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`
`US007786455B2
`
`(12) Ulllted States P316111’,
`Smith
`
`(10) Patent N0.:
`45 Date of Patent:
`
`US 7,786,455 B2
`9
`Au . 31 2010
`
`356/237.1
`9/2001 Fairley ct al.
`6,288,780 B1
`. 315/111.31
`..
`7/2002 lirooks et ztl.
`6,417,625 131*
`9/2004 Lange ................... .. 356./237.2
`6,788,404 B2
`6,956,329 B2* 10/2005 Brooks ct a1.
`........ .. 315/111.31
`7,652,430 B1 ,-,
`1/(20.10 D6133do
`313,633-
`2002./0021508 Al
`2/2002 Ishiham ..
`359/853
`
`
`
`313/_634
`9/3003 Kim “““ "
`2003'/10168982 A]
`................... 362/26.0
`200310231496 A1 ‘ 12/.003 Sato eta].
`20,04/0264512 A1 "‘ 12/2004 Hartlove et al.
`............
`372;’:
`2005/0167618 A1"
`X/2005 1-ioshino ct .11.
`250/504R
`
`2007/0285921 /11"‘ 12/2007 Zulim et al.
`
`362/240
`
`(54) LASER-DRIVEN LIGHT SOURCE
`
`Inventor: Donald K- Sn1ith,Be11110x1t. MA (US)
`(75)
`‘
`N
`_
`‘
`_
`__
`(/3) Assignee:
`lLnerget‘1q"1cchn0]ogy,»Inc., Woburn,
`MA‘ (U5)
`
`( "‘ ) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U-’S.C‘. _154(b) by 820 days.
`
`(21) Appt. No.: 11/695,348
`
`(22)
`
`Filed:
`
`Apr. 2, 2007
`
`(65)
`
`Prior Publication Data
`US 2007./0228300 A1
`Oct. 4. 2007
`
`(comjnued)
`FOREIGN 1)A']‘EN'1‘ DOCUNIENTS
`
`Related U.S. Application Data
`
`JP
`
`51-193353
`
`3,/1935
`
`(63) ContinuaIion—in—part of application No. 11/395,523.,
`filed on Mar. 31, 2006’, now Pat. No. 7,435,982.
`
`OTHER PUBLICATIONS
`Wilbers et :31, “The VUV Emissivity of a Higl1—Pressure Cascade
`Argon Arc from 125 to 200 nm',”./. Quan/. Spez.-/ro.vr:. Radial. Ham'-
`_/2:'r,Vo1.46, 1991, pp. 299-302.
`
`(Contillued)
`Prim” ,.}4,W]m.”er__Bema,rd E Souw
`R (3 Lu,
`(74 H? if
`I
`1
`I
`F.
`P
`k,
`‘
`) ’ “W-" ’ ge" ' 0” ""0" ms “W W’
`(57)
`ABSTRACT
`
`,
`_
`_
`~
`-
`-
`,
`An ,1
`V
`( pparatus for producing light includes a chamber and an
`ignition source ’tl1“a't' ioiiizés’ a gas wit‘h’in"the chaniber. The
`apparatus also includes at least one laser that provides energy
`to the ionized gas within the chamber to produce a high
`brightness. light. The laser can provide a substantially con-
`tinuous amount of energy to the ionized gas to generate 21
`substantially continuous high brightness light.
`
`43 Claims, 8 Drawing Sheets
`
`(51)
`
`(52)
`
`Int CL
`(200601)
`#053 31/26
`(200601)
`601‘, 3/10
`(2006-01)
`G21G 4/00
`(2006.01)
`H01.) 61/28
`250/493.1; 250/504 R;
`"U.S. C1.
`315/111,21; 315/111.71; 315,/111.91; 313/231.31;
`313/231.41; 313/231.71
`(58) Field 0fC1assification Search ............. 250/423 R,
`250/423 1’, 424, 426, 494.1, 493.1, 504 R,
`250/504 H; 315/111.21, 111.71, 111.91;
`313/231.31, 231.41, 231.61, 231.71, 631,
`313/632 633
`z w
`.
`,
`3 ‘ l
`,
`’
`' S“ ‘lpphcangllme fgr colnplem warch mstory‘
`References (med
`U.S. 1’A'1‘EN'I' DOCU1V11:‘N'1‘S
`
`5/‘I978 Samis
`2/’ 1985 Yoshi'z.awaeta1.
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`....... ..
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`
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`
`.. 313/231.51
`315/39
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`RE32,626 E *
`
`(56)
`
`
`
`ASML ‘I501
`
`

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`US 7,786,455 B2
`Page 2
`
`U PA'l"EN'1' DOCUMENTS
`
`............ .. 250/503.1
`2/2009 Smith etal.
`2009/0032740 A|"‘
`OTHER PUBLICATIONS
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`Wilbers et :11, “The Continuum Emission of an Arc Plasma,” J.
`Quant. Specfro.s'c. Radim. Yiwnxfer, vol. 45, No. 1. 1991. pp. 1-10.
`Beck, “Simple Pulse Generator for Pulsing Xenon Arcs with High
`Repetition Rate.” Rev. Sci. InsIrmn., vol. 45, No. 2, Feb. 1974, pp.
`318-319.
`Rai7.e1', “Optical Discharges,” Sov. Plzyy. Usp; 23(l 1), Nov. 1980, pp.
`789-806.
`Fiedorowicz ct 211., “X—Ray Emission form l..aser-Irradiated Gas Pufi‘
`Targets,”Appl. Plzyx. L211. 62 (22), May 31, 1993, pp. 2778-2780.
`Keefer et 2.1., “Experimental Study of _a Stationary Laser-Sustained
`Air Plasma,” Journal ofApplied P/IySi(.‘.S', ‘vol. 46, No. 3. Mar. 1975,
`pp. 1080-1083.
`Jenget .11., “’l‘l1coretical Investig:ition of Laser"-Sustained Argon Plas-
`mz1s,”.7. App]. Pins’. 60 (7). Oct. 1, 1986. pp. 2272-2279.
`Franzen, “CW Gas Breakdown in Argon Using 10.6-pun'Laser Radia-
`tion,”.-tppl. Phys. Lari, vol. 21, No. 2. Jul. 15, 1972, pp. 62-64.
`Moody, “Maintenance of:1 Gas Breakdown in Argon Using 10.6,-pew
`Radiation,” Journal q/‘Applied Physics, Vol. 46, No. 6, Jun. 1975, pp.
`2475-2482,
`
`Generalov et .11., "Experimental Investigation of a Continuous Opti-
`cal Discharge,” Soviet Physics JETP. Vol. 34, No. 4, Apr. 1972, pp.
`763-769.
`
`Generalov et 31., “Continuous Optical Discharge,” Z/JEYIF Pix. Red.
`11,- No. 9, May 5, 1970, pp. 302-304.
`Kozlov et al., “Radiative Losses by Argon Plasma and the Emissive
`Model ofaContinuous Optical Discharge.” Sov. Phys. ./E11-’, vol. 39,
`No. 3, Sop. 1974, pp. 463-468.
`Carlhoff et 01., “Continuous Optical Discharges at Very High Pres-
`sure,” P/Iysica 103C, 1981, pp. 439-447.
`Creiners et 111., “l3\=aluz1tion of the. Continuous Optical Discharge for
`Spectrochenfical Analysis,” Speciroclzinzica Acta, vol. 4013. No. 4.
`1985, pp. 665-679.
`Kozlov et 111., “Sustained Optical Discharges in Molecular Gases,”
`Sov. Phys. 72:011. Phys. 49(l1), Nov. 1979, pp. 1283-1287.
`Keefer, “Laser-Sust:1ined Plasmas,” Laser-ImI'm?ed Plaxmas and
`Applir'(1tion.v, published by Marcel Dekkcr, edited by Radziemski at
`211., 1989, pp. 169-206.
`Ilamamatsu Product Information,“Super—Quiet Xenon Lamp Super-
`Quiet Mercury-Xenon Lamp,” Nov. 2005.
`
`cited by examiner
`
`

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`U.S. Patent
`
`Aug. 31, 2010
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`US 7,786,455 B2
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`1
`I.ASER—I)RIVEN LIGHT SOURCE
`
`RELATED APPLICATIONS
`
`This application‘ is a continuation-in-part ofU.S. Ser. No.
`-1 I/395,523’, filed on Mar. 31;, 2006, now U.S. Pat. No. 7,435,
`982, the entire disclosure of which is incorporated by refer-
`ence herein.
`
`FIELD OF THE INVENTION
`
`The invention relates to methods and apparatus for provid-
`ing a laser-driven light source.
`
`-BACKGROUND OF T INVENTI ON
`
`High brightness light. sources can be used in a variety of
`applications. For example, a high brightness light source can
`be used for inspection, testing or measuring properties asso-
`ciated with semiconductor waters or materials used in the
`fabrication of waters (e.g., reticles and photomasks). The
`electromagnetic energy produced by highbrightness lights
`sources can, alternatively, be used as a source of illumination
`in a lithography system used in the fabrication of wafers, a
`microscopy systems, or a photoresist curing system. The‘
`parameters (e. g., wavelength, power level and brightness) of
`the light vary depending upon the application.
`The state of the art in, .for example, wafer inspection sys-
`tems involves the use of xenon or mercury arc lamps to
`produce light."Ihe arc lamps include an anode and cathode
`that are used to excite xenon or mercury gas located in a
`chamber of the lamp. An electrical discharge is generated
`between ‘the anode and cathode to provide power to the
`excited (e.g., ionized)‘ gas to ‘sustain’ the light emitted by the
`ionized gas during operation of the light source. During
`operation, the anode and cathode become very hot due to
`electrical discharge delivered to the ionized gas located
`between the anode and cathode. As {I result, the anode and/or
`cathode are prone to wear and may emit particles that can
`——contaminate the light—seurce— or result in -failure of the—light-
`source. Also, these are lamps do not provide suilicient bright-
`ness for some applications, especially in the ultraviolet spec-
`trum. Furthcr, the position of the arc can be unstable in these
`lamps.
`Accordingly, a need therefore exists for "improved high
`brightness light sources. A need also exists for improved high
`brightness light" sources that do not rely on an electrical dis-
`charge to maintain a plasma that generates a high brightness
`light.
`
`SUMMARY OF THE INVENTION
`
`The present invention features a light source for generating
`a high brightness light.
`The invention, in one aspect, features a light source having
`a chamber. The light source also includes an ignition source
`for ionizing a gas within the chamber. The light source also
`includes at least one laser for providing energy to the ionized
`gas within the chamber to produce a high brightness light.
`In some embodiments, the at least one laser is a plurality of
`lasers directed at a region from which the high brightness
`light originates. In some embodiments, the light source also
`includes at least one optical element for modifying a property
`of the laser energy provided to the ionized gas. The optical
`element can be. for example, a lens
`an aplanatic lens, an
`achromatic lens, a single element lens, and a frcsncl lens) or
`mirror (e.g., a coated mirror. a dielectric coated mirror.
`21
`
`2
`narrow band mirror, and an ultraviolet transparent infrared
`reflecting mirror). In some embodiments, the optical element
`is one or more fiber optic elements for directing the-laser
`energy to the gas.
`.
`‘
`_
`'
`-
`The chamber can include‘ an ultraviolet transparent region.
`The chamber or a window in. the chamber can include a
`material selected from the group'consisting of quartz, Supra-
`‘sil® quartz (Heraeus Quartz America, LLC, Buford, Ga.),
`sapphire, MgF2, diamond, and Cal72. In some embodiments,
`the chamber is a sealed chamber. In some embodiments, the
`chamber is capable of being actively pumped. In some
`embodiments, the chamber includes a dielectric material
`(e.g., quartz). The chamber can be, for example, a glass bulb._
`In som_e embodiments, the chamber is an ultraviolet transpar-
`ent dielectric chamber.
`
`The gas can be one or more of a noble gas, Xe, Ar, Ne, Kr,
`He. D2, H2, 02, F3, a metal halide, a halogen, Hg, Cd, Zn, Sn,
`Ga, Fe, Li, Na, an excimer forming gas, air, a vapor, ametal
`oxide, an aerosol, a flowing media, or a recycledmedia. The
`gas can be produced by a pulsed laser beam that impacts a
`target (e.g., a solid or liquid) in the ‘chamber. The target can be
`a pool or film of metal. In some embodiments, the target is
`capable of moving. For example, the target may be a liquid
`that is directed to a region from which thehigh brightness
`light originates.”
`In some embodiments, the at least one laser is multiple
`diode lasers coupled into a fiber optic element. In some
`embodiments‘, the at least one laser includes a pulse or con-
`tinuous wave laser. In some embodiments, the at least one
`laser is an IR laser, a diode laser, a fiber laser, an ytterbium
`laser, a C02 laser, a YAG laser, or a gas discharge‘ laser. In
`some embodiments, the at least one laser emits at least one
`wavelength of electromagnetic energy that
`is strongly
`absorbed by the ionizedmedium.
`The ignition source can be or can include electrodes, an
`ultraviolet ignition source, a capacitive ignition source, an
`inductive ignition source, an RF ignition source, a microwave
`ignition source, a flash lamp, a pulsed laser, or a pulsed lamp.
`The igllitionsourcc,ca11.12c.a.continuous Wjavc (CW) <>rp1Il§Cd
`laser impinging on a solid or liquid target in the chamber. The
`ignition source can be external or internal to the chamber.
`The light source caninclude at least one optical element for
`modifying a property ofelectromagnetic radiation emitted by
`the ionized gas. The optical element can be, for example, one
`or more mirrors or lenses. In some embodiments, the optical
`element is configuredto deliverthe electromagnetic radiation
`emitted by the ionized gas to a tool. (eg, a wafer inspection
`tool, a microscope, a mctrology tool, a lithography tool, or an
`endoscopic tool).
`The invention, in another aspect, relates to a method for
`producing light. The method involves ionizing with an igni-
`tion sourcc a gas withinacl1an1ber.The method also involves
`providing laser energy to the ionized gas in the chamber to
`produce a high brightness light;
`In some embodiments, the method also involves directing
`the laser energy through at
`least one optical element for
`modifying a property of the laser energy provided to the
`ionized gas. In some embodiments, the method also involves
`actively pumping the chamber. The ionizable medium can be
`a moving target. In some embodiments,
`the method also
`involves directing the high brightness light through at least
`one optical element to modify a property of the light. In some
`embodiments, the method also involves delivering the high
`brightness light emitted by the ionized medium to a tool (eg,
`a wafer inspection tool, a microscope, a mctrology tool, a
`lithography tool. or an endoscopic tool).
`
`10
`
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`()3
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`US 7,786,455 B2
`
`3
`In another aspect, the invention features a light source. The
`lights source includes a chamber and an ignition source for
`ionizing an ionizable medium Within the chamber. Thelight
`source also includes at least one laser for providing substan-
`tially continuous energy to the .ionized medium within the
`chamber to produce a high brightness light.
`In some embodiments, the at least one laser is a continuous
`wave laser or a high pulse rate laser. In some embodiments,
`the at least one laser is :1 high pulse rate laser tl1at provides
`pulses ofenergy to the ionized medium so the high brightness
`light is substantially continuous. In some embodiments, the
`magnitude ofthe high brightness light does not vary by more‘ '
`than about -90% during operation. In some embodirnents‘, the
`at least one laser provides energy substantially continuously
`to minimize cooling of the ionized medium when energy is
`not provided to the ionized medium.
`In some embodiments, the light source can include at least
`one optical element (c.g., a lens or mirror) for modifying a
`property ofthe laser energy provided to the ionized medium.
`The optical element carrbe, for example, an aplanatic lens, an
`achromatic lens, a single element lens, a fresnel lens, a coated
`mirror, a dielectric coated mirror, a narrow band mirror, or an
`ultraviolet transparent infrared rellecting mirror. In some
`embodiments, the optical element is one or more fiber optic
`elements for directing the laser energy to the ionizable
`medium.
`I
`In some embodiments, the chamber includes an ultraviolet
`transparent region. In some embodiments, the chamber or a
`window in the chamber includes a quartz inatcrial, suprasil
`quartz material, sapphire material, Mgf-'3 material, diamond
`material, or CaI“2 material. In some embodiments, the cham-
`ber is a sealed chamber. The chamber can be capable of being’
`actively pumped.
`In some embodiments,
`the chamber
`includes a dielectric material (c.g., quartz). In some embodi-
`ments, the chamber is a glass bulb. In some embodiments, the
`chamber is an ultraviolet transparent dielectric chamber.
`The ionizable medium can be a solid, liquid or gas. The
`ionizableniedium can include one or more ofa noble gas, Xe,
`Ar, Ne, Kr, I-Ie, D3, H3, 03. F2, a metal halide, a halogen, Hg,
`Cd, 7n, Sn, Ga, Fe, Li, Na, an excimer forming gas, air, a
`vapor, a metal oxide, an aerosol, a flowing media, a recycled
`media, or an evaporating target. In some ernbodiments, the
`ionizable medium is a target in the chamber and the ignition
`source is a pulsed laser that provides a pulsed laser beam that
`strikes the target. The target can be a pool or film of metal. In
`some embodiments, the target is capable of moving.
`In sortie embodiments, the at least one laser is multiple
`diode lasers coupled into a fiber optic element. The at least
`one laser can emit at least one wavelength ofelectromagnetic
`energy that is strongly absorbed by the ionized medium.
`The ignition source can be or can include electrodes. an
`ultraviolet ignition source, a capacitive ignition source, an
`inductive ignition source, an RF ignition source, a microwave
`ignition source, a flash lamp, a pulsed laser, or a pulsed lamp.
`The ignition source can be external or internal to the chamber.
`In some embodiments, the light source includes at least one
`optical element (e.g., a mirror or lens) for modifying a prop-
`erty of electromagnetic radiation emitted by the ionized
`medium. The optical element can be configured to deliver the
`electromagnetic radiation emitted by the ionized medium to a
`tool (eg., a wafer inspection tool, a microscope, a metrology
`tool, a lithography tool, or an endoscopic tool).
`The invention, in another aspect relates to a method for
`producing limit. The method involves ionizing with an igni-
`tion source an ionizable medium within a chamber. The
`
`10
`
`20
`
`30
`
`40
`
`45
`
`v.'.I1
`
`60
`
`65
`
`4
`method also involves providing substantially continuous
`laser energy to the ionized medium in the chamber to produce
`a high brightness light. '
`ht some embodiments, the method also involves directing
`the. laserenergy through at least one optical. element for
`modifying a property of the laser energy provided to the
`ionizable medium. The method also can involve aetivcly
`pumping the chamber. In some embodiments, the ionizable
`medium is a moving target. The ionizable medium can
`include a solid, liquid or gas. In some embodiments, the
`method also involves directing the high brightness light
`through at least one optical element to modify a property of
`the light. In some embodiments, the method also involves
`delivering the. high brightness light emitted by the ionized
`medium to a tool.
`The invention, in another aspect, features a light source
`having a chamber. The light source includes a first ignition
`means for ionizing an ionizable medium within the. chamber.
`The light source also includes a means for providing substan-
`tially continttous laser energy to the ionized medium within
`the chamber.
`The invention, in another aspect, features a light source
`having a chamber that includes a reflective surface. The light
`source also includes an ignition source for ionizing a gas
`within the chamber. The light source also includes a rellector
`that at least substzuttially retlects a first set of predefined
`wavelengths of electromagnetic energy directed toward the
`reflector and at least substantially allows a second set of
`predefined wavelengths of electroiitagnetic energy to pass»
`through the rctlector. The light source also includes at least
`one laser (e.g.. a continuous-wave fiber laser) external to the
`chamber for providing electromagnetic energy to the ionized
`gas within the chamber to produce a plasma that generates a
`high brightness light. A continuous-wave laser emits radia-
`tion continuously or substantially continuously rather than in
`short bursts, as in a pulsed laser.
`I11 some embodiments, at least one laser directs a first set of
`wavelengths of electromagnetic energ 1 through the rellector
`toward the reflective surface (e. g._, inner surface) of the cham-
`ber and the reflective surface directs at least a portion of the
`first set ofwavclengths ofclectromagnetic energy toward the
`plasma. In some embodiments, at least a portion of the high
`brightness light is directed toward the rellective surface of the
`chamber, is reflected toward the rcilector, and is rcllected by
`the reflector toward a tool. In some embodiments, at least one
`laser directs a llrst set of wavelengths of electromagnetic
`energy toward the reflector, the reflector reflects at least a
`portion of the first wavelengths of electromagnetic energy
`towards the reilective surface ofthe chamber, and the rellec-
`tive surface directs a portion ofthe first set ofwavelengtlis of
`electromagnetic energy toward the plasma.
`In some embodiments, at least a portion of the high bright-
`ness light is directed toward the reilcctive surface of the
`chamber, is rellected toward the rellector, and passes through
`the rellector toward an output of the light source.
`I11 some
`crnbodiments, the light source comprises a microscope, ultra-
`violet microscope, wafcr inspection system, reticle inspec-
`tion system or lithography system spaced relative to the out-
`put of the light source to receive the high brightness light. In
`some embodiments, a portion of the high brightness light is
`directed toward the rellective surface of the chamber,
`is
`rellectcd toward the rellcctor, and electromagnetic energy
`comprising the second set ofpredefincd wavelengths ofelec-
`tromagnetic energy passes through the reflector.
`The chamber of the light source can include a window. In
`some embodiments, the chamber is a sealed chamber. In some
`embodiments, the rellective surface of the chamber com-
`
`

`
`US 7,786,455 B2
`
`prises a curved shape, parabolic shape, elliptical shape,
`spherical shape or aspherical shape. In some embodiments,
`K the chamber has a reflective inner surface. In some "embodi-
`ments, a coating or film is located’ on the outside of the
`chamber to produce the reflective surface. In some embodi-.
`ments, a coatingor film is located on the inside ofthe chamber
`to produce the reflective surface. ln some embodiments, the
`reflective surface is a structure or optical element that is
`distinct from the inner surface of the chamber.
`
`The light source can include an optical element disposed
`along a path the electromagnetic energy from the laser travels.
`In some embodiments, the optical element is adapted to pro-
`vide electromagnetic energy from the laser to the plasma over
`a large solid angle. In some embodiments, the reflective stir-
`face of the chamber is adapted to provide electromagnetic
`energy from the laser to the plasma over a large solid angle. In
`some embodiments, the reflective surface of the chamber is
`adapted to collect the high‘ brightness light generated by the
`plasma over a large solid angle. In some embodiments, one or
`more of the reflective surface, reflector and the window
`include (e.g., are coated or include) a material to filter pre-
`defined wavelengths (e.g., infrared wavelengths of electro-
`magnetic energy) of electromagnetic energy.
`The invention, in another aspect, features a light source that
`includes a chamber that has a reflective surface. The light
`source also includes an ignition source for ionizing a gas
`within the chamber. The light source also includes at least one
`laser external to the chamber. for providing electromagnetic
`energy to the ionized gas within the chamber to produce a
`plasma that generates a high brightness light. The light source
`also includes a reflector positioned along a path that the
`electromagnetic energy travels from the at least one laser to
`the reflective surface of the chamber.
`
`10
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`15'
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`20
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`30
`
`'
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`'40
`
`In some embodiments", the reflector is adapted to at least‘ ’
`substantially reflect a first set of predefined wavelengths ‘of
`electromagnetic energy directed toward the reflector and at
`least substantially allow a second set of ‘predefined wave-
`lengtlis of electromagnetic energy to pass through the reflec-
`tor.
`7 H
`K
`7
`The invention, in another aspect, relates to a- method for
`producing light. The method involves ionizing with an igni-
`tion source a gas within a chamber that has a reflective sur-
`face. The method also involves providing laser energy to the
`ionized gas in the chamber to produce a plasma that generates
`a high brightness light.
`In some embodiments, the method involves directing the
`laser energy comprising a first set ofwavelengths of electro-
`magnetic cnergy through a reflector toward the reflective
`surface of the chamber, the reflective surface reflecting at
`least a portion ofthe first set of wavelengths of electromag-
`netic energy toward the plasma. In some embodiments, the
`method involves directing at least a portion of the high bright-
`ness light’ toward the reflective surface of the chamber which
`is reflected toward the reflector and is reflected by the reflec-
`tor toward a tool.
`
`45
`
`U.’.Ii
`
`In some embodiments, the method involves directing the
`laser energy comprising a first set of wavelengths of electro-
`magnetic energy lOV and the reflector, the reflector reflects at
`least a portion of the first wavelengths of electromagnetic
`energy toward the reflective surface of the chamber,
`the
`reflective surface directs a portion of the first set of wave-
`lengths ofclcctrontagnctic cncrgy toward the plasma. In some
`embodiments. the method involves directing a portion of the
`high brightness light toward the reflective surface of the
`chamber which is reflected toward the reflector and, electro-
`
`60
`
`(D
`
`6
`magnetic energy comprising the second set of predefined
`wavelengths of electromagnetic energy passes through the
`reflector.
`V
`'
`_
`The method can involve directing the laser energy through
`an optical element that modifies a property ofthe laser energy
`to direct the laser energy toward the plasma over a large solid
`angle. In some embodiments, the method involves directing
`the laser energy through an optical element that modifies a
`property of the. laser energy to direct the laser energy toward
`the plasma over a solid angle of approximately 0.012 stera-
`dians. In some embodiments, the method involves directing
`thelaser energy through an optical element that modifies a
`property of the laser energy to direct the laser energy toward
`the plasma over .a solid angle of approximately 0.048 stera-
`dians. ln sortie embodiments, the method involves directing
`the laser energy through an optical element that modifies a
`property of the laser energy to direct the laser energy toward
`the plasma over a solid angle of greater than about 2.712 (about
`6.28) steradians. In some embodiments, therellcctive surface
`of the chamberis adapted to provide the laser energy to the
`plasma over a large solid angle. In some embodiments, the
`reflective surfaceof the chamber is adapted to collect the high
`brightness light generated by the plasma over a large solid
`angle.
`The invention, in another aspect, relates to a method for
`producing light. The method involves ionizing with an igni-
`tion source a gas within a chamber that has a reflective sur-
`face. The method also involves directing electromagnetic
`energy from a laser toward" a reflector that at least substan-
`tially rcflects a first set of wavelengths of electromagnetic
`energy toward the ionized gas in the chamber to produce a
`plasma that generates a high brightness light.
`In some embodiments, the electromagnetic energyfrom
`the laser first is reflected by the reflector toward the reflective
`surface of the chamber. In some embodiments, the electro-
`magnetic energy directed toward the reflective surface of the
`chamber. is reflected toward the plasma. In some embodi-
`ments, a portion ofthe high brightness light is directed toward
`the reflective surface of the chamber, reflected toward the
`reflector and —passes~through the reflector. —
`In some embodiments, the electromagnetic energy from
`the laser first passes through the reflector and travels toward
`the reflective surface of the chamber. In some embodiments,
`the electromagnetic energy directed toward the reflective sur-
`face of the chamber is reflected toward the plasma. In some
`embodiments, a portion ofthe high brightness light is directed
`toward the reflective surface of the chamber, reflected toward
`the reflector and reflected by the reflector.
`The invention, in another aspect, features a light source that
`includes a chamber having a reflective surface. The light
`source also includes a means for ioni7.ing a gas within the
`chamber. The light source also includes a means for at least
`substantially reflecting a first set ofpredefined wavelengths of
`electromagnetic energy directed toward the reflector and at
`least substantially allowing a second set of predefined wave-
`lengths of electromagnetic energy to pass through the reflec-
`tor. The light source also includes :1 means for providing
`electromagnetic energy to the ionized gas within the chamber
`to produce a plasma that generates a high brightness light.
`The invention, in another aspect, features a light sourcethat
`includes a sealed chamber. 'l'hc light source also includes an
`ignition source for ioni‘/.ing a gas within the chamber. The
`light source also includes at lcast one laser external to the
`sealed chamber for providing electromagnetic energy to the
`ionized gas within the chamber to produce a plasma that
`generates a high briglitncss light. The light source also
`includes a curved reflective surface disposed external to the
`
`

`
`7
`
`US 7,786,455 B2
`
`8
`FIG. 8B is a schematic block diagram ofthe light source of
`FIG. 8A in which the electromagnetic energy from the laser is
`provided to the plasma over a larger solid angle, according to
`an illustrative embodiment of the invention.
`
`5
`
`DETAILED DESCRIPTION OF ILLUSTRATIVE
`EMBODIMENTS
`
`sealed chamber to receive at leas a portion ofthe high bright-
`ness light emitted by the sealed chamber and reflect the high
`brightness light toward an output of the lightisource. V
`In some embodiments,‘ the light source includes an optical
`element disposed —along a path the electromagnetic energy’
`from the laser travels. In some embodiments,
`the sealed »
`chamber includes a support element that locates the sealed.
`chamber relative to the curved reflective surface. In some
`embodiments, the sealed chamber is a quartz bulb. In some
`embodiments, the light source includes a second curved
`reflective surface disposed internal or external to the sealed
`chamber to receive at least a portion of the laser electromag-
`netic energy and focus the electromagnetic energy on the
`plasma that generates the high brightness light.
`The invention, in another aspect, features a light source that
`includes a sealed chamber and an ignition source for ionizing
`a gas within the chamber. The light source also includes at
`least one laser external to the sealed chamber for providing
`electromagnetic energy. The light source also includes a
`curved rellective surface to receive and rellect at least a por-
`tion of the electromagnetic energy toward the ionized gas
`within the chamber to produce a plasma that generates a high
`brightness light, the curved refiective surface also receives at
`least a portion of the high brightness light emitted by the
`plasma and reflects the high brightness light toward anooutput
`of the light source.
`the curved rcflective surface
`In some embodiments,
`focuses the electromagnetic energy on a region in the cham-
`ber where the plasma is located. In some embodiments, ‘the
`curved reflective surface is located within the chamber. In
`some embodiments, the curved rellective surface is located
`external to the chamber. In some embodiments, the high
`brightness light is ultraviolet light, includes ultraviolet light
`or is substantially ultraviolet light.
`The foregoing and other objects, aspects, features, and
`advantages of the invention will become more apparent from
`the following description and from the claims.
`
`FIG. 1 is a schematic block diagram of a light source 100
`for generating light, that embodies the invention. The light
`source 100 includes a chamber 128 that contains an ionizable
`
`medium (not shown). The light source ‘I 00 provides energy to
`a region 130 of-the chamber 128 having the ionizable medium
`which creates a plasma 132. The plasma 132 generates and
`emits a high brightness light 136 that originates from the
`plasma 132. The light source 100 also includes at least one
`laser source 104 that generates a laser beam that is provided to
`the plasma 132 locatcdin the chamber 128 to initiate and/or
`sustain the high brightness light 136.
`In some embodiments, it is desirable for at least one wave-
`length of electromagnetic energy generated by the laser
`source 104 to be strongly absorbed by the ionirable medium
`in order to maximize the elliciency of the translcr of energy
`from-thevlaser source 104 to the ionizable medium.
`In some embodiments, it is desirable for the plasma 132 to
`be small in size in order to achieve a high brightness light
`source. Brightness is the power radiated by a source oflight
`per unit surface area into a unit solid angle. The brightness of
`the light produced by-a light source determines the ability of
`a system (e.g., ‘a metrology tool) oran operator to see or
`measure things (e.g., features on the surface of a wafer) with
`adequate resolution. It is also desirable for the laser source
`104 to drive and/or wstain the plasma with a high power laser
`beam.
`
`Generating a plasma 132 that is small in size and providing
`the plasma 132 with a high power laser beam leads simulta-
`neously to a high brightness light 136