Secondary radiation mitigation

Abstract

An X-ray source is disclosed, comprising an electron source configured to provide an electron beam; an aperture configured to limit an angular distribution of the electron beam, wherein the electron beam causes secondary X-ray radiation to be emitted from a region around the aperture; a secondary radiation limiting aperture configured to limit an angular distribution of the secondary X-ray radiation, wherein the secondary radiation limiting aperture is arranged downstream from the electron beam limiting aperture; a first deflector configured to deflect the electron beam towards the secondary radiation limiting aperture, a target arranged downstream from the secondary radiation limiting aperture and configured to generate primary X-ray radiation by interaction with the electron beam; and an exit window arranged to transmit the primary X-ray radiation; wherein the secondary radiation is prevented from reaching the exit window by the secondary radiation limiting aperture. A corresponding method is also disclosed.

Description

[0001] SECONDARY RADIATION MITIGATION

[0002] Technical field

[0003] The present disclosure relates to X-ray arrangements and X-ray sources. More particularly, the disclosure relates to mitigation of effects caused by secondary X-ray radiation, i.e. undesired X-ray radiation generated in an X-ray source.

[0004] Background

[0005] X-ray radiation can be generated by directing an electron beam onto a target material. A typical prior art X-ray source thus comprises an electron source to generate the electron beam, and electron optics to direct and focus the electron beam onto the target material. Such electron optics may include magnetic alignment and / or stigmator coils, and electrostatic or magnetic focusing lenses and deflectors. The target material typically comprises a high-Z (e.g. atomic number Z > 20) material that generates X-ray radiation as bremsstrahlung and line emission upon electron impact. The target may comprise a target film, e.g. made from tungsten (W), deposited on a target substrate material, e.g. made from diamond. A typical thickness of the target film may be about 0.5 pm or several micrometers or several tens of micrometers, while a typical thickness of the target substrate may be about 100 pm or several hundreds of micrometers.

[0006] The generated X-ray radiation may be directed for use in, for example, X-ray imaging of a sample or an object.

[0007] Secondary radiation is a well-known problem within the art of X-ray sources. Secondary radiation is generally understood as any radiation other than the desired radiation and can originate, for example, from scattering of X-rays or from unintended electron beam interactions in the system. The source of secondary radiation may be scattering of the primary X-ray radiation, interaction of backscattered electrons with other source components, or interaction between the electron beam and some part of the source located upstream of the target.

[0008] Improved techniques for addressing such secondary X-ray radiation are thus desired. Summary

[0009] The problem solved by the present invention presents itself, for example, when an aperture or diaphragm is used for limiting an angular distribution of the electron beam before it enters electron optics in the X-ray source. Limiting the angular distribution of the electron beam may be required when the desired X-ray spot size, and therefore the desired electron beam spot size on the target, is small. Small, in this context, typically refers to spot sizes below 1 pm in diameter. Furthermore, the solution disclosed herein is most relevant for cases where the dominant source of secondary radiation is a well-defined object upstream from the target, such as an aperture as discussed above. X-ray radiation generated by interaction with such object may show up as distinct artifacts or create a secondary image of an object within the field of view having a different magnification than the image obtained from the primary X-ray source. However, other sources of secondary radiation causing more or less uniform noise in the images may also be mitigated, at least to a degree.

[0010] The present invention is based on the recognition that secondary X-ray radiation can be prevented from reaching a detector used for imaging a sample, or even prevented from escaping the X-ray source altogether. Since the primary source of secondary X-ray radiation is the electron beam limiting aperture, such secondary radiation can be prevented from reaching the detector either by positioning the detector outside of a solid angle at which the secondary radiation escapes an exit window of the X-ray source, or by confining the propagation of secondary radiation inside an enclosure of the X-ray source such that it cannot reach the exit window. The present invention relates to confining the propagation of secondary radiation such that it cannot reach the exit window. For example, by introducing a second aperture acting as a secondary radiation limiting aperture inside the X-ray source through which the electron beam is passed before reaching the target, propagation of secondary radiation beyond the second aperture is confined.

[0011] Generally, the solution proposed herein involves deflecting the electron beam after is has passed the electron beam limiting aperture, i.e. the aperture that is used for limiting the angular distribution of the electron beam, and limiting an angular distribution of any secondary X-ray radiation generated at that aperture. As will be understood, the deflection of the electron beam will not influence the propagation of X-ray radiation, and a secondary radiation limiting element, e.g. an aperture, can therefore be positioned such that no secondary X-ray radiation reaches the exit window of the X-ray source, and thus cannot interfere with a detector used for detecting the primary X-ray radiation generated from interaction of the electron beam with the target. In a typical X-ray source it is only through one or more intentional exit windows that the X-ray radiation can escape / exit the enclosure.

[0012] As used herein, the expressions "upstream" and "downstream" designate locations relative to a propagation direction of the electron beam. Hence, upstream means closer to the electron source, and downstream means further from the electron source.

[0013] An X-ray source according to the principles disclosed herein comprises an electron source configured to provide an electron beam; an aperture configured to limit an angular distribution of the electron beam, wherein the electron beam causes secondary X-ray radiation to be emitted from a region around the aperture; a secondary radiation limiting aperture configured to limit an angular distribution of the secondary X-ray radiation, wherein the secondary radiation limiting aperture is arranged downstream from the electron beam limiting aperture; a first deflector arranged e.g. between the electron beam limiting aperture and the secondary radiation limiting aperture, the first deflector being configured to deflect the electron beam towards the secondary radiation limiting aperture, a target arranged downstream from the secondary radiation limiting aperture and configured to generate primary X-ray radiation by interaction with the electron beam; and an exit window arranged to transmit the primary X-ray radiation; wherein the secondary radiation is prevented from reaching the exit window by the secondary radiation limiting aperture.

[0014] In some other examples, the secondary radiation limiting element comprises an exit window arranged for allowing the primary X-ray radiation to be emitted towards the detector, and wherein the detector is positioned such that a normal to a surface of the detector is at an angle of at least b / l radians to an impact direction of the electron beam at the target, wherein b is a half width of the detector and / is a distance from the target to the detector. Preferably, a surface of the target is arranged parallel to the detector. While the first deflector could be configured to also deflect the electron beam towards a desired location on the target, it is preferred to include a second deflector for this purpose. It may also be preferred to include a third deflector configured to direct the electron beam towards the second deflector, wherein the target is positioned on a straight line defined by the electron source and the electron beam limiting aperture. In this type of embodiments an X-ray source is provided where substantially all of the emitted radiation is primary radiation generated by interactions between the electron beam and the target. In other words, interactions between the electron beam and any other parts of the source, such as the electron beam limiting aperture, do not result in X-ray radiation that is emitted from the source.

[0015] In all embodiments, it is preferred to have at least one deflector that is configured to scan the electron beam over the target. Typically, the target may comprise a substrate and a metal layer configured to generate X-ray radiation upon impact of the electron beam. To achieve a desirable spot size of the electron beam on the target, the arrangement preferably comprises a focus lens to focus the electron beam on the target.

[0016] There is also provided a method at an X-ray source, the method comprising: directing an electron beam towards an electron beam limiting aperture, wherein the electron beam limiting aperture limits an angular distribution of the electron beam, the electron beam causing secondary X-ray radiation to be emitted from a region around said aperture; deflecting the electron beam towards a secondary radiation limiting aperture, wherein the secondary radiation limiting aperture confines propagation of the secondary X-ray radiation to a secondary radiation propagation region preventing the secondary X-ray radiation from reaching an exit window of the X-ray source while being sufficiently large to pass the electron beam; causing the electron beam, after passage through the secondary radiation limiting aperture, to impinge upon a desired location on a target to generate primary X-ray radiation by interaction between the target and the electron beam Causing the electron beam to impinge upon a desired location on the target may comprise deflecting the electron beam towards the target. The method may also involve scanning the electron beam over the target. Furthermore, the primary X-ray radiation emitted through the exit window may be detected by means of a detector. Brief of the

[0017] In the following detailed description of preferred embodiments, reference is made to the accompanying drawings, on which:

[0018] Fig. la schematically shows an example in which secondary X-ray radiation is allowed to escape the X-ray enclosure, but in a direction such that it does not reach the detector;

[0019] Fig. lb illustrates the geometry of the example in Fig. la in more detail;

[0020] Figs. 2-6 schematically show examples in which secondary X-ray radiation is contained within the X-ray enclosure; and

[0021] Fig. 7 illustrates an exemplary method at an X-ray arrangement.

[0022] Detailed description

[0023] A first example of an X-ray arrangement 100 is shown in Fig. la. The arrangement 100 comprises an electron source 102 for generating an electron beam 104 (shown by a dashed line throughout the figures). Primary X-ray radiation is generated by interaction of the electron beam 104 with a target 106. The X-ray arrangement comprises a focus lens 108 for focusing the electron beam to a spot on the target 106, and a deflector 110 for deflecting the electron beam such that the spot is formed at a desired location on the target. Before the electron beam enters electron beam optics, e.g. the focus lens 108, it is passed through an electron beam limiting aperture 112 for limiting the angular distribution of the electron beam. When the angular distribution of the electron beam 104 is restricted using the aperture 112, secondary X-ray radiation will typically be produced by interaction between the electron beam and the aperture structure. The propagation of such secondary radiation is illustrated at reference numeral 114 in Fig. 1. In order for the secondary radiation not to adversely influence measurements made using the primary X-ray radiation, the X-ray arrangement of Fig. 1 is designed such that the secondary radiation does not reach a detector 120 used for detecting X-ray radiation, and preferably also not a sample 116 to be imaged (which may be mounted to a translation stage 118). As shown, the electron source 102, the focus lens 108, the deflector 110 and the aperture 112 are all mounted within an enclosure 122. The target 106 is located at, and may even constitute, an exit window from the enclosure.

[0024] Effects from secondary X-ray radiation in the detected X-ray image are here mitigated by allowing the secondary radiation 114 to escape the enclosure 122 in a solid angle not used by the application, i.e. in a solid angle not containing the detector 120 (and preferably also not containing the sample 116). Although this may put a limitation on how the X-ray tube can be used due to the geometry thereof, this is acceptable for some applications. As illustrated in Fig. la, the target 106 is tilted with respect to the incoming electron beam 104 to enable placement of the sample 116 close to the source, and the target 106 itself acts as a secondary radiation limiting element for the secondary X-ray radiation 114. An added complexity with this example, however, is that the tilted target surface will deform the electron beam cross section, i.e. a circular electron beam impinging the target at a nonnormal angle will result in an elliptic electron beam spot on the target. To compensate for this, it may be preferred to adjust the astigmatism of the incoming electron beam before it impacts the target.

[0025] As will be understood, any secondary radiation generated upstream from the target will follow its propagation path and exit the enclosure in a solid angle determined by the size of the exit window (target) and the size and location of the source of such secondary radiation. By positioning the detector, and preferably also the sample, outside this solid angle occupied by the secondary radiation, influence from the secondary radiation in the detector (and sample) is eliminated.

[0026] Fig. lb shows a schematic view of the target 106 and the detector 120 to illustrate the geometry in more detail. The minimum angle a required for the tilt of the target and the detector relative to the incoming electron beam in order to prevent the secondary radiation from impacting the detector may, in a simplified model where the secondary radiation is considered to be emitted from a point source, be calculated based on congruent triangles. For a detector of width 2b positioned a distance I from the target, the angle should, to a first approximation valid for small angles, be larger than b / l radians. As an example, for a detector of a half width b=100 mm placed at a distance / =500 mm from the target, the angle a should be at least 0.2 radians or 11.5 degrees. The present invention provides improved solutions for mitigating the effects of secondary X-ray radiation. In general, embodiments of the present invention prevent the secondary radiation from reaching an exit window of the X-ray source. As illustrated herein, the exit window may be constituted by the target itself or be arranged in proximity thereof.

[0027] Figs. 2-6 illustrate examples wherein the secondary radiation is contained within the enclosure of the X-ray source of the arrangement, i.e. prevented from reaching an exit window of the X-ray source. The examples are presented in order of increasing number of components.

[0028] Fig. 2 shows an X-ray source 200 where the electron beam 204, generated by the electron source 202, is limited by an electron beam limiting aperture 212. Secondary radiation 214 will be created by interaction between the electron beam 204 and the material defining the electron beam limiting aperture 212. By careful selection of materials and physical dimensions, the emitted secondary radiation may be reduced by self-absorption. However, the edge of the aperture will still emit a non-negligible amount of secondary radiation. Therefore, a deflector 203 is provided downstream of the electron beam limiting aperture 212 for deflecting the electron beam 204 towards a second aperture 213 and the target 206. The second aperture 213 is arranged so as to prevent the secondary radiation 214 from reaching the target 206, and is thus herein referred to as a secondary radiation limiting aperture. Any part of the secondary radiation that is transmitted through the second aperture will instead impact the enclosure 222, which is opaque to X-ray radiation. The second aperture 213 is structured and arranged so that the electron beam 204 does not interact with the material defining the aperture, and thus no secondary radiation is generated there. Since the second aperture has the purpose of limiting the secondary radiation, and not to constrain the electron beam, the diameter of the second aperture can be considerably larger than the electron beam limiting aperture. A focus lens 208 is arranged downstream of the second aperture 213 to focus the electron beam 204 on the target 206 to generate the primary X-ray radiation.

[0029] Fig. 3 shows an example similar to that of Fig. 2. In the X-ray source 300 shown in Fig. 3, the electron beam 304 is limited by an electron beam limiting aperture 312. Secondary radiation 314 will be created by interaction between the electron beam 304 and the material defining the electron beam limiting aperture 312. A first deflector 303 is provided downstream of the electron beam limiting aperture 312 for deflecting the electron beam 304 towards a second aperture 313 and the target 306. The second aperture 313 is arranged so as to prevent the secondary radiation 314 from reaching the target 306. A focus lens 308 is arranged downstream of the second aperture 313 to focus the electron beam 304 on the target 306 to generate the primary X-ray radiation. The difference of this example compared to that shown in Fig. 2 is that a second deflector 315 has been added downstream of the second aperture 313. The purpose of this second deflector 315 is to direct the electron beam onto a desired position on the target and / or to scan the electron beam over the target. While, in principle, the first deflector 303 could perform these tasks of directing and / or scanning the electron beam (e.g., as could be done in the example shown in Fig. 2), it may be more practical to have one component dedicated to directing the electron beam from the electron beam limiting aperture to the second aperture in a static fashion and another component dedicated to moving the electron beam on the target in a dynamic fashion.

[0030] Common to the examples of Figs. 1-3 is that the set-up is angled. Although such angled set-up is comparatively simple and requires fewer components, it is desirable in some applications to have a more linear set-up. Various examples that provide linear, or straight, set-ups are shown in Figs. 4-6.

[0031] Fig. 4 shows a first example of an X-ray arrangement 400 with a straight X-ray tube. The electron source 402 is here placed off-axis with respect to the outgoing X- ray beam 401. The angular distribution of the electron beam 404 is limited by an electron beam limiting aperture 412, downstream of which it is deflected by a first deflector 405 towards a second aperture 413. At the second aperture the electron beam is deflected by a second deflector 403, towards a focus lens 408 and a target 406. To enable movement of the electron beam spot on the target the second deflector 403 would need a constant bias and a control signal on top of that. The electron beam limiting aperture 412 and the second aperture 413 have a similar function as that described in connection with Fig. 3 above. The output primary X-ray radiation 401 will thus be free from any secondary radiation since the geometrical construction is such that any generated secondary X-ray radiation is confined to propagation directions that impact the inside of the enclosure 422 away from the exit window (i.e. the target 406). Thereby, effects from the secondary X-ray radiation on the imaging of a sample 416 using a detector 420 are mitigated. Hence, the example shown in Fig. 4 comprises a second deflector 403 for deflecting the electron beam towards a desired location on the target, and a first deflector 405 for deflecting the electron beam towards the second aperture 413.

[0032] Fig. 5 shows an example similar to that shown in Fig. 4, but where the electron source 402 is on-axis. There may be several reasons for having the electron source placed on-axis, e.g. to facilitate electrical connections. On-axis placement of the electron source is here enabled by adding a third deflector 407 as compared to the example in Fig. 4. This is likely a preferred implementation in many cases since it can be realized with comparatively small changes to prior art designs. Hence, the example shown in Fig. 5 comprises a first deflector 405 for deflecting the electron beam towards the aperture 413, a second deflector 403 for deflecting the electron beam towards a desired location on the target, and a third deflector 407 for deflecting the electron beam towards the second deflector 403.

[0033] Fig. 6 shows another example similar to that of Fig. 5, but the functionality of directing the electron beam towards a desired position on the target and scanning the electron beam over the target have been separated by the addition of a fourth deflector 415. Hence, the example shown in Fig. 6 comprises a first deflector 405 for deflecting the electron beam towards the aperture 413, a second deflector 403 for directing the electron beam towards a desired location on the target, and a third deflector 407 for deflecting the electron beam towards the fourth deflector 415, wherein the fourth deflector 415 deflects the electron beam towards the second deflector 403.

[0034] For each of the examples shown in Figs. 2-6, the electron beam limiting aperture and the second aperture are positioned such that secondary X-ray radiation generated at the electron beam limiting aperture is prevented from escaping the enclosure through the exit window / target 206, 306, 406. Structurally this may be realized by ensuring that there is no direct / straight propagation path (i.e., no line of sight) from the electron beam limiting aperture to the target / exit window, which means that the electron beam has to be deflected at least once after passage of the electron beam limiting aperture in order to pass through the secondary radiation limiting aperture and reach the target / exit window. The electron beam limiting aperture and the secondary radiation limiting aperture are thus non-collinear on a path towards the target / exit window. Thereby, the primary X-ray radiation output from the X-ray source will be substantially free from such secondary radiation. It should be noted that a deflector and an aperture may be arranged in several ways to achieve the intended functionality according to the invention. For example, aperture 413 arranged upstream of deflector 407 in Figs. 5 and 6 may instead be arranged downstream of the deflector. In a further alternative the aperture and the deflector may be integrated so that deflection is performed in a plane defined by the aperture.

[0035] Fig. 7 illustrates a method 700 in an X-ray arrangement for mitigating effects of secondary X-ray radiation. At 701, an electron beam is directed towards an electron beam limiting aperture, wherein the electron beam limiting aperture limits an angular distribution of the electron beam. When the angular distribution of the electron beam is limited by the aperture, secondary X-ray radiation will be generated from a region around the electron beam limiting aperture. At 702, the electron beam is deflected towards a secondary radiation limiting aperture that confines propagation of the secondary X-ray radiation to a secondary radiation propagation region preventing the secondary X-ray radiation from reaching an exit window of the X-ray source. Then, at 703, after passage through the secondary radiation limiting aperture, the electron beam is caused to impinge upon a desired location on a target to generate primary X-ray radiation by interaction between the target and the electron beam. The generated primary X-ray radiation can then be used, for example, to image a sample and may to that effect be detected using a detector, as indicated at 704, arranged to receive X-ray radiation transmitted through the exit window. By having the exit window of the X-ray source located outside of the secondary radiation propagation region (i.e. by letting the secondary radiation limiting aperture prevent the secondary X-ray radiation from reaching the exit window), the primary X-ray radiation emitted through the exit window is substantially free from any interfering secondary X-ray radiation.

[0036] Arrangements, sources, and methods according to the invention may be used for different types of X-ray imaging such as X-ray microscopy, radiography, fluoroscopy, or CT scanning.

Claims

CLAIMS1. An X-ray source (200; 300; 400) comprising: an electron source (202; 302; 402) configured to provide an electron beam (204; 304); an electron beam limiting aperture (212; 312; 412) configured to limit an angular distribution of the electron beam, wherein the electron beam causes secondary X-ray radiation to be emitted from a region around the electron beam limiting aperture; a secondary radiation limiting aperture (213; 313; 413) configured to limit an angular distribution of the secondary X-ray radiation, wherein the secondary radiation limiting aperture is arranged downstream from the electron beam limiting aperture; a first deflector (203; 303; 405) configured to deflect the electron beam towards the secondary radiation limiting aperture, a target (206; 306; 406) arranged downstream from the secondary radiation limiting aperture and configured to generate primary X-ray radiation by interaction with the electron beam; and, an exit window arranged to transmit the primary X-ray radiation; wherein the secondary radiation is prevented from reaching the exit window by the secondary radiation limiting aperture.

2. The X-ray source of claim 1, wherein the first deflector (203) is further configured to deflect the electron beam towards a desired location on the target.

3. The X-ray source of claim 1, further comprising a second deflector (315; 403) configured to deflect the electron beam towards a desired location on the target.

4. The X-ray source of claim 3, further comprising a third deflector (407) configured to direct the electron beam towards the second deflector, wherein the target is positioned on a straight line defined by the electron source and the electron beam limiting aperture.

5. The X-ray source of claim 3, further comprising a third deflector (407) and a fourth deflector (415), wherein the third deflector is configured to deflect the electron beam towards the fourth deflector, and the fourth deflector is configured to deflect the electron beam towards the second deflector (403).

6. The X-ray source of any one of any one of the preceding claims, comprising at least one deflector (203; 315; 403) that is configured to scan the electron beam over the target.

7. The X-ray source of any one of the preceding claims, wherein the target comprises a substrate and a metal layer configured to generate X-ray radiation upon impact of the electron beam.

8. The X-ray source of claim 7, wherein the substrate of the target constitutes the exit window.

9. The X-ray source of any one of the preceding claims, further comprising a focus lens (208; 308; 408) configured to focus the electron beam on the target.

10. An X-ray arrangement comprising an X-ray source according to any one of claims 1-9; and a detector for detecting the primary X-ray radiation.

11. A method at an X-ray source, comprising: directing an electron beam towards an electron beam limiting aperture, wherein the electron beam limiting aperture limits an angular distribution of the electron beam, the electron beam causing secondary X-ray radiation to be emitted from a region around said electron beam limiting aperture; deflecting the electron beam, towards a secondary radiation limiting aperture, wherein the secondary radiation limiting aperture confines propagation of the secondary X-ray radiation to a secondary radiation propagation region preventing the secondary X-ray radiation from reaching an exit window of the X-ray source;causing the electron beam, after passage through the secondary radiation limiting aperture, to impinge upon a desired location on a target to generate primary X-ray radiation by interaction between the target and the electron beam.

12. The method of claim 11, wherein causing the electron beam to impinge upon a desired location on the target comprises deflecting the electron beam towards the target.

13. The method of claim 11 or 12, further comprising scanning the electron beam over the target.

14. The method of any one of claims 11-13, further comprising detecting the primary X-ray radiation using a detector arranged to receive X-ray radiation transmitted through the exit window.

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