Method and system for ribbon axis alignment

Abstract

Methods and systems for zone axis alignment. Methods and systems for aligning a zone axis of a sample with an incident beam are provided. As an example, the alignment can be based on zone axis tilt. The zone axis tilt can be determined based on the position of a direct beam and a zero-order Laue zone in the diffraction pattern. The direct beam position can be determined based on diffraction patterns obtained at different incident angles.

Description

Technical Field

[0001] The present description relates generally to methods and systems for aligning a sample with an incident beam, and more particularly to aligning zone axes of a crystalline material with a charged particle beam using a diffraction pattern. Background Art

[0002] In order to image a crystalline sample with high accuracy in high-resolution charged particle beam microscopy, the charged particle beam must be aligned with the zone axis of the crystalline sample. If the zone axis of the sample is misaligned, such as when the zone axis is not oriented parallel to the incident beam, then the measurement of nanoscale features on the sample may be inaccurate. The process of aligning the sample crystal structure with the incident beam is called zone axis alignment.

[0003] One method of zone axis alignment is to use the diffraction pattern of the sample. For example, when collimated charged particles pass through a thin crystalline sample, the charged particles interfere with each other and form a diffraction pattern on the back focal plane of the objective lens located below the sample. The diffraction pattern consists of a plurality of bright spots. Each bright spot is produced by the diffraction of a charged particle from a set of specific planes within the crystal structure. The alignment between the zone axis and the incident beam can be adjusted based on the distribution of bright spots in the diffraction pattern. Summary of the invention

[0004] In one embodiment, a method for aligning a zone axis of a sample with an incident beam includes obtaining a first diffraction pattern of the sample by directing an incident beam toward the sample at a first angle, obtaining a second diffraction pattern of the sample by directing the incident beam toward the sample at a second angle, determining a position of a direct beam in the first diffraction pattern based on the first diffraction pattern and the second diffraction pattern, and aligning the zone axis of the sample with the incident beam based on the position of the direct beam. In this way, the zone axis of the sample and the incident beam can be automatically aligned based on the position of the direct beam determined from a plurality of diffraction patterns.

[0005] It should be understood that the above summary is provided to introduce a selection of concepts further described in the detailed description in a simplified form. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is uniquely defined by the appended claims. Furthermore, the claimed subject matter is not limited to implementations that address any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a diagram of an imaging system according to an exemplary embodiment of the present invention.

[0007] Figure 2A An example diffraction pattern obtained using a collimated beam is shown.

[0008] Figure 2B An example diffraction pattern obtained using a converging beam is shown.

[0009] Figure 3 is an example method for imaging a sample.

[0010] Figure 4 is an example subroutine for determining the position of a straight beam.

[0011] Figure 5A , 5B and 5C are the diffraction patterns obtained at different tilt angles.

[0012] Figure 5D Based on Figures 5A-5C The combined diffraction pattern generated by the diffraction pattern.

[0013] Figure 6 An example subroutine for determining the position of a zero-order Laue zone (ZOLZ) in a diffraction pattern is shown.

[0014] Fig. 7A , 7B 7C show the process for generating an intensity curve from a diffraction pattern.

[0015] Fig.7D and 7E Example intensity curves are shown.

[0016] Figure 8 Slight adjustments to the position of ZOLZ are shown.

[0017] Fig. 9 The zone axis tilt determined based on the position of the direct beam and ZOLZ in the diffraction pattern is shown.

[0018] Fig.10 The coordinate system used to adjust the sample orientation is shown.

[0019] Like reference numerals refer to corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0020] The following description relates to systems and methods for imaging a sample with an incident beam. For example, the crystal zone axis of the sample can be directly imaged by an imaging system, such as Figure 1 The scanning transmission electron microscope (STEM) system shown in Figure 1 is based on the alignment of the diffraction pattern of the sample with the incident beam.

[0021] Figure 2A and 2BExample diffraction patterns of a crystalline sample obtained using a collimated incident beam and a converging incident beam are shown, respectively. When the incident beam is collimated, the misalignment between the sample's crystal zone axis and the incident beam can be determined based on the position of the straight beam and the zero-order Laue zone (ZOLZ) in the diffraction pattern. The straight beam position can be manually assigned by the operator. However, under certain conditions, the straight beam position may be difficult to identify in the diffraction pattern. For example, the diffraction pattern may include two points with maximum intensity under two beam conditions or when strong beam illumination causes camera saturation. Figure 2A An example diffraction pattern generated with a collimated beam is shown. Figure 2A The direct beam in is not the only point with maximum intensity, nor is it located at the center of the image. When the incident beam is converging, as in a STEM system, the diffraction pattern is different from that generated with a collimated beam. For example, Figure 2B shows the diffraction pattern generated by a converging beam. Figure 2B In FIG. 2 , the area of ​​the direct beam 211 increases as the beam convergence angle increases. In addition, the diffraction pattern 212 along ZOLZ forms a disk and overlaps the direct beam 211. One method for aligning the sample with the converging incident beam is to first align the sample with a collimated beam and then switch the collimated beam to a converging beam. However, this method can be slow and inaccurate due to frequent switching between imaging modes, beam convergence angles, or aperture sizes.

[0022] The above questions can be solved by Figure 3 A method is shown in which the zone axis of the sample can be directly aligned with the converging incident beam. Specifically, the zone axis tilt between the zone axis of the sample and the incident beam can be determined based on the position of the direct beam and ZOLZ in the diffraction pattern. Figure 4 As shown, the direct beam position can be determined from a combined diffraction pattern generated from diffraction patterns obtained at different incident angles. Figures 5A-5D Example diffraction patterns and combined diffraction patterns are shown. Figure 6 is an example method for determining the position of ZOLZ in a diffraction pattern. The position of ZOLZ can be determined based on an intensity curve generated by rotating the diffraction pattern, such as Figures 7A-7E The ZOLZ position can be further Figure 8 Adjust as shown. Fig. 9 The estimated zone axis tilt based on the position of the direct beam and ZOLZ in the diffraction pattern is shown. The coordinates used to adjust the sample orientation in the imaging system are Fig.10 Shown in.

[0023] Go to Figure 1, a STEM system 100 according to an embodiment of the present disclosure is shown. The STEM system 100 includes an electron source 10 that emits charged particles, such as an electron beam 11, toward a focusing column 12. The electron beam can generate high-energy electrons, i.e., electrons with typical energies between about 10 keV and 1,000 keV. In some embodiments, the focusing column 12 may include one or more of a condenser 121, an aperture 122, a scanning coil 123, and an upper objective lens 124. The focusing column 12 focuses the electrons from the electron source 10 into a small spot on the sample 14. The electron beam direction can be adjusted by the scanning coil 123 to scan different positions of the sample. For example, by operating the scanning coil 123, the incident beam 112 can be moved (as shown in dashed lines) to focus on different positions of the sample 14. The sample 14 can be thin enough not to hinder the transmission of most of the electrons in the electron beam 11.

[0024] The optical axis 110 of the imaging system may be the central axis of the electron beam emitted from the electron source 10. The optical axis 110 may also be the central axis of the condenser lens 121. When the incident beam is not moved or scanned (i.e., incident beam 112), the incident beam may be focused at a position where the optical axis 110 intersects the sample 14.

[0025] The sample 14 can be held by a sample holder 13. The sample holder 13 can adjust the sample orientation by tilting and translating the sample. For example, Fig.10 The coordinate system used to adjust the sample orientation is shown in Fig.10 , an incident beam 112 may be focused on a sample 14 along an optical axis 110 of the imaging system. The z-axis may be parallel to the optical axis. The xy plane may be a plane perpendicular to the z-axis. The sample 14 may be tilted relative to the optical axis 110 by rotating about the x-axis or about the y-axis. For example, the direction of rotation about the x-axis may be an α tilt direction 1001, and the direction of rotation about the y-axis may be a β tilt direction 1002. The sample holder may also translate or move the sample 14 along any one of the x-axis, y-axis, and z-axis. In some embodiments, the sample 14 may be rotated about the z-axis.

[0026] Return to Figure 1 As an example, when the sample holder 13 tilts the sample 14 from position 17 (solid line, the sample is perpendicular to the optical axis) to position 16 (dashed line), the incident angle 15 between the axis perpendicular to the sample surface and the optical axis 110 increases. Before scanning or imaging the sample 14, the zone axis of the sample 14 can be adjusted to align with the incident beam 112 by adjusting the sample holder 13. When the zone axis is parallel to the optical axis of the imaging system, the zone axis of the sample can be aligned with the incident beam.

[0027] Electrons 101 passing through sample 14 may enter projector 116. In one embodiment, projector 116 may be a separate part from the focusing column. In another embodiment, projector 116 may be an extension of the lens field from the lens in focusing column 12.

[0028] The projector 116 can be adjusted by the controller 30 so that direct electrons passing through the sample impinge on the disk-shaped bright field detector 115, while diffracted or scattered electrons that are more strongly deflected by the sample are detected by one or more of the high angle annular dark field (HAADF) detector 18 and the annular dark field (ADF) detector 19. The signals from the HAADF and ADF detectors can be amplified by amplifiers 20 and 21, respectively. The signal from the bright field detector 115 can be amplified by amplifier 22. The signals from the amplifiers 20, 21, and 22 can be sent to an image processor 24, which can form an image of the sample 14 from the detected electrons. The HAADF detector 18, the ADF detector 19, and the bright field detector 115 can be scintillator-photomultiplier detectors or solid-state PIN detectors. The STEM system 100 can simultaneously detect signals from one or more of the ADF detector, the ADF detector, and the HAADF detector.

[0029] The zone axis of the sample 14 may be aligned with the incident beam 112 based on a diffraction pattern of the sample 14 obtained when the sample is irradiated with the incident beam 112. In one embodiment, the diffraction pattern may be obtained by obtaining a diffraction pattern formed on a fluorescent screen 141 via a camera 142. During the alignment of the zone axis, the fluorescent screen 141 may be inserted between the projector 116 and the bright field detector 115. For example, the fluorescent screen 141 may be positioned between the HAADF detector 18 and the LAADF detector 19. In another embodiment, the diffraction pattern on the fluorescent screen may be collected via a camera 143 positioned downstream of the bright field detector 115. The camera 143 may be a CCD or CMOS camera or a pixelated STEM detector. The obtained diffraction pattern may be sent to the controller 30 for determining the zone axis tilt.

[0030] The controller 30 may control the operation of the imaging system 100 manually in response to operator instructions or automatically according to computer readable instructions stored in the non-transitory memory 32. The controller 30 may be configured to execute the computer readable instructions and control the various components of the imaging system 100 so as to implement any of the methods described herein. For example, the controller may adjust the scanning position on the sample by operating the scanning coil 123. The controller may adjust the profile of the incident beam by adjusting one or more apertures and / or lenses in the focusing column 12. The controller may adjust the sample orientation relative to the incident beam by adjusting the sample holder 13. The controller 30 may be further connected to a display 31 to display notifications and / or images of the sample. The controller 30 may receive user input from a user input device 33. The user input device 33 may include a keyboard, a mouse, or a touch screen.

[0031] Although STEM systems are described by way of example, it should be understood that the present technique can be used to align the crystal zone axis with a collimated incident beam. The present technique may also be useful when applied to sample alignment in other charged particle beam microscope systems, such as transmission electron microscope (TEM) systems, scanning electron microscope (SEM) systems, and dual beam microscope systems. The current discussion of STEM imaging is provided only as an example of a suitable imaging modality.

[0032] Go to Figure 3 , an example method 300 for imaging a sample is shown. The zone axis tilt between the sample's zone axis and an incident beam is first estimated using a diffraction pattern obtained by directing the incident beam toward the location of the sample at different incident angles. Then, before imaging or scanning the sample, the sample orientation is adjusted based on the zone axis tilt.

[0033] At 302, a sample is loaded into an imaging system, such as Figure 1 For example, a thin sample can be loaded into a sample holder (such as a vacuum chamber) positioned in the imaging system. Figure 1 on the sample holder 13).

[0034] At 304, an incident beam may be directed to the sample along an optical axis of the imaging system. In this context, the incident beam may be a charged particle beam that interacts directly with the sample without its profile or beam path being altered by any component of the imaging system. Figure 1 In the imaging system 100, the incident beam 112 illuminates the sample 14 along the optical axis 110. The incident beam 112 is directed to the sample without being tilted by the scanning coil 123. In one embodiment, directing the incident beam to the sample may include directing the incident beam to a region of interest (ROI) of the sample. The ROI may be a region including a crystalline structure.

[0035] At 306, the incident beam may be used to obtain a diffraction pattern of the sample. In one embodiment, obtaining the diffraction pattern may include inserting a fluorescent screen (e.g., Figure 1 screen 141), and via a camera (such as Figure 1 The camera 142 or the camera 143 (with the aid of the optical system) takes a picture of the diffraction pattern formed on the fluorescent screen. The obtained diffraction pattern is in the form of a two-dimensional image. For example, the diffraction pattern can be as follows: Figure 2A-2B The graph is shown in grayscale. Light pixels correspond to higher signal intensities, while dark pixels correspond to lower signal intensities.

[0036] The diffraction pattern includes a direct beam. The direct beam may appear as a bright dot in the diffraction pattern. The center of the direct beam may be located at the intersection of the optical axis of the imaging system (e.g., optical axis 110) and the plane in which the diffraction pattern is formed. The plane may be a fluorescent screen (e.g., Figure 1 The size or radius of the direct beam can be determined by the convergence angle of the incident beam. For example, when the incident beam is collimated, the direct beam is a small spot. The radius of the direct beam increases as the convergence angle increases. When the diffraction pattern is collected without positioning the sample in the electron beam, the direct beam can be located at the position of the diffraction pattern.

[0037] At 308, method 300 optionally determines whether zone axis alignment is required based on the diffraction pattern of 306. In some embodiments, the diffraction pattern may be displayed on a display and evaluated by an operator. In other embodiments, the diffraction pattern may be automatically analyzed by executing instructions stored in a non-transitory memory to determine whether the zone axis of the sample is aligned with the incident beam. For example, misalignment may be determined based on the centroid of the diffraction pattern 306. If the zone axis alignment is satisfactory and no further alignment is required, then the sample is imaged or scanned at 326. Otherwise, method 300 proceeds to step 310 to align the sample.

[0038] At 310, the position of the direct beam in the diffraction pattern of 306 is determined. Determining the position of the direct beam in the diffraction pattern may include determining the position of the center of the direct beam and the radius of the direct beam. Figure 4 As shown in detail, the position of the direct beam can be determined by combining the diffraction patterns obtained at different incident angles and fitting a circle to the combined diffraction pattern. The diffraction patterns at different incident angles can be obtained, for example, by tilting the sample with respect to the incident beam by the sample holder.

[0039] At 312, the position of the ZOLZ in the diffraction pattern is determined based on the direct beam position. The position of the ZOLZ includes the center position and radius of the ZOLZ. If the crystal zone axis of the sample has not yet been aligned with the incident beam, the position of the ZOLZ in the diffraction pattern obtained at 306 can be determined. If the crystal zone axis of the sample has been aligned with the incident beam, the position of the ZOLZ in the most recently obtained diffraction pattern can be determined at 324. In some embodiments, the diffraction pattern may be a tilted diffraction pattern obtained after tilting the sample at a known tilt angle relative to the incident beam (such as the tilt angle sent to the sample holder by the controller). For example, a tilted diffraction pattern can be obtained at 310 while determining the direct beam position. After determining the ZOLZ position in the tilted diffraction pattern, the ZOLZ position in other diffraction patterns, such as the untilted diffraction pattern obtained at 306 or other tilted diffraction patterns, can be estimated based on the ZOLZ position in the tilted diffraction pattern and the known tilt angle.

[0040] In one embodiment, the ZOLZ position can be determined based on the direct beam position and the centroid of the diffraction pattern. When the misalignment of the zone axis (or the zone axis tilt) is relatively small (e.g., 1 degree), the ZOLZ position can be determined based on the centroid of the diffraction pattern. In one example, the centroid of the diffraction pattern can be determined using image processing procedures such as noise removal, contrast enhancement, and image thresholding.

[0041] In another embodiment, the position of the ZOLZ may be determined by determining an axis passing through the center of the direct beam and the ZOLZ. For example, when the diffraction pattern is rotated relative to the center of the direct beam, the axis passing through the center of the direct beam and the ZOLZ may be determined based on the integrated intensity of the pixel values ​​within the intensity integration region. Details of determining the position of the ZOLZ based on the intensity curve are described in Figure 6 Shown in.

[0042] At 314, the zone axis tilt is estimated based on the position of the direct beam from 310 and the position of the ZOLZ from 312. In one example, the zone axis tilt may include an α tilt angle, a β tilt angle, or a combination of an α tilt angle and a β tilt angle. Fig. 9 The process of estimating the zone axis tilt is shown in detail in FIG.

[0043] At 316, an error in the estimation of the zone axis tilt may optionally be determined and compared to a threshold estimation error. If the estimation error is less than the threshold estimation error, then the method 300 may proceed to 320 and align the sample based on the estimated zone axis tilt. The threshold estimation error may, for example, be 0.5 degrees in each tilt direction of the sample holder (e.g., α tilt direction or β tilt direction). Otherwise, if the estimation error is greater than the threshold estimation error, then at 318, an alignment failure may be reported to the operator via a display. In response to the notification, the operator may select other methods for zone axis alignment.

[0044] The error in the estimation of the crystal zone axis tilt can be determined based on the estimated tilt angle and the commanded tilt angle between the two diffraction patterns (such as the tilt angle sent by the controller to the sample holder). In some embodiments, a first diffraction pattern is obtained at a first incident angle at the location of the sample. By tilting the sample at the commanded tilt angle relative to the incident beam, the incident angle can be adjusted from the first incident angle to a second incident angle. A second diffraction pattern at the same location of the sample can be obtained at the second incident angle. The difference between the first and second incident angles can be estimated based on the position of the direct beam and ZOLZ in each diffraction pattern. The position of the direct beam in the first and second diffraction patterns can be the same as the position of the direct beam in 310. The position of ZOLZ in the diffraction pattern can be based on Figure 6 In one example, the first diffraction pattern is the diffraction pattern obtained at 306, and the second diffraction pattern is the tilted diffraction pattern obtained at 310. The difference between the estimated first and second incident angles can be compared to the commanded tilt angle to determine the error in the zone axis tilt estimate.

[0045] At 320, the estimated absolute value of the zone axis tilt from 314 is compared to a threshold zone axis tilt angle. The threshold zone axis tilt angle may correspond to an acceptable alignment error. For example, the threshold zone axis tilt angle may be 0.5 degrees in each tilt direction. If the estimated absolute value of the zone axis tilt is within the threshold zone axis tilt angle, then the zone axis alignment may be complete and the sample may then be scanned or imaged at 326. If the absolute value of the zone axis tilt is greater than the threshold zone axis tilt angle, then at 322, the zone axis of the sample may be aligned with the incident beam.

[0046] At 322, the sample is aligned with the incident beam by adjusting the incident angle based on the zone axis tilt estimated at 314. In one embodiment, the incident angle can be adjusted by tilting the sample based on the zone axis tilt angle in each tilt direction (such as the α and β tilt directions). In one embodiment, after adjusting the incident angle, the sample can be repositioned by moving or translating in the xy plane and the z axis to ensure that the same feature (or ROI) is imaged before and after adjusting the incident angle. In other words, the sample can be moved to ensure that the same position of the sample is in the field of view before and after adjusting the incident angle. For example, an image of the sample before and after adjusting the incident angle can be obtained by scanning the incident beam over the sample surface. The image obtained can be a dark field image or a bright field image. The sample position can be adjusted by matching the obtained images.

[0047] In some embodiments, after the zone axis alignment process, the sample may be repositioned before scanning the sample at 326 to ensure that the same ROI is imaged. The sample may be repositioned based on images of the sample acquired before and after zone axis alignment.

[0048] In some embodiments, the sample may be scanned after aligning the zone axis at 326. In other embodiments, method 300 may proceed to 324 to further align the zone axis. For example, after acquiring another diffraction pattern of the aligned sample at 324, and again estimating the zone axis tilt based on the newly acquired diffraction pattern.

[0049] At 326, the sample is scanned or imaged. After performing the crystal zone axis alignment, the sample can be scanned or imaged without changing the optical mode or the incident beam convergence angle. In some embodiments, the incident beam is a converging beam, such as in Figure 1 In STEM diffraction mode, a converging incident beam (such as Figure 1 The incident beam 112 of the crystal zone axis alignment is performed. After the crystal zone axis alignment, the sample can be imaged by scanning the incident beam over the sample. In one example, the sample can be scanned under the same imaging mode, incident beam convergence angle, and aperture of the focusing column as during the crystal zone axis alignment. In this way, the crystal zone axis alignment can be automatically performed with high accuracy and robustness in a short time.

[0050] In this way, the zone axis tilt can be automatically estimated based on multiple diffraction patterns obtained at different angles of incidence. The estimation is insensitive to the beam convergence angle and does not require a priori information about the position of the straight beam. As a result, zone axis alignment can be implemented in STEM diffraction mode with minimal or no adjustments to the imaging system.

[0051] Figure 4 The subroutine 400 is used to determine the position of the direct beam in the diffraction pattern. The position of the direct beam includes the position of the center of the direct beam and the size (such as radius) of the direct beam. In one example, multiple diffraction patterns with different incident angles are combined. Because the direct beam position remains the same in the diffraction pattern, the direct beam has the maximum intensity in the combined diffraction pattern and can be identified. The position of the direct beam can then be estimated based on the combined diffraction pattern.

[0052] At 402, the beam incident angle is adjusted. In one embodiment, the incident angle can be adjusted by tilting the sample relative to the incident beam at a tilt angle. In one example, the sample can be tilted in an α tilt direction or a β tilt direction. In another example, the tilt direction can be determined based on the performance of the sample holder. If the sample holder can tilt the sample in the α tilt direction with higher accuracy than in the β tilt direction, then the sample holder can be used to tilt the sample in the α tilt direction. The tilt angle can be predetermined. The accuracy of determining the position of the straight beam increases with the increase of the tilt angle. In some embodiments, the tilt angle can be between 2 degrees and 25 degrees. In one example, the tilt angle is 3 degrees. In other embodiments, the incident angle can be adjusted by tilting the incident beam relative to the optical axis of the system. In some embodiments, after adjusting the incident angle, the sample can be repositioned by moving or translating in the xy plane and refocusing on the z axis. In other words, the sample can be adjusted to ensure that the same ROI of the sample is in the field of view before and after adjusting the incident angle. In one example, if the crystal band axis tilt estimation error is determined at 316, the sample can be repositioned after adjusting the incident angle.

[0053] At 404, a tilted diffraction pattern is obtained. Figure 3 The untilted diffraction pattern and the tilted diffraction pattern obtained at 306 determine the direct beam position.

[0054] At 406, the center and radius of the direct beam in the diffraction pattern are determined. Figure 3 The position of the direct beam is the same in the non-tilted diffraction pattern and the tilted diffraction pattern of 306. In other words, the position of the direct beam in the diffraction pattern is not affected by the change of the incident angle.

[0055] In one embodiment, the center and radius of the direct beam may be determined by detecting the edge of the direct beam in the combined diffraction pattern shown in steps 408-412.

[0056] At 408, each diffraction pattern is normalized to the same dynamic range of pixel values. In particular, the normalization is from Figure 3 Each of the diffraction pattern 306 and the tilted diffraction pattern 404. For example, the dynamic range may be a value between 0 and 255.

[0057] At 410, the normalized diffraction patterns are combined to form a combined diffraction pattern. In one embodiment, combining the diffraction patterns includes taking a running average of the normalized diffraction patterns. For example, at each pixel location, the average of the pixel values ​​in the normalized diffraction patterns is calculated to obtain a pixel value of the combined diffraction pattern at the pixel location.

[0058] At 412, the edge of the direct beam in the combined diffraction pattern can be fitted by a circle representing the perimeter of the direct beam. In some embodiments, before the circle fitting, the combined diffraction pattern can be thresholded to remove pixels with low values. The circle fitting can be performed, for example, by using a circular Hugh transform method. The center of the fitted circle in the diffraction pattern is the position of the direct beam, and the radius of the fitted circle is the radius of the direct beam.

[0059] At 414, subroutine 400 may determine whether additional tilted diffraction patterns are needed. For example, in response to the failure of the circle fit at 406, more diffraction patterns may be needed. If there is not enough signal contrast in the straight beam in the combined diffraction pattern, the circle may not be successfully fitted to the combined diffraction pattern. If more tilted diffraction patterns are needed, the incident angle may be further adjusted at 402, and one or more additional tilted diffraction patterns may be obtained. In one embodiment, the incident angle may first increase a first tilt angle and then decrease a second tilt angle. In another embodiment, during the adjustment of the incident angle, the incident angle may be continuously increased. In one example, the maximum incident angle may be limited by the maximum tilt angle of the sample holder. In another example, the maximum incident angle may be less than 25 degrees. If no more tilted diffraction patterns are needed, subroutine 400 proceeds to 416.

[0060] At 416, the angle of incidence is adjusted back to the angle of incidence that may have been before performing subroutine 400. For example, the angle of incidence may be adjusted by reversing each of the angle of incidence adjustments performed at 402. After adjusting the angle of incidence to the angle of incidence before performing subroutine 400, the sample may be repositioned so that the same feature or location of the sample is in the field of view by aligning the sample in the xy plane and along the z-axis. For example, images of the sample before adjusting the angle of incidence (before performing subroutine 400) and after reversing the angle of incidence adjustments may be obtained. Correlation between the images obtained may be used for sample alignment.

[0061] In this way, the straight beam position can be determined using multiple diffraction patterns obtained at different incident angles. In some embodiments, the number of tilted diffraction patterns can be predetermined. For example, two tilted diffraction patterns can be obtained by tilting the sample holder in a tilt direction and in the opposite direction to the tilt direction. For example, tilted diffraction patterns can be obtained along the α tilt direction at 3 degrees and -3 degrees. The diffraction patterns from the two tilted diffraction patterns can be combined with the diffraction patterns from the sample holder. Figure 3 The untilted diffraction pattern of 306 determines the direct beam position.

[0062] Figures 5A-5C Examples of diffraction patterns obtained at different incident angles are shown. Figure 5D To combine Figures 5A-5C The combined diffraction pattern generated by the diffraction pattern. Figure 5B is the diffraction pattern without adjusting the incident beam angle (as in Figure 3 The untilted diffraction pattern obtained by 306 of FIG. 3 is obtained after tilting the sample stage by -3 degrees and 3 degrees respectively along the α tilt direction. Figure 5A and Figure 5B The position of the direct beam 501 in the diffraction pattern remains unchanged, while the positions of the remaining high-intensity points in the diffraction pattern vary with the tilt angle. Figures 5A-5C The moving average of the diffraction pattern is Figure 5D The signal contrast of the straight beam in the combined diffraction pattern is increased, while the signal contrast of the remaining diffraction patterns is reduced. The edge of the high contrast signal in the combined diffraction pattern can be fitted with a circle 502. The center of the straight beam is the same as the center of the circle 502, and the radius of the straight beam is the same as the radius of the circle 502.

[0063] Figure 6 A subroutine 600 is shown for determining the position of ZOLZ in a diffraction pattern based on the position of a direct beam. The position of ZOLZ includes the center position of ZOLZ and the size of ZOLZ, such as the radius. The position of ZOLZ can be determined by determining an axis passing through the center of the direct beam and ZOLZ. The axis passing through the center of the direct beam and ZOLZ can be determined by rotating the diffraction pattern relative to the center of the direct beam. As the diffraction pattern rotates, the intensity of the pixels in the non-rotating intensity integration area fluctuates, and the position including the center and radius of ZOLZ can be estimated based on the intensity change and the position of the direct beam.

[0064] At 602, an intensity integration region of a diffraction pattern may be determined. Determining the intensity integration region may include determining the location and area of ​​the region. In addition, the shape of the intensity integration region may be determined. In one embodiment, the intensity integration region is rectangular. In other embodiments, the intensity integration region may be other shapes, such as a circle. The distance between the intensity integration region and the center of the direct beam may be greater than the radius of the direct beam and small enough so that a smaller crystal zone axis tilt can be estimated. In one example, the distance between the intensity integration region and the center of the direct beam may be a fixed value. In another example, the distance between the intensity integration region and the center of the direct beam may be determined based on the radius of the direct beam. The distance may increase as the radius of the direct beam increases. For example, the distance may be twice the radius of the direct beam. In yet another embodiment, the distance between the intensity integration region and the center of the direct beam may be adjusted based on the pixel value of the diffraction pattern within the intensity integration region. In one example, the distance increases in response to a total pixel value within the intensity integration region that is above a threshold level. Because when the total pixel value is high, the intensity integration region may include a large portion of the direct beam. In another example, the distance is determined based on the total pixel value in the intensity integration region at different distances from the direct beam. The area of ​​the intensity integration region should be large enough to include enough features of the diffraction pattern. On the other hand, the area of ​​the intensity integration region should be small enough to reflect the spatial variation of the diffraction pattern. In one example, the area of ​​the integration region is one quarter of the area of ​​the integration region.

[0065] Fig. 7A 701 is an example intensity integration region of a diffraction pattern 710. The diffraction pattern is shown in the xy plane. The distance L between the center 702 of the direct beam and the upper boundary 704 of the intensity integration region (i.e., the boundary with the highest y-axis value) is twice the radius of the direct beam. The intensity integration region covers the diffraction pattern 710 below the upper boundary 704 of the intensity integration region (i.e., having a lower y-axis value). In other embodiments, the intensity integration region may cover a portion of the diffraction pattern 710 below the upper boundary 704.

[0066] Return to Figure 6 At 604, an intensity curve may be generated by integrating pixel values ​​in an intensity integration region while the diffraction pattern is rotated relative to the center of the direct beam. The intensity integration region does not rotate while the diffraction pattern is rotated. For example, Figure 7B and Figure 7C Rotated diffraction patterns 720 and 730 are shown, which are rotated in the clockwise direction 703 relative to the straight beam center 702 Fig. 7A Diffraction pattern 710. Figure 7B-7C The diffraction patterns 720 and 730 are respectively relative to Fig. 7AThe diffraction pattern 710 is rotated by A1 and A2 degrees. At each rotation angle, the intensity can be calculated by summing the values ​​of the pixels in the overlapping area between the rotated diffraction pattern and the non-rotated intensity integration area. As the diffraction pattern rotates 360 degrees, an intensity curve can be generated. Fig.7D Shown for Fig. 7A The intensity curve generated by the diffraction pattern 710. The x-axis is the rotation angle, and the y-axis is the intensity calculated at the corresponding rotation angle. The rotation angle represents the angle of rotation clockwise from the y-axis. The intensity curve starts at zero, indicating that Fig. 7A The bright spot (pixel with high value) in the diffraction pattern 710 is outside the intensity integration region 701. As the diffraction pattern 710 rotates clockwise, at the rotation angle A1, the intensity increases from zero, indicating that the bright spot in the diffraction pattern enters the intensity integration region (such as Figure 7B As shown in Figure 1, the intensity curve fluctuates from the rotation angle A1 to A2 and is not zero. The intensity curve reaches zero at A2 and remains zero before the entire rotation (i.e., 360 degrees). This indicates that the bright spot in the diffraction pattern leaves the intensity integration area at the rotation angle A2, as shown in Figure 1. Figure 7C Based on the rotation angles A1 and A2 and the position of the direct beam, the position of ZOLZ can be estimated.

[0067] At 606, a radius of the ZOLZ may be calculated based on the intensity curve. In one embodiment, when the intensity curve intersects a low threshold intensity, the ZOLZ radius may be calculated based on the rotation angle. The low threshold intensity may be determined based on a noise level. If background noise is removed, the low threshold intensity may be zero. In one example, the ZOLZ radius may be determined based on a rotation angle at which the intensity curve intersects the low threshold intensity. In another example, the ZOLZ radius may be determined based on a range of rotation angles at which the intensity curve is not greater than the low threshold intensity. An untilted direction angle corresponding to an untilted direction may be determined based on the rotation angle at which the intensity curve intersects the low threshold intensity. For example, for Fig.7D The intensity curve 741 between the rotation angles A1 and A2 is above the low threshold intensity of zero. The untilted direction angle is calculated as exist Fig. 7E In another exemplary intensity curve 751, the intensity curve is not zero from zero to A3 degrees rotation angle and from A4 to 360 degrees rotation angle. The intensity curve 751 remains zero from rotation angle A3 to A4. Thus, for the intensity curve 751, the non-tilted direction angle α is calculated as Then the ZOLZ radius R can be calculated as Where L is the distance between the center of the direct beam and the intensity integration area, and r is the radius of the direct beam.

[0068] Return to Figure 6At 608, the location of the center of the ZOLZ may be determined based on the intensity curve, the radius of the ZOLZ, and the location of the straight beam. In one embodiment, a misalignment vector from the center of the straight beam to the center of the ZOLZ may be determined based on the untilted direction angle α, and the location of the center of the ZOLZ may be determined based on the misalignment vector. The magnitude of the misalignment vector is proportional to (Rr) / r. The direction of the misalignment vector overlaps with an axis passing through the straight beam and the center of the ZOLZ. The direction of the misalignment vector may be determined based on the untilted direction angle α. In one example, for Fig.7D The direction of the misalignment vector is A degrees clockwise relative to the y-axis. In another example, for Fig. 7E For intensity curve 751, the direction of the misalignment vector is A degrees clockwise relative to the y-axis.

[0069] At 610, the radius and center of the ZOLZ determined at 606 and 608 may optionally be further adjusted. In one embodiment, the position of the ZOLZ center determined from 608 may be adjusted by fitting a ring to cover the area with the highest pixel intensity. In one example, the width of the ring may be a predetermined width, such as the radius of the straight beam. The outer radius of the ring may be the ZOLZ radius determined at 606. The position of the ring may be determined via an optimization process, where the initial position of the center of the ring is the ZOLZ center determined at 608. The radius of the ZOLZ may be adjusted based on the adjusted position of the ZOLZ center and the straight beam position. For example, Figure 8 A diffraction pattern is shown with an adjusted ZOLZ center 801. The ZOLZ center 801 is the center of the ring 802. The position of the ring 802 is obtained via an optimization process by maximizing the total signal intensity covered by the ring.

[0070] In this way, the ZOLZ position in the diffraction pattern can be determined based on the intensity curve generated by integrating the pixel values ​​in the unrotated intensity integration region when the diffraction pattern is rotated relative to the center of the direct beam. The ZOLZ position can be further adjusted to improve accuracy.

[0071] Fig. 9 The determination of the crystal zone axis tilt based on the position of the direct beam and the ZOLZ is shown. The diffraction pattern is plotted in the xy plane. The circumference 904 of the direct beam intersects the circumference 901 of the ZOLZ. The coordinates of the center 902 of the direct beam and the radius 903 can be found in Figure 3 The coordinates of the center 905 of ZOLZ and the radius of ZOLZ can be determined by Figure 3312 is determined. Axis 909 passes through the straight beam center 902 and the ZOLZ center 905. The misalignment vector 908 originates from the straight beam center 902 and points to the ZOLZ center 905. The crystal zone axis tilt can be determined based on the misalignment vector 908. For example, the crystal zone axis tilt may include an α tilt component (in degrees) and a β tilt component (in degrees), which represent the crystal zone axis misalignment in the α and β tilt directions of the sample holder, respectively. For example, the β tilt component may be proportional to the projection of the misalignment vector 908 in the x-axis direction, and the α tilt component may be proportional to the projection of the misalignment vector 908 in the y-axis direction. The magnitude of the crystal zone axis tilt is also proportional to the difference between the ZOLZ radius and the straight beam radius, and is inversely proportional to the straight beam radius. Therefore, the crystal zone axis tilt can be determined based on the β tilt component The β tilt angle can be calculated based on the α tilt component and the convergence angle of the incident beam. The generation and convergence angle of the incident beam is used to calculate the α tilt angle.

[0072] In this way, the zone axis of the sample can be aligned with the incident beam based on the estimated zone axis tilt. The alignment can be performed automatically with minimal or no input from the operator. For example, alignment does not require beam information, such as convergence angle. Because scanning or imaging can be performed after the zone axis is aligned without mode switching or beam profile adjustment, the total imaging time of the sample can be reduced.

[0073] The technical effect of determining the position of a direct beam based on a plurality of diffraction patterns obtained at different incident angles is that the direct beam can be identified and separated from the remaining diffraction patterns. In addition, when the sample is irradiated with a converging incident beam, the position of the direct beam can be determined. The technical effect of determining the position of the ZOLZ by rotating the diffraction pattern is that the axis passing through the direct beam and the center of the ZOLZ can be determined. The center of the ZOLZ can be determined based on the axis passing through the direct beam and the center of the ZOLZ. The technical effect of calculating the intensity curve by summing the pixel values ​​of the rotating diffraction pattern within the intensity integration region is that the radius of the ZOLZ can be calculated based on the rotation angle at which the intensity curve intersects the low intensity level.

Claims

1. A method for aligning a zone axis of a sample with an incident beam, comprising: obtaining a first diffraction pattern of the sample by directing the incident beam toward the sample at a first angle; obtaining a second diffraction pattern of the sample by directing the incident beam toward the sample at a second angle; determining a position of a direct beam in the first diffraction pattern based on the first diffraction pattern and the second diffraction pattern; and aligning the zone axis of the sample with the incident beam based on the position of the direct beam; wherein aligning the zone axis of the sample with the incident beam based on the position of the straight beam comprises: determining a zone axis tilt between the zone axis of the sample and the incident beam directed toward the sample at the first angle based on the position of the straight beam, and aligning the zone axis of the sample with the incident beam based on the zone axis tilt; Wherein determining the zone axis tilt based on the position of the direct beam comprises determining a position of a zero-order Laue zone (ZOLZ) in the first diffraction pattern based on the position of the direct beam, and determining the zone axis tilt based on the position of the ZOLZ. The method according to claim 1 , wherein the incident beam is a converging beam.

3. The method according to claim 1 or 2, wherein determining the position of the direct beam in the first diffraction pattern based on the first diffraction pattern and the second diffraction pattern comprises: A combined diffraction pattern is generated based on the first diffraction pattern and the second diffraction pattern, and the position of the direct beam in the first diffraction pattern is determined based on the combined diffraction pattern.

4. The method of claim 1 or 2, wherein determining the position of the direct beam in the first diffraction pattern comprises determining a center of the direct beam and a radius of the direct beam in the first diffraction pattern.

5. The method of claim 1 , wherein determining the position of ZOLZ in the first diffraction pattern based on the position of the direct beam comprises: The first diffraction pattern is rotated relative to the center of the direct beam, an intensity curve corresponding to pixel values ​​in an overlapping area between an integration region and the rotated first diffraction pattern is generated, and the position of the ZOLZ is determined based on the intensity curve. The method of claim 5 , further comprising determining an intensity integration area based on a radius of the direct beam. 7 . The method of claim 5 , wherein determining the position of the ZOLZ based on the intensity curve comprises determining the position of the ZOLZ based on a rotation angle at which the intensity curve intersects a threshold intensity.

8. A method for aligning a zone axis of a sample with an incident beam, comprising: obtaining a plurality of diffraction patterns of the sample by adjusting an incident angle between the sample and the incident beam; determining a position of the direct beam in the plurality of diffraction patterns based on the obtained plurality of diffraction patterns; and aligning the zone axis of the sample with the incident beam based on the position of the direct beam; wherein aligning the zone axis of the sample with the incident beam based on the position of the straight beam comprises: determining a zone axis tilt between the zone axis of the sample and the incident beam directed toward the sample at a first angle based on the position of the straight beam, and aligning the zone axis of the sample with the incident beam based on the zone axis tilt; Wherein determining the zone axis tilt based on the position of the direct beam comprises determining a position of a zero-order Laue zone (ZOLZ) in a first diffraction pattern based on the position of the direct beam, and determining the zone axis tilt based on the position of the ZOLZ.

9. The method of claim 8, further comprising determining a radius of a zero-order Laue zone (ZOLZ) in one of the plurality of diffraction patterns, and aligning the zone axis of the sample with the incident beam further based on the radius of the ZOLZ.

10. The method according to claim 8 or 9, wherein the incident angle is adjusted by tilting the sample by a sample holder.

11. The method according to claim 8 or 9, wherein obtaining the multiple diffraction patterns of the sample by adjusting the incident angle between the sample and the incident beam comprises obtaining three diffraction patterns by tilting the sample in predetermined steps.

12. The method of claim 8 or 9, wherein the incident beam is a collimated beam.

13. The method of claim 8 or 9, further comprising scanning the sample by moving the incident beam over the sample after aligning the zone axis of the sample with the incident beam.

14. An imaging system comprising: a sample holder for holding samples; a source for generating charged particles; a focusing column for directing the charged particles to the sample; a detector for obtaining a diffraction pattern of the sample; and A controller having computer readable instructions stored on a non-transitory memory configured to: obtaining a first diffraction pattern by directing the charged particles at a first angle to the sample; obtaining a second diffraction pattern by directing the charged particles at a second angle to the sample; determining a position of a direct beam in the first diffraction pattern based on the first diffraction pattern and the second diffraction pattern; estimating a zone axis tilt between a zone axis of the sample and a direction of the charged particles toward the sample based on the position of the direct beam; and aligning the zone axis of the sample with the charged particles based on the estimated zone axis tilt; Wherein estimating the zone axis inclination between the zone axis of the sample and the direction of the charged particles based on the position of the direct beam includes: determining the radius of the zero-order Laue zone (ZOLZ) in the second diffraction pattern by rotating the second diffraction pattern relative to the center of the direct beam, and determining the zone axis inclination based on the radius of the direct beam and the radius of the ZOLZ.

15. The imaging system of claim 14, wherein the second diffraction pattern is obtained after the first diffraction pattern is obtained, and the controller is configured to guide the charged particles from the first angle to the sample to the second angle to the sample by tilting the sample via the sample holder relative to the direction of the charged particles.

16. The imaging system of claim 15 , wherein the controller is further configured to estimate a difference between the first angle and the second angle based on the first diffraction pattern and the second diffraction pattern, and to determine an error in the estimation of the crystal zone axis tilt by comparing the estimated difference between the first angle and the second angle with a tilt angle of the sample.

17. An imaging system according to claim 14, 15 or 16, wherein the imaging system is a scanning transmission electron microscope system, and the controller is further configured to scan the sample without adjusting the convergence angle of the charged particles after aligning the crystal axis of the sample with the charged particles.

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