METHOD AND APPARATUS FOR GENERATING X-RAY OR EUV RADIATION

Abstract: In a method and an apparatus for generating X-ray or EUV radiation, an electron beam is brought to interact with a propagating target jet, typically in a vacuum chamber. The target jet is formed by urging a liquid substance under pressure through an outlet opening. Hard X-ray radiation may be generated by converting the electron-beam energy to Bremsstrahlung and characteristic line emission, essentially without heating the jet to a plasma-forming temperature. Soft X-ray or EUV radiation may be generated by the electron beam heating the jet to a plasma-forming temperature.

Description: METHOD AND APPARATUS FOR GENERATING X-RAY OR EUV RADIATION Technical Field The present invention generally relates to a method and an apparatus for generating X-ray or extreme ultraviolet (EUV) radiation, especially with high brilliance. The generated radiation can for example be used in medical diagnosis, non-destructive testing, lithography, microscopy, materials science, or in some other X-ray or EUV application. Background Art X-ray sources of high power and brilliance are applied in many fields, for instance medical diagnosis, non-destructive testing, crystal structural analysis, surface physics, lithography, X-ray fluorescence, and microscopy. In some applications, X-rays are used for imaging the interior of objects that are opaque to visible light, for example in medical diagnostics and material inspection, where 10-1000 keV X-ray radiation is utilized, i.e. hard X-ray radiation. Conventional hard X-ray sources, in which an electron beam is accelerated towards a solid anode, generate X-ray radiation of relatively low brilliance. In hard X-ray imaging, the resolution of the obtained image basically depends on the distance to the X-ray source and the size of the source. The exposure time depends on the distance to the source and the power of the source. In practice, this makes X-ray imaging a trade-off between resolution and exposure time. The challenge has always been to extract as much X-ray power as possible from as small a source as possible, i.e. to achieve high brilliance. In conventional solid-target sources, X-rays are emitted both as continuous Bremsstrahlung and characteristic line emission, wherein the specific emission characteristics depend on the target material used. The energy that is not converted into X- ray radiation is primarily deposited as heat in the solid target. The primary factor limiting the power, and the brilliance, of the X-ray radiation emitted from a conventional X-ray tube is the heating of the anode. More specifically, the electron-beam power must be limited to the extent that the anode material does not melt. Several different schemes have been introduced to increase the power limit. One such scheme includes cooling and rotating the anode, see for example Chapters 3 and 7 in "Imaging Systems for Medical Diagnostics", E. Krestel, Siemens Aktiengesellschaft, Berlin and Munich, 1990. Although the cooled rotating anode can sustain a higher electron-beam power, its brilliance is still limited by the localized heating of the electron-beam focal spot. Also the average power load is limited since the same target material is used on every revolution. Typically, very high intensity sources for medical diagnosis operate at 100 kW/mm 2 , and state of the art low-power micro-focus devices operate at 150 kW/mm 2 . Applications in the soft X-ray and EUV wavelength region (a few tens of eV to a few keV) include, e.g., next generation lithography and X-ray microscopy systems. Ever since the 1960s, the size of the structures that constitute the basis of integrated electronic circuits has decreased continuously. The advantage thereof is faster and more complex circuits requiring less power. At present, photolithography is used to industrially produce such circuits having a line width of about 0.13 μm. This technique can be expected to be applicable down to about 0.1-0.07 μm. In order to further reduce the line width, other methods will probably be necessary, of which EUV projection lithography is a strong candidate, see for example "International Technology Roadmap for Semiconductors", International SEMATECH, Austin TX, 1999. In EUV φ CQ tr H- 0 PJ H- Φ 3 0 < Ω Ω H- ϋ Φ rt π- Ω H f-i Φ tr 0 H- Pi CQ Φ 0! Ω • rt SD tr ø H l- 1 ■ ! K ø 0 Φ 3 Φ rt s; X Φ 0 tr rt 3 Φ Φ rt H. Φ 0 CQ tQ ii H CQ ■ ; Φ H- <! SJ H- 3 rt Φ o 3 rt t H" . rt- H- Φ ; 0 <1 σ. φ fu P TJ CQ fu ; TJ Φ CQ TJ φ (- ■ H- • H 0 Ω H- 5 > J 1-5 > tT μ> H- Ω H- Lπ LQ 0 H TJ o t 0 H- TJ ΓT rr Φ ■ <; CQ PJ ii 0 \Ω 3 CQ 0 Hi ω Pi H- Hi rt σi 0 — rt 0 f X • J 1 Φ φ TJ H H ø SU tr l-h CQ ^ Φ 0 H- 0 CQ Hi Ω ø Su Φ H- ϋ 0 f- 1 Φ Ω fi rt J 0 Φ J CQ Ω CQ t?a ϋ Φ ^ cj CQ l H- <q Φ 0 o_ rt Hi CQ light into X-ray radiation, necessitating the use of expensive high-power lasers. Quite recently, electron-beam excitation of a gas- jet target has been tested for direct, non-thermal gene- ration of soft X-ray radiation, albeit with relatively low power and brilliance of the resulting radiation, see Ter-Avetisyan et al , Proceedings of the SPIE, No. 4060, pp 204-208, 2000. There are also large facilities such as synchrotron light sources, which produce X-ray radiation with high average power and brilliance. However, there are many applications that require compact, small-scale systems that produce X-ray radiation with a relatively high average power and brilliance. Compact and more inexpen- sive systems yield better accessibility to the applied user and thus are of potentially greater value to science and society. Summary of the Invention It is an object of the present invention to solve or alleviate the problems described above. More specifically, the invention aims at providing a method and an apparatus for generation of X-ray or EUV radiation with very high brilliance in combination with relatively high average power. It is also an object of the invention to provide a compact and relatively inexpensive apparatus for generation of X-ray or EUV radiation. The inventive technique should also provide for stable and uncomplicated generation of X-ray or EUV radiation, with minimum production of debris. A further objective is to provide a method and an apparatus generating radiation suitable for medical diagnosis and material inspection. Still another object of the invention is to provide a method and an apparatus suitable for use in lithography, non-destructive testing, microscopy, crystal analysis, surface physics, materials science, X-ray photo spectroscopy (XPS) , X-ray fluorescence, protein structure determination by X-ray diffraction, and other X-ray application . These and other objectives, which will be apparent from the following description, are wholly or partially achieved by the method and the apparatus according to the appended independent claims . The dependent claims define preferred embodiments . Accordingly, the invention provides a method for generating X-ray or EUV radiation, comprising the steps of forming a target jet by urging a liquid substance under pressure through an outlet opening, which target jet propagates through an area of interaction; and directing at least one electron beam onto the target jet in the area of interaction such that the electron beam interacts with the target jet to generate X-ray or EUV radiation. Depending on the material of the target jet, the temperature, speed and diameter of the jet, as well as on the current, voltage and focal spot size of the electron beam, the inventive method and apparatus allows for operation in either of two modes. In a first mode of operation, hard X-ray radiation is generated by direct conversion of the electron-beam energy to Bremsstrahlung and characteristic line emission, essentially without heating the jet to a plasma-forming temperature. In the second mode of operation, soft X-ray or EUV radiation is generated by heating the jet to a plasma-forming tempera- ture. In either mode of operation, the invention provides significant improvements over prior-art technology. In the first mode of operation, the jet target provides several advantages over the solid anode conventionally used in generation of hard X-ray radiation. More specifically, the liquid jet has a density high enough to allow for high brilliance and power of the generated radiation. Further, the jet is regenerative to its nature so there is no need to cool the target material. In fact, the target material can be destroyed, i.e. heated to a temperature above its melting temperature, due to the regenerative nature of the jet target. Thus, the electron-beam power density at the target may be increased significantly compared to non- regenerative targets. In addition, the jet can be given a very high propagation speed through the area of interaction. Compared to conventional stationary or rotating anodes, more energy can be deposited in such a fast propagating jet due to the correspondingly high rate of material transport into the area of interaction. The combination of these features allows for a significant increase in brilliance of the generated hard X-ray radiation. Thus, the use of a small, high-density, regenerative, high-speed target in the form of a jet, formed by urging a liquid substance under pressure through an outlet opening, should typically allow for a 100-fold increase in brilliance of the generated hard X- ray radiation compared to conventional techniques. In order to achieve the power density allowed for by this novel, regenerative target, the electron beam should preferably be properly focused thereon. Typically, the acceleration voltage used for generating the electron beam will be in the order of 5-500 kV, but might be higher. The beam current will typically be in the order of 10-1000 mA, but might be higher. The second mode of operation emanates from the basic insight that at least one electron beam can be used instead of a laser beam to form a plasma emitting soft X- ray or EUV radiation. Compared to the conventional equipment based on the above-discussed PP concept, the inventive method and apparatus allows for a significant increase in wall-plug conversion efficiency, as well as lower cost and complexity. Other attractive features include low emission of debris, essentially no limitation on repetition rate, and uninterrupted usage. In the second mode of operation, the electron source should typically deliver in the order of 10 10 -10 13 /cm 2 to the area of interaction in order to establish the desired plasma temperature. This could be easily achieved by ope- rating the electron source to generate a pulsed electron beam, wherein the pulse length preferably is matched to the size of the jet. The repetition rate of the electron source then determines the average power of the generated X-ray or EUV radiation. When using a pulsed electron beam, the jet might be disturbed by the discontinuous interaction with the electron beam. To this end, the jet propagation speed should preferably be so high that the jet is capable of stabilizing between each electron-beam pulse. It should be noted that the electron beam can be pulsed or continuous in either of the first and second modes . In both modes of operation, for optimum utilization of the accessible electron beam power, the beam is preferably focused on the jet to essentially match the size of the beam to the size of the jet. In this context it is possible to use a line focus instead of a point focus, the transverse dimensions of the line focus being essentially matched to the transverse dimensions of the jet. The jet is preferably generated with a diameter of about 1-100 μm but may be as large as millimeters. Thereby, the radiation will be emitted with high brilliance from a small area of interaction. To better utilize the generated radiation, the inventive apparatus and method may naturally be used in conjunction with X-ray optics, such as polycapillary lenses, compound refractive lenses or X-ray mirrors. Preferably, the target jet is generated by urging a liquid substance through an outlet opening, such as a nozzle or an orifice, typically by means of a pump and/or a pressurized reservoir yielding a pressure typically in the range of 0.5-500 MPa to bring about a jet propagation ω to to μ> LΠ o LΠ o LΠ O LΠ rt SD Hi Ω μ- tr SU Ω LQ μ- Ω SD a 3 μ- rt μ- tr tr J SU 0 ø 0 H 0 K rt O rt u SD ø tr ø φ J < 0 μ- φ 0 0 CQ 0 rt ^ rt Φ <! Si SD 0 Si M M SD i ø Φ i rt μ- Φ Φ ϋ D H H ø 0 Si ø 0 M 0 tr o tr H ø 0 TJ SD m Su Ω φ 1 o Ω P. Ω φ 0 Φ »τ] SD φ rt i J rt tr tr rt 0 0 Hi rt rt 03 0 Ω rt μ- *<; SD 0 Φ M μ- Ω TJ μ- TJ 03 μ- SD Φ ii rt <i 0 H CQ Φ <! Φ μ- <! 0 0 <! li 0 M rt μ- μ- φ SD SD . 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Thereby, the apparatus can be continuously or semi- continuously operated to generate X-ray or EUV radiation, as will be described below. Further, this approach results in sufficient spatial stability of the jet 5 to permit the focal spot of the electron beam 4 on the jet 5 to be of approximately the same size as the diameter of the jet 5. In the case of a pulsed electron beam 4, this approach also alleviates the need for any temporal synchronization of the electron source 2 with the target generator 3. In some cases, similar advantages can be obtained with jets consisting of separate, spatially continuous portions. It should be emphasized, however, that any formation of condensed matter emanating from a liquid jet can be used as target for the electron-beam within the scope of the invention, be it liquid or solid, spatially continuous, droplets, or a spray of droplets or clusters. By properly adapting the characteristics of the electron beam 4 in relation to the characteristics of the target 5, the interaction of the beam 4 with the jet 5 results, in a first mode of operation, in that radiation is emitted from the area of interaction 9 by direct conversion, essentially without heating the jet 5 to a plasma-forming temperature. In a second mode of operation, these characteristics are adapted such that the jet 5 is heated to a suitable plasma-forming temperature. The choice of mode depends on the desired wavelength range of the generated radiation. A plasma-based operation is most effective for generating soft X-ray and EUV radiation, i.e. in the range from a few tens of eV to a few keV, whereas as an essentially non-plasma, direct conversion operation is more efficient for generation of harder X- rays, typically in the range from about 10 keV to about 1000 keV. In the following, the operation of the apparatus in the first and second modes will be discussed in general terms. Examples of conceivable realizations are also given, without limiting the disclosure to these examples. In the first mode of operation, which is primarily intended for generation of hard X-ray radiation to be used in, inter alia, medical diagnosis, the electron source 2 is controlled in such a manner, in relation to the characteristics of the target 5, that essentially no plasma is formed at the area of interaction 9. Thereby, hard X-ray radiation is obtained via Bremsstrahlung and characteristic line emission. It is preferred that the distance from the outlet opening 8 to the area of interaction 9 is sufficiently long, typically 0.5-10 mm, so that the beam-jet-interaction does not damage the outlet. In one conceivable realization, use is made of a jet 5 of liquid metal having a diameter of about 30 μm and a propagation speed of about 600 m/s, the jet 5 being irradiated about 10 mm away from the outlet opening 8 by means of an electron beam 4 of about 100 mA and 100 keV, the beam 4 being focused on the jet 5 to obtain a power density of about 10 MW/mm 2 in the area of interaction 9. This power density is roughly a factor of 100 better than in conventional solid-target systems, as discussed by way of introduction. By means of the invention, a high- resolution image can be obtained with a low exposure time. In this first mode of operation, the jet 5 is preferably formed from metals heated to a liquid state. In this context, tin (Sn) should be easy to use, although other metals or alloys may be used for generation of radiation in a desired wavelength range. Further, it is also conceivable to use completely different substances for generating the jet 5, such as gases cooled to a liquid state or material dissolved in a carrier liquid. The apparatus operating in the first mode can include a window (not shown) transparent to X-rays for extracting the generated radiation from the chamber 1 to LO ω CO CO H H LΠ o LΠ o LΠ o LΠ ture in the area • of interaction 9. In the second mode of operation, the jet 5 is preferably formed from a noble gas cooled to a liquid state, to avoid coating of sensitive components within the apparatus. For example, it is known from laser-plasma studies that liquefied xenon results in strong X-ray emission in the wavelength range of 10-15 nm (see for example the article "Xenon liquid-jet laser-plasma source for EUV lithography", by Hansson et al, published in Proceedings of the SPIE, vol. 3997, 2000) . Besides liquefied noble gases, it is conceivable to use completely different substances for generating the jet, such as material dissolved in a carrier liquid or liquefied metals. An apparatus operating in the second mode and being designed for use in lithography or microscopy can include a collector system of multi-layer mirrors (not shown) that collects a large portion of the created EUV or soft x-ray radiation and transports it to illumination optics and the rest of the lithography/microscopy system. By using a microscopic target in the form of a jet 5 generated from a liquid substance, the production of debris will be very low. The inventive apparatus operating in the second mode has the potential of providing the same performance as an LPP system but at a lower price since multi kilowatt lasers are very complicated and expensive. Furthermore, the wall-plug conversion efficiency is much higher for electron sources than for lasers. It should also be noted that, when the electron source 2 is operated for first-mode X-ray generation and/or emits pulsed electron radiation, a large portion of the liquid substance may remain unaffected by the electron beam 4 and propagate unhindered through the chamber 1. This would result in an increase of pressure in the vacuum chamber 1 owing to evaporation. This problem can be solved, for instance, by a using a differential pumping scheme, indicated in the drawing, where the jet 5 is collected at a small aperture 10 and then recycled to the reservoir 7 by means of a pump 11 that compresses the collected substance and feeds it back to the reservoir 7. It should be realized that the inventive method and apparatus can be used to provide radiation for 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) , X-ray fluorescence, or in some other X-ray or EUV application.

Claims: CLAIMS 1. A method for generating X-ray or EUV radiation, comprising the steps of forming a target jet (5) by urging a liquid substance under pressure through an outlet opening, the target jet propagating through an area of interaction (9) , and directing at least one electron beam (4) onto the jet (5) in the area of interaction (9) such that the beam (4) interacts with the jet (5) to generate said X-ray or EUV radiation. 2. A method according to claim 1, wherein the substance comprises a solid material, preferably a metal, heated to a liquid state. 3. A method according to claim 1, wherein the sub- stance comprises a gas, preferably a noble gas, cooled to a liquid state. 4. A method according to any one of claims 1-3, comprising the further step of controlling the electron beam (4) to interact with the jet (5) at an intensity such that Bremsstrahlung and characteristic line emission is generated in the hard X-ray region, essentially without heating the jet (5) to a plasma-forming temperature . 5. A method according to any one of claims 1-3, comprising the further step of controlling the electron beam (4) to interact with the jet (5) at an intensity such that the jet is heated to a plasma-forming temperature, such that soft X-ray radiation or EUV radiation is generated. 6. A method according to any one of claims 1-5, wherein the jet (5) is in a solid state in the area of interaction (9) . 7. A method according to any one of claims 1-5, wherein the jet (5) is in a liquid state in the area of interaction (9) . 8. A method according to any one of the preceding claims, wherein the beam (4) interacts with a spatially continuous portion of the jet (5) in the area of interaction (9) . 9. A method according to claim 7, wherein the beam (4) interacts with at least one droplet in the area of interaction (9) . 10. A method according to claim 7 or 9, wherein the beam (4) interacts with a spray of droplets or clusters in the area of interaction (9) . 11. A method according to any one of the preceding claims, wherein electron beam (4) interacts with the jet (5) at a distance in the order of 0.5-10 mm from the outlet opening (8) . 12. A method according to any one of the preceding claims, wherein the electron beam (4) is focused on the jet (5) to essentially match a transverse dimension of the electron beam (4) to a transverse dimension of the jet (5) . 13. A method according to any one of the preceding claims, wherein the jet (5) is generated with a diameter of about 1-10000 μm. 14. A method according to any one of the preceding claims, wherein the electron beam (4) is generated by means of an acceleration voltage in the order of 5-500 kV with a beam current in the order of 10-1000 mA. 15. A method according to any one of the preceding claims, wherein at least one pulsed electron beam (4) is directed onto the jet (5) . 16. A method according to any one of the preceding claims, wherein at least one continuous electron beam (4) is directed onto the jet (5) . 17. An apparatus for generating X-ray or EUV radiation, comprising a target generator (3) arranged to form a target jet by urging a liquid substance under pressure through an outlet opening, the target jet propagating through an area of interaction, and an electron source (2) for providing at least one electron beam (4) and directing the at least one electron beam (4) onto the jet (5) in the area of interaction (9) , said radiation being generated by the beam (4) interacting with the jet (5) . 18. An apparatus according to claim 17, wherein the substance comprises a solid, preferably a metal, heated to a liquid state. 19. An apparatus according to claim 17, wherein the substance comprises a gas, preferably a noble gas, cooled to a liquid state. 20. An apparatus according to any one of claims 17- 19, wherein the electron source (2) is controllable to effect interaction of the electron beam (4) with the jet (5) at an intensity of the electron beam such that Bremsstrahlung and characteristic line emission is generated in the hard X-ray region, essentially without heating the jet (5) to a plasma-forming temperature. 21. An apparatus according to any one of claims 17- 19, wherein the electron source (2) is controllable to effect interaction of the electron beam (4) with the jet (5) at an intensity of the electron beam such that the jet is heated to a plasma-forming temperature, such that soft X-ray radiation or EUV radiation is generated. 22. An apparatus according to any one of claims 17- 21, wherein the target generator (3) is controllable to provide condensed matter in the area of interaction (9) . 23. An apparatus according to any one of claims 17- 22, wherein the target generator (3) is controllable to provide a spatially continuous portion of the jet (5) , at least one droplet, or a spray of droplets or clusters in the area of interaction (9) . 24. An apparatus according to any one claims 17-23, wherein the electron source (2) is controllable to direct the electron beam (4) onto the jet (5) at a distance in the order of 0.5-10 mm from the outlet opening (8) . 25. An apparatus according to any one claims 17-24, wherein the electron source (2) is controllable to essentially match a transverse dimension of the electron beam (4) to a transverse dimension of the jet (5) by focusing the electron beam (4) on the jet (5) . 26. An apparatus according to any one of claims 17- 25, wherein the target generator (3) is adapted to gene- rate the jet (5) with a diameter of about 1-10000 μm. 27. An apparatus according to any one of claims 17- 26, wherein the electron source (2) is controllable to generate the electron beam (4) by means of an acceleration voltage in the order of 5-500 kV, the electron beam (4) having an average beam current in the order of 10-1000 mA. 28. An apparatus according to any one of claims 17- 27, wherein the electron source (2) is controllable for generation of at least one pulsed electron beam (4) . 29. An apparatus according to any one of claims 17- 28, wherein the electron source (2) is controllable for generation of at least one continuous electron beam (4) . 30. Use of the radiation generated by a method according to any one of claims 1-16 or an apparatus according to any one of claim 17-29 for medical diagnosis . 31. Use of the radiation generated by a method according to any one of claims 1-16 or an apparatus according to any one of claim 17-29 for non-destructive testing. 32. Use of the radiation generated by a method according to any one of claims 1-16 or an apparatus according to any one of claim 17-29 for EUV projection lithography. 33. Use of the radiation generated by a method according to any one of claims 1-16 or an apparatus according to any one of claim 17-29 for crystal analysis. 34. Use of the radiation generated by a method according to any one of claims 1-16 or an apparatus according to any one of claim 17-29 for microscopy. 35. Use of the radiation generated by a method according to any one of claims 1-16 or an apparatus according to any one of claim 17-29 for protein structure determination by X-ray diffraction.

Inventors: HEMBERG OSCAR, HERTZ HANS, HEMBERG, OSCAR, HERTZ, HANS
Applicants: HEMBERG OSCAR, HERTZ HANS, JETTEC AB, HEMBERG, OSCAR, HERTZ, HANS
Application year: 2001
Main classification: H05G12
IPC codes: G03F7, G21K5, G21K7, H01J35, H01L21, H05G2