Method for controlling an x-ray source

Abstract

A method for controlling an X-ray source configured to emit, from an X-ray spot on a target, X-ray radiation generated by an interaction between an electron beam and the target, wherein the X-ray spot is determined by the field of view of an X-ray optical system of the X-ray source. The method includes providing the target, providing the electron beam forming an electron spot on the target and interacting with the target to generate X-ray radiation, and adjusting a width and total power of the electron beam such that a maximum of the power density profile in the electron spot is below a predetermined limit, and such that a total power delivered to the target in the X-ray spot is increased.

Claims

1. A method for controlling an X-ray source configured to emit, from an X-ray spot on a target, X-ray radiation generated by an interaction between an electron beam and the target, wherein the X-ray spot is determined by the field of view of an X-ray optical system of the X-ray source, the method comprising the steps of: providing a liquid jet forming the target; providing the electron beam accelerated by an acceleration voltage, forming an electron spot focused on the target by means of a focus current, and arranged to interact with the target to generate X-ray radiation; determining a scale factor for the acceleration voltage and the focus current relating a deflection current to a displacement of the electron beam relative to the target; measuring a quantity indicative of an interaction between the electron beam and the target for a range of displacements of the electron beam; calculating a power density profile of the electron beam based on the quantity; adjusting a width and a total power of the electron beam such that a maximum of the power density profile thereby obtained in the electron spot is below a predetermined limit, and such that a total power delivered to the target in the X-ray spot is increased.

2. The method according to claim 1, wherein the width and total power of the electron beam is further adjusted such that an X-ray source performance indicator is below a predetermined threshold.

3. The method according to claim 2, wherein the X-ray source performance indicator is associated with at least one of: a total vapor generation from the target; a maximum in delivered power per unit area to the target by the electron beam; a maximum surface temperature of the target; and a maximum in delivered power per unit length, by the electron beam, along a width of the target.

4. The method according to claim 1, wherein the step of determining the scale factor comprises at least one of: receiving the scale factor from a scale factor database; displacing the electron beam on the target and measuring a movement of an X-ray spot generated on the target; and displacing the electron beam on a sensor aperture having predetermined aperture dimensions.

5. The method according to claim 1, further comprising determining a target width.

6. The method according to claim 5, wherein the step of determining the target width comprises at least one of: receiving the target width from a target width database; and setting the width of the electron beam to a width smaller than an expected target width, measuring the quantity indicative of the interaction between the target and the electron beam for a range of displacements of the electron beam, and calculating the target width based on the measured quantity.

7. The method according to claim 5, wherein the step of determining the scale factor comprises displacing the electron beam on the target and measuring the quantity indicative of the interaction between the electron beam and the target, and calculating the scale factor based on the quantity and the target width.

8. The method according to claim 1, wherein the quantity indicative of an interaction between the electron beam and the target pertains to detecting backscattered electrons and/or emitted electrons formed by the interaction of the electron beam and the target.

9. The method according to claim 1, wherein the quantity indicative of an interaction between the electron beam and the target pertains to detecting X-ray radiation generated by the interaction of the electron beam and the target.

10. The method according to claim 1, wherein the X-ray source comprises an electron detector arranged downstream of the target in a propagation direction of the electron beam, wherein the quantity indicative of an interaction between the electron beam and the target pertains to: detecting electrons collected by the electron detector for the range of displacements of the electron beam.

11. An X-ray source comprising: a target generator configured to provide a liquid jet forming a target; an electron source configured to provide an electron beam forming an electron spot on the target and interacting with the target to generate X-ray radiation from an X-ray spot on the target; an acceleration aperture arranged for providing an acceleration voltage for accelerating the electron beam; a focusing coil arranged for focusing the electron beam by application of a focus current; a controller; an X-ray optical system having a field of view defining the X-ray spot; and an electron optical system interacting with the electron beam; wherein the controller is configured to determine a scale factor for the acceleration voltage and the focus current relating a deflection current to a displacement of the electron beam relative to the target and further to operate the electron optic system and the electron source to determine a power density profile of the electron beam, and to adjust a width and total power of the electron beam such that a maximum of the power density profile thereby obtained in the electron spot is below a predetermined limit, and such that a total power delivered to the target in the X-ray spot is increased.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description of different embodiments of the present inventive concept, with reference to the appended drawings, wherein:

(2) FIG. 1 schematically illustrates an example of an X-ray source which may utilize the method according to the inventive concept;

(3) FIGS. 2a and 2b illustrate power densities and various regions on a target;

(4) FIG. 2c illustrates a cross section of a target according to the inventive concept;

(5) FIG. 3 schematically illustrates an example of an X-ray source which may utilize the method according to the inventive concept;

(6) FIG. 4 schematically illustrates various power density profiles and how they are affected by electron beam width and total power;

(7) FIG. 5 illustrates a method for controlling an X-ray source in a block diagram. The figures are not necessarily to scale, and generally only show parts that are necessary in order to elucidate the inventive concept, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

(8) An example of an X-ray source 100 which may utilize the method according to the inventive concept will now be described with reference to FIG. 1. The illustrated X-ray source 100 utilizes a liquid jet 110 as a target for the electron beam. However, as is readily appreciated by the person skilled in the art, other types of targets, such as solid targets, are equally possible within the scope of the inventive concept. Further, some of the disclosed features of the X-ray source 100 are merely included as examples, and may not be necessary for the operation of the X-ray source 100.

(9) As indicated in FIG. 1, a low pressure chamber, or vacuum chamber, 102 may be defined by an enclosure 104 and an X-ray transparent window 106 which separates the low pressure chamber 102 from the ambient atmosphere. The X-ray source 100 comprises a liquid jet generator 108 configured to form a liquid jet 110 moving along a flow axis F. The liquid jet generator 110 may comprise a nozzle through which liquid, such as e.g. liquid metal may be ejected to form the liquid jet 110 propagating towards and through an intersecting region 112. The liquid jet 110 propagates through the intersecting region 112, towards a collecting arrangement 113 arranged below the liquid jet generator 108 with respect to the flow direction. The X-ray source 100 further comprises an electron source 114 configured to provide an electron beam 116 directed towards the intersecting region 112. The electron source 114 may comprise a cathode for the generation of the electron beam 116. In the intersecting region 112, the electron beam 116 interacts with the liquid jet 110 to generate X-ray radiation 118, which is transmitted out of the X-ray source 100 via the X-ray transparent window 106. The X-ray radiation 118 is here directed out of the X-ray source 100 substantially perpendicular to the direction of the electron beam 116.

(10) The liquid forming the liquid jet is collected by the collecting arrangement 113, and is subsequently recirculated by a pump 120 via a recirculating path 122 to the liquid jet generator 108, where the liquid may be reused to continuously generate the liquid jet 110.

(11) Still referring to FIG. 1, the X-ray source 100 here comprises an electron detector 128 configured to receive at least part of the electron beam 116 passing the liquid jet 110. The electron detector 128 is here arranged behind the intersecting region 112 as seen from a viewpoint of the electron source 114. It is to be understood that the shape of the electron detector 128 is here merely schematically illustrated, and that other shapes of the electron detector 128 may be possible within the scope of the inventive concept.

(12) Referring now to FIG. 2a, a power density distribution and various regions on a target are illustrated. It should be noted these figures are not necessarily drawn to scale, and that the shapes of the illustrated features are not limiting but merely an example of possible shapes.

(13) Part of a target 210a is shown, wherein an electron spot 230a and an X-ray spot 232a are illustrated. It may be noted that the electron spot 230a and the X-ray spot 232a in this particular example are overlapping. The graphs below the target 210a illustrate properties of the power density distribution along the line A-A indicated on the target 210a.

(14) Below the target 210a in FIG. 2a, a graph illustrating the power density profile 236a of an electron beam is shown. As defined in the present disclosure, the electron spot 230a corresponds to the full width at half maximum I.sub.max. Also, as illustrated by the shaded area 234a, some electrons do not contribute to the generation of X-ray radiation and may in some respects be deemed wasted. The area 234a under the graph 236a reflect the power of electrons that do not contribute to the generation of X-ray radiation.

(15) At the bottom of FIG. 2a, a graph illustrating the power density distribution of electrons interacting with the target 210a within the X-ray spot 232a is shown. Since the amount of useful X-ray radiation generated in the target 210a may be proportional to the electron beam current striking the target within the X-ray spot 232a, the area 240a below the graph 238a may reflect the amount of useful X-ray radiation generated in the X-ray spot 232a. It may be noted that at the edge of the X-ray spot 232a, the power density I.sub.a is equal to half of I.sub.max.

(16) Referring now to FIG. 2b, a power density profile and various regions on a target are illustrated. It should be noted these figures are not necessarily drawn to scale, and that the shapes of the illustrated features are not limiting but merely an example of possible shapes.

(17) Part of a target 210b is shown, wherein an electron spot 230b and an X-ray spot 232b are illustrated. It may be noted that the electron spot 230b exceeds the X-ray spot 232b. In particular, a width 233b of the electron spot 230b is larger than a width 231b of the X-ray spot 232b. Further, the electron spot 230b here has a width 233b being larger than a height 237b of the electron spot 230b. The graphs below the target 210b illustrate properties of the power density profile along the line A-A indicated on the target 210b.

(18) Below the target 210b in FIG. 2b, a graph illustrating the power density profile 236b of an electron beam is shown. As defined in the present disclosure, the electron spot 230b corresponds to the full width at half maximum I.sub.max. It is emphasized that a total power of the electron beam pertaining to the power density profile 236b is higher compared to a total power of the electron beam pertaining to the power density profile 236a illustrated in FIG. 2a. The higher total power may be achieved by e.g. increasing a current applied to the electron source.

(19) Also, as illustrated by the shaded area 234b, some electrons do contribute to the generation of X-ray radiation, but do not generate X-ray radiation in the X-ray spot 232b and may in some respects be deemed wasted. In particular, the area 234b reflects the power of electrons interacting with the target 210b to generate X-ray radiation outside of the X-ray spot 232b. Such X-ray radiation is not emitted by the X-ray source to be utilized in applications such as e.g. imaging or diffraction applications.

(20) The area 239b reflects the power of electrons that do not contribute to the generation of X-ray radiation. Further, the area 235b reflect the power of electrons that do not interact with the target 210b, but instead pass on e.g. the sides of the target 210b. In other words, the area 235b reflects the power of electrons that do not interact with the target 210b to generate X-ray radiation. The sum of the areas 234b, 235b, and 239b reflect the power of electrons that do not contribute to generating X-ray radiation in the X-ray spot 232b.

(21) It may be noted that the sum of the areas 234b, 235b, and 239b is larger than the area 234a of FIG. 2a. In other words, by setting the width of the electron beam such that the electron spot 230b exceeds the X-ray spot 232b, more power may be deemed wasted in the sense that the power of electrons that do not contribute to generating X-ray radiation in the X-ray spot 232b is increased.

(22) At the bottom of FIG. 2b, a graph illustrating the power density distribution of electrons interacting with the target 210b within the X-ray spot 232b is shown. Since the amount of useful X-ray radiation generated in the target 210b may be proportional to the electron beam current striking the target within the X-ray spot 232b, the area 240b below the graph 238b may reflect the amount of useful X-ray radiation generated in the X-ray spot 232b. It may be noted that at the edge of the X-ray spot 232b, the power density I.sub.b is greater than half of I.sub.max. In particular, it may be noted that the area 240b, which reflects the amount of useful X-ray radiation generated in the X-ray spot 232b, is larger than the area 240a of FIG. 2a, which reflects the amount of useful X-ray radiation generated in the X-ray spot 232a. Hence, by setting the width of the electron beam such that the electron spot 230b exceeds the X-ray spot 232b, more useful X-ray radiation may be generated in the X-ray spot 232b, compared to setting the width of the electron beam such that the electron spot is equal to or smaller than the X-ray spot.

(23) The maximum power density I.sub.max of FIGS. 2a and 2b may represent the predetermined limit of the power density. In other words, the maximum power density I.sub.max may correspond to a power density level which is below a level causing the target to vaporize in case of a liquid anode or melt in case of a solid anode. The maximum power density I.sub.max may also correspond to a level which causes the target to assume a surface temperature below a vaporizing temperature of the target material in case of a liquid target or a below a melting point of the target material in case of a solid target. In case the maximum power density I.sub.max of the power density profile is not at the predetermined limit, the power of the electron beam may be adjusted, i.e. increased or decrease, in order to set the maximum power density at the predetermined limit.

(24) FIG. 2c is a cross section of a liquid target 210, orthogonal to a propagation direction of the target material. The width of the electron spot 233 is in this example defined by the width of the electron beam 166 impinging on the target, whereas the width of the X-ray spot 231 is defined by the relative orientation of the target and an X-ray optical component, in this example illustrated by two apertures 250 forming a pinhole configuration; other components and X-ray optical systems, for example comprising focusing mirrors, are however also conceivable. In the present example, the size of the X-ray spot is defined by the direction in which the X-ray radiation 118 is emitted, and thus the geometry of the target, and by the field of view defined by the apertures 250 used for collecting the generated X-ray radiation 118. X-ray radiation 118 lying outside the field of view may not be considered to origin from the X-ray spot, according to the definition used in the context of the present disclosure. It will be appreciated that the electron spot 233 may be defined by the width of the electron beam 166 and/or the size and orientation of the target 210. In case the electron beam 166 is wider than the target 210, the electron spot 233 may have a width corresponding to the width of the target 210. In case the electron beam 166 is narrower than the target 210, as shown in the present example, the electron spot 233 size may be determined by the width of the electron beam 166, such as its full width at half maximum (FWHM).

(25) With reference to FIG. 3, an example of an X-ray source 300 which may utilize the method according to the inventive concept will now be described. The illustrated X-ray source 300 utilizes a liquid jet 310 as a target for the electron beam. However, as is readily appreciated by the person skilled in the art, other types of targets, such as solid targets, are equally possible within the scope of the inventive concept. Further, some of the disclosed features of the X-ray source 300 are merely included as examples, and may not be necessary for the operation of the X-ray source 300.

(26) The X-ray source 300 generally comprises an electron source 314, 346, and a liquid jet generator 308 configured to form a liquid jet 310 acting as an electron target. The components of the X-ray source 300 is located in a gas-tight housing 342, with possible exceptions for a power supply 144 and a controller 347, which may be located outside the housing 342 as shown in the drawing. Various electron-optical components functioning by electromagnetic interaction may also be located outside the housing 342 if the latter does not screen off electromagnetic fields to any significant extent. Accordingly, such electron-optical components may be located outside the vacuum region if the housing 342 is made of a material with low magnetic permeability, e.g., austenitic stainless steel. The electron source generally comprises a cathode 314 which is powered by the power supply 144 an includes an electron emitter 346, e.g. a thermionic, thermal-field or cold-field charged-particle source. Typically, the electron energy may range from about 5 keV to about 500 keV. An electron beam from the electron source is accelerated towards an accelerating aperture 348, at which point it enters an electron-optical system comprising an arrangement of aligning plates 350, lenses 352 and an arrangement of deflection plates 354. Variable properties of the aligning plates 350, lenses 352, and deflection plates 354 are controllable by signals provided by the controller 347. In the illustrated example, the deflection and alignment plates 350, 354 are operable to accelerate the electron beam in at least two transversal directions. After initial calibration, the aligning plates 350 are typically maintained at a constant setting throughout a work cycle of the X-ray source 300, while the deflection plates 354 are used for dynamically scanning or adjusting an electron spot location during use of the X-ray source 300. Controllable properties of the lenses 352 include their respective focusing powers (focal lengths). Although the drawing symbolically depicts the aligning, focusing and deflecting means in a way to suggest that they are of the electrostatic type, the invention may equally well be embodied by using electromagnetic equipment or a mixture of electrostatic and electromagnetic electron-optical components. The X-ray source may comprise stigmator coils 353 which may provide for that a non-circular shape of the electron spot may be achieved.

(27) Downstream of the electron-optical system, an outgoing electron beam 12 intersects with the liquid jet 310 in an intersecting region 312. This is where the X-ray production may take place. X-ray radiation may be led out from the housing 342 in a direction not coinciding with the electron beam. Any portion of the electron beam 12 that continues past the intersecting region 312 may reach an electron detector 328. In the illustrated example, the electron detector 328 is simply a conductive plate connected to earth via an ammeter 356, which provides an approximate measure of the total current carried by the electron beam I.sub.2 downstream of the intersecting region 312. As the figure shows, the electron detector 328 is located a distance D away from the intersecting region 312, and so does not interfere with the regular operation of the X-ray source 300. Between the electron detector 328 and the housing 342, there is electrical insulation, such that a difference in electrical potential between the electron detector 328 and the housing 342 can be allowed. Although the electron detector 328 is shown to project out from the inner wall of the housing 342, it should be understood that the electron detector 328 could also be mounted flush with the housing wall. The electron detector may further be equipped with an aperture arranged so that electron impinging inside the aperture may be registered by the electron detector whereas electrons impinging outside of the aperture may not be detected.

(28) A lower portion of the housing 342, a vacuum pump or similar means for evacuating gas molecules from the housing 342, receptacles and pumps for collecting and recirculating the liquid jet are not shown on this drawing. It is also understood that the controller 347 has access to the actual signal from the ammeter 356.

(29) Referring now to FIG. 4, various power density profiles of an electron beam are shown, and the effect of adjusting width and/or total power of the electron beam is schematically illustrated.

(30) In each power density profile I-VI, the vertical axis represents power per unit length, while the horizontal axis represents position along an arbitrary line of the electron beam.

(31) The power density profiles I-VI are arranged in a relative coordinate system, wherein positive or negative movement along the horizontal axis corresponds to an increase or decrease in electron beam width respectively, and wherein a positive or negative movement along the vertical axis corresponds to an increase or decrease in total power of the electron beam respectively.

(32) Power density profiles I, II and III represent electron beams having equal electron beam width. However, the total power of each of the electron beams associated with each of the power density profiles I, II and III is increased through I to II to III. Accordingly, a maximum power density, and/or a maximum in delivered power per unit length, is increased moving from power density profile I to III along the vertical axis of the drawing.

(33) Power density profiles I, IV and V represent electron beams having equal total power. However, the width of each of the electron beams associated with each of the power density profiles I, IV and V is increased through I to IV to V. Accordingly, a maximum power density, and/or a maximum in delivered power per unit length, is decreased through power density profile I to V along the horizontal axis of the drawing. Further, the spot size, i.e. the full width at half maximum of the power density profile, is increased through power density profile I to V along the horizontal axis of the drawing.

(34) Power density profile VI represent an electron beam having an increased width and total power compared to the electron beam associated with power density profile I. As can be seen, the maximum power density of power density profile VI, and/or the maximum in delivered power per unit length of power density profile VI, is unchanged compared to power density profile I. However, the width of power density profile VI is increased.

(35) A method for controlling an X-ray source according to the inventive concept will now be described with reference to FIG. 5. For clarity and simplicity, the method will be described in terms of ‘steps’. It is emphasized that steps are not necessarily processes that are delimited in time or separate from each other, and more than one ‘step’ may be performed at the same time in a parallel fashion.

(36) The method for controlling an X-ray source configured to emit, from a X-ray spot on a target, X-ray radiation generated by an interaction between an electron beam and the target, comprises the step 558 of providing the target; the step 560 of providing the electron beam arranged to interact with the target to generate X-ray radiation in an interaction region; the step 562 of determining a power density profile of the electron beam; the step 564 of setting a width and a total power of the electron beam such that the electron spot exceeds the X-ray spot in at least one direction; and the step 566 of setting the total power of the electron beam such that a maximum power density in the target is at a predetermined limit.

(37) 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 target or 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 and/or detectors tailored to specific applications exemplified by but not limited to medical diagnosis, non-destructive testing, lithography, crystal analysis, microscopy, materials science, microscopy surface physics, protein structure determination by X-ray diffraction, X-ray photo spectroscopy (XPS), critical dimension small angle X-ray scattering (CD-SAXS), and X-ray fluorescence (XRF). Additionally, variation to the disclosed examples can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 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.

LIST OF REFERENCE SIGNS

(38) 100 X-ray source 102 Vacuum chamber 104 Enclosure 106 X-ray transparent window 108 Liquid jet generator 110 Liquid jet 112 Intersecting region 113 Collecting arrangement 114 Electron source 116 Electron beam 118 X-ray radiation 120 Pump 122 Recirculating path 128 Electron detector 144 Power Supply 210a,b Target 230a,b Electron spot 231b Width of X-ray spot 232a,b X-ray spot 233b Width of electron spot 234a,b Area 235b Area 236a,b Power density profile 237b Height of electron spot 238a,b Power density profile 239b Area 240a,b Area 250 X-ray optical system 300 X-ray source 308 Liquid jet generator 310 Liquid target 312 Intersecting region 314 Cathode 328 Electron detector 342 Housing 346 Electron emitter 347 Controller 350 Aligning plates 352 Lenses 353 Stigmator coils 354 Deflection plates 356 Ammeter 558 Step of providing the target 560 Step of providing the electron beam 562 Step of determining a power density profile 564 Step of setting a width and a total power of the electron beam 566 Step of setting the total power of the electron beam I Power density profile II Power density profile III Power density profile IV Power density profile V Power density profile VI Power density profile F Flow axis D Distance I.sub.2 Electron beam