US007435982B2
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`(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
`
`....... .. 250/504 R
`8/2005 Hoshino et al.
`2005/0167618 A1*
`2007/0228300 A1* 10/2007 Smith ................... .. 250/504R
`
`(75)
`
`Inventor: Donald K. Smith, Belmont, MA (US)
`
`FOREIGN PATENT DOCUMENTS
`
`(73) Assignee: Energetiq Technology, Inc., Woburn,
`MA (US)
`
`JP
`
`61-193358
`
`8/1986
`
`OTHER PUBLICATIONS
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 452 days.
`
`(21) Appl.No.: 11/395,523
`
`(22)
`
`Filed:
`
`Mar. 31, 2006
`
`(65)
`
`Prior Publication Data
`
`US 2007/0228288 A1
`
`Oct. 4, 2007
`
`(51)
`
`Int. C1.
`(2006.01)
`A61N 5/06
`(2006.01)
`G01] 3/10
`(2006.01)
`H05G 2/00
`(52) U.S. Cl.
`............................. .. 250/504 R; 250/423 P;
`250/426; 250/493.1; 438/104; 438/301; 438/513;
`438/156; 252/301.36; 252/301.16; 252/301.4 F;
`385/31; 385/33; 385/38
`(58) Field of Classification Search ........... .. 250/504 R,
`250/423 P, 426, 493.1; 438/104, 301, 513,
`438/156; 252/301.16, 301.36, 301.4 F; 385/31,
`385/33, 38
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6,288,780 B1
`6,788,404 B2
`2004/0264512 A1
`
`.......... .. 356/237.1
`9/2001 Fairley et al.
`9/2004 Lange . . . . . . . . . . . . .
`. . . .. 356/237.2
`
`.............. .. 372/5
`12/2004 Hartlove et al.
`
`Wilbers et al., “The VUV Ernissivity of a High-Pressure Cascade
`Argon Arc from 125 to 200 nm,” 1. Quant. Spectrosc. Radiat. Trans-
`fer, Vol.46, 1991, pp. 299-308.
`Wilbers et al., “The Continuum Emission of Arc Plasma,” J'. Quant.
`Spectrosc. Radiat. Transfer, vol. 45, No. 1, 1991, pp. 1-10.
`Beck, “Simple Pulse Generator for Pulsing Xenon Arcs with High
`Repetition Rate,” Rev. Sci. Ihstrum., vol. 45, No. 2, Feb. 1974, pp.
`3 18-3 19.
`Raizer, “Optical Discharges,”S0v. 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 0fApplied Physics, vol. 46, No. 3, Mar. 1975,
`pp. 1080-1083.
`
`(Continued)
`
`Primary Examiner—Jack I Berman
`Assistant Examiner—Meenakshi S Sahu
`
`(74) Attorney, Agent, or Firm—Proskauer Rose, LLP
`
`(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-
`tinuous amount of energy to the ionized gas to generate a
`substantially continuous high brightness light.
`
`81 Claims, 4 Drawing Sheets
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`140
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`108
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`1 04
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`1 1 2
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`ASML 1118
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`US 7,435,982 B2
`Page 2
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`OTHER PUBLICATIONS
`
`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-[J cw
`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. JEPT, 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,” Lase/"—Ihduced Plasmas and
`Applications, published by Marcel Dekker, edited by Radziemski et
`a1., 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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`U.S. Patent
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`Oct. 14,2008
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`Sheet 1 of4
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`U.S. Patent
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`Oct. 14,2008
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`Sheet 2 of4
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`US 7,435,982 B2
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`Oct. 14,2008
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`Sheet 4 of4
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`US 7,435,982 B2
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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-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 sufiicient 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
`
`be present invention features a light source for generating
`a h'gh 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
`inc udes 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
`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
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`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 ir1to a fiber optic eler11er1t. 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).
`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
`
`

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`US 7,435,982 B2
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`3
`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, O2, 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
`son1e 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
`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.
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`The light source also includes a means for 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 n1ore apparent from
`the following description and from the claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The foregoing and other objects, feature and advantages of
`the invention, as well as the invention itself, will be more fully
`understood from the following illustrative description, when
`read together with the accompanying drawings which are not
`necessarily to scale.
`FIG. 1 is a 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.
`
`DETAILED DESCRIPTION OF ILLUSTRATIVE
`EMBODIMENTS
`
`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 and ioniz-
`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 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.
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`US 7,435,982 B2
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`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.
`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 eler11er1t 108 provides the laser beam to a collima-
`tor 112 that aids in conditioning the output of the diode laser
`by aiding in making laserbearn rays 116 substantially parallel
`to each other. The collimator 112 then directs the laser beam
`116 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 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 light source 100 also includes an
`ignition source 140 dcpictcd as two clcctrodcs (c.g., an anodc
`and cathode located in the chamber 128). The ignition source
`140 generates an electrical discharge in the chamber 128
`(e.g., 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 then 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-
`ments, 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 be, for example, an infrared (IR) laser source, a
`diode laser source, a fiber laser source, an ytterbium laser
`source, a C02 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 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
`efiiciency.
`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 sufiicient 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 sufficient 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 of optical 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) of the laser beam delivered to the cham-
`ber 132. Various properties ofthe laser beam can be modified
`with one or more optical elements (e.g., mirrors 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 and/or 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 of the invention include, coated mir-
`rors, dielectric coated mirrors, narrow band mirrors, and
`ultraviolet transparent infrared reflecting mirrors. By way of
`example, ultraviolet transparent infrared reflecting mirrors
`are used in some embodiments of the invention where it is
`desirable to filter out infrared energy from a laser beam while
`permitting ultraviolet energy to pass through the mirror to be
`delivered to atool (e.g., awafer inspection tool, amicroscope,
`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 more 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 or include one or more of, for example, a dielectric
`material, a quartz material, Suprasil quartz, sapphire, MgF2,
`diamond or CaF2. The type of material may be selected based
`on, for example, the type of ionizable medium used and/or the
`wavelengths of light 136 that are desired to be generated and
`output from the chamber 128. In some embodiments, a region
`of the chamber 128 is transparent to, for example, ultraviolet
`energy. Chambers 128 fabricated using quartz 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 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 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 embodi-
`ments of the invention. For example, the ionizable medium
`can be one or more ofa 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. In some
`embodiments, a solid or liquid target (not shown) in the
`chamber 128 is used to generate an ionizable 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 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). In some embodiments, a first ionizable 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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`US 7,435,982 B2
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`7
`the second ionizable gas is a gas that produces a particular
`wavelength of electromagnetic energy.
`In this embodiment, the 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, for 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 USH-200DP
`quartz bulb manufactured by Ushio (with offices in Cypress,
`Calif.)). In some embodiments, the electrodes are smaller
`and/or 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 witl1ir1 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 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. In one embodiment, no ignition
`source 140 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.
`In somc cmbodimcnts, it is dcsirablc to maintain thc tcm-
`perature of the chamber 128 and the contents of the chamber
`128 during operation of the 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 of the light source 100,
`where the ignition source 140 provides energy to the plasma
`132 in addition to the energy provided by the laser source 1 04.
`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 the chamber 128.
`
`In some embodiments, the light source 100 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 132 (e.g., an
`ionized gas), similarly as described elsewhere herein.
`FIG. 2 is a schematic block diagram of a portion of a 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
`of a 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 where the plasma 132 is located. The
`laser beam 124 provides energy to the plasma 132 to sustain
`and/or 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 reflect a
`substantial amount of the high brightness light 136 toward
`and out of the window 204. In this embodiment, the reflector
`212 is transparent to the emitted light 136 (e.g., at least one or
`more wavelengths of ultraviolet light). In this manner, the
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`emitted light 136 is transmitted out of the chamber 128 and
`directed to, for example, a metrology tool (not shown). In one
`embodiment, the emitted light 136 is first directed towards or
`through additional optical elements before it is directed to a
`tool.
`
`By way of illustration, an experiment was conducted to
`generate ultraviolet light using a light source, according to an
`illustrati