Detection of disturbances

Abstract

A method for detecting disturbances in an electron impact X-ray source is disclosed. The method comprises controlling an electron beam so as to be directed toward an intended target position on a stationary target and to form a spot at the intended target position, wherein the electron beam thereby forms a spot on the stationary target at the actual target position, the stationary target having a first edge between regions of different electron backscattering probabilities, and measuring a first set of values ​​indicating the current absorbed in the stationary target as a function of time, with the electron beam spot directed toward the intended target position and overlapping the first edge, wherein the intended target position is kept stationary while measuring the first set of values, and calculating a first quality measure indicating disturbance at the relative position between the intended target position and the actual target position based on the first set of values. An alternative method in which the electron beam is scanned across the edge and an X-ray source configured to perform the method are also disclosed.

Description

Technical Field

[0001] The present disclosure relates to an X-ray source. In particular, the present disclosure relates to the detection, identification, and estimation of disturbances in an X-ray source.

Background Art

[0002] In an electron impact X-ray source, X-ray radiation is generated by colliding an electron beam with a solid target. Conventionally, the target may comprise a layer of tungsten (W) deposited on a diamond substrate. The quality of the generated X-ray radiation depends, inter alia, on the spot size of the electron beam on the target and the stability of both the electron beam and the target. Any disturbance in an electron impact X-ray source will affect the quality of the generated X-ray radiation. Therefore, there is a general desire to detect, categorize, and / or characterize such disturbances.

[0003] WO2019 / 154994 discloses a method for protecting a liquid jet X-ray source. A monitoring device is used to collect data from various parts of the X-ray source in order to obtain a quality measure indicative of the performance of the liquid jet. From the quality measure, any malfunction of the liquid jet is identified. The monitoring device comprises an acoustic sensor, an accelerometer, an optical sensor, an electronic detector, an X-ray detector, and / or an inductive coil device.

[0004] Solutions for detecting, identifying, or estimating disturbances are also sought, for example, for transmission-type solid target X-ray sources.

Summary of the Invention

[0005] In short, the present invention relates to the detection of disturbances that may be present in an electron-impact X-ray source, such as fluctuations caused by electrical noise and / or mechanical vibrations. By sampling the current absorbed in the target (or at least a quantity indicating the absorbed target current) while directing the electron beam toward the edges between target regions having different electron absorption characteristics or different electron backscatter probabilities, fluctuations caused by the relative movement between the electron beam and the target can be detected by analyzing the sampled current. The type of disturbance can be identified, for example, by analyzing the frequency configuration of the sampled current. Other characteristics of the absorbed target current that can be analyzed to detect, categorize, and / or characterize the disturbance include standard deviation, peak-to-peak value, mean absolute deviation, or similar. Furthermore, in the case of repeatable disturbances, the electron beam can be controlled to cancel out the disturbance.

[0006] Several types of disturbances can contribute to the instability of an X-ray spot. Two distinct main types of such disturbances are mechanically induced disturbances (e.g., vibrations of internal or external origin) and electrically induced disturbances (e.g., those caused by variations in the electron optics, emission current, or accelerating voltage supply). Disturbances can be detected by determining the deviation between the intended position of the electron beam on the target and its actual position.

[0007] While an X-ray source operator may observe the effects of such instability, such as an increased noise level in the captured X-ray image, they may not be able to determine which part of the system is the source of the noise. In fact, the operator may not even notice the instability at all. In the most practical applications, the integration time for the X-ray image is long compared to the timescale of the disturbance, in which case the operator may notice this as an X-ray spot size that is significantly larger than expected according to the system settings. In other words, the spot can be obscured by disturbances in the electron beam spot position. Therefore, it would be advantageous to monitor the stability of the electron beam spot and to be able to internally detect / identify / characterize disturbances within the X-ray source.

[0008] When an electron blast X-ray source is operated, the electron beam is typically controlled to be directed toward a intended target location and to form a spot at that location, where X-ray emission is generated by the interaction between the electron beam and the target. However, due to the presence of disturbances such as mechanical vibrations, the spot may not necessarily form at the intended target location but at a slightly different actual target location. Mechanical disturbances such as vibrations will then manifest as a deviation between the intended and actual target locations, which may vary over time with the frequency of the vibrations. It should be noted that the time scale of such vibrations is often shorter than the time scale of X-ray imaging, meaning that in the resulting X-ray image, the spot may appear stationary at the intended location and somewhat enlarged.

[0009] This invention relies on measuring the absorbed target current (or the quantity indicating the absorbed target current) using an electron beam directed to an edge separating two regions having different electron backscatter probabilities. As understood, different electron backscatter probabilities lead to different numbers of electrons absorbed in the regions, thereby causing different target currents. When the electron beam spot is positioned on the target so as to overlap with the edge separating the two regions, the absorbed target current will be highly sensitive to the position of the electron beam spot relative to the edge. Thus, sampling the target current while keeping the intended electron beam spot position fixed (i.e., without intentionally moving the intended target position) will provide a measure of any unintended relative movement between the actual electron beam spot and the edge. The signal thus generated can be analyzed to determine its frequency spectrum, which can provide guidance on what kind of disturbance is causing the variation. Typically, mechanically induced disturbances will have relatively low frequencies, e.g., around 100 Hz, while electrically induced disturbances may have relatively high frequencies, e.g., in the range of tens of kHz. However, some electrically induced disturbances may have relatively low frequencies, such as those caused by a 50 / 60 Hz mains power supply.

[0010] The intended target position of the electron beam spot is generally not identical to the actual target position, but it is a straightforward task to ensure that the electron beam spot overlaps with the edge when it collides with the actual target position. If the electron beam spot is sufficiently large, it can be reliably assumed that the electron beam spot will overlap with the edge if the intended target position is at the center of the edge. Alternatively, an electron beam scan across the target may be performed, and the overlap between the electron beam spot and the edge may then be determined in any suitable way, for example, from the amount of X-ray emission produced, the amount of electron backscatter from the target, and / or changes in the current absorbed by the target.

[0011] The term “quantity indicating absorbed target current” refers to any quantity that can be measured or determined directly or indirectly and that contains 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 X-ray radiation produced, the number of electrons passing through or 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 cathodoluminescence, for example, and the charge of the target. The quantity may also refer to the brightness of the X-ray radiation produced. Brightness can be measured, for example, as photons per steradian per square millimeter at a given power, or normalized per watt. Alternatively or additionally, the quantity may relate to the bandwidth of the X-ray radiation, i.e., the flux distribution across the wavelength spectrum.

[0012] In the first type of implementation, the electron beam is directed toward an intended target position on a stationary target, thereby forming a spot on the target at an actual target position that may deviate from the intended target position. The target has at least one edge between regions of different electron backscattering probabilities. Maintaining the intended target position in a stationary state, a first set of values ​​representing the current absorbed in the target as a function of time is measured while the electron beam spot overlaps with at least one edge. A quality measure representing the deviation between the intended target position and the actual target position is then calculated based on the first set of values. This type of implementation may be preferred when the electron beam spot is relatively large, so that the edges are reliably overlapped by the electron beam spot, even when there is a deviation between the actual target position and the intended target position.

[0013] In another type of implementation, the intended target position is repeatedly scanned across the edge while measuring a first set of values. For each scan, and based on the values ​​measured during each scan, an edge position is determined where the intended target position coincides with the first edge, and the difference between the edge positions determined for each individual scan is calculated from the average edge position over multiple scans. This type of implementation may be preferred when the electron beam spot is relatively small and therefore the edge cannot be reliably overlapped by the electron beam spot.

[0014] In the embodiments for carrying out the following invention, references are made to the accompanying drawings. [Brief explanation of the drawing]

[0015] [Figure 1] The absorbed target current (i.e., the normalized absorbed current) with respect to the electron beam current is expressed as a function of the displacement of the electron beam spot relative to the edge separating the target layer from the bare substrate. [Figure 2] This shows the normalized absorbed current as a function of time in an ideal example where the disturbance consists of motion at a single frequency and an amplitude smaller than the electron beam spot radius. [Figure 3] This shows the normalized absorbed current as a function of time in an ideal example where the disturbance consists of motion at a single frequency and an amplitude larger than the electron beam spot radius. [Figure 4] This shows the normalized absorbed current as a function of time in a more realistic example, which includes two frequencies of disturbances with different amplitudes, as well as some white noise. [Figure 5] Figure 4 shows the frequency spectrum of the disturbance exemplified therein. [Figure 6] A target having an electron beam directed to the edge between the first and second regions is schematically illustrated. [Figure 7] This flowchart illustrates a method based on the principles disclosed herein. [Figure 8]This is a graph showing the position of the edge between two regions with different backscattering probabilities, as determined according to the principles described herein. [Figure 9] This graph shows the power spectrum of the disturbance located at the position indicated in Figure 8. [Figure 10] This is a graph showing the position of the edge of a circular shape between two regions of different backscattering probabilities, as determined according to the principles described herein. [Figure 11] This graph shows the power spectrum of the disturbance located at the position indicated in Figure 10. [Figure 12] A schematic diagram of an electron blast X-ray source is shown. [Modes for carrying out the invention]

[0016] As an introductory example, consider an X-ray target schematically shown in FIG. 6, which includes a diamond substrate and a tungsten (W) target layer deposited on the substrate. During normal operation of the X-ray source, an electron beam spot is directed towards the target layer to generate X-ray radiation. In this example, the diamond substrate is considered to be thick enough such that either all electrons colliding with the target are backscattered or absorbed, i.e., the electrons do not penetrate the target. This assumption is realistic for most practical implementations since electron penetration through the entire target is usually undesirable. The first region of the substrate is bare, i.e., not covered by the target layer, and thus the diamond substrate is exposed as illustrated on the left side of FIG. 6. The second region is covered by the W layer as illustrated on the right side of FIG. 6. From this, an edge is formed between the first region and the second region. Assuming that the backscattering probability is different between the exposed diamond substrate and the W layer, the variation in the absorbed target current can be calculated by considering the ratio of the electron beam spot that collides with each region of the target. In the case of a circular electron beam spot (illustrated by a circle in FIG. 6), this corresponds to calculating the area of the circular segment. In this simplified example, the electron intensity distribution is assumed to be a perfect top-hat distribution, i.e., the intensity of the electron beam is equal to a constant value inside the circle and zero outside. In a more realistic case, the intensity distribution is approximately Gaussian, and the electron beam spot size can be defined by the full width at half maximum.

[0017] Assuming that the ratio of the electron beam colliding with the target layer is α, the absorbed current is

[0018]

Equation

[0019] can be written as, where I beam is the electron beam current, η t and η sThese are the backscatter probabilities of the target layer and the substrate, respectively. The ratio α can be calculated for the deviation from perfect alignment between the edge and center of the electron beam by a well-known formula for the area of ​​the circular segment.

[0020]

number

[0021] Here, R is the radius of the electron beam spot, and θ is the central angle of the segment, which can then be expressed as a function of the distance x between the center and edge of the electron spot (where |x| is assumed to be less than R).

[0022]

number

[0023] From this, by dividing the segment area by the total electron beam spot area and substituting it into the equation for θ from (3), the ratio is:

[0024]

number

[0025] This can be written as follows. This formula can be generalized to other values ​​of x (as long as the entire electron beam spot is on the target).

[0026]

number

[0027] Figure 1 illustrates the absorbed current normalized to the electron beam current. For large positive values ​​of x, i.e., when the electron beam collides only with the bare diamond substrate, the absorbed current is position-independent and determined by the backscatter probability of the diamond substrate. For large negative values ​​of x, the absorbed current is also position-independent, but instead determined by the backscatter probability of the target layer. When x=0 (i.e., the electron spot has its center on the edge between the two regions), the absorbed current is most sensitive to the electron beam position. Sensitivity will increase as the difference between the backscatter probabilities for the target layer and the substrate increases.

[0028] To illustrate the time series of absorbed target current, Figure 2 shows the normalized target current in an ideal example where the disturbance is motion at only one frequency and has an amplitude smaller than the radius of the electron spot. As can be seen from the figure, the target current will then fluctuate between a maximum and minimum value at the frequency of the disturbance. If the amplitude of the disturbance is larger than the radius of the electron beam spot, there will be periods during which the electron beam collides with only one of the substrate and the target layer, respectively, and during these periods there will be no change in the detected target current due to the disturbance. An example of an ideal time series for the target current in this latter situation is shown in Figure 3.

[0029] With respect to the relative size of the electron beam spot and the magnitude of the spatial disturbance (deviation between the intended target position and the actual target position), two special cases can be identified. In the first case, if the electron beam spot is significantly smaller than the magnitude of the disturbance, the spot will, for most of the time, collide with either the substrate only or the target layer only. The absorbed current as a function of time in this case will alternate between a maximum and a minimum value, as shown in Figure 3. In other words, α will be either 0 or 1 according to equation (5) above. To determine the magnitude of the disturbance in this case, the electron beam spot may be deliberately moved in a direction substantially perpendicular to the edge, and a new time series of absorbed currents may be recorded. If a disturbance is still detected, it can be inferred that the magnitude of the disturbance is greater than the distance the electron beam spot has been moved (i.e., greater than the amplitude of the electron beam movement). By repeating this procedure multiple times, for example using different electron beam displacements, the magnitude of the disturbance (i.e., the deviation between the intended target position and the actual target position) can be determined.

[0030] In the second case, where the electron beam spot is large compared to the magnitude of the disturbance, the ratio x / R in equation (5) above is small.

[0031]

number

[0032] It can be approximated as a first-order inequality. The magnitude of the disturbance can be estimated in this case by examining the peak-to-peak value of the absorbed current as a function of time. From equation (1) above, it is clear that the maximum value that the peak-to-peak current can achieve is given by multiplying the difference between the backscatter probabilities for the target layer and the exposed substrate by the beam current. This current difference can be determined by recording the absorbed current as the electron beam spot collides with the target layer and the substrate, respectively. In the situation where the electron beam spot is crossed by the edges between the target layer and the substrate during the entire time while recording a series of absorbed currents, the peak-to-peak readings will correspond to two displacements x1 and x2 having proportions α1 and α2, respectively, according to equation (5). The peak-to-peak value of the absorbed current can then be obtained from this,

[0033]

number

[0034] This can be written as follows, where the linear expansion of equation (6) above is used in the final equation. From this, it can be inferred that the magnitude of the displacement, i.e., the difference between two displacements corresponding to the peak-to-peak value in the absorbed current, is directly proportional to the ratio between the observed peak-to-peak absorbed current and its maximum attainable value.

[0035]

number

[0036] Furthermore, at the limit where the disturbance is small compared to the electron beam spot size, the sensitivity of the absorbed current to positional displacement can be obtained by inserting (6) into (1) and differentiating with respect to displacement x.

[0037]

number

[0038] This suggests that the larger the difference in backscattering probability, the more sensitive the absorbed current will be to disturbances.

[0039] To quantify the sensitivity of the absorbed current, the backscattering probability η B and electron penetration depth R e The following formula can be used for this:

[0040]

number

[0041]

number

[0042] Here, E is the electron energy in eV units, Z is the atomic number, d is the layer thickness, A is the atomic weight, and ρ is the density. For a thin target layer material deposited on a substrate, the following formula can be used to calculate the effective backscattering probability.

[0043]

number

[0044] Here, η s η is the backscattering probability for the substrate, and B0 η is the bulk backscattering probability for the target layer material, and B This is the backscattering probability for the target layer according to equation (10).

[0045] In the above example, which has a tungsten (W) layer deposited on a diamond substrate at an accelerating voltage of 160kV, a backscattering probability of 7.8% for the exposed diamond can be calculated, while a backscattering probability of 14% can be obtained for the 0.5μm thick W layer on the substrate. From this, the absorbed current is expected to be 86% of the beam current when it collides with the W layer, and 92.2% of the beam current when it collides with the diamond substrate. When the electron beam is positioned to collide with the edge between the target layer and the exposed substrate, a change in the relative absorbed target current of the order of 4% of the position change normalized with respect to the radius of the electron beam spot size can be expected at the limit of small displacement (as calculated by equation (9)). Correspondingly, by differentiating the curve shown in Figure 1 and taking an electron beam spot diameter of 300nm, a current change rate of 0.3‰ per 1nm displacement can be calculated.

[0046] Of particular interest is the frequency configuration of the current absorbed by the electron beam positioned at the edge between the target layer and the exposed substrate. The identification of specific frequencies indicates the presence of systematic disturbances. Such disturbances can be mitigated either by finding and eliminating the source of the disturbance, or by applying compensation to the electron beam by deflecting it to counteract the introduced positional disturbances. This, of course, requires that the compensation has not only the correct frequency but also the correct amplitude and phase (i.e., it starts in sync with the disturbance). An alternative may be to inform the user that, given the circumstances (e.g., externally induced vibrations), the expected X-ray source performance may not be achieved unless appropriate compensation is applied.

[0047] To illustrate this, Figure 4 shows the normalized absorbed target current as a function of time for a disturbance with two frequencies and some white noise. The frequency spectrum of the disturbance is shown in Figure 5. As can be understood, the frequency configuration can be obtained, for example, using the Fast Fourier Transform. A disturbance of a particular frequency can be further characterized by repeated measurements using electron beam spots located at different positions to determine the amplitude of the frequency components (if the electron beam spot is small enough as discussed above). Furthermore, the measurements can be repeated at different sampling frequencies to ensure that the identified frequencies are correct and not the result of aliasing.

[0048] As disclosed herein, when determining a disturbance using the boundary between two regions of different electron backscatter probabilities, only information about the disturbance in a direction normal to the boundary will be obtained. Therefore, it is proposed to perform at least two measurements using two different non-parallel boundaries in order to obtain complete two-dimensional information about any given disturbance.

[0049] To rule out possible sources of variation in the absorbed target current other than the relative motion between the target and the electron beam spot (i.e., the deviation between the intended target position relative to the spot and the actual target position), a time series of the electron beam directed to a uniform region of the target and the registered target current can be obtained. According to equation (5), the absorbed current is then expected to be constant as a function of time (as long as the entire electron beam spot collides with a uniform portion of the target). Nevertheless, if variation in the absorbed current is detected, some other source of variation must be looked for. One reason for such variation could be variation in the emission current of the electron source, and another could be noise in the measuring instrument. If the magnitude of a particular peak in the frequency spectrum of the absorbed current collected with an electron beam directed towards the edge is substantially the same as the magnitude of that particular peak in the frequency spectrum of the absorbed current collected with an electron beam directed towards a uniform portion of the target, then it can be inferred that the source of that frequency component is not the relative motion between the target and the electron beam.

[0050] In other words, as explained above, disturbances can be investigated by comparing measurements taken across the edge with measurements taken while the electron beam is directed to a region of the target having a uniform electron backscatter probability. The characteristics of the time series of the target current registered with the edge-directed electron beam are then compared with the characteristics of the time series of the target current registered with the electron beam directed to a uniform region of the target. Any mechanical disturbances will appear in the time series registered with the edge-directed electron beam, but will not affect the time series registered with the electron beam directed to a uniform region. Therefore, if the characteristics of the two time series (e.g., frequency configuration) are the same, the disturbance can be inferred to be non-mechanical. On the other hand, if the characteristics of the two time series are indeed different, it can be inferred that a mechanical fault condition exists.

[0051] Referring to Figure 7, the method according to the principle disclosed herein comprises controlling an electron beam so as to be directed toward an intended target position on a stationary target and to form a spot at the intended target position S701, thereby forming a spot on the stationary target at the actual target position, the stationary target having a first rim between regions of different electron backscattering probabilities. In a typical example, the target would comprise a diamond substrate on which a tungsten target layer is deposited. The rim may be formed between a bare region of the substrate and a region comprising the target layer. With the electron beam spot directed toward the intended target position and overlapping the first rim, a first set of values ​​indicating the current absorbed in the stationary target as a function of time is measured S702, the intended target position is maintained stationary when the first set of values ​​is measured. Based on the first set of values, a first quality measure indicating the deviation between the intended target position and the actual target position is then calculated S703.

[0052] In some embodiments, in order to obtain two-dimensional information about any displacement between the intended target position and the actual target position, the target has a second edge between regions of different electron backscattering probabilities, and the first and second edges are nonparallel. Two-dimensional information about the displacement between the intended target position and the actual target position can then be obtained by measuring a second set of values ​​indicating the current absorbed in the target as a function of time, with the electron beam spot oriented toward the intended target position so as to overlap with the second edge, S704 and calculating a second quality measure indicating the displacement between the intended target position and the actual target position based on the second set of values, S705.

[0053] Preferably, the method also comprises extracting a frequency spectrum from a first set of values, for example using a fast Fourier transform, S706, and determining the type of disturbance based on a peak in the frequency spectrum having a magnitude significantly greater than the noise level, S707. The method may also comprise controlling the electron beam, S708, to reduce the displacement between the intended target position and the actual target position, i.e., to cancel out the disturbance.

[0054] In an alternative implementation, the method may involve repeatedly scanning an electron beam spot (intended target position) across a region having a boundary separating two regions having different electron backscatter probabilities, while measuring a quantity indicating the absorbed target current. In each such scan, the boundary may be detected, for example, as a point where the derivative of the target current achieves a maximum value. In the ideal case, i.e., when there is no relative motion between the target and the electron beam spot (the intended target position and the actual target position are the same), the boundary will be in substantially the same position for each scan. If disturbance (i.e., deviation between the intended target position and the actual target position) exists, the observed boundary position will fluctuate. The source of the disturbance may be any kind of relative motion between the electron beam and the target. If the time required to perform a single scan is short compared to the relevant time scale for the disturbance, successive scans may be considered samples of boundary positions relative to the scanned region. Deviations from the mean of the samples may be analyzed to quantify and characterize the disturbance in a manner similar to that discussed above. This procedure can, of course, be repeated for a second edge to obtain two-dimensional information, as described elsewhere in this specification.

[0055] In a further alternative, the electron beam may be scanned over some known feature on the target, such as a straight line separating two regions with different backscatter probabilities, or a circle separating an internal region from an external region, while registering the absorbed target current. In this way, an image of the feature can be created based on the measured values. Such a procedure may be advantageous during electron beam alignment and electron beam spot size adjustment. By comparing the edge positions determined for each scan with expected edge positions, disturbances on the target relative to the electron beam can be detected and / or identified. As an example, consider a circle scanned by multiple equidistant line scans. Points identified as being on the edge of the circle can be analyzed to detect disturbances along the scanning direction and disturbances perpendicular to the scanning direction. For each scan across the circle, two edge transitions may be detected, thus giving two positions for each scan. Analyzing the average position of the two transitions for each respective scan, compared to the average position of all scans, provides information about disturbances along the scanning direction. Analyzing the distance between two transitions for each scan, compared to the expected distance around the circle, provides information about disturbances perpendicular to the scanning direction. An alternative method for analyzing the data might be to calculate the distance from each detected edge transition to the average center position for all transitions. These distances provide information about disturbances in all directions, but it is not possible to isolate a single cause of the total disturbance from different directions. A further alternative form of evaluating data from images obtained by scanning the electron beam across a circle might be to convert the image to polar coordinates (centered at the observed center of the circle), determine the circumference of the circle, and analyze how the observed radius varies along the circle. In general, several sources may contribute to such variations, most notably astigmatism, coma aberration, target imperfections, and unintended relative displacements between the target and the electron beam spot during scanning. Different metrics may be applied to distinguish between these causes and characterize the electron beam spot.The results from this type of analysis can be used to isolate and potentially quantify a factor in the relative displacement between the target and the electron beam spot, by deciding to perform measurements using an electron beam directed toward the edge, as discussed above.

[0056] An advantage of not scanning the electron beam across the edge to acquire data on relative disturbances between the target and the electron beam is that imperfections in target production do not affect the measurement. Another advantage may be that the sample rate can be freely selected based on a desired frequency range. In embodiments where the electron beam is scanned during the measurement, the sample rate is limited by the time required for scanning, which can make it difficult to detect high-frequency disturbances.

[0057] Figure 8 illustrates an example in which a straight, slightly angled edge separating two regions with different backscatter probabilities is identified by multiple scans using an electron beam. In this example, the scan interval is set to 100 μs, i.e., every 100 μs, the electron beam completes a right-to-left scan across the edge. This effectively results in a sampling frequency of 10 kHz for edge identification. Between each scan, the electron beam is moved 0.1 μm in a direction perpendicular to the scanning direction, i.e., upward in the figure. Each point in the figure corresponds to the determination of the edge location based on data obtained from a single electron beam scan. In this example, the edge location was calculated by adding a periodic disturbance at position x with an amplitude of 0.1 μm and a frequency of 100 Hz. White noise with an amplitude of 2.5 μm (peak-to-peak of 5 μm) was also added. After subtracting a linear fit from the data to obtain deviations from the straight line, the power spectrum shown in Figure 9 was calculated by applying the Fast Fourier Transform. As can be seen from the figure, periodic disturbances are recovered despite the noise. The effective sampling frequency used in this example is 10 kHz (one scan line per 100 μs), which implies that periodic disturbances up to approximately 5 kHz can be resolved according to the Nyquist-Shannon theorem.

[0058] Figure 10 shows an image obtained in the same manner as in Figure 8. In this case, the edge is a circle surrounding a region with a different backscatter probability than the region outside the circle. The observed position is generated by adding a periodic disturbance with an amplitude of 0.1 μm and a frequency of 100 Hz in the X direction and a periodic disturbance with an amplitude of 0.1 μm and a frequency of 175 Hz in the Y direction. White noise with an amplitude of 0.15 μm (peak-to-peak of 0.3 μm) was also added. The circular marker corresponds to the nominal circle center, and the cross marker corresponds to the center calculated from the observed edge position. A frequency spectrum for the X displacement can be obtained by analyzing the average value of the X position for each pair of edge locations from the same electron beam scan. Since the electron beam scan is fast (repeats every 100 μs) compared to the disturbance (shortest period time 5.7 ms), the edge can be considered fixed during each scan. From this, the displacement of the circle in the X direction can be detected as the displacement of the detected edge compared to the average X position for all scans. The Fast Fourier Transform of this dataset is shown as a solid line in Figure 11, where the 100 Hz disturbance is recovered. A corresponding analysis for the Y direction can be performed by analyzing the distance between two edge positions for each scan. The expected distance can be easily calculated from the knowledge that the shape of the edge is circular. The Fast Fourier Transform of this dataset is shown as a dashed line in Figure 11, where the 175 Hz disturbance is recovered. As can be seen from Figure 11, for the 100 Hz disturbance, i.e., in the X direction, the recovered amplitude of the disturbance is approximately 0.1 μm. The amplitude in the Y direction is slightly smaller than expected, which may be due to a mismatch between the sampling frequency (10 kHz) and the disturbance frequency (175 Hz). This is an example of a potential drawback of embodiments based on scanning the electron beam across the edge, as the selection of the sampling frequency is limited by both the electron optics and the frequency to be detected.

[0059] A further extension of this method may involve performing a disturbance level analysis, as discussed above in relation to Figure 10, whenever an image of some known feature is generated as part of electron beam alignment and / or focusing, and if some predefined limit is exceeded, positioning the electron beam spot to the edge and performing the analysis as discussed above in relation to Figure 4. In this way, the stability of the electron beam position relative to the target is monitored without excessive interference with source operation.

[0060] Figure 12 schematically shows an electron impact X-ray source 120 comprising an electron beam generator 102 configured to provide an electron beam 104. The X-ray source also comprises a target 106 having at least a first edge between regions of different electron backscattering probabilities. An electron optics system 108 is provided to focus and position the electron beam onto the target 106. A detector 110 is provided to detect a value indicating the current absorbed in the target 106. A controller 112 is provided and configured to control the electron optics system 108 to direct the electron beam 104 toward an intended target position on the target 106 and to form a spot at the intended target position, thereby causing the electron beam to form a spot on the target 106 at the actual target position. The controller 112 is further configured to measure a first set of values ​​indicating the current absorbed in the target 106 as a function of time, with the spot directed toward the intended target position and overlapping the first edge, and to calculate a first quality measure indicating the displacement between the intended target position and the actual target position based on the first set of values.

[0061] In some embodiments, the target 106 has both a first and a second edge between regions of different electron backscattering probabilities, and the first and second edges are nonparallel. This allows the displacement between the intended target position and the actual target position to be determined in two dimensions. For example, the edges may take the form of circles.

[0062] The controller may be configured to extract a frequency spectrum from the detected value of the absorbed target current and determine the type of disturbance in the X-ray source based on the frequency spectrum. The controller may also control the electron beam to cancel out the disturbance, i.e., to reduce the displacement between the intended target position and the actual target position.

[0063] In conclusion, an electron-impact X-ray source and a corresponding method for determining the displacement between the intended target position and the actual target position for an electron beam are disclosed. This allows disturbances in the X-ray source to be detected, categorized, and / or characterized. The frequency configuration of such displacements over time can provide useful information about fault conditions in the X-ray source. For periodic disturbances, the X-ray source, e.g., the electron beam, can be controlled to counteract the disturbance. Furthermore, the operator can be alerted to the presence of fault conditions in the X-ray source or its environment.

Claims

1. A method for detecting disturbances in an electron blast X-ray source, Controlling an electron beam so that it is directed toward an intended target position on a stationary target and forms a spot at the intended target position, wherein the electron beam thereby forms a spot on the stationary target at the actual target position, and the stationary target has a first edge between regions of different electron backscattering probabilities. With the electron beam spot oriented toward the intended target position and overlapping with the first edge, a first series of values ​​representing the current absorbed in the stationary target as a function of time is measured, wherein the intended target position is kept stationary when the first series of values ​​are measured. Based on the first set of values, a first quality measure is calculated that indicates a first deviation between the intended target position and the actual target position. A method that includes [a certain feature].

2. The stationary target has a second edge between regions of different electron backscattering probabilities, the first edge and the second edge are nonparallel, and the method is With the electron beam spot oriented toward a second intended target position and overlapping with the second edge, a second series of values ​​representing the current absorbed in the stationary target as a function of time is measured. Based on the second set of values, a second quality measure is calculated that indicates a second deviation between the second intended target position and the actual target position. The method according to claim 1, further comprising:

3. A method for detecting disturbances in an electron blast X-ray source, Controlling an electron beam so that it is directed toward an intended target position on a stationary target and forms a spot at the intended target position, wherein the electron beam thereby forms a spot on the stationary target at the actual target position, and the stationary target has a first edge between regions of different electron backscattering probabilities. Measuring a first set of values ​​representing the current absorbed in the stationary target as a function of time, wherein the intended target position is repeatedly scanned across the first edge when measuring the first set of values. For each of the multiple scans, and based on the values ​​measured during each scan, determine the edge position in which the intended target position coincides with the first edge, The difference between the edge positions determined for each scan is calculated from the average edge position for the multiple scans. This involves calculating a first quality measure that indicates the deviation between the intended target position and the actual target position based on the first set of values. A method that includes [a certain feature].

4. The intended target position is repeatedly scanned over the second edge when measuring the first series of values, and the first quality measure is For each of the plurality of scans, and based on the values ​​measured during each scan, a second edge position is determined in which the intended target position coincides with the second edge. The difference between the second edge positions determined for each scan is calculated from the second average edge position for the plurality of scans. The method according to claim 3, further calculated by...

5. The method according to any one of claims 1 to 4, wherein the first edge is a straight line.

6. The method according to claim 2 or 4, wherein the first edge and the second edge are part of a common circle.

7. Extracting a frequency spectrum from the first series of values, To determine the type of disturbance in the electron blast X-ray source based on the frequency spectrum. The method according to any one of claims 1 to 6, further comprising:

8. The method according to any one of claims 1 to 7, further comprising controlling the electron beam to reduce the detected disturbance.

9. An electron blast X-ray source, An electron beam generator configured to provide an electron beam, A stationary target having a first edge between regions of different electron backscattering probabilities, An electron optical system configured to focus and position the aforementioned electron beam, A controller configured to control the electron blast X-ray source to perform the method described in any one of claims 1 to 8, An electron shock X-ray source equipped with this.

10. The electron shock X-ray source according to claim 9, wherein the edge or each edge is a part of a straight line or a circle.

11. The aforementioned stationary target is circuit board and A target layer is disposed on the substrate, wherein the target layer is configured to generate X-ray emission through interaction with the electron beam. The electron shock X-ray source according to claim 9 or 10, comprising, wherein the edge or each edge separates a first region in which the substrate is exposed from a second region comprising the target layer.

12. The substrate comprises a carbon material such as beryllium or diamond. The electron shock X-ray source according to claim 11, wherein the target layer comprises a material selected from tungsten, rhenium, molybdenum, vanadium, and niobium.

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