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Use of Channel-plate Intensifies in the Field-ion Microscope
Use of Channel-plate Intensifiers in the Field-ion Microscope P. J. TURNER, P. CARTWRIGHTT, E. D. BOYESS and M. J. SOUTHON Department of Metallurgy and Materials Science, University of Cambridge, England
INTRODUCTION This paper describes the use of a channel-plate intensifier for display of the image formed in the field-ion microscope. The specimen is in the form of a very sharp point, having a radius of the order of 100 nm, to which a high positive voltage is applied. The electric field is sufficiently high to cause ionization of the residual inert gas a t the specimen, and the image is formed by the impingement of the resulting positive ions on a phosphor screen. Details of the specimen are revealed by a spatial modulation of the ion current density. A single image point is produced typically by a n ion flux of some lo4 particles/sec. Common image gases are helium, neon, argon and hydrogen. A full review of field-ion microscopy has recently been given by Miiller and Ts0ng.l Records of the field-ion image are usually made by means of a large aperture lens and fast film. The photographic exposure times needed therefore depend on the ion-to-photon energy conversion efficiency of the phosphor, the performance of any intensifier employed, and the statistics of the noise introduced into the system. The efficiency of the phosphor depends inversely on the mass of the exciting ion, and is therefore higher for helium than for neon or argon. The phosphor is also very sensitive to ion damage. Without any intensifier, exposure times are about 1 min for helium images, 2 h for neon and almost impossibly long for argon. The use of these latter two gases is, however, essential to field-ion microscopy since the field required to form images with neon or argon is significantly lower than that required to form a helium-ion image. The field for a helium-ion image is about 50 V/nm, which generates a hydrostatic surface stress a t the specimen of Now at English Electric Valve Company, Chelrnsford, England On leave from General Electrio Company, Wembley, England.
inn
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P. J. TURNER, P. CARTWRIOHT, E. D. BOYES AND .&I
J. SOIJTHON
1011dyneslcm2, which is sufficient to fracture all but the most refractory of materials, e.g. tungsten, iridium. Using neon, images may be obtained from materials such as iron, nickel and gold, whilst using argon, images may be obtained from materials such as aluminium. The number of ions per second per image point is independent of the image gas and if it were possible to record each of these ions directly, an acceptable signal-to-noise ratio could be obtained within a few milliseconds. Coupling a high-gain image intensifier to the microscope is not the best way of achieving this. Although the brightness gain is sufficient, an acceptable signal-to-noise ratio is only preserved in the case of a helium-ion image. After the inefficient ion-to-photon conversion and optical coupling stages, there are still 10- photoelectrons released from the photocathode of the intensifier per ion arriving a t the phosphor, which is sufficient t o give short, noise-free exposures. With photoelectrons per ion and a large argon, this number is reduced t o amount of noise is introduced. This can be expressed in a quantitative manner as follows. The average number of photoelectrons fi released from the intensifier photocathode in time t is related t o the average incident ii on the microscope screen by the expression : number of ions ?
ii=cpF.a, where c i is the quantum efficiency of the photocathode, p is the coupling lens collection efficiency and i? is the phosphor conversion efficiency of the microscope screen expressed as photons per ion. Following Mandel,2 the variance of the photoelectron distribution is then : A2n = r5 p B 6,(5 p i? 1).
+
and i? = lo2 Typical values of these quantities are : 6 = i; for helium ions and for argon i o n s d f three is taken as a usable value of the signal-to-noise ratio ii(A2n)-1/2, then a statistically significant image point requires lo2 helium ions and los argon ione. An external intensifier may therefore be expected to reduce photographic exposure times to no less than minutes for an argon-ion image.
INTERNAL INTENSIFIER SYSTEMS The problem of adverse noise characteristics may be avoided by replacing the phosphor with some device which transforms the ion image into an electron image. This was initially achieved using an oxide-coated mesh.3 Electrons emitted from this were accelerated and focused on to a phosphor having a high energy-conversion efficiency for electrons, and the gain of the system as a whole results from substitution of the high electron-energy conversion efficiency for the poor ion-energy conversion efficiency. The detection efficiency of this system
CHANNEL-PLATE INTENSIFIERS IN FIELD-ION MICROSCOPE
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is nearly unity and is independent of the image gas employed. Additional brightness gain could now be obtained satisfactorily with an external intensifier, but both processes may be combined by using a channel electron-multiplier array as the primary detector. As has already been reported this gives a detection efficiency which is again nearly unity and is approximately independent of the ion specie^.^ Gain arises from the improved phosphor efficiency and the particle multiplication within the array. The statistics of the performance of the oxide-coated mesh system are similar to those of helium using a phosphor as the primary detector. Using a channel-plate system, the variance in the number of secondary electrons emerging from the channel plate released by f i ions is : -2
K=FGG
where G is the gain of the channel plate, and F is the noise factor.= The noise in the resulting image is therefore that of the primary ion beam increased by the noise factor of the channel plate. Images obtained with any image gas may be recorded in milliseconds, and the limit to exposure time is set by the noise in the field-ion signal.
EXPERIMENTAL PERFORMANCE OF THE CHANNELPLATE Figure 1 is a fairly representative field-ion image. Each bright spot represents the position of a surface atom and there are typically some lo4 image points per picture. No intensifier system was used to record the image. All the image points are of differing sizes and intensities, so that to preserve the image characteristics to the greatest possible extent, the display system should possess the highest practicable resolution and contrast. Both proximity and magnetic focusing of the secondary electrons from the channel plate have been used and the latter appears to have the better performance. The layout of the combined microscope and channel-plate section is shown in Fig. 2 and in this case magnetic focusing has been used. The whole system Torr or better, and must is designed to achieve a pressure of therefore be compatible with repeated bake-out cycles. I n operation, the specimen is held a t the appropriate low temperature a t the end of a cryostat and the ion image impinges directly on to the channel plate. The electric field is provided by a series of annuli in the walls of the removable glass envelope and the magnetic field is provided by an external solenoid which may be slid over the envelope. An internal potential divider chain is used to minimize the number of electrical feeds required. This method of focusing preserves the spatial distribution of the secondary electrons to such an extent that, for a channel plate composed of 40-pm-diameter channels, individual channels can
1080 P. J.
TURNER, P. CARTWRIOHT, E. D. BOYES AND M. J. SOUTHON
be resolved on the microscope screen. This is shown in Fig. 3. The resolution capability of the converter system can be expressed objectively using the concept of the modulation transfer function. This is strictly valid only for systems which are both linear and spatially invariant. The channel plate at high gains satisfies neither of these conditions. However, a t low gain and spatial frequencies below half the channel plate lattice fundamental, the m.t.f. is a useful guide to the expected performance. This is shown in Fig. 4, which has been derived from a Fourier analysis of the line-spread function. The resolution of the image is not seriously impaired a t surface densities as low as 16 channels per image point and, in practice, 4 channels per
Pro. 1. A helium-ion micrograph of an iridium specimen. Each bright spot represents an atom a t the surface of the Rpeoimen, and there are typically lo4 image points per micrograph. These points are all of differing sizes, shapes and intensities, and any intensifier system must be capable of preserving all these characteristics to the greatest possible extent. It is estimated that the system will be capable of resolving some 106 elements. (Micrograph kindly supplied by T. F. Pagc.)
CHANNEL-PLATE INTENSIFIERS I N FIELD-ION MICROSCOPE
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annuli and resistor chain
Fro. 2. Schematic layout of thc field-ion mici o~~copt’ uiid channcl-plate intenaifier. The specimen is hrltl at a high positive potential, u p to 30 liV, at the end of a cryostat. The ion-image fall* directly on to the channel plate. In thir caw a magrletlc focuaing arrangement haz been employed, and the whole systrrn 15 capable of achievlng prrsrurer between 10-12 ant1 10-lo Torr of rezitliial paw,.
Fro. 3. The intliviclual 40-pm tliainetrr channels rr.it~lvetlat tho Ycreen. Thir syrtrm preserves all the image points avail~hlron a I-in. diametrr platr.
1082 P.
J . TURNER, P. CARTWRIGHT, E. D. BOYES AND M. J. SOUTHON
image point has been found to be acceptable. With proximity focusing, using the spacings and screen voltages reliably attainable in a demountable system (4 kV across a 1.5-mm gap) the m.t.f. is significantly poorer. A comparison of the images obtained from the two systems is shown in Figs. 5 and 6 (magnetically and proximity focused, respectively).
--.\" ; lI o t
2
'
/
,
,
2
5
1
\I
,\ '
,:, 1
10
20
50
Spatial frequency ( l p / m r n )
FIG.4. The modulation transfer function for the channel plate. The clotted line denotes the region whew the spatial frequency bocomes comparable with the channel spacing.
CHANNELPLATE FATIGUE As mentioned above, apart from the purified image gas admitted Torr, the channel plate is operating to a pressure of the order of in a residual vacuum of better than Torr. Under these conditions, operating in the linear portion of the gain characteristic where saturation effects would not occur, a reduction in the gain of the channel plate due to fatigue has been observed. This is most pronounced in those regions of the plate where the signal is strongest, and is possibly to be associated with a fall in the secondary emission coefficient of the channel walls due to electron impact desorption of the adsorbed gas. Thus, if, after illuminating the channel plate with an image of dark and bright areas, the plate is flooded with uniform illumination, those areas which were bright now appear dark, and those areas which were dark now appear bright. This is illustrated in Fig. I. If a steady image is displayed for more than a few seconds, the contrast starts t o deteriorate, obviously an undesirable feature. Bright spots (denoting different atomic species or differently sited atoms) are quickly reduced tfo an intensity comparable with normally emitting atoms.
CHANNEL-PIATE INTENSIFIERS IT FIELD-IOS MI('HOSC0PE
I083
FIG.5 . A neon-ion micrograph of a nickel specimen imaged using a magnetically focmed 2-in. channel plate. Resolution and contrast losses are not readily detectable by Byt'.
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P. J. TURNER, P. CARTWRIUHT, E. D. BOYES AND M. J . SOVTHON
FIQ.6. An iridium specimen imaged in helium using a 2.in. diameter channel plate in the proximity mode. There is some loss of contrast, and all image points tend to appear circular. (Micrographkindly supplied by N. J. Sanders.)
(4
(b)
FIG.7. (a)Tungsten specimen using helium as the image g a z . The channel plate had not been outgartsxed, and subsequent flooding of the plate wlth uniform ion illumination shows (b) a negative image.
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P. J . TURNER, P. CARTWRIQHT, E. D. BOYES AND M. J. SOUTHON
This effect can be minimized by degassing the channel plate. This is effected by subjecting the plate to uniform electron illumination for a period of about 15 min. An input current of the order of 1 pA has usually been used, and the plate is run a t a differential of 1000 V. Under normal operation, a differential of 800 V or less is used, since contrast reduction takes place more rapidly for higher voltages. It has been found unnecessary to repeat this process after subsequent vacuum cycles. It might further be expected that ion feedback phenomena would occur due to the presence of the image gas. No such effects have been observed. A high image-gas pressure is synonymous with a high input current to the channel plate and, in practice, saturation effects set in a t lower input currents.
CONCLUSIONS The advent of the channel plate has widened very considerably the range of materials which may be studied by field-ion microscopy. Images using any gas may be recorded in times which are limited only by the noise in the primary ion signal. I n particular, argon may conveniently be employed as an image gas, and work is currently proceeding on the applications of argon-ion microscopy to aluminium and its alloys. An argon image of aluminium is shown in Fig. 8.
BIG. 8. An argon-ion image of an aluminium specimon. This material, and many uthers of metallrirgical importance, can now be imaged in a convenient manner using thp channel-plate intensifier.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support of the Science Research Council and U.K.A.E.A. Harwell. All the work described in this paper has been carried out using channel plates manufactured by Mullard Limited. The authors are grateful to Mullard for having made theRe plates available and for their advice on their usage.
REFERENCES 1. Muller, E. W. and Tsong, T. T., “Field-Ion Microscopy”. Elsevier, New York (1969). 2. Mandel, L. L., Br. J . Appl. Phys. 26, 1302 (1959). 3. Turner, P. J., Ph.D. Thesis, Cambridge University (1967). 4. Turner, P. J., Cartwright, P., Southon, M. J., van Oostrom, A., and Manley, B. W., J . Sci. Instrum. ( J . Phys. E . ) ,Series 2 , 2, 731 (1969). 5. Manley, B. W., Guest, A. J. and Holmshaw, R. T., In “Adv. E.E.P.”, Vol. ZSA, p. 471 (1969).
DISCUSSION J. RING : You
mentioned that you need lo5 picture points in the final picture which is comparable to the ast,ronomical usc of image tubes. How do you use all these data? P. CARTWRIGHT : A typical field-ion image consists of roughly lo4 bright spots or image points, each representing an individual atom of the specimen. The use that is mads of these data depends on t,he particular experiment but one is usually not concerned with the brightness or shape of every individual spot, but rather with features of the pattern of spots as a n~hole. For example, deviations of the patt>ernfrom that expected for a perfect crystal may indicate the presence of lat,tice defects, grain boundaries or particlcs of a different phase, all of interest to metallurgists, whereas changes in t.he pattern from one image to the next enable one to follow the progress of cert,nin surface processes. H. J . G . MEYER : You can now observe met’alsother than were previously posvible but has it. now also become possible to st,udy effects due t,o field evaporation of individual atoms? (i.e. by applying short)voltage pulses and making a comparison between the conditions before and after the pulses using comparative colour techniques). P. CARTWRIGHT : The introduction of the channel-plate seems to have satisfied the long-&anding need for image intensification in field-ion microscopy and has widened it,s scope in several directions by great,ly facilitating observation and recording of images. Among tlhe new possibilities is that successive images on a suitable phosphor can be recorded through different colour filters on a single frame of colour film. The resulting photograph indicates sensitively any small changes in brightness or position of image points between exposures, as van Oostrom has shown in Eindhoven, and can usefully form composite micrographs of two-phase materials as Schubort has shown in Cambridge.
INTRODUCTION This paper describes the use of a channel-plate intensifier for display of the image formed in the field-ion microscope. The specimen is in the form of a very sharp point, having a radius of the order of 100 nm, to which a high positive voltage is applied. The electric field is sufficiently high to cause ionization of the residual inert gas a t the specimen, and the image is formed by the impingement of the resulting positive ions on a phosphor screen. Details of the specimen are revealed by a spatial modulation of the ion current density. A single image point is produced typically by a n ion flux of some lo4 particles/sec. Common image gases are helium, neon, argon and hydrogen. A full review of field-ion microscopy has recently been given by Miiller and Ts0ng.l Records of the field-ion image are usually made by means of a large aperture lens and fast film. The photographic exposure times needed therefore depend on the ion-to-photon energy conversion efficiency of the phosphor, the performance of any intensifier employed, and the statistics of the noise introduced into the system. The efficiency of the phosphor depends inversely on the mass of the exciting ion, and is therefore higher for helium than for neon or argon. The phosphor is also very sensitive to ion damage. Without any intensifier, exposure times are about 1 min for helium images, 2 h for neon and almost impossibly long for argon. The use of these latter two gases is, however, essential to field-ion microscopy since the field required to form images with neon or argon is significantly lower than that required to form a helium-ion image. The field for a helium-ion image is about 50 V/nm, which generates a hydrostatic surface stress a t the specimen of Now at English Electric Valve Company, Chelrnsford, England On leave from General Electrio Company, Wembley, England.
inn
1078
P. J. TURNER, P. CARTWRIOHT, E. D. BOYES AND .&I
J. SOIJTHON
1011dyneslcm2, which is sufficient to fracture all but the most refractory of materials, e.g. tungsten, iridium. Using neon, images may be obtained from materials such as iron, nickel and gold, whilst using argon, images may be obtained from materials such as aluminium. The number of ions per second per image point is independent of the image gas and if it were possible to record each of these ions directly, an acceptable signal-to-noise ratio could be obtained within a few milliseconds. Coupling a high-gain image intensifier to the microscope is not the best way of achieving this. Although the brightness gain is sufficient, an acceptable signal-to-noise ratio is only preserved in the case of a helium-ion image. After the inefficient ion-to-photon conversion and optical coupling stages, there are still 10- photoelectrons released from the photocathode of the intensifier per ion arriving a t the phosphor, which is sufficient t o give short, noise-free exposures. With photoelectrons per ion and a large argon, this number is reduced t o amount of noise is introduced. This can be expressed in a quantitative manner as follows. The average number of photoelectrons fi released from the intensifier photocathode in time t is related t o the average incident ii on the microscope screen by the expression : number of ions ?
ii=cpF.a, where c i is the quantum efficiency of the photocathode, p is the coupling lens collection efficiency and i? is the phosphor conversion efficiency of the microscope screen expressed as photons per ion. Following Mandel,2 the variance of the photoelectron distribution is then : A2n = r5 p B 6,(5 p i? 1).
+
and i? = lo2 Typical values of these quantities are : 6 = i; for helium ions and for argon i o n s d f three is taken as a usable value of the signal-to-noise ratio ii(A2n)-1/2, then a statistically significant image point requires lo2 helium ions and los argon ione. An external intensifier may therefore be expected to reduce photographic exposure times to no less than minutes for an argon-ion image.
INTERNAL INTENSIFIER SYSTEMS The problem of adverse noise characteristics may be avoided by replacing the phosphor with some device which transforms the ion image into an electron image. This was initially achieved using an oxide-coated mesh.3 Electrons emitted from this were accelerated and focused on to a phosphor having a high energy-conversion efficiency for electrons, and the gain of the system as a whole results from substitution of the high electron-energy conversion efficiency for the poor ion-energy conversion efficiency. The detection efficiency of this system
CHANNEL-PLATE INTENSIFIERS IN FIELD-ION MICROSCOPE
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is nearly unity and is independent of the image gas employed. Additional brightness gain could now be obtained satisfactorily with an external intensifier, but both processes may be combined by using a channel electron-multiplier array as the primary detector. As has already been reported this gives a detection efficiency which is again nearly unity and is approximately independent of the ion specie^.^ Gain arises from the improved phosphor efficiency and the particle multiplication within the array. The statistics of the performance of the oxide-coated mesh system are similar to those of helium using a phosphor as the primary detector. Using a channel-plate system, the variance in the number of secondary electrons emerging from the channel plate released by f i ions is : -2
K=FGG
where G is the gain of the channel plate, and F is the noise factor.= The noise in the resulting image is therefore that of the primary ion beam increased by the noise factor of the channel plate. Images obtained with any image gas may be recorded in milliseconds, and the limit to exposure time is set by the noise in the field-ion signal.
EXPERIMENTAL PERFORMANCE OF THE CHANNELPLATE Figure 1 is a fairly representative field-ion image. Each bright spot represents the position of a surface atom and there are typically some lo4 image points per picture. No intensifier system was used to record the image. All the image points are of differing sizes and intensities, so that to preserve the image characteristics to the greatest possible extent, the display system should possess the highest practicable resolution and contrast. Both proximity and magnetic focusing of the secondary electrons from the channel plate have been used and the latter appears to have the better performance. The layout of the combined microscope and channel-plate section is shown in Fig. 2 and in this case magnetic focusing has been used. The whole system Torr or better, and must is designed to achieve a pressure of therefore be compatible with repeated bake-out cycles. I n operation, the specimen is held a t the appropriate low temperature a t the end of a cryostat and the ion image impinges directly on to the channel plate. The electric field is provided by a series of annuli in the walls of the removable glass envelope and the magnetic field is provided by an external solenoid which may be slid over the envelope. An internal potential divider chain is used to minimize the number of electrical feeds required. This method of focusing preserves the spatial distribution of the secondary electrons to such an extent that, for a channel plate composed of 40-pm-diameter channels, individual channels can
1080 P. J.
TURNER, P. CARTWRIOHT, E. D. BOYES AND M. J. SOUTHON
be resolved on the microscope screen. This is shown in Fig. 3. The resolution capability of the converter system can be expressed objectively using the concept of the modulation transfer function. This is strictly valid only for systems which are both linear and spatially invariant. The channel plate at high gains satisfies neither of these conditions. However, a t low gain and spatial frequencies below half the channel plate lattice fundamental, the m.t.f. is a useful guide to the expected performance. This is shown in Fig. 4, which has been derived from a Fourier analysis of the line-spread function. The resolution of the image is not seriously impaired a t surface densities as low as 16 channels per image point and, in practice, 4 channels per
Pro. 1. A helium-ion micrograph of an iridium specimen. Each bright spot represents an atom a t the surface of the Rpeoimen, and there are typically lo4 image points per micrograph. These points are all of differing sizes, shapes and intensities, and any intensifier system must be capable of preserving all these characteristics to the greatest possible extent. It is estimated that the system will be capable of resolving some 106 elements. (Micrograph kindly supplied by T. F. Pagc.)
CHANNEL-PLATE INTENSIFIERS I N FIELD-ION MICROSCOPE
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annuli and resistor chain
Fro. 2. Schematic layout of thc field-ion mici o~~copt’ uiid channcl-plate intenaifier. The specimen is hrltl at a high positive potential, u p to 30 liV, at the end of a cryostat. The ion-image fall* directly on to the channel plate. In thir caw a magrletlc focuaing arrangement haz been employed, and the whole systrrn 15 capable of achievlng prrsrurer between 10-12 ant1 10-lo Torr of rezitliial paw,.
Fro. 3. The intliviclual 40-pm tliainetrr channels rr.it~lvetlat tho Ycreen. Thir syrtrm preserves all the image points avail~hlron a I-in. diametrr platr.
1082 P.
J . TURNER, P. CARTWRIGHT, E. D. BOYES AND M. J. SOUTHON
image point has been found to be acceptable. With proximity focusing, using the spacings and screen voltages reliably attainable in a demountable system (4 kV across a 1.5-mm gap) the m.t.f. is significantly poorer. A comparison of the images obtained from the two systems is shown in Figs. 5 and 6 (magnetically and proximity focused, respectively).
--.\" ; lI o t
2
'
/
,
,
2
5
1
\I
,\ '
,:, 1
10
20
50
Spatial frequency ( l p / m r n )
FIG.4. The modulation transfer function for the channel plate. The clotted line denotes the region whew the spatial frequency bocomes comparable with the channel spacing.
CHANNELPLATE FATIGUE As mentioned above, apart from the purified image gas admitted Torr, the channel plate is operating to a pressure of the order of in a residual vacuum of better than Torr. Under these conditions, operating in the linear portion of the gain characteristic where saturation effects would not occur, a reduction in the gain of the channel plate due to fatigue has been observed. This is most pronounced in those regions of the plate where the signal is strongest, and is possibly to be associated with a fall in the secondary emission coefficient of the channel walls due to electron impact desorption of the adsorbed gas. Thus, if, after illuminating the channel plate with an image of dark and bright areas, the plate is flooded with uniform illumination, those areas which were bright now appear dark, and those areas which were dark now appear bright. This is illustrated in Fig. I. If a steady image is displayed for more than a few seconds, the contrast starts t o deteriorate, obviously an undesirable feature. Bright spots (denoting different atomic species or differently sited atoms) are quickly reduced tfo an intensity comparable with normally emitting atoms.
CHANNEL-PIATE INTENSIFIERS IT FIELD-IOS MI('HOSC0PE
I083
FIG.5 . A neon-ion micrograph of a nickel specimen imaged using a magnetically focmed 2-in. channel plate. Resolution and contrast losses are not readily detectable by Byt'.
1084
P. J. TURNER, P. CARTWRIUHT, E. D. BOYES AND M. J . SOVTHON
FIQ.6. An iridium specimen imaged in helium using a 2.in. diameter channel plate in the proximity mode. There is some loss of contrast, and all image points tend to appear circular. (Micrographkindly supplied by N. J. Sanders.)
(4
(b)
FIG.7. (a)Tungsten specimen using helium as the image g a z . The channel plate had not been outgartsxed, and subsequent flooding of the plate wlth uniform ion illumination shows (b) a negative image.
1056
P. J . TURNER, P. CARTWRIQHT, E. D. BOYES AND M. J. SOUTHON
This effect can be minimized by degassing the channel plate. This is effected by subjecting the plate to uniform electron illumination for a period of about 15 min. An input current of the order of 1 pA has usually been used, and the plate is run a t a differential of 1000 V. Under normal operation, a differential of 800 V or less is used, since contrast reduction takes place more rapidly for higher voltages. It has been found unnecessary to repeat this process after subsequent vacuum cycles. It might further be expected that ion feedback phenomena would occur due to the presence of the image gas. No such effects have been observed. A high image-gas pressure is synonymous with a high input current to the channel plate and, in practice, saturation effects set in a t lower input currents.
CONCLUSIONS The advent of the channel plate has widened very considerably the range of materials which may be studied by field-ion microscopy. Images using any gas may be recorded in times which are limited only by the noise in the primary ion signal. I n particular, argon may conveniently be employed as an image gas, and work is currently proceeding on the applications of argon-ion microscopy to aluminium and its alloys. An argon image of aluminium is shown in Fig. 8.
BIG. 8. An argon-ion image of an aluminium specimon. This material, and many uthers of metallrirgical importance, can now be imaged in a convenient manner using thp channel-plate intensifier.
CHANNEL-PLATE INTENSIFIERS IN FIELD-ION MICROSCOPE
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support of the Science Research Council and U.K.A.E.A. Harwell. All the work described in this paper has been carried out using channel plates manufactured by Mullard Limited. The authors are grateful to Mullard for having made theRe plates available and for their advice on their usage.
REFERENCES 1. Muller, E. W. and Tsong, T. T., “Field-Ion Microscopy”. Elsevier, New York (1969). 2. Mandel, L. L., Br. J . Appl. Phys. 26, 1302 (1959). 3. Turner, P. J., Ph.D. Thesis, Cambridge University (1967). 4. Turner, P. J., Cartwright, P., Southon, M. J., van Oostrom, A., and Manley, B. W., J . Sci. Instrum. ( J . Phys. E . ) ,Series 2 , 2, 731 (1969). 5. Manley, B. W., Guest, A. J. and Holmshaw, R. T., In “Adv. E.E.P.”, Vol. ZSA, p. 471 (1969).
DISCUSSION J. RING : You
mentioned that you need lo5 picture points in the final picture which is comparable to the ast,ronomical usc of image tubes. How do you use all these data? P. CARTWRIGHT : A typical field-ion image consists of roughly lo4 bright spots or image points, each representing an individual atom of the specimen. The use that is mads of these data depends on t,he particular experiment but one is usually not concerned with the brightness or shape of every individual spot, but rather with features of the pattern of spots as a n~hole. For example, deviations of the patt>ernfrom that expected for a perfect crystal may indicate the presence of lat,tice defects, grain boundaries or particlcs of a different phase, all of interest to metallurgists, whereas changes in t.he pattern from one image to the next enable one to follow the progress of cert,nin surface processes. H. J . G . MEYER : You can now observe met’alsother than were previously posvible but has it. now also become possible to st,udy effects due t,o field evaporation of individual atoms? (i.e. by applying short)voltage pulses and making a comparison between the conditions before and after the pulses using comparative colour techniques). P. CARTWRIGHT : The introduction of the channel-plate seems to have satisfied the long-&anding need for image intensification in field-ion microscopy and has widened it,s scope in several directions by great,ly facilitating observation and recording of images. Among tlhe new possibilities is that successive images on a suitable phosphor can be recorded through different colour filters on a single frame of colour film. The resulting photograph indicates sensitively any small changes in brightness or position of image points between exposures, as van Oostrom has shown in Eindhoven, and can usefully form composite micrographs of two-phase materials as Schubort has shown in Cambridge.