- Email: [email protected]
Grid shadow pattern analysis of the Shanghai nuclear microprobe
a __
__ I!!3
Nuclear
Instruments
and Methods
in Physics
Research
B 104 (1995) 86-91
E!lilRZ B
Beam Interactions with Materials & Atoms
ELSEVIER
Grid shadow pattern analysis of the Shanghai nuclear microprobe D.N. Jamiesor? *, Jianle Zhub, Yu Maob, Rongrong Lub, Zhishan Wangb, Jieqing Zhub aSchool of Physics, University of Melbourne, Microanalytical Research Centre, Parkville, 3052, Australia b Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204,
Shanghai, 201800, China
Abstract The ion optics of the Shanghai nuclear microprobe system have been investigated with the grid shadow method. The level of parasitic aberration found in the magnetic quadrupole lenses of the probe forming lens system was found to be very low and typical of levels seen in other high-quality systems, such as the Melbourne system. It was found that the parasitic field components were mainly sextupole with a strength of between 0.070% and 0.28% of the main quadrupole field. The small level of parasitic field is not a significant limit to the resolution achievable by the system. A complication in the analysis of the grid shadow patterns was that some of the parasitic field was due to skew sextupole components (non-zero phase angle); however, these could be identified by comparing with simulated shadow patterns. Further experiments with the grid shadow method revealed an improvement by a factor of 4-6 in focused probe intensity, and could be achieved by installation of a beam steerer between the accelerator condenser lens and the beam switcher dipole magnet. The measurements show that the lens system is potentially able to focus probes with 1 pm resolution.
1. Introduction and method High-resolution probes for nuclear microprobe analysis and imaging can only be achieved if the probe forming lens system has very low levels of parasitic aberration. However, even if the probe forming lenses have low levels of aberration, fine probes cannot be achieved if the brightness of the ion source is low, or if the emittance of the accelerator is not well matched to the acceptance of the microprobe beam line itself. An investigation of the ion optical properties of the Shanghai nuclear microprobe system was done [l]. Studies were made of the probe forming lenses and the matching of the accelerator to the beam line. The parasitic aberrations of the lenses were measured by the grid shadow method [2-41. The physical parameters of the Shanghai system appear in Table 1 and the layout of the system is shown in Fig. 1. Each lens of the probe forming system was tested individually and was used to focus, in turn, vertical and horizontal lines. For the grid shadow method, a 2000 mesh grid (period 12.5 pm) was located in the real
*Corresponding author. Tel.: 61393445376, 3 9347 4783, E-mail: [email protected]. 0168-583X/95/$9.50 0 1995 Elsevier SSDI 0168-583X(95)00437-8
Fax:
+
Science B.V. All rights
61
reserved
(converging) image plane. The quadrupole lens was focused onto the grid by observing the shadow pattern and adjusting the lens strength until the characteristic pattern was obtained that consisted of nearly straight parallel lines at right angles to the line focus. The quadrupole lens was rotated so that the line focus made an angle between 0.5” and 1.5” with either the vertical or horizontal grid bars following the procedure outlined below. The grid was fixed in the usual specimen position of the nuclear microprobe. Grid shadow patterns were cast onto the front of a screen located 293 mm downstream of the Gaussian image. A high-sensitivity screen was employed that consisted of powdered ZnSiO,:Mn bound with epoxy on a glass substrate which was mounted on the rear window of the target chamber. The shadow patterns were Ijhotographed from outside the vacuum system by a camera looking upstream through the target chamber rear window. The camera was fitted with a macro lens and 1000 ASA Kodak Tri-X black and white film. Exposure times were typically a few seconds. A 3 MeV H+ beam was employed for all of the images presented here. At the commencement of the study of the system, the image on the shadow screen was observed with the grid displaced out of the beam path. In this situation, it was expected that the beam would fill a uniformly illuminated circular region. However this was not observed, instead
D.N. Jamieson
et al. /Nucl.
Instr. and Meth. in Phys. Res. B 104 (1995) 86-91
ate angle between the alignment of the grid bars and the line focus of the single quadrupole lens under test. This was accomplished by observing the shadow pattern from a lens focused on the grid, whilst the lens was rotated towards alignment with either the vertical or horizontal grid bars. The optimum position was judged to be the one which produced a shadow pattern showing the shadow of four to ten grid bars that covered an area that was nominally square, with a width equal to narrowest dimension of the overall pattern (this is in the converging plane) and centred on the middle of the pattern. Smaller angles produced less grid bar shadows, however, resulting in great distortion of the pattern owing to the influence of high-order parasitic multipole components in the lens magnetic field. This makes the pattern difficult to analyse numerically and should be avoided since only the lowest-order parasitic multipole is of interest. This is the component that is most significant at causing a degradation of the focused probe.
Table 1 Parameters of the Shanghai nuclear microprobe system 5233 mm 2850 mm 63.7 mm 89.5 mm 6.35 mm 2.5 mm 298 mm 293 mm
Object to aperture Aperture to Ql Lens length of Ql and 44 Lens length of 42 and 43 Lens bore radius Lens spacing Image (grid) distance Grid to shadow plane
Lens system
I
I
81
-u
2. Grid shadow pattern results Shadow
I
plane II
Fig. 1. The layout of the Shanghai nuclear microprobe system.
a collection of bright spots occupying less than 25% of the expected area was observed. This suggested that the emittance of the accelerator was not well matched to the acceptance of the microprobe beamline. The situation was greatly improved by the installation of a beam steerer between the condenser lens and the beam switcher magnet. The condenser lens is located half way between the exit of the accelerator and the entrance to the switcher magnet. With the steerer installed the brightness of beam in the target chamber was 9-30 pA Pm -’ mradm2 MeV-’ depending on the choice of object and aperture diaphragms. This represents an increase by a factor of 4-6 compared to the system without the steerer. It was found that the maximum beam divergence that could be accepted by the microprobe beam line for focusing on the target was 0.34 mrad in the xoz-plane and 0.24 mrad in the yoz-plane. This was slightly larger than the actual maximum divergence of the beam from the accelerator. To achieve the best sensitivity with the grid shadow method, which requires the greatest possible beam divergence, the beam angle was scanned over the maximum range of divergences with an AC magnetic steerer located just upstream of the object diaphragm. The scan frequency was fast compared to the exposure time used to photograph the shadow patterns. In addition to the use of a beam with the largest possible divergence, the sensitivity of the measurement was further increased by the use of the smallest appropri-
Representative shadow patterns for the four magnetic quadrupole lenses of the probe forming lens system are shown in Fig. 2. For all of the lenses the dominant parasitic component appeared to be a sextupole field, or in one case, a skew sextupole field. The parasitic octupole component, if present, was negligible compared to the sextupole component. The central grid bar shadow of the patterns was digitised and fitted with a polynomial curve in order to measure the parasitic aberration coefficient of the lens under test. Complete details of this procedure are given in Ref [2]. Since the dominant aberration was from parasitic sextupole field components, a quadratic curve was fitted to the digitised shadow patterns. The linear term, which represents a small displacement of the grid from the real image plane, is ignored. It was found that the appearence of the grid shadow patterns, and’hence the magnitude and phase angle of the parasitic sextupole field component, was not precisely reproducible and appeared to suffer from hysteresis. By phase angle in this context we mean the relative alignment between the quadrupole field and the parasitic sextupole field. As a consequence our measured parasitic sextupole aberration coefficients, listed in Table 2, which were obtained with the assumption that the phase angle of the sextupole component was zero, varied by around a factor of two between grid shadow patterns taken at different times. Despite this the order of the multipolarity itself was certainly reproducible. The measured parasitic aberration coefficient may be converted in to a representative parasitic field component from the reduced parasitic aberration coefficients: s = (x/e”)/(x/e2s),
II. MICROBEAM FORMATION
88
D.N. Jamieson et al. /Nucl. Instr. and Me&. in Phys. Res. B 104 (1995) 86-91
D.N. Jamieson et al. /Nucl. Instr. and Meth. in Phys. Res. B 104 (1995) 86-91
II. MICROBEAM
89
FORMATION
9a
D.N. Jamieson et al. /Nucl. In&. and Meth. in Phys. Rex B 104 (1995) 86-91
Table 2 Analysis of central grid bar shadow from Fig. 2 Lens & sign +1 -1 +2 -2 +3 -3 +4 -4
Dominant aberration
Grid angle (degrees)
Skew sext.(?) Sextupole Sextupole Sextupole Sextupole Sextupole Sextupole Sextupole
1.31(14) 1.21(05) 0.69(15) 0.74(10) 0.76(13) 0.74 (09) 1.66(13) 1.58(14)
(XP) (m/rad)
CW) (m/rad2) -
1.5e - 2.le 6.5e - 3.8e 6.3e - 7.9e 4.2e -
6 3 4 4 3 5 3
- 18.9 - 24.6 37.3 21.1 - 36.2 8.21 - 3.68
(xP3) (m/rad3) 0 0 0 0 0 0 0
Note: A grid angle of 1.58 (14) indicates that the grid angle was measured lo be 1.58” with an uncertainty of 0.14”.
Table 3 Reduced aberration coefficients and field components for the four singlet systems with parasitic sextupole components (%) Lens
Quadrupole field (kG)
1 2 3 4
0.47 0.39 0.48 0.82
W’s) pm/(mrad’%) -
113 116 118 118
% sextupole
(XPn) pm/(mrad3 %)
0.13 0.22 0.28 0.07
136 135 135 139
Note: The uncertainty in the % sextupole field contamination varies from 0.1% in lens 1 to 0.05% in lens 4.
where (x/e”) is the measured parasitic aberration coefficient, (x/0’s) is the reduced parasitic aberration coefficient and S is the percentage parasitic sextupole field on the pole tip of the quadrupole lens. The reduced coefficients were calculated by program PRAM. The resulting field components, listed in Table 3, are typical of those also seen in the Melbourne lenses. It was also observed that the effective grid angle, computed from the period of the grid bar shadows along the x-axis (for a negatively excited lens) or y-axis (for a positively excited lens) appeared to differ for the same lens. There are two probable explanations for this effect. Either the two line focii of the lens were not at 90” or there were parasitic skew sextupole components in the field of the lens. The latter effect can give rise to an apparent rotation of the line focus when the sign of the excitation is reversed because it causes the period of the grid bar shadows to depend on the displacement from the axis owing to the presence of (x/04) aberration coefficients that couple f3 and 4. This latter explanation is
almost certainly correct in this case owing to the presence of the observed parasitic sextupole field components that are invariably associated with skew sextupole field components for the opposite excitation of the lens [S].
3. Conclusion The present measurements may be used to simulate the expected probe achievable with typical operating conditions of the system. Fig. 4 shows that if the two central lenses are assumed to have the largest observed parasitic sextupole component, with a sign chosen to maximise the aberration of the focused probe, then a 1 pm probe, with a useful amount of beam current, can readily be achieved. The results of this experiment with the Shanghai nuclear microprobe showed how to improve the resolution of the probe forming lens system by optimizing the matching of the ion source and accelerator to the microprobe itself. The results also revealed that the lenses of
D.N. Jamieson et al. INucl. Instr. and Meth. in Phys. Rex B 104 (1995) 86-91 Box
edge length
= 2.00
pm
91
Z = 0.0 pn
Fig. 4. Simulated probe for - 100 pA of beam showing the effects of 0.25% parasitic sextupole components in lenses 2 and 3 which overestimates the effect of the parasitic aberration. The FWHM is very close to the first-order demagnified image of the object which is 1.2 km.
the system contained aberration.
remarkably
low levels of parasitic
Acknowledgements This work was supported in part by the National Natural Science Foundation of China under contract no. 19392100 and the Australian Department of Industry, Technology and Commerce under grant no. 91/ 7922.
References [l] J. Zhu, R. Lu, A. Le, Y. Gu, C. Yang, W. Wang, M. Li and Y. Mao, Nucl. Instr. and Meth. B 77 (1993) 422. [2] D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 29 (1987) 544. [3] D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 30 (1988) 235. [4] D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 34 (1988) 411. [S] M.B.H. Breese and D.N. Jamieson, Nucl. Instr. and Meth. B 83 (1993) 394.
II. MICROBEAM FORMATION
__ I!!3
Nuclear
Instruments
and Methods
in Physics
Research
B 104 (1995) 86-91
E!lilRZ B
Beam Interactions with Materials & Atoms
ELSEVIER
Grid shadow pattern analysis of the Shanghai nuclear microprobe D.N. Jamiesor? *, Jianle Zhub, Yu Maob, Rongrong Lub, Zhishan Wangb, Jieqing Zhub aSchool of Physics, University of Melbourne, Microanalytical Research Centre, Parkville, 3052, Australia b Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204,
Shanghai, 201800, China
Abstract The ion optics of the Shanghai nuclear microprobe system have been investigated with the grid shadow method. The level of parasitic aberration found in the magnetic quadrupole lenses of the probe forming lens system was found to be very low and typical of levels seen in other high-quality systems, such as the Melbourne system. It was found that the parasitic field components were mainly sextupole with a strength of between 0.070% and 0.28% of the main quadrupole field. The small level of parasitic field is not a significant limit to the resolution achievable by the system. A complication in the analysis of the grid shadow patterns was that some of the parasitic field was due to skew sextupole components (non-zero phase angle); however, these could be identified by comparing with simulated shadow patterns. Further experiments with the grid shadow method revealed an improvement by a factor of 4-6 in focused probe intensity, and could be achieved by installation of a beam steerer between the accelerator condenser lens and the beam switcher dipole magnet. The measurements show that the lens system is potentially able to focus probes with 1 pm resolution.
1. Introduction and method High-resolution probes for nuclear microprobe analysis and imaging can only be achieved if the probe forming lens system has very low levels of parasitic aberration. However, even if the probe forming lenses have low levels of aberration, fine probes cannot be achieved if the brightness of the ion source is low, or if the emittance of the accelerator is not well matched to the acceptance of the microprobe beam line itself. An investigation of the ion optical properties of the Shanghai nuclear microprobe system was done [l]. Studies were made of the probe forming lenses and the matching of the accelerator to the beam line. The parasitic aberrations of the lenses were measured by the grid shadow method [2-41. The physical parameters of the Shanghai system appear in Table 1 and the layout of the system is shown in Fig. 1. Each lens of the probe forming system was tested individually and was used to focus, in turn, vertical and horizontal lines. For the grid shadow method, a 2000 mesh grid (period 12.5 pm) was located in the real
*Corresponding author. Tel.: 61393445376, 3 9347 4783, E-mail: [email protected]. 0168-583X/95/$9.50 0 1995 Elsevier SSDI 0168-583X(95)00437-8
Fax:
+
Science B.V. All rights
61
reserved
(converging) image plane. The quadrupole lens was focused onto the grid by observing the shadow pattern and adjusting the lens strength until the characteristic pattern was obtained that consisted of nearly straight parallel lines at right angles to the line focus. The quadrupole lens was rotated so that the line focus made an angle between 0.5” and 1.5” with either the vertical or horizontal grid bars following the procedure outlined below. The grid was fixed in the usual specimen position of the nuclear microprobe. Grid shadow patterns were cast onto the front of a screen located 293 mm downstream of the Gaussian image. A high-sensitivity screen was employed that consisted of powdered ZnSiO,:Mn bound with epoxy on a glass substrate which was mounted on the rear window of the target chamber. The shadow patterns were Ijhotographed from outside the vacuum system by a camera looking upstream through the target chamber rear window. The camera was fitted with a macro lens and 1000 ASA Kodak Tri-X black and white film. Exposure times were typically a few seconds. A 3 MeV H+ beam was employed for all of the images presented here. At the commencement of the study of the system, the image on the shadow screen was observed with the grid displaced out of the beam path. In this situation, it was expected that the beam would fill a uniformly illuminated circular region. However this was not observed, instead
D.N. Jamieson
et al. /Nucl.
Instr. and Meth. in Phys. Res. B 104 (1995) 86-91
ate angle between the alignment of the grid bars and the line focus of the single quadrupole lens under test. This was accomplished by observing the shadow pattern from a lens focused on the grid, whilst the lens was rotated towards alignment with either the vertical or horizontal grid bars. The optimum position was judged to be the one which produced a shadow pattern showing the shadow of four to ten grid bars that covered an area that was nominally square, with a width equal to narrowest dimension of the overall pattern (this is in the converging plane) and centred on the middle of the pattern. Smaller angles produced less grid bar shadows, however, resulting in great distortion of the pattern owing to the influence of high-order parasitic multipole components in the lens magnetic field. This makes the pattern difficult to analyse numerically and should be avoided since only the lowest-order parasitic multipole is of interest. This is the component that is most significant at causing a degradation of the focused probe.
Table 1 Parameters of the Shanghai nuclear microprobe system 5233 mm 2850 mm 63.7 mm 89.5 mm 6.35 mm 2.5 mm 298 mm 293 mm
Object to aperture Aperture to Ql Lens length of Ql and 44 Lens length of 42 and 43 Lens bore radius Lens spacing Image (grid) distance Grid to shadow plane
Lens system
I
I
81
-u
2. Grid shadow pattern results Shadow
I
plane II
Fig. 1. The layout of the Shanghai nuclear microprobe system.
a collection of bright spots occupying less than 25% of the expected area was observed. This suggested that the emittance of the accelerator was not well matched to the acceptance of the microprobe beamline. The situation was greatly improved by the installation of a beam steerer between the condenser lens and the beam switcher magnet. The condenser lens is located half way between the exit of the accelerator and the entrance to the switcher magnet. With the steerer installed the brightness of beam in the target chamber was 9-30 pA Pm -’ mradm2 MeV-’ depending on the choice of object and aperture diaphragms. This represents an increase by a factor of 4-6 compared to the system without the steerer. It was found that the maximum beam divergence that could be accepted by the microprobe beam line for focusing on the target was 0.34 mrad in the xoz-plane and 0.24 mrad in the yoz-plane. This was slightly larger than the actual maximum divergence of the beam from the accelerator. To achieve the best sensitivity with the grid shadow method, which requires the greatest possible beam divergence, the beam angle was scanned over the maximum range of divergences with an AC magnetic steerer located just upstream of the object diaphragm. The scan frequency was fast compared to the exposure time used to photograph the shadow patterns. In addition to the use of a beam with the largest possible divergence, the sensitivity of the measurement was further increased by the use of the smallest appropri-
Representative shadow patterns for the four magnetic quadrupole lenses of the probe forming lens system are shown in Fig. 2. For all of the lenses the dominant parasitic component appeared to be a sextupole field, or in one case, a skew sextupole field. The parasitic octupole component, if present, was negligible compared to the sextupole component. The central grid bar shadow of the patterns was digitised and fitted with a polynomial curve in order to measure the parasitic aberration coefficient of the lens under test. Complete details of this procedure are given in Ref [2]. Since the dominant aberration was from parasitic sextupole field components, a quadratic curve was fitted to the digitised shadow patterns. The linear term, which represents a small displacement of the grid from the real image plane, is ignored. It was found that the appearence of the grid shadow patterns, and’hence the magnitude and phase angle of the parasitic sextupole field component, was not precisely reproducible and appeared to suffer from hysteresis. By phase angle in this context we mean the relative alignment between the quadrupole field and the parasitic sextupole field. As a consequence our measured parasitic sextupole aberration coefficients, listed in Table 2, which were obtained with the assumption that the phase angle of the sextupole component was zero, varied by around a factor of two between grid shadow patterns taken at different times. Despite this the order of the multipolarity itself was certainly reproducible. The measured parasitic aberration coefficient may be converted in to a representative parasitic field component from the reduced parasitic aberration coefficients: s = (x/e”)/(x/e2s),
II. MICROBEAM FORMATION
88
D.N. Jamieson et al. /Nucl. Instr. and Me&. in Phys. Res. B 104 (1995) 86-91
D.N. Jamieson et al. /Nucl. Instr. and Meth. in Phys. Res. B 104 (1995) 86-91
II. MICROBEAM
89
FORMATION
9a
D.N. Jamieson et al. /Nucl. In&. and Meth. in Phys. Rex B 104 (1995) 86-91
Table 2 Analysis of central grid bar shadow from Fig. 2 Lens & sign +1 -1 +2 -2 +3 -3 +4 -4
Dominant aberration
Grid angle (degrees)
Skew sext.(?) Sextupole Sextupole Sextupole Sextupole Sextupole Sextupole Sextupole
1.31(14) 1.21(05) 0.69(15) 0.74(10) 0.76(13) 0.74 (09) 1.66(13) 1.58(14)
(XP) (m/rad)
CW) (m/rad2) -
1.5e - 2.le 6.5e - 3.8e 6.3e - 7.9e 4.2e -
6 3 4 4 3 5 3
- 18.9 - 24.6 37.3 21.1 - 36.2 8.21 - 3.68
(xP3) (m/rad3) 0 0 0 0 0 0 0
Note: A grid angle of 1.58 (14) indicates that the grid angle was measured lo be 1.58” with an uncertainty of 0.14”.
Table 3 Reduced aberration coefficients and field components for the four singlet systems with parasitic sextupole components (%) Lens
Quadrupole field (kG)
1 2 3 4
0.47 0.39 0.48 0.82
W’s) pm/(mrad’%) -
113 116 118 118
% sextupole
(XPn) pm/(mrad3 %)
0.13 0.22 0.28 0.07
136 135 135 139
Note: The uncertainty in the % sextupole field contamination varies from 0.1% in lens 1 to 0.05% in lens 4.
where (x/e”) is the measured parasitic aberration coefficient, (x/0’s) is the reduced parasitic aberration coefficient and S is the percentage parasitic sextupole field on the pole tip of the quadrupole lens. The reduced coefficients were calculated by program PRAM. The resulting field components, listed in Table 3, are typical of those also seen in the Melbourne lenses. It was also observed that the effective grid angle, computed from the period of the grid bar shadows along the x-axis (for a negatively excited lens) or y-axis (for a positively excited lens) appeared to differ for the same lens. There are two probable explanations for this effect. Either the two line focii of the lens were not at 90” or there were parasitic skew sextupole components in the field of the lens. The latter effect can give rise to an apparent rotation of the line focus when the sign of the excitation is reversed because it causes the period of the grid bar shadows to depend on the displacement from the axis owing to the presence of (x/04) aberration coefficients that couple f3 and 4. This latter explanation is
almost certainly correct in this case owing to the presence of the observed parasitic sextupole field components that are invariably associated with skew sextupole field components for the opposite excitation of the lens [S].
3. Conclusion The present measurements may be used to simulate the expected probe achievable with typical operating conditions of the system. Fig. 4 shows that if the two central lenses are assumed to have the largest observed parasitic sextupole component, with a sign chosen to maximise the aberration of the focused probe, then a 1 pm probe, with a useful amount of beam current, can readily be achieved. The results of this experiment with the Shanghai nuclear microprobe showed how to improve the resolution of the probe forming lens system by optimizing the matching of the ion source and accelerator to the microprobe itself. The results also revealed that the lenses of
D.N. Jamieson et al. INucl. Instr. and Meth. in Phys. Rex B 104 (1995) 86-91 Box
edge length
= 2.00
pm
91
Z = 0.0 pn
Fig. 4. Simulated probe for - 100 pA of beam showing the effects of 0.25% parasitic sextupole components in lenses 2 and 3 which overestimates the effect of the parasitic aberration. The FWHM is very close to the first-order demagnified image of the object which is 1.2 km.
the system contained aberration.
remarkably
low levels of parasitic
Acknowledgements This work was supported in part by the National Natural Science Foundation of China under contract no. 19392100 and the Australian Department of Industry, Technology and Commerce under grant no. 91/ 7922.
References [l] J. Zhu, R. Lu, A. Le, Y. Gu, C. Yang, W. Wang, M. Li and Y. Mao, Nucl. Instr. and Meth. B 77 (1993) 422. [2] D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 29 (1987) 544. [3] D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 30 (1988) 235. [4] D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 34 (1988) 411. [S] M.B.H. Breese and D.N. Jamieson, Nucl. Instr. and Meth. B 83 (1993) 394.
II. MICROBEAM FORMATION