Determining the operating state of the X-ray source

Abstract

A method is disclosed in an x-ray source comprising an electron source for providing an electron beam and a target having a first working area for generating x-ray radiation upon interaction with the electron beam, the method comprising determining a first amount indicative of a current absorbed by the target at the first working area, moving the electron beam from the first working area to a reference area in response to the first amount deviating from an expected value, determining a second amount indicative of a current absorbed at the reference area, and determining an operational state of the x-ray source based on the first amount and the second amount, the operational state being an electron beam obstruction condition or a target obstruction condition.

Description

[Technical field]

[0001] The present disclosure relates to X-ray sources and methods therein. [Background technology]

[0002] X-ray radiation can be generated by an X-ray source in which an electron beam impinges on a working area on a target. The performance of the X-ray source depends, among other things, on the characteristics of the working area and the interaction between the electron beam and the target. Conventionally, only a portion of the energy of the impinging electron beam is converted into X-ray radiation. The target is therefore often exposed to a relatively high thermal load, leading to thermally induced wear and gradually reduced performance.

[0003] It is therefore important to monitor the performance of the target. This may be done, for example, in a periodically performed calibration process in which an electron beam is scanned across the surface of the target to generate an image of the target's surface and detect visible defects. Alternatively, X-ray radiation generated during the scan is monitored to determine performance at various locations on the target and identify damaged areas.

[0004] Eventually, damaged or malfunctioning targets must be replaced. Since this tends to be a relatively time consuming process that often requires the x-ray source to be opened and taken out of operation, techniques are needed to reduce the time for maintenance and increase the uptime of the x-ray source. Summary of the Invention

[0005] The present disclosure relates to an X-ray source and a method in which the operational state of the X-ray source is determined.

[0006] A typical x-ray source to which the inventive principles disclosed herein may be applied may include an electron source for providing an electron beam and a target having a working area for generating x-ray radiation upon interaction with the electron beam.

[0007] According to an aspect of the invention, a first quantity indicative of a current absorbed by the target in a working region is determined. If the first quantity deviates from an expected value, the electron beam is moved from the working region to a reference region. A second quantity is then determined, the second quantity indicative of a current absorbed in the reference region. The first quantity and the second quantity can then be used to determine an operating condition of the x-ray source.

[0008] More specifically, the first and second quantities are compared to a predetermined criterion, and if the quantities meet the criterion, the operating condition is determined to be an electron beam failure condition. On the other hand, if the first and second quantities do not meet the predetermined criterion, the operating condition is determined to be a target failure condition. Preferably, a ratio between the first and second quantities is calculated and the ratio is compared to a reference range. If the calculated ratio is within the reference range, the predetermined criterion is met, and if the calculated ratio is outside the reference range, the predetermined criterion is not met. Alternatively, only the second quantity is compared to the reference range, and if the second quantity is outside the reference range, the predetermined criterion is met, and if the second quantity is within the reference range, the predetermined criterion is not met.

[0009] The invention is based on the realization that although a deviating target current absorbed in the working area may indicate that there is a problem with the working area, it is not possible to exclude that the deviation has another cause, such as a malfunctioning electron beam. Replacing the target or changing to another working area would therefore be futile, since the problem lies elsewhere. An advantage of the invention is that by moving the electron beam to a reference area in response to a detected deviation of a first amount, the performance of the electron beam can be verified before any action is taken with respect to the allegedly malfunctioning or damaged working area. If the second amount deviates from the expected range, this may indicate that there is a problem with the electron beam and not with the working area. The determined deviation may be caused, for example, by a malfunctioning electron source or poorly calibrated electron optics. From this, the concept of the invention provides a method to verify that a deviation detected in the working area is not related to the electron beam and that it is motivated to change to another working area on the target or to replace the entire target.

[0010] The movement of the electron beam from the first working area to the reference area in order to determine the second quantity and hence the operating state of the X-ray source is generally associated with an interruption of X-ray production. As a result, each movement of the electron beam from the first working area risks increasing the downtime of the X-ray source. Advantageously, the invention allows the X-ray source to be operated according to a scheme in which the electron beam is first moved from the first working area to the reference area when a deviation is detected. This is more advantageous than an operating scheme in which the electron beam is periodically moved between the working area and the reference area, regardless of whether the first quantity deviates from the expected value or not, since such a scheme interrupts X-ray production even when not motivated by any observation. By moving the electron beam according to the deviation of the first quantity, the number of movements of the electron beam to the reference area is reduced and therefore the uptime of the X-ray source can be increased.

[0011] The term "quantity indicative of the current absorbed in the working / reference area" should be understood to be any quantity that can be measured or determined, either directly or indirectly, and that comprises information that can be used to determine or characterize the current absorbed by the target (also called "target current" or "absorbed current"). Examples of such quantities may include the amount of generated X-ray radiation, the number of electrons that pass through or are absorbed by the target, and the number of secondary electrons or electrons backscattered from the target. Further examples include heat generated in the target, light emitted from the target due to, for example, cathodoluminescence, and charging of the target. The quantity may also refer to the brightness of the generated X-ray radiation. The brightness may be measured, for example, as photons per steradian per square millimeter at a particular power or normalized per watt. Alternatively or additionally, the quantity may be related to the bandwidth of the X-ray radiation, i.e., the flux distribution across the wavelength spectrum.

[0012] The first quantity may be determined using a sensor, such as, for example, a current sensor configured to measure the current absorbed by the target when the electron beam interacts with the working region. This quantity may also be determined using an X-ray sensor, or a sensor configured to measure backscattered electrons, secondary electrons, or electrons transmitted through the target. For example, an indirect measure of the target current associated with X-ray radiation or backscattering may require additional information to be known, such as the ratio between the electron beam energy converted to X-ray radiation and the energy absorbed as target current, or the ratio between backscattered and absorbed electrons. This information may be determined, for example, at the time of installation of the X-ray source, from calibration measurements, or as a constant associated with a particular target type. It should further be appreciated that the first quantity may be determined by monitoring a sequence of values ​​over time, either continuously or discretely, to detect trends and deviations over time.

[0013] The second quantity can be determined in the same way as the first quantity, preferably using the same sensor as the first quantity. However, if the reference area does not form part of the target itself, it may be advantageous to use a separate sensor dedicated to such a reference area. The reference area may for example form part of the target holder or may form a separate element, which may preferably have a relatively high electronic absorption.

[0014] The deviation of the first quantity may be determined relative to an expected value or range. The expected value or range may be, for example, a reference value related to the target type and / or setting on which the electron beam is operated. The reference value may thus be defined during the installation process or during a previously performed calibration. In some embodiments, the expected value may be determined based on previous measurements, forming, for example, a sequence of measurements over time. From this, the deviation may be defined as a gradual change of the monitored parameter or deviation from a previously recorded time average of the quantity, such as, for example, a gradual decrease in the generated X-ray radiation or a gradual increase in the absorbed target current. In some embodiments, a slow gradual decrease of said first quantity is expected and may also be compensated by increasing the emission current. The deviation may in this case be defined as an unexpected rate of change, such as a sudden drop from a steady decrease. Preferably, the first quantity is monitored during operation of the X-ray source to assess whether the target quality is consistent.

[0015] If the first parameter is determined to deviate from an expected value, for example by being outside a predetermined range or by changing at a rate exceeding a reference rate, the electron beam spot can be moved to a reference area. The electron beam can be moved to a reference location as soon as a deviation is detected, or operation can continue despite the detected deviation and the electron beam can be moved to a reference location when there is a pause in the operation of the X-ray source. The reference area can form part of the target or can be a separate element that is structurally and physically different from the target. In an example, the reference area is another working area on the target that has known interaction characteristics with the electron beam. Thus, the working area and the reference area can be of similar type and material. Alternatively, the working area can be an area suitable for generating X-ray radiation during operation of the X-ray source, whereas the reference area is selected as an area that does not participate in the generation of X-ray radiation during normal operation of the X-ray source. The reference area can preferably have a lower backscatter coefficient than the working area, for example less than two-thirds of the backscatter coefficient of the working area.

[0016] The target may, for example, comprise a sheet, foil, or substrate having at least two distinct regions that can be used as a working region and a reference region, respectively. The target may be formed from a patterned or etched material suitable for generating X-ray radiation, and the removed portions that define the pattern or geometric structure may form the reference region. The substrate may, for example, be a diamond substrate covered with a patterned layer of tungsten, with the working region disposed on the tungsten layer and the reference region formed by a partially exposed underlying diamond substrate.

[0017] The operating state of the X-ray source may be, for example, a target failure state, in which a suspected malfunction or damage of a working area is indicated, or an electron beam failure state, in which a suspected problem with the electron beam performance is indicated. In the case of a target failure state, the operator may be prompted to select another working area or to replace the target. The new working area may be selected from a list of available working areas, for example, either automatically by the X-ray source itself, manually by the operator, or semi-automatically by the operator being presented with one or more possible working areas. The first working area in which a suspected malfunction or damage was determined may then be indicated as unavailable in an updated version of the list.

[0018] In the event of an electron beam fault condition, the operator may be prompted to begin troubleshooting or adjusting the electron beam settings. The operator may, for example, change to a different electron beam generator, adjust or recalibrate the electron optics, or adjust the electron beam alignment. Additionally, a detected malfunction of the working area or electron beam may cause operation of the x-ray source to be interrupted and a warning or error signal to be generated.

[0019] It should be appreciated that in some options, the above actions in response to a detected fault condition may be performed automatically without operator intervention, and in other options may involve actions performed by an operator.

[0020] In an embodiment, determining the operational state of the X-ray source after a deviation of the first quantity is detected may comprise calculating a ratio between the first quantity and the second quantity and comparing the ratio to a reference range, which may be defined, for example, from a previously performed measurement. The ratio may be measured as part of a calibration procedure. An electron beam failure condition may be indicated in response to the ratio being within the reference range. If the ratio is outside the reference range, a target failure condition may be indicated.

[0021] In another embodiment, the second amount may be compared to a reference range, and an electron beam fault condition is indicated if the second amount is outside the range, and a target fault condition is indicated if the second amount is within the reference range.

[0022] Once the operating condition is determined to be a target fault condition, it is also envisioned to determine whether the target fault condition is caused by the consumable substrate or the consumable target layer. To this end, a third quantity indicative of a current absorbed by the target for a reduced accelerating voltage of the electron beam can be measured.

[0023] It should be appreciated that instead of the above reference range, a reference limit or threshold value can be considered. The disorder status can then be determined based on the ratio between the first amount and the second amount, or the second amount itself exceeding or falling below such a reference limit or threshold value.

[0024] As mentioned above, the method may be implemented in an x-ray source comprising an electron source for providing an electron beam and a target comprising a working region for generating x-ray radiation upon interaction with the electron beam. Accordingly, in another aspect of the invention, an x-ray source is provided comprising an electron optical mechanism for moving the electron beam over the target, a sensor mechanism for determining a quantity indicative of current absorbed in the working region and a second quantity indicative of current absorbed in a reference region, and a controller operatively connected to the electron source, the electron optical mechanism, and the sensor mechanism. The controller may be configured to compare the first quantity with an expected value, and in response to the first quantity deviating from the expected value, operate the electron optical mechanism to move the electron beam from the working region to the reference region. Furthermore, the controller may determine an operating state of the x-ray source based on the first quantity and the second quantity.

[0025] The present invention contemplates different types of x-ray radiation generating targets. The targets may comprise, for example, reflective targets or transmissive targets. The targets may further be provided as stationary or moving (e.g., rotating anode) targets.

[0026] Several modifications and variations are possible within the scope of the invention, as defined in the appended claims. In particular, X-ray sources with more than one target or more than one electron beam are considered within the concept of the invention. The X-ray source may also comprise more than one electron source, such as an additional cathode that can replace the first electron source if the first electron source is indicated as malfunctioning or defective. Furthermore, X-ray sources of the type described herein may be advantageously combined with X-ray optics and / or detectors tailored to specific applications, exemplified by, but not limited to, medical diagnostics, non-destructive testing, lithography, crystallography, microscopy, materials science, microscopy surface physics, protein structure determination by X-ray diffraction, X-ray spectroscopy (XPS), critical dimension small angle X-ray scattering (CD-SAXS), wide angle X-ray scattering (WAXS), and X-ray fluorescence (XRF).

[0027] The following detailed description is presented with reference to the accompanying drawings. [Brief description of the drawings]

[0028] [Figure 1] 1 illustrates a schematic representation of an X-ray source with parts and components relevant to the present disclosure. [Figure 2a] 1 illustrates a target with a working area and a reference area. [Figure 2b] 1 illustrates a target with a working area and a reference area. [Figure 2c] 1 illustrates a target with a working area and a reference area. [Figure 3a] Illustrates damage to the target working area. [Figure 3b] Illustrates damage to the target working area. [Figure 3c]Illustrates damage to the target working area. [Figure 4a] FIG. 1 illustrates the relationship between target thickness and electron backscattering probability. [Figure 4b] FIG. 13 illustrates the relationship between target damage and absorbed target current. [Diagram 5] 1 illustrates a schematic overview of an X-ray source with a controller and a memory. [Figure 6] 1 is a flow chart illustrating a procedure for identifying an operating state of an X-ray source. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] FIG. 1 generally shows an apparatus 100 for generating X-ray radiation, comprising an electron source 110 for providing an electron beam and a target 120 for generating X-ray radiation upon interaction with the electron beam. Furthermore, an electron optical mechanism 140 may be provided for moving the electron beam spot on the target 120. In this example, the electron source 110 comprises a cathode 111 aligned along the optical axis of the electron optical mechanism 140, a grid (also called Wehnelt) 112, and an anode 114. The cathode 111 is configured to emit electrons of an electron beam, which are accelerated towards the anode 114 by an accelerating potential. The Wehnelt 112 may be configured to adjust the cone angle of the electron beam emitted by the cathode 111. The beam direction may be adjusted by the electron optical mechanism 140, which in this example comprises a set of alignment coils 142 and two sets of stigmator coils 144 for adjusting the cross-sectional shape of the electron beam. Additionally, an aperture 146 may be disposed along the beam path to prevent unintended emissions at high angles from reaching the target 120. The electron optics 140 may also include a focusing lens 148 to provide a desired electron beam spot size, and a deflector 149 to move the electron beam spot on the target 120. As described above, the electron source 110 and the electron optics 140 may be disposed along the optical axis of the X-ray source to direct the electron beam onto the target 120, which may also be disposed along the optical axis. Additionally, as illustrated in this figure, the electron source 110 and the electron optics 140 may be disposed within a housing 102, which may define a sealed or actively pumped chamber that protects the electron beam path from the ambient environment. The target 120 may function as an X-ray window for the housing 102, allowing X-ray radiation to be emitted from the chamber. In other embodiments, an X-ray window may be provided in the housing to allow emission of X-ray radiation.

[0030] The X-ray source 100 may further comprise a sensor 130 for measuring a first quantity indicative of the current absorbed by the target in a first working area of ​​the target 120 and, in some examples, a second quantity indicative of the current absorbed in a reference area (see Figures 2a-2c for further details regarding the working area and the reference area). In this example, the same sensor 130 may be configured to measure both the first quantity and the second quantity. However, the second quantity may also be measured by another sensor different from the sensor measuring the first quantity.

[0031] The sensor 130 shown in this figure may be, for example, a current sensor configured to measure a target current generated by the absorption of electrons of the electron beam by the target 120. The current sensor 130 may be, for example, electrically connected to the target 120 and in series with a lower potential, such as ground. The output from the current sensor 130 may thus be a direct reflection of the target current.

[0032] However, other sensor mechanisms are contemplated. In one option, the sensor may be configured to provide an indirect measure of the target current. This may be accomplished, for example, by measuring backscattered electrons or generated X-ray radiation. FIG. 1 shows an example of a backscatter sensor 132 positioned upstream (relative to the electron beam path) of the target 120 to collect backscattered electrons. The backscatter sensor 132 may further comprise an ammeter for measuring an absorbed current generated by the backscattered electrons. FIG. 1 also shows an example of an X-ray sensor 134 configured to generate a signal indicative of a quality measure of the generated X-ray radiation, such as the number of photons generated per second or the brilliance of the X-ray radiation. The output of the sensor(s) may be transmitted to a controller (shown in FIG. 5) configured to determine an operational state of the X-ray source based on the first and second quantities.

[0033] 2a-2c show an example of a target 120 according to some embodiments of the present invention, with FIGS. 2a and 2b showing top views of the target 120 and FIG. 2c showing a cross-sectional view of an enlarged portion of FIG. 2b. The target 120 may comprise a first region 121, i.e. a working region 121, suitable for generating X-ray radiation, and a reference region 122 suitable for verifying the performance of the electron beam. The working region 121 may preferably comprise a high density material, such as tungsten, which is known to generate X-ray radiation upon interaction with impinging electrons. The high density material may be provided in a layer 124 on a substrate 123, which may be formed from a material that is more transparent to impinging electrons compared to the material of the working region 121. The substrate 123 may comprise, for example, diamond or a similar lightweight material having a relatively low atomic number and preferably having a high thermal conductivity. The target layer 124 may be provided with apertures, such as square, octagonal, or circular holes, exposing the underlying substrate 123. These apertures may be formed, for example, by photolithography and etching of the target layer 124 .

[0034] During operation, i.e., during the generation of X-ray radiation, the electron beam may be maintained at a fixed location on the target 120. When adjusting an electron beam characteristic such as the electron beam spot size, the electron beam may be scanned over the target 120, in particular over the edge separating the portion of the target 120 provided with the X-ray generating material (such as tungsten) and the portion where the underlying substrate (such as a diamond substrate) is exposed. Because the electronic absorption coefficient may be different in these two regions, data from the target current sensor 130 shown in FIG. 1 may be used to create an image of the target 120. Furthermore, by measuring the change in absorbed target current as the electron beam is scanned over the edge between the two regions, the electron beam spot size may be calculated.

[0035] Figures 2a and 2b show top views of an illustrative target, with Figure 2b being a magnified portion of target 120. Target layer 124, in this example formed from tungsten, may have a thickness on the order of 0.5 micrometers (μm), and substrate 123, which may be formed, for example, from diamond, may have a thickness in the range of 100-400 μm, e.g., 100-150 μm.

[0036] The overall size of the patterned area or structured target shown in FIG. 2a may be 800×800 μm and the electron beam spot size on the target may be in the range of 100 nm to 20 μm, for example 300 nm to 1200 nm. The pattern of open areas may repeat itself every 60 μm. The pattern may comprise a larger open area 122 and a smaller open area 122′. The larger open area may be used to measure the electron beam throughput, i.e. the total current incident on the target for a particular setting of the electron optics setup. The electron beam throughput may in turn be used to calculate the fraction of the electron beam passing through the aperture 146 and, consequently, the cone angle from the electron source 110. The cone angle may be adjusted by applying a bias potential on the grid 112 to either attract electrons towards the grid 112 (making the cone angle wider) or repel electrons from the grid 112 towards the beam center (making the cone angle narrower).

[0037] The smaller aperture area 122' may be referred to as a calibration point 122' that is used during a calibration procedure to adjust parameters related to, for example, focus, astigmatism, and positioning of the electron beam.

[0038] The region of the target layer 124 between the aperture regions 122, 122′ may be used as a working region 121 for generating X-ray radiation during operation of the X-ray source. The working region 121, which may also be referred to as an operating point, may be defined by an operator indicating a suitable region or may be searched from a list of possible working regions (or coordinates) on the target 120. In general, the working region 121 shown in FIG. 2b may be divided into sub-regions and each sub-region may be used as an operating point, provided that the quality of one sub-region is not affected by the use of a neighboring sub-region as an active working point.

[0039] A first quantity indicative of the current absorbed by the target in the working region 121 may be monitored during the x-ray generation. This may be accomplished, for example, by a target current sensor 130 configured to provide a direct measure of the target current, or a backscatter sensor 132 or an x-ray sensor 134 configured to provide an indirect measure of the target current, as discussed above with reference to FIG. 1. If the target current is determined to be consistent during the x-ray emission or to be within a predetermined or expected range, the x-ray generation may proceed as planned. However, if the monitoring of the first quantity indicates a deviation from the expected value, for example, by gradually changing over time or by being outside a predetermined range, a decision may be made to investigate whether the change in current is caused by a change in the target 120 or by a change in the electron beam. This decision may be made automatically by the system or may be queried to an operator.

[0040] The investigation may involve moving the electron beam from the working area 121 to a reference area, preferably with known electron absorption properties. The reference area may be, for example, one of the larger aperture areas 122 discussed above, or another working area 121 on the target layer 124. A second quantity indicative of the target current absorbed in the reference area may be measured (preferably similar to the first quantity) and used to determine the operating condition of the X-ray source 100. The second quantity may be compared to a reference value, such as, for example, an expected limit or range, or may be compared to the first quantity. In the latter case, a ratio between the first and second quantities may be calculated and compared to a reference range. If this ratio is within the reference range, this indicates that there may be some anomaly in the electron beam. If the ratio is outside the reference range, this indicates that there may be some anomaly in the working area 121. In the latter case, the operator or controller of the X-ray source 100 may decide to select another working area 121 and resume operation, or to replace the target if no suitable working areas are available any more.

[0041] If the reference area is an alternative working area 121 on the target layer 124, the difference in absorbed target current may be caused by different backscattering in the two areas. If the generated X-ray radiation is measured as an indirect measure of the target current, a decrease in X-ray radiation in the first working area may trigger a movement of the electron beam to the second working area, which serves as the reference area. If the generation of X-ray radiation is restored after the movement to the second working area, this indicates a fault in the first working area, and operation may therefore continue in the second working area. On the other hand, if the X-ray output (or absorbed target current) is not restored after the movement to the second working area, this indicates that the decrease in performance is probably not caused by the first working point being damaged or worn out.

[0042] It should be appreciated that the concept of the present invention is not limited to transmission type targets. On the contrary, the concept of determining the operating state of an X-ray source based on a first quantity indicative of target current absorbed in a working region and a second quantity indicative of target current absorbed in a reference region may be implemented for other types of targets as well, such as reflective targets. The embodiments shown in Figures 1 and 2a-2c are merely illustrative examples.

[0043] Below, a detailed example of the relationship between absorbed target current and electron backscattering is discussed with reference to Figures 3a-3c and 4a-4b to clarify a possible implementation of the inventive concept. The discussion applies to an x-ray source and target that may be configured similarly to the illustrative x-ray source 100 and target 120 discussed with reference to Figures 1 and 2a-2c. A theoretical model is proposed to determine the depth of wear-induced damage in the working region 121 caused by the impinging electron beam and its relationship to absorbed target current.

[0044] Figure 3a illustrates an ideal working area having a flat surface, whereas Figures 3b and 3c illustrate wear-induced damage in the form of depressions or dents in the surface of the working area. The depressions have a depth H p and effective radius R p and the impinging electron beam can be characterized by its angle of incidence φ with respect to the normal of the surface it impinges on. Additionally, the backscattered electrons are incident on the surface at a solid angle θ in FIG. p , within which backscattered electrons may escape the target. Thus, FIG. 3c illustrates two distinct regions, the first region defining the electron trajectories of electrons scattered from the cavity, and the second region defining the trajectories of electrons scattered from the bottom of the cavity and absorbed by the cavity walls. Furthermore, FIG. 4a illustrates the backscatter probability as a function of target thickness for tungsten and copper at accelerating voltages of 160 kV and 80 kV. FIG. 4b shows the relationship between target damage and the relative increase in absorbed current for thick tungsten and copper targets for an accelerating voltage of 160 kV.

[0045] It is known that the backscattering probability (BP) of energetic electrons depends on the electron energy (E), chemical composition (atomic number Z) and incidence angle (φ) of the electron beam as well as the geometric shape of the target. When the electron beam interacts with a flat target with recesses or depressions, the solid angle for the backscattered electrons is less than 2π and some of the electrons can therefore be absorbed in the depressions, as illustrated in Figures 3b and 3c. The increase in absorbed current (AC) increases, among other things, with the depth H of the depressions. p and effective radius R p The absorbed current can be calculated as the difference between the oncoming electron beam current and the current of unmeasured electrons scattered outside the recess solid angle.

[0046]

number

[0047] where i abs is the measured AC, and i beam is the approaching electron beam current, and η B (Z,φ,E) is the backscattering probability for electrons for an incidence angle of (π / 2-φ) between the target surface and the electron beam, and η a is the probability for electron absorption in the cavity. In the general case, some of the backscattered electrons may be absorbed by the surrounding parts of the system, contributing to the measured absorbed current. In this case, equation (1) would need to be modified. The probability η a depends on the dimple configuration and can be estimated as follows:

[0048]

number

[0049] Here, ξ p is the fraction of approaching electrons that reach the pit, and ΔΩ pis the solid angle through which a backscattered electron can interact with the cavity wall and thus be absorbed within the cavity. p The analytical expression for can be written as follows:

[0050]

number

[0051]

number

[0052]

number

[0053] Here, θ p (H p ,R p )π / 2-arctg[H p / R p ] is the height of the depression H p and the effective recess radius R p A simplified schematic of the solid angle calculation for normal incidence of the incoming electrons on the target surface is shown in FIG. 3c.

[0054] Equation (5) can be written as follows:

[0055]

number

[0056] Backscattering probability η B can be considered unaffected by the dimple, equations (1) and (6) can be used to approximate the absorbed current ξ a The dependence of the relative increase of on the ratio of the effective dimple radius to the dimple height is given as:

[0057]

number

[0058] The inverse solution for the ratio of pit height to effective pit radius versus the relative increase in absorbed current allows this ratio to be estimated based on the experimentally measured change in absorbed current as follows:

[0059]

number

[0060] Here, ξ a is the measured value.

[0061] A simplified analytical solution for the backscatter probability according to the example shown in FIGS. 3a and 3b can be written as:

[0062]

number

[0063]

number

[0064] where φ is the angle between the electron beam and the normal to the target, and R e is the electron penetration depth in meters (neglecting relativistic corrections) in a material with atomic number Z when the electron has energy EeV, A is the atomic weight of the target material, and ρ is kg / m 3is the density in units of electrons. Some examples of the backscattering probability calculated as a function of target thickness for different electron energies and target materials are shown in FIG. 4a. The assumption underlying equation (9) is that any target substrate has a negligible contribution to the probability for electron backscattering. This assumption would be justified if the backscattering probability for the target layer is close to the value for the bulk material and if the backscattering probability for the substrate material is small relative to the corresponding probability for the target material.

[0065] Equation (8) allows to track the dynamic damage on the target in situ during normal operation of the X-ray system by measuring the relative change in AC. The calculated dependence (8) for thick (30 μm) copper and tungsten targets is shown in FIG. 4b. As can be seen from FIG. 4a, for sufficiently large layer thicknesses, the backscattering probability is independent of the electron energy and the target layer thickness. For this reason, the data for FIG. 4b were calculated with an electron energy of 160 keV, and repeating the calculations at 80 keV would yield practically identical curves. For these calculations, the fraction of incoming electrons interacting with the pits, ξ p =0.9 was assumed.

[0066] The radius of the cavity can be determined by making an assumption about which physical process is dominant. For example, if evaporation of the metal target is the dominant process, the radius of the evaporation cavity can be roughly estimated as follows:

[0067]

number

[0068] Here, C target is a constant that depends on the target material (C Cu ≒0.435), σ xT and σ yTis the standard deviation of the temperature field distribution within the electron spot (assuming the elliptical electron spot is oriented along the X-axis). From this, from equations (8) and (11), the observed relative change in absorbed current can be used to calculate the effective pit radius and pit height.

[0069] In an illustrative embodiment, the target comprises a thick copper layer, such as at least 30 μm. In this case, equation (9) gives an accurate estimate for the backscatter probability and can be used in equation (8) to correlate the relative change in absorbed target current to target damage. If a relative increase in absorbed current beyond a predetermined limit, such as 5%, is detected, the electron beam spot can be moved to some other location on the target that is known to be undamaged. If the absorbed current at this location is restored to, or at least close to, the initial value recorded at the previous site, the operating state of the x-ray system can be set to indicate a target failure condition. On the other hand, if the absorbed current is not restored, the operating state can be set to indicate an electron beam failure condition. Furthermore, monitoring the absorbed target current provides a way to monitor target damage with respect to crater depth and radius by using equations (8) and (11) above. It should be noted that equation (7) was derived under the assumption that the backscatter probability is not affected by target damage, and therefore the change in absorbed current is due to absorption at the crater walls. This assumption is justified for a 30 μm thick copper layer, at least for reasonably small recess depths, as can be seen from equations (9) and (10).

[0070] In another illustrative embodiment, the target comprises a 0.5 μm thick tungsten layer on top of a 150 μm thick diamond substrate. As shown in FIG. 2a-c, a portion of the tungsten layer was removed to expose the diamond substrate. The electron acceleration voltage was set to 160 kV, and the electron beam was positioned to strike the target perpendicular to the target surface. From equation (10), the electron penetration depth in diamond can be calculated to be about 17 μm. Since the thickness of 150 μm is much larger than the penetration depth, the exponential function in equation (9) can be ignored, and the backscattering probability for the substrate can be considered to be equal to the backscattering probability of bulk diamond, i.e., the prefactor in equation (9), which is equal to about 7.8% for diamond. This is a wise design choice, since electrons escaping from the target into the surrounding atmosphere generate ozone. The 0.5 μm thick tungsten layer, however, is quite far from the bulk properties, as can be seen from FIG. 4a. To calculate the resulting backscatter probability for an electron impinging on a tungsten layer, the following formula can be used:

[0071]

number

[0072] Here, η s is the backscattering probability for the diamond substrate, and η B0 is the bulk backscattering probability for the target layer material (48.4% for tungsten), and η Bis the backscattering probability for the target layer according to equation (9). From this, for 0.5 μm of tungsten, which would have a backscattering probability of about 9.6% by itself, on top of the diamond substrate, the effective backscattering probability is about 16%. From this, the backscattering coefficient of the exposed substrate is about 49% of the backscattering coefficient at the target layer. When the electron beam is directed to the working point on the tungsten layer, the corresponding absorbed target current is about 84% of the beam current (because substantially all electrons that are not backscattered are absorbed by the target), whereas when the electron beam is directed to the reference area where the diamond substrate is exposed, the absorbed current is about 92.2% of the beam current. As a result, the expected ratio between the absorbed currents measured at the working point and the reference area, respectively, is about 0.91 (84 / 92.2). It should be noted that for this configuration, the above equation (7) may not be valid, since the backscattering probability decreases as the target material evaporates. For this reason, a more sophisticated model needs to be applied in this case to make a quantitative estimate on the target damage. If the unused part of the tungsten layer is used as the reference, rather than the exposed (i.e. bare, uncovered) part of the substrate, the expected ratio between the two measurements is 1. If the target material evaporates from the working point, the absorbed current at that location will be larger than the absorbed current measured in the unused part of the tungsten layer, since the thinner target layer causes less backscattering and therefore more electron absorption in the substrate.

[0073] As can be seen from equation (10) above, the electron penetration depth depends on the electron energy. Hence, by adjusting the acceleration voltage, different depths of the target can be probed by measuring the absorbed or backscattered current. Electrons subjected to a relatively low acceleration voltage may mainly interact with material close to the target surface, whereas electrons subjected to a relatively high acceleration voltage will interact with material deeper in the target. Modulation of the acceleration voltage can therefore be used as a tool to further investigate the type of target damage that may have occurred. This allows more detailed operating conditions to be determined.

[0074] An example of a target damage condition is that a contaminant, e.g., a hydrocarbon, has been deposited on top of the target layer. A hydrocarbon layer consisting of elements with a relatively low atomic number will have a lower backscattering probability than the target material. This will be particularly evident for lower electron energies. High energy electrons can penetrate the hydrocarbon layer. The result is that for this type of target damage, a characteristic dependence of the absorbed current on the acceleration voltage can be expected. For relatively low electron energies, most of the electrons will be absorbed in the hydrocarbon layer, giving a relatively high absorbed current. For higher energies, the electrons will penetrate the hydrocarbon layer and backscatter from the tungsten layer, thus decreasing the absorbed current. With further increases in energy, the electrons will penetrate the tungsten layer and be absorbed in the diamond layer, leading to an increase in the absorbed current. For even higher energies, the electrons will start to escape through the diamond substrate, thus decreasing the absorbed current. In other words, by observing the voltage dependence of the absorbed current, more detailed operating conditions can be determined. If data from the target current monitoring indicates that a hydrocarbon layer is building up on the target, the operator may be prompted to check any pumping system constructed and arranged to control the atmosphere within the x-ray source enclosure.

[0075] One way to distinguish between the two types of target damage leading to an increase in the absorbed target current could be to measure the absorbed current, or a quantity dependent on the absorbed current, at a relatively low accelerating voltage, for example on the order of a few kV. In the case of an ablated target layer, the absorbed current will be reduced compared to the absorbed current measured at working conditions. This is because at lower energies, fewer electrons penetrate the target layer and the backscattering probability approaches the bulk value for the target material. For higher energies, more electrons will penetrate the target layer and hence the backscattering probability will be reduced. On the other hand, if the observed increase in absorbed current at the working voltage is caused by a hydrocarbon contamination on top of the target layer, a further increase would be expected at lower voltages. The reason is that at lower energies, fewer electrons penetrate the contamination layer and are absorbed instead. A hydrocarbon layer with a lower atomic number compared to the target layer will have a lower backscattering probability, as can be seen from equation (9) above.

[0076] The two types of target damage discussed above (evaporation of the target layer and accumulation of hydrocarbons) both result in an increase in absorbed current. However, if a decrease in absorbed current is observed and reference measurements confirm that a target fault condition should be indicated, another type of target damage can be inferred. A thinner substrate layer directly below the working position may be the reason behind such an observation. This may occur due to evaporation or oxidation of the substrate from outside. If this operating condition is indicated, the operator may be prompted to ensure proper cooling of the target.

[0077] In summary, according to the present invention, three different target failure conditions can be identified: target layer wear, contaminant buildup on top of the target layer, and substrate wear. If a target failure condition is identified, a new working position can be selected to extend the operation of the X-ray source. Furthermore, the operator can be prompted to perform maintenance on different parts of the X-ray source depending on which target failure condition is indicated. Target layer wear can be avoided by reducing the power or power density applied to the target. Contaminant buildup can be mitigated by plasma cleaning. Target substrate wear can be prevented by improving target cooling or by reducing the power or power density applied to the target.

[0078] FIG. 5 is a schematic diagram of a system or X-ray source that may be configured similarly to the X-ray source 100 discussed in connection with FIGS. 1 and 2a-2c. As shown in this figure, a controller 150 may be provided to control the operation of the electron source 110 and / or the electron optics mechanism 140. The controller 150 may further be communicatively connected to a sensor 130, such as a target current sensor, a backscatter sensor, or an X-ray sensor that sends data to the controller indicative of the current absorbed by the target. The controller may be configured to perform the method outlined in the above embodiment by determining a deviation of the first quantity from an expected value, causing the electron optics mechanism 140 to move the electron beam spot to a reference area on the target, and using data from the sensor 130 to determine a second quantity indicative of the current absorbed in the reference area. The expected value may be based, for example, on a previously recorded time average of the measured quantity, which may be retrieved from a memory 160 communicatively connected to the controller. The memory 160 may further store and maintain a list of working areas 121 from which a second working area may be selected depending on an operating condition of the X-ray source that indicates a target fault condition. Preferably, the controller 150 periodically updates the list of working areas to keep track of which working areas are still available and which working areas have been determined to be "consumed," i.e., non-functional or damaged.

[0079] In some embodiments, the controller 150 may be operatively connected to a user interface configured to present information indicative of a fault, such as a target fault or an electron beam fault. The presented information may instruct the operator to select a different working area, change the target, adjust the electron beam settings, or change to a different electron source, depending on the determined operational state of the x-ray source and the available working areas. In an example, the operator may be presented with a list of available alternative working areas.

[0080] An example of a method 600 according to the present invention is illustrated in FIG. 6. As shown in step 601, the absorbed target current (I abs ), or a quantity indicative of the absorbed target current, is monitored during operation. The monitored absorbed target current is then compared to an expected value (I 0 ) and if the magnitude of the difference exceeds a preset limit, it is determined that there is a deviation from the expected value, as shown in step 602. 0 If a deviation from the reference absorbed current (I ref ), or a quantity indicative of the absorbed current, is measured in step 603. Then, in step 604, the absorbed current (I abs ) and the reference current (I ref ) on the other hand, and the expected current value (I 0 ) and the expected reference current value (I ref,0 ) is compared with the ratio between the absorbed current and the reference current. If the ratio between the absorbed current and the reference current is sufficiently close to the expected ratio, the reason for the change in absorbed target current may be attributed to a change in the incoming electron beam, as shown in step 605, and the operating state may be set to the electron beam disturbance state. On the other hand, if the ratio I abs / I ref If the observed target current I deviates from the expected ratio, a target fault condition may be inferred. The method may then continue to infer the type of target fault condition. In step 606, the observed target current I absis the expected current value I 0 and if the difference exceeds a preset limit, it is inferred that the target substrate is depleted, as shown in step 607. If the observed target current is smaller than the expected value, it means that electrons are escaping the target, and the operating state can be set to target substrate depletion from this. If the absorbed target current is instead larger than the expected value, the reason can be either target layer depletion or accumulation of contaminants on the top of the target layer. To distinguish between these two cases, the absorbed current at the reduced acceleration voltage can be measured, as shown in step 608. Then, in step 609, a determination is made whether the absorbed current at the reduced acceleration voltage is larger than the absorbed current at the non-reduced acceleration voltage. If the absorbed current decreases when the acceleration voltage is lowered, it means that more electrons are backscattered, which indicates that the electron beam is impinging on the target layer. From this, the operating state can be set to target layer depletion, as shown in step 610. On the other hand, if the absorbed current increases for a reduced accelerating voltage, it means that material with a low backscatter probability is present at the surface of the target layer, and therefore, the operating condition can be set to contaminant accumulation, as shown in step 611.

[0081] There may also be an extreme case of target layer depletion, corresponding to evaporation of the entire thickness of the target layer at the working position. In this case, the absorbed current will increase as the voltage is reduced. This condition can be recognized from the fact that the absorbed current is equal to that seen with the electron beam directed at the reference location to which the substrate is exposed, for all acceleration voltages. In this case, the operating state can be set to a target layer evaporation state. By continuously monitoring the absorbed target current and having a strong enough reference to investigate potential target damage, complete evaporation of the target layer can be avoided.

Claims

1. A method in an X-ray source (100), comprising: an electron source (110) for providing an electron beam; a target (120) having a first working area (121) for generating X-ray radiation upon interaction with the electron beam; the method comprising: determining a first amount indicative of current absorbed by the target in the first working region; In response to the first amount deviating from an expected value, moving the electron beam from the first working region to a reference region (122); determining a second quantity indicative of current absorbed in the reference region; determining an operational state of the X-ray source based on the first amount and the second amount; and the operational state is an electron beam hazard state when the first amount and the second amount meet a predetermined criterion; The method of claim 1, wherein the operating condition is a target fault condition if the first amount and the second amount do not meet the predetermined criteria.

2. The method of claim 1 , wherein the first amount and the second amount are determined based on an output from a current sensor (130).

3. The method of claim 1 or 2, wherein the expected value is a previously recorded time average of the first quantity.

4. Determining the operational state of the X-ray source comprises: calculating a ratio between the first amount and the second amount; Comparing said ratio to a reference range; wherein the predetermined criterion is met if the ratio is within the reference range; The method according to any one of claims 1 to 3, wherein the predetermined criterion is not met if the ratio is outside the reference range.

5. Determining the operational state of the X-ray source comprises: Comparing said second amount to a reference range. the predetermined criterion is met if the second amount is outside the reference range; The method of any one of claims 1 to 3, wherein the predetermined criterion is not met if the second amount is within the criteria range.

6. 6. The method of claim 4 or 5, further comprising, after indicating the target fault condition, directing the electron beam onto a second working area for generating X-ray radiation upon interaction with the electron beam, the second working area being different from the first working area.

7. The method of claim 6 , wherein the second working area corresponds to a reference location.

8. The method of any one of claims 1 to 7, further comprising selecting the reference area from a list of working areas.

9. The method of claim 8 , further comprising indicating the first workspace in the list as unavailable.

10. The method of any one of claims 1 to 9, further comprising selecting the reference region as a region located on the target.

11. The operational condition is a target fault condition, and the method further comprises: setting the operating state to a target substrate depletion state when the first amount deviates from the expected value corresponding to a decrease in the current absorbed by the target; measuring a third quantity when the first quantity deviates from the expected value corresponding to a decrease in the current absorbed by the target, wherein the third quantity is indicative of a current absorbed by the target due to a reduced acceleration voltage; setting the operating state to a target layer depleted state if the third amount is less than the first amount; and setting the operating state to a contaminant accumulation state if the third amount is greater than the first amount; The method of any one of claims 1 to 10, further comprising:

12. The reference area corresponds to an area where the target substrate is exposed, and the method includes: measuring a fourth quantity indicative of current absorbed in the reference region due to the reduced accelerating voltage; setting the operating state to a target layer evaporation state when the first amount is equal to the second amount and the third amount is equal to the fourth amount; The method of claim 11 further comprising:

13. An X-ray source (100), an electron source (110) for providing an electron beam; a target (120) having a working area for generating X-ray radiation upon interaction with the electron beam; an electron optics mechanism (140) for moving the electron beam onto the target; a sensor mechanism (130) for determining a first quantity indicative of current absorbed in the working region and a second quantity indicative of current absorbed in a reference region; a controller (150) operatively connected to the electron source, the electron optical mechanism, and the sensor mechanism; The controller comprises: comparing the first amount to an expected value; operating the electron optical mechanism to move the electron beam from the working region to the reference region in response to the first amount deviating from the expected value; determining an operational state of the X-ray source based on the first amount and the second amount; and the operational state is an electron beam hazard state when the first amount and the second amount meet a predetermined criterion; The x-ray source, wherein the operational condition is a target fault condition if the first amount and the second amount do not meet the predetermined criteria.

14. The target comprises a substrate (123) and a target layer (124) disposed on the substrate; the working area is formed from a portion of the target layer; the reference region is formed from an exposed portion of the substrate; 14. The x-ray source of claim 13.

15. 15. An X-ray source according to claim 13 or 14, wherein the backscatter coefficient of the reference region is less than two-thirds the backscatter coefficient of the working region.

16. An X-ray source according to any one of claims 13 to 15, wherein the sensor mechanism comprises at least one of a current sensor and an X-ray sensor.

17. An X-ray source according to any one of claims 13 to 16, wherein the target is a transmission target or a reflection target.

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