(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0044629 A1
`Hertz et al.
`(43) Pub. Date:
`Apr. 18, 2002
`
`US 20020044629A1
`
`(54) METHOD AND APPARATUS FOR
`GENERATING X-RAY OR EUV RADIATION
`
`Publication Classification
`
`(76)
`
`Inventors: Hans Martin Hertz, Stocksund (SE);
`Oscar Hemberg; Stockholm (SE); Lars
`Rymell; Stockholm (SE); Bjorn A.M.
`Hansson; Stockholm (SE); Magnus
`Berglund; Axeltorp (SE)
`
`Correspondence Address:
`Benton S. Dufi'ett, Jr.
`BURNS, DOANE, SWECKER & MATHIS,
`L.L.P.
`P.O. Box 1404
`Alexandria, VA 22313-1404 (US)
`
`(21) Appl. No.:
`
`09/974,975
`
`(22)
`
`Filed:
`
`Oct. 12, 2001
`
`Related U.S. Application Data
`
`(63) Non-provisional of provisional
`60/239,896; filed on Oct. 13; 2000.
`
`application No.
`
`(30)
`
`Foreign Application Priority Data
`
`Oct. 13, 2000
`
`(SE) ........................................ .. 0003715-0
`
`(51)
`
`Int. Cl.7 .......................... .. H01J 35/00; H05H 1/00;
`G21G 4/00
`
`(52) U.S. Cl.
`
`.......................................... .. 378/119; 378/120
`
`(57)
`
`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
`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 radia-
`tion 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').
`
`ASML 1026
`ASML 1026
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`Patent Application Publication Apr. 18, 2002 Sheet 1 of 2
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`US 2002/0044629 A1
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`Patent Application Publication Apr. 18, 2002 Sheet 2 of 2
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`US 2002/0044629 A1
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`o-I
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`Intensity[a.u.]'9P990L74:onan
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`0
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`20
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`40
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`so
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`80
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`100
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`Shot ‘no.
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`Fig. 2
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`0
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`.
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`40
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`Shot no.
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`so
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`so
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`US 2002/0044629 A1
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`Apr. 18, 2002
`
`METHOD AND APPARATUS FOR GENERATING
`X-RAY OR EUV RADIATION
`
`TECHNICAL FIELD
`
`[0001] 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 appli-
`cation requiring X-ray or EUV radiation, for example lithog-
`raphy, microscopy, materials
`science,
`or medical
`diagnostics.
`
`BACKGROUND ART
`
`[0002] EUV and X-ray sources of high intensity are
`applied in many fields, for instance surface physics, mate-
`rials testing, crystal analysis, atomic physics, medical diag-
`nostics, 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 rela-
`tively 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.
`
`[0003] 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 industrially produce such cir-
`cuits having a line width of about 0.18 pm with projected
`extension towards 0.10-0.13 ym. 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) objec-
`tive system in the wavelength range around 10-20 nm
`(“EUV Lithography—The Successor to Optical Lithogra-
`phy?” 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).
`
`[0004] 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 elimi-
`nate the effect of debris, i.e., preventing the already pro-
`duced 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 tar-
`gets, or liquid-jet targets.
`
`[0005] 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 Rymell 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 cham-
`ber, 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.
`
`[0006] As an alternative, as known from U.S. Pat. No.
`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 hydrody-
`namic 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 “Cryo-
`genic 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, respec-
`tively, 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 evapo-
`ration rate. Thus, the local gas atmosphere around the jet
`also decreases with the distance from the outlet.
`
`[0007] 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 11° 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
`
`

`
`US 2002/0044629 A1
`
`Apr. 18, 2002
`
`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
`[0008]
`provide a method and an apparatus for stable and uncom-
`plicated generation of X-ray or EUV radiation. More spe-
`cifically, the invention should provide for low pulse-to-pulse
`fluctuations in the generated X-ray or EUV radiation flux,
`low erosion of the jet-generating outlet, as well as low
`self-absorption of the generated radiation.
`
`It is also an object to provide an apparatus for
`[0009]
`generating X-ray or EUV radiation that is compact, inex-
`pensive, generates radiation at a relatively high average
`power and has a minimum production of debris.
`
`[0010] A further object is to provide a method and an
`apparatus which produces X-ray or EUV radiation which is
`suitable for EUV projection lithography and proximity
`lithography.
`
`[0011] One more object of the invention is to permit use of
`the method and the apparatus in microscopy, materials
`science, biomedical and medical diagnostics.
`
`[0012] These and other objects, which will be apparent
`from the following specification, are wholly or partially
`achieved by the method and the apparatus according to the
`independent claims. The dependent claims define preferred
`embodiments.
`
`It has been found that controlling the temperature
`[0013]
`of the outlet, normally heating the same, has the effect of
`considerably improving the directional stability of the gen-
`erated jet. Thus, the invention allows for low pulse-to-pulse
`fluctuations in the generated X-ray or EUV radiation flux,
`increased average power, as well as increased spatial stabil-
`ity of the radiating beam-jet-interaction area. The direction-
`ally stable jet also allows for a large distance between the
`outlet and the beam-j et-interaction area, thereby minimizing
`both erosion of the outlet and self-absorption of the gener-
`ated radiation. A “large distance” in this context is typically
`at least a few millimeters. In view of the large distance made
`possible by the invention, the power of the energy beam
`might be increased without causing undesired heating of the
`target generator. Thus,
`the invention allows for a higher
`X-ray and EUV flux. Further, the invention allows for use of
`several new substances and, thus, for stable generation of
`radiation in new wavelength ranges.
`
`[0014] These advantages are obtained while retaining
`many of the advantages of the prior art technologies, as
`discussed by way of introduction,
`for example a great
`reduction of debris, excellent geometric access, a possibility
`of long-term operation without interruption by providing
`new target material continuously through the jet, a possibil-
`ity of using lasers of high repetition rates, which increases
`the average power of the generated X-ray or EUV radiation.
`
`[0015] The inventive control of the temperature of the
`outlet should preferably be effected with minimum influence
`on the temperature of the substance inside the jet-generating
`outlet, since such influence might cause boiling or a modi-
`fication of the hydrodynamic properties of the substance,
`which potentially might lead to instabilities in the generation
`of the jet, for example potentially undesired spray formation.
`
`[0016] According to one preferred embodiment, the tem-
`perature of the outlet
`is controlled by means of ohmic
`
`heating, for example by applying a voltage to an electrically
`conducting resistive wire arranged around and preferably in
`contact with the outlet, or by applying the voltage to a
`portion of the outlet itself. This embodiment is advantageous
`in its simplicity and ruggedness. By means of thin wires or
`evaporated electrodes it is possible to localize the generated
`heat close to the outlet opening.
`
`[0017] According to another preferred embodiment, the
`temperature of the outlet is controlled by directing radiation
`energy, for example laser radiation or microwaves, onto the
`outlet which is heated by absorption of this radiation energy.
`This embodiment provides for non-intrusive heating of the
`outlet, in that no new material needs to be mechanically
`introduced at the outlet, and can be precisely controlled to
`heat only the 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.
`
`is
`it
`[0018] Without committing oneself to a theory,
`assumed that the results regarding the improved directional
`stability of the generated jet can be explained by the fol-
`lowing 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 mini-
`mize such deposition of frozen material.
`
`[0019] 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 particu-
`larly 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 unac-
`cessible 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
`[0020]
`any suitable state (gaseous, liquid, or solid) when interacting
`with the energy beam.
`
`[0021] 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.
`
`

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`US 2002/0044629 A1
`
`Apr. 18, 2002
`
`[0022] 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.
`
`[0023] 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.
`
`[0024] 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 tem-
`perature, 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.
`
`[0025] The present invention is based on the need for
`compact and intensive X-ray or EUV sources for, inter alia,
`lithography, microscopy, materials science and medical
`diagnostics. Wavelength ranges of particular interest for
`such applications are approximately 1-2 nm (proximity
`lithography), 2.3-4.4 nm (X-ray microscopy), 10-15 nm
`(EUV projection lithography) and 0.01-20 nm (materials
`science, for instance photoelectron spectroscopy or X-ray
`fluorescence, or biomedical applications such as X-ray dif-
`fraction or medical diagnostics). The generation of radiation
`in these wavelength ranges with high conversion efficiency
`by means of laser-produced plasma generally necessitates
`laser
`intensities around 101°-1015 W/cm2.
`In order
`to
`achieve such intensities with compact laser systems,
`the
`laser radiation is focused to a diameter of about five to a few
`hundred micrometers. Thus, the target can be made micro-
`scopic, provided that it is spatially stable. The small dimen-
`sions contribute to effective utilization of the target material,
`which, among other things, results in a drastic reduction of
`debris. By using short-pulsed laser radiation, for example in
`the femtosecond range, harder X-rays may be generated.
`
`[0026] The method and apparatus according to the inven-
`tion is particularly, but not exclusively, suited for EUV
`projection lithography which requires irradiation in the
`wavelength range of approximately 10-15 nm. Such radia-
`tion can be generated, by means of the invention, by using
`xenon as target material.
`
`[0027] By using different target materials in a method or
`an apparatus according to the invention, EUV and X-ray
`radiation can be generated at suitable wavelengths for a
`number of different applications. Examples of such appli-
`cations are X-ray microscopy, materials science (e.g. pho-
`toelectron microscopy and X-ray fluorescence), EUV pro-
`jection lithography, proximity X-ray lithography, medical
`X-ray diagnostics or crystal analysis.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0028] The invention will now be described for the pur-
`pose of exemplification with reference to the accompanying
`schematic drawings, which illustrate a currently preferred
`embodiment and in which
`
`[0029] 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;
`
`[0030] FIG. 2 shows the flux of generated X-ray radiation
`over time in a prior art apparatus, i.e. without stabilization
`heating; and
`
`[0031] FIG. 3 shows the flux of generated X-ray radiation
`over time in the apparatus of FIG. 1, i.e. with stabilization
`heating.
`
`DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`[0032] 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.
`
`[0033] Alaser 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.
`
`[0034] 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 pm 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.
`
`[0035] The laser beam 1‘ is controlled to interact with the
`spatially continuous jet 2. For most liquids, the jet 2 propa-
`gates in the chamber 3 in a given direction towards a
`break-up point (not shown), at which it should spontane-
`ously 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.
`
`[0036] For generation of X-ray emission in the wavelength
`range around 1-5 nm, a laser intensity of about 1012-1015
`W/cm2 is required while 1010-1013 W/cm2 are usually pref-
`
`

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`US 2002/0044629 A1
`
`Apr. 18, 2002
`
`erable for the EUV range. Suitable laser systems for this
`purpose in the visible, ultraviolet and near infra-red wave-
`length range are commercially available with repetition rates
`of 10-10000 Hz, and systems having a higher repetition rate
`are being developed at present.
`
`the main vacuum chamber 3 and inject the jet 2 through a
`very small aperture. In that case, a mechanical chopper or
`electric defiection means (not shown) outside the main
`vacuum chamber 3 can be used to supply merely the desired
`amount of liquid to the main vacuum chamber 3.
`
`the jet may exhibit poor
`[0037] As pointed out above,
`directional stability, thus creating unstable laser-plasma gen-
`eration. 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.
`
`[0038] 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 hydrody-
`namic properties of the flowing liquid xenon inside the
`nozzle 6 are not markedly changed while the orifice tem-
`perature 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 evapo-
`rated 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 radia-
`tion 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.
`
`[0039] The heating as described above results in sufficient
`spatial stability (1 a few micrometers) to permit
`laser-
`plasma production with a laser beam 1‘ focused to approxi-
`mately the same size as the diameter of the jet 2.
`
`[0040] The effectiveness of the invention is further evi-
`denced by the experimental results presented in FIGS. 2 and
`3. FIG. 2 shows the flux of the generated radiation over time
`in an apparatus operating with laser-plasma production from
`a continuous jet of liquid xenon, but without any heating of
`the nozzle orifice. FIG. 3 shows the flux of the generated
`radiation over time in a corresponding apparatus with heat-
`ing of the nozzle orifice. Evidently,
`the heating reduces
`pulse-to-pulse fluctuations in the generated X-ray or EUV
`radiation flux.
`
`In the embodiment shown in FIG. 1, the laser beam
`[0041]
`1‘ is focused with a spherical lens 12 to a point having a
`diameter of about five to a few hundred micrometers. Given
`the speed of the jet 2 of liquid, the main part of the liquid 2
`will thus not be used for laser plasma production, which for
`many liquids results in an increase of pressure in the vacuum
`chamber 3 owing to evaporation. Low pressure is main-
`tained with a vacuum pump 13, which typically keeps the
`chamber pressure at 10‘3-10’4 mbar during operation. Addi-
`tional efforts to keep the pressure low may include, for
`instance, a cold trap (not shown) catching the non-used
`liquid. Another means is the use of a differential pumping
`scheme, which also may include recycling of the xenon gas,
`which is attractive from a cost-of-target perspective. Alter-
`natively (not shown), the nozzle 6 can he positioned outside
`
`It should be noted that the above-description is
`[0042]
`only given for the purpose of illustration, and that many
`modifications are conceivable within the scope of the inven-
`tion. For example, an elongated laser focus might be formed
`over a certain length of the jet, for example by means of one
`or more cylinder lenses (not shown) in combination with one
`or more spherical lenses, resulting in an elongated EUV
`emitting plasma. Semicontinuous or pulsed jets of liquid
`may, within the scope of the invention, be applicable in
`special cases. This type of jet consists of separate, spatially
`continuous portions, which are generated by ejecting the
`liquid through the nozzle during short periods of time only.
`In contrast to some of the methods described above, this
`requires some type of valve (not shown) to supply the liquid
`xenon to the nozzle.
`
`the generation of a directionally stable
`[0043] Above,
`liquid jet of liquefied gas for laser-plasma production of
`X-ray and EUV radition has been described by using xenon
`as an example. Xenon is believed to be especially important
`due to its high Z (resulting in high conversion efficiency), its
`inert noble gas character (resulting in minimum damage to
`sensitive components positioned close to the plasma), and its
`suitable emission spectrum (that matches the requirements
`of EUV projection lithography in the wavelength range of
`10-15 nm wavelength, and proximity lithography in the
`wavelength range of 1-2 nm).
`
`[0044] Without limiting the invention to any examples, it
`should be mentioned that other liquefied gases may be used
`for directionally stable laser-plasma liquid-jet-target opera-
`tion for other specific applications. Liquid nitrogen has a
`suitable emission spectrum for water-window X-ray micros-
`copy, see for example the article “Cryogenic liquid-jet target
`for debris-free laser-plasma soft X-ray generation” by Ber-
`glund et al, published in Rev. Sci. Instrum. 69, p. 2361,
`1998. Argon emits at a few keV, having potential use in
`X-ray absorption and fluorescence studies. Oxygen and
`nitrogen might be used for surface sensitive photoelectron
`spectroscopy. Heavy target elements, especially in combi-
`nation with high peak-power lasers, will result in higher
`energy emission, which may be suitable for, e.g., X-ray
`diffraction for crystallography or protein-structure determi-
`nation.
`
`It should be noted that the directional stability of
`[0045]
`the jet might be improved by other means minimizing the
`deposition of frozen material on, or close to,
`the outlet
`orifice. For example, a protective coating prohibiting any
`deposition (wetting) of target liquid could be provided on the
`outlet, preferably close to the outlet orifice. Alternatively, or
`additionally, the geometrical shape of outlet could be opti-
`mized to prohibit such deposition (wetting), For example,
`the orifice-defining distal end wall of the outlet could be
`inclined to the longitudinal direction of the outlet. In special
`cases, these outlet coating/shaping measures could be used
`without the inventive heating of the outlet.
`
`1. A method of generating X-ray or EUV radiation,
`comprising the steps of:
`
`(i) urging a substance through an outlet to generate a jet
`in a direction from the outlet,
`
`

`
`US 2002/0044629 A1
`
`Apr. 18, 2002
`
`(ii) directing at least one energy beam onto the jet, the
`energy beam interacting with the jet to generate said
`X-ray or EUV radiation, and
`
`(iii) controlling the temperature of said outlet, such that
`the stability of said jet is improved.
`2. A method as set forth in claim 1, wherein said step of
`controlling the temperature comprises effecting ohmic heat-
`ing of the outlet, 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.
`4. A method as set forth in claim 1, wherein the jet leaves
`the outlet in a condensed state.
`5. Amethod as set forth in claim 1, wherein the substance
`comprises a gas which is cooled to a liquid state before being
`urged through said outlet.
`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 claim 1, wherein the energy
`beam is directed onto a spatially continuous portion of the
`jet.
`8. A method as set forth in claim 1, wherein the energy
`beam is directed onto at least one droplet of the jet.
`9. A method as set forth in claim 1, wherein the energy
`beam is directed onto a spray of droplets or clusters formed
`from the jet.
`10. A method as set forth in claim 4, wherein the jet is
`cooled by evaporation to a frozen state, and the energy beam
`is directed onto a frozen portion of the jet.
`11. A method as set forth in claim 1, wherein the energy
`beam comprises pulsed laser radiation which interacts with
`the jet
`to form a plasma emitting said X-ray or EUV
`radiation.
`
`12. A method as set forth in claim 1, wherein said energy
`beam is focused on the jet to essentially match a transverse
`dimen