Scanning transmission electron microscope and aperture alignment method

Abstract

To provide a scanning transmission electron microscope in which the position of an aperture can be easily adjusted.SOLUTION: A scanning transmission electron microscope according to the present invention includes an electron source that generates an electron beam, an optical system having a focusing lens, an aperture, and an objective lens that focuses the electron beam generated by the electron source to form an electron probe, and a control unit that controls the electron source and the optical system, and the optical system is in a state in which an image does not move at the center of a Ronchigram even when an acceleration voltage for accelerating the electron beam is varied, and the control unit performs the following processes: inserting the aperture into the path of the electron beam; acquiring a first STEM image with the acceleration voltage set to a first voltage value while the aperture is inserted; acquiring a second STEM image with the acceleration voltage set to a second voltage value different from the first voltage value while the aperture is inserted; and moving the aperture based on the positional deviation between the first STEM image and the second STEM image.SELECTED DRAWING: Figure 4

Description

Technical Field

[0006] , , ,

[0001] The present invention relates to a scanning transmission electron microscope and a method for aligning an aperture.

Background Art

[0002] A scanning transmission electron microscope (STEM: scanning transmission electron microscope) is a device that focuses an electron beam generated by an electron source to form an electron probe, scans a sample with the electron probe, and detects electrons transmitted through the sample to obtain a scanning transmission electron microscope image (STEM image).

[0003] In a scanning transmission electron microscope, a Ronchigram is used for adjusting an optical system such as axial adjustment and aberration correction. A Ronchigram is a projection image (figure) of a sample formed on a diffraction plane by focusing an electron beam near the sample in a scanning transmission electron microscope.

[0004] In a scanning transmission electron microscope, in order to reduce the aberration of the irradiation system, the center of the aperture of the irradiation system is aligned with the center of the Ronchigram. Such a method for aligning an aperture is disclosed in, for example, Patent Document 1.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0007] One embodiment of the scanning transmission electron microscope according to the present invention is: An electron source that generates an electron beam, Focusing lens, aperture, deflector, and an optical system having an objective lens that focuses the electron beam generated by the electron source to form an electron probe, A control unit that controls the electron source and the optical system, Includes 、 The control unit, A process to adjust the optical system so that the image does not shift at the center of the Ronchigram even when the acceleration voltage for accelerating the electron beam is varied, The process of inserting the aperture into the electron beam path, With the aperture inserted, the process involves acquiring a first STEM image using the acceleration voltage as the first voltage value, With the aperture inserted, the process of acquiring a second STEM image is performed by setting the acceleration voltage to a second voltage value different from the first voltage value, A process to move the aperture based on the positional misalignment between the first STEM image and the second STEM image, To do so.

[0008] With this type of scanning transmission electron microscope, the center of the aperture can be aligned with the center of the ronchigram using the first and second STEM images, eliminating the need to check the ronchigram each time the aperture is aligned. Therefore, aperture alignment can be easily performed with this type of scanning transmission electron microscope.

[0009] One embodiment of the scanning transmission electron microscope according to the present invention is: An electron source that generates an electron beam, Focusing lens, aperture, deflector, and has an objective lens, and generates in the electron source An optical system that focuses an electron beam to form an electron probe, A control unit that controls the optical system, Includes, The control unit, A process of adjusting the optical system so that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens is varied, The process of inserting the aperture into the electron beam path, With the aperture inserted, the process of acquiring a first STEM image using the excitation current as the first current value, With the aperture inserted, the process involves setting the excitation current to a second current value different from the first current value to acquire a second STEM image. A process to move the aperture based on the positional misalignment between the first STEM image and the second STEM image, To do so.

[0010] With this type of scanning transmission electron microscope, the center of the aperture can be aligned with the center of the ronchigram using the first and second STEM images, eliminating the need to check the ronchigram each time the aperture is aligned. Therefore, aperture alignment can be easily performed with this type of scanning transmission electron microscope.

[0011] One aspect of the aperture alignment method according to the present invention is: An electron source that generates an electron beam, Focusing lens, aperture, deflector, and an optical system having an objective lens that focuses the electron beam generated by the electron source to form an electron probe, A method for aligning the aperture in a scanning transmission electron microscope, including, A step of adjusting the optical system so that the image at the center of the ronchigram does not shift even when the accelerating voltage for accelerating the electron beam is varied, The steps include inserting the aperture into the electron beam path, With the aperture inserted, acquiring a first STEM image with the acceleration voltage set to a first voltage value; With the aperture inserted, acquiring a second STEM image with the acceleration voltage set to a second voltage value different from the first voltage value; Based on the misalignment between the first STEM image and the second STEM image, moving the aperture; and including.

[0012] In such an aperture alignment method, since the center of the aperture can be aligned with the center of the Ronchi grating using the first STEM image and the second STEM image, there is no need to check the Ronchi grating every time the aperture is aligned. Therefore, with such an aperture alignment method, the center of the aperture can be easily aligned with the center of the Ronchi grating.

[0013] One aspect of the aperture alignment method according to the present invention is an electron source that generates an electron beam, a focusing lens, an aperture, deflector, and an optical system having an objective lens that focuses the electron beam generated by the electron source to form an electron probe, and the aperture alignment method in a scanning transmission electron microscope including A process of adjusting the optical system so that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens is varied, inserting the aperture into the path of the electron beam, With the aperture inserted, acquiring a first STEM image with the excitation current set to a first current value, and acquiring a second STEM image with the excitation current set to a second current value different from the first current value with the aperture inserted, Based on the misalignment between the first STEM image and the second STEM image, moving the aperture; and including. and including.

[0014] With this aperture alignment method, the center of the aperture can be aligned with the center of the ronchigram using the first and second STEM images, eliminating the need to check the ronchigram each time the aperture is aligned. Therefore, this aperture alignment method makes it easy to align the center of the aperture with the center of the ronchigram. [Brief explanation of the drawing]

[0015] [Figure 1] A diagram showing an example of the configuration of a scanning transmission electron microscope according to the first embodiment. [Figure 2] This diagram shows how an aperture is inserted so that its center aligns with the optical axis of the illumination system, with the optical axis of the illumination system aligned with the center of the ronchigram. [Figure 3] This diagram shows how an aperture is inserted so that its center aligns with the optical axis of the illumination system, when the optical axis of the illumination system is offset from the center of the ronchigram. [Figure 4] A flowchart showing an example of how to align apertures. [Figure 5] Ronchigrams taken before and after changing the accelerating voltage, with the optical axis of the irradiation system aligned with the center of the Ronchigram. [Figure 6] Ronchigrams taken before and after changing the accelerating voltage, with the optical axis of the irradiation system offset from the center of the Ronchigram. [Figure 7] STEM images taken with the aperture center aligned with the center of the ronchigram, before and after changing the accelerating voltage. [Figure 8] STEM images taken before and after changing the accelerating voltage, with the aperture center offset from the center of the ronchigram. [Figure 9] This diagram shows the state where the center of the aperture aligns with the center of the ronchigram. [Figure 10] This diagram shows a state where the center of the aperture is offset from the center of the ronchigram. [Figure 11]A flowchart illustrating an example of aperture alignment processing in the control unit. [Figure 12] A flowchart showing an example of how to align apertures. [Figure 13] A flowchart illustrating an example of aperture alignment processing in the control unit. [Modes for carrying out the invention]

[0016] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. The embodiments described below are not intended to unduly limit the scope of the present invention as described in the claims. Furthermore, not all of the configurations described below are essential components of the present invention.

[0017] 1. First Embodiment 1.1. Scanning transmission electron microscope First, a scanning transmission electron microscope according to the first embodiment will be described with reference to the drawings. Figure 1 shows an example of the configuration of a scanning transmission electron microscope 100 according to the first embodiment.

[0018] The scanning transmission electron microscope 100 is a device for scanning a sample S with an electron probe, detecting electrons that have passed through the sample S, and acquiring a scanning transmission electron microscope image (hereinafter also referred to as a "STEM image"). The scanning transmission electron microscope 100 is equipped with a STEM mode for acquiring STEM images and a TEM mode for acquiring transmission electron microscope images (hereinafter also referred to as "TEM images").

[0019] As shown in Figure 1, the scanning transmission electron microscope 100 includes an electron source 10, an optical system 20, a sample stage 30, a sample holder 32, a detector 40, and a control unit 50.

[0020] The electron source 10 generates an electron beam. The electron source 10 is, for example, an electron gun that accelerates electrons emitted from the cathode at the anode and emits an electron beam. The electron beam generated by the electron source 10 is accelerated by a predetermined acceleration voltage. The acceleration voltage is the voltage used to accelerate the electron beam generated by the electron source 10 and irradiated onto the sample S. The acceleration voltage is controlled by the control unit 50.

[0021] The optical system 20 includes an irradiation system 20a and an imaging system 20b. The irradiation system 20a is an optical system for irradiating the sample S with an electron beam emitted from the electron source 10. In the irradiation system 20a, the electron beam emitted from the electron source 10 is focused to form an electron probe, and the sample S can be scanned with this electron probe.

[0022] The irradiation system 20a includes a focusing lens 21, an aperture 22, a scanning deflector 24, and an objective lens 26. The focusing lens 21 focuses the electron beam emitted from the electron source 10. The focusing lens 21, together with the objective lens 26, focuses the electron beam to form an electron probe.

[0023] The aperture 22 is incorporated into the illumination system 20a. The aperture 22 reduces the aberration of the illumination system 20a by cutting out unwanted electrons far from the optical axis of the illumination system 20a, allowing only electron beams near the optical axis to pass through. The aperture 22 is, for example, a metal plate with a circular opening formed therein to allow the electron beam to pass through. The aperture 22 is, for example, placed inside the focusing lens 21, that is, in the magnetic field that functions as the focusing lens 21.

[0024] The scanning transmission electron microscope 100 has a movement mechanism 23 for moving the aperture 22. The movement mechanism 23 moves the aperture 22 in a plane perpendicular to the optical axis of the irradiation system 20a. By moving the aperture 22 with the movement mechanism 23, the aperture 22 can be inserted into or removed from the path of the electron beam.

[0025] The scanning deflector 24 deflects the electron beam emitted from the electron source 10 in two dimensions. By deflecting the electron beam with the scanning deflector 24, the sample S can be scanned with the electron probe.

[0026] The objective lens 26 forms a TEM image in TEM mode and focuses the electron beam to form an electron probe in STEM mode. The objective lens 26 creates a forward magnetic field in front of the sample S (towards the focusing lens 21) and a back magnetic field behind the sample S (towards the intermediate lens 27). Electron diffraction patterns and Ronchigrams are formed on the back focal plane of the objective lens 26.

[0027] The scanning transmission electron microscope 100 does not have an aberration corrector to correct the spherical aberration of the illumination system 20a. Therefore, the scanning transmission electron microscope 100 cannot correct the spherical aberration of the illumination system 20a.

[0028] The illumination system 20a may also include optical elements other than the focusing lens 21, aperture 22, scanning deflector 24, and objective lens 26, such as lenses, apertures, and deflectors.

[0029] The sample stage 30 holds a sample holder 32 that supports the sample S. The sample stage 30 is equipped with, for example, a moving mechanism for moving the sample S.

[0030] The imaging system 20b guides the electron beam that has passed through the sample S to the detector 40. The imaging system 20b includes an objective lens 26, an intermediate lens 27, and a projection lens 28. The intermediate lens 27 adjusts its excitation current to change its focal length, focusing on the diffraction pattern or TEM image created by the objective lens 26, magnifying them, and forming an image of them on the object plane of the projection lens 28. The projection lens 28 further magnifies the image magnified by the intermediate lens 27 and forms an image on the detector 40.

[0031] The imaging system 20b may also include optical elements other than the objective lens 26, intermediate lens 27, and projection lens 28, such as lenses, apertures, and deflectors.

[0032] Detector 40 detects electrons that have passed through the sample S. Detector 40 is, for example, a bright-field STEM detector. The bright-field STEM detector detects electrons that have passed through the sample S without scattering, and electrons that have been scattered at a small angle. Although not shown in the figures, the scanning transmission electron microscope 100 may also be equipped with an annular dark-field STEM detector that detects electrons scattered at a high angle.

[0033] Although not shown in the diagram, the scanning transmission electron microscope 100 includes an imaging device for capturing ronchigrams. The imaging device is a digital camera capable of recording ronchigrams as two-dimensional digital images.

[0034] The control unit 50 controls various parts of the scanning transmission electron microscope 100. The control unit 50 includes, for example, a processor such as a CPU (Central Processing Unit) and a memory device such as RAM (Random Access Memory) and ROM (Read Only Memory). The memory device stores programs and data for performing various controls. The functions of the control unit 50 can be realized by executing a program on the processor. The control unit 50 may be realized by a general-purpose circuit such as a microcontroller or microprocessor that operates according to a program, or by a dedicated circuit such as an ASIC (application specific integrated circuit).

[0035] The control unit 50 performs processes such as focusing, correcting astigmatism, adjusting the gain and offset of the detector 40, and aligning the aperture 22. Therefore, the scanning transmission electron microscope 100 is equipped with an autofocus function for automatic focusing, an autostigma function for automatic correction of astigmatism, an autogain / offset adjustment function for automatic adjustment of gain and offset, and a function for automatic alignment of the aperture 22.

[0036] The control unit 50 performs focusing, astigmatism correction, and gain and offset adjustment using known methods. The process of aligning the aperture 22 will be described later in "1.3. Aperture Alignment Process".

[0037] The scanning transmission electron microscope 100 is equipped with a STEM mode that functions as a scanning transmission electron microscope and a TEM mode that functions as a transmission electron microscope.

[0038] In STEM mode, the electron beam emitted from the electron source 10 is focused by the focusing lens 21 and the objective lens 26 to form an electron probe on the sample S, which is then deflected by the scanning deflector 24. This causes the sample S to be scanned by the electron probe. The imaging system 20b guides the electron beam that has passed through the sample S to the detector 40, where it is detected. By synchronizing the intensity of the electron beam detected by the detector 40 with the scanning of the electron probe, a STEM image can be obtained.

[0039] In TEM mode, the electron beam emitted from the electron source 10 is focused by the focusing lens 21 and irradiated onto the sample S. The objective lens 26 forms a TEM image with the electron beam that has passed through the sample S. The TEM image formed by the objective lens 26 is then projected onto the imaging device by the intermediate lens 27 and the projection lens 28. This allows a TEM image to be acquired.

[0040] When switching from TEM mode to STEM mode in the scanning transmission electron microscope 100, adjustments are made to the focus, correction of astigmatism, adjustment of the gain and offset of the detector 40, and alignment of the aperture 22. The scanning transmission electron microscope 100 is equipped with an autofocus function, an autostigma function, an auto gain / offset adjustment function, and a function to automatically align the aperture 22, so the adjustment of the optical system 20 when switching from TEM mode to STEM mode can be performed automatically.

[0041] 1.2. Aperture alignment 1.2.1. Principle In aligning aperture 22, the center of aperture 22 is aligned with the center of the Ronchigram. The center of aperture 22 is the center of the opening of aperture 22.

[0042] A ronchigram is a projection image (figure) of a sample S formed on the diffraction plane when an electron beam is focused near the sample S. Specifically, a ronchigram is the diffraction pattern of electrons that have passed through the sample S, formed along the optical axis.

[0043] The Ronchigram is compatible with the electron probe. Therefore, by aligning the center of aperture 22 with the center of the Ronchigram, aberrations can be reduced, and the size of the electron probe can be reduced. This improves the resolution.

[0044] In the method for aligning the aperture 22 in the scanning transmission electron microscope 100, with the optical axis of the irradiation system 20a aligned with the center of the ronchigram, the center of the aperture 22 is aligned with the optical axis of the irradiation system 20a, thereby aligning the center of the aperture 22 with the center of the ronchigram.

[0045] Figure 2 shows the aperture 22 inserted so that its center coincides with the optical axis of the illumination system 20a, with the optical axis of the illumination system 20a aligned with the center of the ronchigram. Figure 3 shows the aperture 22 inserted so that its center coincides with the optical axis of the illumination system 20a, with the optical axis of the illumination system 20a offset from the center of the ronchigram.

[0046] Figure 2 shows an image of the illumination system 20a with its optical axis aligned with the center of the Ronchigram, and an image of the aperture 22 inserted so that its center aligns with the optical axis of the illumination system 20a. Figure 3 shows an image of the illumination system 20a with its optical axis offset from the center of the Ronchigram, and an image of the aperture 22 inserted so that its center aligns with the optical axis of the illumination system 20a. In Figures 2 and 3, the intersection of the two lines represents the optical axis of the illumination system 20a.

[0047] As shown in Figure 2, with the optical axis of the illumination system 20a aligned with the center of the ronchigram, the aperture 22 is moved so that its center coincides with the optical axis of the illumination system 20a. This allows the center of the aperture 22 to be aligned with the center of the ronchigram.

[0048] Furthermore, as shown in Figure 3, when the optical axis of the illumination system 20a is offset from the center of the Ronchigram, even if the aperture 22 is moved so that its center coincides with the optical axis of the illumination system 20a, the center of the aperture 22 will still be offset from the center of the Ronchigram.

[0049] Here, the optical axis of the illumination system 20a is the voltage axis. In the scanning transmission electron microscope 100, when the accelerating voltage is varied, the image expands and contracts concentrically. The center of this expansion and contraction is the voltage axis. When the optical axis of the illumination system 20a is aligned with the center of the ronchigram, when the accelerating voltage is varied, the image expands and contracts concentrically around the center of the ronchigram. Therefore, when the optical axis of the illumination system 20a is aligned with the center of the ronchigram, the image does not move at the center of the ronchigram even when the accelerating voltage is varied. This can be used to align the center of the aperture 22 with the center of the ronchigram. The method for aligning the aperture 22 will be explained in detail below.

[0050] 1.2.2. Method for aligning apertures Figure 4 is a flowchart showing an example of a method for aligning aperture 22.

[0051] First, the optical system 20 is adjusted so that the image does not shift at the center of the ronchigram even when the accelerating voltage is varied, and the optical axis (voltage axis) of the illumination system 20a is aligned with the center of the ronchigram (step S10).

[0052] Specifically, first, the optical system 20 is set up to observe the longchigram, and the center of the longchigram is identified. Next, the longchigram is observed before and after changing the accelerating voltage, and the irradiation system 20a is adjusted so that the image does not shift at the center of the longchigram even when the accelerating voltage is changed. For example, by deflecting the electron beam with a deflector incorporated into the irradiation system 20a, the optical axis of the irradiation system 20a is aligned with the center of the longchigram. By adjusting the irradiation system 20a so that the image does not shift at the center of the longchigram even when the accelerating voltage is changed, the optical axis of the irradiation system 20a can be aligned with the center of the longchigram.

[0053] Figure 5 shows the Ronchigram before and after changing the accelerating voltage, taken with the optical axis of the irradiation system 20a aligned with the center of the Ronchigram. Figure 6 shows the Ronchigram before and after changing the accelerating voltage, taken with the optical axis of the irradiation system 20a offset from the center of the Ronchigram.

[0054] As shown in Figure 5, when the optical axis of the irradiation system 20a is aligned with the center of the Ronchigram, the image does not shift before and after changing the accelerating voltage. In contrast, as shown in Figure 6, when the optical axis of the irradiation system 20a is offset from the center of the Ronchigram, the image shifts before and after changing the accelerating voltage. Therefore, as shown in Figure 5, the irradiation system 20a is adjusted so that the image does not shift before and after changing the accelerating voltage.

[0055] Furthermore, the relative positions of the center of the Ronchigram and the optical axis of the illumination system 20a remain almost unchanged. Therefore, once the optical axis of the illumination system 20a is aligned with the center of the Ronchigram, this operation does not need to be repeated.

[0056] Next, with the optical axis of the irradiation system 20a aligned with the center of the Ronchigram, that is, with the image not shifting at the center of the Ronchigram even when the accelerating voltage is varied, aperture 2 Insert 2 into the electron beam path (step S12).

[0057] Next, the optical system 20 is set up to capture STEM images, and STEM images are taken before and after changing the accelerating voltage (step S14).

[0058] Specifically, first, a STEM image of sample S is taken. The location where the STEM image is taken is not particularly limited, as long as it is a region on sample S that contains a landmark image within the field of view. Next, the accelerating voltage is changed, and a STEM image of sample S is taken after the accelerating voltage has been changed. The conditions for taking the STEM image before changing the accelerating voltage and the conditions for taking the STEM image after changing the accelerating voltage are the same, except for the voltage value of the accelerating voltage.

[0059] Next, the STEM images before and after changing the accelerating voltage are compared to calculate the positional displacement between the STEM images before and after the change in accelerating voltage (step S16). The positional displacement between the STEM images before and after changing the accelerating voltage includes the amount of the displacement and the direction of the displacement. For example, the amount of the positional displacement between the STEM images before and after changing the accelerating voltage can be calculated by determining the cross-correlation between the two STEM images.

[0060] Next, the aperture 22 is moved based on the positional shift between the STEM images before and after the change in acceleration voltage (step S18).

[0061] Specifically, first, the positional displacement between the center of aperture 22 and the center of the ronchigram (i.e., the optical axis of the illumination system 20a) is calculated from the positional displacement between the STEM images before and after changing the accelerating voltage. Here, the illumination system 20a is in a state where the image at the center of the ronchigram does not move even when the accelerating voltage is changed. Therefore, the positional displacement between the STEM images before and after changing the accelerating voltage corresponds to the positional displacement between the center of aperture 22 and the center of the ronchigram. Thus, the positional displacement between the center of aperture 22 and the center of the ronchigram can be calculated from the positional displacement between the STEM images before and after changing the accelerating voltage.

[0062] Next, the amount and direction of movement of aperture 22 so that its center aligns with the center of the Ronchigram are calculated from the positional displacement between the center of aperture 22 and the center of the Ronchigram. Here, the relationship between the distance on the STEM image and the amount of movement of aperture 22 is pre-calibrated. Similarly, the relationship between the direction on the STEM image and the direction of movement of aperture 22 is pre-calibrated. Therefore, the amount and direction of movement of aperture 22 are determined using the calibration results from the positional displacement between the center of aperture 22 and the center of the Ronchigram. Next, aperture 22 is moved according to the determined amount and direction of movement of aperture 22. This allows aperture 22 to be moved so that its center aligns with the center of the Ronchigram.

[0063] Figure 7 shows STEM images taken before and after changing the accelerating voltage, with the center of aperture 22 aligned with the center of the ronchigram. Figure 8 shows STEM images taken before and after changing the accelerating voltage, with the center of aperture 22 offset from the center of the ronchigram.

[0064] As shown in Figure 7, when the center of aperture 22 is aligned with the center of the ronchigram, the image does not shift before and after changing the accelerating voltage. In contrast, as shown in Figure 8, when the center of aperture 22 is offset from the center of the ronchigram, the image shifts before and after changing the accelerating voltage.

[0065] The process of acquiring STEM images before and after changing the acceleration voltage, calculating the positional shift, and moving the aperture 22 described above may be repeated until the positional shift between the STEM images before and after changing the acceleration voltage is eliminated, that is, until the center of aperture 22 aligns with the center of the ronchigram.

[0066] Through the above steps, the center of aperture 22 can be aligned with the center of the ronchigram.

[0067] Figure 9 shows the state where the center of aperture 22 is aligned with the center of the ronchigram. Figure 10 shows the state where the center of aperture 22 is offset from the center of the ronchigram. By using the above method of aligning aperture 22, the center of aperture 22 can be aligned with the center of the ronchigram, as shown in Figure 9.

[0068] 1.3. Aperture alignment process In the scanning transmission electron microscope 100, the optical system 20 is adjusted so that the optical axis (voltage axis) of the irradiation system 20a is at the center of the ronchigram. In other words, in the scanning transmission electron microscope 100, the optical system 20 is configured such that the image does not shift at the center of the ronchigram even when the accelerating voltage is varied.

[0069] Figure 11 is a flowchart showing an example of the aperture 22 alignment process of the control unit 50.

[0070] The control unit 50 determines whether the user has given an instruction to start the aperture 22 alignment process (step S100). Although not shown in the figures, the control unit 50 determines that the user has given an instruction to start the alignment process when the alignment start button on the GUI (Graphical User Interface) of the scanning transmission electron microscope 100 is pressed or when an instruction to start the alignment process is input via an input device.

[0071] If the control unit 50 determines that the user has issued a start command (Yes in step S100), it causes the moving mechanism 23 to insert the aperture 22 into the electron beam path (step S102). At this time, the control unit 50 positions the aperture 22 at a pre-set initial position. The initial position may be, for example, the position of the aperture 22 determined by the previous positioning process of the aperture 22, or it may be any set position.

[0072] Next, with the aperture 22 inserted, the control unit 50 takes a first STEM image using the acceleration voltage as the first voltage value (step S104). The control unit 50 sets the acceleration voltage to the first voltage value and controls the optical system 20 to scan the sample S with the electron probe. This allows the first STEM image to be acquired. The first voltage value can be set to any value. For example, in the case of a scanning transmission electron microscope with an acceleration voltage of 200kV, the first voltage value may be 200kV.

[0073] Next, the control unit 50 changes the acceleration voltage from the first voltage value to the second voltage value (step S106). The second voltage value is different from the first voltage value. The second voltage value can be set to any value as long as it is different from the first voltage value. The rate of change of the second voltage value relative to the first voltage value is, for example, about 0.5%.

[0074] Next, the control unit 50 takes a second STEM image with the aperture 22 inserted (step S108). The control unit 50 controls the optical system 20 to scan the sample S with the electron probe. This allows the second STEM image to be acquired. The imaging conditions when taking the second STEM image are the same as when taking the first STEM image, except that the acceleration voltage is the second voltage value. The shooting conditions are the same as before.

[0075] Next, the control unit 50 calculates the positional displacement between the first STEM image and the second STEM image (step S110). The control unit 50 calculates the amount of positional displacement by calculating the cross-correlation between the first STEM image and the second STEM image.

[0076] The control unit 50 determines whether the calculated displacement is less than or equal to an acceptable value (step S112). The acceptable value is set according to the acceptable displacement between the center of the aperture 22 and the center of the ronchigram. The acceptable value can be set to any value. For example, if the displacement between the first STEM image and the second STEM image is expressed in pixels, the acceptable value is also expressed in pixels.

[0077] If the control unit 50 determines that the amount of misalignment is not less than or equal to the allowable value (No. in step S112), that is, if it determines that the amount of misalignment is greater than the allowable value, it moves the aperture 22 to the moving mechanism 23 based on the amount of misalignment (step S114).

[0078] The memory of the control unit 50 stores the calibration results of the distance on the STEM image and the amount of movement of the aperture 22, and the calibration results of the direction on the STEM image and the direction of movement of the aperture 22. Using these calibration results, the control unit 50 calculates the amount and direction of movement of the aperture 22 from the positional displacement between the first STEM image and the second STEM image.

[0079] The control unit 50 moves the aperture 22 to the moving mechanism 23 by the calculated amount and direction of movement of the aperture 22. After moving the aperture 22 to the moving mechanism 23 (after step S114), the control unit 50 returns to step S104 and takes a first STEM image with the acceleration voltage as the first voltage value (step S104).

[0080] The control unit 50 changes the acceleration voltage from the first voltage value to the second voltage value (step S106), takes a second STEM image (step S108), and calculates the positional displacement between the first STEM image and the second STEM image (step S110).

[0081] The control unit 50 repeats the processes of steps S114, S104, S106, S108, S110, and S112 until it determines that the amount of misalignment is less than or equal to an allowable value.

[0082] If the control unit 50 determines that the amount of misalignment is less than or equal to the allowable value (Yes in step S112), it terminates the alignment process of the aperture 22.

[0083] The control unit 50 performs the above-described aperture 22 alignment process, thereby aligning the center of the aperture 22 with the center of the ronchigram.

[0084] 1.4. Effects The scanning transmission electron microscope 100 includes an electron source 10 that generates an electron beam, an optical system 20 having a focusing lens 21, an aperture 22, and an objective lens 26 that focuses the electron beam generated by the electron source 10 to form an electron probe, and a control unit 50 that controls the electron source 10 and the optical system 20. Furthermore, the optical system 20 is in a state where the image does not move at the center of the ronchigram even when the acceleration voltage is varied. The control unit 50 also performs the following processes: inserting the aperture 22 into the electron beam path; acquiring a first STEM image with the aperture 22 inserted and the acceleration voltage set to a first voltage value; acquiring a second STEM image with the aperture 22 inserted and the acceleration voltage set to a second voltage value different from the first voltage value; and the first STEM Based on the positional misalignment between the image and the second STEM image, the aperture 22 is moved.

[0085] Therefore, in the scanning transmission electron microscope 100, the center of the aperture 22 can be aligned with the center of the ronchigram using the first and second STEM images before and after changing the acceleration voltage. As a result, in the scanning transmission electron microscope 100, it is not necessary to check the ronchigram each time the aperture 22 is inserted and aligned. Thus, the aperture 22 can be easily aligned in the scanning transmission electron microscope 100.

[0086] For example, when inserting aperture 22 while confirming the center of the ronchigram with an imaging device or visually, and aligning the center of aperture 22 with the center of the ronchigram, the ronchigram of the amorphous region of sample S is required to confirm the center of the ronchigram. In addition, the optical system 20 needs to be switched to conditions that allow observation of the ronchigram.

[0087] In contrast, the scanning transmission electron microscope 100 allows for the alignment of the aperture 22 using a STEM image of any region of the sample S. That is, it is not necessary to check the ronchigram each time the aperture 22 is aligned. Therefore, the scanning transmission electron microscope 100 allows for easy alignment of the aperture 22. Furthermore, since it is not necessary to move to the amorphous region of the sample S in order to align the aperture 22, drift associated with the movement of the field of view can be reduced.

[0088] In the scanning transmission electron microscope 100, the user can easily align the aperture 22 by inputting an instruction to start the aperture 22 alignment process, which causes the control unit 50 to align the center of the aperture 22 with the center of the ronchigram.

[0089] The scanning transmission electron microscope 100 is equipped with an autofocus function, an autostigma function, an auto gain / offset adjustment function, and a function to automatically align the aperture 22. Therefore, when switching from TEM mode to STEM mode, the user does not need to adjust the focus, correct astigmatism, adjust the gain and offset of the detector 40, or adjust the position of the aperture 22. Thus, with the scanning transmission electron microscope 100, the user can easily acquire STEM images after switching from TEM mode to STEM mode.

[0090] In the scanning transmission electron microscope 100, the control unit 50 calculates the positional displacement between the center of the aperture 22 and the center of the ronchigram from the positional displacement between the first STEM image and the second STEM image during the process of moving the aperture 22. Therefore, the scanning transmission electron microscope 100 can determine the amount of positional displacement between the center of the aperture 22 and the center of the ronchigram without having to check the ronchigram.

[0091] In the scanning transmission electron microscope 100, the control unit 50 moves the aperture 22 so that its center aligns with the center of the ronchigram. Therefore, the scanning transmission electron microscope 100 can easily align the aperture 22.

[0092] The scanning transmission electron microscope 100 does not have an aberration corrector to correct the spherical aberration of the optical system 20. In a scanning transmission electron microscope without an aberration corrector, coma aberration cannot be observed because spherical aberration is dominant. Therefore, the shape of the ronchigram cannot be observed with such a scanning transmission electron microscope. Consequently, it is difficult to align the center of the aperture 22 with the center of the ronchigram while observing the center of the ronchigram.

[0093] In contrast, with the scanning transmission electron microscope 100, the center of aperture 22 can be aligned with the center of the Ronchigram from the STEM images before and after changing the accelerating voltage, without having to check the Ronchigram. Therefore, with the scanning transmission electron microscope 100, the center of aperture 22 can be easily aligned with the center of the Ronchigram even without an aberration corrector.

[0094] The method for aligning the aperture 22 in a scanning transmission electron microscope 100 includes the steps of: inserting the aperture 22 into the electron beam path when the accelerating voltage for accelerating the electron beam in the optical system 20 is varied, the image at the center of the ronchigram does not move; acquiring a first STEM image with the accelerating voltage set to a first voltage value while the aperture 22 is inserted; acquiring a second STEM image with the accelerating voltage set to a second voltage value different from the first voltage value while the aperture 22 is inserted; and moving the aperture 22 based on the positional misalignment between the first STEM image and the second STEM image.

[0095] Thus, in this aperture 22 alignment method, the center of aperture 22 is aligned with the center of the ronchigram using the first and second STEM images before and after changing the accelerating voltage. Therefore, with this aperture 22 alignment method, it is not necessary to check the ronchigram each time aperture 22 is inserted and aligned. Consequently, aperture 22 can be easily aligned.

[0096] The method for aligning the aperture 22 in the scanning transmission electron microscope 100 includes a step of adjusting the optical system 20 before inserting the aperture 22, so that the image does not shift at the center of the ronchigram even when the accelerating voltage is varied. This step allows the optical system 20 to be aligned with the optical axis of the illumination system 20a at the center of the ronchigram.

[0097] In the method for aligning the aperture 22 in the scanning transmission electron microscope 100, the positional displacement between the center of the aperture 22 and the center of the longigram is calculated from the positional displacement between the first STEM image and the second STEM image during the step of moving the aperture 22. Therefore, in this method for aligning the aperture 22, the positional displacement between the center of the aperture 22 and the center of the longigram can be calculated without checking the longigram.

[0098] In the method for aligning the aperture 22 in the scanning transmission electron microscope 100, the aperture 22 is moved in the step of moving the aperture 22 so that the center of the aperture 22 aligns with the center of the ronchigram. Therefore, the aperture 22 can be easily aligned.

[0099] 2. Second Embodiment 2.1. Scanning transmission electron microscope Next, a scanning transmission electron microscope according to the second embodiment will be described. The configuration of the scanning transmission electron microscope according to the second embodiment is the same as that of the scanning transmission electron microscope 100 according to the first embodiment shown in Figure 1 above, so its description will be omitted.

[0100] 2.2. Aperture Alignment 2.2.1. Principle In the first embodiment described above, the aperture 22 was aligned to the center of the ronchigram by aligning the optical axis of the illumination system 20a with the center of the ronchigram, and then aligning the center of the aperture 22 with the optical axis of the illumination system 20a. Here, in the first embodiment, the optical axis of the optical system 20 was the voltage axis. In contrast, in the second embodiment, the optical axis of the optical system 20 is the current axis.

[0101] When the excitation current of the objective lens 26 is varied, the image expands and contracts concentrically. The center of this expansion and contraction is the current axis. That is, in the second embodiment, the current axis of the illumination system 20a is aligned with the center of the ronchigram. When the optical axis of the illumination system 20a is aligned with the center of the ronchigram, when the excitation current of the objective lens 26 is varied, the image expands and contracts concentrically around the center of the ronchigram. Therefore, when the optical axis of the illumination system 20a is aligned with the center of the ronchigram, the image does not move at the center of the ronchigram even when the excitation current of the objective lens 26 is varied. This can be used to align the center of the aperture 22 with the center of the ronchigram.

[0102] The method for aligning the aperture 22 will be described in detail below. Note that the same points as those described in the first embodiment for aligning the aperture 22 will be omitted from this explanation.

[0103] 2.2.2. Method for aligning apertures Figure 12 is a flowchart showing an example of a method for aligning aperture 22.

[0104] First, the optical system 20 is adjusted so that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens 26 is varied, and the optical axis (current axis) of the illumination system 20a is aligned with the center of the ronchigram (step S20).

[0105] Specifically, first, the optical system 20 is set up to observe the longchigram, and the center of the longchigram is confirmed. Next, the longchigram is observed before and after changing the excitation current of the objective lens 26, and the illumination system 20a is adjusted so that the image does not shift at the center of the longchigram even when the excitation current of the objective lens 26 is changed. For example, by deflecting the electron beam with a deflector incorporated in the illumination system 20a, the optical axis of the illumination system 20a can be aligned with the center of the longchigram. By adjusting the illumination system 20a so that the image does not shift at the center of the longchigram even when the excitation current of the objective lens 26 is changed, the optical axis of the illumination system 20a can be aligned with the center of the longchigram.

[0106] Furthermore, the relative positions of the center of the Ronchigram and the optical axis of the illumination system 20a remain almost unchanged. Therefore, once the optical axis of the illumination system 20a is aligned with the center of the Ronchigram, this operation does not need to be repeated.

[0107] Next, with the optical axis of the illumination system 20a aligned with the center of the ronchigram, that is, with the image remaining at the center of the ronchigram even when the excitation current of the objective lens 26 is varied, the aperture 22 is inserted into the electron beam path (step S22).

[0108] Next, the optical system 20 is set up to capture STEM images, and STEM images are captured before and after changing the excitation current of the objective lens 26 (step S24).

[0109] Specifically, first, a STEM image of sample S is taken. Next, the excitation current of the objective lens 26 is changed, and a STEM image of sample S is taken after the excitation current of the objective lens 26 has been changed. The conditions for taking the STEM image before changing the excitation current of the objective lens 26 and the conditions for taking the STEM image after changing the excitation current of the objective lens 26 are the same except for the current value of the excitation current of the objective lens 26.

[0110] Next, the STEM images before and after changing the excitation current of the objective lens 26 are compared, and the positional displacement between the STEM images before and after changing the excitation current of the objective lens 26 is calculated (step S26). The positional displacement between the STEM images before and after changing the excitation current of the objective lens 26 is calculated as the amount of positional displacement between the STEM images before and after changing the excitation current of the objective lens 26 and the method of positional displacement. Includes direction.

[0111] Next, the aperture 22 is moved based on the positional shift between the STEM images before and after changing the excitation current of the objective lens 26 (step S28).

[0112] Specifically, first, the positional displacement between the center of the aperture 22 and the center of the ronchigram is calculated from the positional displacement between the STEM images before and after changing the excitation current of the objective lens 26. Here, the illumination system 20a is in a state where the image at the center of the ronchigram does not move even when the excitation current of the objective lens 26 is changed. Therefore, the positional displacement between the STEM images before and after changing the excitation current of the objective lens 26 corresponds to the positional displacement between the center of the aperture 22 and the center of the ronchigram. Thus, the positional displacement between the center of the aperture 22 and the center of the ronchigram can be calculated from the positional displacement between the STEM images before and after changing the excitation current of the objective lens 26.

[0113] Next, the amount and direction of movement of aperture 22 are calculated from the positional displacement between the center of aperture 22 and the center of the Ronchigram, so that the center of aperture 22 aligns with the center of the Ronchigram. The calculation of the amount and direction of movement of aperture 22 is performed in the same manner as in step S18 described above.

[0114] The above-described process of capturing STEM images before and after changing the excitation current of the objective lens 26, calculating the positional shift, and moving the aperture 22 may be repeated until the positional shift between the STEM images before and after changing the excitation current of the objective lens 26 is eliminated, that is, until the center of the aperture 22 aligns with the center of the ronchigram.

[0115] Through the above steps, the center of aperture 22 can be aligned with the center of the ronchigram.

[0116] 2.3. Aperture alignment process In the scanning transmission electron microscope 100, the optical system 20 is adjusted so that the optical axis (current axis) of the illumination system 20a is at the center of the ronchigram. In other words, in the scanning transmission electron microscope 100, the optical system 20 is configured such that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens 26 is varied.

[0117] Figure 13 is a flowchart showing an example of the aperture 22 alignment process of the control unit 50. Note that the same points as those described in Figure 11 regarding the aperture 22 alignment process are omitted from the explanation.

[0118] The control unit 50 determines whether the user has given an instruction to start the aperture 22 alignment process (step S200). If the control unit 50 determines that the user has given an instruction to start (Yes in step S200), it causes the moving mechanism 23 to insert the aperture 22 into the electron beam path (step S202).

[0119] Next, with the aperture 22 inserted, the control unit 50 sets the excitation current of the objective lens 26 to a first current value and takes a first STEM image (step S204). The control unit 50 sets the excitation current of the objective lens 26 to a first current value and controls the optical system 20 to scan the sample S with the electron probe. This allows the first STEM image to be acquired. The first current value can be set to any value.

[0120] Next, the control unit 50 changes the excitation current of the objective lens 26 from the first current value to the second current value (step S206). The second current value is different from the first current value. Any value different from the current value can be set to any value.

[0121] Next, the control unit 50 takes a second STEM image with the aperture 22 inserted (step S208). The control unit 50 controls the optical system 20 to scan the sample S with the electron probe. This allows the second STEM image to be acquired. The imaging conditions when taking the second STEM image are the same as those when taking the first STEM image, except that the excitation current of the objective lens 26 is the second current value.

[0122] Next, the control unit 50 calculates the positional displacement between the first STEM image and the second STEM image (step S210), and determines whether the calculated positional displacement is less than or equal to an allowable value (step S212).

[0123] If the control unit 50 determines that the amount of misalignment is not below an acceptable value (No. in step S212), it moves the aperture 22 to the moving mechanism 23 based on the amount of misalignment (step S214).

[0124] After moving the aperture 22 to the moving mechanism 23 (after step S214), the control unit 50 returns to step S204 and takes a first STEM image with the excitation current of the objective lens 26 set to a first current value (step S204). The control unit 50 changes the excitation current of the objective lens 26 from the first current value to a second current value (step S206), takes a second STEM image (step S208), and calculates the positional shift between the first STEM image and the second STEM image (step S210).

[0125] The control unit 50 repeats the processes of steps S214, S204, S206, S208, S210, and S212 until it determines that the amount of misalignment is less than or equal to an allowable value.

[0126] If the control unit 50 determines that the amount of misalignment is less than or equal to the allowable value (Yes in step S212), it terminates the alignment process of the aperture 22.

[0127] The control unit 50 performs the above-described aperture 22 alignment process, thereby aligning the center of the aperture 22 with the center of the ronchigram.

[0128] 2.4. Effects In the scanning transmission electron microscope 100, the optical system 20 is configured such that the image does not move at the center of the ronchigram even when the excitation current of the objective lens 26 is varied. The control unit 50 performs the following processes: inserting the aperture 22 into the electron beam path; acquiring a first STEM image with the excitation current of the objective lens 26 set to a first current value while the aperture 22 is inserted; acquiring a second STEM image with the excitation current of the objective lens 26 set to a second current value different from the first current value while the aperture 22 is inserted; and moving the aperture 22 based on the positional shift between the first STEM image and the second STEM image.

[0129] Therefore, in the scanning transmission electron microscope 100, the center of the aperture 22 can be aligned with the center of the ronchigram by using the first and second STEM images before and after changing the excitation current of the objective lens 26. Thus, the aperture 22 can be easily aligned in the scanning transmission electron microscope 100.

[0130] The method for aligning the aperture 22 in the scanning transmission electron microscope 100 involves inserting the aperture 22 into the electron beam path when the optical system 20 is in a state where the image does not move at the center of the ronchigram even when the excitation current of the objective lens 26 is varied, and the aperture 22 The process includes: acquiring a first STEM image with the excitation current of the objective lens 26 set as a first current value while the aperture 22 is inserted; acquiring a second STEM image with the excitation current of the objective lens 26 set as a second current value different from the first current value while the aperture 22 is inserted; and moving the aperture 22 based on the positional misalignment between the first STEM image and the second STEM image.

[0131] In this method of aligning aperture 22, the center of aperture 22 is aligned with the center of the longigram using the first and second STEM images before and after changing the excitation current of the objective lens 26. Therefore, with this method of aligning aperture 22, it is not necessary to check the longigram each time aperture 22 is inserted and aligned. Thus, aperture 22 can be easily aligned.

[0132] The method for aligning the aperture 22 in the scanning transmission electron microscope 100 includes a step of adjusting the optical system 20 before inserting the aperture 22, so that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens 26 is varied. This step allows the optical system 20 to be aligned with the optical axis of the illumination system 20a at the center of the ronchigram.

[0133] 3. Variations The scanning transmission electron microscope 100 according to the first and second embodiments described above included a STEM mode for acquiring STEM images and a TEM mode for acquiring TEM images, but it may also include only a STEM mode.

[0134] Furthermore, although the first and second embodiments described above described a case in which the scanning transmission electron microscope 100 is not equipped with an aberration corrector for correcting spherical aberration, the scanning transmission electron microscope 100 may be equipped with an aberration corrector for correcting spherical aberration.

[0135] The present invention is not limited to the embodiments described above, and various further modifications are possible. For example, the present invention includes configurations that are substantially identical to those described in the embodiments. A substantially identical configuration is, for example, a configuration that has the same function, method, and result, or a configuration that has the same purpose and effect. The present invention also includes configurations in which non-essential parts of the configuration described in the embodiments are replaced. Furthermore, the present invention includes configurations that produce the same effects or achieve the same purpose as the configuration described in the embodiments. Furthermore, the present invention includes configurations that add known technology to the configuration described in the embodiments. [Explanation of symbols]

[0136] 10...Electron source, 20...Optical system, 20a...Irradiation system, 20b...Imaging system, 21...Focusing lens, 22...Aperture, 23...Movement mechanism, 24...Scanning deflector, 26...Objective lens, 27...Intermediate lens, 28...Projection lens, 30...Sample stage, 32...Sample holder, 40...Detector, 50...Control unit, 100...Scanning transmission electron microscope

Claims

1. An electron source that generates an electron beam, An optical system comprising a focusing lens, an aperture, a deflector, and an objective lens, which focuses the electron beam generated by the electron source to form an electron probe, A control unit that controls the electron source and the optical system, Includes, The control unit, A process to adjust the optical system so that the image does not shift at the center of the Ronchigram even when the acceleration voltage for accelerating the electron beam is varied, The process of inserting the aperture into the electron beam path, With the aperture inserted, the process of acquiring a first STEM image using the acceleration voltage as the first voltage value, With the aperture inserted, the process involves setting the acceleration voltage to a second voltage value different from the first voltage value to acquire a second STEM image. A process to move the aperture based on the positional misalignment between the first STEM image and the second STEM image, A scanning transmission electron microscope (STEM) is used for this purpose.

2. An electron source that generates an electron beam, An optical system comprising a focusing lens, an aperture, a deflector, and an objective lens, which focuses the electron beam generated by the electron source to form an electron probe, A control unit that controls the optical system, Includes, The control unit, A process of adjusting the optical system so that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens is varied, The process of inserting the aperture into the electron beam path, With the aperture inserted, the process of acquiring a first STEM image using the excitation current as the first current value, With the aperture inserted, the process involves setting the excitation current to a second current value different from the first current value to acquire a second STEM image. A process to move the aperture based on the positional misalignment between the first STEM image and the second STEM image, A scanning transmission electron microscope (STEM) is used for this purpose.

3. In claim 1 or 2, The control unit, in the process of moving the aperture, calculates the positional displacement between the center of the aperture and the center of the ronchigram from the positional displacement between the first STEM image and the second STEM image, in a scanning transmission electron microscope.

4. In claim 1 or 2, The control unit moves the aperture in the process of moving the aperture so that the center of the aperture aligns with the center of the ronchigram, in a scanning transmission electron microscope.

5. In claim 1 or 2, The control unit performs a process to determine whether the positional displacement between the first STEM image and the second STEM image is less than or equal to an allowable value. A scanning transmission electron microscope, wherein the control unit performs a process to move the aperture when it determines that the value is not below the tolerance value.

6. In claim 1 or 2, A scanning transmission electron microscope that does not have an aberration corrector for correcting spherical aberration in the optical system.

7. An electron source that generates an electron beam, An optical system comprising a focusing lens, an aperture, a deflector, and an objective lens, which focuses the electron beam generated by the electron source to form an electron probe, A method for aligning the aperture in a scanning transmission electron microscope, including, A step of adjusting the optical system so that the image at the center of the ronchigram does not shift even when the accelerating voltage for accelerating the electron beam is varied, The steps include inserting the aperture into the electron beam path, With the aperture inserted, the process involves acquiring a first STEM image using the acceleration voltage as the first voltage value, With the aperture inserted, the process of acquiring a second STEM image by setting the acceleration voltage to a second voltage value different from the first voltage value, A step of moving the aperture based on the positional misalignment between the first STEM image and the second STEM image, Alignment methods, including those mentioned above.

8. An electron source that generates an electron beam, An optical system comprising a focusing lens, an aperture, a deflector, and an objective lens, which focuses the electron beam generated by the electron source to form an electron probe, A method for aligning the aperture in a scanning transmission electron microscope, including, A process of adjusting the optical system so that the image does not shift at the center of the ronchigram even when the excitation current of the objective lens is varied, The steps include inserting the aperture into the electron beam path, With the aperture inserted, the process involves acquiring a first STEM image using the excitation current as the first current value, With the aperture inserted, the process of acquiring a second STEM image by setting the excitation current to a second current value different from the first current value, A step of moving the aperture based on the positional misalignment between the first STEM image and the second STEM image, Alignment methods, including those mentioned above.

9. In claim 7 or 8, A positioning method for moving the aperture, which involves calculating the positional misalignment between the center of the aperture and the center of the ronchigram from the positional misalignment between the first STEM image and the second STEM image.

10. In claim 7 or 8, A positioning method for moving the aperture, wherein the aperture is moved so that its center aligns with the center of the Ronchigram.

11. In claim 7 or 8, A scanning transmission electron microscope is not equipped with an aberration corrector for correcting spherical aberration in the optical system, and is provided with a positioning method.

Images