Energy filter image generator for electrically charged particles and the use thereof

The invention relates to an imaging energy filter for electrically charged particles, such as electrons and ions, with at least two toroidal energy analyzers arranged one behind the other, at least one energy analyzer each having an aperture in its entrance and exit plane. The invention also relates to the use of such imaging energy filters.

The diaphragms in the entry and exit planes can be slit diaphragms or circular diaphragms extending perpendicular to the respective energy-dispersive plane.

Energy filters are preferably to be understood as imaging or imaging energy filters. The use of imaging filters is particularly advantageous if the image fields processed in parallel contain more than 100 x 100 image elements (pixels). The registration times are then much shorter than in a spectrometer which sequentially scans the sample.

Energy filters are used, for example, in photoelectron spectroscopy, which is one of the most important methods for quantitative element analysis of surfaces. The combination of high spatial resolution together with the measurement of the photoelectron energy distribution is called spectromicroscopy. Two different methods are used to achieve high spatial resolution.

In the first variant, a focused photon beam is scanned over the sample and the photoelectrons coming from the emission spot defined in this way are analyzed for their energy.

In the second method, the photon beam is focused just enough to illuminate the field of view of the objective lens. The intensity distribution of the generated photoelectrons is magnified electron-optically.

In order to derive a map of the element distribution or the chemical bond, the photoelectrons must be analyzed for their kinetic energy. Various techniques have been developed for this purpose in transmission electron microscopy. There are essentially two different principles here:

There are microscopes that use all electrons to create an image, with a small portion of the electrons going through an energy analyzer to generate a spectrum from part of the image. Another part of the microscope uses only a narrow band of energy, but it transports a complete image through the energy analyzer.

The electrons are filtered by electrostatic or magnetic devices that only allow the passage of electrons with a certain energy. The intensity of the resulting beam reflects the concentration of a chemical component present on the sample surface. With this method, the spatial resolution when passing through the monochromator should not be impaired.

A number of energy analyzers have been developed for this mapping function. For energy analysis without image quality, the hemisphere analyzer has been used in commercial devices due to its good transmission and energy resolution.

The possible imaging properties of electrostatic energy analyzers were analyzed many years ago for analyzers with general toroidal fields (B.Wannberg, G.Engdahl, A. Sköllermo, Imaging Properties of electrostatic Energy Analyzers with toroidal fields, J. Electron Spectr. Rel. Phenomen. 9 (1976), 111-127 For a toroidal potential, the radius of curvature in one direction is different than perpendicular to it. As a special case, the spherical capacitor with a radius ratio equal to 1 is included in this generalized representation. A cylindrical capacitor is curved in one direction only, its radius ratio is zero. There are spectrometers where the transition between the field shapes can be set continuously, as described, for example, in K. Jost, Novel Design of a spherical electron spectrometer, J. Phys. E: Sci. Instr. 12, 1979, pp. 1006-1012.

An electron microscope with an energy filter is known from EP 0293924 B1, which comprises a spherical analyzer with a hemispherical structure. In order to improve the imaging quality of the energy filter, a complex lens system is arranged in front of the entrance slit in order to generate electron beams that strike as perpendicularly as possible. For electrons at the mean orbit radius r₀ = x₀ start, should apply α₀ = -α₁, where α₀ the entry angle and α₁ denotes the exit angle from the energy filter.

The entry angles for these electrons are supposed to be transferred exactly to the exit angle, regardless of their energy.

To take advantage of this property, an enlarged image of the sample is not placed in the inlet slot of the analyzer, but in the focal point of a lens that is attached in front of the slit diaphragm of the analyzer. This transforms the position of the image into angles. The entrance slit diaphragm is placed in the focal point of the lens on the image side.

The analyzer outlet slot selects the desired energy range. Another lens behind the analyzer reconstructs a location image that is now filtered according to the energy from the transmitted angle image. This can be further enlarged and with the help of an intensity amplifier, e.g. B. a microchannel plate can be made visible on a fluorescent screen.

An electron spectrometer with a similar arrangement is described in EP 02 46 841 B1. With this energy analyzer of the Torroid condenser type with upstream and downstream lens system, a spatial resolution of up to 2.5 µm is achieved.

However, it was overlooked that the equation α₁ = -α₀ generally just a rough approximation. In Nucl. Instr. Methods A291 (1990) pages 60-66 show that entry and exit angles also depend on the entry and exit location. Entry and exit angles differ significantly if entry and exit positions are different. Then (tan α₀): x₀ = - (tan α₁): x₁.

The aberrations decrease with an enlarged image field, i.e. a possible difference between x₁ and x₀ to. The following example shows their size:

With a field of view of 4 mm in diameter, e.g. B. x₀ = 122 mm and x₁ = 126 mm together with an acceptance angle of α₀ = 5 ° one calculates an angle of reflection of α₁ = 5.16 °. That is 3% deviation from the angle of incidence. With a field of view of 100 µm radius, this results in an imaging error of 3 µm at the edge of the image field.

A different exit position and a different exit angle also result for electrons with the same entry position at a different entry angle <maths id="math0001" num=""><math display="block"><msub><mrow><mi>tan</mi>&lt;mtable /&gt;<mi mathvariant="italic">α</mi></mrow><mn>1</mn></msub><mo>=</mo><msub><mrow><mi>tan</mi>&lt;mtable /&gt;<mi mathvariant="italic">α</mi></mrow><mn>0</mn></msub><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mfrac><mn>2</mn><mrow><msup><mi>cos</mi><mn>2</mn></msup><mo>⁢</mo><msub><mi mathvariant="italic">α</mi><mn>0</mn></msub></mrow></mfrac><mo>⁢</mo><msup><mrow><mo>)</mo></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup></mrow></math><img file="EP1559126B1_D0001.tif" /></maths>

This is described, for example, in T. Sagara et al., Resolution improvements for hemispherical energy analyzers, Rev. Sci. Instr. 71, 2000, 4201-4207.

Another example uses a hemisphere analyzer in a different mode of operation. Here the potentials are chosen so that the electrons move on a hyperbolic orbit in a field that increases square with the radius.

An electrostatic spherical mirror analyzer is described in U.S. Patent 5,185,524. The electrons enter the inner sphere through slots and are brought back to a focus by an opposing field through the inner sphere. The object and picture are inside the inner sphere.

Disadvantages of this arrangement are in Nucl. Instr. Methods 42, 1966, 71-76. There are large slits in the inner sphere, these slits not being in the place of a small beam cross section. To ensure the spherical potential required at these points, nets are attached at this point. The remaining field penetration through the mesh limits the achievable spatial resolution. Each mesh is a small lens. Another disadvantage of networks in the beam path is the triggering of secondary electrons, which leads to an increased background and thereby reduces the contrast that can be represented. The energy-selective gap is located in the electrical field between the hemispheres and is therefore difficult to access for adjustment. The voltages that have to be applied to the outer sphere are significantly higher than with the conventional hemisphere analyzer.

In this design, as in the previous one, there are inherent image errors which are due to the only two-fold symmetry of the structure.

DE 196 33 496 A1 presents a monochromator for electron microscopy, which is constructed with mirror symmetry. The structure in the form of an Ω avoids second order errors and also some third order errors disappear. For the chosen structure, an essential design criterion was the avoidance of an intermediate focus. This should make it possible to monochromatize a primary electron beam with a small diameter and high current density. This requirement leads to a solution that is mechanically complex. The structure consists of eight toroidal sectors that have to be adjusted very precisely to one another. This makes production very costly and adjustment very time-consuming.

A similar mirror-symmetrical arrangement of monochromators is selected in EP 0 470 299 A1. This arrangement also does not contain an intermediate lens, but a straight connecting tube.

The energy-selecting slot is located in the plane of symmetry. Here, too, no use is intended for imaging.

From US 5,466,933 an energy filter is known which consists of a complementary opposite pair of 90 ° sectors which are arranged in relation to one another in such a way that they form an S shape. An aperture diaphragm is arranged between the two sectors. With this energy filter, the incoming parallel electron beams are imaged at the exit of the sector arrangement.

With this arrangement using parallel electron beams, a high-contrast image is achieved at the output of the energy filter, but the intensity arriving at the output is extremely low. An increase in intensity can indeed be achieved in that electrons with an entrance angle α₀ not equal to 0 are permitted, but then the pixels are smeared and the contrast is reduced.

WO 01/61725 A1 describes an emission electron microscope which contains an imaging beam path consisting of an electron-optical imaging system which shifts the electron beam in parallel and analyzes it energetically. It consists of two spherical energy analyzers with an interposed lens. This lens is at the focal point of both analyzers. An intermediate image of the sample or the angular image of the sample is placed in the middle of this lens. Since the magnification is positive for field lenses, image errors that occur when passing through the first deflector are not corrected. The claim of image correction is not made in this document and is not discussed.

DE 30 14 785 A1 describes a double monochromator for charged particles which contains a retardation lens in the form of slit diaphragms between the partial monochromators. The monochromator is described with higher intensities than was previously possible without loss of energy resolution. No lens is specified which improves the imaging properties of the system. Slit diaphragms are also described in US Pat. No. 4,742,223. The imaging properties of the system are not discussed.

An imaging mirror-symmetrical energy filter is described in US Pat. No. 5,448,063, which only compensates for the 2nd and 3rd order image errors. This image error correction can only be achieved with a high expenditure on equipment, by using additional hexapole fields.

The object of the invention is therefore to provide an imaging energy filter with minimal image errors, which ensures both high-contrast imaging with high spatial resolution and high intensity at its output.

This object is achieved with an energy filter, which is characterized in that a transfer lens device is arranged between the exit plane of the first energy analyzer and the entry plane of the second energy analyzer, which has a negative lateral magnification V_L and a negative angular magnification V_w, has an image rotation by the angle γ = β -180 ° and a telescopic beam path, their respective deflection angles φ being the same size and their energy-dispersive planes (33, 43) being rotated relative to one another by the angle β.

The energy analyzers rotate about the axis of the transfer lens device.

A telescopic beam path is understood to mean a beam path in which each parallel beam is converted into a parallel beam regardless of the angle of incidence. The angle γ denotes the rotation of the image from its inverse position. This inverse position is e.g. B. is achieved by means of electrostatic lenses.

The advantage of the energy filter is that it is not absolutely necessary to work with particle beams hitting the entrance plane of the first energy analyzer, that is, the entrance angle α₀ not equal to 0 can be allowed, so that a high-contrast image of high intensity can be generated at the output of the energy filter. The image errors occurring at the output of the first energy analyzer, in particular second-order image errors, are transformed by the transfer lens device to the entry level of the second energy analyzer, so that these image errors are completely eliminated when the charged particles pass through the second energy analyzer.

The imaging quality of the energy filter is essentially only limited by the quality of the transfer lens device.

The transfer lens device is preferably designed such that it forms the intermediate image ZB in the energy-dispersive plane₁ in the exit plane of the first energy analyzer with a linear magnification <maths id="math0002" num=""><math display="inline"><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">L</mi></msub><mo>=</mo><mfrac><msub><mi mathvariant="italic">Eg</mi><mn>2</mn></msub><msub><mi mathvariant="italic">Eg</mi><mn>1</mn></msub></mfrac><mo>&lt;</mo><mn>0</mn></math><img file="EP1559126B1_D0002.tif" /></maths> and with an angular magnification <maths id="math0003" num=""><math display="inline"><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">w</mi></msub><mo mathvariant="normal">=</mo><mfrac><msub><mi mathvariant="normal">α</mi><mn mathvariant="normal">2</mn></msub><msub><mi mathvariant="normal">α</mi><mn mathvariant="normal">1</mn></msub></mfrac><mo mathvariant="normal">&lt;</mo><mn mathvariant="normal">0</mn></math><img file="EP1559126B1_D0003.tif" /></maths> With <maths id="math0004" num=""><math display="inline"><msub><mi>V</mi><mi>W</mi></msub><mo>⁢</mo><msub><mi>V</mi><mi>L</mi></msub><mo>⁢</mo><msqrt><mfrac><msub><mi>E</mi><mn>2</mn></msub><msub><mi>E</mi><mn>1</mn></msub></mfrac></msqrt><mo>=</mo><mn>1</mn></math><img file="EP1559126B1_D0004.tif" /></maths> rotated by the angle γ = β-180 ° on the entry plane of the second energy analyzer as an intermediate image of the size ZB₂ maps, where α₁ the exit angle of the charged particles from the exit plane of the first energy analyzer, α₂ the entry angle into the entry plane of the second energy analyzer, E₁ the kinetic energy of the charged particles in the exit plane of the first energy analyzer and E₂ denotes the kinetic energy of the charged particles in the entry plane of the second energy analyzer and wherein the charged particles pass through a telescopic beam path in the transfer lens device.

For an electrostatic transfer lens device, β = 180 ° and V_L and V_W are negative. For a magnetic transfer lens device, V_L and V_W negative and an image rotation can occur, so that β ≠ 180 ° must be selected.

The energy analyzers and the transfer lens device are preferably arranged point-symmetrically around the center Z of the transfer lens device. This means that the energy analyzers have the same size and that V_L = -1 and V_W = -1.

However, the energy analyzers can also have different sizes, which creates a quasi-point-symmetrical arrangement in which V_W and V_L are less than 0.

V_w = V_L = -1 can be set by coordinating the radii and the pass energies accordingly.

In the context of the invention, it is fundamentally free which type of deflection fields are used for the analyzers. In addition to magnetic fields that can be generated permanently or by currents, the use of electrostatic fields is particularly preferred.

The toroidal energy analyzers are preferably spherical sectors or spark detectors, in particular with a deflection angle of more than 90 °.

In particular, hemispherical analyzers with deflection angles of Φ = 180 ° are preferred because they have a particularly large energy dispersion at their exit.

In a spherical field where the potential energy of the particle ~ <maths id="math0005" num=""><math display="inline"><msub><mi>V</mi><mi>W</mi></msub><mo>⁢</mo><msub><mi>V</mi><mi>L</mi></msub><mo>⁢</mo><msqrt><mfrac><msub><mi>E</mi><mn>2</mn></msub><msub><mi>E</mi><mn>1</mn></msub></mfrac></msqrt><mo>=</mo><mn>1</mn></math><img file="EP1559126B1_D0005.tif" /></maths> the charged particles move on closed elliptical orbits. All particles starting at a point with different energy and also at different angles have exactly returned to their starting position after a rotation of 360 °, so that no image errors occur at the exit. In such a closed spherical analyzer, the two energy-dispersive planes of the two hemispheres would by definition include the angle β = 0. After a rotation of 180 °, the particles with different energies have a maximum radial distance from each other. If an aperture is installed here that does not disturb the radial field, only particles of a desired energy are allowed to pass through. However, there would be no space in such a closed ball analyzer to accommodate, for example, an entry lens and a detector or the transfer lens.

In the case of two hemispherical analyzers which are arranged with respect to one another in such a way that their energy-dispersive planes are rotated by the angle β, the properties of a complete spherical analyzer can be retained by the imaging properties of the transfer lens device, so that error-free images are also produced at the output of the energy filter. The transfer lens device ensures that the paths are retained with exact imaging, only the entry point in the first hemisphere analyzer being spatially separated from the exit point in the second hemisphere analyzer.

The result is a spherical spherical capacitor. It is known that non-relativistic particles run through closed periodic elliptical orbits. After a complete revolution, the angle and position are retained. This is independent of the starting position, the entry angle or the energy of the charged particles.

The energy-dispersive plane of the hemisphere analyzers is preferably arranged rotated by β = 180 ° around the axis of the transfer lens device, so that the beam path has an S-shaped shape. This arrangement has the advantage that a particularly simple transfer lens device can be used.

For practical reasons, the angle β between the dispersion planes is preferably selected from the angle range from 5 ° to 355 °, in particular from 15 ° to 340 °.

The energy analyzers can be designed differently, which in turn must be taken into account when designing the transfer lens device. The lateral magnification and the angular magnification must be different for different energy analyzers, for example for different pass energies E₁ and E₂, be adjusted when mapping the intermediate image on the entry level of the second energy analyzer. The Lagrangian-Helmholtz relationship is therefore important when designing the transfer lens device<maths id="math0006" num=""><math display="inline"><msub><mi>Eg</mi><mn>1</mn></msub><mo>⋅</mo><msub><mi mathvariant="normal">α</mi><mn>1</mn></msub><mo>⋅</mo><msub><msqrt><mi>E</mi></msqrt><mn>1</mn></msub><mo>=</mo><mi>const</mi><mn>.</mn><mo>=</mo><msub><mi>Eg</mi><mn>2</mn></msub><mn>.</mn><msub><mi mathvariant="normal">α</mi><mn>2</mn></msub><mo>⋅</mo><msub><msqrt><mi>E</mi></msqrt><mn>2</mn></msub></math><img file="EP1559126B1_D0006.tif" /></maths> consider.

From a cost point of view, however, it is advantageous if identical energy analyzers are used and are operated with the same pass energy. In this case, the lateral enlargement of the transfer lens device V_L = -1 and the angular magnification of the transfer lens device V_W = -1.

The transfer lens device preferably comprises at least one electrostatic lens, in particular an electrostatic tubular lens, which is used in particular in connection with two hemispherical analyzers whose energy-dispersive planes are preferably arranged rotated relative to one another by the angle β = 180 °.

The transfer lens device can comprise at least one magnetic lens. Magnetic lenses have the advantage that they have smaller aberrations than electrostatic lenses. They are therefore preferably used when the intermediate image ZB is located on the basis of energy analyzers rotated by the angle β₁ must also be shown rotated by the angle γ = β -180 °.

The transfer lens device preferably has at least two lenses.

The exit plane of the first energy analyzer is advantageously arranged in the focal point of the first lens and the entry plane of the second energy analyzer is arranged in the focal point of the second lens, the distance between the two lenses being 2F and F denoting the focal length of the two lenses.

The transfer lens device can also comprise at least one electrostatic or magnetic multipole lens. Multipole lenses have the advantage that they can be used for imaging without any spherical aberrations. A multipole lens is arranged between the two energy analyzers in such a way that a point-symmetrical structure results.

One possible use provides that the energy filter is arranged in the imaging beam path of an imaging electron-optical system. The task of the energy filter is to select electrons of certain energies from the beam path by appropriately adjusting the diaphragms of the energy filter. It is irrelevant whether an intermediate image of the sample to be examined, the Fourier-transformed intermediate image or another intensity distribution of the imaging beam path is placed in the entry slot of the energy analyzer. By changing the energy window, the energies of these charged particles as well as the energy width of the detected beam path can be determined and varied.

A preferred use relates to use in electron microscopy. The energy filter is used to image the electrons emitted or reflected by the object. These electrons are naturally wide in energy. The contrast can be improved by using electrons from a narrow energy band. By selectively setting the energy window, certain signals can be successively picked out and amplified, while others can be weakened. In this way, specific highlighting of certain information is possible.

Another preferred use of the energy filter relates to the use for time-resolved measuring devices. The advantage of the energy filter is that time-of-flight differences that occur in the first energy analyzer are also eliminated by the transfer lens device in conjunction with the passage through the second energy analyzer.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawings.

Show it:<dl id="dl0001"><dt>Figure 1</dt><dd>1 shows a schematic representation of an energy filter with two hemispherical analyzers,</dd><dt>Figure 2</dt><dd>1 shows a schematic illustration of a transfer lens device,</dd><dt>Figure 3</dt><dd>1 shows an embodiment of the arrangement shown in FIG. 1,</dd><dt>Figure 4</dt><dd>3 shows a perspective illustration of the illustration shown in FIG. 3,</dd><dt>Figure 5</dt><dd>4 shows another embodiment which differs from the embodiment shown in FIG. 4 by a different angle of rotation,</dd><dt>Figure 6</dt><dd>1 shows a schematic representation of a further embodiment with spherical sectors as energy analyzers,</dd><dt>Figure 7</dt><dd>another embodiment with a total of four toroid sectors,</dd><dt>Figure 8</dt><dd>an energy filter with cylinder analyzers,</dd><dt>Figure 9</dt><dd>a transfer lens device with multipole lenses and</dd><dt>Figure 10</dt><dd>a transfer lens device with magnetic lenses.</dd></dl>

1 shows a schematic cross section through an energy filter which has two hemisphere analyzers 30 and 40, between which a transfer lens device 20 is arranged. The two energy analyzers 30, 40 together with the transfer lens device 20 are arranged in such a way that the beam path lies in one plane and has an S-shaped shape.

The overall arrangement is point-symmetrical with respect to the center Z of the transfer lens device 20, the point symmetry being two-fold.

Here, the electrons in the first energy analyzer 30 traverse a left curve and after passing through the transfer lens device 20 in the second energy analyzer 40, a right curve. This means that the two energy-dispersive planes 33, 43 of the two energy analyzers are rotated relative to one another by the angle β = 180 ° (see FIG. 4).

In FIG. 1, only the middle beam paths 4 and 7 of the electrons in the first and second energy analyzers are shown. The energy filter has imaging properties while avoiding second and higher order aberrations.

The surface 1 'of the sample 1 is located at a distance g from the first lens system 2, which images the electrons emerging from the surface 1' onto the entry plane 3 of the first hemisphere analyzer 30.

The object distance g can also be equal to the focal length of the lens system 2, so that the image distance b becomes approximately equal to infinity. In this case, the entry plane 3 of the first energy analyzer 30 is preferably arranged in the eliminated focal plane of the lens system 2.

A first energy-defining slit diaphragm 25, which extends perpendicular to the plane of the drawing and has the width B, is located in the entry plane 3₁ has (see also Figure 3).

The hemisphere analyzer 30 images the electrons entering through the slit diaphragm 25 with aberrations in the exit plane 5, where a second slit diaphragm 26 of width B2 is located.

Since the electrons in the slit diaphragm 25 located in the entry plane 3 at different entry angles α₀ occur, they also occur at different exit angles α when leaving the deflection field of the first energy analyzer₁ out.

The second slit diaphragm 26 extends perpendicular to the plane of the drawing, in which the unia focus of the astigmatic intermediate image ZB₁ 23 lies. The energy dispersion takes place in the plane of the drawing, which means a deviation from the central beam path 4 by a value that is proportional to the energy deviation. By changing the slot width B₂ (see also FIG. 3), the energy width of the electrons let through the slit diaphragm 26 can be selectively adjusted or changed. As a result, only those electrons reach the intermediate image ZB₁ 23, which are within this energy range. As a result, the electron beam is monochromatic.

Downstream of this exit plane 5 is a transfer lens device 20, which consists of two identical convergent lenses 21 and 22 and the first intermediate image 23 generated in the exit plane 5 as an inverted second intermediate image ZB₂ 24, ie V_L = -1, on the entry level 6 of the second energy analyzer 40.

The transfer lens device 20 forms the intermediate image ZB₁ 23 not only vice versa, but also with an inversion of the angle onto the entry plane 6, so that for the entry angle α₂ in the entry level 6 of the second energy analyzer 40 α₂ = - α₁ applies.

In the second energy analyzer 40, by reversing the astigmatism of the intermediate image, ZB₁ 23 in connection with the reversal of the curvature of the path in comparison to the first energy analyzer 30, the image errors are eliminated, so that an energy-filtered stigmatic image 29 is produced in the exit plane 8, which image can be projected onto a detector 10 by means of the lens system 9.

In this embodiment, the second energy analyzer 40 also has a slit diaphragm 27 with the width B.₃ in the entrance plane 6 and a slit diaphragm 28 with the width B.₄ in the exit level 8.

In the event that the surface 1 'of the sample 1 or an enlarged or reduced image is positioned at a distance from the focal length of the lens system 2, the lens system 9 is also at a distance of the focal length from the exit plane 8 and the detector 10 is at a distance from the focal length of the Lens system 9 arranged.

Diffraction images are then located in the entry and exit planes of the two energy analyzers 30, 40 instead of real images. If the lens systems 2 and 9 are operated asymmetrically, the diffraction pattern of sample 1 can be obtained with the energy filter while avoiding second and higher order aberrations. The lens systems are called "asymmetrically operated" if either the surface of the sample is placed in the entrance plane 3 with the aid of the lens system 2 and the lens system 9 is adjusted so that the intermediate image 29 lies in the focal length of the lens system 9, or vice versa the lens system 2 is set so that the sample surface (or its intermediate image) lies in the focal plane of the lens and, at the same time, the plane 8 with the lens system 9 is imaged sharply on the detector 10.

The diffraction pattern of the sample is then placed in the entrance plane 3 with the aid of the lens system 2. This diffraction pattern is energetically filtered and reaches the exit plane 8. From there it is imaged on the detector 10 with the lens system 9.

The beam path in the transler lens device 20 is shown schematically in FIG. The two identical, electrostatic converging lenses 21, 22 have an F-2F-F arrangement, where F denotes the focal length of the lenses 21, 22. Because of this lens arrangement, the first intermediate image ZB₁ 23 in the exit plane 5 with the lateral enlargement V_L = -1 and the beam with the angle magnification V_W = -1 on entry level 6 as second intermediate image ZB₂ 24 shown. The beam path is point symmetrical and telescopic.

In the non-dispersive plane, when using other lenses, for example electron-optical cylindrical lenses, the angular and lateral magnification can also be + 1:

FIG. 3 shows a possible embodiment of the arrangement schematically shown in FIG. 1 with three possible electron orbits E.₀, E₁ and E₂. A section through the energy-dispersive levels is shown.

The electrons start from the surface 1 'of the sample 1 and pass through the slit 25 with the width B.₁ into the first hemisphere analyzer 30, in which an electrostatic deflection field is applied between the inner shell 31 and the outer shell 32.

If the electrons are perpendicular to X₀ Enter through the slit 25, they describe the web E.₀, which describes a semicircle in the first and second hemisphere analyzer.

Since the web E₀ impinges on the axis 200 of the transfer lens device 20, it is also at location X₀ the slit diaphragm 27 of the second hemisphere analyzer 40 is shown and runs in the second hemisphere analyzer through a path that is point symmetrical to the point Z.

The electrons on orbit E₁ start at location X₁ the slit diaphragm 25 of the first hemisphere analyzer 30 with different energy and the entry angle α_0.1while the electrons of orbit E₂ in place X₁ with the entry angle -α_0.2 start. The electrons become point X₂ deflected in the second slit diaphragm 26, each describing elliptical orbits. The exit angles are α_1,1 and α_1,2, where in this example | α_1,1| = | α_1,2| was chosen.

The pixel X₀ of the first intermediate image ZB₁ in the slit diaphragm 26 with the lateral magnification -1 and with the angular magnification -1 in the plane 6 at location X₃ as the pixel of the second intermediate image ZB₂ pictured. Therefore α applies to the angles_1.2= -α_2.2 and α_1.1 = -α_2.1.

In the second energy analyzer 40, an equally strong electrostatic deflection field is present between the inner shell 41 and the outer shell 42, so that the electron paths E₁ and E₂ Traverse elliptical trajectories that correspond to the elliptical trajectories in the first energy analyzer 30. The electrons occur at x₄ at the angles α₃₁ and α_3.2 from, which in turn the angles α_0.1 and α_0,2 correspond. The deviation of the angle α_1,1 and α_1,2 is compensated for by the second pass through the energy analyzer 40. Also regarding the location x₄ applies x₄= x₁. An energy-filtered image of sample 1 without imaging errors is thus obtained in the plane of the slit diaphragm 28.

In FIG. 4, the embodiment shown in FIG. 3 is shown in perspective. The energy-dispersive planes 33 and 43 and the slit diaphragms 25, 26, 27 and 28 are shown in the hemisphere analyzers 30 and 40. The second hemisphere analyzer 40 is arranged rotated about the axis 200 of the transfer lens device 20 through the slit diaphragm 27 by the angle β = 180 °.

FIG. 5 shows a further embodiment in which the second hemisphere analyzer 40 is only rotated by the angle β = 90 ° about the axis 200 running through the slit diaphragm 27.

FIG. 6 shows an embodiment corresponding to FIG. 3, wherein instead of the hemispherical analyzers 30, 40, spherical sectors 30 'and 40' with inner shells 31 ', 41' and outer shells 32 ', 42' are used, whose deflection angle Φ ≤ 180 ° be. The arrangement of the diaphragms 25, 26 and 27 differs from the arrangement in FIG. 3 in that they are not placed in the entry or exit planes of the spherical sectors. This embodiment also shows a two-fold point symmetry with respect to the point Z.

In FIG. 7, the arrangement shown in FIG. 6 is supplemented by two further toroidal sectors 50a and 50b, the toroidal sector 50a being arranged in front of the first spherical sector 30 'and the toroidal sector 50b being arranged behind the second spherical sector 40'. These additional toroid sectors 50a and 50b serve to correct higher order aberrations.

Figure 8 shows an energy filter consisting of two cylinder analyzers 30 "and 40" with inner shells 31 ", 41" and outer shells 32 "and 42" and a transfer lens device 20. The axis 200 of the transfer lens system 20 is not collinear to the cylinder axes 34 and 44, but in the direction of the middle tracks 4 ', 7' through the cylinder analyzers, which enclose an angle of 42.3 ° with the cylinder axes 34, 44.

FIGS. 9a, b show a transfer lens device 20 which avoids both the spherical aberration and the coma error. This can be achieved by a combination of electrical or magnetic round lenses (21, 22) with two sextupol lenses 121 and 122. The axis 200 of the transfer lens device corresponds to the direction z.

Figure 9a shows an axially perpendicular section through a sextupole segment. The force F on a particle changes its direction between two neighboring electrodes, which are alternately occupied with the voltages U and -U with respect to the axis potential.

In part 9b, the course of the axially distant orbits of two electrons incident in the xy section parallel to the axis in the case of switched off (dashed) and excited (solid line) sextupoles 121 and 122 is shown. The axis-near path is influenced very little by the sextupoles.

The sextupoles are in the exit or entry level 5, 6 of the energy analyzers.

FIG. 10 shows a magnetic transfer lens device 20 ′ analogous to the electrostatic lenses of FIG. 2 in a schematic representation. The magnetic field of the lenses 22 'and 21' is generated by coils. The main difference to an electrostatic transfer lens device is an additional rotation of the image by the angle γ, where γ is based on the position of the image for<maths id="math0007" num=""><math display="inline"><msub><mi mathvariant="normal">V</mi><mi mathvariant="normal">L</mi></msub><mo>=</mo><mfrac><msub><mi mathvariant="italic">Eg</mi><mn>2</mn></msub><msub><mi mathvariant="italic">Eg</mi><mn>1</mn></msub></mfrac></math><img file="EP1559126B1_D0007.tif" /></maths> is related.

Reference numerals

<dl id="dl0002" compact="compact"><dt>1</dt><dd>sample</dd><dt>1'</dt><dd>Sample surface</dd><dt>2</dt><dd>first lens system</dd><dt>3</dt><dd>Entrance level</dd><dt>4, 4'</dt><dd>Middle beam path in the first energy analyzer</dd><dt>5</dt><dd>Exit level</dd><dt>6</dt><dd>Entrance level</dd><dt>7, 7'</dt><dd>Middle beam path in the second energy analyzer</dd><dt>8</dt><dd>Exit level;</dd><dt>9</dt><dd>Lens system</dd><dt>10</dt><dd>detector</dd><dt>20, 20'</dt><dd>Transfer lens device</dd><dt>21,21'</dt><dd>First transfer lens</dd><dt>22,22'</dt><dd>Second transfer lens</dd><dt>23</dt><dd>First intermediate picture</dd><dt>24</dt><dd>Second intermediate picture</dd><dt>25</dt><dd>First slit diaphragm</dd><dt>26</dt><dd>Second slit diaphragm</dd><dt>27</dt><dd>Third slit diaphragm</dd><dt>28</dt><dd>Fourth slit diaphragm</dd><dt>29</dt><dd>image</dd><dt>30, 30', 30"</dt><dd>First toroidal energy analyzer</dd><dt>31, 31', 31 "</dt><dd>Inner shell</dd><dt>32, 32', 32"</dt><dd>Outer shell</dd><dt>33</dt><dd>energy dispersive level</dd><dt>34</dt><dd>axis</dd><dt>40,40', 40"</dt><dd>Second toroidal energy analyzer</dd><dt>41, 41', 41"</dt><dd>Inner shell</dd><dt>42, 42', 42"</dt><dd>Outer shell</dd><dt>43</dt><dd>energy dispersive level</dd><dt>44</dt><dd>axis</dd><dt>50a, b</dt><dd>Toroidal sector</dd><dt>121</dt><dd>Sextupole lens</dd><dt>122</dt><dd>Sextupole lens</dd><dt>200</dt><dd>Axis of the transfer lens device</dd></dl>