Electron beam characterization

Abstract

A method for characterizing an electron beam in a liquid metal jet X-ray source is disclosed. The method comprises: providing the electron beam and directing the electron beam to an interaction region; providing an electron beam reservoir connected to a ground potential for receiving the electron beam after the electron beam passes through the interaction region; scanning the electron beam over at least a portion of the interaction region; measuring X-ray radiation generated by the interaction between the electron beam and the electron beam reservoir during the scanning to obtain an X-ray distribution curve; and calculating electron beam characteristics based on the X-ray distribution curve. A corresponding liquid metal jet X-ray source is also disclosed.

Description

Technical Field

[0001] The invention disclosed herein generally relates to characterizing electron beams in liquid metal jet X-ray sources. Background Art

[0002] X-ray radiation can be generated by impinging an electron beam on a target material. The X-ray radiation can be generated as bremsstrahlung or characteristic line emission from the target material. The performance of an X-ray source depends inter alia on the characteristics of the focal spot of the X-ray radiation generated by the interaction between the electron beam and the target. In general, efforts are being made to obtain higher brightness and smaller focal spots of X-ray radiation, which requires improved control of the electron beam and its interaction with the target. In particular, several attempts have been made to more accurately determine and control the spot size and shape of the electron beam impinging on the target.

[0003] WO 2012 / 087238 discloses a technique for determining and controlling the width of an electron beam at its point of interaction with a target. This prior art involves the use of a sensor with a charge sensitive region. The measurement of the width of the electron beam is performed by deflecting the electron beam over a sensor region while an electron target is present and partially obscures the sensor region. Because the electron target obscures or partially obscures a portion of the sensor region, the recorded sensor signal will exhibit a transition between minimum attenuation of the beam (unobstructed sensor region) and maximum attenuation (behind the target). The beam width can be derived from this information, in particular from the width of the transition. However, for measurement purposes, the sensor region cannot be electrically grounded. Therefore, this technique presents challenges, such as how to avoid short circuits and arcs at the edges of the sensor. Summary of the invention

[0004] The present invention provides an improvement in an electron beam impinging on a liquid metal jet X-ray source and is based on the idea of ​​characterizing the electron beam by measuring the X-ray radiation generated by the electron beam. More specifically, the generated X-ray radiation can be measured while scanning the electron beam, thereby obtaining an X-ray profile. One or more characteristics of the electron beam can then be calculated based on the obtained X-ray profile.

[0005] Thus, in an embodiment of the present invention, the characteristics of an electron beam are determined by detecting X-ray radiation generated therefrom during a scan of the electron beam. Since there is no need to measure the current through a portion of the X-ray source (the "electron reservoir") (i.e., the portion that the electron beam strikes after passing through the interaction region), this portion can be grounded (i.e., electrically grounded). By relying on X-ray measurements rather than current measurements, the electron beam reservoir does not need to be electrically isolated from its surroundings, thereby eliminating the risk of short circuits caused by droplets of material, such as from a liquid jet, being deposited on the edge of the electron reservoir. In addition, image distortions caused by droplet deposition on the surface of the electron reservoir can be significantly reduced, since such deposited material is typically transparent to X-rays or acts as another X-ray source, depending on the configuration.

[0006] One characteristic of the electron beam of interest is its cross-sectional extension (width). The width or cross-sectional extension of the electron beam can be appropriately defined as the full width at half maximum (FWHM). This is sometimes referred to as the "spot size" of the beam. The width of the electron beam in the interaction region (where the electron beam hits the liquid metal jet target) is an important factor affecting the X-ray generation process. Embodiments of the present invention can be used to determine the width of the electron beam in the interaction region by the following operations: using the liquid metal jet as a shield on which the electron beam is scanned, or scanning the electron beam on the interaction region and letting the electron beam pass through the aperture before detecting it. In the latter case, the width of the electron beam is determined at the aperture, while the width in the interaction region is determined mathematically by a direct geometric transformation. Other characteristics of the electron beam can, for example, include intensity distribution curves and alignment.

[0007] In some embodiments of the present invention, a liquid metal jet is used as an object to shield the electron beam from the electron beam accumulator. The electron beam is scanned between a first position where the electron beam impinges on the electron beam accumulator that is not shielded by the liquid metal jet and a second position where the liquid metal jet maximally shields the electron beam accumulator and a set of suitable intermediate positions. The X-ray radiation generated by the interaction between the electron beam and the electron beam accumulator is measured during the scan to obtain an X-ray distribution curve that maps the scan position to the generated X-ray radiation, that is, the X-ray distribution curve can be regarded as a function of the deflection setting during the scan. Therefore, the transition between the unshielded position and the shielded position can be identified, and the width of this transition corresponds to the width of the electron beam measured at the liquid metal jet. As will be understood, if the displacement of the electron beam at the liquid metal jet is known for each scan position, the width determined in terms of the scan position can be easily converted into length units.

[0008] In embodiments where the electron beam is scanned over a liquid metal jet, the distance required to move the beam from one side of the jet to the other can be considered a measure of the width of the liquid metal jet itself. Additionally, the position of the liquid metal jet can be obtained from the position where the electron beam is obscured by the liquid metal jet. Changes in the width and / or position of the liquid metal jet can be considered an indicator of the stability of the process that produces the liquid metal jet.

[0009] In some embodiments, scanning can be performed between a first position where at least half of the electron beam passes through a first side of the liquid metal jet before impinging on the electron beam reservoir and a second position where at least half of the electron beam passes through a second side of the liquid metal jet before impinging on the electron beam reservoir. As the electron beam is scanned from the first side to the other side, the width of the electron beam can be extracted from the variation in the generated X-ray radiation. In this way, the width of the electron beam can be measured beyond the width of the liquid metal jet.

[0010] In some embodiments, shields other than liquid metal jets are used. Various shields may be used as long as they absorb and / or reflect electrons so that they do not reach the electron beam reservoir.

[0011] In other embodiments, the X-ray distribution curve is determined not only based on the X-ray radiation generated by the interaction between the electron beam and the electron beam accumulator, but also based on the X-ray radiation generated by the interaction between the electron beam and the liquid metal jet itself. In such an embodiment, the electron beam accumulator is mainly used as a feature for processing charge. An X-ray detector is used to measure the X-ray radiation generated by the interaction between the electron beam and the liquid metal jet during scanning of the electron beam. As can be understood, when the electron beam hits the liquid metal jet, the X-ray detector will be able to detect X-ray radiation, but when the electron beam does not hit the liquid metal jet, no X-ray radiation will be detected. At a certain scanning position of the electron beam, the generation of X-ray radiation will be the largest, and similar to what has been described above, the width of the electron beam can be determined based on the relationship between the scanning position and the detected X-ray radiation (i.e., based on the X-ray distribution curve).

[0012] In some embodiments, the generated X-ray radiation is detected by an X-ray detector after passing through the pin hole.Such use of a pin hole provides an imaging capability that can be used to determine properties such as the cross-sectional extension of the electron beam.

[0013] Accordingly, a method and an apparatus are provided as set out in the independent claims. The dependent claims define advantageous embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the disclosed invention will be described in the following detailed description, with reference to the accompanying drawings, in which:

[0015] Figure 1 is a flow chart showing a method according to the present invention;

[0016] Figure 2 is a schematic perspective view of a liquid metal jet X-ray source according to some embodiments of the present invention;

[0017] Figure 3 A first embodiment of a liquid metal jet X-ray source according to the present invention is schematically shown;

[0018] Figure 4 A second embodiment of a liquid metal jet X-ray source according to the present invention is schematically shown;

[0019] Figure 5 schematically shows a third embodiment of a liquid metal jet X-ray source according to the present invention; and

[0020] Figure 6 A fourth embodiment of a liquid metal jet X-ray source according to the present invention is schematically shown.

[0021] In the drawings, corresponding features are indicated by the same reference numerals throughout. DETAILED DESCRIPTION

[0022] Embodiments of the present invention provide for characterization of an electron beam used to generate X-ray radiation in a liquid metal jet X-ray source. To characterize the electron beam, the electron beam is scanned over a shield and the X-ray radiation generated during such scanning is detected. The shield may be an orifice, a liquid metal jet, or some reference object.

[0023] From a general perspective, preferably, the measurement can be made at the focal plane of the electron beam, since the focal plane is generally where the electron beam will interact with the liquid metal jet target during operation to produce X-rays. However, it is also conceivable to make measurements at some other plane along the electron beam and then mathematically transform the results so that they reflect the conditions at the focal plane. In some embodiments, it is conceivable that such a transformation can be established as part of a factory calibration process.

[0024] In a preferred embodiment of the present invention, an obstruction along the path of the electron beam is used to characterize the electron beam. When the electron beam is scanned over at least a portion of the interaction region, the path of the electron beam will at least partially intersect the obstruction in some scanning directions, while the electron beam is not obstructed in other scanning directions. Therefore, characteristics of the electron beam, such as its cross-sectional size or shape, etc., can be obtained based on the degree to which the electron beam is obstructed in different scanning directions (which in turn can be derived by measuring the X-ray radiation generated during scanning of the electron beam).

[0025] The shield may be located in a variety of positions. For example, an aperture may be provided in front of the electron beam reservoir so that only electrons passing through the aperture are detected at the beam reservoir. This approach is useful in embodiments where the measured X-ray radiation is generated by interaction between the electron beam and the electron beam reservoir. Alternatively or additionally, the shield may be placed in the interaction region so that the shield intersects the electron beam at or near the location where the electron beam interacts with the liquid metal jet during operation of the X-ray source.

[0026] In some preferred embodiments, the liquid metal jet itself is used as a shield (ie edge) over which the electron beam is scanned, and the measured X-ray radiation may then be the radiation resulting from the interaction between the electron beam and the liquid metal jet.

[0027] In other embodiments, a reference object is inserted into the beam path of the electron beam when measurements are made, and then removed prior to normal operation of the X-ray source. Such a reference object may provide edges in more than one direction, facilitating measurements of, for example, electron beam astigmatism. Measurements performed using reference objects are typically part of a factory calibration or maintenance procedure.

[0028] Similar measurements can also be made “in the field” for diagnostic purposes. Some quantities can then be determined and compared to pre-set limit values ​​without having to calculate the actual characteristics of the electron beam. If diagnostic measurements indicate that the electron beam is not within specification, the system can adjust the settings of the electron optical system until the measured quantity is within limits, or alternatively alert the operator that maintenance is required to bring the system within specification.

[0029] First reference Figure 2 A general introduction is given, which is a schematic perspective view of a liquid metal jet X-ray source 200 according to some embodiments of the present invention. The illustrated X-ray source 200 utilizes a liquid metal jet 210 as a target for an electron beam. It should be noted that some of the illustrated features of the X-ray source 200 are included only as possible examples and may not necessarily be present or necessary for the operation of all embodiments.

[0030] The X-ray source 200 includes electron sources 214, 246 and a liquid jet generator 208 configured to form a liquid jet 210 that serves as an electron target. The components of the X-ray source 200 are located in a hermetic housing 242. However, some components, such as a power supply 244 and a controller 247, may be located outside the hermetic housing 242. It is also contemplated that various electro-optical components that work through electromagnetic interactions may be located outside the housing if the housing 242 is not shielded to a large extent from electromagnetic fields (e.g., austenitic stainless steel).

[0031] The electron source generally includes a cathode 214 powered by a power supply 244, and includes an electron emitter 246, such as a thermionic, hot field or cold field charged particle source. Typically, the electron energy can be in the range of about 5keV to about 500keV. The electron beam from the electron source is accelerated toward an accelerating orifice 248, at which point the electron beam enters an electron-optical system, which includes an arrangement of alignment plates 250, a lens 252, and an arrangement of deflection plates 254. The variable characteristics of the alignment plates 250, the lens 252, and the deflection plates 254 can be controlled by signals provided by the controller 247. In the example shown, the deflection plates 254 and the alignment plates 250 are operable to accelerate the electron beam in at least two lateral directions. After initial calibration, the alignment plates 250 are typically kept at a constant setting throughout the operating cycle of the X-ray source 200, while the deflection plates 254 are used to dynamically scan or adjust the position of the electron spot during the use of the X-ray source 200. Controllable properties of lenses 252 include their respective focusing capabilities (ie, focal lengths). Figure 2 The alignment, focusing and deflection devices are depicted symbolically in a manner that indicates that they are of electrostatic type, but the invention may be equally well embodied using electromagnetic devices or a mixture of electrostatic and electromagnetic electro-optical components. The X-ray source 200 may also include an astigmatism corrector coil 253 that may provide for adjustment of the cross-sectional shape of the electron spot.

[0032] Downstream of the electron optical system, in the interaction region 212, the emitted electron beam I2 intersects the liquid metal jet 210. The interaction region is where X-rays may be generated. X-ray radiation can be drawn from the housing 242 in a direction that does not coincide with the propagation direction of the electron beam. Any portion of the electron beam I2 that continuously passes through the interaction region 212 can reach the electron beam accumulator 228 that is electrically connected to the ground. As shown in the figure, the electron beam accumulator 228 can be located at a certain distance D from the interaction region 212 so that it does not interfere with the normal operation of the X-ray source 200. An aperture ( Figure 2 (not shown in the figure), the aperture is arranged so that electrons passing through the aperture impinge on the electron beam accumulator 228, while electrons not passing through the aperture do not impinge.

[0033] Figure 1 A method according to the invention is presented.

[0034] The method for characterizing an electron beam in a liquid metal jet X-ray source according to the present invention comprises the following steps: providing S110 an electron beam and directing the electron beam to an interaction region; providing S120 an electron beam accumulator connected to a ground potential for receiving the electron beam after the electron beam passes through the interaction region; scanning S130 the electron beam over at least a portion of the interaction region; measuring S140 the X-ray radiation generated during the scan to obtain an X-ray distribution curve; and calculating S150 electron beam characteristics based on the X-ray distribution curve.

[0035] In a preferred embodiment, the step S140 of measuring the X-ray radiation generated during the scan comprises measuring the X-ray radiation generated by the interaction between the electron beam and the electron beam store. For example, the cross-sectional extension (width) of the electron beam may be determined by scanning the electron beam over the interaction region, and thereby over the electron beam store, and simultaneously measuring the generated X-ray radiation. An aperture may be provided so that X-ray radiation is generated at the electron beam store only by any portion of the electron beam that passes through the aperture. For example, a method such as Figure 3 The aperture is shown, wherein only those electrons that reach the electron beam reservoir surface 124 contribute to the detected X-ray radiation. By correlating the direction of the electron beam (e.g., in terms of the voltage applied to the corresponding deflection plates) with the detected X-ray radiation, an X-ray distribution curve is obtained, which can be used to calculate the cross-sectional extension of the electron beam in the scanning direction. By scanning the electron beam through the aperture in more than one direction, the entire cross-sectional extension of the electron beam can be calculated.

[0036] Alternatively, rather than relying on an aperture that limits the amount of electrons reaching the electron beam reservoir, an object that partially intersects the path of the electron beam can be provided. Any object that absorbs and / or reflects electrons can be used. In this context, it may be preferred to use a liquid metal jet target to achieve this purpose. The electron beam is then scanned across an obstacle that acts as an inverted aperture because it prevents electrons from reaching the electron beam reservoir. When the electron beam strikes the electron beam reservoir that is not blocked by the object, the maximum amount of X-ray radiation will be generated. When the electron beam is scanned over the object, the electron beam will become partially blocked, and the amount of X-ray radiation generated at the electron beam reservoir will decrease until the electron beam is blocked to the greatest extent by the object. Again, an X-ray distribution curve that can be used to calculate the cross-sectional extension of the electron beam is obtained.

[0037] In other embodiments, the method involves measuring X-ray radiation generated by the interaction between the electron beam and the liquid metal jet target during scanning of the electron beam.

[0038] In such Figures 4 to 6 In the schematically shown liquid metal jet X-ray source, one or more X-ray detectors are positioned to detect X-ray radiation generated by the interaction between the electron beam and the liquid metal jet, and therefore, step S140 of measuring the X-ray radiation generated during the scan to obtain the X-ray distribution curve includes measuring the X-ray radiation generated by the interaction between the electron beam and the liquid metal jet.

[0039] The self-absorption effect of X-ray radiation in the liquid metal jet target can be reduced by using two X-ray detectors 128a and 128b located on both sides of the interaction region, such as Figure 5 Schematically shown. Then, the step S140 of measuring the X-ray radiation generated during the scan to obtain the X-ray distribution curve may include taking into account the total X-ray radiation detected at the two detectors, for example by summing the outputs from the two detectors. If the two detectors are symmetrically positioned on either side of the interaction region, the sum of the outputs will compensate for any asymmetry caused by self-absorption in the X-ray distribution curve recorded by one of the detectors, because a corresponding higher or lower level of X-ray radiation will be detected at the other detector. If the detectors are not symmetrically positioned, the appropriate weights may be applied to the respective outputs before summing.

[0040] The self-absorption effect of X-ray radiation in the liquid metal jet can be further reduced by measuring the generated X-ray radiation from the same side of the liquid metal jet as the electron beam impinges on it. Figure 6 Schematically shown. In this arrangement, there will not be any self-absorption affecting the generated X-ray radiation between the interaction region and the X-ray detector. This may be particularly useful for alignment purposes, as the detector will "see" where the electron beam strikes the liquid metal jet. The X-ray detector may be provided with imaging capabilities, for example by including a CCD array, and a pin hole may be provided between the CCD array and the interaction region (where the electron beam strikes the liquid metal jet) to enhance imaging.

[0041] An X-ray detector including, for example, a CCD array and pinhole similar to those described above is used in Figure 4 This may also be useful in the setup shown. The X-ray detector will then "see" the X-ray radiation produced.

[0042] Alternatively, during characterization, some other object capable of producing X-ray radiation upon electron impact may be positioned at the interaction region, and in such an embodiment, the liquid metal jet may therefore be absent during characterization.

[0043] Figure 3 A liquid metal jet X-ray source 300 according to a first embodiment of the present invention is schematically shown. The source 300 includes an electron source / cathode 110 that emits electrons toward an anode 114. An accelerating potential 112 can be applied between the cathode 110 and the anode 114 to accelerate the emitted electrons. Downstream of the anode 114, one or more alignment coils 116 are arranged for aligning the electron beam. One or more focusing lenses 118 and deflection plates 120 are also arranged along the electron beam path to focus and guide the electron beam toward an interaction region where the electron beam can interact with a liquid metal jet target 122. During normal operation, useful X-ray radiation is generated by the interaction between the electron beam and the liquid metal jet 122 in the interaction region. The X-ray source 300 also includes an electron beam accumulator 124 that is struck by electrons that have passed through the interaction region. The electron beam accumulator 124 is electrically connected to the ground so that the electrons that strike it are placed, that is, accumulated.

[0044] exist Figure 3 In an embodiment of the present invention, the electron beam reservoir 124 is arranged so that X-ray radiation is generated when electrons impinge thereon. An X-ray detector 128 is provided to detect the X-ray radiation generated from the electron beam reservoir 124. The detector 128 can be arranged so that it only detects X-ray radiation generated from the electron beam reservoir (and does not detect radiation generated, for example, from the interaction between the electron beam and the liquid metal jet 122). Using such a configuration, the alignment and focusing process can be performed in a manner similar to that of a conventional electron beam reservoir in which the current through the beam reservoir is measured, for example as discussed in WO 2012 / 087238 mentioned above. However, since the X-ray radiation generated from the beam reservoir 124 is used in the process rather than the current through the beam reservoir, there is no need to maintain the beam reservoir 124 at a particular potential. Instead, in an embodiment of the present invention, the electron beam reservoir 124 is electrically connected to ground, such as Figure 3 126 in FIG. Therefore, if, for example, a metal droplet adheres to the edge of the beam reservoir 124, no harmful short circuit will occur, and the deposited metal droplet will also generate X-rays when electrons strike, so that the function will not be disabled.

[0045] In some embodiments, the design may be optimized in the sense that the material of the electron beam reservoir 124 provides a similar X-ray generation cross section for all relevant orientations of the electron beam. Embodiments may, for example, include flat surfaces arranged at appropriate angles relative to the impact direction of the electron beam. In other embodiments, the electron beam reservoir 124 may include a cylindrical surface, wherein the radius of the cylinder is large compared to the distance the electron beam moves on the surface during scanning of the electron beam across the aperture of the electron beam reservoir.

[0046] Preferably, the electron beam reservoir is provided with suitable cooling means in order to handle the heat load associated with the impingement of the electron beam.

[0047] In an embodiment of the present invention, the electron beam accumulator 124 is electrically connected to the ground. Thereby, the charge accumulation at the beam accumulator 124 is effectively prevented, and the prior art problems related to the short circuit between the beam accumulator and other parts of the device are avoided. However, it should be noted that the electron beam accumulator 124 may not need to be grounded all the time. It is conceivable that the grounding is activated intermittently (for example, when the threshold potential at the beam accumulator is reached) so that the accumulated charge can be accumulated to the ground optionally through a suitable current limiting device (such as a resistor). However, a preferred embodiment causes the electron beam accumulator 124 to be electrically grounded all the time, so that the electron beam accumulator is maintained at the ground potential. It is conceivable to create a virtual ground potential for the capsule and the electron beam accumulator within the scope of the present invention, that is, these components can be actively maintained at a specific potential that is not necessarily equal to zero. This type of embodiment may have design advantages in some cases, but does not affect the general concept of the present invention.

[0048] Any suitable type of detector may be used for the X-ray detector 128, such as a cadmium telluride (CdTe) diode in a tungsten (W) housing.

[0049] exist Figure 3 In the illustrated embodiment, the X-ray profile during scanning of the electron beam is obtained by measuring the X-ray radiation generated by the interaction between the electron beam and the electron beam reservoir 124. Optionally, an object such as the liquid metal jet target 122 may be present to partially block the electron beam during scanning. The X-ray detector 128 is positioned so that it detects only the X-ray radiation from the electron beam reservoir and does not detect any radiation generated by the interaction between the electron beam and the liquid metal jet or any other part of the system (e.g., the housing or an aperture placed between the interaction region and the electron beam reservoir).

[0050] Other embodiments may also rely on detecting X-ray radiation generated by the interaction between the electron beam and the liquid metal jet of the source 100 or between the electron beam and a reference object placed in the electron beam path. Figure 4An embodiment of an X-ray source 400 is schematically shown, wherein an X-ray detector 128 is positioned to detect X-rays generated in the interaction region. The X-ray sensor for detecting X-rays generated in the interaction region is preferably a second sensor dedicated to this purpose. The X-ray detector 128 can then be conveniently placed outside the vacuum chamber of the X-ray source and detect X-ray radiation passing through an X-ray transparent window. A typical X-ray source according to the present invention may include one or more X-ray transparent windows or ports through which the generated X-ray radiation is extracted. The detector 128 can be conveniently positioned at one such port. Thus, the detector 128 will be able to detect X-ray radiation when the electron beam impinges on a liquid metal target (or a suitably placed reference object), but will not detect any X-ray radiation when the electron beam does not impinge on the target. Therefore, by sampling the X-ray detector 128 while scanning the electron beam on the target, an X-ray intensity distribution curve can be obtained, which can be used to derive characteristics such as the cross-sectional size of the electron beam. It is conceivable that self-absorption in the target may cause some deviation in the measured distribution curve, but this can be compensated by subtracting the varying background or by using only X-ray radiation of such energy that is reduced by self-absorption in the target (e.g., only detecting X-ray radiation with energies well above the X-ray absorption edge of the target material).

[0051] Figure 5 Another embodiment of a liquid metal jet X-ray source 500 according to the invention is schematically shown, wherein self-absorption is compensated by using two X-ray detectors 128a, 128b, each placed at a different angle relative to the liquid metal jet 122. Thus, by taking into account the total X-ray radiation detected at these two detectors, a compensated measurement result can be obtained.

[0052] Figure 6 Yet another embodiment is schematically shown, in which an X-ray detector 128 is positioned in the line of sight of the interaction region but below (or above) the electron beam. As long as the X-ray detector has a sufficiently narrow field of view, any radiation generated by electrons impinging on the electron beam reservoir 124 can be prevented from being detected by the X-ray detector 128. By positioning the X-ray detector in this way, artifacts caused by self-absorption in the target can be reduced.

[0053] As will be understood, it is also possible to combine Figures 3 to 6 For example, the detector placement shown in Figure 3 The X-ray source shown and / or the corresponding method involving measuring the X-ray radiation generated by the interaction between the electron beam and the electron beam reservoir can be used in conjunction with Figures 4 to 6Any of the measurements shown in the figure are combined with (i.e., supplemented by) the X-ray radiation generated by the interaction between the electron beam and the liquid metal jet or other obstruction. It is also conceivable that the X-ray radiation generated by the interaction between the electron beam and the electron beam reservoir is not taken into account when determining the X-ray distribution curve, and thus only relies on the reference Figures 4 to 6 One or more of the detection schemes described.

[0054] In various embodiments of the present invention, an X-ray profile is acquired by scanning an electron beam across a liquid metal jet, a reference object, an orifice, etc., and the X-ray profile thus obtained can be used when calibrating or adjusting the focus of the electron beam. In order to align the electron beam along the optical axis of the system, a second sensor can be used, such as a sensor that detects backscattered electrons. However, the use of such a backscatter sensor may be less advantageous for non-flat targets because the backscatter coefficient will change as the electron beam is scanned across the target.

[0055] If metal droplets are deposited somewhere between the target and the X-ray detector, this may reduce the amount of X-ray radiation reaching the detector, but this does not render the function inoperable; although the intensity is slightly reduced, an X-ray intensity profile is still obtained that can be used to determine the electron beam width.

[0056] Provided an X-ray detector with sufficient imaging capability is used (e.g. by providing it with pinholes and / or comprising a CCD array), the extension of the focal spot in this direction can be obtained by detecting the amount of radiation reaching the detector while scanning the electron beam in a direction substantially parallel to the liquid metal jet. The distance that the electron beam spot must move in order for the X-ray signal to go from full signal to zero, or some other appropriately defined limit, will then correspond to the size of the beam spot.

[0057] In the above-described embodiments, the detection of X-ray radiation is direct (e.g., by using a diode-based detector). However, the detection of X-ray radiation may also be indirect, i.e., the X-ray radiation is first converted into radiation having a lower frequency and then the low-frequency radiation is detected (e.g., using a scintillator and a visible light detector). In all embodiments, the X-ray detector is preferably shielded or positioned so that only radiation from the intended source is detected. As described above, such shielding can be implemented by a CdTe diode placed within a W housing. In a preferred embodiment, the X-ray detector has a CdTe diode placed at an appropriate depth inside the W cylinder to shield unwanted X-ray radiation. Other types of collimators for limiting the field of view of the X-ray detector are conceivable within the scope of the present invention.

[0058] In summary, embodiments of the present invention provide methods for determining properties such as the cross-sectional extension of an electron beam. An electron beam is directed to an interaction region. After passing through the interaction region, the electron beam impinges on an electron beam accumulator and the charge is placed to electrical ground. The electron beam is scanned over at least a portion of the interaction region, and the X-ray radiation generated during the scan is measured to obtain an X-ray distribution curve that relates the measured X-ray radiation to the electron beam direction. Electron beam properties, such as its cross-sectional extension, are then calculated based on the generated X-ray distribution curve.

[0059] In some embodiments, the X-ray radiation is generated by the interaction between the electron beam and the beam accumulator, and the electron beam reaches the electron beam accumulator after passing through the aperture. Only those portions of the electron beam that pass through the aperture can reach the electron beam accumulator and therefore contribute to the generation of the X-ray radiation. Therefore, the X-ray distribution curve can be used to calculate the cross-sectional extension of the electron beam. It is even conceivable that the aperture is embodied as an extension of the electron beam accumulator itself. In other embodiments, the aperture is embodied as an opening in the wall of the liquid metal jet X-ray source, as schematically shown in the accompanying drawings. As will be understood, in embodiments utilizing such an aperture, it is necessary to scan the electron beam at a sufficiently large angle to reach the edge of the aperture.

[0060] In other embodiments, an object is provided that partially intersects the path of the electron beam during scanning. Such an object can take many different forms, as long as it has properties that absorb and / or reflect electrons so that fewer electrons reach the electron beam reservoir when the object partially intersects the path of the electron beam. The object that intersects the path can be a jet of liquid metal present in the interaction region.

[0061] Although some example embodiments have been described herein, those skilled in the art will not be limited to these examples when practicing embodiments of the present invention. On the contrary, within the scope of the appended claims, many modifications and variations are possible. In particular, within the scope of the present invention, it is conceivable to include an X-ray source including more than one target or more than one electron beam. In addition, an X-ray source of the type described herein can be advantageously combined with X-ray optics and / or detectors customized for specific applications, such as but not limited to the following: medical diagnosis, non-destructive testing, lithography, crystal analysis, microscopy, materials science, surface physics, X-ray diffraction method to determine protein structure, X-ray spectroscopy (XPS), critical size small angle X-ray scattering (CD-SAXS) and X-ray fluorescence spectroscopy. After reading and understanding the present disclosure in conjunction with the accompanying drawings, those skilled in the art will be able to implement various embodiments.

Claims

1. A method for characterizing an electron beam in a liquid metal jet X-ray source, the method comprising: providing the electron beam and directing the electron beam to an interaction region; providing an electron beam reservoir connected to a ground potential for receiving said electron beam after said electron beam passes through said interaction region; scanning the electron beam over at least a portion of the interaction region; measuring X-ray radiation generated by interaction between the electron beam and the electron beam reservoir during the scan to obtain an X-ray distribution curve; as well as Electron beam characteristics are calculated based on the X-ray distribution curve.

2. The method of claim 1, further comprising An object is provided to partially intersect the path of the electron beam during said scanning, wherein: The object absorbs and / or reflects electrons.

3. The method of claim 2, wherein: The object is a liquid metal jet present in the interaction zone.

4. The method of claim 3, further comprising calculating a property of the liquid metal jet based on the X-ray distribution curve.

5. The method according to any one of claims 2 to 4, wherein: The step of measuring X-ray radiation to obtain an X-ray profile further comprises measuring X-ray radiation generated by interaction between the electron beam and the object.

6. The method according to any one of claims 1 to 4, further comprising An aperture is provided between the interaction region and the electron beam reservoir, the aperture being arranged such that only electrons passing through the aperture contribute to the X-ray radiation measured during the scan.

7. The method of claim 6, wherein: Scanning the electron beam over the interaction region includes scanning the electron beam over the aperture.

8. The method of claim 5, further comprising providing a pin hole, and wherein, Measuring a profile of X-ray radiation generated during the scan includes detecting X-ray radiation that has passed through the pin hole.

9. A liquid metal jet X-ray source, comprising: an electron source arranged to provide an electron beam and direct said electron beam to an interaction region; an electron beam reservoir connected to ground potential, the electron beam reservoir being arranged to receive the electron beam after it has passed through the interaction region; scanning means capable of scanning said electron beam over at least a portion of said interaction region; an X-ray sensor positioned and arranged to detect X-ray radiation generated by interaction between the electron beam and the electron beam reservoir; as well as Circuitry is operatively connected to the scanning device and the X-ray sensor, the circuitry being configured to determine an X-ray profile during scanning of the electron beam.

10. The liquid metal jet X-ray source according to claim 9, wherein: The circuitry is further configured to calculate a characteristic of the electron beam based on the X-ray profile.

11. A liquid metal jet X-ray source as claimed in claim 9 or 10, further comprising a reference object removably arranged to partially intersect the path of the electron beam during said scanning, wherein The object absorbs and / or reflects electrons.

12. The liquid metal jet X-ray source of claim 9 or 10, further comprising a collimator for limiting the field of view of the X-ray sensor.

13. A liquid metal jet X-ray source as claimed in claim 9 or 10, further comprising an aperture between the interaction region and the electron beam reservoir, the aperture being arranged so that only electrons passing through the aperture contribute to the X-ray radiation measured during the scan.

14. A liquid metal jet X-ray source as claimed in claim 9 or 10, further comprising a second X-ray sensor positioned and arranged to detect X-ray radiation generated in the interaction region.

15. The liquid metal jet X-ray source of claim 14, further comprising a pin hole positioned and arranged such that X-ray radiation detected by the second X-ray sensor passes through the pin hole.

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