INSTRUMENTATION, OPTICS AND X-RAY SPECTROSCOPY T. Mulvey

To cite this version:

T. Mulvey. INSTRUMENTATION, ELECTRON OPTICS AND X-RAY SPECTROSCOPY. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-149-C2-154. ￿10.1051/jphyscol:1984233￿. ￿jpa-00223946￿

HAL Id: jpa-00223946 https://hal.archives-ouvertes.fr/jpa-00223946 Submitted on 1 Jan 1984

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INSTRUMENTATION, ELECTRON OPTICS AND X-RAY SPECTROSCOPY

T. Mulvey

Department of Physios, The University of Aston in Birmingham, B4 7ET, U.K.

Résumé - Les développements récents dans le champ d'instrumentation du micro­ analyseur à rayons X sont revus, ainsi que les canons électroniques, les systèmes des lentilles et les détecteurs des électrons rétrodiffusés (Z contraste). Les amé­ liorations souhaitables pour l'analyse des échantillons minces à l'aide de la spec- trométrie à dispersion en énergie sont aussi discutées.

Abstract - Recent instrumental developments in the field of electron probe instrumentation are reviewed. These include electron guns, lens systems and back-scattered electron (Z contrast) detectors. Desirable changes in instrumental design are discussed for the analysis of thin specimens by the use of energy dispersive spectrometers.

1 - INTRODUCTION The present symposium is concerned with recent developments in all aspects of electron instrumentation of electron probe X-ray microanalysis. It is perhaps interesting to note that the initial stimulus to develop the electron probe micro- analyser was the early discussions between Castaing and Guinier in the period 1947-1948 as to whether it was possible to analyse by X-ray spectrometry the fine detail visible at that time in thin metallic specimens in the electron microscope. It was of course necessary to begin with the more modest resolution set by the light microscope. After a period of some thirty years of development, microanalysis of this type has now reached a high stage of technological development especially with the recent introduction of microprocessor and mini-computer control. Nevertheless, further development is still possible, especially in the choice and control of X-ray spectrometers. On the other hand, recent progress in atomic resolution transmission electron microscopy (TEH) and the development of high resolution analytical scanning transmission microscopes (STEM) has made it desirable to review the instrumental techniques used in X-ray analysis of thin specimens at high spatial resolution ( = 2 nm) .

These include, among others, the elimination from the detector of all X-rays not emanating from the specimen area under investigation and the handling of the greatly increased amount of analytical data produced by the modern analytical STEM. In addition, it is desirable to be able to correlate the analytical information from the characteristic X-ray emission with that from Auger and energy loss spectroscopy and perhaps from convergent beam diffraction, which combines the high spatial reso­ lution of electron probe analysis with crystallographic information not available with any other X-ray diffraction technique,

The increasing computer automation that is available today also calls for an approach to instrumentation that eliminates, as far as possible, intervention by the operator in setting up and aligning the instrument and the recognition of instrumental artefacts arising during the analysis.

Improved methods of detection of light elements in low concentrations and the increased ability to examine thin surface layers have recently emphasized the need to improve, by a few orders of magnitude, the traditional poor vacuum ( - 10 mbar)

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of many X-ray microanalysers. Here the introduction of turbo-molecular pumps, the availability of improved diffusion pump oils, and the insertion of efficient traps in backing pump lines has done much to reduce the hydrocarbon content of the residual gases in the system. Such improved vacuum systems also make it possible to replace the traditional filament by, for example, a hexaboride cathode with a substantial improvement both in electron optical brightness and filament life. STEM instruments need field emission cathodes and hence will be designed for a vacuum pressure in the region of lo-'' mbar. Such guns were originally designed for the production of nanometre probes and so are not efficient for producing an adequate current in the larger probes needed for analytical work. Further development is however possible.

A valuable ancillary development is the high energy backscattered electron or '2 contrast' detector whose output is proportional to the average atomic number of of the specimen. When made in the form of four adjacent quadrants, so as to eliminate topographical contrast, a rapid preliminary analysis can be made of the average atomic number of the main constituents of the specimen.

2 - ELECTRON GUNS The current density 0 of emission from the heated cathode wire is given by the Richardson equation

(1) 0 = A T~ exp (-e @ /k T) , where A is a constant for a given emitting material, T is the absolute temperature, k is Boltzmann's constant and @ is the '' of the material expressed in 'electron volts'. The maximum brightness (Richtstrahlwert) B of the resulting electron beam after acceleration through a potential difference V is given approxi- mately by

The work function of tungsten is inconveniently large, approximately 4.5 eV, but this can be reduced by the Schottky effect, by which a strong electric field of strength E at thef cathode surface enhances the emission current density 0 by a factor exp(0.44 E /T). Here E is measured in volts/metre. This field can be most conveniently produced by forming a fine point on the emitting surface. Thus Van der Mast et al. /1/ succeeded in producing a laser-heated Schottky thermionic source that is comparable in performance with a field emission electron gun, but does not need a particularly good vacuum. In addition it is capable of delivering the larger probe currents that are needed for analytical work. In this source (Fig. 1) a thin tungsten wire of some 10 w in diameter is heated locally at its tip to a temperature near its . An electrode at a positive potential positioned near the source creates the electric field needed for enhanced Schottky emission. Under the influence of this field and surface tension a stable emitter of fixed small radius of curvature is produced. As the cathode wire is evaporated the wire is fed into the laser focus to provide the necessary replacement. The measured brightness of this source is in the region of 8 X 10' A/cm2/sterad. at 25 kV, comparable with that of a field emission gun. This electron gun is not yet available commercially but its performance indicates that expremely high brightness can be produced in electron guns with tungsten filaments /2,3/ if technological problems can be solved. It should perhaps be pointed out that the gun brightness as measured under operating conditions is not necessarily the maximum brightness as defined in Equation (2) but rather a 'mean brightness' /4/ as limited, for example, by the total cathode current that may be drawn from the power supply or a particular arrangement of lens apertures. In general the brightness of an electron gun improves as the work function and radius of the cathode is reduced /5/, as the total current is increased and as the vacuum is made better. These requirements are not easily satisfied simultaneously and economically in commercial equipment. At present the trend is towards the use of lanthanum hexaboride as a substitute for the tungsten cathode. A comprehensive review of the underlying theory and measurement lUNGSTEN WIRE FEED MECHANISM WEHNELT

FOCUSED LASER BEAM

ANODE /I Fig.1 Schottky-assisted thermionic emission cathode with laser heating HIGH BRIGHTNESS I ELECTRON BEAM

MODULATOR GRAPHITE BLOCK

SHIELD

LABs ROD HEAT SINK

Fig.2 Schematic diagram of LaB6 Fig.3 Schematic diagram of rod cathode mechanical clamping of LaB6 crystal between graphite blocks

CARBON ARCH

LAB6 CRYSTAL\

@ CARBON ARCH' RHENIUM FILAMENT

BASE

Fig.4 Schematic diagram of LaB6 Fig.5 LaB6 crystal arc-bonded to crystal sintered to two rhenium wire filament carbon arches C2-152 JOURNAL DE PHYSIQUE

of the performance of triode thermionic guns has recently been given by Lauer /6/.

3 - LANTHANUM HEXABORIDE CATHODES Lanthanum hexaboride was suggested as a cathode material by Lafferty /7/ more than thirty years ago. LaB6 has a low work function (: 2.2 eV) and is capable of producing a current density in excess of 50 A/cm2. In general LaB6 cathodes can produce an order of magnitude increase in brightness and life compared with a conventional tungsten filament. The chief difficulties lie in the fact that LaB6 is chemically reactive and therefore difficult to mount in a cathode block. In addition it requires a considerably better vacuum (lo-' mbar) than is usually a available in an X-ray microanalyzer if stable operation is to be achieved. Moreover, existing filament current supplies are not usually designed specifically for the special requirements of such cathodes. The technical and scientific problems associated with the construction of LaB6 cathodes have recently been reviewed by Crawford /8/ together with an extensive bibliography. The first cathode construction that succeeded in producing a higher current density than that of conventional tungsten cathodes was that of Broers /9/ shown schematically in Figure 2. A long rod of sintered or single crystal LaB6 is supported in a cooled support and heated by radiation and electron bombardment heating. Thus the hot LaB6 is not in contact with any other material. Considerable development has taken place /lo, 11, 12/ in the last few years, resulting in an optimised design. The chief advantage of this design is that the cathode rod is rigidly mounted and does not react chemically with the mounting block. The large volume of LaB6 also ensures a long life. Either polycrystalline or single crystal rods can be employed. Poly- crystalline material can yield the desired tenfold increase of brightness over that of tungsten. Single crystal cathodes appear to achieve high brightness more consistently /3/. The disadvantage of this design is that the heating arrangement is complicated both mechanically and electrically and consumes considerably more power, (10-30 watts depending on the design), than that of other possible arrange- ments. This in turn may cause outgassing problems in the cathode region. Ideally the electron optical column should be designed from the beginning to accept this type of gun. Where this has been done, as for example in the Cambridge high resolution high voltage microscope /14/ the expected tenfold improvement of electron- optical brightness of lanthanum hexaboride has been achieved.

Since heated LaB6 does not react appreciably with hot graphite it is possible to clamp a small single crystal between two graphite blocks as shown schematically in Figure 3/15, 16, 17/. Heating current is passed through the vitreous carbon blocks of low thermal and electrical conductivity and through the base of the single crystal filament. The power requirements are modest (2-3 watts), comparable with that of a conventional tungsten filament. Considerable care must be taken with the mechanical and thermal design of the crystal holder and with the choice of materials. Such a cathode assembly has the advantage that it uses only a small volume of expensive LaB6 single crystal material. In addition it can be made as a direct replacement for a standard tungsten cathode. However care must be taken with the electrical supplies to avoid accidental overheating of the crystal. For example, the hot resistance of the graphite blocks is lower, perhaps by one third, than the room temperature resistance. It would be an advantage if the voltage across the blocks and the current through them could be measured in order to control electronically the power reaching the filament. Such a control system, or even the direct sensing of the temperature of the emitting tip would be of considerable benefit for all types of cathodes. However, it would require extra wires in the core of the high voltage cable. This may well become necessary in future when fully automatic operation of X-ray microanalysers is required including switching on, and alignment of the column.

A design /8/ which allows an LaB6 crystal to be mounted on a standard tungsten filament base is shown schematically in Figure 4. An accurately machined block of LaB6 is mounted between two parallel arched carbon strips. The mechanical joint is made by heating the carbon so as to allow local interdiffusion of carbon and LaB6. In this design the skill of the manufacturer eliminates the need for a complicated mechanical clamping device. The power requirements are again comparable with that of the tungsten filament that it replaces. The main disadvantage of this design is its fragility /8/ and the absence of any means of adjusting the orientation of the crystal, should this become necessary after the initial activation operation. It is interesting that that LaB6 can now be succeSSfully arc-bonded to a rhenium wire /18/ at a temperature of 2000' C. A d.c. arc is struck between the LaB6 and the rhenium wire in an argon atmosphere of 10-100 mbar. The resulting joint is subse- quently stable, at least for temperatures up to 1550° C. The form of this cathode construction is shown schematically in Figure 5. This cathode, mounted on a standard tungsten filament base, closely resembles a pointed tungsten cathode in appearance and has similar electron optical properties but with higher brightness and longer life, assuming that the vacuum pressure is sufficiently low.

A morerecentlydeveloped method of construction is to embed a small single crystal of LaB6 into a glassy carbon filament /19/. A heater power of some 7 watts is sufficient to produce a tip temperature of about 1600' C. The authors have carried out careful measurements with this system on the poisoning effects of oxygen, water vapour and hydrocarbons andconclude that previous authors may have underestimated the effects of residual vapours and gases on the chemical stability of the LaB6 surface and the trme required to overcome poisoning effects if the vacuum is poor. These ideas have been strengthened by the work of Sewell and Ramachandran /20/ who have investigated the effects of the vitreous films that form the Wehnelt cylinder aperture when operating LaB6 sources in the 1700' C - 1800' C range. Such films lead to unacceptable beam instabilities. It seems likely that these films are not caused by direct evaporation of LaB6 but are oxidation products caused by ion bombardment following ionization of the residual gases. To eliminate these effects for sources operating above 1600' C a pressure of lo-' mbar or better is necessary. We may conclude therefore, that there is now sufficient scientific and technological information available to enable michoanalyzers to be adapted for operation, at choice, with either single crystal LaB6 cathodes or perhaps pointed tungsten cathodes.

4 - FIELD EMISSION GUNS Of all electron guns, the field emission gun has the highest brightness and the lowest energy spread. Because of its small effective source size (10-15 nm) and the aberrations of the first accelerating electrode, it is superior to other sources only for electron probes of this order of magnitude. For analytical instruments it is necessary to improve present designs so that a corresponding superiority should be possible for much larger probe sizes. The whole subject has recently been reviewed in detail by Kasper /21/ and van der Mast /22/, both articles containing extensive bibliographies. It is perhaps sufficie~~tto pin-point important areas for further development. Apart from the difficulty of maintaining the best possible vacuum, (better than lo-'' mbar), the chief problem, as in thermionic cathodes is to lower the work function in a stable manner. This may be achieved in practice with either oxygen processing /23/ or with zirconium oxide processing /24/. The next step is to reduce the effective aberration of the accelerating system by immersing the tip in a strong magnetic field (see /22/). These are now mainly technological problems perhaps involving some compromise between the ideal arrangement of the lens and the stringent requirements of the vacuum system. It is possible that at least some of these problems can be solved by the computer assisted processing of the activation of the emitting tip, and of any 'remoulding' operation that may be necessary to produce the optimum shape and radius of the tip.

5 - IMPROVED '2-CONTRAST' DETECTORS The standard Everhart-Thornley scintillation detector /25/ is an efficient wide-band detector but until recently /26/ not analysed in detail+++New scintillation crystals, especially yttrium aluminium garnet (YAG - Ce )/27/ enable this detector to be used also as a high energy backscatter (Z constrast) detector /28/ at TV rates. Since this crystal can be machined it is possible to use it in the construction of quadrant detectors of the Volbert and Reimer /29/ type in which C2-154 JOURNAL DE PHYSIQUE

topographical and atomic number contrast can be displayed separately at TV rates. Such a detector represents a substantial improvement over all previous detectors.

6 - X-RAY SPECTRA FROM THIN FILM SPECIMENS For the highest spatial resolution, thin film specimens must be used. Because of the low counting rates energy dispersive spectrometers are generally employed. To extract the maximum information from such a specimen it is essential that the X-rays passing into the detector should originate at the point under the probe and not from elsewhere. The design principles of eliminating such spurious radiation have recently been established by Nicholson et a1 /30/. These mainly concern the design of components in the vicinity of the specimen stage. Such refinements allow areas of about 2 nm in diameter to be analysed.

7 - ACKNOWLEDGEMENTS The author would like to thank Dr R Autrata and his colleagues from the Institute of Scientific Instruments of the Czech Academy of Sciences at Brno for stimulating discussions on YAG crystals and Ing Salyk of the same Institute and Dr C Crawford of Kimball Physics USA for valuable discussions on LaB6 cathodes.

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