(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2007/0228288 Al
`Smith
`(43) Pub. Date:
`Oct. 4, 2007
`
`US 20070228288Al
`
`(54) LASER-DRIVEN LIGHT SOURCE
`
`Publication Classification
`
`(75)
`
`Inventor: Donald K. Smith, Belmont, MA (US)
`
`Correspondence Address:
`PROSKAUER ROSE LLP
`
`ONE INTERNATIONAL PLACE
`BOSTON MA 02110 US
`’
`(
`)
`(73) Assigneez Energetiq Technology Inc” Wobuma
`MA
`
`(21) App]. No‘;
`
`11/395,523
`
`(22)
`
`Filed;
`
`Mar, 31, 2006
`
`(51)
`
`Int. Cl.
`(2006.01)
`H01J 27/24
`(52) U.S. C1.
`..................................... .. 250/426; 250/423 P
`
`ABSTRACT
`(57)
`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
`continuous amount of energy to the ionized gas to generate
`a substantially continuous high brightness light.
`
`112
`
`108
`
`104
`
`ASML 1131
`
`ASML 1131
`
`

`
`Patent Application Publication Oct. 4, 2007 Sheet 1 of 4
`
`US 2007/0228288 A1
`
`V.O_n_
`
`8F
`
`.2:
`
`

`
`Patent Application Publication Oct. 4, 2007 Sheet 2 of 4
`
`US 2007/0228288 A1
`
`wmm
`
`

`
`Patent Application Publication Oct. 4, 2007 Sheet 3 of 4
`
`US 2007/0228288 Al
`
`O(
`
`\l1’
`
`Oo‘
`
`-
`
`co?
`/m
`
`C0
`-
`
`LI.
`
`O
`\o
`on
`
`o
`$
`
`5
`3
`0 ea
`
`3
`_:
`
`3
`
`‘o
`(\l
`
`o
`
`O¢D(DVl'(\lOO0(Dfl‘(\lO
`(\]1-\—1—v-1-
`
`(USZUJUJ/M) (wu oov-ooz) sseuxufiua
`N‘.-
`0')
`
`5
`
`30
`
`CL
`L
`cu
`8
`.1
`9'
`
`0’
`5,
`.c
`&
`.9’
`In
`>
`3
`
`

`
`Patent Application Publication Oct. 4, 2007 Sheet 4 of 4
`
`US 2007/0228288 A1
`
`om?
`
`2:2v8
`
`8
`
`_m>>on_$3..
`
`V.O_..._
`
`cow
`
`
`
`

`
`US 2007/0228288 A1
`
`Oct. 4, 2007
`
`LASER-DRIVEN LIGHT SOURCE
`
`FIELD OF THE INVENTION
`
`[0001] The invention relates to methods and apparatus for
`providing a laser-driven light source.
`
`BACKGROUND OF THE INVENTION
`
`[0002] 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 associated with semiconductor wafers or materi-
`als 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.
`
`[0003] The state of the art in, for example, wafer inspec-
`tion systems 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 gener-
`ated 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
`brightness for some applications, especially in the ultraviolet
`spectrum. Further, the position of the arc can be unstable in
`these lamps.
`
`[0004] 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 discharge to maintain a plasma that generates a
`high brightness light.
`
`SUMMARY OF THE INVENTION
`
`[0005] The present invention features a light source for
`generating a high brightness light.
`
`[0006] 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
`[0007]
`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 modi-
`fying 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 embodi-
`
`ments, the optical element is one or more fiber optic ele-
`ments for directing the laser energy to the gas.
`
`[0008] 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, Suprasil® 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 transparent dielectric chamber.
`
`[0009] The gas can be one or more ofa 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, 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.
`
`the at least one laser is
`In some embodiments,
`[0010]
`multiple diode lasers coupled into a fiber optic element. In
`some embodiments, the at least one laser includes a pulse or
`continuous wave laser. In some embodiments, the at least
`one laser is an IR laser, a diode laser, a fiber laser, an
`ytterbium laser, a CO2 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.
`
`[0011] The ignition source can be or can include elec-
`trodes, 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.
`
`[0012] The light source can include at least one optical
`element for modifying a property of electromagnetic radia-
`tion emitted by the ionized gas. The optical element can be,
`for example, one or more mirrors or lenses. In some embodi-
`ments,
`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).
`
`relates to a
`in another aspect,
`[0013] The invention,
`method for producing light. The method involves ionizing
`with an ignition 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
`[0014]
`directing the laser energy through at least one optical ele-
`ment 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
`
`

`
`US 2007/0228288 Al
`
`Oct. 4, 2007
`
`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 micro-
`scope, a metrology tool, a lithography tool, or an endoscopic
`tool).
`
`In another aspect, the invention features a light
`[0015]
`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
`substantially 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
`[0016]
`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 ion-
`ized medium when energy is not provided to the ionized
`medium.
`
`In some embodiments, the light source can include
`[0017]
`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
`[0018]
`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 chamber is a sealed chamber. The cham-
`ber can be capable of being actively pumped. In some
`embodiments, the chamber includes a dielectric material
`(e.g., quartz). In some embodiments, the chamber is a glass
`bulb. In some embodiments, the chamber is an ultraviolet
`transparent dielectric chamber.
`
`[0019] 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 embodi-
`ments, 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.
`
`the at least one laser is
`In some embodiments,
`[0020]
`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.
`
`[0021] The ignition source can be or can include elec-
`trodes, 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
`[0022]
`least one optical element (e.g., a mirror or lens) for modi-
`fying a property 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 micro-
`scope, a metrology tool, a lithography tool, or an endoscopic
`tool).
`
`[0023] The invention, in another aspect relates to a method
`for producing light. The method involves ionizing with an
`ignition 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
`[0024]
`directing the laser energy through at least one optical ele-
`ment 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.
`
`[0025] 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 pro-
`viding substantially continuous laser energy to the ionized
`medium within the chamber.
`
`[0026] The foregoing and other objects, aspects, features,
`and advantages of the invention will become more apparent
`from the following description and from the claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`feature and
`[0027] The foregoing and other objects,
`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
`[0028]
`source, according to an illustrative embodiment of the
`invention.
`
`FIG. 2 is a schematic block diagram ofa portion of
`[0029]
`a light source, according to an illustrative embodiment of the
`invention.
`
`FIG. 3 is a graphical representation of UV bright-
`[0030]
`ness as a function of the laser power provided to a plasma,
`using a light source according to the invention.
`
`FIG. 4 is a graphical representation of the trans-
`[0031]
`mission of laser energy through a plasma generated from
`mercury, using a light source according to the invention.
`
`

`
`US 2007/0228288 A1
`
`Oct. 4, 2007
`
`DETAILED DESCRIPTION OF ILLUSTRATIVE
`EMBODIMENTS
`
`[0032] 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 ionizable medium (not shown). The light source 100
`provides energy to a region 130 of the chamber 128 having
`the ionizable medium which creates a plasma 132. The
`plasma 132 generates and emits a high brightness light 136
`that originates from the plasma 132. The light source 100
`also includes at least one laser source 104 that generates a
`laser beam that is provided to the plasma 132 located in the
`chamber 128 to initiate and/or sustain the high brightness
`light 136.
`
`In some embodiments, it is desirable for at least
`[0033]
`one wavelength 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
`[0034]
`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.
`
`[0035] Generating a plasma 132 that is small in size and
`providing the plasma 132 with a high power laser beam
`leads simultaneously 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 sus-
`tained plasma 132 yield increased radiation at shorter wave-
`lengths 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 electromagnetic spectrum according to Planck’s
`radiation law. The wavelength of maximum radiation is
`inversely proportional 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 wavelength is expected for laser sustained
`plasmas having a temperature 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 of
`[0036]
`the invention to maintain the temperature of the plasma 132
`during operation of the light source 100 to ensure that a
`sufiiciently 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
`[0037]
`laser that outputs a laser beam via a fiberoptic element 108.
`The fiber optic element 108 provides the laser beam to a
`
`collimator 112 that aids in conditioning the output of the
`diode laser by aiding in making laser beam rays 116 sub-
`stantially 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 of the 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).
`
`the light source 100 also
`In this embodiment,
`[0038]
`includes an ignition source 140 depicted as two electrodes
`(e.g., an anode and cathode located in the chamber 128). The
`ignition source 140 generates an electrical discharge in the
`chamber 128 (e.g., the region 130 of the chamber 128) to
`ignite the ionizable 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 generated by the light source 100 is then directed out of
`the chamber to, for example, a wafer inspection system (not
`shown).
`
`[0039] Alternative laser sources are contemplated accord-
`ing to illustrative embodiments of the invention. In some
`embodiments, neither the collimator 112, the beam expander
`118, or the lens 120 may be required. In some embodiments,
`additional 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 ytter-
`bium laser source, a CO2 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
`embodiments, 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 efficiency.
`
`In some embodiments, the laser source 104 is a
`[0040]
`high pulse rate laser source that provides substantially
`continuous laser energy to the light source 100 sufficient to
`produce the high brightness light 136. In some embodi-
`ments, 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 continuous energy provided to the plasma 132
`is sufficient to minimize cooling of the ionized medium to
`maintain a desirable brightness of the emitted light 136.
`
`In this embodiment, the light source 100 includes
`[0041]
`a plurality of optical elements (e.g., a beam expander 118, a
`lens 120, and fiber optic element 108) to modify properties
`(e.g., diameter and orientation) of the laser beam delivered
`to the chamber 132. Various properties of the 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 orienta-
`tion. In some embodiments, optical elements modify the
`
`

`
`US 2007/0228288 A1
`
`Oct. 4, 2007
`
`wavelength of the laser beam and/or filter out certain wave-
`lengths of electromagnetic energy in the laser beam.
`
`[0042] 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 mirrors, dielectric coated mirrors, narrow band mir-
`rors, and ultraviolet transparent infrared reflecting mirrors.
`By way of example, ultraviolet transparent infrared reflect-
`ing 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 a tool (e.g., a wafer inspection tool,
`a microscope, a lithography tool or an endoscopic tool).
`
`In this embodiment, the chamber 128 is a sealed
`[0043]
`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 fabricated 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 wave-
`lengths of electromagnetic energy of as long as about 2
`microns to pass through walls of the chamber. Sapphire
`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 cham-
`[0044]
`ber 128 to be a sealed chamber capable of sustaining high
`pressures and temperatures. For example, in one embodi-
`ment, the ionizable 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 trans-
`mission of the ultraviolet light 136 generated by the plasma
`132 of the light source 100 through the chamber 128 walls.
`
`[0045] Various ionizable media can be used in alternative
`embodiments of the invention. For example, the ionizable
`medium 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. 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 introduced 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 embodi-
`
`ment, the first ionizable gas is a gas that is more easily
`ignited using the ignition source 140 and the second ioniz-
`able gas is a gas that produces a particular wavelength of
`electromagnetic energy.
`
`In this embodiment, the ignition source 140 is a
`[0046]
`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.
`
`ignition
`types and configurations of
`[0047] Various
`sources are also contemplated, however, that are within the
`scope of the present invention. In some embodiments, the
`ignition source 140 is external
`to the chamber 128 or
`partially internal and partially external to the chamber 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 embodi-
`ment, 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 some embodiments, it is desirable to maintain
`[0048]
`the temperature 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 104. In this manner, the ignition source
`140 is used to maintain (or maintain at an adequate level) the
`temperature of the chamber 128 and the contents of the
`chamber 128.
`
`source 100
`the light
`In some embodiments,
`[0049]
`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 ofa portion of
`[0050]
`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.
`
`[0051] A laser source 104 (not shown) provides a laser
`beam 216 that is directed through a lens 208 to produce laser
`
`

`
`US 2007/0228288 A1
`
`Oct. 4, 2007
`
`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 parabo-
`[0052]
`loid shape and an inner surface 228 that is reflective. The
`paraboloid 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 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.
`
`[0053] By way of illustration, an experiment was con-
`ducted to generate ultraviolet light using a light source,
`according to an illustrative embodiment of the invention. A
`model L6724 quartz bulb manufactured by Hamamatsu
`(with offices in Bridgewater, N.J.) was used as the chamber
`of the light source (e.g., the chamber 128 of the light source
`100 of FIG. 1) for experiments using xenon as the ionizable
`medium in the chamber. A model USH-200DP quartz bulb
`manufactured by Ushio (with offices in Cypress, Calif.) was
`used as the chamber of the light source for experiments
`using mercury as the ionizable medium in the chamber. FIG.
`3 illustrates a plot 300 of the UV brightness of a high
`brightness light produced by a plasma located in the cham-
`ber as a function of the laser power (in watts) provided to the
`plasma. The laser source used in the experiment was a 1.09
`micron, 100 watt CW laser. The Y-Axis 312 of the plot 300
`is the UV brightness (between about 200 and about 400 mm)
`in watts/mmz steradian (sr). The X-Axis 316 of the plot 300
`is the laser beam power in watts provided to the plasma.
`Curve 304 is the UV brightness of the high brightness light
`produced by a plasma that was generated using xenon as the
`ionizable medium in the chamber. The plasma in the experi-
`ment using xenon was between about l m and about 2 mm
`in length and about 0.1 mm in diameter. The length of the
`plasma was controlled by adjusting the angle of convergence
`of the laser beam. A larger angle (i.e.,
`larger numerical
`aperture) leads to a shorter plasma because the converging
`beam reaches an intensity capable of sustaining the plasma
`when it is closer to the focal point. Curve 308 is the UV
`brightness of the high brightness light produced by a plasma
`that was generated using mercury as the ionizable medium
`in the chamber. The plasma in the experiment using mercury
`was about 1 mm in length and about 0.1 mm in diameter.
`
`[0054] By way of illustration, another experiment was
`conducted to generate ultraviolet using a light source
`according to an illustrative embodiment of the invention. A
`model USH-200DP quartz bulb manufactured by Ushio
`(with offices in Cypress, Calif.) was used as the chamber of
`the light source for experiments using mercury as the
`ionizable medium in the chamber (e.g., the chamber 128 of
`the light source 100 of FIG. 1). The laser source used in the
`experiment was a 1.09 micron, 100 watt ytterbium doped
`fiber laser from SPI Lasers PLC (with offices in Los Gatos,
`
`Calif.). FIG. 4 illustrates a plot 400 of the transmission of
`laser energy through a plasma located in the chamber
`generated from mercury versus the amount of power pro-
`vided to the plasma in watts. The Y-Axis 412 of the plot 400
`is the transmission coefficient in non-dimensional units. The
`
`X-Axis 416 of the plot 400 is the laser beam power in watts
`provided to the plasma. The curve in the plot 400 illustrates
`absorption lengths of 1 mm were achieved using the laser
`source. The transmission value of 0.34 observed at 100 watts
`
`corresponds to a l/e absorpti