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US-12626884-B2 - Method and scanning transmission charged-particle microscope

US12626884B2US 12626884 B2US12626884 B2US 12626884B2US-12626884-B2

Abstract

A computer-implemented method of processing image data according to the present disclosure comprises: receiving the image data, wherein the image data is scanning transmission charged-particle microscope (STCPM) image data representing a STCPM scan obtained at a first focus depth; and processing a system of equations expressing the image data as a sum of contributions from a plurality of slices of the sample at a plurality of focus depths, wherein each equation of the system of equations relates at least a portion of the image data to: at least one of a plurality of contrast transfer functions of the STCPM, each contrast transfer function of the STCPM being determined at a different respective focus depth; and at least one set of unknown objects of the STCPM, each unknown object in a set being at a different respective focus depth. The step of processing comprises solving the system of equations to obtain at least one of the plurality of unknown objects of the STCPM.

Inventors

  • Eric Bosch
  • Ivan Lazic

Assignees

  • FEI COMPANY

Dates

Publication Date
20260512
Application Date
20230419
Priority Date
20220428

Claims (17)

  1. 1 . A computer-implemented method of processing image data, the method comprising: receiving the image data, wherein the image data is scanning transmission charged-particle microscope (STCPM) image data representing a STCPM scan obtained at a first focus depth; processing a system of equations expressing the image data as a sum of contributions from a plurality of virtual slices of a sample at a plurality of focus depths, wherein each equation of the system of equations relates at least a portion of the image data to: at least one of a plurality of contrast transfer functions of the STCPM, each contrast transfer function of the STCPM being determined at a different respective focus depth from the first focus depth; and at least one set of representations of unknown structures of the STCPM, each unknown structure in a set being at a different respective focus depth from the first focus depth; wherein the step of processing comprises solving the system of equations to obtain at least one of the plurality of representations of the unknown structures of the STCPM; and generating, from the obtained representations, reconstructed image data respectively corresponding to the plurality of virtual slices of the sample at the plurality of focus depths.
  2. 2 . The method of claim 1 , wherein the step of processing comprises solving the system of equations to obtain the plurality of image data representing unknown structures of the STCPM at different focus depths.
  3. 3 . The method of claim 1 , wherein the step of processing comprises solving the system of equations by singular value decomposition (SVD).
  4. 4 . The method of claim 1 , wherein the step of processing comprises regularising the system of equations before solving the system of equations.
  5. 5 . The method of claim 1 , wherein: one of the sets of representations of unknown structures comprises sin φ n (r p ) and/or one of the sets of representations of unknown structures comprises 1−cos φ n (r p ); where r p represents a scan position of the image data and φ n represents a phase shift induced by an n-th virtual slice of the plurality of virtual slices of the sample.
  6. 6 . The method of claim 1 , wherein the system of equations is of the form A u = v , where A is a matrix representing the plurality of contrast transfer functions of the STCPM, u is a vector representing the at least one set of representations of unknown structures of the STCPM, and v is a vector representing the image data.
  7. 7 . The method of claim 6 , wherein the system of equations satisfies at least one of conditions (i), (ii) and/or (iii): A = ( CTF S , 0 , 0 CTF C , 0 , 0 CTF S , 0 , 1 CTF C , 0 , 1 … CTF S , 0 , N CTF C , 0 , N CTF S , 1 , 0 CTF C , 1 , 0 CTF S , 1 , 1 CTF C , 1 , 1 … CTF S , 1 , N CTF C , 1 , N ⋮ ⋮ ⋮ ⋮ ⋱ ⋮ ⋮ CTF S , M , 0 CTF C , M , 0 CTF S , M , 1 CTF C , M , 1 … CTF S , M , N CTF C , M , N ) ( i ) where CTF S/C,M,N are contrast transfer functions of the STCPM for the representations sin φ n (r p ) and 1−cos φ n (r p ) respectively and where r p represents a scan position of the image data, wherein the plurality of virtual slices comprises N virtual slices and the image data is segmented STCPM image data from a STCPM detector having M segments; u _ = ( ℱ ⁢ { sin ⁢ φ 0 } ℱ ⁢ { 1 - cos ⁢ φ 0 } ℱ ⁢ { sin ⁢ φ 1 } ℱ ⁢ { 1 - cos ⁢ φ 1 } ⋮ ℱ ⁢ { sin ⁢ φ N } ℱ ⁢ { 1 - cos ⁢ φ N } ) ( ii ) where {sin φ N } and {1−cos φ N } for n=0, 1, . . . , N are components of Fourier transforms of the representations sin φ n (r p ) and 1−cos φ n (r p ), where r p represents a scan position of the image data and wherein the plurality of virtual slices comprises N virtual slices; and/or v _ = ( ℱ ⁢ { I 0 STEM } ℱ ⁢ { I 1 STEM } ⋮ ℱ ⁢ { I M STEM } ) ( iii ) where ℱ ⁢ { I M STEM } , for s=0,1, . . . , M, are components of Fourier transforms of the image data, wherein the image data is segmented STCPM image data from a STCPM detector having M segments.
  8. 8 . The method of claim 1 , wherein solving the system of equations comprises obtaining components of Fourier transforms of at least one of the plurality of representations of unknown structures of the STCPM and obtaining the at least one of the plurality of representations of unknown structures of the STCPM by inverse Fourier transformation.
  9. 9 . The method of claim 1 , wherein solving the system of equations comprises solving the system of equations for each of a plurality of k-vectors of a Fourier transform of the image data.
  10. 10 . The method of claim 1 , wherein the image data is segmented STCPM image data.
  11. 11 . The method of claim 1 , wherein: the plurality of virtual slices comprises N virtual slices and the image data is segmented STCPM image data from a STCPM detector having M segments, wherein NM N<<0.5MN<0.5MN<0.4MN<0.3MN<0.2MN<0.1MN<0.05MN<0.01M a ratio of N/M has a value in a range from greater than 0 to less than 0.5.
  12. 12 . The method of claim 1 , wherein λ is the wavelength of charged particles of the STCPM and α is a beam opening angle of the STCPM, wherein the focus depths of the plurality of virtual slices differ by a distance of: greater than or equal to a value between λ α 2 ⁢ 2 ⁢ λ α 2 ⁢ 4 ⁢ λ α 2 ⁢ 6 ⁢ λ α 2 and 8 ⁢ λ α 2 ; and/or, 10 ⁢ λ α 2 ⁢ 8 ⁢ λ α 2 ⁢ 6 ⁢ λ α 2 ⁢ 4 ⁢ λ α 2 ⁢ 2 ⁢ λ α 2 .
  13. 13 . The method of claim 1 , further comprising determining one or more parameters of at least one of the plurality of contrast transfer functions of the STCPM by iteratively adjusting the one or more parameters, with respect to a quality criterion, on at least one of the plurality of representations of unknown structures of the STCPM.
  14. 14 . The method of claim 1 , wherein: the plurality of virtual slices comprises N virtual slices and N is greater than a value in a range of between 2 and 10.
  15. 15 . The method of claim 1 , wherein λ is the wavelength of charged particles of the STCPM and α is a beam opening angle of the STCPM, wherein the focus depths of the plurality of slices differ by a distance of: less than or equal to a value in a range of between 10 ⁢ λ α 2 ⁢ and ⁢ 2 ⁢ λ α 2 .
  16. 16 . A method of obtaining a through-focus series (TFS) of a sample using a scanning transmission charged-particle microscopy (STCPM), the method comprising: performing a plurality of STCPM scans on the sample at a plurality of different respective focus depths to obtain a first TFS comprising a plurality of first TFS image data sets; performing the method of any preceding claim on each of the first TFS image data sets to obtain, for each first TFS image data set, at least one representation of an unknown structure of the STCPM; and generating a second TFS comprising, for each first TFS image data set, reconstructed image data generated from the at least one representation of the unknown structure.
  17. 17 . A scanning transmission charged-particle microscope (STCPM) comprising: a charged-particle source; particle optics configured to direct charged particles from the charged-particle source towards a sample to cause the charged particles to pass through the sample; a detector configured to detect charged particles passing through the sample to provide image data; and a processor configured to cause the STCPM to: receive the image data, wherein the image data is scanning transmission charged-particle microscope (STCPM) image data representing a STCPM scan obtained at a first focus depth; process a system of equations expressing the image data as a sum of contributions from a plurality of virtual slices of the sample at a plurality of focus depths, wherein each equation of the system of equations relates at least a portion of the image data to: at least one of a plurality of contrast transfer functions of the STCPM, each contrast transfer function of the STCPM being determined at a different respective focus depth from the first focus depth; and at least one set of representations of unknown structures of the STCPM, each unknown structure in a set being at a different respective focus depth from the first focus depth; wherein at least one of the plurality of representations of the unknown structures of the STCPM is obtained through solving the system of equations; and generate, from the obtained representations, reconstructed image data respectively corresponding to the plurality of virtual slices of the sample at the plurality of focus depths.

Description

FIELD This disclosure concerns scanning transmission charged-particle microscopes (STCPMs), methods of processing STCPM image data and computer programs for processing STCPM image data. BACKGROUND Charged-particle microscopy, and particular electron microscopy, is a well-known and increasingly important technique for imaging microscopic objects. Historically, the basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM). Moreover, various sub-species have been developed, such as so-called “dual-beam” tools (e.g. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID), for example. In a SEM, irradiation of a specimen by a scanning electron beam causes emanation of “auxiliary” radiation from the specimen, in the form of secondary electrons, backscattered electrons, X-rays and photoluminescence (infrared, visible and/or ultraviolet photons). One or more components of this emanating radiation is/are then detected and used for image accumulation purposes, and/or spectroscopic analysis (as in the case of EDX (Energy-Dispersive X-Ray Spectroscopy), for example). In a TEM, an electron beam used to irradiate the specimen is chosen to be of a high-enough energy to penetrate the specimen (which, to this end, will generally be thinner than in the case of a SEM specimen). The flux of transmitted electrons emanating from the specimen can then be used to create an image, or produce a spectrum (as in the case of Electron Energy-Loss Spectroscopy, EELS). If such a TEM is operated in scanning mode (thus becoming a STEM), the image/spectrum in question will be accumulated during a scanning motion of the irradiating electron beam. As an alternative to the use of electrons as an irradiating beam, charged-particle microscopy can also be performed using other species of charged particle. In this respect, the phrase “charged particle” should be broadly interpreted as encompassing electrons, positive ions (e.g. Ga or He ions), negative ions, protons and positrons, for instance. In addition to imaging and/or spectroscopy, a charged-particle microscope (CPM) may also have other functionalities, such as examining diffractograms, performing (localized) surface modification (e.g. milling, etching, deposition), etc. In any case, scanning transmission charged-particle microscopes (STCPMs) usually comprise at least the following components: A radiation source, such as a Schottky electron source or ion gun.An illuminator, which manipulates a “raw” radiation beam from the source and performs certain operations on the beam, such as focusing, aberration mitigation, cropping (with a stop/iris/condensing aperture), filtering, etc. An illuminator will generally comprise one or more charged-particle lenses, and may also comprise other types of particle-optical components. If desired, the illuminator can be provided with a deflector system that can be used to cause its output beam to perform a scanning motion across the specimen being investigated.A specimen holder, on which a specimen under investigation can be held and positioned (e.g. tilted, rotated). If desired, the holder can be moved to effect a scanning motion of the beam with respect to the specimen. In general, such a specimen holder will be connected to a positioning system such as a mechanical stage.An imaging system, which takes charged particles that are transmitted through a specimen (plane) and directs (focuses) the charged particles onto analysis apparatus, such as a detection/imaging device, spectroscopic apparatus, etc. As with the illuminator, the imaging system may also perform other functions, such as aberration mitigation, cropping, filtering, etc., and it will generally comprise one or more charged-particle lenses and/or other types of particle-optical components.A detector, which may be unitary or compound/distributed in nature, and which can take many different forms, depending on the radiation/entity being recorded. Such a detector may, for example, be used to register an intensity value, to capture an image, or to record a spectrum. Examples include photomultipliers (including solid-state photomultipliers, SSPMs), photodiodes, (pixelated) CMOS detectors, (pixelated) CCD detectors, photovoltaic cells, etc., which may, for example, be used in conjunction with a scintillator film, for instance. For X-ray detection, use is typically made of a so-called Silicon Drift Detector (SDD), or a Silicon Lithium (Si(Li)) detector, for example. Typically, an STCPM will comprise several detectors, of various types. EP-3,082,150-B1, which is commonly assigned with the present disclosure and incorporated herein by reference, discloses a method of performing a through-focus series (TFS), that