Improved scanning electron microscope and methods of use thereof

Abstract

The present disclosure relates to improved scanning electron microscopes and methods of using the same. 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 is arranged on an optical axis of the scanning electron microscope. The projector is movable between a first operative position in which the projector is positioned downstream of the sample holder and between the sample holder and the first detector along the optical axis, and a second retracted position in which the projector is positioned away from the optical axis. The invention also provides a method for imaging a sample by using the scanning electron microscope.

Description

Technical Field

[0001] The present invention as claimed relates to a scanning electron microscope (SEM) enabling coaxial diffraction imaging with higher resolution. 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 types of electron microscopes have evolved into many well-known equipment categories such as transmission electron microscopes (TEMs), scanning electron microscopes (SEMs), and scanning transmission electron microscopes (STEMs), and also into various sub-categories such as so-called "dual-beam" devices (such as, for example, FIB-SEMs) which additionally employ a "machining" focused ion beam (FIB), thus allowing, for example, supportive activities such as ion beam milling or ion beam induced deposition (IBID). Those skilled in the art will be familiar with the different categories of charged particle microscopy.

[0003] In an SEM, the irradiation of a sample by a scanned electron beam causes "secondary" radiation to emanate from the sample in the form of secondary electrons, backscattered electrons, X-rays, and cathodoluminescence (infrared, visible, and / or ultraviolet photons). One or more components of this emitted radiation are then detected and used for image accumulation purposes and / or spectral analysis (as in the case of, for example, EDX (energy-dispersive X-ray spectroscopy)). If the sample is thin enough, the transmitted electrons can also be detected, and this is referred to as SEM-based scanning transmission electron microscopy (SEM-based STEM). In an SEM, the scanned electron beam typically has an energy ranging from 0.2 keV to 30 keV.

[0004] In a TEM, the electron beam used to irradiate the sample is chosen to have a high enough energy to penetrate the sample (for this purpose, the sample will typically be thinner than in the case of an SEM sample). Then, the transmitted electron current emanating from the sample can be used to create an image, or to generate a spectrum (as in the case of electron energy loss spectroscopy (EELS)). If such a TEM is operated in a scanning mode (i.e., in TEM-based STEM), the image / spectrum under consideration will be accumulated during the scanning movement of the irradiating electron beam. In a TEM, the electron beam typically has an energy ranging from 60 keV to 300 keV.

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

[0006] It is known that a TEM can be advantageously switched between its normal imaging mode and diffraction mode to analyze defects and crystal structures. Diffraction techniques within an SEM typically involve electron backscattering techniques. The concept of coaxial transmission diffraction in an SEM remains relatively new. Transmission diffraction techniques within an SEM include ptychography and transmission Kikuchi diffraction (TKD). Recently, pixelated detectors have provided new possibilities for detecting coaxial transmission diffraction patterns using an SEM. However, such techniques are limited by the achievable resolution. Summary of the Invention

[0007] According to a first aspect of 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 is arranged on the optical axis of the scanning electron microscope. The projector is movable between a first operating position and a second retracted position, in the first operating position the projector is positioned downstream of the sample holder and along the optical axis between the sample holder and the first detector, and in the second retracted position the projector is positioned 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 arranged (i.e., aligned / coaxial / centered therewith). In use, a primary electron beam generated by the electron source is emitted along the optical axis of the SEM from the electron source. Throughout this specification, the indexing of upstream / downstream refers to the path of the electron beam through the SEM. The path of the electrons is from the electron source towards the sample holder, such that, for example, the electron source is upstream of the sample holder.

[0009] A component being arranged or positioned along the optical axis means that the optical axis passes through the component, preferably, the component is centered / coaxial / aligned with the optical axis.

[0010] The projector in a known TEM magnifies the diffracted beam onto the detector and improves the resolution of the resulting diffraction pattern. However, known projectors within a TEM typically comprise multiple magnetic lenses having a significant weight and a total length of about 50 cm, thereby precluding 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 claimed invention, a projector positioned in a first operating position downstream of a sample holder and along the optical axis of the SEM between the sample holder and a first detector is combined to improve the resolution of the coaxial transmission diffraction pattern of a thin sample. Such thin samples can have a thickness ranging from 1 atomic layer to several hundreds of 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 also be used to image thicker samples using a backscattered electron 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 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 the transmitted electrons emitted from the sample will pass through the projector and then be transmitted to the first detector. Accordingly, the first detector can detect transmitted coaxial 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 discussed above, the indexing of "positioned along the optical axis" in this context means that the optical axis bisects or preferably is concentric / 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 directly located between the sample holder and the first detector such that there are no other intervening components therebetween.

[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 the transmitted electrons emitted from the sample will not pass through the projector. This enables imaging of thicker samples because when the projector is positioned away from the optical axis of the SEM, there is more space along the optical axis between the sample holder and the detector to accommodate a thicker sample.

[0014] The SEM can include one or more second detectors. The second detector can be a backscattered electron (BSE) detector and / or a secondary electron (SE) detector. BSE and SE detectors and their arrangement within the SEM are known in the art. As is known in the art, the BSE detector and / or the SE detector can be positioned upstream of the sample holder. The BSE detector can 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 located 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 can be coupled to the projector and can move with the projector as the projector moves between its first operating position and its second retracted position. The first detector can be fixed relative to the projector and spaced apart from the projector such that the projector is between the sample holder and the first detector when the projector is in the first operating position. The first detector can be spaced apart from the projector such that the first detector is downstream of the projector. The first detector can be spaced apart from the projector in a direction parallel to or along the optical axis of the projector (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 passes through both the first detector and the projector. In other words, the first detector can overlap the projector and be downstream of the projector such that electrons passing through the projector strike the first detector. This configuration means that when the projector is in the first operating position, the first detector can detect transmitted electrons emitted from the sample. The first detector can be fixed relative to the projector such that the effective 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 effective area of the detector is aligned with the projector optical axis (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 pass through the projector or the first detector. Moving the projector and the first detector away from the optical axis provides more space for accommodating thicker samples. The SEM can then be operated in its normal imaging mode, whereby the thicker sample is imaged based on secondary electrons and backscattered electrons emitted from the sample. In this arrangement, when imaging a thicker sample, a second detector such as a BSE or SE detector can be used, as is known in the art. By way of example, such a thicker sample can have any thickness so long as it can be geometrically adapted to the sample holder of the SEM according to the typical use of the SEM. For example, a thicker sample can have a thickness of up to several centimeters.

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

[0018] The scanning electron microscope may also include an arm operable to move a projector between a first operating position and a second retracted position. The arm may 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 may 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 between a first end and a second end in a plane orthogonal to the optical axis of the SEM, wherein the first end is coupled to the projector. The arm may have a fixed length and may be rigid such that it does not deform in use. The arm may 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 along the length of the arm and the axis along which the arm moves. The longitudinal axis may be perpendicular to the optical axis of the SEM.

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

[0021] Optionally, the scanning electron microscope further includes a slider configured to transfer linear motion to the arm. The slider may be coupled to the second end of the arm. The slider may include a movable carriage and a track. The second end of the arm may include a mounting configured to couple the arm to the movable carriage. The second end of the arm may be fixed relative to the movable carriage such that linear movement of the movable carriage within the track causes linear movement of the arm.

[0022] The projector and / or the first detector can be cooled. Cooling the projector and the first detector prevents heat accumulation, which otherwise would lead to increased noise and reduced stability, thereby degrading the performance of the projector and the first detector. Specifically, 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 is configured to cool the projector and / or the first detector. The projector and the first detector can be thermally coupled together. The arm can be configured to conduct heat away from the projector and / or the first detector. The arm can include a thermally conductive material, such as copper. The arm can be configured to conduct heat from the projector and / or the first detector along its length to a second end of the arm at its first end. The second end of the arm can include 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. Cooling the second end of the arm results in a thermal gradient between the first end and the second end of the arm to conduct heat away from the projector and / or the first detector. The cooling component can 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 employ a pump or a fan for the circulation of coolant therein. The cooling component can be, for example, a heat exchanger, a radiator, or a closed-loop system that recirculates coolant therein.

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

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

[0025] The objective lens generates a magnetic field referred to as the objective lens magnetic field, which 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, the one or more projector lenses may be electromagnetic lenses. Optionally, the one or more projector lenses may be multipole lenses. Optionally, the one or more projector lenses may be circular lenses. Specifically, the projector may include a single lens (i.e., one lens), more specifically consisting of a single lens, thereby minimizing the weight, size, and cost of the projector, which is particularly important for incorporation into an SEM. Specifically, the single lens may be an electromagnetic lens. Minimizing the size is particularly important because in an SEM system, the space within the chamber that houses the sample (referred to herein as the first chamber) is restricted, thereby limiting the height of the projector. The claimed invention achieves enhanced optical performance in an SEM without compromising size, weight, and cooling capabilities. In contrast, in a TEM, there are no size or weight restrictions on the projector because the projector will be placed outside the chamber that houses the sample.

[0027] In an embodiment where the projector has a single lens, the single lens may be an electromagnetic multipole lens. In such an embodiment, the projector lens may have a housing that houses projector lens coils circumferentially arranged about 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, optionally where the upper projector pole piece and the lower projector pole piece include cylindrical inlet portions upstream of frustoconical portions, further optionally where 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 can be used with a projector coil excitation of, for example, 1000 to 2000 ampere-turns, preferably 1500 to 2500 ampere-turns.

[0029] The first detector can be a pixelated detector. The pixelated detector can include a pixel grid having a plurality of pixels. It is particularly advantageous to use a projector within an SEM having a pixelated detector because the pixelated detector enables the need to magnify the diffracted beam to 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 ptychographic coherent diffraction imaging, strain analysis, micro-ED, and TKD. Since such pixelated detectors generate a large 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 including an electron source, a sample holder, a projector, and a first detector, wherein the electron source and the sample holder are arranged 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 projecting the first image onto the first detector using the projector in its first operating position. In the first operating position, the projector is positioned downstream of the sample holder and along the optical axis between the sample holder and the first detector, and in 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 onto the first detector, i.e., spreading the electrons that have transmitted through the sample and been diffracted by the sample.

[0032] The teachings of the above-described features of the scanning electron microscope apply equally to the method.

[0033] As described above, the scanning electron microscope may further include a second detector, wherein the second detector is arranged upstream of the sample holder, and wherein the second detector is configured to detect backscattered electrons and / or secondary electrons emitted from the sample. The method may further include: replacing 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; and receiving the backscattered electrons and / or secondary electrons at the second detector to form an image of the second sample.

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

[0035] For example, the first sample can have a thickness of from 1 atomic layer thick 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 thickness of up to 1×10 -1 m.

[0036] Thus, the method enables imaging of thinner samples in coaxial transmission diffraction imaging mode with improved resolution due to the projector in an SEM, and imaging of thicker samples in normal SEM mode (using backscattered electrons and secondary electrons).

[0037] Alternatively, the method may not involve the step of replacing the first sample with a second sample, and may image the first sample first using the projector and the first detector and subsequently using the second detector. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a schematic diagram of the optical components of an SEM 100 according to the claimed invention and the path of the electron beam passing therethrough, the SEM 100 having an electron optical column 110, a sample holder 50, a projector 60, and a first detector 70, wherein the projector 60 is arranged in a first operating position.

[0039] Figure 2 is a schematic diagram of a part of an SEM 100 according to the claimed invention, wherein the housing 200 has been partially removed from the view and the projector 60 is in its first operating position. Figure 2 Includes a slider mechanism 300 and an arm 90 for moving the projector 60 of the SEM 100 between its first operating position and its second retracted position. For clarity, Figure 1 the electron optical column 110 of the SEM 100 shown in Figure 2 has been omitted from

[0040] Figure 3 is similar to Figure 2 , except that the projector 60 is in its second retracted position.

[0041] Figure 4 is a perspective view of an SEM 100 according to the claimed invention, wherein the projector 60 is in its second retracted position. Similar to Figure 2 and Figure 3 and Figure 4Includes a slider 300 and an arm 90 for moving the projector between its first operating position and its second retracted position. For clarity, the optical column 110 and the sample holder 50 are omitted. For clarity, parts of the housing enclosing the optical column 110, the sample holder 50, the projector 60, and the first detector 70 are also omitted.

[0042] Figure 5 Is a plot of the distance from the optical axis (r = 0 mm) versus the distance from the lower pole piece of the optical electron column (z = 0 mm) for electrons diffracted from a sample within an SEM at different angles (θ). The SEM has a detector arranged along the optical axis of the SEM but does not have a projector arranged along the optical axis of the SEM. Each line on the plot represents electrons diffracted from the sample at a certain angle (θ) with respect to the optical axis of the SEM. The paths of these electrons are determined by simulation calculations. 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), and the coaxial detector position starts at z = 40 mm (40 mm from the lower pole piece of the electron optical column).

[0043] Figure 6 Is a plot of the distance from the optical axis (r = 0 mm) versus the distance from the lower pole piece of the optical electron column (z = 0 mm) for electrons diffracted from a sample within an SEM at different angles (θ). The SEM has a detector arranged along the optical axis of the SEM but does not have a projector arranged along the optical axis of the SEM. Each line on the graph represents electrons diffracted from the sample at a certain angle (θ) with respect to the optical axis of the SEM. The paths of these electrons are determined by simulation calculations. In this particular example, the sample is positioned at z = 1 mm (i.e., 1 mm from the lower pole piece of the electron optical column), and the coaxial detector position starts at z = 40 mm (40 mm from the lower pole piece of the electron optical column). Figure 6 And Figure 5 Differs in that a shorter working distance is employed (1 mm instead of 4 mm).

[0044] Figure 7 Is a plot demonstrating the path of electrons from the lower pole piece of the electron optical column 110 through the projector 60 to the first detector 70 when the projector 60 is in the first operating position in the SEM 100 (i.e., along the optical axis 101 of the SEM 100). The paths of these electrons are determined by simulation calculations. In this plot, r is the distance from the optical axis of the SEM 100 (at r = 0 mm), and z is the distance from the lower pole piece of the electron optical column 110, i.e., the distance from the downstream end of the objective lens 40 (at z = 0 mm). The peak of the axial magnetic field and the cross-sections of the lower pole piece of the electron optical column 110, the sample holder 40, and the projector 60 are superimposed on this plot.

[0045] Figure 8 is Figure 7 an enlarged version of a part of.

[0046] Figure 9 is a graph of the projector lens coil excitation versus the radial position of electrons on the detector, where the radial position is measured in mm from the SEM optical axis at r = 0 mm. Each line on the graph represents electrons diffracted by the sample at an angle (θ) with respect to the optical axis (θ) of the SEM. The paths of these electrons are determined by simulation calculations.

[0047] Figures 10 to 12 is an exemplary diffraction image obtained using the claimed SEM of the present invention (where the projector 60 is in the first operating position). These diffraction images are obtained under the same SEM conditions. These diffraction images are taken from the same region of the sample. The sample is crocidolite asbestos fibers having a thickness of approximately 100 nm. These images differ due to the excitation of the projector lens coil. For Figure 10 , the excitation of the projector lens coil is 0 ampere-turns. For Figure 11 , the excitation of the projector lens coil is 1600 ampere-turns. For Figure 12 , the excitation of the projector lens coil is 2500 ampere-turns. Detailed Description

[0048] Figure 1 is a schematic diagram of the imaging components of the claimed SEM 100 of the present invention and the electron path through the SEM 100. The SEM has an optical axis 101.

[0049] The SEM 100 includes a housing 200 (as Figure 2 shown), which has a first housing portion 200a defining a first chamber 201 that contains an electron source 10, a condenser module 20, a scanning coil 30, an objective 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, in use, the electron beam is directed from the electron source 10 towards the sample holder 50 in the downstream direction. The path of the electron beam within the SEM is schematically shown by the thin solid line in Figure 1 .

[0050] Both the electron source 10 and the sample holder 50 are arranged on the optical axis of the SEM, i.e., coaxial with the optical axis (centered along the optical axis).

[0051] The electron source 10 can 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 generally lies between 0.2 eV and 40 keV, preferably between 0.2 eV and 30 keV. The primary beam emitted from the electron source 10 is aligned with the optical axis 101 of the SEM 100, i.e., coaxial with this optical axis.

[0052] Downstream of the electron source is the condenser module 20, which manipulates the primary electron beam from the electron source 10. The condenser module 20 includes at least one condenser lens that converges the electron beam as it travels towards the sample holder 40. The condenser lens is typically an electromagnetic lens that uses a magnetic field to control the direction of electrons within the electron beam. The condenser module 20 may include other lenses or other particle optical components that can perform, for example, aberration reduction, cropping (using a diaphragm / aperture / condenser aperture), filtering, etc.

[0053] Downstream of the condenser module 20 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), as is known in the art. The scanning coils 30 are controlled by a controller (not shown).

[0054] Downstream of the scanning coils 30 is the 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 that can be an electromagnetic lens. Specifically, the objective lens can be an electromagnetic immersion lens.

[0055] 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 can be aligned with the optical axis 101 of the SEM 100, i.e., centered on this optical axis. The electron optical column 110 can be mounted to the inner surface of the first part 200a of the housing, thereby fixing the electron optical column 110 in place within the first chamber 201. The downstream end of the objective lens 40 forms the lower pole piece of the electron optical column 110.

[0056] Downstream of the electron optical column is a sample holder 50 that supports a sample. The sample holder 50 can be tilted or rotated under the control of a controller and a motor (not shown) coupled to the sample holder 50, as is known in the art. The sample holder 50 is aligned with / centered on / disposed on the optical axis 101 of the SEM 100. The optical axis 101 of the SEM 100 passes through the sample holder 50, specifically, through the center of the sample holder 50. The sample holder 50 can be positioned in a first portion 200a of the housing 200 of the SEM 100. The sample holder 50 can be fixed relative to the first portion 200a of the housing 200. … The sample holder 50 can be held in alignment with the optical axis 101 of the SEM 100 such that the optical axis 101 still passes through the sample holder despite tilting or rotation of the sample holder 50.

[0057] The SEM 100 can optionally include a second detector (not shown), which can be a backscattered electron detector and / or a secondary electron detector. The backscattered electron detector can 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 can be arranged to be offset from the optical axis 101 of the SEM 100 and upstream of the sample holder 50, as is known in the art. The backscattered electron detector can be a scintillation detector or a solid state detector. The secondary electron detector can be, for example, an Everhart - Thornley detector employing a scintillator within a Faraday cage.

[0058] The claimed SEM of the present invention differs from known SEMs in that it employs a projector 60 and a first detector 70 downstream of the projector 60. When the projector 60 is in a first operating position as shown Figure 1 in the figure, the projector 60 is disposed downstream of the sample holder 40 and directs transmitted electrons emitted 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 and project the diffraction pattern onto the first detector 70. The projector lenses can be electromagnetic lenses, and the magnetic field strength thereof can be adjusted to adjust their focal length and thus the magnification. Projectors within known TEMs are used to perform magnification. However, such projectors are large, heavy, and include multiple lenses. The projector 60 in the claimed present invention can advantageously employ a single projector lens, optionally an electromagnetic lens, thereby minimizing weight and size. In other words, the projector 60 can consist of a single projector lens.

[0059] In an embodiment where the projector has a single projector lens, the single projector lens can be an electromagnetic multipole lens.Figure 7 A cross-section of an example of such a projector lens is shown. The projector lens in this example has a housing / yoke 61 that houses a projector lens coil 62 arranged circumferentially around the projector optical axis.

[0060] As shown in this figure, the projector lens may have an upper projector pole piece 60a and a lower projector pole piece 60b separated by a gap (referred to herein as the projector gap 63). The projector gap 63 may be defined between the upper projector pole piece 60a and the lower projector pole piece 60b and is defined by the projector lens coil 62 extending circumferentially around the projector gap 63. The projector gap 63 is centered on the projector optical axis. The upper projector pole piece 60a defines a first path for the electron beam, and the lower projector pole piece 60b defines a second path for the electron beam. These paths are joined together through the projector gap 63. The projector gap 63 reduces spherical aberration and thus improves image resolution.

[0061] 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 is in 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, that is, the upper projector pole piece 60a and the lower projector pole piece 60b have diameters that increase in the direction from upstream to downstream. Optionally, the frustoconical portion of the lower projector pole piece 60b has a greater height and / or is inclined at a greater angle than the frustoconical portion of the upper projector pole piece 60a.

[0062] In this exemplary embodiment, each of the upper projector pole piece 60a and the lower projector pole piece 60b may be defined by the walls of the yoke 61. The upstream wall of the yoke 61 may define the upper pole piece 60a, and the downstream wall of the yoke 61 may define the lower pole piece 60b. The walls of the yoke 61 may be tapered to produce the frustoconical portions of the upper pole piece 60a and the lower pole piece 60b.

[0063] This exemplary configuration of the projector lens in the described projector 60 is by way of example and not intended to be limiting. Other shapes of multipole lenses and other types of lenses such as circular lenses may alternatively be employed in the projector 60.

[0064] As Figure 1As shown, 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 that can detect individual electrons incident on the detector to generate a diffraction image. The pixelated detector may include a two-dimensional pixel arrangement. Pixelated detectors typically utilize semiconductor diode arrays and complementary metal oxide semiconductor (CMOS) technology. These devices comprise a pixelated detector array (e.g., typically 256×256 pixels, but other sizes are possible) closely coupled to a CMOS preamplifier array. The small capacitance of each pixel (typically only about 55μm×55μm in size) and the close coupling of the preamplifier to each individual pixel allow the detector to pulse count the individual electrons arriving at each pixel. The large number (e.g., 64,000) of independent pixels in the detector array allows for extremely high counting rates. Examples of such detectors include MEDIPIX RTM and TIMEPIX RTM detectors in the series. Such pixelated detectors may be referred to as active pixelated detectors.

[0065] In Figure 1 an embodiment, the projector 60 is in a first operating position such that the optical axis of the SEM passes through both the projector 60 and the first detector 70. Specifically, the projector 60 is centered on the optical axis of the SEM 100 and thus on 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 axis 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 and emitted from the electron source along the optical axis of the SEM 100, then the electron beam is focused by the condenser module 20 and the objective lens generated by the objective 40, and then scanned over the sample in a plane orthogonal to the optical axis under the control of the scanning coils 30. The electrons transmitted through the sample on the sample holder 50 then pass through the projector lens 60 and strike the first detector 70. The projector 60 magnifies the resulting diffraction pattern onto the first detector 70.

[0066] As shown by comparing Figure 2 and Figure 3 the projector 60 is capable of moving between a first operating position (as shown in Figure 2 ) and a second retracted position (as shown in Figure 3 ). The projector 60 is capable of moving relative to the sample holder 50, the electron source 10, the condenser module 20, and the objective 40 of the SEM 100.

[0067] In Figure 2 and Figure 3In the illustrated embodiment, the first detector 70 is optionally fixed relative 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 (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 (usually through the center of the projector 60). In other words, the detector 60 is fixed downstream of the projector 60 such that the electrons passing through the projector 60 strike / impact the first detector 70. More specifically, the first detector 70 is fixed relative to the projector 60 such that the effective area of the first detector 70 configured to receive electrons thereon (in the case of a pixelated detector, this effective area contains pixels) overlaps with the projector optical axis and is preferably centered on the projector optical axis. The first detector 70 is capable of moving 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.

[0068] As Figure 3 shown, when the projector 60 is in the second retracted position, both the projector 60 and the first detector 70 are positioned away from the optical axis 101 of the SEM 100, i.e., 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 such that any electrons transmitted through the sample do not pass through the projector 60 or the first detector 70. Specifically, when the projector 60 is in the second retracted position, the projector 60 is offset from the optical axis 101 of the SEM 100 such 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 used with the projector 60 in the second retracted position and the first detector 60 fixed relative to the projector 60 and located downstream thereof, the 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 for imaging the sample.

[0069] When the projector 60 is in the second retracted position such that 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 can be imaged based on secondary electrons and backscattered electrons emitted from the sample and detected using a second detector (not shown). As described above, the second detector can be a BSE detector and / or an SE detector. Imaging the sample using secondary electrons and backscattered electrons emitted from the sample can be referred to herein as the normal SEM imaging mode. Thus, by moving the projector 60 (along with the first detector 70 fixed relative to the projector 60) between the first operating position and the second retracted position, the SEM can be switched from the STEM coaxial diffraction mode (scanning transmission electron microscope coaxial diffraction mode) for thinner samples, such as those having a thickness ranging from 1 atomic layer up to 1×10 -7 m thick, to the normal SEM imaging mode for thicker samples, such as those having a thickness limited only by the geometric constraints of the sample holder. In the normal SEM imaging mode, there is more space along the optical axis 101 of the SEM to accommodate a thicker sample because the projector 60 and the first detector 70 are away from the optical axis.

[0070] As Figure 2 and Figure 3 shown, the projector 60 is capable of moving from its first operating position (coaxial position) to its second retracted position (off-axis position) in a plane orthogonal to the optical axis 101 of the SEM 100. Specifically, the projector 60 is capable of linearly moving between the first operating position and the second retracted position.

[0071] In Figure 2 and Figure 4 the illustrated embodiment, the projector 60 is optionally coupled to an arm 90 that is operable to move the projector 60 between the first operating position and a 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 can have a fixed length and be rigid such that it does not deform during use.

[0072] The arm 90 can be configured to move linearly when moving the projector 60 between the first operating position and the second retracted position. The axis along which the arm extends and along which the arm moves is referred to herein as the longitudinal axis 93 and is perpendicular to the optical axis 101 of the SEM100. The arm 90 can be fixed relative to the projector 60 such that movement of the arm 90 along the longitudinal axis 93 causes the projector 60 to move along or parallel to the longitudinal axis 93 between the first operating position and the second retracted position. More specifically, the arm can be 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 can be driven by an actuator (not shown).

[0073] The first end 91 of the arm 90 can be coupled to the projector 60 and the first detector 70 such that the projector 60 does not move relative to the arm 90, as Figures 2 to 4 shown. The arm can be configured to support the projector 60. The arm 90 can be configured to support both the projector and the first detector 70. Optionally, the first end 91 of the arm 90 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 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. The projector 60 and the first detector 70 being 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 employ one or more sockets or clamps for receiving and holding the projector 60 and the first detector 70. The holder 94 can hold (i.e., hold and support) the first detector 70 spaced apart from and 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 can optionally be integrally formed. In such an embodiment, the first detector 70 can be attached to the holder 94, for example, using fixing means such as screws. Such an arrangement enables replacement or repair of the detector 70.

[0074] As discussed in further detail below, the arm can be thermally coupled to the projector 60 and / or the first detector 70 and is configured to conduct heat away from the projector 60 and / or the first detector 70. The arm can be configured to conduct heat thermally from the first end of the arm along the length of the arm to the second end 92 of the arm 90, thereby removing heat from the projector 60 and / or the first detector 70. Optionally, as Figures 2 to 4 shown, 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 and the second end of the arm. The cooling component 97 can be a heat exchanger, a radiator, a closed-loop system, or other components configured to absorb or otherwise remove heat. The cooling component 96 can be an active cooling component that employs a pump or a fan to drive a cooling fluid to flow therein. Specifically, the cooling component 96 can be an actively cooled heat exchanger. The heat exchanger can be actively cooled by an actively circulating closed-loop cooling water circuit.

[0075] Optionally, the longitudinal portion of the arm 90 extending between the first end 91 and the second end 92 of the arm 90 can be formed by one or more rods 95, as Figures 2 to 4 shown. In other words, the arm 90 can include a first end 91, a second end 92, and one or more rods 95 extending therebetween. In Figure 2 andFigure 3 In the illustrated embodiment, the arm 90 optionally includes two rods 95 that are parallel to the longitudinal axis 93 and extend between the first end 91 and the second end 92 of the arm 90. Although two rods 95 are shown in the Figure 1 and Figure 2 embodiment, any number of rods 95 can be employed in 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. Employing at least two rods 95 instead of a single rod 95 facilitates increased heat transfer from the projector 60 and the first detector 70. Employing at least two rods 95 also facilitates reducing the bending of the arm 90 due to the weight of the projector 60.

[0076] The rods can be coupled to the first end 91 of the arm and the second end 92 of the arm 90 in a variety of ways. For example, the rods 95 can be welded 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 can include corresponding receivers that are complementary to the respective ends of the rods 95. The corresponding receivers can be, for example, threaded channels. As other alternatives, the rods 95 can be integrally formed with the first end 91 and the second end 92 of the arm 90.

[0077] The second end 92 of the arm 90 can be configured to be coupled to the slider 300, as Figures 2 to 4 shown. More specifically, the second end 92 of the arm can be fixed relative to a portion of the slider 300 such that movement of the slider 300 is transmitted to the arm 90. The slider can be configured to transmit linear movement to the arm such that the arm moves along the longitudinal axis. As described above, movement of the arm 90 along the longitudinal axis 93 causes movement of the projector 60 between its first operating position and its second retracted position.

[0078] The slider 300 is optionally a linear slider. Any suitable slider that can transmit linear motion to the arm 90 can be employed. Movement of the slider 300 can be controlled by an actuator. The actuator can 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 Figure 2 and Figure 3In the illustrated embodiment, the slider 300 optionally includes a movable carriage 310 and a track 320, wherein 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 configured to be received within the linear track 320 and move linearly relative to the linear track 320. In other words, the movable carriage 310 can be configured to move along the track in a direction parallel to the longitudinal axis. The linear track 320 may have a channel or recess or groove arranged to guide the movement of the movable carriage 310 therein. The track 320 is fixed in place (fixed relative to the housing 200 of the SEM and thus fixed relative to the electron source 10 and the sample holder 50 within the housing 200).

[0079] The movable carriage 310 can be configured to be coupled to the second end 92 of the arm 90. The movable carriage 310 can be fixed relative 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 a variety of ways. The second end 92 of the arm 90 may have a mounting member 96 for mounting the arm 90 to the movable carriage 310 of the slider 300. In an embodiment where the arm has one or more rods 95 between its first end 91 and second end 92, the mounting member 96 may include a corresponding receiver 96a for receiving and holding the rod 95 therein. The receiver 96a can be formed as a channel extending parallel to the longitudinal axis 93.

[0080] The movable carriage 310 may include a protruding portion 311 that is received within the channel of the track 320 such that the movement of the movable carriage 310 is restricted by the linear track 320. The movable carriage 310 may include a support portion 312 configured to be coupled to the second end 92 of the arm 90. Specifically, the mounting member 96 of the second end 92 of the arm 90 can be mounted to the support portion 312 of the movable carriage 310. The support portion 312 can be a support planar surface parallel to the channel of the track 320. The protruding portion 311 and the support portion 312 of the movable carriage 310 can be integrally formed.

[0081] The movable carriage 310 can be held within the track in a variety of ways, for example, by employing lips or abutting portions that define the edges of the track. As Figure 4As shown, the movable carriage 310 can 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 can have a mounting portion 313 with a through-hole therein that extends parallel to the longitudinal axis 93, i.e., parallel to the longitudinal direction of the track 320. The through-hole can be configured to slidably receive the support rod 330 therein such that the carriage 310 is capable of sliding along (i.e., relative to) the support rod 330. In this arrangement, when linearly moving along the track 320, the carriage 310 can slide along the support rod 330. The support rod 330 can be configured to support the weights of both the carriage 310 and the second end 92 of the arm 90 mounted on the carriage 310.

[0082] The movement of the movable carriage 310 can be restricted between a first position along the linear track 320 and a second position along the linear track 320, where the projector 60 is in a first operating position when the movable carriage 310 is in the first position and the projector 60 is in a second operating position when the movable carriage 310 is in the second position. The movement of the movable carriage 310 along the linear track 320 can be restricted between the first position and the second position of the movable carriage 310 by abutting against a stop (not shown) on the linear track 320 or on the support rod 330. When in the first position and when in the second position, the movable carriage 310 can be fixed in place.

[0083] 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 also be disposed outside the first chamber 201, i.e., outside the first housing portion 200a. The arm may 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 the space within the first chamber 201 is maximized and the weight within the chamber 201 is minimized because the slider 300 and the cooling component 97 will be disposed outside the first chamber 201. This configuration also minimizes contamination within the first chamber 201 where imaging of the sample is performed. When the projector 60 is in the first operating position or the second retracted position, the projector 60 and the first detector 70 will remain within the first chamber 201, thereby keeping these components under vacuum. The port 210 may include a seal to seal 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 when a portion of the arm 90 is translated therethrough. The arm 90 may be slidably movable within the port 210 as the arm 90 moves along the longitudinal axis.

[0084] The second portion 200b of the housing 200 that defines the second chamber 202 may be connected to the first portion 200a of the housing via a vacuum flange 220, thereby keeping both the first chamber and the second chamber under vacuum. Components of the SEM 100 that are outside the first chamber 201 may be housed within the second chamber 202, i.e., 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. The port within the housing through which the arm may be translated may be formed within the vacuum flange 220.

[0085] In an embodiment where the arm 90 has a rod 95 extending between the first end 91 and the second end 92, the rod 95 may be received within a port 210 formed in the housing 200 (specifically, within the vacuum flange 220). When the arm 90 is configured with multiple rods 95 such as Figure 2 and Figure 3When the two rods are in it, each rod 95 can be received within a corresponding port 210 formed in the housing 200. Each port 210 can be an orifice formed in the wall of the housing 200 and sized such that the corresponding rod 95 can pass therethrough. Each port 210 includes a seal configured to seal around the rod 95 so as to maintain the vacuum of the first chamber defined by the first part 200a of the housing 200 even in the case where the rod 95 translates therethrough. Specifically, each port 210 can be an orifice formed in the vacuum flange 220.

[0086] As described above, the arm 90 can be configured to conduct heat away from the projector 60 and / or the first detector 70.

[0087] In an embodiment where the arm includes one or more rods 95 extending between its first end and second end, the rods 95 can have conduits extending therethrough for receiving a coolant therein. The rods 95 can be thermally coupled to the projector 60 and the first detector 70 such that the coolant flowing therein conducts heat away from the projector 60 and the first detector 70. The rods 95 can include a thermally conductive material such as copper or be made of the thermally conductive material. Each rod 95 can include a heat pipe extending longitudinally therethrough. For example, each heat pipe can have a wicking material on its inner surface. Each heat pipe can be made of a thermally conductive material such as copper. The wicking material can be a sintered material, such as sintered copper powder. Each heat pipe can include a working fluid therein, as is known in the art.

[0088] As will be apparent to those skilled in the art, in operation, the heat received at the first end portion 91 of the arm due to the heating of the projector 60 and / or the first detector 70 will be conducted to the heat pipes of the rods 95 and into the wicking material, causing the evaporation of the working fluid within the heat pipes to form a vapor. Then, the vapor will flow to the relatively cooler portion of the heat pipe proximal to the second end portion 92 of the arm, where the vapor condenses on the wick, thereby releasing heat. The condensed liquid will flow back through the heat pipe towards the hotter end proximal to the projector 60 by capillary action. The heat released proximal to the second end portion 92 of the arm 90 can be absorbed by the cooling component 97 or otherwise radiated away from the second end portion 92 of the arm 90. The cooling component 97 can actively cool the second end portion 92 of the arm 90, thereby increasing the thermal gradient across the heat pipes within the rods 95 in order to more efficiently remove heat from the projector 60 and / or the first detector 70.

[0089] Now reference will be made to Figures 5 to 9 explain the influence of the projector 60 on the electron beam in the SEM of the claimed invention when the projector is in the first operating position.

[0090] Figure 5It is a plot of the distance (r, where r = 0 mm is the optical axis) from the optical axis of the SEM to the electrons diffracted from the sample at different angles (θ) 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. For this plot, the SEM has a coaxial detector arranged along the optical axis similar to the above-mentioned first detector 70, but does not have a projector arranged along the optical axis. Each line on this plot is symmetric about r = 0 mm and represents the electrons diffracted by the sample at a certain angle (θ) with respect to the optical axis 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 paths of the electrons are determined by simulation calculations, where the SEM operates in an ultra-high resolution mode in which the objective lens is an immersion magnetic lens and the electron paths are independent of energy.

[0091] Figure 5 It can be used as a comparison plot to demonstrate the effect of using the projector 60 along the optical axis of the SEM 100. As shown in this figure, when the SEM 100 operates in the UHR mode, the electron beam contracts / narrows due to the immersion magnetic field. 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 coaxial detector position starts at z = 40 mm (40 mm from this lower pole piece).

[0092] For Figure 5 the plot shown, the electron optical column is configured similarly to the above-mentioned electron optical column 110 having an objective lens and a condenser module. The objective lens employed produces an objective lens that is a magnetic immersion lens. As Figure 5 shown, this magnetic immersion lens is an immersion magnetic field. When the SEM operates in the ultra-high resolution mode (UHR mode), this immersion magnetic field is emitted from the lower magnetic pole piece of the electron optical column and immerses the sample therein. In this arrangement (i.e., in the absence of the projector 60), the downstream end of the objective lens acts as the upper pole piece of a two-pole magnetic lens, and the part of the housing that encloses the electron optical column (usually formed of a conductive material such as iron) acts as the lower pole piece of the two-pole magnetic lens. In this arrangement, the magnetic field leaving the downstream end of the objective lens penetrates the gap (at z = 4 mm) where the sample is positioned and is contained by the part of the housing that encloses the electron optical column.

[0093] There are two peaks in the immersion magnetic field. The strongest peak in 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 part of the electron beam diffracted below the sample. This weaker peak acting as a weak lens is undesirable because it limits the diffracted beam, thereby reducing the resolution of the diffracted beam on the detector.

[0094] Figure 6 differs from Figure 5 in that a smaller working distance is employed (the sample is closer to the objective lens). As can be seen from Figure 6 , in the case of a smaller working distance, the situation is more complex. 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 coaxial 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 focal points below the sample. As Figure 6 shows, in the UHR mode, due to the immersion magnetic field, the electrons in the electron beam are chaotic.

[0095] When the projector 60 is employed and positioned in the first operating position along the optical axis, the projector 60 controls the electron beam downstream of the sample. By using the projector 60 to control the electron beam downstream of the sample, rather than relying only on the weaker peak in the immersion magnetic field generated by the objective lens 40, the magnetic field that focuses the diffracted beam below the sample can be tuned to move the focal point closer to below the sample. This results in an increase in the size of the diffraction pattern on the first detector 70. In this embodiment of the present invention, the objective lens 40 serves as the upper pole piece of a two-pole magnetic lens, and the projector 60 serves as the lower pole piece of the two-pole magnetic lens.

[0096] Figure 7 is a drawing demonstrating the path of electrons passing through the projector 60 from the sample 50 when the projector 60 is in the first operating position in the SEM 100 (i.e., along the optical axis 101 of the SEM 100). 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 peaks of the axial magnetic field are superimposed on this drawing. As can be seen from 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 starting from z = 32 mm is due to the residual magnetic field.

[0097] The cross-section of an exemplary embodiment of the lower pole piece formed by the downstream end of the objective lens 40, the sample holder 50, and the projector 60 of the electron optical column 110 is also superimposed on this drawing. 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, which is referred to herein as the projector lens, and this projector lens is an electromagnetic multipole lens. As discussed in detail above, the projector lens has a housing / yoke 61 that houses the projector lens coil 62 arranged circumferentially around the projector optical axis. As mentioned above, the projector 60 has an upper projector pole piece 60a upstream of the lower projector pole piece 60b, where the upper projector pole piece 60a and the lower projector pole piece 60b are separated by a projector gap 63.

[0098] As Figure 7 visible in and discussed in more detail below, the electrons are first focused on the sample by the field of the immersion magnetic lens formed in the gap between the lower pole piece of the objective lens 40 (serving as the upper pole piece of the immersion magnetic lens) and the upper projector pole piece 60a (serving as the lower pole piece of the immersion magnetic lens). The electrons diffracted by the sample are then further focused by the projector 60 (when the projector is in the first position and turned on) at or proximal to the projector gap 63 and then dispersed. The position where the electrons are focused by the projector 60 depends on the projector coil excitation, where a stronger (higher) excitation results in a shorter focal length. For example, in Figure 7 , the projector coil excitation is 1600 ampere-turns (At), resulting in the electrons being focused at approximately z = 12 mm. In Figure 8 , the projector coil excitation is 1000 At, resulting in the electrons being focused at approximately z = 17 mm.

[0099] As Figure 8 (This figure is Figure 7 an enlarged version of a part of) more clearly shown, the strong peak in the magnetic field near the projector gap 63 (i.e., at z = 12 mm) focuses the electrons with an energy of 25 kV at z = 17 mm near but downstream of the projector gap 63, such that they can reach a wider spread (up to r = 6 mm from the optical axis) from the optical axis on the first detector 70 located at approximately z = 40 mm compared to the situation shown for the electrons at the same angle (θ). Figure 5 In

[0100] The use of the projector can result in up to a 4-fold magnification of the diffraction pattern. The use of the projector can result in a resolution greater than 160 px / DS (160 pixels per diffraction spot).

[0101] As Figure 7 and Figure 8As shown below, an explanation of the path of electrons entering, passing through, and leaving the projector 60 is provided. In use, the projector lens coil 62 wound around the circumference of the projector optical axis within the yoke 61 has a current passing through it during use, thereby generating a magnetic flux in the yoke 61. Due to the presence of the projector gap 63, the magnetic field from the upper projector pole piece 60a leaks into the vacuum within the yoke 61. This leakage magnetic field has a curvature and acts as a lens for converging the electron beam. Its intensity can be varied by changing the current in the projector lens coil 62. The leakage magnetic field in the projector gap 63 between the upper projector pole piece 60a and the lower projector pole piece 60b leaks into the upstream end of the lower projector pole piece 60b, thereby generating a strong magnetic lens, as Figure 7 indicated by the peak in the axial magnetic field at z = 12 mm. The peak in the axial magnetic field near the projector gap 63 focuses the electron beam slightly below the projector gap 63, as Figure 8 best shown in, and then the electron beam diverges at a high angle from the optical axis as it travels through the lower pole piece 60b. The diverging (spreading) electron beam results in magnification of the resulting diffraction pattern on the detector at z = 40 mm, thereby increasing its resolution.

[0102] 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., the radial position on the detector, where r = 0 mm is the optical axis of the SEM). Each line on this graph represents electrons diffracted by the sample at a certain angle (θ) relative to the optical axis of the SEM, where 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 Figure 9 shown, when the projector 60 is off (labeled as mode 0 in the plot), the spread of electrons is lower than when the projector 60 is on with a primary coil excitation of 2200 At (labeled as projector mode 2). In fact, r varies from 0.5 mm to 2 mm in mode 0 and from 1.5 to 4.5 in mode 2. This achieves the maximum magnification 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.

[0103] Therefore, there is an optimal projector lens coil excitation at which the spread of the electron beam is maximum at approximately 1500 to 2500 ampere-turns, preferably maximum at 2000 ampere-turns. There is also an optimal size for the projector lens coil and the yoke / housing to achieve the optimal excitation while minimizing mass and heating due to the high coil current as described above. The optimal design enables the diffraction magnification of the electron beam to be quadrupled using the optical excitation of the projector coil.

[0104] Figures 10 to 12An exemplary diffraction pattern obtained using the claimed SEM of the present invention, where the projector 60 is in the first operating position. These diffraction images were acquired from the same area of the sample under the same SEM conditions. The sample is crocidolite asbestos fibers with a thickness of approximately 100 nm. The diffraction pattern is seen according to its orientation. The acceleration voltage employed is 30 kV, and the electron beam current employed is several tens of pA. The images differ due to the excitation of the projector lens coil applied, and thus are magnified more and more. For Figure 10 , the excitation of the projector lens coil is 0 ampere-turns. For Figure 11 , the excitation of the projector lens coil is 1600 ampere-turns. For Figure 12 , the excitation of the projector lens coil is 2500 ampere-turns. From the comparison of these figures, it can be seen that the diffraction patterns in the figures are the same, but are magnified as the excitation of the projector lens coil increases. The diffraction images are rotated in software post-processing to remove the lens-induced image rotation between them.

[0105] It should be understood that the embodiments described above are for illustrative purposes only and the present invention is not limited thereto. Those skilled in the art will envision modifications and alternatives that fall within the scope of the claims.

[0106] All aspects and / or features disclosed in this specification can be combined in any combination, except for combinations where at least some of such features and / or steps are mutually exclusive. Specifically, the preferred features of this disclosure apply to all aspects and embodiments of this disclosure and can be used in any combination. Similarly, features described in non-essential combinations can be used alone (not in combination).

[0107] In the description and claims of this disclosure, the words "comprising", "including", "having", and "containing" and variations of these words, e.g., "comprising" or like words, mean "including but not limited to", and are not intended to (and do not) exclude other components.

[0108] Unless otherwise stated, the use of any and all examples or exemplary language (such as "for example", "such as" and like language) herein is only intended to better illustrate the present invention and does not limit the scope of the present invention. No language in the specification should be construed as indicating that any unclaimed element is essential for practicing the present invention.

[0109] Although the projector 60 is described as having a single lens to minimize its weight and size (i.e., only one lens), the projector 60 may alternatively have multiple lenses.

[0110] Although the projector lens is mainly described as an electromagnetic multipole lens, other types of projector lenses, such as circular lenses, may also be employed.

[0111] Although Figure 2 and Figure 3 In the embodiment shown, the first detector 70 is fixed relative to the projector 60 such that the first detector 70 moves with the projector 60. However, the first detector 70 may alternatively be fixed in a position below the sample holder 50 along the optical axis of the SEM. In this arrangement, the projector 60 may move relative to the first detector 70 such that when the projector 60 is in the first operating position, the first projector 60 is between the sample holder 50 and the first detector 70 along the optical axis of the SEM. In this arrangement, even if the projector 60 has been retracted, the first detector 70 can still be used to image electrons transmitted through the sample.

[0112] Although Figure 2 and Figure 3 the embodiment shown employs two rods 95 having a cylindrical cross-section within the arm 90, any number of rods 95 having any cross-section 90 may be employed.

[0113] Although Figure 2 and Figure 3 the embodiment shown employs an arm 90 having rods 95 and first and second ends 91 and 92 joined together, the rods 95 and first and second ends 91 and 92 of the arm 90 may be integrally formed.

[0114] Although Figure 2 and Figure 3 the embodiment shown includes a slider 300 for transmitting linear motion to the arm 90, other means for linearly moving the arm along the longitudinal axis 93 may be contemplated.

[0115] The above description relates to the use of a retractable (cooled) projector 60 in the SEM 100. However, the projector 60 may equivalently be used in other types of electron microscopes. It should be understood that the teachings above regarding the projector 60 and the first detector 70 may equivalently be applied to such other electron microscopes, but are not stated herein again to avoid repetition.

[0116] For example, the retractable projector 60 (optionally together with the first detector 70) may be used in a dual-beam system such as a dual-beam FIB-SEM.

[0117] As another example, the retractable projector 60 (optionally together with the first detector 70) may be used in a transmission electron microscope. More specifically, a transmission electron microscope is contemplated that includes:

[0118] an electron source;

[0119] a sample holder for holding a sample to be analyzed;

[0120] A projector; and

[0121] A first detector;

[0122] Each of the electron source and the sample holder is arranged on the optical axis of the transmission electron microscope,

[0123] Characterized in that the projector is movable between a first operating position and a second retracted position, in the first operating position the projector is downstream of the sample holder and positioned along the optical axis between the sample holder and the first detector, and in the second retracted position the projector is positioned away from the optical axis.

[0124] There is also contemplated a method of imaging a sample using a transmission electron microscope, the transmission electron microscope including an electron source, a sample holder, a projector and a first detector, wherein the electron source and the sample holder are arranged on the optical axis of the transmission electron microscope, the method comprising:

[0125] Placing a first sample on the sample holder of the microscope;

[0126] Moving the projector from the second retracted position to the first operating position;

[0127] Wherein in the first operating position, the projector is downstream of the sample holder and positioned along the optical axis between the sample holder and the first detector,

[0128] Wherein in the second retracted position, the projector is positioned away from the optical axis;

[0129] Scanning a first electron beam generated by the electron source along the first sample to generate a first image; and

[0130] Using the projector in its first operating position to project the first image onto the first

[0131] Detector;

[0132] Wherein the first image is an electron diffraction pattern formed by electrons transmitted through the sample and diffracted by the sample.

[0133] The above description of the projector and the first detector can be equivalently applied to the transmission electron microscope. For example, the projector and the first detector can be coupled together 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 can 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 configured to hold a sample to be analyzed; a projector; and a first detector; each of the electron source and the sample holder is arranged on the optical axis of the scanning electron microscope, characterized in that the projector is movable between a first operating position and a second retracted position, in the first operating position the projector is positioned downstream of the sample holder and along the optical axis between the sample holder and the first detector, and in the second retracted position the projector is positioned away from the optical axis.

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 any one of the preceding claims, wherein the projector is movable linearly between the first operating position and the second retracted position.

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

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

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

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

8. The scanning electron microscope according to any one of the preceding claims, 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 scanning electron microscope according to claim 8, wherein the first detector is fixed relative to the projector and spaced apart from the projector such that the projector is between the sample holder and the first detector in the first operating position.

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

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 scanning electron microscope according to any one of the preceding claims, wherein the projector comprises one or more projector lenses, optionally wherein the one or more projector lenses are electromagnetic lenses, and further optionally wherein the one or more projector lenses are multipole lenses.

13. The scanning electron microscope according to any one of the preceding claims, wherein the first detector is a pixelated detector.

14. The scanning electron microscope according to any one of the preceding claims, wherein the scanning electron microscope further comprises a second detector, optionally wherein the second detector is arranged upstream of the sample holder, and further optionally wherein the second detector is a BSE detector or a secondary electron detector.

15. 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, wherein the electron source and the sample holder are arranged 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; Wherein in the first operating position, the projector is positioned downstream of the sample holder and along the optical axis between the sample holder and the first detector, Wherein in the second retracted position, the projector is positioned away from the optical axis; Scanning a first electron beam generated by the electron source along the first sample to generate a first image; And Projecting the first image onto the first detector using the projector in its first operating position; Wherein the first image is an electron diffraction pattern formed by electrons transmitted through the sample and diffracted by the sample.

16. The method according to claim 15, wherein the scanning electron microscope further comprises a second detector, wherein the second detector is arranged upstream of the sample holder, and wherein the second detector is configured to detect backscattered electrons and / or secondary electrons emitted from the sample; The method further comprises: Replacing 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; and Receiving the backscattered electrons and / or secondary electrons at the second detector to form an image of the second sample.

Images