Image acquisition method and scanning transmission electron microscope

Abstract

To provide an image acquisition method capable of reducing influence of drift of a sample.SOLUTION: An image acquisition method according to the present invention is an image acquisition method for a crystalline sample in a scanning transmission electron microscope including an electron source that emits an electron beam, an irradiation system including a focusing lens, an aperture, and an irradiation-system deflector that deflects the electron beam having passed through the aperture, a sample stage that supports a sample, an imaging device capable of capturing a Ronchigram formed on a diffraction plane, and an imaging-system deflector that deflects an electron beam incident on the imaging device. The image acquisition method includes: a step of aligning a center of the Ronchigram with a center of a detection surface of the imaging device; a step of aligning a shadow of the aperture on the diffraction plane with a zone axis of the sample to align an incident direction of the electron beam with respect to the sample to the zone axis; and a step of deflecting the electron beam by the imaging-system deflector to align the electron beam with the center of the detection surface of the imaging device.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an image acquisition method and a scanning transmission electron microscope. [Background technology]

[0002] A scanning transmission electron microscope (STEM) is a device that scans a sample with a focused electron beam and detects the electrons that pass through the sample to obtain a scanning transmission electron microscope image (STEM image).

[0003] When observing a crystalline sample with a scanning transmission electron microscope, the incident direction of the electron beam is aligned with the crystal zone axis, as disclosed in Patent Document 1. Here, in a crystal, a group of planes parallel to a certain direction is called a crystal zone, and that direction is called the crystal zone axis. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2024-39605 Summary of the Invention [Problem to be solved by the invention]

[0005] To align the incident direction of the electron beam with the crystal zone axis, the specimen is tilted on the specimen stage, but tilting the specimen on the specimen stage causes drift in the specimen due to effects such as backlash. [Means for solving the problem]

[0006] One aspect of the image acquisition method according to the present invention includes: an electron source that emits an electron beam; an illumination system including a focusing lens, an aperture, and an illumination system deflector that deflects the electron beam that has passed through the aperture; a sample stage for supporting the sample; an imaging device capable of photographing a Ronchigram formed on a diffractive surface; an imaging system deflector that deflects an electron beam incident on the imaging device; 1. A method for imaging a crystalline sample in a scanning transmission electron microscope, comprising: aligning the center of the Ronchigram with the center of the detection surface of the imaging device; a step of aligning a shadow of the aperture on the diffraction plane with a zone axis of the sample, and aligning an incident direction of the electron beam with the zone axis of the sample; deflecting the electron beam with the imaging system deflector to align the electron beam with the center of the detection surface of the imaging device; Includes.

[0007] In this image acquisition method, the direction of incidence of the electron beam on the sample can be aligned with the zone axis by moving the shadow of the aperture in the diffraction plane, thereby reducing sample drift caused by tilting the sample stage.

[0008] One aspect of the image acquisition method according to the present invention includes: One aspect of the scanning transmission electron microscope according to the present invention is an electron source that emits an electron beam; an illumination system including a focusing lens, an aperture, and an illumination system deflector that deflects the electron beam that has passed through the aperture; a sample stage for supporting the sample; an imaging device capable of photographing a Ronchigram formed on a diffractive surface; an imaging system deflector that deflects an electron beam incident on the imaging device; a control unit for controlling the illumination system and the imaging system deflector; Including, The control unit a process of deflecting the electron beam with the imaging system deflector to align the center of the Ronchigram with the center of the detection surface of the imaging device; a process of aligning the shadow of the aperture on the diffraction plane with a zone axis of the sample, and aligning the incident direction of the electron beam with the zone axis of the sample; a process of deflecting the electron beam with the imaging system deflector to align the electron beam with the center of the detection surface of the imaging device; Do the following.

[0009] In such a scanning transmission electron microscope, the direction of incidence of the electron beam on the sample can be aligned with the zone axis by moving the shadow of the aperture in the diffraction plane to align it with the zone axis, thereby reducing sample drift caused by tilting the sample stage.

[0010] One aspect of the scanning transmission electron microscope according to the present invention is an electron source that emits an electron beam; an illumination system for scanning the sample with an electron beam; a sample stage for supporting the sample; a detector for detecting electrons transmitted through the sample; a control unit that controls the irradiation system; Including, the control unit performs processing to scan the sample with an electron beam and obtain a scanned image; In the process of acquiring the scanned image, the direction in which the scanning line is drawn is set to be perpendicular to the tilt direction of the sample.

[0011] In such a scanning transmission electron microscope, even if the sample is tilted, the change in height of the sample while drawing one scan line can be reduced. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a scanning transmission electron microscope. [Figure 2] 1 is a flowchart showing an example of a method for acquiring a scanning transmission electron microscope image. [Figure 3] A Ronchigram image taken with an imaging device. [Figure 4] An image showing the state in which the focusing aperture is inserted onto the optical axis of the optical system. [Figure 5]FIG. 1 is a schematic diagram of a scanning transmission electron microscope when a focusing aperture is inserted with the center of the Ronchigram aligned with the center of the detection surface. [Figure 6] Image showing the shadow of the focusing aperture aligned with the zone axis in the diffraction plane. [Figure 7] FIG. 1 is a schematic diagram showing a scanning transmission electron microscope when the focusing aperture is aligned with the zone axis. [Figure 8] Image showing the state where the shadow of the focusing aperture is located outside circle A in the diffraction plane. [Figure 9] An image obtained when the electron beam is deflected by the imaging deflector. [Figure 10] FIG. 1 is a schematic diagram of a scanning transmission electron microscope when the electron beam is deflected. [Figure 11] FIG. 1 is a diagram showing a schematic diagram of an electron beam scanning a sample tilted relative to the optical axis. [Figure 12] FIG. 1 is a diagram showing a schematic diagram of an electron beam scanning a sample tilted relative to the optical axis. [Figure 13] Schematic diagram showing how an electron probe scans a sample tilted relative to the optical axis while maintaining a constant focus. [Figure 14] FIG. 1 is a diagram showing a schematic diagram of an electron probe scanning a sample tilted relative to the optical axis by changing the focus. [Figure 15] FIG. 10 is a diagram showing an example of an image of a change in a Ronchigram. [Figure 16] FIG. 10 is a diagram for explaining an example of a method for determining the center of a Ronchigram. [Figure 17] 10 is a flowchart showing an example of an image acquisition process by a control unit. [Figure 18] FIG. 10 is a diagram for explaining the relationship between the tilt of a sample and chromatic aberration. DETAILED DESCRIPTION OF THE INVENTION

[0013] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. Note that the embodiments described below do not unduly limit the content of the present invention as defined in the claims. Furthermore, not all of the configurations described below are necessarily essential components of the present invention.

[0014] 1. Scanning Transmission Electron Microscopy First, a scanning transmission electron microscope according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram showing an example of the configuration of a scanning transmission electron microscope 100 according to this embodiment.

[0015] The scanning transmission electron microscope 100 is a device for scanning a sample S with an electron beam (electron probe) and detecting electrons that have transmitted through the sample S to obtain a scanned image (scanning transmission electron microscope image, hereinafter also referred to as a "STEM image").

[0016] As shown in FIG. 1, the scanning transmission electron microscope 100 includes an optical system 10, a sample stage 20, an imaging device 30, a detector 40, and a control unit 50.

[0017] The optical system 10 includes an electron source 11, a focusing lens 12, a focusing aperture 13 (an example of an aperture), an illumination system deflector 14, an aberration corrector 15, an objective lens 16, an intermediate lens 17, and an imaging system deflector 18.

[0018] The electron source 11 emits an electron beam. The electron source 11 is, for example, an electron gun that accelerates electrons emitted from a cathode by an anode to emit an electron beam. An accelerating voltage is applied between the cathode and the anode.

[0019] The condenser lens 12 focuses the electron beam emitted from the electron source 11. Although not shown, the condenser lens 12 may be made up of a plurality of electron lenses.

[0020] The focusing aperture 13 is disposed inside the focusing lens 12. The focusing aperture 13 is an aperture for determining the aperture angle of the electron beam and the irradiation amount of the beam.

[0021] The irradiation system deflector 14 deflects the electron beam irradiated onto the sample S, tilting the electron beam with respect to the optical axis of the irradiation system 2. The irradiation system deflector 14 may be incorporated into the aberration corrector 15. The irradiation system deflector 14 is disposed, for example, between the focusing lens 12 and the aberration corrector 15. Note that the position of the irradiation system deflector 14 is not particularly limited as long as it is incorporated into the irradiation system 2.

[0022] The aberration corrector 15 corrects aberrations in the illumination system 2. The aberration corrector 15 is disposed between the focusing lens 12 and the objective lens 16. The aberration corrector 15 is, for example, a spherical aberration corrector that corrects spherical aberrations in the illumination system 2.

[0023] The irradiation system 2 is an optical system arranged in front of the sample S for irradiating the sample S with an electron beam. In the scanning transmission electron microscope 100, the irradiation system 2 is made up of a focusing lens 12, a focusing aperture 13, an aberration corrector 15, and an objective lens 16 (a forward magnetic field of the objective lens 16). The electron beam is focused by the system 2 to form an electron probe. The electron probe is the focal point where the electron beam is most focused. The irradiation system 2 includes a scanning coil (not shown) for deflecting the electron beam and scanning the sample S with the electron probe.

[0024] The objective lens 16 focuses the electron beam to form an electron probe. An electron diffraction pattern, Kikuchi pattern, Ronchigram, etc. are formed on the back focal plane of the objective lens 16, i.e., the diffraction plane. A Ronchigram is a projection image (figure) of a sample that is formed on the diffraction plane when an electron beam is focused near the sample in a scanning transmission electron microscope.

[0025] The intermediate lens 17 magnifies and transfers the electron diffraction pattern, Kikuchi pattern, and Ronchigram formed on the back focal plane of the objective lens 16. The imaging system deflector 18 is disposed before the imaging device 30. The imaging system deflector 18 is disposed between the intermediate lens 17 and the imaging device 30. The imaging system deflector 18 deflects the electron beam that passes through the sample S and enters the imaging device 30. The position of the imaging system deflector 18 is not particularly limited as long as it is incorporated into the imaging system 4.

[0026] In the scanning transmission electron microscope 100, the rear magnetic field of the objective lens 16, the intermediate lens 17, and the imaging system deflector 18 constitute an imaging system 4. The imaging system 4 is an optical system arranged behind the sample S, and is an optical system for forming an image of the sample S with an electron beam that has passed through it.

[0027] The optical system 10 may include optical elements other than the above-mentioned lenses and diaphragms.

[0028] The sample stage 20 supports the sample S. The sample S supported by the sample stage 20 is positioned between the forward magnetic field of the objective lens 16 and the backward magnetic field of the objective lens 16. The sample S is positioned by the sample stage 20. The sample stage 20 has a movement mechanism that moves the sample S in the height direction, a movement mechanism that moves the sample S in the horizontal direction, and a tilt mechanism that tilts the sample S. The height direction of the sample S is the direction along the optical axis of the illumination system 2.

[0029] The imaging device 30 is disposed on the back focal plane of the objective lens 16 or on a plane conjugate to the back focal plane of the objective lens 16. The imaging device 30 can capture Ronchigrams, electron diffraction patterns, Kikuchi patterns, and the like formed on the diffraction plane. The imaging device 30 is, for example, a digital camera capable of recording Ronchigrams and the like as two-dimensional digital images.

[0030] The center of the detection surface 32 of the imaging device 30 (center of the sensor) is located on the optical axis of the optical system 10. The center of the detection surface 32 of the imaging device 30 also corresponds to the center of the image captured by the imaging device 30.

[0031] The detector 40 detects electrons that have transmitted through the sample S. The detector 40 is a detector for acquiring a STEM image. The detector 40 is disposed on the optical axis of the optical system 10. Although not shown, the scanning transmission electron microscope 100 may be provided with an annular detector for acquiring a high-angle-scattering dark-field image (HAADF-STEM image) as a detector for detecting electrons that have transmitted through the sample S.

[0032] The control unit 50 (computer) controls each component of the scanning transmission electron microscope 100. The control unit 50 controls the optical system 10 and the sample stage 20. The control unit 50 includes, for example, a processor such as a CPU (Central Processing Unit) and a storage device such as a RAM (Random Access Memory) and a ROM (Read Only Memory). The storage device stores programs and data for performing various controls. The functions of the control unit 50 can be realized by executing the programs in the processor.

[0033] 2. Image acquisition method Next, a method for acquiring a scanning transmission electron microscope image will be described. Here, a case where a high-resolution STEM image of a crystalline sample is acquired will be described. Figure 2 is a flowchart showing an example of the method for acquiring a STEM image.

[0034] First, the center of the Ronchigram is aligned with the center of the detection surface 32 of the imaging device 30 (step S100).

[0035] FIG. 3 shows an image of a Ronchigram captured by the imaging device 30. Circle A in FIG. 3 indicates the region of uniform intensity in the Ronchigram. The center of the Ronchigram is the center of the region of uniform intensity in the Ronchigram, and coincides with the center of circle A. In this way, the center of the Ronchigram can be confirmed from the image of the Ronchigram captured by the imaging device 30. Alternatively, as will be explained in "3. Method for Determining the Center of the Ronchigram" below, the center of the Ronchigram may be determined from an image of the change in the Ronchigram due to a change in the relative positional relationship between the sample and the electron probe. By deflecting the electron beam with the imaging system deflector 18, the center of the Ronchigram can be aligned with the center of the detection surface 32 of the imaging device 30.

[0036] Next, the angle of the zone axis of the sample S is roughly adjusted (step S102). Specifically, first, the sample S is moved to the desired imaging area using the sample stage 20. Next, the sample S is tilted using the sample stage 20 so that the inclination of the zone axis relative to the optical axis of the irradiation system 2 becomes smaller. For example, while checking the zone axis using a Kikuchi pattern or electron diffraction pattern captured by the imaging device 30, the inclination of the zone axis relative to the optical axis is reduced by tilting the sample S using the sample stage 20 so that the zone axis approaches the center of the image. Through the above steps, the zone axis can be roughly adjusted.

[0037] Next, the focusing diaphragm 13 is inserted onto the optical axis of the illumination system 2 (step S104). Fig. 4 is an image showing the state in which the focusing diaphragm 13 is inserted onto the optical axis of the illumination system 2. Fig. 4 also shows the center O of the detection plane 32 of the imaging device 30. By inserting the focusing diaphragm 13, the shadow of the focusing diaphragm 13 can be confirmed on the diffraction plane, as shown in Fig. 4.

[0038] It should be noted that step S104 of introducing the focusing aperture 13 may be performed before step S102 or after step S106.

[0039] 5 is a schematic diagram showing the scanning transmission electron microscope 100 when the focusing aperture 13 is inserted with the center of the Ronchigram aligned with the center O of the detection surface 32. In the state shown in FIG. 5, the sample S is tilted with respect to the optical axis, and the electron beam is incident on the sample S along the optical axis. Therefore, the incident direction of the electron beam is not aligned with the zone axis.

[0040] Next, the incident direction of the electron beam is aligned with the zone axis by aligning the shadow of the focusing aperture 13 with the zone axis on the diffraction plane (step S106). As shown in FIG. 4, the Kikuchi pattern that appears on the diffraction plane indicates that the incident direction of the electron beam is offset from the zone axis. The position of the zone axis can be confirmed from the Kikuchi pattern that appears on the diffraction plane. Therefore, while checking the shadow of the focusing aperture 13 in the image captured by the imaging device 30, the electron beam is tilted by the irradiation system deflector 14 so that the center of the circle formed by the shadow of the focusing aperture 13 coincides with the zone axis. This allows the shadow of the focusing aperture 13 to be aligned with the zone axis on the diffraction plane.

[0041] Fig. 6 is an image showing a state in which the shadow of the focusing aperture 13 on the diffraction plane is aligned with the zone axis Z. Fig. 7 is a diagram schematically showing the scanning transmission electron microscope 100 when the electron beam is tilted by the illumination system deflector 14 to align the shadow of the focusing aperture 13 with the zone axis Z.

[0042] As shown in Fig. 7, by tilting the electron beam with the irradiation system deflector 14, the shadow of the focusing aperture 13 moves on the diffraction plane, and the shadow of the focusing aperture 13 on the diffraction plane can be aligned with the zone axis Z, as shown in Fig. 6. This allows the incident direction of the electron beam on the sample S to be aligned with the zone axis Z.

[0043] Here, the shadow of the focusing aperture 13 is aligned with the crystal zone axis Z by tilting the electron beam with the irradiation system deflector 14, but the shadow of the focusing aperture 13 may also be aligned with the crystal zone axis Z by mechanically moving the focusing aperture 13.

[0044] Next, it is determined whether the tilt of the zone axis with respect to the optical axis of the irradiation system 2 is within the valid range (step S108).

[0045] Here, circle A in Figure 6 indicates the region where the intensity of the Ronchigram is uniform. The region where the intensity of the Ronchigram is uniform is an angular range without aberration. Therefore, by placing the shadow of the focusing aperture 13 within this circle A, the focusing aperture 13 can select an electron beam within an angular range without aberration. This allows the sample S to be irradiated with an electron beam within an angular range without aberration.

[0046] For example, if the inclination of the zone axis with respect to the optical axis is large, the shadow of the focusing aperture 13 may be positioned outside the circle A when the shadow of the focusing aperture 13 is aligned with the zone axis Z.

[0047] Fig. 8 is an image showing a state in which, on the diffraction plane, the shadow of the focusing aperture 13 is located outside the circle A. As shown in Fig. 8, when the shadow of the focusing aperture 13 is located outside the circle A, the focusing aperture 13 cannot select an electron beam within an angle range free of aberration.

[0048] Therefore, in step S108, it is determined whether the magnitude of the tilt of the zone axis is within the valid range. Specifically, when the shadow of the focusing aperture 13 is aligned with the zone axis, it is determined whether the magnitude of the tilt of the zone axis is within the valid range based on whether the shadow of the focusing aperture 13 is located within circle A. Here, when the shadow of the focusing aperture 13 is located within circle A, this refers to when the entire circle formed by the shadow of the focusing aperture 13 is included within circle A.

[0049] The diameter of circle A may be found by examining a region where the intensity is uniform in a Ronchigram captured in advance by imaging device 30. The diameter of circle A may also be calculated from the performance of optical system 10 including aberration corrector 15.

[0050] If the tilt of the zone axis is outside the valid range (No in step S108), that is, if the shadow of the focusing aperture 13 is not located within the circle A as shown in Fig. 8, the process returns to step S102, and the sample S is tilted on the sample stage 20 to perform coarse adjustment of the angle of the zone axis. Then, steps S104, S106, and S108 are performed.

[0051] If the tilt of the zone axis is within the valid range (Yes in step S108), that is, if the shadow of the focusing aperture 13 is located within the circle A as shown in FIG. 6, the imaging system deflector 18 deflects the electron beam to align it with the center O of the detection surface 32 of the imaging device 30 (step S110).

[0052] 6, the electron beam is deviated from the optical axis of the optical system 10, and therefore cannot be detected by the detector 40. Therefore, the electron beam is deflected by the imaging system deflector 18, and the shadow of the focusing aperture 13 is moved to the center O of the detection surface 32, so that the electron beam is made incident on the center O of the detection surface 32.

[0053] 9 shows an image when the electron beam is deflected back by the imaging system deflector 18. FIG. 1 is a diagram schematically illustrating a scanning transmission electron microscope 100 when the electron beam is deflected back by a deflector 18.

[0054] As shown in FIG. 10, by deflecting the electron beam using the imaging system deflector 18, the shadow of the focusing aperture 13 and the zone axis Z can be moved to the center O of the detection surface 32 of the imaging device 30, as shown in FIG.

[0055] By the above steps, the incident direction of the electron beam on the sample S can be aligned with the zone axis.

[0056] Next, the sample S is scanned with an electron beam to obtain a scanned image (STEM image) (step S112).

[0057] 11 and 12 are diagrams showing a state in which a sample S tilted with respect to the optical axis of the illumination system 2 is scanned with an electron probe. FIG. 11 is a plan view showing a scanning area Sa of the sample S. FIG. 12 is a cross-sectional view showing the sample S. Note that FIGS. 11 and 12 show an X-axis, a Y-axis, and a Z-axis which are orthogonal to each other. The Z-axis is parallel to the optical axis of the illumination system 2.

[0058] In the scanning transmission electron microscope 100, a STEM image of the scan area Sa is obtained by raster scanning, in which an electron probe draws multiple scan lines L in the scan area Sa. The direction in which the scan lines L are drawn is perpendicular to the tilt direction of the sample S. In the example shown in FIGS. 11 and 12, the sample S is tilted in the X direction. Therefore, the scan lines L are drawn along the Y axis. This reduces the change in height of the sample S while drawing one scan line L. Therefore, the focus can be kept constant while drawing one scan line L. In other words, the position in the Z direction of the electron probe, which is the focal point of the electron beam, can be kept constant. For example, if the direction in which the scan lines L are drawn is not perpendicular to the tilt direction of the sample S, the height of the sample S changes while drawing one scan line L, causing the focus to shift and resulting in a blurred STEM image.

[0059] Here, the magnification of the STEM image is WD / WS, where WS is the width of the scanning region Sa and WD is the display width of the STEM image. Therefore, as shown in FIG. 12, when the sample S is tilted by an angle θ in the X direction with respect to the optical axis, the magnification of the STEM image in the X direction becomes cosθ times. In other words, the STEM image shrinks in the X direction. Therefore, as shown in FIG. 11, the width WS of the scanning region Sa in the X direction is multiplied by cosθ to set the width of the scanning region Sa to WS × cosθ. This prevents the STEM image from shrinking in the direction of the tilt of the sample S.

[0060] Note that, assuming that the width of the scanning region Sa is WS, the reduction in the X direction of the acquired STEM image may be corrected by image processing, i.e., the X direction of the STEM image may be multiplied by 1 / cosθ.

[0061] Fig. 13 is a diagram schematically illustrating how the electron probe scans the sample S tilted with respect to the optical axis of the illumination system 2 while keeping the focus constant. Fig. 14 is a diagram schematically illustrating how the electron probe scans the sample S tilted with respect to the optical axis of the illumination system 2 while changing the focus depending on the degree of tilt of the sample S.

[0062] As shown in Fig. 13, when the focus is kept constant, a focus deviation occurs at the end in the +X direction and the end in the -X direction of the scanning area Sa. Therefore, as shown in Fig. 14, the focus is changed by an amount corresponding to the magnitude of the tilt (angle θ) of the sample S each time a scanning line L is drawn. Specifically, if the interval between drawing the scanning lines L is D, the focus is changed by D × tan θ each time a scanning line L is drawn. This allows the focus deviation due to the tilt of the sample S to be reduced. can be reduced.

[0063] The focus can be changed by changing the acceleration voltage, the excitation of the objective lens 16, or the height of the sample S. Note that controlling the focus by controlling the acceleration voltage can increase the response speed. For example, the objective lens 16 has a slow response speed because the focus is changed by changing the excitation current flowing through the coil. Also, the height of the sample S is changed by mechanically operating the sample stage 20, so the response speed is slow. In contrast, the acceleration voltage is a voltage applied to the cathode and anode of the electron source 11, so the response speed is fast.

[0064] As described above, acquiring a STEM image requires information on the tilt direction and magnitude (angle θ) of the sample S. Therefore, information on the tilt direction and magnitude (angle θ) of the sample S is calculated based on the deflection direction and deflection amount of the electron beam by the irradiation system deflector 14 when the shadow of the focusing aperture 13 is aligned with the zone axis.

[0065] For example, calibration data is obtained in advance that indicates the relationship between the control amount (e.g., excitation current) of the irradiation system deflector 14 and the deflection direction and deflection amount of the electron beam. Using this calibration data, the tilt direction and tilt amount of the sample S are calculated from the control amount of the irradiation system deflector 14 when the focusing aperture 13 is aligned with the zone axis.

[0066] In the image of the diffraction plane captured by the imaging device 30, the tilt direction and the tilt magnitude (angle θ) of the sample S may be calculated from the distance between the center of the Ronchigram and the zone axis.

[0067] 3. How to determine the center of the Ronchigram Next, a method for determining the center of the Ronchigram will be described.

[0068] The center of the Ronchigram can be determined from an image of the change in the Ronchigram taken by the imaging device 30 while changing the excitation of the objective lens 16. The image of the change in the Ronchigram can be acquired, for example, by maintaining the imaging device 30 in an exposed state while changing the excitation of the objective lens 16. The image of the change in the Ronchigram is a single image that records the change in the Ronchigram due to the change in the excitation of the objective lens 16 (change in the defocus amount).

[0069] Figure 15 is an example of an image of a Ronchigram change. The pattern of radial lines in the image shown in Figure 15 is observed centered on the center of the magnification change, i.e., the center of the Ronchigram. Therefore, ideally, the point where these multiple lines intersect coincides with the center of the Ronchigram.

[0070] FIG. 16 is a diagram for explaining an example of a method for determining the center of a Ronchigram from an image of a change in the Ronchigram.

[0071] When determining the center of the Ronchigram from an image of the Ronchigram change, first, a band-pass filter is applied to image IA of the Ronchigram change to remove noise components (see image IB). Next, edge portions of the image are extracted from image IB of the Ronchigram change from which the noise components have been removed (see image IC). Next, line components are detected from image IC of the Ronchigram change from which the edge portions have been extracted (see image ID). Next, in image ID from which the line components have been detected, multiple pairs of line components that intersect with each other are extracted, and the intersections formed by each pair are determined. Next, as shown in image IE, the center of gravity of the multiple intersections determined is calculated, and the position of this center of gravity is set as the center of the Ronchigram. In this way, by determining multiple intersections and setting the positions of their centers of gravity as the center of the Ronchigram, the Ronchigram can be determined. The position of the center can be determined with high accuracy.

[0072] Note that an image of the change in the Ronchigram may be obtained from a plurality of Ronchigram images acquired under conditions where the relative positional relationship between the sample and the electron probe is different. That is, an image of the change in the Ronchigram may be obtained by accumulating or averaging a plurality of Ronchigram images acquired under conditions where the excitation amount of the objective lens 16 is different to generate a single image.

[0073] In the above, the excitation of the objective lens 16 was changed to obtain an image of the change in the Ronchigram, but instead of the excitation of the objective lens 16, the acceleration voltage may be changed, or the height of the sample S may be changed.

[0074] 4. Processing FIG. 17 is a flowchart showing an example of the image acquisition process of the control unit 50.

[0075] The control unit 50 aligns the center of the Ronchigram with the center O of the detection surface 32 of the imaging device 30 (step S200). The control unit 50 acquires an image of the change in the Ronchigram and determines the center of the Ronchigram from the image of the change in the Ronchigram. The control unit 50 controls the imaging system deflector 18 so that the center of the Ronchigram aligns with the center O of the detection surface 32.

[0076] Next, the control unit 50 roughly adjusts the angle of the zone axis (step S202). The control unit 50 identifies the position of the zone axis in the diffraction plane from the Kikuchi pattern or electron diffraction pattern captured by the imaging device 30, and tilts the sample S by operating the sample stage 20 so that the inclination of the zone axis with respect to the optical axis becomes small.

[0077] Next, the control unit 50 inserts the focusing aperture 13 onto the optical axis (step S204). Next, the control unit 50 aligns the shadow of the focusing aperture 13 on the diffraction plane with the zone axis, and aligns the incident direction of the electron beam with the zone axis (step S206). The control unit 50 operates the illumination system deflector 14 so that the center of the circle formed by the shadow of the focusing aperture 13 coincides with the zone axis in the image of the diffraction plane captured by the imaging device 30. The control unit 50 calculates the position of the zone axis from the image of the Kikuchi pattern. The position of the zone axis may also be specified by the user. By the processing of step S206, the incident direction of the electron beam with respect to the sample S can be aligned with the zone axis.

[0078] The control unit 50 may mechanically move the focusing aperture 13 to align the shadow of the focusing aperture 13 with the zone axis on the diffraction plane.

[0079] Next, the control unit 50 determines whether the tilt of the zone axis is within the valid range (step S208). The control unit 50 captures an image of the diffraction surface with the imaging device 30, and determines whether the circle formed by the shadow of the focusing aperture 13 in the captured image is located within circle A, which indicates the region where the intensity of the Ronchigram is uniform. If the control unit 50 determines that the circle formed by the shadow of the focusing aperture 13 is located within circle A, it determines that the tilt of the zone axis is within the valid range. The diameter of circle A is set in advance.

[0080] If the control unit 50 determines that the tilt of the zone axis is outside the valid range (No in step S208), it returns to step S202 and tilts the sample S on the sample stage 20 to perform coarse adjustment of the angle of the zone axis (step S202).Then, the control unit 50 performs the process of step S204, the process of step S206, and the process of step S208.

[0081] In this manner, the control unit 50 repeats the process of steps S202, S204, S206, and S208 until it is determined that the tilt of the zone axis is within the valid range.

[0082] When the control unit 50 determines that the tilt of the zone axis is within the valid range (Yes in step S208), it causes the imaging system deflector 18 to deflect the electron beam so that the electron beam is incident on the center O of the detection surface 32 of the imaging device 30 (step S210). The control unit 50 captures an image of the diffraction surface with the imaging device 30, and operates the imaging system deflector 18 based on the captured image so that the shadow of the focusing aperture 13 coincides with the center O of the detection surface 32. This allows the electron beam to be incident on the detector 40.

[0083] Next, the control unit 50 acquires a STEM image (step S212). The control unit 50 calculates the direction and magnitude of tilt of the sample S from the excitation amount of the irradiation system deflector 14 in the process of step S206, which aligns the incident direction of the electron beam with the zone axis. The control unit 50 then sets scanning conditions based on the direction and magnitude of tilt of the sample S. The scanning conditions include the direction of drawing the scanning line L, the width of the scanning area Sa, and the amount of focus change for each scanning line L. Specifically, the direction of drawing the scanning line L is set to be perpendicular to the direction of tilt of the sample S. The width of the scanning area Sa in the tilt direction of the sample S is also set according to the amount of tilt of the sample S. The amount of focus change for each scanning line L is also set according to the amount of tilt of the sample S. The control unit 50 controls the optical system 10 and the acceleration voltage based on the scanning conditions to acquire a STEM image.

[0084] After acquiring the STEM image, the control unit 50 ends the image acquisition process.

[0085] 5. Effects The image acquisition method according to this embodiment is a method for acquiring an image of a crystalline sample in a scanning transmission electron microscope 100, which includes an electron source 11 that emits an electron beam, an irradiation system 2 that includes a focusing lens 12, a focusing aperture 13, and an irradiation system deflector 14 that deflects the electron beam that has passed through the focusing aperture 13, a sample stage 20 that supports a sample S, an imaging device 30 that can capture a Ronchigram formed on the diffraction plane, and an imaging system deflector 18 that deflects the electron beam that is incident on the imaging device 30. The image acquisition method according to this embodiment also includes the steps of aligning the center of the Ronchigram with the center O of a detection plane 32 of the imaging device 30, aligning the shadow of the focusing aperture 13 on the diffraction plane with a zone axis of the sample S so that the incident direction of the electron beam with respect to the sample S is the zone axis, and deflecting the electron beam with the imaging system deflector 18 to align the electron beam with the center O of the detection plane 32 of the imaging device 30.

[0086] As described above, in the image acquisition method according to this embodiment, by moving the shadow of the focusing aperture 13 on the diffraction plane to align it with the zone axis, the incident direction of the electron beam on the sample S can be aligned with the zone axis without tilting the sample S on the sample stage 20. For example, if the incident direction of the electron beam is aligned with the zone axis by tilting the sample S on the sample stage 20, drift of the sample S will occur. In contrast, in the image acquisition method according to this embodiment, sample drift caused by tilting the sample stage 20 can be reduced.

[0087] The image acquisition method according to this embodiment includes, after a step of aligning the incident direction of the electron beam with the zone axis, a step of determining whether the magnitude of the tilt of the zone axis with respect to the optical axis of the irradiation system 2 is within a predetermined range (effective range), and, if it is determined that the magnitude of the tilt of the zone axis is not within the predetermined range, a step of tilting the sample S on the sample stage 20 to reduce the tilt of the zone axis with respect to the optical axis of the irradiation system 2. Therefore, with the image acquisition method according to this embodiment, even if the sample S is tilted with respect to the optical axis, it is possible to acquire a STEM image with reduced aberration (no aberration).

[0088] In the image acquisition method according to this embodiment, in the step of determining whether the magnitude of the tilt of the zone axis with respect to the optical axis of the illumination system 2 is within a predetermined range, this is determined by whether the circle formed by the shadow of the focusing aperture 13 on the diffraction plane is within a region where the intensity of the Ronchigram is constant. Therefore, in the image acquisition method according to this embodiment, a STEM image is acquired in a state where aberrations are reduced (a state where there is no aberration). You can gain.

[0089] In the image acquisition method according to this embodiment, in the step of aligning the incident direction of the electron beam with the crystal zone axis, the focusing aperture 13 is aligned with the crystal zone axis by tilting the electron beam with the illumination system deflector 14. Therefore, in the image acquisition method according to this embodiment, the incident direction of the electron beam with respect to the sample S can be aligned with the crystal zone axis without tilting the sample S with the sample stage 20.

[0090] In the image acquisition method according to this embodiment, in the step of aligning the incident direction of the electron beam with the zone axis, the focusing aperture 13 is aligned with the zone axis by moving the focusing aperture 13. Therefore, in the image acquisition method according to this embodiment, the incident direction of the electron beam with respect to the sample S can be aligned with the zone axis without tilting the sample S on the sample stage 20.

[0091] In the image acquisition method according to this embodiment, in the step of aligning the center of the Ronchigram with the center O of the detection surface 32 of the imaging device 30, an image of the change in the Ronchigram due to a change in the relative positional relationship between the sample S and the electron probe is acquired, and the center of the Ronchigram is determined based on the image of the change in the Ronchigram. Therefore, with the image acquisition method according to this embodiment, the center of the Ronchigram can be determined from an amorphous region contained in a general sample without preparing a sample with a distinctive shape. Therefore, for example, there is no need to change the sample to adjust the optical system, and the center of the Ronchigram can be easily detected.

[0092] The image acquisition method according to this embodiment includes a step of acquiring a STEM image by scanning the sample S with an electron beam, and in the step of acquiring the STEM image, the direction in which the scanning line L is drawn is orthogonal to the direction of tilt of the sample S. Therefore, in the image acquisition method according to this embodiment, it is possible to reduce the change in height of the sample S while one scanning line L is drawn.

[0093] In the image acquisition method according to this embodiment, in the process of acquiring a STEM image, the focus is changed by an amount corresponding to the tilt of the sample S each time a scanning line L is drawn. Therefore, in the image acquisition method according to this embodiment, it is possible to reduce the shift in focus due to the tilt of the sample S.

[0094] In the image acquisition method according to this embodiment, in the step of acquiring a STEM image, the size of the scanning area Sa in the tilt direction of the sample S is determined based on the tilt direction and the magnitude of the tilt of the sample S. Therefore, in the image acquisition method according to this embodiment, it is possible to prevent the STEM image from shrinking in the tilt direction of the sample S.

[0095] The image acquisition method according to this embodiment includes a step of correcting the size of the STEM image in the direction corresponding to the tilt direction of the sample, based on the tilt direction and the tilt magnitude of the sample S. Therefore, the image acquisition method according to this embodiment can correct a STEM image that has been reduced in size in the tilt direction of the sample S.

[0096] The image acquisition method according to this embodiment includes a step of calculating the direction and magnitude of tilt of the sample S based on the direction and amount of deflection of the electron beam by the irradiation system deflector 14. Therefore, the image acquisition method according to this embodiment makes it possible to easily acquire information about the direction and magnitude of tilt of the sample S.

[0097] In the scanning transmission electron microscope 100, the control unit 50 performs the following processes: deflecting the electron beam with the imaging system deflector 18 to align the center of the Ronchigram with the center O of the detection plane 32 of the imaging device 30; aligning the shadow of the focusing aperture 13 on the diffraction plane with the zone axis of the sample S to align the incident direction of the electron beam with the zone axis; and deflecting the electron beam with the imaging system deflector 18 to align the electron beam with the center O of the detection plane 32 of the imaging device 30. Therefore, in the scanning transmission electron microscope 100, sample drift caused by tilting the sample stage 20 can be reduced.

[0098] 6. Variations In the above-described embodiment, in step S108 of determining whether the tilt of the zone axis is within the valid range, whether the tilt of the zone axis is within the valid range is determined based on whether the circle formed by the shadow of the focusing aperture 13 is located within circle A, which indicates a region where the intensity of the Ronchigram is uniform.

[0099] However, the criteria for determining whether the tilt of the zone axis is within the valid range in step S108 are not limited to this. For example, whether the tilt of the zone axis is within the valid range may be determined based on whether the circle formed by the shadow of the focusing aperture 13 is located within a circle indicating an area free from the effects of chromatic aberration.

[0100] Fig. 18 is a diagram for explaining the relationship between the tilt of the sample and chromatic aberration. Fig. 18 shows electron diffraction patterns obtained when the tilt of the zone axis relative to the optical axis is 0 mrad (without tilt), when the tilt of the zone axis is 13 mrad, when the tilt of the zone axis is 32 mrad, and when the tilt of the zone axis is 65 mrad, as well as the electron diffraction patterns obtained when a silicon single crystal is used. <110> 18 shows a STEM image taken from the direction of the focusing aperture 13. In the example shown in Fig. 18, the region of uniform intensity in the Ronchigram extends up to a focusing half angle of 80 mrad, and the focusing half angle of the focusing aperture 13 is 26.1 mrad.

[0101] In the example shown in Figure 18, even if the zone axis is tilted up to 55.9 mrad (82 mrad - 26.1 mrad), the shadow of the focusing aperture 13 is a region where the intensity of the Ronchigram is uniform, and it is thought that there is no effect of aberration. However, as shown in Figure 18, depending on the magnitude of the tilt of the zone axis, a lack of electron diffraction spots in one direction was observed. This is due to the effect of the tilt of the electron beam and chromatic aberration.

[0102] Here, the effect of chromatic aberration depends on the magnitude of the inclination of the electron beam. Therefore, in the electron diffraction pattern shown in FIG. 18, the greater the inclination of the electron beam, the larger the chipping of the electron diffraction spot. Also, in the STEM image shown in FIG. 18, as the inclination of the electron beam increases, the more the electron diffraction spot becomes chipped with respect to the silicon single crystal. <110> The dumbbell structure becomes unclear when the electron beam is incident from the direction. The magnitude of the inclination of the electron beam to obtain a good quality STEM image without being affected by this chromatic aberration is thought to be about 30 mrad.

[0103] Therefore, whether the tilt of the zone axis is within the effective range is determined by whether the circle formed by the shadow of the focusing aperture 13 is located within the focusing half angle of 30 mrad. This allows the focusing aperture 13 to select an electron beam within an angular range free of chromatic aberration, and the electron beam within this angular range free of chromatic aberration can be irradiated onto the sample S.

[0104] The above-described embodiment and modifications are merely examples, and the present invention is not limited to these. For example, the embodiments and modifications can be combined as appropriate.

[0105] The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the present invention includes configurations that are substantially identical to the configurations described in the embodiments. A substantially identical configuration means, for example, a configuration with the same function, method, and result, or a configuration with the same purpose and effect. The present invention also includes configurations in which non-essential parts of the configurations described in the embodiments are replaced. The present invention also includes configurations that achieve the same effects or purposes as the configurations described in the embodiments. The present invention also includes configurations in which publicly known technology is added to the configurations described in the embodiments. [Explanation of symbols]

[0106] 2...Illumination system, 4...Imaging system, 10...Optical system, 11...Electron source, 12...Converging lens, 13...Converging aperture, 14...Illumination system deflector, 15...Aberration corrector, 16...Objective lens, 17...Intermediate lens , 18... imaging system deflector, 20... sample stage, 30... imaging device, 32... detection surface, 40... detector, 50... control unit, 100... scanning transmission electron microscope

Claims

1. an electron source that emits an electron beam; an illumination system including a focusing lens, an aperture, and an illumination system deflector that deflects the electron beam that has passed through the aperture; a sample stage for supporting the sample; an imaging device capable of photographing a Ronchigram formed on a diffractive surface; an imaging system deflector that deflects an electron beam incident on the imaging device; 1. A method for imaging a crystalline sample in a scanning transmission electron microscope, comprising: aligning the center of the Ronchigram with the center of the detection surface of the imaging device; a step of aligning a shadow of the aperture on the diffraction plane with a zone axis of the sample, and aligning an incident direction of the electron beam with the zone axis of the sample; deflecting the electron beam with the imaging system deflector to align the electron beam with the center of the detection surface of the imaging device; An image acquisition method comprising:

2. In claim 1, a step of determining whether or not the magnitude of the inclination of the zone axis with respect to the optical axis of the irradiation system is within a predetermined range after the step of aligning the incident direction of the electron beam with the zone axis; tilting the sample on the sample stage to reduce the tilt of the zone axis relative to the optical axis of the illumination system when it is determined that the magnitude of the tilt of the zone axis is not within a predetermined range; An image acquisition method comprising:

3. In claim 1, In the step of aligning the incident direction of the electron beam with the crystal zone axis, the electron beam is tilted by the irradiation system deflector to align the shadow of the aperture with the crystal zone axis.

4. In claim 1, In the step of aligning the incident direction of the electron beam with the crystal zone axis, the diaphragm is moved to align the shadow of the diaphragm with the crystal zone axis.

5. In claim 1, In the step of aligning the center of the Ronchigram with the center of the detection surface of the imaging device, acquiring an image of a Ronchigram change due to a change in the relative positional relationship between the sample and the electron beam; and determining the center of the Ronchigram based on an image of the Ronchigram change.

6. In any one of claims 1 to 5, scanning the sample with an electron beam to obtain a scanned image; In the step of acquiring the scanned image, a direction in which a scanning line is drawn is orthogonal to a direction in which the sample is tilted.

7. In claim 6, In the step of acquiring the scanned image, the focus is changed by an amount corresponding to the tilt of the sample each time a scanning line is drawn.

8. In claim 6, An image acquisition method, wherein in the step of acquiring the scanned image, a width of a scanning area in the tilt direction of the sample is determined based on the tilt direction and the magnitude of the tilt of the sample.

9. In claim 6, An image acquisition method comprising a step of correcting the size of the scanned image in a direction corresponding to the direction of tilt of the sample based on the direction of tilt of the sample and the magnitude of the tilt of the sample.

10. In claim 6, An image acquisition method comprising the step of calculating a direction and a magnitude of tilt of the sample based on a direction and an amount of deflection of the electron beam by the illumination system deflector.

11. an electron source that emits an electron beam; an illumination system including a focusing lens, an aperture, and an illumination system deflector that deflects the electron beam that has passed through the aperture; a sample stage for supporting the sample; an imaging device capable of photographing a Ronchigram formed on a diffractive surface; an imaging system deflector that deflects an electron beam incident on the imaging device; a control unit for controlling the illumination system and the imaging system deflector; Including, The control unit a process of deflecting the electron beam with the imaging system deflector to align the center of the Ronchigram with the center of the detection surface of the imaging device; a process of aligning the shadow of the aperture on the diffraction plane with a zone axis of the sample, and aligning the incident direction of the electron beam with the zone axis of the sample; a process of deflecting the electron beam with the imaging system deflector to align the electron beam with the center of the detection surface of the imaging device; Scanning transmission electron microscope.

12. In claim 11, The control unit a process of aligning the incident direction of the electron beam with the zone axis, and then determining whether or not the magnitude of the inclination of the zone axis with respect to the optical axis of the irradiation system is within a predetermined range; a process of tilting the sample on the sample stage to reduce the tilt of the zone axis with respect to the optical axis of the illumination system when it is determined that the magnitude of the tilt of the zone axis is not within a predetermined range; Scanning transmission electron microscope.

13. In claim 11, In the process of aligning the incident direction of the electron beam with the crystal zone axis, the control unit tilts the electron beam with the irradiation system deflector to align the shadow of the aperture with the crystal zone axis.

14. In claim 11, The control unit, in the process of aligning the center of the Ronchigram with the center of the detection surface of the imaging device, acquiring an image of a Ronchigram change due to a change in the relative positional relationship between the sample and the electron beam; The scanning transmission electron microscope determines the center of the Ronchigram based on an image of the Ronchigram change.

15. In any one of claims 11 to 14, the control unit performs processing to scan the sample with an electron beam and obtain a scanned image; In the process of acquiring the scanned image, the direction of the scanning line is set perpendicular to the tilt direction of the sample. Scanning transmission electron microscope.

16. In claim 15, The control unit changes the focus by an amount corresponding to the tilt of the sample each time a scan line is drawn in the process of acquiring the scanned image.

17. In claim 15, The control unit determines the width of the scanning region in the tilt direction of the sample based on the tilt direction and the magnitude of the tilt of the sample in the process of acquiring the scanned image.

18. an electron source that emits an electron beam; an illumination system for scanning the sample with an electron beam; a sample stage for supporting the sample; a detector for detecting electrons transmitted through the sample; a control unit that controls the irradiation system; Including, the control unit performs processing to scan the sample with an electron beam and obtain a scanned image; In the process of acquiring the scanned image, the direction of drawing the scanning line is set perpendicular to the tilt direction of the sample.

Images