US007786455B2
`
`(12) Ulllted States Patent
`Smith
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,786,455 B2
`Aug. 31, 2010
`
`6,288,780 B1
`6,417,625 B1*
`
`9/2001 Fairley etal.
`7/2002 Brooks et a1.
`
`.......... .. 356/237.1
`........ .. 315/111.31
`
`9/2004 Lange ................... .. 356/237.2
`6,788,404 B2
`6,956,329 B2 * 10/2005 B
`k
`tal.
`315/111.31
`7,652,430 B1*
`1/2010 D:i;iii: .. ...
`. ... .. 313/633
`2002/0021508 A1
`2/2002 Ishihara . . . . . .
`. . . . .. 359/853
`
`
`
`9/2003 Kim ......................... .. 313/634
`2003/0168982 A1
`12/2003 Sato et a1.
`................. .. 362/268
`2003/0231496 A1
`2004/0264512 A1* 12/2004 Hartlove et al.
`.............. .. 372/5
`2005/0167618 A1*
`8/2005 Hoshino et al.
`....... .. 250/504 R
`
`2007/0285921 A1* 12/2007 Zulim et al.
`
`.............. .. 362/240
`
`(54) LASER-DRIVEN LIGHT SOURCE
`
`(75)
`Inventor: Donald K- Smith,Be1n10nt, MA (US)
`(73) Assignee Energetiq Teeh“°1°gY= Ine-= W°b“m=
`MA (US)
`
`(,1) Notice:
`
`Subject to any disclaimer, the term Ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(1)) by 820 days.
`
`(21) App1.N0.Z 11/695,348
`
`(22)
`
`Filed:
`
`Apr. 2, 2007
`
`(65)
`
`Prior Publication Data
`
`(Continued)
`
`Related U.S. Application Data
`
`JP
`
`51.193353
`
`3/1935
`
`(63) Continuation-in-part of application No. 11/395,523,
`filed on Mar. 31, 2006, now Pat. No. 7,435,982.
`
`(51)
`
`Int CL
`(200601)
`H053 31/26
`(200601)
`G01‘, 3/10
`(200601)
`GZIG 4/00
`(2006.01)
`H01J 61/28
`(52) U.S. Cl.
`............................. .. 250/493.1; 250/504 R;
`315/111.21; 315/111.71; 315/111.91; 313/231.31;
`313/23141; 313/23171
`(58) Field of Classification Search ........... .. 250/423 R,
`250/423 P, 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,
`,
`1,
`fil f
`1
`h
`3/632’ 633
`ee app lcanon
`e or Comp ete Seam lstory‘
`References Cited
`
`S
`
`(56)
`
`OTHER PUBLICATIONS
`.,
`ty of a High-Pressure Cascade
`“The VUV Emissivi
`'
`Wilbers et al
`Argon Arc from 125 to 200 nm,” .1. Quant. Spectrosc. Radial. Trans-
`fer, vol. 46, 1991, pp. 299-308.
`
`(Continued)
`Primar Examl.neriBemardE Souw
`(74) Azforne A em or Firm—Proskauer Rose LLP
`y’ g
`’
`
`(57)
`
`ABSTRACT
`
`An apparatus for producing light includes a chamber and an
`ignition source that ionizes a gas within the chamber. 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-
`Unueus emeum ef energy.” the lemzeegas te generate a
`Substamlany eemmueus hlgh bnghmess hghe
`
`43 Claims, 8 Drawing Sheets
`
`U-S- PATENT DOCUMENTS
`4,088,966 A *
`5/1978 Samis
`................. .. 313/231.51
`4,498,029 A *
`2/1985 Yoshizawa etal.
`.......... .. 315/39
`4,646,215 A
`2/1987 Levin et a1.
`....... ..
`. 362/296
`
`.......... .. 315/39
`RE32,626 E *
`3/1988 Yoshizawa et al.
`
`228
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`ASML 1325
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`ASML 1325
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`

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`US 7,786,455 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`2009/0032740 A1*
`
`2/2009 Smith etal.
`
`............ .. 250/503.1
`
`OTHER PUBLICATIONS
`
`Wilbers et al., “The Continuum Emission of an Arc Plasma,” ./'.
`Quant. Spectrosc. Radial. Transfer, vol. 45, No. 1, 1991, pp. 1-10.
`Beck, “Simple Pulse Generator for Pulsing Xenon Arcs with High
`Repetition Rate,” Rev. Sci. Instrum., vol. 45, No. 2, Feb. 1974, pp.
`3 18-3 19.
`Raizer, “Optical Discharges,”Sov. Phys. Usp. 23(11), Nov. 1980, pp.
`789-806.
`Fiedorowicz et al., “X-Ray Emission form Laser-Irradiated Gas Puff
`Targets,”Appl. Phys. Lett. 62 (22), May 31, 1993, pp. 2778-2780.
`Keefer et al., “Experimental Study of a Stationary Laser-Sustained
`Air Plasma,” Journal ofApplied Physics, vol. 46, No. 3, Mar. 1975,
`pp. 1080-1083.
`Jeng et al., “Theoretical Investigation ofLaser-Sustained Argon Plas-
`mas,” .1. Appl. Phys. 60 (7), Oct. 1, 1986, pp. 2272-2279.
`Franzen, “CW Gas Breakdown in Argon Using 10.6-um Laser Radia-
`tion,”Appl. Phys. Lett., vol. 21, No. 2, Jul. 15, 1972, pp. 62-64.
`Moody, “Maintenance ofa Gas Breakdown in Argon Using 10 .6-ucw
`Radiation,” Journal ofAppliedPhysics, vol. 46, No. 6, Jun. 1975, pp.
`2475-2482.
`
`Generalov et al., “Experimental Investigation of a Continuous Opti-
`cal Discharge,” Soviet Physics JETP, vol. 34, No. 4, Apr. 1972, pp.
`763-769.
`
`Generalov et al., “Continuous Optical Discharge,” ZhETF Pis. Red.
`11, No. 9, May 5, 1970, pp. 302-304.
`Kozlov et al., “Radiative Losses by Argon Plasma and the Emissive
`Model of a Continuous Optical Discharge,” Sov. Phys. JETP, vol. 39,
`No.3, Sep. 1974, pp. 463-468.
`Carlhoff et al., “Continuous Optical Discharges at Very High Pres-
`sure,” Physica 103C, 1981, pp. 439-447.
`Cremers et al., “Evaluation of the Continuous Optical Discharge for
`Spectrochemical Analysis,” Spectrochimica Acta, vol. 40B, No. 4,
`1985, pp. 665-679.
`Kozlov et al., “Sustained Optical Discharges in Molecular Gases,”
`Sov. Phys. Tech. Phys. 49(11), Nov. 1979, pp. 1283-1287.
`Keefer, “Laser-Sustained Plasmas,” Laser—Induced Plasmas and
`Applications, published by Marcel Dekker, edited by Radziemski et
`al., 1989, pp. 169-206.
`Hamamatsu Product Information, “Super-Quiet Xenon Lamp Super-
`Quiet Mercury-Xenon Lamp,” Nov. 2005.
`
`* cited by examiner
`
`

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`Aug. 31, 2010
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`

`
`1
`LASER-DRIVEN LIGHT SOURCE
`
`RELATED APPLICATIONS
`
`US 7,786,455 B2
`
`2
`
`This application is a continuation-in-part of U.S. Ser. No.
`ll/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 THE INVENTION
`
`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 wafers or materials used in the
`
`fabrication of wafers (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 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. The 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 a result, the anode and/or
`cathode are prone to wear and may emit particles that can
`contaminate the light source or result in failure of the light
`source. Also, these arc lamps do not provide sufficient bright-
`ness for some applications, especially in the ultraviolet spec-
`trum. Further, 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 (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
`
`5
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`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 CaF2. 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 some 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, F2, a metal halide, a halogen, Hg, Cd, Zn, Sn,
`Ga, Fe, Li, Na, an excimer 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 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 the high 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 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 a continuous wave (CW) or pulsed
`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 can include at least one optical element for
`modifying a property of electromagnetic 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 configured to deliver the electromagnetic radiation
`emitted by the ionized gas to a tool (e.g., 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 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 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 (e.g.,
`a wafer inspection tool, a microscope, a metrology tool, a
`lithography tool, or an endoscopic tool).
`
`

`
`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. 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.
`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 a high pulse rate laser that provides
`pulses of energy to the ionized medium so the high brightness
`light is substantially continuous. In some embodiments, the
`magnitude of the 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
`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 (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 reflecting mirror. In some
`embodiments, the optical element is one or more fiber optic
`elements for directing the laser energy to the ionizable
`medium.
`
`In 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, MgF2 material, diamond
`material, or CaF2 material. In some embodiments, the cham-
`ber is a sealed chamber. The chamber can be capable ofbeing
`actively pumped.
`In some embodiments,
`the chamber
`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 of a noble gas, Xe,
`Ar, Ne, Kr, He, D2, H2, 02, F2, a metal halide, a halogen, Hg,
`Cd, Zn, 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 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. The target can be a pool or film of metal. In
`some embodiments, the target is capable of moving.
`In some 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 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 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 (e.g., 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 light. The method involves ionizing with an igni-
`tion source an ionizable medium within a chamber. The
`
`10
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`
`method also involves providing substantially continuous
`laser energy to the ionized medium 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
`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 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 continuous 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 reflector
`that at least substantially reflects 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 electromagnetic energy to pass
`through the reflector. 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.
`In some embodiments, at least one laser directs a first set of
`wavelengths of electromagnetic energy through the reflector
`toward the reflective surface (e.g., inner surface) ofthe cham-
`ber and the reflective surface directs at least a portion of the
`first set of wavelengths of electromagnetic energy toward the
`plasma. In some embodiments, at least a portion of the high
`brightness light is directed toward the reflective surface of the
`chamber, is reflected toward the reflector, and is reflected by
`the reflector toward a tool. In some embodiments, at least one
`laser directs a first 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 reflective surface of the chamber, and the reflec-
`tive surface directs a portion of the first set of wavelengths of
`electromagnetic energy toward the plasma.
`In some embodiments, at least a portion of the high bright-
`ness light is directed toward the reflective surface of the
`chamber, is reflected toward the reflector, and passes through
`the reflector toward an output of the light source. In some
`embodiments, the light source comprises a microscope, ultra-
`violet microscope, wafer 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 reflective surface of the chamber,
`is
`reflected toward the reflector, and electromagnetic energy
`comprising the second set of predefined wavelengths of elec-
`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 reflective surface of the chamber com-
`
`

`
`US 7,786,455 B2
`
`5
`prises a curved shape, parabolic shape, elliptical shape,
`spherical shape or aspherical shape. In some embodiments,
`the chamber has a reflective inner surface. In some embodi-
`
`6
`magnetic energy comprising the second set of predefined
`wavelengths of electromagnetic energy passes through the
`reflector.
`
`ments, a coating or film is located on the outside of the
`chamber to produce the reflective surface. In some embodi-
`ments, a coating or film is located on the inside ofthe chamber
`to produce the reflective surface. In 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 sur-
`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.
`
`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-
`lengths of electromagnetic energy to pass through the reflec-
`tor.
`
`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 of wavelengths of electro-
`magnetic energy through a reflector toward the reflective
`surface of the chamber, the reflective surface reflecting at
`least a portion of the first set of wavelengths of electromag-
`netic energy toward the plasma. In some embodiments, the
`method involves directing at least a portion ofthe 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.
`
`In some embodiments, the method involves directing the
`laser energy comprising a first set of wavelengths of electro-
`magnetic energy toward 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 ofelectromagnetic energy 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-
`
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`25
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`35
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`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
`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 0.048 stera-
`dians. 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 greater than about 275 (about
`6.28) steradians. In some embodiments, the reflective surface
`of the chamber is adapted to provide the laser energy to the
`plasma over a large solid angle. In some embodiments, the
`reflective surface ofthe 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 reflects 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 energy from
`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 ionizing 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 a 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 source that
`includes a sealed 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 external to the
`sealed 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 curved reflective surface disposed external to the
`
`

`
`US 7,786,455 B2
`
`7
`sealed chamber to receive at leas a portion of the high bright-
`ness light emitted by the sealed chamber and reflect the high
`brightness light toward an output of the light source.
`In some embodiments, the light source includes an optical
`element disposed along a path the electromagnetic energy 5
`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
`
`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.
`
`DETAILED DESCRIPTION OF ILLUSTRATIVE
`EMBODIMENTS
`
`10
`
`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 100 provides energy to
`a region 130 ofthe 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 located in 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 ionizable medium
`in order to maximize the efficiency 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 order 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 of a wafer) with
`adequate resolution. It is also desirable for the laser source
`104 to drive and/or 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 simulta-
`neously to a high brightness light 136. The light 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 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
`of electromagnetic energy, for example, 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, in a general sense, is distributed over the elec-
`tromagnetic spectrum according to Planck’s radiation law.
`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.
`Most conventional arc lamps are, however, 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 112 that aids in conditioning the output of the diode laser
`
`embodiments, the sealed chamber is a quartz bulb. In some
`embod