Vapour monitoring

Abstract

A method for generating X-ray radiation, the method including providing a liquid target in a chamber, directing an electron beam towards the liquid target such that the electron beam interacts with the liquid target to generated X-ray radiation, estimating a number of particles produced from the interaction between the electron beam and the liquid target by measuring a number of positively charged particles in the chamber and eliminating a contribution from scattered electrons to the estimated number of particles, and controlling the electron beam, and/or a temperature in a region of the liquid target in which the electron beam interacts with the target, such that the estimated number of particles is below a predetermined limit. Also, a corresponding X-ray source.

Claims

1. A method for generating X-ray radiation, comprising: providing a liquid target in a chamber; directing an electron beam towards said liquid target such that the electron beam interacts with the liquid target to generate X-ray radiation; estimating a number of particles produced from the interaction between the electron beam and the liquid target by measuring a number of positively charged particles in the chamber and eliminating a contribution from scattered electrons to the estimated number of particles by measuring a current generated by the scattered electrons; and controlling said electron beam, and/or a temperature in a region of the liquid target in which the electron beam interacts with said target, such that the estimated number of particles is below a predetermined limit.

2. The method according to claim 1, wherein the estimated number of particles produced from the interaction between the electron beam and the liquid target is a measure of a vaporisation rate of the liquid target.

3. The method according to claim 1, wherein the estimated number of particles produced from the interaction between the electron beam and the liquid target is a measure of an amount of liquid target material present as particles in the chamber.

4. The method according to claim 1, wherein the step of controlling the electron beam comprises varying at least one of a current, a spot size, and focus of the electron beam.

5. The method according to claim 1, comprising forming the liquid target as a jet.

6. The method according to claim 5, wherein the step of controlling the temperature of the liquid target in the interaction region comprises varying a speed of the jet.

7. An X-ray source comprising: a chamber; a liquid target source configured to provide a liquid target in the chamber; an electron source adapted to provide an electron beam directed towards the liquid target such that the electron beam interacts with the liquid target to generate X-ray radiation; and an arrangement adapted to measure a number of particles produced from the interaction between the electron beam and the liquid target, the arrangement comprising: a particle sensor adapted to measure a number of positively charged particles in the chamber; and means for measuring a current generated by scattered electrons in the chamber and based on said current, eliminating a contribution from scattered electrons to the measured number of positively charged particles, wherein: the electron source is controllable, during operation, such that the estimated number of particles is below a predetermined limit, and/or the liquid target source is operable to control a temperature in a region of the liquid target, in which region the electron beam interacts with said target, such that the estimated number of particles is below a predetermined limit.

8. The X-ray source according to claim 7, wherein the particle sensor comprises: a particle trap adapted to collect positively charged particles produced from the interaction with the liquid target; a particle repeller adapted to be connected to a positive electric potential so as to deflect positively charged particles produced from the interaction with the liquid target; a measuring device for measuring a trap current (I.sub.T) generated by the positively charged particles interacting with the particle trap, and for measuring a repeller current (I.sub.R) generated by the scattered electrons interacting with the particle repeller; and a processing device configured to estimate the number of particles based on the trap current and the repeller current.

9. The X-ray source according to claim 8, wherein the particle trap is adapted to be connected to a negative electric potential so as to attract positively charged particles.

10. The X-ray source according to claim 8, wherein the particle trap and the particle repeller are arranged along a path of the electron beam.

11. The X-ray source according to claim 8, further comprising an aperture enclosing the path of the electron beam, wherein the particle repeller is arranged between the electron source and the particle trap and the aperture is arranged between the electron source and the particle repeller.

12. The X-ray source according to claim 11, wherein a surface at least partly surrounding the aperture, and/or a surface of the particle repeller, is coated with an electron-absorbing material.

13. The X-ray source according to claim 12, wherein the electron-absorbing material is graphite.

14. The X-ray source according to claim 7 further comprising a controller adapted to control said electron beam and/or said liquid target source based on the measured number of particles.

15. The X-ray source according to claim 7, wherein the liquid target is provided in the form of a liquid jet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described for the purpose of exemplification with reference to the accompanying drawings, on which:

(2) FIG. 1 is a schematic, cross sectional side view of an X-ray source according to some embodiments of the present invention;

(3) FIG. 2 is a partial view of an X-ray source according to FIG. 1 wherein the effect of backscattered electrons is illustrated;

(4) FIG. 3 is a cross sectional perspective view of an aperture, particle trap and particle repeller according to an embodiment;

(5) FIG. 4 is a schematic illustration of a system according to an aspect; and

(6) FIG. 5 schematically illustrates a method for generating X-ray radiation according to an embodiment of the present invention.

(7) All figures are schematic, not necessarily to scale, and generally only show parts that are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION OF EMBODIMENTS

(8) An X-ray source 100 according to an embodiment of the invention will now be described with reference to FIG. 1. As indicated in FIG. 1, a vacuum chamber 120 may be defined by an enclosure 122 and an X-ray transparent window 124 that separates the vacuum chamber 120 from the ambient atmosphere. The X-rays 134 may be generated from an interaction region T, in which electrons from an electron beam 132 may interact with a target J.

(9) The electron beam 132 may be generated by an electron source 130, such as an electron gun 130 comprising a high-voltage cathode, directed towards the interaction region T. The electron beam 132 may follow a trajectory, or path, between the electron source 130 and the interaction region T, wherein the trajectory may be adjusted by electron-optical means and/or the configuration of the electron source. The electron source may further be controllable so as to allow for parameters of the electron beam to be adjusted, such as e.g. beam current, intensity, width, height and electron energy. Furthermore, the electron source may be arranged to provide a plurality of electron beams.

(10) According to the present embodiment, the target may e.g. be formed of a liquid jet J intersecting the interaction region T. The liquid jet J may be generated by a target generator 110 comprising a nozzle through which e.g. fluid, such as e.g. liquid metal may be expelled to form the jet J propagating towards and through the interaction region T. Alternatively, the liquid target J may be formed of e.g. multiple jets, a liquid reservoir or pool, which may be stationary or rotating, or a liquid curtain or sheet that may float on a surface or freely within the chamber. In some examples, the jet J may be collected by a reservoir or pool.

(11) The X-ray source 100 may further comprise a closed loop circulation system (not shown) located between a collection reservoir 112 for collecting the material of the liquid jet J and the target generator 110. The closed-loop system may be adapted to circulate the collected liquid metal to the target generator 110 by means of a high-pressure pump adapted to raise the pressure to at least 10 bar, preferably at least 50 bar or more, for generating the target jet J.

(12) Further, the X-ray source may comprise a particle sensor for measuring a number of particles present in the chamber and/or produced from the liquid target. The particle sensor may e.g. be implemented as one or several electrical sensors measuring a current, and/or as a sensor for measuring an amount of material deposited on a specific surface within the chamber. In the present figure, several examples of implementations of the particle sensor are indicated. Each one of the illustrated examples may be used separately or combined with each other. In a first example, the particle sensor comprises a particle trap 140 for collecting particles present in the chamber 120. The particle trap 140 may e.g. be formed of an electrically conductive element that may be connected to a voltage source 160 for applying an electric potential, such as e.g. a negative electric potential difference, to the particle trap 140. FIG. 1 shows a cross section of a particle trap 140 formed as a plate with an aperture that is arranged to enclose the electron beam 132 and thereby capture charged particles, such as e.g. positively charged debris and vapour from the interaction region T, on their way towards the electron source 130. The particles may be accelerated towards a surface of the particle trap 140, at which they may be deposited or adsorbed. The plate may e.g. be formed of stainless steel or other electrically conductive materials.

(13) In a second example of the particle sensor, a particle repeller 150 may be provided. The particle repeller may be formed of an electrically conductive element operating at an electrical potential for deflecting or repelling positively charged particles in the vicinity of the repeller 150. The repeller may in some examples be similarly configured as the particle trap 140, i.e., comprising a plate with an aperture enclosing the electron beam 132, and may preferably be used in combination with the particle trap 140. Such an example is illustrated in the present figure, in which the repeller is located along the path of the electron beam 132 and between the particle trap 140 and the electron source 130. Similar to the particle trap 140, the repeller may be electrically connected to a voltage source 160 producing the electric potential difference required for achieving the particle repelling effect. The repeller may e.g. be formed of stainless steel or other electrically conductive materials.

(14) The particle repeller may be combined with an aperture means 190, which may be arranged in a plate or wall element 192 delimiting the chamber region 120 and the cathode region 121 of the X-ray source 130, to protect the electron source 130 from particles (such as debris and vapour) generated in the chamber 120. Thus, the particle repeller 150 may be arranged between the aperture 190 and the particle trap so as to prevent particles that manage to pass the particle trap from reaching the aperture 190 (and eventually the electron source 130).

(15) A further embodiment may include an aperture between the particle repeller and the particle trap to reduce the number of electrons backscattering from the ion repeller that reaches the ion trap. The aperture may hence act as a means for eliminating a contribution from at least some scattered electrons to the measurements of the ions. An electric field may be provided for guiding the ions towards the ion trap, and may be modified accordingly to provide for a large ionic current in the ion trap.

(16) In a third example of the particle sensor, a measuring element 172 may be provided for measuring an amount of deposited material formed by particles produced in the interaction region T. The measuring element 172 may e.g. be an oscillating device, such as e.g. a crystal monitoring device, for which the resonance frequency may be varied according to a thickness (or amount) of the deposited material. In the present example illustrated in FIG. 1, the measuring element 172 may be a quartz crystal monitoring device (QCM) arranged in the vicinity of the X-ray window 124 and facing the interaction region J to provide an indication of the amount of material that may have deposited on the X-ray window 124, and thus an indication of when it is time to replace or clean the window 124. The measuring element 172 may be used instead of the particle trap 140 and the particle repeller 150, or in combination with these elements.

(17) Although the particle trap 140, the particle repeller 150 and the aperture 190 are aligned along the path of the electron beam 132 in the present figure, other configurations are conceivable as well. Alternative (or additional) locations of the particle trap 140 and/or repeller 150 may e.g. include the close vicinity of the X-ray window 124, or the interaction region T.

(18) The voltage source 160 may be arranged outside the chamber 120 and connected to the particle trap 140 and the particle repeller 150 via electrical feedthroughs. The voltage source 160 may be common to both the particle trap 140 and the particle repeller 150, and capable of supplying both with the required voltage, or comprise two separate and preferable individually controllable voltage sources 160one for the particle trap 140, and one for the particle repeller 150. The voltage source 160 may be operated by a controller circuitry (not shown) adapted to generate a desired electric potential difference at the particle trap 140 and particle repeller 150, respectively. The electric potential difference may be varied based on e.g. the rate at which the particles are generated in the chamber, and the type and amount of material captured by the trap.

(19) The X-ray source 100 may further comprise (or be operably connected to) means, such as e.g. ammeters 170, for measuring a trap current I.sub.T generated in the particle trap 140, and a repeller current I.sub.R generated in the particle repeller 150. The trap current I.sub.T may be used as a measure of the number of particles (such as positively charged particles or ions) that are captured by the particle trap 140, and thus give an indication of the amount of vapour (or number of particles) currently present or generated in the chamber 120. The repeller current I.sub.R, on the other hand, may be used as a measure of the number of backscattered electrons that are attracted and captured by the positively biased particle repeller 150. This measure can be used for determining a correction factor that corresponds to the contribution from backscattered electrons to the trap current I.sub.T and can be used for a more accurate estimation of the number of particles in the chamber 120. In other words, the repeller current I.sub.R may be used to eliminate or at least reduce a contribution from scattered electrons to the estimated number of particles. It will be appreciated that the voltage source 160 and the ammeter 170 may be combined in a common unit. In one example, the voltage source 160 may be configured keep the particle trap 140 and/or the repeller 150 at a relatively constant bias. This allows for the trap current I.sub.T and/or the repeller current I.sub.R to be detected as fluctuations or disturbances in the bias caused by the impinging particles and/or electrons.

(20) FIG. 2 illustrates the effects of backscattered electrons BS that are present in the chamber 120 and, in their turn, are backscattered against surfaces of the particle repeller 150 and the aperture 190 in an X-ray source 100 that may be similarly configured as the one described above with reference to FIG. 1. The influx of backscattered electrons BS may be considered as a current I.sub.BS, which can be used to estimate the contribution of electrons to the measured trap current I.sub.T and repeller current I.sub.R. The measured trap current I.sub.T may be estimated as the sum of the positive current I.sub.ion generated by ions trapped in the particle trap 150, and the negative contribution k.sub.1.Math.I.sub.BS of electrons originating from backscattered electrons BS that are backscattered again from the particle repeller 150 and interacts with the particle trap 140. The factor k.sub.1 represents in this case the fraction of those electrons BS that are backscattered again from the particle repeller and captured by the trap. Thus, the trap current I.sub.T may be expressed as:
I.sub.T=I.sub.ion+k.sub.1.Math.I.sub.BS
Further, the repeller current I.sub.R may be estimated by considering the number of backscattered electrons BS that are absorbed by the particle repeller 150, denoted k.sub.2.Math.I.sub.BS, and the number of backscattered electrons BS that backscatter at the surface 192 surrounding the aperture 190 and are absorbed by the repeller. This contribution may be denoted k.sub.3.Math.I.sub.BS. Thus, the repeller current I.sub.R may be expressed as:
I.sub.R=(k.sub.2+k.sub.3).Math.I.sub.BS
where k.sub.2 is the fraction of backscattered electrons that is absorbed in the particle repeller 150 and k.sub.3 the fraction that is backscattered from the aperture means 190 and then absorbed in the particle repeller.

(21) The estimation of the trap current I.sub.T may be improved by reducing the fraction of electrons that are backscattered from the particle repeller 150, i.e., k.sub.1. This allows for the relative contribution from the positive current I.sub.ion to be increased compared to the contribution from electrons backscattered from the particle repeller 150. This may be achieved by providing an electron-absorbing material 152, e.g. in the form of a coating, on the particle repeller 150. As a consequence, the factor k.sub.2, representing the fraction of backscattered electrons that is absorbed by the particle repeller, may be increased.

(22) The estimation of the trap current I.sub.T may also be further improved by reducing k.sub.3 relative to k.sub.2. This may be achieved by arranging electron-absorbing material 194 on the aperture means 190 so that the fraction of backscattered electrons BS that are backscattered from the aperture means 190 may be reduced.

(23) The above examples disclose direct measurements of the effect of backscattered electrons BS. It is however appreciated that the contribution of electrons to the measured trap current I.sub.T and repeller current I.sub.R may, according to other examples, be provided by means of reference data, which for example be retrived by means of a lookup table. The reference data may for example be based on previous measurements or calibrations.

(24) FIG. 3 show a portion of the X-ray source discussed above in connection with FIG. 1, illustrating an example of the particle trap 140, the particle repeller 150 and the aperture means 190 in further detail. According to the present embodiment, the aperture means 190 may comprise a housing or wall portion 192 for supporting the particle trap 140 and the particle repeller 150 which may be aligned with the aperture 190 along the path of the electron beam. The particle repeller 150 and/or the particle trap 140 may e.g. be ring-shaped or plate-shaped, and may form an aperture or opening arranged around the electron beam. The particle trap 140 and the particle repeller 150 may further be electrically connected to a respective voltage source and current measuring device (not shown) by means of electrical connectors, such as e.g. conduits 162, 164. As indicated in the present example, the particle trap 140 may be geometrically hidden from the line of sight of the interaction region T. This may e.g. be achieved by means of a flange or aperture structure arranged between the particle trap 140 and the interaction region. By arranging the particle trap 140 at such a position, it may be less exposed to backscattered electrons originating from the interaction region T. Further, the particle trap 140 may be provided with a relatively small surface area, especially as compared to the particle repeller 150, so as to further reduce the exposure to electrons and hence increase the quality of the measured particle trap current I.sub.T. In an embodiment, the particle trap 140 may be connected to a negative electric potential so as to attract charged particles even though it has a relatively small surface area and even though it is arranged at a somewhat hidden position relative to the line of sight from the interaction region T.

(25) FIG. 4 schematically illustrates a system for generating X-rays, comprising an X-ray source 100 according to the embodiments described above in connections with the previous figures, a processing device (or processing circuitry) 180 and a controller (or controlling circuitry) 182. The processing device 180 may be configured to receive information from the measuring device 170 and/or measuring element 172 (shown in FIG. 1), such as e.g. an estimated trap current I.sub.T and repeller current I.sub.R, and process the received data in order to estimate e.g. a number of particles present in the chamber. The estimation may e.g. comprise calculations using the correction factors as discussed above in connection with FIG. 2.

(26) The result from the processing device 180 may then be outputted to the controller 182, which may be configured to control the electron source accordingly. The controller may e.g. control the intensity of the electron beam or the temperature of the liquid target to reduce the number of generated particles in case the estimated number of particles e.g. exceeds a predetermined limit. The system may operate according to a feedback loop, in which vapour generated by the interaction between the electron beam and the metal jet of the X-ray source 100 may be determined by the processing device 180 and used by the controller 182 for adjusting the operation of the X-ray source. The adjusted operation may result in a change in the rate of the vapour production, which may be determined by the processing device 180 and transmitted to the controller 182, etcetera.

(27) FIG. 5 is an outline of a method for generating X-ray radiation according to an embodiment of the present invention. The method may e.g. be performed by means of the controller 182 and the processing device 180 described above for FIG. 4 and used for controlling an X-ray source 100 that may be similarly configured as any one of the above embodiments. The method comprises providing 10 the liquid target and directing 20 the electron beam 132 towards the liquid target such that the electron beam 132 interacts with the liquid target to generate the X-ray radiation 134. The method further comprises estimating 30 a number of particles produced from the interaction between the electron beam and the liquid target and controlling 40 the electron beam such that the estimated number of particles are below a predetermined limit.

(28) In the specific example disclosed in the present figure, the step of estimating 30 the number of particles may comprise applying 31 a negative electrical potential to the particle trap 140, and applying 33 a positive electrical potential to the particle repeller 150. By then measuring 32 the trap current I.sub.T generated by positively charged particles interacting with the particle trap, and measuring 34 the repeller current I.sub.R generated by scattered electrons interacting with the particle repeller, the number of particles in the chamber 120 may be estimated based on the trap current I.sub.T and the repeller current I.sub.R. The number of particles may be used as input to the controller 182 for controlling 40 e.g. the current, focus or spot size of the electron beam 132, or the temperature of the liquid target J such that the vaporisation rate is kept at a relatively low level.

(29) According to an embodiment, the step of estimating 30 the number of particles may (in addition, or as an alternative) comprise measuring 36 an amount of deposited material on e.g. an oscillating measuring element, wherein the deposited material is formed by the particles produced in the interaction region.

(30) According to an embodiment, the step of estimating 30 the number of particles may (in addition, or as an alternative) comprise measuring 36 an amount of deposited material on e.g. a part of inner wall by measuring an electrical resistance between two electrodes arranged on said wall. Provided the deposited material forms a film on an insulating surface connecting the two electrodes the resistance will be inversely proportional to the film thickness and thus the amount of deposited material. In cases where material leaving the target is in the form of droplets these may deposit on the electrodes and thus create a path for conduction between the electrodes effectively making the electrical resistance approach zero (within the measurement accuracy).

(31) The person skilled in the art realises that the present invention by no means is limited to the examples and configurations described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the particle trap and the particle repeller may be arranged in other geometric positions. The particle trap and the particle repeller may e.g. be used for protecting the X-ray window from being contaminated, or for protecting other parts and elements within the chamber, in combination with the above described method for estimating the number of particles in the chamber. Further, the applied voltages to the particle trap and particle repeller need not be constant, but may be varied in different ways provided it is effective in limiting or controlling the mobility of particles and/or measuring the number of contaminants. In particular, time-varying electric potentials may be realised, which may provide for more sophisticated ways of diverting particles from unsafe regions (e.g. the vicinity of the aperture or the window) and estimated the rate at which they are produced. Furthermore, means for actively ionizing debris or particles generated from the interaction between the electron beam and the liquid target may be included, thus increasing the fraction of debris or particles directed to the ion trap. An X-ray source utilising such an ionisation tool is disclosed in applicant's European application no. 16175573.1, which is hereby incorporated by reference. Furthermore, X-ray sources and systems comprising more than one liquid jet or more than one electron beam are conceivable within the scope of the present inventive concept. Furthermore, X-ray sources of the type described herein may advantageously be combined with X-ray optics and/or detectors tailored to specific applications exemplified by but not limited to medical diagnosis, non-destructive testing, lithography, crystal analysis, microscopy, materials science, microscopy surface physics, protein structure determination by X-ray diffraction, X-ray photo spectroscopy (XPS), critical dimension small angle X-ray scattering (CD-SAXS), and X-ray fluorescence (XRF). Additionally, variations to the disclosed examples can be understood and effected by the skilled person in practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.