Illumination aperture for extending sample life in helical tomography

Abstract

To provide electron beam imaging, in particular, spiral tomography.SOLUTION: An opening with a reference edge is arranged to demarcate a sample irradiation zone and a shielding zone. The sample irradiation zone includes a portion close to the shielding zone that is conjugated to a detector. The sample can be left unexposed until the sample is scanned from the shielding zone to the sample irradiation zone and as a result is properly positioned with respect to the detector for imaging. Irradiation exposure of the sample is reduced to allow for better imaging.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to imaging by an electron beam, particularly tomosynthesis.

Background Art

[0002] High-resolution images can be obtained using a transmission electron microscope (TEM) to reveal important details of many types of samples, including biological samples. In a typical application, a sample of interest is placed on an electron-transparent lamella for support, and the lamella and sample are exposed to an electron beam. Many samples of interest can be damaged or otherwise modified by exposure to the electron beam, and the dose (total charge) that can be applied without damage is limited. In some cases, multiple images of the sample are required, such as in electron beam tomography. Even when a low dose is used for each image, acquiring multiple images can damage the sample, and as a result, the ultimately acquired images may not correspond to the original sample structure. Furthermore, low-dose exposure can result in noisy images. An improved approach is needed that enables satisfactory imaging without causing unacceptable sample damage.

Summary of the Invention

[0003] The method involves defining an irradiation zone using a beam limiting aperture and positioning a detector to receive radiation from a detector zone within the irradiation zone, the detector zone being conjugate to the detector active region. A first portion of the sample is positioned within the detector zone, and at least a second portion of the sample is positioned outside the irradiation zone. A first image of the first portion of the sample is generated based on the radiation received by the detector when the first portion of the sample is positioned within the detector zone. The sample is translated so that a second portion of the sample is positioned within the detector zone, and a second image of the second portion of the sample is generated based on the radiation received by the detector when the second portion of the sample is positioned within the detector zone. In the example, the sample is translated so that a third portion of the sample, positioned outside the irradiation zone, is moved into the detector zone, and a third image of the third portion of the sample is generated based on the radiation received by the detector when the third portion of the sample is positioned within the detector zone.

[0004] In a typical example, multiple translations of the sample are applied to position each of several additional parts of the sample located outside the irradiation zone within the detector zone. An image of each additional part is generated based on the radiation received by the detector when each additional part of the sample is positioned within the detector zone. In some examples, the radiation is generated by a charged particle beam. In a typical example, the detector has at least one edge, and a beam limiting aperture defines the conjugate edge of the detector zone, and the first and second parts of the sample are translated into the detector zone at the conjugate edge. The beam limiting aperture may include an arc-shaped peripheral portion that defines the portion of the irradiation zone extending beyond the detector zone. At least one part of the sample can be scanned from the detector zone to the portion of the irradiation zone extending beyond the detector zone. A shielding region can be conjugate to a portion of the detector active region. Multiple images of at least the first and second parts of the sample can be generated at multiple rotation angles. The first and second parts of the sample can step into the detector zone or scan continuously into the detection zone.

[0005] A charged particle beam system includes a charged particle beam source operable to generate a charged particle beam and an aperture plate defining a beam-limiting aperture having a straight edge. The aperture plate is positioned to receive the charged particle beam and generate a sample irradiation zone corresponding to the beam-limiting aperture. A detector is positioned to receive radiation from a detector zone in the irradiation zone. The detector zone is conjugate to the detector active region and has a detector zone edge conjugate to the straight edge of the beam-limiting aperture. The detector is positioned to generate an image of the sample portion introduced into the detector zone at the detector zone edge. In the example, the charged particle beam is an electron beam, and the detector is operable to generate an image based on the portion of the electron beam scattered, reflected, or transmitted by the sample. The beam-limiting aperture may have an arc-shaped edge terminating at the straight edge. The sample irradiation zone may include a region extending beyond the detector zone. The detector may be operable to generate an image when a first portion of the sample is introduced into the detector zone and a second portion of the sample exits the detector zone. The detector can be positioned such that the detector zone edge is conjugate to the edge of the detector active region. The linear edge is positioned to define the shielding region so that the sample is introduced from the shielding region into the detector zone. The sample stage can be operated to translate the sample into the detector zone and rotate the sample about at least one axis, and the detector can be operated to generate images in multiple or rotational directions.

[0006] The method includes defining the shielding zone and the sample irradiation zone by the reference edge of the beam limiting aperture, and positioning the detector such that at least a portion of the sample irradiation zone is conjugated to the detector. The image of the sample is acquired by using the detector to introduce the sample from the shielding zone to the sample irradiation zone at the reference edge.

[0007] 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]

[0008] [Figure 1] This figure shows a typical TEM system that includes a beam limiting aperture with a protective edge based on the shape of the electron detector. [Figure 1A] This figure shows a beam limiting aperture as shown in Figure 1. [Figure 2A] This figure shows sample exposure using a beam-limiting aperture (BLA). [Figure 2B] This figure shows sample exposure using a beam limiting aperture. [Figure 2C] This figure shows sample exposure using a beam limiting aperture. [Figure 2D] This figure shows sample exposure using a beam limiting aperture. [Figure 3A] This diagram shows a typical beam limiting aperture. [Figure 3B] This diagram shows a typical beam limiting aperture. [Figure 3C] This diagram shows a typical beam limiting aperture. [Figure 3D] This diagram shows a typical beam limiting aperture. [Figure 3E] This diagram shows a typical beam limiting aperture. [Figure 3F] This diagram shows a typical beam limiting aperture. [Figure 4] This figure shows a TEM system configured to align beam liming apertures that are positioned and configured based on the shape of the electron detector. [Figure 5] This diagram shows typical methods. [Figure 6] This figure shows a typical optical system in which the BLA is optically imaged in a manner conjugated to the detector. [Modes for carrying out the invention]

[0009] Introduction Methods and apparatus for reducing radiation exposure to a sample, thereby reducing damage or alteration of the sample and enabling the acquisition of multiple images, are described below. These approaches are particularly useful for electron beam imaging of biological samples that cannot tolerate exposure to the electron beam. In some applications, such samples are columnar and require multiple image exposures to produce a satisfactory image. By protecting the unimaging portion of the sample from radiation exposure, longer exposures can be used to obtain more excellent images. The examples below are based on image acquisition with a TEM for electron beam tomography for ease of explanation. Tomographic imaging may require multiple images of samples at many different tilt angles, and consequently, dose reduction is particularly useful in this application. The disclosed methods and apparatus can be used in other charged particle beam (CPB) imaging systems or in imaging systems using electromagnetic radiation such as ion beams or X-rays, ultraviolet, visible light, infrared, or other optical radiation provided by lasers, LEDs, or other sources. Imaging can be based on reflected or scattered radiation (including secondary electron emission in CPB systems), but transmission imaging is used for clarity in the explanation. Examples are disclosed, typically with reference to a beam limiting aperture (BLA) located within an optical system such as a TEM electron beam system, which defines the range of the beam incident on the sample, and therefore the portion of the detector that receives the beam portion from the sample. Radiation refers to electromagnetic radiation as described above, as well as neutral particle beams and charged particle beams.

[0010] As used herein, "image" refers to a visual image presented for viewing by a technician or another person using a display device, or stored data associated with a visual image, such as data stored in JPG, TIFF, BMP, RAW, or other formats. Images are typically rectangular, but other shapes may be used, and images may have curves or other borders and various shapes.

[0011] The detectors used in the disclosed approaches are generally selected based on the type of radiation to be detected. Often, detector arrays containing a rectangular array of radiation-sensitive elements are available. The detector generally includes a radiation-sensitive region, as defined by the array of radiation-sensitive elements, and may include circuits for the operation of the detector elements, such as bias or amplifier circuits, the relevant parts of which are generally not sensitive to incident radiation. Unless otherwise specified, the detector active region as used herein refers to the portion of the detector that responds to incident radiation. While integrated detector arrays are convenient, single-element or other detector array configurations can also be used.

[0012] In these examples, the aperture is defined within a substrate such as a metal substrate (gold, platinum, tungsten, or other metals), a semiconductor (silicon, etc.), or an insulator (ceramic, etc.). When used with a CPB system, semiconductor or insulating substrates generally require a conductive layer to eliminate or reduce static charge. The material and thickness of the aperture substrate are selected to significantly attenuate incident radiation. When used herein, the aperture substrate reduces the radiation intensity by a factor of at least 0.75, 0.5, 0.25, 0.1, 0.05, 0.01, 0.005, 0.001, or greater. Greater attenuation allows for superior sample shielding.

[0013] In these examples, the image is generally a projection image obtained by directing a beam of radiation onto the sample and measuring the transmittance passing through the sample without additional imaging elements such as lenses. As used herein, “sample scanning” refers to a continuous, stepwise, or other iterative (usually translational) movement of the sample into the irradiation zone to expose previously shielded areas of the sample for imaging. While scanning the sample is generally convenient, the irradiation zone can be scanned as well, and “irradiation zone scanning” refers to a continuous, stepwise, or other iterative (usually translational) movement of the irradiation zone relative to the sample to expose previously shielded areas of the sample for imaging. Images are typically acquired continuously, periodically, or at other times during scanning. The sample may also be tilted or rotated during scanning for tomographic imaging.

[0014] Where used herein, planes or other locations being imaged relative to each other are referred to as conjugate to each other, or more simply as conjugate. In some examples, features of one or more objects, such as apertures or edges, are imaged at corresponding conjugate locations, and these features are referred to as conjugate. Conjugate locations can be established by imaging using one or more optical elements placed between the locations. Locations from which the silhouette of an object is generated may also be referred to as conjugate. For example, an aperture that is collimated or illuminated by a diverging radiation beam can be positioned to generate silhouettes of the aperture at multiple conjugate locations.

[0015] Samples of various shapes can be imaged as disclosed herein. For example, the sample may be lamellar, but other shapes such as cylinders, columnar, or needles can also be imaged.

[0016] Example 1 Referring to FIG. 1, a representative transmission electron microscope system 100 includes an electron beam source and associated condenser optics 102 that direct an electron beam 104 at an aperture substrate 106 that defines a beam limiting aperture 108 along axis 103. The electron beam 104 is shown as a divergent beam for purposes of illustration, but can diverge, converge, or be collimated during propagation. At the aperture substrate 106, the beam limiting aperture 108 defines a transmitted electron beam 110 that propagates to a sample 112 disposed on a stage 114 that can scan and tilt the sample with respect to the transmitted electron beam 110. Thus, the transmitted electron beam 110 defines an irradiation zone in the sample 112, i.e., a region arranged to receive radiation as limited by the beam limiting aperture 108. Next, the transmitted electron beam 110 propagates toward a detector 116 after interacting with the sample 112. In a typical embodiment, a stage controller 118 is operable to scan the sample 112 in a direction 120 through the transmitted electron beam 110. In the case of tomographic imaging, the stage controller 118 can also provide a range of tilts, which are not shown in FIG. 1.

[0017] In one example, the stage controller 118 moves the stage 114 to translate the sample 112 in a stepwise or continuous manner in the direction 120 and, at each step, acquire a series of sample images at a range of tilt angles. The aperture 108 is arranged such that a first aperture edge 122 (referred to as the “front edge”) corresponds to a detector edge 124. The aperture edge 122 can be referred to as being “projected” onto the detector edge 124, or the aperture edge 122 and the detector edge 124 can be referred to as being conjugate or optically conjugate. In this configuration, the sample portion 126 scanned by the transmitted beam portion 110 is not exposed for imaging until it is positioned with respect to the detector 124 while the portion 129 is exposed. Any exposure prior to the imaging exposure does not contribute to the imaging but can cause damage to the sample. The detector 116 is coupled to an image processor 138 that can control the acquired images in terms of storage, synthesis, processing, communication, or other modalities.

[0018] In this example, the transmission electron beam 110 includes an imaging portion 130 corresponding to the detector 116 and a post-imaging portion 132 that is incident on the sample portion 128 that has previously been scanned for imaging. These beam portions irradiate the sample in the irradiation zone 131. The additional exposure by the post-imaging portion 132 can modify the sample, but the imaging of the sample portion 128 has already been completed, and this sample damage does not degrade the sample image.

[0019] The first aperture edge 122 is preferably arranged to correspond to the edge 124 of the detector 116. A typical array detector has a straight boundary, and as a result, the first aperture edge can be made straight. FIG. 1A is a schematic view of the aperture substrate 106 along the axis 103 showing the relative positions of the sample 112, the aperture 108, and the detector 116. Through the aperture 108, the exposed portion 129 of the sample is visible, while the portion 126 to be exposed and the previously exposed portion 129 are hidden. The position of the detector 116 is shown, indicating that only a part of the exposed portion 129 of the sample is imaged and the remaining part has been imaged previously.

[0020] Example 2 Figures 2A to 2D show the irradiation of a sample 206 scanned through an irradiation zone 204 defined by an aperture in the aperture substrate 200. The irradiation zone 204 includes a detector zone 205, which corresponds to a detector active region 207 that is imaged into the irradiation zone by one or more optical elements or by close focusing, or otherwise arranged and configured such that a sample located in the detector zone 205 is imaged by the detector. As used herein, the detector zone 205 of the irradiation zone 204 is referred to as conjugate to the detector active region, and is referred to as a conjugate region or detector conjugate region. In this example, the detector zone 205 corresponds substantially to the entire detector region, and the aperture edge 208 corresponds to the edge of the detector active region. In other examples, a portion of the detector zone is shielded by the aperture substrate 200 and does not extend into the irradiation zone 204. For convenience of explanation, the irradiation zone 204 can be referred to as corresponding to or conjugate to the detector plane, and the detector plane includes a portion corresponding to the detector active region. In this example, the aperture substrate 200 includes a notch 210 or other features that can be used to align the aperture edge 208 with respect to the detector edge.

[0021] In Figure 2A, sample 206 (scanned in direction 212) has a portion 206B that extends into the detector zone 205 of the irradiation zone 204 and a portion 206A that is shielded by the aperture substrate 200. In Figure 2B, sample 206 is further scanned into the irradiation zone 204, introducing the previously shielded portion, resulting in a larger portion 206B within the detector region 204 and a smaller shielded portion 206A. In Figure 2C, sample 206 is further scanned into the irradiation zone 204, so that portion 206B extends across the entire detector zone 205, and portion 206C extends beyond the detector zone 205, scanning from the detector zone to a portion of the irradiation zone that extends beyond the region conjugate to the detector. Portion 206C can be imaged as it passes through the detector zone 205, and radiation-induced damage or changes generated during additional exposure do not degrade the sample image. In Figure 2D, sample 206 is scanned such that portion 206D is shielded by the aperture substrate, while imaging of portion 206B within detector zone 205 can continue. Sample 206 can be scanned to pass through detector zone 205 completely, but such scanning is not shown.

[0022] Example 3 Figures 3A–3E show typical apertures that can provide reduced dose during imaging. For ease of explanation, the location of sample 302 is shown, which is conjugate to the detector active region and includes location 304, currently exposed location 305, and location 306 to be exposed relative to the detector zone 307 having a margin 308, as if it had been previously exposed. In these examples, the exposure zone 309 is larger than the conjugate detector region, but in other examples, the exposure zone can be selected to coincide with the detector region.

[0023] Referring to Figure 3A, the aperture plate 310 defines an aperture 312 having a straight edge 313 (used as the leading edge in sample exposure) and an arcuate edge 314, which in this example is part of arc A. Notches 319 or other reference markings or features can be provided to allow alignment of the straight edge 313 with the detector edge 308. Figure 3B shows the aperture plate 310 offset so that the edge 313 is inclined relative to the detector zone 307. Nevertheless, this arrangement can allow for dose reduction.

[0024] Referring to Figure 3C, the aperture plate 320 defines a polygonal (hexagonal) aperture 322 having a straight edge (front edge) 323 (used as the front edge in sample exposure) and a polygonal edge 324. In this example, the edge 308 of the detector zone 307 is parallel to the front edge 323 but is displaced from the front edge 323. Referring to Figure 3D, the aperture plate 330 defines an irregular aperture 332 having a straight edge 333 (used as the front edge in sample exposure) and an irregular edge 334. In this example, the edge 308 of the detector zone 307 is not precisely aligned with the front edge 333. Referring to Figure 3E, the aperture plate 340 defines an irregular aperture 342 having a serrated edge (front edge) 343 (used as the front edge in sample exposure) and an irregular edge 344. In this example, the edge 308 of the detector zone 307 is not aligned with the leading edge 343.

[0025] Referring to Figure 3F, the aperture plate 350 defines a polygonal aperture 352 having a straight front edge 353 and a straight rear edge 354. In this example, the edges 353 and 354 correspond to the edges 308 and 311 of the detector zone 307.

[0026] Example 4 Referring to Figure 4, a typical electron-optical system 400 includes an electron source / focusing optical system 402 that directs an electron beam 404 along axis 401 to an aperture plate 406. The aperture plate 406 defines a beam-limiting aperture 408 that restricts the electron beam 404 to produce an aperture electron beam 410. The aperture electron beam 410 propagates onto the sample 412. Sample portions 414, 416 are not exposed due to limitation by the aperture 408 defining a shielding zone 470. The sample is exposed within the irradiation zone defined by the aperture beam 410. As used herein, the irradiation zone is the zone irradiated in the plane of the sample. The propagation of a peripheral beam portion 405 is shown along with the propagation of this beam portion in the absence of an aperture plate, as shown in 407. Only sample portions 418, 419 are exposed, but only the exposed portion 418, corresponding to the imaging portion 422 of the aperture electron beam 410, is imaged by the detector 426. The sample 412 can be scanned or stepped in direction 428, for example, using the stage 430, so that exposure of the sample portion to the electron beam begins when it is properly positioned relative to the detector 426 for imaging.

[0027] The beam can be shaped and aligned with the detector 426 using beam deflectors, electron lenses, or other beam manipulation elements such as those shown in 440 and 442. For example, the alignment control unit 464 can be used to control elements 440 and 442 to align the aperture electron beam 410. Additionally or alternatively, the alignment control unit may be coupled to a stage 450 that moves the aperture plate 406, a sample stage 430 coupled to move the sample 412, or a detector stage 452 that can position the detector 426. A controller 460 is coupled to the sample stage 430 to scan and tilt the sample 412 for imaging, and an image processor 462 is coupled to the detector 426 to receive and process the sample image.

[0028] Example 5 Referring to Figure 5, a typical method 500 includes selecting a beam liming aperture (BLA) having a reference edge, at least in 502. The reference edge of the BLA is selected to reduce the sample radiation dose before imaging. In 504, the BLA is defined within an aperture substrate such as a metal or conductive coated insulator. In 506, the BLA is positioned to be conjugate to a detector, for example, in a charged particle beam imaging system. In 508, the reference edge of the BLA is mechanically aligned with the detector by translation of one or more of the BLA, BLA stage, sample substrate stage, or detector, or is aligned using a charged particle beam optics system. In 510, the sample is scanned and imaged.

[0029] Example 6 The above example illustrates dose control by BLA without the use of optical elements to re-image the BLA at the sample location. Referring to Figure 6, a typical optical system 600 (using charged particles or electromagnetic radiation) positioned along axis 601 includes an aperture plate 604 that defines an aperture 605 positioned to receive a beam 602 from a suitable light source (not shown in Figure 6). A lens 606 re-images the aperture 605 to form an exposure zone 615, and for illustrative purposes, the corresponding image 614 of the aperture plate 604 is shown. Sample portion 616 is exposed, while sample portions 617, 619 are not. The exposed sample portion 616 (or part thereof) is imaged onto a detector 624 by a lens 620, and images 622, 626 correspond to the aperture plate 604 and the exposed sample portion 616, respectively. As discussed above, the sample can be scanned and / or tilted as shown in 613 to produce an image suitable for tomographic reconstruction. Typical ray paths 610 and 611 are shown for illustrative purposes. This optical configuration can be made up of a charged particle beam optical element, an optical microscope, or other optical configurations.

[0030] In light of the numerous possible embodiments to which the principles of the disclosed technology may be applied, it should be understood that the embodiments shown are merely examples and should not be considered as limiting the scope of the invention. Accordingly, all that is contained in the appended claims and ideas is claimed.

Claims

1. A method for implementing a charged particle beam system, The steps include defining the irradiation zone using a beam limiting aperture, The steps include: positioning a detector so as to receive radiation from the detector zone of the irradiation zone, wherein the detector zone is projected onto the detector active region; The steps include positioning a first portion of the sample within the detector zone, wherein at least a second portion of the sample is positioned outside the irradiation zone, A step of generating a first image of the first portion of the sample based on the radiation received by the detector when the first portion of the sample is positioned within the detector zone, The steps include translating the sample such that the second portion of the sample is positioned within the detector zone, A step of generating a second image of the second portion of the sample based on the radiation received by the detector when the second portion of the sample is positioned within the detector zone, Methods that include...

2. The above method further, A step of translating the sample so that a third portion of the sample located outside the irradiation zone is moved to the detector zone, A step of generating a third image of the third portion of the sample based on the radiation received by the detector when the third portion of the sample is positioned within the detector zone, The method according to claim 1, including the method described in claim 1.

3. The above method further, The steps include applying multiple translations of the sample in order to position each of the multiple additional portions of the sample located outside the irradiation zone within the detector zone, The steps include generating an image of each of the plurality of additional parts based on the radiation received by the detector when each additional part of the sample is positioned within the detector zone, The method according to claim 1 or 2, including the method according to claim 1 or 2.

4. The detector has at least one edge, The beam limiting aperture defines the edge that is projected onto the detector zone, The first and second portions of the sample are translated into the detector zone at the projected edges. The method according to any one of claims 1 to 3.

5. The beam limiting aperture includes an arc-shaped peripheral portion that defines the portion of the irradiation zone that extends beyond the detector zone, The method according to claim 4.

6. At least a portion of the sample is scanned from the detector zone to the portion of the irradiation zone that extends beyond the detector zone. The method according to claim 5.

7. The shielding region of the irradiation zone is projected onto a part of the detector active region. The method according to claim 6.

8. The above method further, A step of generating multiple images of at least the first and second portions of the sample at multiple rotation angles, The method according to any one of claims 1 to 7, including the method described in any one of claims 1 to 7.

9. The first and second portions of the sample are stepped into the detector zone or continuously scanned into the detector zone. The method according to claim 8.

10. The above method further, The step of directing a charged particle beam towards the beam limiting aperture in order to irradiate the irradiation zone, Based on the radiation from the charged particle beam, the first image and the second image are generated. The method according to any one of claims 1 to 9.

11. A charged particle beam system, A charged particle beam source capable of generating a charged particle beam, An aperture plate defining a beam limiting aperture having a straight edge, wherein the aperture plate is positioned to receive the charged particle beam and generate a sample irradiation zone onto which the beam limiting aperture is projected, A detector arranged to receive radiation from a detector zone of the sample irradiation zone, wherein the detector zone is projected onto a detector active region and has a detector zone edge onto which the linear edge of the beam limiting aperture is projected, and the detector is arranged to generate an image of the portion of the sample introduced into the detector zone at the detector zone edge, A system equipped with these features.

12. The beam limiting aperture has an arc-shaped edge that terminates at the straight edge, The straight edge portion is positioned such that it defines the shielding region so that the sample is introduced from the shielding region into the detector zone. The system according to claim 11.

13. The first portion of the sample is introduced into the detector zone. When the second portion of the sample leaves the detector zone, the detector is capable of generating the image. The system according to claim 11 or 12.

14. The detector is positioned such that the edge of the detector zone is projected onto the edge of the detector active region. The system according to any one of claims 11 to 13.

15. The aforementioned system further, The sample stage comprises a sample stage that is operable to translate the sample into the detector zone and rotate the sample about at least one axis, The detector is capable of generating images in multiple or rotating directions. The system according to any one of claims 11 to 14.

Images