Semiconductor X-ray target

Abstract

A solid X-ray target for generating X-ray radiation is disclosed. The X-ray target includes at least one material selected from a list including trivalent elements; and at least one material selected from a list including pentavalent elements, wherein a first one of the materials is capable of generating the X-ray radiation upon interaction with an electron beam, and a second one of the materials forms a compound with the first one of the materials. An X-ray source including such an X-ray target and an electron source is also disclosed.

Claims

1. An X-ray source, comprising: an X-ray target; an electron source operable to generate an electron beam interacting with the X-ray target to generate X-ray radiation with an energy within the range 9 to 12 key; wherein the X-ray target comprises: a first element selected from a list consisting of trivalent elements; a second element selected from a list consisting of pentavalent elements forming a compound with said first element; a first region including the compound formed of the first and second material; and a second region supporting the first region; wherein the first region generates X-ray radiation upon interaction with the electron beam, and heat conduction between the first and second region is dominantly phonon heat conduction.

2. The X-ray source according to claim 1, wherein the first region is at least one of: provided in the form of a layer on the second region, and at least partly embedded in the second region.

3. The X-ray source according to claim 1, wherein said list of trivalent elements comprises boron, gallium, and indium.

4. The X-ray source according to claim 1, wherein said list of pentavalent elements comprises nitrogen, arsenic, and phosphorous.

5. The X-ray source according to claim 1, wherein said compound is selected from a list including gallium nitride, boron arsenide, indium arsenide, gallium phosphide, indium gallium nitride and gallium arsenide.

6. The X-ray source according to claim 1, wherein the second region comprises beryllium oxide or diamond.

7. The X-ray source according to claim 1, wherein the first region and the second region is separated by an edge, wherein said X-ray source further comprises: an electron-optical means for scanning the electron beam over the edge; a sensor adapted to measure a time evolution of a quantity indicative of the interaction between the electron beam and the first region and between the electron beam and the second region as the electron beam is being scanned over the edge; and a controller operably connected to the sensor and the electron-optical means and adapted to determine a lateral extension of the electron beam along the scanning direction, based on the measured time evolution of the quantity and a scanning speed of the electron beam.

8. The X-ray source according to claim 1, further comprising a target holder arranged to fixate said target.

9. The X-ray source according to claim 8, wherein said target holder comprises a path for a coolant arranged to remove excess heat from said target.

10. The X-ray source according to claim 9, wherein said target holder further comprises at least one of a heat exchanger, a cooling flange, a Peltier element, and a fan arranged to remove heat from a coolant.

11. The X-ray source according to claim 1, further comprising an X-ray optic arranged to form a monochromatic X-ray beam directed to a sample position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of portion of an X-ray target according to the invention.

(2) FIG. 2 shows a flow chart of a process for manufacturing an X-ray target according to some embodiments.

(3) FIG. 3a is a cross section of an X-ray target according to an embodiment of the invention.

(4) FIG. 3b shows an alternative implementation of an X-ray target of the type shown in FIG. 2a.

(5) FIG. 3c shows a top view of an X-ray target similar to the types shown in FIGS. 2a and b.

(6) FIG. 4 is a perspective view of an X-ray source for generating X-ray radiation, comprising an X-ray target of the sort shown in any one of the previous figures.

(7) FIG. 5a schematically shows a cross section of a target holder according to embodiments of the invention.

(8) FIG. 5b schematically shows a top view of a target holder according to embodiments of the invention.

(9) FIG. 5c shows a top view of a target according to embodiments of the invention.

(10) Unless otherwise indicated, the drawings are schematic and not to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

(11) FIG. 1 schematically shows a compound formed of a first material 101 selected from a list including trivalent elements, such as e.g. boron, aluminium, gallium, indium and thallium, and a second material 102 selected from a list including pentavalent materials, such as e.g. nitrogen, phosphorous, arsenic, antimony and bismuth. In the specific, illustrative example of FIG. 1, the first material 101 is represented by gallium and the second element 102 represented by nitrogen. Thus, the illustrated compound is gallium nitride (GaN). GaN is a binary III-V semiconductor material arranged in a tetrahedral crystal structure. The predominant heat conducting mechanism may be phonon based.

(12) Upon interaction with an impinging electron beam, the gallium may contribute to the X-ray generation by emitting a characteristic X-ray radiation of 9.3 keV, whereas the nitrogen may contribute to improved thermal properties by having formed a compound with the gallium. As already mentioned, by forming a compound such as GaN, the relatively low melting point of gallium (303 K) may be increased to about 2773 K.

(13) FIG. 2 is a flow chart illustrating a process for forming an X-ray target comprising a first region 110, including a compound formed by a trivalent material and a pentavalent material as described above in connection with FIG. 1, and a second region 120 for supporting the first region 110. In the present example, the compound may be formed of gallium nitride, GaN, and the first region 110 may thus be capable of generating X-rays upon interaction with impinging electrons. The second region 120 may be formed of a material primarily selected for its ability to dissipate heat from the first region 110, such as e.g. diamond. However, the skilled person understands that the process described hereinbelow is not limited to gallium nitride and diamond, but may also be useful in embodiments comprising the other compounds and materials disclosed herein.

(14) Gallium nitride (GaN) may be deposited on diamond by a process starting with a commercially available GaN-on-Si wafer, comprising GaN deposited on a silicon substrate 130. In a first step, a temporary carrier 140 may be deposited onto the GaN surface. The temporary carrier 140 may be any suitable material known in the art, such as silicon. Then, the silicon substrate 130 may be removed from the GaN layer by any suitable process, such as chemical etching, leaving one side of the GaN layer exposed. Onto the exposed side of GaN a diamond layer may be deposited by for example chemical vapour deposition (CVD), such as microwave assisted chemical vapour deposition. The diamond may be deposited onto the GaN in an epitaxial manner. Other methods for depositing the diamond may also be used, such as physical vapour deposition (PVD). Optionally, a thin dielectric layer may be deposited onto the GaN before the diamond layer is deposited. Following the deposition of the diamond substrate onto the GaN region, the temporary carrier 140 may be removed by means known in the art, such as chemical etching.

(15) FIG. 3a shows a cross sectional a portion of an X-ray target, which may be similarly configured as the target discussed above in connection with FIG. 2. The first region 110, as indicated in the present example of FIG. 3a, may form a layer that may be about 500 nm thick and provided with apertures, such as square, octagon, or circle shaped holes, exposing the underlying substrate 122 forming the second region 120. 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 micrometres thick. The substrate may e.g. comprise diamond, beryllium oxide, or similar light material with low atomic number and preferably high thermal conductivity.

(16) As illustrated in FIG. 3a, the first region 110 may comprise an aperture or open region exposing the underlying diamond substrate 122, thereby forming the second region 120 of the target 100.

(17) FIG. 3b shows another embodiment of a target that may be similarly configured as the one in FIG. 3a, 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.

(18) FIG. 3c 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.

(19) FIG. 4 shows an X-ray source or system 1 for generating X-ray radiation, generally comprising a solid X-ray target 100 of the type described above in connection with the previous figures, and an electron source 200 for generating an electron beam I. 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.

(20) 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 thermionic, 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.

(21) Downstream of the electron-optical means 300, the outgoing electron beam I may intersect with the X-ray target 100. 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.

(22) FIGS. 5a and b schematically shows an embodiment where the target 100, comprising a first region 110 (e.g. GaN) and a second region 120 (e.g. diamond) is arranged in a target holder 501. The target holder 501 may have any suitable shape or form, such as circular or quadrangular, for providing mechanical support and/or thermal managment. The target holder 501 may be adapted to hold the target 100 such that it can be inpinged by the electron beam I. The target 100 may be positioned in the target holder 501 such that the target is directed towards the inpinging electron beam I. The target may e.g. be positioned at an angle towards the inpinging electron beam I. The target holder may comprise cooling means 505 (illustrated herein as a coolant flow 505) for cooling the target 100. Such cooling means may e.g. employ a circulating cooling fluid (such as water, air or another fluid) in thermal contact with a rear surface of the target 100. The cooling fluid may e.g. be circulated through an inlet and an outlet of the target holder. Alternatively, or additionally the cooling means may comprise a heat sink or heat dissipating means cooling the target by means of e.g. conduction or convection.

(23) The target holder may comprise metal or alloy such as brass or steel. To secure the target to the target holder the substrate may be provided with a metallized surface 503 that may be brazed onto the target holder. The design of the target holder may further be adapted so as to accept thermally induced shape changes of the target without compromising the joint between the target and the target holder. In an exemplary embodiment shown in FIG. 5a the target holder comprises a thin walled tube with a recess adapted to the target dimensions. The displacements caused by thermal expansion of the target will deform the tube slightly without breaking the joint between the target and the target holder.

(24) A compact X-ray detector (not shown) may be included to monitor and continuously optimise the position of the electron focal spot. This may be a small solid state detector or other X-ray detecting device.

(25) The X-ray source or system 1 may in some embodiments comprise an X-ray focusing and/or deflection device (not shown). The system encompasses an X-ray focusing device located close to the source to provide a magnified image of the focal spot at controlled varying distances from the source. Options for the X-ray focusing systems may include

(26) 1. Micromirrors, using specular reflectivity from a gold or similar coating of highly controlled smoothness (around 10 Å rms), from a circularly symmetric profile. The micromirrors may also have a ellipsoidal profile which gives focused beams of X-rays (such as 300 μm diameter 600 mm from the focal spot). Ellipsoidal profile provides a measured insertion gain of <150 (could be <250). The reason for close coupling is so that a large solid angle of radiation may be collected, but also that the focusing elements may form a magnified image of the focal spot at the sample (low beam divergence but high insertion gain). The micromirrors may also have a paraboloidal profile, which provides a nearly parallel beam, yielding a expected insertion gain of >200.

(27) 2. Kirkpatrik-Baez type, comprising Bent plates arranged in combinations of elliptical or parabolic or combination. Allows simple change of mirror profiles to suit different applications.

(28) 3. Other possibilities include zone plates, Bragg Fresnel optics and/or multilayer optics.

(29) The distance between the focusing device and the source on the target 100 may be small, such as less than 20 mm, preferably about 10 mm, to ensure close coupling.

(30) The energy spectrum of the generated X-ray radiation will typically comprise both characteristic X-ray radiation (also referred to as line radiation) and Brehmsstralung. Whereas characteristic X-ray radiation comprise discrete energies the Brehmsstralung comprises a broad range of energies. Thus, it may be advantageous to select a focusing device that attenuate X-rays with other energies than the discrete energies of the characteristic radiation capable of projecting a monochromatic X-ray beam onto the sample. Such a focusing device may be realized as a curved multilayer mirror where the distance between the planes are adjusted along the curvature so that the Bragg condition for reflection is fulfilled for the particular X-ray wavelength along the mirrors curvature. This type of mirror may be referred to as a Gobel mirror. When two such mirrors are arranged side by side perpendicular to each other the arrangement may be referred to as a Montel mirror. By providing a curvature with the shape of a paraboloid a collimated or parallel monochromatic beam may be produced. By providing a curvature with the shape of an ellipsoid a focused monochromatic beam may be produced.

(31) FIG. 5c 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. This estimated information may e.g. be used for calibration of the electron optics along these three directions.

(32) An electron beam spot Ai may be traversed across a surface of a target 100 in a certain direction. The target may be similarly configured as the targets discussed in connection with FIGS. 3 a-c and 5 c 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 (not shown) between the first region 110 and the second region 120 towards the second region 120 of the target 100. Further, the beam spot Ai may continue over the second region 120 towards a second edge 113, perpendicular to the first edge 112, at which the beam spot Ai enters the first region 110 again. The scanning motion may be controlled by a controller and a electron-optical means (not shown).

(33) Since the material of the first region 110 and the second region 120 generally interact differently with impinging electrons—GaN, 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 capability—the 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. The quantity Q may be measured by a sensor.

(34) The resulting quantity Q may be visualized as a sensor signal indicating the measured quantity Q as a function of the travelled distanced 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 a second edge (not shown), in a direction perpendicular to the first edge, the rate of increase in sensor signal is proportional to a second width W.sub.x of the beam spot A.sub.l.

(35) 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 of the target 100.