(12) United States Patent
`Smith et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 7,307,375 B2
`Dec. 11, 2007
`
`US007307375B2
`
`11/2002 Smith et al.
`4/2003 Partlo et al.
`4/2003 Smith et al.
`
`......... .. 219/121.57
`250/504 R
`219/121.43
`
`219/121.57
`5/2003 Smith et al.
`250/504 R
`5/2003 Rauch et al.
`............. .. 355/67
`1/2004 Van Elp et al.
`......... .. 378/119
`10/2004 Schriever et al.
`11/2004 Melnychuk et al.
`250/504 R
`11/2004 del Pueito et al.
`........ .. 700/245
`1/2005 Bakker et al.
`......... .. 250/492.2
`3/2005 Holber et al.
`........ .. 219/121.52
`
`
`
`(54)
`
`INDUCTIVELY-DRIVEN PLASMA LIGHT
`SOURCE
`
`(75)
`
`Inventorsz Donald K_ smith’ Behnoma MA (US);
`Stephen E Horne, Chelmsforda MA
`.
`(US)’ Matthew M‘ Beseni And°Ver’
`MA(US)3 Pa“1A-B1a°kb°“’“’=
`Cambridge, MA(US)
`
`.
`(73) Asslgneei Energetiq Technology 1119-, Woburn,
`MA (US)
`.
`.
`.
`.
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`USC 15409) by 211 days.
`
`.
`( * ) Notice:
`
`6,486,431 B1
`6,541,786 B1
`6,552,296 B2
`
`6,559,408 B2
`6,566,668 B2
`6,678,037 B2
`6,804,327 B2
`6,815,700 B2
`6,826,451 B2
`6,838,684 B2
`6,872,909 B2
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`56125882
`2/1981
`
`JP
`
`(21) Appl. No.: 10/888,795
`
`(Continued)
`
`OTHER PUBLICATIONS
`Atwood, EUV Source Candidates for Clean, Collectable 13-14 nm
`Wavelength Radiation, class illustrations for Chapter 6 of Soft
`X-rays and Extreme Ultraviolet Radiation: Principles and Applica-
`tions, Course AST210, UC Berkeley, (Jan. 2004).
`
`(Continued)
`
`Primary Examiner—J0seph Wllhams
`(74) Attorney, Agent. or Fzrm—Pr0skauer Rose LLP
`
`(57)
`
`ABSTRACT
`
`An apparatus for producing light includes a chamber that has
`a plasma discharge region and that contains an ionizable
`medium. The apparatus also includes a magnetic core that
`surrounds a portion of the plasma discharge region. The
`a
`1
`1
`1
`1
`f
`1
`th
`1'
`1“
`Ze115:f1ng°§Zwp§ ifa:)la:1Ii1i:rT0]1inOe:d i1i:tlie121l3T2I11seni:1 dliifiargi
`~
`~
`~
`~
`~
`region. The plasma has a localized high intensity zone.
`
`51 Claims, 8 Drawing Sheets
`
`(22)
`
`(65)
`
`Filed;
`
`Jul_ 9, 2004
`.
`.
`.
`Pnor Pubhcatlon Data
`Us 2006/0006775 A1
`Jan. 12, 2006
`
`(51)
`
`Int‘ Cl‘
`(2006.01)
`H01J 19/42
`(52) U.s. Cl.
`....................... .. 313/31;315/248;313/279
`(58) Field of Classification Search .............. .. 378/119;
`313/31; 315/248
`See application file for complete search history.
`
`(56)
`
`References Cited
`US. PATENT DOCUMENTS
`
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`ggggg gfith er :1.
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`219/121.57
`7/2002 Cox et al.
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`7/2002 McGeoch ................. .. 378/119
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`
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`4,042,848 A
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`(cid:34)(cid:52)(cid:46)(cid:45)(cid:1)(cid:18)(cid:18)(cid:20)(cid:24)
`
`ASML 1137
`
`

`
`US 7,307,375 B2
`Page 2
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`Mohanty et al., “A novel fast capillary discharge system emitting
`intense EUV radiation,” Microelectronic Engineering, vol. 65
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`O’Sullivan et al., “Spectroscopy of a 13.5 nm Laser Plasma Source”
`International SEMATECH EUVL Source Workshop (Oct. 2002) pp.
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`
`Pouvesle et al., “Discharge-based sources of XUV-X radiations:
`development and applications,” Plasma Sources Science and Tech-
`nology vol. 12 (2003) pp. S43-S50.
`Teramoto et al., “High repetition rate MPC generator-driven capil-
`lary Z-pinch EUV source,” SPIE 29‘h Annual International Sympo-
`sium on Microlithography, Santa Clara, CA (Feb. 22-27, 2004) pp.
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`
`Teramoto et al., “Radiation Characteristics of a Capillary Z—Pinch
`EUV Source,” 2“ International EUVL Symposium, Antwerp, Bel-
`gium (Sep. 30-Oct. 2, 2003) pp. 1-18.
`Wheeler et al., “The high-power constricted plasma discharge col.
`1. Theoretical analysis,” .1. Phys. D.‘ Appl. Phys, vol. 3 (1970) pp.
`1374-1380.
`
`II.
`Wheeler, “The high-power constricted plasma discharge col.
`Experimental investigation,” J. Phys. D.‘ Appl. Phys, vol. 4 (1971)
`pp. 400-408.
`Invitation to Pay Additional Fees (Form PCT/ISN206) for PCT/
`US2005/024095 (Dec. 21, 2005).
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`boiling for non-uniform heating conditions in a swirl tube,” Fusion
`Engineering and Design, vol. 28, 1995, pp. 53-58.
`A. Hassanein et al., “Candidate Plasma-Facing Materials for EUV
`Lithography Source Components,” Emerging Lithographic Tech-
`nologies VII, Proceedings of the SPIE, vol. 5037, 2003, pp. 358-
`369.
`
`M. McGeoch et al., “Star Pinch Scalable EUV Source,” Emerging
`Lithographic Technologies VII, Proceedings ofthe SPIE, vol. 5037,
`2003, pp. 141-146.
`International Search Report for corresponding PCT application No.
`PCT/US05/024095, dated Mar. 30, 2006 (12 pages).
`
`4/2005 Ahmad ................. .. 250/504 R
`6,881,971 B2
`5/2005 Ahmad et al.
`..
`.. 250/504 R
`6,894,298 B2
`8/2002 Shan et al.
`..
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`2002/0101167 A1
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`2002/0163313 A1
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`2002/0167282 A1
`12/2002 McGeoch ....... ..
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`2002/0186814 A1
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`
`
`
`..
`.
`
`
`
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`WO 90/13136 A
`
`11/1990
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`and Significant Void Flow Regions of
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`capillary-tube discharge, positive colunm plasma,” J. Phys. D.‘ Appl.
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`30-31, 2003).
`Lebert et al., “A gas discharged based radiation source for EUV-
`lithography,” Microelectronic Engineering, vol. 46 (1999) pp. 449-
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`Liberman et al., Physics of High—Density Z—Pinch Plasmas,
`Springer-Verlag, New York, (1999) pp. 1-277.
`McGeoch, “Progress on the Astron EUV Source,” presented at the
`International SEMATECH 2001 Next Generation Lithography
`Workshop (Aug. 29, 2001) pp. 1-9.
`
`

`
`U.S. Patent
`
`Dec. 11,2007
`
`Sheet 1 of 8
`
`US 7,307,375 B2
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`2:
`
`

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`U.S. Patent
`
`Dec. 11, 2007
`
`Sheet 2 of 8
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`US 7,307,375 B2
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`©_.N
`
`
`
`N.O_u_SN
`
`

`
`U.S. Patent
`
`Dec. 11,2007
`
`Sheet 3 of 8
`
`US 7,307,375 B2
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`

`
`U.S. Patent
`
`Dec. 11,2007
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`Sheet 4 of 8
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`US 7,307,375 B2
`
`EN
`
`v.O_u_
`
`o9.
`
`

`
`

`
`

`
`U.S. Patent
`
`Dec. 11,2007
`
`Sheet 7 of 8
`
`US 7,307,375 B2
`
`FIG.6
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`U.S. Patent
`
`Dec. 11, 2007
`
`Sheet 8 of 8
`
`US 7,307,375 B2
`
`N.0_u_
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`won
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`oom
`
`

`
`US 7,307,375 B2
`
`1
`INDUCTIVELY-DRIVEN PLASMA LIGHT
`SOURCE
`
`FIELD OF THE INVENTION
`
`The invention-relates to methods and apparatus for gen-
`erating a plasma, and more particularly,
`to methods and
`apparatus for providing an inductively-driven plasma light
`source.
`
`BACKGROUND OF THE INVENTION
`
`Plasma discharges can be used in a variety of applications.
`For example, a plasma discharge can be used to excite gases
`to produce activated gases containing ions, free radicals,
`atoms and molecules. Plasma discharges also can be used to
`produce electromagnetic radiation (e.g., light). The electro-
`magnetic radiation produced as a result of a plasma dis-
`charge can itself be used in a variety of applications. For
`example, electromagnetic radiation produced by a plasma
`discharge can be a source of illumination in a lithography
`system used in the fabrication of semiconductor wafers.
`Electromagnetic radiation produced by a plasma discharge
`can alternatively be used as the source of illumination in
`microscopy systems, for example, a soft X-ray microscopy
`system. The parameters (e.g., wavelength and power level)
`of the light vary widely depending upon the application.
`The present state of the art in (e.g., extreme ultraviolet and
`x-ray) plasma light sources consists of or features plasmas
`generated by bombarding target materials with high energy
`laser beams, electrons or other particles or by electrical
`discharge between electrodes. A large amount of energy is
`used to generate and project the laser beams, electrons or
`other particles toward the target materials. Power sources
`must generate voltages large enough to create electrical
`discharges between conductive electrodes to produce very
`high temperature, high density plasmas in a working gas. As
`a result, however, the plasma light sources generate unde-
`sirable particle emissions from the electrodes.
`It
`is therefore a principal object of this invention to
`provide a plasma source. Another object of the invention is
`to provide a plasma source that produces minimal undesir-
`able emissions (e.g., particles,
`infrared light, and visible
`light). Another object of the invention is to provide a high
`energy light source.
`Another object of the invention is to provide an improved
`lithography system for
`semiconductor
`fabrication. Yet
`another object of the invention is to provide an improved
`microscopy system.
`
`SUMMARY OF THE INVENTION
`
`The present invention features a plasma source for gen-
`erating electromagnetic radiation.
`The invention, in one aspect, features a light source. The
`light source includes a chamber having a plasma discharge
`region and containing an ionizable medium. The light source
`also includes a magnetic core that surrounds a portion of the
`plasma discharge region. The light source also includes a
`pulse power system for providing at
`least one pulse of
`energy to the magnetic core for delivering power to a plasma
`formed in the plasma discharge region. The plasma has a
`localized high intensity zone.
`The plasma can substantially vary in current density along
`a path of current flow in the plasma. The zone can be a point
`source of high intensity light. The zone can be a region
`where the plasma is pinched to form a neck. The plasma can
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`be a non-uniform plasma. The zone can be created by, for
`example, gas pressure, an output of the power system, or
`current flow in the plasma.
`The light source can include a feature in the chamber for
`producing a non-uniformity in the plasma. The feature can
`be configured to substantially localize an emission of light
`by the plasma. The feature can be removable or, altema-
`tively, be permanent. The feature can be located remotely
`relative to the magnetic core. In one embodiment the feature
`can be a gas inlet for producing a region of higher pressure
`for producing the zone. In another embodiment the feature
`can be an insert located in the plasma discharge region. The
`feature can include a gas inlet. In some embodiments of the
`invention the feature or insert can include cooling capability
`for cooling the insert or other portions of the light source. In
`certain embodiments the cooling capability involves pres-
`surized subcooled flow boiling. The light source also can
`include a rotating disk that is capable of alternately uncov-
`ering the plasma discharge region during operation of the
`light source. At least one aperture in the disk can be the
`feature that creates the localized high intensity zone. The
`rotating disk can include a hollow region for carrying
`coolant. A thin gas layer can conduct heat from the disk to
`a cooled surface.
`
`In some embodiments the pulse of energy provided to the
`magnetic core can form the plasma. Each pulse of energy
`can possess different characteristics. Each pulse of energy
`can be provided at a frequency of between about 100 pulses
`per second and about 15,000 pulses per second. Each pulse
`of energy can be provided for a duration of time between
`about 10 ns and about 10 us. The at least one pulse of energy
`can be a plurality of pulses.
`In yet another embodiment of the invention the pulse
`power system can include an energy storage device, for
`example, at least one capacitor and/or a second magnetic
`core. A second magnetic core can discharge each pulse of
`energy to the first magnetic core to deliver power to the
`plasma. The pulse power system can include a magnetic
`pulse-compression generator, a magnetic switch for selec-
`tively delivering each pulse of energy to the magnetic core,
`and/or a saturable inductor. The magnetic core of the light
`source can be configured to produce at least essentially a
`Z-pinch in a charmel region located in the chamber or,
`alternatively, at
`least a capillary discharge in a channel
`region in the chamber. The plasma (e.g., plasma loops) can
`form the secondary of a transformer.
`The light source of the present invention also can include
`at least one port for introducing the ionizable medium into
`the chamber. The ionizable medium can be an ionizable fluid
`
`(i.e., a gas or liquid). The ionizable medium can include one
`or more gases, for example, one or more of the following
`gases: Xenon, Lithium, Nitrogen, Argon, Helium, Fluorine,
`Tin, Ammonia, Stannane, Krypton or Neon. The ionizable
`medium can be a solid (e.g., Tin or Lithium) that can be
`vaporized by a thermal process or sputtering process within
`the chamber or vaporized externally and then introduced
`into the chamber. The light source also can include an
`ionization source (e.g., an ultraviolet lamp, an RF source, a
`spark plug or a DC discharge source) for pre-ionizing the
`ionizable medium. The ionization source can also be induc-
`
`tive leakage current that flows from a second magnetic core
`to the magnetic core surrounding the portion of the plasma
`discharge region.
`The light source can include an enclosure that at least
`partially encloses the magnetic core. The enclosure can
`define a plurality of holes in the enclosure. A plurality of
`plasma loops can pass through the plurality of holes when
`
`

`
`US 7,307,375 B2
`
`3
`the magnetic core delivers power to the plasma. The enclo-
`sure can include two parallel (e.g., disk-shaped) plates. The
`parallel plates can be conductive and form a primary wind-
`ing around the magnetic core. The enclosure can,
`for
`example, include or be formed from a metal material such as
`copper, tungsten, aluminum or one of a variety of copper-
`tungsten alloys. Coolant can flow through the enclosure for
`cooling a location adjacent the localized high intensity zone.
`In some embodiments of the invention the light source
`can be configured to produce light for different uses. In other
`embodiments of the invention a light source can be config-
`ured to produce light at wavelengths shorter than about 100
`nm when the light source generates a plasma discharge. In
`another embodiment of the invention a light source can be
`configured to produce light at wavelengths shorter than
`about 15 nm when the light source generates a plasma
`discharge. The light source can be configured to generate a
`plasma discharge suitable for semiconductor fabrication
`lithographic systems. The light source can be configured to
`generate a plasma discharge suitable for microscopy sys-
`tems.
`
`The invention, in another aspect, features an inductively-
`driven light source.
`In another aspect of the invention, a light source features
`a chamber having a plasma discharge region and containing
`an ionizable material. The light source also includes a
`transformer having a first magnetic core that surrounds a
`portion of the plasma discharge region. The light source also
`includes a second magnetic core linked with the first mag-
`netic core by a current. The light source also includes a
`power supply for providing a first signal (e.g., a voltage
`signal) to the second magnetic core, wherein the second
`magnetic core provides a second signal (e.g., a pulse of
`energy) to the first magnetic core when the second magnetic
`core saturates, and wherein the first magnetic core delivers
`power to a plasma formed in the plasma discharge region
`from the ionizable medium in response to the second signal.
`The light source can include a metallic material for con-
`ducting the current.
`In another aspect of the invention, a light source includes
`a chamber having a charmel region and containing an
`ionizable medium. The light source includes a magnetic core
`that surrounds a portion of the channel region and a pulse
`power system for providing at least one pulse of energy to
`the magnetic core for exciting the ionizable medium to form
`at least essentially a Z-pinch in the channel region. The
`current density of the plasma can be greater than about 1
`KA/cmz. The pressure in the channel region can be less than
`about 100 mTorr.
`
`In yet another aspect of the invention, a light source
`includes a chamber containing a light emitting plasma with
`a localized high-intensity zone that emits a substantial
`portion of the emitted light. The light source also includes a
`magnetic core that surrounds a portion of the non-uniform
`light emitting plasma. The light source also includes a pulse
`power system for providing at least one pulse of energy to
`the magnetic core for delivering power to the plasma.
`In another aspect of the invention, a light source includes
`a chamber having a plasma discharge region and containing
`an ionizable medium. The light source also includes a
`magnetic core that surrounds a portion of the plasma dis-
`charge region. The light source also includes a means for
`providing at least one pulse of energy to the magnetic core
`for delivering power to a plasma formed in the plasma
`discharge region. The plasma has a localized high intensity
`zone.
`
`10
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`25
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`4
`
`In another aspect of the invention, a plasma source
`includes a chamber having a plasma discharge region and
`containing an ionizable medium. The plasma source also
`includes a magnetic core that surrounds a portion of the
`plasma discharge region and induces an electric current in
`the plasma sufficient to form a Z-pinch.
`In general, in another aspect the invention relates to a
`method for generating a light signal. The method involves
`introducing an ionizable medium capable of generating a
`plasma into a chamber. The also involves applying at least
`one pulse of energy to a magnetic core that surrounds a
`portion of a plasma discharge region within the chamber
`such that the magnetic core delivers power to the plasma.
`The plasma has a localized high intensity zone.
`The method for generating the light signal can involve
`producing a non-uniformity in the plasma. The method also
`can involve localizing an emission of light by the plasma.
`The method also can involve producing a region of higher
`pressure to produce the non-uniformity.
`The plasma can be a non-uniform plasma. The plasma can
`substantially vary in current density along a path of current
`flow in the plasma. The zone can be a point source of high
`intensity light. The zone can be a region where the plasma
`is pinched to form a neck. The zone can be created with a
`feature in the chamber. The zone can be created with gas
`pressure. The zone can be created with an output of the
`power system. Current flow in the plasma can create the
`zone.
`
`The method also can involve locating an insert in the
`plasma discharge region. The insert can define a necked
`region for localizing an emission of light by the plasma. The
`insert can include a gas inlet and/or cooling capability. A
`non-uniformity can be produced in the plasma by a feature
`located in the chamber. The feature can be configured to
`substantially localize an emission of light by the plasma. The
`feature can be located remotely relative to the magnetic core.
`The at least one pulse of energy provided to the magnetic
`core can form the plasma. Each pulse of energy can be
`pulsed at a frequency of between about 100 pulses per
`second and about 15,000 pulses per second. Each pulse of
`energy can be provided for a duration of time between about
`10 ns and about 10 us. The pulse power system can an
`energy storage device, for example, at least one capacitor
`and/or a second magnetic core.
`In some embodiments, the method of the invention can
`involve discharging the at least one pulse of energy from the
`second magnetic core to the first magnetic core to deliver
`power to the plasma. The pulse power system can include,
`for example, a magnetic pulse-compression generator and/or
`a saturable inductor. The method can involve delivering each
`pulse of energy to the magnetic core by operation of a
`magnetic switch.
`In some embodiments, the method of the invention can
`involve producing at least essentially a Z-pinch or essen-
`tially a capillary discharge in a channel region located in the
`chamber. In some embodiments the method can involve
`
`introducing the ionizable medium into the chamber via at
`least one port. The ionizable medium can include one or
`more gases, for example, one or more of the following gases:
`Xenon, Lithium, Nitrogen, Argon, Helium, Fluorine, Tin,
`Ammonia, Stannane, Krypton or Neon. The method also can
`involve pre-ionizing the ionizable medium with an ioniza-
`tion source (e.g., an ultraviolet lamp, an RF source, a spark
`plug or a DC discharge source). Alternatively or addition-
`ally, inductive leakage current flowing from a second mag-
`netic core to the magnetic core surrounding the portion of
`the plasma discharge region can be used to pre-ionize the
`
`

`
`US 7,307,375 B2
`
`5
`ionizable medium. In another embodiment, the ionizable
`medium can be a solid (e.g., Tin or Lithium) that can be
`vaporized by a thermal process or sputtering process within
`the chamber or vaporized externally and then introduced
`into the chamber.
`In another embodiment of the invention the method can
`
`involve at least partially enclosing the magnetic core within
`an enclosure. The enclosure can include a plurality of holes.
`A plurality of plasma loops can pass through the plurality of
`holes when the magnetic core delivers power to the plasma.
`The enclosure can include two parallel plates. The two
`parallel plates can be used to form a primary winding around
`the magnetic core. The enclosure can include or be formed
`from a metal material, for example, copper, tungsten, alu-
`minum or copper-tungsten alloys. Coolant can be provided
`to the enclosure to cool a location adjacent the localized high
`intensity location.
`The method can involve alternately uncovering the
`plasma discharge region. A rotating disk can be used to
`alternately uncover the plasma discharge region and alter-
`nately define a feature that creates the localized high inten-
`sity zone. A coolant can be provided to a hollow region in
`the rotating disk.
`In another embodiment the method can involve producing
`light at wavelengths shorter than about 100 nm. In another
`embodiments the method can involve producing light at
`wavelengths shorter than about 15 nm. The method also can
`involve generating a plasma discharge suitable for semicon-
`ductor fabrication lithographic systems. The method also
`can involve generating a plasma discharge suitable for
`microscopy systems.
`The invention, in another aspect, features a lithography
`system. The lithography system includes at least one light
`collection optic and at least one light condenser optic in
`optical communication with the at least one collection optic.
`The lithography system also includes a light source capable
`of generating light for collection by the at least one collec-
`tion optic. The light source includes a chamber having a
`plasma discharge region and containing an ionizable
`medium. The light source also includes a magnetic core that
`surrounds a portion of the plasma discharge region and a
`pulse power system for providing at
`least one pulse of
`energy to the magnetic core for delivering power to a plasma
`formed in the plasma discharge region. The plasma has a
`localized high intensity zone.
`In some embodiments of the invention, light emitted by
`the plasma is collected by the at least one collection optic,
`condensed by the at least one condenser optic and at least
`partially directed through a lithographic mask.
`The invention, in another aspect, features an inductively-
`driven light source for illuminating a semiconductor wafer
`in a lithography system.
`In general, in another aspect the invention relates to a
`method for illuminating a semiconductor wafer in a lithog-
`raphy system. The method involves introducing an ionizable
`medium capable of generating a plasma into a chamber. The
`method also involves applying at least one pulse of energy
`to a magnetic core that surrounds a portion of a plasma
`discharge region within the chamber such that the magnetic
`core delivers power to the plasma. The plasma has a local-
`ized high intensity zone. The method also involves collect-
`ing light emitted by the plasma, condensing the collected
`light; and directing at
`least part of the condensed light
`through a mask onto a surface of a semiconductor wafer.
`The invention, in another aspect, features a microscopy
`system. The microscopy system includes a first optical
`element for collecting light and a second optical element for
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`projecting an image of a sample onto a detector. The detector
`is in optical communication with the first and second optical
`elements. The microscopy system also includes a light
`source in optical communication with the first optical ele-
`ment. The light source includes a chamber having a plasma
`discharge region and containing an ionizable medium. The
`light source also includes a magnetic core that surrounds a
`portion of the plasma discharge region and a pulse power
`system for providing at least one pulse of energy to the
`magnetic core for delivering power to a plasma formed in
`the plasma discharge region. The plasma has a localized high
`intensity zone.
`In some embodiments of the invention, light emitted by
`the plasma is collected by the first optical element
`to
`illuminate the sample and the second optical element
`projects an image of the sample onto the detector.
`In general, in another aspect the invention relates to a
`microscopy method. The method involves introducing an
`ionizable medium capable of generating a plasma into a
`chamber. The method also involves applying at least one
`pulse of energy to a magnetic core that surrounds a portion
`of a plasma discharge region within the chamber such that
`the magnetic core delivers power to the plasma. The plasma
`has a localized high intensity zone. The method also
`involves collecting a light emitted by the plasma with a first
`optical element and projecting it through a sample. The
`method also involves projecting the light emitted through
`the sample to a detector.
`The foregoing and other objects, aspects, features, and
`advantages of the invention will become more apparent from
`the following description and from the claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The foregoing and other objects, feature and advantages
`of the invention, as well as the invention itself, will be more
`fully understood from the following illustrative description,
`when read together with the accompanying drawings which
`are not necessarily to scale.
`FIG. 1 is a cross-sectional view of a magnetic core
`surrounding a portion of a plasma discharge region, accord-
`ing to an illustrative embodiment of the invention.
`FIG. 2 is a schematic electrical circuit model of a plasma
`source, according to an illustrative embodiment of the
`invention.
`
`FIG. 3 is a cross-sectional view of two magnetic cores and
`a feature for producing a non-uniformity in a plasma,
`according to another illustrative embodiment of the inven-
`tion.
`
`FIG. 4 is a schematic electrical circuit model of a plasma
`source, according to an illustrative embodiment of the
`invention.
`
`FIG. 5A is an isometric view of a plasma source, accord-
`ing to an illustrative embodiment of the invention.
`FIG. 5B is a cutaway view of the plasma source of FIG.
`5A.
`
`FIG. 6 is a schematic block diagram of a lithography
`system, according to an illustrative embodiment of the
`invention.
`
`FIG. 7 is a schematic block diagram of a microscopy
`system, according to an illustrative embodiment of the
`invention.
`
`

`
`7
`DETAILED DESCRIPTION OF ILLUSTRATIVE
`EMBODIMENTS
`
`US 7,307,375 B2
`
`8
`source 100. In other embodiments, the pulse of energy can
`be delivered to the magnetic core when, for example, a
`saturable inductor becomes saturated.
`
`The plasma source 100 also may include a means for
`generating free charges in the chamber 104 that provides an
`initial ionization event that pre-ionizes the ionizable medium
`to ignite the plasma loops 116a and 116b in the chamber 104.
`Free charges can be generated in the chamber by an ioniza-
`tion source, such as, an ultraviolet light, an RF source, a
`spark plug or a DC discharge source. Alternatively or
`additionally, inductive leakage current flowing from a sec-
`ond magnetic core in the power system 136 to the magnetic
`core 108 can pre-ionize the ionizable medium. In certain
`embodiments, the ionizable medium is pre-ionized by one or
`more ionization sources.
`
`The ionizable medium can be an ionizable fluid (i.e., a gas
`or liquid). By way of example, the ionizable medium can be
`a gas, such as Xenon, Lithium, Tin, Nitrogen, Argon,
`Helium, Fluorine, Ammonia, Stannane, Krypton or Neon.
`Alternatively, the ionizable medium can be finely divided
`particle (e.g., Tin) introduced through at least one gas port
`into the chamber 104 with a carrier gas, such as helium. In
`another embodiment, the ionizable medium can be a solid
`(e.g., Tin or Lithium) that can be vaporized by a thermal
`process or sputtering process within the chamber or vapor-
`ized externally and then introduced into the chamber 104. In
`certain embodiments,
`the plasma source 100 includes a
`vapor generator (not shown) that vaporizes the metal and
`introduces the vaporized metal into the chamber 104. In
`certain embodiments, the plasma source 100 also includes a
`heating module for heating the vaporized metal
`in the
`chamber 104. The chamber 104 may be formed, at least in
`part, from a metallic material such as copper, tungsten, a
`copper-tungsten alloy or any material suitable for containing
`the ionizable medium and the plasma and for otherwise
`supporting the operation of the plasma source 100.
`Referring to FIG. 1, the plasma loops 116a and 116b
`converge in a charmel region 132 defined by the magnetic
`core 108 and the winding 140. In one exemplary embodi-
`ment, pressure in the channel region is less than about 100
`mTorr. Energy intensity varies along the path of a plasma
`loop if the cross-sectional area of the plasma loop varies
`along the length of the plasma loop. Energy intensity may
`therefore be altered along the path of a plasma loop by use
`of features or forces that alter cross-sectional area of the
`
`plasma loop. Altering the cross-sectional area of a plasma
`loop is also referred to herein as constricting the flow of
`current in the plasma or pinching the plasma loop. Accord-
`ingly, the energy intensity is greater at a location along the
`path of the plasma loop where the cross-sectional area is
`decreased. Similarly, the energy intensity is lower at a given
`point along the path of the plasma loop where the cross-
`sectional area is increased. It is therefore possible to create
`locations with higher or lower energy intensity.
`Constricting the flow of current
`in a plasma is also
`sometimes referred to as producing a Z-pinch or a capillary
`discharge. A Z-pinch in a plasma is characterized by the
`plasma decreasing in cross-sectional area at a specific loca-
`tion along the path of the plasma. The plasma decreases in
`cross-sectional area as a result of the current that is flowing
`through the cross-sectional area of the plasma at the specific
`location. Generally, a magnetic field is generated due to the
`current in the plasma and, the magnetic field confines and
`compresses the plasma. In this case, the plasma carries an
`induced current along the plasma path and a resulting
`magnetic field surrounds and compresses the plasma. This
`effect
`is strongest where the cross-sectional area of the
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`FIG. 1 is a cross-sectional View of a plasma source 100 for
`generating a plasma that embodies the invention. The 5
`plasma source 100 includes a chamber 104 that defines a
`plasma discharge region 112. The chamber 104 contains an
`ionizable medium that is used to generate a plasma (shown
`as two plasma loops 116a and 116b) in the plasma discharge
`region 112. The plasma source 100 includes a transformer
`124 that induces an electric current into the two plasma
`loops 116a and 116b (generally 116) formed in the plasma
`discharge region 112. The transformer 124 includes a mag-
`netic core 108 and a prim