United States Patent [191
`
`[11] Patent Number:
`
`4,866,517
`
`Mochizuki et al.
`
`[45] Date of Patent:
`
`Sep. 12, 1989
`
`[54] LASER PLASMA X-RAY GENERATOR
`CAPABLE OF CONTINUOUSLY
`GENERATING X-RAYS
`
`[75]
`
`Inventors: Takayasu Mochizuki, Tokyo; Chiyoe
`Yamanaka, 11-1, Nishiyama-cho,
`Ashiya-shi, Hyogo-ken, both of
`Japan
`
`[73] Assignees: Hoya Corp., Tokyo; Chiyoe
`Yamanaka, Hyogo, both of Japan
`
`[21] Appl. No.: 95,414
`
`[22] Filed:
`
`Sep. 10, 1987
`
`Foreign Application Priority Data
`[30]
`Sep. 11, 1986 [JP]
`Japan ...................... 61-214734
`
`Int. Cl.‘ ........................................ I-I05H 1/24
`[51]
`[52] U.S. C1. ......................... .. 378/119; 378/120;
`378/160
`[58] Field of Search .................. 378/119, 120, 34, 160
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`4,700,371 10/1987 Forsyth et a1.
`4,723,262
`2/1988 Noda et al.
`
`378/34
`378/119
`
`Primary Examiner—Carolyn E. Fields
`Assistant Examiner-—David P. Porta
`Attorney, Agent, or Fz'rm——Roberts, Spiecens & Cohen
`
`[57]
`
`ABSTRACI‘
`
`In a laser plasma X-ray device for use in generating
`X-rays by bombarding a target material by a pulsed
`laser beam, the target material is selected from materials
`which are in a gas phase at the room temperature and
`which are cooled in a selected one of liquid and solid
`phases. Such a selected phase of the target material is
`continuously supplied to a focal point of the pulsed laser
`beam to be subjected to bombardment and to generate
`the X-rays. On generation of the X-rays, the target
`material is rendered into the gas phase to be recycled
`into the selected phase. The X-rays are guided outside
`of the chamber through an X-ray gate unit opened in
`synchronism with a repetition frequency of the pulsed
`laser beam.
`
`4,408,338 10/1983 Grobman ............................ 378/119
`
`12 Claims, 3 Drawing Sheets
`
`
`
`56
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`ONTROLLER
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`6 5
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`ASML 1128
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`ASML 1128
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`U.S. Patent
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`Sep.12,1989
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`Sheet 1 013
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`4866517
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`F I
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`1
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`PRIOR ART
`
`

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`U.S. Patent
`
`Sep.12,1989
`
`Sheet 2 of3
`
`4,866,517
`
`ENERGY(J/srkeVEL)
`
`X-RAYRADIATION
`
`PHOTON ENERGY (keV)
`
`F|G.3
`
`PHOTON ENERGY (keV)
`
`F|G.4
`
`

`
`U.S. Patent
`
`Sep. 12,1989
`
`Sheet 3 of3
`
`4,866,517
`
`
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`FlG.5
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`F|G.6
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`

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`1
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`4,866,517
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`LASER PLASMA X-RAY GENERATOR CAPABLE
`OF CON'I'INUOUSLY GENERATING X-RAYS
`
`BACKGROUND OF THE INVENTION:
`
`This invention relate to a laser plasma X-ray genera-
`tor which is mainly used as an X-ray source for X-ray
`lithography, X-ray microscopy, and the like.
`In a conventional laser plasma X-ray generator of the
`type described, X-rays are generated by emitting a
`pulsed laser beam of high power onto a target placed in
`a chamber and by producing plasma due to emission of
`the target by the pulsed laser beam. Such plasma has a
`high temperature and a high density. The chamber is
`usually kept at a low pressure in comparison with ambi-
`ent pressure.
`It is common that the target is formed by a target
`material in a solid-state and may be called a solid-state
`target. For X-ray lithography or the like, it is preferable
`that energy of X-rays is between 0.1 keV and 3 keV.
`Taking this into account, either copper or aluminum is
`frequently used as the target material. Such a target of
`a metal is subjected to emission or bombardment of the
`pulsed laser beam. In this event, the pulsed laser beam is
`focused on a focal point to bombard the target at the
`focal point and to generate the X-rays. As a result of
`bombardment of the pulsed laser beam onto the target,
`evaporation of the target material takes place at and
`near the focal point and undersirably roughens the sur-
`face of the target with a crater. Such roughness of the
`target makes convergence of the pulsed laser beam
`objectionably unstable on the target surface. Accord-
`ingly, the pulsed laser beam must always converge onto
`a pure or unbombarded surface of the target. To this
`end, the target is formed into a conical or a cylindrical
`shape and is rotated around an axis of the target.
`In addition, the X-rays are derived from the chamber
`through an outlet portion which comprises a thin film
`of, for example, beryllium which is attached to an outlet
`and which has a good transmissivity to the X-rays.
`As is readily understood from the above, the solid-
`state target is eventually worn out and can not be re-
`used. This means that a new solid-state target must be
`exchanged for a used one after a predetermined time of,
`for example, one hour. Such exchange of the solid-state
`target is troublesome and results in interruption of oper-
`ation or processing carried out by the use of the laser
`plasma X-ray generator.
`Consideration might be made about automatic ex-
`change of the solid-state target. However, the auto-
`matic exchange makes the laser plasma X-ray generator
`undersirably large in size and brings about an increase
`of an expense for faulities.
`Moreover, the target material is evaporated during
`the emission of the pulsed laser beam onto the solid-
`state target, as mentioned before. Such evaporation
`results in deposition of the target material onto an inter-
`nal wall surface of the chamber and the thin film of the
`outlet portion. The deposition of the target material on
`the thin film reduces the transmissivity of the X-rays
`and objectionably attenuates an intensity of the X-rays.
`Therefore, use of the solid-state target makes it difficult
`to keep the X-rays stable in intensity.
`In addition, it should be considered that the X-rays
`are also inevitably attenuated by the thin film of beryl-
`lium or the like attached to the outlet, although the
`beryllium itself exhibits a good transmissivity to the
`X-rays. It is mentioned here that the thin film must be
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`thick when a difference of pressures is large between an
`inside and an outside of the chamber and when an area
`of the outlet is wide. Under the circumstances, the at-
`tenuation of the X-rays becomes serious due to the thin
`film with an increase of thickness of the thin film.
`In order to avoid the attenuation of the X-rays, the
`outlet might be opened at the outlet portion with the
`thin film removed. However, this makes it difficult to
`keep the chamber at a predetermined degree of vacuum
`and brings about leakage of a vapored target material.
`Such a vapored target material might be deposited onto
`an object, such as an X-ray mask, to be processed by the
`X-rays. As a result, the object might be mechanically
`destructed or degraded in its characteristics.
`In Unexamined Japanese Patent Publication No. Syé
`60-7130, namely, 7130/1985, proposal is made as re-
`gards a device wherein an object, such as a mask, a
`substrate with a resist layer, is located within a vacuum
`chamber to avoid attenuation of X-rays. However, un-
`desirable deposition of a vapored target material onto
`the object and the resultant destruction of the object
`can not be avoided in the proposed device.
`In Unexamined Japanese Patent Publication No. Syé
`58-225636, namely, 225636/ 1983, an X-ray emission
`device is disclosed which comprises a first chamber
`kept at a high degree of vacuum, a second chamber
`filled with a helium gas having a low X-ray attenuation
`factor, and an intermediate chamber between the first
`and the second chambers. X-rays are generated in the
`first chamber by bombarding a solid-state target by a
`pulsed laser beam and is guided through the intermedi-
`ate chamber to the second chamber. The second cham-
`ber is kept at a pressure substantially equal to the ambi-
`ent pressure and serves to allow the X-rays to pass
`through an outlet. With this structure, it is possible to
`make a thickness of a film on the outlet considerably
`thin and to widen an area irradiated by the X-rays.
`However, maintenance of a high degree of vacuum in
`the first chamber encounters with a difficulty because
`the first chamber which must be kept at a high degree of
`vacuum is associated with the second chamber of a high
`pressure through the intermediate chamber. In addition,
`attenuation A of the X-rays is unavoidable due to the
`helium gas filled in the second chamber, which might
`cancel an effect of making the film thin.
`In order to avoid undersirable deposition of a target
`material, a radio frequency (RF) oscillation coil is dis-
`posed outside of a chamber in Unexamined Japanese
`Patent Publication No.
`Syo
`58-40757,
`namely,
`40757/ 1983. Specifically, the RF oscillation coil is ener-
`gized with a gas of, for example, chlorine filled in the
`chamber. The gas is rendered into plasma by energiza-
`tion of the RF oscillation coil. A deposited target mate-
`rial, such as aluminum, is gasified by the plasma into a
`predetermined gas, which may become Al2C13 accord-
`ing to the description.
`In this event, it is necessary to introduce the gas into
`the chamber and to cause the plasma to occur in the
`chamber each time on deposition of the target material.
`Such an operation for removing the undesirable deposi-
`tion is very combersome and inevitably suspends a con-
`tinuos run of operation.
`Furthermore, a device is disclosed in Unexamined
`Japanese Patent Publication No. Syé
`58-158842,
`namely, 158842/1983, and uses a target formed by a
`target material such that a product which appears after
`occurrence of plasma is in a gas phase. Such a product
`
`

`
`4,866,517
`
`3
`may take place as a result of either reaction of the prod-
`uct with an atmosphere or dissolution of the product
`itself. Such a target material may be ice, an solid phase
`of ammonia, dry ice, or the like. At any rate, the target
`is in a solid-state before occurrence of the plasma. With
`this structure, it is possible to avoid deposition of the
`target material or a chamber.
`However, the target material must be replaced by
`another one after it is .wom out. In other words, it is
`difficult to continuously introduce the target material
`into the chamber and to continuously generate the X-
`rays for a long time without any interruptions. In addi-
`tion, no suggestion is made about avoiding attenuation
`of the X-rays which occurs at an outlet portion.
`SUMMARY OF THE INVENTION
`
`4
`portion formed in the evacuative chamber. The X-ray
`gating member comprises a first disk member having at
`least one first through hole, a second disk member hav-
`ing at least one second through hole which is related in
`number to the first through hole, a first axle member for
`rotatably supporting the first disk member so that the
`first through hole periodically faces the X-ray outlet
`portion, a second axle member for rotatably supporting
`the second disk member with the first and the second
`disk members partially superposed on each other to
`periodically register the second through hole with the
`first through hole, first driving means coupled to the
`first axle member for driving the first axle member to
`rotate the first disk member at a first speed of rotation,
`seconddriving means coupled to the second axle mem-
`ber for driving the second axle member to rotate the
`second disk member at a second speed of rotation which
`is selected so that a difference between the first and the
`second speeds of rotation is substantially equal to the
`predetermined repetition frequency, the first and the
`second through holes being thereby periodically
`opened relative to the X-ray outlet portion in synchro-
`nism with generation of the X-ray.
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIG. 1 is a schematic view of a conventional laser
`plasma X-ray device;
`FIG. 2 is a view of a laser plasma X-ray device ac-
`cording to a first embodiment of this invention;
`FIG. 3 is a graphical representation for use in describ-
`ing a relationship among photon energy, X-ray radia-
`tion energy, and atomic numbers so as to estimate the
`above-mentioned relationship in a target material ac-
`cording to this invention;
`_.
`FIG. 4 is another graphical representation for use in
`describing dependency of photon energy on the atomic
`number to estimate the target material according to this
`invention;
`FIG. 5 is a plan view of an X-ray gate unit used in the
`laser plasma X-ray device illustrated in FIG. 2; and
`FIG. 6 is a partial view of a laser plasma X-ray device
`according to a second embodiment of this invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS:
`
`Referring to FIG. 1, a conventional laser plasma
`X-ray device comprises a vacuum chamber 11 defining
`a hollow space and having a laser inlet portion 12 and an
`X-ray outlet portion 13. The vacuum chamber 11 is
`evacuated to a predetermined degree of vacuum
`through a valve 14 by a vacuum pump (not shown). A
`window plate 15 is fixed to the chamber 11 at the laser
`input portion 12. A pulsed laser beam 16 is generated by
`a laser source (not shown) and is incident onto the win-
`dow plate 15 through an optical system 17. The pulsed
`laser beam 16 is a succession of laser pulses.
`A solid-state target 20 of a target material, as of metal,
`is disposed within the hollow space of the chamber 11
`and is formed into a rotationally symmetric shape which
`has an axis of rotation and which may be cylindrical or
`conical. In this event, the pulsed laser beam 16 is fo-
`cused on the solid-state target 20 so that the pulsed laser
`beam 16 has a focal point on the cylindrical or conical
`surface of the solid-state target 20.
`When the solid-state target 20 is bombarded by the
`pulsed laser beam 16 in the chamber 11 kept at the
`predetermined degree of vacuum, the target material is
`rendered into plasma and, as a result, generation of
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`It is an object of this invention to provide a laser
`plasma X-ray generator which can generate X-rays for
`a long time without any interruption.
`It is another object of this invention to provide a laser
`plasma X-ray generator of the type described, which is
`capable of avoiding undesirable deposition of a target
`material on a chamber.
`It is a further object of this invention to provide a
`laser plasma X-ray generator of the type described,
`which can avoid attenuation of X-rays due to deposition
`of the target material on an outlet portion.
`It is still another object of this invention to provide a
`laser plasma X-ray generator of the type described,
`wherein it is possible to prevent undersirable leakage of 30
`X-rays.
`It is yet another object of this invention to provide a
`laser plasma X-ray generator of the type described,
`which can keep a degree of vacuum substantially invari-
`able in the chamber.
`It is another_object of this invention to provide a laser
`plasma X-ray generator of the type described, wherein
`an object to be processed can be protected from de-
`struction which might occur due to leakage of the X-
`rays.
`According to an aspect of this invention, there is
`provided a laser plasma X-ray generator which com-
`prises an evacuative chamber having a laser inlet por-
`tion and an X-ray outlet portion and defining a hollow
`space in the chamber, laser emitting means for emitting
`a pulsed laser beam of a predetermined repetition fre-
`quency into the hollow space through the laser inlet
`portion so that the pulsed laser beam converges at a
`focal point predetermined within the hollow space, and
`target material supplying means for continuously sup-
`plying a target material into the hollow space. The
`target material is in a selected one of a liquid phase and
`a solid phase that is evaporable by emission of the
`pulsed laser beam to generate X-rays. The generator
`further comprises conveying means placed within the
`hollow space for continuously conveying the target
`material through the focal point and gating means adja-
`cent to the X-ray outlet portion for gating the X-ray
`outlet portion is synchronism with the predetermined
`repetition frequency of the pulsed laser beam to intro-
`duce the X-rays outside of the hollow space.
`According to another aspect of this invention, there is
`provided an X-ray gating member or unit which is for
`use in gating X-rays which are produced by bombard-
`ing a target material in an evacuative chamber by the
`use of a succession of laser pulses each of which has a
`predetermined pulse width and a predetermined repeti-
`tion frequency and which exits through an X-ray outlet
`
`

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`4,866,517
`
`5
`X-rays takes place. The X-rays are emitted from the
`chamber 11 through a thin film of, for example, beryl-
`lium placed in the X-ray outlet portion 13.
`With this structure, it is possible to stably focus the
`pulsed laser beam 16 onto the surface of the solid-state
`target 20 and to readily keep the chamber 11 at the
`predetermined degree of vacuum. Therefore, the illus-
`trated X-ray device is practically very useful.
`However, the X-ray device has a lot of disadvantages
`as pointed out in the preamble of the present specifica-
`tion.
`_
`Referring to FIG. 2, a laser plasma X-ray device
`according to a first embodiment of this invention com-
`prises a metallic chamber 11’ defining a hollow space.
`Like in FIG. 1, the chamber 11’ has a laser inlet portion
`12 having a window plate 15 fitted to a side surface of
`the chamber 11’. It is to be noted here that an X-ray
`outlet portion 13 is opened without any thin film 22 as
`shown in FIG. 1 but serves to substantially keep the
`hollow space at a predetermined degree of vacuum, as
`will later be described. In this connection, the X-ray
`outlet portion 13 has an outlet or passage 23 formed on
`a front wall of the chamber 11' without any thin film.
`The hollow space in the chamber 11' is evacuated by
`the use of an exhausting unit 25 to a predetermined
`degree of vacuum which may be less than 10 Torr.
`A pulsed laser beam 30 is emitted from a laser beam
`source 31 and is introduced into the hollow space
`through an optical system 32 and a window plate 33 of
`the laser inlet portion 12. Preferably, the laser beam
`source 31 has an output of 10 Watts and produces the
`pulsed laser beam 30 focused by the optical system 32
`on a focal point placed in the hollow space. A combina-
`tion of the laser beam source 31 and the optical system
`32 may be called a laser emitting section for emitting the
`pulsed laser beam 30 into the hollow space of the cham-
`ber 11’.
`The pulsed laser beam 30 is composed of a succession
`of laser pulses having a predetermined wavelength,
`predetermined output power, a predetermined pulse
`width, and a predetermined repetition frequency, all of
`which are selected in consideration of photon energy of
`X-rays generated by the X-ray device. Preferably, the
`pulse width and the repetition frequency are between 10
`and 0.1 nanoseconds and between 10 and 1000 Hz, re-
`spectively. The laser beam source 31 may have an out-
`put not lower than 10 Watts. In the example being illus-
`trated, the predetermined wavelength, output power,
`pulse width, and repetition frequency are selected at
`0.53 micron meter, about 50 Watts, 0.1 nanosecond, and
`100 Hz, respectively, so as to generate the X-rays hav-
`ing the photon energy between 0.1 and 0.3 keV. In this
`event, the pulsed laser beam 30 had a spot size diameter
`between 100 and 500 micron meters and an optical in-
`tensity of 1013 Watts/cm? at the focal point.
`A target material supply unit 35 is coupled to the
`hollow space in order to supply a target material 36 into
`the hollow space. Herein, it is to be noted that the target
`material 36 is given in the form of either a liquid phase
`or a solid phase of a natural or genuine material which
`is chemically inert and which is in a gas phase at a room
`temperature. The gas phase, as herein called, may be
`whichever of the gas phase in the narrow sense and a
`vapour phase. In the illustrated example,
`the target
`material 36 is assumed to be in the liquid phase. Specifi-
`cally, the natural material may be readily gasified at a
`temperature equal to or higher than minus 50° C. and
`may have a liquefying point and a solidifying point both
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`of which are lower than minus 50° C. According to the
`inventors’ experimental studies, it has been found out
`that the natural material may be, for example, krypton,
`xenon, argon, or the like. Anyway, the natural material
`may be selected from a‘ group consisting of rare gases
`and inert gases as will become clear as the description
`proceeds.
`The illustrated target material supply unit 35 is also
`coupled to the exhausting unit 25 to recycle the target
`material in a manner to be described later in detail.
`
`The target material supply unit 35 comprises a reser-
`voir 38 placed outside of the chamber 11’ to stock and
`comprises the target material 36 given through an in-
`coming port 39 and a conduit 40 extended through
`another side wall of the chamber 11’ into the hollow
`space. A cooling unit 41 is coupled to the reservoir 38 to
`cool or refrigerate the target material 36 to a tempera-
`ture less than the liquefying temperature of the target
`material 36. The illustrated cooling unit 41 serves to
`recover the target material exhausted from the chamber
`11’ through the exhausting unit 25. In this connection,
`the target material supplied through the incoming port
`35b may be in the gas phase.
`The target material 36 of the liquid phase is con-
`ducted through the conduit 40 into an open end of the
`conduit 40 and dropped through the open end onto a
`conveyor unit 45 located in the hollow space.
`The conveyor unit 45 is operable to continuously
`convey the target material 36 to the focal point of the
`pulsed laser beam 30. The conveyor unit 45 comprises
`an endless belt 46 supported and carried on a pair of
`rolls 47 and 48 spaced apart from each other. Either one
`of the rolls 47 and 48 is driven or rotated by a motor
`(not shown) at a predetermined speed in a direction
`illustrated by an arrowhead while the other acts as an
`idler roll. Thus, the rolls 47 and 48 serve as a driving
`member for driving the endless belt 46. The endless belt
`46 and the rolls 47 and 48 are positioned in the hollow
`space so that the endless belt 46 passes through the focal
`point of the pulsed laser beam 30.
`When the target material 36 of the liquid phase is
`dropped on the endless belt 46 and is successively solidi-
`fied or frozen on the endless belt 46 into a mass of the
`target material which is adhered to the endless belt 46.
`The mass of the target material is cooled as mentioned
`above and may therefore be referred to as a cryo-target.
`In any event, the mass of the target material is succes-
`sively conveyed to the focal point to be subjected to
`emission or bombardment of the pulsed laser beam 30.
`Thus, the endless belt 46 is operable to continuously
`transfer themass of the target material to the focal
`point.
`When the mass of the target material, namely, the
`cryo-target is bombarded by the pulsed laser beam 30 at
`the focal point, it is found out that plasma appears from
`the cryo-target in the hollow space and brings about
`occurrence of X-rays which have photon energy avail-
`able for the X-ray lithography. In particular, it has been
`found out that the photon energy of the X-rays falls
`within a range which is between 1.0 and 3.0 keV and
`which is optimum for the X-ray lithography, when
`kryton, xenon, or argon is used as the target material.
`The X-rays are guided through the outlet 23 and a
`gating unit 50 which will later be described in detail.
`Temporarily referring to FIGS. 3 and 4, consider-
`ation will be made as regards X-rays generated from the
`above-mentioned cryo-target. In FIG. 3, a three-dimen-
`sional coordinate is illustrated which has an x-axis rep-
`
`

`
`7
`resentative of photon energy (keV) of the X-rays, a
`y-axis representative of X-ray radiation energy (Jou-
`le/Sr.keV EL), and a z-axis representative of the atomic
`number, where Sr represents a solid angle of the X-rays
`in steradian, and EL represents laser energy. The X-ray
`radiation energy may be regarded as conversion effi-
`ciency between a pulsed laser beam and the X-rays
`while the photon energy may be regarded as an X-ray
`spectrum.
`The X-ray radiation energy is measured about vari-
`ous kinds of materials, such as carbon (C), aluminum
`(Al), titanium (Ti), copper (Cu), germanium (Ge), neo-
`dymium (Nb), molybdenum (Mo), silver (AS). tin (Sn),
`gadolinium (Gd), tantalum (Ta), and gold (Au) which
`have the atomic numbers 6, 13, 22, 29, 32, 41, 42, 49, 50,
`64, 73, and 79, respectively.
`As shown in FIG. 3, the exemplified materials exhibit
`maximum X-ray radiation energy» namely, maximum
`conversion efficiency within a range of the photon en-
`ergy between 0.1 and 3 keV. For example, the carbon
`exhibits the maximum conversion efficiency within the
`range of the photon energy between 0.1 and 0.5 keV
`while the gadolinium has the maximum conversion
`efficiency within another range of the photon energy
`between 1.3 and 2.0 keV.
`In FIG. 4, illustration is made about a relationship
`between the photon energy (keV) taken along an ab-
`scissa and a square of the atomic number taken along the
`z-axis. The enumerated materials have high conversion
`efficiency zones, as shown by solid lines in FIG. 4. For
`example, the high conversion efficiency zones of alumi-
`num, germanium, tin, and gadolinium resides between
`1.5 and 2.0 keV, between 1.4 and 2.1 keV, between 0.6
`and 1.1 keV, and between 0.2 and 0.5 keV, respectively.
`In addition, the high conversion efficiency zones are
`classified and_can be supposed in consideration of elec-
`tron orbits of the atom. More particularly, classification
`is possible by considering groups which have K-shell,
`L-shell, M-shell, and N-shell of the electron orbits and
`each of which has the high conversion efficiency zone
`distributed along a broken line determined for each
`shell. Such high conversion efficiency zones become
`high in photon energy with an increase of the square of
`the atomic number, as known by Moseley’s law.
`Under the circumstances,
`it
`is readily possible to
`determine high conversion efficiency zones of argon,
`krypton, and xenon, as indicated in FIG. 4. Accord-
`ingly, the target material can be selected with reference
`to high conversion efficiency zones necessary for the
`X-ray lithography and X-ray microscopy.
`Referring back to FIG. 2, the X-rays 50 which are
`generated in the hollow space in the above-mentioned
`manner are guided through the outlet 23 of the chamber
`11’ outside of the chamber 11’. On generation of the
`X-rays, the cryo-target is returned back to the natural
`material, namely, gas phase because the cryo-target is
`heated by bombardment of the pulsed 1ase_r beam 30 to
`a temperature higher than the liquefying temperature of
`the target material. The natural material is evacuated by
`the exhausting unit 25 to be sent back to the cooling unit
`41 and to be compressed within the cooling unit 41. As
`a result of compression, the natural material of the gas
`phase is changed to the target material of the liquid
`phase. Thus, the target material 36 is successively recy-
`cled through the exhausting unit 25 and the cooling unit
`41 into the reservoir 38. Therefore,
`it is possible to
`continuously operate the X-ray device by only compen-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4,866,517
`
`8
`sating the target material of the gas phase through the
`incoming port 39.
`In addition, the target material 36 is in the gas phase
`at the room temperature and is therefore never depos-
`ited on a wall of the chamber 11’. From this fact, it is
`seen that no contamination occurs on the wall of the
`chamber 11’ and that a thin film of, for example, beryl-
`lium may be therefore attached to the outlet 23.
`However, attenuation of the X-rays 50 inevitably
`appears by the use of the thin film, as pointed out in the
`preamble of the instant specification.
`Referring to FIG. 5 afresh and FIG. 2 again, an X-ray
`gate unit 51 is placed outside of the chamber 11’ in the
`vicinity of the outlet 23 formed in the chamber 11’, with
`a gap left between the chamber 11’ and the X-ray gate
`unit 51. The gap may be, for example, 1 mm long. The
`X-ray gate unit 51 is intermittently opened in synchro-
`nism with the pulsed laser beam 30 to expose the outlet
`23 to the atmosphere. As a result, the X-ray gate unit 51
`serves to make the X-rays 50 exit from the chamber 11’
`and may be called an X-ray exit member.
`The illustrated X-ray gate unit 51 is partially accom-
`modated in an additional chamber 53 attached to the
`chamber 11’ and comprises first and second disk mem-
`bers 56 and 57 both of which are partially superposed
`on each other, as illustrated in FIG. 5 and which are
`rotatable around axes of rotation. The first and the sec-
`ond disk members 56 and 57 juxtaposed with each other
`with a slight interval left therebetween and have first
`and second through holes 58 and 59 which are spaced
`apart from the axes of rotation and each of which is, for
`example, 1 cm2 in area. Distances between the axes of
`rotation and the through holes 58 and 59 may be be-
`tween 5 cm and 10 cm.
`_ .
`The first and the second disk members 56 and 57 are
`rotatably supported at the axes of rotation by first and
`second axle members 61 and 62, respectively. The first
`and the second axle members 61 and 62 are located in
`parallel with a spacing left between the first and the
`second axle members 58 and 59. Such a spacing is se-
`lected so that the first and the second through holes 58
`and 59 are registered with each other when the first disk
`member 56 is superposed on the second disk member 57.
`Consequently, the first and the second through holes 58
`and 59 are periodically aligned with the outlet 23, as
`shown in FIGS. 2 and 5, when the first and the second
`disk members 56 and 57 are rotated as indicated at ar-
`rowheads in FIG. 5.
`The first and the second axle members 56 and 57 are
`driven by first and second motors 63 and 64 so as to
`rotate the first and the second disk members 56 and 57
`at first and second speeds of rotation, respectively. The
`first and the second speeds of rotation may be regarded
`as first and second rotational frequencies fl and f2,
`respectively,
`in consideration of the repetition fre-
`quency of the pulsed laser beam 30.
`In the example being illustrated, it is assumed that the
`first and the second rotational frequencies fl and f2 are
`equal to 600 Hz and 500 Hz, respectively, when the
`pulsed laser beam 30 has the repetition frequency of 100
`Hz. In other words, the first and the second rotational
`frequencies fl and f2 are decided so that an absolute
`value of a difference V between the first and the second
`rotational frequencies fl and f2 becomes equal to the
`repetition frequency of the pulsed laser beam 30. In
`order to rotate the first and the second disk members 56
`and 57 at the first and the second rotational frequencies
`fl and f2 as mentioned above, the first and the second
`
`

`
`9
`motors 63 and 64 are controlled by a controller 65 and
`the first and the second speeds of rotation are monitored
`by first and second rotation detectors 66 and 67, respec-
`tively.
`'
`Accordingly, the first and the second disk members
`56 and 57 are automatically controlled by the use of the
`controller 65 and the first and the second rotation detec-
`tors 66 and 67 so that the difference V between the
`rotational frequencies fl and f2 becomes equal to the
`repetition frequency of the pulsed laser beam 30. As a
`result, the first and the second through holes 58 and 59
`are aligned with the outlet 23 in synchronism with rota-
`tion of the first and the second disk members 56 and 57
`and are opened relative to the outlet 23. Thus, the first
`and the second through holes 58 and 59 are periodically
`superposed on the outlet 23.
`Under the circumstances, it is possible to determine
`an opening time of the X-ray gate unit 51 by selecting
`the first and the second rotational frequencies fl and t2,
`the sizes of the first and the second through holes 58 and
`59, and the distances between the axes of rotation and
`the first and the second through holes 58 and 59. Specif-
`ically, it is assumed that the first and the second through
`holes 58 and 59 have the areas of 1 cm2 and are spaced
`apart from the axes of rotation by the distances between
`5 cm and 10 cm and that the first and the second rota-
`tional frequencies fl and f2 are equal to 600 Hz and 500
`Hz, respectively. In this event, the opening time be-
`comes 64 microseconds.
`In the illustrated example, the laser pulses of the
`pulsed laser beam 30 have the repetition frequency of
`100 Hz, namely, a repetition period of 10 milliseconds,
`and the pulse width of 10 nanoseconds. Therefore, it is
`readily understood that the above-mentioned opening
`time is very shorter than the repetition period of the
`laser pulses and is remarkably longer than the pulse
`width of the laser pulses.
`Inasmuch as generation of the X-rays is synchronized
`with each of the laser pulses, the X-rays are passed
`through the X-ray gate unit 51 during the opening time
`of the X-ray gate unit 51.
`The gaps between the chamber 11’ and the first disk
`member 56 and between the first and the second disk
`members 56 and 57 may be between 1 and 2 millimeters.
`Since only extremely short gaps are left between the
`chamber 11' and the first disk member 56 and be