Off-plane imaging mode for laser phase plates

Abstract

This invention provides a method for charged particle beam (CPB) phase contrast imaging. [Solution] In a CPB system, the phase contrast image is obtained by directing a sample-modulated CPB to the phase plate so that it has a diffraction plane displaced from the phase plate. Based on the interference fringes of the image, the lateral displacement of the CPB relative to the phase plate can be estimated and adjusted.

Description

[Technical Field]

[0001] This disclosure relates to phase-dependent charged particle beam imaging. [Background technology]

[0002] While phase-contrast microscopy offers many advantages for imaging, implementing the necessary phase plates in charged particle beam (CPB) imaging can be challenging. Several developed phase plates add additional material to the CPB path to provide the required phase difference. So-called Zernike phase plates based on thin films with a central aperture can provide suitable phase shifts with appropriate thin film thicknesses. Such phase plates and other phase plates based on the insertion of thin film layers into the CPB tend to result in undesirable scattering, exhibit damage or phase changes in response to CPB exposure, and / or generate phase changes based on phase plate charging.

[0003] One approach to generating the necessary phase shift in CPB imaging, which can circumvent these difficulties, is based on a laser phase plate in which a periodic optical field of sufficient intensity is established to generate a suitable CPB phase shift based on the ponderomotive potential associated with the periodic optical field. In most examples, the periodic optical field is established by oriented the laser beam into a high-finesse Fabry-Perot resonator to establish a standing wave optical field. Such a system is described, for example, in Axelrod, “A Laser Phase Plate,” available at https: / / arxiv.org / abs / 2403.10670, submitted March 15, 2024. "for Transmission Electron Microscopy," and Axelrod et al., "Modern Approaches to Transmission Electron Microscopy," submitted on January 22, 2024, available at https: / / arxiv.org / abs / 2401.11678. This is described in “Improving Phase Contrast Electron Microscopy,” both of which are incorporated herein by reference. For imaging with a laser phase plate to be successful, the phase plate (i.e., the standing wave field) and the CPB must be well aligned, and the alignment must be stable. Unfortunately, maintaining stable alignment of the CPB within a portion of the standing wave field's wavelength is difficult, and in conventional approaches, imaging is interrupted to allow for realignment. This introduces further complexity, delay, and inefficiency to phase imaging, and an alternative approach is needed. [Overview of the project]

[0004] The disclosed methods and apparatus are generally illustrated with reference to CPB, and typical practical applications include phase-dependent electron beam imaging such as TEM. Disclosed are phase-contrast CPB imaging methods and apparatus in which a phase plate, such as a laser phase plate, is positioned to be displaced from a diffraction plane associated with a sample. In some examples, the diffraction plane is the back focal plane of the objective lens used to image the sample. This imaging state is referred to as off-plane imaging. In methods and systems implementing off-plane imaging, the alignment of the CPB with respect to the phase plate can be determined and corrected as necessary. This is particularly useful in applications involving a laser phase plate that provides periodic phase modulation with a period equal to λ / 2 (where λ is the laser wavelength).

[0005] The above and other features and advantages of the technology of this disclosure will become apparent from the following detailed description, which will proceed with reference to the attached drawings. [Brief explanation of the drawing]

[0006] [Figure 1A] This example illustrates how diffraction planes associated with a charged particle beam (CPB) optical system can be positioned at the phase plate or at a location axially displaced from the phase plate. [Figure 1B]This example illustrates how diffraction planes associated with a charged particle beam (CPB) optical system can be positioned at the phase plate or at a location axially displaced from the phase plate. [Figure 1C] This example illustrates the periodic phase distribution generated using a laser phase plate corresponding to a standing wave optical field. [Figure 1D] Figures 1A to 1B show the phase distribution applied to the CPB by a laser phase plate having a diffraction surface positioned at the laser phase plate (Figure 1D) or at a location displaced from the laser phase plate (Figure 1E). In Figures 1D to 1E, the CPB alignment with respect to the standing wave light field is superimposed. [Figure 1E] Figures 1A to 1B show the phase distribution applied to the CPB by a laser phase plate having a diffraction surface positioned at the laser phase plate (Figure 1D) or at a location displaced from the laser phase plate (Figure 1E). In Figures 1D to 1E, the CPB alignment with respect to the standing wave light field is superimposed. [Figure 1F] We illustrate with an example a CPB diffraction plane positioned at a location displaced axially from the laser phase plate by a lateral offset selected to change the image contrast. [Figure 1G] This example illustrates the breaking of Friedel symmetry with zero lateral offset. [Figure 2A] Sample images obtained using the CPB diffraction plane associated with a sample positioned at a location axially displaced from the laser phase plate are illustrated as an example. [Figure 2B] As an example, the sum of the image intensity of the sample images in Figure 2A is used to illustrate the position along the direction associated with the periodic phase associated with the standing wave optical field generated by the laser phase plate. [Figure 3A] This illustrates simulated image modulation related to the changing CPB diffraction plane displacement from the location of the laser phase plate. In Figure 3A, the displacement increases from left to right and from top to bottom, starting at Δ=0 and increasing in constant increments. [Figure 3B]Figure 3A illustrates the increase in stripe visibility with increasing Δ value, illustrating the total image intensity corresponding to the simulated image. [Figure 3C] This example illustrates CPB alignment for phase plates with periodic phases at various locations for selecting image contrast. [Figure 3D] An example of an image obtained using the alignment method shown in Figure 3C will be illustrated. [Figure 4] This paper provides an example of a typical CPB imaging method using a laser phase plate. [Figure 5] An example is described of a CPB imaging apparatus that includes a laser phase plate, has displacement from the laser phase plate, and is operable to position the CPB diffraction plane to control the lateral offset of the phase plate based on an acquired image. [Figure 6] Figure 5 illustrates a typical computing environment for controlling and operating a device. [Figure 7A] This section provides examples of typical locations where phase plates can be added. [Figure 7B] This section provides examples of typical locations where phase plates can be added. [Figure 8] This paper illustrates a typical method for adjusting images in response to Friedel symmetry breaking. [Modes for carrying out the invention]

[0007] The present disclosure relates to a method and apparatus for maintaining alignment of a CPB, specifically, a CPB diffractive surface and a phase plate, using an image acquired with the phase plate. Embodiments are described with reference to a laser phase plate in which a phase change is based on a ponderomotive force associated with a standing light field that can conveniently supply a laser beam directed to a Fabry-Perot optical resonator. Generally, it is more convenient to adjust the CPB position rather than adjusting the position of the standing light field, and for this reason, the following embodiments describe systems and methods in which the CPB position is adjusted, although in other embodiments, the standing light field can be repositioned, for example, by repositioning an associated Fabry-Perot resonator or by adjusting the laser wavelength. The disclosure is described with reference to a phase plate that generates a spatially varying phase shift based on the periodic intensity and field variations of the standing light field, although the disclosed methods and apparatus can be used with other phase plates that provide a spatially varying phase.

[0008] The standing light field generated by the laser phase plate generates a periodically varying phase where maximum phase modulation is provided at the antinodes and minimum modulation (usually no modulation) is provided at the nodes. For a standing light field of wavelength λ, the associated phase modulation that depends on the light intensity has a period of λ / 2. In a typical example, the standing light field is provided by a laser such as a Nd:YAG laser and one or more fiber optical amplifiers or other laser systems operating at or near 1064 nm. Using this wavelength used to generate the standing light field, the CPB positioning with respect to this standing light field is preferably selected and maintained within λ / 10, λ / 20 or less. The disclosed method enables establishing a suitable alignment to the phase plate and, if necessary, maintaining such alignment without interrupting imaging.

[0009] Furthermore, as used herein, a laser phase plate system refers to a system used to generate a suitable standing wave optical field, and includes a laser light source, a fiber amplifier, a Fabry-Perot interferometer, and other optical elements and systems required to generate, control, and stabilize the standing wave optical field, as illustrated in the Axelrod reference above. When referring to CPB propagation in an optical column, the laser phase plate refers to the region where the standing wave optical field exists, typically the region defined by the Fabry-Perot resonator. Generally, it is preferable to direct the CPB to a portion of this standing wave optical field, typically located in a Gaussian optical beam having a circular cross-section, but other beam shapes and positions can be used, located within the Rayleigh range of the beam waist. The reference to the standing wave optical field as a phase plate will be understood to mean that the location of the phase plate is defined by the location of the standing wave optical field and not by the presence of any physical device. The disclosed methods and apparatus can also be used with other types of phase plates, but for illustrative purposes, the use of a laser phase plate system is detailed herein.

[0010] General terminology As used herein, "image" refers to a visual representation for viewing on a display device, such as by an engineer, operator, or other person, a projection on a surface such as a projection screen, or otherwise presented for viewing. "Image" also refers to a numerical representation of a visible image, such as a JPG, TIFF, BMP, or other format of image file. Such numerical representation can include intensity values or can be processed to generate them as a function of position I(X,Y), where X and Y are coordinates along linearly independent (typically, orthogonal) axes. In the examples described herein, intensity is presented as a single value without reference to color as seen by an observer. However, intensity values can be assigned to one or more spectral components such as red, green, and blue for viewing, or other image values such as hue, saturation, and value, or color coordinates (e.g., LAB, CYMK, RGB) can be used. In an example, the image of interest is a charged particle beam (CPB) image and a single intensity value is appropriate.

[0011] The numerical representation of an image can refer to sample locations based on intensity values I(X,Y) over a range of spatial coordinates. Alternatively, an image can be represented as an array or other set of intensity values I(J,K), where J, K are non-negative integers indicating pixel locations corresponding to sample locations. The mapping of pixel locations (J,K) to image coordinates (X,Y) depends on the array of samples and the magnification of the image. In some cases, an array of intensities I(J,K) is obtained using an array detector, but a non-array detector can also be used. Intensity is generally associated with charge or current in an individual pixel and can be a linear or other function of charge or current. Examples are described with reference to a right-handed orthogonal coordinate system where the Z-axis is generally associated with the CPB optical axis, and the X-direction and Y-direction are orthogonal to the Z-axis and thus associated with lateral displacement.

[0012] As described above, the examples are described with reference to a laser phase plate in which a standing wave optical field is established to generate a suitable CPB phase shift based on a ponderomotive potential associated with a periodic optical field. As considered above, in most examples, the periodic optical field is established by directing a laser beam into a high-finesse Fabry-Perot resonator to establish a standing wave optical field with sufficient intensity to generate a periodic CPB phase shift of sufficient amplitude. In some examples, the amplitude of the phase shift can be set as π / 2, but other values ​​can be selected and this value can be used for convenience. The amplitude of the standing wave optical field in the Fabry-Perot resonator is described as a Gaussian beam, preferably in the lowest mode, having a beam waist w0 that can be selected to provide a beam intensity suitable for generating a ponderomotive potential. A convenient value is typically 2 μm to 20 μm (including both ends). In addition, w0 can be selected to provide a sufficiently wide Rayleigh range that allows for a Gaussian beam spread with a standard value of 50 μm to 200 μm (including both endpoints).

[0013] As used herein, a CPB is described as being oriented along an axis toward a detector such as a sample, a phase plate, and an image detector. The axial displacement of the CPB diffraction plane from the location of the phase plate is referred herein to as the “diffraction plane displacement,” denoted as Δ, and is generally illustrated along the Z-axis. Typically, the diffraction plane is located at the focal plane (such as the back focal plane or front focal plane) of a CPB lens or set of CPB lenses, or a plane optically conjugate to such a focal plane. Imaging with Δ=0 may be referred to as “on-plane imaging,” while imaging with a non-zero value of Δ is referred to as “off-plane imaging.” The displacement of the peak value or other specified value of the phase (or the antinode or other part of the standing wave optical field in the laser phase plate) in a direction perpendicular to the axis is referred herein to as the lateral offset. In embodiments, the lateral offset is typically, for the sake of convenience, associated with the peak value of the phase associated with the phase plate (such as the antinode). The standing wave optical field defines a laser phase plate having nodes and antinodes that are periodic along the X-direction traversing the CPB axis. The direction in which the standing wave optical field exhibits periodic variation between the nodes and antinodes is referred to herein as the “phase axis.” In the embodiments, the phase axis is typically along the X-axis of the coordinate system used for illustrative purposes. Generally, a diffraction plane displacement Δ from the rear or front focal plane of the objective lens is used, but other lenses can be used to generate a suitable conjugate plane. Generally, it is convenient to set the diffraction plane displacement Δ using one or more CPB optical components, but the phase plate can be translated as needed. For illustrative purposes, embodiments are shown in which the laser phase plate is positioned relative to the rear focal point of the objective lens receiving the CPB from the sample. However, it may be convenient to position the laser phase plate relative to the conjugate plane using a transfer optical system that provides a magnification of 1 to 5.

[0014] As used herein, the term "orthogonal" refers to an angle that is 90 degrees ± 1 degree, ± 5 degrees, ± 10 degrees, or ± 15 degrees.

[0015] As used in this application and claims, the singular forms "a," "an," and "the" include the plural form unless otherwise explicitly indicated. Furthermore, the term "includes" means "comprises." Furthermore, the term "coupled" does not exclude the existence of intermediate elements between coupled items.

[0016] The systems, apparatus, and methods described herein should not be construed as limiting. Rather, this disclosure covers all novel and non-obvious features and aspects of the various disclosed embodiments, both individually and in various combinations or secondary combinations. The disclosed systems, methods, and apparatus are not limited to any particular aspects or features or combinations thereof, and the disclosed systems, methods, and apparatus do not require the existence of any one or more particular advantages or the resolution of any problem. Any theories of operation are provided for the sake of explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

[0017] Some of the operations of the disclosed methods are described in a specific sequential order for the sake of presentation; however, it should be understood that this style of description includes sorting unless a specific ordering is required by the specific wording set forth below. For example, operations described sequentially may, in some cases, be performed in a different order or simultaneously. Furthermore, for the sake of simplification, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and apparatus may be used with other systems, methods, and apparatus. In addition, the description may use terms such as “generate” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the particular implementation and will be readily recognizable to those skilled in the art.

[0018] In some examples, values, procedures, or devices are referred to as “minimum,” “best,” “smallest,” etc. Such descriptions are intended to show that a choice can be made from among many usable functional alternatives, and it will be understood that such a choice does not need to be better, smaller, or otherwise preferable than other choices. Examples are described with reference to directions indicated as “above,” “below,” “upper,” “lower,” etc. These terms are used for convenience and do not imply any particular spatial orientation. Specific coordinate systems are used for illustrative purposes only. [Examples]

[0019] Example 1 Figure 1A illustrates a portion of a charged particle beam optics (CPB) system 100 that generates a CPB 102 directed towards a sample 104 along axis 101. The CPB 102 is typically generated, focused, scanned, and manipulated by additional CPB optics contained within an optical column, which is not shown in Figure 1A. After interacting with the sample 104, the CPB 102 is directed along axis 101 to a diffraction plane 132 located at or near the CPB phase plate 108 (circularly shown in the Gaussian optical beam profile) using a CPB lens 106. While a point focus at the diffraction plane 132 is illustrated, the CPB 102 includes portions focused to different lateral locations along the X-axis and Y-axis of coordinate system 191 associated with different spatial frequency components of the sample 104. The objective lens 110 receives the CPB 102 from the sample 104 and the phase plate 108 and generates an image that depends on the phase applied by the phase plate 108. In this embodiment, the phase plate 108 is positioned at the back focal plane of the CPB lens 106.

[0020] The arrangement in Figure 1A requires that the CPB 102 maintain alignment with the phase plate 108. Otherwise, the misalignment of the phase plate 108 and CPB 102 will appear as a feature in the sample image, which is a suboptimal representation of the sample structure. Stable alignment of the phase plate is generally difficult. Stable alignment is particularly difficult for phase plates that exhibit rapid phase changes as a function of position. For example, in the case of a laser phase plate, the phase change is periodic with a period based on the wavelength of the standing wave optical field, such as 1064 nm. The alignment of the CPB and the phase plate should be stable within a portion of this wavelength, for example, within 5%, 2.5%, or 1% or less, i.e., within approximately 52 nm, 27 nm, or 13 nm or less. Not only is it difficult to align the CPB and the phase plate with such strict precision, but maintaining the alignment during imaging can also be challenging. Referring to Figure 1B, the CPB lens 106 is positioned to generate the CPB beam focus 133 with a diffraction plane displacement Δ displaced axially from the phase plate 108 (i.e., along the Z-axis parallel to axis 101, either upstream or downstream, or at an optically conjugate location). In this configuration, the CPB 102 has a beam width BW of the phase plate 108 such that different portions of the CPB 102 undergo different phase shifts due to the changing phase associated with the phase plate 108.

[0021] Figure 1C illustrates a periodic phase change 118 with period P associated with a typical phase plate, such as a laser phase plate 108, where the phase change is based on a standing wave optical field of wavelength λ. For the laser phase plate, period P = λ / 2. For illustrative purposes, the laser phase plate 108 is shown as an envelope of a Gaussian beam with an exaggerated velocity of beam expansion away from the beam waist, and the amplitude of the periodic phase change 118 is substantially constant, as shown in Figure 1C. For example, the periodic phase change typically has a period based on the wavelength (about 1 μm) of the optical beam used to generate the periodic phase change 118, while the Gaussian beam has a Rayleigh range of about 150 μm. Figure 1D illustrates the configuration of Figure 1A for this periodic phase change, and Figure 1E illustrates the configuration of Figure 1B, showing a phase change that is received by a CBP 102 with a diffraction plane displacement Δ, but aligned with respect to a CPB 102. Figure 1F illustrates a phase plate 108 having a lateral offset ΔX of the phase plate. In the configuration of Figure 1F, the image generated by the objective lens 110 includes image features that can be used to evaluate, correct, or establish the alignment of the CPB / phase plate, as will be discussed in detail below.

[0022] Detection and compensation of lateral offset of phase plate using sample images Figure 2A is a sample image 200 obtained using a CPB / phase plate, including image portions such as representative image portions 202 associated with particles or other sample features. For convenience, the sample image 200 can be divided into an N×M pixel array, where each pixel is associated with an image intensity I and coordinates (J,K) to generate an image intensity I(J,K), where J, K, N, and M are non-negative integers such that J ≤ M and K ≤ N. The array can be positioned to extend along the orthogonal X-axis and Y-axis, respectively, corresponding to total distances ΔX and ΔY in the sample 200, based on the orientation and magnification of the sample. In this embodiment, the phase direction associated with the laser phase plate is a function of the distance in the X-direction such that the phase shift generated by the phase plate is a function of X. The image intensity I(J,K) is generated by different phase shifts given to the CPB due to different spatial frequency components in the sample, resulting in a phase contrast image I PC (J,K) and the intensity change I generated by the phase change φ(J) across the CPB beam, which is due to the displacement of the diffraction plane from the phase plate. φ (J) and, including, if the orientation of image 200 is selected such that the X-direction (or J value) is associated with the direction of phase change in the laser phase plate, φ (J) is a function of J only, and not a function of K. For other image alignments, I φ This can be a function of either or both of J and K (or the X-direction and Y-direction projected onto the sample). In conventional approaches to imaging using laser phase plates, the diffraction plane is the phase plate and the phase changes φ=0 and I φIt is positioned at (J,K)=1. However, a CPB with a diffraction plane displacement Δ from the phase plate has a beam width BW at the phase plate. In the case of a CPB oriented along an axis with a lateral offset from the antinode or node of a standing wave optical field, portions of the CPB on different sides of the CPB (i.e., different spatial frequency components) may undergo an asymmetric phase shift, and when these different phase portions are combined in the image, they generate intensity modulation in the image. This intensity modulation is periodic with a lateral offset ΔX, and the image is a ronchigram image showing laser fringes. As used herein, such intensity modulation is referred to as laser fringes. The magnitude of the fringe modulation in the image depends on the diffraction plane / phase plate displacement Δ, as shown in the following examples.

[0023] Referring further to Figure 2A, sample image 200 includes side portions 212 and 213 of the image, along the left and right sides which are relatively darker than the central portion 218, as a result of the phase change φ(X) generated by the diffraction plane displacement Δ. The relative positions of the CPB and standing wave optical fields can be determined from sample image 200 using these intensity changes, as follows. A window, such as a typical window 220, is selected to extend to include some portions of the side portions 212 and 213 and the central portion 218 of the image. In this embodiment, window 220 extends to cover all pixels along the X-axis and ΔN pixels along the Y-direction, where ΔN can be selected to span the Y-direction of sample image 200 or to include only some portions. The intensity I(J,K) of all K values ​​within window 220 is summed for each J value within the window. As an example, for a window width (X-axis or phase direction) that includes pixels within range J (JMIN, JMAX) and pixels associated with the Y-axis (orthogonal to the phase axis) within range KMIN, KMAX, the sum I for each value of J is calculated. SUM (J) is calculated.

number

[0024] The total intensity I SUM (J) peaks at the J - value associated with the peak of the phase modulation associated with the laser phase plate, i.e., at the antinode locations of the standing - wave light field. This is shown in FIG. 2B as the peak 262 of the curve fit 260 to the total intensity I SUM (J). Based on the location of this peak, any displacement of the CPB from the antinode location can be determined and corrected. Generally, it is more convenient to adjust the CPB position in the laser phase plate, and one or more CPB lenses or deflectors operate to correct the measured displacement. In this way, the CPB can be maintained at the location of the selected laser phase plate without operator intervention while imaging. Additionally, the total intensity I SUM (J) can be used to compensate for changes in image intensity due to CPB focus shift by adjusting the image intensity I(J,K) by a factor proportional to, or otherwise 1 / I SUM (J) as a function of each J.

[0025] For some values ​​of the phase shift φ (i.e., some diffraction plane displacement Δ), the image intensity may be too low for useful imaging due to noise, and it will be understood that low-intensity correction is unlikely to be particularly useful. However, for at least a 90-degree phase shift φ, the procedure described above may be successful. In the examples in Figures 2A-2B, the CPB is generally oriented toward the antinode location of the laser phase plate. The CPB location and correction, as well as intensity compensation, can be performed using a CPB oriented to a location other than the antinode, such as the node, but generally, an alignment of less than 50 nm from the antinode is preferred.

[0026] Sample images with various phase plate lateral offsets Figure 3A includes a set of 302 simulated sample images obtained for various values ​​of diffraction plane displacement Δ, where Δ increases in a constant increment from left to right in the image, then from top to bottom, with the top left image associated with Δ=0. Figure 3B includes a set of 312 simulated total intensities along the phase shift direction, corresponding to the diffraction plane displacement Δ of the simulated sample images in Figure 3A. The sample images and simulated total intensities show that the fringes become more pronounced as the diffraction plane displacement Δ increases. The total intensities in Figure 3B can be used to determine the location of the CPB relative to the periodic phase of the laser phase plate, and the CPB (or laser phase plate) can be shifted so that it incidents on the phase plate at a selected location, which does not need to be a node or antinode, but can be any location. However, it is advantageous if the maximum intensity is still visible in the image, otherwise the image quality may be so poor that it may be difficult to determine the corrections that need to be made to return the beam to the antinode. For example, the total intensity peak 322 associated with image 320 at the bottom right corresponds to the location of the antinode of the periodic phase modulation and can be used to verify or adjust the alignment of the CPB / phase plate.

[0027] In some examples, CPB / phase plate alignment is selected and controlled to maintain CPB incidence to the antinode of the periodic phase provided by the phase plate in order to provide contrast to low spatial frequency features in the sample. However, other alignments may be selected. As shown in Figure 3C, the CPB can be incident on a periodic phase change 348 of period P at the antinode 350, at node 356 offset by P / 2 from the antinode, at position 352 offset by P / 4 from the antinode, or at position 354 offset by P / 8 from the antinode. Incidence at antinode 350 produces an image as shown in Figure 3A, incidence at position P / 4 offset from the antinode produces an image with little or no intensity change due to phase contrast, incidence at position P / 8 offset from the antinode produces an image with reduced phase contrast, and incidence at node 356 produces an image with the opposite contrast to that produced using CPB directed to the antinode. Figure 3D includes representative images 360, 362, 364, and 366, associated with CPB lateral offsets from antinodes 0, P / 8, P / 4, and P / 2, respectively, with diffraction plane displacement Δ=0.

[0028] Phase plate / CPB alignment Referring to Figure 4, a typical method 400 includes establishing a standing wave optical field in 402 to generate a laser phase plate that generates a periodic phase shift on the CPB. In 404, the CPB focus and laser phase plate are offset to generate a selected CPB / phase alignment, typically such that the CPB is oriented to the antinode of the periodic phase provided by the phase plate. In 406, the CPB diffraction plane displacement Δ is established by adjusting the CPB beam focus or a suitable positioning of the phase plate. As already mentioned, it is generally more convenient to adjust the CPB focus and position than to adjust the laser phase plate. In 408, an image is acquired. In 410, the acquired image is, for example, the sum of the acquired image intensities I as considered above. SUM Based on this, the intensity changes associated with the striped areas appearing in the acquired image can be removed or reduced. In 412, it is determined whether additional images should be acquired, and if not, the process ends in 414. Otherwise, in 416, the acquired image is typically I SUM Based on this, the lateral offset between the CPB and the phase plate is determined. Based on the determined lateral offset, if the position error is known in 418, the lateral offset can be adjusted in 420 as needed, and the process is returned to 408 for acquisition of additional images.

[0029] In the example above, the initial lateral offset of the CPB and phase plate can be established based on one or more images, with or without operator intervention. The images do not need to be corrected to remove the fringe effect, but can be evaluated solely for monitoring and correcting the lateral offset. The diffraction plane displacement Δ can also be selected and adjusted as needed. Some or all images can be evaluated to monitor the lateral offset of the CPB / phase plate, or images can be selected randomly, periodically, or in response to operator intervention.

[0030] Typical CPB imaging system with a laser phase plate Referring to Figure 5, a typical CPB imaging system 500 includes a deflector 506 and a CPB source 502 that generates a CPB 510 directed to a sample S by an additional CPB optical system 508 such as a deflector, lens, aperture, astigmatism corrector, or other optical elements. After modulation by the sample S, the objective lens 510 directs the CPB through a laser phase plate system 516 to image the sample S with an image sensor 526 or other detector or detection system. The objective lens 510 establishes the diffraction plane 514 shown in this example as the back focal plane of the objective lens 510. The laser phase plate system 516 is based on a Fabry-Perot resonator established by mirrors 518, 519 and a laser system such as a laser and fiber amplifier 523, which is controlled by a control system 525 and is operable to generate a light beam 521 directed to the Fabry-Perot optical resonator to establish a standing wave light field 522. The control system 525 can stabilize the standing wave field 522 by measuring and adjusting the laser system 523 and other parameters. In addition, the relative lateral position of the standing wave field 522 with respect to the CPB along the X-axis and Y-axis of the coordinate system 593 can be adjusted using one or more actuators (not shown in Figure 5) connected to the control system 525, but generally, it is more convenient to control the CPB position using a beam deflector such as a CPB deflector 506 or other CPB optical elements. For example, a CPB deflector 524 can be positioned downstream from the sample S to adjust the CPB position relative to the standing wave field 522. Such a CPB beam deflector can be positioned upstream or downstream of the objective lens 510. In some cases, additional transfer optical elements can be provided, and one or more CPB beam deflectors can be positioned to adjust the CPB position relative to the standing wave field 522.

[0031] The standing wave field 522 is schematically represented to indicate the locations of the nodes and antinodes. The spatial range of the standing wave field 522 along the Z-axis corresponds to the beam modes, typically the lowest-order Gaussian beam modes characterized by the beam waist. The standing wave field 522 is positioned so as to be displaced by a distance Δ from the diffraction plane 514.

[0032] CPB imaging is illustrated by representative CPB propagation directions 540 and 541, shown as dotted lines, illustrating the interaction between CPB 510 and sample S. Selected spatial frequency components generated by sample S are illustrated by representative CPB propagation directions 530 and 531, shown as dashed lines, associated with foci on the diffraction plane 514. For illustrative purposes, CPB propagation directions 530 and 531 can be associated with the focusing of the zero-order diffraction portions of the CPB portions of other spatial frequency components at different locations on the diffraction plane 514.

[0033] For example, I SUM Image processing to determine the lateral offset of the CPB / phase plate can be performed by one or more logic processors, such as the CPU 530, by calculating, adjusting the lateral offset and diffraction plane displacement of the CPB / phase plate, and adjusting the image to compensate for intensity changes caused by fringes. The processed or unprocessed image can be oriented to a display device 532 for user viewing, or the image can be communicated via a network or other connection for remote viewing, analysis, and any additional processing.

[0034] Typical CPB control and processing systems Figure 6 and the following discussion are intended to provide a brief and general description of an exemplary computing environment in which the disclosed technology may be implemented. Specifically, some or all parts of this computing environment may be used in conjunction with the above-mentioned methods and apparatus to, for example, control the positioning, focus, and diffraction plane displacement of a CPB, process images to determine the lateral offset of the CPB / phase plate, correct or compensate for images to remove or mitigate intensity changes caused by fringes, adjust the phase plate, and generally control and process data associated with an apparatus such as the one shown in Figure 5. Although not essential, the disclosed technology may be used with a personal computer. This document describes computer executable instructions, such as program modules, that are executed by a computer (PC). Generally, a program module includes routines, programs, objects, components, data structures, etc., that perform a specific task or implement a specific abstract data type. Furthermore, the disclosed technology may be implemented in other computer system configurations, including handheld devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, etc., and in FPGAs, ASICs, coupled programmable logic devices (CPLDs), or other dedicated processors. The disclosed technology may also be practiced in a distributed computing environment, where tasks are performed by remote processing devices linked over a communication network. In a distributed computing environment, program modules may reside in both local and remote memory storage devices. As used herein, memory storage and storage devices refer to physical devices, not temporary memory storage or signals.

[0035] Referring to Figure 6, an exemplary system implementing the disclosed technology includes a general-purpose computing device, which is an exemplary conventional PC 600, comprising one or more processing units 602, a system memory 604, and a system bus 606 connecting the one or more processing units 602 to various system components including the system memory 604. The system bus 606 may be one of several types of bus structures, including a memory bus or memory controller, a peripheral bus, and a local bus, using any of various bus architectures. The exemplary system memory 604 may include read-only memory (ROM) 608 and random access memory (RAM) 610, and a basic input / output system (BIOS) containing basic routines that facilitate the transfer of information between elements within the PC 600 may be stored in the ROM 608.

[0036] An exemplary PC600 further includes one or more storage devices 630, such as a hard disk drive for reading and writing from a hard disk, a magnetic disk drive for reading and writing from a removable magnetic disk, and an optical disk drive for reading and writing from a removable optical disk (such as a CD-ROM or other optical medium). Such storage devices may be connected to the system bus 606 by hard disk drive interfaces, magnetic disk drive interfaces, and optical drive interfaces, respectively. The drives and associated computer-readable media provide non-volatile storage for computer-readable instructions, data structures, program modules, and other data for the PC600. Other types of computer-readable media, such as magnetic cassettes, flash memory cards, digital video discs, CDs, DVDs, RAM, and ROMs, may store data accessible by the PC and may be used in the exemplary operating environment. In the example in Figure 6, one or more memories or other storage devices 690 each include portions 671, 672, 673, and 674 that store processor-executable instructions for image processing to determine the lateral offset, set the CPB diffraction plane displacement Δ, compensate the image based on the fringe pattern, and compensate the image for breaking of Friedel symmetry.

[0037] Some program modules may be stored in a storage device 630 containing the operating system, one or more application programs, other program modules, and program data. For example, port location data may be stored in the storage device. The user may input commands and information to the PC 600 via one or more input devices 640, such as a keyboard and a pointing device such as a mouse. Other input devices may include a digital camera, microphone, joystick, gamepad, satellite receiver, or scanner. These and other input devices are often connected to one or more processing units 602 via a serial port interface connected to the system bus 606, but may also be connected by other interfaces such as a parallel port, game port, or Universal Serial Bus (USB). A monitor 646 or other type of display device is also connected to the system bus 606 via an interface such as a video adapter. Other input / output devices 635, such as digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), may be included to provide control signals such as focus or other signals and to receive signals from the CPB imaging device.

[0038] The PC600 can operate in a network environment using logical connections to one or more remote computers, such as remote computers 660. In some examples, one or more network or communication connections 650 include wired or wireless connections, as well as data acquisition and control devices such as digital-to-analog and analog-to-digital converters. The remote computer 660 may be another PC, server, router, network PC, or peer device, or other common network node, and typically includes many or all of the elements described above with respect to the PC600, although Figure 6 illustrates only the memory storage device 662. The personal computer 600 and / or remote computer 660 may be connected to a logical local area network (LAN) and a wide area network (WAN). Such networking environments are common in offices, enterprise-scale computer networks, intranets, and the internet.

[0039] When used in a LAN networking environment, the PC600 is connected to the LAN via a network interface. When used in a WAN networking environment, the PC600 typically includes a modem or other means for establishing communication over the WAN, such as the Internet. In a network environment, program modules or parts thereof illustrated in relation to the personal computer 600 may be stored in a remote memory storage device or other location on the LAN or WAN. The network connections shown are illustrative, and other means may be used to establish communication links between computers.

[0040] Location of additional typical phase plates The laser phase plate can be positioned at other locations, as illustrated in Figures 7A and 7B. Referring to Figure 7A, the CPB 704 from sample 702 is directed to a lens 706 that focuses the CPB to 708. The CPB focus 708 is positioned optically upstream of the phase plate 710 by a distance Δ, in contrast to examples such as in Figure 1B where the CPB focus is downstream of the phase plate. Referring to Figure 7B, the CPB 754 from sample 752 is directed to a CPB objective lens 756 having a rear focus at 758. The CPB focus 758 is re-imaged by another CPB optical system 764, and the phase plate is displaced by a distance Δ from a location 759 that is optically conjugate to the rear focus 758. In Figure 7B, the optically conjugate location 759 is optically downstream of the phase plate 760, but in other embodiments it is positioned upstream. Generally, the phase plate is positioned downstream of the sample, at a distance Δ optically shifted upstream or downstream of the CPB focus, or at a location conjugate to such a location.

[0041] Phase imaging with Friedel symmetry breaking Certain spatial frequencies in the sample give rise to so-called +1 and -1 CPB scattering components, referred to as "Friedel pairs." When the sample moves to the camera, the Friedel symmetry is broken when the +1 and -1 components acquire different phase shifts. In some cases, the CPB is transmitted by the laser phase plate in the optical axis of the laser but is not sufficiently aligned to the antinode (i.e., the region of highest intensity) of the standing wave optical field, thereby breaking the Friedel symmetry of some parts of the scattered wave. This symmetry breakdown can occur for any value of the transverse offset ΔX, including ΔX=0. The diffracted beams from different parts of the sample will be directed to different phase shift regions. As shown in Figure 1G, the beams diffracted at sample locations 151-153 interact with different parts of the periodic optical field 118. For example, the -1 diffraction 161 associated with sample location 151 crosses the periodic optical field 118 at a different location than the -1 diffraction 162 associated with sample location 152. Therefore, correction can be applied to images that nominally have no lateral offset ΔX.

[0042] The one-dimensional image intensity I(x) along the x-axis, which is parallel to the phase direction and associated with a single Fourier coefficient k of the sample image intensity without a phase plate, can be written as follows:

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[0043] Referring to Figure 8, a typical method 800 for restoring the image to compensate for the effects of Friedel symmetry breaking involves obtaining the image in 802 using a laser phase plate whose phase axis is aligned in the x-direction. Measured image I M(x,y) is typically a two-dimensional image, and a line image associated with any value of y can be selected, and in 804, the Fourier transform of the selected line image is determined. In 808, the left phase shift and right phase shift are determined based on the diffraction plane displacement Δ, transverse offset ΔX, and period P of the phase modulation associated with the standing wave optical field. In 808, the origin of the Fourier transform is the difference between the left phase shift and the right phase shift (i.e.,

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[0044] Typical disclosure paragraphs Paragraph 1 describes a method for CPB phase contrast imaging, comprising: positioning a CPB phase plate irradiated by a charged particle beam (CPB) from a sample; positioning the CPB phase plate such that it has a displacement Δ from the diffraction plane; and obtaining a sample image based on at least a portion of the CPB received from the CPB phase plate.

[0045] Paragraph 2 includes the subject matter of Paragraph 1 and further includes compensating the obtained sample image based on the diffraction plane displacement Δ.

[0046] Paragraph 3 includes the subject matter of any of paragraphs 1-2 and further includes determining the lateral displacement of the CPB relative to the CPB phase plate based on the obtained sample images.

[0047] Paragraph 4 includes the subject matter of any of paragraphs 1-3 and further includes adjusting the lateral position of the CPB relative to the CPB phase plate based on the determined lateral displacement.

[0048] Paragraph 5 includes the subject matter of any of paragraphs 1-4 and further specifies that adjusting the lateral position of the CPB relative to the CPB phase plate is done by deflecting the CPB.

[0049] Paragraph 6 includes the subject matter of any of paragraphs 1-5, and further specifies that the diffraction plane displacement Δ is selected such that the diffraction plane is optically downstream from the CPB phase plate.

[0050] Paragraph 7 contains the subject matter of any of paragraphs 1-6, further specifying that the CPB phase plate is a laser phase plate, and that the lateral displacement is associated with the separation of the CPB axis relative to the antinode of the standing wave optical field established by the laser phase plate.

[0051] Paragraph 8 contains the subject of any of paragraphs 1-7, further specifying that the lateral displacement is less than or equal to P / 8, where P is the period of the standing wave light field.

[0052] Paragraph 9 includes the subject matter of any of paragraphs 1 to 8 and further includes identifying at least a portion of the laser fringes in the obtained sample image, wherein the lateral displacement of the CPB relative to the laser phase plate is determined based on the portion of the laser fringes.

[0053] Paragraph 10 includes a subject from any of paragraphs 1-9 and further includes adjusting the intensity of at least a selected portion of the sample image based on the amplitude of the laser fringes.

[0054] Paragraph 11 further specifies that the diffraction plane displacement is selected such that it includes a subject from any of paragraphs 1-10, and that a portion of the laser fringes corresponds to less than P / 2.

[0055] Paragraph 12 includes the subject matter of any of paragraphs 1 to 11, further specifying that the standing wave optical field generated by the laser phase plate extends along an axis substantially orthogonal to the CPB axis and is periodic, has a period based on the wavelength of the standing wave optical field, and is selected such that the lateral displacement of the CPB axis from the antinode of the standing wave optical field is less than 1 / 10 of the period P of the standing wave optical field.

[0056] Paragraph 13 includes a subject from any of paragraphs 1 to 12 and further includes adjusting the intensity of at least a selected portion of the sample image based on the amplitude of the laser fringes.

[0057] Paragraph 14 includes the subject of any of paragraphs 1 to 13, further specifying that the CPB phase plate is a laser phase plate that defines a standing wave optical field, and further includes generating an image intensity profile by combining sample image values ​​in an image window along a direction orthogonal to the phase axis of the CPB phase plate, and determining the lateral displacement of the CPB axis from the antinode of the standing wave optical field based on the image intensity profile.

[0058] Paragraph 15 describes a charged particle beam (CPB) apparatus comprising a CPB phase plate positioned to receive a CPB from a sample, and an objective lens positioned to define a diffraction plane displaced at a distance Δ from the CPB phase plate and to form a sample image in a CPB detector, the sample image being based on the phase applied to the CPB by the CPB phase plate.

[0059] Paragraph 16 includes the subject matter of paragraph 15 and further includes an image processor capable of operating to compensate for a sample image based on the diffraction plane displacement Δ.

[0060] Paragraph 17 includes the subject of any of paragraphs 1 to 16 and further includes a controller connected to receive a sample image and determine the lateral displacement of the CPB relative to the CPB phase plate based on the sample image.

[0061] Paragraph 18 includes the subject of any of paragraphs 1-17 and further specifies that the controller is connected to the CPB deflector and is operable to adjust the lateral position of the CPB relative to the CPB phase plate using the CPB deflector based on the determined lateral displacement.

[0062] Paragraph 19 contains the subject matter of any of paragraphs 1 through 18 and further specifies that the controller is linked to determine the lateral displacement based on the sum of image intensities along a direction perpendicular to the phase axis of the CPB phase plate.

[0063] Paragraph 20 further specifies that the subject of paragraphs 1 through 19 is included, the sum of image intensities is obtained within a defined window, and the lateral displacement is obtained based on the location of the antinode of the standing wave light field determined by the sum of image intensities.

[0064] Paragraph 21 contains the subject matter of any of paragraphs 1-20, and the diffraction plane defined by the objective lens is the back focal plane or front focal plane of the objective lens.

[0065] Paragraph 22 includes the subject matter of any of paragraphs 1-21 and further specifies that the diffraction plane is located on the optically conjugate back focal plane or front focal plane of the objective lens.

[0066] Paragraph 23 includes the subject matter of any of paragraphs 1 to 22 and further includes a controller coupled to receive a sample image and adjust the sample image based on the phase difference associated with the CPB lateral offset from the node or antinode of the optical standing wave field generated by the laser phase plate.

[0067] In view of the numerous possible embodiments to which the principles of this disclosure may apply, it should be recognized that the illustrated embodiments are merely preferred examples and should not be construed as limiting the scope of this disclosure.

Claims

1. A method for charged particle beam (CPB) phase contrast imaging, Position the CPB phase plate so that the CPB from the sample crosses it, The CPB phase plate is positioned such that it has a displacement Δ from the diffraction plane, A method comprising obtaining a sample image based on at least a portion of the CPB received from the CPB phase plate.

2. The method according to claim 1, further comprising compensating the obtained sample image based on the diffraction plane displacement Δ.

3. The method according to claim 1, further comprising determining the lateral displacement of the CPB relative to the CPB phase plate based on the obtained sample image.

4. The method according to claim 3, further comprising adjusting the lateral position of the CPB relative to the CPB phase plate based on the determined lateral displacement.

5. The method according to claim 4, wherein the adjustment of the lateral position of the CPB with respect to the CPB phase plate is performed by deflecting the CPB.

6. The method according to claim 1, wherein the diffraction plane displacement Δ is selected such that the diffraction plane is optically downstream or upstream of the CPB phase plate.

7. The method according to claim 3, wherein the CPB phase plate is a laser phase plate, and the lateral displacement is associated with the separation of the CPB axis with respect to the antinode of the standing wave optical field established by the laser phase plate.

8. The method according to claim 7, wherein the lateral displacement is P / 8 or less, and in the formula, P is the period of the standing wave light field.

9. The method according to claim 7, further comprising identifying at least a portion of the laser fringes in the obtained sample image, wherein the lateral displacement of the CPB relative to the laser phase plate is determined based on the portion of the laser fringes.

10. The method according to claim 9, further comprising adjusting the intensity of at least selected portions of the sample image based on the amplitude of the laser fringes.

11. The method according to claim 10, wherein the diffraction plane displacement is selected such that the portion of the laser fringe corresponds to less than P / 2.

12. The method according to claim 7, wherein the standing wave light field generated by the laser phase plate extends along an axis substantially orthogonal to the CPB axis and is periodic, has a period based on the wavelength of the standing wave light field, and is selected such that the lateral displacement of the CPB axis from the antinode of the standing wave light field is less than 1 / 8 of the period P of the standing wave light field.

13. The method according to claim 12, further comprising adjusting the intensity of at least selected portions of the sample image based on the amplitude of the laser fringes.

14. The CPB phase plate is a laser phase plate that defines a standing wave light field, The process involves combining sample image values ​​within the image window along a direction perpendicular to the phase axis of the CPB phase plate to generate an image intensity profile, The method according to claim 1, further comprising determining the lateral displacement of the CPB axis from the antinode of the standing wave light field based on the image intensity profile.

15. A charged particle beam (CPB) device, A CPB phase plate positioned to receive CPB from the sample, An apparatus comprising: at least one CPB lens that defines a diffraction plane displaced by a distance Δ from the CPB phase plate and is positioned to form a sample image in a CPB detector, wherein the sample image is based on the phase applied to the CPB by the CPB phase plate.

16. The apparatus according to claim 15, further comprising an image processor capable of operating to compensate the sample image based on the diffraction plane displacement Δ.

17. The apparatus according to claim 15, further comprising a controller connected to receive the sample image and determine the lateral displacement of the CPB relative to the CPB phase plate based on the sample image.

18. The apparatus according to claim 17, wherein the controller is connected to a CPB deflector and is operable to adjust the lateral position of the CPB relative to the CPB phase plate using the CPB deflector based on the determined lateral displacement.

19. The apparatus according to claim 18, wherein the controller is connected to determine the lateral displacement based on the sum of image intensities along a direction perpendicular to the phase axis of the CPB phase plate.

20. The apparatus according to claim 19, wherein the sum of the image intensities is obtained within a defined window, and the lateral displacement is obtained based on the location of the antinode of the standing wave optical field determined by the sum of the image intensities.

21. The apparatus according to claim 15, wherein the diffraction plane defined by the at least one CPB lens is the back focal plane of the at least one CPB lens.

22. The apparatus according to claim 15, wherein the diffraction plane is positioned in a plane optically conjugate to the back focal plane of the objective lens.

23. The apparatus according to claim 15, further comprising a controller connected to receive the sample image and adjust the sample image based on a phase difference associated with a CPB lateral offset from a node or antinode of an optical standing wave field generated by a laser phase plate.

Images