Electron detector for an electron microscope

Abstract

The invention relates to an electron detector (22) for an electron microscope (10) which generates a primary electron beam (16) in order to illuminate a sample (18), said electron detector having a tubular portion (32) with an internal volume (34). The primary electron beam (16) can pass through the internal volume (34) in order to illuminate the sample (18). The electron detector (22) has a detector element (26) which is arranged outside the tubular portion (32) and which is designed to detect electrons which, coming from the sample (18), move in a second direction (42) counter to the direction of the primary electron beam (16). A filter grid (28) is arranged in front of the detector element (26) in the second direction (42), said filter grid extending in a third direction (60) which runs transversely to the beam direction of the electron beam (16). The electron detector (22) has an insulator (36) which electrically insulates the filter grid (28) and the tubular portion (32) from one another such that the filter grid (28) can be at a negative electrical potential relative to the housing (24). A plurality of electrodes (48, 50, 52) made of an electrically conductive material are arranged outside the tubular portion (32) and are designed to produce a predominantly homogeneous electric field (56) between the filter grid (28) and the detector element (26). The electric field (56) has equipotential lines (66) which run in the third direction (60), predominantly parallel to the filter grid (28). The electrodes (48, 50, 52) are also designed to distort the electric field (56) in the region of the insulator (36) in a targeted manner such that the equipotential lines (66) have an inhomogeneous, predominantly falling profile in a defined region (68) between the filter grid (28) and the detector element (26). The defined region (68) extends in the second direction (42) proceeding from the filter grid (28) and in the third direction (60) proceeding from the tubular portion (32).

Description

Electron detector for an electron microscope

[0001] The present invention relates to an electron detector for an electron microscope, which generates a primary electron beam to illuminate a sample, in particular a scattering or backscattering electron detector, comprising a tubular section with an internal volume through which the primary electron beam can pass to illuminate the sample, wherein the primary electron beam has a beam direction defining a first direction, comprising a detector element arranged outside the tubular section and configured to detect electrons moving from the sample in a second direction, comprising a filter grid arranged in the second direction in front of the detector element and extending in a third direction transverse to the first direction, and comprising an insulator electrically isolating the filter grid and the tubular section from each other.so that the filter grid can be at a negative electrical potential relative to the tubular section, and with a plurality of electrodes made of an electrically conductive material arranged outside of the tubular section, the electrodes being configured to produce a predominantly homogeneous electric field between the filter grid and the detector element, and wherein the electric field has equipotential lines that run predominantly in the third direction.

[0002] Such an electron detector is known in principle from EP 1 605492 A1 or the priority-equal US 8,203,119 B2.

[0003] An electron microscope allows the examination of a sample by directing an electron beam precisely onto it and recording the interactions of the electrons striking the sample. In a scanning electron microscope, the electron beam is typically moved relative to the sample to scan the sample surface in successive steps along a line or area. Accordingly, the sample is illuminated by the electron beam in a location-dependent manner. Typical interactions that are recorded and analyzed for sample examination include electrons that are backscattered from the sample (so-called backscatter electrons) and electrons that originate from a near-surface region of the sample material and are, in effect, ejected from the sample by the electron beam (so-called secondary electrons).Backscattered electrons and secondary electrons are produced as a result of illumination with the primary electron beam, which is why some of its electrons are referred to as primary electrons. Furthermore, the typical interactions recorded for sample analysis include radiation emitted by the sample, such as X-rays or cathode luminescence.

[0004] The detection of backscattered electrons makes it possible to visualize material contrasts within the sample, since chemically heavier elements cause stronger backscattering than lighter elements. Therefore, the detection of backscattered electrons allows the generation of a material contrast image of the sample, in which different materials and material combinations become visible. In contrast, the detection of secondary electrons enables, among other things, the determination of the surface topography of the sample.

[0005] Both backscattered electrons and secondary electrons move essentially in the opposite direction to the primary electron beam away from the sample surface, and it is desirable to detect the backscattered electrons separately from the secondary electrons in order to extract the different information from the to obtain a sample. This is possible because backscattered electrons typically have a higher energy than secondary electrons. It is therefore known to arrange a filter grid in an electron detector. With the help of the filter grid, an electric braking field is generated that is directed against the direction of motion of the backscattered and secondary electrons and thus decelerates them. The electric field can advantageously be chosen such that only the higher-energy backscattered electrons can overcome the electric braking field to reach the detector element.

[0006] The filter grating is typically set to a different electrical potential than the detector element to generate the braking field. This potential typically corresponds to that of a detector housing, and the filter grating must therefore be electrically isolated from the detector element or housing. This isolation is achieved by using a high-resistance material to which the filter grating is attached. Accordingly, the term "insulator" here refers to such a high-resistance material. A disadvantage is that the high-resistance material can become electrically charged by incident backscattered or secondary electrons. This charge affects the primary electron beam and can lead to undesirable effects during image acquisition in the electron microscope, such as image distortion and loss of resolution.

[0007] The aforementioned EP 1 605492 A1 discloses an electron beam microscope with an electron detector for detecting backscattered or secondary electrons. The electron detector has a filter grating arranged in the direction of motion of the backscattered and secondary electrons in front of a detector element in the form of a scintillator to generate an electric braking field. The electric braking field should be as homogeneous as possible, with equipotential lines parallel to the filter grating and minimal stray field regions. To achieve this and to shield the primary electron beam from the backscattered or secondary electrons, EP 1 605492 A1 proposes arranging electrodes in the region of the filter grating. In one embodiment, EP 1 605492 A1 discloses six ring-shaped metallic electrodes arranged at symmetrical intervals in front of and behind the filter grating.In another embodiment, EP 1 605492 A1 discloses two high-resistance cylindrical electrodes made of a ceramic material that extends in front of and behind the filter grid in the direction of electron movement. The aforementioned electrical... The known electron detector cannot prevent the charging of the insulating material layers in the area of ​​the detector element.

[0008] Against this background, it is an object of the present invention to provide an electron detector in which the aforementioned disadvantages are reduced, if not avoided altogether. In particular, it is an object of the invention to provide an electron detector that reduces and, if possible, avoids image distortions and / or resolution losses when used in a scanning electron microscope.

[0009] According to a first aspect of the present invention, an electron detector of the type mentioned above is proposed to solve these problems, wherein the electrodes are further configured to selectively distort the electric field in the region of the insulator such that the equipotential lines in a defined region between the filter grid and the detector element exhibit a predominantly downward slope, the defined region extending in the second direction from the filter grid and in the third direction from the tubular section. Accordingly, the electric field in the defined region is selectively distorted and, compared to the electric field outside the region, is predominantly or even strongly inhomogeneous.

[0010] According to a second aspect of the present invention, to solve these problems, an electron detector for an electron microscope is proposed, which generates a primary electron beam to illuminate a sample, in particular a scattering or backscattering electron detector, comprising a tubular section with an internal volume through which the primary electron beam can pass to illuminate the sample, wherein the primary electron beam has a beam direction defining a first direction, comprising a detector element arranged outside the tubular section and configured to detect electrons moving from the sample in a second direction, comprising a filter grating arranged in the second direction upstream of the detector element and extending in a third direction transverse to the first direction, and comprising an insulator.which electrically insulates the filter grid and the tubular section from each other, so that the filter grid is at a negative electrical potential relative to the tubular section. a section can be located, and with a plurality of electrodes made of an electrically conductive material, which are arranged outside of the tubular section, wherein the electrodes are configured to produce a predominantly homogeneous electric field between the filter grid and the detector element, wherein the filter grid is arranged at a defined distance from the detector element, wherein the filter grid is attached to the insulator via a first electrode from the plurality of electrodes, wherein the first electrode has a first electrode section which lies in the first direction between the detector element and the filter grid and has a defined length in the first direction, and wherein the defined length is 10%-40% of the defined distance.

[0011] Preferably, the detector element is attached to the insulator via a further electrode from the plurality of electrodes, wherein the further electrode has a further defined length in the first direction, and wherein the further defined length is a maximum of 10% of the defined distance.

[0012] Preferably, the electrodes are made of a metallic material. In some preferred embodiments, the electrodes comprise a layer of gold, copper, silver, aluminum, and / or platinum deposited on the insulator, for example, by thermal evaporation, sputtering, and / or a (PE-)CVD process.

[0013] In preferred embodiments, the filter grid extends largely orthogonally to the first beam direction, which is defined by the primary electron beam. It is preferably at a negative potential relative to the detector element or the housing of the electron detector and acts as a suppressor grid for the electrons moving in the second direction, and thus essentially opposite to the beam direction of the primary electron beam. In preferred embodiments, the voltage between the filter grid and the housing is selected such that backscattered electrons can pass through the suppressor field and thus reach the detector element, while lower-energy secondary electrons originating from the sample material cannot overcome the suppressor field. For example, the electrical voltage between the filter grid and the housing can be greater than or equal to 5000 V. to select backscattered electrons with an energy greater than 50 eV. In a preferred embodiment, the electrical voltage between the filter grid and the housing is greater than 8000 V.

[0014] The equipotential lines in the defined region run in the opposite direction to the second direction and preferably monotonically decreasing, or even strictly monotonically decreasing, if one considers the course of the equipotential lines starting from the tubular section with the insulator in the third direction and restricts the view to the region between the filter grid and the detector element. This results in the electric field lines of the braking field leading away from the insulator in the defined region when viewed in the direction of motion of the electrons coming from the sample (backscattered electrons or secondary electrons). Since these electrons follow the electric field lines in their motion, they are guided away from the insulator in one direction in the defined region, thus being selectively deflected. In other words, the trajectories of the electrons in the defined region lead away from the insulator.This prevents the electrons in question from colliding with the insulator on their path in the second direction and negatively charging it. At the very least, it reduces the number of electrons reaching the insulator in this direction. Consequently, the electrical charging of the insulator is reduced, if not completely avoided, and the resulting effects, such as image distortion and / or resolution loss in an electron microscope, are minimized. The problem described above is thus fully solved.

[0015] In a preferred embodiment of the invention, the filter grid has a defined grid length in the third direction and the defined area extends in the third direction over a length that is at most 30% of the defined grid length.

[0016] This design limits the defined area in the third direction to a maximum of one-third of the detection range of the electron detector and has the advantage that the electric braking field can be homogeneous or at least largely homogeneous in the majority of the electron detector. This design thus advantageously contributes to detecting as many backscattered electrons as possible and achieving high resolution and detection efficiency. Preferably, the defined area in the second direction extends over a length that is a maximum of 50% of the detection range. Stands between the filter grid and the detector element are important because this helps to efficiently reduce electrical discharges in the electron detector.

[0017] In a further embodiment, the electrodes are arranged to distort the electric field in such a way that the equipotential lines in the defined area, as well as further equipotential lines in a further area that lies in the second direction in front of the filter grid, run in a mirror-symmetrical manner to the filter grid.

[0018] In this configuration, the electric field in the region of the tubular section appears funnel-shaped and distorted compared to the predominantly homogeneous distribution of the equipotential lines outside the defined area. The central axis of the funnel essentially coincides with the filter grid. This design contributes to obtaining the most homogeneous possible electric braking field in the electron detector and simultaneously utilizes the advantageous deflection of the electrons away from the insulator.

[0019] In a further embodiment, the electrodes are arranged to further distort the electric field in such a way that further equipotential lines in a further area, which lies in front of the filter grid in the second direction, starting from the tubular section, have a predominantly rising course along the third direction, wherein the rising course has a slope that is smaller in magnitude than that of the predominantly falling course.

[0020] In this embodiment, the targeted distortion of the electric braking field is not symmetrical about a plane passing through the filter grid in the third direction. Rather, the equipotential lines (and thus the electric field lines, which are known to be perpendicular to the equipotential lines) are less distorted in the region in front of the filter grid than in the defined region after the filter grid. The electrons moving in the second direction have a different velocity and energy in front of the filter grid than after it and are therefore less affected in this embodiment. This embodiment thus advantageously contributes to a further improvement in resolution and detection efficiency.

[0021] In a further embodiment, the filter grid is attached to the insulator via a first electrode from the plurality of electrodes. Preferably, in this embodiment, the filter grid is electrically conductively connected to the electrode.

[0022] The design enables stable mounting of the filter grid in the new electron detector and therefore facilitates the advantageous field distribution.

[0023] In a further embodiment, the electron detector has an entrance grid which is arranged in the second direction in front of the filter grid, wherein the entrance grid is attached to the insulator via a second electrode from the plurality of electrodes.

[0024] This design contributes advantageously to a homogeneous field distribution and high detection efficiency.

[0025] In another embodiment, the first electrode is longer than the second electrode in the first direction. Alternatively or additionally, the first electrode is thicker than the second electrode in the third direction.

[0026] These configurations offer a particularly efficient way to practically realize the advantageous equipotential line profile and associated field distribution described above. However, the first electrode should not be too long, as this would excessively shorten the insulation distance on the insulator and could lead to discharges during detector operation. Furthermore, the detector's detection efficiency may decrease because incident electrons are reflected back. Therefore, in some embodiments, it is preferred for the first electrode to be a factor longer than the second electrode, with the factor being between 1.25 and 5, preferably between 1.25 and 3.5.

[0027] In a further embodiment, the detector element is attached to the insulator via a third electrode from the multitude of electrodes.

[0028] This design allows for simple and cost-effective mounting of the detector element in the new electron detector. Preferably, the aforementioned entrance grating in an electron microscope is set to the same potential as the previously mentioned electrodes, resulting in largely symmetrical voltage conditions around the filter grating. This advantageously contributes to the desired high degree of homogeneity of the bremsstrahlung field.

[0029] In a further embodiment, the first electrode is longer in the first direction than the third electrode and / or thicker in the third direction.

[0030] This design is a particularly efficient way to practically implement the advantageous distribution of the equipotential lines of the braking field and the associated field distribution described above. In some embodiments, it is preferred that the first electrode be longer than the third electrode by a factor between 1.25 and 5, preferably between 1.25 and 3.5. In some embodiments, the second and third electrodes are largely the same length in the first and second directions, respectively, while the first electrode at the filter grid is longer by a factor greater than 2.

[0031] In a further embodiment, the insulator has a first insulator section which is arranged in the first direction in front of the filter grid, and the insulator has a second insulator section which is arranged in the first direction after the filter grid, wherein the filter grid is attached to the first insulator section via an upper electrode from the plurality of electrodes, and wherein the filter grid is attached to the second insulator section via a lower electrode from the plurality of electrodes.

[0032] In this embodiment, the filter grid is held to the insulator by two electrodes. In some preferred embodiments, the upper and lower electrodes have different thicknesses. In particular, the upper electrode can be thicker than the lower electrode in the third direction, or vice versa. Advantageously, the insulator can be made in one piece. or be designed in two or more parts. These designs allow for simple and therefore cost-effective mounting of the filter grid in the new electron detector. Electrodes of varying thicknesses offer a further degree of freedom for optimization and thus contribute advantageously to minimizing the impact of electrons moving in the second direction on the insulator. In principle, additional electrodes can also be arranged between the first and second electrodes and / or between the first and third electrodes to further optimize the field distribution.

[0033] In another embodiment, the upper electrode is longer than the lower electrode in the first direction.

[0034] This design enables an asymmetrical field distribution in the area in front of and behind the filter grid and very efficiently prevents the insulator from charging in front of and behind the filter grid.

[0035] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0036] Exemplary embodiments of the invention are shown in the drawing and are explained in more detail in the following description. They show: Fig. 1 shows a schematic representation of an embodiment of a scanning electron microscope with the new electron detector. Fig. 2 shows a schematic representation of an embodiment of the new electron detector for the microscope from Fig. 1. Fig. 3 shows the equipotential lines in an electron detector (here only the right half) with a filter grating and an unfavorable dimensioning of the electrodes, Fig. 4 shows the equipotential lines in an electron detector (here only the right half) with a filter grid and an advantageous dimensioning of the electrodes according to a first embodiment. Fig. 5 shows the equipotential lines in an electron detector (here only the right half) with a filter grid and an advantageous dimensioning of the electrodes according to a further embodiment. Fig. 6 Trajectories of backscattered electrons at the electron detector according to Fig. 3, and Fig. 7 Trajectories of backscattered electrons at the electron detector according to Fig. 4.

[0037] Fig. 1 shows the essential structure of an electron microscope, and in this case a scanning electron microscope, with an embodiment of the new electron detector. The electron microscope as a whole is designated here by the reference numeral 10. The electron microscope 10 has, in a manner known per se, a cathode 12 and an anode 14, which are at different electrical potentials during operation. Therefore, an electrical voltage is applied between the cathode 12 and the anode 14, so that electrons emitted from the cathode are accelerated and focused into an electron beam 16. The electron beam 16 illuminates a sample 18, which is arranged here on a sample holder 20, and defines a (first) beam direction, which is indicated here by a symbolic arrow at the end of the electron beam 16.

[0038] The electron microscope 10 has an electron detector, which is designated here in its entirety by the reference numeral 22. The electron detector 22 has a housing 24 in which a detector element 26 and a filter grating 28 are arranged. Furthermore, in this embodiment, the electron detector 22 has an entrance grating 30, which is arranged here at the front end of the housing 24, closer to the sample (viewed in the first beam direction). The housing 24 is here partially cylindrical and The sample is conically shaped and has a tubular section 32 with an internal volume 34 through which the primary electron beam 16 can pass to illuminate the sample 18. In some embodiments, the tubular section 32 can be wholly or partially a component or section of the scanning electron microscope 10. In the simplified representation in Fig. 1, the tubular section 32 is shown as the lower section of a tube that is part of or connected to the anode 14. In other embodiments, the tubular section 32 can be an integral component of the housing 24 of the electron detector 22 and, in particular, separate from the anode 14. As will be explained in more detail below, the electron detector 22 also has an insulator 36, which can be implemented, for example, as a sleeve and / or coating on the outer surface of the tubular section 32 facing away from the internal volume 34.

[0039] In this embodiment, the electron microscope 10 has several electrical coils, in particular lens coils 37, 38. In the case of a scanning electron microscope, deflection coils (not shown here) may also be included, which are configured to deflect the electron beam 16 and move it laterally relative to the sample 18. Alternatively or additionally, in some embodiments, the sample holder 20 can be movable relative to the housing 40 of the electron microscope 10 in order to move the electron beam 16 relative to the sample 18 in a lateral and / or orthogonal direction and / or to allow further degrees of freedom of movement, such as height adjustment, rotation, tilting, etc.

[0040] As is known, the primary electron beam 16 generates interactions with the sample material 18 upon impact. Besides X-rays, which are produced when an electron from the electron beam 16 knocks a nucleus electron of the sample material out of its shell, these interactions primarily consist of backscattered electrons and secondary electrons from the outermost layer of the sample material. In Fig. 1, a dashed arrow 42 indicates that these electrons move in a second direction, which is essentially opposite to the first beam direction.

[0041] Fig. 2 shows the electron detector 22 from Fig. 1 with further details in a simplified representation. The same reference numerals denote the same elements as in Fig. 1. Previously. In preferred embodiments, the detector element 26 includes a scintillator arranged in a disc-like form and largely concentric to the tubular section 32 in the housing 24 of the electron detector 22 (see Fig. 1). On the outer surface of the tubular section 32, facing away from the inner volume 34, a first insulator section 44 and a second insulator section 46 are arranged coaxially in the first beam direction such that the filter grid 28 is held by the first insulator section 44 and the second insulator section 46. In some embodiments, the filter grid 28 can be clamped between the first insulator section 44 and the second insulator section 46. Furthermore, the insulator sections 44 and 46 can be mechanically connected to one another, manufactured integrally from a single piece, or arranged at a distance from each other.The first insulator section 44 has a length in the first beam direction, which is here designated L1. The second insulator section 46 has a length in the first beam direction, which is here designated L2. The lengths L1 and L2 can be the same or different from each other. Alternatively, instead of the lengths of the insulator sections 44 and 46, in some embodiments the length L1 can denote the distance between the detector element 26 and the filter grid 28, and the length L2 can denote the distance between the filter grid 28 and the entrance grid 30.

[0042] As shown in Fig. 2, the filter grid 28 is attached to the insulator 36 with sections 44 and 46 via a first electrode 48. In the illustrated embodiment, the first electrode 48 comprises a lower electrode section 48a located further away from the detector element 26 and an upper electrode section 48b located closer to the detector element 26, between which the filter grid 28 is held. The length of the lower electrode section 48a in the first beam direction is designated L48a. The length of the upper electrode section 48b in the first beam direction is designated L48b. The total length of the first electrode 48 in the first beam direction is the sum of the lengths L48a and L48b. In some preferred embodiments, the length L48a of the lower electrode section (see Fig. 2) is 10%–40% of the length L2.Preferably, the length L48b of the upper electrode section is 10%–40% of the length L1. In some embodiments, the filter grid 28 can be clamped between the electrode sections 48a and 48b. Alternatively or additionally, the filter grid 28 can be... The electrode 48 may be welded, soldered, and / or bonded. In some embodiments, the electrode 48, or the electrode sections 48a, 48b, surround the tubular section 32 (see Fig. 1) in a ring-like manner. Generally, the electrode 48 can be implemented as a sleeve or a sleeve-like coating. In preferred embodiments, the electrode 48 is made of a metallic material, in particular containing gold, copper, platinum, silver, and / or aluminum.

[0043] The entrance grid 30 is attached to the insulator section 46 via a second electrode 50. Similarly, in this embodiment, the detector element 26 is attached to the insulator section 44 via a third electrode 52. In preferred embodiments, the second and third electrodes 50 and 52 are made of the same metallic material as the first electrode 48. The length of the second electrode 50 in the first beam direction is designated L50. The length of the third electrode 52 in the first beam direction is designated L52. In some preferred embodiments, the length L50 is a maximum of 10% of the length L2. In some preferred embodiments, the length L52 is a maximum of 10% of the length L1.

[0044] As shown in Fig. 2, in some applications of the electron detector 22, the filter grid 28 can be at an electrical potential that is negative with respect to the electrical potential at the detector element 26 and / or the electrical potential at the entrance grid 30. Furthermore, in preferred applications of the electron detector 22, the electrical potential at the filter grid 28 is negative with respect to the electrical potential at the housing 40 of the electron microscope. For example, the filter grid 28 can be at a potential of -200 V with respect to the housing 40 of the electron microscope, and the detector element 26 and / or the entrance grid 30 can be at a potential of +8000 V with respect to the housing 40. Preferably, the potential at the filter grid is below the accelerating voltage for the primary beam.Due to the insulator 36 with the insulator sections 44, 46, the filter grid 28 can be held at a potential with a large voltage difference despite its spatial proximity to the aforementioned components of the electron microscope, such as the anode.

[0045] In Fig. 2, reference numeral 54 denotes a backscattered electron moving in the second direction. On its way to the detector element 26, the backscattered electron 54 first passes the entrance grid 30 and then the filter grid 28, provided its kinetic energy is sufficient to overcome the electric braking field 56 in front of the filter grid 28, which is only schematically indicated here.

[0046] Fig. 3 shows the equipotential lines 58 of an electric braking field 56 in an electron detector, showing only a section through one half of the detector. Accordingly, the first electrode 48 is shown on the filter grid 28, the second electrode 50 on the entrance grid 30, and the third electrode 52 on the detector element 26 in the left part of Fig. 3. In the illustrated case, the electrodes 48, 50, and 52 are approximately the same length in the first direction. The filter grid 28 extends in a third direction 60, which, in preferred embodiments, is orthogonal to the first direction of the primary electron beam 16. It has been found that with such an electrode configuration, a largely homogeneous electric braking field can be generated. However, some electrons moving in the second direction near the insulator may reach the insulator 36 or...The electrons strike the insulator 36 and thereby negatively charge it. Such an electrical charge can affect the primary electron beam 16 and lead to further undesirable effects in the electron microscope, such as image distortion and loss of resolution.

[0047] Figure 6 shows exemplary trajectories 62 of backscattered electrons in an electron detector with an electric field distribution according to Figure 3. Reference numeral 64 indicates the trajectory of a backscattered electron that strikes the insulator 36 and negatively charges it.

[0048] Fig. 4 shows the equipotential lines 66 of an electric braking field 56 in an electron detector similar to Fig. 3. However, the electric braking field – which remains predominantly homogeneous – is deliberately distorted in a defined region 68, so that the equipotential lines 66 in the defined region 68 are predominantly downward sloping and preferably monotonically downward sloping when viewed from the first electrode 48 in the third direction 60. As a result of this slope, The electric braking field in the defined region 68 is intentionally made inhomogeneous compared to the majority of the electric braking field, as shown by the equipotential lines 66. Region 68 extends in the third direction over a length L3, which is preferably shorter than the length L1.

[0049] The electric field lines (not shown here) are known to run perpendicular to the equipotential lines 66 and therefore, in the defined region 68, have a direction that leads away from the insulator 36 when considering the electric field lines in the direction of motion of a backscattered electron. On its way to the detector element 26, the backscattered electron essentially follows the course of the electric field lines and is therefore deflected away from the insulator 36, as can be seen in Fig. 7 from the trajectories 70.

[0050] The electron detector according to Fig. 4 has a first electrode 48 on the filter grid 28, which is intentionally longer than the second electrode 50 on the entrance grid 30 and also longer than the third electrode 52 on the detector element 26 (viewed in the first and second directions, respectively). In some embodiments, the second electrode 50 on the entrance grid 30 and the third electrode 52 on the detector element 26 are approximately the same length, while the first electrode 48 on the filter grid 28 is more than twice as long in order to distort the electric braking field as shown in Fig. 4. In some advantageous embodiments, the first electrode 48 is longer in the first direction by a factor of between 1.25 and 5 than the second and / or third electrodes 50, 52. In some embodiments, the first electrode 48, viewed in the first direction, is at least as long as the combined length of the second and third electrodes 50, 52.

[0051] As can be further seen in Fig. 4, the equipotential lines 66 in the defined region 68, as well as further equipotential lines in a further region 70, which lies in the second direction in front of the filter grid 28, run in a mirror-symmetrical manner to the filter grid 28. The electric field of this configuration appears here in the region of the tubular section 52 or in the region of the first electrode 48 to be funnel-shaped in comparison to the predominantly homogeneous course of the equipotential lines 66. outside the defined area 68. The central axis of the funnel essentially coincides with the filter grid 28.

[0052] Fig. 5 shows the equipotential lines 72 of another embodiment of the electron detector 22. In this case, the first electrode 48 in the region of the filter grid 28 is asymmetrical with respect to the central axis of the filter grid 28 in the third direction 60. In other words, the first electrode 48 in this embodiment includes an upper electrode section 48b, which lies downstream of the filter grid 28 in the second direction, and a lower electrode section 48a, which lies upstream of the filter grid 28 in the second direction. The upper electrode section 48b is longer than the lower electrode section 48a in the second direction. As a result, the equipotential lines 72 are not mirror-symmetrical with respect to the filter grid 28.Rather, the magnitude of the slope of the equipotential lines in the defined region 68 above the filter grid 28 is greater than the magnitude of the slope of the equipotential lines in the further region 70 below the filter grid 28. In other words, the equipotential lines in the defined region 68 decrease in magnitude more sharply than the equipotential lines in the further region 70 increase when considering the course in the third direction 60.

[0053] The electrodes 48, 50, 52 can alternatively or additionally be designed with different material thicknesses and / or with varying material thicknesses in the third direction in order to selectively distort the predominantly homogeneous electric braking field, especially between the filter grid 28 and the detector element 26 in the vicinity of the insulator 36, and thus to avoid or at least reduce the impact of backscattered electrons and / or secondary electrons on the insulator 36.

Claims

Patent claims 1. Electron detector for an electron microscope (10) generating a primary electron beam (16) to illuminate a sample (18), in particular a scattering or backscattering electron detector, comprising a tubular section (32) with an internal volume (34) through which the primary electron beam (16) can pass to illuminate the sample (18), wherein the primary electron beam (16) has a beam direction defining a first direction, comprising a detector element (26) arranged outside the tubular section (32) and configured to detect electrons moving from the sample (18) in a second direction (42), comprising a filter grating (28) arranged in the second direction (42) in front of the detector element (26) and extending in a third direction (60) transverse to the first direction, comprising an insulator (36),which electrically insulates the filter grid (28) and the tubular section (32) from each other, so that the filter grid (28) can be at a negative electrical potential relative to the tubular section (32), and with a plurality of electrodes (48, 50, 52) made of an electrically conductive material, which are arranged outside of the tubular section (32), wherein the electrodes (48, 50, 52) are configured to produce a predominantly homogeneous electric field (56) between the filter grid (28) and the detector element (26), and wherein the electric field (56) has equipotential lines (66) which extend predominantly in the third direction (60), characterized in that the electrodes (48, 50, 52) are further configured to selectively distort the electric field (56) in the region of the insulator (36) in such a way as tothat the equipotential lines (66) in a defined area (68) between the filter grid (28) and the detector element (26) have a predominantly downward slope, wherein the defined area (68) extends from the filter grid (28) in the second direction (42) and from the tubular section (32) in the third direction (60).

2. Electron detector according to claim 1, characterized in that the filter grating (28) has a defined grating length in the third direction (60), and that the defined area (68) extends in the third direction (60) over a length that is at most 30% of the defined grid length.

3. Electron detector according to claim 1 or 2, characterized in that the electrodes (48, 50, 52) are arranged to further distort the electric field (56) in such a way that the equipotential lines (66) in the defined region (68) and further equipotential lines in a further region (70) which lies in the second direction in front of the filter grid (28) run in a mirror-symmetric manner to the filter grid (28).

4. Electron detector according to claim 1 or 2, characterized in that the electrodes (48, 50, 52) are arranged to further distort the electric field (56) such that further equipotential lines in a further region (70), which lies in the second direction in front of the filter grid (28), starting from the tubular section (32), have a predominantly rising course along the third direction (60), wherein the rising course has a slope that is smaller in magnitude than the predominantly falling course.

5. Electron detector according to one of claims 1 to 4, characterized in that the filter grid (28) is attached to the insulator (36) via a first electrode (48) from the plurality of electrodes.

6. Electron detector according to one of claims 1 to 5, characterized by an entrance grid (30) arranged in the second direction in front of the filter grid (28), wherein the entrance grid (30) is attached to the insulator (36) via a second electrode (50) from the plurality of electrodes.

7. Electron detector according to claim 6, characterized in that the first electrode (48) is longer in the first direction than the second electrode (50).

8. Electron detector according to claim 6 or 7 characterized in that the first electrode (48) is thicker in the third direction than the second electrode (50).

9. Electron detector according to one of claims 1 to 7, characterized in that the detector element (26) is attached to the insulator (36) via a third electrode (52) from the plurality of electrodes.

10. Electron detector according to claim 9, characterized in that the first electrode (48) is longer in the first direction than the third electrode (52) and / or is thicker in the third direction than the third electrode (52).

11. Electron detector according to one of claims 1 to 10, characterized in that the insulator (36) has a first insulator section (44) which is arranged in the first direction upstream of the filter grid (28), and that the insulator (36) has a second insulator section (46) which is arranged in the first direction downstream of the filter grid (28), wherein the filter grid (28) is attached to the first insulator section (44) via a first electrode section (48b) from the plurality of electrodes, and wherein the filter grid (28) is attached to the second insulator section (46) via a second electrode section (48a) from the plurality of electrodes.

12. Electron detector according to claim 11, characterized in that the first electrode section (48b) is longer in the first direction than the second electrode section (48a).

13. Electron detector for an electron microscope (10) generating a primary electron beam (16) to illuminate a sample (18), in particular a scattering or backscattering electron detector, comprising a tubular section (32) with an internal volume (34) through which the primary electron beam (16) can pass to illuminate the sample (18), wherein the primary electron beam (16) has a beam direction defining a first direction, comprising a detector element (26) arranged outside the tubular section (32) and configured to detect electrons moving from the sample (18) in a second direction (42), comprising a filter grating (28) arranged in the second direction (42) in front of the detector element (26). extending in a third direction (60) that is transverse to the first direction, with an insulator (36) that electrically insulates the filter grid (28) and the tubular section (32) from each other, so that the filter grid (28) can be at a negative electrical potential relative to the tubular section (32), and with a plurality of electrodes (48, 50, 52) made of an electrically conductive material that are arranged outside of the tubular section (32), wherein the electrodes (48, 50, 52) are configured to produce a predominantly homogeneous electric field (56) between the filter grid (28) and the detector element (26), in particular according to one of claims 1 to 12, characterized in that the filter grid (28) is arranged at a defined distance (L1) from the detector element (26), wherein the filter grid (28) is attached to the insulator (36) via a first electrode (48) from the plurality of electrodes,wherein the first electrode (48) has a first electrode section (48b) which lies in the first direction between the detector element (26) and the filter grid (28) and has a defined length (L48b) in the first direction, and wherein the defined length (L48b) is 10%-40% of the defined distance (L1).

14. Electron detector according to claim 13, characterized in that the detector element (26) is attached to the insulator (36) via a further electrode (52) from the plurality of electrodes, wherein the further electrode (52) has a further defined length (L52) in the first direction, and wherein the further defined length (L52) is a maximum of 10% of the defined distance (L1).

15. Electron microscope with an electron detector according to one of claims 1 to 14.

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