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High energy implantation by ion projection
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 41/42 (1998) 257-260
High Energy Implantation by Ion Projection J. Meijer and A.Stephan Physik mit Ionenstrahlen Ruhr-Universit/it Bochum, 44780 Bochum, Germany A new type of ion projector is proposed to structurize a wafer at a resolution below 200 nm reducing the typically used lithography steps to a large extent. Basically, this technique aims to project a mask onto a wafer using an ion optical lens system allowing a demagnification up to 1:50 for ions with energies between 100 keV and some MeV. The center piece of this device is a single magnetic lens consisting of a superconducting solenoid. The achievable lateral resolution depends on lens errors, stability of the stencil mask and straggling of the implanted ions. A variation of the implantation energy would allow three dimensional structurization.
1. I n t r o d u c t i o n Recently, industries asks for ion implanters for two different applications: Very low energy implanters with energies in the range of a few keV are required for implantations at a depth of a few 10 nm as requested for the 0.3 #m technology. High energy implanters are necessary to produce buried layers or retrograded wells. In both energy ranges focused ion beams are suited to replace several conventional mask process steps, but they are used only occasionally up to now in industrial production processes. The main reason restraining the industrial operation of maskless implantation using microbeams is the sequential nature of the writing process. Thus, writing of large structures is very time consuming. A solution to overcome this problem is to project a stencil mask illuminated by ions onto the substrate. A well known version of such an implanter is based on electrostatic lenses [1]. The processed field area is in the range of a few mm 2, the projector is able to focus ions of 50 keV with a demagnification factor of 5-10 [2]. Due to the low demagnification factor, the stencil mask must have a very fine pattern (openings 0.5-1tim). These structures can be produced using thin material only, thus reducing the mechanical stability and thermal conductivity of the mask itself. Ions with energies in the MeV range may traverse the mask material thus producing an unwanted background implantation. We propose a new concept to focus energetic ions,
0167-9317/98/$19.00© ElsevierScienceB.~ All rightsreserved. Plh S0167-9317(98)00059-8
allowing imaging of the mask at a demagnification factor in the range of 1:20 - 1:50. As ions with energies up to MeV cannot be focused using electrostatic lenses, a single superconducting solenoid having superior ion optical properties is used. A single beam focus on the target avoids beam broadening effects due to space charges, a problem frequently occurring in low energy devices. The partial imaging of the stencil mask in combination with an electrostatic multipole system allows the correction of non axial aberrations. The project developed in the following is based on experiences with a MeV microprobe [3] using a superconducting solenoid as final lens.
2. I o n p r o j e c t o r u s i n g a s u p e r c o n d u c t i n g solenoid
A schematic view of the proposed ion projector is shown in fig 1. The beam of energetic ions (1) enters the system through a defining aperture (2) with a diameter of typically 2 mm. A first set of deflection plates (3) scans the beam on the stem cil mask (4). As the beam should hit the mask perpendicularly a second set of deflection plates (5) serves to that effect. Behind the mask a third set of plates (6) deflects the beam to the center of the solenoid lens (7) thus avoiding large aberrations. A diaphragm (9) in the center of the solenoid is implemented to define the divergence angle. This aperture might be split in several segments to allow the measurement of the beam ira-
258
J Mei]er, A. Stephan / Microelectronic Engineering 41/42 (1998) 257-260
I
!i
ion projector prototype under construction in this laboratory. Thus in the following all calculations are based on the well known ion optical properties of this lens. 2.1. Lateral r e s o l u t i o n l i m i t s The achievable resolution of the projector system is mainly limited by the following factors:
Figure 1. Schematic view of the high energy ion projector.
• axial and parasitic aberrations, • non axial aberrations, • lateral straggling.
pinging to each segment. This would help to align the beam to the lens center. The beam is focused on the target (10) by a superconducting solenoid, assisted by an electrostatic multipole system (8) allowing active correction of non axial imaging errors. The multipole unit has a length of 20cm and an inner diameter of 3 cm. The eight poles and the scanning units are independently driven by a computer controlled power supply (11). The main optical parameters of the system in first order are defined by object length, image length and the focusing power of the solenoid. Object length and image length yield the demagnification factor. To achieve a high demagnification of the mask and to reduce lens aberrations a small image length is advantageous, resonable values being in the range of a few 10 cm. A system with an object length of 6 m and an image length of 10-30 cm results in demagnification factors of 20-50. The focusing power depends on width and strength of the magnetic lens field. The field width determines in first order the minimum focal length. Under the assumption that the field distribution can be described by the Glaser's bell shape formula [4], the focusing power of a 16 T field of 6 cm width can be calculated to 80 u MeV/q 2 for a focal length f = 30cm or 25 u MeV/q 2 for f = 10cm, where u is the atomic mass unit and q the charge of the ion. The lens of the present microprobe has a field width of 6 cm and achieves a maximum field strength of 8 T resulting in a focusing power of 20 u MeV/q 2 for f -=- 30 cm and 10 u MeV/q 2 for f = 10 cm. Such a lens will be installed in the setup of an
In third order the axial aberrations are the axial astigmatism, the chromatic aberration and the spherical aberration. The axial astigmatism stems from minute inaccuracies of the lens winding. For the present solenoid this effect produces a field error of -~- = 14 ppm, measured in a circle of 5 mm radius around the field axis. This results in an additional quadrupole component, causing an axial astigmatism of 1.8#m for a beam with a divergence angle of 7 = 0.1 mrad and a focus length of f = 30 cm. Using a stigmator the axial astigmatism is easily corrected. Contrary the correction of the chromatic and spherical aberration is a very difficult task [5]. To minimize these aberrations one needs an accelerator with a very well defined energy (small AE) and a lens with low spherical aberration coefficient. For an accelerator with - ~ ~ 2 x 10 -5 the expected lens error induced by axial chromatic aberration is Ad = 30nm. The spherical aberration of the present lens system is found to be Ad ---- 15nm (same divergence and focal length as above). Under the condition of high demagnification the ion optical resolution is mainly affected by nonaxial aberrations. Fig. 2 shows the contributions of different non-axial aberrations versus the axial distance at the image position, calculated for a beam with incidence angle 70 -- 0.1mrad, an object length of 6.1m and an image length of 30 cm. The isotropic and anisotropic non axial aberrations are summarized to one number [4]. The main contributions are the field curvature and the astigmatism. The distortion is independent of 70 and does not affect the resolution but
I Meijer, A. Stephan/Microelectronic Engineering 41/42 (1998) 257-260
10" asttgrnatisrn
._----- .J--
400
, , ~ , curvature
+ field
7
-~- 55CI z
//f
--
transv
chromatic
'7
7~
/l'
0
" . . . .
i
-
,
\s
-
P ,/
~'
so L z :-~ o 100 L ~2 ~: 5 0
distortion
!/
{lO 2 ~ . . . . .
-
~r 200 -
10 ° 10 ]
~
&sco
coma
= 2so
I0'
~-'-"-
259
d
0 2
4 6 ~x:s rhst~.~ce from]
8
I0
Figure 2. Dependence of various non-axial aberration diameters on the axis distance for an image length of 30 cm and an object length of 6.1 m. The calculations are performed under the condition that the beam passes through the center of the lens with a divergence angle of "f = 0.1 mrad. The energy width of the beam is assumed to A E / E = 10 -s.
the position of the beam, the coma being negligible at % = 0.1 mrad. The transverse chromatic aberration is proportional to the energy resolution A E / E of the beam and results in a deviation of the beam in one direction. This kind of lens error will be reduced, if the ion projector is served by an accelerator of small energy width. The calculation assumes an energy width of - ~ = 10 -5, Fig. 2 demonstrates that an axis distance on the target of more than l m m results in huge aberrations. Fortunately, these aberrations can be corrected as will be shown in the next section. However, the physical limit for lateral structuring by implantation is given by lateral beam straggling. Fig. 3 shows the expected lateral straggling of B, P and As implanted in Si versus the ion range calculated by TRIM [6]. The implantation of e.g. 1 MeV P ions at an implantation depth of 1.2 #m corresponds to a lateral straggling radius of about 200 nm. For projectile energies below 400 keV (implantation depth below 600 nm) the expected straggling is about 100 nm.
. . . . . . . . . . . 0 500 '000 ])1" , j ~ ( t ( ' / I
. . . . . . . 1500 2000 2500 lOll'A(' [ till]
SO00
Figure 3. Projected ion range versus lateral straggling radius for B,P and As-Ions in Si, calculated using the TRIM code.
2.2. C o r r e c t i o n of a b e r r a t i o n s u s i n g a n electrostatic multipole s y s t e m As the beam is scanned over the stencil mask the beam position in the image plane is always known. This gives the possibility to correct nonaxial lens aberrations by adding specific multipole components to the focusing field. The field curvature is caused by the effect that the focal point of non-axial rays is not in the image plane. This can be compensated for using the defocusing effect of an additional rotational symmetric electrostatic field superimposed to the focusing magnetic field. (This kind of correction is possible, if the ion beam is divergent inside the multipole unit.) The isotropic and anisotropic distortion can be corrected by additional dipole components and the isotropic and anisotropic astigmatism are corrected by quadrupole fields. An additional spherical aberration, produced by a deflection of the beam, can compensate the isotropic and anisotropic coma [7]. In the proposed setup of the ion projector the coma would be very small and a correction is not necessary. The correction of the transverse chromatic aberration is important, if the energy width ( A E / E ) of the beam is larger than 10 -4 . Such correction could be performed using a combination of magnetic and elec-
I Meij'er, A. Stephan/Microelectronic Engineering 41/42 (1998) 257-260
260
500
,
, , , ,
400~
•
~%
•
i
300 = o
,
-
2oo
J
100
0
1
2
3
axis distance [mm]
tern is able to correct the non-axial aberration of the lens thus resulting in a beam resolution better than 100 nm for a field of 8 mm x 8 mm exposed simultaneously at a demagnification of 20. The energy mass product at this image length is 20 u MeV/q 2. For implantation energies in the range of a few MeV, the lateral resolution of implanted structures is limited by lateral straggling. The setup allows a fast and easy change of the implantation energy, so that three dimensional structures become feasible.
4
Figure 4. Expected aberration diameters versus the axis distance using different correction fields produced by a multipole device. The calculations are performed assuming an ion projector with an object length of 6.1 m and an image length of 30 cm. The line marked by * shows the expected uncorrected aberration, x correction of the field curvature, A additional correction of the astigmatism and + additional correction of the coma.
trostatic dipole fields (so-called anti-Wien filter). Fig. 4 shows the expected aberrations assuming different multipole field components for an ion beam with a divergence angle of 0.1 mrad, an object length of 6.1 m and an image length of 30 cm. It is evident that an aberration of 500 nm can be reduced to less than 100 nm for an axis distance of the beam of 4 mm at the target. The calculation shows that one unit will be able to correct the field curvature, the astigmatism and the distortion of the ion projector. It allows to correct the axial astigmatism of the lens and it is able to produce an additional deflection field to align the projected pattern on the target [8].
3. Conclusions We describe a new type of high energy ion projector. The main element of the setup is a superconducting solenoid with good focusing properties in combination with an electrostatic multipole. Calculations show that the multipole sys-
4. Acknowledgments This work was supported by the Ministry of science and research of Nordrhein Westfalen (Germany). For further assistance we would like to thank Dr. H.H. Bukow (Institut ffir Physik mit Ionenstrahlen), Dr. K. Grosse (Uni-Kontakt) and Mr, K. Brand (Dynamitron Tandem Laboratory, Bochum). REFERENCES 1. A. Chalupka, J. Fegerl, R. Fischer, G. Lammer, H. LSscher, L. Malek, R. Nowak, G. Stengl, C. Traher and P. Wolf, Microelec. Eng. 17 (1992) 229. 2. E. Cekan, W. Fallmann, W. Friza, F. Paschke, G. Stangl, P. Hudek, E. Bayer, H. Krans, Microelec. Eng. 17 (1992) 241. 3. A. Stephan, J. Meijer, H.H Bukow and C. Rolls, Nucl. Instr. and Meth. B104 (1995) 31. 4. W. Glaser, Grundlagen der Elektronenoptik, Wien, 1952. 5. M. Reichenbach and H. Rose, Optik 28 (1968/69) 475. 6. J . F . Ziegler, TRIM, IBM 1991. 7. A.V. Crewe and N.W. Parker, Optik 46 (1976) 183. 8. J. Meijer, A. Stephan, H.H. Bukow, C. Rolls and F. Bruhn, Nucl. Instr. and Meth. B 104 (1995) 77.
ELSEVIER
Microelectronic Engineering 41/42 (1998) 257-260
High Energy Implantation by Ion Projection J. Meijer and A.Stephan Physik mit Ionenstrahlen Ruhr-Universit/it Bochum, 44780 Bochum, Germany A new type of ion projector is proposed to structurize a wafer at a resolution below 200 nm reducing the typically used lithography steps to a large extent. Basically, this technique aims to project a mask onto a wafer using an ion optical lens system allowing a demagnification up to 1:50 for ions with energies between 100 keV and some MeV. The center piece of this device is a single magnetic lens consisting of a superconducting solenoid. The achievable lateral resolution depends on lens errors, stability of the stencil mask and straggling of the implanted ions. A variation of the implantation energy would allow three dimensional structurization.
1. I n t r o d u c t i o n Recently, industries asks for ion implanters for two different applications: Very low energy implanters with energies in the range of a few keV are required for implantations at a depth of a few 10 nm as requested for the 0.3 #m technology. High energy implanters are necessary to produce buried layers or retrograded wells. In both energy ranges focused ion beams are suited to replace several conventional mask process steps, but they are used only occasionally up to now in industrial production processes. The main reason restraining the industrial operation of maskless implantation using microbeams is the sequential nature of the writing process. Thus, writing of large structures is very time consuming. A solution to overcome this problem is to project a stencil mask illuminated by ions onto the substrate. A well known version of such an implanter is based on electrostatic lenses [1]. The processed field area is in the range of a few mm 2, the projector is able to focus ions of 50 keV with a demagnification factor of 5-10 [2]. Due to the low demagnification factor, the stencil mask must have a very fine pattern (openings 0.5-1tim). These structures can be produced using thin material only, thus reducing the mechanical stability and thermal conductivity of the mask itself. Ions with energies in the MeV range may traverse the mask material thus producing an unwanted background implantation. We propose a new concept to focus energetic ions,
0167-9317/98/$19.00© ElsevierScienceB.~ All rightsreserved. Plh S0167-9317(98)00059-8
allowing imaging of the mask at a demagnification factor in the range of 1:20 - 1:50. As ions with energies up to MeV cannot be focused using electrostatic lenses, a single superconducting solenoid having superior ion optical properties is used. A single beam focus on the target avoids beam broadening effects due to space charges, a problem frequently occurring in low energy devices. The partial imaging of the stencil mask in combination with an electrostatic multipole system allows the correction of non axial aberrations. The project developed in the following is based on experiences with a MeV microprobe [3] using a superconducting solenoid as final lens.
2. I o n p r o j e c t o r u s i n g a s u p e r c o n d u c t i n g solenoid
A schematic view of the proposed ion projector is shown in fig 1. The beam of energetic ions (1) enters the system through a defining aperture (2) with a diameter of typically 2 mm. A first set of deflection plates (3) scans the beam on the stem cil mask (4). As the beam should hit the mask perpendicularly a second set of deflection plates (5) serves to that effect. Behind the mask a third set of plates (6) deflects the beam to the center of the solenoid lens (7) thus avoiding large aberrations. A diaphragm (9) in the center of the solenoid is implemented to define the divergence angle. This aperture might be split in several segments to allow the measurement of the beam ira-
258
J Mei]er, A. Stephan / Microelectronic Engineering 41/42 (1998) 257-260
I
!i
ion projector prototype under construction in this laboratory. Thus in the following all calculations are based on the well known ion optical properties of this lens. 2.1. Lateral r e s o l u t i o n l i m i t s The achievable resolution of the projector system is mainly limited by the following factors:
Figure 1. Schematic view of the high energy ion projector.
• axial and parasitic aberrations, • non axial aberrations, • lateral straggling.
pinging to each segment. This would help to align the beam to the lens center. The beam is focused on the target (10) by a superconducting solenoid, assisted by an electrostatic multipole system (8) allowing active correction of non axial imaging errors. The multipole unit has a length of 20cm and an inner diameter of 3 cm. The eight poles and the scanning units are independently driven by a computer controlled power supply (11). The main optical parameters of the system in first order are defined by object length, image length and the focusing power of the solenoid. Object length and image length yield the demagnification factor. To achieve a high demagnification of the mask and to reduce lens aberrations a small image length is advantageous, resonable values being in the range of a few 10 cm. A system with an object length of 6 m and an image length of 10-30 cm results in demagnification factors of 20-50. The focusing power depends on width and strength of the magnetic lens field. The field width determines in first order the minimum focal length. Under the assumption that the field distribution can be described by the Glaser's bell shape formula [4], the focusing power of a 16 T field of 6 cm width can be calculated to 80 u MeV/q 2 for a focal length f = 30cm or 25 u MeV/q 2 for f = 10cm, where u is the atomic mass unit and q the charge of the ion. The lens of the present microprobe has a field width of 6 cm and achieves a maximum field strength of 8 T resulting in a focusing power of 20 u MeV/q 2 for f -=- 30 cm and 10 u MeV/q 2 for f = 10 cm. Such a lens will be installed in the setup of an
In third order the axial aberrations are the axial astigmatism, the chromatic aberration and the spherical aberration. The axial astigmatism stems from minute inaccuracies of the lens winding. For the present solenoid this effect produces a field error of -~- = 14 ppm, measured in a circle of 5 mm radius around the field axis. This results in an additional quadrupole component, causing an axial astigmatism of 1.8#m for a beam with a divergence angle of 7 = 0.1 mrad and a focus length of f = 30 cm. Using a stigmator the axial astigmatism is easily corrected. Contrary the correction of the chromatic and spherical aberration is a very difficult task [5]. To minimize these aberrations one needs an accelerator with a very well defined energy (small AE) and a lens with low spherical aberration coefficient. For an accelerator with - ~ ~ 2 x 10 -5 the expected lens error induced by axial chromatic aberration is Ad = 30nm. The spherical aberration of the present lens system is found to be Ad ---- 15nm (same divergence and focal length as above). Under the condition of high demagnification the ion optical resolution is mainly affected by nonaxial aberrations. Fig. 2 shows the contributions of different non-axial aberrations versus the axial distance at the image position, calculated for a beam with incidence angle 70 -- 0.1mrad, an object length of 6.1m and an image length of 30 cm. The isotropic and anisotropic non axial aberrations are summarized to one number [4]. The main contributions are the field curvature and the astigmatism. The distortion is independent of 70 and does not affect the resolution but
I Meijer, A. Stephan/Microelectronic Engineering 41/42 (1998) 257-260
10" asttgrnatisrn
._----- .J--
400
, , ~ , curvature
+ field
7
-~- 55CI z
//f
--
transv
chromatic
'7
7~
/l'
0
" . . . .
i
-
,
\s
-
P ,/
~'
so L z :-~ o 100 L ~2 ~: 5 0
distortion
!/
{lO 2 ~ . . . . .
-
~r 200 -
10 ° 10 ]
~
&sco
coma
= 2so
I0'
~-'-"-
259
d
0 2
4 6 ~x:s rhst~.~ce from]
8
I0
Figure 2. Dependence of various non-axial aberration diameters on the axis distance for an image length of 30 cm and an object length of 6.1 m. The calculations are performed under the condition that the beam passes through the center of the lens with a divergence angle of "f = 0.1 mrad. The energy width of the beam is assumed to A E / E = 10 -s.
the position of the beam, the coma being negligible at % = 0.1 mrad. The transverse chromatic aberration is proportional to the energy resolution A E / E of the beam and results in a deviation of the beam in one direction. This kind of lens error will be reduced, if the ion projector is served by an accelerator of small energy width. The calculation assumes an energy width of - ~ = 10 -5, Fig. 2 demonstrates that an axis distance on the target of more than l m m results in huge aberrations. Fortunately, these aberrations can be corrected as will be shown in the next section. However, the physical limit for lateral structuring by implantation is given by lateral beam straggling. Fig. 3 shows the expected lateral straggling of B, P and As implanted in Si versus the ion range calculated by TRIM [6]. The implantation of e.g. 1 MeV P ions at an implantation depth of 1.2 #m corresponds to a lateral straggling radius of about 200 nm. For projectile energies below 400 keV (implantation depth below 600 nm) the expected straggling is about 100 nm.
. . . . . . . . . . . 0 500 '000 ])1" , j ~ ( t ( ' / I
. . . . . . . 1500 2000 2500 lOll'A(' [ till]
SO00
Figure 3. Projected ion range versus lateral straggling radius for B,P and As-Ions in Si, calculated using the TRIM code.
2.2. C o r r e c t i o n of a b e r r a t i o n s u s i n g a n electrostatic multipole s y s t e m As the beam is scanned over the stencil mask the beam position in the image plane is always known. This gives the possibility to correct nonaxial lens aberrations by adding specific multipole components to the focusing field. The field curvature is caused by the effect that the focal point of non-axial rays is not in the image plane. This can be compensated for using the defocusing effect of an additional rotational symmetric electrostatic field superimposed to the focusing magnetic field. (This kind of correction is possible, if the ion beam is divergent inside the multipole unit.) The isotropic and anisotropic distortion can be corrected by additional dipole components and the isotropic and anisotropic astigmatism are corrected by quadrupole fields. An additional spherical aberration, produced by a deflection of the beam, can compensate the isotropic and anisotropic coma [7]. In the proposed setup of the ion projector the coma would be very small and a correction is not necessary. The correction of the transverse chromatic aberration is important, if the energy width ( A E / E ) of the beam is larger than 10 -4 . Such correction could be performed using a combination of magnetic and elec-
I Meij'er, A. Stephan/Microelectronic Engineering 41/42 (1998) 257-260
260
500
,
, , , ,
400~
•
~%
•
i
300 = o
,
-
2oo
J
100
0
1
2
3
axis distance [mm]
tern is able to correct the non-axial aberration of the lens thus resulting in a beam resolution better than 100 nm for a field of 8 mm x 8 mm exposed simultaneously at a demagnification of 20. The energy mass product at this image length is 20 u MeV/q 2. For implantation energies in the range of a few MeV, the lateral resolution of implanted structures is limited by lateral straggling. The setup allows a fast and easy change of the implantation energy, so that three dimensional structures become feasible.
4
Figure 4. Expected aberration diameters versus the axis distance using different correction fields produced by a multipole device. The calculations are performed assuming an ion projector with an object length of 6.1 m and an image length of 30 cm. The line marked by * shows the expected uncorrected aberration, x correction of the field curvature, A additional correction of the astigmatism and + additional correction of the coma.
trostatic dipole fields (so-called anti-Wien filter). Fig. 4 shows the expected aberrations assuming different multipole field components for an ion beam with a divergence angle of 0.1 mrad, an object length of 6.1 m and an image length of 30 cm. It is evident that an aberration of 500 nm can be reduced to less than 100 nm for an axis distance of the beam of 4 mm at the target. The calculation shows that one unit will be able to correct the field curvature, the astigmatism and the distortion of the ion projector. It allows to correct the axial astigmatism of the lens and it is able to produce an additional deflection field to align the projected pattern on the target [8].
3. Conclusions We describe a new type of high energy ion projector. The main element of the setup is a superconducting solenoid with good focusing properties in combination with an electrostatic multipole. Calculations show that the multipole sys-
4. Acknowledgments This work was supported by the Ministry of science and research of Nordrhein Westfalen (Germany). For further assistance we would like to thank Dr. H.H. Bukow (Institut ffir Physik mit Ionenstrahlen), Dr. K. Grosse (Uni-Kontakt) and Mr, K. Brand (Dynamitron Tandem Laboratory, Bochum). REFERENCES 1. A. Chalupka, J. Fegerl, R. Fischer, G. Lammer, H. LSscher, L. Malek, R. Nowak, G. Stengl, C. Traher and P. Wolf, Microelec. Eng. 17 (1992) 229. 2. E. Cekan, W. Fallmann, W. Friza, F. Paschke, G. Stangl, P. Hudek, E. Bayer, H. Krans, Microelec. Eng. 17 (1992) 241. 3. A. Stephan, J. Meijer, H.H Bukow and C. Rolls, Nucl. Instr. and Meth. B104 (1995) 31. 4. W. Glaser, Grundlagen der Elektronenoptik, Wien, 1952. 5. M. Reichenbach and H. Rose, Optik 28 (1968/69) 475. 6. J . F . Ziegler, TRIM, IBM 1991. 7. A.V. Crewe and N.W. Parker, Optik 46 (1976) 183. 8. J. Meijer, A. Stephan, H.H. Bukow, C. Rolls and F. Bruhn, Nucl. Instr. and Meth. B 104 (1995) 77.