X-ray target

Abstract

A transmission-type target for an X-ray source is disclosed. The X-ray target comprises a substrate made from a first material, the substrate having a flat top face, and an embedding layer made from a second material, the embedding layer being provided on the flat top face of the substrate. A target feature made from a third material operative to generate X-ray radiation upon electron impact is at least partly provided within the embedding layer without reaching into the substrate. The target feature has a first extension in a direction orthogonal to the flat top face of the substrate and a second extensions in a direction parallel to the flat top face of the substrate, and the embedding layer has a thickness equal to or larger than the first extension of the target feature. An X-ray source comprising such X-ray target is also disclosed.

Description

[0001]X-RAY TARGET Technical field The present invention relates to targets for X-ray generation. Moreparticularly, the invention relates to solid targets for electron-impact X-ray sources,the solid targets comprising target features that generate X-ray radiation upon exposure to an electron beam. Background Electron-impact X-ray sources are generally known. In such X-ray sources, X-ray radiation is generated by interaction between atoms in a target material andincident electrons from an electron beam source. The X-ray radiation is primarilygenerated as bremsstrahlung, although characteristic emission lines also contribute.Only a relatively small fraction of the energy contained in the incident electron beam is converted into X-ray radiation. Excess heat generated in the target must therefore be handled. To this end, it is known in the art to have X-ray targets comprising an X-ray generating material, such as tungsten, deposited on a substrate having high thermal conductivity, such as diamond. Attempts have been made to embed the X-ray generating material in the substrate in order to increase thermal conduction into the substrate. In this context,US 2011 / 058655 discloses an X-ray target comprising a target portion (e.g. madefrom tungsten, gold, platinum, or the like) deposited in a bottomed hole formed in a diamond substrate. However, it has proven to be notoriously difficult to embed e.g. tungsten in adiamond substrate. Forming holes of sufficient depth in the diamond substrate isnot straightforward, and a comparatively thin embedded target feature may resultin less X-ray radiation than desired. Furthermore, the difference in thermalexpansion between the target portion (e.g. tungsten) and the diamond substratemay increase the risk of defects / cracks forming in the target portion and / or thesubstrate during operation. Also, more generally, a good thermal contact betweenthe target portion and the substrate may be impeded leading to overheating of thetarget portion. Hence, there is a need for improved X-ray targets that address the above issues. An X-ray target according to the present invention comprises a substrate made from a material with good thermal properties, preferably diamond. The substrate is selected to have a sufficiently high thermal conductivity so that heatgenerated by an incident electron beam may be distributed from the point of impactto avoid local overheating and ensuing target damage. In general, the substratematerial should have a low X-ray yield, to not contribute with background noise, andlow X-ray absorption to ensure that the generated radiation is emitted. However, forsome applications selective X-ray absorption in the substrate may be preferred, e.g.,because a material that absorbs low energy X-ray radiation will have a monochromatizing effect, a beneficial effect for dose sensitive samples. In this casesubstrates made from e.g. aluminum may be considered. Furthermore, the targetcomprises at least one target feature in which X-ray radiation is generated upon exposure to the electron beam. As will be understood, a plurality of target features may conveniently be provided in the target. The target features are made from a material with a high X-ray yield, i.e., a comparatively large fraction of electronsimpacting on the target feature contributes to generation of X-ray radiation, eitheras bremsstrahlung or characteristic emission lines. Typically, an element with higher atomic number (Z) will have a higher X-ray yield. A preferred material for the target features is tungsten but other elements like copper, rhenium, rhodium, palladium,molybdenum, vanadium, and niobium, or alloys comprising these elements may alsobe considered. Provided that the electron beam impacts the entire target feature (i.e., that the electron beam focus at the target feature is equal to or larger than the target feature), the size of the target feature will define the size of the X-ray spot. To increase the thermal load that the target feature is able to withstand, ithas been proposed, e.g., in US 2011 / 058655, to embed the target feature within thesubstrate, thus providing for more surface contact between the target feature andthe substrate than would be the case if the target feature were provided on a topface of the substrate. However, in practice it is difficult to provide embedded target features with the preferred material choices.The present invention therefore proposes an improved solution for atransmission-type X-ray target, wherein an embedding layer is provided on top of aflat top face of the substrate and the, or each, target feature is provided in, andpreferably embedded within, this embedding layer without reaching into thesubstrate. As generally understood in the art, a transmission-type X-ray target is a target which is thin enough to allow the generated X-ray radiation to pass through the target and be emitted downstream of the target relative to the incoming electron beam. The material in the embedding layer can be selected to provide good adhesion to the substrate and the target material, to enable formation of suitable holes or indentations for the target features, and to provide for good thermalconduction from the target feature(s) to the substrate. The preferred choice ofmaterial for the embedding layer is silicon carbide, a material that is usedextensively in the semiconductor industry enabling embedding of target featureswith desired shapes and sizes. Alternatively, materials like beryllium, boron nitride,boron carbide, aluminum nitride, silicon nitride, and silicon boride may be used. Inembodiments of the present invention, the materials used for the substrate, the embedding layer, and the target feature(s) are all different. The present invention also provides a method of producing an X-ray target. A substrate having a top face is provided, an embedding layer is formed on the topface of the substrate, and one or more target features are formed in the embeddinglayer. The, or each, target feature is embedded in the embedding layer withoutreaching into the substrate, thereby avoiding the need to make holes in the substrate. In embodiments, a target feature has an extension in a direction orthogonal to the top face of the substrate that is larger than an extension of the target featurein a direction parallel to the top face. Herein, the extension of the target feature in adirection orthogonal to the top face of the substrate may be referred to as the firstextension of the target feature, and the extension of the target feature in a direction parallel to the top face of the substrate may be referred to as the second extension of the target feature. Various geometries for the target feature are described herein,such as conical / pyramid or frustum, for which the extension of the target feature isdifferent for different slices thereof parallel and orthogonal to the top face of the substrate. The extension of the target feature should thus be understood in itsnatural sense to mean the largest overall extension of the target feature parallel andorthogonal, respectively, to the top face of the substrate. The invention is particularly advantageous for target features that have anaxial (first) extension that is long in relation to a radial (second) extension of thetarget feature. In this case thermal transport out of the target feature will, to a largeextent, be in a radial direction and hence benefit from being provided in a materialwith a high heat conduction ability. For a “flat” target feature, i.e., a target featurewhere the axial (first) extension is small in relation to the radial (second) extension, most of the thermal transport will be in the axial direction and embedding thelateral part of the target feature may not contribute significantly to the thermal loadthat the target feature is able to withstand. Another way of stating this condition isthat the lateral surface area of the target feature should be larger than its top orbase area, or more preferable be larger by some factor e.g., 2.To avoid risking thermal failure with the introduction of an embedding layer,it is advantageous that coefficients of thermal expansion for the materials arematched. This is particularly the case for the interface between the target featureand the embedding layer, where the thermal load is expected to be at its highest.Not matching the coefficients of thermal expansion would imply that stress iscreated at the interface whenever the temperature changes. In a preferredembodiment, the embedding layer comprises silicon carbide and the target feature comprises tungsten, and it can be noted that these two materials advantageouslyhave similar thermal expansion coefficients of about 4 · 10-6 K-1.A further advantage may be attained by providing a plurality of targetfeatures on a common substrate. Should one feature, for some reason, be worn outor otherwise fail, the electron beam may be moved to another feature and operation may be continued without extensive maintenance work. It is alsoconceivable to have features with different characteristics, e.g., different lateralextensions, to provide for a wider range of X-ray source performance using differentselected target features. Features of different lengths in the propagation direction of the electron beam may be provided to achieve optimum results for each applied acceleration voltage. To understand the performance, and limits, of this type of target somesimplified physical models may be considered. From a general perspective, to increase the X-ray flux generated by an incoming electron beam more target material should be provided. In the case of a transmission type target (i.e., a targetwhich is thin enough to allow X-ray radiation to pass through the target and beemitted downstream of the target relative to the incoming electron beam) thethickness of the target layer is limited both by the ability of the electrons to penetrate the target material and by the ability of the generated X-ray radiation to escape from within the target layer. Furthermore, it is well known that an electron beam will broaden due to scattering when propagating through a material. Thus, although it may be desired to have a target layer that covers a larger surface area than the actual desired X-ray spot size, which may promote a higher X-ray flux,increasing also the target layer thickness entails an increase of the spot size due tobroadening of the electron beam in the target material. A solution to this, accordingto some embodiments of the present invention, is to provide a target feature with alateral extension equal to the desired X-ray spot size. In other words, the target feature is made sufficiently small relative to the electron beam such that the spot size is determined by the extension of the target feature rather than by the extension of the electron beam. In this way the electrons comprised in the broadened part of the electron beam do not contribute to the production of X-ray radiation. In principle, electrons will to some extent interact with any material they travel through and produce some X-ray radiation, but elements with low atomic number have a low X-ray yield so this contribution will only create some background noise when compared to the X-ray radiation created by interactions between the electrons and the target feature. Nevertheless, the electrons will still scatter within the target feature and some electrons will scatter out of the target feature and not contribute to further X-ray radiation. Making the target feature thicker thus onlymakes sense up to some value, after which additional target feature material willnot result in more X-ray flux due to the diminishing density of the electron beam.The optimal thickness will typically be a function of the electron energy, where ahigher acceleration voltage enables use of a thicker target feature. For the case where a target layer with an area considerably larger that theelectron beam spot is provided, the optimal target layer thickness may be calculatedeither from Monte Carlo simulations or from a simple model where the incomingelectron beam is exponentially attenuated over a first characteristic length as itprogresses into the target layer and the generated X-ray radiation is attenuated inthe target layer over a second characteristic length. The optimal target layer thickness as a function of energy may be approximated as linear for low electron energies and as a power law if larger energies are considered. For the case wherethe X-ray spot is defined by the target feature, electrons scattered out of thatfeature do not contribute to the generated X-ray radiation and neither do electronsimpacting outside of the target feature. For a given total available power, it thusmakes sense to provide an electron beam spot equal in size to the target feature since otherwise some of the available power will not contribute to X-ray production. However, when the electron beam spot size and the target feature extension arecomparable, broadening of the electron beam in the target material will reduce thenumber of electrons that contribute to X-ray production. The result of this is that theoptimal target thickness for this case will be smaller than the corresponding valuefor a target layer with an extension considerably larger than the electron beam spotsize. If the electron beam spot size is considerably larger than the target feature,broadening of the beam may not impact the amount of X-ray radiation produced but then again electrons impacting outside the target feature do not contribute to X-ray production. To illustrate these limitations a simple model based on a paper by Gauvin and Rudinsky from 2016 (“A universal equation for computing the beam broadening of incident electrons in thin films”, Ultramicroscopy, 167, pp.21-30) is employed. A typical thickness of a continuous target layer of tungsten (W) is about 0.5 µm. Spot sizes used for such a target are typically larger than 300 nm. Target features with adiameter of 300 nm (in a plane orthogonal to the electron beam propagationdirection) and thickness up to 3 µm will be considered below, i.e. a 10:1 ratiobetween axial extension and diameter. This is for illustrative purposes only andshould not be considered as limiting the scope of the invention. As evident from thedisclosure herein other target feature diameters and / or thicknesses may be used toan advantage. The diameter may be less than 1 µm, such as less than 500 nm, suchas less than 300 nm. For a non-cylindrical, e.g. conical, pyramid, or otherwisetapered target feature, the diameter may be understood as the largest diameter ofthe target feature in a plane parallel to the top face of the substrate along the lengthof the target feature. The thickness may be less than 10 µm, such as less than 5 µm,such as less than 3 µm. The diameter of an electron beam traversing a material may be written as ^(^) = ^^ ^^ + ^(^)^ (1)where D0 is the diameter of the incoming electron beam, and b(^) is the broadeningof the electron beam at the depth ^ into the material. The broadening of theelectron beam may, in the limit where the thickness of the film is considerably largerthan the mean free path of the electrons, be written as where b and ^ are in m, Z is the atomic number of the target material, ρ is thedensity in g / cm3, A is the atomic weight in g / mol, and E0 is the electron energy inkeV. The mean free path of electrons (λ) may be calculated in cm from where θ0 is the characteristic scattering angle, e is the electron charge in CGS units,and N0 is Avogadro’s number. For 160 kV electrons impacting on W this computes toabout 330 nm. Thus, it seems justified to use the above equation for electron beambroadening, considering that only thicknesses above 0.5 µm are of interest.By considering an incoming electron beam having a diameter of 300 nm, thebeam diameter as a function of propagation depth into the target material, i.e. thematerial of the target feature, may thus be estimated from the equations above.This is an approximation in the sense that only scattering of electrons from W isconsidered. In a more realistic setting, electrons leaving the target feature shouldscatter against the atoms in the embedding layer. However, capturing this effect inthe calculations would require Monte Carlo simulations or the like, which goesbeyond the needs and purposes here. The general characteristics of this simplifiedmodel are expected to hold also for a more complete calculation. Fig.1 shows a plotof the electron beam diameter as a function of propagation depth in tungsten forthree different electron energies as described by equation (1) above. As expected,scattering is more pronounced for low energy electrons and the beam broadening is consequently larger. Since electrons scattered out of the target feature will not contribute to theuseful X-ray production, there is an energy dependent limit beyond which increasingthe target thickness does not increase the useful X-ray output. This is illustrated inFig. 2, where the fractional yield of X-ray generation is plotted as a function ofpropagation depth into tungsten for three electron energies. The data has been normalized with the theoretical maximum that the entire electron beam is withinthe target feature up to 3 µm depth (i.e., no beam broadening). As can be seen, thecurves flatten out well before the 10:1 ratio is reached, i.e. when the depth equals 3µm for the incoming electron beam diameter of 300 nm. Although a more completecalculation would consider the electron beam intensity profile and not just the area, the general characteristics of the present model are expected to be preserved also in a more elaborate scheme. One reason to let the target feature define the X-ray spot, or in other wordsto provide a target feature that is smaller than the electron beam spot size, is toattain a small X-ray spot. However, for a cylindrical target feature, even a smallmisalignment between the viewing direction and the target will result in an apparentenlargement of the X-ray spot. This is because X-ray radiation will be emitted notonly from the end surface of the cylindrical target feature but also from the lateral surface. At perfect alignment only the radiation emitted from the end surface willreach an observer, whereas if there is any misalignment also radiation emitted fromthe lateral surface will reach the observer. If instead a tapered, e.g. conical, targetfeature is provided this enlargement effect may be suppressed. For applications where only one operational X-ray spot at the time is desired, embodiments having more than one X-ray generating target feature are preferably designed such that electrons directed to one target feature do not result in X-ray generation from neighboring features. This may be accomplished by providing sufficient distance between the target features. A lower limit on this distance can be expressed as the mean free path of the electrons according to Equation (3) above. However, electrons travelling one mean free path will typically have undergone one scattering event and thus still carry sufficient energy to generate X-ray radiation. Therefore, a more stringent requirement would be to have a distance between target features at least equal to the broadening according to Equation (2). Other applications may rely on X-ray radiation being emitted from more thanone target feature at the time, e.g., multiple X-ray source points used for Talbot-Lauinterferometry or linear accumulation. In such cases the distance between target features may be smaller than the limits imposed by Equation (2) or (3). In embodiments where target features having different depths are desired, the target may be configured with multiple layers of SiC on top of a diamondsubstrate. In such cases, the embedding layer thus comprises multiple sub-layers.Since the proposed materials for the embedding layer are typicallysemiconductors, charge accumulation may present itself as a potential issue. Toprevent charge accumulation, a conducting layer on top of the embedding layer and / or between the embedding layer and the substrate may be provided. If thislayer is sufficiently thin, the interference with the electron beam and / or thegenerated X-ray radiation can be made negligible. Another approach may be to dope the embedding layer to provide a conductive path for removal of charge carriers. Brief description of the drawings In this description, reference is made to the accompanying drawings, on which: Fig.1 shows a plot of electron beam broadening as a function of propagation depth into tungsten for three different electron energies; Fig. 2 shows a plot of the fractional yield of X-ray radiation as a function ofpropagation depth into tungsten for three different electron energies; Fig. 3a-c schematically show X-ray targets having a target feature provided inan embedding layer on top of a substrate; Fig.4 schematically shows an X-ray target having a plurality of target featuresprovided in a multilayered embedding layer;Fig.5 schematically shows an X-ray source including a target according to the present invention. On the drawings, like parts are designated by like numerals throughout. Detailed X-ray targets according to the principles disclosed herein are schematically shown in Fig.3a-c. All X-ray targets shown in Fig.3 comprise a substrate 30, an embedding layer32 provided on a top face of the substrate, and a target feature 34 provided in theembedding layer. The target feature 34 is provided in the embedding layer 32without reaching into the substrate 30, which means that no holes or recesses need to be formed in the substrate. Although the target feature 34 may protrude from the embedding layer on a side facing away from the substrate, it is generally preferredto have the target feature 34 flush with or buried in the embedding layer 32.In a preferred embodiment, the substrate 30 is made from diamond, which is a durable material having excellent thermal properties. Furthermore, diamond has a low X-ray yield (thereby causing only low background noise) and has good transparency for the X-ray radiation generated in the target feature 34. In apreferred embodiment, the embedding layer 32 is made from silicon carbide and thetarget feature is made from a material having an atomic number above 20, mostpreferably tungsten. Conveniently, tungsten and silicon carbide have similar thermal expansion coefficients of about 4 · 10-6K-1which leads to low thermal stress at the interface between the target feature and the embedding layer. As will be understood, if the X-ray spot defined by the target feature 34 is viewed from an inclined angle, there will be an apparent broadening of the X-ray spot since some radiation exiting the target feature through the lateral sides then also contribute. Therefore, it may be advantageous to implement the target feature to have a conical or tapered shape, as schematically shown in Fig.3b. As long as the X-ray spot is viewed from an angle that is smaller than the taper angle, there will be no apparent broadening of the X-ray spot. Such conical or tapered target featuremay be truncated, as shown in Fig. 3b, or end at an apex. Although the targetfeature in Fig. 3b has a decreasing cross section towards the substrate, the oppositeconfiguration where the target feature is tapered away from the substrate is alsoconceivable. The latter configuration may be preferable from a thermal point of view whereas the former may be more attractive from a manufacturing point ofview. Such conical / tapered shape can be used in any embodiment, as will be readilyunderstood. Fig.3c schematically shows an X-ray target comprising a conducting layer 36provided on top of the embedding layer 32. Such conducting layer is useful forpreventing charge build-up caused by the incident electron beam. Other implementations of conducting layers for carrying away excess charges are also possible, and may also be provided, for example, between the target feature 34 andthe embedding layer or between the embedding layer and the substrate. It is alsoconceivable to have an embedding layer that is doped, such that charge carriers can be removed through the embedding layer. In this latter case, a separate conducting layer may not be required. Typically, the cross section of the target feature in a plane parallel to the top face of the substrate will be circular, although other cross-sectional shapes are also conceivable. The target features are conveniently formed in connection with forming the embedding layer. Thus, in embodiments where target features having different depths are required, the target may be configured with multiple layers of theembedding material, for example silicon carbide, as schematically shown in Fig.4. Asshown, the embedding layer comprises a plurality of sub-layers 32-1, 32-2, 32-3 provided on the substrate 30. A first group of target features 34-1 have the largestdepth and reach through all of the sub-layers of the embedding layer; a secondgroup of target features 34-2 have a smaller depth than the first group, and reach through the two top-most sub-layers 32-2, 32-3, and a third group of target features 34-3 have the smallest depth of the groups and only reach through the top-most sub-layer 34-3. In this example, each group of target features comprises threeindividual target features, but it is understood that this is merely an example, andthat more or fewer individual target features can be provided. The advantage of enhanced cooling capacity in embodiments of the present invention is most pronounced for a target feature where an extension (thickness) of the target feature along a direction perpendicular to the top face of the substrate is longer than an extension (diameter) of the target feature in the lateral direction, i.e. a direction parallel to the top face. Hence, the present invention proposes that thethickness of the target feature is at least as large as its diameter. By instead applyingthe condition that the lateral surface area should be larger than the top surface area a less strict requirement may be imposed on the minimum thickness, and for acylindrical target feature the minimum thickness may in this case be one quarter ofthe diameter, or one half of the diameter if the lateral surface area should be atleast twice the top surface. For a given diameter of the target feature (below denoted d0) the thicknessof the target feature (below denoted t) may be selected so that the amount ofproduced X-ray radiation is sufficiently large. As the electrons penetrate into thetarget the electron beam broadens, and thus fewer electrons are available forinteraction with the target feature. A low thickness of the target feature means thatelectrons available for interaction with the target feature penetrate into thesubstrate material without further contributing to the produced X-ray radiation. A reasonable engineering option may be to select the thickness of the target feature such that at least two thirds of the electrons have scattered out of the target feature before the electron beam has penetrated to a depth corresponding to the thickness of the target feature. The thickness should in this case be larger than a limit thickness that may be written as Another option could be to ensure that a larger fraction of the electronscontributes to the production of X-ray radiation, and the criterion may then insteadbe set so that 90% of the electrons have scattered out of the target feature. Theexpression for the limit thickness may then instead be written as It is noted that the limit thickness depends on the electron energy. Thus, aplurality of target features may be provided with different thicknesses adapted tocorresponding different electron energies. A controller provided in an X-ray source may be configured to direct the electron beam to a target feature suitable for the set acceleration voltage.Making the target feature considerably thicker than the limits introduced inEquations (4) and (5) above may not increase the X-ray output significantly as indicated in Fig.2. Increasing the thickness of the target feature will also result inmore self-absorption, i.e. reabsorption of X-ray radiation in the target feature, andthus a decrease in X-ray output for thicknesses in excess of an energy and geometrydependent optimum thickness. An upper limit for the thickness of the target featuremay be imposed by considering the upper limit for the penetration depth of electrons into the target material. The upper limit for electron penetration depth may written as (6)Making the target feature thicker than this will only result in more self-absorption and not in more X-ray production. It should be acknowledged, however, that some degree of self-absorption (which is preferential to low-energy photons) may in someinstances be desired due to the monochromatizing effect it has on the generated X-ray radiation. Taking all the above aspects into consideration, the requirements on thethickness of the target feature for a given diameter thereof to reach the level of twothirds of the beam being scattered out of the target feature may be written as where d0 and t are in m. The corresponding range for the 90% scattering level maybe written as Considering that there will always be some self-absorption, it may be preferable forimplementations to operate in the lower part of the ranges allowed according toEquations (7) and (8).For the particular case where the target feature comprises tungsten,Equations (7) and (8) yield the following allowed ranges for the thickness of thetarget feature 5.84 ⋅ 10^^ ⋅ ^^.^^ (9) max ^^^ ^ < ^ < 5.84 ⋅ ^^ ^.^^7.38 ∙ 10^^ ∙ (^^^)^⁄ ^ 10 ⋅ ^ (10)where Equation (9) corresponds to the two thirds scattering level and Equation (10) to the 90% scattering level. For target features with diameters 100 nm, 300 nm, and 500 nmcorresponding thickness ranges for electron energies equal to 80 keV, 160 keV, and240 keV are tabulated below. d0 E 80 keV 160 keV 240 keV100 nm (2 / 3) 180 nm < t < 9 µm 280 nm < t < 28 µm 370 nm < t < 55 µm100 nm (90%) 300 nm < t < 9 µm 470 nm < t < 28 µm 610 nm < t < 55 µm300 nm (2 / 3) 370 nm < t < 9 µm 590 nm < t < 28 µm 770 nm < t < 55 µm300 nm (90%) 610 nm < t < 9 µm 980 nm < t < 28 µm 1.28 µm < t < 55 µm500 nm (2 / 3) 520 nm < t < 9 µm 820 nm < t < 28 µm 1.10 µm < t < 55 µm500 nm (90%) 860 nm < t < 9 µm 1.37 µm < t < 28 µm 1.80 µm < t < 55 µmAs a comparison the optimal thickness (topt) for a continuous tungsten targetlayer may, according to Sofiienko et al ("Electron range evaluation and X-ray conversion optimization in tungsten transmission-type targets with the aid of wide electron beam Monte Carlo simulations." 11th European Conference on Nondestructive Testing. ECNDT.2014.), be written as^ = 4.8 ∙ ^^ ^.^^^^^ 10 ^ m. (11)This translates to 3.1 for 80 keV, 8.8 for 160 keV, and 16 for 240 keVrespectively. As discussed above the optimum thickness will be smaller for the casewhere the target feature and the electron beam spot are comparable in size.Furthermore, as discussed above, it may be beneficial to use a somewhat thicker target feature than that corresponding to maximum X-ray production due to the beam hardening effect that may be achieved. For the particular case where the target feature comprises tungsten of athickness t the diameter of the target feature should be less than 107000^^⁄ ^ / ^ mor less than the target feature thickness, whichever is smallest, to achieve a beambroadening such that two thirds of the electrons have scattered out of the targetfeature. To reach a situation where 90% of the electrons scatter out of the targetfeature the diameter should be less than 50000^^⁄ ^ / ^ m or less than the targetfeature thickness, whichever is smallest. As a set of non-limiting examples considerelectron energies of 80 keV, 160 keV, and 240 keV and thicknesses of 0.5 µm, 1 µm,and 2 µm. This would imply corresponding upper limits of the diameter of therespective target feature at the respective scattering level according to the tablebelow. tE 80 keV 160 keV 240 keV0.5 µm (2 / 3) 470 nm 240 nm 160 nm0.5 µm (90%) 220 nm 110 nm 70 nm1 µm (2 / 3) 1 µm 670 nm 450 nm1 µm (90%) 620 nm 310 nm 210 nm2 µm (2 / 3) 2 µm 1.89 µm 1.26 µm2 µm (90%) 1.76 µm 880 nm 590 nmFig.5 schematically shows an example of an X-ray source including a targetaccording to the present invention. The X-ray source comprises an electron source110 with a cathode 112, coupled to a power supply 114. Electrons emitted by the cathode 112 are accelerated towards a ring-shaped anode 116 to form an electronbeam, indicated by I in Fig. 5. Beam aligning means 118 and electron optics 120 are provided to shape and direct the electron beam towards the target 130. As described above, the target 130 comprises a substrate 132 and one or more targetfeatures provided in an embedding layer provided on a top face 134 of the substratefacing the incoming electron beam. As shown, the electron source (and thecathode / anode), the aligning means, the electron optics and the target are provided in a low-pressure (vacuum) environment enclosed by a housing 140. Further, at least the aligning means and the electron optics are operatively connected to a controller 150. In conclusion, a target for an X-ray source is disclosed herein. The X-ray target comprises a substrate having a top face, and an embedding layer provided onthe top face of the substrate. A target feature operative to generate X-ray radiationupon electron impact is at least partly provided in the embedding layer withoutreaching into the substrate. Preferably, an extension of the target feature in a direction orthogonal to the top face is larger than an extension of the target feature in a direction parallel to the top face. An X-ray source comprising such X-ray target isalso disclosed. Arrangements, sources, and methods according to the invention maybe used for different types of X-ray imaging such as X-ray microscopy, radiography,fluoroscopy, laminography, ptychography, or CT scanning.

Claims

CLAIMS1. A transmission-type X-ray target, comprisinga substrate made from a first material, the substrate having a flat top face; an embedding layer made from a second material, the embedding layer being provided on the flat top face of the substrate; and atarget feature made from a third material, the target feature having a firstextension in a direction orthogonal to the flat top face of the substrate and a secondextension in a direction parallel to the flat top face of the substrate,wherein the target feature is at least partly provided within the embeddinglayer without reaching into the substrate;wherein the embedding layer has a thickness equal to or larger than the firstextension of the target feature; andwherein the first, second, and third materials are different.

2. The X-ray target of claim 1, wherein the first extension is larger than thesecond extension.

3. The X-ray target of claim 1 or 2, wherein the first material comprisesdiamond.

4. The X-ray target of any one of the preceding claims, wherein the thirdmaterial comprises a material selected from copper, tungsten, rhenium, rhodium, palladium, molybdenum, vanadium, and niobium.

5. The X-ray target of any one of the preceding claims, wherein the secondmaterial comprises a material selected from silicon carbide, beryllium, boron nitride, boron carbide, aluminum nitride, silicon nitride, and silicon boride.

6. The X-ray target of claim 1 or 2, wherein the third material comprisestungsten, the second material comprises silicon carbide, and the first material comprises diamond.

7. The X-ray target of any one of the preceding claims, wherein the targetfeature has a conical or frustum shape.

8. The X-ray target of any one of the preceding claims, further comprising aconduction layer for removal of charge carriers.

9. The X-ray target of any one of the preceding claims, wherein the secondmaterial comprises a doped semiconductor material for removal of charge carriers.

10. The X-ray target of any one of the preceding claims, wherein the embeddinglayer comprises a plurality of sub-layers, and wherein target features of differentextensions in the direction orthogonal to the top face are provided in different sub-layers.

11. An X-ray source comprising:an electron source arranged to provide an electron beam comprisingelectrons with an energy E keV;a target according to any one of claims 1 to 10 wherein the target featurecomprises an element with atomic number Z, atomic weight A g / mol, and density^^g / cm3^ wherein the second extension is d0 m; and wherein the first extension is at least m, and preferably atleast12. The X-ray source according to claim 11, wherein the first extension is lessthan 2.82 ⋅ 10^^ ⋅ ^⋅^^.^^^⋅^^.^^m.

13. The X-ray source according to claim 11 or 12, wherein the target feature comprises tungsten; the first extension is t m; andthe second extension is less than 107000^^⁄ ^ / ^ m, and preferably less than50000^^⁄ ^ / ^ m.

14. The X-ray source according to any one of claims 11 to 13, wherein the firstextension is less than 10 µm, such as less than 5 µm, such as less than 3 µm.

15. The X-ray source according to any one of claims 11 to 14, wherein the secondextension is less 1 µm, such as less than 500 nm, such as less than 300 nm.

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