Reduction of secondary radiation

Abstract

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

Description

Technical Field

[0001] The present disclosure relates to X-ray arrangements and X-ray sources. More specifically, the present disclosure relates to reducing the effects caused by secondary X-ray radiation, i.e., unwanted X-ray radiation generated at the X-ray source.

Background Art

[0002] X-ray radiation can be generated by directing an electron beam onto a target material. Thus, a typical prior art X-ray source comprises an electron source for generating an electron beam and an electron optical system for directing and focusing the electron beam onto the target material. Such an electron optical system may include magnetic alignment and / or stigmatol 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, for example, a target film made of, for example, tungsten (W) deposited on a target substrate material made of diamond. The typical thickness of the target film can be about 0.5 μm or several micrometers or tens of micrometers, while the typical thickness of the target substrate can be about 100 μm or several hundred micrometers.

[0003] The generated X-ray radiation can, for example, be intended for use in X-ray imaging of a sample or object.

[0004] Secondary radiation is a well-known problem within the technical field of X-ray sources. Secondary radiation is generally understood as any radiation other than the desired radiation and can, for example, result from the scattering of X-rays or from unintentional electron beam interactions in the system. The source of secondary radiation can be the scattering of the primary X-ray radiation, the interaction of backscattered electrons with other source components, or the interaction between the electron beam and a part of the source located upstream of the target.

[0005] Therefore, improved techniques are desired to address such secondary X-ray emission. [Overview of the project]

[0006] The problems solved by the present invention arise, for example, when an aperture or diaphragm is used to restrict the angular distribution of an electron beam before the electron beam enters the electron optical system in the X-ray source. Restricting the angular distribution of the electron beam may be necessary when the desired X-ray spot size, and therefore the desired electron beam spot size on the target, is small. In this context, small typically refers to a spot size smaller than 1 μm in diameter. Furthermore, the solutions disclosed herein are most relevant when the primary source of secondary radiation is a clearly defined object upstream of the target, such as the aperture described above. X-ray radiation generated by interaction with such objects may appear as a distinct artifact or produce a secondary image of the object in the field of view with a different magnification than the image obtained from the primary X-ray source. However, the fact that other sources of secondary radiation may cause more or less uniform noise in the image can also be mitigated, at least to some extent.

[0007] This invention is based on the recognition that secondary X-ray emission can be prevented from reaching a detector used to image a sample, or even completely prevented from escaping from the X-ray source. Since the primary source of secondary X-ray emission is the electron beam limiting aperture, such secondary emission can be prevented from reaching the detector either by positioning the detector outside the solid angle through which the secondary emission escapes from the X-ray source's exit window, or by confining the propagation of the secondary emission within the X-ray source's enclosure so that the secondary emission cannot reach the exit window. This invention relates to confining the propagation of secondary emission so that it cannot reach the exit window. For example, by introducing a second aperture inside the X-ray source that acts as a secondary emission limiting aperture through which the electron beam passes before reaching the target, the propagation of secondary emission beyond the second aperture is confined.

[0008] Generally, the solutions proposed herein involve deflecting the electron beam after it has passed through an electron beam limiting aperture, i.e., an aperture used to limit the angular distribution of the electron beam, and limiting the angular distribution of any secondary X-ray emission generated in that aperture. As is understood, the deflection of the electron beam does not affect the propagation of the X-ray emission, and therefore the secondary emission limiting element, e.g., aperture, can be positioned such that no secondary X-ray emission reaches the exit window of the X-ray source and therefore cannot interfere with the detector used to detect the primary X-ray emission generated from the interaction between the electron beam and the target. In a typical X-ray source, the X-ray emission can only escape / emit from the enclosure through one or more intentional exit windows.

[0009] As used herein, the terms “upstream” and “downstream” refer to positions relative to the direction of electron beam propagation. Therefore, upstream means closer to the electron source, and downstream means further away from the electron source.

[0010] An X-ray source according to the principle disclosed herein comprises: an electron source configured to provide an electron beam; an aperture configured to limit the angular distribution of the electron beam, wherein the electron beam is configured to cause secondary X-ray emission to be emitted from a region surrounding the aperture, thereby limiting the angular distribution of secondary X-ray emission, wherein the secondary X-ray emission limiting aperture is located downstream from the electron beam emission limiting aperture, for example, between the electron beam emission limiting aperture and the secondary X-ray emission limiting aperture, wherein the first deflector is configured to deflect the electron beam toward the secondary X-ray emission limiting aperture, and comprises: a target located downstream from the secondary X-ray emission limiting aperture and configured to generate primary X-ray emission through interaction with the electron beam; and an exit window arranged to transmit primary X-ray emission, wherein secondary emission is prevented from reaching the exit window by the secondary X-ray emission limiting aperture.

[0011] In some other examples, the secondary emission limiting element includes an exit window positioned to allow primary X-ray emission to be emitted toward the detector, and the detector is positioned such that the normal to the detector surface is at an angle of at least b / l radians with respect to the direction of electron beam collision at the target, where b is the full width at half maximum of the detector and l is the distance from the target to the detector. Preferably, the surface of the target is positioned parallel to the detector.

[0012] While the first deflector may also be configured to deflect the electron beam to a desired position on the target, it is preferable to include a second deflector for this purpose. It is also preferable to include a third deflector configured to direct the electron beam toward the second deflector, with the target positioned on a straight line defined by the electron source and the electron beam limiting aperture. In this type of embodiment, an X-ray source is provided in which substantially all of the emitted radiation is primary radiation generated by the interaction between the electron beam and the target. In other words, the interaction between the electron beam and any other part of the source, such as the electron beam limiting aperture, does not result in X-ray radiation emitted from the source.

[0013] In all embodiments, it is preferable to have at least one deflector 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 emission upon impact with the electron beam. To achieve a desired spot size of the electron beam on the target, this arrangement preferably includes a focusing lens for focusing the electron beam onto the target.

[0014] A method in an X-ray source is also provided, which comprises directing an electron beam toward an electron beam limiting aperture, wherein the electron beam limiting aperture restricts the angular distribution of the electron beam, causing the electron beam to be deflected toward a secondary emission limiting aperture, wherein the secondary emission limiting aperture is large enough to allow the electron beam to pass through, but confines the propagation of the secondary X-ray emission to a secondary emission propagation region, preventing the secondary X-ray emission from reaching the exit window of the X-ray source, and causing the electron beam to strike a desired position on the target such that, after passing through the secondary emission limiting aperture, the interaction between the target and the electron beam generates primary X-ray emission. Strikes a desired position on the target may comprise deflecting the electron beam toward the target. The method may also involve scanning the electron beam on the target. Furthermore, the primary X-ray emission emitted through the exit window can be detected by a detector.

[0015] In the following detailed description of preferred embodiments, reference will be made to the accompanying drawings. [Brief explanation of the drawing]

[0016] [Figure 1a] Figure 1a schematically illustrates an example in which secondary X-ray emission is allowed to escape from the X-ray enclosure, but escapes in a direction that prevents it from reaching the detector. [Figure 1b] Figure 1b illustrates the geometric shape of the example in Figure 1a in more detail. [Figure 2] Figure 2 schematically shows an example in which secondary X-ray emission is contained within an X-ray enclosure. [Figure 3] Figure 3 schematically shows an example in which secondary X-ray emission is contained within an X-ray enclosure. [Figure 4] Figure 4 schematically shows an example in which secondary X-ray emission is contained within an X-ray enclosure. [Figure 5]Figure 5 schematically shows an example in which secondary X-ray emission is contained within an X-ray enclosure. [Figure 6] Figure 6 schematically shows an example in which secondary X-ray emission is contained within an X-ray enclosure. [Figure 7] Figure 7 illustrates an exemplary method in an X-ray apparatus. [Modes for carrying out the invention]

[0017] A first example of an X-ray apparatus 100 is shown in Figure 1a. This apparatus 100 includes an electron source 102 for generating an electron beam 104 (shown as a dashed line throughout the drawing). Primary X-ray emission is generated by the interaction between the electron beam 104 and a target 106. The X-ray apparatus includes a focusing lens 108 for focusing the electron beam onto a spot on the target 106, and a deflector 110 for deflecting the electron beam so that the spot is formed at a desired location on the target. The electron beam is passed through an electron beam limiting aperture 112 to restrict the angular distribution of the electron beam before entering the electron beam optical system, such as the focusing lens 108. Once the angular distribution of the electron beam 104 is limited using the aperture 112, secondary X-ray emission is generated, typically by the interaction between the electron beam and the aperture structure. The propagation of such secondary emission is illustrated by reference numeral 114 in Figure 1. To prevent secondary radiation from adversely affecting measurements performed using primary X-ray radiation, the X-ray apparatus in Figure 1 is also designed so that secondary radiation does not reach the detector 120 used to detect the X-ray radiation, and preferably, the sample 116 to be imaged (which may be mounted on the translation stage 118). As shown, the electron source 102, focusing lens 108, deflector 110, and aperture 112 are all mounted within the enclosure 122. The target 106 is located at an exit window from the enclosure, and may even constitute an exit window.

[0018] The influence of secondary X-ray emission on the detected X-ray image is mitigated here by allowing the secondary emission 114 to escape from the enclosure 122 at a solid angle not used in the application, i.e., a solid angle that does not include the detector 120 (and preferably also does not include the sample 116). This may impose limitations on how the X-ray tube can be used due to the geometric shape of the X-ray tube, but this is acceptable in some applications. As illustrated in Figure 1a, the target 106 is inclined with respect to the incident electron beam 104 to allow the sample 116 to be placed close to the source, and the target 106 itself acts as a secondary emission limiting element for the secondary X-ray emission 114. However, an additional complexity in this example is that the inclined target surface deforms the electron beam cross-section, i.e., a circular electron beam striking the target at a non-perpendicular angle results in an elliptical electron beam spot on the target. To compensate for this, it may be preferable to adjust the astigmatism of the incident electron beam before it strikes the target.

[0019] As can be understood, any secondary radiation generated upstream from the target follows its propagation path and exits the enclosure at a solid angle determined by the size of the exit window (target) and the size and position of the source of such secondary radiation. The detector and preferably the sample are also positioned outside this solid angle occupied by the secondary radiation, thereby eliminating the influence of secondary radiation on the detector (and sample).

[0020] FIG. 1b shows a schematic view of the target 106 and the detector 120 to illustrate the geometric shape in more detail. To prevent secondary radiation from hitting the detector, the minimum angle α required for the inclination of the target and the detector with respect to the incident electron beam can be calculated based on congruent triangles in a simplified model where secondary radiation is considered to be emitted from a dotted-line source. For a detector of width 2b located at a distance l from the target, the angle must be greater than b / l radians for a first-order approximation valid for small angles. As an example, for a detector with a half-value width b = 100 mm installed at a distance l = 500 mm from the target, the angle α must be at least 0.2 radians, i.e., 11.5 degrees.

[0021] The present invention provides an improved solution for reducing the influence of secondary X-ray radiation. Generally, embodiments of the present invention prevent secondary radiation from reaching the exit window of the X-ray source. As illustrated herein, the exit window can be constituted by the target itself or can be arranged in proximity to the target.

[0022] FIGS. 2-6 illustrate examples where secondary radiation is confined within the enclosure of the X-ray source of the apparatus, i.e., prevented from reaching the exit window of the X-ray source. These examples are presented in ascending order of the number of components.

[0023] FIG. 2 shows an X-ray source 200 in which an electron beam 204 generated by an electron source 202 is restricted by an electron beam restricting aperture 212. Secondary emission 214 will result from an interaction between the electron beam 204 and the material defining the electron beam restricting aperture 212. By carefully selecting the material and physical dimensions, the emitted secondary emission can be reduced by self-absorption. However, the edges of the aperture will still emit a non-negligible amount of secondary emission. Therefore, downstream of the electron beam restricting aperture 212, a deflector 203 is provided for deflecting the electron beam 204 towards a second aperture 213 and a target 206. The second aperture 213 is arranged to prevent the secondary emission 214 from reaching the target 206 and is thus referred to herein as a secondary emission restricting aperture. Any portion of the secondary emission that passes through the second aperture will instead impinge on an enclosure 222 that is opaque to X-ray radiation. The second aperture 213 is structured and arranged such that the electron beam 204 does not interact with the material defining the aperture and thus no secondary emission is generated there. Since the second aperture has the purpose of restricting secondary emission and not restricting the electron beam, the diameter of the second aperture can be considerably larger than that of the electron beam restricting aperture. A focusing lens 208 is arranged downstream of the second aperture 213 to focus the electron beam 204 onto the target 206 to generate primary X-ray radiation.

[0024] Figure 3 shows an example similar to the example in Figure 2. In the X-ray source 300 shown in Figure 3, the electron beam 304 is limited by the electron beam limiting aperture 312. Secondary emission 314 is produced by the interaction between the electron beam 304 and the material defining the electron beam limiting aperture 312. A first deflector 303 is located downstream of the electron beam limiting aperture 312 to deflect the electron beam 304 toward the second aperture 313 and the target 306. The second aperture 313 is positioned to prevent secondary emission 314 from reaching the target 306. A focusing lens 308 is located downstream of the second aperture 313 to focus the electron beam 304 onto the target 306 to produce primary X-ray emission. The difference in this example compared to the example shown in Figure 2 is that a second deflector 315 is added downstream of the second aperture 313. The purpose of this second deflector 315 is to direct the electron beam to a desired position on the target and / or scan the electron beam on the target. In principle, while the first deflector 303 can perform these tasks of directing and / or scanning the electron beam (as can be done, for example, in the example shown in Figure 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 manner, and another component dedicated to moving the electron beam on the target in a dynamic manner.

[0025] A common feature of the examples in Figures 1-3 is that the setups are angled. While such angled setups are relatively simple and require fewer components, in some applications, a more linear setup is preferable. Various examples providing linear or linear setups are shown in Figures 4-6.

[0026] Figure 4 shows a first example of an X-ray apparatus 400 having a linear X-ray tube. The electron source 402 is positioned off-axis with respect to the output X-ray beam 401. The angular distribution of the electron beam 404 is limited by an electron beam limiting aperture 412, and downstream of the electron beam limiting aperture 412, the electron beam 404 is deflected toward a second aperture 413 by a first deflector 405. In the second aperture, the electron beam is deflected toward a focusing lens 408 and a target 406 by a second deflector 403. To allow movement of the electron beam spot on the target, the second deflector 403 requires a constant bias and, in addition, a control signal. The electron beam limiting aperture 412 and the second aperture 413 have similar functions to those described in relation to Figure 3 above. The geometric structure is such that any generated secondary X-ray emission is confined in a propagation direction that collides with the interior of the enclosure 422, away from the exit window (i.e., target 406). Therefore, the output primary X-ray emission 401 does not contain any secondary emission. This reduces the influence of secondary X-ray emission on imaging of the sample 416 using the detector 420. Accordingly, the example shown in Figure 4 includes a second deflector 403 for deflecting the electron beam to a desired position on the target and a first deflector 405 for deflecting the electron beam towards a second aperture 413.

[0027] Figure 5 shows an example similar to that shown in Figure 4, but with the electron source 402 on-axis. There may be several reasons for orienting the electron source on-axis, for example, to facilitate electrical connections. The orthogonal orientation of the electron source is made possible here by adding a third deflector 407 compared to the example in Figure 4. This is likely to be a preferred embodiment in many cases, as it can be achieved with relatively small changes to the prior art design. Thus, the example shown in Figure 5 comprises a first deflector 405 for deflecting the electron beam toward the aperture 413, a second deflector 403 for deflecting the electron beam toward a desired position on the target, and a third deflector 407 for deflecting the electron beam toward the second deflector 403.

[0028] Figure 6 shows another example similar to the example in Figure 5, but the functions of directing the electron beam to a desired position on the target and scanning the electron beam on the target are separated by the addition of a fourth deflector 415. Thus, the example shown in Figure 6 includes a first deflector 405 for deflecting the electron beam toward the aperture 413, a second deflector 403 for directing the electron beam toward a desired position on the target, and a third deflector 407 for deflecting the electron beam toward the fourth deflector 415, the fourth deflector 415 for deflecting the electron beam toward the second deflector 403.

[0029] In each of the examples shown in Figures 2 to 6, the electron beam limiting aperture and the second aperture are positioned to prevent secondary X-ray emission generated in the electron beam limiting aperture from escaping the enclosure through the exit windows / targets 206, 306, and 406. Structurally, this can be achieved by ensuring that there is no direct / linear propagation path from the electron beam limiting aperture to the target / exit window (i.e., no line of sight), meaning that the electron beam must be deflected at least once after passing through the electron beam limiting aperture before reaching the target / exit window. Therefore, the electron beam limiting aperture and the secondary emission limiting aperture are not collinear on the path toward the target / exit window. This ensures that the primary X-ray emission output from the X-ray source substantially does not contain such secondary emission. It should be noted that the deflectors and apertures can be arranged in several ways to achieve the intended function according to the present invention. For example, the aperture 413, which is located upstream of the deflector 407 in Figures 5 and 6, could instead be located downstream of the deflector. In a further alternative, the aperture and the deflector could be integrated so that the deflection occurs within the plane defined by the aperture.

[0030] Figure 7 illustrates method 700 in an X-ray apparatus for mitigating the effects of secondary X-ray emission. In 701, the electron beam is directed toward an electron beam limiting aperture, which limits the angular distribution of the electron beam. When the angular distribution of the electron beam is limited by the aperture, secondary X-ray emission is generated from the region surrounding the electron beam limiting aperture. In 702, the electron beam is deflected toward a secondary emission limiting aperture, which confines the propagation of secondary X-ray emission to a secondary emission propagation region, preventing the secondary X-ray emission from reaching the exit window of the X-ray source. Then, in 703, after passing through the secondary emission limiting aperture, the electron beam is directed toward a desired position on the target so that the interaction between the target and the electron beam generates primary X-ray emission. The generated primary X-ray emission can then be used, for example, to image a sample, and in that sense can be detected using a detector positioned to receive the X-ray emission transmitted through the exit window, as shown in 704. By positioning the X-ray source's exit window outside the secondary radiation propagation region (i.e., by preventing secondary X-ray radiation from reaching the exit window using a secondary radiation limiting aperture), the primary X-ray radiation emitted through the exit window substantially contains no interfering secondary X-ray radiation.

[0031] The arrangement, radiation source, and method according to the present invention may be used for different types of X-ray imaging, such as X-ray microscopy, radiography, fluoroscopy, or CT scanning.

Claims

1. X-ray sources (200; 300; 400), wherein the X-ray sources are 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 the angular distribution of the electron beam, wherein the electron beam causes secondary X-ray emission to occur from the region surrounding the electron beam limiting aperture. A secondary emission limiting aperture (213; 313; 413) configured to limit the angular distribution of the secondary X-ray emission, wherein the secondary emission limiting aperture is located downstream of the electron beam limiting aperture. A first deflector (203; 303; 405) configured to deflect the electron beam toward the secondary emission limiting aperture, A target (206; 306; 406) is located downstream from the secondary emission limiting aperture and is configured to generate primary X-ray emission through interaction with the electron beam, An exit window arranged to allow the aforementioned primary X-ray radiation to pass through, Equipped with, An X-ray source in which the secondary X-ray emission is prevented from reaching the exit window by the secondary emission limiting aperture.

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

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

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

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

6. The X-ray source according to any one of claims 1 to 5, comprising at least one deflector (203; 315; 403) configured to scan the electron beam over the target.

7. The X-ray source according to any one of claims 1 to 6, wherein the target comprises a substrate and a metal layer configured to generate X-ray emission when the electron beam collides with it.

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

9. The X-ray source according to any one of claims 1 to 8, further comprising a focusing lens (208; 308; 408) configured to focus the electron beam onto the target.

10. An X-ray source according to any one of claims 1 to 9, A detector for detecting the aforementioned primary X-ray emission, An X-ray apparatus equipped with the following features.

11. A method using an X-ray source, The electron beam is directed toward an electron beam limiting aperture, where the electron beam limiting aperture restricts the angular distribution of the electron beam, causing secondary X-ray emission from the region surrounding the electron beam limiting aperture. The electron beam is deflected toward a secondary emission limiting aperture, wherein the secondary emission limiting aperture confines the propagation of the secondary X-ray radiation to the secondary emission propagation region, preventing the secondary X-ray radiation from reaching the exit window of the X-ray source. The electron beam, after passing through the secondary emission limiting aperture, is to strike a desired position on the target in order to generate primary X-ray emission through interaction between the target and the electron beam. A method that includes [a certain feature].

12. The method according to claim 11, wherein causing the electron beam to strike a desired position on the target is to deflect the electron beam toward the target.

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

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

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