Transmission electron microscope with variable effective focal length

Abstract

The invention relates to a transmission electron microscope with a variable effective focal length. A transmission electron microscope (TEM) includes: a sample holder configured to hold a sample; an electron source configured to provide an electron beam toward the sample holder; and a primary lens configured to receive the electron beam after exiting the sample holder and generate a diffraction pattern in a first diffraction plane.

Description

Technical Field

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

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

[0003] The contrast of an image produced by interfering electrons depends on the angle at which the electrons are scattered and the amount of defocus applied. Object features with a specific spatial frequency scatter the beam at a specific angle, and the scattering angle is proportional to that spatial frequency. When the phase object being imaged is exactly in focus, the contrast is close to zero for all spatial frequencies. When defocus is applied, for low spatial frequencies, the scattering angle is close to zero and the contrast remains close to zero. For higher spatial frequencies, the contrast fluctuates between positive and negative contrast depending on the spatial frequency. This contrast fluctuation is described by the so-called contrast transfer function (CTF), which oscillates like a sine function, starting from zero for zero spatial frequency. Since the CTF is close to zero for low spatial frequencies, large structures are not visible in the image.

[0004] In 1947, Boersch described that the introduction of a phase plate would result in the following CTF, where low spatial frequencies show a maximum and large structures can thus be imaged better, see "über die Kontraste von Atomen im Elektronenmikroskop", H. Boersch, Z. Naturforschung 2A (1947), pp. 615 - 633. Such phase plates have been successfully introduced into TEMs.

[0005] The phase plate is a structure placed in the plane (so-called back focal plane) where the parallel beam irradiating the sample is focused by the objective lens after passing through the sample. A diffraction pattern is formed in this plane; all the unscattered electrons are focused at one point, while the scattered electrons reach other positions. The phase plate induces a phase shift, such as π / 2, between the scattered electrons and the unscattered electrons, thus converting the sine-like behavior of the CTF into a cosine-like behavior.

[0006] The phase plate can also be placed in a plane conjugate to this first diffraction plane. In such conjugate planes, the diffraction pattern can be magnified (reduced) by a factor M. Such planes have a so-called effective focal length f eff , which is defined as M times the focal length of the objective lens. Advantageously, a phase plate is used in such conjugate planes to create more space for placing the phase plate and to have the freedom to select the optimal value of f eff . A large f eff value generally improves the onset ("cut-on frequency") of the cosine-like behavior of the CTF.

[0007] Phase plates based on CW lasers (laser phase plates, LPPs) are currently being investigated for use in transmission electron microscopes, but their use in the objective lens region is challenging due to the size of the LPP hardware near the electron beam and the risk of contamination of the sensitive laser optics near the sample region. In addition, LPPs require a large f eff value to have a sufficiently low cut-on frequency. Therefore, it is attractive to place the LPP in a plane conjugate to the back focal plane.

[0008] In addition to using phase plates in TEMs, it is known to incorporate image correctors that correct the aberrations of the imaging optics, thereby improving the image quality of the TEM.

[0009] For example, the correction system can be placed between the objective lens and the projection system of the TEM. The correction system can include a transmission system in the form of a doublet lens to image the back focal plane of the objective lens onto a hexapole lens. The hexapole lens then images onto another hexapole lens through a pair of lenses, 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] In these prior art arrangements, there is generally little or no flexibility in terms of the magnification factor that any conjugate diffraction plane can achieve. The design of the microscope is customized to achieve a specific magnification.

[0011] The phase plate may also include an amplitude mask to partially or completely block the electron wave at the mask region. Such an amplitude mask can help improve the phase contrast in the image. Thus, the phase plate is used to manipulate the electron exit wavefront in a way that optimizes the phase contrast at the detector plane or enables the optimization of the recovery of the phase contrast after some image processing steps. Summary of the Invention

[0012] In this context, 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, thus alleviating problems related to device size or device contamination. Additionally, by selecting an optimal value for the effective focal length, the first diffraction plane can be magnified and better matched to the size of the phase shift region provided by the wavefront manipulation device. This results in a lower cut-off frequency of the spatial frequency of the sample with enhanced contrast obtained by using the wavefront manipulation device. Furthermore, by providing at least three lenses and two ports configured to receive the wavefront manipulation device, a single transmission electron microscope operating in different configurations can be used, and a series of different magnifications can be achieved using different lens excitations.

[0014] In this way, by providing a modest amount of additional hardware in the form of additional ports for the wavefront manipulation device, along with associated lens control, a more versatile TEM capable of providing various different magnifications of the first diffraction plane is produced. Brief Description of the Drawings

[0015] Embodiments of the present disclosure are now described with reference to the following drawings, in which:

[0016] Figure 1 A schematic diagram of a transmission electron microscope according to the prior art is shown;

[0017] Figure 2 A schematic diagram of the components of a transmission electron microscope including a phase plate according to the prior art is shown;

[0018] Figure 3 A schematic diagram of the components of a transmission electron microscope according to the prior art is shown, which is similar to Figure 2 the components of and includes additional lenses;

[0019] Figure 4 A schematic diagram of the components of a transmission electron microscope according to a first aspect of the present disclosure and including a plurality of wavefront manipulation device receiving positions is shown;

[0020] Figure 5Schematic diagram showing components of a transmission electron microscope according to a second aspect of the present disclosure and including a correction optical device plus a plurality of wavefront manipulation device receiving positions;

[0021] Figure 6 Shows a first operating mode of a transmission electron microscope according to a first aspect of the present disclosure;

[0022] Figure 7 Shows a second operating mode of a transmission electron microscope according to a first aspect of the present disclosure;

[0023] Figure 8 Shows a third operating mode of a transmission electron microscope according to a first aspect of the present disclosure;

[0024] Figure 9 Shows a first operating mode of a transmission electron microscope according to a second aspect of the present disclosure;

[0025] Figure 10 Shows a second operating mode of a transmission electron microscope according to a second aspect of the present disclosure. Detailed Description

[0026] Figure 1 Schematic diagram showing a TEM system 100 according to the prior art, the TEM system including an electron source 10 that emits an electron beam 11 along an optical axis 110 toward condenser optics 12. The electron source 10 can generate high-energy electrons, i.e., electrons having a typical energy between about 10 keV and 1,000 keV. The condenser optics 12 can include one or more condenser lenses and one or more apertures. A deflector 19 positioned downstream of the condenser optics 12 deflects and / or tilts the electron beam relative to the optical axis 110. An objective lens 16 in front of the sample 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 in a specimen plane 111 by a sample stage 13. In some examples, the sample is located on a TEM grid attached to the sample stage. The sample stage 13 can adjust the sample position by tilting the sample relative to the optical axis and / or translating the sample within the specimen plane. The unscattered electrons and scattered electrons leaving the sample 14 sequentially pass through the post-sample objective lens 123 and the projection system 21 and are collected by a detector 25 positioned on the opposite side of the sample 14 relative to the electron source 10. The projection system 21 operates differently 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.

[0027] Figure 1 Shows a TEM system 100 operating in SA (selected aperture) imaging mode and SA diffraction mode (although in practice only one mode is possible at any given time). Dashed line 41 shows the beam path of unscattered electrons from the sample 14 to the detector 25 in SA diffraction mode. In the case of parallel illumination of the sample 14, the unscattered electrons are focused in the back focal plane 43, and the scattered electrons can form an electron diffraction (ED) pattern in this plane. In SA diffraction mode, the projector system 21 images the back focal plane 43 of the sample post-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, while the scattered electrons can still reach the detector. Dashed line 42 shows the beam path of scattered electrons from the sample 14 to the detector 25 in SA imaging mode. In SA imaging mode, the specimen 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 in the SA plane 44. Alternatively, the condenser aperture in the condenser optics 12 can act as a beam limiting aperture. In another example, an image deflector 45 can be positioned between the sample and the detector for deflecting and tilting the electrons transmitted through the sample back onto the optical axis so that the ED pattern remains centered on the detector during beam tilt and the image remains centered on 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 unscattered beam.

[0028] The controller 30 can manually or automatically control the operation of the TEM system 100 in response to operator instructions or according to computer-readable instructions stored in a non-transitory memory (or computer-readable medium) 32. The controller 30 can include a processor and is configured to execute the computer-readable instructions and 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 apertures, the intensity of the objective lens 123, the beam stopper 17, and the projector system 21. The controller 30 can adjust the beam position and / or the beam incident angle on the sample by adjusting the deflector 19. The controller 30 can adjust the electron dose rate on the sample by adjusting one or more settings of the electron source or the illumination optics. The controller 30 can adjust the magnification by adjusting the projector system 21. The controller 30 can also be coupled to a display 31 to display notifications and / or signals detected by the detector 25. The 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.

[0029] Figure 2 FIG. 1 shows a schematic view of a component of a TEM system according to the prior art, the component including a correction system 330 and a phase plate 340.

[0030] The correction system 330 can be used to correct the aberration of the objective lens 305 and also to correct some aberrations introduced by the correction system.

[0031] The sample is positioned on the optical axis 301 in the sample plane 302 and imaged by the objective lens 305. The sample is placed near the front focal plane of the objective lens 305. Two principal rays are shown in this figure, namely, an axial ray 303 that emanates from the sample center at an angle at the position where the microscope optical axis intersects the sample, and a field ray 304 that is parallel to the optical axis and emanates from an off-axis point of the sample. The ray 304 intercepts the optical axis in the rear focal plane 306 of the objective lens 305. The correction system 330 is arranged around the optical axis 301.

[0032] The last lens of the correction system 330 can be used to form a diffraction plane at the position of the phase plate 340. This will result in a first image plane 314 after 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.

[0033] Note that, for convenience, the objective lens 305 is shown here (and also in Figure 1 FIG. 2) as a thin lens, where the sample plane 302 is outside the lens itself. Many TEMs use a thick lens as the objective lens 305, where the sample position is placed within the (magnetic) field of the objective lens 305.

[0034] In Figure 2 the arrangement of FIG. 2, the correction system 330 is entirely placed 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.

[0035] The additional lens 341 can be used to form an image of the plane 314 at the plane 323, which serves as the SA plane in front of the projection system.

[0036] Figure 3 FIG. 3 shows a schematic view of a component of a TEM system according to the prior art, Figure 2 the correction system 330 and the phase plate 340 of FIG. 3, and a lens 343 added between the correction system 330 and the phase plate 340. In this arrangement, an image of the sample is formed at the plane 342.

[0037] Another method is provided for changing the magnification of the rear focal plane on the plane of the phase plate.

[0038] The present disclosure recognizes that a greater range of magnifications can be provided by having a series of lenses that can be selectively deployed and more than one port for receiving a phase plate such that the phase plate can be moved between multiple positions.

[0039] Figure 4 A transmission electron microscope according to a first aspect of the present disclosure is shown and includes a plurality of phase plate receiving positions 442, 443, 444, 445, 446. Each phase plate receiving position 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 a phase plate at the phase plate receiving position 442, 443, 444, 445, 446 on the optical axis 110 at a position conjugate to the first diffraction plane 430. Although these are referred to as phase plate receiving ports 442, 443, 444, 445, 446, it should be noted that they can be configured to receive any wavefront manipulation device 440 and can thus be referred to as wavefront manipulation device receiving ports 442, 443, 444, 445, 446.

[0040] The arrangement includes an electron source (not shown in Figure 4 ) configured to generate a particle beam along the optical axis 110. The arrangement 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. The arrangement also includes an intermediate lens assembly 408 located between the objective lens 405 and the projection assembly 412. The arrangement also includes illumination optics (not shown).

[0041] The intermediate lens assembly 408 includes a plurality of lenses 410, 411. The first lens 410 of the intermediate lens assembly 408 is located in a first lens region. The second lens 411 of the intermediate lens assembly 408 is located in a second lens region.

[0042] The projection assembly 412 occupies a projection lens region. The projection assembly 412 may include a plurality of lenses (not shown).

[0043] In use, a sample is positioned in the sample holder 402 in a sample plane on the optical axis 110 in a region near the front focal plane of the objective lens 405. The sample is imaged by the objective lens 405 and a diffraction pattern is generated in the first diffraction plane 430. The intermediate lens assembly 408 is configured to receive the electron beam after leaving the primary lens and is configured to generate a magnified image of the sample present in the sample holder. The projection assembly 412 is configured to receive the magnified image of the sample.

[0044] The intermediate lens assembly 408 includes at least some of the plurality of phase plate receiving ports 442, 443, 444, 445, 446.

[0045] Each phase plate receiving port 442, 443, 444, 445, 446 may include an inlet and an outlet. This is because, for a laser-based wavefront manipulation device 440 suitable for use with the present disclosure, the outlet port facilitates diagnostic output of the laser.

[0046] For a non-laser-based wavefront manipulation device, it may be more straightforward to deploy a double prism using a port that has only an inlet and no outlet.

[0047] The arrangement may also include an additional (optional) lens 409 located between the objective lens 405 and the intermediate lens assembly 408.

[0048] The additional lens 409 may optionally be deployable such that it can be deployed in some modes and not deployed in other modes.

[0049] Just as not all lenses may be required, similarly, not all of the phase plate receiving ports 442, 443, 444, 445, 446 may be required. However, the minimum number of phase plate receiving ports 442, 443, 444, 445, 446 is two to allow for more than one option for the location of the inserted phase plate 440 or other wavefront manipulation device 440.

[0050] In this way, by providing more than one option for placing the wavefront manipulation device 440, where each option is 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 will be further explained below.

[0051] Figure 5 A transmission electron microscope is shown that includes a plurality of phase plate receiving ports 442, 443, 444 according to a second aspect of the present disclosure. Many 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.

[0052] In Figure 5 the arrangement, one or more lenses of the intermediate lens assembly 408 are replaced by a correction optical assembly 450, and an additional lens 409 may be required. Figure 5 the arrangement includes at least two phase plate receiving ports 442, 443, 444.

[0053] In this way, by providing more than one option for placing the phase plate 440 (or other wavefront manipulation device 440), where each option is spaced apart, and by appropriate deployment of the lenses in the lenses 409, 410 or the correction optical assembly 450, a series of different magnifications can be achieved, as will be further explained below.

[0054] Figure 6 shows Figure 4 a first operating mode of the transmission electron microscope arrangement shown in. In this mode, the additional lens 409 is not deployed, two lenses 410, 411 of the intermediate lens assembly 408 are deployed, and the phase plate 440 is deployed in the phase plate receiving position 445 immediately after the second lens 411 of the intermediate lens assembly 408. The diffraction pattern is guided from the first diffraction plane into the second diffraction plane that coincides with the phase plate 440, and in this configuration, the phase plate is located in the phase plate receiving position 445. In this particular arrangement, a small magnification of the first diffraction plane, such as 2x, can be achieved.

[0055] Figure 7 shows Figure 4 a second operating mode of the transmission electron microscope arrangement shown in. In this mode, the additional lens 409 is not deployed, two lenses 410, 411 of the intermediate lens assembly 408 are deployed, and the phase plate 440 (or alternative wavefront manipulation device 440) is deployed in the phase plate receiving position 446 immediately before the projection assembly 412. The diffraction pattern is guided from the first diffraction plane into the third diffraction plane that coincides with the phase plate 440, and in this configuration, the phase plate is located in the phase plate receiving position 446. In this particular arrangement, an intermediate magnification, such as 4x, can be achieved.

[0056] Figure 8 shows Figure 4 a third operating mode of the transmission electron microscope arrangement shown in. In this mode, the additional lens 409 is deployed, two lenses 410, 411 of the intermediate lens assembly 408 are deployed, and the phase plate 440 is deployed in the phase plate receiving position 443 immediately after the first lens 410 of the intermediate lens assembly 408. The diffraction pattern is guided from the first diffraction plane into the first diffraction plane that coincides with the phase plate 440, and in this configuration, the phase plate is located in the phase plate receiving position 443. In this particular arrangement, a large magnification, such as 6x, can be achieved.

[0057] In this way, it is obvious that the same device can be directly reconfigured to achieve different levels of magnification. Furthermore, even with fewer components (e.g., without the additional lens 409), according to Figure 6 and Figure 7 , a certain degree of flexibility can still be achieved.

[0058] Figure 9 shows Figure 5 a first operating mode of the transmission electron microscope arrangement shown in Figure 5 . In this mode, an additional lens 409 is deployed, a correction optical component 450 is deployed, and a lens 410 of an intermediate lens assembly 408 is deployed between the correction optical component 450 and a projection component 412. A phase plate 440 or other wavefront manipulation device 440 is located in a phase plate receiving position 442 immediately before the lens 410 of the intermediate lens assembly 408. In this particular arrangement, a relatively small magnification can be achieved.

[0059] Figure 10 shows Figure 5 a second operating mode of the transmission electron microscope arrangement shown in Figure 5 . In this mode, an additional lens 409 is deployed, a correction optical component 450 is deployed, and a phase plate 440 or other wavefront manipulation device 440 is located in a phase plate receiving position 444 immediately before the projection component 412. In this particular arrangement, a relatively large magnification can be achieved.

[0060] As will be apparent to those skilled in the art, the specific arrangement of the lenses, as well as optional features such as correction optics, and the specific magnifications given are not necessary for a particular implementation. The core of the present disclosure is 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 to achieve different magnifications by deploying different lens combinations relative to the location where the phase plate is deployed.

Claims

1. A transmission electron microscope (TEM), comprising: a sample holder configured to hold a sample; an electron source configured to provide an electron beam toward the sample holder; a primary lens configured to receive the electron beam after leaving the sample holder and to produce a diffraction pattern in a first diffraction plane; an intermediate lens assembly configured to receive the electron beam after exiting the primary lens and to form an image of a sample present in the sample holder, wherein the intermediate lens assembly includes, in sequence, 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 being configured to receive a wavefront manipulation device for manipulating a wavefront of a beam, wherein: Each of the first port and the second port is located in a different one of: the projection assembly area, the first lens area, and the second lens area; and a controller configured to control actuation of the first lens and the second lens, wherein: In a first mode, the controller is configured to control the first lens and the second lens to direct the diffraction pattern into a second diffraction plane, wherein the second diffraction plane coincides with the first port plane; and In a second mode, the controller is configured to control the first lens and the second lens to direct the diffraction pattern into a third diffraction plane, wherein the third diffraction plane coincides with the second port plane.

2. The TEM according to claim 1, comprising: A wavefront manipulation device is configured to be received in the first port and is configured to be received in the second port.

3. A TEM according to claim 1 or claim 2, wherein the first port and the second port each comprise an inlet and an outlet.

4. The TEM according to any preceding claim, wherein at least one of the first port and the second port is located between the first lens and the second lens.

5. A TEM according to any preceding claim, wherein the wavefront manipulation means comprises a phase manipulation means, such as a phase plate.

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

7. A TEM according to any preceding claim, wherein the primary lens is configured to immerse the sample holder in a magnetic field.

8. The TEM according to any preceding claim, further comprising a secondary lens located between the primary lens and the intermediate lens assembly, wherein the secondary lens is configured not to immerse the sample holder in a magnetic field.

9. A TEM according to any preceding claim, wherein the first port is located in 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. The TEM of claim 9, wherein the first port is located between the first lens of the intermediate lens assembly and the second lens of the intermediate lens assembly.

12. The TEM according to any one of claims 1 to 8, wherein the first port is located in the second lens region.

13. The TEM of claim 12, wherein the first port is located between the first lens of the intermediate lens assembly and the second lens of the intermediate 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. A TEM according to any preceding claim, wherein the second port is located in 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. A TEM according to any preceding claim, further comprising a corrective optical assembly.

18. The TEM of claim 17, wherein the corrective optical assembly is located between the first lens of the intermediate lens assembly and the second lens of the intermediate 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. The TEM of claim 19, wherein the first port is located between the corrective optics assembly and the second lens of the intermediate lens assembly.

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

22. The TEM according to any preceding claim, wherein the projection assembly comprises a first projection assembly lens, and wherein the projection assembly area is an area of ​​the first projection assembly lens.

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