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

Abstract

Systems and methods for simultaneous phase-contrast imaging and electron energy loss spectroscopy. The method and system for imaging a sample with charged particles include guiding charged particles toward the sample along a principal axis, and simultaneously detecting a first portion and a second portion of the charged particles transmitted through the sample using a first detector and a second detector, 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 the direction of the transmitted charged particle and the principal axis. The exit angle of the first portion of the transmitted charged particle overlaps with the exit angle of the second portion of the transmitted charged particle. In this way, complementary information, such as structural and compositional information, can be obtained simultaneously.

Description

TECHNICAL FIELD

[0001] This disclosure describes methods and systems generally related to imaging a sample using a charged particle system, and more specifically, to performing phase contrast imaging and energy loss spectroscopy simultaneously on a sample using a charged particle system. BACKGROUND

[0002] Electron energy loss spectroscopy (EELS) can reveal compositional information by measuring the energy loss of electrons that pass through a thin sample. In EELS, the electrons transmitted through the sample are dispersed spatially by a spectrometer based on 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 microscopy (STEM) image showing the sample structure and an EELS dataset containing chemical information for each scan position can be obtained within a single scan. Simultaneously acquiring the STEM and EELS signals ensures that the acquired structural and compositional information is recorded spatially. However, in certain conditions, the STEM image acquired with the HAADF detector cannot show all sample structures. For example, when both light and heavy atoms are present in the sample, the HAADF image does not have sufficient signal-to-noise to show the light atoms.

[0003] EELS cannot be performed simultaneously with other types of TEM or STEM using its full capacity based on imaging techniques because electrons with high scattering angles are blocked by the detector positioned upstream of the spectrometer. Differential phase contrast (DPC) STEM imaging and integrated differential phase contrast (iDPC) STEM imaging, for example, can achieve sub-Ångstrom resolution. DPC-STEM uses, for example, a segmented detector positioned downstream of the sample to record a two-dimensional spatial wave amplitude distribution in the image plane. iDPC-STEM imaging is introduced 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 14 / 629,387 by Lazic et al., filed February 23, 2015. The above disclosures and patent application 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 enables imaging of both light and heavy atoms in a sample. With existing / current detector configurations, neither DPC-STEM nor iDPC-STEM can be performed with EELS because the segmented DPC / iDPC detector blocks the electrons needed to detect and interpret EELS datasets. The same problem also arises in differential phase contrast after differentiation (dDPC) STEM, which is obtained by differentiating (applying a divergence operator to) DPC-STEM vector images. The present invention proposes methods and systems that allow simultaneous acquisition of the above-described EELS and TEM / STEM-based images. SUMMARY

[0004] In one embodiment, a method for imaging a sample with charged particles includes directing charged particles along a principal axis toward the sample, and simultaneously detecting first and second portions of the charged particles transmitted through the sample with a first detector centered on the principal axis and a second detector downstream of the first detector, respectively, wherein each of the transmitted charged particles exits the sample at an exit angle between a direction of the transmitted charged particle at a backside of the sample and the principal axis, and the exit angle of the first portion of the transmitted charged particles overlaps the exit angle of the second portion of the transmitted charged particles. In this way, structural and compositional information of the sample can be acquired simultaneously using the first and second detectors. The structural information can be shown in a sample image, and the compositional information can be shown in spectral form. High scattering angle electrons 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. Spectra acquired before and after the beam rotation can be combined to generate a combined spectrum covering 360 degrees of azimuthal angle of the transmitted charged particles at the backside of the sample.

[0005] It is to be understood that the above overview is provided merely for purposes of summarizing some of the concepts of the detailed description that follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined solely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any or all of the disadvantages of any of the problems in the background. BRIEF DESCRIPTION OF DRAWINGS

[0006] Figure 1 A charged particle microscope for combined phase contrast imaging and electron energy loss spectroscopy (EELS) is shown in accordance with some embodiments.

[0007] Figure 2 A detector for imaging a sample structure is shown relative to a charged particle beam.

[0008] Figure 3A An example detector for imaging a sample structure is shown having four connected detection segments.

[0009] Figure 3B is the contrast transfer function (CTF) of the detector using Figure 3A .

[0010] Figure 3C is an integrated differential phase contrast (iDPC-STEM) image acquired with the detector using Figure 3A .

[0011] Figure 3D is a Fourier transform of Figure 3C .

[0012] Figure 4A An example detector with four separate detection segments for imaging a sample structure is shown.

[0013] Figure 4B is the CTF of the detector used Figure 4A

[0014] Figure 4C is an iDPC-STEM image acquired with the detector of Figure 4A

[0015] Figure 4D is the Fourier transform of Figure 4C

[0016] Figure 5A An ideal CTF using a camera is shown.

[0017] Figure 5B and 5C An example detector with eight connected detection segments and its corresponding CTF is shown.

[0018] Figure 5D and 5E An example detector with eight separate detection segments and its corresponding CTF is shown.

[0019] Figure 6A and 6B An example detector with four and eight detection segments separated by broken openings is shown.

[0020] Figure 7 An example method for simultaneously acquiring sample images and EELS data using a system of Figure 1

[0021] Another example method for simultaneously acquiring sample images and EELS data using a system of Figure 8 Figure 1 Another example method for simultaneously acquiring sample images and EELS data using a system of

[0022] Figure 9 Figure 1 Another example method for simultaneously acquiring sample images and EELS data using a system of

[0023] The same reference numbers are used throughout the drawings to represent corresponding or like parts. DETAILED DESCRIPTION

[0024] The following description relates to using a charged particle imaging system, such as Figure 1 ​​​​​The system and method illustrated herein are for simultaneously acquiring structural and compositional / bonding information of a sample using a charged particle imaging system. Structural information can be obtained via 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 via 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 charged particles based on particle energy. The first detector is located at the center of the principal axis of the charged particle beam. In response to the charged particle beam irradiating the front side of the sample, some of the scattered charged particles are transmitted through the sample and exit from the back side. The direction of each transmitted charged particle at the back side of the sample is defined by the exit angle and azimuth angle, such as... Figure 2 As shown in the diagram. The exit angle is the angle between the direction of the transmitted charged particle and the principal axis. The azimuth angle is in a plane perpendicular to the principal axis. In one example, the principal axis is aligned with the z-axis. The azimuth angle is the angle between the projection of the direction of the transmitted charged particle in the xy-plane and the x-axis. A first detector and a second detector simultaneously detect a first portion and a second portion 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 particle to pass through the first detector without being detected by it. The second portion of the transmitted charged particle may not interact with the first detector, or may have a negligible interaction with it. Therefore, the presence of the first detector does not affect the detection of the second portion of the transmitted charged particle detected by the second detector. In one example, one or more charged particle transparent regions in the first detector may be openings. In another example, one or more charged particle transparent regions may be thin layers of material that do not interfere with the energy distribution of the charged particles impacting them. The exit angle of the first portion of the transmitted charged particle detected by the first detector overlaps with the exit angle of the second portion of the transmitted charged particle detected by the second detector. Therefore, some of the transmitted charged particles received by the first and second detectors have the same exit angle but different azimuth angles. The maximum exit angle of the transmitted charged particles detected by the second detector can be the same as that 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.

[0025] In one example, the first detector detects a first portion of the transmitted charged particles having an exit angle within a first range [A1A2], where A1 is the smallest exit angle detected and A2 is the largest exit angle detected, and where A1 > 0 degrees and A2 < 90 degrees. The second detector detects a second portion of the transmitted charged particles having an exit angle within a second range [A3A4], where A3 is the smallest exit angle detected and A4 is the largest exit angle detected, and where 0 < A3 < A2 and A4 < 90 degrees. The first and second ranges overlap each other. The overlap range can be [A1A4] or [A1A2]. In one example, the largest exit angles detected by the first and second detectors are the same. That is, A2 = A4. In another example, A4 > ½ A2, such that the second detector is sensitive to high scattering angle electrons. The first and second detectors can be capable of detecting any exit angle within the first and second ranges, respectively. In other examples, the first or second detector can be capable of detecting selective exit angles within its respective range.

[0026] In another example, at a particular exit angle within the overlap range, the first and second detectors are sensitive to transmitted electrons having different azimuthal angles. The first detector can include at least one charged particle transparent region that extends less than 360 degrees in the azimuthal direction along a radius of the first detector. The region allows a portion of the transmitted electrons to pass through the first detector. The first detector does not affect (or has a negligible effect on) the electrons that pass through the opening. That is, the energy and direction of the electrons that pass through the charged particle transparent region are not changed (or have a negligible change) by the first detector.

[0027] In some examples, the first detector includes a plurality of detection segments and at least one opening arranged between at least two of the detection segments. The signals detected by each of the detection segments can be passed to the controller via a separate amplifier. As Figure 4A 、 5D As shown in 6A and 6B, the first detector (e.g., a DPC / iDPC detector) can be in the form of a "windmill," where multiple detection segments of the same shape are evenly circularly spread out in the azimuthal direction relative to a main axis. Two edges of each detection segment (or blades of the windmill) extend in the radial direction from the main axis. The detection segments are separated from each other by openings. For a detector with four detection segments, although the detection area of the detector is reduced compared to a conventional four-segment DPC / iDPC detector with connected detection segments, the quality of the iDPC-STEM images acquired using the detector with broken detection segments is similar to the image quality acquired with the conventional DPC / iDPC detector, as shown in Figures 3A to 3Dand shown in FIGS. 4A-4D. This is because the contrast transfer function (CTF) of a detector with a reduced detection area is similar to a conventional DPC / iDPC detector. In some examples, the first detector can include a central opening that allows transmissive charged particles with low exit angles to enter the central opening of the spectrometer. In other examples, the first detector can not have a central opening, as shown in FIG. 4A. The first detector and the second detector receive transmissive charged particles with azimuthal angles that do not overlap each other. Figures 6A to 6B The first detector and the second detector receive transmissive charged particles with azimuthal angles that do not overlap each other.

[0028] As shown in FIG. 4A, the first detector can include a central opening that allows transmissive charged particles with low exit angles to enter the central opening of the spectrometer. In other examples, the first detector can not have a central opening, as shown in FIG. 4A. Figure 9 As shown in FIG. 4A, the first detector can include a central opening that allows transmissive charged particles with low exit angles to enter the central opening of the spectrometer. In other examples, the first detector can not have a central opening, as shown in FIG. 4A.

[0029] In some embodiments, the transmissive charged particle beam is rotated in azimuthal angle about the primary axis relative to the first detector so that transmissive charged particles covering full 360 degrees of azimuthal angle and a large range of exit angles can be acquired by each of the first detector and the second detector. The second detector can capture all transmissive charged particles with exit angles that do not exceed the maximum detectable exit angle of the first detector. For example, the second detector obtains a first spectrum and a second spectrum before and after, respectively, rotating the transmissive charged particles relative to the first detector. The first and second spectra complement each other. That is, the first and second spectra correspond to different portions of the transmissive charged particles at the scan location. By combining the first and second spectra, the combined spectrum contains the same information as a spectrum acquired without the first detector, that is, without blocking the electrons from entering the EELS spectrometer. The transmissive charged particles can be rotated relative to the first detector by rotating any one of the sample, the first detector, or the transmissive charged particle beam between the sample and the first detector relative to the primary axis. The transmissive charged particle beam can be rotated by adjusting the excitation of at least one lens disposed in a projection lens between the sample and the first detector, as disclosed in Freitag et al. U.S. Patent Application 15 / 803,642, filed November 3, 2017, which is incorporated by reference herein. Adjusting the projection lens is faster and does not introduce sample shift compared to rotating the sample or the first detector. Figures 7 to 8 Two methods of generating a combined spectrum are shown. In Figure 7 the unrotated and rotated spectra at a scan location are acquired before deflecting the charged particle beam to another scan location. Alternatively, Figure 8A method of performing a first scan and acquiring a first spectrum is shown. After the rotating the transmitted charged particle beam, a second scan is performed and a second spectrum is acquired over the same region of interest (ROI). The structural images acquired during each scan can be used to combine the first and second spectra.

[0030] Turning to Figure 1 The drawing shows a highly schematic depiction of an embodiment of a charged particle microscope 100 in which the present invention can be implemented. More specifically, it shows an embodiment of a transmission microscope, which can be an electron microscope. Within a vacuum housing 2, a charged particle (e.g. electron) source 4 generates a beam 111 of electrons, which propagates along a main axis 110 and traverses an electron optical illuminator 6, to direct / concentrate the electrons onto a selected portion of a sample 60, which can be e.g. (locally) thinned / flattened. Also depicted is a deflector 8, which can be used, inter alia, to effect a scanning motion of the beam 111.

[0031] The sample 60 is held by a positioning device / stage A on a sample holder H, which can be positioned in multiple degrees of freedom, the positioning device / stage moving a base A' to which the holder H is (detachably) attached; the sample holder H can comprise, for example, fingers, which can be moved, inter alia, in the XY plane (see depiction of Cartesian coordinate system; typically, motion parallel to the Z direction and tilting around the X / Y directions will also be possible). Such movements allow different portions of the sample 60 to be illuminated / imaged / inspected by the electron beam 111 travelling along the main axis 110 in the Z direction (and / or allow a scanning motion to be performed, as an alternative to beam scanning). The sample holder H can also rotate the sample with respect to the main axis 110. If desired, an optional cooling device (not depicted) can be introduced in thermal contact with the sample holder H, in order to maintain it (and the sample 60 thereon) at a low temperature, for example.

[0032] The electron beam 111 interacts with the sample 60 in a manner such that various types of "stimulated" radiation emanate 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 by means of a detector 22, which can be a combined scintillator / photomultiplier tube or an EDX (energy dispersive X-ray spectroscopy) module, for example. Electrons can be transmitted (through) the sample 60, exit / emanate from the backside of the sample and continue to propagate along the main axis 110 (generally, but typically with some deflection / scattering). Such transmitted electron flux enters a projection lens 24. The projection lens 24 can comprise various electrostatic / magnetic lenses, deflectors, correctors (like stigmators), etc.

[0033] Multiple detectors may be positioned downstream of a projection lens 24 for detecting and analyzing transmitted electrons with various exit angles at the back side of the sample. A first detector and a second detector are used to simultaneously detect the transmitted electrons. In response to illumination, 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 the second detector positioned downstream of the first detector. The first and second detectors may be any of a high-angle annular dark field (HAADF) detector 26, a DPC / iDPC detector 30, a TEM / STEM detector 32, and a spectroscopic device 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, such that the transmitted electrons received by the DPC / iDPC detector 30 and the EELS detector 36 of the spectroscopic device 34 have overlapping exit angles. An example of a DPC / iDPC detector is shown in Figure 2 In step 6, the TEM / STEM detector 32 may optionally be positioned downstream of the HAADF detector 26 and the spectroscopic device 34. The TEM / STEM detector 32 may be retracted / withdrawn (as schematically indicated by arrow 32') to allow transmitted electrons into the spectroscopic device 34, such as an electron energy loss spectroscopy (EELS) module. The spectroscopic device may include a spectrometer 35 that disperses charged particles based on their energy, and an EELS detector 36 that senses the spectrum formed by the dispersed charged particles. The spectrum may be an electron energy loss spectrum.

[0034] In another example, the first detector is a camera comprising a large number of small segments called pixels. The camera includes one or more openings (such as a transparent set of pixels) that allow a second portion of a charged particle to pass through the camera and be detected by the second detector. Centroid (COM)-STEM and / or Integrated Centroid (iCOM)-STEM images can be acquired by the camera. In yet another example, the first detector is a localization-sensitive detector (PSD) comprising one or more openings (such as a transparent set of pixels) that allow a second portion of a charged particle to pass through the PSD and be detected by the second detector.

[0035] The signal detected from the first detector can be used to form an image showing the structure of the sample in response to scanning the charged particle beam over a plurality of scan positions 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 when the detector is a PSD or a camera (ideally, a camera with pixels) made up of a relatively large number of smaller segments, so that the COM or center of illumination can be calculated more accurately. The first detector image can also be any other type of STEM image, such as DPC-STEM, dDPC-STEM, COM-STEM, dCOM-STEM, BF-STEM, ABF-STEM, ADF-STEM, HAADF-STEM, single segment STEM, etc. At each scan position, the signal detected from the second detector can contain chemical or compositional information of the sample.

[0036] The controller 50 is connected to the various illustrated components via control lines (buses) 20'. The controller 50 can include a processor 54 and a non-transitory memory 55 for storing computer readable instructions. By executing the computer readable instructions stored in the non-transitory memory, the controller can implement the various methods disclosed herein. The controller 50 can provide various functions, such as synchronizing actions, providing set points, processing signals, performing calculations, receiving operator input from a user input device 53, and displaying messages / information on a display device 51. For example, the controller 50 can be configured to process signals received from various detectors and generate images showing the structure and / or composition of a sample. The controller 50 can be (partially) inside or outside the housing and, as necessary, can have a unitary or composite structure.

[0037] In some embodiments, the first and second detectors can be used for other types of imaging. As an example, the first detector can be used for chemical / compositional information acquisition and the second detector can be used for structural information acquisition. For example, the first detector is an EELS detector and the second detector is a DPC / iDPC detector. In another example, both the first and second detectors can be used for structural information or compositional information acquisition. In some examples, one or more additional detectors with a charged particle transparent region can be positioned upstream of the second detector.

[0038] While transmission electron microscopy is described by way of example, it should be understood that the imaging system can be other types of charged particle microscopy systems, such as dual beam tools, such as a focused ion beam combined with scanning electron microscopy (FIB-SEM).

[0039] Figure 2is a magnified view showing the configuration of the first and second detectors with respect to the principal 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 disconnected detection segments arranged around the principal axis 110 in the azimuthal direction 252. The electron beam 111 illuminates the sample front side 61 of the sample 60 along the Z direction and impinges on a location 63 on the sample 60. The electron beam 111 can be a focused beam. Electrons transmitted through the sample 60 exit the sample back side 62 in different directions, such as directions 221, 222, and 223. The direction of the transmitted charged particles exiting the sample back side is defined by the exit angle and the azimuthal angle. The exit angle of a transmitted electron is the angle between the electron direction and the principal axis. For example, for a transmitted electron with direction 221, the exit angle is angle 210. The exit angle is in the range of zero to 90 degrees. A large exit angle corresponds to a transmitted electron with a high scattering angle. The azimuthal angle is the angle between the electron direction and the X-axis, that is, the angle between the electron direction projected on the sample plane and the X-axis. The azimuthal angle is in the range of 0 to 360 degrees. For example, the detector 250 is positioned in the X-Y plane downstream of the sample 60. The azimuthal angle of direction 221 is angle 240 between the X-axis and its projected direction 241 on the X-Y plane. The first detector 250 has an outer periphery 251 that allows the detector to detect transmitted charged particles with a maximum exit angle 230.

[0040] In response to the illumination by the electron beam 111, a portion of the transmitted charged particles, i.e., a first portion, as indicated by arrows 264 and 265, impinge on 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, as indicated by arrows 261, 262, and 263, pass through the first detector via one or more openings of the first detector and are detected by the second detector 270 positioned downstream of the first detector. In some embodiments, the second detector 270 can not be centered along the principal axis, as shown in FIG. 2B. For example, the second detector 270 is an EELS detector within a spectroscopy apparatus, and the entrance of the spectroscopy apparatus is aligned with respect to the principal axis. Figure 2

[0041] Figure 3A ​A top view of a conventional DPC / iDPC detector 300 is shown. The detector 300 includes four identical detection segments 301, 302, 303, and 304 arranged around a center (or geometric center) of the detector 300. Each of the detection segments is directly connected with two other detection segments. The detection segments cover 360 degrees in an azimuthal direction 306. The detector 300 can optionally have a circular opening 305 at the center to allow electrons with low exit angles to pass through the detector. The radius of the circular opening can be less than one fifth of the radius of the outer perimeter of the detector. The detector 300 captures all transmitted electrons with exit angles in a range [A1 A2], where A1 > 0 degree and A2 < 90 degree.

[0042] Figure 4A A top view of a DPC / iDPC detector 400 according to the present disclosure is shown. The detector 400 includes four identical detection segments 401, 402, 403, and 404 evenly spread around a center (or geometric center) 430 of the detector 300. The detection segments are separated from each other by openings in an azimuthal direction 413. For example, the detection segments 401 and 402 are separated by an opening 406. Each detection segment has two edges (such as edges 410 and 411) extending in a radial direction, and one edge (such as edge 412) arranged along an outer perimeter (dashed line) of the detector. The detector 400 has a circular opening 405 at the center. The circular opening 405 is combined with each of the openings 406, 407, 408, and 409. The openings 406, 407, 408, and 409 are arranged around the center 430. The detector 400 detects half of the transmitted electrons with exit angles in a range [A3 A4], where A3 is greater than 0 degree and determined by the diameter of the circular opening. In one example, A3 < 0.2 A4. Assuming the radius of the circular opening is r1, and the radius of the outer perimeter is r2, the azimuthal angle covered by the detector 400 is less than 360 degrees at any radius between r1 and r2. The detector 400 and a downstream EELS detector (such as the EELS detector 36 of Figure 1 can simultaneously detect transmitted electrons with overlapping exit angles but non-overlapping azimuthal angles. For example, the detector 36 can detect transmitted electrons with exit angles in a range [0 A4].

[0043] In one example, the shape of each opening can be the same as each detection segment. Each detection segment and each opening covers 360 / 8 degrees in the azimuthal direction relative to the center of the detector. The total azimuthal range occupied by the detection segments is 180 degrees, which is the same as the total azimuthal range occupied by the openings. This allows at least half of the transmitted electrons with high take-off angles, such as take-off 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 can be different from each of the openings between the detection segments. In other words, the total azimuthal range covered by the detection segments can be greater or less than the total azimuthal range covered by the openings.

[0044] In Figures 3A to 3D and Figures 4A to 4D the performance of the detector 300 and the detector 400 are compared. The same SrTi03sample is imaged by both detectors. The detectors have the same outer perimeter and central opening (openings 305 and 405). The total detection area of the detector 400 is half of the detection area of the detector 300. Figure 3B and 4B are the CTFs of the detector 300 and the detector 400, respectively. Figure 3C and 4C are the iDPC-STEM images acquired by the detector 300 and the detector 400, respectively. Figure 3D and 4D are the FFTs of Figure 3C and 4C Even with the reduced total detection area, the CTF of the detector 400 is similar to the CTF of the detector 300. There is no significant change in the resolution of the iDPC-STEM images or the iDPC-STEM images.

[0045] Figures 5B to 5E A detector with eight detection segments and its corresponding CTF are shown. Figure 5B is a top view of a conventional detector 500. The detector 500 has eight identical detection segments (such as detection segment 501) arranged similarly to the detector 300. The detection segments are spread out around the center of the detector. There are no openings between the detection segments in the azimuthal direction 503 relative to the center of the detector. The detection segments cover the entire 360 degrees of azimuth. A central opening 502 is arranged at the center of the detector 500. Figure 5C A CTF of the detector 500 is shown.

[0046] Figure 5D A detector 510 according to the present application is shown. Each detection segment and each opening covers 360 / 16 degrees in the azimuthal direction relative to the center of the detector. The detection segments are separated from each other by openings. The detector 510 has a circular opening 512 at the center. Figure 5A ,5C FIGS. 5E and 5F show the CTFs of ideal detectors (detector 500 and detector 510), respectively. Compared to detectors 300 and 400 with four detection segments, the detectors with eight segments have a CTF more similar to Figure 5A the ideal CTF of a camera in FIG. 4. A camera can be composed of a large number of small segments called pixels, allowing the center of mass (COM) or barycenter of the illumination of the detector signal to be calculated to form an ideal iDPC-STEM image called an iCOM-STEM image (which is the CTF shown).

[0047] In some embodiments, the central opening can not be circular. For example, the opening at the center of the primary axis can be square, hexagonal, or octagonal. In some embodiments, the opening can be covered by an electron-transparent material. In some embodiments, the number of detection segments can be any positive integer. In some embodiments, a detector with detection segments separated by openings can not have a windmill configuration. The detection segments and / or openings can have different shapes within one detector. The detector can be fabricated using semiconductor device fabrication techniques.

[0048] Figure 6A and 6B show an example detector for simultaneous phase contrast and EELS imaging. Figure 6A and Figure 6B The detectors in FIGS. 6A and 6B have four and eight detection segments, respectively. Unlike the detectors in FIGS. 5A and 5B, Figure 4A and Figure 5D the detectors in FIGS. 6A and 6B, Figure 6A and 6B the detection segments in FIGS. 6A and 6B are connected at the center of the detector. There is no opening at the center of the detector. In one example, each detection segment coincides with each opening. At any radius smaller than the radius of the outer perimeter, the total azimuthal angle covered by the detection segments is the same as the total azimuthal angle covered by the openings.

[0049] Figure 7 shows a method 700 for simultaneously acquiring structural and compositional information using a microscope 100 of Figure 1 The sample image showing the sample structure is acquired by scanning a focused electron beam at a plurality of scan positions within the 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.

[0050] 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, such as electron beam parameters, scan parameters, and display parameters, are also set.

[0051] At 704, the charged particle beam is focused onto the scanning position determined at 702. At 706, in response to illumination, transmitted charged particles exiting from the back side 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 angle of the transmitted charged particles detected by the first detector overlaps with the exit angle of the transmitted charged particles detected by the second detector. In one example, the first portion of the transmitted charged particles has an exit angle within a first range. The second portion of the transmitted charged particles has an exit angle within a second range. The first and second ranges overlap each other.

[0052] At position 708, adjust the projection lens between the sample and the first detector (e.g., ...). Figure 1 The projection lens 24 rotates relative to the main axis in the azimuth angle to transmit a beam of charged particles. After the beam is rotated and transmitted, at least a portion of the charged particles detected by the first detector at 706 can pass through the first detector and be detected by the second detector. Similarly, at least a portion of the charged particles passing through the first detector at 706 can 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 sections. For a detector with four detection sections, the rotation angle can be 45 degrees.

[0053] At position 710, the first detector detects the second signal, and the second detector detects the second spectrum. The first and second signals correspond to transmitted electrons with different azimuth angles. Similarly, the first and second spectra correspond to transmitted electrons with different azimuth angles. By rotating the transmitted electron beam, transmitted electrons from the sample exiting from the solid angle can be sensed by the second detector. The solid angle has a vertex at the scan position and is symmetrical with respect to the principal axis.

[0054] At 712, method 700 checks whether all scan positions have been imaged. If the answer is no, then method 700 guides the charged particle beam to the next scan position at 714 and continues data acquisition. Otherwise, method 700 stops scanning and moves to 716.

[0055] At position 716, for each scan location, the first and second spectra are combined to generate a combined spectrum for that scan location. The sample composition is analyzed based on the combined spectrum. Each combined spectrum contains transmitted electrons with a wide range of emission angles. The amount of transmitted electrons forming the combined spectrum is practically the same as the amount collected by a spectroscopic device without a DPC / iDPC detector. Therefore, chemical analysis based on the combined spectrum can be reliably performed.

[0056] At 718, a sample image is generated. The sample image representing the sample structure can be generated based on a first signal and / or a second signal received by a first detector at each scan position. The sample image can be a phase-contrast image, such as a DPC-STEM or iDPC-STEM image. Furthermore, the sample image can be displayed together with compositional information extracted from the combined spectrum at 716. Due to the simultaneous signal detection of DPC-STEM / iDPC-STEM and EELS, structural and compositional information is recorded spatially.

[0057] Figure 8 Showing the use of Figure 1 Another method 800 uses a microscope 100 to simultaneously acquire sample structure and composition information. It acquires the same amount of data as method 700. However, unlike method 700, in this method, the ROI is scanned twice. Furthermore, the projection lens is adjusted between the two scans rather than during a single scan. By adjusting the projection lens between ROI scans, the overall data acquisition time is reduced. Sample displacement between the two scans is corrected based on the sample image acquired using the first detector.

[0058] At 802, similar to step 702 of method 700, prepare the microscope for imaging. Select the ROI and set the system parameters.

[0059] At 804, similar to step 704 of method 700, the focused charged particle beam is guided to the scan position of the ROI. In response to 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 positions have been imaged for the first scan. If the answer is no, then at 810 the charged particle beam is guided to the next scan position. Otherwise, the first scan is complete, and method 800 moves to 812.

[0060] At 812, similar to step 708 of method 700, the projection lens positioned between the sample and the first detector is adjusted to rotate the transmitted charged particle beam relative to the first detector. A second scan is performed using the adjusted projection lens.

[0061] At 814, the charged particle beam is guided to the initial scan position within the ROI to begin the second scan. At 816, the second signal is acquired using the first detector, and the second spectrum is acquired using the second detector. At 818, method 800 checks whether all scan positions have been imaged. If the answer is no, then at 820, the charged particle beam is guided to the next scan position. Otherwise, the second scan is complete, and method 800 moves to 822.

[0062] At position 822, a sample image is formed. The first sample image can be formed by processing a first signal acquired by a first detector during a first scan. The second sample image can be formed by processing a second signal acquired by the first detector during a second scan. The first and second sample images can be DPC-STEM or iDPC-STEM images. In some instances, a combined sample image can be generated based on the first and second signals.

[0063] 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 can 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 can be extracted by analyzing the combined spectrum. The sample image can be displayed as a single image along with the compositional information.

[0064] In some embodiments, the sample or the first detector can be rotated, rather than as... Figures 7 to 8 The projection lens is adjusted as shown in the figure.

[0065] Figure 9 Showing the use of Figure 1 Another method 900, using a microscope 100, to simultaneously acquire information about the structure and composition of a sample. Unlike... Figures 7 to 8 Methods 700, 800, and 900 do not rotate the transmitted charged particles entering the spectrometer. Each of the first and second detectors acquires a portion of the transmitted electrons at the solid angle at the back side of the sample. The quality of the EELS spectrum is lower than the combined spectrum formed in methods 700 and 800. However, because the EELS spectrum obtained in method 900 contains electrons with high scattering angles, chemical composition analysis can still be reliably performed on the EELS spectrum obtained by the second detector.

[0066] At 902, similar to step 702 of method 700, prepare the microscope for imaging. Select the ROI and set the system parameters.

[0067] At 904, similar to step 704 of method 700, the focused charged particle beam is guided to the scan position of the ROI. In response to illumination, a signal is acquired using a first detector (such as a DPC / iDPC detector), and a spectrum is acquired using a second detector (such as an EELS detector). At 908, method 900 checks whether all scan positions have been imaged. If the answer is no, then at 910 the charged particle beam is guided to the next scan position. Otherwise, the scan is complete, and method 900 moves to 912.

[0068] At 912, a sample image is generated with the signal detected 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 composition information. The structural and composition information can be stored together at 914 or displayed in a combined phase contrast and EELS image.

[0069] The technical effect of detecting the transmitted electrons with the first and second detectors simultaneously is that structural and composition information at the sample location can be acquired. Further, the composition information can be aligned / processed based on the structural information. The technical effect of using first and second detectors for detecting transmitted charged particles with overlapping exit angles is that high scattering angle electrons can be detected for EELS analysis. The technical effect of rotating the transmitted electrons with respect to the first detector is that a combined spectrum corresponding to transmitted electrons with large azimuthal angular coverage can be obtained. The technical effect of using a windmill detector for iDPC-STEM imaging is that the image quality is satisfactory even with a reduced detection area compared to conventional DPC / iDPC detectors. Further, high scattering angle electrons can pass through the openings between the detection segments and into the downstream EELS equipment.

[0070] In one embodiment, a method for imaging a sample with charged particles includes directing charged particles along a principal axis toward the sample; and simultaneously detecting first and second portions of the charged particles transmitted through the sample with a first detector centered on the principal axis and a second detector positioned downstream of the first detector, respectively, wherein each of the transmitted charged particles exits the sample at an exit angle between a direction of the transmitted charged particle and the principal axis, and the exit angles of the first and second portions of the transmitted charged particles overlap. In a first instance of the method, the method further includes wherein the first detector includes one or more regions transparent to the charged particles, thereby allowing the second portion of the charged particles to pass through the first detector and be detected by the second detector. A second instance of the method optionally includes the first instance, and further includes wherein the first detector includes charged particle transparent regions extending less than 360 degrees in an azimuthal direction relative to the principal axis along a radius of the first detector. A third instance of the method optionally includes one or more of the first through second instances, and further includes wherein the directions of the transmitted electrons are further defined by an azimuthal angle in a plane perpendicular to the principal axis, and the first and second detectors detect transmitted charged particles having the same exit angle but different azimuthal angles. A fourth instance of the method optionally includes one or more of the first through third instances, and further includes wherein the exit angles of the first portion of the transmitted charged particles are within a first range, and the exit angles of the second portion of the transmitted charged particles are within a second range, and the first range overlaps the second range. A fifth instance of the method optionally includes one or more of the first through fourth instances, and further includes wherein a maximum exit angle of the second range is no less than one fifth of a maximum exit angle of the first range. A sixth instance of the method optionally includes one or more of the first through fifth instances, and further includes wherein the second portion of the transmitted charged particles is dispersed based on particle energy between the first detector and the second detector, 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 instance of the method optionally includes one or more of the first through sixth instances, and further includes scanning a region of interest of the sample with the charged particles and forming a sample image representing a structure of the sample based on the transmitted charged particles detected by the first detector. An eighth instance of the method optionally includes one or more of the first through seventh instances, and further includes, after simultaneously detecting the first and second portions of the transmitted charged particles, rotating the transmitted charged particles relative to the first detector in an azimuthal direction relative to the principal axis, 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; generating a combined spectrum by combining the first and second spectra; and analyzing a composition of the sample based on the combined spectrum.A ninth example of the method optionally includes one or more of the first through eighth examples, and further includes, wherein the first spectrum and the second spectrum are combined based on the transmitted charged particles detected by the first detector.

[0071] In one embodiment, a charged particle imaging system includes a source to generate charged particles, an illuminator to direct the charged particles along a principal axis toward a sample, a first detector positioned downstream of the sample and at a center of the principal axis, a second detector positioned downstream of the first detector, and a controller having non-transitory instructions and, by executing the instructions, the controller is configured to: simultaneously detect, with the first detector and the second detector, respectively, a first portion and a second portion of the charged particles transmitted through the sample in response to illuminating a location of the sample with the charged particles, where each of the transmitted charged particles exits the sample with an exit angle between a direction of the transmitted charged particle and the principal axis, the exit angle of the first portion of the transmitted charged particles overlapping the exit angle of the second portion of the transmitted charged particles. In a first instance of the system, the first detector includes a region transparent to the charged particles extending less than 360 degrees in an azimuthal direction relative to the principal axis along a radius of the first detector. A second instance of the system optionally includes the first instance and further includes where the first detector includes a plurality of detection segments, where at least two of the plurality of detection segments are separated by a region transparent to the charged particles. A third instance of the system optionally includes one or more of the first through second instances and further includes where each of the plurality of detection segments has two edges, and each of the two edges extends in a radial direction relative to the principal axis. A fourth instance of the system optionally includes one or more of the first through third instances and further includes a spectrometer positioned between the first detector and the second detector to disperse the charged particles based on particle energy. A fifth instance of the system optionally includes one or more of the first through fourth instances and further includes, where the controller is further configured to: direct the charged particles to a plurality of locations of the sample; generate a sample image representing a structure of the sample 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 instance of the system optionally includes one or more of the first through fifth instances and further includes a projection lens positioned between the sample and the first detector, and the controller is further configured to: adjust the projection lens to rotate the transmitted charged particles upstream of the first detector about the principal axis. A seventh instance of the system optionally includes one or more of the first through sixth instances and further includes, where the controller is further configured to: direct the charged particles to a first location of the sample and acquire a first spectrum using the second detector; adjust the projection lens; acquire a second spectrum using the second detector with the adjusted projection lens; and generate a combined spectrum of the first location by combining the first spectrum and the second spectrum.An eighth example of the system optionally includes one or more of the first through seventh examples, and further includes wherein the controller is further configured to: scan a plurality of locations of the sample with the charged particles and acquire a first spectrum at each of the plurality of sample locations using the first detector; adjust the projection lens; scan the plurality of locations of 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 at each of the plurality of sample locations, generate a combined spectrum by combining the first spectrum and the second spectrum. A ninth example of the system optionally includes one or more of the first through eighth examples, and further includes wherein the controller is further configured to: generate a first sample image based on the transmitted charged particles detected by the first detector prior to 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 wherein combining the first spectrum and the second spectrum includes combining the first spectrum and the second spectrum based on the first sample image and the second sample image.

Claims

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

2. The method of claim 1, wherein the first detector includes one or more regions transparent to the charged particles, allowing 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 extending less than 360 degrees in an azimuthal direction relative to the principal axis along a radius of the first detector.

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

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 the second range.

6. The method of claim 5, wherein a maximum exit angle of the second range is no less than one fifth of a maximum exit angle of the first range.

7. The method of claim 1, wherein the second portion of the transmitted charged particles is dispersed between the first and second detectors 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. The method of claim 7, further comprising scanning a region of interest of the sample with the charged particles and forming a sample image representing sample structure based on the transmitted charged particles detected by the first detector.

9. The method of any one of claims 1-8, further comprising: after simultaneously detecting the first and second portions of the transmitted charged particles, rotating the transmitted charged particles relative to the first detector in an azimuthal direction relative to the principal axis, 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; generating a combined spectrum by combining the first and second spectra; and analyzing 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 charged particle imaging system, comprising: a source to generate charged particles; an illuminator to direct the charged particles along a principal axis toward a sample; a first detector positioned downstream of the sample and at a center of the principal axis; a second detector positioned downstream of the first detector; and a controller having non-transitory instructions and, by executing the instructions, the controller is configured to: in response to illuminating a location of the sample with the charged particles, simultaneously detect, with the first detector and the second detector, respectively, first and second portions of the charged particles transmitted through the sample, wherein each of the transmitted charged particles exits the sample with an exit angle between a direction of the transmitted charged particle and the principal axis, the exit angle of the first portion of the transmitted charged particles overlapping 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 region transparent to the charged particles extending less than 360 degrees in an azimuthal direction relative to the principal axis along a radius of the first detector.

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

14. The charged particle imaging system of claim 13, wherein each of the plurality of detection segments has two edges, and each of the two edges extends in a radial direction relative to the principal axis.

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

16. The charged particle imaging system of claim 15, wherein the controller is further configured to: direct the charged particles to a plurality of locations of the sample; generate a sample image representing 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.

17. The charged particle imaging system of any of claims 11-16, further comprising a projection lens positioned between the sample and the first detector, and the controller is further configured to adjust the projection lens to rotate the transmitted charged particles upstream of the first detector about the principal axis.

18. The charged particle imaging system of claim 17, wherein the controller is further configured to: direct the charged particles to a first location of the sample and acquire a first spectrum using the second detector; ​ adjusting the projection lens; acquiring a second spectrum using the second detector with the adjusted projection lens; and generating a combined spectrum for the first location by combining the first spectrum and the second spectrum.

19. The charged particle imaging system of claim 17, wherein the controller is further configured to: scan a plurality of locations of the sample with the charged particles and acquire a first spectrum at each of the plurality of sample locations using the second detector; adjust the projection lens; scan the plurality of locations of 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 at each of the plurality of sample locations by combining the first spectrum and the second spectrum.

20. The charged particle imaging system of claim 19, wherein the controller is further configured to: generate a first sample image based on the transmitted charged particles detected by the first detector prior to 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 wherein combining the first spectrum and the second spectrum includes combining the first spectrum and the second spectrum based on the first sample image and the second sample image.

Images