Small target X-ray system

Abstract

An X-ray system is disclosed, the system comprising: an X-ray source; sample position; a detector arranged to detect X-ray radiation downstream of the sample location; and a controller. The X-ray source comprises: an electron source arranged to provide an electron beam; a target arranged to generate X-ray radiation at an electron beam spot when impacted by the electron beam, where the target comprises a substrate and a target layer, and where the target layer presents an aperture having a diameter smaller than a diameter of the electron beam spot; and a deflector for directing the electron beam to a first location on the target, where the electron beam spot is formed at a uniform portion of the target layer remote from the aperture, and directing the electron beam to a second location on the target, where the electron beam spot is formed at a portion of the target presenting the aperture. The controller is configured to acquire, using the detector, a first image in which the electron beam is directed to the first position and a second image in which the electron beam is directed to the second position, and generate a difference image between the first image and the second image. A corresponding method is also disclosed.

Description

Technical Field

[0001] The present disclosure relates to electron impact x-ray sources. 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 includes an electron gun for generating an electron beam, and electron optics for directing and focusing the electron beam onto a target material. Such electron optics may include magnetic alignment and / or astigmatism corrector coils, and electrostatic or magnetic focusing lenses and deflectors. The target material typically includes a high Z (e.g., atomic number Z>20) material that generates X-rays by bremsstrahlung under electron impact. The target may include a target film (e.g., made of tungsten (W)) deposited on a target substrate material (e.g., made of diamond). The typical thickness of the target film may be about 0.5 μm or a few microns or tens of microns, while the typical thickness of the target substrate may be about 100 μm or hundreds of microns.

[0003] The generated X-ray radiation may be directed for example for X-ray imaging of a sample or object.

[0004] One type of electron impact X-ray source is a so-called transmission X-ray source. In such an X-ray source, the generated X-ray radiation is transmitted through the target and thus generally propagates in the same direction as the electron beam. Therefore, the target in such an X-ray source is often referred to as a transmission target (as opposed to a reflection target, in which the generated X-ray radiation does not penetrate the target material, but is instead collected by "reflecting" from the target).

[0005] Transmission targets generally have the advantage that a smaller X-ray spot can be achieved in the target, thereby potentially achieving higher imaging resolution. Summary of the invention

[0006] It is often desirable to make the X-ray spot smaller. The reason for doing this may be to improve resolution or to reduce image blurring generated by any finite size spot itself. The present disclosure is based on the recognition that for any given system, making the electron beam spot on the target smaller will be limited by the limited current that can pass through the focusing coil and the inherent repulsion between the electrons that form the electron beam. Various system defects (e.g., lens aberrations, voltage jitter, vibration, etc.) may also limit the achievable spot size. Then, a possible alternative may be to construct the target film so that it presents a small feature from which the X-ray radiation is generated (and wherein the amount of X-ray radiation generated outside the small feature is negligible or acceptable). In principle, then, the size of the X-ray spot will become independent of the size of the electron beam spot at the target, and instead, will be defined by the size of the target feature. However, with this approach, the spot size will be limited by the thermal power that can be removed from the small target feature, and to some extent, by the background X-ray radiation generated by the target substrate adjacent to the target feature that is struck by the electron beam.

[0007] Therefore, the known methods of reducing the X-ray spot size in a transmission type X-ray source have some limitations. When using electron optics to focus the electron beam into a smaller spot, there are inherent limitations due to the repulsive nature of the available current and electron interactions. In addition, although the spot size is usually defined by the full width at half the maximum intensity, the electron distribution will extend beyond this width and promote the widening of the X-ray spot. The interaction between the electron and the substrate will also promote the widening of the X-ray spot. Since the target layer is usually relatively thin in order to avoid the widening of the electron spot caused by scattering, most of the incident electrons will penetrate the target layer and further scatter in the relatively thick substrate. Therefore, the electron distribution will widen and produce a secondary X-ray radiation source with lower intensity (because the substrate material usually has a lower atomic number than the target material, thereby having a lower X-ray output) and a wider distribution (because the electrons will experience multiple scattering events in the thick substrate). The resulting X-ray irradiation of the object to be imaged will be the sum of a narrow high intensity distribution (giving a relatively clear image) and a wide low intensity distribution (giving a blurred image).

[0008] As briefly discussed above, in X-ray sources where the target includes X-ray generating features disposed on or embedded in a substrate (i.e., a target area that is irradiated with the electron beam as a whole so that the radiated X-ray spot is defined by its size and geometry), the performance of the electron optics is not an issue because the electrons will not produce the expected X-rays unless they strike the X-ray generating features. However, in this case, the interaction between the electrons and the substrate will also produce a broad, low-intensity X-ray contribution that will illuminate the object and cause blurring of the resulting image. The relative magnitude of this contribution is related to the electron energy and the design of the target, and is independent of the exposure time.

[0009] Therefore, the present invention proposes to use a target that presents a small hole in an originally uniform target layer. Then, an effective spot size having the size of the hole in the target layer can be obtained by acquiring a first image using an electron beam directed toward a uniform target away from the hole, and acquiring a second image using an electron beam directed toward the hole, and then subtracting the second image from the first image. Thus, very high imaging resolution can be achieved by making the hole small enough. Another advantage is that the impact of X-ray radiation generated by the interaction between the electron beam and the substrate may be greatly reduced, because the contribution of radiation generated in the substrate when the electron beam is directed toward the uniform target layer is roughly the same as the contribution of radiation when the electron beam is directed toward the hole. Therefore, when the two images are subtracted from each other, the contribution will cancel. The trade-off of this approach is that subtracting the two images will produce a final image with greater noise, which means that the signal-to-noise ratio will be reduced compared to assuming a source with a smaller electron spot or embedded features of corresponding size. However, this trade-off can be compensated by a longer exposure time.

[0010] The invention is defined in the accompanying claims, wherein the dependent claims are directed to preferred embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In the following detailed description, reference is made to the accompanying drawings, in which:

[0012] Figure 1 Schematically illustrates an X-ray system according to the principles disclosed herein;

[0013] Figure 2 The use of a target presenting holes is illustrated;

[0014] Figure 3 shows the results of simulations of the principles disclosed herein;

[0015] Figure 4 shows the results of experimental testing of the principles disclosed herein; and

[0016] Figure 5 A method according to the principles disclosed herein is outlined. DETAILED DESCRIPTION

[0017] By introduction, Figure 1An exemplary X-ray system including an X-ray source 100 is schematically shown. The X-ray source as shown includes an electron gun (or electron source) 102 configured to form an electron beam 114. The electron beam emitted from the electron gun 102 is aligned and shaped using an alignment coil 104 and an astigmatism corrector coil 106, respectively, before reaching a beam-limiting element in the form of an aperture 108. Such an aperture 108 is intended to limit the geometric spread of the electron beam before the electron beam enters the downstream (relative to the propagation direction of the electron beam) beam shaping optical device. The diameter (i.e., the opening size) of the aperture 108 is typically about 1 mm. After passing through the aperture 108, the electron beam reaches the electron optics, which typically includes one or more focusing lenses 110 and one or more deflectors 112. The electron optics is used to guide and focus the electron beam to a desired position on a target 116.

[0018] Target 116 typically comprises a high Z (e.g., atomic number Z>20) material that generates X-rays 118 when impacted by electrons. A typical target material in this type of X-ray source is tungsten (W), which is present as a film on a substrate material made, for example, of diamond. More generally, the target layer may comprise a material selected from tungsten, rhenium, molybdenum, vanadium, and niobium, and the substrate may comprise beryllium or a carbon material such as diamond. These components of the X-ray source are enclosed within a low pressure (vacuum) housing 120, although the electron optics implemented as coils may be located outside of the housing. The generated X-ray radiation 118 may be used to image a sample or object 130 located at a sample location using a detector 140.

[0019] Figure 1 The exemplary X-ray source shown in also includes a controller 150, which is configured to use the detector 140 to record a first image of the electron beam being directed to a first position on the target 116, record a second image of the electron beam being directed to a second position on the target 116, and generate a difference image between the first image and the second image, for example, by subtracting the corresponding pixel values ​​of the second image from the pixel values ​​of the first image. The controller 150 may also be responsible for controlling the lens 110 and the deflector 112 to direct the electron beam to the position. As shown in FIG. Figure 1 As shown, controller 150 may be operatively coupled to electron optics 110 , 112 and detector 140 .

[0020] Thus, an X-ray imaging system according to an embodiment includes an X-ray source 100 as discussed above, a sample position (determined by Figure 1130) and a detector 140. The X-ray source 100 includes an electron source 102 arranged to provide an electron beam, a target 116 arranged to generate X-ray radiation when struck by the electron beam, a device such as electron optical devices 110, 112 for directing the electron beam to a first position and a second position on the target, and a controller 150, which is configured to use the detector to acquire a first image in which the electron beam is directed to the first position, acquire a second image in which the electron beam is directed to the second position, and generate a difference image between the first image and the second image.

[0021] According to the principles disclosed herein, the target layer of the target exhibits a hole having a diameter that is smaller than the diameter of the electron beam spot formed on the target by the electron optics 110, 112. Typically, the diameter of the hole may be smaller than the smallest electron beam spot achievable using the electron optics 110, 112. The diameter of the hole may be, for example, about 300 nm or less, such as less than 200 nm or less than 100 nm.

[0022] Although it is preferred that the holes in the target layer are circular, other geometries are contemplated in some embodiments. In this case, it is preferred that the size of the hole along its largest dimension is less than 300 nm (e.g., less than 200 nm or less than 100 nm). For example, the hole may have an elliptical shape, a rectangular shape, or a square shape.

[0023] The imaging system may further comprise an actuator (not shown) arranged to move the target 116 relative to the electron beam 114, such that directing the electron beam to the first position and the second position may involve steering / deflecting the electron beam using electron optics and / or moving the target. In principle, embodiments in which the electron beam is directed to the first position and the second position on the target by moving the target may leave the electron beam, the sample and the detector fixed, which may be preferred in some cases for simplicity. The imaging system may further comprise a manipulator for moving the X-ray source and the sample position relative to each other. Such a manipulator may, for example, be a translation stage 135 to which the sample may be mounted, such as Figure 1 Preferably, the manipulator is also operatively connected to the controller 150, such as Figure 1Schematically illustrated by the dashed lines in . Typically, the sample manipulator is operated so that the first image and the second image of the sample overlap on the detector, that is, the image of the object is recorded by the same detector pixels for the first image and the second image. In other words, in the first image and the second image, the object located at the sample position is imaged by the detector similarly (i.e., at substantially the same position on the detector), or the image of the object is at substantially the same position on the detector for the first image and the second image. In this case, further alignment of the images may not be required before performing the subtraction, although alignment for fine-tuning may still be implemented. For many applications in which the distance from the sample position to the detector is larger than the distance between the X-ray source and the sample, the sample can be moved in the same way as the electron beam to achieve the desired effect, although in principle the sample motion should be scaled by the ratio between the distance from the sample to the detector and the distance from the source to the detector.

[0024] exist Figure 2 The inventive concept is schematically illustrated in FIG. A portion of a target is shown at (a) where the target layer 210 is uniform, thus covering the target substrate 220, and a portion of a target is shown at (b) where the target layer 210 exhibits pores, i.e., areas where no target material is present. The pores may be empty, exposing the underlying substrate 220, or may be filled with a suitable low-Z material. Figure 2 The top part of the graph indicates the intensity of the incident electron beam, while Figure 2 The intensity of the X-ray spot is indicated at the bottom portion of the . When the electron beam is directed to a portion of the target having a uniform target layer, such as Figure 2 As shown in (a) in the figure, the X-ray spot will have a smooth, Gaussian-like energy distribution. When the electron beam is directed to a portion of the target layer presenting a hole, as shown at (b), the energy distribution in the X-ray spot will have a corresponding indentation or depression. By subtracting the image obtained using the X-ray distribution affected by the hole as shown at (b) from the image obtained using the X-ray distribution obtained from the uniform portion of the target, the result will be similar to the result achieved using a target that generates X-ray radiation only from the portion of the target corresponding to the hole, as shown in the X-ray distribution shown at (c). Therefore, an effective X-ray spot with the size of the hole can be achieved. It should be understood that the electron beam can have the same size and intensity curve and nominal power when incident on a uniform portion of the target (as shown at (a)) and when incident on a portion of the target presenting a hole (as shown at (b)).

[0025] However, in some embodiments, the electron beam may have a different nominal power when directed to the first target location compared to the second target location.The acquired image may then be scaled, for example to take into account the dynamic range, before performing the subtraction to obtain the difference image.

[0026] Further, as will be appreciated, it may be preferable to ensure that the electron beam is blanked (to prevent X-ray generation) during displacement from the first position to the second position, and / or to gate the detector to avoid artifacts in the acquired image.

[0027] In an embodiment of the present invention, the diameter of the hole is preferably less than 300 nm. As discussed above, the reason for making the effective X-ray spot smaller is to achieve higher imaging resolution, i.e., to be able to resolve small details. The size of the details that can be resolved will ultimately be limited by diffraction. Therefore, in order to avoid diffraction effects, the object can be positioned close to the source. For a transmission target, the distance between the source and the object is in principle limited by the thickness of the target (target layer and target substrate). Therefore, it may be advantageous to reduce the target thickness and thereby reduce the distance from the source to the object.

[0028] The dimensionless number that characterizes diffraction is the Fresnel number where d is the diameter of the object (or source), t is the distance from the source to the object, and λ is the wavelength. F When it is less than 1, diffraction effects (Fraunhofer diffraction) will dominate. Therefore, for an object diameter of 100 nm and an X-ray wavelength of 0.1 nm (corresponding to a photon energy of 12 keV), the source-to-object distance should preferably be less than 100 μm. Therefore, in order to be able to exploit the advantages offered by an effective X-ray spot size of 100 nm, when it comes to resolving small details, the target thickness (which generally imposes a lower limit on the source-to-object distance) should preferably be less than about 100 μm.

[0029] When capturing a first image and a second image according to the principles disclosed herein, the first image and the second image may not always coincide on the detector because different portions of the target layer are used to generate X-ray radiation and because the sample may have been moved between the two images. Therefore, in some embodiments, the controller is further configured to align the first image and the second image (by shifting and / or rotating) before generating the difference image. Such alignment of the images can be implemented by image processing techniques known per se.

[0030] In some embodiments, it may also be useful to include other kinds of image processing in image acquisition. For example, a captured image may be cropped to eliminate portions of the image that are not of interest. Other image processing techniques that may become useful in image acquisition are noise reduction, scaling, compression, thresholding, image formatting, etc. For example, thresholding (where relative intensity values ​​below a threshold are set to zero or relative values ​​above a threshold are set to one) may be useful for eliminating background artifacts outside of an area of ​​interest in an image.

[0031] Figure 3Shown are simulation results of an X-ray image of a gold cube with sides of 100 nm, located at a distance of 100 μm from a 10 keV X-ray source. The simulation has an ideal detector with 100 μm pixels placed one meter from the source. Figure 3 The field of view in is 2 μm×2 μm, and the images are shown in black and white after appropriate thresholding. Figure 3 (a) shows a simulated image acquired using a 100nm wide top-hat source spot profile, and as expected, the gold cube is clearly visible. Phase contrast dominates over absorption, and diffraction makes the cube appear round rather than square in the image. Using a 300nm Gaussian spot, Figure 3 (b) shows the image. Note that the cube is still discernible, although the contrast is lower. Using a 300nm Gaussian spot with a top-hat disk with the center 100nm missing from the radiation, we obtain Figure 3 (c) shows an image with reversed contrast. Figure 3 (b) Subtract the image Figure 3 (c) image, we get Figure 3 The image shown in (d) is, as expected, Figure 3 The image shown in (a) is very similar. However, Figure 3 The image shown in (d) is slightly noisier. The signal-to-noise ratio obtained by this subtraction method is less than half of that obtained with a 100 nm spot; therefore, the subtraction method using holes in the target layer is expected to require approximately 4 times the exposure time to achieve the same image quality.

[0032] Figure 4 Results from experimental tests of the technique proposed herein are shown.In these experimental tests, the size of the electron beam spot on the target was about 12 μm (full width at half maximum). Figure 4 The field of view in is approximately 850 μm and the image is shown in black and white after appropriate thresholding. Figure 4 The image in (a) is obtained using a uniform target layer to generate X-ray radiation. The image is obtained using an electron beam directed towards a portion of the target layer presenting a hole with a diameter of 3 μm. Figure 4 Finally, by Figure 4 Subtract the image in (a) Figure 4 (b) to obtain Figure 4 Image in (c).

[0033] Figure 4 The final difference image in (c) shows the improvement in resolution achieved using the techniques of the present invention. Figure 4 (a) and Figure 4It is not obvious in the image of (b) that the eight rectangular shapes consist of five parallel strips (6 μm wide and 6 μm apart), but in Figure 4 This can be clearly seen in the difference image in (c).

[0034] When getting Figure 4 When taking the X-ray images shown, the sample was not moved between the images, but instead the images were shifted by a distance corresponding to the difference in electron beam position. Therefore, artifacts appearing in the background of the difference image (blurred diagonal lines) may be due to secondary X-ray radiation generated upstream of the electron beam deflector. Instead, by moving the sample between the two image exposures so that the image of the sample does not move on the detector, the appearance of such artifacts as an effect of this secondary radiation can be eliminated or at least greatly reduced.

[0035] It can also be noted that, compared with the two actually acquired X-ray images, Figure 4 The difference image in (c) has more background noise. This is expected because when adding or subtracting two random variables, the variance of the result will be the sum of the variances of the two variables. This means that the standard deviation of the difference image should be expected to be larger than the standard deviation of one of the images. times. To improve the signal-to-noise ratio of the final difference image, the exposure time of the two acquired X-ray images can be increased. The signal-to-noise ratio is expected to increase with the square root of the number of detected X-ray photons and, therefore, with the square root of the integration time, i.e., to double the signal-to-noise ratio, the exposure time should be increased fourfold.

[0036] Despite Figure 4 A relatively large target hole with a diameter of 3 μm was used in the experimental tests, but it clearly shows the increased resolution obtained using the technology disclosed in this article.

[0037] Figure 5The X-ray imaging method according to the present invention is schematically illustrated. As discussed above, the method is performed using an X-ray source having a target arranged to generate X-ray radiation at an electron beam spot when struck by an electron beam, wherein the target includes a substrate and a target layer, and wherein the target layer presents a hole, the diameter of the hole being smaller than the diameter of the electron beam spot formed on the target layer. The method includes step 510: directing the electron beam to a first position on the target, wherein the electron beam spot is formed at a uniform portion of the target layer away from the hole. At step 512, a first X-ray image is acquired using a detector while directing the electron beam to the first position. At step 514, the electron beam is directed to a second position on the target, wherein the electron beam spot is formed at a portion of the target presenting the hole, and at step 516, a second X-ray image is acquired using a detector while directing the electron beam to the second position. Finally, at step 518, a difference image between the first image and the second image is generated.

[0038] As will be understood, the order in which the first image and the second image are acquired can be reversed. Of course, it is not important whether one image is acquired before the other image or after the other image.

[0039] The steps of directing the electron beam, acquiring images, and generating difference images are typically performed under the control of one or more processors in the X-ray system. Therefore, the present invention may also be implemented as computer software, which includes instructions (e.g., stored on a non-transitory computer readable medium) that, when executed, cause one or more processors in the X-ray system to perform steps 510 to 518 discussed above.

Claims

1. An X-ray system, comprising: X-ray source; Sample location; a detector arranged to detect X-ray radiation downstream of the sample location; Controller; Wherein, the X-ray source comprises: an electron source arranged to provide an electron beam; a target arranged to generate X-ray radiation at an electron beam spot when struck by the electron beam, wherein the target comprises a substrate and a target layer, and wherein the target layer exhibits holes having a diameter smaller than a diameter of the electron beam spot; a deflector for directing the electron beam to a first position on the target, wherein the electron beam spot is formed at a uniform portion of the target layer away from the hole, and directing the electron beam to a second position on the target, wherein the electron beam spot is formed at a portion of the target presenting the hole; The controller is configured to use the detector to acquire a first image in which the electron beam is directed to the first position and a second image in which the electron beam is directed to the second position, and to generate a difference image between the first image and the second image.

2. The X-ray system of claim 1, wherein: The diameter of the pores is less than 300 nm.

3. The X-ray system according to claim 1 or 2, wherein: The target thickness is less than where d is the diameter of the hole, and λ is the wavelength of the X-ray radiation.

4. The X-ray system according to any one of claims 1 to 3, wherein: The controller is further configured to align the first image and the second image before generating the difference image.

5. The X-ray system of any one of claims 1 to 4, further comprising a sample manipulator, and wherein, The controller is further configured to move the sample position using the sample manipulator so that an object located at the sample position is imaged at substantially the same position on the detector in the first image and the second image.

6. The X-ray system according to any one of claims 1 to 5, wherein: The target layer includes a material selected from the group consisting of tungsten, rhenium, molybdenum, vanadium, and niobium.

7. The X-ray system according to any one of claims 1 to 6, wherein: The substrate includes beryllium or a carbon material such as diamond.

8. A method for performing X-ray imaging using an X-ray source, the X-ray source comprising: an electron source arranged to provide an electron beam; as well as a target arranged to generate X-ray radiation at an electron beam spot when struck by the electron beam, wherein the target comprises a substrate and a target layer, and wherein the target layer exhibits holes having a diameter smaller than a diameter of the electron beam spot; The method includes: directing the electron beam to a first location on the target, wherein the electron beam spot is formed at a uniform portion of the target layer away from the aperture; acquiring a first X-ray image using a detector while directing the electron beam to the first location; directing the electron beam to a second location on the target, wherein the electron beam spot is formed at a portion of the target exhibiting the aperture; acquiring a second X-ray image using the detector while directing the electron beam to the second location; and A difference image between the first image and the second image is generated.

9. The method of claim 8, wherein: The diameter of the pores is less than 300 nm.

10. The method according to claim 8 or 9, wherein: The target thickness is less than where d is the diameter of the hole, and λ is the wavelength of the X-ray radiation.

11. The method of any one of claims 8 to 10, further comprising aligning the first image and the second image before generating the difference image.

12. The method of any one of claims 8 to 11, further comprising moving the sample position so that an object located at the sample position is imaged at substantially the same position on the detector in the first image and the second image.

13. The method according to any one of claims 8 to 12, wherein: Acquiring the X-ray image comprises applying image processing to the first and / or the second X-ray image before generating the difference image.

14. The method of claim 13, wherein: The image processing may involve any of noise reduction, scaling, compression, thresholding, formatting and / or cropping.

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