Cathodoluminescence electron microscope

Abstract

A scanning electron microscope with an electron column positioned to guide an electron beam onto a sample, the electron column comprising: a vacuum housing; an electron source; and an electromagnetic objective positioned within the vacuum housing, the electromagnetic objective comprising a housing having an inlet aperture on its top surface and an outlet aperture on its bottom surface; an electromagnetic coil radially positioned within the housing; an optical objective positioned within the housing and comprising a concave mirror having a first axial aperture and a convex mirror having a second axial aperture; and an electron beam deflector positioned within the housing and comprising a first set of deflectors and a second set of deflectors positioned below the first set of deflectors, wherein the second set of deflectors is positioned below the first axial aperture and the first set of deflectors is positioned above the second set of deflectors.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to U.S. Provisional Application No. 62 / 845,722, filed May 9, 2019, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to scanning electron microscopes including electromagnetic lenses. In particular, but not exclusively, this invention relates to scanning cathode ray luminescence microscopes. The invention also relates to measuring devices and methods of operating the aforementioned microscopes. Background Technology

[0004] Unlike optical microscopes, electron microscopes use accelerated electrons as a source of illumination. Because electrons have wavelengths that are up to 1 / 100,000th the wavelength of visible light photons, electron microscopes have a higher resolution than optical microscopes and can reveal the structure of smaller objects.

[0005] Scanning electron microscopy (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused electron beam. Electrons interact with atoms in the sample, generating various detectable signals that contain information about the sample's surface morphology, structure, and composition. The electron beam is typically scanned in a raster scanning mode, but other scanning techniques can also be used, and the position of the electron beam is combined with the detected signals to produce an image. SEM can achieve resolutions better than 1 nanometer. The types of signals produced by SEM include secondary electrons (SE), backscattered electrons (BSE), characteristic X-rays, light (cathodic emission, CL), sample current, and transmitted electrons (TEM).

[0006] The light emitted by a sample when bombarded by electrons is called cathodoluminescence. Cathodoluminescence measurements can be performed in a scanning electron microscope by scanning the sample surface with a highly focused electron beam probe and recording the cathodoluminescence signal intensity as a function of the electron beam's position on the sample. This generates cathodoluminescence maps that provide higher resolution spectral information than wide-field photoluminescent images obtained through optical microscopy.

[0007] Typically, electron microscopes are aligned in secondary electron mode. In this mode, the intensity of electrons (secondary electrons) extracted when the sample is bombarded is recorded as a function of the electron probe position. The contrast of the secondary electron image is primarily related to the surface morphology of the sample. Secondary electron images can be generated in real time and can be used to minimize the probe size of the electron microscope. It is preferable to measure the cathodoluminescence image after the probe size has been minimized.

[0008] Today, most commercial cathodoluminescence measurement solutions are third-party accessories for existing electron microscopes. They consist of at least a light-collecting device and a light detector. Figure 1 A cross-section of a conventional electron microscope is illustrated schematically. The first element of the collecting optics 3, used to redirect the electron beam 4, is typically a parabolic or elliptical reflector (mirror), inserted between the last electromagnetic lens 5 (called the electron objective) of the electron optics column and the sample 7 (also called the specimen 7). The collecting optics includes a central aperture through which the electron beam 9 passes. The focal point of the parabolic reflector 3 coincides with the focal point of the electron beam on the sample. This system has three disadvantages:

[0009] First, reflector 3 must be large to maintain good optical and mechanical properties, such as high reflectivity, good wavefront uniformity, a large field of view, and good mechanical stability. A larger reflector results in a longer working distance, i.e., a greater distance between sample 7 and the last lens 5 of the electron microscope. However, electron optics have enabled smaller electron beam probes at shorter working distances (a few millimeters). Therefore, a trade-off must be found between the size of the small electron beam probe and good optical and mechanical properties for cathodoluminescence detection.

[0010] Secondly, reflector 3 shields or blocks the electric field used to extract secondary electrons, thus hindering optimal collection of secondary electrons. This reduces the signal-to-noise ratio in the secondary electron mode, makes it more difficult to optimize the electron probe size, and reduces the achievable optimal spatial resolution.

[0011] Third, the field of view of parabolic / elliptical reflectors used in conventional cathodoluminescence collection systems is limited to a few micrometers. The field of view corresponds to the area around the reflector's focal point, which can be imaged onto the detector without light loss. Therefore, to obtain a cathodoluminescence map without light loss, the sample must be positioned very precisely at the focal point, and the electron probe must scan the sample within the reflector's field of view. However, it is difficult to position the sample at the reflector's focal point while ensuring the electron probe is of optimal size and scans the correct area. Tedious and frequent alignment is required, which is only feasible when the sample emits a large amount of light. In practice, such repetitive work cannot be performed properly, reducing collection efficiency. Medium to large area scans (above a few micrometers) suffer from significant non-uniformity in collection efficiency (vignetting) and cannot produce quantitative cathodoluminescence maps.

[0012] Most of the problems mentioned above can be solved by using a compound reflecting objective lens that includes a concave mirror, a convex mirror, and a plane mirror, such as... Figure 2This is a schematic illustration. It represents a known solution in electron beam probe microscopy, where a reflecting objective is used to image the sample surface. In most embodiments, a Cassegrain-like reflecting objective is used. A Cassegrain-like reflecting objective consists of two spherical mirrors positioned such that they provide a much larger field of view than a single parabolic or elliptical mirror. A large concave spherical mirror M1 is positioned above the sample so that it reflects light from the sample onto a small convex spherical mirror M2 positioned between the sample and M1. Mirror M2 is arranged to redirect the light along the optical axis of the electromagnetic objective 5, and the plane mirror M3 redirects the beam to the output. All three mirrors, M1, M2, and M3, have apertures for the electron beam path to pass through, thus ensuring the electron beam is not obstructed.

[0013] Cassegrain-like reflecting objectives have a large working distance and can be mounted within the electron objective of an electron microscope. The field of view is significantly large (typically several hundred micrometers), and this arrangement can also be used as an objective for optical microscopes. Because there is no reflecting mirror below the electron objective, secondary electrons are not shielded, and the working distance can be as short as needed to achieve high spatial resolution (small electron probe size).

[0014] However, this solution is primarily used for imaging samples illuminated by strong light sources. It is unsuitable for measuring weak cathode ray emission signals that emit only a few photons per second or per minute. Therefore, the numerical aperture (NA) of currently used reflecting objectives is very low (typically below 0.3), making it unsuitable for achieving high collection efficiency under low light intensity conditions. Notably, it is not yet known how to embed high numerical aperture reflecting objectives (NA>0.4) within the electron objective of a scanning electron microscope while maintaining the ability to scan large sample areas.

[0015] For further disclosures regarding systems that incorporate light collection into a particle beam column, the reader may refer to U.S. Patent No. 3,845,305, U.S. Publications Nos. 2013 / 0335817 and 2019 / 0103248, and French Patent No. 2173436. One problem these disclosures attempt to address is how to combine all the various particles and light using optical elements in a small area to achieve a short working distance. For example, U.S. Publication No. 2019 / 0103248 and French Patent FR 2173436 place the optical elements below the electromagnetic elements. However, this results in a relatively large working distance, causing the electron beam to spread due to the mutual repulsion of electrons within the beam. Summary of the Invention

[0016] To provide a basic understanding of some aspects and features of the invention, the following summary of the invention is included in the specification. This summary is not a broad overview of the invention and is therefore not intended to specifically identify essential or critical elements of the invention or to define the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that follows.

[0017] One object of the present invention is to overcome the aforementioned problems associated with scanning electron microscopy. In particular, the object of the present invention is to demonstrate how to integrate high numerical aperture reflecting objectives into the electromagnetic electron objectives of a scanning electron microscope without degrading the performance of the electron microscope, including spatial resolution, field of view, etc.

[0018] According to the disclosed aspects, an integrated quantitative cathode ray luminescence scanning electron microscope is provided, wherein optical optical elements are coaxially positioned within a space occupied by electromagnetic optical elements.

[0019] In the disclosed embodiments, the collecting mirrors are positioned within the electromagnetic objectives of the electron column. Each collecting mirror has a central aperture that allows the electron beam to pass through. Beam scanning elements are also placed within the electromagnetic objectives. The beam scanning elements are positioned within the "shadow" of the collecting mirrors to avoid interfering with light collection. In some cases, some or all of the beam scanning elements may be placed at least partially within one or two apertures of the collecting mirror.

[0020] The advantage of the disclosed embodiments is that the electron beam deflection device (beam scanning element) can be placed very close to the sample, increasing the scanning area compared to conventional scanning electron microscopes. Furthermore, since the beam scanning elements are located within the microscope, they do not cause any further obstruction to light. By ensuring that the electron beam passes through the center of the electromagnetic objective (located at the intersection of the optical axis of the electromagnetic objective and the point where the generated magnetic field has its maximum value), any optical aberrations caused by sample scanning can be minimized. Attached Figure Description

[0021] The accompanying drawings, which are included in and form part of this specification, illustrate embodiments of the invention and, together with the specification, serve to explain and illustrate the principles of the invention. The drawings are intended to illustrate the main features of exemplary embodiments in a illustrative manner. The drawings are not intended to depict every feature of an actual embodiment or the relative dimensions of the depicted elements, and are not drawn to scale.

[0022] Other features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments with reference to the accompanying drawings, wherein:

[0023] - Figure 1 It is a schematic cross-sectional view of the lower part of an electron microscope based on existing technology;

[0024] - Figure 2 This is a schematic cross-sectional view of the lower part of another electron microscope based on existing technology;

[0025] - Figure 3 This is a schematic cross-sectional view of the lower part of a cathode ray luminescence scanning electron microscope according to one embodiment;

[0026] - Figure 4 This is a cross-section of an integrated quantitative cathode ray luminescence scanning electron microscope according to an embodiment;

[0027] - Figure 5a and 5b This is a simplified schematic cross-sectional view of the lower part of a scanning electron microscope, illustrating the function of the electron beam deflector;

[0028] - Figures 6a to 6d This is a schematic diagram illustrating various geometries of the reflecting objective lens used in the scanning electron microscope according to the present invention;

[0029] - Figure 7 This is a schematic cross-sectional view of the lower part of a cathode ray luminescence scanning electron microscope according to another embodiment of the present invention; and

[0030] - Figure 8 This is a flowchart illustrating an example of operating a scanning electron microscope according to the present invention. Detailed Implementation

[0031] Some embodiments of the invention are described in more detail below with reference to the accompanying drawings. Identical functional and structural elements appearing in different drawings are assigned the same reference numerals.

[0032] Embodiments of the cathode ray luminescence scanning electron microscope of the present invention will now be described with reference to the accompanying drawings. Different embodiments or combinations thereof may be used for different applications or to achieve different benefits. Depending on the desired outcome, the different features disclosed herein may be used partially or entirely, alone or in combination with other features, balancing advantages with requirements and constraints. Therefore, reference will be made to different embodiments to highlight certain benefits, but not to the disclosed embodiments. That is, the features disclosed herein are not limited to the embodiments in which they are described, but may be “mixed and matched” with other features and included in other embodiments, even if these are not explicitly described herein.

[0033] Figure 3 A cross-sectional view of the lower part of a cathode ray luminescence scanning electron microscope together with the sample or specimen 7 is shown. Figure 4 The lower part, resembling a microscope, is shown in a cross-sectional view. (Example) Figure 4 As shown, the microscope as a whole includes an electron column 41 housed within a vacuum housing 10 and an imaging element 42 in an atmospheric environment. Figure 4The integrated microscope shown can generate electron beam images, beam images, cathodoluminescence (CL) images, and CL spectral images. The imaged CL emission can be correlated with the structure and quality of nanoscale sample materials. CL data can reveal material stresses, impurities, crystallinity, and subsurface defects that are not visible using other imaging modalities. Importantly, CL imaging is a non-destructive method for sample inspection.

[0034] The electron column includes an electron source 1 that emits electrons, such as a thermionic source or a field emission source. The emitted electrons are formed into an electron beam 9 by various particle-optical elements, such as an electromagnetic lens 5', an electromagnetic objective lens 5, and an aperture disk (sometimes called an aperture stop) 6. Note that any aperture disk 6 can function as an electrostatic lens by applying a potential to it. Figure 4 In this example, pulse 2 indicates that electron emission from source 1 occurs in pulses, but this is not always the case. For example, emission can be continuous, or intermittent emission can be generated using a shutter.

[0035] exist Figure 3 The electromagnetic object lens 5, also simply referred to as the electromagnetic objective lens, is shown in more detail below. A coil 11 is provided in a known manner to generate a magnetic field, which in this figure is located on the optical axis z( of the electromagnetic objective lens 5). Figures 6a to 6d The magnetic field is essentially horizontal at the level of the output or exit aperture 13. Most of the magnetic field can be located at the level of the output or exit aperture 13, or most of the magnetic field can be outside the region between the lens and the sample. Since the electromagnetic objective 5 can be considered a thick lens, it has a first principal plane and a second principal plane. Figure 3 In the diagram, the first principal plane will be the upper plane, and the second principal plane will be the lower plane. In this configuration, the second principal plane, where the electron beam is deflected, is where the electron beam focusing occurs. In the following description, it is assumed that most of the magnetic field is located at the intersection of the second principal plane and the optical axis of the electromagnetic objective 5. This intersection point is axially located between the sample 7 and the lower end of the first deflector 17, as explained later.

[0036] The purpose of the magnetic field is to generate a converging electron beam 9, which can be focused onto the surface of sample 7. In this example, the electron beam 9 generated by electron emitter 1 propagates downwards from the top of the figure. The electron beam span can be modified by a beam-focusing device (e.g., lens 5') so that the electron beam can diverge, collimate, or converge. The beam-focusing device can be placed below the electron emitter. The electron beam typically has a width in the range of several millimeters, for example, between 2 and 3 mm.

[0037] Electromagnetic objective 5 is rotationally symmetrical about its optical axis, which substantially coincides with the path of electron beam 9. Electromagnetic objective 5 is designed such that an electron beam emitted from source 1 at a finite or infinite distance (collimated beam) to pass through the center of electromagnetic objective 5 will be focused onto the focal plane, as explained later. The position of the focal plane, more specifically its height above the sample, can be adjusted by varying the strength of the magnetic field flowing through electromagnetic objective 5, although the lens is optimized to produce a minimal probe size when the focal plane is positioned approximately 5 mm below the center of electromagnetic objective 5.

[0038] Lens 5 has a hollow interior along its optical axis so that the electron beam 9 can pass through. The hollow portion (channel or gap) is wide enough so that light emitted or reflected from sample 7 (as explained later) can also pass through without obstruction. Since the output aperture 13 of the electromagnetic objective lens 5 is preferably kept as small as possible to maintain good electro-optical performance, the system is preferably constructed to keep the working distance small.

[0039] As can be seen, a reflecting objective is provided within the electromagnetic objective 5 for imaging the surface of sample 7. A Schwarzschild reflecting objective is used in this example. The Schwarzschild objective is a double-mirror reflecting objective that is rotationally symmetric about the optical axis z (essentially coinciding with the path of the electron beam, see [link]). Figures 6a to 6d The objective lens is apomorphically corrected and infinity-corrected. In the context of geometric optics, an objective lens is infinity-corrected if all light rays entering the objective lens parallel to the optical axis are focused at the same focal point to form a diffraction-limited spot, or conversely, if all light rays emanating from the focal point and passing through the objective lens form a beam of light parallel to the optical axis or an equivalent collimated output beam. The electromagnetic objective lens 5 and the reflecting objective lens can have the same focal plane.

[0040] An objective lens is amorphous if it satisfies both Abbe's sine law and the Lagrange condition (also known as Fermat's principle). For an infinity-corrected objective, Abbe's sine law states that a ray of light passing through the focal point and forming an angle α with the optical axis will be parallel to the optical axis and leave the objective at a distance y' from the optical axis, therefore y' = m × sin(α), where m is a real constant. This law should satisfy at least near the focal point. For an infinity-corrected objective, the Lagrange condition states that the path of light rays between the focal point and a plane orthogonal to the optical axis and located within the collimated beam is constant, regardless of the value of angle α.

[0041] The advantages of amorphous objectives can be evaluated using optical aberration theory. Aberrations describe deviations of an imaging optical system from an ideal state. An ideal imaging optical system is (1) aastigmatic and (2) achromatic, that is, (1) the image of a point located at a finite or infinite distance from the optical system is a point (or more precisely, the size of the spot is limited by the diffraction of light), and (2) the position of the image point is independent of the wavelength of light. The effect of aberrations on the imaging characteristics of an optical system is to distort the diffraction-limited image of a point object. Seidel developed a method (among others) to describe monochromatic aberrations. Here, aberrations are classified according to their relative importance (as a term for odd power series) and their effect on the image of a point source through the imaging optical system. The third-order terms of the power series describe the most fundamental aberrations (called third-order aberrations), such as spherical aberration and coma, astigmatism, field curvature, and distortion.

[0042] Objectives that correct spherical aberration using spherical aberration and coma. Spherical aberration affects the image of a point object, regardless of whether the object is on-axis or off-axis. On-axis and off-axis should be understood here as rays emitted from an object on the optical axis and rays emitted from an object at a certain distance from the optical axis, respectively. Objectives affected by spherical aberration focus the incident light rays at different positions along an axis parallel to the optical axis, depending on the angle formed by the incident light rays and the optical axis, thus causing deviations from the diffraction-limited spot size of the image. When the object is off-axis, coma affects the image of a point object. Objectives affected by coma focus the incident light rays at different positions in a plane perpendicular to the optical axis, depending on the angle between the incident light rays and the optical axis and the position of the point object relative to the optical axis. Coma transforms the originally diffraction-limited circular symmetrical spot into a comet-shaped spot.

[0043] Working with an infinity-corrected objective has advantages. First, because the rays leaving the objective are parallel to the optical axis, an image of the object can be formed at any axial distance along the optical axis, for example, if another lens is added. Second, optical elements such as plates, polarizers, spatial and spectral filters can be added without modifying the image position. Schwarzschild reflective objectives are calculated to have the desired optical properties at their focal point. Schwarzschild reflective objectives are also achromatic, meaning these properties do not change significantly when leaving the focal point. Schwarzschild reflective objectives will maintain relatively good optical performance over a large field of view (up to several degrees), meaning that light emitted off-axis from a point within the focal plane will produce an almost collimated output beam. Conversely, a collimated beam entering the objective at a relatively large angle (at most a few degrees) will form a relatively small spot (almost diffraction-limited) on the focal plane.

[0044] It should be noted that the teachings of this invention also apply when using non-infinity corrected reflecting objectives. For example, electromagnetic objectives can be used well with Schwarzschild objectives with a back focal plane for focusing objects.

[0045] from Figure 3 and Figure 4 As can be seen, the reflecting mirrors within the electromagnetic objective 5 include: a first reflecting mirror M1, also called the primary reflecting mirror, which in this example is spherical and concave; and a second reflecting mirror M2, also called the secondary reflecting mirror, which in this example is spherical and convex. The diameter of the first reflecting mirror M1 is larger than the diameter of the second reflecting mirror M2. The first reflecting mirror M1 is located above the second reflecting mirror M2 and is arranged to reflect light from the sample 7 due to the electron beam 9 striking the surface of the sample 7, and to guide the light to the second reflecting mirror M2, which is placed between the sample and the first reflecting mirror M1. The second reflecting mirror M2 is arranged to redirect the light along the optical axis of the electromagnetic objective (in... Figure 3 (Upward), and the third reflector M3 (planar in this example) is arranged to redirect beam 4 to the output, as shown. Figure 4 As shown. In this example, the third reflector M3 is at a 45° angle to the electron beam 9 axis to redirect the light outside the vacuum enclosure 10. All three reflectors M1, M2, and M3 have apertures or openings in the electron beam path so that the electron beam is unobstructed.

[0046] Figure 3 and Figure 4 The microscope shown also includes: a first electron beam deflection device 17, referred to as a first deflector element or simply the first deflector 17; and a second electron beam deflection device 15, referred to as a second deflector element or simply the second deflector 15. It can be seen that in... Figure 3 and 4 In the first embodiment shown, the first deflector 17 is at least partially located within the aperture of the second reflector M2, while the second deflector 15 is at least partially located within the aperture of the first reflector M1. In other words, the first deflector 17 is positioned radially inward from the aperture of the second reflector M2 and is at least partially axially aligned with that aperture, while the second deflector 15 is positioned radially inward from the aperture of the first reflector M1 and is at least partially axially aligned with that aperture. The positions of the deflectors 15 and 17 ensure that they do not obstruct the propagation of the electron beam 9 or the reflected beam.

[0047] Each deflector may include, for example, four longitudinal electrical conductors (quadrupoles), also known as electrodes. They may also include four magnetic coils (magnetic deflectors). However, the number of electrodes or magnetic poles may not be four. For example, there may be eight electrodes or magnetic poles (octupoles) instead of four, which can also correct for astigmatism in the electron beam. In this case, the four electrodes or magnetic poles are called astigmatism correctors and are used to correct for astigmatism.

[0048] Electron beam characteristics are affected by electromagnetic elements along its path. Spherical aberration and chromatic aberration of the focusing lens 5', possibly located after the emitter, limit the quality of beam formation, thus limiting the spot size. If the aperture diameter in the electron optics is not circular, or if it is displaced or tilted relative to the optical axis, it can lead to potentially severe forms of astigmatism in the focused spot. In these cases, the elliptical shape of the resulting electric or magnetic field will result in an aberration called ellipticity astigmatism. The cross-section of the resulting focused spot will be elliptical rather than circular on the first order. This form of astigmatism can be corrected using an astigmatism corrector, the simplest form of which is an n-pole element with opposite electron or magnetic fields arranged around the beam. The same n-pole element can be used to deflect the beam and correct astigmatism.

[0049] The electrodes are positioned along the electron beam trajectory; however, the electrodes allow the electron beam to pass through the channels in the electromagnetic objective 5 and the apertures in the mirrors M1, M2, and M3 without obstructing the electron beam. Deflectors are used to deflect the trajectory of the electron beam so that the electron beam can scan the surface of the sample 7. The sample 7 is positioned on the sample holder 47 at the focal point or focal plane of the electromagnetic objective 5 such that the distance from the first deflector 17 to the sample 7 is less than twenty times the focal length of the electromagnetic objective 5.

[0050] Due to limitations of the electromagnetic objective 5, and more specifically, the need for a large hollow interior to allow light to pass through, conventional electromagnetic objectives typically suffer from relatively strong off-axis aberrations, causing the scan on the sample surface to deviate from the optical axis of the electromagnetic objective. Therefore, to minimize this off-axis aberration in this invention, the scan is designed to intersect the electron beam 9 with the electromagnetic objective 5 at its center, wherein the optical axis of the electromagnetic objective 5 intersects with the second principal plane. In this example, the second principal plane coincides with the position of the output aperture 13. This is in Figure 5a and 5b The diagram is schematically shown. A second deflector 15 is configured to deflect the electron beam 9 in one direction, while a first deflector 17 is configured to counteract the effect of the second deflector, allowing the electron beam to pass through the center of the electromagnetic objective 5. In this example, two electrostatic octodes are used to deflect the electron beam, allowing it to be deflected in any direction and also correcting for astigmatism. The optimal pivot point for the electron beam 9 is obtained by varying the voltage ratio applied to the two deflectors 15, 17 and selecting a scanning voltage that minimizes aberrations. A fixed pivot point for the electron beam 9 can be maintained at the center of the lens by carefully maintaining the voltage ratio between the two deflectors 15, 17 during scanning. In the illustrated example, the pivot point is located at the exit aperture 13.

[0051] The farther the deflector is from the second principal plane of the electromagnetic lens, the stronger the electric field required to deflect the electron beam. It should be noted that the electron beam 9 is typically not in contact with deflectors 15 and 17. To minimize cost and technical issues, such as better electrical isolation, cooling, expensive power supplies, or slower final scanning speeds associated with the manufacture of high-voltage deflectors, the deflectors are advantageously positioned as close as possible along the optical axis to the second principal plane of the electromagnetic objective 5. This means that the high-voltage deflector is advantageously at least partially inserted into the reflecting objective.

[0052] The positions, shapes, and sizes of deflectors 15 and 17 are chosen such that they do not obstruct any light, or only a portion of the light emitted by sample 7 that would otherwise be obstructed by the reflecting objective. The first deflector 17 is positioned within the shadow cone generated by the second reflecting mirror M2, i.e., within an imaginary cone defined by the intersection of the outer diameter of the second reflecting mirror M2 and the optical axis on the sample surface. The shadow cone can also be defined as an imaginary cone defined by the aperture diameter of the primary reflecting mirror M1 and the intersection of the optical axis on the sample surface. Deflector 17 is typically positioned within the shadow cone.

[0053] In order to position the two deflectors 15, 17 within the reflecting objective, in this embodiment, the inner radius of the second reflecting mirror M2 is designed to be large enough to accommodate the first deflector 17, and the outer radius of the first reflecting mirror M1 is kept small enough to keep the size and cost of the electromagnetic objective 5 reasonable. These are essentially two parameters that determine the shape and size of the dual-mirror rotationally symmetric reflecting objective. The second deflector 15 can be positioned at least partially within the aperture of the first reflecting mirror M1, or, as in this embodiment, extend above the reflecting objective. Therefore, the second deflector 15 in this embodiment is also within the shadow cone. Thus, deflectors 15 and 17 do not obstruct any light that can be collected by the collecting objective.

[0054] The outer radius of the first reflecting mirror M1 can be in the range of 40 mm to 160 mm, or in the range of 50 mm to 70 mm. In this particular example, the outer radius of the primary reflecting mirror is approximately 60 mm, which ensures a reasonable manufacturing cost for the electromagnetic objective 5. The inner radius of the second mirror M2 can be in the range of 1 mm to 4 mm, or in the range of 1.50 mm to 3 mm. In this particular example, the inner radius of the second reflecting mirror M2 is approximately 2 mm, allowing a deflector with a diameter of 4 mm to be inserted into the aperture of the second reflecting mirror M2. Once the focal point is selected, these two constraints uniquely define the geometry of the Schwarzschild reflecting objective used in embodiments of the invention. It should be noted that the radial cross-section of the reflecting mirror aperture can be non-circular. If the radial cross-section of the reflecting mirror aperture is not circular, the aforementioned radius should be understood as half of the maximum cross-sectional dimension of the aperture. The aperture of the first reflecting mirror M1 has a larger radial cross-section than that of the second reflecting mirror M2, allowing light reflected by the secondary reflecting mirror M2 to pass along the optical axis.

[0055] The first and second mirrors can be mechanically connected to each other, but in a manner that does not obstruct any light or at least causes minimal obstruction. For example, the second mirror M2 can be secured to the first mirror M1 by a bracket consisting of four very thin legs designed to obstruct light as little as possible. The two mirrors and the mechanical connectors can be made of, for example, identical aluminum or copper castings, so that the mirrors are non-thermal, i.e., they retain their optical properties at all temperatures as long as the mirrors are heated. Aluminum and copper have the advantage of high thermal conductivity and rapid heating. The mirrors can be coated with various reflective materials, such as metals or dielectrics, to improve reflectivity over a given spectral range. Any wiring leading to the first and second mirrors M1 and M2 is also designed not to create further light obstruction.

[0056] The precise geometry of a given Schwarzschild objective is uniquely defined by specifying the positions of a point on the surface of the first mirror M1 and a point on the surface of the second mirror M2 (relative to their focal points). In practice, it is easier to specify the positions of the two mirror vertices relative to the objective focal point. A mirror vertex is the intersection of the mirror surface (or an imaginary extension due to the mirror aperture) with the optical axis z. The shape and position of the mirrors within a Schwarzschild objective can be calculated analytically or numerically. Since the calculation depends on only two parameters, such as the positions of the two mirror vertices relative to the objective focal point, an infinite number of possible Schwarzschild objectives can be calculated by changing these two parameters. Figures 6a to 6d The diagrams illustrate the geometry of various reflective lenses that can be applied. These diagrams will be discussed in more detail below.

[0057] For practical reasons, it is impossible to collect light emitted from sample 7 over the entire solid angle, and the collection angle is constrained to a receiving cone whose axis of symmetry is the optical axis z and has a half-apex angle α. The half-apex angle α determines the numerical aperture (NA) of the Schwarzschild reflecting objective, where NA is defined as NA = n × sin(α) (where n = 1, because the objective is designed to be in a vacuum). In other words, the numerical aperture is constrained to the sine of the maximum collection half-angle α, i.e., the angle formed by the optical axis z and the most extreme collection angle.

[0058] Limiting the maximum collection angle also limits the maximum outer radius of mirrors M1 and M2, such as Figures 6a to 6d As shown. A ray emitted from focus O at the maximum collection half-angle α will be reflected at point P1 on the first mirror M1 and at point P2 on the second mirror M2. P1 and P2 define the maximum extent of M1 and M2, respectively, and the overall size of the Schwarzschild objective. It can be seen that M2 obstructs some emitted light. Therefore, a ray emitted from focus O and passing through P2 will be reflected at point P3 on M1 and at point P4 on M2. Point P2 defines the ray with the minimum emission angle that the Schwarzschild objective can collect. Any light with a smaller emission angle (relative to the optical axis) will be obstructed by the secondary mirror M2, thus defining the shadow cone.

[0059] The outer radius is defined here as the radial distance from the optical axis z to point P1 or P2. Similarly, the inner radius is defined as the radial distance from the optical axis z to point P3 or P4. The surface of the first mirror M1, defined by the optical axis and P3 (P4), is not optically active because it is obscured by the second mirror M2. For a given numerical aperture, the overall obstruction of the second mirror M2 can be adjusted by changing the positions of the two vertices. Similarly, the inner and outer radii of the two mirrors can be adjusted. The second mirror M2 can be designed such that the obstruction half-angle β is less than 30°, and in some embodiments less than 20° or 15°. The solid angle corresponding to the light obstruction angle (defined here by the focal point of the reflecting objective and the outer diameter of the second mirror M2) is at least 0.3 sqrtdecats smaller than the solid angle corresponding to the maximum light collection angle (defined here by the focal point of the reflecting objective and the maximum outer diameter of the first mirror M1). This will ensure sufficient collection of the faint light emitted by the cathode rays.

[0060] The fraction of light emitted at focal point O that is ultimately collected by the reflecting objective depends not only on the numerical aperture of the objective and the obstruction caused by the second reflecting mirror M2, but also on the angular emission intensity of the emitted light. The fraction of light collected can be determined by integrating the angular emission intensity of the light collected by the reflecting objective over the solid angle. In many cases, the light emitted by sample 7 at focal point O follows a Lambert radiation diagram and obeys Lambert's cosine law. This means that the radiation intensity observed from the surface of sample 7 is proportional to the cosine of the angle α between the observer's line of sight and the surface normal.

[0061] The collection efficiency of the Lambertian emitter increases slowly at low numerical apertures and accelerates rapidly with increasing numerical aperture (up to a numerical aperture of 0.71). If sample 7 emits photons at a very low rate, it is important to collect as many photons as possible and use a large numerical aperture. According to the invention, the numerical aperture can be greater than 0.35. In some variations, it is between 0.5 and 0.9, while in others it is between 0.6 and 0.8. In some further embodiments, it is between 0.65 and 0.75. In this particular example, it is 0.72 (i.e., the maximum collection angle relative to the optical axis is 45°), thus allowing the collection of 50% of the light emitted by the Lambertian emitter. By choosing a Schwarzschild geometry that does not cover a large solid angle, the obstruction caused by the presence of the second mirror M2 can be mitigated. This can be done by moving the vertex of mirror M2 as close as possible to the vertex of mirror M1. In practice, if the two vertices are very close to each other, the objective lens becomes very difficult to manufacture. While maintaining good manufacturability, the barrier level can typically be kept below 4% of the light.

[0062] Figure 7 Another embodiment is shown. In this embodiment, the geometry of the Schwarzschild reflector differs from that in the other embodiments. According to this embodiment, the geometry of the reflector corresponds to... Figure 6bThe geometry shown is illustrated. It can be seen that in this embodiment, the first deflector 17 is located entirely below the second mirror M2, i.e., between the second mirror M2 and the output aperture 13. Similarly, deflectors 15 and 17 do not further obstruct light because the first deflector 17 is positioned within the obstruction cone created by the second mirror. The obstruction cone can be defined by the angle between the optical axis of the electromagnetic objective and the light ray emitted from the sample and reaching the innermost reflecting surface of the first mirror M1; that is, the light ray is an imaginary line drawn from the point where the electron beam strikes the sample to the inner diameter of the aperture of the first mirror M1. The position of the deflector 17 can also be defined within an imaginary cone extending from the aperture of the mirror M1 to the focal point of the electromagnetic coil 11, which is also the point where the electron beam strikes the sample. As long as the deflector 17 is positioned within this imaginary cone, the deflector will not interfere with light collection.

[0063] In the disclosed embodiment, the first deflector 17 is positioned within a volume region axially located between the first central aperture and the focal plane, and within a solid angle enclosed by the intersection of the first central aperture at the focal plane of the electromagnetic objective and the optical axis of the electromagnetic objective. The optical axis of the electromagnetic objective may substantially coincide with the optical axis of the reflecting objective. Therefore, the optical axis may be the rotational axis of the electromagnetic objective. It should be noted that the solid angle is not limited to a conical shape.

[0064] Figure 8 The flowchart summarizes the method of operating a scanning electron microscope according to an example, where some or all of the steps mentioned may occur simultaneously or in any other desired order. In step 31, the electron emitter 1 or probe generates a pulsed electron beam 9. In step 32, the electron beam generated by the electron emitter is converted into a collimated, focused, or diverging electron beam by a collimating lens 5' located in the trajectory of the electron beam. In step 33, the electromagnetic objective 5 generates a magnetic field at its output aperture 13 to focus the electron beam onto the sample 7. In step 34, deflectors 15, 17 deflect the electron beam 9, thereby enabling it to perform a scan on the sample 7. In step 35, a reflecting objective mounted along the axis of the electron beam 9 and within the electromagnetic objective 5 collects light from the sample 7. The light collected from the sample is redirected to a third reflecting mirror M3, which further redirects the light outside the vacuum housing 10. In step 36, the collected light passes through a monochromator to spatially separate the light into its wavelength components. In step 37, the intensity of the light at the selected wavelength is detected.

[0065] return Figure 4The light reflected by mirror M3 is focused by lens 22 onto imaging monochromator 43. In this example, two imagers are provided: a CCD camera 45 and a detector 46, such as an InGaAs or PMT detector. If mirror 24 is a semi-reflective mirror, the two imagers can be operated simultaneously. Conversely, mirror 24 can be a flip mirror, allowing one imager to be activated at a time. With this arrangement, detector 46 can be used to detect the intensity of light at a specific wavelength, while the CCD camera can be used to detect the intensity of light at multiple wavelengths simultaneously.

[0066] To generate an optical image of the sample, light source 26 can be operated to generate a light beam, which is reflected by flip mirror 27 onto lens 22, and then reflected by mirrors M2 and M1 onto mirror M3 facing the sample. The reflected light then enters the CCD camera via a reverse path. In this operating mode, the mirror arrangement including three mirrors M1, M2, and M3 is used to guide light from light source 26 to sample 7 and collect the light reflected from sample 7 and guide it to CCD detector 45.

[0067] exist Figure 4 In one embodiment, an electron detector 19 is provided to detect secondary electrons emitted from the sample, or backscattered electrons reflected from the sample. The signal from this detector can be used to generate scanning electron microscope (SEM) images. Furthermore, in Figure 4 In one embodiment, the sample holder 47 is in the form of a low-temperature stage, which keeps the sample at a low temperature, thereby avoiding noisy light emission.

[0068] The embodiments disclosed herein include an electromagnetic lens comprising: a housing having an inlet aperture on its top surface and an outlet aperture on its bottom surface; an electromagnetic coil radially positioned within the housing; a light-collecting optical element located within the housing; and an electron beam deflector located within the housing; wherein the light-collecting optical element includes a concave mirror having a first axial aperture; and a convex mirror having a second axial aperture; and the electron beam deflector includes a first set of deflectors at least partially positioned within the first axial aperture and a second set of deflectors positioned below the first set of deflectors.

[0069] The disclosed embodiments provide an electron microscope for generating electron beam images, beam images, cathode ray emission (CL) images, and CL spectral images, comprising: a vacuum housing; an electron source located at the top position within the vacuum housing; an electromagnetic objective located at the bottom position within the vacuum housing, the electromagnetic objective including a housing having an inlet aperture on its top surface and an outlet aperture at its bottom; an electromagnetic coil radially positioned within the housing; an optical objective located within the housing and including a concave mirror having a first axial aperture and a convex mirror having a second axial aperture; and an electron beam deflector located within the housing and including a first set of deflectors and a second set of deflectors, the first set of deflectors... The system comprises a rotating head and a second set of deflectors that cooperate to scan the electron beam across the sample; a deflecting mirror that receives the light collected by the optical objective and deflects it to the outside of the vacuum chamber; a light source that generates a beam of light directed toward the deflecting mirror; an electronic sensor that is positioned to detect electrons emitted from the sample; an imaging sensor; an imaging monochromator that splits the light collected by the optical objective into different wavelengths; and a controller that receives the output signal from the electronic sensor and thereby generates an electronic image, receives the output signal from the imaging sensor and thereby generates an optical image, and receives the output signal from the imaging monochromator and thereby generates a cathodoluminescence (CL) image and a CL spectral image.

[0070] The disclosed embodiments provide a scanning electron microscope, comprising: an electron emitter for providing an electron beam (9); an electromagnetic objective (5) configured to generate a magnetic field for focusing the electron beam onto a focal plane of the electromagnetic objective (5), the focal plane being substantially perpendicular to the optical axis of the electromagnetic objective; a first electron beam deflection device (17) for deflecting the electron beam (9) such that the electron beam (9) scans on a sample (7); and a reflecting objective disposed within the electromagnetic objective (5) around the path of the electron beam (9) for collecting light from the sample (7) or for focusing light onto the sample (7), the reflecting objective including a distance from the electromagnetic objective (5) of [missing information]. A first reflecting mirror (M1) is located at a first distance from the output aperture (13), and a second reflecting mirror (M2) is located at a second distance from the output aperture (13), the first distance being greater than the second distance. The first reflecting mirror (M1) and the second reflecting mirror (M2) have a first central aperture and a second central aperture, respectively, so that the electron beam (9) can pass through the first and second reflecting mirrors (M1, M2). A first electron deflection device (17) is positioned axially within a volume region between the first central aperture and the focal plane, and within the solid angle enclosed by the intersection of the focal plane of the electromagnetic objective and the optical axis of the electromagnetic objective (5) and the first central aperture. The first electron deflection device (17) can be radially inwardly positioned from the second central aperture, and the first electron deflection device (17) can be coaxially positioned with the second central aperture. The first electron deflection device (17) can be axially positioned between the second reflecting mirror (M2) and the output aperture (13) of the electromagnetic objective (5).

[0071] The microscope may also include a second electron beam deflection device (15) for deflecting the electron beam (9), the second electron beam deflection device being axially located between the first electron beam deflection device (17) and the electron emitter.

[0072] The electromagnetic objective (5) may have an optical axis perpendicular to the magnetic field, wherein the first electron beam deflection device (17) and the second electron beam deflection device (15) are configured to scan the electron beam so that the electron beam (9) passes through the optical axis in the electromagnetic field generated by the electromagnetic objective (5).

[0073] The electromagnetic objective (5) may have a first principal plane and a second principal plane, one of which is configured to focus the electron beam (9). A first electron beam deflection device (17) and a second electron beam deflection device (15) are configured to scan the electron beam, such that the electron beam (9) passes through the optical axis at the principal plane configured to deflect the electron beam (9). The second electron beam deflection device (15) may be coaxial with the first central aperture and radially inwardly positioned.

[0074] The reflecting objective can be achromatic and can be a Schwarzschild or Head reflecting objective. In addition, the electromagnetic objective (5) and the reflecting objective can have the same focal plane.

[0075] The numerical aperture NA of the reflecting objective is defined by NA = n × sin(α), where α is the maximum half-angle of the light cone that can enter or exit the first reflecting mirror (M1), and n is the refractive index of the medium in which the reflecting objective is located, which can be at least 0.35 or at least 0.7.

[0076] The maximum radial dimension of the second center aperture can be between 2.5 mm and 12 mm.

[0077] The half-angle (“β”) of the solid angle defined by the focal point of the reflecting objective and the maximum external dimension (M2) of the second reflecting mirror can be up to 20 degrees.

[0078] The microscope may also include an electron beam shaping device for shaping the electron beam (9) from the electron emitter before it reaches the electromagnetic objective (5).

[0079] The solid angle defined by the focal point of the reflecting objective and the maximum external dimension (M2) of the second reflecting mirror can be at least 0.3 sqrtions smaller than the solid angle defined by the focal point of the reflecting objective and the maximum external dimension of the first reflecting mirror (M1).

[0080] Furthermore, a method for operating a scanning electron microscope is provided, comprising: energizing an electron emitter to generate an electron beam (9); energizing an electromagnetic objective (5) to generate a magnetic field for focusing the electron beam onto a focal plane of the electromagnetic objective (5), the focal plane being substantially perpendicular to the optical axis of the electromagnetic objective; energizing a first electron beam deflection device (17) to deflect the electron beam (9) so that the electron beam (9) scans on a sample (7); and arranging a reflecting objective within the electromagnetic objective (5) around the path of the electron beam (9) to collect light from the sample (7), the reflecting objective including an output aperture at a distance from the electromagnetic objective (5). A first reflecting mirror (M1) within a first distance from the output aperture (13) and a second reflecting mirror (M2) within a second distance from the output aperture (13), the first distance being greater than the second distance, the first reflecting mirror (M1) and the second reflecting mirror (M2) having a first central aperture and a second central aperture respectively, for enabling the electron beam (9) to pass through the first and second reflecting mirrors (M1, M2), wherein a first electron deflection device (17) is positioned in a volume region axially located between the first central aperture and the focal plane, and located within the solid angle enclosed by the intersection of the focal plane of the electromagnetic objective and the optical axis of the electromagnetic objective (5) to the first central aperture.

[0081] While the invention has been described and illustrated in detail with reference to the accompanying drawings and the foregoing description, such illustrations and descriptions are intended to be illustrative or exemplary and not restrictive, and the invention is not limited to the disclosed embodiments. Those skilled in the art, upon studying the drawings, the disclosure, and the appended claims, will understand and implement other embodiments and variations in practicing the claimed invention.

[0082] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The fact that different features are stated in mutually different dependent claims does not imply that combinations of these features cannot be advantageously used. Any reference numerals in the claims should not be construed as limiting the scope of the invention.

Claims

1. An electromagnetic objective lens, comprising: The housing has an inlet aperture on its top surface and an outlet aperture on its bottom surface; An electromagnetic coil radially positioned within the housing; A light-collecting optical element, which is positioned within a housing and includes a concave mirror having a first axial aperture and a convex mirror having a second axial aperture; An electron beam deflector, located within a housing, includes a first set of deflectors and a second set of deflectors located below the first set of deflectors, wherein the second set of deflectors is positioned within a region defined by an imaginary cone extending from a first axial aperture to the focal point of an electromagnetic coil.

2. The electromagnetic objective lens as described in claim 1, wherein, The second set of deflectors is at least partially positioned within the second axial aperture.

3. The electromagnetic objective lens as described in claim 1, wherein, The second set of deflectors is positioned below the convex lens.

4. The electromagnetic objective lens as described in claim 1, wherein, The second set of deflectors is positioned within the obstruction cone of the convex mirror.

5. The electromagnetic objective lens as described in claim 1, wherein, The first set of deflectors is at least partially positioned within the first axial aperture.

6. The electromagnetic objective lens as described in claim 5, wherein, The second set of deflectors is at least partially positioned within the second axial aperture.

7. The electromagnetic objective lens as described in claim 5, wherein, The second set of deflectors is positioned below the convex lens.

8. The electromagnetic objective lens as described in claim 1, wherein, Each of the first and second sets of deflectors includes one of four or eight poles.

9. The electromagnetic objective lens as described in claim 1, wherein, The light-collecting optics include the Schwarzschild reflector.

10. The electromagnetic objective lens as claimed in claim 1, wherein, The light-collecting optics include an aberration-corrected and infinity-corrected reflecting objective.

11. The electromagnetic objective lens as claimed in claim 1, further comprising an astigmatism corrector positioned within the electromagnetic objective lens.

12. The electromagnetic objective lens of claim 1, further comprising a connector for mechanically attaching the convex lens to the concave lens.

13. The electromagnetic objective lens as claimed in claim 1, wherein, The solid angle defined by the focal point of the light-collecting optical element and the outer diameter of the convex mirror is at least 0.3 steradian smaller than the solid angle defined by the focal point of the light-collecting optical element and the outer diameter of the concave mirror.

14. A scanning electron microscope, comprising: Sample rack; An electron column, positioned to guide an electron beam onto a sample positioned on a sample holder; as well as A light imager, positioned to receive a beam of light from an electron column; The electron column includes: Vacuum enclosure; Electronic source; An electromagnetic objective lens positioned within a vacuum enclosure, the electromagnetic objective lens comprising a housing having an inlet aperture on its top surface and an outlet aperture on its bottom surface; An electromagnetic coil radially positioned within the housing; An optical objective lens, positioned within a housing, includes a concave mirror having a first axial aperture and a convex mirror having a second axial aperture. An electron beam deflector, located within a housing, includes a first set of deflectors and a second set of deflectors located below the first set of deflectors, wherein the second set of deflectors is located below a first axial aperture and the first set of deflectors is located above the second set of deflectors.

15. The scanning electron microscope as described in claim 14, wherein, The optical imager includes an imaging monochromator positioned outside the vacuum housing and an optical sensor positioned to receive light from the imaging monochromator.

16. The scanning electron microscope as claimed in claim 15, wherein, The sample rack includes a low-temperature stage.

17. The scanning electron microscope of claim 16, wherein, The electron source includes a pulsed electron source.

18. The scanning electron microscope as claimed in claim 14, wherein, The first set of deflectors is at least partially positioned within the first axial aperture.

19. The scanning electron microscope as claimed in claim 18, wherein, The second set of deflectors is at least partially positioned within the second axial aperture.

20. The scanning electron microscope of claim 14, wherein, The second set of deflectors is positioned within an area defined by an imaginary cone extending from the first aperture to the focal point of the electromagnetic coil.

21. The scanning electron microscope of claim 14, further comprising a connector for mechanically attaching a convex mirror to a concave mirror.

22. The scanning electron microscope as claimed in claim 14, wherein, The solid angle defined by the focal point of the light-collecting optical element and the outer diameter of the convex mirror is at least 0.3 steradian smaller than the solid angle defined by the focal point of the light-collecting optical element and the outer diameter of the concave mirror.

Images