(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2006/0192152 A1
`(43) Pub. Date:
`Aug. 31, 2006
`Ershov et al.
`
`US 20060192152A1
`
`(54) LPP EUV LIGHT SOURCE DRIVE LASER
`SYSTEM
`
`(75) Inventors: Alexander I. Ershov, San Diego, CA
`(US); Alexander N. Bykanov, San
`Diego, CA (US); Oleh Khodykin, San
`Diego, CA (US); Igor V. Fomenkov,
`San Diego, CA (US)
`
`Correspondence Address:
`William C. Cray
`Cymer, Inc.
`Legal Dept., MS/4-2C
`17075 Thornmint Court
`San Diego, CA 92127-2413 (US)
`
`(73) Assignee: Cymer, Inc., San Diego, CA
`
`(21) Appl. No.:
`
`11/217,161
`
`(22) Filed:
`
`Aug. 31, 2005
`
`Related US. Application Data
`
`(63) Continuation-in-part of application No. 11/174,299,
`?led on Jun. 29, 2005.
`
`(60) Provisional application No. 60/657,606, ?led on Feb.
`28, 2005.
`
`Publication Classi?cation
`
`(51) Int. Cl.
`(2006.01)
`G01] 1/00
`(52) U.S. c1. ........................................................ ..250/503.1
`
`(57)
`
`ABSTRACT
`
`An apparatus and method is disclosed Which may comprise
`a laser produced plasma EUV system Which may comprise
`a drive laser producing a drive laser beam; a drive laser
`beam ?rst path having a ?rst axis; a drive laser redirecting
`mechanism transferring the drive laser beam from the ?rst
`path to a second path, the second path having a second axis;
`an EUV collector optical element having a centrally located
`aperture; and a focusing mirror in the second path and
`positioned Within the aperture and focusing the drive laser
`beam onto a plasma initiation site located along the second
`axis. The apparatus and method may comprise the drive
`laser beam is produced by a drive laser having a Wavelength
`such that focusing on an EUV target droplet of less than
`about 100 pm at an effective plasma producing energy if not
`practical in the constraints of the geometries involved uti
`liZing a focusing lens. The drive laser may comprise a C02
`laser. The drive laser redirecting mechanism may comprise
`a mirror.
`
`[66
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`Patent Application Publication Aug. 31, 2006 Sheet 1 0f 8
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`Patent Application Publication Aug. 31, 2006 Sheet 2 0f 8
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`Patent Application Publication Aug. 31, 2006 Sheet 3 0f 8
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`US 2006/0192152 A1
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`Patent Application Publication Aug. 31, 2006 Sheet 4 0f 8
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`Patent Application Publication Aug. 31, 2006 Sheet 5 0f 8
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`Patent Application Publication Aug. 31, 2006 Sheet 6 0f 8
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`Patent Application Publication Aug. 31, 2006 Sheet 8 0f 8
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`US 2006/0192152 A1
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`

`
`US 2006/0192152 A1
`
`Aug. 31, 2006
`
`LPP EUV LIGHT SOURCE DRIVE LASER
`SYSTEM
`
`RELATED APPLICATIONS
`
`[0001] The present application is a Continuation-in-Part of
`patent application Ser. No. 11/174,299, ?led on Jun. 29,
`2005, Which is related to US. patent application Ser. No.
`11/021,261, ?led on Dec. 22, 2004, entitled EUV LIGHT
`SOURCE OPTICAL ELEMENTS, Attorney Docket No.
`2004-0023-01; Ser. No. 11/067,124, entitled METHOD
`AND APPARATUS FOR EUV PLASMA SOURCE TAR
`GET DELIVERY, ?led on Feb. 25, 2005, Attorney Docket
`No. 2004-0008-01; and Ser. No. 10/979,945, entitled EUV
`COLLECTOR DEBRIS MANAGEMENT, ?led on Nov. 1,
`2004, Attorney Docket No. 2004-0088-01; and Ser. No.
`10/979,919, entitled EUV LIGHT SOURCE, ?led on Nov.
`1, 2004, Attorney Docket No. 2004-0064-01; and Ser. No.
`10/803,526, entitled A HIGH REPETITION RATE LASER
`PRODUCED PLASMA EUV LIGHT SOURCE, ?led on
`Mar. 17, 2004, Attorney Docket No. 2003-0125-01; Ser. No.
`10/900,839, entitled EUV LIGHT SOURCE, ?led on Jul.
`27, 2004, Attorney Docket No. 2004-0044-01, and Ser. No.
`11/067,099, entitled SYSTEMS FOR PROTECTING
`INTERNAL COMPONENTS OF AN EUV LIGHT
`SOURCE FROM PLASMA-GENERATED DEBRIS, ?led
`on Feb. 25, 2005, Attorney Docket No. 2004-0117-01; and
`60/657,606, entitled EUV LPP DRIVE LASER, ?led on
`Feb. 28, 2005, Attorney Docket No. 2004-0107-01; and
`Attorney Docket No. 2004-0086-01, the disclosures of all of
`Which are hereby incorporated by reference.
`
`FIELD OF THE INVENTION
`
`[0002] The present invention related to laser produced
`plasma (“LPP”) extreme ultraviolet (“EUV”) light sources.
`
`BACKGROUND OF THE INVENTION
`
`[0003] C02 laser may be used for laser produced plasma
`(“LPP”) extreme ultraviolet (“EUV”), i.e., beloW about 50
`nm and more speci?cally, e.g., at around 13.5 nm. Such
`systems may employ a drive laser(s) to irradiate a plasma
`formation material target, e.g., target droplets formed of a
`liquid containing target material, e.g., molten metal target
`material, such as lithium or tin.
`
`[0004] C02 has been proposed as a good drive laser
`system, e.g., for tin because of a relatively high conversion
`ef?ciency both in terms of ef?ciency in converting laser light
`pulse photon energy into EUV photons and in terms of
`conversion of electrical energy used to produce the drive
`laser pulses for irradiating a target to form a plasma in Which
`EUV light is generated and the ultimate Wattage of EUV
`light generated.
`[0005] Applicants propose an arrangement for delivering
`the drive laser pulses to the target irradiation site Which
`addresses certain problems associated With certain types of
`drive lasers, e.g., CO2 drive lasers.
`[0006] Pre-pulses from the same laser as the main pulse
`(e.g., at a different Wavelength than the main pulse may be
`used, e.g., With a YAG laser (355 nmimain and 532
`nmipre-pulse, for example). Pre-pulses from separate
`lasers for the pre-pulse and main pulse may also be used.
`Applicants propose certain improvements for providing a
`
`pre-pulse and main pulse, particularly useful in certain types
`of drive laser systems, such as CO2 drive laser systems.
`[0007] Applicants also propose certain improvements to
`certain types of drive lasers to facilitate operation at higher
`repetition rates, e.g., at 18 or more kHZ.
`
`SUMMARY OF THE INVENTION
`
`[0008] An apparatus and method is disclosed Which may
`comprise a laser produced plasma EUV system Which may
`comprise a drive laser producing a drive laser beam; a drive
`laser beam ?rst path having a ?rst axis; a drive laser
`redirecting mechanism transferring the drive laser beam
`from the ?rst path to a second path, the second path having
`a second axis; an EUV collector optical element having a
`centrally located aperture; and a focusing mirror in the
`second path and positioned Within the aperture and focusing
`the drive laser beam onto a plasma initiation site located
`along the second axis. The apparatus and method may
`comprise the drive laser beam is produced by a drive laser
`having a Wavelength such that focusing on an EUV target
`droplet of less than about 100 pm at an effective plasma
`producing energy if not practical in the constraints of the
`geometries involved utiliZing a focusing lens. The drive
`laser may comprise a C02 laser. The drive laser redirecting
`mechanism may comprise a mirror. The focusing mirror may
`be positioned and siZed to not block EUV light generated in
`a plasma produced at the plasma initiation site from the
`collector optical element outside of the aperture. The redi
`recting mechanism may be rotated and the focusing mirror
`may be heated. The apparatus and method may further
`comprise a seed laser system generating a combined output
`pulse having a pre-pulse portion and a main pulse portion;
`and an amplifying laser amplifying the pre-pulse portion and
`the main pulse portion at the same time Without the pre-pulse
`portion saturating the gain of the ampli?er laser. The ampli
`fying laser may comprise a C02 laser. The pre-pulse portion
`of the combined pulse may be produced in a ?rst seed laser
`and the main pulse portion of the combined pulse may be
`produced in s second seed laser or the pre-pulse and main
`pulse portions of the combined pulse being produced in a
`single seed laser. The apparatus and method may further
`comprise a seed laser producing seed laser pulses at a pulse
`repetition rate X ofat least 4 kHZ, e.g., 4, 6, 8, 12 or 18 kHZ;
`and a plurality of N ampli?er lasers each being ?red at a rate
`of X/N, positioned in series in an optical path of the seed
`laser pulses and each amplifying in a staggered timing
`fashion a respective Nth seed pulse are a pulse repetition rate
`of X/N. Each respective ampli?er laser may be ?red in time
`With the ?ring of the seed producing laser such that the
`respective Nth output of the seed producing laser is Within
`the respective ampli?er laser. The seed laser pulse may
`comprise a pre-pulse portion and a main pulse portion.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0009] FIG. 1 shoWs a schematic block diagram illustra
`tion of a DPP EUV light source system in Which aspects of
`embodiments of the present invention are useful;
`
`[0010] FIG. 2 shoWs a schematic block diagram illustra
`tion of a control system for the light source of FIG. 1 useful
`With aspects of embodiments of the present invention;
`
`[0011] FIG. 3 shoWs schematically an example of a pro
`posed drive laser delivery system utiliZing a focusing lens;
`
`

`
`US 2006/0192152 A1
`
`Aug. 31, 2006
`
`[0012] FIG. 4 illustrates schematically a drive laser deliv
`ery system according to aspects of an embodiment of the
`present invention;
`[0013] FIG. 5 shoWs schematically a drive laser delivery
`system according to aspects of an embodiment of the present
`invention;
`[0014] FIG. 6 shoWs schematically in block diagram form
`an LPP EUV drive laser system according to aspects of an
`embodiment of the present invention;
`[0015] FIG. 7 shoWs schematically in block diagram form
`an LPP EUV drive laser system according to aspects of an
`embodiment of the present invention;
`[0016] FIG. 8 shoWs schematically in block diagram form
`an LPP EUV drive laser system according to aspects of an
`embodiment of the present invention;
`[0017] FIG. 9 shoWs a drive laser ?ring diagram accord
`ing to aspects of an embodiment of the present invention;
`[0018] FIG. 10 shoWs schematically in block diagram
`form an LPP EUV drive laser system according to aspects of
`an embodiment of the present invention;
`[0019] FIG. 11 shoWs schematically in block diagram
`form an LPP EUV drive laser system according to aspects of
`an embodiment of the present invention;
`
`[0020] FIG. 12 shoWs a schematically an illustration of
`aspects of a further embodiment of the present invention.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`[0021] Turning noW to FIG. 1 there is shoWn a schematic
`vieW of an overall broad conception for an EUV light source,
`e.g., a laser produced plasma EUV light source 20 according
`to an aspect of the present invention. The light source 20
`may contain a pulsed laser system 22, e.g., a gas discharge
`laser, e.g., an excimer gas discharge laser, e.g., a KrF or ArF
`laser or a CO2 laser operating at high poWer and high pulse
`repetition rate and may be a MOPA con?gured laser system,
`e.g., as shoWn in Us. Pat. Nos. 6,625,191, 6,549,551, and
`6,567,450. The laser may also be, e.g., a solid state laser,
`e.g., a YAG laser. The light source 20 may also include a
`target delivery system 24, e.g., delivering targets in the form
`of liquid droplets, solid particles or solid particles contained
`Within liquid droplets. The targets may be delivered by the
`target delivery system 24, e.g., into the interior of a chamber
`26 to an irradiation site 28, otherWise knoWn as an ignition
`site or the sight of the ?re ball. Embodiments of the target
`delivery system 24 are described in more detail beloW.
`[0022] Laser pulses delivered from the pulsed laser system
`22 along a laser optical axis 55 through a WindoW (not
`shoWn) in the chamber 26 to the irradiation site, suitably
`focused, as discussed in more detail beloW in coordination
`With the arrival of a target produced by the target delivery
`system 24 to create an ignition or ?re ball that forms an X-ray
`(or soft X-ray (EUV) releasing plasma, having certain char
`acteristics, including Wavelength of the X-ray light produced,
`type and amount of debris released from the plasma during
`or after ignition, according to the material of the target.
`
`[0023] The light source may also include a collector 30,
`e.g., a re?ector, e.g., in the form of a truncated ellipse, With
`an aperture for the laser light to enter to the ignition site 28.
`
`Embodiments of the collector system are described in more
`detail beloW. The collector 30 may be, e.g., an elliptical
`mirror that has a ?rst focus at the ignition site 28 and a
`second focus at the so-called intermediate point 40 (also
`called the intermediate focus 40) Where the EUV light is
`output from the light source and input to, e.g., an integrated
`circuit lithography tool (not shoWn). The system 20 may also
`include a target position detection system 42. The pulsed
`system 22 may include, e.g., a master oscillator-poWer
`ampli?er (“MOPA”) con?gured dual chambered gas dis
`charge laser system having, e.g., an oscillator laser system
`44 and an ampli?er laser system 48, With, e.g., a magnetic
`reactor-sWitched pulse compression and timing circuit 50 for
`the oscillator laser system 44 and a magnetic reactor
`sWitched pulse compression and timing circuit 52 for the
`ampli?er laser system 48, along With a pulse poWer timing
`monitoring system 54 for the oscillator laser system 44 and
`a pulse poWer timing monitoring system 56 for the ampli?er
`laser system 48. The pulse poWer system may include poWer
`for creating laser output from, e. g., a YAG laser. The system
`20 may also include an EUV light source controller system
`60, Which may also include, e.g., a target position detection
`feedback system 62 and a ?ring control system 65, along
`With, e.g., a laser beam positioning system 66. The system
`could also incorporate several ampli?ers in cooperation With
`a single master oscillator.
`
`[0024] The target position detection system may include a
`plurality of droplet imagers 70, 72 and 74 that provide input
`relative to the position of a target droplet, e.g., relative to the
`ignition site and provide these inputs to the target position
`detection feedback system, Which can, e. g., compute a target
`position and trajectory, from Which a target error cam be
`computed, if not on a droplet by droplet basis then on
`average, Which is then provided as an input to the system
`controller 60, Which can, e.g., provide a laser position and
`direction correction signal, e.g., to the laser beam position
`ing system 66 that the laser beam positioning system can
`use, e.g., to control the position and direction of he laser
`position and direction changer 68, e.g., to change the focus
`point of the laser beam to a different ignition point 28.
`
`[0025] The imager 72 may, e.g., be aimed along an imag
`ing line 75, e.g., aligned With a desired trajectory path of a
`target droplet 94 from the target delivery mechanism 92 to
`the desired ignition site 28 and the imagers 74 and 76 may,
`e.g., be aimed along intersecting imaging lines 76 and 78
`that intersect, e.g., alone the desired trajectory path at some
`point 80 along the path before the desired ignition site 28.
`[0026] The target delivery control system 90, in response
`to a signal from the system controller 60 may, e.g., modify
`the release point of the target droplets 94 as released by the
`target delivery mechanism 92 to correct for errors in the
`target droplets arriving at the desired ignition site 28.
`
`[0027] An EUV light source detector 100 at or near the
`intermediate focus 40 may also provide feedback to the
`system controller 60 that can be, e. g., indicative of the errors
`in such things as the timing and focus of the laser pulses to
`properly intercept the target droplets in the right place and
`time for effective and ef?cient LPP EUV light production.
`
`[0028] Turning noW to FIG. 2 there is shoWn schemati
`cally further details of a controller system 60 and the
`associated monitoring and control systems, 62, 64 and 66 as
`shoWn in FIG. 1. The controller may receive, e.g., a plurality
`
`

`
`US 2006/0192152 A1
`
`Aug. 31, 2006
`
`of position signal 134, 136 a trajectory signal 136 from the
`target position detection feedback system, e. g., correlated to
`a system clock signal provided by a system clock 116 to the
`system components over a clock bus 115. The controller 60
`may have a pre-arrival tracking and timing system 110
`Which can, e.g., compute the actual position of the target at
`some point in system time and a target trajectory computa
`tion system 112, Which can, e.g., compute the actual trajec
`tory of a target drop at some system time, and an irradiation
`site temporal and spatial error computation system 114, that
`can, e.g., compute a temporal and a spatial error signal
`compared to some desired point in space and time for
`ignition to occur.
`
`[0029] The controller 60 may then, e.g., provide the tem
`poral error signal 140 to the ?ring control system 64 and the
`spatial error signal 138 to the laser beam positioning system
`66. The ?ring control system may compute and provide to a
`resonance charger portion 118 of the oscillator laser 44
`magnetic reactor-sWitched pulse compression and timing
`circuit 50 a resonant charger initiation signal 122 and may
`provide, e.g., to a resonance charger portion 120 of the PA
`magnetic reactor-sWitched pulse compression and timing
`circuit 52 a resonant charger initiation signal, Which may
`both be the same signal, and may provide to a compression
`circuit portion 126 of the oscillator laser 44 magnetic
`reactor-sWitched pulse compression and timing circuit 50 a
`trigger signal 130 and to a compression circuit portion 128
`of the ampli?er laser system 48 magnetic reactor-sWitched
`pulse compression and timing circuit 52 a trigger signal 132,
`Which may not be the same signal and may be computed in
`part from the temporal error signal 140 and from inputs from
`the light out detection apparatus 54 and 56, respectively for
`the oscillator laser system and the ampli?er laser system.
`The Pa could also possibly be a CW or CO2 laser.
`
`[0030] The spatial error signal may be provided to the
`laser beam position and direction control system 66, Which
`may provide, e.g., a ?ring point signal and a line of sight
`signal to the laser bean positioner Which may, e.g., position
`the laser to change the focus point for the ignition site 28 by
`changing either or both of the position of the output of the
`laser system ampli?er laser 48 at time of ?re and the aiming
`direction of the laser output beam.
`
`[0031] In order to improve the total conversion e?iciency
`(“TCE”), including the drive laser conversion e?iciency
`(“DLCE”) relating to the conversion of drive laser light
`pulse energy into EUV photon energy and also the electrical
`conversion e?iciency (“ECE”) in converting electrical
`energy producing the drive laser pulses to EUV light energy,
`and also to reduce the drive laser overall costs, as Well as
`EUV system costs, according to aspects of an embodiment
`of the present invention, applicants propose to provide for
`the generation of both a drive laser pre-pulse and a drive
`laser main pulse from the same CO2 laser. This can also have
`a positive impact on laser light focusing optics lifetimes and
`drive laser light input WindoW lifetime.
`
`energy into EUV. HoWever, drivel lasers such as CO2 drive
`lasers suffer from a rather signi?cant inability to properly
`focus such drive lasers as opposed to, e.g., solid state lasers
`like NdzYAG lasers or excimer lasers such as XeF or XeCl
`lasers. The CO2 laser output pulse light at 10.6 um radiation
`is difficult to focus tightly at the required dimensions.
`
`[0033] A typical siZe of a plasma formation material target
`droplet 94 may be on the order of from 10-100 microns,
`depending on the material of the plasma source and also
`perhaps the drive laser type, With smaller generally being
`better, e.g., from a debris generation and consequent debris
`management point of vieW. With currently proposed focus
`ing schemes, e.g., as illustrated schematically and not to
`scale in FIG. 3, e.g., utiliZing a focusing lens 160 a drive
`laser beam 152 of diameter DD (e.g., about 50 mm) and
`focal distance LL (, e.g., about 50 cm, to focus 10.6 micron
`Wavelength radiation into, e.g., even the largest end of the
`droplet range, e.g., at about 100 microns, the divergence of
`a laser should be less than 2*10'4 radian. This value is less
`than diffraction limit of l22*10.6"‘l0_6/50*l0_3=2.6*l0-4
`(e.g., for an aperture of 50 mm). Therefore, the focus
`required cannot be reached, and, e.g., laser light energy Will
`not enter the target droplet and CE is reduced.
`
`[0034] To overcome this limitation either focal distance
`has to be decreased or the lens 160 and laser beam 151
`diameter has to be increased. This, hoWever, can be coun
`terproductive, since it Would then require a large central
`opening in a EUV collector 30, reducing the EUV collection
`angle. The larger opening also results in limiting the effect
`of the debris mitigation offered by the drive laser delivery
`enclosure 150, as that is explained in more detail in one or
`more of the above referenced co-pending applications. This
`decrease in effectiveness, among other things can result in a
`decrease in the laser input WindoW lifetime.
`
`[0035] According to aspects of an embodiment of the
`present invention applicants propose an improved method
`and apparatus for the input of drive laser radiation as
`illustrated schematically and not to scale in FIGS. 4 and 5.
`For, e.g., a C02 laser it is proposed to use internal re?ecting
`optics With high NA and also, e.g., using deposited plasma
`initiation source material, e.g., Sn as a re?ecting surface(s).
`The focusing scheme may comprise, e.g., tWo re?ecting
`mirrors 170, 180. Mirror 170 may, e.g., be a ?at or curved
`mirror made, e.g., of molybdenum. The ?nal focusing mirror
`180 can, e.g., focuses CO2 radiation in a C02 drive laser
`input beam 172, redirected by the redirecting mirror 170 into
`the focusing mirror 180 to form a focused beam 176
`intersecting the target droplets 92 at the desired plasma
`initiation site 28.
`
`[0036] The focal distance of mirror 180 may be signi?
`cantly less than 50 cm, e.g., 5 cm but not limited by this
`number. Such a short focal distance mirror 180 can, e.g.,
`alloW for the focus of the CO2 radiation on, e.g., 100 micron
`or less droplets, and particularly less than 50 um and doWn
`to even about 10 pm.
`
`[0032] Applicants have recently determined through much
`investigation, experimentation and analysis that the use of a
`C02 drive laser for LPP EUV can have certain very bene?
`cial results, e.g., in the case of a Sn-based EUVLPP plasma
`source material. By Way of example a relatively high DLCE
`and ECE and thus also TCE number can be reached for
`conversion of electrical energy and also drive laser light
`
`[0037] Applicants also propose to use heating, e.g., With
`heaters 194, e.g., a Mo-ribbon heater, Which can be placed
`behind the mirror 180' according to aspects of an embodi
`ment illustrated schematically and not to scale in FIG. 5.
`Heating to above the Sn melting point and rotation, using,
`e.g., spinning motor 192 for the mirror 180', Which may be
`a brushless loW voltage motor, e.g., made by MCB, Inc.
`
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`under the name LB462, and may be encased in a stainless
`steel casing to protect it from the environment of the plasma
`generation chamber 26, and a similar motor 190 for the
`mirror 170', can be employed. Re?ection of the laser radia
`tion Will be, e.g., from a thin ?lm of the plasma source
`material, e.g., Sn, coating the mirrors 170, 180, due to
`deposition from the LPP debris. Rotation can be used if
`necessary to create smooth surface of the molten plasma
`source material, e.g., Sn. This thin ?lm of liquid Sn can form
`a self-healing re?ective surface for the mirror 170, 180.
`Thus, plasma source material deposition, e.g., Sn deposition
`on the mirror 170, 180 can be utiliZed as a plus instead of a
`negative Were the focusing optics in the form of one or more
`lenses. The requirements for roughness (lambda/l0) for 10.6
`um radiation can be easily achieved. The mirrors 170, 180
`can be steered and/or positioned With the motors 192, 192.
`
`[0038] Re?ectivity of the liquid Sn can be estimated from
`Drude’s formula Which gives a good agreement With experi
`mental results for the Wavelengths exceeding 5 um.
`
`[0039] Rzl-2/\/(S*T), Where S is the conductivity of the
`metal (in CGS system) and T is the oscillation period for the
`radiation. For copper the formula gives estimation of re?ec
`tivity for 10.6 um about 98.5%. For Sn the re?ectivity
`estimate is 96%.
`
`[0040] Heating of, e.g., the mirror 180' of FIG. 5 above
`required melting point may also be performed With an
`external heater (not shoWn) installed behind the rotating
`mirror 180' With a radiative heat transfer mechanism or by
`self-heating due to, e.g., about 4% radiation absorption from
`the drive laser light and/or proximity to the plasma genera
`tion site 28.
`
`[0041] As shoWn schematically in FIGS. 4 and 5, the
`laser radiation 172 may be delivered into the chamber
`through a side port and therefore not require an overly large
`aperture in the central portion of the collector 30. For
`example With approximately the same siZe central aperture
`as is effective for certain Wavelengths, e.g., in the excimer
`laser DUV ranges, but ineffective for a focusing lens for
`Wavelengths such as CO2, the focusing mirror arrangement
`according to aspects of an embodiment of the present
`invention can be utiliZed. In addition the laser input WindoW
`202, Which may be utiliZed for vacuum sealing the chamber
`26 and laser delivery enclosure 300 are not in direct line of
`vieW of plasma initiation site and debris generation area, as
`is the case With the delivery system of FIG. 3. Therefore, the
`laser delivery enclosure With its associated apertures and
`purge gas and counter ?oW gas, as described in more detail
`in at least one of the above noted co-pending applications,
`can be even more effective in preventing debris from reach
`ing the WindoW 202. Therefore, even if the focusing of the
`LPP drive laser light as illustrated according to aspects of the
`embodiment of FIG. 5, e.g., at the distal end of the drive
`laser delivery enclosure 200, needs to be relatively larger,
`e.g., for a C02 drive laser, the indirect angle of the debris
`?ight path from the irradiation site 28 to the distal end of the
`enclosure 200 alloWs for larger or no apertures at the distal
`end, Whereas the enlargement or removal of the apertures at
`the distal end of the enclosure 150 illustrated in the embodi
`ment of FIG. 3 could signi?cantly impact the ability of the
`enclosure 150 to keep debris from, e.g., the lens 160 (Which
`could also in some embodiments serve as the chamber
`WindoW or be substituted for by a chamber WindoW). Thus,
`
`Where debris management is a critical factor, the arrange
`ment of FIGS. 4 and 5 may be utiliZed to keep the drive
`laser input enclosure off of the optical axis of the focused
`LPP drive laser beam 152, 176 to the irradiation site 28.
`
`[0042] According to aspects of an embodiment of the
`present invention, for example, the laser beam 172 may be
`focused by external lens and form a converging beam 204
`With the open ori?ce of the drive laser input enclosure cone
`200 located close to the focal point. For direct focusing
`scheme When external lens, e.g., lens 160 of FIG. 3, focuses
`the beam on the droplets 94 the cone tip Would have to be
`located at some relatively distance, e. g., 20-50 mm from the
`focal point, i.e., the plasma initiation site 28, for intersection
`With the droplet target 94 at about the focal point of the lens
`160. This can subject the distal end to a signi?cant thermal
`load, With essentially all of the drive laser poWer being
`absorbed by the target in the formation of the plasma and
`being released in or about the plasma. For the suggested
`optical arrangement according to aspects of an embodiment
`of the present invention With intermediate focus the cone tip
`can be approached to the focal point (at distance of feW
`millimeters) and output ori?ce of the cone can be very small.
`This alloWs us to increase signi?cantly the gas pressure in
`the gas cone and reduce signi?cantly the pressure in the
`chamber With other parameters (WindoW protection e?i
`ciency, pumping speed of the chamber) keeping the same.
`Re?ecting optics may be utiliZed, e.g., for a C02 laser.
`
`[0043] Referring noW to FIG. 6, there is shoWn schemati
`cally and in block diagram form a drive laser system 250,
`e.g., a C02 drive laser, according to aspects of an embodi
`ment of the present invention, Which may comprise a
`pre-pulse master oscillator (“MO”) 252 and a main pulse
`master oscillator (“MO”) 254, each of Which may be a C02
`gas discharge laser or other suitable seed laser, providing
`seed laser pulses at about 10.6 um in Wavelength to a poWer
`ampli?er (“PA”) 272, Which may be a single or multiple pass
`CO2 gas discharge laser, lasing at about 10.6 um. The output
`of the MO 252 may form a pre-pulse, having a pulse energy
`of about 1% to 10% of the pulse energy of the main pulse,
`and the output of the MO 254 may form a main pulse having
`a pulse energy of about l><l01O Watts/cm2, With Wavelengths
`that may be the same or different.
`
`[0044] The output pulse from the MO 255 may be
`re?ected, e.g., by a mirror 260, to a polariZing beam splitter
`262, Which Will also re?ect all or essentially all of the light
`of a ?rst selected polarity into the PA 272 as a seed pulse to
`be ampli?ed in the PA 272. The output of the MO 252 of a
`second selected polarity can be passed through the polariZ
`ing beam splitter 262 and into the PA 272 as another seed
`pulse. The outputs of the MO 252 and MO 254 may thus be
`formed into a combined seed pulse 270 having a pre-pulse
`portion from the MO 252 and a main pulse portion from the
`MO 254.
`
`[0045] The combined pulse 270 may be ampli?ed in the
`PA 272 as is knoWn in the art of MOPA gas discharge lasers,
`With pulse poWer supply modules as are sold by Applicants’
`Assignee, e.g., as XLA 100 and XLA 200 series MOPA laser
`systems With the appropriate timing betWeen gas discharges
`in the MO’s 252, 254 and PA 272 to insure the existence of
`an amplifying lasing medium in the PA as the combined
`pulse 270 is ampli?ed to form a drive laser output pulse 274.
`The timing of the ?ring of the MO 254 and the MO 252, e.g.,
`
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`US 2006/0192152 A1
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`Aug. 31, 2006
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`such that the MO 254 is ?led later in time such that its gas
`discharge is, e.g., initiated after the ?ring of the MO 252, but
`also Within about a feW nanoseconds of the ?ring of the MO
`252, such that the pre-pulse Will slightly precede the main
`pulse in the combined pulse 270. It Will also be understood
`by those skilled in the art that the nature of the pre-pulse and
`main pulse, e. g., the relative intensities, separation of peaks,
`absolute intensities, etc. Will be determined from the desired
`e?“ect(s) in generating the plasma and Will relate to certain
`factors, e.g., the type of drive laser and, e. g., its Wavelength,
`the type of target material, and e.g., its target droplet siZe and
`so forth.
`
`[0046] Turning noW to FIG. 7 there is shoWn in schematic
`block diagram form aspects of an embodiment of the present
`invention Which may comprise a drive laser system 250,
`e.g., a C02 drive laser system, e.g., including an M0 gain
`generator 280, formed, e.g., by a laser oscillator cavity
`having a cavity rear mirror 282 and an output coupler 286,
`With a Q-sWitch 284 in