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High current ion implanter aimed at clean and dust-free production
Nuclear Instruments and Methods in Physics Research B21 (1987) 235-238 North-Holland, Amsterdam
235
HIGH C U R R E N T ION IMPLANTER AIMED AT CLEAN A N D DUST-FREE P R O D U C T I O N K. K O M A T S U , K. T A K A G I , O. T S U K A K O S H I , T. K A T A G A W A , E. K O U N O a n d S. K O M I Y A
UL VA C Corporation, 2500, Hagisono, Chigasaki, Kanagawa 253, Japan
A new high current, high throughput ion implanter, ULVAC Model IMH-2200, has been developed based on a new design concept. The IMH-2200 has a small floor area (8.2 m2) and a total weight of - 7.2 t. And yet it has a high implantation capability: 12.5 mA As + beam at 200 keV. Computer aided design has been carried out throughout ion beam optics. The mass analysed ion beam is bent by 25 ° upwards by a compact deflection magnet and then postaccelerated to the rotating wafer disk which is inclined about 25 o. A high throughput end station adopts the cassette to cassette load lock system. A specially designed linear pulse motor for wafer transport in vacuum reduces wafer damages and contributes to a high level of throughput. Complete cryopumping is capable of oil-free clean implantations. The new design principle has enabled us to realize a system free from dust particles and organic vapor contaminations.
1. Introduction
2. Ion source and extraction system
The ion implanter, ULVAC Model IMH-2200, is a new, 200 keV, 12.5 mA system designed for long maintenance-free operations in a production line. The IMH2200 has the following outstanding features: (1) Wafer handlings are carried out in vacuum to minimize dust particle contaminations; (2) Wafers are stable while they are transported in vacuum by making the transport system tilted; (3) Only cryopumps are used for the vacuum system and oil-free clean implantations are possible; (4) A reliable high current ion source with long lifetime and a small divergence angle extraction system make high dose implantations possible; (5) A new compact acceleration tube is capable of handling high current ion beam in the postacceleration; (6) The distance from the acceleration tube to the wafer is minimized by placing the electron shower, Faraday flag and the isolation gate valve in a plane parallel to the disk; (7) The volume of the machine is minimized by compact designs of component systems especially on the gas box and power supplies; and (8) The whole process of ion implantations is completely computerized. A schematic diagram of the ion beam system and the end station is shown in fig. 1. In what follows, each component system is described in more detail. The reader can find more details on the end station elsewhere [1].
New types of ion sources and their associated extraction systems have been developed. A customer can choose either an ordinary gas-type Freeman-type ion source or, if the ion species is P or As, a new solid type ion source. The electrodes are carefully designed so that the ion beam does not hit any part of the extraction system. In order to obtain long lifetime of the filament, a thick thoriated tungsten wire is used. Low outgassing materials are used for all parts of the ion source and the extraction electrodes. The exhausting system consists of a 12 in. cryopump only, which realizes oil-free vacuum conditions. The crucible and its heater of the solid-type ion source are placed outside the vessel. The crucible is protected from the heat of the arc chamber by reflectors and a thick aluminum wall which is cooled by liquid freon. With this configuration, precise temperature controls between room temperature and 500 ° C are possible without the effect of heat radiation from the arc chamber. Extensive computer simulations have been carried out to find the best configuration of the extraction system. Calculations have also been done for the design of electrodes and for optimum spacings of them corresponding to different ion species. An ion beam with a small divergence angle can be extracted by forming a concave plasma meniscus at the exit of the arc chamber. To obtain high transmission of the ion beam through the analyser tube, the divergence of the beam in the longitudinal direction must be small at the exit of the
0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
IV. NEW EQUIPMENT AND SYSTEMS
236
K. Komatsu et al. / High current ion implanter Acceleration Tube
\
ue,ioing ~\1 Magnet
"J
Flog
-.
Sector
Faraday
Faraday /
/
B e a m Sensor
t ~Electr°de
/Cup ' / "
Deflection ~ , ~ d " ~ % ' ~ Magnet~ ' ~ ' ~ ' ~ - ~ : ~
-
9 too
~-(3)
o (1990
I
Aperture
Defining Aperture
I
B~~
~
J
_N ~E rr
~
Z
Ion Source
Plasma Electrode Extraction Electrode
/
,
0.98q _
i
5 INNEREDGE
.
.
_
,
.
.
.
.
0
*
5
CENTER
.
.
t
OUTER EDGE
RADIAL DISTANCE (cm)
Fig. 1. Schematic view of the ion beam transport system IMH-2200.
'
|
/
/
r-~ UJ
O
Ion /
. . . .
--~-----(2)
z I 'Sensing
i
0.999
:E
/
. . . .
LLLLI1.000 I-ILl
i/\,~ I \
i
Ground Electrode
Fig. 2. Calculated ion beam trajectories in the extraction system of a Freeman-type ion source.
extraction system, since the separation gap of the sector magnet is limited. Numerical investigations using a 3-dimensional code suggested that the angle can be made small by employing a shallow ridge angle to the plasma electrode as shown in fig. 2. The electrodes are designed according to these numerical simulations. When the operational conditions such as ion species, extraction energy and dose are specified, the electrodes are placed at the optimum locations by a computer controlled mechanism.
Fig. 3. Distribution of the magnetic field on the median plane of the sector magnet (1) with shim, experimental plot, (2) with shim, calculated curve, and (3) with flat pole pieces, calculated curve.
pieces and the gap between them are 160 and 60 mm, respectively. The magnets are designed to have good uniformity in the magnetic field despite of the fact that the gap between them is quite large considering their compact sizes. According to an ion optical analysis, the width of a magnetic field with a uniformity of 1 × 10-3 should be at least 80 mm. Numerical simulations showed that such uniformity can be obtained by putting shims at both edges of each pole piece. The simulations indicated that the uniformity is guaranteed over a width of 100 mm by placing the pole pieces with a distance of 60 mm between the shims of 2 mm in height. Fig. 3 shows the magnetic field which was measured by using a Hall element with a sufficiently low thermal coefficient. There is another small magnet attached to the ion beam system. The mass-analysed ion beam is deflected upward by 25 ° with this magnet and is focused to the defining aperture. Two sets of compact beam sensors that have magnetic suppressors are located surrounding the ion beam just behind the defining aperture. The xand y-direction output signals of the beam sensors are fed back to the power supplies of the magnets. With this feedback, the fluctuations in the focused beam position is always corrected, resulting in a stable ion beam to the target with very little drift even for long time operations of implantation.
4. Acceleration tube and EFG assembly 3. Mass analyser and magnetic deflector The ion beam extracted from the ion source is massanalysed by the sector magnet as shown in fig. 1. The deflection angle of the beam is 90 ° and the radius of the circular orbit is 300 mm. The width of the pole
A compact ceramic acceleration tube with a high transmission rate is used in the post acceleration. The contours of the ion beams going through the acceleration tube are simulated by taking the space charge effect into account. For the case of low acceleration energy
237
K. Komatsu et al. / High current ion implanter
ELECTRODE I
\
L CONTOUR.~
\
lill
,
SHIELD
,
" --
0.05 Ib=4rnA
-o.o5
Eb_-7Oke v
\
Fig. 6. Electron shower and calculated orbits of the electrons.
!
!
.
,
,
DISTANCt=
i
{era)
li /
' ,, I"l,lli,' , I
','
Fig. 4. Calculated ion beam contour in the acceleration tube.
I
1.0
I
~.5 52O ®
O.8
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I
5.1 8~ ® ~ 3.6
®
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~_ 06 < 0.4 rr V--
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00
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I
I
50 100 150 BEAM ENERGY ( KeV )
200
ries of argon ions through the acceleration tube. The transmission rate of argon ions has a favourable flat dependence on the beam energy as plotted in fig. 5. The electron shower was designed by considering the following two requirements; (1) The filament is not exposed directly to the wafers so that the wafers are not contaminated with alkaline metal vapors coming from the heated filament. (2) The electron energy at the target is less than 25 eV. Fig. 6 shows a schematic diagram of the electron shower and the simulated electron beam trajectories. Electrons coming out from the flag-shaped electrode reach the wafer surface taking curved trajectories. It is desirable to reduce the distance between the acceleration tube and the target in order to have a high transmission rate of ion beams and to make the machine compact. For that purpose, an efficient configuration for the electron shower (E), Faraday flag (F) and isolation gate valve (G) was employed. These components are arranged in the same plane which is perpendicular to the ion beam. This arrangement requires a small space and helps to reduce the distance just mentioned before. When either one of them is needed, it is set on the optical axis of ion beam. This arrangement is called EFG Assembly.
Fig. 5. Experimentally observed ion beam transmission through the acceleration tube vs the final ion beam energy.
5. End station and Faraday cup and high ion current, the numerical results indicated that the beam divergence can be made small by increasing the potential gradient along the optical axis regardless of the electrode configuration. The acceleration tube consists of five aluminum electrodes and is designed to have a high potential gradient. The electrodes are easy to remove from the acceleration tube for cleanings. The insulator is made of ceramics (A1203) in order to get clean vacuum conditions. Fig. 4 shows trajecto-
A compact and high throughput end station has been developed based on mechanical scanning disk and a cassette to cassette load lock system. The end station is designed from the standpoint to minimize dust particle contamination of wafers and clean room floor area and to maximize reliability and speed of wafer handling. A bird's eye view of the end station is shown in fig. 7. A wafer cassette placed on the stage by an operator is transported to the process system through a IV. NEW EQUIPMENT AND SYSTEMS
238
K. Komatsu et aL
/ High current ion implanter
Process ,Rotating o Chomber~/Disk , Cassette Lonm ~ ' //\~Valve(load)v Loodir'~. Wafer ~ k ~ AUnl°ad'ng / / Interface V a l v e ~ ~.~ (unload)
'~~¢toad)
~ Buffer ~ ]x'~l~ Chamber/ Rotationcfl Transport
\
Tunnet
Load-lock \?Mun!p~atOrchamber
~:-
LinearIndexing Carriage LinearPulse Motor
Fig. 7. ULVAC IMH-2200 end station. Magnetic
t
/ /
/
/
put to an unload cassette keeping the original address. Sliding, grasping and knocking are carefully avoided in wafer handling to prevent the generation of dust particles in vacuum. A wafer cooling system employing new low thermal resistance wafer pads has been designed and operated successfully during the higher power ion implantation. The wafer temperature is kept below 100 ° C during implantations with beam powers of 2.5 kW. The ordinary type of rotating disk has a diameter which is large enough to cover the Faraday cup all the time. The diameter, therefore, is larger than necessary just to hold wafers. The new disk, however, has a minimum diameter just large enough to hold the wafers as shown in fig. 8. Instead, the Faraday cup is covered by a plate with a narrow silt. This slit is narrower than that of the rotating disk and a signal which consists of dc and pulse components is observed as the rotating disk moves over the Faraday cup. Monitoring of the total beam current is done by measuring these pulsepeak heights. Calibrations of the beam current is carried out by scanning the Faraday cup in the direction perpendicular to the slit length while placing the disk at a position where the beam is not interfered by the disk.
. . . . . .
6. Summary
wof~--.
\\
/
Ion B e a m /
z tD
B n 5 0
%!11] TIME
Fig. 8. Faraday cup and output signal during ion implantation.
clean tunnel. After the process, wafers are returned to the stage maintaining the same addresses in the cassette. The sequence time for handling 13 wafers from loading to unloading is 90 s. The wafer transport system in vacuum is inclined by about 25 ° from the vertical direction. Thereby wafers are kept stable by the gravitational force while they are transported. Moreover, the wafers are kept face down and the front surfaces are free from dust particle contaminations. A load cassette is transported in vacuum just beneath the disk, and wafers are lifted by a wafer fork driven by a vacuum linear pulse motor. The processed wafers are demounted by another lifter and
A compact, high throughput and high current implanter IMH-2200 has been developed. Clean implantations are possible by employing cryopumps only for its vacuum system. The mass-analysed ion beam is kept stable even for long-time operations by monitoring the beam with beam sensors. The high throughput end station consists of the mechanical scanning disk and the cassette to cassette load lock system and is completely computerized. Wafer handlings are carried out always in vacuum and face down so that contamination due to dust particles is minimized. A new linear pulse motor is used to drive the high speed wafer transport system in vacuum, which contributes to realize high throughput of the whole process. The authors wish to express their sincere gratitude to K. Yui, T. Terasawa and K. Niikura for their cooperation and contributions throughput this work. The authors are also greatly indebted to Dr. C. Hayashi for his valuable discussions and encouragement, and for permission to publ;sh the present report. Reference
[1] K. Komatsu, K. Yui, T. Terasawa, K. Niikura, E. Kouno and S. Komiya, these Proceedings (Ion Implantation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 235.
235
HIGH C U R R E N T ION IMPLANTER AIMED AT CLEAN A N D DUST-FREE P R O D U C T I O N K. K O M A T S U , K. T A K A G I , O. T S U K A K O S H I , T. K A T A G A W A , E. K O U N O a n d S. K O M I Y A
UL VA C Corporation, 2500, Hagisono, Chigasaki, Kanagawa 253, Japan
A new high current, high throughput ion implanter, ULVAC Model IMH-2200, has been developed based on a new design concept. The IMH-2200 has a small floor area (8.2 m2) and a total weight of - 7.2 t. And yet it has a high implantation capability: 12.5 mA As + beam at 200 keV. Computer aided design has been carried out throughout ion beam optics. The mass analysed ion beam is bent by 25 ° upwards by a compact deflection magnet and then postaccelerated to the rotating wafer disk which is inclined about 25 o. A high throughput end station adopts the cassette to cassette load lock system. A specially designed linear pulse motor for wafer transport in vacuum reduces wafer damages and contributes to a high level of throughput. Complete cryopumping is capable of oil-free clean implantations. The new design principle has enabled us to realize a system free from dust particles and organic vapor contaminations.
1. Introduction
2. Ion source and extraction system
The ion implanter, ULVAC Model IMH-2200, is a new, 200 keV, 12.5 mA system designed for long maintenance-free operations in a production line. The IMH2200 has the following outstanding features: (1) Wafer handlings are carried out in vacuum to minimize dust particle contaminations; (2) Wafers are stable while they are transported in vacuum by making the transport system tilted; (3) Only cryopumps are used for the vacuum system and oil-free clean implantations are possible; (4) A reliable high current ion source with long lifetime and a small divergence angle extraction system make high dose implantations possible; (5) A new compact acceleration tube is capable of handling high current ion beam in the postacceleration; (6) The distance from the acceleration tube to the wafer is minimized by placing the electron shower, Faraday flag and the isolation gate valve in a plane parallel to the disk; (7) The volume of the machine is minimized by compact designs of component systems especially on the gas box and power supplies; and (8) The whole process of ion implantations is completely computerized. A schematic diagram of the ion beam system and the end station is shown in fig. 1. In what follows, each component system is described in more detail. The reader can find more details on the end station elsewhere [1].
New types of ion sources and their associated extraction systems have been developed. A customer can choose either an ordinary gas-type Freeman-type ion source or, if the ion species is P or As, a new solid type ion source. The electrodes are carefully designed so that the ion beam does not hit any part of the extraction system. In order to obtain long lifetime of the filament, a thick thoriated tungsten wire is used. Low outgassing materials are used for all parts of the ion source and the extraction electrodes. The exhausting system consists of a 12 in. cryopump only, which realizes oil-free vacuum conditions. The crucible and its heater of the solid-type ion source are placed outside the vessel. The crucible is protected from the heat of the arc chamber by reflectors and a thick aluminum wall which is cooled by liquid freon. With this configuration, precise temperature controls between room temperature and 500 ° C are possible without the effect of heat radiation from the arc chamber. Extensive computer simulations have been carried out to find the best configuration of the extraction system. Calculations have also been done for the design of electrodes and for optimum spacings of them corresponding to different ion species. An ion beam with a small divergence angle can be extracted by forming a concave plasma meniscus at the exit of the arc chamber. To obtain high transmission of the ion beam through the analyser tube, the divergence of the beam in the longitudinal direction must be small at the exit of the
0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
IV. NEW EQUIPMENT AND SYSTEMS
236
K. Komatsu et al. / High current ion implanter Acceleration Tube
\
ue,ioing ~\1 Magnet
"J
Flog
-.
Sector
Faraday
Faraday /
/
B e a m Sensor
t ~Electr°de
/Cup ' / "
Deflection ~ , ~ d " ~ % ' ~ Magnet~ ' ~ ' ~ ' ~ - ~ : ~
-
9 too
~-(3)
o (1990
I
Aperture
Defining Aperture
I
B~~
~
J
_N ~E rr
~
Z
Ion Source
Plasma Electrode Extraction Electrode
/
,
0.98q _
i
5 INNEREDGE
.
.
_
,
.
.
.
.
0
*
5
CENTER
.
.
t
OUTER EDGE
RADIAL DISTANCE (cm)
Fig. 1. Schematic view of the ion beam transport system IMH-2200.
'
|
/
/
r-~ UJ
O
Ion /
. . . .
--~-----(2)
z I 'Sensing
i
0.999
:E
/
. . . .
LLLLI1.000 I-ILl
i/\,~ I \
i
Ground Electrode
Fig. 2. Calculated ion beam trajectories in the extraction system of a Freeman-type ion source.
extraction system, since the separation gap of the sector magnet is limited. Numerical investigations using a 3-dimensional code suggested that the angle can be made small by employing a shallow ridge angle to the plasma electrode as shown in fig. 2. The electrodes are designed according to these numerical simulations. When the operational conditions such as ion species, extraction energy and dose are specified, the electrodes are placed at the optimum locations by a computer controlled mechanism.
Fig. 3. Distribution of the magnetic field on the median plane of the sector magnet (1) with shim, experimental plot, (2) with shim, calculated curve, and (3) with flat pole pieces, calculated curve.
pieces and the gap between them are 160 and 60 mm, respectively. The magnets are designed to have good uniformity in the magnetic field despite of the fact that the gap between them is quite large considering their compact sizes. According to an ion optical analysis, the width of a magnetic field with a uniformity of 1 × 10-3 should be at least 80 mm. Numerical simulations showed that such uniformity can be obtained by putting shims at both edges of each pole piece. The simulations indicated that the uniformity is guaranteed over a width of 100 mm by placing the pole pieces with a distance of 60 mm between the shims of 2 mm in height. Fig. 3 shows the magnetic field which was measured by using a Hall element with a sufficiently low thermal coefficient. There is another small magnet attached to the ion beam system. The mass-analysed ion beam is deflected upward by 25 ° with this magnet and is focused to the defining aperture. Two sets of compact beam sensors that have magnetic suppressors are located surrounding the ion beam just behind the defining aperture. The xand y-direction output signals of the beam sensors are fed back to the power supplies of the magnets. With this feedback, the fluctuations in the focused beam position is always corrected, resulting in a stable ion beam to the target with very little drift even for long time operations of implantation.
4. Acceleration tube and EFG assembly 3. Mass analyser and magnetic deflector The ion beam extracted from the ion source is massanalysed by the sector magnet as shown in fig. 1. The deflection angle of the beam is 90 ° and the radius of the circular orbit is 300 mm. The width of the pole
A compact ceramic acceleration tube with a high transmission rate is used in the post acceleration. The contours of the ion beams going through the acceleration tube are simulated by taking the space charge effect into account. For the case of low acceleration energy
237
K. Komatsu et al. / High current ion implanter
ELECTRODE I
\
L CONTOUR.~
\
lill
,
SHIELD
,
" --
0.05 Ib=4rnA
-o.o5
Eb_-7Oke v
\
Fig. 6. Electron shower and calculated orbits of the electrons.
!
!
.
,
,
DISTANCt=
i
{era)
li /
' ,, I"l,lli,' , I
','
Fig. 4. Calculated ion beam contour in the acceleration tube.
I
1.0
I
~.5 52O ®
O.8
IF=3.3mA
I
5.1 8~ ® ~ 3.6
®
3.~
5.8 • 5.3
®
3~
z Q
~_ 06 < 0.4 rr V--
Ta=TL
0.2
00
I
I
I
50 100 150 BEAM ENERGY ( KeV )
200
ries of argon ions through the acceleration tube. The transmission rate of argon ions has a favourable flat dependence on the beam energy as plotted in fig. 5. The electron shower was designed by considering the following two requirements; (1) The filament is not exposed directly to the wafers so that the wafers are not contaminated with alkaline metal vapors coming from the heated filament. (2) The electron energy at the target is less than 25 eV. Fig. 6 shows a schematic diagram of the electron shower and the simulated electron beam trajectories. Electrons coming out from the flag-shaped electrode reach the wafer surface taking curved trajectories. It is desirable to reduce the distance between the acceleration tube and the target in order to have a high transmission rate of ion beams and to make the machine compact. For that purpose, an efficient configuration for the electron shower (E), Faraday flag (F) and isolation gate valve (G) was employed. These components are arranged in the same plane which is perpendicular to the ion beam. This arrangement requires a small space and helps to reduce the distance just mentioned before. When either one of them is needed, it is set on the optical axis of ion beam. This arrangement is called EFG Assembly.
Fig. 5. Experimentally observed ion beam transmission through the acceleration tube vs the final ion beam energy.
5. End station and Faraday cup and high ion current, the numerical results indicated that the beam divergence can be made small by increasing the potential gradient along the optical axis regardless of the electrode configuration. The acceleration tube consists of five aluminum electrodes and is designed to have a high potential gradient. The electrodes are easy to remove from the acceleration tube for cleanings. The insulator is made of ceramics (A1203) in order to get clean vacuum conditions. Fig. 4 shows trajecto-
A compact and high throughput end station has been developed based on mechanical scanning disk and a cassette to cassette load lock system. The end station is designed from the standpoint to minimize dust particle contamination of wafers and clean room floor area and to maximize reliability and speed of wafer handling. A bird's eye view of the end station is shown in fig. 7. A wafer cassette placed on the stage by an operator is transported to the process system through a IV. NEW EQUIPMENT AND SYSTEMS
238
K. Komatsu et aL
/ High current ion implanter
Process ,Rotating o Chomber~/Disk , Cassette Lonm ~ ' //\~Valve(load)v Loodir'~. Wafer ~ k ~ AUnl°ad'ng / / Interface V a l v e ~ ~.~ (unload)
'~~¢toad)
~ Buffer ~ ]x'~l~ Chamber/ Rotationcfl Transport
\
Tunnet
Load-lock \?Mun!p~atOrchamber
~:-
LinearIndexing Carriage LinearPulse Motor
Fig. 7. ULVAC IMH-2200 end station. Magnetic
t
/ /
/
/
put to an unload cassette keeping the original address. Sliding, grasping and knocking are carefully avoided in wafer handling to prevent the generation of dust particles in vacuum. A wafer cooling system employing new low thermal resistance wafer pads has been designed and operated successfully during the higher power ion implantation. The wafer temperature is kept below 100 ° C during implantations with beam powers of 2.5 kW. The ordinary type of rotating disk has a diameter which is large enough to cover the Faraday cup all the time. The diameter, therefore, is larger than necessary just to hold wafers. The new disk, however, has a minimum diameter just large enough to hold the wafers as shown in fig. 8. Instead, the Faraday cup is covered by a plate with a narrow silt. This slit is narrower than that of the rotating disk and a signal which consists of dc and pulse components is observed as the rotating disk moves over the Faraday cup. Monitoring of the total beam current is done by measuring these pulsepeak heights. Calibrations of the beam current is carried out by scanning the Faraday cup in the direction perpendicular to the slit length while placing the disk at a position where the beam is not interfered by the disk.
. . . . . .
6. Summary
wof~--.
\\
/
Ion B e a m /
z tD
B n 5 0
%!11] TIME
Fig. 8. Faraday cup and output signal during ion implantation.
clean tunnel. After the process, wafers are returned to the stage maintaining the same addresses in the cassette. The sequence time for handling 13 wafers from loading to unloading is 90 s. The wafer transport system in vacuum is inclined by about 25 ° from the vertical direction. Thereby wafers are kept stable by the gravitational force while they are transported. Moreover, the wafers are kept face down and the front surfaces are free from dust particle contaminations. A load cassette is transported in vacuum just beneath the disk, and wafers are lifted by a wafer fork driven by a vacuum linear pulse motor. The processed wafers are demounted by another lifter and
A compact, high throughput and high current implanter IMH-2200 has been developed. Clean implantations are possible by employing cryopumps only for its vacuum system. The mass-analysed ion beam is kept stable even for long-time operations by monitoring the beam with beam sensors. The high throughput end station consists of the mechanical scanning disk and the cassette to cassette load lock system and is completely computerized. Wafer handlings are carried out always in vacuum and face down so that contamination due to dust particles is minimized. A new linear pulse motor is used to drive the high speed wafer transport system in vacuum, which contributes to realize high throughput of the whole process. The authors wish to express their sincere gratitude to K. Yui, T. Terasawa and K. Niikura for their cooperation and contributions throughput this work. The authors are also greatly indebted to Dr. C. Hayashi for his valuable discussions and encouragement, and for permission to publ;sh the present report. Reference
[1] K. Komatsu, K. Yui, T. Terasawa, K. Niikura, E. Kouno and S. Komiya, these Proceedings (Ion Implantation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 235.