METHOD AND APPARATUS FOR GENERATING X-RAY OR EUV RADIATION

Abstract: A method and an apparatus is designed to produce X-ray or EUV radiation for use in lithography, microscopy, materials science, or medical diagnostics. The radiation is produced by urging a substance through an outlet (6) to generate a microscopic jet (2) in a direction from the outlet (6), and by directing at least one energy beam (1') onto the jet (2), wherein the energy beam (1') interacts with the jet (2) to produce the X-ray or EUV radiation. The temperature of the outlet (6) is controlled to increase the directional stability of the jet (2). The thus-achieved directional stability of the jet (2) provides for reduced pulse-to-pulse fluctuations of the produced radiation, improved spatial stability of the radiation source, as well as high average power since the energy beam (1') can be tightly focused on the jet (2), even at a comparatively large distance from the jet-generating outlet (6). The large distance provides for low erosion of the outlet (6), even when using a high-power energy beam (1').

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 EUV radiation, i.e. radiation in the wavelength region of approximately 0.01-100 nm. The generated radiation can be used in any application requiring X-ray or EUV radiation, for example lithography, microscopy, materials science, or medical diagnostics . Background Art EUV and X-ray sources of high intensity are applied in many fields, for instance surface physics, materials testing, crystal analysis, atomic physics, medical diagnostics, lithography and microscopy. Conventional X-ray sources, in which an electron beam is brought to impinge on an anode, generate a relatively low X-ray intensity. Large facilities, such as synchrotron light sources, produce a high average power. However, there are many applications that require compact, small-scale systems which produce a relatively high average power. Compact and more inexpensive systems yield better accessibility to the applied user and thus are of potentially greater value to science and society. An example of an application of particular industrial importance is future narrow-line-width lithography 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 complicated circuits needing less power. At present, photolithography is used to industri- ally produce such circuits having a line width of about 0.18 μm with projected extension towards 0.10-0.13 μm. In order to further reduce the line width, other methods will probably be necessary, of which EUV projection lithography is a very interesting candidate and X-ray lithography may become interesting for certain technological niches. In EUV projection lithography, use is made of a reducing extreme ultraviolet (EUV) objective system in the wavelength range around 10-20 nm ("EUV Lithography - The Successor to Optical Lithography?" by Bjorkholm, published in Intel Technology Journal Q3'98) . Proximity X- ray lithography, employing a contact copy scheme, is carried out in the wavelength range around 1 nm (see for instance the article "X-ray Lithography" by Maldonado, published in J. Electronic Materials 19, p. 699, 1990). Laser plasmas are attractive table-top X-ray and EUV sources due to their high brightness, high spatial stability and, potentially, high-repetition rate. However, with conventional bulk or tape targets, the operating time is limited, especially when high- repetition-rate lasers are used, since fresh target material cannot be supplied at a sufficient rate. Furthermore, such conventional targets produce debris which may destroy or coat sensitive components such as X- ray optics or EUV multilayer mirrors positioned close to the plasma. Several methods have been designed to eliminate the effect of debris, i.e., preventing the already produced debris from reaching the sensitive components. As an alternative, the amount of debris actually produced can be limited by replacing conventional solid targets with for example gas targets, gas-cluster targets, liquid-droplet targets, or liquid-jet targets. Targets in the form of microscopic liquid droplets, such as disclosed in the article "Droplet target for low- debris laser-plasma soft X-ray generation" by Ry ell and Hertz, published in Opt. Commun. 103, p. 105, 1993, are attractive low-debris, high-density targets potentially capable of high repetition-rate laser-plasma operation with high-brightness emission. Such droplets are generated by stimulated breakup of a liquid jet which is formed at a nozzle in a low-pressure chamber. However, the hydrodynamic properties of certain fluids result in unstable drop formation. Furthermore, the operation of the laser must be carefully synchronized with the droplet formation. Another problem may arise in the use of liquid substances with rapid evaporation, namely that the jet freezes immediately upon generation so that drops cannot be formed. Such substances primarily include media that are in a gaseous state at normal pressure and temperature and that are cooled to a liquid state for generation of the droplet targets. To ensure droplet formation, it is necessary to provide a suitable gas atmosphere in the low-pressure chamber, or to raise the temperature of the jet above its freezing temperature by means of an electric heater provided around the jet, such as disclosed in the article "Apparatus for producing uniform solid spheres of hydrogen" by Foster et al . , published in Rev. Sci. Instrum. 6, pp 625-631, 1977. ' As an alternative, as known from US-A-6 002 744, which is incorporated herein by reference, the laser radiation is instead focused on a spatially continuous portion of a jet which is generated by urging a liquid substance through an outlet or nozzle. This liquid-jet approach alleviates the need for temporal synchronization of the laser with the generation of the target, while keeping the production of debris equally low as from droplet targets. Furthermore, liquid substances having unsuitable hydrodynamic properties for droplet formation can be used in this approach. Another advantage over the droplet-target approach is that the spatially continuous portion of the jet can be allowed to freeze. Such a liquid-jet laser-plasma source has been further demonstrated in the article "Cryogenic liquid-jet target for debris-free laser-plasma soft x-ray generation" by Berglund et al, published in Rev. Sci. Instrum. 69, p. 2361, 1998, and the article "Liquid-jet target laser- plasma sources for EUV and X-ray lithography" by Rymell et al, published in Microelectronic Engineering 46, p. 453, 1999, by using liquid nitrogen and xenon, respectively, as target material. In these cases, a high- density target is formed as a spatially continuous portion of the jet, wherein the spatially continuous portion can be in a liquid or a frozen state. Such laser-plasma sources have the advantage of being high-brightness, low- debris sources capable of continuous high-repetition-rate operation, and the plasma can be produced far from the outlet orifice, thereby limiting plasma-induced erosion of the outlet. Such erosion may be a source of damaging debris. Further, by producing the plasma far from the outlet, self-absorption of the generated radiation can be minimized. This is due to the fact that the temperature around the jet decreases with the distance from the outlet, resulting in a correspondingly decreasing evaporation rate. Thus, the local gas atmosphere around the jet also decreases with the distance from the outlet. However, many substances, and in particular liquid substances formed by cooling normally gaseous substances, yield a jet that experiences stochastic changes in its direction from the jet-generating outlet. Typically the change in direction can be as large as about ±1° and occurs a few times per minute to a few times per second. This in turn results in a spatial instability at the focus of the laser beam, i.e. at the desired area of beam-jet-interaction, which should be as far away from the outlet orifice as possible for the reasons given above. The spatial instability leads to high pulse-to- pulse fluctuations in the emitted X-ray and EUV radiation flux and spatial instability of the radiating plasma. Furthermore, the average power is significantly lowered. Summary of the Invention It is therefore an object of the present invention to provide a method and an apparatus for stable and uncomplicated generation of X-ray or EUV radiation. More specifically, the invention should provide for low pulse- to-pulse fluctuations in the generated X-ray or EUV ^ H- ø 0 y- μ- tQ Φ if J μ- μ- H TJ rt o o φ μ- SD rt CQ rt y- 03 SD SD μ- CQ H SD H 0 0 P Hi SD 0. 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SD ti Ω SD Pi p. ι-3 * ! 0 CQ rt ϋ tr ø ø Hi rt if H - - μ- H O rt 0 φ tr rt rt rt tr SD Φ 0 0 rt CQ 3 LQ μ- ti H LQ ø SD 0 if φ CQ CQ μ- if φ μ- SD i O X J μ- rt Φ rt 0 0 SD H SD LQ (3- y- Φ £ 0 ø Φ Pi CQ TJ 0 Hi SD Φ Ω P. if 0 μ- ø J Φ CQ μ- > Hi ti LQ TJ 0 H SD φ Φ Φ 0 LQ rt SD 3 SD o TJ tr y- 0 0 3 rt ø ii 0 if μ- rt SD rt rt H rt Φ * if μ- CQ rt SD 0 0 μ- ø" μ- SD μ- 3 Φ Φ φ if rt rt 0 • ! 0 Φ 0 rt ø μ- CQ Φ Φ tr 0 ø CQ Ω P. Φ outlet opening, if desired. Preferably, the outlet is treated for enhanced and/or more localized absorption of the radiation energy, for example by providing an absorbing or conducting arrangement, such as a coating or an antenna, on the outlet. Without committing oneself to a theory, it is assumed that the results regarding the improved directional stability of the generated jet can be explained by the following model. When the substance leaves the outlet, the thus-formed jet, as well as any liquid wetting the outlet, undergoes evaporative cooling. This results in the outlet being cooled, leading to uncontrolled deposition of frozen material on, or close to, the outlet orifice. Such frozen material could induce the stochastic directional instability described above. Heating of the outlet is believed to minimize such deposition of frozen material . According to a further preferred embodiment of the invention, the jet leaves the outlet in a condensed, i.e. liquid or frozen, state. This allows for collimated transport of target material far from the outlet . To form such a jet of condensed matter, it is preferred to urge a substance in a liquid state through the jet-forming outlet. The substance used in the invention could be a medium which is in a liquid state both at room temperature and the temperature prevailing at the generation of the jet. This medium could also be a solution comprising solids and a suitable carrier fluid. In a particularly preferred embodiment, however, the substance is a medium which is in a gaseous state at room temperature, but which is cooled to a liquid state before being urged through the outlet to form the jet. This type of medium can, by means of the invention, be used in an uncomplicated way for stable generation of X-ray or EUV radiation at previously unaccessible wavelengths. By using an inert gas, in particular a noble gas, the damages caused by debris can be reduced significantly. It should be noted, however, that the jet can be in any suitable state (gaseous, liquid, or solid) when interacting with the energy beam. According to one embodiment of the invention, the energy beam is directed onto a spatially continuous portion of the jet. This can be achieved, for instance, by generating a spatially completely continuous jet, and by directing the energy beam onto the actual jet before it spontaneously breaks up into droplets or a spray. Alternatively, it is conceivable to generate a pulsed or semicontinuous jet consisting of separate, spatially continuous portions each having a length that significantly exceeds the diameter. In both cases, the jet might be frozen due to evaporative cooling before interacting with the energy beam. An alternative embodiment include directing the energy beam onto one or more droplets, or a spray, which could be formed spontaneously, or by stimulation, from the jet. Thus, the energy beam can, within the scope of the invention, interact with any formation emanating from the jet, be it gaseous, liquid or solid, spatially continuous, droplets, or a spray of droplets or clusters. The energy beam is preferably a beam of electro- magnetic radiation, such as laser radiation, which interacts with the jet and heats it to a plasma-forming temperature. It is also conceivable to use a beam of electrons as energy beam, wherein EUV or X-ray radiation is generated either by the electrons heating the jet to a plasma-forming temperature, or by direct conversion of the electron-beam energy to Bremsstrahlung and characteristic line emission, essentially without the need of heating the jet to a plasma-forming temperature. The present invention is based on the need for com- pact and intensive X-ray or EUV sources for, inter alia, lithography, microscopy, materials science and medical diagnostics. Wavelength ranges of particular interest for to μ* μ> LΠ o LΠ o LΠ o LΠ TJ H Φ Hi Φ H i Φ i φ CJ 1 0 P. μ- 3 Φ r øt SD 0 P. μ- ø $ if μ- Ω if Fig. 1 illustrates a preferred embodiment of the inventive apparatus for generating X-ray or EUV radiation from a stable jet in a low-pressure chamber employing ohmic heating close to the nozzle orifice; Fig. 2 shows the flux of generated X-ray radiation over time in a prior art apparatus, i.e. without stabilization heating; and Fig. 3 shows the flux of generated X-ray radiation over time in the apparatus of Fig. 1, i.e. with stabi- lization heating. Description of Preferred Embodiments The method and the apparatus according to the invention are basically illustrated in Fig. 1. Below, xenon is used as an example but the invention may be operated with many other substances, of which liquefied gases are believed to be the most important, as discussed in the end of this section. A laser 1 generates one or more pulsed laser beams 1' that are focused from one or more directions on a jet 2 of liquefied xenon liquid, which serves as target. For reasons of clarity, only one laser beam 1' is shown in Fig. 1. The beam-jet-interaction produces a plasma P emitting the desired X-ray and EUV radiation. The actual production of X-rays usually takes place at low pressure, to prevent emitted soft X-ray or EUV radiation from being absorbed. For certain X-ray or EUV wavelengths, the laser-plasma production may be operated in a gaseous environment. This gaseous environment may be local around the area of beam-jet-interaction. Low pressure is often preferable to eliminate laser-induced breakdowns in front of the jet 2 or to reduce self-absorption of the emitted radiation. The microscopic jet 2 of liquid xenon is spatially continuous and is formed in a vacuum chamber 3, as shown in Fig. 1. In general, liquid xenon 4 is urged under high pressure (usually 5-500 atmospheres) from a pump (not shown) or a pressure vessel 5 through a small nozzle 6 having an orifice diameter which usually is smaller than about 100 μm and typically a few to up to a few tens of micrometers. This results in a microscopic jet 2 of liquid of essentially the same diameter as the orifice diameter and with a speed of about ten m/s to a few hundred m/s. In one arrangement (not shown), the liquid xenon is produced separately and then forced through the nozzle by a pump or other pressure-producing arrangement. Alternatively, as shown in Fig. 1, the pressure of gaseous xenon 7 itself is used as the driver. Here, xenon gas is forced at a pressure of about 5-100 bar into the pressure vessel 5 which is cooled to about 160-200 K by a Gifford-McMahon-type or other cold head 8. The glass capillary nozzle 6 is attached directly to the pressure vessel 5, producing the microscopic jet 2 of liquid xenon in the low-pressure chamber 3. The laser beam 1' is controlled to interact with the spatially continuous jet 2. For most liquids, the jet 2 propagates in the chamber 3 in a given direction towards a break-up point (not shown) , at which it should spontaneously separate into droplets or a spray. In the exemplifying embodiment shown in Fig. 1, the laser beam 1' is focused on the jet 2 upstream of any such break-up point. For many cryogenic liquids, such as xenon, the jet 2 is rapidly cooled by evaporation as it leaves the nozzle 6, and in many cases the jet 2 freezes close to the nozzle 6 so that no droplets are formed. In that case, the laser beam 1' could be focused on a spatially continuous portion of the thus frozen jet 2. For generation of X-ray emission in the wavelength range around 1-5 nm, a laser intensity of about 10 12 - 10 15 W/cm 2 is required while 10 10 -10 13 W/cm 2 are usually preferable for the EUV range. Suitable laser systems for this purpose in the visible, ultraviolet and near infra- red wavelength range are commercially available with repetition rates of 10-10000 Hz, and systems having a higher repetition rate are being developed at present. As pointed out above, the jet may exhibit poor directional stability, thus creating unstable laser- plasma generation. To this end, the tip 10 of the nozzle 6 is heated locally, i.e. close to the nozzle orifice, by some means described below. The heating of the nozzle tip 10 is preferably local since a general heating of the liquid xenon could result in a spray being formed instead of a spatially continuous jet 2. There are several methods to induce such local heating. One is to resistively heat the tip 10 by applying a thin resitive wire 11 (diameter of a few microns or larger) at the nozzle tip 10 and supply a current through the wire 11. The current, and thus the heating, should be tuned so that the hydrodynamic properties of the flowing liquid xenon inside the nozzle 6 are not markedly changed while the orifice temperature is raised sufficiently to improve the directional stability of the jet 2, presumably by removing evaporation-produced frozen material or fragments on the nozzle orifice. Alternatively, the resistive wire 11 can be exchanged for electrodes (not shown) with proper resistivity that are evaporated at the nozzle tip 10. Other methods of heating the tip 10 include local absorption of cw laser light or other electromagnetic radiation that is focussed on the tip 10. In this context, it is preferred that the outlet is treated to enhance and/or further localize the absorption of the radiation energy, for example by providing an absorbing or conducting arrangement, such as a coating or an antenna (not shown), on the tip 10. Pulsed laser light may also be used, and in this case it cannot be ruled out that any improved directional stability also depends on other factors than just heating, for example ablation of frozen material. The heating as described above results in sufficient spatial stability (± a few micrometers) to permit laser- ω t to μ 1 μ 1 LΠ o LΠ o LΠ o LΠ 3 0 0 Ω Pi SD Ω rt Ω fi SD Ω Pi rt r 1 0 TJ rt o Pi μ- Hi Hi 0 Ω rt 3 TJ 0 SD Φ SD TJ 0 0 Hi if Φ TJ tr tr O Φ SD O ø *< 0 Hi fi tr μ- μ- 03 y- y- fi 0 tr μ- fi <! 0 <1 TJ y- i y- SD Hi Φ SD Φ 03 Ω P. rt s; fi TJ € 0 Φ <! SD 0 ø μ- ii φ rt o Φ P. μ- TJ SD μ- *< y- 3 fi 3 rt ■ ^ μ- Ω μ- μ- TJ P. 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Claims: CLAIMS 1. A method of generating X-ray or EUV radiation, comprising the steps of urging a substance through an outlet (6) to generate a jet (2) in a direction from the outlet (6), and directing at least one energy beam (1') onto the jet (2), the energy beam (1') interacting with the jet (2) to generate said X-ray or EUV radiation, charac teri zed by the further step of controlling the temperature of said outlet (6) such that the stability of the jet (2) is improved. 2. A method as set forth in claim 1, wherein said step of controlling the temperature comprises effecting ohmic heating of the outlet (6) , preferably at an orifice thereof . 3. A method as set forth in claim 1, wherein said step of controlling the temperature comprises directing radiation energy onto said outlet (6) . 4. A method as set forth in any one of the preceding claims, wherein the jet (2) leaves the outlet (6) in a condensed state . 5. A method as set forth in any one of the preceding claims, wherein the substance comprises a gas which is cooled to a liquid state before being urged through said outlet (6) . 6. A method as set forth in claim 5, wherein the gas is an essentially inert gas, such as a noble gas. 7. A method as set forth in any one of claims 1-6, wherein the energy beam (1') is directed onto a spatially continuous portion of the jet (2) . 8. A method as set forth in any one of claims 1-6, wherein the energy beam (1') is directed onto at least one droplet of the jet (2) . 9. A method as set forth in any one of claims 1-6, wherein the energy beam (1') is directed onto a spray of droplets or clusters formed from the jet (2) . 10. A method as set forth in any one of the preceding claims, wherein the jet (2) is cooled by evaporation to a frozen state, and the energy beam (1') is directed onto a frozen portion of the jet (2) . 11. A method as set forth in any one of the preceding claims, wherein said energy beam (1') comprises pulsed laser radiation which interacts with the jet (2) to form a plasma (P) emitting said X-ray or EUV radiation. 12. A method as set forth in any one of the preceding claims, wherein said energy beam (1') is focused on the jet (2) to essentially match a transverse dimension of the energy beam (1') to a transverse dimension of the jet (2) . 13. A method as set forth in claim 7, wherein the energy beam (1') is focused to essentially coincide with said spatially continuous portion over a length thereof. 14. An apparatus for generating X-ray or EUV radiation, comprising an energy source (1) arranged to emit at least one energy beam (1'), a target generator (4-11) arranged to urge a substance through an outlet (6) to generate a target in the form of a jet (2) in a direction from the outlet (6) , and a beam controller (12) operative to direct the energy beam (1') emitted by the energy source onto the target jet (2) generated by the target generator, said X-ray or EUV radiation being generated by the energy beam (1') interacting with the jet (2), characteri zed in that the target generator (4-11) comprises a means (11) for controlling the temperature of said outlet (6) such that the stability of the jet (2) is improved. 15 An apparatus as set forth in claim 14, wherein the temperature-controlling means comprises a resistive element (11) arranged in association with said outlet (6) , and a power supply connected to the resistive element (11) to heat the outlet (6) by ohmic heating. 16. An apparatus as set forth in claim 14, wherein the temperature-controlling means (11) comprises a radiation heater directing radiation energy onto said outlet (6) . 17. An apparatus as set forth in claim 16, wherein the outlet (6) comprises means providing for enhanced and/or confined absorption of the radiation energy. 18. An apparatus as set forth in any one of claims 14-17, wherein the target generator (4-11) is adapted to generate the jet (2) such that it is in a condensed state when leaving the outlet (6) . 19. An apparatus as set forth in any one of claims 14-18, wherein the substance comprises a gas, the target generator (4-11) being adapted to cool said gas to a liquid state before urging it through the outlet (6) . 20. An apparatus as set forth in claim 19, wherein the gas is an essentially inert gas, such as a noble gas. 21. An apparatus as set forth in any one of claims 14-20, wherein the target generator (4-11) is controllable to provide a spatially continuous portion, at least one droplet, or a spray of droplets or clusters for the energy beam (1') to interact with. 22. An apparatus as set forth in any one of claims 14-21, wherein the outlet (6) is arranged to generate the jet (2) in a chamber (3) , and wherein a conditioning means (13) is arranged to control the atmosphere in the chamber (3) such that the jet (2) is cooled by evaporation to a frozen state on entry into the chamber (3) . 23. An apparatus as set forth in any one of claims 14-22, wherein the energy source comprises a laser (1) emitting at least one beam (1') of pulsed laser radiation, said beam (1') / when directed onto the jet, interacting therewith to form a plasma (P) emitting said X-ray or EUV radiation. 24. An apparatus as set forth in any one of claims 14-23, wherein the beam controller (12) is adapted to focus said energy beam (1') on the jet (2) to essentially match a transverse dimension of the energy beam (!') to a transverse dimension of the jet (2) . 25. An apparatus as set forth in claim 21, wherein the beam controller (12) is adapted to focus said energy beam (1') to essentially coincide with said spatially continuous portion over a length thereof. 26. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for X-ray microscopy. 27. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for proximity lithography. 28. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for EUV projection lithography. 29. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for photoelectron spectroscopy. 30. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for X-ray fluorescence. 31. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for X-ray diffraction. 32. Use of radiation generated by a method as set forth in any one of claims 1-13 or an apparatus as set forth in any one of claims 14-25 for medical diagnostics.

Inventors: BERGLUND MAGNUS, HANSSON BJOERN A M, HEMBERG OSCAR, HERTZ HANS MARTIN, RYMELL LARS, BERGLUND, MAGNUS, HANSSON, BJOERN, A., M., HEMBERG, OSCAR, HERTZ, HANS, MARTIN, RYMELL, LARS
Applicants: BERGLUND MAGNUS, HANSSON BJOERN A M, HEMBERG OSCAR, HERTZ HANS MARTIN, JETTEC AB, RYMELL LARS, BERGLUND, MAGNUS, HANSSON, BJOERN, A., M., HEMBERG, OSCAR, HERTZ, HANS, MARTIN, RYMELL, LARS
Application year: 2001
Main classification: H05G2
IPC codes: G03F7, H01J35, H05G2