Energy spectrometer with dynamic focus

Abstract

To provide an energy spectrometer with dynamic focus for a transmission electron microscope (TEM).SOLUTION: An energy spectrometer 204 and a TEM 202 at least include a charged particle column including a projection system 208 arranged after a sample plane, where the projection system is operated in a first configuration. The energy spectrometer 204 includes a dispersive element 212, a bias tube 214, optics for magnifying the energy loss spectrum and for correcting aberrations, and a detector 222 arranged conjugately to a spectrum plane of the energy spectrometer 204. The energy spectrometer further includes an optical element 234 electrically biased to refocus at least a portion of a spectrum on the detector 222. The value of the electrical bias is at least partially based on the first configuration of the charged particle column.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates generally to charged particle microscopes, and more particularly to energy spectrometers included in charged particle microscopes that provide dynamic refocusing of the electron beam based on changes in the operating parameters of the microscope. [Background technology]

[0002] Charged particle microscopy, such as transmission electron microscopy, uses a range of detection techniques to obtain various information about a sample. One such technique is called electron energy loss spectroscopy (EELS), in which an electron beam emerging from a sample is dispersed into an energy spectrum, and one or more bands of energy are individually imaged. However, in current iterations of EELS, there is a loss of energy resolution due to various issues, such as microscope misalignment, chromatic aberration, and unintended lensing effects. All of these issues combine to result in a loss of energy resolution in the EELS detector. This loss of resolution may be due, at least in part, to the desired portion of the electron beam not being focused on the spectral plane of the EELS system due to these issues. For example, an electron beam transmitted through a sample may have a different focal point exiting the objective lens due to the chromatic aberration of the objective lens. This change in focus along the optical axis translates into a change in the position of the intersection plane in the electron energy loss spectrometer (or simply "energy spectrometer") relative to the spectral plane. This change in position relative to the spectral plane can lead to defocusing of the electron beam at the EELS detector, thereby affecting the energy resolution of the EELS detector. These spectral defocusing effects can be (partially) compensated for by appropriate (re)focusing of the energy spectrometer. However, changes in the microscope and energy spectrometer operating parameters, such as changing the magnification or selecting a different energy band of the electron beam, can change this defocusing at the detector, thereby changing the resolution loss at the EELS detector and causing previously obtained spectrometer focus settings to no longer be optimal. Therefore, the extent of the energy resolution loss depends on the operating parameters, and solutions may be required for each permutation of the operating parameters. Summary of the Invention

[0003] An energy spectrometer with dynamic focus for a transmission electron microscope (TEM) is disclosed herein. The exemplary energy spectrometer and TEM include at least a charged particle column including a projection system disposed after a sample lane, the projection system operating in a first configuration, and an energy spectrometer coupled to the charged particle column to acquire one or more energy loss spectra, the energy spectrometer including a dispersive element, a bias tube, optics for expanding the energy loss spectrum and correcting aberrations, and a detector disposed conjugate with a spectral plane of the energy spectrometer, the energy spectrometer further including an optical element that is electrically biased to refocus at least a portion of the energy loss spectrum onto the detector, and a value of the electrical bias is based at least in part on the first configuration of the charged particle column.

[0004] Another example includes at least a light source coupled to provide an electron beam toward a sample, a projection system coupled to collect the electron beam after passing through the sample and direct the electron beam along an optical axis, the projection system configured to operate at first operating parameters, and an energy spectrometer coupled to receive the electron beam from the projection system, the energy spectrometer including a dispersive element with a bias tube, a dispersive element coupled to direct the electron beam into multiple energy bands, a detector positioned conjugate to a spectral plane of the energy spectrometer, multiple lenses or multipoles downstream of the dispersive element and positioned to direct the energy bands to the detector, and an electrically biased optical element positioned to refocus the energy bands onto the detector, where the level of the electrical bias is based on the first operating parameters. [Brief explanation of the drawings]

[0005] [Figure 1] 1 is an exemplary illustration of some of the microscope systems currently known in the art. [Figure 2] 1 is a charged particle microscope according to one embodiment of the present disclosure. [Figure 3] 1 is an illustrative energy spectrometer including a refocusing optic, according to one embodiment of the present disclosure. [Figure 4A] 1A and 1B are end and side views, respectively, of an optical element according to one embodiment of the present disclosure. [Figure 4B] 1A and 1B are end and side views, respectively, of an optical element according to one embodiment of the present disclosure. [Figure 5] 1 is a quadrupole according to one embodiment of the present disclosure. [Figure 6] 1 includes two detector readings obtained by an energy spectrometer according to one embodiment of the present disclosure.

[0006] Like reference numerals refer to corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION OF THE INVENTION

[0007] Embodiments of the present invention are described below in the context of a charged particle microscope including an energy spectrometer for acquiring an energy spectrum of a sample, where the energy spectrometer includes additional optical elements that reduce or eliminate defocusing in a dynamic manner. For example, depending on the operating parameters of the microscope and / or the energy spectrometer, the optical elements may be electrically biased to refocus the electron beam. However, it should be understood that the methods described herein are generally applicable to a wide range of different tomography methods and apparatus, including both scanning probe systems and parallel illumination systems, and are not limited to any particular apparatus type, beam type, object type, length scale, or scanning trajectory.

[0008] As used in this application and the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "comprises" means "comprises." Furthermore, the term "coupled" does not exclude the presence of intermediate elements between the coupled items.

[0009] The systems, devices, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and subcombinations with one another. The disclosed systems, methods, and devices are not limited to any particular aspect or feature or combination thereof, nor do the disclosed systems, methods, and devices require that any one or more particular advantages exist or problems be solved. While any theory of operation is for ease of explanation, the disclosed systems, methods, and devices are not limited to such theory of operation.

[0010] Although some operations of the disclosed methods are described in a particular order for convenience of presentation, it should be understood that this description style encompasses reordering unless a specific order is required by specific terminology described below. For example, operations described sequentially may, in some cases, be reordered or performed simultaneously. Moreover, for simplicity, the accompanying figures may not show the various ways in which the disclosed systems, methods, and apparatuses can be used with other systems, methods, and apparatuses. Additionally, this specification sometimes uses terms such as "generate" and "provide" to describe the disclosed methodologies. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and will be readily discernible to those skilled in the art.

[0011] In some instances, values, procedures, or devices are referred to as "lowest," "best," "smallest," etc. Such descriptions are intended to indicate that a selection from among many functional options may be made, and it will be understood that no such selection is necessarily better, lesser, or otherwise desirable than other options.

[0012] Electron energy loss spectroscopy (EELS) is a detection technique typically incorporated into transmission electron microscopes (TEMs) to analyze the composition and other chemical properties of samples. EELS is traditionally performed using an energy spectrometer attached to the end of the microscope's column. Such energy spectrometers are sometimes called post-column filters (PCFs). Energy spectrometers are used to obtain energy spectra of transmission data. For example, a sample in a TEM is interrogated with an electron beam of a desired energy, and a portion of the electrons exiting the sample are reduced in energy characteristic of various properties of the sample. These properties may be chemical or crystalline, for example. The energy spectrometer can then be used to view the energy distribution of the emerging electron beam and search for peaks at various energy levels that provide data about the sample material. Given current TEM and detector technology, peaks can be highly resolved, for example, down to the millielectronvolt level. However, microscope imperfections can blur the data such that the desired resolution is reduced or lost. The loss of resolution can be due to defocusing of the electron beam as it propagates through parts of the microscope and energy spectrometer. This loss of focus may not be static and may vary depending on the microscope settings. As higher resolution energy spectra are desired, defocusing becomes more of a problem. Therefore, a solution to the problem is highly desirable.

[0013] 1 is an exemplary diagram of a portion of a microscope system 100 currently known in the art. The microscope system 100 includes a TEM 102 and a portion of an energy spectrometer 104. The microscope system 100 can be used to acquire an energy spectrum of a sample 106, but may suffer from defocusing of the electron beam 124 on the detector 122 due to operating parameters of the microscope 102 and the energy filter 104.

[0014] To illustrate, as the electron beam 124 emerges from the sample 106, it follows a trajectory along the optical axis of the microscope 102. This trajectory depends on the energy of the electrons in the beam and, in turn, on the operating parameters of the microscope 102. The operating parameters include, for example, the operating parameters of the projection system 108, such as adjusting the microscope's magnification setting relative to the magnification from the sample 106 to the intersection at the end of the microscope 102. This magnification determines how much the cone of the electron beam 124 exiting the sample is compressed toward the energy spectrometer, so this freedom to change magnification is desirable for users of the microscope system. More compression results in a larger portion of the cone of the electron beam 124 being accepted by the entrance aperture of the spectrometer. The operating parameters determine the excitation settings of optical elements, such as lenses, multipole, and quadrupole elements, of the projection system 108. Note that the operating parameters vary depending on the energy of the primary electron beam, e.g., the energy of the electron beam used to interrogate the sample. However, due to the sample-electron interaction, the electron beam emerging from the sample contains some energy loss. However, measuring the loss over a range of energies is desirable because that is what is measured by the energy spectrometer 104. However, the energy loss also affects the intersection position of the electron beam at the microscope 102 and the energy spectrometer 104. This change in intersection results in a defocusing of the electron beam spectrum on the detector 122.

[0015] For example, an electron beam propagating through the microscope 102 without energy loss may have an intersection location, e.g., a focal point, as shown. However, an electron beam with some energy loss will have a different intersection location, a ΔZ offset, as shown, propagating through the microscope 102 and the energy spectrometer 104.

[0016] The energy spectrometer 104 includes a lens 110, a dispersive element 112 that includes a bias tube 114, lenses 116 and 118, multiple optical elements (lenses, quadrupoles, and multipoles) 120, and a detector 122. The dispersive element 112 is energized to disperse, or "fan out," the beam at different energy bands. The multiple optical elements 120 focus and expand this band onto the detector 122. Typically, this energy band is smaller than (does not encompass) the entire range of the EELS spectrum that an operator is interested in and wants to record. Therefore, a bias tube 114 is provided that can be electrically biased to add various amounts of energy to the electron beam 124, for example, up to 2 keV. This can be used to shift the band of energies recorded by detector 122 (such a shift of energy band could, of course, also be achieved by adjusting the excitation of the dispersive element or by adjusting the operating potential of the electron microscope, but such methods have the disadvantage of suffering from relatively long settling times and / or magnetic hysteresis). Due to its electrostatic nature, the liner tube can establish a shift from one energy band to another in a fraction of a millisecond, thus allowing near-simultaneous recording of multiple energy bands (e.g., as described in US10832901B2, incorporated herein for all purposes). Detector 122 can be positioned at the conjugate of the spectral plane of the spectrometer so that the electron beam is focused onto detector 122.

[0017] As with the microscope 102, the various components 110-120 may be configured with operating parameters based on the primary electron beam energy rather than the energy loss beam. It should also be noted that the ends 114A and 114B of the liner tube 114 may form unintended lenses due to differences in electrical bias between the bias tube 114 and a grounded element coupled to the bias tube 114, such as the liner tube encompassing the optical axis. Because the energy spectrometer 104 is configured to operate based on the energy of the primary electron beam, any defocusing of the energy loss beam occurring in the microscope 102 propagates to the energy spectrometer 104, resulting in a proportional defocus amount ΔZ. A change in focus moves the crossover point from the spectral plane to a position before or after the spectral plane. Such a change in the crossover point results in a defocused energy spectrum at the detector 122.

[0018] One solution to the above problem involves adding additional optical elements to the path of the electron beam to refocus it onto the spectral plane and ultimately onto the detector. The additional optical elements can be inserted anywhere from after the sample to just before the detector of the energy spectrometer. The optical elements can be electrically biased and energized, for example, depending on the operating parameters of the microscope and / or energy spectrometer, and changes in the operating parameters result in changes to the electrical bias levels of the optical elements. In this way, as EELS experiments are performed in different spectral bands centered around different energy losses, the entire system can be dynamically focused to account for different operating parameters while maintaining the desired focus.

[0019] The optical element can be any form of charged particle optics, such as a lens or a multipole element. In some examples, the optical element can take the form of two opposing plates, similar to an electrostatic shutter. The two opposing plates can be contained within a circular envelope, with the opposing plates biased as needed and the envelope grounded. An example of a multipole device is a quadrupole, in which two opposing members are biased while the other member is grounded. However, as one skilled in the art will appreciate, multipole devices offer greater flexibility for beam manipulation, and thus can not only refocus the electron beam, but can also be biased to partially adjust the astigmatism (i.e., apply a small amount of cylindrical lensing) if necessary, for example, to compensate for small roundness of the unintended lenses 114A and 114B. Other higher-order multipoles are also contemplated herein, providing greater flexibility for beam manipulation.

[0020] 2 is an example of a charged particle microscope 200 according to an embodiment of the present disclosure. The CPM 200 may be a TEM 202 in some examples and includes an energy spectrometer 204. The energy spectrometer may be used, for example, to acquire an EEL spectrum. The CPM 200 may include optical elements to reduce or counteract defocusing problems that occur during acquisition of the spectrum. In some examples, the optical elements may be biased to a level based on operating parameters of the microscope and / or the energy spectrometer, such that refocusing is dynamic.

[0021] The TEM 202 includes a light source 228, an illumination system 230, a projection system 208, and various detectors 232, all of which may be controlled by a controller 226. The light source 228 may be an electron source, such as a Schottky source or a (cold) field emission gun (CFEG), that provides an electron beam that propagates along the optical axis of the TEM 202 and interacts with the sample 206. The illumination system 230 includes multiple electron optical components for conditioning the electron beam for delivery to the sample 206. Conditioning the electron beam may include collimation, astigmatism adjustment, and focusing the electron beam onto the sample plane. As mentioned above, the projection system 208 may include various electrostatic / magnetic lenses, deflectors, correctors (such as astigmatism adjusters), etc., used to focus the electron beam emerging from the sample 206 onto one of the various detectors 232. In some examples, the projection system 208 focuses and conditions the electron beam for delivery to the energy spectrometer 204. The various detectors 232 can be individually moved in and out of the optical path to provide different detection schemes for the TEM 202. The detectors 232 can include an imaging screen, a TEM camera, and a STEM camera.

[0022] The energy spectrometer 204 includes a dispersive element 212 (with bias tube 214), an optical element 234, a plurality of optical systems 220, and a detector 222. Many of the components of the energy spectrometer 204 have been described with respect to FIG. 1 and will not be revisited for brevity. Note that other components of the energy spectrometer 104 not specifically shown in the energy spectrometer 204 are included but are excluded from the drawing. An additional component, namely, the optical element 234, is included in the energy spectrometer 204 to refocus the electron beam as described. While FIG. 2 shows the optical element 234 downstream of the dispersive element 212, this is not the only location for placing the optical element. See FIG. 3 for alternative locations. In general, the optical element 234 can be located upstream of the dispersive element 212 (but downstream of the sample 206), or can be located downstream of the dispersive element 212, such as before the plurality of optical elements 220, within the optical system 220, or after the optical system 220.

[0023] The controller 226 may include one or more processing cores and memory that stores executable code. The controller 226 may also provide operating voltages to some of the components of the system 200, or may be coupled to a voltage supply (not shown) that provides operating voltages in response to control signals provided by the controller 226. For example, the controller 226 may provide control and / or voltages to the illumination system 230, the projection system 208, and the optical elements 234. Additionally, the controller 226 may control the operation of and / or receive data from the detector 222. Generally, the controller 226 sets operating parameters of the system 200 and adjusts the electrical bias of the optical elements 234 to dynamically focus the electron beam onto the detector 222 in response to changes in operating conditions of the system 200, such as magnification.

[0024] The optical element 234 may be formed from a multipole element including two or more conductive elements. In some examples, the optical element 234 may be formed from two opposing conductive elements, similar to a deflector or shutter-type arrangement housed in an electrical conductor. In other examples, the optical element 234 is formed from a quadrupole or higher-order multipole element. Regardless of the physical example, the optical element may be electrically biased to a level based on the operating parameters of the TEM 202 and / or energy filter 204. For example, the desired minimum or maximum collection angle of the electron cone exiting the sample determines the desired magnification from the sample to the intersection, and therefore the excitation energy of the projection optics 208 in the TEM 202. The desired bandwidth of the EELS spectrum (which depends on the chemical composition of the sample and the resulting characteristics of the EELS spectrum the operator wishes to record) determines the electrical bias applied to the bias tube 214 included in the dispersive element 212, and therefore the unintended lenses (not shown) at the entrance and exit positions of the bias tube 214, similar to the unintended lenses 114A and 114B shown in FIG. 1. Such operating parameters may determine the bias level provided to the optical element 234, which may be provided or controlled by the controller 226. For example, the higher the bias to the bias tube 214, the higher the bias level provided to the optical element 234. In a shutter-type optical element configuration, the same electrical bias may be provided to the two opposing conductive elements, which may be between +10 volts and +10 volts. Again, the level depends on the operating parameters of the CPM 200.

[0025] During operation, an electron beam generated by the source 228 at a primary energy is projected toward the sample 206, where it interacts with the sample. The interaction may result in some electrons losing energy by an amount related to the various material properties of the sample 206. The electron beam emerging from the sample 206 may then contain electrons of different energies across a range of energies. The emerging electron beam may then propagate on a different trajectory than the primary energy electrons, which affects the focus, e.g., intersection, of the electron beam along the remaining optical path, including the energy filter 204 and the TEM 202. As the emerging electron beam passes through the dispersive element 212, it is guided (sequentially) by the dispersive element 212 and bias tube 214 into different energy bands and recorded by the detector 222. However, due to changes in the focus of the different energies, one or more bands may not be focused in the spectral plane to which the detector is located conjugate. This defocusing problem results in reduced resolution. To refocus the electron beam, the optical element 234 is biased based on the current operating parameters. Biasing the optical element 234 adjusts the locus of the bands to align the intersection of the bands with the spectral plane and therefore with the detector 222 .

[0026] 3 is an illustrative energy spectrometer 304 including refocusing optics, according to one embodiment of the present disclosure. The energy spectrometer 304 may be included as an analytical add-on to a TEM, such as in system 200, and used to obtain an energy spectrum of a sample. Thus, the energy spectrometer 304 may receive an electron beam from the TEM 302 and direct the electron beam (sequentially) into multiple bands based on the energy of each of the bands. These bands may then be directed to a detector.

[0027] The energy spectrometer 304 includes many of the elements disclosed with respect to FIG. 1 and will not be described again for brevity. However, the energy spectrometer 304 further includes an additional optical element 334 for refocusing the electron beam 324 and / or various energy bands onto the detector 322. The optical element 334 may be a multipole element including two or more conductive elements for manipulating the trajectory of the electron beam 324. As shown here, the optical element 334 also includes electrical contacts so that the optical element 334 can be electrically biased. In the example of two conductive elements in the optical element 334, both conductive elements may be biased with the same voltage, which may range, for example, from −10 V to +10 V. For optical elements with more conductive elements, various biasing schemes, as known in the art, may be used to refocus the electron beam 324 and, in some cases, adjust for astigmatism.

[0028] Although the optical element 334 is shown in position A in FIG. 3 , other positions are possible. For example, the optical element 334 could instead be located in positions B, C, D, E, and F. While it is possible to locate the optical element 334 in any of positions A through F, position A may be the simplest in terms of design and manufacturing. However, locations other than A may affect electrical biasing requirements. Regardless of the position, electrically biasing the optical element 334 based on the operating parameters of the TEM 302 and / or energy spectrometer 304 reduces or eliminates defocusing of the electron beam 324, e.g., refocuses the electron beam 324, so that the electron beam 324 is focused on the spectral plane and ultimately on the detector 322.

[0029] 4A and 4B are end and side views, respectively, of an optical element 434 according to an embodiment of the present disclosure. The optical element 434 includes two opposing conductive elements 436 and 438 disposed within a housing 440. Electrical contacts 442 and 444 may be included to provide electrically biasing the conductive elements 436, 438. The housing 440 may also be electrically contacted to provide a ground or bias voltage, as needed. The two opposing conductive elements may be disposed on opposite sides of the optical axis 450. FIG. 4 shows two opposing elements disposed above and below the optical axis. In another embodiment, these two elements are disposed to the left and right of the optical axis 450.

[0030] Optical element 434 may be included in an energy spectrometer, such as energy spectrometer 304, and may be positioned in the optical path and at any of positions A through F. In some examples, optical element 434 may be positioned inside a liner tube of the energy spectrometer. In other examples, optical element 434 may form a length of liner tube.

[0031] 5 illustrates a quadrupole 534 according to an embodiment of the present disclosure. The quadrupole 534 is an example of a multipole of the optical element 334 and includes four conductive elements 446. As is known in the art, each element 446 can be individually electrically biased to refocus an electron beam traveling along its optical axis. The quadrupole 534 can be included in an energy spectrometer, such as the energy spectrometer 304, at any of positions A through F.

[0032] FIG. 6 includes two detector readings 601 and 603 obtained by an energy spectrometer in accordance with an embodiment of the present disclosure. Detector reading 601 illustrates detection of an electron beam or sub-band without using a refocusing element, as described herein. However, detector reading 603 illustrates detection of an electron beam or sub-band with additional optics biased according to the operating parameters of the TEM and / or energy spectrometer. In this example, the optics are biased to 7 V. A clear improvement in focus is shown in FIG. 6B.

[0033] The embodiments discussed herein to illustrate the disclosed technology should not be considered limiting and merely provide examples of implementations. For example, the optical elements disclosed herein can be formed from any number of conductive elements so that the electron beam is not only refocused but also the astigmatism is adjusted. Those skilled in the art will recognize myriad other ways in which the disclosed technology can be implemented that are contemplated herein and within the scope of this disclosure.

Claims

1. An apparatus comprising: A charged particle column including a projection system disposed behind a sample plane, wherein the projection system operates in a first configuration; An energy spectrometer coupled to the charged particle column for obtaining one or more energy loss spectra, the energy spectrometer comprising: A dispersive element, a bias tube, an optical system for magnifying an energy loss spectrum and correcting aberrations, and a detector disposed conjugate to a spectral plane of the energy spectrometer, the energy spectrometer further comprising an optical element electrically biased to refocus at least a portion of the energy loss spectrum onto the detector, the value of the electrical bias being at least partially based on the first configuration of the charged particle column; an energy spectrometer.

2. The apparatus according to claim 1, wherein the optical element is formed from an outer housing having two opposing inner plates, the two opposing plates being electrically biased and the outer housing being electrically grounded.

3. The apparatus according to claim 2, wherein the two opposing plates are equally electrically biased.

4. The apparatus according to claim 1, wherein the optical element is formed from a multipole optical element.

5. The apparatus according to claim 1, wherein the first configuration includes a magnification setting of the charged particle column, the magnification setting determining an operating parameter of the projection system.

6. The apparatus according to claim 1, wherein the energy spectrometer is configured to operate in a second configuration, the second configuration setting operating parameters of the dispersive element, the bias tube, and the optical system.

7. The apparatus according to claim 6, wherein the value of the electrical bias is further based on the second configuration.

8. The device according to claim 1, wherein the optical element is electrically biased with a voltage in the range of -10 volts to 10 volts.

9. The device according to claim 1, wherein the optical system includes a plurality of lenses, quadrupoles, and / or multipoles for adjusting an electron beam for detection.

10. The device according to claim 1, wherein the optical element is disposed downstream of the bias tube.

11. The device according to claim 1, wherein the optical element is disposed upstream of the bias tube.

12. An electron microscope, a light source coupled to provide an electron beam toward a sample, a projection system coupled to collect the electron beam after passing through the sample and direct the electron beam along an optical axis, wherein the projection system is configured to operate with a first operating parameter, the projection system; an energy spectrometer coupled to receive the electron beam from the projection system, wherein the energy spectrometer, a dispersion element having a bias tube, the dispersion element being coupled to direct the electron beam continuously into a plurality of energy bands, the dispersion element, a detector disposed conjugate to a spectral plane of the energy spectrometer, a plurality of lenses or multipoles disposed downstream of the dispersion element and configured to direct the energy bands toward the detector, and an optical element disposed to refocus the energy bands onto the detector and electrically biased, wherein a level of the electrical bias is based on the first operating parameter, the energy spectrometer including the optical element.

13. The electron microscope according to claim 12, wherein the first operating parameter includes at least a magnification setting.

14. The energy spectrometer is configured to operate based on a second operating parameter, and the level of the electrical bias is further adjusted in response to the second operating parameter. The electron microscope according to claim 12.

15. The optical element is a multipole optical element. The electron microscope according to claim 12.

16. The optical element is formed from an outer casing having two opposing inner plates, the two opposing plates are electrically biased, and the outer casing is electrically grounded. The electron microscope according to claim 12.

17. The two opposing plates are equally electrically biased. The electron microscope according to claim 16.

18. The optical element is electrically biased at a voltage in the range of -10 volts to 10 volts. The electron microscope according to claim 12.

19. The optical element is arranged downstream of the dispersive element. The electron microscope according to claim 12.

20. The optical element is arranged upstream of the dispersive element. The electron microscope according to claim 12.

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