Improved scanning electron microscope and method of using the same

Abstract

To provide a scanning electron microscope that can achieve higher resolution on-axis diffraction imaging.SOLUTION: According to the present invention, there is provided a scanning electron microscope comprising: an electron source; a sample holder for holding a sample to be analyzed; a projector; and a first detector. Each of the electron source and the sample holder are arranged upon an optical axis of the scanning electron microscope. The projector is movable between a first, operational position, in which the projector is located along the optical axis downstream of the sample holder and between the sample holder and the first detector, and a second, retracted position, in which the projector is located away from the optical axis. There is also provided a method of imaging a sample with the scanning electron microscope.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The invention according to the claims relates to a scanning electron microscope (SEM) capable of achieving higher resolution on-axis diffraction imaging. diffraction imaging) to achieve a scanning electron microscope (scanning electron microscope, SEM).

Background Art

[0002] Charged particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic attributes of electron microscopes have evolved into several well-known device types such as Transmission Electron Microscopes (TEMs), Scanning Electron Microscopes, and Scanning Transmission Electron Microscopes (STEMs), and are also used in various sub-types such as so-called "dual beam" devices that employ a "mechanical" Focused Ion Beam (FIB) as an adjunct, such as Ion-Beam-Induced Deposition (IBID), for example, in the case of an FIB-SEM. Those skilled in the art will be familiar with different types of charged particle microscopy.

[0003] In SEM, upon irradiation of a specimen by a scanning electron beam, "secondary" radiation is emitted from the specimen in the form of secondary electrons, backscattered electrons, X-rays, and photoluminescence (infrared, visible, and / or ultraviolet photons). Next, one or more components of this emitted radiation are detected and used for purposes of image accumulation and / or spectroscopic analysis (such as in the case of EDX (Energy-Dispersive X-Ray Spectroscopy)). When the sample is sufficiently thin, transmission electrons can also be detected, which is called SEM-based scanning transmission electron microscopy (SEM-based STEM). In SEM, the scanning electron beam typically has an energy in the range of 0.2 keV to 30 keV.

[0004] In TEM, the electron beam used to irradiate the specimen is selected to be of sufficiently high energy to penetrate the specimen (for this purpose, it is generally thinner than in the case of an SEM specimen). Using the transmitted electron beam emitted from the specimen, an image can be created or a spectrum can be generated (similar to the case of Electron Energy-Loss Spectroscopy, EELS). When such a TEM operates in a scanning mode, i.e., TEM-based STEM, the image / spectrum in question can be accumulated during the scanning motion of the irradiating electron beam. In TEM, the electron beam typically has an energy in the range of 60 to 300 keV.

[0005] In TEM-based STEM, the electron beam typically has a much higher energy, thereby resulting in lower chromatic aberration and lower spherical aberration compared to SEM-based STEM. However, in SEM-based STEM, thin STEM specimens can be prepared in situ from bulk specimens (i.e., within the SEM chamber).

[0006] The TEM is known to be advantageously switchable between a normal imaging mode and a diffraction mode for analyzing defects and crystal structures. Diffraction techniques within the SEM typically relate to electron backscattering techniques. The concept of on-axis transmission diffraction in the SEM is still relatively new. Transmission diffraction techniques within the SEM includetychography and transmission Kikuchi diffraction (TKD). Recently, pixelated detectors have provided new possibilities for detecting on-axis transmission diffraction patterns using the SEM. However, such techniques are limited by the achievable resolution. Summary of the Invention

[0007] According to a first aspect of the present invention, a scanning electron microscope is provided, the scanning electron microscope comprising an electron source, a sample holder for holding a sample to be analyzed, a projector, and a first detector. Each of the electron source and the sample holder is disposed on the optical axis of the scanning electron microscope. The projector is movable between a first operating position where the projector is located downstream of the sample holder and along the optical axis between the sample holder and the first detector, and a second retracted position where the projector is located away from the optical axis.

[0008] The optical axis of the SEM is the axis on which the sample holder and the electron source are disposed, i.e., aligned / coaxial / centered with the sample holder and the electron source. In use, the primary electron beam generated by the electron source is emitted from the electron source along the optical axis of the SEM. References to upstream / downstream throughout this specification refer to the path of the electron beam through the SEM. The path of the electrons from the electron source towards the sample holder is, for example, a path such that the electron source is upstream of the sample holder.

[0009] A component disposed or located along the optical axis refers to the optical axis intersecting that component, and preferably the component is centered / coaxial / aligned with the optical axis.

[0010] In a TEM, there is known a projector that magnifies the diffracted beam onto a detector and improves the resolution of the produced diffraction pattern. However, known projectors within a TEM typically include multiple magnetic lenses having a significant weight and an overall length of about 50 cm, thereby preventing their use in known SEM systems where the available space between the sample holder and the detector is typically less than 4 cm.

[0011] In the invention according to the claims, by incorporating a projector that can be positioned at a first operating position downstream of the sample holder and along the optical axis of the SEM between the sample holder and the first detector, the projector can improve the resolution of the on-axis transmission diffraction pattern of a thin sample. Such a thin sample can have a thickness ranging from 1 atomic layer to several hundred nanometers. However, since the projector can be moved from this first operating position along the optical axis of the SEM to a second retracted position away from the optical axis of the SEM, the scanning electron microscope can still be used to image thicker samples using a backscattered or secondary electron detector upstream of the sample holder. In fact, when the projector is in the second retracted position, there is sufficient space along the optical axis of the SEM, such that a thicker sample can be accommodated on the sample holder and imaged using the BSE detector or SE detector of the SEM.

[0012] When the projector is in the first operating position, the sample holder, the projector, and the first detector are positioned along the optical axis of the SEM such that transmitted electrons diverging from the sample pass through the projector before reaching the first detector. Thus, the first detector can detect off-axis diffraction (electrons transmitted through and diffracted by the sample), and the projector can increase the magnification of the diffraction pattern on the first detector. As noted above, the reference to being "positioned along" the optical axis in this context means that the optical axis bisects, or preferably is centered / coaxial with, the sample holder, the projector, the first detector, and the electron source. When the projector is in the first operating position, the projector can be positioned directly between the sample holder and the first detector such that there are no other intervening components between the sample holder and the first detector.

[0013] In the second retracted position, the projector is positioned away from the optical axis of the SEM. The projector is offset from the optical axis of the SEM such that transmitted electrons diverging from the sample do not pass through the projector. This allows for imaging of thicker samples because there is more space along the optical axis between the sample holder and the detector when the projector is in a position away from the optical axis of the SEM.

[0014] The SEM can include one or more second detectors. The second detectors can include a back-scattered electron (BSE) detector and / or a secondary electron It may also be a (backscattered electron, BSE) detector or a secondary electron (SE) detector. The BSE detector and SE detector in the SEM and their arrangements are known in the art. As is known in the art, the BSE detector and / or SE detector can be positioned upstream of the sample holder. The BSE detector may be an annular detector arranged concentrically with the optical axis of the SEM. The SE detector can be arranged offset from the optical axis of the SEM. In such an embodiment, when the projector is in the second retracted position, a thicker sample can be imaged in an imaging mode of the SEM in which backscattered electrons and / or secondary electrons are detected by the second detector.

[0015] The first detector may be coupled to the projector and movable with the projector as the projector moves between its first operating position and its second retracted position. The first detector may be fixed relative to the projector and spaced apart from the projector such that when the projector is in the first operating position, the projector is between the sample holder and the first detector. The first detector may be spaced apart from the projector such that the first detector is downstream of the projector. The first detector may be spaced apart from the projector in a direction parallel to or along the optical axis of the projector, which is referred to herein as the projector optical axis. The projector optical axis is the axis along which the electron beam travels through the projector, typically through the center of the projector.

[0016] The first detector can be fixed relative to the projector such that when the projector is in the first operating position, the projector is between the sample holder and the first detector and the optical axis of the SEM intersects both the first detector and the projector. In other words, the first detector may overlap the projector and be downstream of the projector such that electrons passing through the projector collide with the first detector. Such a configuration means that when the projector is in the first operating position, the first detector can detect the transmitted electrons emitted from the sample. The first detector can be fixed relative to the projector such that the active area of the detector (i.e., the area of the first detector configured to receive electrons) is always aligned with the projector optical axis. When the projector is in the second retracted position, the first detector can still be aligned with the projector such that the active area of the detector is aligned with (i.e., overlaps, preferably centered on) the projector optical axis, but the projector (and thus the first detector) can be positioned away from the optical axis of the SEM. This means that the optical axis of the SEM does not intersect the projector or the first detector. Moving both the projector and the first detector away from the optical axis provides more space for accommodating thicker samples. The SEM can then operate in its normal imaging mode, whereby thicker samples are imaged based on secondary electrons and backscattered electrons emitted from the sample. In such an arrangement, a second detector, such as a BSE detector or an SE detector, can be used when imaging thicker samples, as is known in the art. As an example, such thicker samples can have any thickness, provided that they can geometrically conform to the SEM sample holder in accordance with typical use of the SEM. For example, the thicker sample may have a thickness up to several centimeters.

[0017] The projector may be movable between the first operating position and the second retracted position in a plane orthogonal to the optical axis of the SEM. The projector may be linearly movable between the first operating position and the second retracted position.

[0018] The scanning electron microscope can further include an arm operable to move the projector between a first operating position and a second retracted position. The arm can be configured to move in a plane orthogonal to the optical axis of the SEM when moving the projector between the first operating position and the second retracted position. The arm can be configured to move linearly when moving the projector between the first operating position and the second retracted position.

[0019] The arm may extend longitudinally in a plane orthogonal to the optical axis of the SEM between a first end and a second end, and the first end is coupled to the projector. The arm may have a fixed length and may be rigid so as not to deform during use. The arm can be configured to move linearly when moving the projector between the first operating position and the second retracted position. The longitudinal axis referred to herein may be an axis along the length of the arm along which the arm moves. The longitudinal axis may be perpendicular to the optical axis of the SEM.

[0020] The arm can be configured to support the projector. The arm can be configured to support both the projector and the first detector. The first end of the arm can be coupled to both the projector and the first detector. The first end of the arm can hold both the first detector and the projector such that the first detector is fixed relative to the projector. The first end of the arm may include a holder configured to hold the first detector and the projector such that the first detector is fixed relative to the projector. The holder can hold the first detector spaced from the first detector in a direction parallel to or along the optical axis of the projector (i.e., hold and support). The holder can hold the first detector fixed relative to the projector, and in particular, the holder can hold the first detector downstream of the projector.

[0021] Optionally, the scanning electron microscope further comprises a slider configured to transmit linear motion to the arm. The slider may be coupled to the second end of the arm. The slider may comprise a movable carriage and a track. The second end of the arm can comprise a mount configured to couple the arm to the movable carriage. The second end of the arm can be fixed relative to the movable carriage such that linear motion of the movable carriage within the track causes linear motion of the arm.

[0022] The projector and / or the first detector may be cooled. Cooling the projector and the first detector prevents heat accumulation that would otherwise result in increased noise and decreased stability, thereby degrading the performance of the projector and the first detector. In particular, an arm operable to move the projector between a first position and a second position can be thermally coupled to the projector and / or the first detector and configured to cool the projector and / or the first detector. The projector and the first detector may be thermally coupled to each other. The arm can be configured to thermally conduct heat from the projector and / or the first detector. The arm can comprise a thermally conductive material such as copper. The arm can be configured to thermally conduct heat from the projector and / or the first detector at its first end along its length to the second end of the arm. The second end of the arm can comprise a cooling component configured to cool the second end of the arm. In such an arrangement, the arm can thermally couple the projector and / or the first detector to the cooling component. By cooling the second end of the arm, a thermal gradient is created between the first end and the second end of the arm, and heat is conducted from the projector and / or the first detector. The cooling component may be an active cooling component such that the second end of the arm is actively cooled by the cooling component. Such an active cooling component can use a pump or a fan to circulate a coolant therein. The cooling component may be, for example, a heat exchanger, a heat sink, or a closed loop system in which a coolant is recirculated.

[0023] The arm may comprise one or more rods extending between a first end and a second end of the arm. In other words, the one or more rods may extend along the longitudinal axis of the arm. The rods may be rigid and of a fixed length. The rods may be hollow, i.e., may have a cavity therein. The cavity within the rod may be configured to receive a coolant, such as water. The rod may be thermally coupled to the projector and / or the first detector via the first end of the arm. The rod may thermally couple the projector and / or the first detector to a cooling component. Optionally, the one or more rods may comprise one or more heat pipes. A portion of the one or more heat pipes proximate the second end of the arm may be actively cooled by a cooling component. Providing a corresponding detector (first detector), which is both retractable and cooled, to the projector is particularly advantageous for performing on-axis transmission diffraction imaging in a SEM.

[0024] The scanning electron microscope may comprise a housing having a first housing portion that defines a first chamber therein, and within the first chamber, a first detector, a sample holder, a projector, an objective lens, a condenser module, and an electron source (and a second detector if present) are located. The first chamber may be under vacuum. The arm may extend through one or more ports within the first housing portion such that the first end of the arm (and the holder if present) is positioned within the first chamber and the second end of the arm is positioned outside the first chamber. The second end of the arm may be coupled to a slider, which may be positioned outside the first chamber. In an arrangement where a cooling component is employed, the cooling component may also be positioned outside the first chamber. The housing may further comprise a second housing portion coupled to the first housing portion via a vacuum flange that includes one or more ports. The second housing portion may define a second chamber that includes the second end of the arm and, if present, the slider and the cooling component. The second chamber may be under vacuum.

[0025] The objective lens generates a magnetic field called an objective lens that may optionally be an electromagnetic immersion lens.

[0026] The projector may include one or more projector lenses. The projector lenses may be referred to herein as projection lenses. Optionally, one or more projector lenses may be electromagnetic lenses. Optionally, one or more projection lenses may be multi-pole lenses. Optionally, one or more projection lenses may be circular lenses. In particular, the projector can include a single lens (i.e., one lens), and more specifically can consist of a single lens, thereby minimizing the weight, size, and cost of the projector, which is particularly important for integration into an SEM. In particular, the single lens may be an electromagnetic lens. In an SEM system, the space within the chamber (referred to herein as the first chamber) that houses the sample is limited, thereby limiting the height of the projector, so minimizing the size is particularly important. The invention according to the claims achieves an improvement in optical performance in an SEM without sacrificing size, weight, and cooling capacity. In contrast, in a TEM, the projector is placed outside the chamber that houses the sample, so there is no limitation on the size or weight of the projector.

[0027] In embodiments where the projector has a single lens, the single lens may be an electromagnetic multi-pole lens. In such embodiments, the projector lens can have a shell that houses a projector lens coil arranged circumferentially around the projector optical axis. The projector lens may have an upper projector pole piece and a lower projector pole piece separated by a projector gap, and optionally, the upper projector pole piece and the lower projector pole piece may include a cylindrical inlet portion upstream of a frustoconical portion, and further optionally, the frustoconical portion of the lower projector pole piece has a greater height and / or is inclined at a greater angle than the frustoconical portion of the upper projector pole piece.

[0028] The projector may be used, for example, with a projector coil excitation of 1000 to 2000 ampere-turns, preferably 1500 to 2500 ampere-turns.

[0029] The first detector may be a pixelated detector. The pixelated detector may comprise a pixel grid having a plurality of pixels. Using a projector within a SEM having a pixelated detector is particularly advantageous because the pixelated detector has a need to expand the diffracted beam onto the detector to increase the resolution of the diffraction pattern. The combination of the projector and the pixelated detector within the SEM facilitates advanced SEM-based STEM diffraction techniques including tycography, strain analysis, micro-ED, and TKD. Since such a pixelated detector generates a significant amount of heat, it is particularly advantageous to provide a mechanism for cooling and retracting the projector and the first detector.

[0030] According to the present invention, there is also provided a method of imaging a sample using a scanning electron microscope, the scanning electron microscope comprising an electron source, a sample holder, a projector, and a first detector, the electron source and the sample holder being disposed on the optical axis of the scanning electron microscope, the method comprising placing a first sample on the sample holder of the microscope, moving the projector from a second retracted position to a first operating position, scanning a first electron beam generated by the electron source along the first sample to generate a first image, and using the projector at the first operating position to project the first image onto the first detector. At the first operating position, the projector is positioned along the optical axis downstream of the sample holder and between the sample holder and the first detector, and at the second retracted position, the projector is positioned away from the optical axis. The first image is an electron diffraction pattern formed by electrons transmitted through the sample and diffracted by the sample.

[0031] The step of projecting the first image onto the first detector includes magnifying the image on the first detector, i.e., spreading the electrons transmitted through the sample and diffracted by the sample.

[0032] The above teachings regarding the features of a scanning electron microscope are equally applicable to this method.

[0033] As described above, the scanning electron microscope may further include a second detector, which is disposed upstream of the sample holder, and the second detector is configured to detect backscattered electrons and / or secondary electrons emitted from the sample. This method may further include exchanging a first sample with a second sample on the sample holder of the microscope, moving the projector from a first operating position to a second retracted position, scanning a second electron beam generated by an electron source along the second sample to generate backscattered electrons and / or secondary electrons, and receiving the backscattered electrons and / or secondary electrons with the second detector to form an image of the second sample.

[0034] The second sample may be thicker than the first sample.

[0035] For example, the first sample can have a thickness ranging from 1 atomic layer to 1×10 -7 m thick. The second sample can have a thickness limited only by the geometric constraints of the sample holder. For example, the second sample can have a maximum thickness of 1×10 -1 m.

[0036] Thus, this method enables imaging of thinner samples in an on-axis transmission diffraction imaging mode with improved resolution by the projector using an SEM, and imaging of thicker samples in a normal SEM mode (using backscattered electrons and secondary electrons).

[0037] Alternatively, this method may not involve the step of exchanging the first sample with the second sample, and the first sample may first be imaged using the projector and the first detector, and then imaged using the second detector.

Brief Description of the Drawings

[0038]

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Mode for Carrying Out the Invention

[0039] FIG. 1 is a schematic diagram of the imaging components of the SEM 100 of the invention according to the claim and the electron path through the SEM 100. The SEM has an optical axis 101.

[0040] The SEM100 includes a housing 200 (shown in FIG. 2) having a first housing portion 200a that defines a first chamber 201 including an electron source 10, a capacitor module 20, a scanning coil 30, an objective lens 40, a sample holder 50, a projector 60, and a first detector 70. The first chamber 201 is under vacuum to protect these components from contamination, vibration, etc. As used herein, upstream and downstream refer to the direction of the electron beam when the SEM is in use. For example, during use, the electron beam is directed in the downstream direction from the electron source 10 toward the sample holder 50. The path of the electron beam within the SEM is schematically shown by the thin solid line in FIG. 1.

[0041] Both the electron source 10 and the sample holder 50 are disposed on the optical axis of the SEM, i.e., they are coaxial (centered along the optical axis).

[0042] The electron source 10 may be, for example, a Schottky electron source or an ion gun. The electron source 10 generates a primary electron beam. The energy of the primary beam emitted from the electron source 10 is typically from 0.2 eV to 40 keV, preferably from 0.2 eV to 30 keV. The primary beam emitted from the electron source 10 is aligned with the optical axis 101 of the SEM100, i.e., it is coaxial.

[0043] Downstream of the electron source, there is a capacitor module 20 that operates on the primary electron beam from the electron source 10. The capacitor module 20 includes at least one capacitor lens that converges the electron beam as the electron beam travels toward the sample holder 40. The capacitor lens is typically an electromagnetic lens that uses a magnetic field to control the direction of electrons within the beam. The capacitor module 20 can include other lenses or other particle optical components that can perform, for example, aberration mitigation, cropping (by aperture / diaphragm / condenser aperture), filtering, etc.

[0044] Downstream of the condenser module 20, as is known in the art, there are one or more scanning coils 30 that deflect / rasterize the electron beam across the sample, i.e., in a plane orthogonal to the optical axis. The scanning coils 30 are controlled by a controller.

[0045] Downstream of the scanning coils 30 there is an objective lens 40. The condenser module 20, the scanning coils 30, and the objective lens 40 form the electron optical column 110 of the SEM, as is known in the art. The downstream end of the objective lens 40 forms the lower pole piece of the electron optical column 110. The objective lens 40 generates an objective lens, which may be an electromagnetic lens. In particular, the objective lens may be an immersion electromagnetic field lens.

[0046] The electron source 10, the condenser module 20, the scanning coils 30, and the objective lens 40 together form the electron optical column 110 of the SEM 100. The operation of these components within the SEM is known in the art. The electron optical column 110 is aligned with the optical axis 101 of the SEM 100, i.e., the centers may be aligned. The electron optical column 110 may be mounted on the inner surface of the first part 200a of the housing, thereby fixing the electron optical column 110 in a fixed position within the first chamber 201. The downstream end of the objective lens 40 forms the lower pole piece of the electron optical column 110.

[0047] Downstream of the electron optical column, there is a sample holder 50 for supporting a sample. As is known in the art, the sample holder 50 can be tilted or rotated so as to be controlled by a controller and a motor (not shown) coupled to the sample holder 50. The sample holder 50 is aligned / centered / arranged thereon with the optical axis 101 of the SEM100. The optical axis 101 of the SEM100 intersects the sample holder 50, particularly at the center of the sample holder 50. The sample holder 50 can be positioned within the first portion 200a of the housing 200 of the SEM100. The sample holder 50 can be fixed to the first portion 200a of the housing 200. The sample holder 50 may remain aligned with the optical axis 101 of the SEM100 such that the optical axis 101 intersects the sample holder, regardless of the tilt or rotation of the sample holder 50.

[0048] The SEM100 can optionally include a second detector (not shown) that can be a backscattered electron detector and / or a secondary electron detector. The backscattered electron detector may be disposed upstream of the sample holder 50 between the sample holder 50 and the objective lens 40. The backscattered electron detector is known in the art and is typically annular in shape and positioned concentrically with the optical axis 101 of the SEM. The secondary electron detector may be offset from the optical axis 101 of the SEM100 and disposed upstream of the sample holder 50, as is known in the art. The backscattered electron detector may be a scintillation detector or a solid state detector. The secondary electron detector may be, for example, an Everhart-Thornley detector using a scintillator within a Faraday cage.

[0049] The SEM according to the claimed invention uses a projector 60 and a first detector 70 downstream of the projector 60, which is different from known SEMs. When the projector 60 is in the first operating position as shown in FIG. 1, the projector 60 is disposed downstream of the sample holder 40 and guides the transmitted electrons diverging from the sample to the first detector 70. The projector 60 has a projector optical axis that is aligned with the optical axis of the SEM when the projector 60 is in the first operating position. The projector 60 includes one or more projector lenses that magnify the diffraction pattern and project it onto the first detector 70. The projector lens may be an electromagnetic lens, and the intensity of its magnetic field may be adjusted to adjust its focal length, and thus its magnification. Projectors within a TEM for performing magnification are known. However, such projectors are large, heavy, and include multiple lenses. The projector 60 according to the claimed invention advantageously can use a single projector lens, optionally an electromagnetic lens, thereby minimizing weight and size. In other words, the projector 60 may be composed of a single projector lens.

[0050] In an embodiment where the projector has a single projector lens, the single projector lens may be an electromagnetic multipole lens. A cross-section of an example of such a projector lens is shown in FIG. 7. The projector lens in this example has a shell / yoke 61 that houses a projector lens coil 62 arranged circumferentially around the projector optical axis.

[0051] As shown in this figure, the projector lens can have an upper projector pole piece 60a and a lower projector pole piece 60b separated by a gap, which is referred to herein as the projector gap 63. The projector gap 63 can be defined by a projector lens coil 62 that extends circumferentially around the projector gap 63 between the upper and lower projector pole pieces 60a, 60b. The projector gap 63 is centered on the projector optical axis. The upper projector pole piece 60a defines a first path of the electron beam, and the lower projector pole piece 60b defines a second path of the electron beam. These paths are coupled to each other by the projector gap 63. The projector gap 63 reduces spherical aberration, and as a result, improves the image resolution.

[0052] The upper projector pole piece 60a and the lower projector pole piece 60b are coaxial with the projector optical axis. In this exemplary embodiment, each of the upper projector pole piece 60a and the lower projector pole piece 60b has a funnel shape with a cylindrical inlet portion upstream of a frustoconical portion. For each of the upper projector pole piece 60a and the lower projector pole piece 60b, the frustoconical portion has a downstream opening that is wider than the upstream opening, i.e., the upper projector pole piece 60a and the lower projector pole piece 60b have a diameter that increases in the upstream to downstream direction. The frustoconical portion of the lower projector pole piece 60b optionally has a greater height and / or is inclined at a greater angle than the frustoconical portion of the upper projector pole piece 60a.

[0053] In this exemplary embodiment, each of the upper projector pole piece 60a and the lower projector pole piece 60b may be defined by a wall of the yoke 61. The upstream wall of the yoke 61 can define the upper pole piece 60a, and the downstream wall of the yoke 61 can define the lower pole piece 60b. The walls of the yoke 61 may taper to form the frustoconical portions of the upper pole piece 60a and the lower pole piece 60b.

[0054] Note that the configuration of the projector lens in the projector 60 described above is an example and is not limited thereto. Other types of lenses such as multi-pole lenses and circular lenses of other shapes may be used in the projector 60 instead.

[0055] As shown in FIG. 1, the first detector 70 is downstream of the projector 60. The first detector 70 is optionally a pixelated detector and is optionally controlled by a controller (not shown). The pixelated detector includes a plurality of pixels capable of detecting individual electrons incident on the detector and generating a diffraction image. The pixelated detector can include a two-dimensional arrangement of pixels. The pixelated detector utilizes an array of semiconductor diodes and complementary metal-oxide-semiconductor (CMOS) technology. These devices have an array of pixelated detectors (e.g., typically 256×256 pixels, but other sizes are possible) closely coupled to an array of CMOS preamplifiers. Due to the small capacitance of each pixel (typically having a size of only about 55 pm×55 pm) and the close coupling of each individual pixel of the preamplifier, the detector can pulse-count the individual electrons reaching each pixel. A large number of independent pixels (e.g., 64,000) in the detector array enable a very high counting rate to be achieved. Examples of such detectors include MEDIPIX RTM and TIMEPIX RTM family of detectors. Such a pixelated detector may be referred to as an active pixelated detector.

[0056] In the embodiment of FIG. 1, the projector 60 is in the first operating position such that the optical axis of the SEM intersects both the projector 60 and the first detector 70. In particular, the projector 60 is centered with the optical axis of the SEM 100, and thus is centered with the sample holder 50 and the electron source 10. With the projector 60 in the first operating position, the primary electron beam generated by the electron source is aligned / coaxial with the optical axes of the projector 60 and the first detector 70. When used with the projector 60 in the first operating position, the primary electron beam is generated by the electron source 10, emitted from the electron source 10 along the optical axis of the SEM 100, and the electron beam is focused by the objective lens generated by the condenser module 20 and the objective lens 40 before being scanned over the sample in a plane orthogonal to the optical axis under the control of the scanning coil 30. The electrons that have passed through the sample on the sample holder 50 enter the first detector 70 through the projector lens 60. The projector 60 magnifies the obtained diffraction pattern onto the first detector 70.

[0057] As shown by comparing FIGS. 2 and 3, the projector 60 is movable between a first operating position (as shown in FIG. 2) and a second retracted position (as shown in FIG. 3). The projector 60 is movable relative to the sample holder 50, the electron source 10, the condenser module 20, and the objective lens 40 of the SEM 100.

[0058] In the embodiments shown in FIGS. 2 and 3, the first detector 70 is optionally fixed to the projector 60 and is spaced from the projector 60 in the downstream direction of the projector 60, more specifically, in a direction parallel to or along the optical axis of the projector 60, which is referred to herein as the projector optical axis. The projector optical axis is the axis along which the electron beam travels through the projector 60, typically through the center of the projector 60. In other words, the detector 60 is fixed downstream of the projector 60 such that electrons passing through the projector 60 impinge on the first detector 70. More specifically, the first detector 70 is fixed to the projector 60 such that the active region of the first detector 70 (the active region including pixels in the case of a pixelated detector) configured to receive electrons overlaps and preferably is centered on the projector optical axis. The first detector 70 is movable together with the projector 60 relative to the sample holder 50, the electron source 10, the condenser module 20, the objective lens 40, and the housing 200 of the SEM 100. When the projector 60 is moved from the first operating position to the second retracted position, the first detector 70 remains aligned with the projector 60.

[0059] As shown in FIG. 3, when the projector 60 is in the second retracted position, both the projector 60 and the first detector 70 are located away from the optical axis 101 of the SEM 100, that is, in an off-axis position. When the projector 60 is in the second retracted position, the projector 60 and the first detector 70 are offset from the optical axis 101 of the SEM 100 so that electrons transmitted through the sample do not pass through the projector 60 or the first detector 70. In particular, when the projector 60 is in the second retracted position, the projector 60 is offset from the optical axis 101 of the SEM 100 so that there is no overlap between the sample holder 50 and the projector 60. When the projector 60 is in the second retracted position, there is also no overlap between the sample holder 50 and the first detector 70. When the projector 60 is in the second retracted position and the first detector 60 is used with the first detector 70 fixed downstream of the projector 60, electrons transmitted through the sample do not pass through the projector 60 or the first detector 70. In fact, when the projector 60 is in the second retracted position, neither the projector 60 nor the first detector 70 is used to image the sample.

[0060] When the projector 60 is in the second retracted position and both the projector 60 and the first detector 70 are offset from the optical axis, the first detector 70 is not used to detect electrons transmitted through the sample. Instead, a thicker sample may be imaged based on secondary electrons and backscattered electrons that are emitted from the sample and detected using a second detector (not shown). As described above, the second detector may be a BSE detector and / or an SE detector. Imaging the sample using secondary electrons and backscattered electrons emitted from the sample is sometimes referred to in this specification as the normal SEM imaging mode. Therefore, by moving the projector 60 between the first operating position and the second retracted position (together with the first detector 70 fixed to the projector 60), the SEM can, for example, image samples with thicknesses ranging from 1 atomic layer to 1×10 -7From the STEM-axis diffraction mode (scanning transmission electron microscope axis diffraction mode) for thinner samples having a thickness up to m thickness, it is possible to switch to the normal SEM imaging mode for thicker samples having a thickness limited only by, for example, the geometric constraints of the sample holder. In the normal SEM imaging mode, since the projector 60 and the first detector 70 are away from the optical axis, there is more space along the optical axis 101 of the SEM to accommodate thicker samples.

[0061] As shown in FIGS. 2 and 3, the projector 60 is movable in a plane orthogonal to the optical axis 101 of the SEM 100 from its first operating position (on-axis position) to its second retracted position (off-axis position). In particular, the projector 60 is linearly movable between the first operating position and the second retracted position.

[0062] In the embodiment shown in FIGS. 2 and 4, the projector 60 is optionally coupled to an arm 90 operable to move the projector 60 between the first operating position and the second retracted position. The arm 90 extends longitudinally between a first end 91 of the arm 90 and a second end 92 of the arm 90. The arm 90 may be of fixed length and rigid so as not to deform during use.

[0063] The arm 90 may be configured to move linearly when moving the projector 60 between the first operating position and the second retracted position. The axis is along which the arm extends longitudinally and along which the arm moves, and is referred to herein as the longitudinal axis 93 and is perpendicular to the optical axis 101 of the SEM 100. The arm 90 may be fixed to the projector 60 such that movement of the arm 90 along the longitudinal axis 93 moves the projector 60 along or parallel to the longitudinal axis 93 between the first operating position and the second retracted position. More specifically, the arm is configured to move in a plane orthogonal to the optical axis 110 of the SEM 100. Movement of the arm 90 along the longitudinal axis 93 may be driven by an actuator (not shown).

[0064] As shown in FIGS. 2 to 4, the first end portion 91 of the arm 90 may be coupled to the projector 60 and the first detector 70 so that the projector 60 does not move relative to the arm 90. The arm may be configured to support the projector 60. The arm 90 may be configured to support both the projector and the first detector 70. The first end portion 91 of the arm 90 optionally holds both the projector 60 and the first detector 70 such that the first detector 70 is fixed relative to the projector 60. The first end portion 91 of the arm 90 optionally includes a holder 94 configured to hold the first detector 70 and the projector 60 such that the first detector 70 is fixed relative to the projector 60. That the projector 60 and the first detector 70 are fixed relative to each other means that the projector 60 and the first detector 70 do not move relative to each other. The holder 94 can use one or more sockets or clamps for receiving and holding the projector 60 and the first detector 70. The holder 94 is spaced apart from the projector 60 and can hold (i.e., keep and support) the first detector 70 downstream of the projector 60. The holder 94 can hold the first detector 70 spaced apart from the projector 60 in a direction parallel to or along the projector optical axis. In one embodiment, the projector 60 and the holder 94 may optionally be integrally formed. In such an embodiment, the first detector 70 can be attached to the holder 94 using fixing means such as screws, for example. Such an arrangement allows for replacement or repair of the detector 70.

[0065] As will be described in more detail below, the arm can be thermally coupled to the projector 60 and / or the first detector 70 and configured to dissipate heat from the projector 60 and / or the first detector 70. The arm can be configured to conduct heat from a first end of the arm along the length of the arm to a second end 92 of the arm 90, thereby removing heat from the projector 60 and / or the first detector 70. Optionally, as shown in FIGS. 2-4, the second end 92 of the arm 90 can include a cooling component 97 configured to cool the second end 92 of the arm 90, thereby creating a thermal gradient between the first end of the arm and the second end of the arm. The cooling component 97 can be a heat exchanger, a heat sink, a closed loop system, or other component configured to absorb or otherwise remove heat. The cooling component 96 can be an active cooling component that uses a pump or fan to drive the flow of a cooling fluid therein. In particular, the cooling component 96 can be an actively cooled heat exchanger. The heat exchanger can be actively cooled by a closed loop cooling water circuit that is actively circulated.

[0066] Optionally, the longitudinal portion of the arm 90 extending between the first end 91 and the second end 92 of the arm 90 may be formed from one or more rods 95, as shown in FIGS. 2-4. In other words, the arm 90 can comprise the first and second ends 91, 92 and one or more rods 95 extending therebetween. In the embodiments shown in FIGS. 2 and 3, the arm 90 optionally comprises two rods 95 extending between the first end 91 and the second end 92 of the arm 90, parallel to the longitudinal axis 93. Although two rods 95 are shown in the embodiments of FIGS. 1 and 2, any number of rods 95 can be used for an arm having any cross-sectional shape. Each rod 95 is rigid and has a fixed length extending parallel to the longitudinal axis 93 of the arm 90. Using at least two rods 95 instead of a single rod 95 is advantageous for enhancing heat transfer from the projector 60 and the first detector 70. Using at least two rods 95 is also advantageous for reducing the bending of the arm 90 due to the weight of the projector 60.

[0067] The rods may be coupled to the first end 91 of the arm and the second end 92 of the arm 90 in several ways. For example, the rod 95 may be soldered to the first end 91 of the arm and the second end 92 of the arm 90. Alternatively or additionally, the first end 91 of the arm and the second end 92 of the arm 90 may comprise corresponding receptacles complementary to the respective ends of the rod 95. The corresponding receptacles may be, for example, threaded channels. As a further alternative, the rod 95 may be integrally formed with the first and second ends 91, 92 of the arm 90.

[0068] As shown in FIGS. 2 to 4, the second end 92 of the arm 90 can be configured to couple to the slider 300. More specifically, the second end 92 of the arm may be fixed to a part of the slider 300 such that the movement of the slider 300 is transmitted to the arm 90. The slider can be configured to transmit linear motion to the arm such that the arm moves along the longitudinal axis. As described above, the movement of the arm 90 along the longitudinal axis 93 moves the projector 60 between its first operating position and its second retracted position.

[0069] The slider 300 is optionally a linear slider. Any suitable slider capable of transmitting linear motion to the arm 90 can be used. The movement of the slider 300 may be controlled by an actuator. The actuator may be a pneumatic actuator. A controller (not shown) can be configured to control the operation of the actuator to move the slider such that the projector 60 moves between its first operating position and its second retracted position. In the embodiments shown in FIGS. 2 and 3, the slider 300 optionally includes a movable carriage 310 and a track 320, and the movable carriage 310 is movably coupled to the track 320. The track extends longitudinally in a direction parallel to the longitudinal axis 93. The movable carriage 310 is received within the linear track 320 and configured to move linearly with respect to the linear track 320. In other words, the movable carriage 310 can be configured to move in a direction parallel to the longitudinal axis along the track. The linear track 320 may have channels or recesses or grooves arranged to guide the movement of the movable carriage 310 therein. The track 320 is fixed in a stationary position (fixed to the housing 200 of the SEM and thus fixed to the electron source 10 and the sample holder 50 within the housing 200).

[0070] The movable carriage 310 can be configured to be coupled to the second end 92 of the arm 90. The movable carriage 310 may be fixed to the second end 92 of the arm 90 such that the movement of the movable carriage 310 is transmitted to the arm 90. The movable carriage 310 can be fixed to the second end 92 of the arm 90 in several ways. The second end 92 of the arm 90 can have a mount 96 for attaching the arm 90 to the movable carriage 310 of the slider 300. In embodiments where the arm has one or more rods 95 between its first end 91 and second end 92, the mount 96 can include corresponding receptacles 96a for receiving and holding the rods 95. The receptacle 96a may be formed as a channel extending parallel to the longitudinal axis 93.

[0071] The movable carriage 310 may include a protruding portion 311 received within the channel of the track 320 such that the movement of the movable carriage 310 is constrained by the linear track 320. The movable carriage 310 can include a support portion 312 configured to couple to the second end 92 of the arm 90. In particular, the mount 96 at the second end 92 of the arm 90 may be attached to the support portion 312 of the movable carriage 310. The support portion 312 may be a support plane parallel to the channel of the track 320. The protruding portion 311 and the support portion 312 of the movable carriage 310 may be integrally formed.

[0072] The movable carriage 310 may be held within the track by several methods, for example, by employing a lip or abutting portion that defines the edge of the track. As shown in FIG. 4, the movable carriage 310 may be held within the track by coupling the movable carriage 310 to a support rod 330 that extends parallel to the track 320, i.e., parallel to the longitudinal axis 93. The movable carriage 310 may have a mounting portion 313 having a through hole that extends parallel to the longitudinal axis 93, i.e., parallel to the longitudinal direction of the track 320. The through hole may be configured to slidably receive the support rod 330 therein such that the carriage 310 is slidable along the support rod 330 (i.e., slidable with respect to the support rod 330). In such an arrangement, the carriage 310 can slide along the support rod 330 when moving linearly along the track 320. The support rod 330 may be configured to support the weights of both the carriage 310 and the second end 92 of the arm 90 attached to the carriage 310.

[0073] The movement of the movable carriage 310 may be restricted between a first position along the linear track 320 and a second position along the linear track 320. When the movable carriage 310 is in the first position, the projector 60 is in a first operating position, and when the movable carriage 310 is in the second position, the projector 60 is in a second operating position. The movement of the movable carriage 310 along the linear track 320 may be restricted by abutment against a stop (not shown) on the linear track 320 or on the support rod 330 between the first and second positions of the movable carriage 310. The movable carriage 310 may be fixed in place when in the first position and when in the second position.

[0074] The slider 300 may be disposed outside the first chamber 201, i.e., outside the first housing portion 200a. The first chamber 201 is a chamber that encloses the optoelectronic column 110, the sample holder 50, the projector 60, the first detector 70, and the second detector (if present). Similarly, the cooling component 97 (if present) may be disposed outside the first chamber 201, i.e., outside the first housing portion 200a. The arm can extend through a port 210 formed in the housing 200 such that the second end 92 of the arm 90 is positioned outside the first chamber 201 and the first end 91 of the arm 90 is positioned inside the first chamber 201. This means that since the slider 300 and the cooling component 97 are disposed outside the first chamber 201, the space within the first chamber 201 is maximized and the weight within the chamber 201 is minimized. This configuration also minimizes contamination within the first chamber 201 where imaging of the sample takes place. When the projector 60 is in the first operating position or the second retracted position, the projector 60 and the first detector 70 remain within the first chamber 201, thereby maintaining these components under vacuum. The port 210 may include a seal that seals around the portion of the arm 90 that extends therethrough. The seal maintains the vacuum of the first chamber 201 defined by the first portion 200a of the housing even though a portion of the arm 90 translates therethrough. As the arm 90 moves along the longitudinal axis, the arm 90 may be slidably moved within the port 210.

[0075] The second part 200b of the housing 200 that defines the second chamber 202 may be connected to the first part 200a of the housing via a vacuum flange 220, thereby maintaining both the first chamber and the second chamber under vacuum. Components of the SEM 100 that are outside the first chamber 201, namely the slider 300, the cooling component 97, the second end 92 of the arm, and the portion of the arm that extends outside the first chamber 201, may be housed within the second chamber 202. Ports within the housing through which the arm can translate may be formed within the vacuum flange 220.

[0076] In an embodiment where the arm 90 has a rod 95 that extends between the first end 91 and the second end 92, the rod 95 may be received within a port 210 formed within the housing 200, particularly within the vacuum flange 220.

[0077] When the arm 90 is configured with a plurality of rods 95, for example, two rods as shown in FIGS. 2 and 3, each rod 95 may be received within a corresponding port 210 formed in the housing 200. Each port 210 may be an opening formed in the wall of the housing 200 and may be sized such that each respective rod 95 can pass through. Each port 210 is provided with a seal configured to seal around the rod 95, thereby maintaining the vacuum in the first chamber defined by the first part 200a of the housing 200 despite the translation of the rod 95 passing through it. In particular, each port 210 may be an opening formed in the vacuum flange 220.

[0078] As described above, the arm 90 can be configured to thermally conduct heat from the projector 60 and / or the first detector 70.

[0079] In an embodiment where the arm comprises one or more rods 95 extending between its first and second ends, the rod 95 may have a conduit extending therethrough for receiving a coolant therein. The rod 95 can be thermally coupled to the projector 60 and the first detector 70 such that the coolant flowing therethrough conducts heat from the projector 60 and the first detector 70. The rod 95 can include or be formed of a thermally conductive material such as copper. Each rod 95 can comprise a heat pipe extending longitudinally therethrough. For example, each heat pipe may have a wicking material on their inner surfaces. Each heat pipe can be formed of a thermally conductive material such as copper. The wicking material may be a sintered material such as sintered copper powder. As is known in the art, each heat pipe can contain a working fluid therein.

[0080] As will be apparent to those skilled in the art, during operation, the heat received at the first end 91 of the arm due to heating of the projector 60 and / or the first detector 70 is conducted to the heat pipes of the rod 95 and into the wicking material, resulting in the evaporation of the working fluid within the heat pipes to form vapor. This vapor then flows to a relatively cooler portion of the heat pipe proximate the second end of the arm 92, where it condenses on the wick and releases heat. The condensed liquid flows back through the heat pipe by capillary action towards the warmer end of the heat pipe proximate the projector 60. The heat released proximate the second end 92 of the arm 90 can be absorbed by the cooling component 97 from the second end 92 of the arm 90 or otherwise radiated. The cooling component 97 can actively cool the second end 92 of the arm 90, thereby increasing the thermal gradient across the heat pipes within the rod 95 for more efficient heat removal from the projector 60 and / or the first detector 70.

[0081] The effect of the projector 60 on the electron beam in the SEM of the invention according to the claims when the projector is in the first operating position will be described with reference to FIGS. 5 to 9.

[0082] Figure 5 is a plot of the distance (r, where r = 0 mm is the optical axis) from the optical axis of the SEM for electrons diffracted from a sample at different angles (theta), and the distance (z, where z = 0 mm is the downstream end of the objective lens) from the lower pole piece of the optical electron column. In this plot, the SEM has an on-axis detector similar to the above-described first detector 70 arranged along the optical axis, but does not have a projector arranged along the optical axis. Each line on the plot is symmetric with respect to r = 0 mm and represents electrons diffracted at a specific angle by the sample with respect to the optical axis (theta) of the SEM. The electrons diffracted at the minimum angle (10 degrees) form the line closest to r = 0 mm, and the electrons diffracted at the maximum angle form the line farthest from r = 0 mm. The electron path is determined through simulation calculations by an SEM operating in an ultra-high resolution mode using an objective lens that forms a lens which is an immersion magnetic lens, and the electron path is independent of energy.

[0083] Figure 5 is useful as a comparison figure for showing the effect of using the projector 60 along the optical axis of the SEM 100. As shown in this figure, the electron beam is contracted / narrowed by the immersion magnetic field while the SEM 100 is operating in the UHR mode. In this particular example, the sample is positioned at z = 4 mm (i.e., 4 mm from the lower pole piece of the electron optical column 110), and the on-axis detector position starts at z = 40 mm (40 mm from the lower pole piece).

[0084] In the plot shown in FIG. 5, the electron optical column is configured in the same manner as the above-described electron optical column 110 having an objective lens and a condenser module. The objective lens used generates an objective lens that is a magnetic immersion lens. As shown in FIG. 5, the magnetic immersion lens is an immersion magnetic field. When the SEM is operating in the ultra-high resolution mode (UHR mode), this immersion magnetic field is emitted from the lower pole piece of the electron optical column and immerses the sample therein. In this arrangement (i.e., when the projector 60 is not present), the downstream end of the objective lens functions as the upper pole piece, and the portion of the housing that encloses the electron optical column (usually formed of a conductive material such as iron) functions as the lower pole piece of the two-pole magnetic lens. In such an arrangement, the magnetic field emerging from the downstream end of the objective lens penetrates the gap where the sample is positioned (at z = 4 mm) and is confined by the portion of the housing that encloses the electron optical column.

[0085] The immersion magnetic field has two peaks. The strongest peak of the immersion magnetic field is strong and narrow (i.e., more concentrated) and acts as an immersion lens that focuses the beam onto the sample near the lower pole piece of the electron optical column 110 (i.e., near the objective lens). The weaker peak in the immersion magnetic field is wide and weak (i.e., more diffused) and acts as a weak lens that focuses the portion of the electron beam diffracted under the sample. This weak peak acting as a weak lens is undesirable because it limits the diffracted beam and thereby reduces the resolution of the diffracted beam on the detector.

[0086] FIG. 6 is different from FIG. 5 in that a smaller working distance is used (the sample is closer to the objective lens). As can be seen from FIG. 6, the smaller the working distance, the more complex the situation becomes. In this particular example, the sample is positioned at z = 1 mm, i.e., 1 mm from the downstream end of the objective lens, and the on-axis detector position starts at z = 40 mm, i.e., 40 mm from the downstream end of the objective lens. In this situation, there are many foci under the sample. As shown in FIG. 6, the electrons in the electron beam are perturbed due to the immersion magnetic field in the UHR mode.

[0087] When the projector 60 is used and positioned at its first operating position along the optical axis, the projector 60 controls the electron beam downstream of the sample. Instead of relying only on the weaker peak within the immersion magnetic field generated by the objective lens 40, by using the projector 60 to control the electron beam downstream of the sample, the magnetic field that focuses the diffracted beam under the sample can be adjusted to move the focus closer to under the sample. As a result, the size of the diffraction pattern on the first detector 70 increases. In this embodiment of the present invention, the objective lens 40 functions as the upper magnetic pole piece of the two-pole magnetic lens, and the projector 60 functions as the lower magnetic pole piece.

[0088] FIG. 7 is a plot showing the path of electrons from the sample 50 through the projector 60 when the projector 60 is in the first operating position within the SEM 100 (i.e., along the optical axis 101 of the SEM 100) according to the invention claimed. In this figure, r is the distance from the optical axis 101 of the SEM 100 at r = 0 mm, and z is the distance from the downstream end of the objective lens 40 at z = 0 mm. The peak of the axial magnetic field is superimposed on this plot. As can be seen in this figure, the axial magnetic field has a strong peak near the sample at z = 1 mm and a strong peak near the projector gap 63 at z = 12 mm. The increase in the axial magnetic field from z = 32 mm is due to the residual magnetic field.

[0089] The cross-section of the lower pole piece of the electron optical column 110 formed by an exemplary embodiment of the downstream end of the objective lens 40, the sample holder 50, and the projector 60 is also superimposed on this plot. The projector starts at z = 3 mm and the sample is positioned at z = 2 mm. As can be seen from this figure, in this exemplary embodiment, the projector 60 has a single lens, an electromagnetic multipole lens, which is referred to herein as the projector lens. As described in detail above, the projector lens has a shell / yoke 61 that houses a projector lens coil 62 arranged circumferentially around the projector optical axis. As described above, the projector 60 has an upper projector pole piece (upstream of the lower projector pole piece 60b), and the upper and lower projector pole pieces 60a, 60b are separated by a projector gap 63.

[0090] As seen in FIG. 7 and described in more detail below, the electrons are first focused onto the sample by a magnetic field acting as an immersion magnetic lens formed in the gap between the lower pole piece of the objective lens 40 (acting as the upper pole piece of the immersion magnetic lens) and the upper projector pole piece 60a (acting as the lower pole piece of the immersion magnetic lens). Next, the electrons diffracted by the sample are further focused by the projector 60 at or near the projector gap 63 before being diffused (when the projector is in the first position and turned on). The position at which the electrons are focused by the projector 60 depends on the projector coil excitation, and stronger (higher) excitation results in a shorter focal length. For example, in FIG. 7, the projector coil excitation is 1600 Ampere-turn (At), resulting in the focusing of electrons at approximately z = 12 mm. In FIG. 8, the projector coil excitation is 1000 At, resulting in the focusing of electrons at approximately z = 17 mm.

[0091] As is clearly shown by FIG. 8, which is an enlarged version of a part of FIG. 7, a strong peak of the magnetic field near the projector gap 63 (i.e., z = 12 mm) focuses electrons having an energy of 25 kV near but downstream of the projector gap 63 at z = 17 mm. As a result, the electrons can reach a wider spread from the optical axis up to r = 6 mm on the first detector 70 positioned at about z = 40 mm compared to those shown in FIG. 5 for electrons at the same angle (theta).

[0092] The use of the projector can provide magnification up to four times the diffraction pattern. The use of the projector can provide a resolution greater than 160 px / DS (160 pixels per diffraction spot).

[0093] The following provides an explanation of the path of electrons entering, passing through, and exiting the projector 60, as shown in FIGS. 7 and 8. In use, a current is passed through the projector lens coil 62 wound around the projector optical axis inside the yoke 61 to generate a magnetic flux within the yoke 61. The magnetic field from the upper projector pole piece 60a leaks into the vacuum within the yoke 61 due to the presence of the projector gap 63. The leakage magnetic field has a curvature and acts as a lens to converge the electron beam. This intensity can be changed by varying the current in the projector lens coil 62. The leakage magnetic field within the projector gap 63 between the upper and lower projector pole pieces 60a, 60b leaks to the upstream end of the lower projector pole piece 60b, generating a strong magnetic lens as indicated by the peak of the axial magnetic field at z = 12 mm in FIG. 7. As best shown in FIG. 8, the peak of the axial magnetic field near the projector gap 63 focuses the electron beam slightly below the projector gap 63 before the electron beam spreads at a high angle from the optical axis as it travels through the lower pole piece 60b. The branched (spread) electron beam results in an enlargement of the diffraction pattern on the detector at z = 40 mm, thereby increasing its resolution.

[0094] Figure 11 is a graph of the projector coil excitation versus the distance from the optical axis on the detector in the radial direction, i.e., r = 0 mm is the radial position on the detector at the optical axis of the SEM. Each line on the graph represents electrons diffracted by the sample at a specific angle (theta) with respect to the optical axis of the SEM. The minimum angle (1 degree) forms the bottom line of the plot, and the maximum angle (8 degrees) forms the top line of the plot. As shown in FIG. 9, when the projector 60 is switched off (labeled as mode 0 on the plot), the spread of the electrons is lower than when the projector 60 is switched on with a primary coil excitation of 2200 At (labeled as projector mode 2). In fact, r varies from 0.5 to 2 mm in mode 0 and from 1.5 to 4.5 in mode 2. This achieves the maximum expansion of the electron beam on the detector. When the projector coil excitation is 1000 At (labeled as projector mode 1), diffracted electrons with angles up to 8 degrees reach the detector.

[0095] Therefore, there exists an optimal projector lens coil excitation at which the spread of the electron beam is maximized at approximately 1500 - 2500 ampere - turns, preferably 2000 ampere - turns. Also, as described above, there exist optimal dimensions for the projector lens coil and the yoke / shell to reach the optimal excitation while minimizing heating due to mass and high coil current. The optimal design enables quadrupling of the electron beam diffraction with optical excitation of the projector coil.

[0096] Figures 10 to 12 are exemplary diffractograms obtained using SEM of the invention according to the claims where the projector 60 is in the first operating position. The diffraction images were acquired under the same SEM conditions from the same region of the sample. The sample was asbestos crocidolite fibers with a thickness of about 100 nm. The diffraction pattern is seen according to its orientation. The acceleration voltage was 30 kV and the electron beam current was several tens of pA. The images vary depending on the excitation of the applied projector lens coil and thus become increasingly magnified. In Figure 10, the excitation of the projector lens coil was 0 ampere-turns. In Figure 11, the excitation of the projector lens coil was 1600 ampere-turns. In Figure 12, the excitation of the projector lens coil was 2500 ampere-turns. As can be seen from the comparison of these figures, the diffraction patterns in the figures are the same, but they are magnified as the excitation of the projector lens coil increases. The diffraction images were rotated by software post-processing to remove the lens-induced image rotation between them.

[0097] It will be understood that the above-described embodiments are for illustrative purposes only and that the present invention is not so limited. Those skilled in the art will envision various modifications and alternative forms included within the scope of the claims.

[0098] All aspects and / or features disclosed herein can be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. In particular, the preferred features of the present disclosure are applicable to all aspects and embodiments of the present disclosure and can be used in any combination. Similarly, features described in non-essential combinations can be used separately (rather than in combination).

[0099] Throughout the specification and claims of the present disclosure, words such as "comprise", "including", "having", and "contain", and variations of the words, such as "comprising" and "comprises" or the like, mean "including but not limited to", and are not intended to exclude other components. but not limited to) and are not intended to exclude other components.

[0100] Any and all examples, or the use of exemplary language (such as the language of "for instance", "such as", "for example", etc.) provided in this specification are merely for better explaining the present invention and do not indicate a limitation of the scope. Unless otherwise claimed, they are within the scope of the present invention. No word in this specification should be construed as indicating an element essential for the implementation of the present invention.

[0101] The projector 60 is described as having a single lens (i.e., only one lens) to minimize its weight and size, but alternatively, the projector 60 may have a plurality of lenses.

[0102] The projector lens is mainly described as an electromagnetic multi-pole lens, but other types of projector lenses, such as circular lenses, may be used.

[0103] The embodiment shown in FIGS. 2 and 3 employs a first detector 70 fixed to the projector 60 such that the first detector 70 moves with the projector 60. However, the first detector 70 may instead be fixed at a position along the optical axis of the SEM under the sample holder 50. In such an arrangement, when the projector 60 is in the first operating position, the projector 60 can move relative to the first detector 70 such that the first projector 60 is between the sample holder 50 and the first detector 70 along the optical axis of the SEM. In such an arrangement, electrons that have passed through the sample can still be imaged using the first detector 70 even if the projector 60 is retracted.

[0104] The embodiment shown in FIGS. 2 and 3 uses two rods 95 within an arm 90 having a cylindrical cross-section, but any number of rods 95 of any cross-section 90 can be used.

[0105] The embodiment shown in FIGS. 2 and 3 employs an arm 90 having rods 95 coupled to each other and first and second ends 91, 92, but the rods 95 of the arm 90 and the first and second ends 91, 92 may be integrally formed.

[0106] The embodiment shown in FIGS. 2 and 3 includes a slider 300 for transmitting linear motion to the arm 90, but other means for linearly moving the arm along the longitudinal axis 93 may be contemplated.

[0107] The above description relates to using a retractable (cooled) projector 60 in the SEM 100. However, the projector 60 can be used similarly in other types of electron microscopes. It should be understood that the teachings regarding the above projector 60 and first detector 70 can be similarly applied to such other electron microscopes, but are not described in this section to avoid repetition.

[0108] For example, the retractable projector 60 can be used in a dual beam system such as a dual beam FIB-SEM (optionally together with the first detector 70).

[0109] As a further example, the retractable projector 60 may be used (optionally together with the first detector 70) in a transmission electron microscope. More specifically, a transmission electron microscope comprising: an electron source, a sample holder for holding a sample to be analyzed, a projector, a first detector, and wherein each of the electron source and the sample holder is disposed on the optical axis of the transmission electron microscope, the projector is movable between a first operating position where the projector is located downstream of the sample holder and along the optical axis between the sample holder and the first detector, and a second retracted position where the projector is located away from the optical axis, is contemplated.

[0110] Also, a method of imaging a sample using a transmission electron microscope, wherein the transmission electron microscope comprises an electron source, a sample holder, a projector, and a first detector, and the electron source and the sample holder are disposed on the optical axis of the transmission electron microscope, the method comprising: placing a first sample on the sample holder of the microscope, moving the projector from the second retracted position to the first operating position, wherein in the first operating position, the projector is located downstream of the sample holder and along the optical axis between the sample holder and the first detector, in the second retracted position, the projector is located away from the optical axis, scanning a first electron beam generated by the electron source along the first sample to generate a first image, using the projector in its first operating position to project the first image onto the first detector. A method is contemplated that includes projecting an electron diffraction pattern formed by electrons that have passed through and diffracted from a sample.

[0111] The above description of the projector and the first detector can be similarly applied to this transmission electron microscope. For example, the projector and the first detector may be coupled to each other such that the projector and the first detector move together between the first operating position and the second retracted position as described above. The transmission electron microscope may also employ one or more second detectors that can be used to image the sample when the projector and the first detector are in the second retracted position.

Claims

1. A scanning electron microscope comprising: an electron source; a sample holder for holding a sample to be analyzed; a projector; and a first detector, wherein each of the electron source and the sample holder is disposed on the optical axis of the scanning electron microscope, the projector is movable between a first operating position where the projector is located downstream of the sample holder and along the optical axis between the sample holder and the first detector, and a second retracted position where the projector is located away from the optical axis. Scanning electron microscope, characterized in that.

2. The scanning electron microscope according to claim 1, wherein the projector is movable between the first operating position and the second retracted position in a plane orthogonal to the optical axis.

3. The scanning electron microscope according to claim 1 or 2, wherein the projector is linearly movable between the first operating position and the second retracted position.

4. The scanning electron microscope according to any one of claims 1 to 3, further comprising an arm operable to move the projector between the first operating position and the second retracted position.

5. The arm is configured to move linearly when moving the projector between the first operating position and the second retracted position, and optionally, the scanning electron microscope further comprises a slider configured to transmit linear motion to the arm. The scanning electron microscope according to claim 4.

6. The scanning electron microscope according to claim 4 or 5, wherein the arm extends longitudinally in a plane orthogonal to the optical axis between a first end and a second end, and the first end is coupled to the projector.

7. The arm is thermally coupled to the projector and is configured to cool the projector, and optionally, the arm includes one or more rods having cavities configured to receive a coolant therein, and further optionally, each rod includes a heat pipe. The scanning electron microscope according to any one of claims 4 to 6.

8. The scanning electron microscope according to any one of claims 1 to 7, wherein the first detector is coupled to the projector and is movable with the projector between the first operating position and the second retracted position.

9. The first detector is fixed relative to the projector and is spaced apart from the projector such that the projector is between the sample holder and the first detector at the first operating position. The scanning electron microscope according to claim 8.

10. The arm includes a holder configured to hold the projector and the first detector such that the first detector is fixed relative to the projector. The scanning electron microscope according to claim 8 or 9 when dependent on any one of claims 4 to 7.

11. The scanning electron microscope according to claim 10 when dependent on claim 7, wherein the arm is thermally coupled to the projector and the first detector and is configured to cool the projector and the first detector.

12. The projector includes one or more projector lenses. Optionally, the one or more projector lenses are electromagnetic lenses. Further optionally, the one or more projector lenses are multipole lenses. The scanning electron microscope according to any one of claims 1 to 11.

13. The first detector is a pixelated detector. The scanning electron microscope according to any one of claims 1 to 12.

14. The scanning electron microscope further includes a second detector. Optionally, the second detector is disposed upstream from the sample holder. Further optionally, the second detector is a BSE detector or a secondary electron detector. The scanning electron microscope according to any one of claims 1 to 13.

15. A method of imaging a sample using a scanning electron microscope, the scanning electron microscope including an electron source, a sample holder, a projector, and a first detector, the electron source and the sample holder being disposed on an optical axis of the scanning electron microscope, the method comprising: placing a first sample on the sample holder of the microscope; moving the projector from a second retracted position to a first operating position, wherein at the first operating position, the projector is located along the optical axis downstream of the sample holder and between the sample holder and the first detector; and moving the projector to a position away from the optical axis at the second retracted position. ​ To generate a first image, scanning a first electron beam generated by the electron source along the first sample; using the projector in its first operating position to project the first image onto the first detector, wherein the first image is an electron diffraction pattern formed by electrons transmitted through the sample and diffracted by the sample. **Claim 16** The scanning electron microscope further includes a second detector, the second detector is disposed upstream of the sample holder, and the second detector is configured to detect backscattered electrons and / or secondary electrons diverging from the sample; The method exchanging the first sample with a second sample on the sample holder of the microscope; moving the projector from the first operating position to the second retracted position; scanning a second electron beam generated by the electron source along the second sample to generate backscattered electrons and / or secondary electrons; receiving the backscattered electrons and / or secondary electrons by the second detector to form an image of the second sample. The method according to claim 15, further comprising:

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