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Computer simulations in the design of ion beam deflection systems
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
Research
BlO/ll
817
(1985) 817-821
COMPUTER SlMULATIONS IN THE DESIGN OF ION BEAM DEFLECTION SYSTEMS J.E. BOER&
R.V. BRICK
Varran / Extrion
Division. Blackburn Industrial
and D.L. HUTTON Park,
Gloucester, MA 01930,
USA
Computer codes for the simulation of ion beams and Laplace fields have been employed for the analysis of ion implanter deflection systems. As implanter energies have increased to over 400 kV the deflection systems needed to scan the beam over ever larger wafers with more stringent implant uniformity requirements have become rather large and difficult to design. The computer codes have permitted the optimization of these systems to minimize both the size and distortions while maximizing the deflections obtained. The simulations have led to the design of optimized deflection systems utilizing straight or curved electrodes which are shorter and in most cases easier to manufacture. Design iterations can be evaluated in hours on the computer rather than weeks required for the precision laboratory testing to obtain equivalent information.
1. Introduction
neling in the wafer and prevent high energy neutral particles in the beam from getting to the wafer while keeping equipment overall size within reason. Obtaining the required deflections at energies below 200 kV has been routine, but with energies of 400 kV the horizontal deflection plates become quite large and must be either bent or curved to permit use of existing scan supplies and amplifiers. The vertical deflection can usually be obtained with relatively short flat or nearly flat plates using existing scan supplies and amplifiers due to the relatively low deflection angle requirements. Field uniformity between the plates is also important
A typical deflection system for a moderate current implanter system is shown in fig. 1. As in most ion implanters the ion beam is typically offset at a 7” angle (usually in the horizontal plane) when it is at the center of the wafer. This permits two wafers to be in the scanning positions at + 7” or -7”. The beam is typically scanned as much as 3’ to either side and up and down from the center of the wafer in a raster to produce uniform implantation in a region up to 150 mm in diameter. The 7’ angle was selected to minimize chan-
NEUTRAL BEAM Y
/ SCAN OFFSET ANGLE 0, 7. , /‘I’
/’ -WAFER
HORIZONTAL
(;i,
A’
-
I ..:\-,_,’
;
TARGET SURFACE
VERTICAL SCAN ANGLE
IOti BEAM
Fig. 1. Ion beam scanning
of wafer using electrostatic
deflection
0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
plates.
VIII. ACCELERATOR TECHNOLOGY
to assure scan linearity and thus control dose uniformity, This is especially important in the horizontal deflection plates, since the beam has already been deflected in the vertical plane and another dimension has been introduced to the system. The Laplace simuiation is carried out in both horizontal and vertical systems to assure linearity in both planes. The fields at the ends of the plates are also investigated for their effects on distortions within the beam which show up in the emittance plots taken at the wafer plane.
2. The computer programs The programs employed in these simulations were developed for the analysis of particle beams in either axisymmetric or rectangular matrix configurations. SNOW [l] is used for the simulation of the ion beam passing through the deflection plates. and FEARS [2] is employed for the investigation of the uniformity of the fiefds between the plates. Both programs employ basic relaxation techniques to solve either Poisson’s or Laplace’s equation in two dimensions each at right angles to the other to obtain a three dimensional extrapolation.
3. The deflection system simulations The first simulation shown is for the deflection system used on a Varian/Extrion ion implanter (with up to a 200 kV beam) which has been in use for many years. The horizontal deflection plates are shown in fig. 2. Here the beam is seen to scrape the plates at this extreme voltage level, a fact that has been observed from damage on the plates. Even at this voltage level. the horizontal deflection plates are necessarily bent in order to obtain the deflection required. The quality of the beam can be seen in fig. 3 where the emittance plots at entrance and exit planes are shown. The particles are not as uniformly distributed in the y-y’ plot as they were at the input plane. This nonuniformity could contribute to variations in the implant uniformity in the wafer. An initial deflection system for the 400 kV beam is shown in fig. 4. Here the plates are bent in order to obtain maximum deflection in the shortest possible length. In the horizontal plane visible distortion of the ion beam takes place as the beam passes near, indeed scrapes, electrodes. The final horizontal and vertical design arrived at is seen in fig. 5; where the plates, the gaps between them, and the apertures have all been reduced. The horizontal emittance plot for this final design is shown in fig. 6, where the relative uniformity of the ion beam has been maintained throughout the deflection system.
819
VIII. ACCELERATOR
TECHNOL~Y
Fig. 5. Final design for 400 kV horizontal
deflection
-_----
system in horizontal
x
AND
____--
planes.
EQUIPOTENTIALS
-____
(top) and vertical (bottom)
TRAJECTORIES
_ ._
__:: _J
823
EMITTClNCE ,RMS
Emittance
~0.054
PLOT
EQiJIPOTENTiALS
pitmmtmrad
yjlmml Fig. 6. Emittance plot at exit plane for 400 kV deflection system.
Fig. 7. Representative Laplace simulation for deflection system cross section. The cross sections of the deflection electrodes are simulated with the Laplace simulation code. Fig. 7 shows such a cross section. The equipotentials should be parallel and uniformly spaced over the region between the plates through which the beam will pass. Here they are seen to be nearly perfect over the central two-thirds of the region.
tion systems needed for high uniformity implantation systems, The ability to see details of the effects of electrode changes on the deflected beam permits accurate designs which could not be obtained in any other way without prohibitively costly and time consuming experimentation and analysis.
4. Conclusions
References
The use of computer programs for the simulation of ion beams has been shown to be a powerful tool in the design of high energy deflection systems. The programs permit the detailed analysis needed to design minimum Iength, minimum djstortio~, yet highly accurate deflec-
[l] J.E. Beers, SNOW - A Digital Computer Program for the Simulation of Ion Beam Devices, SAND79-1027. Sandia Laboratories, Albuquerque, New Mexico 87185 (Dec. 1982). [Z] J.E. Beers, Proc. 11th Symp. on Electron, Ion, and Laser Beam Technology, ed., R.F.M. Thomley (San Francisco Press, San Francisco, CA 1971) p. 167.
VIII. ACCELERATOR TECHNOLOGY
in Physics
Research
BlO/ll
817
(1985) 817-821
COMPUTER SlMULATIONS IN THE DESIGN OF ION BEAM DEFLECTION SYSTEMS J.E. BOER&
R.V. BRICK
Varran / Extrion
Division. Blackburn Industrial
and D.L. HUTTON Park,
Gloucester, MA 01930,
USA
Computer codes for the simulation of ion beams and Laplace fields have been employed for the analysis of ion implanter deflection systems. As implanter energies have increased to over 400 kV the deflection systems needed to scan the beam over ever larger wafers with more stringent implant uniformity requirements have become rather large and difficult to design. The computer codes have permitted the optimization of these systems to minimize both the size and distortions while maximizing the deflections obtained. The simulations have led to the design of optimized deflection systems utilizing straight or curved electrodes which are shorter and in most cases easier to manufacture. Design iterations can be evaluated in hours on the computer rather than weeks required for the precision laboratory testing to obtain equivalent information.
1. Introduction
neling in the wafer and prevent high energy neutral particles in the beam from getting to the wafer while keeping equipment overall size within reason. Obtaining the required deflections at energies below 200 kV has been routine, but with energies of 400 kV the horizontal deflection plates become quite large and must be either bent or curved to permit use of existing scan supplies and amplifiers. The vertical deflection can usually be obtained with relatively short flat or nearly flat plates using existing scan supplies and amplifiers due to the relatively low deflection angle requirements. Field uniformity between the plates is also important
A typical deflection system for a moderate current implanter system is shown in fig. 1. As in most ion implanters the ion beam is typically offset at a 7” angle (usually in the horizontal plane) when it is at the center of the wafer. This permits two wafers to be in the scanning positions at + 7” or -7”. The beam is typically scanned as much as 3’ to either side and up and down from the center of the wafer in a raster to produce uniform implantation in a region up to 150 mm in diameter. The 7’ angle was selected to minimize chan-
NEUTRAL BEAM Y
/ SCAN OFFSET ANGLE 0, 7. , /‘I’
/’ -WAFER
HORIZONTAL
(;i,
A’
-
I ..:\-,_,’
;
TARGET SURFACE
VERTICAL SCAN ANGLE
IOti BEAM
Fig. 1. Ion beam scanning
of wafer using electrostatic
deflection
0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
plates.
VIII. ACCELERATOR TECHNOLOGY
to assure scan linearity and thus control dose uniformity, This is especially important in the horizontal deflection plates, since the beam has already been deflected in the vertical plane and another dimension has been introduced to the system. The Laplace simuiation is carried out in both horizontal and vertical systems to assure linearity in both planes. The fields at the ends of the plates are also investigated for their effects on distortions within the beam which show up in the emittance plots taken at the wafer plane.
2. The computer programs The programs employed in these simulations were developed for the analysis of particle beams in either axisymmetric or rectangular matrix configurations. SNOW [l] is used for the simulation of the ion beam passing through the deflection plates. and FEARS [2] is employed for the investigation of the uniformity of the fiefds between the plates. Both programs employ basic relaxation techniques to solve either Poisson’s or Laplace’s equation in two dimensions each at right angles to the other to obtain a three dimensional extrapolation.
3. The deflection system simulations The first simulation shown is for the deflection system used on a Varian/Extrion ion implanter (with up to a 200 kV beam) which has been in use for many years. The horizontal deflection plates are shown in fig. 2. Here the beam is seen to scrape the plates at this extreme voltage level, a fact that has been observed from damage on the plates. Even at this voltage level. the horizontal deflection plates are necessarily bent in order to obtain the deflection required. The quality of the beam can be seen in fig. 3 where the emittance plots at entrance and exit planes are shown. The particles are not as uniformly distributed in the y-y’ plot as they were at the input plane. This nonuniformity could contribute to variations in the implant uniformity in the wafer. An initial deflection system for the 400 kV beam is shown in fig. 4. Here the plates are bent in order to obtain maximum deflection in the shortest possible length. In the horizontal plane visible distortion of the ion beam takes place as the beam passes near, indeed scrapes, electrodes. The final horizontal and vertical design arrived at is seen in fig. 5; where the plates, the gaps between them, and the apertures have all been reduced. The horizontal emittance plot for this final design is shown in fig. 6, where the relative uniformity of the ion beam has been maintained throughout the deflection system.
819
VIII. ACCELERATOR
TECHNOL~Y
Fig. 5. Final design for 400 kV horizontal
deflection
-_----
system in horizontal
x
AND
____--
planes.
EQUIPOTENTIALS
-____
(top) and vertical (bottom)
TRAJECTORIES
_ ._
__:: _J
823
EMITTClNCE ,RMS
Emittance
~0.054
PLOT
EQiJIPOTENTiALS
pitmmtmrad
yjlmml Fig. 6. Emittance plot at exit plane for 400 kV deflection system.
Fig. 7. Representative Laplace simulation for deflection system cross section. The cross sections of the deflection electrodes are simulated with the Laplace simulation code. Fig. 7 shows such a cross section. The equipotentials should be parallel and uniformly spaced over the region between the plates through which the beam will pass. Here they are seen to be nearly perfect over the central two-thirds of the region.
tion systems needed for high uniformity implantation systems, The ability to see details of the effects of electrode changes on the deflected beam permits accurate designs which could not be obtained in any other way without prohibitively costly and time consuming experimentation and analysis.
4. Conclusions
References
The use of computer programs for the simulation of ion beams has been shown to be a powerful tool in the design of high energy deflection systems. The programs permit the detailed analysis needed to design minimum Iength, minimum djstortio~, yet highly accurate deflec-
[l] J.E. Beers, SNOW - A Digital Computer Program for the Simulation of Ion Beam Devices, SAND79-1027. Sandia Laboratories, Albuquerque, New Mexico 87185 (Dec. 1982). [Z] J.E. Beers, Proc. 11th Symp. on Electron, Ion, and Laser Beam Technology, ed., R.F.M. Thomley (San Francisco Press, San Francisco, CA 1971) p. 167.
VIII. ACCELERATOR TECHNOLOGY