d e l berg M P t a n d e m . T h e a u t h o r s are i n d e b t e d to the late G. H o r t i g w h o initiated this w o r k . T h e successful o p e r a t i o n o f the source owes m u c h to K . H . H e m b e r g e r . T h e e m i t t a n c e m e a s u r e m e n t s w o u l d n o t h a v e been p o s s i b l e w i t h o u t the c o l l a b o r a t i o n o f H. B a u m a n n . T h e s u p p o r t
Transmission (%)
48 5l 53 46 47 43
and a d v i c e given by the a c c e l e r a t o r staff is g r a t e f u l l y appreciated.
References t) R. Middletron and C.T. Adams, Nucl. Instr. and Metb. 118 (1974) 329. 2) G. Ihmels, Thesis (MPI Heidelberg, 1975). 3) G. Doucas and H. R. Mck. Hyder, Nucl. Instr. and Meth. 119 (1974) 413. 4) G. Doucas and H. R. Mck. Hyder, Nucl. Instr. and Meth. 124 (1975) 11. s) H. Wegener, Brookhaven National Laboratory, private communication. 6) A. van Steenbergen, Nucl. Instr. and Meth. 51 (1967) 245. 7) H. Bauman and G. Klein, Annual Report MPI Heidelberg (1974). 8) A.B. Wittkower and H. D. Betz, At. Data5 (1973) 113. 9) R. Repnow et al., Nucl. Instr. and Meth. 122 (I974) 179