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An improved scanning system for a high-voltage electron microscope
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1977 J. Phys. E: Sci. Instrum. 10 502 (http://iopscience.iop.org/0022-3735/10/5/024) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 141.209.100.60 This content was downloaded on 03/10/2015 at 11:45
Please note that terms and conditions apply.
An improved scanning system for a high-voltage electron microscope
A Strojnik? and T G Sparrow Cavendish Laboratory, University of Cambridge, Cambridge CB2 3RQ Receiced 29 December 1976 Abstract The 750 kV Cavendish electron microscope has been adapted for scanning transmission operation. Disturbing magnetic fields at 50 and 100 Hz and at 8 kHz have been tracked carefully and successively removed to produce an electron beam without cross modulation. Further attention to detail in HV and lens electronics as well as in deflection systems led to an electron probe smaller than 3 nm in diameter. By operating the last demagnifying (objective) lens at very high excitation and by introducing a new electron detector, less than 4 nm point-to-point resolution has been obtained at 500 kV. 1 Introduction Superior selective penetration/resolution in thick specimens (Strojnik 1975a),combined with electronic and electron optical contrast and gamma control, and a possibility to simultaneously extract information from both elastically and inelastically scattered electrons, make the scanning transmission mode (STEM) an attractive supplement and a possible future alternative to the fixed-beam (TEM)imaging. If the electron optical performance of the objective or the last demagnifying lens is identical, the same theoretical resolving power could be expected in TEM and in STEM. In practice, however, STEMS using thermionic electron sources show rather poor resolution. In the best 100 kV commercial TEMS, converted into the STEM mode, the resolution is almost an order of magnitude inferior in STEM as compared to TEM. The major reason is insufficient electron optical brightness of the available thermionic guns. The electron optical brightness, all other parameters remaining equal, however, increases linearly with the (relativistically corrected) accelerating voltage, giving HV microscopes in principle a better chance for obtaining high resolution in the STEM mode. While there is only one HV scanning microscope in operation today (Strojnik 1972), a number of HV TEMS exist and attempts have been made to adapt them for STEM operation. The presence of the diffraction, intermediate and projector lenses as installed in all HV TEMS offers a powerful electron optical contrast and alignment control, not available in pure scanning instruments. During the early stages of the development of the Cavendish 750 kV microscope (Smith et al 1966), a brief attempt was
t Present address: Institut fur Experimentelle Kernphysik, Karlsruhe, Weberstrasse 5, West Germany. 502
made to convert it for scanning operation (Smith and Considine 1968, Considine 1969). Beam alignment coils in the region between the second condenser and the objective lens were used for scanning the electron beam and the specimen was placed approximately in the middle of the objective lens, allowing operation at some 350 kV. A piece of plastic scintillator was introduced from the side just under the objective lens. Resolution better than 100 nm was obtained. Recently, Darlington (1974) used the same arrangement, except that he placed the scintillator at the bottom end of the column under the photographic chamber and obtained a point-to-point resolution of some 80nm. The electron probe diameter was observed to have strong cross modulation at 50 and 100 Hz and at 8 kHz (the fundamental HT power supply frequency), which could not be eliminated. Excessive HV ripple was also noticed and it was believed to contribute to the broadening of the electron probe. The practical resolution of the Cavendish 750 kV microscope, when properly maintained and operated, is better than 2 nm in the TEM mode. A systematic investigation of possible causes of the poor STEM performance was carried out and corrective measures were taken to close the gap. As similar problems and solutions are likely to appear when converting other HV instruments to STEM, a brief report of the work done and the results obtained is given below. 2 The optical system The injector for the accelerator originally consisted of a 5 kV thermionic gun with an operating emission of about 60 pA of which only 1-5 p A pass through a limiting aperture. The total length of the injector, including an auxiliary magnetic lens, amounts to approximately 1 m and it requires careful magnetic shielding because of the high sensitivity of relatively slow 5 kV electrons travelling along that length. The HT power supply has now been redesigned to produce 11 kV, the highest possible voltage within the space and equipment available, and the gun replaced by another similar to the one introduced in the Arizona HV STEM (Strojnik 1975b). The first condenser is strongly excited and sends an almost parallel beam, restricted by one of the adjustable apertures in the condenser 2, into the objective lens. Condenser 2 is normally switched off, but it can be used as a continuous objective aperture control. The objective lens is now excited at the highest current available and has a (calculated) focal length of approximately 3.8 mm at 500 kV. The specimen sits deep in the lens and some difficulty has been experienced in moving the long specimen holder through the airlock. The intermediate and projector lenses can be used to change the detector angle continuously. The bore of the intermediate lens allows electrons scattered at an angle of up to 0.1 rad to be collected. The combination of the control of the projector and intermediate lens permits the size and the angular spread of the transmitted beam to be optimized for the detector, independently of the detector acceptance angle. Some practical difficulties have been experienced in removing disturbing AC magnetic fields from the vicinity of the objective lens. In particular, the A c power of the diffusion pump, located at exactly the level of the objective lens and some 60 cm away, had to be replaced by a DC installation. The area under the photographic chamber has also been shielded to prevent AC fields from interfering with the detector/photomultiplier unit installed there. The deflection coils available in the region between the second condenser lens and the objective lens could be used directly for scanning the beam. Their inductance, however, limits the line frequency to 200 Hz. A new magnetic octopole stigmator had to be designed and built into the existing deflection coil system, because the stigmator of condenser 2 did
An improved scanning system for a high-voltage electron microscope not provide enough control of the beam cross section. The new stigmator is energized by the existing pair of power operational amplifiers, controlled by sine/cosine potentiometers and thus separating the adjustment of the azimuth and intensity of the correction. 3 Electronics Scanning electronic equipment already available in the laboratory was used and had to be adapted to the electronic equipment built into the microscope. A low-distortion survey picture of a whole specimen grid can be produced by switching off the objective lens and producing a probe (less than 1 pm in diameter) by exciting both condenser lenses, but the limited power output of the operational amplifiers driving the deflection coils restricts the lowest magnification in the ‘highresolution mode’ (i.e. with the objective lens on) to some 2 0 0 0 ~at 500 kV. The upper limit (600 OOOx) is at present given by the electron noise in the probe. It was also found that the signal-to-noise ratio in the deflection power amplifiers was poor at high magnifications; consequently the circuits were redesigned by tightening the feedback loop and decreasing the transconductance of the amplifiers.
non-transparent for the transmission of the light generated in the upper layers. Aluminizing the surface should give only a moderate improvement. With some 2 kg m-2 of the equivalent phosphor thickness the luminous output in reflection is considerably higher than in transmission (Bril-Klasens 1953) and this fact represents the basis for the new detector (figure 2). An electron beam entering through the 0.3 cm hole strikes a 0.1 cm thick layer of P47 phosphor. Part of the light is transmitted through the remaining thickness of the phosphor and penetrates a 0.3 cm thick, highly transparent sheet of Perspex
Figure 2 Electron detector. Electron beam passes through the 3 mm bore (1) into the aluminized (2) Perspex box (3) and strikes the phosphor (4). Light reaches the face of the photomultiplier (5) either directly through the bottom of the box or indirectly through reflections from the aluminized top (and walls) of the box. The phosphor is earthed by a 1 mm wide band of evaporated aluminium
Figure 1 Video amplifier connecting the EM1 9524 photomultiplier with the ‘contrast’ attenuator at the grids of the CRTS. Integration time constant adjustable with C1, DC control with RI
A simple video amplifier was designed (figure 1) to match the high-impedance output of the photomultiplier tube to the 10 kQ ‘contrast’ attenuators at the grids of both observation and recording CRTS. The potentiometer RI serves as a DC control and the variable capacitor C1 as the integration timeconstant adjustment. The combination of the DC control and either of the contrast controls (voltage supply for the photomultiplier tube or the attenuator at the grids of the CRTS) gives at least one order of magnitude wider contrast control than is possible with photographic material in the TEM mode, thus leaving the detector acceptance angle as a free parameter to be independently selected by the operator. The current stability of all deflection systems has been found to be of paramount importance. A stability of the order of 10 ppm is required for high-resolution operation. 4 A new electron detector Efficient electron recording at the 10-10-10-13 A level presents some problems at high accelerating voltages. Plastic scintillators do not appear to ofer enough resistance to the electron beam and are less efficient than some recently introduced fastdecay phosphors. The optimum thickness of the phosphor layer facing the usual photomultiplier tube varies with voltage. The penetration of 500 kV-1 MV electrons in a material of density g=2OOO kg m-3 is, according to von Borries (1942), about 0-1 cm. This thickness however makes the phosphor almost
Fiere 3 Gdd-shadowed 0.79 CLm diameter PolYstYrene sphere. The inset shows details in the penumbra at higher magnification, 500 kV, 60 s, illumination angle 3 x 10-3 rad, detector angle 1o-2 rad
503
A Strojnik and T G Sparrow to strike the face of the photomultiplier. A major part of the light is, however, emitted backwards and strikes the mirror at the top of the Perspex box and eventually also passes through the bottom (0.3 cm) Perspex sheet. Assuming that the angular distribution of the back-emitted light obeys Lambert’s law, the improvement in collecting efficiency amounts theoretically to a factor of 3, and more than x 2 has been experimentally verified. It should be noted that the detector described here does not represent an optimized design. A parabolic, or similar, mirror and larger dimensions should give considerably greater collecting efficiency for high-energy electrons. 5 Results Figure 3 shows a 0.8 pm diameter gold-shadowed polystyrene sphere, and a highly magnified part of the penumbra, in which a resolution of less than 4 nm is demonstrated at several places. The microscope was operated at 500 kV and pictures were taken from a lm-~ine CRT in approximate~y 60 S. The resOlution at this stage is believed to be limited by the accelerator instability. Contamination is noticeable at high resolution but it does not limit the resolution. A direct quantitative evaluation of micrographs, only possible in STEM, is demonstrated in figure 4, which shows a through-focus series of an AI single crystal taken at 500 kV, approximately 1.5 p m in thickness (left) and they modulation traces along a selected horizontal line (right). Interferences due to Bragg reflections are clearly visible on the traces and are
Figure 4 AI single crystal, 1.5 pm thick. Through-focus series showing phase reversal of the ‘node’. Total defocusing 60 pm (objective lens current increasing from the top to the bottom). Width of micrographs 10 pm. Detector angle 10-3 rad, the rest of the data as in figure 3. Right-hand side shows y modulation across the indicated horizontal line, directly available to the computer or tape recorder for a quantitative evaluation.
504
immediately available for numerical processing. The relatively large ‘defocusing’ of 60 pm at a focal length of less than 4 mm should be noted. Acknowledgments The conversion of the Cavendish 750 kV microscope into STEM was suggested by Dr V E Cosslett as an exploratory investigation for possible future applications in high-resolution HV microscopes. One of the authors (A S) was provided with financial support as a Senior Visiting Fellow by the Science Research Council, on leave from Arizona State University. References von Berries B 1942 2.phys. 119 498-501 Bri1-Klasens 1953 philips Tech. Rund. 14 398-401 C o d d i n e K T 1969 PhD Thesis University of Cambridge Darlington E H 1974 Proc. 3rd Znt. ConJ on HV Electron Microscopy, Oxford (London: Academic Press) pp 136-9 Smith K C A and Considine K 1968 Proc. 4th Europ. Regional ConJ on Electron Microscopy, Rome (Rome: Tipografia Poliglotta Vaticana) v ~ 1l pp 7 3 4 Smith K C A, Considine K T and Cosslett V E 1966 Proc. 6th Znt. Congr. on Electron Microscopy, Kyoto (Tokyo: Maruzen Co.) pp 99-100 Strojnik A 1972 Proc. 5th Ann. SEM Symp. (Chicago: IITRI) pp 215-23 Strojnik A 1975a Proc. 4th Znt. Congr. on HV Electron Microscopy, Toulouse, eds B Jouffrey and P Favard (Paris: Societ6 Franpise de Microscopie Electronique) pp 27-30 Strojnik A 1975b Proc. 33rd A. EMSA Meeting, Las Vegas, ed G W Bailey (Baton Rouge: Claitor) pp 116-7
Journal of Physics E: Scientific Instruments 1977 Volume 10 Printed in Great Britain 0 1977
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My IOPscience
An improved scanning system for a high-voltage electron microscope
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1977 J. Phys. E: Sci. Instrum. 10 502 (http://iopscience.iop.org/0022-3735/10/5/024) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 141.209.100.60 This content was downloaded on 03/10/2015 at 11:45
Please note that terms and conditions apply.
An improved scanning system for a high-voltage electron microscope
A Strojnik? and T G Sparrow Cavendish Laboratory, University of Cambridge, Cambridge CB2 3RQ Receiced 29 December 1976 Abstract The 750 kV Cavendish electron microscope has been adapted for scanning transmission operation. Disturbing magnetic fields at 50 and 100 Hz and at 8 kHz have been tracked carefully and successively removed to produce an electron beam without cross modulation. Further attention to detail in HV and lens electronics as well as in deflection systems led to an electron probe smaller than 3 nm in diameter. By operating the last demagnifying (objective) lens at very high excitation and by introducing a new electron detector, less than 4 nm point-to-point resolution has been obtained at 500 kV. 1 Introduction Superior selective penetration/resolution in thick specimens (Strojnik 1975a),combined with electronic and electron optical contrast and gamma control, and a possibility to simultaneously extract information from both elastically and inelastically scattered electrons, make the scanning transmission mode (STEM) an attractive supplement and a possible future alternative to the fixed-beam (TEM)imaging. If the electron optical performance of the objective or the last demagnifying lens is identical, the same theoretical resolving power could be expected in TEM and in STEM. In practice, however, STEMS using thermionic electron sources show rather poor resolution. In the best 100 kV commercial TEMS, converted into the STEM mode, the resolution is almost an order of magnitude inferior in STEM as compared to TEM. The major reason is insufficient electron optical brightness of the available thermionic guns. The electron optical brightness, all other parameters remaining equal, however, increases linearly with the (relativistically corrected) accelerating voltage, giving HV microscopes in principle a better chance for obtaining high resolution in the STEM mode. While there is only one HV scanning microscope in operation today (Strojnik 1972), a number of HV TEMS exist and attempts have been made to adapt them for STEM operation. The presence of the diffraction, intermediate and projector lenses as installed in all HV TEMS offers a powerful electron optical contrast and alignment control, not available in pure scanning instruments. During the early stages of the development of the Cavendish 750 kV microscope (Smith et al 1966), a brief attempt was
t Present address: Institut fur Experimentelle Kernphysik, Karlsruhe, Weberstrasse 5, West Germany. 502
made to convert it for scanning operation (Smith and Considine 1968, Considine 1969). Beam alignment coils in the region between the second condenser and the objective lens were used for scanning the electron beam and the specimen was placed approximately in the middle of the objective lens, allowing operation at some 350 kV. A piece of plastic scintillator was introduced from the side just under the objective lens. Resolution better than 100 nm was obtained. Recently, Darlington (1974) used the same arrangement, except that he placed the scintillator at the bottom end of the column under the photographic chamber and obtained a point-to-point resolution of some 80nm. The electron probe diameter was observed to have strong cross modulation at 50 and 100 Hz and at 8 kHz (the fundamental HT power supply frequency), which could not be eliminated. Excessive HV ripple was also noticed and it was believed to contribute to the broadening of the electron probe. The practical resolution of the Cavendish 750 kV microscope, when properly maintained and operated, is better than 2 nm in the TEM mode. A systematic investigation of possible causes of the poor STEM performance was carried out and corrective measures were taken to close the gap. As similar problems and solutions are likely to appear when converting other HV instruments to STEM, a brief report of the work done and the results obtained is given below. 2 The optical system The injector for the accelerator originally consisted of a 5 kV thermionic gun with an operating emission of about 60 pA of which only 1-5 p A pass through a limiting aperture. The total length of the injector, including an auxiliary magnetic lens, amounts to approximately 1 m and it requires careful magnetic shielding because of the high sensitivity of relatively slow 5 kV electrons travelling along that length. The HT power supply has now been redesigned to produce 11 kV, the highest possible voltage within the space and equipment available, and the gun replaced by another similar to the one introduced in the Arizona HV STEM (Strojnik 1975b). The first condenser is strongly excited and sends an almost parallel beam, restricted by one of the adjustable apertures in the condenser 2, into the objective lens. Condenser 2 is normally switched off, but it can be used as a continuous objective aperture control. The objective lens is now excited at the highest current available and has a (calculated) focal length of approximately 3.8 mm at 500 kV. The specimen sits deep in the lens and some difficulty has been experienced in moving the long specimen holder through the airlock. The intermediate and projector lenses can be used to change the detector angle continuously. The bore of the intermediate lens allows electrons scattered at an angle of up to 0.1 rad to be collected. The combination of the control of the projector and intermediate lens permits the size and the angular spread of the transmitted beam to be optimized for the detector, independently of the detector acceptance angle. Some practical difficulties have been experienced in removing disturbing AC magnetic fields from the vicinity of the objective lens. In particular, the A c power of the diffusion pump, located at exactly the level of the objective lens and some 60 cm away, had to be replaced by a DC installation. The area under the photographic chamber has also been shielded to prevent AC fields from interfering with the detector/photomultiplier unit installed there. The deflection coils available in the region between the second condenser lens and the objective lens could be used directly for scanning the beam. Their inductance, however, limits the line frequency to 200 Hz. A new magnetic octopole stigmator had to be designed and built into the existing deflection coil system, because the stigmator of condenser 2 did
An improved scanning system for a high-voltage electron microscope not provide enough control of the beam cross section. The new stigmator is energized by the existing pair of power operational amplifiers, controlled by sine/cosine potentiometers and thus separating the adjustment of the azimuth and intensity of the correction. 3 Electronics Scanning electronic equipment already available in the laboratory was used and had to be adapted to the electronic equipment built into the microscope. A low-distortion survey picture of a whole specimen grid can be produced by switching off the objective lens and producing a probe (less than 1 pm in diameter) by exciting both condenser lenses, but the limited power output of the operational amplifiers driving the deflection coils restricts the lowest magnification in the ‘highresolution mode’ (i.e. with the objective lens on) to some 2 0 0 0 ~at 500 kV. The upper limit (600 OOOx) is at present given by the electron noise in the probe. It was also found that the signal-to-noise ratio in the deflection power amplifiers was poor at high magnifications; consequently the circuits were redesigned by tightening the feedback loop and decreasing the transconductance of the amplifiers.
non-transparent for the transmission of the light generated in the upper layers. Aluminizing the surface should give only a moderate improvement. With some 2 kg m-2 of the equivalent phosphor thickness the luminous output in reflection is considerably higher than in transmission (Bril-Klasens 1953) and this fact represents the basis for the new detector (figure 2). An electron beam entering through the 0.3 cm hole strikes a 0.1 cm thick layer of P47 phosphor. Part of the light is transmitted through the remaining thickness of the phosphor and penetrates a 0.3 cm thick, highly transparent sheet of Perspex
Figure 2 Electron detector. Electron beam passes through the 3 mm bore (1) into the aluminized (2) Perspex box (3) and strikes the phosphor (4). Light reaches the face of the photomultiplier (5) either directly through the bottom of the box or indirectly through reflections from the aluminized top (and walls) of the box. The phosphor is earthed by a 1 mm wide band of evaporated aluminium
Figure 1 Video amplifier connecting the EM1 9524 photomultiplier with the ‘contrast’ attenuator at the grids of the CRTS. Integration time constant adjustable with C1, DC control with RI
A simple video amplifier was designed (figure 1) to match the high-impedance output of the photomultiplier tube to the 10 kQ ‘contrast’ attenuators at the grids of both observation and recording CRTS. The potentiometer RI serves as a DC control and the variable capacitor C1 as the integration timeconstant adjustment. The combination of the DC control and either of the contrast controls (voltage supply for the photomultiplier tube or the attenuator at the grids of the CRTS) gives at least one order of magnitude wider contrast control than is possible with photographic material in the TEM mode, thus leaving the detector acceptance angle as a free parameter to be independently selected by the operator. The current stability of all deflection systems has been found to be of paramount importance. A stability of the order of 10 ppm is required for high-resolution operation. 4 A new electron detector Efficient electron recording at the 10-10-10-13 A level presents some problems at high accelerating voltages. Plastic scintillators do not appear to ofer enough resistance to the electron beam and are less efficient than some recently introduced fastdecay phosphors. The optimum thickness of the phosphor layer facing the usual photomultiplier tube varies with voltage. The penetration of 500 kV-1 MV electrons in a material of density g=2OOO kg m-3 is, according to von Borries (1942), about 0-1 cm. This thickness however makes the phosphor almost
Fiere 3 Gdd-shadowed 0.79 CLm diameter PolYstYrene sphere. The inset shows details in the penumbra at higher magnification, 500 kV, 60 s, illumination angle 3 x 10-3 rad, detector angle 1o-2 rad
503
A Strojnik and T G Sparrow to strike the face of the photomultiplier. A major part of the light is, however, emitted backwards and strikes the mirror at the top of the Perspex box and eventually also passes through the bottom (0.3 cm) Perspex sheet. Assuming that the angular distribution of the back-emitted light obeys Lambert’s law, the improvement in collecting efficiency amounts theoretically to a factor of 3, and more than x 2 has been experimentally verified. It should be noted that the detector described here does not represent an optimized design. A parabolic, or similar, mirror and larger dimensions should give considerably greater collecting efficiency for high-energy electrons. 5 Results Figure 3 shows a 0.8 pm diameter gold-shadowed polystyrene sphere, and a highly magnified part of the penumbra, in which a resolution of less than 4 nm is demonstrated at several places. The microscope was operated at 500 kV and pictures were taken from a lm-~ine CRT in approximate~y 60 S. The resOlution at this stage is believed to be limited by the accelerator instability. Contamination is noticeable at high resolution but it does not limit the resolution. A direct quantitative evaluation of micrographs, only possible in STEM, is demonstrated in figure 4, which shows a through-focus series of an AI single crystal taken at 500 kV, approximately 1.5 p m in thickness (left) and they modulation traces along a selected horizontal line (right). Interferences due to Bragg reflections are clearly visible on the traces and are
Figure 4 AI single crystal, 1.5 pm thick. Through-focus series showing phase reversal of the ‘node’. Total defocusing 60 pm (objective lens current increasing from the top to the bottom). Width of micrographs 10 pm. Detector angle 10-3 rad, the rest of the data as in figure 3. Right-hand side shows y modulation across the indicated horizontal line, directly available to the computer or tape recorder for a quantitative evaluation.
504
immediately available for numerical processing. The relatively large ‘defocusing’ of 60 pm at a focal length of less than 4 mm should be noted. Acknowledgments The conversion of the Cavendish 750 kV microscope into STEM was suggested by Dr V E Cosslett as an exploratory investigation for possible future applications in high-resolution HV microscopes. One of the authors (A S) was provided with financial support as a Senior Visiting Fellow by the Science Research Council, on leave from Arizona State University. References von Berries B 1942 2.phys. 119 498-501 Bri1-Klasens 1953 philips Tech. Rund. 14 398-401 C o d d i n e K T 1969 PhD Thesis University of Cambridge Darlington E H 1974 Proc. 3rd Znt. ConJ on HV Electron Microscopy, Oxford (London: Academic Press) pp 136-9 Smith K C A and Considine K 1968 Proc. 4th Europ. Regional ConJ on Electron Microscopy, Rome (Rome: Tipografia Poliglotta Vaticana) v ~ 1l pp 7 3 4 Smith K C A, Considine K T and Cosslett V E 1966 Proc. 6th Znt. Congr. on Electron Microscopy, Kyoto (Tokyo: Maruzen Co.) pp 99-100 Strojnik A 1972 Proc. 5th Ann. SEM Symp. (Chicago: IITRI) pp 215-23 Strojnik A 1975a Proc. 4th Znt. Congr. on HV Electron Microscopy, Toulouse, eds B Jouffrey and P Favard (Paris: Societ6 Franpise de Microscopie Electronique) pp 27-30 Strojnik A 1975b Proc. 33rd A. EMSA Meeting, Las Vegas, ed G W Bailey (Baton Rouge: Claitor) pp 116-7
Journal of Physics E: Scientific Instruments 1977 Volume 10 Printed in Great Britain 0 1977
