- Email: [email protected]
New ion implantation system with advanced process capabilities
280
Nuclear Instruments and.Methods in Physics Research B21 (1987) 280-284 North-Holland. Amsterdam
NEW ION IMPLANTATION SYSTEM WITH ADVANCED PROCESS CAPABILITIES M.T. W A U K Applied Materials Implant Division, Horsham, West Sussex, England RHI3 5PY
A new ion implantation system has been developed with several significant IC wafer processing improvements resulting from innovations in beam optics, target system, wafer handling, and control electronics design. This paper will concentrate on the process results that have been established thus far with this new system. Implanted wafers are subjected to temperatures of less than approximately 40 ° C even with beams of 30 mA at 120 keV. This significantly reduces residual lattice damage due to partial annealing, and also significantly reduces photoresist blistering and flow. Wafers are completely flat and planar during implant, providing a constant implant angle and reduced uniformity and depth profile variations due to channeling, as well as providing more uniform wafer temperature. Low ion beam density and a high speed wafer scanning system provides reduced charging of electrically floating semiconductor structures. Data will be presented on implant uniformity, wafer temperature and charging.
I. Introduction
~HAMEER .....
OPERATOR CONTROL VD.U. WAFER LOAOER
The process design objectives of this system development were to provide improved characteristics of wafer temperature control for photoresist survival and low lattice damage, improved uniformity of implanteddose, improved uniformity of depth of channeling, lower wafer charging, low cross-contamination and other contamination, and very low particulates, over a wide dose range down to 1 × 1011/cm2. A second objective was to provide increased throughput with higher beam currents. The first issue to be addressed was that of implantation of devices with beam currents much higher than previously used. It is important to note that much of the published ion implantation work has been done under conditions of poor wafer cooling, and any effect due purely to high current is yet to be revealed. While the beam current is high in this work, the ion current density is actually lower than other systems owing to the large (nominal 6 × 6 cross section) area beam.
2. System description The system uses a fixed ion beam with a unique design mechanically scanned spinning wheel (patent pending). Fig. 1 shows a diagram of the system. Fig. 2 shows a photo of the implantation wheel for 150 mm wafers. A single row of wafers are held individually on water-cooled heat sinks that are tilted out of the plane of the wheel 14 °. This gives a component of centrifugal force normal to the wafer sufficient to hold it against the heat sink without the need for mechanical clamps. The wheel is relatively large so as to provide greatly 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
PROCESSOR CRYO-PUMP$
/
WAFER LOADER ELECTRONICS RACK
SYSTEM :'r POWER UTILITIES~
......
(
PUMP TUllE
SEAM LiNE ELECTRONICS RACK
DIFFUSION PUMP AND BAFFLE CHAMBER
ClllINEl
BEAMLINE
CONTROt MODULE
SOURCE ELECTRONK:S TERMINAL ELECTRONICS RACK
Fig. 1. Schematic diagram of precision implant 9000 system.
improved heat dissipation and temperature control compared to previous ion implanters, to provide high centrifugal force for wafer holding, and to provide low device charging characteristics. The vertical wheel is scanned through the beam horizontally by a scan arm located in the vacuum chamber.
3. Implant angle variation, channeling, and uniformity It is well known that the incident angle of beam to wafer is set to some angle (for instance, 7 o ) in order to reduce channeling. Wafer azimuth control is also effective in reducing channeling, or more specifically the variation in channeling over the wafer [1]. In modem implanters there are several sources of variations over the wafer in beam incident angle perpendicular to the
281
M.T. Wauk / New ion implantation system
"
)/._
¢
fz,
.; JJ
-;
)(
Fig. 2. Photograph of the 9000 implantation wheel for 150 mm wafers. Each wafer heat sink is water cooled. Wafer temperature is < 40°C at 3600 W.
7 o angle. In spinning wheel implanters, where the wafer is tilted out of the plane of the wheel, these are: a) tilt of the wafer normal to 7 o angle as the wheel spins, b) tilt of the wafer normal to 7 ° angle as the wheel is scanned, c) domed heat sink causing variation over the wafer, and d) divergence of beam (ignored here). In scanned-beam systems, these sources are: a) deflection
7
6
o1
_o
Scanned-Beam Implanter (2m Drift)
_~,
"~,,.
~,. ~ ~ . Dom.dw i n ,
of beam in order to scan the wafer, b) domed heat sink, and c) divergence of beam. Fig. 3 shows a comparison of the maximum angle variation in several types of systems. Five cases are shown: a) " s t a n d a r d " 7 ° wheel with domed heat sinks, b) " s t a n d a r d " 0 ° (wafers in plane of wheel) with domed heat sinks, c) a scanned-beam system (2 m drift length) with domed heat sinks, and d, e) Precision Implant 9000 designs of 14 ° (for 7 ° implant) and 7 ° (for 0 ° implant.) D o m e d heat sinks are assumed to have 1 m radius of curvature. Note that the Precision Implant 9000 design gives significantly lower error angle than any other system because of the plane wafer geometry and be-
s 4
ol c
<
3
I.-
S2 k
o 175
7 C~, NO Dome Wheel ~ (0" Implant)
~:====::====:=: 9OOO
Ill
I
I
I
I
II
I
I
250
325
400
475
550
625
700
775
Wheel Radius
A
B
mm
Fig. 3. Comparison of wafer tilt (angle between incident beam and wafer normal, perpendicular to 7 ° angle) during ion implantation, for various implanter designs.
Fig. 4. Sheet resistance contour maps for 120 keV P+ implants with a dose of 1 x 1014/cm 2. 125 mm bare wafers (A): wafer not rotated, 322.5 12/sq. st. dev. = 1.24%, 1.00% contours. (B): wafer rotated 20 ° , 337.3 ~2/sq. st. dev. = 0.57, 1.00% contours. IV. NEW EQUIPMENT AND SYSTEMS
282
M.T. Wauk / New ion implantation system --I --
"~1+
I'Nd
III
•
I
I
I
cause the wheel radius is so large. As in [1], planar channeling effects can be observed in this system, however they appear to be smaller than shown in the reference for other implanters. Fig. 4 shows a sheet resistance profile map of a wafer implanted in this system. The uniformity is indicated in a) a standard deviation value of 1.2%. This is a worst case example. Fig. 4b shows a similar profile with the wafer rotated 20 ° , with standard deviation of 0.5%. The data shown in this study is based on Si (100), 5_15 cm, bare wafers implanted, then annealed in a furnace at 950 or 1100°C for 30 min in N 2. Wafers were then probed using a Prometrix Omnimap 111.
I I II-- -
O4 4
re =
3
•.t
2
0
1.0
E
0.e
a) I--
0.6 0.4
0.3 0.2 n•
0.1
-
+~
-
~
E = 120 K e V
-
R.r = 30 I
II I I I I I I I
30
I
C ~ Wlcm2 I II
100
I I IIII 1000 i
i
B e a m Width x W h e e l Radius, ~m2~
4. Wafer temperature control Fig. 5. Calculated wafer temperature rise per mA of beam current during ion implantation, as function of product of wheel radius and beam width, for various values of slow scan speed. Beam width is along wheel radius.
Wafer temperature is kept very low in the 9000 by specific mechanical design. A model was developed for wafer temperature based on conductive heat transfer.
2600 °o
...%
1950 "-. f/)
E "
O
_ " ; - ~-. ~V-.~ -..'q"=
I = 8mA E = 100KeV
1300
o
650
105
210
315
420
315
420
Channel Number A 2400
1800
C
"
O
1200
Y
o
•~.~
.~:
600
½ 0
105
210 Channel Number B
Fig. 6. RBS spectrum of sample implanted with 100 keV As + to dose of 1.4 × 1016/cm 2. (A) beam current of 8 mA. (B) 29 mA.
283
M.T. Wauk / New ion implantation system
Numerical methods were used based on independent measurements in vacuum of heat sink static thermal resistance. Each point on a wafer sees the beam in pulse bursts of a duration that depends on slow-scan speed. The temperature rises during each burst towards a steady-state value with a time constant approximately given by C,, D R t (approximately 5 s), where C~ is the wafer specific heat, D is the wafer thickness, and R t is the thermal resistance. For the case of zero slow scan speed (U0), steady-state maximum temperature rise can be written as: A T:
IER t 2~rRoD2Q o '
where I is the beam current, E is beam energy, R t is thermal resistance, R 0 is wheel radius, D2 is beam size along wheel radius, and Q0 is electron charge. For the higher scan speeds, the temperature rise is limited to a value much less than the above because the beam is shared over the full scanned area. The curves in fig. 5 show wafer temperature rise in ° C / m A of beam versus the product of beam width and wheel radius. A comparison with other "standard" systems shows that the combination of large wheel, larger beam, and higher scan speed provides 10-20 X reduction in temperature rise per mA. At full power (30 mA at 120 keV) the maximum expected wafer temperature is 38 ° C (assuming 20 ° C coolant). Temperature measurements were made using temperature conversion indicators placed on the rear of the wafers at the edge, and away from the elastomer. Using beams of up to 30 mA at 120 keV, it is repeatedly found that wafer temperature does not exceed 40 ° C. Rutherford backscattering measurements have been done to see if any difference in damage is observable. Fig. 6 shows two RBS spectra for Arsenic, 100 keV, 1.4 × 1016 implants carried out with 8 mA and 29 mA beam current. No difference can be observed in the amorphous region channeled spectrum for the two cases.
A
Fig. 7. Sheet resistance contour maps. 125 mm bare wafers (A) 75 keV As + dose of 1 × 1014/cm2. st. dev. = 0.30%, 1% contours. (B) 90 keV B÷ with dose of 1 × 1016/cm2. st. dev. = 0.18%, 1% contours.
~ 2 . 3'01 1 X 10 '3 0.74% 2.6 . ~ o ~ & ~ = 2.4
1.2
i
5 X 1013 o.,o%
O.15 0.4 0.
3 X 10" 0.09% I
I
I
t
I
303234363840
I x
I
I
I
I
x 424446485052
I
I x
I
I
I
I
x 858789919498
I x
Run Number Wafer One
+ Wafer Two
Fig. 8. Measured sheet resistance repeatability for successive runs of 80 keV B+. (A) dose of 1 x 1013/cm2. st. dev. = 0.74% (B) dose of 5 × 1013/cm2, st. dev. = 0.40%. (C) dose of 3 × 1014/cm2, st. dev. = 0.69%.
100 E
10
uniformity Q.
Implantation uniformity is quite good in this system. Four-point probe measurements across a wafer diameter in the fast spin direction generally give sigma values of 0.1% to 0.2%. Fig. 7 (a and b), show uniformity profile maps for wafers implanted in the 9000. The system is capable of doses as low as 1 x 10 ll. Fig. 8 shows run-to-run repeatability for three series of repeated implants at 1 X 1013 and 5 × 1013 and 3 × 1014. Standard deviation values of r u n - r u n variation for these two cases were found to be 0.74, 0.40 and 0.69%, respectively.
80 KeV B +
g 2.2
d 5. Wafer
B
mE tO
1.0
~
S
4) o.
0.1 0.1
1 10 100 "Standard" Implanter Current, mA
Fig. 9. Beam current for equivalent device yield compared with other "standard" systems in production. • with flood gun; O without flood gun. IV. NEW EQUIPMENT AND SYSTEMS
284
M.T. Wauk / New ion implantation system
6. Charging For design reasons similar to those explained above for temperature control (especially in regard to beam width and wheel radius) plus high rate, the charging behavior of this system is similarly favorable. Fig. 9 shows data collected over a period of time from customer tests comparing the maximum current that can be used in the Precision Implant 9000 system to achieve the customer's standard yields, with the current they use in their present systems to achieve the same results. Charging damage of floating-gate structures appears to be equivalent at approximately three to six times higher current in this system.
charging. Data collected to date supports the conclusion that the design was successful in achieving significant improvements in these areas. This paper covers a few aspects of wide range, far reaching, professional, and personally rewarding five year project to which many people at Applied Materials contributed. The author is greatly indebted to Mr. Len Robinson, with whom the target system wafer cooling was designed; Dr. Steve Moffatt, who carried out much of the early process feasibility work and prototype implantation evaluations; Dr. Babak Adibi, who did the more recent wafer uniformity measurements shown in this paper; and Dr. Sarko Cherekjian, who provided the charging, RBS and temperature test data.
7. Summary The design criteria of this system development effort were specifically chosen to address and solve major implantation issues o wafer temperature control, implant angle variation, channeling, dose uniformity, and
Reference
[1] M.I. Current, N.L. Turner, T.C. Smith and D. Crane, Proc. 5th Int. Conf. Ion Implantation Equipment and Techniques, Nucl. Instr. and Meth. B6 (1985) 336.
Nuclear Instruments and.Methods in Physics Research B21 (1987) 280-284 North-Holland. Amsterdam
NEW ION IMPLANTATION SYSTEM WITH ADVANCED PROCESS CAPABILITIES M.T. W A U K Applied Materials Implant Division, Horsham, West Sussex, England RHI3 5PY
A new ion implantation system has been developed with several significant IC wafer processing improvements resulting from innovations in beam optics, target system, wafer handling, and control electronics design. This paper will concentrate on the process results that have been established thus far with this new system. Implanted wafers are subjected to temperatures of less than approximately 40 ° C even with beams of 30 mA at 120 keV. This significantly reduces residual lattice damage due to partial annealing, and also significantly reduces photoresist blistering and flow. Wafers are completely flat and planar during implant, providing a constant implant angle and reduced uniformity and depth profile variations due to channeling, as well as providing more uniform wafer temperature. Low ion beam density and a high speed wafer scanning system provides reduced charging of electrically floating semiconductor structures. Data will be presented on implant uniformity, wafer temperature and charging.
I. Introduction
~HAMEER .....
OPERATOR CONTROL VD.U. WAFER LOAOER
The process design objectives of this system development were to provide improved characteristics of wafer temperature control for photoresist survival and low lattice damage, improved uniformity of implanteddose, improved uniformity of depth of channeling, lower wafer charging, low cross-contamination and other contamination, and very low particulates, over a wide dose range down to 1 × 1011/cm2. A second objective was to provide increased throughput with higher beam currents. The first issue to be addressed was that of implantation of devices with beam currents much higher than previously used. It is important to note that much of the published ion implantation work has been done under conditions of poor wafer cooling, and any effect due purely to high current is yet to be revealed. While the beam current is high in this work, the ion current density is actually lower than other systems owing to the large (nominal 6 × 6 cross section) area beam.
2. System description The system uses a fixed ion beam with a unique design mechanically scanned spinning wheel (patent pending). Fig. 1 shows a diagram of the system. Fig. 2 shows a photo of the implantation wheel for 150 mm wafers. A single row of wafers are held individually on water-cooled heat sinks that are tilted out of the plane of the wheel 14 °. This gives a component of centrifugal force normal to the wafer sufficient to hold it against the heat sink without the need for mechanical clamps. The wheel is relatively large so as to provide greatly 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
PROCESSOR CRYO-PUMP$
/
WAFER LOADER ELECTRONICS RACK
SYSTEM :'r POWER UTILITIES~
......
(
PUMP TUllE
SEAM LiNE ELECTRONICS RACK
DIFFUSION PUMP AND BAFFLE CHAMBER
ClllINEl
BEAMLINE
CONTROt MODULE
SOURCE ELECTRONK:S TERMINAL ELECTRONICS RACK
Fig. 1. Schematic diagram of precision implant 9000 system.
improved heat dissipation and temperature control compared to previous ion implanters, to provide high centrifugal force for wafer holding, and to provide low device charging characteristics. The vertical wheel is scanned through the beam horizontally by a scan arm located in the vacuum chamber.
3. Implant angle variation, channeling, and uniformity It is well known that the incident angle of beam to wafer is set to some angle (for instance, 7 o ) in order to reduce channeling. Wafer azimuth control is also effective in reducing channeling, or more specifically the variation in channeling over the wafer [1]. In modem implanters there are several sources of variations over the wafer in beam incident angle perpendicular to the
281
M.T. Wauk / New ion implantation system
"
)/._
¢
fz,
.; JJ
-;
)(
Fig. 2. Photograph of the 9000 implantation wheel for 150 mm wafers. Each wafer heat sink is water cooled. Wafer temperature is < 40°C at 3600 W.
7 o angle. In spinning wheel implanters, where the wafer is tilted out of the plane of the wheel, these are: a) tilt of the wafer normal to 7 o angle as the wheel spins, b) tilt of the wafer normal to 7 ° angle as the wheel is scanned, c) domed heat sink causing variation over the wafer, and d) divergence of beam (ignored here). In scanned-beam systems, these sources are: a) deflection
7
6
o1
_o
Scanned-Beam Implanter (2m Drift)
_~,
"~,,.
~,. ~ ~ . Dom.dw i n ,
of beam in order to scan the wafer, b) domed heat sink, and c) divergence of beam. Fig. 3 shows a comparison of the maximum angle variation in several types of systems. Five cases are shown: a) " s t a n d a r d " 7 ° wheel with domed heat sinks, b) " s t a n d a r d " 0 ° (wafers in plane of wheel) with domed heat sinks, c) a scanned-beam system (2 m drift length) with domed heat sinks, and d, e) Precision Implant 9000 designs of 14 ° (for 7 ° implant) and 7 ° (for 0 ° implant.) D o m e d heat sinks are assumed to have 1 m radius of curvature. Note that the Precision Implant 9000 design gives significantly lower error angle than any other system because of the plane wafer geometry and be-
s 4
ol c
<
3
I.-
S2 k
o 175
7 C~, NO Dome Wheel ~ (0" Implant)
~:====::====:=: 9OOO
Ill
I
I
I
I
II
I
I
250
325
400
475
550
625
700
775
Wheel Radius
A
B
mm
Fig. 3. Comparison of wafer tilt (angle between incident beam and wafer normal, perpendicular to 7 ° angle) during ion implantation, for various implanter designs.
Fig. 4. Sheet resistance contour maps for 120 keV P+ implants with a dose of 1 x 1014/cm 2. 125 mm bare wafers (A): wafer not rotated, 322.5 12/sq. st. dev. = 1.24%, 1.00% contours. (B): wafer rotated 20 ° , 337.3 ~2/sq. st. dev. = 0.57, 1.00% contours. IV. NEW EQUIPMENT AND SYSTEMS
282
M.T. Wauk / New ion implantation system --I --
"~1+
I'Nd
III
•
I
I
I
cause the wheel radius is so large. As in [1], planar channeling effects can be observed in this system, however they appear to be smaller than shown in the reference for other implanters. Fig. 4 shows a sheet resistance profile map of a wafer implanted in this system. The uniformity is indicated in a) a standard deviation value of 1.2%. This is a worst case example. Fig. 4b shows a similar profile with the wafer rotated 20 ° , with standard deviation of 0.5%. The data shown in this study is based on Si (100), 5_15 cm, bare wafers implanted, then annealed in a furnace at 950 or 1100°C for 30 min in N 2. Wafers were then probed using a Prometrix Omnimap 111.
I I II-- -
O4 4
re =
3
•.t
2
0
1.0
E
0.e
a) I--
0.6 0.4
0.3 0.2 n•
0.1
-
+~
-
~
E = 120 K e V
-
R.r = 30 I
II I I I I I I I
30
I
C ~ Wlcm2 I II
100
I I IIII 1000 i
i
B e a m Width x W h e e l Radius, ~m2~
4. Wafer temperature control Fig. 5. Calculated wafer temperature rise per mA of beam current during ion implantation, as function of product of wheel radius and beam width, for various values of slow scan speed. Beam width is along wheel radius.
Wafer temperature is kept very low in the 9000 by specific mechanical design. A model was developed for wafer temperature based on conductive heat transfer.
2600 °o
...%
1950 "-. f/)
E "
O
_ " ; - ~-. ~V-.~ -..'q"=
I = 8mA E = 100KeV
1300
o
650
105
210
315
420
315
420
Channel Number A 2400
1800
C
"
O
1200
Y
o
•~.~
.~:
600
½ 0
105
210 Channel Number B
Fig. 6. RBS spectrum of sample implanted with 100 keV As + to dose of 1.4 × 1016/cm 2. (A) beam current of 8 mA. (B) 29 mA.
283
M.T. Wauk / New ion implantation system
Numerical methods were used based on independent measurements in vacuum of heat sink static thermal resistance. Each point on a wafer sees the beam in pulse bursts of a duration that depends on slow-scan speed. The temperature rises during each burst towards a steady-state value with a time constant approximately given by C,, D R t (approximately 5 s), where C~ is the wafer specific heat, D is the wafer thickness, and R t is the thermal resistance. For the case of zero slow scan speed (U0), steady-state maximum temperature rise can be written as: A T:
IER t 2~rRoD2Q o '
where I is the beam current, E is beam energy, R t is thermal resistance, R 0 is wheel radius, D2 is beam size along wheel radius, and Q0 is electron charge. For the higher scan speeds, the temperature rise is limited to a value much less than the above because the beam is shared over the full scanned area. The curves in fig. 5 show wafer temperature rise in ° C / m A of beam versus the product of beam width and wheel radius. A comparison with other "standard" systems shows that the combination of large wheel, larger beam, and higher scan speed provides 10-20 X reduction in temperature rise per mA. At full power (30 mA at 120 keV) the maximum expected wafer temperature is 38 ° C (assuming 20 ° C coolant). Temperature measurements were made using temperature conversion indicators placed on the rear of the wafers at the edge, and away from the elastomer. Using beams of up to 30 mA at 120 keV, it is repeatedly found that wafer temperature does not exceed 40 ° C. Rutherford backscattering measurements have been done to see if any difference in damage is observable. Fig. 6 shows two RBS spectra for Arsenic, 100 keV, 1.4 × 1016 implants carried out with 8 mA and 29 mA beam current. No difference can be observed in the amorphous region channeled spectrum for the two cases.
A
Fig. 7. Sheet resistance contour maps. 125 mm bare wafers (A) 75 keV As + dose of 1 × 1014/cm2. st. dev. = 0.30%, 1% contours. (B) 90 keV B÷ with dose of 1 × 1016/cm2. st. dev. = 0.18%, 1% contours.
~ 2 . 3'01 1 X 10 '3 0.74% 2.6 . ~ o ~ & ~ = 2.4
1.2
i
5 X 1013 o.,o%
O.15 0.4 0.
3 X 10" 0.09% I
I
I
t
I
303234363840
I x
I
I
I
I
x 424446485052
I
I x
I
I
I
I
x 858789919498
I x
Run Number Wafer One
+ Wafer Two
Fig. 8. Measured sheet resistance repeatability for successive runs of 80 keV B+. (A) dose of 1 x 1013/cm2. st. dev. = 0.74% (B) dose of 5 × 1013/cm2, st. dev. = 0.40%. (C) dose of 3 × 1014/cm2, st. dev. = 0.69%.
100 E
10
uniformity Q.
Implantation uniformity is quite good in this system. Four-point probe measurements across a wafer diameter in the fast spin direction generally give sigma values of 0.1% to 0.2%. Fig. 7 (a and b), show uniformity profile maps for wafers implanted in the 9000. The system is capable of doses as low as 1 x 10 ll. Fig. 8 shows run-to-run repeatability for three series of repeated implants at 1 X 1013 and 5 × 1013 and 3 × 1014. Standard deviation values of r u n - r u n variation for these two cases were found to be 0.74, 0.40 and 0.69%, respectively.
80 KeV B +
g 2.2
d 5. Wafer
B
mE tO
1.0
~
S
4) o.
0.1 0.1
1 10 100 "Standard" Implanter Current, mA
Fig. 9. Beam current for equivalent device yield compared with other "standard" systems in production. • with flood gun; O without flood gun. IV. NEW EQUIPMENT AND SYSTEMS
284
M.T. Wauk / New ion implantation system
6. Charging For design reasons similar to those explained above for temperature control (especially in regard to beam width and wheel radius) plus high rate, the charging behavior of this system is similarly favorable. Fig. 9 shows data collected over a period of time from customer tests comparing the maximum current that can be used in the Precision Implant 9000 system to achieve the customer's standard yields, with the current they use in their present systems to achieve the same results. Charging damage of floating-gate structures appears to be equivalent at approximately three to six times higher current in this system.
charging. Data collected to date supports the conclusion that the design was successful in achieving significant improvements in these areas. This paper covers a few aspects of wide range, far reaching, professional, and personally rewarding five year project to which many people at Applied Materials contributed. The author is greatly indebted to Mr. Len Robinson, with whom the target system wafer cooling was designed; Dr. Steve Moffatt, who carried out much of the early process feasibility work and prototype implantation evaluations; Dr. Babak Adibi, who did the more recent wafer uniformity measurements shown in this paper; and Dr. Sarko Cherekjian, who provided the charging, RBS and temperature test data.
7. Summary The design criteria of this system development effort were specifically chosen to address and solve major implantation issues o wafer temperature control, implant angle variation, channeling, dose uniformity, and
Reference
[1] M.I. Current, N.L. Turner, T.C. Smith and D. Crane, Proc. 5th Int. Conf. Ion Implantation Equipment and Techniques, Nucl. Instr. and Meth. B6 (1985) 336.