Systems and methods for simultaneous phase contrast imaging and electron energy loss spectroscopy

Abstract

To provide methods allowing simultaneous acquisition of EESL and TEM / STEM based images.SOLUTION: A method and system for imaging a sample 60 with charged particles comprises directing charged particles 111 towards the sample along a primary axis 110, and simultaneously detecting a first portion and a second portion of the charged particles transmitted through the sample with a first detector 250 and a second detector 270, respectively. The second detector is positioned downstream of the first detector. Each of the transmitted charged particles exits the sample at an exit angle between a direction of the transmitted charged particle and the primary axis. Exit angles of the first portion of the transmitted charged particles overlap with exit angles of the second portion of the transmitted charged particles. In this way, complimentary information, such as structural information and compositional information, may be obtained simultaneously.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present description relates generally to methods and systems for imaging a sample using a charged particle system, and more particularly to methods and systems for simultaneously performing phase contrast imaging and energy loss spectroscopy on a sample using a charged particle system. [Background technology]

[0002] Electron energy loss spectroscopy (EELS) can reveal compositional information by measuring the energy loss of electrons passing through a thin sample. In EELS, electrons transmitted through the sample are spatially dispersed by a spectrometer based on their electron energy and collected as a spectrum by a detector downstream of the spectrometer. EELS can be performed simultaneously with dark-field transmission electron microscopy (TEM). The TEM signal can be acquired with a high-angle annular dark-field (HAADF) detector positioned between the sample and the spectrometer. By scanning a focused electron beam over the sample, a scanning transmission electron microscope (STEM) image showing the sample structure and an EELS dataset containing chemical information at each scan location can be obtained in a single scan. Simultaneous acquisition of the STEM and EELS signals ensures that the acquired structural and compositional information are spatially registered. However, under certain conditions, STEM images acquired with an HAADF detector may not display all sample structures. For example, HAADF images lack a sufficient signal-to-noise ratio to display light atoms when both light and heavy atoms are present in the sample.

[0003] EELS cannot be performed simultaneously with other types of TEM- or STEM-based imaging techniques at its full capacity. This is because electrons with large scattering angles are blocked by detectors positioned upstream of the spectrometer. For example, differential phase contrast (DPC) STEM imaging and integrated differential phase contrast (iDPC) STEM imaging can reach sub-Å resolution. DPC-STEM, for example, uses a segmented detector positioned downstream of the sample to record two-dimensional spatial wave amplitude distributions within the image plane. iDPC-STEM imaging has been described by I. Lazic and E. G. T. Bosch in Ultramicroscopy 160 (2016) 265–280, I. Lazic and E. G. T. Bosch in Advances in Imaging and Electron Physics 199 (2017) 75–184, and E. Yucelen, I. Lazic, and E. G. T. Bosch in Scientific Reports 8, 2676 (2018). iDPC-STEM imaging is also disclosed in U.S. Patent Application No. 14 / 629,387, filed February 23, 2015, by Lazic et al. The above publications and patent applications are incorporated herein by reference. iDPC-STEM images are generated by integrating DPC-STEM vector images acquired by a segmented (DPC / iDPC) detector or camera. iDPC-STEM can image both light and heavy atoms in a sample. With existing / current detector configurations, neither DPC-STEM nor iDPC-STEM can be performed in conjunction with EELS because the segmented DPC / iDPC detector blocks electrons necessary for detecting and interpreting EELS data sets. The same problem also occurs with differentiated differential phase contrast (dDPC) STEM, which is obtained by differentiating (applying a divergence operator) a DPC-STEM vector image. The present invention presents a method and system that enables the above-mentioned simultaneous acquisition of EELS and TEM / STEM-based images. Summary of the Invention

[0004] In one embodiment, a method for imaging a sample using charged particles includes directing charged particles toward the sample along a primary axis and simultaneously detecting a first portion and a second portion of the charged particles transmitted through the sample using a first detector centered on the primary axis and a second detector downstream of the first detector, respectively, where each of the transmitted charged particles exits the sample at an exit angle between the direction of the transmitted charged particles at the back side of the sample and the primary axis, and the exit angle of the first portion of the transmitted charged particles overlaps with the exit angle of the second portion of the transmitted charged particles. In this manner, structural and compositional information of the sample can be simultaneously obtained using the first and second detectors. The structural information can be displayed in a sample image, and the compositional information can be displayed in the form of a spectrum. Electrons at high scattering angles can reach the second detector without significantly sacrificing the quality of the phase-contrast image. In some embodiments, the transmitted charged particle beam can be rotated relative to the first detector at a location upstream of the first detector. The spectra acquired before and after beam rotation can be combined to produce a combined spectrum covering 360 degrees of azimuthal coverage of the transmitted charged particles at the backside of the sample.

[0005] It should be understood that the foregoing summary is provided to introduce in a simplified form a selection of concepts that are further described in the detailed description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Moreover, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 illustrates a charged particle microscope for combined phase contrast imaging and electron energy loss spectroscopy (EELS) according to some embodiments. [Figure 2]FIG. 2 shows the position of the detector for imaging the sample structure relative to the charged particle beam. [Figure 3A] FIG. 3A shows an example detector with four connected detection segments for imaging a sample structure. [Figure 3B] FIG. 3B is the contrast transfer function (CTF) using the detector of FIG. 3A. [Figure 3C] FIG. 3C is an integral-differential phase contrast (iDPC-STEM) image acquired with the detector of FIG. 3A. [Figure 3D] FIG. 3D is the Fourier transform of FIG. 3C. [Figure 4A] FIG. 4A shows an example detector with four separated detection segments for imaging sample structures. [Figure 4B] FIG. 4B is the CTF using the detector of FIG. 4A. [Figure 4C] FIG. 4C is an iDPC-STEM image acquired with the detector of FIG. 4A. [Figure 4D] FIG. 4D is the Fourier transform of FIG. 4C. [Figure 5A] FIG. 5A shows the ideal CTF using a camera. [Figure 5B] 5B and 5C show an example detector with eight connected detector segments and their corresponding CTFs. [Figure 5C] 5B and 5C show an example detector with eight connected detector segments and their corresponding CTFs. [Figure 5D] 5D and 5E show an example detector with eight separated detection segments and their corresponding CTFs. [Figure 5E] 5D and 5E show an example detector with eight separated detection segments and their corresponding CTFs. [Figure 6A] 6A and 6B show example detectors with four and eight detector segments separated by cut openings. [Figure 6B] 6A and 6B show example detectors with four and eight detector segments separated by cut openings. [Figure 7] FIG. 7 shows an exemplary method for simultaneously acquiring sample images and EELS data using the system of FIG. [Figure 8] FIG. 8 illustrates another exemplary method for simultaneously acquiring sample images and EELS data using the system of FIG. [Figure 9] FIG. 9 illustrates another exemplary method for simultaneously acquiring sample images and EELS data using the system of FIG.

[0007] Like reference numbers refer to corresponding parts throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION

[0008] The following description relates to a system and method for simultaneously obtaining structural and compositional / bonding information of a sample using a charged particle imaging system, such as the charged particle imaging system shown in Figure 1. Structural information can be obtained by phase-contrast imaging, such as differential phase-contrast (DPC) scanning transmission electron microscopy (STEM) imaging or integrated differential phase-contrast (iDPC) STEM imaging, via a first detector positioned downstream of the sample. Compositional, bonding, or electronic structure information can be obtained by electron energy loss spectroscopy (EELS) via a second detector positioned downstream of the first detector. A spectrometer is positioned between the first and second detectors to spatially disperse the charged particles based on particle energy. The first detector is centered on the primary axis of the charged particle beam. In response to irradiation of the front surface of the sample with the charged particle beam, a portion of the scattered charged particles transmits through the sample and is emitted from the back surface of the sample. The direction of each transmitted charged particle at the back surface of the sample is defined by an exit angle and an azimuthal angle, as shown in Figure 2. The exit angle is the angle between the direction of the transmitted charged particles and the primary axis. The azimuthal angle is in a plane perpendicular to the primary axis. In one example, the primary axis is aligned with the z-axis. The azimuthal angle is the angle between the projection of the direction of the transmitted charged particles onto the x-y plane and the x-axis. The first and second detectors simultaneously detect the first and second portions of the transmitted charged particle beam, respectively. The first detector includes one or more charged particle transparent regions that allow the second portion of the transmitted charged particles to pass through the first detector without being detected by the first detector. The second portion of the transmitted charged particles may not interact with the first detector or may have negligible interaction. Therefore, the presence of the first detector does not affect the detection of the second portion of the transmitted charged particles detected by the second detector. In one example, the one or more charged particle transparent regions in the first detector may be apertures. In another example, one or more charged particle transparent regions can be a thin layer of material that does not perturb the energy distribution of impinging charged particles.The exit angle of the first portion of the transmitted charged particles detected by the first detector overlaps with the exit angle of the second portion of the transmitted charged particles detected by the second detector. Thus, some of the transmitted charged particles received by the first and second detectors have the same exit angle but different azimuthal angles. The maximum exit angle of the transmitted charged particles detected by the second detector may be the same as that of the transmitted charged particles detected by the first detector, so that both the first and second detectors can detect the full range of angular momentum of the transmitted charged particles.

[0009] In one example, a first detector detects a first portion of the transmitted charged particles having exit angles in a first range [A1 A2], where A1 is the minimum exit angle to be detected and A2 is the maximum exit angle to be detected, where A1 ≥ 0 degrees and A2 < 90 degrees. A second detector detects a second portion of the transmitted charged particles having exit angles in a second range [A3 A4], where A3 is the minimum exit angle to be detected and A4 is the maximum exit angle to be detected, where 0 ≤ A3 ≤ A2 and A4 < 90 degrees. The first and second ranges overlap. The overlapping ranges can be [A1 A4] or [A1 A2]. In one example, the maximum exit angles detected by the first and second detectors are the same, i.e., A2 = A4. In another example, A4 > 1 / 2A2, so the second detector is sensitive to electrons at high scattering angles. The first and second detectors may be capable of detecting any exit angle within the first and second ranges, respectively. In other examples, the first or second detectors may be capable of detecting selective exit angles within their respective ranges.

[0010] In another example, at certain exit angles within the overlapping range, the first and second detectors are sensitive to transmitted electrons having different azimuthal angles. The first detector may include at least one charged particle transparent region extending azimuthally for less than 360 degrees along the radius of the first detector. This region allows a portion of the transmitted electrons to pass through the first detector. The first detector has no or minimal effect on electrons passing through the aperture. That is, the energy and direction of electrons passing through the charged particle transparent region are not (or are minimally) altered by the first detector.

[0011] In some examples, the first detector includes multiple detector segments and at least one aperture disposed between at least two of the detector segments. Signals detected by each detector segment can be transmitted to the controller via separate amplifiers. As shown in Figures 4A, 5D, 6A, and 6B, the first detector (e.g., a DPC / iDPC detector) can be in the form of a "pinwheel," in which multiple identically shaped detector segments are evenly spaced in a circular pattern azimuthal to a primary axis. Two edges of each detector segment (or pinwheel blade) extend radially from the primary axis. The detector segments are separated from each other by apertures. In the case of a detector with four detector segments, the detector's detection area is reduced compared to a conventional four-segment DPC / iDPC detector with connected detector segments. However, the quality of iDPC-STEM images acquired using a detector with disconnected detector segments is similar to that acquired with a conventional DPC / iDPC detector, as shown in Figures 3A-3D and 4A-4D. This is because the contrast transfer function (CTF) of the detector with a reduced detection area is similar to that of a conventional DPC / iDPC detector. In some examples, the first detector may include a central aperture that allows transmitted charged particles with a low exit angle to enter the spectrometer. In other examples, the first detector may not have a central aperture, as shown in Figures 6A and 6B. The first detector and the second detector receive transmitted charged particles with azimuthal angles that do not overlap each other.

[0012] As shown in Figure 9, a sample structure image can be obtained from the first detector by scanning a focused charged particle beam across multiple scan positions. Composition information at each scan position is extracted based on the spectrum obtained by the second detector. The first and second detectors are sensitive to high-scattering charged particles with different azimuthal angles.

[0013] In some embodiments, the transmitted charged particle beam is rotated azimuthally about the primary axis relative to the first detector, so that the transmitted charged particles covering the entire 360-degree azimuth angle and a wide range of exit angles can be acquired by each of the first and second detectors. The second detector can capture all of the transmitted charged particles at exit angles equal to or less than the maximum detectable exit angle of the first detector. For example, the second detector acquires a first spectrum and a second spectrum, respectively, before and after rotating the transmitted charged particles relative to the first detector. The first and second spectra are complementary to each other; that is, the first and second spectra correspond to different portions of the transmitted charged particles at the scan position. By combining the first and second spectra, the combined spectrum contains the same information as a spectrum acquired without the first detector (i.e., without blocking electrons entering the EELS spectrometer). The transmitted charged particles can be rotated relative to the first detector by rotating either the sample, the first detector, or the transmitted charged particle beam between the sample and the first detector about a primary axis. As disclosed in U.S. Patent Application No. 15 / 803,642 by Freitag et al., filed November 3, 2017, which is incorporated herein by reference, the transmitted charged particle beam can be rotated by adjusting the excitation of at least one lens of a projection lens disposed between the sample and the first detector. Compared to rotating the sample or the first detector, adjusting the projection lens is faster and does not cause sample shift. Figures 7 and 8 show two methods for generating a combined spectrum. In Figure 7, non-rotated and rotated spectra are acquired at a scan location before deflecting the charged particle beam to another scan location. Alternatively, Figure 8 shows a method for performing a first scan and acquiring a first spectrum. After rotating the transmitted charged particle beam, a second scan is performed on the same region of interest (ROI) to acquire a second spectrum. The structural images acquired during each scan can be used to combine the first and second spectra.

[0014] Turning to FIG. 1 , that figure shows a highly schematic depiction of an embodiment of a charged particle microscope 100 in which the present invention can be implemented. More specifically, an embodiment of a transmission microscope is shown, which may be an electron microscope. Within a vacuum enclosure 2, a charged particle (e.g., electron) source 4 generates a beam of electrons 111 propagating along a primary axis 110 and traversing an electron-optical illuminator 6, which serves to direct / focus the electrons onto a selected portion of a sample 60 (e.g., to (locally) thin / planarize) the sample. A deflector 8 is also shown, which may be used to (among other things) effect a scanning motion of the beam 111.

[0015] The sample 60 is held on a sample holder H, which can be positioned with multiple degrees of freedom by a positioning device / stage A (e.g., the sample holder H can include fingers that can move (among other things) in the XY plane (see the illustrated Cartesian coordinate system, typically with motion parallel to the Z direction and tilting about the X / Y directions also possible)), which moves the cradle A' to which the sample holder H is (removably) mounted. Such movement allows different portions of the sample 60 to be illuminated / imaged / inspected by the electron beam 111 traveling in the Z direction along the primary axis 110 (and / or can perform a scanning motion instead of a beam scan). The sample holder H can also rotate the sample relative to the primary axis 110. If desired, an optional cooling device (not depicted) can be in thermal contact with the sample holder H, thereby maintaining the sample holder H (and the sample 60 thereon) at, for example, a cryogenic temperature.

[0016] The electron beam 111 interacts with the sample 60 to cause various types of “stimulated” radiation to be emitted from the sample 60, including (for example) secondary electrons, backscattered electrons, X-rays, and optical radiation (cathodoluminescence). If desired, one or more of these radiation types can be detected using a detector 22, which may be, for example, a combined scintillator / photomultiplier tube or an EDX (energy dispersive X-ray spectroscopy) module. The electrons can transmit (pass) through the sample 60, exit / enter / exit the backside of the sample, and continue propagating along the primary axis 110 (substantially, but typically with some deflection / scattering). This transmitted electron flux enters the projection lens 24, which may include various electrostatic / magnetic lenses, deflectors, correctors (such as stigmators), and the like.

[0017] To detect and analyze transmitted electrons with various exit angles at the backside of the sample, multiple detectors can be positioned downstream of the projection lens 24. A first detector and a second detector are used to simultaneously detect the transmitted electrons. In response to irradiation, a first portion of the transmitted electrons is detected by the first detector. A second portion of the transmitted electrons passes through one or more charged particle transparent regions of the first detector and is detected by a second detector positioned downstream of the first detector. The first and second detectors can be any one of a high-angle annular dark-field (HAADF) detector 26, a DPC / iDPC detector 30, a TEM / STEM detector 32, and a spectrometer 34. In one example, the first detector is a DPC / iDPC detector 30, and the second detector is an EELS detector. The DPC / iDPC detector 30 is configured to allow a portion of the high-scattering-angle electrons to pass through, so that the transmitted electrons received by the DPC / iDPC detector 30 and the EELS detector 36 of the spectrometer 34 have overlapping exit angles. Examples of DPC / iDPC detectors are shown in FIGS. 2-6. A TEM / STEM detector 32 may optionally be disposed downstream of the HAADF detector 26 and the spectrometer 34. The TEM / STEM detector 32 may be retracted (as indicated diagrammatically by arrow 32′) to allow the transmitted electrons to enter the spectrometer 34, such as an electron energy loss spectroscopy (EELS) module. The spectrometer may include a spectrometer 35 that disperses the charged particles based on their energy and an EELS detector 36 for sensing the spectrum formed by the dispersed charged particles. The spectrum may be an electron energy loss spectrum.

[0018] In another example, the first detector is a camera that includes small segments of a mass called pixels. The camera includes one or more apertures (e.g., a set of transparent pixels) that allow a second portion of the charged particles to pass through the camera and be detected by the second detector. Center-of-mass (COM)-STEM and / or integrated center-of-mass (iCOM)-STEM images can be acquired with the camera. In yet another example, the first detector is a position sensitive detector (PSD) that includes one or more apertures (e.g., a set of transparent pixels) that allow a second portion of the charged particles to pass through the PSD and be detected by the second detector.

[0019] In response to scanning the charged particle beam across multiple scan positions on the sample, the signal detected from the first detector can be used to form an image showing the structure of the sample. The first image can be an iDPC-STEM image or an iCOM-STEM image. An iCOM-STEM image is an ideal version of an iDPC-STEM image obtained when the detector is either a PSD or a camera composed of a relatively large number of small segments (ideally a pixelated camera), allowing for more accurate calculation of the COM or illumination center of gravity. The first detector image can also be several other types of STEM images, such as DPC-STEM, dDPC-STEM, COM-STEM, dCOM-STEM, BF-STEM, ABF-STEM, ADF-STEM, HAADF-STEM, and single-segment STEM. At each scan position, the signal detected from the second detector can contain chemical or compositional information about the sample.

[0020] A controller 50 is connected to the various illustrated components via control lines 20′. (Bus) Controller 50 may include a processor 54 and non-transitory memory 55 for storing computer-readable instructions. Executing the computer-readable instructions stored in the non-transitory memory enables the controller to implement the various methods disclosed herein. Controller 50 may provide various functions, such as synchronizing actions, providing set points, processing signals, performing calculations, receiving operator input from user input device 53, and displaying messages / information on display device 51. For example, controller 50 may be configured to process signals received from various detectors and generate an image indicative of the structure and / or composition of the sample. Controller 50 may adjust the transmitted charged particle beam by adjusting one or more lenses in projection lens 24. Controller 50 may be (partially) inside or outside the housing, and may be of unitary or composite construction, as desired.

[0021] In some embodiments, the first detector and the second detector can be used for other types of imaging. As an example, the first detector can be used to acquire chemical / compositional information, and the second detector can be used to acquire structural information. For example, the first detector is an EELS detector, and the second detector is a DPC / iDPC detector. In another example, both the first detector and the second detector can be used to acquire structural or compositional information. In some examples, one or more additional detectors having charged particle transparent regions can be positioned upstream of the second detector.

[0022] Although a transmission electron microscope system is described as an example, it will be appreciated that the imaging system may be other types of charged particle microscope systems, such as dual beam tools such as a focused ion beam combined with a scanning electron microscope (FIB-SEM).

[0023] FIG. 2 is an enlarged view showing the arrangement of the first and second detectors relative to the primary axis of the charged particle beam. As an example, the first detector 250 can be a DPC / iDPC detector. The first detector can include four cut-away detector segments arranged around the primary axis 110 in an azimuthal direction 252. The electron beam 111 illuminates the front surface 61 of the sample 60 along the Z direction and strikes a location 63 on the sample 60. The electron beam 111 can be a focused beam. Electrons transmitted through the sample 60 exit the back side 62 of the sample in different directions (e.g., directions 221, 222, and 223). The direction of the transmitted charged particles exiting the back side of the sample is defined by the exit angle and the azimuthal angle. The exit angle of the transmitted electrons is the angle between the electron direction and the primary axis. For example, for transmitted electrons in direction 221, the exit angle is angle 210. The exit angle ranges from 0 to 90 degrees. A large exit angle corresponds to transmitted electrons with a large scattering angle. The azimuthal angle is the angle between the electron direction and the X-axis, i.e., the angle between the electron direction projected onto the sample plane and the X-axis. The azimuthal angle ranges from 0 to 360 degrees. For example, detector 250 is positioned in the XY plane downstream of sample 60. The azimuthal angle of direction 221 is the angle 240 between the X-axis and its projection direction 241 in the XY plane. The first detector 250 has a periphery 251 that allows the detector to detect transmitted charged particles at the maximum exit angle 230.

[0024] In response to irradiation with the electron beam 111, a portion of the transmitted charged particles (i.e., a first portion), such as the transmitted charged particles indicated by arrows 264 and 265, strike one or more detection segments of the first detector 250 and are acquired by the first detector. A portion of the transmitted charged particles (i.e., a second portion), such as the transmitted charged particles indicated by arrows 261, 262, and 263, pass through the first detector through one or more apertures of the first detector and are detected by a second detector 270 positioned downstream of the first detector. In some embodiments, the second detector 270 may not be centered along the primary axis as shown in FIG. 2 . For example, the second detector 270 is an EELS detector in a spectrometer, and the entrance of the spectrometer is aligned with the primary axis.

[0025] FIG. 3A shows a top view of a conventional DPC / iDPC detector 300. The detector 300 includes four identical detector segments 301, 302, 303, and 304 arranged around the center (or geometric center) of the detector 300. Each detector segment is directly connected to the other two detector segments. The detector segments cover 360 degrees in the azimuthal direction 306. The detector 300 can optionally have a round aperture 305 in the center to allow electrons with low exit angles to pass through the detector. The radius of the round aperture may be less than one-fifth the radius of the detector's outer periphery. The detector 300 captures all transmitted electrons with exit angles in the range [A1 A2], where A1 is ≧0 degrees and A2 is <90 degrees.

[0026] FIG. 4A shows a top view of a DPC / iDPC detector 400 in accordance with the present invention. Detector 400 includes four identical detector segments 401, 402, 403, and 404 evenly distributed around a center (or geometric center) 430 of detector 300. The detector segments are separated from one another in the azimuthal direction 413 by apertures. For example, detector segments 401 and 402 are separated by aperture 406. Each detector segment has two radially extending edges (such as edges 410 and 411) and one edge (such as edge 412) disposed along the detector's outer periphery (dashed lines). Detector 400 has a central round aperture 405. Round aperture 405 is connected to each of apertures 406, 407, 408, and 409. Apertures 406, 407, 408, and 409 are disposed around center 430. Detector 400 detects half of the transmitted electrons at exit angles in the range of [A3 A4], where A3 is greater than 0 degrees and is determined by the diameter of the circular aperture. In one example, A3 ≦ 0.2 · A4. Assuming the radius of the circular aperture is r1 and the radius of the outer periphery is r2, the azimuthal angle covered by detector 400 at any radius between r1 and r2 is less than 360 degrees. Detector 400 and a downstream EELS detector (such as EELS detector 36 in FIG. 1) can simultaneously detect transmitted electrons with overlapping exit angles but non-overlapping azimuthal angles. For example, detector 36 can detect transmitted electrons at exit angles in the range of [0 A4].

[0027] In one example, the shape of each aperture may be the same as that of each detection segment. Each detection segment and each aperture covers 360 / 8 degrees in the azimuthal direction relative to the center of the detector. The total azimuthal range occupied by the detection segment is 180 degrees, which is the same as the total azimuthal range occupied by the aperture. This allows at least half of the transmitted electrons with high exit angles (e.g., exit angles greater than A3) to pass through the detector 400 and be detected by the second detector. In another example, the area of each detection segment may be different from each of the apertures between the detection segments. In other words, the total azimuthal range covered by the detection segment may be greater or smaller than the total azimuthal range covered by the aperture.

[0028] The performance of detectors 300 and 400 is compared in Figures 3A-3D and 4A-4D. Similar SrTiO3 samples were imaged with both detectors. The detectors have the same perimeter and central apertures (apertures 305 and 405). The total detection area of detector 400 is half that of detector 300. Figures 3B and 4B are the CTFs of detectors 300 and 400, respectively. Figures 3C and 4C are iDPC-STEM images acquired with detectors 300 and 400, respectively. Figures 3D and 4D are FFTs of Figures 3C and 4C, respectively. Even though the total detection area is reduced, the CTF of detector 400 is similar to that of detector 300. There is no significant change in the resolution of either the iDPC-STEM or iDPC-STEM images.

[0029] 5B-5E show a detector with eight detection segments and their corresponding CTFs. FIG. 5B is a top view of a conventional detector 500. Detector 500 has eight identical detection segments (such as detection segment 501) arranged similarly to detector 300. The detection segments are spread around the center of the detector. There are no openings between the detection segments in the azimuthal direction 503 relative to the detector center. The detection segments cover a full 360 degrees of azimuthal coverage. A central opening 502 is located at the center of detector 500. FIG. 5C shows the CTF of detector 500.

[0030] FIG. 5D shows a detector 510 according to the present invention. Each detector segment and each aperture covers 360 / 16 degrees azimuthally relative to the center of the detector. The detector segments are separated from each other by apertures. Detector 510 has a central round aperture 512. FIGS. 5A, 5C, and 5E show the CTFs of ideal detectors, detector 500 and detector 510, respectively. Compared to detectors 300 and 400, which have four detector segments, the detector with eight segments has a CTF more similar to the ideal CTF of the camera in FIG. 5A. The camera may be composed of many small segments, called pixels, which allows the detector signal centroid (COM) or illumination centroid to be calculated to form an ideal iDPC-STEM image, called an iCOM-STEM image (whose CTF is shown).

[0031] In some embodiments, the central opening may not be circular. For example, the opening about the primary axis may be square, hexagonal, or octagonal. In some embodiments, the opening may be covered with an electronically transparent material. In some embodiments, the number of detection segments may be any positive integer. In some embodiments, a detector with detection segments separated by openings may not have a pinwheel configuration. The detection segments and / or openings may have different shapes within a single detector. The detector may be fabricated using semiconductor device fabrication techniques.

[0032] 6A and 6B show exemplary detectors for simultaneous phase contrast and EELS imaging. The detectors in FIGS. 6A and 6B have four and eight detector segments, respectively. Unlike the detectors in FIGS. 4A and 5D, the detector segments in FIGS. 6A and 6B are connected to the center of the detector. There is no aperture in the center of the detector. In one example, each detector segment is identical to each aperture. At radii smaller than the outer radius, the total azimuthal angle covered by the detector segments is the same as the total azimuthal angle covered by the aperture.

[0033] 7 shows a method 700 for simultaneously acquiring structural and compositional information using the microscope 100 of FIG. 1. A sample image showing the sample structure is acquired by scanning a focused electron beam at multiple scan positions within an ROI. At each scan position, two spectra are acquired before and after adjusting a projection lens positioned between the sample and a first detector (such as a DPC / iDPC detector).

[0034] At 702, the microscope is prepared for imaging. After loading the sample into the imaging chamber, a low-resolution overview image can be acquired to determine the ROI. Various system parameters are also set, such as electron beam parameters, scanning parameters, and display parameters.

[0035] At 704, the charged particle beam is focused at the scan position determined at 702. At 706, in response to the irradiation, transmitted charged particles exiting the backside of the sample are simultaneously detected by a first detector (i.e., a DPC / iDPC detector) and a second detector downstream of the first detector (i.e., an EELS detector). A first portion of the transmitted charged particles is detected by the first detector as a first signal, and a second portion of the transmitted charged particles is detected by the second detector as a first spectrum. The exit angles of the transmitted charged particles detected by the first detector overlap with the exit angles of the transmitted charged particles detected by the second detector. In one example, the first portion of the transmitted charged particles have exit angles within a first range. The second portion of the transmitted charged particles have exit angles within a second range. The first range and the second range overlap with each other.

[0036] At 708, a projection lens (such as projection lens 24 in FIG. 1 ) between the sample and the first detector is adjusted to rotate the transmitted charged particle beam azimuthally relative to the primary axis. After rotating the transmitted charged particle beam, at least a portion of the charged particles detected by the first detector at 706 may pass through the first detector and be detected by a second detector. Similarly, at least a portion of the charged particles that pass through the first detector at 706 may be detected by the first detector. The rotation angle depends on the configuration of the first detector. For example, for a detector with a "windmill" configuration, the rotation angle depends on the number of detection segments. For a detector with four detection segments, the rotation angle may be 45 degrees.

[0037] At 710, the first detector detects a second signal, and the second detector detects a second spectrum. The first signal and the second signal correspond to transmitted electrons with different azimuthal angles. Similarly, the first and second spectra correspond to transmitted electrons with different azimuthal angles. By rotating the transmitted electron beam, transmitted electrons exiting the sample from a solid angle can be sensed by the second detector. The solid angle has an apex at the scan position and is symmetrical about the primary axis.

[0038] At 712, method 700 checks whether all scan positions have been imaged. If the answer is no, method 700 directs the charged particle beam to the next scan position at 714 and continues data acquisition. Otherwise, method 700 stops the scan and moves to 716.

[0039] At 716, for each scan location, the first spectrum and the second spectrum are combined to generate a combined spectrum for the scan location. The sample composition is analyzed based on the combined spectrum. Each combined spectrum includes transmitted electrons with a wide range of exit angles. The amount of transmitted electrons forming the combined spectrum is substantially the same as the amount of transmitted electrons collected by a spectrometer without a DPC / iDPC detector. As a result, chemical analysis based on the combined spectrum can be reliably performed.

[0040] At 718, a sample image is generated. A sample image representing the sample structure may be generated based on the first signal and / or the second signal received by the first detector at each scan position. The sample image may be a phase-contrast image, such as a DPC-STEM or iDPC-STEM image. Additionally, the sample image may be displayed along with the compositional information extracted from the combined spectrum at 716. Simultaneous signal detection of DPC-STEM / iDPC-STEM and EELS spatially registers the structural and compositional information.

[0041] FIG. 8 illustrates another method 800 for simultaneously acquiring structural and compositional information of a sample using the microscope 100 of FIG. 1. The same amount of data is acquired as in method 700. However, unlike method 700, here the ROI is scanned twice. Additionally, the projection lens is adjusted between two scans instead of one scan. By adjusting the projection lens between ROI scans, the total data acquisition time can be reduced. Sample shift between the two scans is corrected based on the sample image acquired using the first detector.

[0042] At 802, the microscope is prepared for imaging, similar to step 702 of method 700. An ROI is selected and system parameters are set.

[0043] At 804, similar to step 704 of method 700, a focused charged particle beam is directed to a scan location of the ROI. In response to the illumination, a first signal is acquired with a first detector (such as a DPC / iDPC detector) and a first spectrum is acquired with a second detector (such as an EELS detector). At 808, method 800 checks whether all scan locations have been imaged for the first scan. If the answer is no, then the charged particle beam is directed to the next scan location at 810. Otherwise, the first scan is complete and method 800 moves to 812.

[0044] At 812, a projection lens positioned between the sample and the first detector is adjusted to rotate the transmitted charged particle beam relative to the first detector, similar to step 708 of method 700. A second scan is performed using the adjusted projection lens.

[0045] At 814, the charged particle beam is directed to a first scan location within the ROI to begin a second scan. At 816, a second signal is acquired with the first detector and a second spectrum is acquired with the second detector. At 818, method 800 checks whether all scan locations have been imaged. If the answer is no, the charged particle beam is directed to the next scan location at 820. Otherwise, the second scan is complete and method 800 moves to 822.

[0046] At 822, sample images are formed. The first sample image may be formed by processing a first signal acquired with a first detector during a first scan. The second sample image may be formed by processing a second signal acquired with the first detector during a second scan. The first and second sample images may be DPC-STEM or iDPC-STEM images. In some examples, a combined sample image may be generated based on the first and second signals.

[0047] At 824, at each scan position, the acquired first and second spectra are combined to generate a combined spectrum for the scan position. To correct for sample shift between the first and second scans, the first and second spectra may be combined based on the first and second sample images. In one example, the sample shift is determined by comparing the first and second sample images. The first and second spectra corresponding to the same scan position of the sample are then combined (e.g., added) to generate a combined spectrum for the scan position. Compositional information may be extracted by analyzing the combined spectrum. The sample image may be displayed as a single image together with the compositional information.

[0048] In some embodiments, instead of adjusting the projection lens as shown in FIGS. 7 and 8, the sample or the first detector can be rotated.

[0049] FIG. 9 illustrates another method 900 for simultaneously acquiring structural and compositional information about a sample using the microscope 100 of FIG. 1. Unlike methods 700 and 800 of FIGS. 7 and 8, method 900 does not rotate the transmitted charged particles entering the spectrometer. The first and second detectors each acquire transmitted electrons over a portion of a solid angle at the back side of the sample. The quality of the EELS spectrum will be lower than the combined spectrum formed by methods 700 and 800. However, because the EELS spectrum acquired by method 900 includes electrons with high scattering angles, chemical composition analysis can still be reliably performed on the EELS spectrum acquired by the second detector.

[0050] At 902, the microscope is prepared for imaging, similar to step 702 of method 700. An ROI is selected and system parameters are set.

[0051] At 904, similar to step 704 of method 700, the focused charged particle beam is directed to a scan location of the ROI. In response to the illumination, a signal is acquired with a first detector (such as a DPC / iDPC detector) and a spectrum is acquired with a second detector (such as an EELS detector). At 908, method 900 checks whether all scan locations have been imaged. If the answer is no, the charged particle beam is directed to the next scan location at 910. Otherwise, the scan is complete and method 900 moves to 912.

[0052] At 912, a sample image is generated from the detected signal from the first detector. The sample image can be a DPC-STEM or iDPC-STEM image. The spectrum acquired with the second detector is analyzed to extract compositional information. The structural and compositional information can be stored or displayed together at 914 by combining the phase contrast and EELS images.

[0053] The technical advantage of simultaneously detecting transmitted electrons with the first and second detectors is that structural and compositional information can be obtained at the sample location. Furthermore, the compositional information can be aligned and processed based on the structural information. The technical advantage of using the first and second detectors to detect transmitted charged particles with overlapping exit angles is that electrons at high scattering angles can be detected for EELS analysis. The technical advantage of rotating the transmitted electrons relative to the first detector is that a combined spectrum corresponding to transmitted electrons with a large azimuthal range can be obtained. The technical advantage of using a pinwheel detector for iDPC-STEM imaging is that satisfactory image quality can be achieved even with a reduced detection area compared to conventional DPC / iDPC detectors. Furthermore, electrons at high scattering angles can pass through the aperture between the detector segments and enter the downstream EELS device.

[0054] In one embodiment, a method for imaging a sample with charged particles includes directing charged particles toward the sample along a primary axis and simultaneously detecting a first portion and a second portion of the charged particles transmitted through the sample, respectively, with a first detector centered on the primary axis and a second detector positioned downstream of the first detector, where each of the transmitted charged particles exits the sample at an exit angle between the direction of the transmitted charged particles and the primary axis, and the exit angle of the first portion of the transmitted charged particles overlaps with the exit angle of the second portion of the transmitted charged particles. In a first example of the method, the method further includes the first detector including one or more regions transparent to the charged particles that allow the second portion of the charged particles to pass through the first detector and be detected by the second detector. A second embodiment of the method optionally includes the first embodiment and further includes the first detector including a charged particle transparent region extending less than 360 degrees azimuthal along a radius of the first detector relative to the primary axis. A third embodiment of the method optionally includes one or more of the first and second embodiments and further includes the direction of the transmitted electrons further defined by an azimuthal angle in a plane perpendicular to the primary axis, and the first detector and the second detector detecting transmitted charged particles at the same exit angle but at different azimuthal angles. A fourth embodiment of the method optionally includes one or more of the first through third embodiments and further includes the exit angle of a first portion of the transmitted charged particles within a first range and the exit angle of a second portion of the transmitted charged particles within a second range, the first range overlapping the second range. A fifth embodiment of the method optionally includes one or more of the first through fourth embodiments, and further includes: the maximum exit angle of the second range is greater than or equal to one-fifth of the maximum exit angle of the first range. A sixth embodiment of the method optionally includes one or more of the first through fifth embodiments, and further includes: a second portion of the transmitted charged particles are dispersed between the first detector and the second detector based on particle energy, and the method further includes forming a first spectrum based on the second portion of the transmitted charged particles detected by the second detector.A seventh embodiment of the method optionally includes one or more of the first through sixth embodiments, further including scanning a region of interest of the sample with the charged particles and forming a sample image representative of the sample structure based on the transmitted charged particles detected by the first detector. An eighth embodiment of the method optionally includes one or more of the first through seventh embodiments, further including, after simultaneously detecting the first and second portions of the transmitted charged particles, rotating the transmitted charged particles azimuthally relative to the primary axis relative to the first detector, simultaneously detecting the second and first portions of the transmitted charged particles with the first and second detectors, respectively, forming a second spectrum based on the first portion of the transmitted charged particles detected by the second detector, combining the first and second spectra to generate a combined spectrum, and analyzing the sample composition based on the combined spectrum. A ninth embodiment of the method optionally includes one or more of the first to eighth embodiments, and further includes: the first spectrum and the second spectrum are combined based on the transmitted charged particles detected by the first detector.

[0055] In one embodiment, a charged particle imaging system includes a source for generating charged particles, an illuminator for directing the charged particles toward a sample along a primary axis, a first detector positioned downstream of the sample and centered about the primary axis, a second detector positioned downstream of the first detector, and a controller having non-transitory instructions configured to, in response to illuminating a location of the sample with the charged particles, simultaneously detect, with the first and second detectors, first and second portions of the charged particles transmitted through the sample, respectively, where each of the transmitted charged particles exits the sample at an exit angle between the direction of the transmitted charged particles and the primary axis, and where the exit angle of the first portion of the transmitted charged particles overlaps with the exit angle of the second portion of the transmitted charged particles. In a first example of the system, the first detector includes a region transparent to the charged particles that extends azimuthally along a radius of the first detector by less than 360 degrees relative to the primary axis. A second embodiment of the system optionally includes the first embodiment, further including: the first detector includes a plurality of detection segments, at least two of the plurality of detection segments being separated by a charged particle transparent region. A third embodiment of the system optionally includes one or more of the first and second embodiments, further including: each of the plurality of detection segments having two edges, each of the two edges extending radially relative to the primary axis. A fourth embodiment of the system optionally includes one or more of the first through third embodiments, further including a spectrometer positioned between the first and second detectors to disperse the charged particles based on particle energy. A fifth embodiment of the system optionally includes one or more of the first through fourth embodiments, further including: the controller is further configured to direct the charged particles to a plurality of locations on the sample; generate a sample image representing the sample structure based on the transmitted charged particles detected by the first detector; and generate a spectrum at each of the plurality of locations based on the transmitted charged particles detected by the second detector.A sixth embodiment of the system optionally includes one or more of the first through fifth embodiments, further including a projection lens positioned between the sample and the first detector, wherein the controller is further configured to adjust the projection lens to rotate the transmitted charged particles upstream of the first detector about a primary axis.A seventh embodiment of the system optionally includes one or more of the first through sixth embodiments, further including the controller is further configured to: direct the charged particles to a first location on the sample, acquire a first spectrum using a second detector, adjust the projection lens, acquire a second spectrum using the second detector with the adjusted projection lens, and combine the first spectrum and the second spectrum to generate a combined spectrum for the first location. An eighth embodiment of the system optionally includes one or more of the first through seventh embodiments, and further includes: the controller is further configured to: scan a plurality of locations on the sample with the charged particles and acquire a first spectrum at each of the plurality of sample locations using a second detector; adjust the projection lens; scan a plurality of locations on the sample with the charged particles and acquire a second spectrum at each of the plurality of sample locations using the second detector with the adjusted projection lens; and generate a combined spectrum by combining the first spectrum and the second spectrum at each of the plurality of sample locations. A ninth embodiment of the system optionally includes one or more of the first to eighth embodiments, and further includes: the controller is configured to generate a first sample image based on the transmitted charged particles detected by the first detector before adjusting the projection lens; and generate a second sample image based on the transmitted charged particles detected by the first detector after adjusting the projection lens; and further includes: combining the first spectrum and the second spectrum based on the first sample image and the second sample image.

Claims

1. 1. A method for imaging a sample with charged particles, comprising: directing the charged particles toward the sample along a primary axis; and simultaneously detecting, with a first detector centered on the primary axis and a second detector positioned downstream of the first detector, first and second portions of the charged particles transmitted through the sample, respectively, wherein each of the transmitted charged particles exits the sample at an exit angle between a direction of the transmitted charged particles and the primary axis, and wherein the exit angle of the first portion of the transmitted charged particles overlaps with the exit angle of the second portion of the transmitted charged particles.

2. 2. The method of claim 1, wherein the first detector includes one or more regions transparent to the charged particles that allow the second portion of the charged particles to pass through the first detector and be detected by the second detector.

3. The method of claim 2 , wherein the first detector includes a charged particle transparent region that extends less than 360 degrees azimuthally along a radius of the first detector relative to the primary axis.

4. 4. The method of claim 1, wherein a direction of transmitted electrons is further defined by an azimuthal angle in a plane perpendicular to the primary axis, and wherein the first detector and the second detector detect the transmitted charged particles at the same exit angle but different azimuthal angles.

5. 5. The method of claim 1, wherein the exit angle of the first portion of the transmitted charged particles is within a first range and the exit angle of the second portion of the transmitted charged particles is within a second range, and the first range overlaps with the second range.

6. The method of claim 5 , wherein the maximum exit angle of the second range is at least one-fifth the maximum exit angle of the first range.

7. 7. The method of claim 1, wherein the second portion of the transmitted charged particles is dispersed between the first detector and the second detector based on particle energy, and the method further comprises forming a first spectrum based on the second portion of the transmitted charged particles detected by the second detector.

8. 8. The method of claim 7, further comprising scanning a region of interest of the sample with the charged particles and forming a sample image representative of a sample structure based on the transmitted charged particles detected by the first detector.

9. after simultaneously detecting the first and second portions of the transmitted charged particles, rotating the transmitted charged particles azimuthally about the primary axis relative to the first detector and simultaneously detecting the second and first portions of the transmitted charged particles with the first and second detectors, respectively; forming a second spectrum based on the first portion of the transmitted charged particles detected by the second detector; combining the first spectrum and the second spectrum to generate a combined spectrum; The method of claim 7 , further comprising: analyzing a sample composition based on the combined spectrum.

10. The method of claim 9 , wherein the first spectrum and the second spectrum are combined based on the transmitted charged particles detected by the first detector.

11. a source for generating charged particles; an illuminator for directing the charged particles along a primary axis toward a sample; a first detector positioned downstream of the sample and centered on the primary axis; a second detector positioned downstream of the first detector; A controller having non-transient instructions, the instructions being executed to cause the controller to: and a controller configured to, in response to irradiating the sample location with the charged particles, simultaneously detect, using the first detector and the second detector, first and second portions of the charged particles transmitted through the sample, respectively, wherein each of the transmitted charged particles exits the sample at an exit angle between a direction of the transmitted charged particles and the primary axis, and the exit angle of the first portion of the transmitted charged particles overlaps with the exit angle of the second portion of the transmitted charged particles.

12. The charged particle imaging system of claim 11 , wherein the first detector includes a plurality of detection segments, at least two of the plurality of detection segments being separated by a charged particle transparent region.

13. The charged particle imaging system of claim 12 , wherein each of the plurality of detector segments has two edges, each of the two edges extending radially relative to the primary axis.

14. 14. The charged particle imaging system of claim 11, further comprising a spectrometer positioned between the first detector and the second detector to disperse charged particles based on particle energy.

15. 15. The charged particle imaging system of claim 11, further comprising a projection lens positioned between the sample and the first detector, wherein the controller is further configured to adjust the projection lens to rotate the transmitted charged particles upstream of the first detector about the primary axis.

Images