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Proton and nuclear microprobe developments
Nuclear Instruments and Methods 197 (1982) 243-253 North-Holland Publishing Company
243
Part IV. Workshop on nuclear microprobe developments PROTON AND NUCLEAR MICROPROBE DEVELOPMENTS G.J.F. L E G G E School of Physics, University of Melbourne, Parkville, Vic., Australia
Recent developments with proton and nuclear microprobes are reviewed and various systems of focussing and construction are compared. Present limitations and difficulties, and techniques of alignment, spot measurement, scanning and data collection are discussed. The requirements for future development are briefly summarized.
1. Introduction This review of progress in the construction and operation of proton or nuclear microprobes was compiled from the records of a workshop conducted at N a m u r as part of the International Conference on Microanalysis. The intention of the workshop was to share the experiences of those who have faced such problems and to offer some guidance to those who may be embarking upon a similar course. This review has the same aims and, as a summary of varied and sometimes divergent ideas and opinions, it may be tempered slightly by the knowledge and experience of the author but hopefully not by his prejudices. Participants in the workshop are identified in the references and are, in a very real sense, contributors to this paper, but can not be held responsible if they are misquoted. Where appropriate, reference is made to a single speaker but this frequently represents a body of opinion or experience. In m a n y cases a reference refers to the work of a group rather than a statement from a representative of that group. It has not been possible to refer to all the published literature where some of these topics have been discussed; however most of the review is concerned with technical details which, though important in practice, have not previously been discussed in print. The one type of instrument is referred to in the literature as both a proton microprobe and a nuclear microprobe. The term proton microprobe is most readily understood in analogy with the electron microprobe, and most groups employ protons to induce X-ray emission - an atomic interaction. Nevertheless, the same instrument is some-
times used to induce nuclear interactions (often with a beam of heavier ions) and those groups which frequently employ such interactions often use the term nuclear microprobe. Both terms will probably remain in use. In this paper both may be understood when the terms microprobe or probe are used.
2. Magnetic quadrupole lenses There are now about 27 focussed microprobes and the great majority of these employ a combination of magnetic quadrupole lenses. A wide variety of such combinations is possible, but all those reported so far are represented in table 1. The minimum spot size achieved for each system is noted, but should only be taken as a rough guide, since various methods of measurement and criteria were used. Within a category the systems are listed in approximate order of construction and for many the attainable resolution has improved with time. However the best resolution performance so far has been the well documented 1/~m achieved by the relatively new triplet assembly of Oxford [1]. The triplet, unlike other configurations so far investigated experimentally, has a crossover in one plane. Darmstadt have also used a triplet. Harwell sometimes use a triplet, but in their case only for maximum brightness in a rectangular focus. The doublet configuration as used at Heidelberg is essentially unsymmetric but can give large, though unequal, demagnification factors with a relatively short compound lens [2]. It has an obvious advantage in only two lens currents to optimize and stabilize. The Zurich configuration, one
0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
IV. WORKSHOP
Lens configuration
doublet
2 doublets
achromatic doublet
Microprobe location
Heidelberg M.P.I. Karlsruhe K.F.K. Amsterdam Vrije Univ. Hamburg Univ.
Zi~rich E.T.H.
Worcester Polytech
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3.3
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Demag factor
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Resolution Scanning achieved method (tt m)
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EN tandem V de G
6 MV tandem V de G 3 MV KN V de G 55 in. AVF cyclotron 10-30 MeV cyclotron
Accelerator
biol., S.C.
geol., S.C.
biol., mat. met.
biol.
mat.
biol., geol.
Field of Application Biol. Geol. Mat. Met. S.C. Arch. Cor. Cat.
Table 1 Construction, characteristics and applications of microprobes which employ a magnetic quadrupole multiplet lens to focus the beam. Abbreviations: Demag.--demagnification, elec. = electrostatic, mag = magnetic, T.M. - turbomolecular, O.D. = oil diffusion, I -- ion, Ti S. = titanium sublimation, pre &post = pre- and post-lens, biol. - biology and medicine, g e o l . - g e o l o g y and mineralogy, mat. = material science and nuclear materials, m e t . - m e t a l l u r g y , S . C . - s e m i c o n d u c t o r s and solid state, c o r . - c o r r o s i o n , cat. =catalysis, a r c h . -
0
0
7:
Bochum Ruhr Univ. Eindhoven Univ.
C.E.A.
asymmetric quadruplet antistigmatic quadruplet
Russian quadruplet
Harwell A.E.R.E.
Melbourne Univ. Studsvik A.B. Atomenergi Lower Hutt I.N.S. N am u r (LARN) Univ. Lurid L.I.T. Saclay
triplet
Oxford Univ. Darmstadt G.S.I. Guildford Surrey Univ.
4.6
8
38
5.6
4.5
50
20
10
4.6
50
38
5
6
13.6
40
5
5
52
20
5.6
15.1
10
16 or 20
9
steel girder
4.6
38
5.6
steel frame
4.5
38
steel box girder steel
steel box girder aluminium girder aluminium channel light frame steel frame steel frame
concrete
3
4
concrete
4 X 4 to 20 X 20 8X 18
6
30
15 X66
5X 10 8 10 6
T.M.
10 9
10 s
3 × 10 7
10 s
10 -6
T.M.
TiS.&I.
O.D.
D&T.M.
T.M.
O.D.
10 7
10 6
O.D.
I.
10 6
2 x 10 8
10 -6
T.M., l cryo O.D.
O.D.
3
20
2
-
15
5 x 10
10
5
3
2×
30
3
1
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post
post plates
specimen
pre plates plates
post coils specimen
post plates
post plates pre coils pre plates
tandem Dynamitron cyclotron
5 MV Pelletron 6 MV V de G 3 MV KN V de G 2.5 MV V de G 3 MV Pelletron 4 MV V de G
3 MV V deG
tandem VdeG 300 kV or 10 M e V / a m u 2MV VdeG
biol.
biol.
biol., geol., mat., met.
biol., geol.
geol., arch., cor., met. met., arch.
biol., geol., mat., met., S.C. arch., cor., cat. biol., geol., S.C. biol.
biol., S.C.
biol.. S.C.
biol.
t_n
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246
G.J.F. Legge / Microprobe developments
of the earliest, comprises two doublet stages of demagnification [3]; but present limitations are the quality of the magnets and the multipurpose nature of the beamlines. Magnetic lenses suffer from inherently large chromatic aberrations. The same is generally true of electrostatic lenses; but at Worcester an achromatic doublet has been developed with superimposed electrostatic and magnetic quadrupoles [4]. This development could be of great value to those accelerators with poor energy stability, but all current probes are limited by chromatic aberrations. The Russian Quadruplet configuration as used on the first focussed probe at Harwell [5] is the most popular configuration, partly because of its symmetry and its orthomorphic character which permits the use of circular object diaphragms. However Bochum have successfully departed from this symmetry in order to gain a larger demagnification factor [6]. More work, experimental and theoretical, needs to be done with all of these lens configurations and other untried ones, such as the high excitation modes of a Russian Quadruplet [7,8]. The Melbourne [9] and Oxford probes were purposely built with this in mind and the four quadrupole magnets in each case are freely movable along a 3 m optical bench. Bochum have rebuilt their system and also employ an optical bench for the lenses. Hopefully, with more data from these and other probes of variable geometry, it will be possible to make more direct comparisons between the many possible lens configurations. The use of cyclotrons [10-12] with microprobes is an interesting development. The beam optics of such an accelerator are not ideal for microprobe use, but some improvements may be had by deconvoluting the intensity profile of the beam from a measured distribution [10]. This is only possible on an accelerator whose selected beam profile is relatively constant.
solenoid at Los Alamos [13] which has attained a spot size of 5/~m. The phase acceptance of such a lens is high and is limited mainly by the chromatic aberration coefficient. This must limit the performance of the M I T superconducting lens which is on a poorly stabilized accelerator [14]. These lenses represent a very important development. It has enabled the cylindrical lens to challenge the quadrupole lens in this area of high energy proton microscopy, in contrast with the history of electron microscopy where the quadrupole lens has so far not been able to displace the basically simpler and more easily set up cylndrical lens, except in the role of aberration corrector [15,18]. The electrostatic quadrupole, as developed at Bell Laboratories [16] where a triplet is used to obtain a resolution of 12 /,m, provides a very compact lens and this is its main advantage. It also is limited by chromatic aberrations, which can probably only be overcome by the introduction of a magnetic quadrupole as at Worcester. The electrostatic radial field annular lens developed at New South Wales [17] has cylindrical symmetry but very large aberrations. Consequently, the beam acceptance must be limited to a thin annulus. However the lenses are very simple, and positive and negative elements may be combined in series to correct for spherical and also chromatic aberration. The most recent innovation in probe lenses is the plasma lens introduced at Oregon [18]. This lens, which employs a trapped electron distribution to focus a positive ion beam which traverses it, has so far achieved a resolution of 20 lain. It is of interest, not only because it focusses even heavy ions without requiring large applied electric field gradients, but because the aberrations in theory can be adjusted by shaping the plasma. It is to be hoped that all of these lenses may be further developed in the next few years so that the practical choices for high resolution probe development are clarified.
3. Other probe-forming lenses 4. Collimated probes Magnetic quadrupole multiplets are by no means the only type of lens employed for probe formation. Others must be mentioned, although without the benefit of comments from representatives of the laboratories concerned. The most successful of these has been the superconducting
In general, the microprobes mentioned so far employ an object diaphragm or pair of slits followed by an aperture diaphragm or pair of slits to limit the divergence angle of the beam accepted by the lens. An exception to this arrangement is the
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Karlsruhe system which uses the second diaphragm merely as an antiscattering slit [19]. In this system, the divergence is controlled by a zoom lens - two pairs of quadrupole lenses preceding the object slits. A fundamental principle of all these systems is that no collimator is used after the lens, where it could give rise to background radiation or scattered beam. However there are several systems which employ no lens, the probe diameter being defined by collimation. These are represented by the systems in table 2. The spot diameters reported here are merely an indication of the diaphragm used and depend upon the degree of scattered beam that can be tolerated. They are related to the type of application envisaged for the probe. In many cases, these collimated beams are extracted from the vacuum system as noted in the table, and the scattering associated with this process has discouraged attempts to use an elaborate lens system. Such probes are needed for the examination of hygroscopic material and large priceless articles [29]. The open geometry also allows free use of detectors [30]. Highly collimated systems are also used for channeling [31]. For all such systems where aberrations play no part, it is advantageous to compensate for the reduction in beam on collimation by employing high current ion sources and accelerators, such as a Dynamitron capable of accelerating 1 mA beams [32].
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It is apparent from table 1 that many ion optical designs have been used in the construction of proton or nuclear microprobes, since the first focussed probe was set up at Harwell [5]. Even for probes with the same configuration of quadrupole lenses, there is a wide variation in bore diameters and in overall system lengths. There is no consensus on what might constitute an optimum design; however, it is possible to summarize the general principles on which decisions have been based. For a magnetic system of given object size, acceptance angle, demagnification factor and ampere turns, all dimensions can be scaled together to obtain a larger working distance. The tolerances are then less critical for machining the large bore lenses [33]. But shorter systems are less IV. WORKSHOP
24~
G.J. F Legye / Microprobe det:elopments
susceptable to vibration and stray fields [34[. If complete scaling is abandoned and the magnets are merely given a larger bore in order to decrease the tolerances, this also results in a smaller fraction of the bore being used. Then the effect of undesired sextapole or higher order components of the field is greatly reduced [33]. However this requires greater power input to the magnets, and water cooling of these is to be avoided because of the vibrations which can be generated by the nonlaminar flow [35]. It is true that the fringing fields become more important when the gap to length ratio is increased, but calculations suggest that these only affect the 3rd order aberration coefficients [33]. The opposite approach is to use a fairly small bore but a long system so that the lenses can be short with a long working distance and still retain a fairly large demagnification factor [36]. Another argument favouring the long system is the increased distance of the specimen and detector from any diaphragms or slits [37]. Notwithstanding all of these arguments, Heidelberg have shown that a successful short system in possible and this may be favoured for convenience and space in the future [38].
6. Aberrations and alignment of the lens components If the effects of vibrations and external ac fields are disregarded, the important aberrations are: Parasitic."
I. rotational (about beam axis) 1st order: 2. other Parasitic (associated with lens construction). Intrinsic:
3. chromatic (momentum spread and lens current instability) 1st order; 4. spherical 3rd order. The rotational aberrations are very important and provision should be made to correct them, but it is possible to effectively eliminate them. Although very critical, they are additive and, with approximately correct alignment of lenses, the residual aberrations can be eliminated by rotation of only one lens [33]. The correction can also be made with a small current through a 45 ° weak iron-free quadrupole field [39]. Rotation about other axes is not critical. The effect of displacements off axis is to cause
steering of the beam. With most groups, alignment in X and Y directions (transverse to the beam axis) is performed for each lens by obtaining a line focus and moving the lens to eliminate steering as the lens current is changed. It can also be performed by moving the beam with small dipole magnets rather than moving the lenses [35]. Spherical aberrations increase rapidly if the acceptance angle (aperture) of the lens is increased: but conversely they decrease rapidly if this angle is decreased, so that chromatic aberrations are the most important and limiting aberrations for present microprobes once the parasitic aberrations are overcome [33]. The spherical aberrations will become more important if the chromatic aberration effects are reduced by improvements to accelerator energy stability; but calculations by Jamieson have shown that the spherical aberrations can be cancelled by the insertion of only weak octapoles [36]. Below the 1 micron level, all these aberrations will be important, and particularly the parasitic aberrations. It may be necessary to adjust all poles of the lens independently to compensate for mechanical or magnetic material inequalities [38]. There has been no investigation of magnet steels in this field, important as it may be. The Harwell, Namur, Surrey, Manchester and Saclay lenses were made to the same specifications from the one batch of Swedish iron and performed similarly on initial test. The Oxford lenses are likewise of Swedish iron. The Melbourne lenses are of Australian iron. The Bochum lenses are old (1946) ones refabricated. The Heidelberg lenses, of 50% Fe/50% Co, gave a good performance but twice showed an effect of aging not in magnetic properties, but in movement of the material. In each case, it was corrected by reassembly [34]. Unless relieved after machining, there are great stresses in the lens components. After annealing, they should only be ground; and failure of the factory to observe this direction probably led to the Heidelberg problem [40}.
7. Vibrations and stray fields The problem of vibration is a serious one because, although these microprobes do not work at the resolution level of electron microscopes, they are rather bulky and difficult to mechanically isolate from their surroundings which tend to be
G.J.F. Legge / Microprobe developments
noisy. Apart from the problem of traffic noise, the beam line always remains as a necessary link to the accelerator. There is no problem if the probe moves as a unit (a rigid pendulum) and it is wise to build on a heavy concrete base, as many have done, to dampen movement [35]. The finger is a surprisingly sensitive detector of vibration for amplitudes well below a micrometer according to electron microscopists [41]. However it is still possible that some critical parts of the probe (the lens or the specimen) may vibrate with an undetected large amplitude because of resonances [34]. Vibrations, particularly of critical components, in all three modes (X, Y and Z) should be measured with an a accelerometer. Modern instruments readily record down to 0.1 /~m over a range of frequencies [42]. One measurement is not sufficient; one must always be on guard against vibrations appearing as a result of actions taken by others in the laboratory and the same may be said of stray fields. Experience has shown that both may creep in, for example from the replacement of a coupling or the installation of equipment containing a solenoid valve [43]. Ac fields can provide a serious problem whether caused by hum in the lens current supply or stray fields from some distant electrical device. Several of the longer lines are shielded with mumetal. The effects of a stray dc field are just to shift the beam and cause steering in the lenses. The lenses must always be ultimately aligned magnetically, with removal of steering as the criterion, so that the lenses are on the beam axis [33]. However the beam axis will move if the probe operating energy is changed [44]. Likewise, if the lenses are mounted on an optical bench, they will probably not then stay on axis when moved; and it should also be remembered that the direction and strength of the local magnetic field can vary with movement of steel members in the vicinity [36]. It is more satisfactory to elliminate such fields and on the 16m long line of Z0rich, along which the earth's field could deflect the beam by 2cm, Helmholtz coils are employed to cancel the local magnetic field, in addition to the use of two layers of magnetic shielding [45]. 8. Observation and measurement of the beam spot
There is a problem in determining the size of a focussed beam spot - how are its dimensions to be
249
measured? For an accurate statement of resolution, one needs to define a width at half height of a Gaussian profile or whatever the shape might be. This is often done by scanning the beam with respect to a sharp edge. This, of course, must be done in two directions. It is not easy to produce a suitable edge and some form of visualization is desirable to find the focal spot. It is also necessary to have a quick means of estimating the approximate spot size when making a lengthy series of measurements to determine the values of aberration coefficients. Measurements down to 3/~m (and possibly down to 1/~m) can be made with a beam focussed on a thin glass microscope cover slip, providing the optical microscope used has a high magnification ( x 200) and a short depth of focus [36]. The microscope must be focussed precisely on the front surface of the glass where the beam enters. The halo of scattered beam and light which forms further in is then out of focus and can be ignored. It is also possible to measure spot size by scanning a pair of crossed Walliston wires of 2 ktm diameter [37]. In the case of a 26 MeV proton beam, such a remote method is essential [46]. An interesting method of observing aperture aberrations optically is to insert an aperture screen with a regular array of holes in front of the lens [35]. This will produce a similar pattern of spots on the scintillating surface observed when the probe is not focussed. However, as the probe is brought to a focus, these spots should coalesce to a central point, if the aberrations are small. As the resolution improves, one will be forced to use electronic techniques and one can use the secondary electron image of a scanned rough surface. There are always structures smaller than the beam spot diameter which will test the resolution [41]. Electron microscopy suppliers stock gold shadowed 0.1 /~m spheres which could possibly be used [33]. Scanning a small speck of material (smaller than the beam spot) is the best method of measuring not only the beam spot dimonsions, but also the precise profile and halo [36]. One is effectively scanning the beam spot and its halo with the material speck, but one needs to have a two-dimensional quantitative scanning and data handling system. With this one can produce a complete map of the beam profile. The same information can not be obtained by scanning a hole - the strength of a weak halo can not be measured with a hole, except as an integral count with the beam centred. IV. WORKSHOP
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G.J.I( Legge/ Microprobedevelopments
9. Beam scanning Either electrostatic plates or magnetic 'coils may be used to deflect the microbeam. There are few quantitative measurements of aberrations due to beam scanning. Obviously, sweeping the beam off axis as it enters the lens must produce some increase in the spot size but it will depend strongly on the beam optics and will be negligible for some systems with their present resolution. The Karlsruhe probe employs a double deflection pre-lens scanner in one plane similar to that used in scanning electron microscopes to tilt the beam about the centre of the lens [35]. Post-lens deflection might appear safer if the space is available; however, if deflection plates are partly immersed in the magnetic field of the lens, the deflection per Volt can be affected [43]. Aberrations can arise from very short scanning plates or coils, due to the strength of the fringing fields [35]. Some quantitative measurements of aberrations induced by postlens electrostatic scanning have been made at Harwell and the necessity to maintain the median plane at ground potential, by using equal and opposite voltages on the two plates, has been discussed by Cookson [20]. A theoretical discussion of aberrations caused by perturbing fields is included in the extensive aberration calculations of Meyer [21 ]. On several probes, the beam is fixed and all movement is of the specimen itself. This technique is necessary when a microbeam is formed by collimation or is extracted into air through a very small aperture. However there are some advantages even for a focussed beam within the vacuum system - there are no scanning aberrations, the beam always remains focussed at the same point in the field of view of an optical microscope and beam geometry is fixed with respect to any detectors [44]. The disadvantages of such a mechanical movement are the restriction to relatively low scanning frequencies and possible limitations in the accuracy or repeatability of the mechanical movement. Whether or not relatively slow scanning frequencies are acceptable depends on the approach to data collection and this in turn may depend on the fields of application. The digitization of scans enables all data to be treated quantitatively and all elements to be analysed simultaneously by a computer. The scan may be generated inside or outside the computer,
internal generation probably being favoured for slow digital scans but not suitable for very fast scans of high resolution. However there are two basically different approaches to the control of the scan and the collection of the data. Subsequent data handling could be similar, though in practice there are again differences. One system of operation, favoured by several groups, is to generate the scan digitally as a continuous series of points along sequential parallel lines traversed alternately in opposite directions, but to control the length of dwell time for each point so that the charge distribution over the scanned area is uniform [3]. This method is conceptually straightforward and involves the least hardware and computer software in the data collection. The other system of operation is to generate the scan with two uncoupled free running oscillators and, using event-by-event (multiparameter) mode, to store the energy and address of each event as it arises [22]. These events will arrive in random order from points over the scanned area but can be sorted by the computer. This method can employ rapid scanning and so very quickly produces a complete picture whose statistics are steadily improved. However, the major issues are those of beam current measurement and of specimen damage. The rapid continuous scanning technique relies partly on averaging over beam fluctuations and therefore the two scanning generator frequencies must be unrelated to beam fluctuation frequencies as well as to each other. Digitized beam charge pulses can also be stored but these relate to the entire scanned area. The slow sequential digitized scanning technique relies upon an accurate current measurement for each point. This in turn is not without problems, because of the low beam currents and the fluctuations. With thick specimens, it may require the shielding of an isolated specimen chamber which is acting as a Faraday cup [35]. As an alternative technique, Rutherford scattering from a beam interruptor has shown unexplained differences of up to 20% in repeatability [34]. With thin specimens, the beam current may be measured with a Faraday cup behind the specimen. However, with non-uniform specimens, the variations in thickness are important. In such cases one is more interested in the product of beam charge and specimen thickness for normalizing the data for each point. This can be determined from the bremsstrahlung yield collected in the X-ray spectrum or the yields of
G.J.F. Legge / Microprobe developments" [19] D. Heck, Beitr. electronenmikroskop. Direktabb. Oberfl. (1979) p. 259. [20] J.A. Cookson, Nucl. Instr. and Meth. 165 (1979) 477. [21] W.E. Meyer reviewed by P.W. Hawkes, Quadrupole optics (Springer, Heidelberg, 1966). [22] G.J.F. Legge and I. Hammond, J. Microsc. 117 (1979) 201. [23] A.J.J. Bos, R.D. Vis, F. van Langenvelde, F. Ullings and H. Verheul, these Proceedings, p. 139. [24] D. Heck, Report KFK 2734 (1978). [25] G.E. Coote and R.J. Sparks, N.Z.J. Archaeol. 3 (1981) 21. [26] C.J. Maggiore, IEEE Trans. Nucl. Sci. NS-28 (1981) 1417. [27] T.B. Pierce and J. Huddleston, Nucl. Instr. and Meth. 144 (1977) 231. [28] S.A. Ingarfield, C.D. McKenzie, K.T. Short and J.S. Williams, Nucl. Instr. and Meth. 191 (1981) 521. [29] C.P. Swann, Bartol Research Foundation, Delaware University, U.S.A. [30] K.M. Barfoot, Queen's University, Kingston, Canada. [3 I] E.G. Earwaker, Birmingham University, England. [32] C.C. Trail, Brooklyn College, New York, U.S.A. [33] F. Watt, Oxford University, England. [34] K. Traxel, Heidelberg Universit~it (and M.P.I.), West Germany. [35] D. Heck, Karlsruhe, K.F.K., West Germany.
253
[36] G.J.F. Legge, Melbourne University, Australia. [37] B. Gonsior, Ruhr Universitgt, Bochum, West Germany. [38] F.W. Martin, Microscope Associates Inc. (and Worcester Polytech. Inst), Mass., U.S.A. [39] M. Prins, Eindhoven Universite~t, The Netherlands. [40] R. Nobiling, Heidelberg Universit~,t (and M.P.I.), West Germany. [41] B.E. Fisher, Darmstadt G.S.I., West Germany. [42] G. Deconninck, Namur Universitaires, Belgium. [43] J.A. Cookson, A.E.R.E., Harwetl, England. [44] R.D. Vis, Vrije Universiteit, Amsterdam, The Netherlands. [45] G. Bonani, Zfirich E.T.H., Switzerland. [46] H. Brfickmann, Hamburg UniversitSt, West Germany. [47] F. Bodart, Namur Universitaires, Belgium. [48] Ch. Engelmann, Saclay CEN, France. [49] P. Hemment, Surrey University, Guildford, England. [50] L.E. Carlsson, Lund Inst. Tech., Sweden. [51] G.E. Coote, Lower Hutt, I.N.S., New Zealand. [52] U. Lindh, Upsala University (and A.B. Atomenergi, Studsvik), Sweden. [53] J.W. McMillan~ A.E.R.E., Harwell, England. [54] M.S.A.L. A1-Ghazi, Manitoba University, Winnipeg, Canada.
IV. WORKSHOP
G.J.F. Legge / Microprobe developments
major elements collected with X-ray, back-scatter or forward-scatter detectors [36]. Alternatively, it may be better to use the method of Boss et al. [23]. The slow sequential digitized scanning technique was developed for work in the fields of mineralogy, solid state and nuclear physics [3], material research [24], geology, material and corrosion [25] and semiconductors [26]. In contrast, the rapid continuous scanning technique was developed specifically for work in the fields of biology and medicine [22], though it is also equally applicable to other fields. With biological and medical work, elemental losses due to beam heating of the sample must be minimized; so dwell times are to be avoided as is slow scanning from one edge of a sample to the other. This problem will become more serious with better resolution as the current density increases. Also it is wise to store individual events in the order of collection or at least to monitor any change in elemental yields. Thus there are many points to consider when deciding on a beam scanning system for microprobe work, not least of which is the type of work to be undertaken. The majority of microprobes are connected to minicomputers for on-line data handling. This is to be expected with an instrument whose major property is its sensitivity and ability to rapidly collect large amounts of quantitative data. Data handling techniques vary widely and now include the use of colour mapping [46].
10. Specimen chamber Specimen chambers take many forms - cylindrical, rectangular, octagonal and trapezoidal and it is often said that they cannot be too large, there being a need for specimen movements, microscope objectives, mirrors, Faraday cup, X-ray filters, surface barrier detectors, secondary electron detector and in the future probably special stages for heating or cooling specimens. Some chambers must be designed to handle large specimens [47] or multiple specimen holders [48]. Some applications require a goniometer [49]. Thick samples may require an electron suppressor for charge measurement [50] but uncoated highly insulating specimens may require an electron flood gun [51]. The space immediately in front of the specimen is likely to be congested because of the close
251
proximity of the beam lens and because of the large number of detectors which may be required to look at the front surface of the specimen; however, since all may not be required simultaneously, these detectors can be mounted on removeable front 45 ° cones [43]. Bulky NaI(TI) detectors are often placed at forward angles, for example at 0 ° [52]. Stereo microscopes with large working distances are popular for front viewing; but a high power microscope objective with long working distance is available in the so-called mirror objective. This type of lens has been coupled to a television camera for continuous viewing of the specimen [47].
11. Applications and resolution requirements The applications listed for the microprobes in tables 1 and 2 cover many fields. The group at Harwell now has much experience in the use of nuclear reactions to detect and map the distributions of very light elements in metals [27]. In contrast, those groups dealing with biological materials have more use for PIXE analysis to identify important heavy metal trace elements. The very heavy elements are commonly detected by means of L shell radiation, and there are problems with interference between L and K lines or low production cross sections. This specific area can now be covered with the advent of high energy proton microprobes [46]. The 10-30 MeV protons available from the Hamburg isochronous cyclotron will give a high cross section of high Z K shell ionization simultaneously with a relatively low energy loss in a thin biological (predominantly low Z ) specimen. Another high energy cyclotron is to be equipped with a microprobe in Winnipeg [54]. The resolution required of a microprobe will depend on the field of application, In biology one needs a resolution of about 1 # m to study the structure of cells and one would like 0.1 gm. Often one does not wish to see this fine structure but rather to average over a much larger area for which 20 ~m resolution is adequate [46]. There are many problems, involving functional groups of cells, for which present resolutions are adequate and it is better to study these problems than subcellular ones for which adequate resolution is not yet available [37]. In theory the resolution can be improved indefinitely be deconvoluting the IV. WORKSHOP
243
Part IV. Workshop on nuclear microprobe developments PROTON AND NUCLEAR MICROPROBE DEVELOPMENTS G.J.F. L E G G E School of Physics, University of Melbourne, Parkville, Vic., Australia
Recent developments with proton and nuclear microprobes are reviewed and various systems of focussing and construction are compared. Present limitations and difficulties, and techniques of alignment, spot measurement, scanning and data collection are discussed. The requirements for future development are briefly summarized.
1. Introduction This review of progress in the construction and operation of proton or nuclear microprobes was compiled from the records of a workshop conducted at N a m u r as part of the International Conference on Microanalysis. The intention of the workshop was to share the experiences of those who have faced such problems and to offer some guidance to those who may be embarking upon a similar course. This review has the same aims and, as a summary of varied and sometimes divergent ideas and opinions, it may be tempered slightly by the knowledge and experience of the author but hopefully not by his prejudices. Participants in the workshop are identified in the references and are, in a very real sense, contributors to this paper, but can not be held responsible if they are misquoted. Where appropriate, reference is made to a single speaker but this frequently represents a body of opinion or experience. In m a n y cases a reference refers to the work of a group rather than a statement from a representative of that group. It has not been possible to refer to all the published literature where some of these topics have been discussed; however most of the review is concerned with technical details which, though important in practice, have not previously been discussed in print. The one type of instrument is referred to in the literature as both a proton microprobe and a nuclear microprobe. The term proton microprobe is most readily understood in analogy with the electron microprobe, and most groups employ protons to induce X-ray emission - an atomic interaction. Nevertheless, the same instrument is some-
times used to induce nuclear interactions (often with a beam of heavier ions) and those groups which frequently employ such interactions often use the term nuclear microprobe. Both terms will probably remain in use. In this paper both may be understood when the terms microprobe or probe are used.
2. Magnetic quadrupole lenses There are now about 27 focussed microprobes and the great majority of these employ a combination of magnetic quadrupole lenses. A wide variety of such combinations is possible, but all those reported so far are represented in table 1. The minimum spot size achieved for each system is noted, but should only be taken as a rough guide, since various methods of measurement and criteria were used. Within a category the systems are listed in approximate order of construction and for many the attainable resolution has improved with time. However the best resolution performance so far has been the well documented 1/~m achieved by the relatively new triplet assembly of Oxford [1]. The triplet, unlike other configurations so far investigated experimentally, has a crossover in one plane. Darmstadt have also used a triplet. Harwell sometimes use a triplet, but in their case only for maximum brightness in a rectangular focus. The doublet configuration as used at Heidelberg is essentially unsymmetric but can give large, though unequal, demagnification factors with a relatively short compound lens [2]. It has an obvious advantage in only two lens currents to optimize and stabilize. The Zurich configuration, one
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IV. WORKSHOP
Lens configuration
doublet
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Worcester Polytech
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Accelerator
biol., S.C.
geol., S.C.
biol., mat. met.
biol.
mat.
biol., geol.
Field of Application Biol. Geol. Mat. Met. S.C. Arch. Cor. Cat.
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0
0
7:
Bochum Ruhr Univ. Eindhoven Univ.
C.E.A.
asymmetric quadruplet antistigmatic quadruplet
Russian quadruplet
Harwell A.E.R.E.
Melbourne Univ. Studsvik A.B. Atomenergi Lower Hutt I.N.S. N am u r (LARN) Univ. Lurid L.I.T. Saclay
triplet
Oxford Univ. Darmstadt G.S.I. Guildford Surrey Univ.
4.6
8
38
5.6
4.5
50
20
10
4.6
50
38
5
6
13.6
40
5
5
52
20
5.6
15.1
10
16 or 20
9
steel girder
4.6
38
5.6
steel frame
4.5
38
steel box girder steel
steel box girder aluminium girder aluminium channel light frame steel frame steel frame
concrete
3
4
concrete
4 X 4 to 20 X 20 8X 18
6
30
15 X66
5X 10 8 10 6
T.M.
10 9
10 s
3 × 10 7
10 s
10 -6
T.M.
TiS.&I.
O.D.
D&T.M.
T.M.
O.D.
10 7
10 6
O.D.
I.
10 6
2 x 10 8
10 -6
T.M., l cryo O.D.
O.D.
3
20
2
-
15
5 x 10
10
5
3
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30
3
1
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post plates
specimen
pre plates plates
post coils specimen
post plates
post plates pre coils pre plates
tandem Dynamitron cyclotron
5 MV Pelletron 6 MV V de G 3 MV KN V de G 2.5 MV V de G 3 MV Pelletron 4 MV V de G
3 MV V deG
tandem VdeG 300 kV or 10 M e V / a m u 2MV VdeG
biol.
biol.
biol., geol., mat., met.
biol., geol.
geol., arch., cor., met. met., arch.
biol., geol., mat., met., S.C. arch., cor., cat. biol., geol., S.C. biol.
biol., S.C.
biol.. S.C.
biol.
t_n
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246
G.J.F. Legge / Microprobe developments
of the earliest, comprises two doublet stages of demagnification [3]; but present limitations are the quality of the magnets and the multipurpose nature of the beamlines. Magnetic lenses suffer from inherently large chromatic aberrations. The same is generally true of electrostatic lenses; but at Worcester an achromatic doublet has been developed with superimposed electrostatic and magnetic quadrupoles [4]. This development could be of great value to those accelerators with poor energy stability, but all current probes are limited by chromatic aberrations. The Russian Quadruplet configuration as used on the first focussed probe at Harwell [5] is the most popular configuration, partly because of its symmetry and its orthomorphic character which permits the use of circular object diaphragms. However Bochum have successfully departed from this symmetry in order to gain a larger demagnification factor [6]. More work, experimental and theoretical, needs to be done with all of these lens configurations and other untried ones, such as the high excitation modes of a Russian Quadruplet [7,8]. The Melbourne [9] and Oxford probes were purposely built with this in mind and the four quadrupole magnets in each case are freely movable along a 3 m optical bench. Bochum have rebuilt their system and also employ an optical bench for the lenses. Hopefully, with more data from these and other probes of variable geometry, it will be possible to make more direct comparisons between the many possible lens configurations. The use of cyclotrons [10-12] with microprobes is an interesting development. The beam optics of such an accelerator are not ideal for microprobe use, but some improvements may be had by deconvoluting the intensity profile of the beam from a measured distribution [10]. This is only possible on an accelerator whose selected beam profile is relatively constant.
solenoid at Los Alamos [13] which has attained a spot size of 5/~m. The phase acceptance of such a lens is high and is limited mainly by the chromatic aberration coefficient. This must limit the performance of the M I T superconducting lens which is on a poorly stabilized accelerator [14]. These lenses represent a very important development. It has enabled the cylindrical lens to challenge the quadrupole lens in this area of high energy proton microscopy, in contrast with the history of electron microscopy where the quadrupole lens has so far not been able to displace the basically simpler and more easily set up cylndrical lens, except in the role of aberration corrector [15,18]. The electrostatic quadrupole, as developed at Bell Laboratories [16] where a triplet is used to obtain a resolution of 12 /,m, provides a very compact lens and this is its main advantage. It also is limited by chromatic aberrations, which can probably only be overcome by the introduction of a magnetic quadrupole as at Worcester. The electrostatic radial field annular lens developed at New South Wales [17] has cylindrical symmetry but very large aberrations. Consequently, the beam acceptance must be limited to a thin annulus. However the lenses are very simple, and positive and negative elements may be combined in series to correct for spherical and also chromatic aberration. The most recent innovation in probe lenses is the plasma lens introduced at Oregon [18]. This lens, which employs a trapped electron distribution to focus a positive ion beam which traverses it, has so far achieved a resolution of 20 lain. It is of interest, not only because it focusses even heavy ions without requiring large applied electric field gradients, but because the aberrations in theory can be adjusted by shaping the plasma. It is to be hoped that all of these lenses may be further developed in the next few years so that the practical choices for high resolution probe development are clarified.
3. Other probe-forming lenses 4. Collimated probes Magnetic quadrupole multiplets are by no means the only type of lens employed for probe formation. Others must be mentioned, although without the benefit of comments from representatives of the laboratories concerned. The most successful of these has been the superconducting
In general, the microprobes mentioned so far employ an object diaphragm or pair of slits followed by an aperture diaphragm or pair of slits to limit the divergence angle of the beam accepted by the lens. An exception to this arrangement is the
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Karlsruhe system which uses the second diaphragm merely as an antiscattering slit [19]. In this system, the divergence is controlled by a zoom lens - two pairs of quadrupole lenses preceding the object slits. A fundamental principle of all these systems is that no collimator is used after the lens, where it could give rise to background radiation or scattered beam. However there are several systems which employ no lens, the probe diameter being defined by collimation. These are represented by the systems in table 2. The spot diameters reported here are merely an indication of the diaphragm used and depend upon the degree of scattered beam that can be tolerated. They are related to the type of application envisaged for the probe. In many cases, these collimated beams are extracted from the vacuum system as noted in the table, and the scattering associated with this process has discouraged attempts to use an elaborate lens system. Such probes are needed for the examination of hygroscopic material and large priceless articles [29]. The open geometry also allows free use of detectors [30]. Highly collimated systems are also used for channeling [31]. For all such systems where aberrations play no part, it is advantageous to compensate for the reduction in beam on collimation by employing high current ion sources and accelerators, such as a Dynamitron capable of accelerating 1 mA beams [32].
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It is apparent from table 1 that many ion optical designs have been used in the construction of proton or nuclear microprobes, since the first focussed probe was set up at Harwell [5]. Even for probes with the same configuration of quadrupole lenses, there is a wide variation in bore diameters and in overall system lengths. There is no consensus on what might constitute an optimum design; however, it is possible to summarize the general principles on which decisions have been based. For a magnetic system of given object size, acceptance angle, demagnification factor and ampere turns, all dimensions can be scaled together to obtain a larger working distance. The tolerances are then less critical for machining the large bore lenses [33]. But shorter systems are less IV. WORKSHOP
24~
G.J. F Legye / Microprobe det:elopments
susceptable to vibration and stray fields [34[. If complete scaling is abandoned and the magnets are merely given a larger bore in order to decrease the tolerances, this also results in a smaller fraction of the bore being used. Then the effect of undesired sextapole or higher order components of the field is greatly reduced [33]. However this requires greater power input to the magnets, and water cooling of these is to be avoided because of the vibrations which can be generated by the nonlaminar flow [35]. It is true that the fringing fields become more important when the gap to length ratio is increased, but calculations suggest that these only affect the 3rd order aberration coefficients [33]. The opposite approach is to use a fairly small bore but a long system so that the lenses can be short with a long working distance and still retain a fairly large demagnification factor [36]. Another argument favouring the long system is the increased distance of the specimen and detector from any diaphragms or slits [37]. Notwithstanding all of these arguments, Heidelberg have shown that a successful short system in possible and this may be favoured for convenience and space in the future [38].
6. Aberrations and alignment of the lens components If the effects of vibrations and external ac fields are disregarded, the important aberrations are: Parasitic."
I. rotational (about beam axis) 1st order: 2. other Parasitic (associated with lens construction). Intrinsic:
3. chromatic (momentum spread and lens current instability) 1st order; 4. spherical 3rd order. The rotational aberrations are very important and provision should be made to correct them, but it is possible to effectively eliminate them. Although very critical, they are additive and, with approximately correct alignment of lenses, the residual aberrations can be eliminated by rotation of only one lens [33]. The correction can also be made with a small current through a 45 ° weak iron-free quadrupole field [39]. Rotation about other axes is not critical. The effect of displacements off axis is to cause
steering of the beam. With most groups, alignment in X and Y directions (transverse to the beam axis) is performed for each lens by obtaining a line focus and moving the lens to eliminate steering as the lens current is changed. It can also be performed by moving the beam with small dipole magnets rather than moving the lenses [35]. Spherical aberrations increase rapidly if the acceptance angle (aperture) of the lens is increased: but conversely they decrease rapidly if this angle is decreased, so that chromatic aberrations are the most important and limiting aberrations for present microprobes once the parasitic aberrations are overcome [33]. The spherical aberrations will become more important if the chromatic aberration effects are reduced by improvements to accelerator energy stability; but calculations by Jamieson have shown that the spherical aberrations can be cancelled by the insertion of only weak octapoles [36]. Below the 1 micron level, all these aberrations will be important, and particularly the parasitic aberrations. It may be necessary to adjust all poles of the lens independently to compensate for mechanical or magnetic material inequalities [38]. There has been no investigation of magnet steels in this field, important as it may be. The Harwell, Namur, Surrey, Manchester and Saclay lenses were made to the same specifications from the one batch of Swedish iron and performed similarly on initial test. The Oxford lenses are likewise of Swedish iron. The Melbourne lenses are of Australian iron. The Bochum lenses are old (1946) ones refabricated. The Heidelberg lenses, of 50% Fe/50% Co, gave a good performance but twice showed an effect of aging not in magnetic properties, but in movement of the material. In each case, it was corrected by reassembly [34]. Unless relieved after machining, there are great stresses in the lens components. After annealing, they should only be ground; and failure of the factory to observe this direction probably led to the Heidelberg problem [40}.
7. Vibrations and stray fields The problem of vibration is a serious one because, although these microprobes do not work at the resolution level of electron microscopes, they are rather bulky and difficult to mechanically isolate from their surroundings which tend to be
G.J.F. Legge / Microprobe developments
noisy. Apart from the problem of traffic noise, the beam line always remains as a necessary link to the accelerator. There is no problem if the probe moves as a unit (a rigid pendulum) and it is wise to build on a heavy concrete base, as many have done, to dampen movement [35]. The finger is a surprisingly sensitive detector of vibration for amplitudes well below a micrometer according to electron microscopists [41]. However it is still possible that some critical parts of the probe (the lens or the specimen) may vibrate with an undetected large amplitude because of resonances [34]. Vibrations, particularly of critical components, in all three modes (X, Y and Z) should be measured with an a accelerometer. Modern instruments readily record down to 0.1 /~m over a range of frequencies [42]. One measurement is not sufficient; one must always be on guard against vibrations appearing as a result of actions taken by others in the laboratory and the same may be said of stray fields. Experience has shown that both may creep in, for example from the replacement of a coupling or the installation of equipment containing a solenoid valve [43]. Ac fields can provide a serious problem whether caused by hum in the lens current supply or stray fields from some distant electrical device. Several of the longer lines are shielded with mumetal. The effects of a stray dc field are just to shift the beam and cause steering in the lenses. The lenses must always be ultimately aligned magnetically, with removal of steering as the criterion, so that the lenses are on the beam axis [33]. However the beam axis will move if the probe operating energy is changed [44]. Likewise, if the lenses are mounted on an optical bench, they will probably not then stay on axis when moved; and it should also be remembered that the direction and strength of the local magnetic field can vary with movement of steel members in the vicinity [36]. It is more satisfactory to elliminate such fields and on the 16m long line of Z0rich, along which the earth's field could deflect the beam by 2cm, Helmholtz coils are employed to cancel the local magnetic field, in addition to the use of two layers of magnetic shielding [45]. 8. Observation and measurement of the beam spot
There is a problem in determining the size of a focussed beam spot - how are its dimensions to be
249
measured? For an accurate statement of resolution, one needs to define a width at half height of a Gaussian profile or whatever the shape might be. This is often done by scanning the beam with respect to a sharp edge. This, of course, must be done in two directions. It is not easy to produce a suitable edge and some form of visualization is desirable to find the focal spot. It is also necessary to have a quick means of estimating the approximate spot size when making a lengthy series of measurements to determine the values of aberration coefficients. Measurements down to 3/~m (and possibly down to 1/~m) can be made with a beam focussed on a thin glass microscope cover slip, providing the optical microscope used has a high magnification ( x 200) and a short depth of focus [36]. The microscope must be focussed precisely on the front surface of the glass where the beam enters. The halo of scattered beam and light which forms further in is then out of focus and can be ignored. It is also possible to measure spot size by scanning a pair of crossed Walliston wires of 2 ktm diameter [37]. In the case of a 26 MeV proton beam, such a remote method is essential [46]. An interesting method of observing aperture aberrations optically is to insert an aperture screen with a regular array of holes in front of the lens [35]. This will produce a similar pattern of spots on the scintillating surface observed when the probe is not focussed. However, as the probe is brought to a focus, these spots should coalesce to a central point, if the aberrations are small. As the resolution improves, one will be forced to use electronic techniques and one can use the secondary electron image of a scanned rough surface. There are always structures smaller than the beam spot diameter which will test the resolution [41]. Electron microscopy suppliers stock gold shadowed 0.1 /~m spheres which could possibly be used [33]. Scanning a small speck of material (smaller than the beam spot) is the best method of measuring not only the beam spot dimonsions, but also the precise profile and halo [36]. One is effectively scanning the beam spot and its halo with the material speck, but one needs to have a two-dimensional quantitative scanning and data handling system. With this one can produce a complete map of the beam profile. The same information can not be obtained by scanning a hole - the strength of a weak halo can not be measured with a hole, except as an integral count with the beam centred. IV. WORKSHOP
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G.J.I( Legge/ Microprobedevelopments
9. Beam scanning Either electrostatic plates or magnetic 'coils may be used to deflect the microbeam. There are few quantitative measurements of aberrations due to beam scanning. Obviously, sweeping the beam off axis as it enters the lens must produce some increase in the spot size but it will depend strongly on the beam optics and will be negligible for some systems with their present resolution. The Karlsruhe probe employs a double deflection pre-lens scanner in one plane similar to that used in scanning electron microscopes to tilt the beam about the centre of the lens [35]. Post-lens deflection might appear safer if the space is available; however, if deflection plates are partly immersed in the magnetic field of the lens, the deflection per Volt can be affected [43]. Aberrations can arise from very short scanning plates or coils, due to the strength of the fringing fields [35]. Some quantitative measurements of aberrations induced by postlens electrostatic scanning have been made at Harwell and the necessity to maintain the median plane at ground potential, by using equal and opposite voltages on the two plates, has been discussed by Cookson [20]. A theoretical discussion of aberrations caused by perturbing fields is included in the extensive aberration calculations of Meyer [21 ]. On several probes, the beam is fixed and all movement is of the specimen itself. This technique is necessary when a microbeam is formed by collimation or is extracted into air through a very small aperture. However there are some advantages even for a focussed beam within the vacuum system - there are no scanning aberrations, the beam always remains focussed at the same point in the field of view of an optical microscope and beam geometry is fixed with respect to any detectors [44]. The disadvantages of such a mechanical movement are the restriction to relatively low scanning frequencies and possible limitations in the accuracy or repeatability of the mechanical movement. Whether or not relatively slow scanning frequencies are acceptable depends on the approach to data collection and this in turn may depend on the fields of application. The digitization of scans enables all data to be treated quantitatively and all elements to be analysed simultaneously by a computer. The scan may be generated inside or outside the computer,
internal generation probably being favoured for slow digital scans but not suitable for very fast scans of high resolution. However there are two basically different approaches to the control of the scan and the collection of the data. Subsequent data handling could be similar, though in practice there are again differences. One system of operation, favoured by several groups, is to generate the scan digitally as a continuous series of points along sequential parallel lines traversed alternately in opposite directions, but to control the length of dwell time for each point so that the charge distribution over the scanned area is uniform [3]. This method is conceptually straightforward and involves the least hardware and computer software in the data collection. The other system of operation is to generate the scan with two uncoupled free running oscillators and, using event-by-event (multiparameter) mode, to store the energy and address of each event as it arises [22]. These events will arrive in random order from points over the scanned area but can be sorted by the computer. This method can employ rapid scanning and so very quickly produces a complete picture whose statistics are steadily improved. However, the major issues are those of beam current measurement and of specimen damage. The rapid continuous scanning technique relies partly on averaging over beam fluctuations and therefore the two scanning generator frequencies must be unrelated to beam fluctuation frequencies as well as to each other. Digitized beam charge pulses can also be stored but these relate to the entire scanned area. The slow sequential digitized scanning technique relies upon an accurate current measurement for each point. This in turn is not without problems, because of the low beam currents and the fluctuations. With thick specimens, it may require the shielding of an isolated specimen chamber which is acting as a Faraday cup [35]. As an alternative technique, Rutherford scattering from a beam interruptor has shown unexplained differences of up to 20% in repeatability [34]. With thin specimens, the beam current may be measured with a Faraday cup behind the specimen. However, with non-uniform specimens, the variations in thickness are important. In such cases one is more interested in the product of beam charge and specimen thickness for normalizing the data for each point. This can be determined from the bremsstrahlung yield collected in the X-ray spectrum or the yields of
G.J.F. Legge / Microprobe developments" [19] D. Heck, Beitr. electronenmikroskop. Direktabb. Oberfl. (1979) p. 259. [20] J.A. Cookson, Nucl. Instr. and Meth. 165 (1979) 477. [21] W.E. Meyer reviewed by P.W. Hawkes, Quadrupole optics (Springer, Heidelberg, 1966). [22] G.J.F. Legge and I. Hammond, J. Microsc. 117 (1979) 201. [23] A.J.J. Bos, R.D. Vis, F. van Langenvelde, F. Ullings and H. Verheul, these Proceedings, p. 139. [24] D. Heck, Report KFK 2734 (1978). [25] G.E. Coote and R.J. Sparks, N.Z.J. Archaeol. 3 (1981) 21. [26] C.J. Maggiore, IEEE Trans. Nucl. Sci. NS-28 (1981) 1417. [27] T.B. Pierce and J. Huddleston, Nucl. Instr. and Meth. 144 (1977) 231. [28] S.A. Ingarfield, C.D. McKenzie, K.T. Short and J.S. Williams, Nucl. Instr. and Meth. 191 (1981) 521. [29] C.P. Swann, Bartol Research Foundation, Delaware University, U.S.A. [30] K.M. Barfoot, Queen's University, Kingston, Canada. [3 I] E.G. Earwaker, Birmingham University, England. [32] C.C. Trail, Brooklyn College, New York, U.S.A. [33] F. Watt, Oxford University, England. [34] K. Traxel, Heidelberg Universit~it (and M.P.I.), West Germany. [35] D. Heck, Karlsruhe, K.F.K., West Germany.
253
[36] G.J.F. Legge, Melbourne University, Australia. [37] B. Gonsior, Ruhr Universitgt, Bochum, West Germany. [38] F.W. Martin, Microscope Associates Inc. (and Worcester Polytech. Inst), Mass., U.S.A. [39] M. Prins, Eindhoven Universite~t, The Netherlands. [40] R. Nobiling, Heidelberg Universit~,t (and M.P.I.), West Germany. [41] B.E. Fisher, Darmstadt G.S.I., West Germany. [42] G. Deconninck, Namur Universitaires, Belgium. [43] J.A. Cookson, A.E.R.E., Harwetl, England. [44] R.D. Vis, Vrije Universiteit, Amsterdam, The Netherlands. [45] G. Bonani, Zfirich E.T.H., Switzerland. [46] H. Brfickmann, Hamburg UniversitSt, West Germany. [47] F. Bodart, Namur Universitaires, Belgium. [48] Ch. Engelmann, Saclay CEN, France. [49] P. Hemment, Surrey University, Guildford, England. [50] L.E. Carlsson, Lund Inst. Tech., Sweden. [51] G.E. Coote, Lower Hutt, I.N.S., New Zealand. [52] U. Lindh, Upsala University (and A.B. Atomenergi, Studsvik), Sweden. [53] J.W. McMillan~ A.E.R.E., Harwell, England. [54] M.S.A.L. A1-Ghazi, Manitoba University, Winnipeg, Canada.
IV. WORKSHOP
G.J.F. Legge / Microprobe developments
major elements collected with X-ray, back-scatter or forward-scatter detectors [36]. Alternatively, it may be better to use the method of Boss et al. [23]. The slow sequential digitized scanning technique was developed for work in the fields of mineralogy, solid state and nuclear physics [3], material research [24], geology, material and corrosion [25] and semiconductors [26]. In contrast, the rapid continuous scanning technique was developed specifically for work in the fields of biology and medicine [22], though it is also equally applicable to other fields. With biological and medical work, elemental losses due to beam heating of the sample must be minimized; so dwell times are to be avoided as is slow scanning from one edge of a sample to the other. This problem will become more serious with better resolution as the current density increases. Also it is wise to store individual events in the order of collection or at least to monitor any change in elemental yields. Thus there are many points to consider when deciding on a beam scanning system for microprobe work, not least of which is the type of work to be undertaken. The majority of microprobes are connected to minicomputers for on-line data handling. This is to be expected with an instrument whose major property is its sensitivity and ability to rapidly collect large amounts of quantitative data. Data handling techniques vary widely and now include the use of colour mapping [46].
10. Specimen chamber Specimen chambers take many forms - cylindrical, rectangular, octagonal and trapezoidal and it is often said that they cannot be too large, there being a need for specimen movements, microscope objectives, mirrors, Faraday cup, X-ray filters, surface barrier detectors, secondary electron detector and in the future probably special stages for heating or cooling specimens. Some chambers must be designed to handle large specimens [47] or multiple specimen holders [48]. Some applications require a goniometer [49]. Thick samples may require an electron suppressor for charge measurement [50] but uncoated highly insulating specimens may require an electron flood gun [51]. The space immediately in front of the specimen is likely to be congested because of the close
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proximity of the beam lens and because of the large number of detectors which may be required to look at the front surface of the specimen; however, since all may not be required simultaneously, these detectors can be mounted on removeable front 45 ° cones [43]. Bulky NaI(TI) detectors are often placed at forward angles, for example at 0 ° [52]. Stereo microscopes with large working distances are popular for front viewing; but a high power microscope objective with long working distance is available in the so-called mirror objective. This type of lens has been coupled to a television camera for continuous viewing of the specimen [47].
11. Applications and resolution requirements The applications listed for the microprobes in tables 1 and 2 cover many fields. The group at Harwell now has much experience in the use of nuclear reactions to detect and map the distributions of very light elements in metals [27]. In contrast, those groups dealing with biological materials have more use for PIXE analysis to identify important heavy metal trace elements. The very heavy elements are commonly detected by means of L shell radiation, and there are problems with interference between L and K lines or low production cross sections. This specific area can now be covered with the advent of high energy proton microprobes [46]. The 10-30 MeV protons available from the Hamburg isochronous cyclotron will give a high cross section of high Z K shell ionization simultaneously with a relatively low energy loss in a thin biological (predominantly low Z ) specimen. Another high energy cyclotron is to be equipped with a microprobe in Winnipeg [54]. The resolution required of a microprobe will depend on the field of application, In biology one needs a resolution of about 1 # m to study the structure of cells and one would like 0.1 gm. Often one does not wish to see this fine structure but rather to average over a much larger area for which 20 ~m resolution is adequate [46]. There are many problems, involving functional groups of cells, for which present resolutions are adequate and it is better to study these problems than subcellular ones for which adequate resolution is not yet available [37]. In theory the resolution can be improved indefinitely be deconvoluting the IV. WORKSHOP