Small Target X-ray System

Abstract

An X-ray system is disclosed that includes an X-ray source, a sample position, a detector positioned to detect X-ray radiation downstream of the sample position, and a controller. The X-ray source includes an electron source positioned to provide an electron beam, a target positioned to generate X-ray radiation when the electron beam strikes an electron beam spot, and a deflector for directing the electron beam to a first position on the target where the electron beam spot is formed in a uniform portion of the target layer away from the hole and to a second position on the target where the electron beam spot is formed in a portion of the target exhibiting the hole. The controller is configured to use the detector to acquire a first image with the electron beam directed at the first position, acquire a second image with the electron beam directed at the second position, and generate a difference image between the first and second images. Corresponding methods are also disclosed.

Description

[Technical Field]

[0001] The present disclosure relates to electron impact x-ray sources. [Background technology]

[0002] X-ray radiation can be generated by directing an electron beam toward a target material. Thus, a typical prior art X-ray source includes an electron gun that generates an electron beam and electron optics that direct and focus the electron beam toward the target material. Such electron optics may include magnetic alignment and / or stigmator coils, electrostatic or magnetic focusing lenses, and deflectors. The target material typically includes a high-Z (e.g., atomic number Z greater than 20) material that generates X-rays by bremsstrahlung upon electron impact. The target may include a target film, for example, made of tungsten (W), deposited on a target substrate material, for example, made of diamond. A typical thickness of the target film may be about 0.5 μm, or a few micrometers, or tens of micrometers, and a typical thickness of the target substrate may be about 100 μm, or a few hundred micrometers.

[0003] The generated X-ray radiation may be intended for use in, for example, 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 this type of X-ray source, the generated X-ray radiation passes through the target and therefore propagates in approximately 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, where the generated X-ray radiation does not penetrate the target material but is "reflected" off the target and collected).

[0005] Transmission targets generally have the advantage that a smaller x-ray spot can be achieved at the target, thus potentially enabling higher imaging resolution. Summary of the Invention

[0006] A smaller X-ray spot is often desirable. This may be to increase resolution or to reduce image blurring inherent in a limited-size spot. This disclosure recognizes that achieving a smaller electron beam spot on the target for any given system is limited by the finite current that can be drawn through the focusing coil and the inherent repulsion between the electrons that form the electron beam. Various system imperfections (e.g., lens aberrations, voltage jitter, vibrations, etc.) may also limit the achievable spot size. In that case, a possible alternative may be to structure the target film so that it exposes small features from which the X-ray radiation originates (and a negligible or acceptably small amount of X-ray radiation occurs outside the small features). In principle, in this case, the X-ray spot size would be independent of the electron beam spot size at the target and would instead be defined by the size of the target feature. However, using this approach, the spot size is 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 impinging on the electron beam adjacent to the target feature.

[0007] Thus, known methods for reducing the X-ray spot size in transmission X-ray sources have several limitations. When using electron optics to focus the electron beam into a smaller spot, there are inherent limitations due to the available current and the repulsive nature of electron interactions. Furthermore, while the spot size is often defined by the full width at half maximum of the maximum intensity, the electron distribution extends beyond this width, contributing to X-ray spot broadening. Electron interaction with the substrate also contributes to X-ray spot broadening. Because the target layer is typically relatively thin to avoid scattering-induced electron spot broadening, a large proportion of the incident electrons penetrate the target layer and are further scattered within the relatively thick substrate. Thus, the electron distribution is broadened, resulting in a secondary source of X-ray radiation with lower intensity (because the substrate material typically has a lower atomic number and therefore lower X-ray yield compared to the target material) and a wider distribution (because the electrons undergo multiple scattering events within the thick substrate). The resulting X-ray radiation of the imaged object is the sum of a narrow, high-intensity distribution, which provides a relatively sharp image, and a broad, low-intensity distribution, which provides a blurred image.

[0008] For x-ray sources in which the target comprises x-ray-generating features mounted on or embedded within a substrate (i.e., a target area entirely exposed to the electron beam, thereby defining an emitted x-ray spot by its size and geometry), the performance of the electron optics is less of an issue, as discussed briefly above, since electrons that do not strike the x-ray-generating features do not contribute to the intended x-ray generation. However, even in this case, the interaction of the electrons with the substrate will cause broad, low-intensity x-ray contributions that irradiate the object and blur the resulting image. The relative magnitude of this contribution is a function of electron energy and target design, and is independent of exposure time.

[0009] Therefore, the present invention proposes using a target exhibiting a small hole in an otherwise uniform target layer. In this case, an effective spot size equal to the size of the hole in the target layer can be obtained by acquiring a first image with the electron beam directed toward the uniform target away from the hole, acquiring a second image with the electron beam directed toward the hole, and then subtracting the second image from the first. This allows very high imaging resolution to be achieved by making the hole sufficiently small. A further advantage is that the contribution of X-ray radiation resulting from the interaction of the electron beam with the substrate can be substantially reduced, since the contribution of radiation generated within the substrate is approximately the same when the electron beam is directed toward the uniform target layer as when it is directed toward the hole. Therefore, this contribution cancels out when the two images are subtracted from each other. The tradeoff is that subtracting the two images implies a reduced signal-to-noise ratio compared to a hypothetical source with a smaller electron spot or a correspondingly sized embedded feature, resulting in a noisier final image. However, this tradeoff can be compensated for by a longer exposure time.

[0010] The invention is defined in the accompanying claims, with the dependent claims relating to preferred embodiments.

[0011] In the following detailed description, reference is made to the accompanying drawings. [Brief explanation of the drawings]

[0012] [Figure 1] 1 illustrates a schematic diagram of an X-ray system according to principles disclosed herein. [Figure 2] 1 illustrates the use of targets that present holes. [Figure 3] 1 shows results from a simulation of the principles disclosed herein. [Figure 4] 1 shows the results of an experimental test of the principles disclosed herein. [Figure 5]A method according to the principles disclosed herein will now be outlined. DETAILED DESCRIPTION OF THE INVENTION

[0013] By way of introduction, FIG. 1 schematically illustrates an exemplary X-ray system comprising an X-ray source 100. The illustrated X-ray source 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 a stigmator coil 106, respectively, before reaching a beam-limiting element in the form of an aperture 108. Such aperture 108 has the purpose of limiting the geometric spread of the electron beam before it enters downstream beam-shaping optics (relative to the electron beam propagation direction). The aperture 108 typically has a diameter, or opening size, on the order of 1 mm. After passing through the aperture 108, the electron beam reaches electron optics, which typically include one or more focusing lenses 110 and one or more deflectors 112. The electron optics are used to direct and focus the electron beam to an intended location on a target 116.

[0014] The target 116 typically comprises a high-Z (e.g., atomic number Z greater than 20) material that generates X-rays 118 upon electron impact. A typical target material in this type of X-ray source is tungsten (W) 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) enclosure 120, although the electron optics, realized as a coil, may be located outside the enclosure. 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.

[0015] 1 also includes a controller 150 configured to use the detector 140 to record a first image with the electron beam directed to a first location on the target 116, record a second image with the electron beam directed to a second location on the target 116, and generate a difference image between the first and second images, e.g., by subtracting pixel values ​​of the second image from corresponding pixel values ​​of the first image, in accordance with the principles disclosed herein. The controller 150 may also be responsible for controlling the lens 110 and the deflector 112 to direct the electron beam to the location. As illustrated in FIG. 1, the controller 150 may be operably coupled to the electron-optics systems 110, 112 and the detector 140.

[0016] Thus, an X-ray imaging system according to one embodiment comprises the X-ray source 100 described above, a sample position (exemplified by sample 130 in FIG. 1 ), and a detector 140. The X-ray source 100 comprises an electron source 102 arranged to provide an electron beam, a target 116 arranged to produce X-ray radiation when struck by the electron beam, means such as electron optics 110, 112 for directing the electron beam to first and second positions on the target, and a controller 150 configured to use the detector to acquire a first image with the electron beam directed at the first position, acquire a second image with the electron beam directed at the second position, and generate a difference image between the first and second images.

[0017] According to the principles disclosed herein, the target layer of the target exhibits a hole having a diameter smaller than the diameter of the electron beam spot formed on the target layer by the electron-optics 110, 112. The diameter of the hole may typically be smaller than the smallest electron beam spot achievable using the electron-optics 110, 112. The diameter of the hole may be about 300 nm or less, such as, for example, less than 200 nm or less than 100 nm.

[0018] The holes in the target layer are preferably circular, although other geometric shapes are possible in some implementations. In such cases, the size of the holes along their largest dimension is preferably less than 300 nm (e.g., less than 200 nm or less than 100 nm). For example, the holes may have an oval, rectangular, or square shape.

[0019] The imaging system may also include an actuator (not shown) arranged to move the target 116 relative to the electron beam 114, so that directing the electron beam to the first and second positions 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 and second positions on the target by moving the target may have fixed electron beam, sample, and detector, which may be preferred for simplicity in some cases. The imaging system may also include a manipulator for moving the X-ray source and sample positions relative to each other. Such a manipulator may be, for example, a translation stage 135 to which the sample may be attached, as shown in FIG. 1 . Preferably, the manipulator is also operatively connected to the controller 150, as schematically illustrated by the dashed lines in FIG. 1 . The sample manipulator is typically operated so that the first and second images of the sample overlap on the detector, i.e., so that the image of the object is recorded by the same detector pixels in the first and second images. In other words, an object located at the sample position is imaged by the detector in the same way (i.e., at substantially the same location on the detector) in the first and second images, or the position of the image of the object on the detector is substantially the same in the first and second images. In this case, further alignment of the images may not be necessary before subtraction, although fine-tuning alignment may still be performed. In many applications where the distance from the sample position to the detector is large compared to 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 movement of the sample should be scaled by the ratio of the sample-to-detector distance to the source-to-detector distance.

[0020] The concept of the present invention is illustrated schematically in FIG. 2. (a) shows a portion of a target where the target layer 210 is uniform and thus covers the target substrate 220, and (b) shows a portion of a target where the target layer 210 exhibits holes, i.e., areas where no target material is present. The holes may be empty, exposing the underlying substrate 220, or may be filled with a suitable low-Z material. The intensity of the incident electron beam is shown in the upper portion of FIG. 2, and the intensity of the X-ray spot is shown in the lower portion of FIG. 2. When the electron beam is directed at a portion of a target having a uniform target layer, as shown in FIG. 2(a), the X-ray spot will have a smooth, Gaussian-like energy distribution. When the electron beam is directed at a portion of a target exhibiting holes in the target layer, as shown in FIG. 2(b), the energy distribution of the X-ray spot will have a corresponding depression or dip. By subtracting the image acquired using the X-ray distribution affected by the hole shown in (b) from the image acquired using the X-ray distribution obtained from the uniform portion of the target shown in (a), the result is similar to that which would have been achieved using a target generating X-ray radiation only from the portion of the target corresponding to the hole, as exemplified by the X-ray distribution shown in (c). Thus, an effective X-ray spot having the size of the hole can be achieved. It will be appreciated that the electron beam can have the same size and intensity profile and nominal power when incident on both the uniform portion of the target shown in (a) and the target portion exhibiting the hole shown in (b).

[0021] However, the electron beam may, in some implementations, have a different nominal power when directed at a first target position compared to a second target position, in which case the acquired images may be scaled, e.g., to account for dynamic range, before subtraction to obtain the difference image.

[0022] Furthermore, as will be appreciated, it may be preferable to ensure that the electron beam is blanked (to prevent x-ray generation) during the shift from the first position to the second position, and / or that the detector is gated to avoid artifacts appearing in the acquired image.

[0023] In an embodiment of the present invention, the diameter of the hole is preferably less than 300 nm. As mentioned above, the reason for making the effective X-ray spot small is to achieve high imaging resolution, i.e., to be able to resolve small details. The size of the details that can be resolved is ultimately limited by diffraction. Therefore, to avoid diffraction effects, the object can be positioned close to the source. In the case of 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 can be advantageous to reduce the thickness of the target, thereby reducing the source-object distance.

[0024] The dimensionless number that characterizes diffraction is the Fresnel number

number

[0025] When the first and second images are captured according to the principles disclosed herein, they may not always coincide on the detector because different portions of the target layer are used to generate the 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 and second images (by shifting and / or rotating them) before generating the difference image. Such image alignment can be performed by image processing techniques known per se.

[0026] In some implementations, it may also be useful to include other types of image processing in acquiring an image. 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 be useful in acquiring an image include noise reduction, scaling, compression, thresholding, image formatting, etc. Thresholding, for example, where relative intensity values ​​below a threshold are set to zero or relative values ​​above a threshold are set to one, may be useful to eliminate background artifacts outside the region of interest in the image.

[0027] Figure 3 shows the simulated X-ray image of a gold cube with 100 nm sides located 100 μm away from a 10 keV X-ray source. In the simulation, an ideal detector with 100 μm pixels was placed 1 meter from the source. The field of view in Figure 3 is 2 μm x 2 μm, and the image is shown in black and white after appropriate thresholding. Figure 3(a) shows a simulated image acquired using a 100 nm wide top-hat source spot distribution. As expected, the gold cube is clearly visible. Phase contrast dominates over absorption, and diffraction causes the cube to appear circular rather than square in the image. Using a 300 nm Gaussian spot, the image shown in Figure 3(b) was obtained. It can be noted that although the contrast is lower, the cube is still recognizable. Using a 300 nm Gaussian spot with the central 100 nm top-hat disk missing from the radiation, the image shown in Figure 3(c) was obtained, exhibiting inverted contrast. Finally, subtracting the image in Figure 3(c) from the image in Figure 3(b) yields the image shown in Figure 3(d), which, as expected, is very similar to the image shown in Figure 3(a). However, the image in Figure 3(d) is slightly noisier. The signal-to-noise ratio obtained with this subtraction method is approximately half that obtained with the 100 nm spot. Therefore, subtraction methods using holes in the target layer would require approximately four times the exposure time to achieve the same image quality.

[0028] The results of experimental tests of the technique proposed herein are shown in FIG. 4. In these experimental tests, the size of the electron beam spot on the target was approximately 12 μm (full width at half maximum). The field of view in FIG. 4 is approximately 850 μm, and the images are shown in black and white after suitable thresholding. The image in FIG. 4(a) was acquired with the electron beam directed at a normal point of impact on the target, i.e., a uniform target layer was used to generate X-ray radiation. The image in FIG. 4(b) was acquired with the electron beam directed at a portion of the target layer exhibiting a hole with a diameter of 3 μm. Finally, the image in FIG. 4(c) was obtained by subtracting the image in FIG. 4(b) from the image in FIG. 4(a).

[0029] The final difference image in Figure 4(c) shows the improvement in resolution achieved using the techniques of the present invention. While it is not clear from the images in Figures 4(a) and 4(b) that the eight rectangular shapes comprise five parallel bars (6 μm wide with 6 μm spacing), this is clearly visible in the difference image in Figure 4(c).

[0030] When acquiring the X-ray images shown in Figure 4, the sample was not moved between images; instead, the images were shifted by a distance corresponding to the difference in electron beam position. Therefore, the artifacts (faint diagonal lines) that appear in the background of the difference image likely result from secondary X-ray radiation generated upstream of the electron beam deflector. Instead, by moving the sample between the two image exposures so that the sample image on the detector does not move, the appearance of such artifacts as the effect of this secondary radiation can be eliminated, or at least significantly reduced.

[0031] It can also be noted that the difference image in Figure 4(c) has more background noise than the two actually acquired X-ray images. This is expected because when adding or subtracting two random variables, the variance of the result is the sum of the variances of the two variables. This means that the standard deviation of the difference image should be expected to be √2 ≈ 1.4 times larger than the standard deviation of either image. 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 as the square root of the number of detected X-ray photons and therefore as the square root of the integration time; i.e., to double the signal-to-noise ratio, the exposure time should be increased by a factor of four.

[0032] The experimental test in Figure 4 used a relatively large target hole, 3 μm in diameter, and clearly demonstrates the increased resolution obtainable using the techniques disclosed herein.

[0033] A method of X-ray imaging according to the present invention is illustrated schematically in FIG. 5. As described above, the method is performed using an X-ray source having a target arranged to generate X-ray radiation when an electron beam strikes an electron beam spot, the target comprising a substrate and a target layer, the target layer exhibiting a hole having a diameter smaller than the diameter of the electron beam spot formed on the target layer. The method includes step 510 of directing an electron beam to a first position on the target where the electron beam spot is formed on a uniform portion of the target layer away from the hole. In step 512, a first X-ray image is acquired using a detector while the electron beam is directed to the first position. In step 514, the electron beam is directed to a second position on the target where the electron beam spot is formed on the portion of the target exhibiting the hole, and in step 516, a second X-ray image is acquired using a detector while the electron beam is directed to the second position. Finally, in step 518, a difference image between the first and second images is generated.

[0034] As will be appreciated, the order in which the first and second images are acquired can be reversed, and it is of course immaterial whether one of the images is acquired before the other or vice versa.

[0035] The steps of directing the electron beam, acquiring images, and generating difference images are typically performed under the control of one or more processors within the x-ray system. Thus, the present invention may also be implemented as computer software comprising instructions stored on, for example, a non-transitory computer-readable medium that, when executed, causes one or more processors within the x-ray system to perform steps 510-518 described above.

Claims

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

2. The x-ray system of claim 1 , wherein the holes have a diameter of less than 300 nm.

3. The thickness of the target is [Equation 1] where d is the diameter of the hole and λ is the wavelength of the X-ray radiation.

3. An X-ray system according to claim 1 or 2.

4. 4. The X-ray system of claim 1, wherein the controller is further configured to align the first image and the second image before generating the difference image.

5. 5. The X-ray system of claim 1, further comprising a sample manipulator, 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 location on the detector in the first image and the second image.

6. The x-ray system of any one of claims 1 to 5, wherein the target layer comprises a material selected from tungsten, rhenium, molybdenum, vanadium, and niobium.

7. The x-ray system of any one of claims 1 to 6, wherein the substrate comprises beryllium or a carbon material such as diamond.

8. 1. A method for x-ray imaging using an x-ray source, the x-ray source comprising: an electron source arranged to provide an electron beam; a target positioned to produce X-ray radiation upon impact of the electron beam at an electron beam spot; the target comprises a substrate and a target layer, the target layer presenting a hole having a diameter smaller than a diameter of the electron beam spot; The method comprises: directing the electron beam at a first location on the target where the electron beam spot is formed in a uniform portion of the target layer away from the hole; acquiring a first x-ray image using a detector while the electron beam is directed at the first location; directing the electron beam to a second location on the target where the electron beam spot is formed in the portion of the target exhibiting the hole; acquiring a second x-ray image using the detector while the electron beam is directed at the second location; generating a difference image between the first x-ray image and the second x-ray image; A method comprising:

9. The method of claim 8 , wherein the holes have a diameter of less than 300 nm.

10. The thickness of the target is [Equation 2] where d is the diameter of the hole and λ is the wavelength of the X-ray radiation.

10. The method according to claim 8 or 9.

11. The method of any one of claims 8 to 10, further comprising registering the first and second X-ray images before generating the difference image.

12. 12. The method of claim 8, further comprising moving the sample position such that an object located at the sample position is imaged at substantially the same location on the detector in the first X-ray image and the second X-ray image.

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

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

Images