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Development of a high current ion source for ion implantation
Nuclear Instruments and Methods in Physics Research BlO/ll North-Holland, Amsterdam
767
(1985) 767-770
DEVELOPMENT OF A HIGH CURRENT ION SOURCE FOR ION IMPLANTATION E. GHANBARI, Variun /Exrrion,
J. BOERS,
R. LIEBERT,
Gloucester, Massachusetts 01930.
L. AYERS
and P. BAZELEY
USA
A duopigatron ion source, based on Sandia National Laboratories design, has been designed and constructed. Some of the preliminary operating parameters of the source using argon, BF, and ASH, have been obtained. At an arc current of 8 A, the total unanalyzed beam current is 13.5 mA at 30 kV extraction potential. The total unanalyzed beam currents are 19.5 mA using BF, gas and 32 mA using ASH,.
1. Introduction A reliable ion source capable of producing a stable ion beam is highly desirable for high-current ion implantation. The source should provide high intensity and good emittance beams of common dopant species (B+, P+, As+, and Sb+). In addition, the source should be easily accessible for maintenance and have a long lifetime. Several types of ion sources are in use in both research and commercial ion implanters today and many comprehensive reviews on different ion sources are available in the literature [l-4]. In this paper a duopigatron ion source, for possible use in ion implanters, is investigated and some of its characteristics are presented.
3. Experimental measurements
2. Duopigatron ion source Duopigatron ion sources are now among those most widely used for intense beam applications to either accelerators or CTR [5-71. The duopigatron ion source configuration under investigation at Varian/Extrion, shown in fig. 1, is based on the Sandia National Laboratories design [7]. All of the source electrodes are watercooled to prevent damage due to the high heat load during operation. The intermediate electrode with 6.35 mm exit aperture, is made from iron and is surrounded by a water-cooled electromagnet. The cathode is a 0.76 mm coiled tungsten wire mounted on two molybdenum feedthrough posts. The gas is fed into the source through the filament mounting flange. The stainless steel anode has a molybdenum insert in the center to reduce the erosion of this electrode by the arc discharge. Similarly, the aluminum secondary cathode has a molybdenum insert in the center. Apertures were optimized by substitutions of molybdenum inserts of different aperture diameters. The molybdenum insert in the secondary cathode extends into the iron plasma expansion cup to 0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
minimize the erosion of this electrode. Two alumina spacers, shown in fig. 1, provide electrical insulation between the source electrodes. A schematic sketch of the source electronic circuitry is also shown in fig. 1. The filament requires approximately 300 W heating power for complete emission. A 300 D resistor connects the anode to the intermediate electrode in order to start and maintain a stable arc discharge. The electromagnet requires 4 A to produce 0.15 T axial magnetic field near the intermediate electrode aperture and the field strength reduces to 4 mT near plasma expansion cup exist aperture. The ion extraction geometry, shown in fig. 1, is the geometry used for the preliminary source experiments. The optimization work on the ion extraction and suppression electrode is given in the following section.
B.V.
The source performance data was obtained in a stainless steel chamber pumped by a 152.4 mm diffusion pump. We were able to achieve base pressure of approximately 5 x 10m6 Torr and a pressure of 5 X lo-’ Torr during source operation. The axial magnetic field profile, shown in fig. 2, was obtained by moving an axial hall probe along the source axis. The maximum axial field on the curve is approximately 0.15 T and occurs near the exit aperture of the intermediate electrode. The preliminary operating parameters of the source were obtained using argon gas. Fig. 3 shows the arc current and voltage dependence on the argon gas influx. This data indicates that at least 9.25 SCCM is requireed to maintain a 15 A, 170 V stable arc discharge. The magnetic field required, at a constant approximately 90 V arc voltage, to maintain a stable arc discharge is given in fig. 4. At an arc current of 8 A, the total unanalyzed beam current is 13.5 mA at 30 kV extraction potential. Fig. 5 illustrates the total argon ion current dependence on the arc current. VIII. ACCELERATOR TECHNOLOGY
E. Ghanhurt ef al. / De~el5p~~etlf of u high currettlton source
768
SECONARY
._
CATHODE
1 COOLING
CHANNEL
-i
1
I
/------
-EXTRACTION
FOCUS
ELECTRODE
ELECTRODE
rCATHODEJ w
Ir-
+
__ -f-
u
INSERT
ARC POWER SUPPLY
Fig. 1. Schematic drawing of duopigatron ion source. electronic circuitry and extraction electrodes.
Fig. 2. Axial magnetic field profile for the duopigatron ion source
,v..s
769 21)96. 24. 22.
Fig. 3. Arc current and arc voltage dependence on the argon pas flow.
I 0
: 0
:
:
2
:
:
AsHs and the arc discharge is very stable. For example, at an arc current of 4 A, the total unanalyzed beam current is 32 mA at 35 kV extraction potentiai. To maintain a stable arc discharge the source requires approximately 10 SCCM influx of ASH, gas. This is due to the high concentration of hydrogen presence in the gas. The source tends to run hotter using RF, gas; because higher arc voltage is necessary to maintain a stabie arc discharge - the reason being that molecular dissociation and ionization of RF, gas require higher bombarding electron energy. Using BF, gas, the total unanalyzed beam current is 19.5 mA at 10 A arc current and 40 kV extraction potential. Examination of the source components indicated
:
:
:..:
: 10
IARC
We have tested the source using dopant material RF, and ASH,. The source runs very smoothly using
:
4
: 12
:
:
.j
I4
(‘AMPS)’
Fig. 5. Total argon ion beam current dependence on the arc current.
that the two insulator rings, see fig. I, were coated with sputtered material. The rings had to be cleaned to avoid shorting out the electrodes due to the deposition of sputtered metal on them. The filament had to be replaced frequently due to the damages received to one of its legs and the mounting post. This probiem was alleviated by changing the filament mounting mechanism. The total extracted beam current given above were obtained using the extraction geometry shown in fig. 1. This geometry is not optimized for high-current extraction using dopant material. Using the SNOW [8] computer simulation code, we are able to predict the proper extraction geometry for our ion source. For example, fig. 6 shows a typical evaluation of a proposed extraction geometry using SNOW’s computer simulation.
4. Conehrsions
1
-I 1
MAGNETIC
2
3
FlflQf
4 ARBITRARY
5
The duopigatron ion source is among various ion sources under investigation at our laboratory. In order to evaiuate the source capability, we are in the process of the following: (1) mass analyze the total beam eurrent to find the percentage of each species available, (2) optimize the extraction geometry for higher current output, (3) obtain the source plasma parameters using a Langmuir probe [9] and (4) investigate the source lifetime by shielding the insulator rings and lengthening the filament lifetime. At this stage, we feel that, at least empirically. we understand the operation of this source.
UNIT 1
Fig. 4. Arc current dependence at a constant arc voltage on the magnetic fidd.
VIII. ACCEtERATOR TECHNOLOGY
E. Ghunhuri
er al. /
Deuelopnlenr
TRAJECTORIES
Fig. 6. A typical
extraction
electrodes
geometry
AND
design using computer
References (1) R.G. Wilson and G.R. Brewer, Ion Beams (Wiley, New York, 1973) p. 1. f2] A. Sidenius. Low Energy ion Beams, ed., KG. Stephen (1977). [3] D. Aitken, Ion Implantation Techniques, eds.. H. Ryssel and H. Glawischnig (Springer, Heidelberg, 1982) p. 23. [4] KG. Stephens, Ion Implantation Science and Technology, ed.. J.F. Ziegler (Academic Press, New York, 1984) p. 375. [S] Proc. 2nd Symp. on Jon Sources and Formation of Beams,
of o high rurrenl
m
source
EQUIPOTENTIALS
simulation
code SNOW
[Xl.
Lawrence Berkeley and Livermore Laboratories, California, Lawrence Berkeley Lab. Rep LBL-3399 (1974). I61 F.M. Bacon. R.W. Bickes, Jr.. and J.B. O’Hagen, Rev. Sci. Instr. 4 (1977) 435. Rev. Sci. Instr. 53 171 R.W. Bickes, Jr. and J.B. O’Hagan, ((1982) 585. Division. private PI J.E. Boers, Varian Associates/Extrion communication (1984). et al.. to be presented at APS Division of 191 E. Ghanbari Plasma Physics, 26th Annual Meeting. Boston (1984).
767
(1985) 767-770
DEVELOPMENT OF A HIGH CURRENT ION SOURCE FOR ION IMPLANTATION E. GHANBARI, Variun /Exrrion,
J. BOERS,
R. LIEBERT,
Gloucester, Massachusetts 01930.
L. AYERS
and P. BAZELEY
USA
A duopigatron ion source, based on Sandia National Laboratories design, has been designed and constructed. Some of the preliminary operating parameters of the source using argon, BF, and ASH, have been obtained. At an arc current of 8 A, the total unanalyzed beam current is 13.5 mA at 30 kV extraction potential. The total unanalyzed beam currents are 19.5 mA using BF, gas and 32 mA using ASH,.
1. Introduction A reliable ion source capable of producing a stable ion beam is highly desirable for high-current ion implantation. The source should provide high intensity and good emittance beams of common dopant species (B+, P+, As+, and Sb+). In addition, the source should be easily accessible for maintenance and have a long lifetime. Several types of ion sources are in use in both research and commercial ion implanters today and many comprehensive reviews on different ion sources are available in the literature [l-4]. In this paper a duopigatron ion source, for possible use in ion implanters, is investigated and some of its characteristics are presented.
3. Experimental measurements
2. Duopigatron ion source Duopigatron ion sources are now among those most widely used for intense beam applications to either accelerators or CTR [5-71. The duopigatron ion source configuration under investigation at Varian/Extrion, shown in fig. 1, is based on the Sandia National Laboratories design [7]. All of the source electrodes are watercooled to prevent damage due to the high heat load during operation. The intermediate electrode with 6.35 mm exit aperture, is made from iron and is surrounded by a water-cooled electromagnet. The cathode is a 0.76 mm coiled tungsten wire mounted on two molybdenum feedthrough posts. The gas is fed into the source through the filament mounting flange. The stainless steel anode has a molybdenum insert in the center to reduce the erosion of this electrode by the arc discharge. Similarly, the aluminum secondary cathode has a molybdenum insert in the center. Apertures were optimized by substitutions of molybdenum inserts of different aperture diameters. The molybdenum insert in the secondary cathode extends into the iron plasma expansion cup to 0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
minimize the erosion of this electrode. Two alumina spacers, shown in fig. 1, provide electrical insulation between the source electrodes. A schematic sketch of the source electronic circuitry is also shown in fig. 1. The filament requires approximately 300 W heating power for complete emission. A 300 D resistor connects the anode to the intermediate electrode in order to start and maintain a stable arc discharge. The electromagnet requires 4 A to produce 0.15 T axial magnetic field near the intermediate electrode aperture and the field strength reduces to 4 mT near plasma expansion cup exist aperture. The ion extraction geometry, shown in fig. 1, is the geometry used for the preliminary source experiments. The optimization work on the ion extraction and suppression electrode is given in the following section.
B.V.
The source performance data was obtained in a stainless steel chamber pumped by a 152.4 mm diffusion pump. We were able to achieve base pressure of approximately 5 x 10m6 Torr and a pressure of 5 X lo-’ Torr during source operation. The axial magnetic field profile, shown in fig. 2, was obtained by moving an axial hall probe along the source axis. The maximum axial field on the curve is approximately 0.15 T and occurs near the exit aperture of the intermediate electrode. The preliminary operating parameters of the source were obtained using argon gas. Fig. 3 shows the arc current and voltage dependence on the argon gas influx. This data indicates that at least 9.25 SCCM is requireed to maintain a 15 A, 170 V stable arc discharge. The magnetic field required, at a constant approximately 90 V arc voltage, to maintain a stable arc discharge is given in fig. 4. At an arc current of 8 A, the total unanalyzed beam current is 13.5 mA at 30 kV extraction potential. Fig. 5 illustrates the total argon ion current dependence on the arc current. VIII. ACCELERATOR TECHNOLOGY
E. Ghanhurt ef al. / De~el5p~~etlf of u high currettlton source
768
SECONARY
._
CATHODE
1 COOLING
CHANNEL
-i
1
I
/------
-EXTRACTION
FOCUS
ELECTRODE
ELECTRODE
rCATHODEJ w
Ir-
+
__ -f-
u
INSERT
ARC POWER SUPPLY
Fig. 1. Schematic drawing of duopigatron ion source. electronic circuitry and extraction electrodes.
Fig. 2. Axial magnetic field profile for the duopigatron ion source
,v..s
769 21)96. 24. 22.
Fig. 3. Arc current and arc voltage dependence on the argon pas flow.
I 0
: 0
:
:
2
:
:
AsHs and the arc discharge is very stable. For example, at an arc current of 4 A, the total unanalyzed beam current is 32 mA at 35 kV extraction potentiai. To maintain a stable arc discharge the source requires approximately 10 SCCM influx of ASH, gas. This is due to the high concentration of hydrogen presence in the gas. The source tends to run hotter using RF, gas; because higher arc voltage is necessary to maintain a stabie arc discharge - the reason being that molecular dissociation and ionization of RF, gas require higher bombarding electron energy. Using BF, gas, the total unanalyzed beam current is 19.5 mA at 10 A arc current and 40 kV extraction potential. Examination of the source components indicated
:
:
:..:
: 10
IARC
We have tested the source using dopant material RF, and ASH,. The source runs very smoothly using
:
4
: 12
:
:
.j
I4
(‘AMPS)’
Fig. 5. Total argon ion beam current dependence on the arc current.
that the two insulator rings, see fig. I, were coated with sputtered material. The rings had to be cleaned to avoid shorting out the electrodes due to the deposition of sputtered metal on them. The filament had to be replaced frequently due to the damages received to one of its legs and the mounting post. This probiem was alleviated by changing the filament mounting mechanism. The total extracted beam current given above were obtained using the extraction geometry shown in fig. 1. This geometry is not optimized for high-current extraction using dopant material. Using the SNOW [8] computer simulation code, we are able to predict the proper extraction geometry for our ion source. For example, fig. 6 shows a typical evaluation of a proposed extraction geometry using SNOW’s computer simulation.
4. Conehrsions
1
-I 1
MAGNETIC
2
3
FlflQf
4 ARBITRARY
5
The duopigatron ion source is among various ion sources under investigation at our laboratory. In order to evaiuate the source capability, we are in the process of the following: (1) mass analyze the total beam eurrent to find the percentage of each species available, (2) optimize the extraction geometry for higher current output, (3) obtain the source plasma parameters using a Langmuir probe [9] and (4) investigate the source lifetime by shielding the insulator rings and lengthening the filament lifetime. At this stage, we feel that, at least empirically. we understand the operation of this source.
UNIT 1
Fig. 4. Arc current dependence at a constant arc voltage on the magnetic fidd.
VIII. ACCEtERATOR TECHNOLOGY
E. Ghunhuri
er al. /
Deuelopnlenr
TRAJECTORIES
Fig. 6. A typical
extraction
electrodes
geometry
AND
design using computer
References (1) R.G. Wilson and G.R. Brewer, Ion Beams (Wiley, New York, 1973) p. 1. f2] A. Sidenius. Low Energy ion Beams, ed., KG. Stephen (1977). [3] D. Aitken, Ion Implantation Techniques, eds.. H. Ryssel and H. Glawischnig (Springer, Heidelberg, 1982) p. 23. [4] KG. Stephens, Ion Implantation Science and Technology, ed.. J.F. Ziegler (Academic Press, New York, 1984) p. 375. [S] Proc. 2nd Symp. on Jon Sources and Formation of Beams,
of o high rurrenl
m
source
EQUIPOTENTIALS
simulation
code SNOW
[Xl.
Lawrence Berkeley and Livermore Laboratories, California, Lawrence Berkeley Lab. Rep LBL-3399 (1974). I61 F.M. Bacon. R.W. Bickes, Jr.. and J.B. O’Hagen, Rev. Sci. Instr. 4 (1977) 435. Rev. Sci. Instr. 53 171 R.W. Bickes, Jr. and J.B. O’Hagan, ((1982) 585. Division. private PI J.E. Boers, Varian Associates/Extrion communication (1984). et al.. to be presented at APS Division of 191 E. Ghanbari Plasma Physics, 26th Annual Meeting. Boston (1984).