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`US00?435982B2
`
`(12) United States Patent
`Smith
`
`(10) Patent No.:
`
`(45; Date of Patent:
`
`US 7,435,982 B2
`Oct. 14, 2008
`
`(54) LASER-DRIVEN LIGHT SOURCE
`
`2005r'0l67(1l8 Al"‘
`200'?1’0228300 Al‘
`
`852005 Hoshinoetal.
`I0.-‘"2007 Smith
`
`250.-' 504 R
`250.-"504 R
`
`(751
`
`Inventor: Donald K. Smith. Belmollt. MA (US)
`
`FOREIGN PATENT DOCUMENTS
`
`(73) Assignee: Encrgctiq Technology, Inc.. Woburn.
`MA (US)
`
`JP
`
`6 I - I 93 3 5 8
`
`S." l 936
`
`OTHER PU Bl.lCATJONS
`
`( * ) Notice:
`
`Subject to any disclaimer. tl1e term of this
`patent is extended or adjusted under 35
`U.S.(T. 154(1)) by 452 days.
`
`(21) Appl.No.: 111395.523
`
`(22)
`
`Filed:
`
`Mar. 31, 2006
`
`(65)
`
`Prior Publication Data
`US 2007110228288 Al
`Oct. 4. 2007
`
`(51)
`
`Int. Cl.
`(2006.01)
`/16IN 5/06
`(2006.01)
`GOIJ 3/10
`(2006.01)
`H056" 2./00
`250504 R; 2501423 1’;
`(52) U.S. (fl.
`2501’-426; 250f493.l: 43811104; 4381301; 438E513;
`438E156‘. 252r‘30l.36: 25211301. 16: 252f3()l.4 1-‘;
`38513] : 385.133; 38588
`(58) Field of Classification Search . .......... .. 250504 R,
`250.1423 1’. 426. 493.1: 438104. 301. 513.
`438056; 252E30].16. 301.36.301.41‘: 38551.
`38583. 38
`
`See application file for complete search history.
`
`(56)
`
`References Cited
`U .S. PA'l‘}.'iN"1' 1')O(TUM[7.N’l‘S
`
`6.288.T8(l Bl
`6.'I88.4tl4 B2
`2004.-50264512 Al
`
`9.12001 Fairleyetal.
`‘).-'2004 Lange
`..
`I2-"2004 Ha.i'tl0ve elal.
`
`356-"23?.l
`356.-"2312
`372.55
`
`Wilbers 01 al.. "The VUV Emissivity ofa High-Pressure Cascade
`Argon Arc from 125 to 200 nm.” J. Quaut. S};ecrm.rc. Rodiar. Tram‘-
`fi::'.1ml. 45. 1001. pp. 299-302.
`Wilbers ct a1.. “"1110 Continuum Emission ofluc Plasma.” J. Qmzm.
`.S‘p(’¢1‘r.I1o.\‘r.'. Rad.1'a.r.
`i'7"aH.§'fe’r'. vol. 45. No. 1. I991. pp.
`l~ I0.
`Beck. "Simple Pulse Generator lbr P111.-ring Xenon Arcs with High
`Repetition Rate." Rev. Sci. 1’1t.m'um.. vol. 45. =.\':o. 2. Feb. 1914. pp.
`318-319.
`Rztizer. “Optical Discharges.“ S011 P)'1_1=3. Us-p. 23(1 1). Nov. 1980. pp.
`739306.
`Fiedorowicz et 211.. “X—Ray Emission form l.aser—Irradiated Gas Pull‘
`'1'a.1'gcts." Appl. Phys. Lerr. 62[22}. May 31. 1993. pp. 2778-2780.
`Keefer et al.. “Experimental Study of a Stationary Laser-Sustained
`Air Plasma.” .!o1m:m’ of.-1'pp!1'ed PJ‘.=_1:rr'cs. vol. 46. No. 3. Mar. 1915.
`pp. 1080-1083.
`
`((Tontir1L1ed)
`
`Prfmar_1' E.\'a.m:'ner—Jack I Bertnan
`/1ss:'.9.=‘on1‘ I§.\'a:11ir1er Mccnakshi S Sahu
`
`(74) .4t1‘orm3}'. Agent. or 1"i'rm Pmskeittcr Rose. LLP
`
`(57)
`
`ABSTRACT
`
`An apparatus for producing light includes a chamber and an
`ignition source that ionianes a gas withill the Cl1fl1'I1l')(.’.I'. The
`apparalus also includes at least one laser that provides energy
`to the ionized gas within tl1e chamber to produce a high
`brightness light. The laser can provide a substantially con-
`tinuous anlount of energy to the ionized gas to generate a
`substantially continuous high brightness light.
`
`81 Claims, 4 Drawing Sheets
`
`140
`
`
`
`108
`
`104
`
`112
`
`100
`
`(cid:36)(cid:54)(cid:48)(cid:47)(cid:3)(cid:20)(cid:20)(cid:21)(cid:20)
`
`ASML 1121
`
`

`
`US 7,435,982 B2
`Page 2
`
`OTHER PUBLICATIONS
`
`Jeng et al.. “Theoretical lnvcstigation of Laser-Sustained Argon Plas-
`mas." J’. Appl. Phys. 6U('I). Oct. 1. 1986. pp. 22’F2-2279.
`Franzen. “CW Gas Breakdown in)-\rgon Using 10.6-pm Laser Radia-
`tion." Appt'. Phys. Lem, vol. 21. No. 2. Jul. 15. 1972, pp. 62-64.
`Moody. “Maintenance ofa Gas Breakdown in Arggn Using 10.6-p cw
`Radiation.” Jormm.’ of.-tppf."ed P.r‘.=ySi€.t', vol. 46. No. 6. Jun. I9't'5. pp.
`2475-2482.
`
`Generalov ct ai.. "Experimental Investigation ot'a Continuous Opti-
`cal Dis-;:ha.rge.“ Soviet’ Physics JETP. vol. 34. No. 4. Apr. 1972. pp.
`763369.
`
`Generalov et al.. “Continuous Optical Discharge.” ZhETFPr'_t'. Red.
`1 1. No. 9. May 5, I970, pp. 302-304.
`
`Kozlov et al.. “Radiative Losses by Argon Plasma and the Emissive
`Model ofa Continuous Optical Discharge.“ Sov. Phys. JEPT. vol. 3-9.
`No. 3. Sop. 1974. pp. 463-468.
`Carlhoff et a].. “Contintlotts Optical Discharges at Very High Pres-
`sure.” Pit)-‘sica 103C. I981. pp. 439-447.
`Cremers et .11.. “Evaluation Of the Continuous Optical Discharge for
`Spcctrochcmica] Analysis,” Specnvclrirrtfm Acra, vol. 4013. No. 4,
`I985. pp. 665-679.
`Kozlov :21 al.. “Sustained Optical Discharges in Molecular Gases.”
`Sov. Phys. Tech. Phys. 49(l 1). .\lov. 1979. pp. 1283-1287.
`Keefer. “Laser-Sustained Plasmas." Laser‘-Irtduced Pfasirras and
`Appficatiotts. published by Marcel Dekker. edited by Radziornski 01
`a]., 1989. pp. 169-206.
`Ha.Ina.matsu Product Information. "Super—Quiet Xenon lamp Super-
`Qnict Mercury-Xenon Lamp.” Nov. 2005.
`
`* cited by examiner
`
`

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`U.S. Patent
`
`Oct. 14,2003
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`Sheet] of4
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`US 7,435,982 B2
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`U.S. Patent
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`on. 14, 2008
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`Oct. 14, 2008
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`
`1
`LASER-DRIVEN LIGHT SOURCE
`
`FIELD OF THE INVISNTION
`
`The invention relates to methods and apparatus for provid-
`ing a laser-driven light source.
`
`5
`
`BACKGROUND OF THE INVENTION
`
`l-liglr 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 wafers (e.g.. reticles and plrotorrrasks). The
`electrornagnetie cnerg! produced by high brightness lights
`sources can. alternatively. be used as a source of illumination
`in a lithography system used in the fabrication of wafers. :1
`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 rrtercury art: lamps 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 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 a result‘. the anode andfor
`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.
`Accordirrgly. a need tlrerteforc 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 clramber. 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 ernbodiments. 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 modifyinga property
`of the laser energy provided to the ionized gas. The optical
`element cart be. for example. a lens (e.g._. an aplanatic lens. an
`achrornatic lens. a single element lens, and a Fresnel lens} or
`mirror (e.g., a coated mirror, a dielectric coated mirror. a
`narrow band rrrirror. and an ultraviolet transparent infrared
`reflecting mirror). In sortie embodiments. the optical elenrent
`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 (i-leraeus Quartz America, LLC. Buford. Ga.)_.
`sapphire. Mgl"2. diamond, and Caliz. 111 some embodiments.
`the chamber is a sealed chamber. In some embodiments. the
`
`3o
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`50
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`60
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`US ?,435,982 B2
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`2
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`In some
`chamber is capable of being actively pumped.
`embodiments.
`the chamber includes a dielectric material
`[e.g.. quartz). The chamber can be, for example, 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. KI.
`I-Ie, D2, H2. 02, F2, a metal halide. a halogen, Hg, Cd, Zn. Sn.
`Ga. I-‘c. Li. Na. an excirner forming gas. air. a vapor. a metal
`oxide, an aerosol, a flowing 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 rrretal. ln sorrre embodiments. the target is
`capable of moving. For example. the target may be a liquid
`that is directed to a region from which the high brightness
`light originates.
`In some embodiments, the at least one laser is multiple
`diode lasers coupled into a Iiber optic element. In some
`embodiments. the at least one laser includes a pulse or con-
`tinuous wave laser. In sorrre embodiments. the at least one
`laser is an IR laser. a diode laser. a fiber laser. an ytterbiurn
`lase , a C03 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 ionized medium.
`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. The
`ignition source can he external or internal to the chamber.
`The light source can include at least one optical element for
`modifying a property ofelectronragnetic radiation emitted by
`the ionized gas. The optical element can be. for example, one
`or more mirrors or lenses. in sorrre embodiments. the optical
`element is configured to deliver the electromagnetic radiation
`entitled by the ionized gas to a tool (e.g., a wafer inspection
`tool. a microscope, a metrology tool. a lithography tool, oran
`endoscopic tool).
`The invention, in another aspect. relates to a method tor
`producing light. The method involves ionizing with an igni-
`tion sourcc 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 erribodiments. 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
`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 elerrrent to modify a property of the light. In some
`ernbodirircnts, the method also involves delivering the high
`brightness light 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).
`In another aspect. the invention features a light source. The
`lights source includes a chamber and an ignition source for
`ionizing an ionizable rnedirun within the chamber. The light
`source also includes at least one laser for providing substan-
`tially continuous energy to the ionized rrredimn within the
`chamber to produce a high brightness light.
`In some cnrbodiments. 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 a high pulse rate laser that provides
`pulses o fenergy to the ionized medium so the high brightness
`light is substantially oontinuous. In some embodiments, the
`magnitude ofthe high brightness light does not vary by more
`than about 90% during operation. In some embodiments. the
`at least one laser provides energy substantially continuously
`
`

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`3
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`4
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`US ?,435,982 B2
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`to minimize cooling of the ionized medium when energy is
`not provided to the ionized medium.
`In sortie embodiments. the light source can include at least
`one optical element (e.g., a lens or mirror) for modifying a
`property of the laser energy provided to the ionized medium.
`The optical element can 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 rellccting mirror. ht some
`embodiments, the optical element is one or more fiber optic
`elements for directing the laser energy to the ionizable
`medium.
`111 some embodiments. the chamber includes an ultraviolet
`transparent region. In some embodiments, the chamber or a
`window in the chamber includes a quartz material, suprasil
`quartz material. sapphire material, Mg!-‘E material. diamond
`material. or C aF2 material. In some embodiments. the cham-
`ber is a sealed chamber. The chamber can be capable ofbeing
`actively pumped.
`In some embodiments.
`the cliamber
`includes a dielectric material (e.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
`ionizable medium can include one or more ofa noble gas. Xe.
`Ar. Ne. Kr. He, 1):, H2, 02, F3. a metal halide. a halogen, Hg_._
`C(1, Zn, Sn, Ga. Fe, [.i. Na. an excimer forming gas. air. a
`vapor, a metal oxide. an aerosol, a flowing media, a recycled
`media. or art evaporating target. In some embodiments. 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. ’I'hc target can be a pool or film of metal. In
`some embodiments. the target is capable of moving.
`In some ernbodiments_. the at least one laser is multiple
`diode lasers coupled into 3 Fiber optic element. The at least
`one laser can emit at least one wavelength of electromagnetic
`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 1 i ght 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
`mediuni. The optical element can be configured to deliver the
`electromagnetic radiation emitted by the ionized meditun to a
`tool (eg. a water 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 an ionirable medium within a chamber. The
`method also involves providing substantially continuous
`laser energy to the ionized medium in the chamber to produce
`a high brightttess light.
`In some embodiments. the method also involves directing
`the laser energy through at least one optical element for
`ntodilyittg a property of the laser energy provided to the
`ionizable medium. The method also can involve actively
`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 entbodinients. 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.
`
`‘-4-:
`
`1D
`
`The light source also includes a means lbr 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 and from the claims.
`
`l'3Rll:il-‘ [)1-iSCRIP'l‘ION OI-' TI-ll':i [)RAWIN(}S
`
`The foregoing and other objects, feature and advantages of
`the invention. as well as the invention itself. will be more fully
`understood fmm the following illustrative description. when
`read together with the accompanying drawings which are not
`necessarily to scale.
`1-‘ I(i. 1 is a schematic block diagram of a light source.
`according to an illustrative embodiment of the invention.
`FIG. 2 is a schematic block diagram of a portion of a light
`source. according to an illustrative embodiment of the inven-
`tion.
`
`FIG. 3 is a graphical representation of UV brightness as a
`function ofthe 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.
`
`Dl5.TAII..l'?.D Dl.iS(.‘RIPTION OF II..LUS'I‘RATI\/Ii.
`EMBODIMENTS
`
`3t]
`
`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-
`
`35
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`4f]
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`45
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`50
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`65
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`able medium (not shown}. The 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
`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 located in the chamber 128 to
`initiate atidfor 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 ionizable medium
`in order to rnaxirnizc the elliciency of the transfer of energy
`from the laser source 104 to the ionizable medium.
`
`In some embodiments, it is desirable for the plasma 132 to
`be small in size in under to achieve a high brightness 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 metrology 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
`I04 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 high power laser beam leads simult. -
`neously to a high brightness light 136. The light source 100
`produces a high brightness light 136 because most of the
`power introduced by t.he laser source 104 is then radiated
`from a small volume, high temperature plasma 132. The
`plasma 132 temperature will rise due to heating by the laser
`beam 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
`ol'electromagnctic energy. lbrexatnple. ultraviolet energy. In
`one experiment. temperatures between about 10,000 K and
`about 20.000 K have been observed. The radiation of the
`plasma 132. i11 a general sense. is distributed over the elec-
`tromagnetic spectrum according to l’lanclt’s radiation law.
`
`

`
`US N-135,982 B2
`
`5
`
`The wavelength of maximum radiation is inversely propor-
`tional to the temperature of a black body according to Wien's
`displacement law. While the laser sustained plasma is not a
`black body.
`it behaves similarly and as 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.
`Conventiorial arc lamps are. however, tntable to operate at
`these temperatures.
`It is therefore desirable in some embodiments of the 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 lipdit 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 112 that aids in conditioning the output of the diode laser
`by aiding in making laser beam rays 116 substantially parallel
`to each other. The collimator 112 then directs the laser beam
`US to a beam expander 118. The beam expander 118 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 smaller diznneter 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 light source 100 also includes an
`ignition source 140 depicted as two electrodes (e.g._. an anode
`and cathode located ir1 the chamber 128). The ignition source
`140 generates an electrical discharge in the chamber 128
`(eg. 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 136 gener-
`ated by the light source 100 is t.l1cn directed out of 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 embodiments, addi-
`tional or alternative optical elements can be used. The laser
`source can
`for example. an infrared (IR) laser source. a
`diode laser source, a fiber laser source. an ytterbium laser
`source. a C0: 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 sortie er11bodi-
`tncnts. multiple lasers [e.g.. diode lasers) are coupled to one
`or more fiber optic elements (e.g._. the fiber optic element
`108). In some embodiments. fiber laser sources and direct
`semiconductor laser sources are desirable for 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
`efliciency.
`In some embodiments. the laser source 104 is a high 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 suliicient to mini-
`mize cooling of 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. a 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 (e.g.._ r11irrors or lenses).
`For example. one or more optical elements can be used to
`modify the portions 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 cart be used in various ernhodiments of the
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`invention include. aplanatic lenses. aclnomatic lenses. single
`element lenses, and fresnel lenses. Mirrors that cart be used in
`various embodiments of the invention include. coated mir-
`rtors, dielectric coated mirrors, narrow band mirrors, and
`ultraviolet transparent infrared refiecting mirrors. By way of
`example. ultraviolet transparent infrared refiecting mirrors
`are used in some embodiments of the invention where it is
`desirable to filter out infrared energy from a laser beam while
`pertnitting ultraviolet energy to pass through the mirror to be
`delivered to atool (e.g., a wafer inspectiontoo], 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 of being actively pumped where one or Inore 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. forexample. a dielectric
`material. a quartz material, Snprasil quart7._ sapphire. Mgi-‘2.
`diamond or C aF2. The type ofrttaterial may be selected based
`on. for example, the type of ionizable medium used andfor the
`wavelengths of light 136 that are desired to be generated and
`output from the chamber 128. In some ernboditnents, a region
`of the chamber 128 is transparent to. for example, ultraviolet
`energy. Chambers 123 Jabricatcd using quart‘/. will generally
`allow wavelengths of electromagnetic energy of as 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 of sustaining high pressures
`and tcntperatures. For example. in one embodiment. ll1e 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 10 to
`about 200 atmospheres and operating at about 900 degrees
`Centigrade. The quartz bulb also allows for transmission of
`the ultraviolet light 136 generated by the plasma 132 of the
`light source 100 through the chamber 128 walls.
`Various ionizable media can be used in alternative er11bodi—
`mcnts of the invention. For example, the ionizablc medium
`can be one or more ofa noble gas. Xe. Ar. Ne. Kr. I Ic. 1):, 1-12.
`()2. F2. a metal halide. a halogen. Hg. Cd. Zn_. Sn. Ga. Fe, Li.
`Na. an excimcr fonning gas. air. a vapor. a metal oxide. an
`aerosol, a flowing media. or a recycled media.
`In sortie
`embodirnents. a solid or liquid target (not shown) in the
`chamber 128 is used to generate an ionirable gas in the
`chamber 123. The laser source 104 (or an alternative laser
`source} cart be used to provide energy to the target to generate
`the ionizable gas. The target can be. for 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 of the chamber
`128). hi some enibodinieuts. a first iouizablc 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 llrst ioni‘/able gas is a gas
`that is more easily ignited using the ignition source 148 and
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`US ?,435,982 B2
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`the second iortizable gas is a gas that produces a particular
`wavelength of electromagnetic energy.
`In this embodiment. 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 charriber
`128. A single electrode can be used with. for example, an RF
`ignition source or a microwave ignition source. In some
`embodiments. tl1e electrodes available iii a conventional arc
`lamp bulb are the ignition source (e.g., a model USH-200l)P
`quartz bulb manufactured by Ushio (with oflices in Cypress.
`Calif.)). In some embodiments. the electrodes are smaller
`andfor 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.
`
`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 extemal to the chamber 128 or partially internal and
`partially external to the chamber 128. Alternative types of
`ignition sources 140 that can be used in the light source 100
`include ultraviolet
`ignition sources. capacitive discharge
`ignition sources.
`inductive ignition sources. RF ignition
`sources, a microwave ignition sources, flash lamps, pulsed
`lasers. and pulsed lamps. 111 one embodiment. no ignition
`source 1 40 is required and instead the laser source 104 is used
`to ignite the ionizable medium and to generate the plasma 132
`and to sustain the plasma and the high brightness light 136
`emitted by the plasma 132.
`ill some ernbodiments. it is desirable to maintain the tem-
`perature ol the chamber 128 and the contents of tile cltamber
`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 140 provides energy to the plasma
`132 in addition to the energy provided by the la scr source 104.
`In this manner. the ignition source 140 is used to niaintairi (or
`maintain at an adequate level) the temperature ofthe charn her
`128 and the contents of the chamber 128.
`
`In sortie embodiments, the light source 1 00 includes at least
`one optical element [e.g.. at least one mirror or lens) for
`modifying a property of the electromagnetic energy (e.g._. 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 window 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 reflector 212 directs the laser
`beam 124 on region 130 wherethe 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 reflective. The parabo-
`loid shape and the reflective surface cooperate to reliect a
`substantial amount of the high brightness light 136 toward
`and out of the window 204. In this embodi1nent_. the reflector
`212 is transparent to the emitted light 136 (c.g., at least one or
`more wavelengths of ultravi