Methods and systems for imaging three-dimensional features

Abstract

Methods and systems for imaging three-dimensional features. A method and system for milling and imaging a sample based on multiple references at different sample depths includes: forming a first reference on a first sample surface at a first sample depth; milling at least a portion of the sample surface to expose a second sample surface at a second sample depth; forming a second reference on the second sample surface; and milling at least a portion of the second sample surface to expose a third sample surface including a region of interest (ROI) at a third sample depth. The position of the ROI relative to the first reference at the third sample depth can be calculated based on images of the ROI and the second reference, and the relative position between the first and second references.

Description

Technical Field

[0001] The present invention generally relates to methods and systems for imaging three-dimensional features within a sample, and more specifically, to using charged particle microscopy systems to generate three-dimensional feature profiles. Background Technology

[0002] Charged particle microscopy systems can be used to inspect microfabrication apparatus. Dual-beam systems, including focused ion beam (FIB) and electron beam, can analyze defects and faults during microfabrication to troubleshoot, adjust, and improve the microfabrication process. For example, dual-beam systems can be used to reconstruct three-dimensional features within a sample using a slice-and-view process, where FIB is used to remove material layers from the sample to expose multiple sample surfaces, and scanning electron microscopy (SEM) is used to acquire images of the exposed sample surfaces. However, displacement of features in SEM images, attributable to factors including sample shift and beam drift, can affect the accuracy of the reconstruction.

[0003] A method for determining the location of features within a sample based on slice view data is illustrated in U.S. Patent US7,348,556 B2 to Chitturi et al., in which a reference mark (referred to as a datum) is created on the sample surface immediately adjacent to the feature of interest. Each SEM image acquired during the slice view process includes the exposed sample surface and the surface datum. The location of the data point on each SEM image is measured based on the distance between the data point and the datum in the SEM image. However, the applicants recognized that when the feature is deep within the sample, it may be impossible to accurately measure the distance between the feature and the surface datum in the SEM image. Summary of the Invention

[0004] In one embodiment, a method includes: forming a first reference on a first sample surface at a first sample depth; removing at least a portion of the first sample surface to expose a second sample surface; forming a second reference on the second sample surface; removing at least a portion of the second sample surface to expose a third sample surface including a region of interest (ROI) at a third sample depth; acquiring a first sample image including the second reference and the ROI at the third sample depth; and determining the position of the ROI at the third sample depth relative to the first reference based on a first position of the first reference relative to the second reference and a second position of the ROI at the third sample depth relative to the second reference in the first sample image. In this manner, the position of features deep within the sample can be accurately measured.

[0005] It should be understood that the above overview is provided to introduce some concepts further described in the detailed description in a simplified form. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is uniquely defined by the claims following the detailed description. Furthermore, the claimed subject matter is not limited to embodiments that address any shortcomings pointed out above or in any part of this disclosure. Attached Figure Description

[0006] Figure 1 Demonstrating a charged particle microscopy system.

[0007] Figure 2 A method for reconstructing features within a sample.

[0008] Figure 3 This describes multiple reference points on the sample surface used to determine the layer thickness.

[0009] Figure 4A and 4B Explanation passed Figure 2 The sample was processed using the method described above.

[0010] Figure 5A Through Figure 2 Scanning electron microscopy images of samples processed by the method.

[0011] Figure 5B The display is formed by stitching together multiple images. Figure 5A Examples.

[0012] Figure 6 Is using Figure 2 The method is used to reconstruct the three-dimensional contours of channels in a 3D-NAND sample.

[0013] Figures 7A to 7B This section shows examples of charged particle beams and sample locations.

[0014] Figures 8A to 8B This shows another example location of the charged particle beam and sample.

[0015] Throughout the various views of the diagram, the same reference numerals refer to the corresponding parts. Detailed Implementation

[0016] The following description relates to systems and methods for locating and reconstructing features within a sample based on images acquired using slice-view techniques. These methods can be used... Figure 1The charged particle microscopy system shown is used to mill and image samples. The charged particle microscopy system can be a dual-beam system comprising a focused ion beam (FIB) for milling the sample and an electron beam (e-beam) for acquiring high-resolution images of the sample. In one example, after milling or removing material from the sample using the FIB to expose the sample surface or cross-section, scanning electron microscopy (SEM) images are acquired through the exposed surface using the electron beam. By repeatedly milling and imaging the sample in the sample depth direction, a series of SEM images of the sample surface (or cross-section) at various sample depths can be acquired. The three-dimensional volume of the sample can be reconstructed based on the SEM images. To align the SEM images in a plane perpendicular to the sample depth direction, reference markers or datums can be located on or formed on the top surface of the sample. In one example, each SEM image of the sample surface may include a surface datum for alignment. In another example, the electron beam position can be calibrated based on the surface datum before acquiring the SEM images. However, for features that extend a long distance in the depth direction of the sample, such as channels in a 3D-NAND sample, if the depth of the exposed sample surface exceeds the focal length of the electron beam, the surface reference or feature on the exposed sample surface may be out of focus in the SEM image. As a result, it is impossible to accurately determine the distance between the feature and the surface reference from the SEM image.

[0017] Figure 2A method for solving the above problems is presented. Multiple references are formed at different sample depths during the removal of a material layer in the depth direction of the sample. The removed layer extends in a plane perpendicular to the depth direction. The top surface of the sample may be in the sample plane (the XY plane according to the sample axis). For example, a first reference, such as a surface reference, is located or formed at a location different from the region of interest (ROI) on the top surface of the sample. Here, the ROI (or volume of interest) is defined as the 3D volume of the sample. The area of ​​the ROI in the sample plane may vary with sample depth. The ROI may include a feature of interest. The feature may be a profile or an area. Surface references can be formed by FIB deposition or milling. Alternatively, surface references can be located by assigning unique features on the top surface of the sample as surface references. A second reference is located or formed after removing at least a portion of the first sample surface to expose the ROI on a second sample surface at a second sample depth. The removed portion of the first sample surface does not include the first reference. The second reference is located in a different position in the sample plane than the first reference. In one example, the second reference includes a boundary (or edge) formed between the second sample surface and the remainder of the first sample surface. In another example, the second reference is etched and / or deposited on the second sample surface using a fibrillated electron beam (FIB). At least a portion of the second sample surface is then removed to expose the region of interest (ROI) at a third sample depth on the third sample surface. A first SEM image including both the second reference and the ROI at the third sample depth is acquired using an electron beam at a first focal depth. The position of the ROI at the third sample depth relative to the second reference can be obtained from the first SEM image. In one example, the first SEM image is acquired by scanning an electron beam within a single field of view (FOV). Here, the FOV of the SEM image is the sample region scanned by the electron beam in a single scan. In another example, the first SEM image is stitched together from multiple SEM images acquired at a first focal depth. The FOV of any one of the multiple SEM images overlaps with the FOV of at least another SEM image, such that the multiple SEM images can be accurately stitched together using matching signals in the overlapping region. In this manner, a continuous region within a sample plane, including a second reference and a Region of Interest (ROI) at a third sample depth, is imaged in the first SEM image. The position of the ROI at the third sample depth relative to the first reference is determined based on the position of the ROI relative to the second reference and the position of the second reference relative to the first reference within the sample plane. The position of the second reference relative to the first reference can be obtained from a second SEM image including both the first and second references. The first and second SEM images are obtained with different depths of focus, such that both the reference and the ROI in the SEM images are focused for accurate feature localization and high-resolution feature imaging. The second SEM image can be obtained before milling the second sample surface or after exposing the third sample surface.The difference between the first sample depth and the second sample depth can be determined based on the focal length of the electron beam or experimentally by imaging multiple references on the sample surface, for example. Figure 3 As shown in the image. More benchmarks can be created to expose the ROI deep within the sample. The milled sample resembles a staircase with multiple steps, for example... Figures 4A to 4B and Figure 5A The milled sample shown is illustrated. The step height can be the same for each step, and each reference corresponds to one step. In this way, the relative position of the ROI at a greater sample depth from the surface reference can be accurately determined with minimal post-processing.

[0018] In another example, forming each step in the sample involves removing multiple layers to expose multiple sample surfaces. Multiple SEM images of the exposed sample surfaces are acquired to reconstruct features within the sample. Specifically, a first reference is formed at or located at a first sample depth, such as on the top surface of the sample. Multiple first sample images of the exposed sample surfaces are acquired as material is milled or removed along a depth direction from the first sample depth to a second, greater sample depth. Each of the first sample images includes the first reference and a region of interest (ROI) at the corresponding sample depth. The first sample images are acquired using a first depth of focus of an electron beam. The position of the ROI between the first and second sample depths relative to the first reference can be determined from the first sample images. Once the depth of the exposed sample surfaces (e.g., the total milling depth) reaches the second sample depth, a second reference is located. In one example, the second reference can be formed on a second sample surface at the second sample depth using a charged particle beam. In another example, the second reference can be formed while milling the sample from the first sample depth to the second sample depth. The second reference can be an edge or boundary formed while milling the sample from the first depth to the second sample depth. The depth difference between the first sample depth and the second sample depth is the step height. The step height is determined experimentally or based on system parameters. Multiple second sample images of the exposed sample surface, including the ROI at various sample depths, are acquired while milling or removing material along the depth direction from the second sample depth to a third, larger sample depth. The second reference is outside the milled area in the sample plane. Second sample images are acquired using an electron beam at a second depth of focus. Each of the second sample images includes the second reference and the ROI. In one example, the position of the second reference relative to the first reference in a plane perpendicular to the sample depth is determined based on one or more of the second sample images including the first reference. In another example, the position of the second reference relative to the first reference in a plane perpendicular to the sample depth is determined based on a sample image including both the first and second references. Because both the second reference and the ROI are in focus in the second sample images, the position of the ROI relative to the second reference can be accurately determined. In this way, the position of the ROI relative to the first reference at a depth between the second and third sample depths is determined based on the position of the ROI relative to the second reference and the position of the second reference relative to the first reference.

[0019] More steps can be formed to image the ROI located deep within the sample. Each reference corresponds to a step or a range of sample depths, wherein all SEM images of the exposed sample surface within said range include their corresponding reference. Furthermore, each range of sample depths or each step corresponds to a different depth of focus of the electron beam used to acquire the SEM image. Thus, in each SEM image, the ROI and at least one reference are focused and imaged at high resolution. In some instances, an SEM image of the exposed sample surface is generated by stitching together multiple SEM images acquired at the same electron beam depth of focus and having overlapping fields of view (FOV). The FOVs of the multiple SEM images overlap such that the FOV of any one of the multiple SEM images overlaps with the FOV of at least another of the multiple SEM images. The step height may depend on the nature of the electron beam. The position of the ROI at a specific sample depth relative to a first reference is determined based on the position of the ROI from the most recently formed reference and the position of the most recently formed reference relative to a previously formed reference. The most recently formed reference may be the reference closest to the ROI on the exposed sample surface in a plane perpendicular to the sample depth. In one example, a Region of Interest (ROI) can be reconstructed by aligning a SEM image in 3D space. The SEM image can be aligned based on at least one reference focused in the corresponding SEM image and its corresponding sample depth. In another example, the ROI can be reconstructed based on its position relative to a surface reference at various sample depths. In yet another example, 3D features within the reconstructed ROI can be extracted. For example, such as... Figure 6 As demonstrated, the contours of multiple channels in a 3D-NAND sample at various sample depths are reconstructed by aligning the ROI, which includes multiple channels, based on the distance of the ROI from the surface reference.

[0020] Turn Figure 1 , Figure 1 A highly schematic depiction of an embodiment of a dual-beam charged particle microscope (CPM) in which the present invention is implemented, and more specifically, an embodiment of a FIB-SEM, is shown. The system axis is shown as axis 110. The microscope 100 includes a particle beam 1 that generates a beam of charged particles 3 (in this case, an electron beam) propagating along a particle beam axis 101. The particle beam axis 101 can be aligned with the Z-axis of the system. The beam 1 is mounted on a vacuum chamber 5, which includes a sample holder 7 for holding / positioning a sample 6 and one or more associated actuators 8. The vacuum chamber 5 is emptied using a vacuum pump (not depicted). The sample holder 7 or at least the sample 6 can be biased (floated) to a potential relative to ground, if desired, by means of a voltage power supply 17. A vacuum port 9 is also depicted, which can be opened to introduce or remove articles (components, samples) into or from the interior of the vacuum chamber 5. The microscope 100 may include multiple such ports 9 if desired.

[0021] Column 1 (in the present case) includes an electron source 10 and an illuminator 2. This illuminator 2 includes lenses 11 and 13 for focusing the electron beam 3 onto the sample 6, and a deflection unit 15 (for performing beam guidance / scanning of the beam 3). The microscope M further includes a controller / computer processing device 25 for controlling, in particular, the deflection unit 15, lenses 11 and 13, and detectors 19 and 21, and displays the information collected from detectors 19 and 21 on a display unit 27.

[0022] Detectors 19 and 21 are selected from a variety of possible detector types that can be used to examine different types of "stimulated" radiation emitted from sample 6 in response to irradiation by (impact) beam 3. Detector 19 may be a solid-state detector (e.g., a photodiode) for detecting cathodic emission emitted from sample 6. For example, the detector may alternatively be an X-ray detector, such as a silicon drift detector (SDD) or a silicon-lithium (Si(Li)) detector. For example, detector 21 may be an electron detector in the form of a solid-state photomultiplier tube (SSPM) or a vacuum photomultiplier tube (PMT). This can be used to detect backscattered and / or secondary electrons emitted from sample 6. Those skilled in the art will understand that many different types of detectors can be selected in, for example, the setup depicted, including, for example, ring / segmented detectors. Stimulated radiation, including, for example, X-rays, infrared / visible / ultraviolet light, secondary electrons (SE), and / or backscattered electrons (BSE), is emitted from sample 6 by scanning beam 3 across sample 6. Because such stimulated emission is position-sensitive (due to the scanning motion), the information obtained from detectors 19 and 21 will also be position-dependent. This fact allows, for example, the signal from detector 21 to be used to generate a BSE image of sample 6 (partially), which is essentially a mapping of the signal that varies with the position of the scan path on sample 6.

[0023] Signals from detectors 19 and 21 are transmitted along control lines (buses) 25, processed by controller 26, and displayed on display unit 27. Such processing may include operations such as combination, integration, subtraction, pseudo-coloring, edge enhancement, and other processing known to those skilled in the art. Furthermore, automatic identification processes (e.g., for particle analysis) may be included in such processing. The controller includes a processor and non-transitory memory for storing computer-readable instructions. The methods disclosed herein can be implemented by executing the computer-readable instructions in the processor.

[0024] In addition to the electron column 1 described above, the microscope 100 also includes an ion beam column 31. This includes an ion source 39 and an illuminator 32, which generate / guide an ion beam 33 along an ion beam axis 34. To facilitate access to the sample 6 on the holder 7, the ion axis 34 is tilted relative to the electron axis 101. As described above, such an ion (FIB) column 31 can be used, for example, to perform processing / machining operations on the sample 6, such as cutting, milling, etching, deposition, etc. Alternatively, the ion column 31 can be used to produce an image of the sample 6. It should be noted that, for example, if the ion source 39 is implemented as a so-called NAIS source, the ion column 31 may be able to generate a variety of different kinds of ions at will; therefore, a reference to the ion beam 33 should not be considered as specifying a particular kind of ion in the beam at any given time. In other words, the beam 33 may include ion type A for operation A (e.g., milling) and ion type B for operation B (e.g., implantation), where types A and B can be selected from a variety of possible options.

[0025] A gas injection system (GIS) 43 is also described, which can be used for the local injection of gases such as etching or precursor gases for the purpose of performing gas-assisted etching or deposition. For example, such gases can be stored / buried in a reservoir 41 and applied through a narrow nozzle 42 so as to be ejected near the intersection of shafts 101 and 34.

[0026] It should be noted that many improvements and alternatives to such setups will be known to those skilled in the art, such as the use of controlled environments within (relatively large volume) microscopes 100, for example, maintaining a background pressure of several mbar (as used in ambient SEMs or low-pressure SEMs).

[0027] Figure 2 Demonstrating the use of charged particle microscopy systems (e.g.) Figure 1 Method 200 for reconstructing features within a sample using a charged particle microscopy system. Multiple references are formed at or located at different sample depths to accurately determine the location of features or ROIs at greater sample depths. In one example, references are formed after creating steps by removing or milling multiple layers of the sample using a FIB. The height of the steps is determined experimentally or based on imaging beam parameters (e.g., electron beam parameters). The sample surface is exposed after each layer is removed or milled. The exposed surface corresponding to the same step is imaged with the same electron beam depth of focus. In other words, the sample surface corresponding to different steps is imaged with different electron beam depths of focus.

[0028] At 202, the step height is optionally determined experimentally on the sample. The sample can be the same sample used for the following slice-viewing process or a different sample. The step height can be determined by imaging multiple references on the sample surface using electron beams at various depths of focus. For example, such as Figure 3 As shown in the SEM images, three references 301, 302, and 303 are formed by FIB deposition / milling of material on the top surface 304 of the sample. The position of each reference in the SEM image is determined by the position of its center (i.e., the center of the crosshair). The three references are imaged multiple times at different depths of focus using the same field of view (FOV). At each depth of focus, the distance between at least two of the references is measured from the corresponding SEM image. For example, a first distance 310 between references 301 and 303 and a second distance 320 between references 301 and 302 are measured. As the three references move in and out of focus while adjusting the depth of focus, the accuracy of the reference position estimation changes. The reference positions can be determined more accurately when the references are focused and imaged. As a result, the first and second distance measurements change with the depth of focus. The range of depths of focus within which the distance between at least two of the references can be set as a stepped height. For example, when focusing on the sample surface or multiple references, the first distance 310 and the second distance 320 are first measured in the SEM image. Next, the depth of focus of the electron beam is increased to determine a depth of focus L1, where a change in either the first or second distance measured from the focus measurement at that depth of focus exceeds a threshold distance change. Similarly, the depth of focus of the electron beam is decreased to determine a depth of focus L2, where a change in either the first or second distance measured from the focus measurement at that depth of focus exceeds a threshold distance change. The step height is the difference between L1 and L2.

[0029] At position 204, the Region of Interest (ROI) including the features of interest is identified. The ROI is the 3D volume of the sample. Based on the location of the ROI, the position of the surface reference on the sample surface can be determined. In one instance, the maximum milling depth for milling and imaging can be determined based on the estimated size and structure of the ROI. In another instance, the maximum milling depth can be determined based on a cross-sectional image of the ROI. Additionally, at position 204, system parameters for milling and imaging are set. System parameters may include beam parameters, such as the beam energy and beam current of the FIB and electron beams. The FIB beam parameters can be determined based on the sample type and sample material. The Field of View (FOV) of the SEM image can be determined based on the image resolution. If the step height has not already been determined at position 202, the system parameters may also include the step height. The step height can be determined based on the focal length of the electron beam. For example, the step height is proportional to the focal length of the electron beam.

[0030] At 206, a reference is formed on or located on the sample surface. If step 206 is performed for the first time, the surface reference is formed near the ROI on the top surface of the sample. The surface reference may be a feature on the top surface of the sample. Alternatively, a surface reference similar to the reference at 202 can be formed by using FIB milling or material deposition. If step 206 has been performed previously and the sample surface is an exposed sample surface produced by FIB milling, a reference can be formed or positioned. In one instance, a reference can be formed on a newly exposed sample surface by FIB milling or deposition. In another instance, a reference can be formed during a milling process that creates a step. For example, as shown in SEM image Figure 5, the reference may be a portion of boundary 505 formed when milling from sample surface 501 to expose sample surface 502. Step 206 further includes adjusting the depth of focus of the electron beam by cycles 210-212-214-216 for imaging the exposed sample surface corresponding to the subsequently formed step. The depth of focus of the electron beam can be adjusted so that the newly formed reference is in focus. Adjusting the depth of focus can include increasing the depth of focus in the sample depth direction. The increment can be a step height.

[0031] At position 208, if multiple benchmarks have been formed on or located on the sample, SEM images including the two most recently formed benchmarks are optionally acquired. The two benchmarks are located at different sample depths. For example, the depth difference between the two benchmarks is a step height. The relative positions between the two benchmarks can be determined based on the SEM images. In some instances, the SEM images may also include the Region of Interest (ROI).

[0032] Steps 210, 212, 214, and 216 form a loop for flattening multiple layers until steps are formed or the maximum milling depth is reached. After removing each layer, a SEM image including the ROI and the most recently formed baseline is generated at point 206.

[0033] At 210, the sample is milled to remove the layer and expose the sample surface, including the ROI. At 206, SEM images of the ROI on the exposed sample surface and the most recently formed reference (i.e., the current reference) are acquired. In one example, SEM images are acquired using the ROI and reference within the same FOV. In another example, SEM images are generated by stitching together multiple SEM images with overlapping FOVs. Each of the multiple SEM images is acquired using the same electron beam depth of focus. The thickness of the removed layer can be determined based on a graph of the stage current / image signal versus time or a predetermined cutting width. Based on the thickness of the removed layer, the milling depth within the current step and the total milling depth (i.e., the milling depth from the top surface of the sample) are updated. Additionally, the sample depth and sample image of the exposed sample surface are determined.

[0034] At 212, method 200 compares the total milling depth with the maximum milling depth. The maximum milling depth can be determined at 204. Alternatively, the maximum milling depth can be determined based on the SEM image acquired at 210. If the total milling depth is less than the maximum milling depth, method 200 proceeds to 214. If the exposed surface depth is equal to or greater than the maximum milling depth, the slice view data collection is complete and method 200 proceeds to 222.

[0035] At 214, the milling depth in the current step is compared with the step height determined at 202 or 204. If the milling depth of the current layer is less than the step height, method 200 continues the milling and SEM imaging process at 216. Otherwise, the current step is completed and method 200 proceeds to 218 to form a new step. Additionally, at 218, the milling depth in the current step is set to zero.

[0036] At 222, the sample image can be aligned in the sample plane based on a reference. By aligning the sample image, the 3D volume of the ROI can be reconstructed. In one example, aligning the sample image includes aligning the ROI at various sample depths. The ROI can be aligned based on its position relative to a surface reference in the XY sample plane. The position of the ROI relative to the surface reference at the sample depth can be determined based on the SEM image of the exposed sample surface acquired at the sample depth and the relative position of the reference. For example, as shown in Figure 5, the position of ROI 520 relative to surface reference 510 is determined based on the position of ROI 520 relative to reference 512, the position of reference 512 relative to reference 511, and the position of reference 511 relative to reference 510 in the SEM image of sample surface 504. In another example, the sample image is aligned by shifting the sample image in the XY plane so that the positions of the same references are aligned.

[0037] Furthermore, one or more features within the ROI can be reconstructed at 222. Features can be regions or contours segmented from the SEM image. In one instance, features are reconstructed by extracting features from the reconstructed 3D volume of the ROI. In another instance, features are first extracted from the extent of the SEM image and reconstructed by aligning the extracted features in 3D space. Extracted features can be aligned in the XY sample plane based on their position relative to a surface reference. Extracted features can also be aligned along the Z-direction (sample depth) based on the sample depth of the corresponding sample surface. Figure 6 This demonstrates the reconstructed profiles of multiple channels in a 3D-NAND sample. At a specific sample depth, the profile of channel 601 is circular. Due to the accurate alignment of the channel profiles, the displacement of each channel in the XY sample plane at different sample depths can be accurately measured.

[0038] In this way, sample features (or ROIs) can be accurately reconstructed with minimal image post-processing. High-precision alignment of features or SEM images at various sample depths can be achieved by accurately locating references in SEM images based on focusing reference imaging. The XY position of the ROI at greater sample depths is estimated based on the position of the ROI in the XY sample plane relative to the nearest located reference and the relative position within the references. The relative position within the references is measured based on the positions of at least two references focused in the SEM image.

[0039] Figures 4A to 4B The display is by Figure 2 An example of a step created by a slice-view process. The sample extends in the XY sample plane and has a sample depth along the Z-axis. The sample depth increases in the Z-axis direction. Here, the top surface 401 of the sample has a sample depth of zero. A surface reference 404 is formed on the top surface 401. A first step 410 is formed by repeatedly removing a plurality of first layers having a layer thickness 406. After removing each of the first layers, the sample surface 430 is exposed for SEM imaging with an electron beam at a first focal depth. Each of the SEM images includes the surface reference 404 and the exposed sample surface 430 at a sample depth not greater than d1. The first step has a step height d1 determined experimentally or based on electron beam properties. After forming the first step, a second sample surface 402 is exposed, and a reference 405 is formed on the second sample surface 402. A second step is formed by removing a plurality of second layers. A plurality of sample surfaces 420 and a third sample surface 403 are exposed. An electron beam at a second focal depth can be used to image the SEM image of the second reference 405 along with either the sample surface 420 or the third sample surface 403. The thickness of each second layer can be the same as the thickness of each first layer. The third reference 408 can be formed on the third surface 403 to mill the sample at a greater sample depth and to image the sample. As a result of the milling process, the sample has... Figure 4B The stepped form shown in the figure. In some embodiments, instead of forming references on the second and third sample surfaces, boundaries 441 and 442 may be used as the second and third references, respectively.

[0040] Figure 5ASEM image 500 of a milled sample in the XY sample plane is shown. Three references 510, 511, and 512 are formed on a first sample surface 501, a second sample surface 502, and a third sample surface 503. The sample depth increases from the first sample surface 501 to a fourth sample surface 504. Each sample surface extends in the XY sample plane. Focused imaging is performed on the fourth sample surface 504 and the third reference 512. The position of the ROI 520 relative to the first reference 510 on the fourth sample surface can be determined based on the position of the ROI 520 relative to the third reference 512 and the position of the third reference 512 relative to the first reference 510. The position of the ROI 520 relative to the third reference 512 is determined based on the SEM image of the ROI and the third reference, including the third focal depth. The position of the third reference 512 relative to the first reference is determined based on the first position of the third reference 512 relative to the second reference 511 and the second position of the second reference 511 relative to the first reference 510. A first position is determined based on an SEM image including a third reference and a second reference acquired at a second focal depth, and a second position is determined based on an SEM image including the second reference and the first reference acquired at a first focal depth. The third focal depth is greater than the second focal depth. The second focal depth is greater than the first focal depth. In some embodiments, instead of forming a reference on the sample surface, the reference may be inherently formed during milling. For example, a region 505 including a portion of a boundary 506 formed when milling from the first sample surface to the second sample surface may be used as the second reference.

[0041] This can be obtained by stitching together multiple SEM images with overlapping fields of view (FOV). Figure 5A SEM images in [the image / image]. For example, such as... Figure 5B As shown, this can be obtained by stitching together three SEM images 551, 552, and 553 with overlapping FOV. Figure 5A The SEM images are shown. SEM images 551 and 552 overlap in region 521, and SEM images 552 and 553 overlap in region 522. By matching the signals in the overlapping regions 521 and 522, the three SEM images can be accurately stitched together.

[0042] Figures 7A to 7B and Figures 8A to 8B This shows an example of the FIB and electron beam positions relative to the sample. The sample orientation is indicated by axis 701, and the orientation of the dual-beam system is indicated by system axis 110. Figure 7AThe sample surface 703 is tilted at an angle 704 relative to the XY system plane 705 of the dual-beam system. The sample surface 703 is irradiated with a FIB 33 generated from the ion source 39. The incident angle of the FIB is less than 90 degrees. For example, the sample is milled with an FIB beam orthogonally oriented towards the sample surface. After milling and exposing the sample surface 707, the sample is tilted so that the sample surface 707 is in the XY system plane. The Z-axis (sample depth direction) of the sample is perpendicular to the XY system plane 705. SEM images are acquired with an electron beam 3 orthogonally oriented towards the exposed sample surface 707.

[0043] exist Figures 8A to 8B In the FIB milling process, sample 802 is oriented such that the Z-axis of the sample axis has an angle 804 relative to the XY system plane 705. Sample surface 803 is milled using an FIB beam 33 with an incident angle greater than 90 degrees to expose sample surface 805. For the sample, the FIB is oriented with a grazing angle less than 10 degrees relative to sample surface 803. Sample 802 is then oriented such that the Z-axis of sample axis 801 is aligned with the Z-axis of system axis 101. Sample surface 805 is imaged using an electron beam 3, wherein the electron beam is orthogonally oriented toward sample surface 805.

[0044] The technical effect of using multiple references at different sample depths is that the position of the ROI relative to a surface reference can be deduced based on the relative positions of the references and an SEM image including at least one of the ROI and the reference, where the ROI and the reference are in focus in the SEM image. Additionally, the position of features deep within the sample can be accurately measured. The technical effect of focusing SEM imaging of the ROI and the reference is that the position between the ROI and the reference can be accurately measured.

Claims

1. A method for imaging a sample, comprising: Position the first reference point on the surface of the first sample at the first sample depth; Remove at least a portion of the surface of the first sample to expose the surface of the second sample; A second reference is formed on the surface of the second sample; At least a portion of the second sample surface is removed to expose a third sample surface including the region of interest (ROI) at a third sample depth; Acquire a first sample image including the second reference and the ROI at the third sample depth; as well as The position of the ROI at the third sample depth relative to the first reference is determined based on the first position of the first reference relative to the second reference and the second position of the ROI at the third sample depth relative to the second reference in the first sample image.

2. The method according to claim 1, further comprising: Acquire a second sample image including the first reference and the second reference; And determine the first position of the first reference relative to the second reference based on the second sample image.

3. The method of claim 2, wherein the first sample image is acquired using a charged particle beam at a first focal depth, and the second sample image is acquired using the charged particle beam at a lower second focal depth.

4. The method of claim 3, further comprising: The charged particle beam at the second focal depth acquires a third sample image at a fourth sample depth between the first sample depth and the second sample depth, wherein the third sample image includes the ROI at the fourth sample depth and the first reference. The position of the ROI at the fourth sample depth relative to the first reference is determined based on the third sample image; as well as The ROI at the fourth sample depth and the third sample depth is reconstructed based on the position of the ROI at the third sample depth relative to the first reference and the position of the ROI at the fourth sample depth relative to the first reference.

5. The method according to any one of claims 1 to 4, wherein at least a portion of the surface of the first sample is removed with a first charged particle beam, an image of the first sample is acquired with a second charged particle beam, and the method further comprises determining the difference between the depth of the first sample and the depth of the second sample based on the focal length of the second charged particle beam.

6. The method according to any one of claims 1 to 4, wherein at least a portion of the first sample surface is removed with a first charged particle beam, an image of the first sample is acquired with a second charged particle beam, and the method further includes determining the second sample depth by imaging the distance between at least two references of a plurality of depths of focus of the second charged particle beam before removing the at least a portion of the first sample surface.

7. The method according to any one of claims 1 to 4, wherein the first sample surface is the top surface of the sample, and the first reference is a feature on the top surface.

8. The method according to any one of claims 1 to 4, further comprising forming the first reference by depositing or etching the surface of the first sample with a focused ion beam.

9. The method according to any one of claims 1 to 4, wherein the second reference is formed when at least a portion of the first sample surface is removed to expose the second sample surface.

10. The method of claim 9, wherein the second reference includes a portion of the boundary between the first sample surface and the second sample surface.

11. A method for imaging a sample, comprising: Position the first reference point on the surface of the first sample at the first sample depth; The sample is milled from the first sample depth to the second sample depth to expose a plurality of second sample surfaces, and a plurality of first sample images including the first reference and the region of interest (ROI) are acquired on the exposed plurality of second sample surfaces using a charged particle beam at a first focal depth, wherein each of the plurality of first sample images corresponds to one of the plurality of second sample surfaces from the first sample depth to the second sample depth. Positioning a second reference point; The sample is milled from the second sample depth to the third sample depth to expose a plurality of third sample surfaces including the ROI, and a plurality of second sample images including the second reference and the ROI are acquired on the exposed plurality of third sample surfaces using the charged particle beam at the second focal depth, wherein each of the plurality of second sample images corresponds to one of the plurality of third sample surfaces from the second sample depth to the third sample depth. as well as Features in the ROI are reconstructed by aligning the plurality of first sample images and the plurality of second sample images based on the position of the first reference in the plurality of first sample images, the position of the second reference in the plurality of second sample images, and the position of the first reference relative to the second reference.

12. The method of claim 11, further comprising acquiring a third sample image including the first reference and the second reference after milling the sample from the first sample depth to the second sample depth; and determining the position of the first reference relative to the second reference based on the third sample image.

13. The method of claim 11, wherein at least one of the plurality of second sample images includes the first reference and the second reference, and the method further includes determining the position of the first reference relative to the second reference based on the at least one of the plurality of second sample images.

14. The method of any one of claims 11 to 13, wherein reconstructing the feature comprises extracting the feature from the ROI in each of a plurality of aligned first sample images and a plurality of second sample images, and reconstructing the feature based on the extracted feature and its corresponding sample depth.

15. The method according to any one of claims 11 to 13, further comprising: Positioning a third reference point; The sample is milled from the third sample depth to the fourth sample depth to expose a plurality of fourth sample surfaces, and a plurality of third sample images including the third reference and the ROI are acquired on the plurality of exposed fourth sample surfaces using the charged particle beam at the third focal depth, wherein each of the plurality of third sample images corresponds to one of the plurality of fourth sample surfaces from the third sample depth to the fourth sample depth. as well as The features are reconstructed by further aligning the plurality of third sample images with the plurality of first sample images.

16. The method according to any one of claims 11 to 13, wherein one or more of the first sample image and the second sample image from a plurality of images are stitched together, wherein the field of view of any one of the plurality of images overlaps with the field of view of at least another of the plurality of images.

17. A charged particle microscopy system, comprising: Sample rack, used to hold samples; An ion beam source for directing an ion beam toward the sample held by the sample holder; An electron beam source, used to direct an electron beam toward the sample; as well as A controller includes a non-transitory memory for storing computer-readable instructions that, when executed, cause the controller to: Position the first reference point on the surface of the first sample at the first sample depth; Remove at least a portion of the surface of the first sample to expose the surface of the second sample; Positioning a second reference point; At least a portion of the second sample surface is removed to expose a third sample surface including the region of interest (ROI) at a third sample depth; Acquire a first sample image including the second reference and the ROI at the third sample depth; as well as The position of the ROI at the third sample depth relative to the first reference is determined based on the first position of the first reference relative to the second reference and the second position of the ROI in the sample image relative to the second reference.

18. The system of claim 17, wherein the controller is further configured to: acquire a second sample image including the first reference and the second reference; and determine the first position of the first reference relative to the second reference based on the second sample image.

19. The system of claim 18, wherein the second sample image further includes the ROI on the surface of the second sample, the second sample image is acquired before removing at least a portion of the surface of the second sample, and the controller is further configured to reconstruct the ROI based on the first sample image and the second sample image.

20. The system according to any one of claims 17 to 19, wherein the first sample image is acquired by scanning the electron beam within the field of view.

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