STRUCTURED X-RAY TARGET

Abstract

A system and method for generating X-ray radiation. The system includes an electron source operable to generate an electron beam and an X-ray target for generating X-ray radiation upon interaction with the electron beam. The method includes moving the electron beam over an edge separating a first region and a second region of the X-ray target, wherein the first region and the second region have different capability to generate X-ray radiation upon interaction with the electron beam. The system allows for a lateral extension of the electron beam to be determined based on a change in a quantity indicative of the interaction between the electron beam and the first region and between the electron beam and the second region, and the movement of the electron beam.

Claims

1. A method in a system comprising: an electron source operable to generate an electron beam; and a stationary X-ray target for generating X-ray radiation upon interaction with the electron beam, the target comprising a first target region and a second target region; wherein: the first target region and the second target region have different capability to generate X-ray radiation; the first target region and the second target region are separated by a first interface and a second interface oriented at an angle relative each other; each of the first target region and the second target region has a size allowing it to accommodate an entire cross section of the electron beam; and the first target region and the second target region are arranged on a common substrate; the method comprising: moving the electron beam in a first direction over the first interface and into the second target region, such that the entire cross section of the electron beam is arranged within the second target region; followed by moving the electron beam over the second target region, over the second interface and into the first target region, such that the entire cross section of the electron beam is arranged within the first target region; the method further comprising: measuring, as the electron beam is moved over the first interface, a change in a quantity indicative of the interaction between the electron beam and the first target region and between the electron beam and the second target region; measuring, as the electron beam is moved over the second interface, a change in the quantity indicative of the interaction between the electron beam and the second target region and between the electron beam and the first target region; and determining a width of the electron beam along the first direction and the second direction, respectively, based on the measured change in the quantity and the movement of the electron beam.

2. The method according to claim 1, wherein the quantity is at least one of: an amount of X-ray radiation, an amount of secondary electrons or backscattered electrons, and an amount of electrons absorbed in the target.

3. The method according to claim 1, wherein said first interface is substantially perpendicular to said second interface.

4. The method according to claim 1, comprising varying a focus of the electron beam in the first target region and the second target region.

5. The method according to claim 1, further comprising adjusting, based on the determined width, at least one of: an intensity of the electron beam such that a power density supplied to the target is maintained below a predetermined limit, and a spot size of the electron beam.

6. The method according to claim 1, further comprising directing the electron beam to a specific location on the target based on at least one of: the determined width, and a desired wavelength of the X-ray radiation.

7. The method according to claim 1, wherein the first interface and/or the second interface comprises a surface step of the X-ray target.

8. The method according to claim 1, wherein said first direction is substantially perpendicular to said first interface and said second direction is substantially perpendicular to said second interface.

9. The method according to claim 1, further comprising: moving the electron beam in a third direction over a third interface separating the first target region from the second target region wherein the first direction, second direction, and third direction are different; measuring a change in the quantity indicative of the interaction between the electron beam and the second target region and between the electron beam and the first target region as the electron is being moved over the third interface; and determining, based on the measured change in the quantity and the movement of the electron beam, a major axis, a minor axis, and an angular orientation of an electron beam spot having an elliptic shape.

10. The method according to claim 9, further comprising adjusting, based on the determined major axis, minor axis, and angular orientation of the electron beam spot, at least one of: a spot shape of the electron beam or a spot orientation of the electron beam.

11. A system adapted to generate X-ray radiation, comprising: an electron source operable to generate an electron beam; a stationary X-ray target for generating X-ray radiation upon interaction with the electron beam, comprising a first target region and a second target region, wherein the first target region and the second target region have different capability to generate X-ray radiation and are separated by a first interface and a second interface oriented at an angle relative each other, wherein each of the first target region and the second target region has a size allowing it to accommodate an entire cross section of the electron beam, and wherein the first target region and the second target region are arranged on a common substrate; an electron-optical means for moving the electron beam in a first direction over the first interface and into the second target region, such that the entire cross section of the electron beam is arranged within the second target region, and then moving the electron beam over the second target region, over the second interface and into the first target region, such that the entire cross section of the electron beam is arranged within the first target region; a sensor adapted to measure, as the electron beam is moved over the first interface, a change in a quantity indicative of the interaction between the electron beam and the first target region and between the electron beam and the second target region, and to measure, as the electron beam is moved over the second interface, a change in the quantity indicative of the interaction between the electron beam and the second target region and between the electron beam and the first target region; and a controller operably connected to the sensor and the electron-optical means and adapted to determine a width of the electron beam along the first direction and the second direction, respectively, based on the measured change in the quantity and the movement of the electron beam.

12. A system adapted to generate X-ray radiation, comprising: an electron source operable to generate an electron beam; a stationary X-ray target for generating X-ray radiation upon interaction with the electron beam, comprising a first target region and a second target region, wherein the first target region and the second target region are separated by a first interface and a second interface oriented at an angle relative each other, wherein each of the first target region and the second target region has a size allowing it to accommodate an entire cross section of the electron beam, and wherein the first target region and the second target region are arranged on a common substrate; an electron-optical means for moving the electron beam in a first direction over the first interface and into the second target region, such that the entire cross section of the electron beam is arranged within the second target region, and then moving the electron beam over the second target region, over the second interface and into the first target region, such that the entire cross section of the electron beam is arranged within the first target region; a sensor adapted to measure, as the electron beam is moved over the first interface, a change in a quantity indicative of the interaction between the electron beam and the first target region and between the electron beam and the second target region, and to measure, as the electron beam is moved over the second interface, a change in the quantity indicative of the interaction between the electron beam and the second target region and between the electron beam and the first target region; and a controller operably connected to the sensor and the electron-optical means and adapted to determine a width of the electron beam along the first direction and the second direction, respectively, based on the measured change in the quantity and the movement of the electron beam; wherein: the first target region and the second target region of the X-ray target are arranged to provide a contrast of at least two percent in said quantity.

13. The system according to claim 11, wherein the first target region has a varying thickness as seen in the direction of propagation of the electron beam.

14. The system according to claim 11, wherein the first target region of the X-ray target forms part of a layer and the second target region forms part of the substrate, and wherein the layer is arranged on the substrate.

15. The system according to claim 11, wherein the first target region is at least partly embedded in the second target region.

16. The system according to claim 11, wherein the first target region and the second target region are formed of different materials, the second target region comprising a material having at least one of: a higher transparency to the electron beam and X-ray radiation as compared to the first target region, and an atomic number that is lower than an atomic number of a material of the first target region.

17. The system according to claim 11, wherein the first target region comprises a material selected from a list including tungsten, rhenium, molybdenum, vanadium, and niobium, and wherein the second target region comprises beryllium or carbon, such as diamond.

18. The system according to claim 11, wherein the first target region and the second target region are separated by a plurality of interfaces forming a shape conforming to at least one octagon.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0042] Embodiments of the present invention will now be described with reference to the accompanying drawing, on which:

[0043] FIG. 1a is a perspective view of a system for generating X-ray radiation in accordance with an embodiment of the invention;

[0044] FIGS. 1b and 1c show alternative implementations of the system shown in FIG. 1a;

[0045] FIG. 2a is a cross section of an X-ray target according to an embodiment of the invention;

[0046] FIG. 2b shows an alternative implementation of a target of the type shown in FIG. 2a;

[0047] FIG. 2c-e show top views of targets similar to the types shown in FIGS. 2a and b;

[0048] FIG. 3a shows, in the plane of scanning, a location of an electron beam being scanned over a first and a second region of a target in accordance with an embodiment of the invention;

[0049] FIG. 3b shows a plot of a sensor signal against different positions of the electron beam on the target.

[0050] Unless otherwise indicated, the drawings are schematic and not to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

[0051] FIG. 1 shows a system 1 for generating X-ray radiation, generally comprising an X-ray target 100, an electron source 200 for generating an electron beam I, and a sensor arrangement 400 for measuring a quantity Q indicative of the interaction between the electron beam I and the target 100. This equipment may be located inside a housing 600, with possible exceptions for a voltage supply 700 and a controller 500, which may be located outside the housing 600 as shown in the drawing. Various electron-optical means 300 functioning by electromagnetic interaction may also be provided for controlling and deflecting the electron beam I.

[0052] The electron source 200 generally comprises a cathode 210 which is powered by the voltage supply 700 and includes an electron source 220, e.g., a therm ionic, thermal-field or cold-field charged-particle source. An electron beam I from the electron source 200 may be accelerated towards an accelerating aperture 350, at which point the beam I enters the electron-optical means 300 which may comprise an arrangement of aligning plates 310, lenses 320 and an arrangement of deflection plates 340. Variable properties of the aligning means 310, deflection means 340 and lenses 320 may be controllable by signals provided by the controller 500. In this embodiment, the deflection and aligning means 340, 310 are operable to accelerate the electron beam I in at least two transversal directions.

[0053] Downstream of the electron-optical means 300, the outgoing electron beam I may intersect with the X-ray target 100, which will be described in further detail below. This is where the X-ray production takes place, and the location may also be referred to as the interaction region or interaction point. X-rays may be led out from the housing 600, via e.g. an X-ray window 610, in a direction not coinciding with the electron beam I.

[0054] According to the present embodiment, a portion of the electron beam I may continue past the interaction region and reach the sensor 400. The sensor may e.g. be a conductive plate connected to ground via an ammeter 410, which provides an approximate measure of the total current carried by the electron beam I downstream of the target 100. It is understood that the controller 500 has access to the actual signal from the ammeter 410.

[0055] FIG. 1b shows another embodiment, largely similar to that shown in FIG. 1a, but in which the sensor 400 and the target 100 are differently implemented. In this embodiment, there is no separate sensor arrangement. Rather, the ammeter 410 is used for determining the amount of charge absorbed by the target 100 and is thus directly connected to the target.

[0056] FIG. 1c shows a further embodiment of the invention, also this largely similar to that shown in FIG. 1a, but in which a backscattering sensor 400 is arranged upstream of the interaction region. The backscattering sensor 400 may e.g. comprise an electrically conducting plate or grid connected to an ammeter (not shown) to provide an approximate measure of the amount of electrons that are backscattered from the target 100. As indicated in the present figure, the system 1 may be operated in a transmission configuration, wherein the generated X-rays emanate from the side of the target 100 that is opposite to the side on which the electron beam I impacts. In case the target 100 is arranged at, or even incorporated with, the housing 600, the X-ray window 610 shown in FIGS. 1a and b may be omitted and the generated X-rays exiting the housing 600 directly through the target 100.

[0057] The above embodiments are merely examples of possible implementations of sensors adapted to measure a quantity Q indicative of the interaction between the electron beam I and the X-ray target 100. As shown in those examples, the quantity Q may refer to the number of electrons that passes through the target, the number of electrons that are absorbed in (or charge) the target, and the number of electrons that are backscattered from the target. Other quantities are however conceivable, and may e.g. relate to the local heating of the target, the amount of generated X-rays, the amount of generated visible light, and the energy of the electrons that are not absorbed by the target.

[0058] FIG. 2a shows a cross sectional portion of an X-ray target according to an embodiment of the invention. The target 100 comprises a first region 110 and a second region 120, wherein the interface between the first region 110 and the second region 120 forms an edge or step 112. The first region 110 may be formed of a material capable of generating X-rays upon interaction with impinging electrons, and may e.g. include such a dense material like tungsten. The tungsten region 110 may be provided in a layer that may be evaporated onto a substrate 122. The layer may e.g. be about 500 nm thick and provided with apertures, such as square, octagon, or circle shaped holes, exposing the underlying substrate 122. The apertures may e.g. be formed by means of photo lithography and etching. The substrate may be formed of a material that compared to the material of the first region 110 is more transparent to impinging electrons, and may e.g. be about 100 micrometers thick. The substrate may e.g. comprise diamond or similar light material with low atomic number and preferably high thermal conductivity.

[0059] As illustrated in FIG. 2a, the tungsten layer 110 may comprise an aperture or open region exposing the underlying diamond substrate 122, thereby forming the second region 120 of the target 100.

[0060] FIG. 2b shows another embodiment of a target that may be similarly configured as the one in FIG. 2a, but in which the first regions 110 are at least partly embedded in the substrate 122 and have a thickness, in the direction of propagation of the electron beam, that varies along the surface of the target 100. Alternatively, a first region 110 may have a constant thickness that differs from other first regions 110.

[0061] FIG. 2c is a top view of a target 100 similar to the ones of FIGS. 2a and 2b. In this embodiment, the second regions 120 are formed as five rectangles or squares having edges 112 that extend in two substantially perpendicular directions.

[0062] FIG. 2d is a top view of similar target 100 as in FIGS. 2a-c, wherein the first region 110 is formed as a circle that is enclosed by a second region 120. A second region 120 may also be arranged within the first region 110, forming a circular edge between the different regions 110, 120. The circular edge allows for the lateral extension of the beam spot to be determined in any direction.

[0063] FIG. 2e shows a portion of a target 100, comprising a plurality of first regions 110 shaped as octagons, squares and rectangles. The octagons may be used for measuring the size of the beam spot in at least three directions, such as 0, 45 and 90, thereby allowing for ellipticity of the beam spot (and hence astigmatic effects) to be estimated. By measuring along three directions the length of the major and minor axes as well as the angular orientation of an elliptic spot may be determined. This estimated information may e.g. be used for calibration of the electron optics along these three directions. It might for example be advantageous to orient the major axis of an elliptic spot in a particular direction or alternatively it may be advantageous to obtain a circular spot. Thus one way of using the estimated information is to adjust the electron optics to obtain a desired beam spot.

[0064] FIG. 3a shows, in the plane of scanning, a location of an electron beam spot A.sub.l that is traversed across a surface of a target 100 in the direction indicated by the arrow. The target may be similarly configured as the targets discussed in connection with FIGS. 2a-e. The beam spot A.sub.l, which may have a width W.sub.x in a first direction and W.sub.y in a second direction, may be scanned from a first region 110 of the target, over a first edge 112 between the first region 110 and the second region 120 towards the second region 120 of the target 100. Further, the beam spot A.sub.l may continue over the second region 120 towards a second edge 113, perpendicular to the first edge 112, at which the beam spot A.sub.l enters the first region 110 again. The scanning motion may be controlled by the controller and the electron-optical means (not shown).

[0065] Since the material of the first region 110 and the second region 120 generally interact differently with impinging electronstungsten, which may form the first region 110, tends to generate X-rays whereas diamond, which may form the second region 120, tends to have a lower X-ray generating capabilitythe location of the electron beam spot may be determined by observing its interaction with the target 100. The interaction may e.g. be monitored by measuring a quantity Q such as the amount of generated X-ray radiation, or by measuring a number of electrons that pass through the target 100 or backscatter.

[0066] The resulting quantity Q is shown in FIG. 3b, which shows a plot of a sensor signal indicating the measured quantity Q as a function of the travelled distance d on the surface of the target 100 for backscattered electrons or generated X-rays. The travelled distance d, or position on the surface of the target 100, may e.g. be determined by the particular deflector settings used for deflecting the electron beam. In the present example, the rate of change in the sensor signal (e.g. indicating the amount of X-ray radiation generated at different locations on the target) from a first, relatively constant level to a reduced or near-zero sensor signal is proportional to a first width W.sub.y of the beam spot A.sub.l. As the beam spot A.sub.l then crosses the second edge 113, in a direction perpendicular to the first edge 112, the rate of increase in sensor signal is proportional to a second width W.sub.x of the beam spot A.sub.l.

[0067] A similar procedure may be used for determining the correlation between the settings of the electron-optical means, such as the deflector, and the position of the electron beam relative to the target. This may be done by observing the sensor signal, as described above, for different settings of the electron-optical means and correlate the settings with the electron beam passing over the edges 112, 113 of the target 100.

[0068] The person skilled in the art by no means is limited to the example embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. In particular, X-ray sources and systems comprising more than one electron beam are conceivable within the scope of the present inventive concept. Furthermore, X-ray sources of the type described herein may advantageously be combined with X-ray optics tailored to specific applications (many examples of this are well known within the field of X-ray technology). In particular, the ability to deflect the electron beam to different locations on the target may be used to align the X-ray source with the optics. Additionally, variation to the disclosed embodiments can be understood and effected by the skilled person in practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.