Transmission electron microscope with variable effective focal length

Abstract

To provide a transmission electron microscope (TEM) with a variable effective focal length.SOLUTION: In the TEM, each of a first port and a second port is configured to receive a wave front manipulating device for manipulating the wave front of a beam. Each of the first port and the second port is located in a different one of a projection assembly region, a first lens region and a second lens region. A controller is configured to control excitation of a first lens and a second lens. In a first mode, the controller controls the first and second lenses to direct the diffraction pattern into a second diffraction plane where the second diffraction plane is coincident with a first port plane. In a second mode, the controller is configured to control the first and second lenses to direct the diffraction pattern into a third diffraction plane, where the third diffraction plane is coincident with a second port plane.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present disclosure relates to the field of transmission electron microscopy.

Background Art

[0002] In a transmission electron microscope (TEM), a beam of electrons generated by an electron source is formed into a beam of electrons that illuminates a sample. The sample is very thin, and thus, some of the electrons pass through the sample without being scattered, and some of the electrons are scattered by the sample. Some of the scattered electrons are scattered elastically within the sample, while other scattered electrons are scattered inelastically, and the scattered electrons exit the sample at an angle different from the angle at which they enter the sample. By imaging the sample on a detector such as a fluorescent plate or a CCD camera, the intensity changes in the image plane can be recorded. The intensity fluctuations are partly due to the absorption of some of the scattered electrons within the microscope, and partly due to the interference between the scattered electrons and the unscattered electrons. In thin low-impedance materials such as biological tissues, phase contrast is dominant, and this contrast is mainly caused by an interference process involving elastically scattered electrons. Such materials are usually called phase objects.

[0003] The contrast of the image resulting from the interference of electrons depends on the angle at which the electrons are scattered and the amount of focus blur applied. Object features having a particular spatial frequency scatter the beam at a particular angle, and the scattering angle is proportional to the spatial frequency. When imaging a phase object that is exactly in focus, the contrast is close to zero for all spatial frequencies. When applying focus blur, at low spatial frequencies, the scattering angle is close to zero and the contrast remains close to zero. At higher spatial frequencies, the contrast varies between positive and negative contrast depending on the spatial frequency. This contrast variation is described by the so-called Contrast Transfer Function (CTF), which oscillates like a sine function and starts at zero for zero spatial frequency. Since the CTF is close to zero for low spatial frequencies, large structures do not appear well in the image.

[0004] In 1947, Boersch explained that introducing a phase plate results in a CTF where low spatial frequencies show a maximum value, and thus large structures can be imaged better. See "Ueber die Kontraste von Atomen im Elektronenmikroskop", H. Boersch, Z. Naturforschung 2A (1947), p. 615 - 633. Such phase plates have been successfully introduced into TEMs.

[0005] The phase plate is a structure placed in the so-called back focal plane, where the parallel beam illuminating the sample is focused by the objective lens after passing through the sample. A diffraction pattern is formed in this plane. All unscattered electrons are focused to a single point, but scattered electrons reach other positions. The phase plate, for example, causes a phase shift of π / 2 between scattered and unscattered electrons, thereby converting the sine-like behavior of the CTF to a cosine-like behavior.

[0006] The phase plate can also be arranged in a plane conjugate to this first diffraction plane. In such a conjugate plane, the diffraction pattern can be enlarged (reduced) by a factor M. Such a plane is defined as M times the focal length of the objective lens, the so-called effective focal length f eff which has. Using the phase plate within such a conjugate plane to create more space for the placement of the phase plate and having the freedom in selecting the optimal value of f eff can be advantageous. A large value of f eff usually improves the onset of the cosine-like behavior of the CTF (the "cutoff frequency").

[0007] Currently, phase plates based on CW lasers (Laser Phase Plates; LPPs) are being investigated for use in transmission electron microscopes, but due to the size of the LPP hardware near the electron beam and the risk of contamination of the delicate laser optical elements near the sample region, their use within the objective lens region has become difficult. Furthermore, LPPs require a large value of f eff to have a sufficiently low cutoff frequency. Therefore, it is attractive to place the LPP in a plane conjugate to the back focal plane.

[0008] In addition to the use of phase plates within the TEM, it is known to incorporate an image corrector, which corrects the aberrations of the imaging optical system and thereby improves the image quality of the TEM.

[0009] For example, the correction system can be arranged between the objective lens and the projection system of the TEM. The correction system can include a transfer system in the form of a lens doublet, which images the back focal plane of the objective lens onto a hexapole lens. The hexapole lens is then imaged onto another hexapole lens by the lens doublet, and finally, an adapter lens is used to form an image of the object at the entrance of the projection system of the TEM.

[0010] Typically, there is little or no flexibility with respect to the magnification achievable for any of these prior art conjugate diffraction planes. The microscope design is adjusted to achieve one particular magnification.

[0011] The phase plate can also incorporate an amplitude mask in order to partially or completely block the electron wave in the masked area. Such an amplitude mask can serve to improve the phase contrast in the image. Thus, the phase plate is used to manipulate the electron exit wavefront so as to optimize the phase contrast in the detector plane or to enable optimal recovery of the phase contrast after several image processing steps. SUMMARY OF THE INVENTION

[0012] In view of this background, a transmission electron microscope according to claim 1 is provided.

[0013] In this way, the first diffraction plane can be relayed to one of a plurality of alternative diffraction planes where a wavefront manipulation device such as a laser phase plate can be deployed, reducing problems related to device size or device contamination. Further, the first diffraction plane can be magnified and better matched to the size of the phase shift region as provided by the wavefront manipulation device by selecting an optimum value of the effective focal length. This results in a lower cut-off frequency of the spatial frequency of the sample for which contrast enhancement is obtained by the use of the wavefront manipulation device. Further, by providing at least three lenses and two ports configured to receive the wavefront manipulation device, a series of different magnifications can be achieved using a single transmission electron microscope operating in different configurations using different lens excitations.

[0014] Thus, in addition to a modest addition of hardware in the form of an additional port for the wavefront manipulation device, a more versatile TEM can be obtained that can provide a variety of different magnifications of the first diffraction plane by providing associated lens control. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Here, embodiments of the present disclosure will be described with reference to the following drawings.

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Embodiments for Carrying Out the Invention

[0016] FIG. 1 shows a schematic diagram of a prior art TEM system 100 including an electron source 10 that emits an electron beam 11 toward a condenser optical system 12 along an optical axis 110. The electron source 10 can generate high energy electrons, i.e., electrons having a typical energy of about 10 keV to 1,000 keV. The condenser optical system 12 can include one or more condenser lenses and one or more apertures. A deflector 19 positioned downstream of the condenser optical system 12 shifts and / or tilts the electron beam with respect to the optical axis 110. A front sample objective lens 16 positioned downstream of the deflector 19 collimates the electron beam and directs the electron beam onto a sample 14. The sample 14 can be held by a sample stage 13 within a sample plane 111. In some examples, the sample is positioned on a TEM grid attached to the sample stage. The sample stage 13 can adjust the sample position by tilting the sample with respect to the optical axis and / or by translating the sample within the sample plane. The unscattered electrons and scattered electrons exiting the sample 14 sequentially pass through a post-sample objective lens 123 and a projector system 21 and are collected by a detector 25 positioned on the opposite side of the sample 14 with respect to the electron source 10. The projection system 21 performs different operations in an imaging mode and a diffraction mode. The detector 25 can detect the received electrons and send a signal to an image processor 24 to form an image. The detector 25 can include an amplifier for amplifying the signal before sending the signal to the image processor 24. In one example, the detector 25 can be a CCD camera or a CMOS camera. In some embodiments, different detectors can be used for diffraction pattern acquisition and sample image acquisition.

[0017] FIG. 1 shows a TEM system 100 operating in an SA (selected aperture) imaging mode and an SA diffraction mode (however, in reality, only one mode is possible at a time). The dashed line 41 indicates the beam path of the unscattered electrons from the sample 14 to the detector 25 in the SA diffraction mode. When the sample 14 is illuminated in parallel, the unscattered electrons are focused on the back focal plane 43, and the scattered electrons can form an electron diffraction (ED) pattern on this plane. In the SA diffraction mode, the projector system 21 images the back focal plane 43 of the post-sample objective lens 123 onto the detector 25. The beam stopper 17 can be inserted into the optical axis 110 to block the unscattered beam and protect the detector from damage by this strong beam, but the scattered electrons can still reach the detector. The dashed line 42 indicates the beam path of the scattered electrons from the sample 14 to the detector 25 in the SA imaging mode. In the SA imaging mode, the sample plane 111 is imaged onto the SA plane 44, and the projector system 21 images the SA plane 44 onto the detector 25. The beam stopper 17 is retracted from the optical axis 110. In one example, an SA aperture can be inserted into the beam path. The SA aperture can be positioned within the SA plane 44. Alternatively, the condenser aperture within the condenser optics 12 can function as a beam-limiting aperture. In another example, an image deflector 45 is positioned between the sample and the detector to shift and tilt the electrons transmitted through the sample back to the optical axis so that the ED pattern remains at the center of the detector during beam tilt and the image remains at the center of the detector during beam shift. The image deflector 45 can be positioned between the back focal plane 43 and the SA plane 44. In some embodiments, the TEM system does not include a beam stopper, and the detector receives the non-scattered beam.

[0018] Controller 30 can control the operation of the TEM system 100 manually in response to an operator command or automatically according to computer-readable instructions stored in a non-transitory memory (or computer-readable medium) 32. Controller 30 includes a processor and can be configured to execute computer-readable instructions to control various components of the TEM system 100 in order to implement any of the methods described herein. For example, the controller can adjust the TEM system to operate in different modes by adjusting one or more of the apertures, the strength of the objective lens 123, the beam stopper 17, and the projector system 21. Controller 30 can adjust the beam position and / or the beam incident angle on the sample by adjusting the deflector 19. Controller 30 can adjust the electron dose rate on the sample by adjusting one or more of the settings of the electron source or the illumination optics. Controller 30 can adjust the magnification by adjusting the projector system 21. Controller 30 can be further coupled to a display 31 to display notifications and / or signals detected by the detector 25. Controller 30 can receive user input from a user input device 33. The user input device 33 can include a keyboard, a mouse, or a touch screen.

[0019] FIG. 2 shows a schematic diagram of a portion of a prior art TEM system including a correction system 330 and a phase plate 340.

[0020] The correction system 330 can be used to correct the aberrations of the objective lens 305 and can also correct some of the aberrations introduced by the corrector system.

[0021] The sample is positioned on the optical axis 301 within the sample plane 302 and imaged by the objective lens 305. The sample is disposed near the front focal plane of the objective lens 305. Two principal rays are shown in this figure. The axial ray 303 is incident at an angle from the sample center where the microscope optical axis intersects the sample, and the field ray 304 is incident from an off-axis point on the sample parallel to the optical axis. The ray 304 blocks the optical axis at the rear focal plane 306 of the objective lens 305. The correction system 330 is disposed around the optical axis 301.

[0022] Using the last lens of the correction system 330, a diffraction plane can be formed at the position of the phase plate 340. This results in a first image plane 314 behind the corrector. An additional lens 341 can be used to transfer this image plane to the SA plane 323 in front of the projection system.

[0023] Note that the objective lens 305 is shown here (and also in FIG. 1) for the sake of convenience as a thin lens, and the sample plane 302 is outside the lens itself. Many TEMs use a thick lens as the objective lens 305, and the sample position is disposed within the field (magnetic field) of the objective lens 305.

[0024] In the configuration of FIG. 2, the correction system 330 is disposed entirely between the phase plate 340 and the objective lens 305. The correction system 330 is configured to form an image of the rear focal plane of the objective lens 305 on the phase plate.

[0025] An additional lens 341 can be used to form an image of the plane 314 on the plane 323, and the plane 323 serves as the SA plane in front of the projection system.

[0026] FIG. 3 shows a schematic view of a part of a TEM system having an additional lens 343 between the correction system 330 and the phase plate 340, with the correction system 330 and the phase plate 340 of the prior art in FIG. 2. In this configuration, an image of the sample is formed on the plane 342.

[0027] Provide a further method for varying the magnification of the rear focal plane on the plane of the phase plate.

[0028] It is recognized that the present disclosure can be provided by having a series of lenses in which a larger range of magnifications can be selectively developed and two or more ports for receiving the phase plate such that the phase plate can be movable between a plurality of locations.

[0029] FIG. 4 shows a transmission electron microscope including a plurality of phase plate receiving locations 442, 443, 444, 445, 446 according to a first aspect of the present disclosure. Each phase plate receiving location 442, 443, 444, 445, 446 includes a phase plate receiving port 442, 443, 444, 445, 446. Each phase plate receiving port 442, 443, 444, 445, 446 is configured to receive the phase plate receiving location 442, 443, 444, 445, 446 on the optical axis 110 at a position conjugate to the first diffraction plane 430. These are referred to as phase plate receiving ports 442, 443, 444, 445, 446, but it should be noted that these can be configured to receive any wavefront operation device 440 and can thus be referred to as wavefront operation device receiving ports 442, 443, 444, 445, 446.

[0030] This configuration includes an electron source (not shown in FIG. 4) configured to generate a beam of particles along the optical axis 110. This configuration includes a sample holder 402 perpendicular to the optical axis 110, a primary lens 405 (also referred to as an objective lens 405), and a projection assembly 412. This configuration further includes an intermediate lens assembly 408 between the objective lens 405 and the projection assembly 412. This configuration also includes an illumination optical system (not shown).

[0031] The intermediate lens assembly 408 includes a plurality of lenses 410, 411. The first lens 410 of the intermediate lens assembly 408 is located within the first lens region. The second lens 411 of the intermediate lens assembly 408 is located within the second lens region.

[0032] The projection assembly 412 occupies the projection lens region. The projection assembly 412 may include a plurality of lenses (not shown).

[0033] In use, the sample is positioned within the sample holder 402 in the sample plane on the optical axis 110 near the front focal plane region of the objective lens 405. The sample is imaged by the objective lens 405, and a diffraction pattern is generated within the first diffraction plane 430. The intermediate lens assembly 408 is configured to receive the electron beam after it exits the primary lens and is configured to generate an enlarged image of the sample present within the sample holder. The projection assembly 412 is configured to receive the enlarged image of the sample.

[0034] The intermediate lens assembly 408 includes at least some of a plurality of phase plate receiving ports 442, 443, 444, 445, 446.

[0035] Each phase plate receiving port 442, 443, 444, 445, 446 may include an inlet and an outlet. This is because in the case of a laser-based wavefront manipulation device 440 suitable for use with the present disclosure, the outlet port is convenient for diagnosing the output laser light.

[0036] In the case of a non-laser-based wavefront manipulation device, it may be easier to deploy the biprism using a port that has only an inlet and no outlet.

[0037] This configuration may further include an additional (optional) lens 409 between the objective lens 405 and the intermediate lens assembly 408.

[0038] The additional lens 409 may be selectively deployable such that it may be deployed in some modes and not deployed in other modes.

[0039] Just as not all lenses are required, not all of the phase plate receiving ports 442, 443, 444, 445, 446 are necessarily required either. However, the minimum number of phase plate receiving ports 442, 443, 444, 445, 446 is two, allowing for two or more options for inserting the phase plate 440 or other wavefront manipulation device 440.

[0040] In this way, by providing two or more options for the placement of the wavefront manipulation device 440, with each option being spaced apart, and by appropriate deployment of the lenses 409, 410, 411, a series of different magnifications of the first diffraction plane can be achieved, as further explained below.

[0041] FIG. 5 shows a transmission electron microscope including a plurality of phase plate receiving ports 442, 443, 444 according to a second aspect of the present disclosure. Many of the components of the second aspect of the present disclosure are the same as those of the first aspect of the present disclosure, and thus, the description of these components will not be repeated.

[0042] In the configuration of FIG. 5, one or more of the lenses of the intermediate lens assembly 408 are replaced with a correction optical assembly 450, and an additional lens 409 may be required. The configuration of FIG. 5 includes at least two phase plate receiving ports 442, 443, 444.

[0043] In this way, by providing two or more options for the placement of the phase plate 440 (or other wavefront manipulation device 440), with each option being spaced apart, and by appropriate deployment of the lenses 409, 410, or the lenses within the correction optical assembly 450, a series of different magnifications can be achieved, as further explained below.

[0044] Figure 6 shows the first operation mode of the transmission electron microscope configuration shown in Figure 4. In this mode, the additional lens 409 is not deployed, the two lenses 410 and 411 of the intermediate lens assembly 408 are deployed, and the phase plate 440 is deployed within the phase plate receiving location 445 immediately behind the second lens 411 of the intermediate lens assembly 408. The diffraction pattern is directed from the first diffraction plane to a second diffraction plane that coincides with the phase plate 440 (in this configuration, located within the phase plate receiving location 445). In this particular configuration, a small magnification of the first diffraction plane, for example, 2x, can be achieved.

[0045] Figure 7 shows the second operation mode of the transmission electron microscope configuration shown in Figure 4. In this mode, the additional lens 409 is not deployed, the two lenses 410 and 411 of the intermediate lens assembly 408 are deployed, and the phase plate 440 (or an alternative wavefront manipulation device 440) is deployed within the phase plate receiving location 446 immediately before the projection assembly 412. The diffraction pattern is directed from the first diffraction plane to a third diffraction plane that coincides with the phase plate 440 (in this configuration, located within the phase plate receiving location 446). In this particular configuration, an intermediate magnification, for example, 4x, can be achieved.

[0046] Figure 8 shows the third operation mode of the transmission electron microscope configuration shown in Figure 4. In this mode, the additional lens 409 is deployed, both lenses 410 and 411 of the intermediate lens assembly 408 are deployed, and the phase plate 440 is deployed within the phase plate receiving location 443 immediately behind the first lens 410 of the intermediate lens assembly 408. The diffraction pattern is directed from the first diffraction plane to a first diffraction plane that coincides with the phase plate 440 (in this configuration, located within the phase plate receiving location 443). In this particular configuration, a large magnification, for example, 6x, can be achieved.

[0047] It is clear that in this way, the same apparatus can be easily reconfigured to achieve different levels of magnification. Furthermore, even with fewer components (for example, with respect to Figures 6 and 7, some flexibility can still be achieved without the additional lens 409.

[0048] Figure 9 shows the first operating mode of the transmission electron microscope configuration shown in Figure 5. In this mode, an additional lens 409 is deployed, a correction optical assembly 450 is deployed, and a lens 410 of the intermediate lens assembly 408 is deployed between the correction optical assembly 450 and the projection assembly 412. The phase plate 440 or other wavefront manipulation device 440 is located within a phase plate receiving location 442 immediately in front of the lens 410 of the intermediate lens assembly 408. With this particular configuration, a relatively small magnification can be achieved.

[0049] Figure 10 shows the second operating mode of the transmission electron microscope configuration shown in Figure 5. In this mode, an additional lens 409 is deployed, a correction optical assembly 450 is deployed, and the phase plate 440 or other wavefront manipulation device 440 is located within a phase plate receiving location 444 immediately in front of the projection assembly 412. With this particular configuration, a relatively large magnification can be achieved.

[0050] As will be apparent to those skilled in the art, the particular configuration of optional features such as lenses, and correction optics, and the given particular magnification are not essential for implementation. The core of the present disclosure lies in the flexibility provided by the ability to deploy a phase plate (or alternative wavefront manipulation device 440) at two or more different locations within the TEM and achieve different magnifications by deploying different combinations of lenses relative to the locations where the phase plate is deployed.

Claims

1. A transmission electron microscope (TEM), a sample holder configured to hold a sample; an electron source configured to provide a beam of electrons toward the sample holder; a primary lens configured to receive the beam of electrons after exiting the sample holder and generate a diffraction pattern at a first diffraction plane; an intermediate lens assembly configured to receive the beam of electrons after exiting the primary lens and form an image of a sample present in the sample holder, the intermediate lens assembly including, in order, a first lens occupying a first lens area and a second lens occupying a second lens area; a projection assembly configured to receive the image of the sample, the projection assembly occupying a projection assembly area; a first port in a first port plane and a second port in a second port plane, each of the first port and the second port configured to receive a wavefront manipulation device for manipulating a wavefront of the beam; a first port and a second port, each of the first port and the second port being located in a different region of the projection assembly region, the first lens region, and the second lens region; a controller configured to control excitation of the first lens and the second lens; In a first mode, the controller is configured to control the first lens and the second lens to direct the diffraction pattern onto a second diffraction plane, the second diffraction plane coinciding with the first port plane; A transmission electron microscope (TEM), wherein in a second mode, the controller is configured to control the first lens and the second lens to direct the diffraction pattern onto a third diffraction plane, the third diffraction plane coinciding with the second port plane.

2. The TEM of claim 1 comprising a wavefront manipulation device configured to be received in the first port and configured to be received in the second port.

3. The TEM of claim 1 , wherein the first port and the second port each comprise an inlet and an outlet.

4. The TEM of claim 1 , wherein at least one of the first port and the second port is between the first lens and the second lens.

5. The TEM of claim 1 , wherein the wavefront manipulation device comprises a phase manipulation device, such as a phase plate.

6. The TEM of claim 5 , wherein the phase plate comprises a laser phase plate.

7. The TEM of claim 1 , wherein the primary lens is configured to place the sample holder in a magnetic field.

8. The TEM of claim 1 further comprising a secondary lens between the primary lens and the intermediate lens assembly, the secondary lens configured to keep the sample holder out of a magnetic field.

9. The TEM of claim 1 , wherein the first port is located within the first lens region.

10. The TEM of claim 9 , wherein the first port is located between the primary lens and the first lens of the intermediate lens assembly.

11. 10. The TEM of claim 9, wherein the first port is located between the first lens of the mid lens assembly and the second lens of the mid lens assembly.

12. The TEM of claim 1 , wherein the first port is located within the second lens region.

13. The TEM of claim 12 , wherein the first port is located between the first lens of the mid lens assembly and the second lens of the mid lens assembly.

14. The TEM of claim 12 , wherein the first port is located between the second lens of the intermediate lens assembly and the projection assembly.

15. The TEM of claim 1 , wherein the second port is located within the projection assembly region.

16. The TEM of claim 15 , wherein the second port is located between the second lens of the intermediate lens assembly and the projection assembly.

17. The TEM of claim 1 further comprising a corrective optical assembly.

18. 20. The TEM of claim 17, wherein the corrective optical assembly is located between the first lens of the mid lens assembly and the second lens of the mid lens assembly.

19. The TEM of claim 18 , wherein the first port is located in the second lens region and the second port is located in the projection assembly region.

20. 20. The TEM of claim 19, wherein the first port is located between the corrective optical assembly and the second lens of the intermediate lens assembly.

21. 20. The TEM of claim 19, wherein the second port is located between the second lens of the intermediate lens assembly and the projection assembly.

22. The TEM of any one of claims 1 to 21, wherein the projection assembly includes a first projection assembly lens, and the projection assembly region is the region of the first projection assembly lens.

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