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Instrumentation, Electron Optics and X-Ray Spectroscopy T INSTRUMENTATION, ELECTRON 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 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C2, supplément au n°2, Tome 45, février 1984 page C2-149 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) Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984233 JOURNAL DE PHYSIQUE 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 tungsten filament by, for example, a lanthanum 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 'work function' 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 melting point. 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.
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