EP0013003B1 - Electron beam investigation process and electron impact spectrometer therefor - Google Patents

Description

The invention relates to a method for electron beam analysis, in particular of solid bodies, in which the electrons cathodically emitted and electron-optically bundled by an emission system are subjected to at least one energy selection in a cylinder capacitor deflection unit and are subsequently detected with a detector. It also includes an electron impact spectrometer for performing the method with electrostatic cylindrical capacitor deflection units as energy-dispersive units, which is designed in particular for impact energies between 1 and 1000 eV.

Electron impact spectrometers (also known as "electron energy loss spectrometers" or abbreviated "electron spectrometers") are used for the analysis of gases and solids, whereby the relevant information is obtained in the form of characteristic energy losses after the electrons are impacted with gas molecules or a solid sample Application for recording vibration spectra of adsorbates and thus the use in catalysis research of particular interest. To do this, the energy resolution of the spectrometers used must be in the range of ΔE = 5-10 meV. In this application in particular, the highest possible current is sought for a given resolution ΔE.

It is a special characteristic of such examinations that the essential part of the electrons are reflected by the specimen. This also applies to electrons that have lost energy due to excitation of adsorbate vibrations (H. Ibach, J. Vac. Sci. Technol. 9, 713 (1972) and E. Evans and DL Mills, Phys. Rev. B5, 4126 ( 1972)). Due to these physical conditions, the presence of the sample - apart from a beam deflection - does not affect the beam path with regard to the focusing conditions and the comparison of different types of spectrometers can be done by comparing their properties in direct penetration without the presence of the sample.

der zugleich ein Maß für die Güte des Spektrometers darstellt. Bei Einstellung der höchstmöglichen Auflösung wird ein zusätzlicher Abfall des transmittierten Stromes durch verstärkte Bildfehler bei niedrigen Elektronenenergien bedingt. Die erzielbare Auflösung ΔE_min (üblicherweise gemessen als Energiebreite bei halbem Signalstrom; englisch »FWHM«), bei der der Strom noch Gleichung (1) folgt, ist deshalb ebenfalls ein Maß für die Spektrometerqualität.It is known that the current is limited by space charge effects (H. Ibach, Applications of Surf. Sci. 1, 1 (1979)). This results in a dependency of the transmitted current at the detector 1 ₀ proportional to ΔE5 / 2. Different embodiments of electrostatic electron impact spectrometers differed in the pre-factor g obtained in the equation

which also represents a measure of the quality of the spectrometer. If the highest possible resolution is set, an additional drop in the transmitted current is caused by increased image errors at low electron energies. The achievable resolution ΔE _min (usually measured as the energy width at half the signal current; English »FWHM«), at which the current still follows equation (1), is therefore also a measure of the spectrometer quality.

Electron impact spectrometers for the applications described have been implemented using various energy-dispersive elements. In particular, cylindrical capacitors, spherical capacitors and so-called cylinder mirrors have become known. The corresponding values for g and the best half-width achieved (as a measure of the resolution) are listed in the table below, provided information on the current at the detector has been made in previous work.

Then particularly high resolutions are achieved with spherical capacitor or cylindrical mirror deflection units. However, since the manufacture and handling of cylinder capacitor deflection units are considerably simpler, it is the object of the invention to improve electron impact spectrometers with cylinder capacitor deflection units in such a way that more favorable values for g and ΔE _min are obtained.

This aim is achieved according to the invention by a method of the type mentioned at the outset, which is characterized in that the electrons in the plane perpendicular to the cylinder capacitor axis are focused on the inlet diaphragm (3 or 3 ') of the capacitor (1 or 2) while parallel the cylinder capacitor axis is focused on the detector (12).

Such a beam guidance electron impact spectrometer with an emission system comprising a cathode and a linden system for an electron current focused on the input aperture of the monochromator, which enters the cylindrical capacitor monochromator for energy selection of the electrons, which come from the monochromator and fall onto the sample after reflection and after reflection on the same Via a lens system in a cylinder capacitor analyzer and after energy selection and passage through the output aperture of the analyzer hit a detector, are characterized by a differently designed emission system perpendicular or parallel to the monochromator cylinder axis in such a way that the electrons perpendicular to the cylinder axis in a known manner to be focused on the input aperture of the monochromator while focusing on the detector parallel to the cylinder axis, and by means of a lens system between the monochrome ator and analyzer with a focusing effect perpendicular to the cylinder axis, but without a focusing effect parallel to the cylinder axis.

For this purpose, the emission system preferably comprises a repeller on the cathode, the focusing surface of which has different radii of curvature parallel and perpendicular to the monochromator cylinder axis, the radius of the curvature profile being greater in the plane passing through the monochromator cylinder axis than perpendicular to it.

Furthermore, the height h of the slots in the inlet and outlet panels is preferably greater than the root of the path radius r and slot width s.

The system-related disadvantage of cylindrical capacitors, which consists in the fact that these energy-dispersive elements focus only in one plane, is compensated for by the invention. As a result, the values for g and ΔE _min achieved, as explained in the exemplary embodiment, are better than in the constructions which have hitherto become known, the additional advantage being the comparatively simple manufacture of cylinder capacitor systems.

• Elektronenstcßspektrometer enthalten mindestens je ein energiedispersives System als Morechromator 1 und als Analysator 2. Solche energiedispersiven Systeme sind selbstfokussierend, das seißt Elektronen der erwünschten Energie werden von der Eintrittsblende 3, 3' auf die Austrittsblende 4, 4' abgebildet. Im Falle von Zylinderkondensatoren als energiedispersive Elemente sind Ein- und Ausgangsblende üblicherweise in der Form von Längsschlitzen (»Schlitzblenden«) ausgebildet und die Selbstfokussierung erfolgt nur in der Ebene senkrecht zur Zylinderachse, (Aufsichtebene in Fig. 4; im folgenden als Spektrometerebene bezeichnet.)

For a more detailed explanation of the invention, the function of the typical components of an electron impact spectrometer is first described with reference to FIG. 4:

• Electron impact spectrometers each contain at least one energy-dispersive system as a Morechromator 1 and as an Analyzer 2. Such energy-dispersive systems are self-focusing, that is, electrons of the desired energy are imaged from the entrance aperture 3, 3 'to the exit aperture 4, 4'. In the case of cylinder capacitors as energy-dispersive elements, the input and output diaphragms are usually designed in the form of longitudinal slits (“slit diaphragms”) and self-focusing takes place only in the plane perpendicular to the cylinder axis (view plane in Fig. 4; hereinafter referred to as the spectrometer plane).

An electron impact spectrometer also contains a system for beam generation 5 (hereinafter referred to as an emission system) (with an emitting cathode 6, repeller 7 and possibly focusing elements 8), and a lens system 9 or 10 between monochromator 1 and sample 11 or sample 11 and analyzer 2, This serves to guide the beam and to image the electrons (otherwise reflected on the target 11) from the exit slit 4 of the monochromator 1 into the entry slit 3 'of the analyzer 2. The electrons are then detected in the detector 12.13, which denotes a supply unit.

In the previously known electron impact spectrometers based on cylindrical capacitors, the emission system and lens system were either circularly symmetrical to the beam axis (D. Roy and J. Carette in "Electron spectroscopy for surface analysis", ed. By H. Ibach, Springer 1977) or else any focusing perpendicular to the plane drawn in FIG. 4 was dispensed with (N. Propst and Th. C. Piper, J. Vac. Sci. Technology 4, 53 (1967) and H. lbaαh, J. Vac. Sci. Technology 9, 713 (1972)).

• Beim zirkularsymmetrischen System kann zum Beispiel durch geeignete Wahl der Spannungen ein Fokus der Kathode in den Eintrittsspalt des Monochromators gelegt werden. Offensichtlich entsteht dann aber wegen der Zirkularsymmetrie ein Bündel, welches nach dem Fokus nicht nur in der Spektrometerebene, sondern auch senkrecht dazu divergiert. Da der Zylinderkondensator senkrecht zur Spektrometerebene nicht fokussiert, geht der überwiegende Teil der Elektronen der Nutzung im Detektor verloren. Entsprechendes gilt für die weiteren abbildenden Einheiten. Der Verzicht auf jede abbildende Wirkung senkrecht zu der aus Fig. 4 ersichtlichen Spektrometerebene führt offensichtlich ebenfalls zu großen Verlusten.

Both of the above-mentioned designs are obviously not optimally adapted to the peculiarities of the cylinder capacitors, focusing only in the plane perpendicular to the cylinder axis:

• In the case of the circularly symmetrical system, for example, a suitable choice of the voltages allows a focus of the cathode to be placed in the entry gap of the monochromator. Obviously, however, because of the circular symmetry, a bundle arises which, after the focus, not only diverges in the spectrometer plane, but also perpendicular to it. Since the cylindrical capacitor does not focus perpendicular to the spectrometer level, the majority of the electrons are lost in the detector. The same applies to the other imaging units. Dispensing with any imaging effect perpendicular to the spectrometer plane shown in FIG. 4 obviously also leads to great losses.

According to the invention, in contrast to that defined by the above-defined design of the emission system perpendicular to the spectrometer plane, a parallel beam is formed, with the rest of the lens systems used, which map the exit spa of the monochromator to the entrance slit of the analyzer in the spectrometer plane, but not the beam perpendicular to the spectrometer plane influence, whereby the focus formed by the cathode system in this direction in the detector remains unaffected.

Obviously, such an emission and lens system is optimally adapted to the imaging properties of cylindrical capacitors. According to the invention, the corresponding properties of the emission system and lens system are achieved by a corresponding design of the electrodes.

wobei s und h Schlitzbreite und -höhe von Eingangs- und Ausgangsblende (3, 3' und 4,4' in Fig. 4), r d Radius des Zylinderkondensators, E die Energie der Elektronen im Zylinderkondensator (1, 2) und α d Winkeldivergenz senkrecht zur Zylinderachse sind. Der zweite Term der Gleichung ergibt sich, wer Strahlen den Zylinderkondensator derart durchsetzen, daß sie von einem Punkt am oberen Rand de Eingangsschlitzes (3) zu einem Punkt am unteren Rand des Ausgangsschlitzes (4) gelangen könne Durch die erfindungsgemäße Strahlführung werden solche Elektronenbahnen ausgeschlosse wodurch der zweite Term entfällt mit dem Resultat einer entsprechend verbesserten Auflösung b gegebener Energie der Elektronen im Monochromator. Da, wie dem Fachmann bekannt ist, die: Energie nicht beliebig erniedrigt werden kann infolge der örtlichen Inhomogenität d Oberflächenpotentials, bedeutet der Wegfall des Terms 2 in Gleichung (2) auch einen grundsätzliche Vorteil im Hinblick auf die maximal erzielbare Auflösung. Dieser theoretische Befund läßt sie experimentell bestätigen. Zur Erzielung einer maximalen Auflösung sollten im übrigen loka Inhomogenitäten des Oberflächenpotentials möglichst klein gehalten werden. Dazu wurden t einigen bisher bekanntgewordenen Spektrometern zusätzlich Ausheizvorrichtungen oder Beschic tungen mit Edelmetallen vorgesehen (Phys. Rev. 173, 222 (1968)). Zur Vermeidung von Aufladungen d Elektroden und zur Reduktion der Sekundärelektronenproduktion ist ferner die Beschichtung n Acetylenruß bekanntgeworden (J. A. Prested, J. Phys. E. Scientific Instruments 6, 661 (1973)).The prescribed beam guidance not only leads to a good current yield at the detector, as explained above, but also brings advantages for the resolution of the system: The energetic resolution of a cylinder capacitor is given by

where s and h slot width and height of the input and output diaphragm (3, 3 'and 4,4' in Fig. 4), rd radius of the cylindrical capacitor, E the energy of the electrons in the cylindrical capacitor (1, 2) and α d angular divergence are perpendicular to the cylinder axis. The second term of the equation results from who beams pass through the cylindrical capacitor in such a way that they can get from a point on the upper edge of the input slot (3) to a point on the lower edge of the output slot (4) the second term is omitted with the result of a correspondingly improved resolution b of the given energy of the electrons in the monochromator. Since, as is known to the person skilled in the art, the: energy cannot be reduced arbitrarily as a result of the local inhomogeneity of the surface potential, the omission of term 2 in equation (2) also means a fundamental advantage with regard to the maximum achievable resolution. This theoretical finding allows them to be confirmed experimentally. In order to achieve maximum resolution, local inhomogeneities in the surface potential should be kept as small as possible. For this purpose, in some spectrometers that have become known, additional heating devices or coatings with precious metals have been provided (Phys. Rev. 173, 222 (1968)). To avoid charging the electrodes and to reduce secondary electron production, the coating n acetylene black has also become known (JA Prested, J. Phys. E. Scientific Instruments 6, 661 (1973)).

Such a coating with carbon also proves to be expedient in the spectrometer according to the invention, and surprisingly a particularly favorable behavior of the system is achieved if the carbon coating is provided in the form of graphite, such as, in particular, by immersing the electrodes in a suspension of colloidal graphite u brief "baking" of the coating is obtained.

• Fig. 1 ein Emissionssystem;
• Fig. 2 dessen Linsenprofil;
• Fig. 3 Kurven für den gemessenen Strom am Detektor in Abhängigkeit von der Energiebreite ΔE; u
• Fig. 4 den Aufbau eines Spektrometers;

The invention is described below using an exemplary embodiment with reference to the drawings; it shows schematically:

• 1 shows an emission system;
• Fig. 2 whose lens profile;
• 3 curves for the measured current at the detector as a function of the energy width ΔE; u
• 4 shows the structure of a spectrometer;

The electron impact spectrometer sketched in FIG. 4 and described above comprises an emission system, the different focal lengths of which are realized by appropriate shaping (electrodes as can be seen in FIG. 1), which shows a vertical section above and a horizontal section through the system below different designs in the two directions, in particular the special shape of the repeller 7 with a curved repeller surface 7 'with radii of curvature r ₁ , r ₂ u which are different in both directions and a lens system 8 which is matched thereto. The lenses 9 used in the spectrometer between the monochromator 1 and analyzer 2 10 have an elongated lens profile with blunted corners, as can be seen in Fig. 2. The lenses 8 of the emission system have an analog profile.

In the selected example, the radius of the cylindrical capacitors is r = 35 mm and the slot width s = 0.15 mm (√r · s≃2.3 mm). For investigations on single-crystal specimens of limited size, the slit height h = 4 mm was selected. The angular half width is α = 3 °. To test the properties of the spectrometer, the current in the sample position and at the detector function of the energy width ΔE (half-value width) was measured in direct penetration (see FIG. 3). The energy of the electrons on the sample was kept fixed at 5 eV. The energy resolution of the monochromator and analyzer was the same in each case.

As can be seen from FIG. 3, the current at the detector follows the theoretical relationship (equals 1), without a deteriorated image due to deviation from the theoretical power law being obvious even at ΔE = 5 meV. The g factor resulting from the curve is 3.5 · 10 ^-6 A / (eV) ^5/2 . A maximum resolution of ΔE _min = 5 meV was achieved. The comparison Table 1 shows that, as a result of the described invention, resolutions in the range of 5 meV with acceptable current (g factor) were realized for the first time. Furthermore, the invention enables the use of particularly simple to manufacture cylindrical capacitors as energy-dispersive elements without loss of current and resolution.

Claims (7)

1. Method for electron-beam analysis, especially of solid bodies in which the electrons, which are cathodically emitted from an emission system, and are electron-optically bundled, are subjected to at least one energy-selection in a cylinder condenser deflection unit and are finally detected by means of a detector, characterised in that the electrons are focussed onto the entry aperture (3 or 3') of the condenser (1 or 2) in the plane normal to the axis of the cylinder condenser, while a focussing onto the detector (12) takes place parallel to the axis of the cylinder condenser.

2. Electron-impact spectrometer for carrying out the method according to claim 1, with an emission system, comprising a cathode and a lens system, for a stream of electrons which is focussed onto the entry aperture of the monochromator, this stream of electrons entering the cylinder condenser monochromator for energy selection of the electrons, and in which spectrometer the selected electrons arriving bundled from the monochromator fall on the specimen and, after reflection at the specimen, enter a cylinder condenser analyser via a lens system, and, following energy-selection and passage through the exit aperture of the analyser, strike a detector, characterised by an emission system (6, 8) which is differently formed normal and parallel to the cylinder-axis of the monochromator in such a way that normal to the cylinder-axis the electrons are focussed onto the entry aperture (3) of the monochromator (1), while a focussing effect onto the detector (12) takes place parallel to the cylinder-axis, this spectrometer also being characterised by a lens system (9, 10) between monochromator (1) and analyser (2), this lens system having a focussing effect normal to the cylinder-axis, but no focussing effect parallel to this axis.

3. Electron-impact spectrometer according to claim 2, characterised by a repeller (7) at the cathode (6), the surface (7') of this repeller having a focussing effect exhibiting dissimilar radii of curvature (r_i, r₂) parallel to and normal to the cylinder-axis of the monochromator, the radius r₁ of the curvature-profile in the plane passing through the cylinder-axis of the monochromator being greatei than the radius in the plane normal to this axis.

4. Electron-impact spectrometer according to claim 2 or 3, characterised in that the height h of the slits in the entry and exit apertures (3, 3', 4, 4') is greater than the square root of the product of the path-radius r and the slit-width s.

5. Electron-impact spectrometer according to one of claims 2 to 4, characterised in that the element: involved in guiding the beam possess a carbon coating.

6. Electron-impact spectrometer according to claim 5, characterised in that the elements involved ir guiding the beam, which possess a carbon coating, comprise the apertures (3, 3', 4,4'), the lenses (7 tc 10) and the deflecting plates of the condensers (1, 2).

7. Process for manufacturing elements involved in guiding the beam in an electron-impaci spectrometer according to claim 5 or 6, characterised in that these elements are dipped into ε suspension of colloidal graphite.

Images