(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0068012 A1
`Ahmad et al.
`(43) Pub. Date:
`Apr. 10, 2003
`
`US 20030068012A1
`
`(54) ARRANGEMENT FOR GENERATING
`EXTREME ULTRAVIOLET (EUV)
`RADIATION BASED ON A GAS DISCHARGE
`
`(57)
`
`ABSTRACT
`
`(75)
`
`Inventors:
`
`Imtiaz Ahmad, Goettingen (DE);
`Guido Schriever, Goettingen (DE);
`Juergen Kleinschmidt, Weissenfels
`(DE)
`
`Correspondence Address:
`Gerald H. Kiel, Esq.
`REED SMITH LLP
`
`599 Lexington Avenue
`New York, NY 10022-7650 (US)
`
`(73) Assignee: XTREME technologies GmbH;
`
`(21) Appl. No.:
`
`10/267,373
`
`(22)
`
`Filed:
`
`Oct. 9, 2002
`
`(30)
`
`Foreign Application Priority Data
`
`Oct. 10, 2001
`
`(DE) ................................... .. 101 51 080.2
`
`Publication Classification
`
`Int. Cl.7 ..................................................... ..H05H 1/00
`(51)
`(52) U.S. Cl.
`............................................................ .. 378/119
`
`The invention is directed to a method and an arrangement for
`generating extreme ultraviolet (EUV) radiation, i.e., radia-
`tion of high-energy photons in the wavelength range from 11
`to 14 nm, based on a gas discharge. The object of the
`invention, to find a novel possibility for generating EUV
`radiation in which an extended life of the system is achieved
`with stable generation of a dense, hot plasma column, is met
`according to the invention in that a preionization discharge
`is ignited between two parallel disk-shaped flat electrodes
`prior to the main discharge by a surface discharge along the
`superficies surface of a cylindrical insulator with a plasma
`column generated through the gas discharge with pulsed
`direct voltage, which preionization discharge carries out an
`ionization of the working gas in the discharge chamber by
`means of fast charged particles. The preionization discharge
`is triggered within a first electrode housing and the main
`discharge takes place between a narrowed output of the first
`electrode housing and a part of the second electrode housing
`close to the outlet opening of the discharge chamber. The
`plasma develops in a part of the second electrode housing
`covered by a tubular insulator and, as a result of the
`current-induced magnetic field, contracts to form a dense,
`hot plasma column, one end of which is located in the
`vicinity of the outlet opening of the second electrode hous-
`1ng.
`
`ASML 1027
`ASIVIL 1027
`
`to the first
`cotlecting optical
`system
`
`'
`
`9
`
`1 ;
`
`72 E
`
`II
`
`

`
`Patent Application Publication Apr. 10, 2003 Sheet 1 of 5
`
`US 2003/0068012 A1
`
`\1t._m
`
`.\
`
`000061
`
`..
`
`xx\
`
`Wa4Wu.i.«%&§_W.1Ax\\N,\
`
`VZ_
`
`M///mifizzzz/r/4u
`_,,\,.
`
`.,,////142
`
`733
`
`21:
`
`FIJIIIL
`
`72"";
`
`
`
`._r../..r/i,wr;l....I...;.l...-.m.M
`
`i
`
`/,..
`
`.»,/
`
`._”.MI/W /
`
`4|.24!2324|3PO554|.1:Al.47222\,,_.7.,,Ri...\\..
`W,/.m5/mmm/rtV4F”ge
`./,....,J...,...\.,i.,,_...i../W,,,/..,_._\_.-M.N\
`////ittt SWV//////////4m\\\
`
`Fig. 1
`
`
`
`

`
`Patent Application Publication Apr. 10, 2003 Sheet 2 of 5
`
`US 2003/0068012 A1
`
`8‘
`
`'\
`
`815 53 545152
`/
`,/
`
`5
`
`/
`
`
`16 3 » 12
`ww‘ wzllf
`‘\“‘-§' Q I 1
`ii“ v::_,__‘
`“
`
`14
`W X 1 5
`4 6
`
`““~~\7
`
`
`
`
`
`g
`
`3
`4
`
`my
`’/////zfdé
`
`/i
`.
`/
`afl///////A
`Z
`26 "X ,:r////f
`af/J/to
`VIII ’
`1:; 1’; V’ I
`(/1291;
`/I‘
`1/
`;:j I \ é¢,,,m M
`24
`71 W///////»'\i-j,;_*J1_\
`I
`_»,,.i..i,.»»'/“Q:
`\‘‘~\.\,I
`................_._..__._...
`32 ~
`/
`\
`‘“
`31
`to the first \
`9 [I 1
`system
`
`_,..r—*
`
`/".»’
`
`,./’
`
`/
`/
`
`/i
`
`’
`
`collecting optical
`
`‘I
`
`\ “‘\\
`
`~21
`“Z2
`
`-
`
`

`
`Patent Application Publication Apr. 10, 2003 Sheet 3 of 5
`
`US 2003/0068012 A1
`
`9?
`8j553§45152
`_..\S...
`....i.
`'I
`,.,/ / //
`61 \ - x 3“; I
`
`
`I“
`
`f “ 1,: 1 /'
`
`4/
`
`I
`
`6
`
`/
`
`‘
`
`3
`
`r////./
`
`9111/
`
`3 '
`I
`7-Y.....[
`
`1 1 1
`
`«,(i,\
`
`\\
`
`16
`
`3
`
`4
`
`x1""
`‘\\\
`
`III
`
`1
`
`Fig. 3
`
`to the firsn \ :
`CONECUHQ optxcai
`E
`system
`«TI
`8/’: 27» ”
`17
`
`532
`
`
`
`'
`
`. 9
`
`72
`
`\\\
`
`\\\\\\\\\\\\V§\=\
`
`\\y\\\\\w&“J
`
`1
`> N
`‘ ‘xv/1.’! /// ,
`/*g i1
`\\
`O
`\\\‘\\\
`xx\&\
`§\\\
`!,_'_Tfii\/g
`
`¢,,,,,;
`//lm, Cathode” ,26
`C?“ if;
`\
`‘
`
`/,..-»
`
`-»
`
`iiiiii~~21
`-22
`
`\\
`
`\
`\\\
`
`
`
`/
`7/1
`/2
`I, «.&
`
`_ ._
`
`’"
`
`.
`
`
`
`

`
`Patent Application Publication Apr. 10, 2003 Sheet 4 of 5
`
`US 2003/0068012 A1
`
`ommiEmE:.
`
`
`
`Va ug iueuno afiieqosgg
`
`ommgEmEF
`
`
`
`M u; a5eJ,|o/\ uonezguogmd
`
`

`
`Patent Application Publication Apr. 10, 2003 Sheet 5 of 5
`
`US 2003/0068012 A1
`
`.
`I
`:
`»
`.
`3
`i
`1
`E
`j,
`1
`O ~wF,./\.,,....---g.,;....-r-..._,_\‘ ‘
`.
`5
`~,‘
`
`E
`‘
`
`I
`
`—
`
`r
`Q
`I
`3
`5
`. AV 9 e.
`>
`
`V
`
`5
`
`i
`»
`:
`‘.
`~
`
`~
`
`‘A
`
`,
`
`:
`:
`2
`;
`
`‘
`
`'
`2
`:
`‘
`3’
`
`E
`«
`
`4
`Z
`2
`
`E
`-
`1
`;
`.
`,.~
`2
`I
`.4"
`‘
`'
`i / \., ‘
`.
`=
`.
`;
`tr
`..
`,
`»n «~
`I
`..
`.,,.,“*;~*—-'-3*‘-..a«~..s-......»_~,..:,......a«......;
`.
`2
`)
`I
`.
`Z
`
`'
`‘*'"”“"i.~““*
`0.8
`
`i l i
`
`0.6
`
`"3
`
`0.0
`
`‘i
`
`“’
`
`if
`
`0.2
`
`;
`’:
`'““i’"‘""*“‘f"" *
`0.4
`
`L
`i
`
`'
`
`Time in psec
`
`.
`
`1
`1
`
`1
`3
`
`E
`
`1.0
`
`
`
`
`
`DischargevoitageinKV
`
`
`
`
`
`EUVsignaiinarbitraryunits
`
`2
`
`7
`‘
`
`“I
`
`—'—‘-\_—;,.»i—'u ~/—-2.-.2:/"-——-—-~ ‘
`‘
`‘
`
`iL
`
`i i i
`
`4 V
`1
`
`}
`
`,
`:
`
`i
`
`i
`
`” §’
`
`‘
`
`'
`
`2
`
`* ~»-
`
`
`
`1
`
`i
`
`.
`
`i
`
`i
`
`S
`’
`
`1
`
`I
`
`0.4
`
`0.5
`
`0.3
`
`1.0
`
`Time in p.SeC
`
`W:
`
`-5.
`
`.3 C:
`
`Q5 C
`
`3
`
`;
`g
`3
`
`2
`
`¥
`
`«
`
`/:
`
`i
`i
`
`P.._iN?
`
`" T“ ‘ ""'””;' " ' M“ j
`I
`.
`,
`§
`1
`i
`E
`0 — /\Vx’\f‘\r~.”§:fi'\‘\,'<7'1‘-v—\%‘;~«%W\ ~ -
`V‘ /_,«""“\';‘\‘;‘/~\;;f/5/\"\y
`e
`E
`3
`M,
`/~ \.
`g
`:
`i
`
`Fig. 5
`
`

`
`US 2003/0068012 A1
`
`Apr. 10, 2003
`
`ARRANGEMENT FOR GENERATING EXTREME
`ULTRAVIOLET (EUV) RADIATION BASED ON A
`GAS DISCHARGE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] This application claims priority of German Appli-
`cation No. 101 51 080.2, filed Oct. 10, 2001, the complete
`disclosure of which is hereby incorporated by reference.
`
`BACKGROUND OF THE INVENTION
`
`[0002]
`
`a) Field of the Invention
`
`[0003] The invention is directed to an arrangement for
`generating extreme ultraviolet (EUV) radiation based on a
`gas discharge, i.e., radiation of high-energy photons in the
`wavelength range from 11 to 14 nm (EUV=extreme ultra-
`violet range).
`
`[0004]
`
`b) Description of the Related Art
`
`integrated circuits on chips
`structures of
`[0005] As
`become increasingly smaller in the future, radiation of
`increasingly shorter wavelength will be needed in the semi-
`conductor industry to expose these structures. Lithography
`machines with excimer lasers which attain their shortest
`
`wavelength at 157 nm and in which transmission optics or
`catadioptic systems are employed are currently in use.
`
`[0006] Therefore, radiation sources which further increase
`resolution with even shorter wavelengths for imaging will
`have to be available in the future (around the year 2007).
`However,
`the optical systems must comprise reflection
`optics at wavelengths below 157 nm because there are no
`available materials which are transparent for these wave-
`lengths. When using reflection optics, the numerical aperture
`is limited and the diversity of optical elements is restricted.
`The lower numerical aperture of the optics results in
`decreased resolution of the system which can only be
`compensated by an even shorter wavelength (by about an
`order of magnitude).
`
`In principle, both laser-induced plasmas and gas
`[0007]
`discharge plasmas are suited for generating EUV radiation.
`Laser-induced plasma requires an energy conversion in two
`stages: from electrical energy to laser radiation energy and
`from laser energy to EUV radiation energy. This twofold
`conversion results in reduced conversion efficiency com-
`pared to gas discharge.
`
`[0008] With respect to gas discharges, different concepts
`are pursued in plasma focus devices, capillary discharge
`devices, hollow cathode discharge devices and Z-pinch
`devices.
`
`[0009] Compared to the other concepts, the plasma focus
`method has the disadvantage of poor spatial stability
`because of the formation of plasma. In this connection, U.S.
`Pat. No. 5,763,930 suggests a variant using a noble gas with
`lithium as working gas. However, this leads to additional
`contamination of the surroundings, particularly the insulator.
`
`[0010] Another competing concept, capillary discharge,
`has only a short life and, consequently, limited applicability.
`
`[0011] The Z-pinch concept exhibits good characteristics
`compared to other gas discharge concepts and laser-induced
`plasmas. In a published technical solution according to U.S.
`
`Pat. No. 5,504,795, preionization by means of R-F (radio-
`frequency) discharge is realized in an insulator tube in which
`the plasma is likewise ignited subsequently. This high-
`frequency preionization system is directly coupled to the
`discharge system and is therefore exposed to plasma radia-
`tion and to bombardment by charged particles resulting in a
`shorter life of the insulator tube in particular.
`
`OBJECT AND SUMMARY OF THE INVENTION
`
`It is the primary object of the invention to find a
`[0012]
`novel possibility for generating EUV radiation in which an
`extended life of the system is achieved with stable genera-
`tion of a dense, hot plasma column.
`
`[0013] According to the invention, in an arrangement for
`generating extreme ultraviolet (EUV) radiation based on a
`gas discharge with a discharge chamber which is enclosed
`by a first electrode housing and a second electrode housing
`and through which a working gas flows under a determined
`pressure, the two electrode housings being arranged coaxial
`to one another and having cylindrical superficies surfaces
`which face the discharge chamber and which are isolated
`from one another by an insulator layer so as to resist
`puncture or breakthrough, and an outlet opening for the
`EUV radiation which is provided axially in the second
`electrode housing, the above-stated object is met in that a
`coaxially arranged preionization unit having parallel, sub-
`stantially flat electrodes at a distance from one another
`axially is provided in the interior of the first electrode
`housing, wherein the flat electrodes are substantially circular
`and a cylindrical insulator in which at least one electric line
`is inserted is arranged between the flat electrodes, so that a
`sliding discharge is generated along the superficies surface
`of the cylindrical insulator when a sufficiently high voltage
`is applied to the flat electrodes, in that the first electrode
`housing has a narrowed output in the direction of the second
`electrode housing, and in that the cylindrical superficies
`surface of the second electrode housing is covered by a
`tubular insulator at least in the immediate vicinity of the
`narrowed output of the first electrode housing.
`
`[0014] An end face of the first electrode housing is advan-
`tageously provided as one of the flat electrodes of the
`preionization unit. The preionization unit with the cylindri-
`cal insulator is inserted into the rear end face of the first
`
`electrode housing and the line for the other flat electrode is
`guided into the interior of the cylindrical insulator.
`
`[0015] The flat electrodes of the preionization unit are
`connected to a preionization pulse generator which advis-
`ably generates high-voltage pulses with short rise times.
`
`[0016] The line for the other flat electrode of the preion-
`ization unit is preferably constructed as a metal tube which
`is provided at the same time as a flow tube for the working
`gas. The tube can be used at the same time as a leadthrough
`for arranging a radiation detector for measuring the EUV
`radiation which is radiated back by the plasma column.
`
`[0017] The tube of the preionization unit can advanta-
`geously communicate with a regulated gas supply system as
`a gas inlet for the working gas. Avacuum system connected
`to the outlet opening for the EUV radiation is provided as
`gas outlet.
`
`the tube of the
`In another construction variant,
`[0018]
`preionization unit is connected to a vacuum system as a gas
`
`

`
`US 2003/0068012 A1
`
`Apr. 10, 2003
`
`outlet for the working gas, wherein gas inlets communicat-
`ing with a regulated gas supply system are provided in the
`second electrode housing for supplying gas. The gas inlets
`are advisably arranged so as to be evenly distributed in a
`plane about the axis of symmetry of the discharge chamber.
`The gas inlets can be inserted in an end face of the second
`electrode housing comprising the outlet opening or in the
`cylindrical superficies surface of the second electrode hous-
`ing. In both cases, the gas inlets are introduced radially in the
`discharge chamber so that the working gas flows into the
`second electrode housing as uniformly as possible.
`
`[0019] The working gas is preferably a noble gas such as
`xenon, krypton, argon or neon. However, oxygen, nitrogen
`or lithium vapor can also be used. Also, to enhance conver-
`sion, gas mixtures of xenon or helium with added hydrogen
`or deuterium can advantageously be used, or, when using
`lithium vapor, helium or neon can advantageously be used as
`added gas.
`
`[0020] The cylindrical insulator of the preionization unit is
`preferably produced from a material with a high dielectric
`constant, preferably lead zirconium titanate (PZT),
`lead
`borsilicate or lead zinc borsilicate. It is advisably manufac-
`tured in such a way that it has channels through which a
`coolant can flow.
`
`In order to achieve a reliable insulation of the
`[0021]
`output of the first electrode housing relative to the superfi-
`cies surface of the second electrode housing, the tubular
`insulator in the second electrode housing is advantageously
`extended into the first electrode housing,
`the narrowed
`output of the first electrode housing projecting into the
`interior of the tubular insulator. The tubular insulator advis-
`
`ably comprises a highly insulating ceramic, particularly
`Si3N4, A1203, AlZr, AlTi, BeO, SiC or sapphire.
`
`[0022] The tubular insulator preferably completely covers
`the cylindrical superficies surface of the second electrode
`housing.
`
`In order to generate the gas discharge (main dis-
`[0023]
`charge), the first electrode housing is advisably connected to
`a high-voltage pulse generator as cathode and the second
`electrode housing is preferably connected to a high-voltage
`pulse generator as anode. In another advantageous construc-
`tion, the first electrode housing is connected as anode and
`the second electrode housing is connected as cathode.
`
`[0024] The pulse generator is advisably operated by a
`thyratron circuit which contains a single-stage or multistage
`compression module with magnetically saturable cores.
`Alternatively, it can also be constructed exclusively from
`semiconductor components.
`
`[0025] The pulse generator is advantageously adjustable
`to a repetition frequency in the range of 1 Hz to 20 kHz and
`to a voltage which is sufficient for igniting the gas discharge
`and generating a plasma column with high density and high
`temperature.
`
`[0026] Because of the high current load and thermal stress,
`the electrode housings are advisably made from materials
`with high proportions of tungsten, tantalum or molybdenum,
`at least in the area of the outputs. Tungsten-copper alloys,
`particularly 90% W and 10% Cu or 80% W and 20% Cu
`(B3C) are preferably used.
`
`[0027] As another step for reducing wear, the electrode
`housings have cavities which communicate with a coolant
`reservoir via oppositely located connections. Additional
`cooling fins can be provided in the cavities for increasing the
`inner surface for heat transfer.
`
`In a method for generating extreme ultraviolet
`[0028]
`(EUV) radiation based on a gas discharge in which a main
`discharge is triggered by direct voltage pulses in a substan-
`tially cylindrical discharge chamber which is enclosed by a
`first electrode housing and a coaxial second electrode hous-
`ing and through which a working gas flows under a defined
`pressure, wherein the main discharge is supported by means
`of preionization, and a plasma column resulting from the
`main discharge along the axis of symmetry of the discharge
`chamber emits the EUV radiation through an outlet opening
`of the discharge chamber, the above-stated object according
`to the invention is met in general in that prior to the main
`discharge a preionization discharge is ignited between two
`parallel disk-shaped flat electrodes by means of a surface
`discharge along the superficies surface of a cylindrical
`insulator, which preionization discharge,
`in addition to a
`radiation emission in the wavelength range of ultraviolet to
`x-ray radiation, generates fast charged particles which cause
`an ionization of the working gas in the discharge chamber,
`in that the preionization discharge is triggered within a first
`electrode housing, and in that the main discharge takes place
`between a narrowed output of the first electrode housing and
`a portion of a second electrode housing near the outlet
`opening of the discharge chamber, wherein the plasma
`causes a progressing ionization of the working gas in one of
`the two electrode housings.
`
`[0029] By means of the arrangement according to the
`invention and the method implemented by means of this
`arrangement, it is possible to generate an EUV radiation in
`the range of 11 to 14 nm with reproducible generation of a
`dense, hot plasma column and an extended system life.
`
`[0030] The invention will be described more fully in the
`following with reference to embodiment examples.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0031]
`
`In the drawings:
`
`[0032] FIG. 1 shows a schematic view of the arrangement
`according to the invention;
`
`[0033] FIG. 2 shows a construction variant of the inven-
`tion with an EUV radiation outlet from the anode;
`
`[0034] FIG. 3 shows another construction variant of the
`invention similar to FIG. 2, but with radiation outlet from
`the cathode and opposite flow direction of the working gas;
`
`[0035] FIG. 4 shows a voltage-time diagram of the
`preionization pulse generator and a current-time diagram of
`the high-voltage pulse generator; and
`
`[0036] FIG. 5 shows diagrams of the discharge voltage
`and EUV radiation.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`the basic arrangement
`[0037] As is shown in FIG. 1,
`according to the invention comprises a first electrode hous-
`ing 1 and a second electrode housing 2 which together form
`
`

`
`US 2003/0068012 A1
`
`Apr. 10, 2003
`
`a discharge chamber 3, the two electrode housings 1 and 2
`being insulated relative to one another against breakthrough
`by an insulating layer 4 and a tubular insulator 22 in the
`interior of the second electrode housing 2, a preionization
`unit 5 which is arranged coaxially inside the first electrode
`housing 1 and communicates with the preionization pulse
`generator 6, a high-voltage pulse generator 7 to which the
`two electrode housings 1 and 2 are connected, a gas supply
`system 8 for feeding working gas into the discharge chamber
`3 so as to be regulated in a defined manner, and a vacuum
`system 9.
`
`[0038] The two electrode housings 1 and 2 are arranged
`coaxially one over the other and have inner cylinder super-
`ficies surfaces 11 and 21 which define the discharge chamber
`3 radially around the axis of symmetry 31. The first electrode
`housing 1 has a narrowed output 12 in the direction of the
`second electrode housing 2 and has a plane rear end face 13
`at which the preionization unit 5 projects coaxially into the
`interior.
`
`[0039] At its cylindrical superficies surface 21, the second
`electrode housing 2 is covered toward the discharge cham-
`ber 3 by the tubular insulator 22 which, together with the
`insulation layer 4 which is arranged along the surface in
`lateral direction to the axis of symmetry 31 of the discharge
`chamber 3, electrically isolates the electrode housings 1 and
`2 from one another. This prevents an electric discharge
`between the first electrode housing 1 and the adjoining parts
`(including essential parts of the cylindrical superficies sur-
`face 21) of the second electrode housing 2, and the discharge
`takes place in a defined manner in the interior of the tubular
`insulator 22 between the narrowed output 12 of the first
`discharge chamber 1 and the end (not insulated) of the
`second electrode housing 2.
`
`[0040] Because of its narrowed output 12, the first elec-
`trode housing 1 has a relatively small opening toward the
`second electrode housing 2. In this way, a separate room in
`which preionization takes place is formed in the first elec-
`trode housing 1. The preionization unit 5 contains a cylin-
`drical insulator of highly insulating ceramic (hereinafter
`referred to as ceramic cylinder 51) which is guided coaxially
`into the interior of the first electrode housing 1 through the
`rear end face 13, and coaxial disk-shaped flat electrodes 52
`which are arranged concentrically outside the ceramic cyl-
`inder 51 on the one hand and on its end face in the interior
`
`of the first electrode housing 1 on the other hand. The
`electric connection of the flat electrodes 52 to a preioniza-
`tion pulse generator 6 is carried out
`inside the ceramic
`cylinder 51.
`
`[0041] Aworking gas which is admitted by a regulated gas
`supply system 8 under defined pressure flows through the
`discharge chamber 3, wherein a vacuum (in the range of 1
`to 20 Pa) is realized in the entire discharge chamber 3 by
`means of an oppositely connected vacuum system 9.
`
`[0042] When the preionization pulse generator 6 sends a
`sufficient voltage pulse to the flat electrodes 52, a sliding
`discharge 61 forms along the surface of the ceramic cylinder
`51. In addition to radiation in the range of ultraviolet to
`x-ray, this sliding discharge 61 generates fast charged par-
`ticles resulting in a progressing ionization of the working
`gas in the entire discharge chamber. The main discharge is
`then ignited by means of the high-voltage pulse generator 7
`via the first and second electrode housings 1 and 2 and leads
`
`to the formation of gas discharges and cylindrical plasma
`between the narrowed output 12 of the first electrode hous-
`ing 1 and the front side 23 of the second electrode housing
`2. The considerable flow of current generates a tangential
`magnetic field of a magnitude such that the plasma contracts
`on the axis of symmetry 31 of the discharge chamber 3 so
`that there is formed in the second electrode housing 2 a
`dense, hot plasma column 71 whose emitted EUV emission
`72 exits through the outlet opening 32 of the discharge
`chamber 3 located in the end face 23 of the second electrode
`
`housing 2 and is focused by a first collecting optical system
`(not shown).
`[0043]
`In FIG. 2, the preionization unit 5 is simplified in
`that one of the flat electrodes 52 is combined with the end
`
`face 13 of the first electrode housing 1. The construction of
`the preionization unit 5 is accordingly simplified in that only
`one (central)
`line is required for the flat electrode 52
`remaining at the end of the ceramic cylinder 51. In this case,
`the latter can be a metallic tube 53 which is provided at the
`same time as a gas inlet 81 for the flow of working gas
`through the discharge chamber 3. The metallic tube 53 is
`accordingly connected to the preionization pulse generator 6
`which communicates with the first electrode housing 1. In
`this example, the first electrode housing 1 functions as the
`cathode and the second electrode housing 2 is the anode. The
`electrode housings 1 and 2 are connected to the high-voltage
`pulse generator 7 which can supply electric pulses with
`repetition frequencies between 1 Hz and 20 kHz. For pho-
`tolithographic purposes in the semiconductor industry, rep-
`etition frequencies between 4 and 8 kHz must be set in order
`to achieve sufficient exposure per surface and time and low
`intensity variations.
`[0044]
`In the example according to FIG. 2, the working
`gas is introduced through the metallic tube 53 of the preion-
`ization unit 5. The pressure of the working gas is held
`constant via the gas supply system 8 which leads to an
`optimal gas flow in the discharge chamber 3. Apreionization
`pulse is applied between the first electrode housing 1 (which
`is also the cathode for the main discharge) and the flat
`electrode 52. For this purpose, the disk-shaped flat electrode
`52 is electrically connected with the preionization pulse
`generator 6 via the tube 53. The preionization pulse genera-
`tor 6 generates electric pulses with a typical rise time of 1011
`V/s and voltages which are sufficient for generating a surface
`discharge (sliding discharge 61) at the outer surface of the
`ceramic cylinder 51.
`In addition to radiation from the
`ultraviolet to the x-ray range, this sliding discharge 61 (at the
`given electrode polarity) generates, above all, fast electrons
`which produce sufficient ionization of the working gas of the
`entire discharge chamber 3. The main discharge pulse then
`ignites a gas discharge between the output 12 of the first
`electrode housing 1 and the end face 23 of the second
`electrode housing 2. The plasma forms virtually in the entire
`interior space of the tubular insulator 22. The peak current
`of the high-voltage pulse generator 7 is about 54 kA with a
`pulse duration of 330 ns. The plasma which is initially
`shaped as a cylinder “implodes” due to the magnetic forces
`induced by the electrical current through the gas discharge
`to the axis of symmetry 31 of the discharge chamber 3
`formed by the electrode housings 1 and 2 and forms a
`plasma column 71 of high density (with a length from 1 mm
`to 50 mm and a diameter from 0.2 to 4 mm) directly in front
`of the outlet opening 22 of the second electrode housing 2
`(anode). The high-voltage pulse generator 7 contains a
`
`

`
`US 2003/0068012 A1
`
`Apr. 10, 2003
`
`thyratron circuit with a single or multiple compression stage
`based on magnetically saturable cores (as disclosed, e.g., in
`U.S. Pat. No. 6,226,307 B1). However, a high-voltage pulse
`generator 7 containing semiconductor components can also
`be used.
`
`[0045] The main discharge takes place a few microsec-
`onds
`later than the surface discharge of the preionization
`unit 5. To illustrate this, the signals of the preionization
`voltage, the discharge current, the discharge voltage and a
`photodiode signal of the EUV emission 72 are shown in
`FIGS. 4 and 5.
`
`[0046] The selected type of preionization guarantees the
`homogeneous triggering of the discharge by the main dis-
`charge pulse. The decisive advantage of this preionization is
`that the preionization unit 5 is not directly exposed to the
`radiation from the plasma and a long useful life is accord-
`ingly achieved.
`
`[0047] The tubular insulator 22 at the inner cylindrical
`superficies surface 21 of the second electrode housing 2 is
`made from Si3N4 and has proven to be a very durable
`material with a life of 2x106 pulses in continuous operation
`without destruction. Instead of Si3N4, various other insulat-
`ing materials such as A1203, AlN, AlZr, AlTi, SiC or
`sapphire can also be used.
`
`[0048] The electrode housings 1 and 2 are produced in
`such a way that a continuous flow of coolant can flow around
`the discharge chamber 3 in each cooling channel 14 and 24.
`In order to increase the transfer of heat, cooling fins 15 and
`25 are incorporated in the cooling channels 14 and 24 of the
`first and second electrode housings 1 and 2. The coolant can
`accordingly absorb heat on an enlarged surface and cooling
`power is improved. The coolant
`is provided by coolant
`reservoirs 17 and 27 and supplied to and removed from the
`electrode housings 1 and 2 via oppositely located connec-
`tions 16 and 26. This design is necessary because an EUV
`source for industrial applications must be operated continu-
`ously for several weeks. If not cooled, the electrodes would
`reach extremely high temperatures due to the current and
`radiation. Cooling is also provided in the preionization unit
`5 via channels 54. In both cases, liquids with low viscosity
`such as oil (e.g., Galden) or distilled or deionized water are
`used as coolants.
`
`[0049] The arrangement according to the invention can
`also be operated with reversed polarity of the high voltage.
`In this connection, FIG. 3 shows a corresponding view in
`which the electrode polarity of the first and second electrode
`housing 1 or 2 is changed. Compared to the preionization
`unit 5 described in the preceding example, with the polarity
`of the preionization voltage likewise being reversed, only
`the generation of fast charged particles is changed in such a
`way that instead of electrons only ions are released in the
`sliding discharge 61. However, they cause a preionization of
`the working gas in the discharge chamber 3 in the same way.
`Aside from the changed direction of the discharge current
`between the first and second electrode housings 1 and 2,
`however, the generation of the plasma, the formation of the
`plasma column 71 and the emission of the EUV radiation 72
`arc achieved in the same manner as described in FIG. 2.
`
`It is also important to note the modified gas feed in
`[0050]
`the embodiment form according to FIG. 3 which has gas
`inlets 82 in the second electrode housing 2 in this example.
`
`This design has the advantage of uniform flow through the
`second electrode housing 2 in particular and the main
`discharge is ignited homogeneously. For this purpose, the
`gas inlets 82 at the second electrode housing are arranged so
`as to be uniformly distributed (or are arranged symmetri-
`cally in pairs) and ensure a uniform flow of gas into the
`discharge chamber 3. On the other side of the discharge
`chamber 3 in the first electrode housing 1, the tube 53 for the
`through-flow of working gas is provided in a manner analo-
`gous to the construction of the preionization unit 5 according
`to FIG. 2 and, in this case, is connected to the vacuum
`system 9. However, the vacuum required in the discharge
`chamber 3 for the discharge processes should be sup-
`ported—as indicated in FIG. 2—by another connection of a
`main vacuum system 91 after the outlet opening 32 of the
`discharge chamber 3.
`
`[0051] Other design variants of the invention are possible
`without departing from the framework of the present inven-
`tion.
`In the preceding examples, an aspect ratio of the
`electrode housings 1 and 2 (diameter to length) of approxi-
`mately 1:1 was assumed, but substantially different ratios are
`also permissible as long as the described discharge processes
`(preionization and main discharge) take place in the manner
`described above. The geometric shapes of the electrode
`housings 1 and 2 can also be substantially modified with
`respect to their axial separation into two chambers, wherein
`the characteristics of the EUV source are changed, but
`without departing from the principle of the generation of a
`reproducible stable plasma with spatially isolated preioniza-
`tion.
`
`[0052] While the foregoing description and drawings rep-
`resent the present invention, it will be obvious to those
`skilled in the art that various changes may be made therein
`without departing from the true spirit and scope of the
`present invention.
`
`[0053] Reference Numbers
`
`[0054]
`
`1 first electrode housing
`
`[0055]
`
`2 cylindrical superficies surface
`
`[0056]
`
`12 narrowed output
`
`[0057]
`
`13 end face
`
`[0058]
`
`14 cooling channels
`
`[0059]
`
`15 fins
`
`[0060]
`
`16 connections
`
`[0061]
`
`17 coolant reservoir
`
`[0062]
`
`2 second electrode housing
`
`[0063]
`
`21 cylindrical superficies surface
`
`[0064]
`
`22 tubular insulator
`
`[0065]
`
`23 end face
`
`[0066]
`
`24 cooling channels
`
`[0067]
`
`[0068]
`
`[0069]
`
`25 cooling fins
`26 connections
`
`27 coolant reservoir
`
`[0070]
`
`3 discharge chamber
`
`[0071]
`
`31 axis of symmetry
`
`

`
`US 2003/0068012 A1
`
`Apr. 10, 2003
`
`[0072]
`
`32 outlet opening
`
`[0073]
`
`4 insulator layer
`
`[0074]
`
`5 preionization unit
`
`[0075]
`
`51 ceramic cylinder (cylindrical insulator)
`
`[0076]
`
`52 flat electrodes
`
`[0077]
`
`53 tube
`
`[0078]
`
`54 channels
`
`[0079]
`
`6 preionization pulse generator
`
`[0080]
`
`61 sliding discharge
`
`[0081]
`
`7 high-voltage pulse generator
`
`[0082]
`
`[0083]
`
`71 plasma column
`72 EUV radiation
`
`[0084]
`
`8 regulated gas feed system
`
`[0085]
`
`[0086]
`
`81 gas inlet (in the first electrode housing)
`
`82 gas inlets (in the second electrode housing)
`
`[0087]
`
`9 vacuum system
`
`[0088]
`
`91 main vacuum system
`
`What is claimed is:
`
`1. An arrangement for generating extreme ultraviolet
`(EUV) radiation based on a gas discharge comprising:
`
`a discharge chamber which is enclosed by a first electrode
`housing and a second electrode housing and through
`which a working gas flows under a defined pressure;
`
`said two electrode housings being arranged coaxial to one
`another and having cylindrical superficies surfaces
`which face the discharge chamber and which are iso-
`lated from one another by an insulator layer so as to
`resist breakthrough;
`
`an outlet opening for the EUV radiation which is provided
`axially in the second electrode housing;
`
`a coaxially arranged preionization unit having two paral-
`lel, substantially flat electrodes at a distance from one
`another axially being provided in the interior of the first
`electrode housing;
`
`said flat electrodes being substantially circular;
`
`a cylindrical insulator in which at least one electric line is
`inserted being arranged between said flat electrodes, so
`that a sliding surface discharge is generated along the
`superficies surface of the cylindrical insulator when a
`sufficiently high voltage is applied to the flat electrodes;
`
`said first electrode housing having a narrowed output in
`the directi