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`US007435982B2
`
`(12) United States Patent
`Smith
`
`(10) Patent No.:
`
`(45) Date of Patent:
`
`US 7,435,982 B2
`Oct. 14, 2008
`
`(54) LASER-DRIVEN LIGIIT SOURCE
`
`2005-0|6?(!18 .-—\l"‘
`200150228300 Al "‘
`
`Iloshinoetal.
`8:"20t}5
`1052007 Smith
`
`350.-5041!
`2.'i(J."504 R
`
`(751
`
`Inventor: Donald K. Smith. Iielmont, MA (US)
`
`FOREIGN PATENT DOCUMENTS
`
`(73) Assignee: Energetiq Technology, Ine.. Woburn.
`MA (US)
`
`JP
`
`61-193358
`
`8 I986
`
`()'l'lI1'iR l"UB1.1('.‘A'l“1(')NS
`
`( * } Notice:
`
`Subject to any disciairner. tl1e tern1 ofthis
`patent is extended or adjusted under 35
`u.s.(:. 154(b) by 452 days.
`
`(21) App1.No.: 111395.523
`
`(22)
`
`Filed:
`
`Mar. 31.. 2006
`
`(65)
`
`Prior Pulllieation Data
`US 200710228288 Al
`Oct. 4. 2007
`
`(51)
`
`Int. Cl.
`(2006.01)
`A6IN 5/06
`(2006.01)
`GOIJ 3X10
`(2006.01)
`H056 2.r’1Wl
`(52) U.S. (fl.
`............................. .. 250504 R: 250:’-123 P:
`2501426;25014931;43811043381301:4381513:
`4381156; 252r‘30l.36: 2521301. 16: 252:’3()l.4 1-‘;
`385131: 385133; 385138
`Field of(?lassifieati0n Search
`250504 R.
`250.1423 1’. 426. 493.1: 4381104. 301. 513.
`4381156; 252E3()1.16. 301.36.301.41’: 385131.
`385133. 38
`
`(58)
`
`See application file for complete search l1istor_v.
`
`(56)
`
`References Cited
`U.S. PA’l‘l'iN’1‘ I)()(‘UMi-1N’l‘S
`
`6.288.’r’8tt Bl
`t‘S.?$\'8.404 B2
`200430264512 At
`
`1-'air1ey et al.
`9.-"3001
`Imlge
`9.-"2004
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`
`........... . 356"23?.|
`356-"2312
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`Wilbers et a1.. "'l'l1e \«'LI\r' Ifirnissivity ofa lligh-Pressure Cascade
`Argon Art: 1mm 125 to 200 nm.” J. Quam. 5}')er.‘.'r'(J.t'r". Radial’. Tr‘r:rr.\'—
`fer‘. ml. 46. I991. pp. 299—308.
`Wilbcrs et al.. “The Contirnlum I.-"mission ofAre l-‘1asn1a.”J. Qmmt.
`.'5'p('(‘r.IU.\'('. Rrm’.-"at. Tr(ur.g')"er'. vol. 45. No. 1. I991. pp. 1-10.
`Beck. "Simple Pulse Generator lbr Pulsing Xenon Ares with High
`Repetition Rate." Rev, Sr'.". fu.m'mrt.. vol. 45. =.\1o. 2. Feb. 1914. pp.
`3 l8—3 I9.
`Rztizer. “tL)ptiea| l_)ise11:|.rges.“Son PI:_1'.s'. {--':.p. 23( l 1). Nov. 1980. pp.
`?8‘)—806.
`Fiedornwiez el al .. “X—Ray Flnission form I.aser—Irr:tdiated Gas Pull‘
`"targets." Appl. Phys. Lert. 6202}. May 31. 1993. pp. 2778-2180.
`Keefer et aI.. “l;'xperimenta| Study of a Stationary Laser-Stlstained
`Air I’I;lsrn;1.”.J'0m'.I:m’ of.‘1'ppIr'ed .”i1:_t‘.\'r'('.\'. vol. 46. N0. 3. Mar. 19'.-"5.
`pp. I080-1083.
`
`((‘nnlir1ued)
`
`Prirttarji‘ E.\'artii'r.=cr—Jack I Bernlan
`Assi.9tant I§.\'a:triner Meenakshi S $211111
`
`(74} .-1ttorne_1'. /lger.-'1. or 1'-irm Pmskauer Rose. 1.1 .P
`
`(57)
`
`A BS TRAC T
`
`An apparatus for producing light includes a cllamher and an
`ignitimi source that ionires at gas within the eliamber. The
`&lppaI'dll.lS also includes at least one laser that provides energy
`to the ionized gas witliin the chamber to produce a high
`brightness light. The laser can provide a stlbstantially eon-
`tinuotls anlount of energy to the ionized gas to generate a
`substantially continuous high brightness light.
`
`81 Claims. 4 Drawing Sheets
`
`140
`
`108
`
`104
`
`112
`
`
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`(cid:36)(cid:54)(cid:48)(cid:47)(cid:3)(cid:20)(cid:19)(cid:21)(cid:19)
`
`ASML 1020
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`

`
`US 7,435,982 B2
`Page 2
`
`OTI-IER PUBLICATIONS
`
`Jeng et al.. "Theoretical Investigation of I.aser-Sustained Argon Plas-
`mas.“ J’. Appf. Phys. 60(7). Oct. 1. 1986. pp. 22't2-ZBT9.
`Franzen. “CW Gas Breakdown in Argon Using 10.6-pm Laser Radia-
`tion."Appt'. Phys. Lem, vol. 21. No. 2. Jul. 15. 19732. pp. 62-64.
`Moody. “Maintenance ofa Gas Breakdown in Argon Using 10.6-p cw
`Radiation.” .Icmmm' q,f}tppt."cd Ptrysmu-, vol. 46. No. 6. Jun. 1915. pp.
`2475-2482.
`
`Generalov ct .11.. "Experimental Investigation ot'a Continuous Opti-
`cal Discharge.“ Sam-r Pt.)-'.vir.'s JETP. vol. 34. No. 4. Apr. 1972. pp.
`763-769.
`
`Generalov el al.. “Continuous Optical Discharge.” ZIIETF Pis. Red.
`II. No. 9. May 5. I970, pp. 302-304.
`
`Kozlov el al.. “Radiative Losses by Argon Plasma and the Emissive
`Model of :1 Continuous Optical Discharge.“ Sov. Phys. JEPT. Vol.3-9.
`No. 3. Sep. 1974. pp. 463-468.
`Carlhoff et a].. “(Iontin1Ious Optical Discharges at Very High Pres-
`sure.” Pirysica lC|I3C. I981. pp. 439-447.
`Crelners at 211.. “F.\ra]ual ion nflhe Continuous Optical Discharge for
`Spcctrochcrnieal Analysis,” Specrrpcl:i'rtIfc:z Acta, vol. 4013. No. 4.
`I985. pp. 665-679.
`Kozlov et al.. “Sustained Optical Discharges in Molecular Gases.”
`Sov. P:'.=ys.
`i'b(.'lr. Phys. 49(l I ). Nov. 191“). pp. 1283-1281
`Keefer. “l_aseI-Sustained Plasmas.“ !.::se:'-lridtaced Ptmmas and
`A;)p!i'cati'om'. published by Marcel Dekker. edited by Radziemski et
`a].. 1989. pp. I69-206.
`HzLma.mats1.I Product Inftlrmalion. "Super—Qui(-:1 Xenon Lamp Super-
`Quiet .-Vlercury-Xenon Lamp.” Nov. 2005.
`
`* cited by examiner
`
`

`
`U.S. Patent
`
`Oct. 14,2003
`
`Sheet 1 of4
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`US 7,435,982 B2
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`U.S. Patent
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`Oct. 14, 2003
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`Sheet 2 of4
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`Oct. 14,2003
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`
`1
`LASER-DRIVEN LIGHT SOURCE
`
`FIELD OF THE INVENTION
`
`The invention relates to methods and apparatus for provid-
`ing a laser-drivctt light Source.
`
`BACKGROUND OF THE INVENTION
`
`l-ligh 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 wafers or materials used in the
`fabrication of waters (e.g.. reticles and photomasks). The
`electromagnetic energy produced by high brightness lights
`sources can. alternatively. be used as a source of illumination
`in a lithography system used in the fabrication ol' wafers. a
`ruicroscopy 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 tl1 -‘ art in, for example, wafer inspection sys-
`tems involves tl1e use of xeno11 or mercury art:
`lar11ps to
`produce light. The are 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 etuitted 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 a restill. the ar1odc andftir
`cathode are prone to wear and may emit particles that can
`contaminate the light source or result in failure ofthe light
`source. Also. these are lamps do not provide su llicient bright-
`ness for some applications. especially in the ultraviolet spec-
`trum. Further_. the position of the arc can be unstable in these
`lamps.
`Accordirtgly, a need lltertefortc 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 or 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 ligtt.
`In some ernbodimcnts. the at least one laser is a plurality of
`lasers directed at a region front which the high brightness
`light originates. In some embodiments, the light source also
`includes at least one optical element tor modifyinga property
`of the laser energy provided to the ionized gas. The optical
`element can be. for example. a lens (e.g._. an aplanatic lens, an
`achromatic lens, a single element lens, and a Fresnel lens) or
`mirror (e.g., a coated mirror, a dielectric coated mirror. a
`narrow band ruirror. and an ultraviolet transparent infrared
`reflecting mirror). In some embodiruents. the optical element
`is one or more Iiber optic elements for directing the laser
`energy to the gas.
`The chamber can include an it] traviolet transparent region.
`The chamber or a window in the chamber can include a
`material selected from the group consisting of quartz. Supra-
`silifi‘ quartz (Heraeus Quartz America. LLC. Buford. Ga).
`sapphire. Mgl"2. diamond, and (.'al’2. In some embodiments.
`the chamber is a sealed chaunbcr. In some embodirncnts. the
`
`US 7,435,982 B2
`
`2
`
`In some
`chamber is capable of being actively pumped.
`embodiments.
`the cheuubcr includes a dielectric material
`(e.g.. quartz). The chamber can be, for exarnplc. a glass bulb.
`In some embodiments. the chamber is an ultraviolet transpar-
`ent dielectric chamber.
`The gas can be one or more ofa noble gas. Xe. Ar. Ne. Kr.
`I-le, D2, H2. 02, F2. a metal halide. a lralogen, Hg, Cd. Zn. Sn.
`Ga. Fe. Li. Na. an excimcr forming gas. air. a vapor. a metal
`oxide, an aerosol. a [lowing media. or a recycled media. The
`gas can be produced by a pulsed laser beam that impacts at
`target (eg. a solid or liquid) in the chamber. The target can be
`a pool or film of metal. in sortie embodiments. the target is
`capable of ntovinn. For example. the target may be a liquid
`that is directed to a region from which the high briglttriess
`light originates.
`in some embodiments. the at least one laser is multiple
`diode lasers coupled into a fiber optic element. In some
`crnbodirnerrts, the at least one laser includes a pulse or cort-
`tinuous wave laser. in sortie embodirner1ts_. the at least one
`laser is an IR laser. a diode laser. a fiber laser. an ytterbiutn
`laser, a C03 laser. a YAG laser. or a gas discharge laser. In
`sortie embodirnents. the at least one laser emits at least one
`wavelength of electromagnetic energy that
`is
`strongly
`absorbed by the ionized mcditun.
`The ignition source can be or can include electrodes. an
`ultraviolet ignition source. a capacitive ignition source. an
`inductive ignition source. an RI’ ignition source. a microwave
`ignition source. a flash lamp, a pulsed laser, or a pulsed lamp.
`The ignition source can be a continuous wave (CW) or pulsed
`laser impinging on a solid or liquid target in the chamber. Tlte
`ignition source can be external or intcmal to the chamber.
`The light source can include at least one optical element for
`modifying a property ofclectromagnetic radiation emitted by
`the ionized gas. The optical elenterrt can be. for example, one
`or more mirrors or lenses. in sortie embodiments. the optical
`cletnent is configured to deliver the eleetrotnagnctic radiation
`emitted by the ionized gas to a tool [e.g.. a wa fer 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 light. The method involves ionizing with an igni-
`tion source a gas within a chamber. 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 embodirnertts. 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 {c.g..
`a wafer inspection tool. a microscope. a metrology tool. a
`lithography tool. or an endoscopic tool).
`In another aspect. the invention features a light source. The
`lights source includes a chamber and an ignition source for
`ionizing an ionizablc medium within the chamber. The light
`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.
`lr1 some embodiments. the at least one laser is a continuous
`wave laser or a high pulse rate laser. In some enrbodiments.
`the at least one laser is a high pulse rate laser that provides
`pulses o fenergy 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% durittg operation. In sortie crnbodiments. the
`at least one laser provides energy substantially continuously
`
`5
`
`1U
`
`3U
`
`bi '.v.
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`45
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`50
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`60
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`65
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`

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`3
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`4
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`US ?,435,982 B2
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`Ia:
`
`1U
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`3U
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`to minimize coolittg ofthe ionized medium wlten energy is
`not provided to the ionized medium.
`In sonte entbodintents. the light source can include at least
`one optical element (e.g.. a lens or mirror) for ntodifying a
`property pfthe laser energy provided to the ionized medium.
`The optical element cart be. 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 rcllecting mirror.
`Itt some
`embodirnents. tlte optical element is one or ntore fiber optic
`elements for directing the laser energy to the ionizable
`medium.
`I11 some embodiments. the chamber includes an ultraviolet
`transparent region. In some embodiments. the chamber or a
`window in tlte chantber includes a quartz material. suprasil
`quartz material. sapphire material, Mgliz, material. diamond
`tttaterial. or CaF2 material. In some emboditttents. the cham-
`ber is a sealed cltamber. The chamber can be capable ofbeing
`actively pumped.
`Irt
`sortte embodiments.
`the chamber
`includes a dielectric material (e.g.. quartz). In some entbodi-
`ments. the chamber is a glass bulb. In some embodiments. the
`chamber is an ultraviolet transparent dielectric chamber.
`The ionizable medium cart be a solid. liquid or gas. The
`ionizable Ittedium cart include orte or more ofa noble gas. Xe.
`Ar. Ne. Kr. I-le_. D3, I-I2. 02, F3. a metal halide. a ha logeu. l--lg.
`Cd. Zn. Sn. Ga. Fe, Li. Na. art eitcitner forming gas. air. a ~
`vapor, a metal oxide. an aerosol, a flowing media. a recycled
`media. or an evaporating target. In some embodiments. the
`ionizable medium is a target irt the chamber and the ignition
`source is a pulsed laser tltat provides a pulsed laser beam that
`strikes tlte target. The target can be a pool or film of metal. In
`some embodiments. tl1e target is capable of moving.
`In sonte 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 o feleetrornaguetie
`energy that is strongly absorbed by the ionized medium.
`The ignition source can be or can include electrodes. art
`ultraviolet ignition source. a capacitive ignition source. art
`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 extemal or internal to the chantber.
`In some entbodiments. the l i ght source includes at least one
`optical elentertt (eg._. a mirror or lens} for ntodifying a prop-
`erty of electromagnetic radiation entitled by the ionized
`ntediunt. Tlte optical elentent can be configured to deliver the
`electromagnetic radiation emitted by the ionized medium to a
`tool (e.g.. a wafer inspection tool. a microscope. a metrology
`tool. a lithography tool. or an endoscopic tool).
`The invention.
`itt another aspect relates to a method for
`producing light. The method involves ionizing with an igni-
`tion source an iorti'/able medium within a chamber. The
`method also involves providing substantially continuous
`laser energy to the ionized medium in tlte chamber to produce
`a high briglttness ligltt.
`In seine embodiments. the method also involves directing
`the laser energy through at least one optical element for
`modifying a propeny of the laser energy provided to the
`ionizable medium. The method also can involve actively
`pumping the chamber. I.l1 sonte embodiments. the ionizable
`medium is a moving target. The ionizable medium cart
`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 seine embodiments. the method also involves
`delivering the high brightness light emitted by the ionized
`medium to a tool.
`
`45
`
`50
`
`60
`
`The invention. in another aspect. features a light source
`having a chamber. The light source includes a first ignition
`means for ionizing an ionizable mediunt within the chamber.
`
`65
`
`The light source also includes a means tbr providing substan-
`tially continuous laser energy to the ionized medium within
`the chamber.
`
`The foregoing and other objects. aspects. features. and
`advantages of the invention will become more apparent from
`the following description artd from the claims.
`
`ISRIIEI-' l)l ESCRIPTION OI’ Tl-IIE DRAWINGS
`
`The foregoing and other obj ects, 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.
`1-‘ I6. 1 is a schematic block diagram of a light source.
`according to art illustrative embodiment ofthe invention.
`FIG. 2 is a schematic block diagram ofa portion ofa light
`source. according to an illustrative embodiment of the invert-
`tion.
`
`FIG. 3 is a graphical representation of UV brightness as a
`function oftlte laser power provided to a plasma. using a light
`source according to the invention.
`FIG. 4 is a graphical representation of the transmission of
`laser energy through a plasma generated from mercury. using
`a light source according to the invention.
`
`l)l_E"II’\II..l".I) I.)l-IS(.‘RIl’TI()N O1" I[..I_.USTR.«\"l"I\«’l.i
`EMBODIMENTS
`
`FIG. 1 is a schematic block diagram ofa light source 100
`for generating light. that embodies the invention. The light
`source 100 includes a chamber 128 that contains and ioniz-
`
`able medium {not shown}. Tlte light source 100 provides
`energy to a region 130 of the chamber 128 having the ioniz-
`able medium which creates a plasma 132. The plasma 132
`generates and emits a high brightness light 136 that originates
`frottt the plasma I32. The ligltt source llltl also includes at
`least one laser sottrce 104 that generates a laser beam that is
`provided to the plasma 132 located in the chamber 128 to
`initiate andfor sustain the high brightness light 136.
`In sortie entbodintents, it is desirable for at least one wave-
`length of electromagnetic energy generated by the laser
`source 104 to be strongly absorbed by the ionizable medium
`in order to rnaximize the eliiciency of the transfer o f energy
`from the laser source 104 to the ionizable medium.
`
`I11 sortte embodiments, it is desirable for the plasma 132 to
`be small in size in order to acliicve a high br-iglttness light
`source. Brightness is the power radiated by a source of light
`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 ntetrology tool) or an operator to see or
`measure things (e.g.. features on the surface ofa water) with
`adequate resolution. It is also desirable for the laser source
`104 to drive andfor sustain the plasma with a high power laser
`beam.
`
`Generating a plasma 132 that is small in size and providing
`the plasma 132 with a ltiglt power laser beartt leads sintulta-
`rteously to a high brightness ligltt 136. The ligltt source 100
`produces a high brightness light 136 because most of the
`power introduced by the laser source 104 is then radiated
`from a small volunte. high temperature plasma 132. The
`plasma 132 temperature will rise due to heating by the laser
`beartt until balanced by radiation and other processes. The
`high temperatures that are achieved in the laser sustained
`plasma 132 yield increased radiation at shorter wavelengths
`ofelectromagnetic energy. for example, ultraviolet energy. In
`one experitnertt. temperatures between about 10.000 K and
`about 20.000 K ltave been observed. The radiation of the
`plasma 132. in a general scttse. is distributed over the elec-
`tromagnetic spectrum according to l’lanck’s radiation law.
`
`

`
`The wavelength of niaximuni radiation is inversely propor-
`tional to the tentpemttlre ofa black body according to Wien's
`displacement law. While the laser sustained plasma is not a
`black body.
`it behaves similarly and
`such. the highest
`brightness in the ultraviolet range at around 300 mm wave-
`length is expected for laser sustained plasmas having a tem-
`perature of between about 10,000 K and about 15.000 K.
`Conventional art: lamps are.
`liovt-‘ever. unable to operate at
`these temperatures.
`It is therefore desirable in some embodiments ofthe inven-
`tion to maintain the temperature of the plasma 132 during
`operation of the light source 100 to ensure that a sufficiently
`bright light 136 is generated and that the light emitted is
`substantially continuous during operation.
`In this embodiment. the laser source 104 is a diode laser
`that outputs a laser beam via a fiberoptic element 108. The
`fiber optic element 108 provides the laser beam to a collima-
`tor 1 12 that aids in conditioning the output of the diode laser
`by aiding in niaking laser beam rays 116 substantially parallel
`to each other. The collimator 112 then directs the laser beam
`I I 6 to a beam expander 118. The beam expander I18 expands
`the size ofthe laser beam 116 to produce laser beam 122. The
`beam expander 118 also directs the laser beam 122 to an
`optical lens 120. The optical lens 120 is configured to focus
`the laser beam 122 to produce a smal lcr diameter laser beam
`124 that is directed to the region .130 of the chamber 128 _
`where the plasma 132 exists (or where it is desirable for the
`plasma 132 to be generated and sustained).
`In this embodiment. the llglt source 100 also includes an
`ignition source 140 depicted as two electrodes [e.g.. an anode
`and cathode located in the chamber 128). The ignition source
`140 generates an electrical discharge in the chamber 128
`(tag. the region 130 of the chamber 128) to ignite the ioniz-
`able medium. The laser then provides laser energy to the
`ionized medium to sustain or create the plasma 132 which
`generates the high brightness light 136. The light I36 gener-
`ated by the light source 100 is then directed out ol‘ the cham-
`ber to. for example. a wafer inspection system (not shown).
`Alternative laser sources are contemplated according to
`illustrative embodiments of the invention. In some embodi-
`merits. neither the collimator 112. the beam expander 118. or
`the lens 120 may be required. In some embodirnents. addi-
`tional or alternative optical elements can be used. The laser
`source can be. for example. an infrared (IR) laser source. a
`diode laser source. a fiber laser source, an ytterbium laser
`source. a C 03 laser source. a YAG laser source. or a gas
`discharge laser source.
`In some embodiments.
`the laser
`source 104 is a pulse laser source (e.g.. a high pulse rate laser
`source) or a continuous wave laser source. In some embodi-
`ments. multiple lasers [e.g.. diode lasers) are coupled to one
`or more fiber optic elements (e.g._. the liber optic element
`108). Ill some embodiments. fiber laser sources and direct
`semiconductor laser sources are desirable liir use as the laser
`source 104 because they are relatively low in cost, have a
`small form factor or package size. and are relatively high in
`elliciency.
`In some embodiments. the laser source 104 is a l1i gh pulse
`rate laser source that provides substantially continuous laser
`energy to the light source 100 sullicient to produce the high
`brightness light 136. In some embodiments. the emitted high
`brightness light 136 is substantially continuous where. for
`example. magnitude (e.g. brightness or power) of the high
`brightness light does not vary by more than about 90% during
`operation. In some embodiments. the substantially continu-
`ous energy provided to the plasma 132 is sufiicicnt to mini-
`mize cooling o I‘ the ionized medium to maintain a desirable
`brightness of the emitted light 136.
`In this embodiment, the light source 100 includes a plural-
`ity ofoptical elements [e.g.. a beam expander 118. at lens 120.
`and fiber optic element 108) to modify properties {e.g.. diam-
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`eter and orientation) ofthe laser beam delivered to the cham-
`ber 132. Various properties of the laser beam can be modified
`with one or more optical elements (eg. mirrors or lenses}.
`For example. one or more optical elements can be used to
`modify the ponions of. or the entire laser beam diameter.
`direction. divergence. convergence. and orientation. In some
`embodiments. optical elements modify the wavelength ofthe
`laser beam andfor filter out certain wavelengths of electro-
`magnetic energy in the laser beam.
`Lenses that can be used in various embodiments of the
`
`invention include. aplanatic lenses. achromatic lenses. single
`element lenses. and Fresnel lenses. Mirrors that can be used in
`various embodiments ol‘ the invention include. coated mir-
`rors. dielectric coated mirrors_. narrow band mirrors. and
`ultraviolet transparent infrared refiecting mirrors. By way of
`example. ultraviolet transparent infrared rellecting mirrors
`are tlsed in some embodiments of the invention where it is
`desirable to filter out infrared energy front a laser beam while
`permitting ultraviolet‘ energy to pass through the min'or to be
`delivered to atool (e.g.. a wafer inspection tool, a microscope.
`a lithography tool or an endoscopic tool).
`In this embodiment. the chamber 128 is a sealed chamber
`initially containing the ionizable medium (e. g. . a solid. liquid
`or gas). In some embodiments. the chamber 128 is instead
`capable o'|' being actively pumped where one or Iuore gases
`are introduced into the chamber 128 through a gas inlet (not
`shown). and gas is capable of exiting the chamber 128
`through a gas outlet (not shown). The chamber can be fabri-
`cated from orinclude one or more of. lorexample. a dielectric
`material. a quamz material_. Suprasil quart':r._. sapphire. Mgliz.
`diamond or C al-'2. The type of material may be selected based
`on. for example, the type of ionizable medium used andfor the
`wavelengths oflight 136 that are desired to be generated and
`output from the chamber 1 28. In some embodiments. a region
`ofthc chamber 128 is transparent to. for example. ultraviolet
`energy. Chainbers I28 fabricated using quart): will generally
`allow wavelengths of electromagnetic energy ofas long as
`about 2 microns to pass through walls of the chamber. Sap-
`phire chamber walls generally allow electromagnetic energy
`of as long as about 4 microns to pass through the walls.
`In some embodiments. it is desirable for the chamber 128
`to be a sealed chamber capable o l‘ sustaining high pressures
`and temperatures. For example. in one embodiment. the ion-
`izable medium is mercury vapor. To contain the mercury
`vapor during operation. the chamber 128 is a sealed quartz
`bulb capable of sustaining pressures between about
`ID to
`about 200 atmospheres and operating at about 900 degrees
`ccntigrade. The quartz bulb also allows for transmission 0|"
`the ultraviolet light [36 generated by the plasma 132 of the
`light source 100 through the chamber 128 walls.
`Various ionizable media can be used in alternative e111bodi—
`ments of the invention. For example. the ionizable medium
`can be one or more o la noble gas. Xe. Ar. Ne. Kr. I Ic. D2. lI2.
`()3. F2. :1 metal halide. a halogen. Hg. ("d. Zn. Sn. (ia. Fe, Li.
`Na. an excimer tbrming gas. air. a vapor. a tnetal oxide. an
`aerosol, a flowing media. or a recycled media.
`In some
`embodiments. a solid or liquid target (not shown) in the
`chamber 128 is used to generate an ionirable gas in the
`chamber 128. The laser source 104 (or an alternative laser
`source} can be used to provide energy to the target to generate
`the ionizable gas. The target can bc. lbr example. a pool or
`film of metal. In some embodiments. the target is a solid or
`liquid that moves in the chamber (e.g., in the form of droplets
`of a liquid that travel through the region 130 ol‘ the cliarnber
`128). In some embodiments. :1 first ioni'/able gas is first intro-
`duced into the chamber 128 to ignite the plasma 132 and then
`a separate second ionizable gas is introduced to sustain the
`plasma 132. In this embodiment. the first ionizable gas is a gas
`that is more easily ignited using the ignition source 140 and
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`tl1e second ionjzable gas is a gas that produces a particular
`wavelength of electromagnetic energy.
`In this emboditnent. tl1e ignition source 140 is a pair of
`electrodes located in the chamber 128. In some embodiments.
`the electrodes are located on the same side of the chamber
`128. A single electrode can be used with. tor example, an RF
`ignition source or a microwave ignition source. In some
`embodiments. the electrodes available in a conventional arc
`lamp bulb are the ignition source (e.g._. a model U Sl'l-2UOI)l-’
`quartz bulb ntanulactured by Ushio (with olliccs in Cypress.
`("alil‘.)). In some embodiments. the electrodes are smaller
`andlor spaced further apart than the electrodes used in a
`conventional arc lamp bulb because the electrodes are not
`required for sustaining the high brightness plasma in the
`chamber 128.
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`Various types and configurations of ignition sources are
`also contemplated. however. that are within the scope of the
`present invention. In some embodiments, the ignition source
`140 is external to the chamber 128 or partially internal and
`partially external to the chamber I28. Alternative types of
`ignition sources 140 that can be used in the light source 100
`include ultraviolet
`ignition sources_. capacitive discltarge
`ignition sources.
`inductive ignition sources. RF ignition
`sources. a microwave ignition sources, llash lamps. pulsed
`lasers. and pulsed lamps. In one embodiment. no ignition
`source I40 is required and instead the laser source 104 is used _
`to ignite the ionirable medium and to generate the plasma I 3 2
`and to sustain the plasma and the high brightness light 136
`emitted by the plasma 132.
`In some embodiments. it is desirable to maintain the tem-
`perature of the chamber 128 and the contents ofthe charnber
`128 during operation ofthe light source 100 to ensure that the
`pressure of gas or vapor within the chamber 128 is maintained
`at a desired level. In some embodiments. the ignition source
`140 can be operated during operation ofthe light source 100.
`where the ignition source I40 provides energy to die plasma
`132 in addition to the energy provided by the laser source 104.
`In this manner. the ignition source 140 is used to maintain (or
`maintain at an adequate level) the temperature ofthe chamber
`128 and the contents of tl1e chamber 128.
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`In some embodiments. the light source 1 00 includes at least
`one optical element (e.g.. at
`least one mirror or lens) for
`Inodi lying a property to I the electromagnetic energy (eg._. the
`high brightness light 136) emitted by the plasma 13 2 (eg. an
`ionized gas), similarly as described elsewhere herein.
`FIG. 2 is a schematic block diagram ofa portion ofa light
`source 200 incorporating principles of the present invention.
`The light source 200 includes a chamber 128 containing an
`ionizable gas and has a wittdow 204 that maintains a pressure
`within the chamber 128 while also allowing electromagnetic
`energy to enter the chamber 128 and exit the chamber 128. In
`this embodiment. the chamber 128 has an ignition source {not
`shown) that ignites the ionizable gas {e.g.. mercury or xenon)
`to produce a plasma 132.
`A laser source 104 (not shown) provides a laser beam 216
`that is directed through a lens 208 to produce laser beam 220.
`The lens 208 focuses the laser beam 220 on to a surface 224
`ofa thin film reflector 212 that reflects the laser beam 220 to
`produce laser beam 124. The rellector 212 directs the laser
`beam 124 on region 130 whcrethe plasma 132 is located. The
`laser beam 124 provides energy to the plasma 132 to sustain
`andlor generate a high brightness light 136 that is emitted
`from the plasma 132 in the region 130 of the chamber 128.
`In this embodiment. the chamber 128 has a paraboloid
`shape and an inner surface 228 that is relleetive. The parabo-
`loid shape and the relloctive surface cooperate to rcllect a
`substantial amount o