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`Transmitted herewith for filing under 35 U.S.C. 111(a) and 37 C.F.R. 1.53(b) is a new utility patent application for an
`invention entitled:
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`I PP EUV LIGHT SOURCE DRIVE LASER SYSTEM
`
`and invented by:
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`lllllllllllllll
`062905
`Illlllllllllllll
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` - lexander I. Ershov, Alexander N. Bykanov, Oleh Khodykin, and Igor V. Fomenkov
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`CYMER Docket No. 2005-0044-01
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`Patent Application ‘5
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`LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM
`
`Inventors
`
`'Alexander.I. Ershov
`
`Alexander N. Bykanov
`Oleh Khodykin
`Igor V. Fomenkov
`
`CYMER, Inc.
`Legal Dept., MS/4-2C
`17075 Thommint Court‘
`
`San Diego, CA 92127-2413
`
`#EL 994704195 US
`"Express Mail" Label Number
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`Date of Deposit
`June 29 2005
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`I hereby certify thatthis paper or fee is being deposited with the United
`States Postal Service "Express Mail Post Ofiice to Addressee" services
`under 37 C.F.R. 1.10 on the date indicated above and is addressed to the
`Commissioner for Patents, P.O. Box 1450, Alexandria, VA 22313-1450.
`
`Annabeth Monforte
`Typed Name of Person Mailing Paper or Fee
`
`Sign
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`e
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`5
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`

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`PATENT APPLICATION
`CYNIER Docket No.: 2005-0044-O1
`
`Express Mail Number: EL 994704195 US
`
`TITLE
`
`LPP EUV Light Source Drive Laser System
`
`Alexander I. Ershov
`
`Alexander Bykauov
`Oleh Khodykin
`_ Igor V. Fomenkov
`
`FIELD OF THE INVENTION
`
`The present invention related to laser produced plasma (“LPP”) extreme
`
`ultraviolet (“EUV”) light sources.
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`20
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`The present application is related to United States Patent Application Ser.
`
`RELATED APPLICATIONS
`
`Nos. 11/021,261, filed on December 22, 2004, entitled EUV LIGHT SOURCE
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`OPTICAL ELEMENTS, Attorney Docket No. 2004-0023-01; 11/067,124, entitled
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`1 METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET
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`DELIVERY, filed on February 25, 2005, Attorney Docket No. 2004-0008-O1; and
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`10/979,945, entitled EUV COLLECTOR DEBRIS MANAGEMENT, filed on
`November 1, 2004, Attorney Docket No. 2004-0088-01; and 10/979,919, entitled
`
`EUV LIGHT SOURCE, filed on November 1, 2004, Attorney Docket No. 2004-
`
`0064-01; and 10/803,526, entitled A HIGH REPETITION RATE LASER
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`PRODUCED PLASMA EUV LIGHT SOURCE, filed on March 17, 2004, Attorney
`Docket No. 2003-0125-01; 10/900,839, entitled EUV LIGHT SOURCE, filed on
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`30
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`July 27, 2004, Attorney Docket No. 2004-0044:01, and 11/067,099, entitled
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`’ SYSTEMS FOR PROTECTING INTERNAL COMPONENTS OF AN EUV
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`LIGHT SOURCE FROM PLASMA-GENERATED DEBRIS, filed on February 25,
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`A
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`2005, Attorney Docket No. 2004-0117-01; and 60/657,606, entitled EUV LPP
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`DRIVE LASER, filed on February 28, 2005, Attorney Docket No. 2004-0107-01;
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`20054044-01
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`and Attorney Docket No. 2004-0086-01, the disclosures of all of which are hereby
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`incorporated by reference.
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`BACKGROUND OF THE INVENTION
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`CO2 laser may be used for laser produced plasma (“LPP”) extreme
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`ultraviolet (“EUV”), i.e., below about 50nm and more specifically, e.g., at around
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`13.5 nm. Such systems may employ a drive laser(s) to irradiate a plasma formation
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`material target, e.g., target droplets formed of a liquid containing target material,
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`e.g., molten metal target material, such as lithium or tin.
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`10
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`CO2 has been proposed as a good drive laser system, e.g., for tin because of a
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`relatively high conversion efiiciency both in terms of efficiency in converting laser
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`light pulse photon energy into EUV photons and in terms of conversion of electrical
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`energy used to produce the drive laser pulses for irradiating a target to form a plasma
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`in which EUV light is generated and the ultimate wattage of EUV light generated.
`Applicants propose an arrangement for delivering the drive laser pulses to
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`the target irradiation site which addresses certain problems associated with certain
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`types of drive lasers, e.g., CO2 drive lasers.
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`Pre-pulses from the same laser as the main pulse (e.g., at a different
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`wavelength than the main pulse may be used, e.g., with a YAG laser (355nm — main
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`and 532nm — pre-pulse, for example). Pre‘-pulses from separate lasers for the pre-
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`pulse and main pulse may also be used. Applicants propose certain improvements
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`for providing a pre-pulse and main pulse, particularly useful in certain types of drive
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`laser systems, such as CO; drive laser systems.
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`Applicants also propose certain improvements to certain types of drive lasers
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`to facilitate operation at higher repetition rates, e.g., at 18 or more kHz.
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`SUMMARY OF THE INVENTION
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`An apparatus and method is disclosed which may comprise alaser produced
`plasma EUV system which may comprise a drive laser producing a drive laser beam;
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`a drive laser beam first path having a first axis; a drive laser redirecting mechanism
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`transferring the drive laser beam from the first path to a second path, the second path
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`having a second axis; an EUV collector optical element having a centrally located
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`aperture; and a focusing mirror in the second path and positioned within the aperture
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`and focusing the drive laser beam onto a plasma initiation site located along the
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`second axis. The apparatus and method may comprise the drive laser beam is
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`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
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`practical in the constraints of the geometries involved utilizing a focusing lens. The
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`drive laser may comprise a C02 laser. The drive laser redirecting mechanism may
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`comprise a mirror. The focusing mirror may be positioned and sized to not block
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`EUV light generated in a plasma produced at the plasma initiation site from the
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`collector optical element outside of the aperture. The redirecting mechanism may be
`rotated and the focusing mirror may be heated. The apparatus and method may
`fiirther comprise a seed laser system generating a combined output pulse having a
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`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 amplifier laser. The amplifying laser may
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`comprise a C0; laser. The pre-pulse portion of the combined pulse may be
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`produced in a first seed laser and the main pulseportion of the combined pulse may
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`be produced in 5 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
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`20
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`may further comprise a seed laser producing seed laser pulses at a pulse repetition
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`rate X of at least 4 kHz, e.g., 4, 6, 8, 12 or 18 kHz; and a plurality of N amplifier
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`lasers each being fired at a rate of X/N, positioned in series in an optical path of the
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`seed ‘laser pulses and each amplifying in a staggered timing fashion a respective Nth
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`seed pulse are a pulse repetition rate of X/N. Each respective amplifier laser may be
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`fired in time with the firing of the seed producing laser such that the respective Nth
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`output of the seed producing laser is within the respective amplifier laser. The seed
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`laser pulse may comprise a pre-pulse portion and a main pulse portion.
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`BRIEF DESCRIPTION OF
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`DRAWINGS
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`FIG. 1 shows a schematic block diagram illustration of a DPP EUV light
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`source system in which aspects of embodiments of the present invention are useful;
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`"-1 FIG. 2 shows a schematic block diagram illustration of a control system for
`the light source of FIG. 1 useful with aspects of embodiments of the present
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`invention;
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`FIG. 3 shows schematically an example of a proposed drive laser delivery
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`system utilizing a focusing lens;
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`FIG. 4 illustrates schematically a drive laser delivery system according to
`aspects of an embodiment of the present invention;
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`FIG. 5 shows schematically a drive laser delivery system according to
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`aspects of an embodiment of the present invention;
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`FIG. 6 shows schematically in block diagram form an LPP EUV drive laser
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`system according to aspects of an embodiment of the present invention;
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`FIG. 7 shows schematically in block diagram form an LPP EUV drive laser
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`system according to aspects of an embodiment of the present invention;
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`FIG. 8 shows schematically in block diagram form an LPP EUV drive laser
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`system according to aspects of an embodiment of the present invention;
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`FIG. 9 shows a drive laser firing diagram according to aspects of an
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`embodiment of the present invention;
`FIG. 10 shows schematically in block diagram form an LPP EUV drive laser
`system according to aspects of an embodiment ofthe present invention;
`FIG. 11 shows schematically in block diagram form an LPI’ EUV drive laser
`system according to aspects of an embodiment of the present invention;
`I
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`FIG. 12 shows a schematically an illustration of aspects of a further
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`embodiment of the present invention.
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`DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
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`Turning now to FIG. 1 there is shown a schematic view of an overall broad
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`conception for an EUV light source, e.g., a laser produced plasma EUV_ light source
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`20 according to an aspect of the present invention. The light source 20 may contain
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`a pulsed laser system 22, e.g., a gas discharge laser, e.g., an excimer gas discharge
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`laser, e.g., a KrF or ArF laser or a C0; laser operating at high power and high pulse
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`repetition rate and may be a MOPA configired laser system, e.g., as shown in
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`United States Patents Nos. 6,625,191,-16,549,551, 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
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`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 262 to
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`an irradiation site 28, otherwise known as an ignition site or the sight of the fire ball.
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`Embodiments of the target delivery system 24 are described in more detail below.
`Laser pulses delivered from the pulsed laser system 22 along a laser optical
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`axis 55 through a window (not shown) in the chamber 26 to the irradiation site,
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`suitably focused, as discussed in more detail below in coordination with the arrival
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`of a target produced by the target delivery system 24 to create an ignition or fire ball
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`that forms an x-ray (or soft x-ray (EUV) releasing plasma, having certain
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`characteristics, including wavelength of the x-ray light produced, type and amount
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`of debris released from the plasma during or afier ignition, according to the material
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`of the target.
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`The light source may also include a collector 30, e.g., a reflector, e.g., in the
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`form of a truncated ellipse, with an aperture for the laser light to enter to the ignition
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`site 28. Embodiments of the collector system are described in more detail below.
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`The collector 30 may be, e.g., an elliptical mirror that has a first focus at the ignition
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`“ site 28 and a second focus at the so-called intermediate point 40 (also called the
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`_
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`intermediate focus 40) where the EUV light is output from the light source and input
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`to, e.g., an integrated circuit lithography tool (not shown). The system 20 may also
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`include a target position detection system 42. The pulsed system 22 may include,
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`e.g., a master oscillator-power amplifier (“MOPA”) configured dual chambered gas
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`discharge laser system having, e.g., an oscillator laser system 44 and an amplifier
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`laser system 48, with, e.g., a magnetic reactor-switched.pulse compression and
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`timing circuit 50 for the oscillator laser system 44 and a magnetic reactor-switched
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`pulse compression and timing circuit 52 for the amplifier laser system 48, along with
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`a pulse power timing monitoring system 54 for the oscillator laser system 44 and a
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`pulse power timing monitoring system 56 for the amplifier laser system 48. The
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`pulse power system may include power for creating laser output from, e.g., a YAG
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`laser. The system 20 may also include an EUV light source controller. system 60,
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`which may also include, e.g., a target position detection feedback system 62 and a
`firing control system 65, along with, e.g., a laser beam positioning system 66. The
`system could also incorporate several amplifiers in cooperation with a single master
`oscillator.
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`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,
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`e.g., relative to the ignition site and provide these inputs to the target position
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`detection feedback system, which can, e.g., compute a target position and trajectory,
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`from which a target error cam be computed, if not on a droplet by droplet basis then
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`on average, which is then provided as an input tothe system controller 60, which
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`can, e.g., provide a laser position and direction correction signal, e.g., to the laser
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`beam positioning 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.,
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`to change the focus point of the laser beam to a different ignition point 28.
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`The imager 72 may, e.g., be aimed along anirnaging line 75, e.g., aligned
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`with a desired trajectory path of a target droplet 94 from the target delivery
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`mechanism 92 to the desired ignition site 28 and the imagers 74 and 76 may, e.g., be
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`aimed along intersecting imaging lines 76 and 78 that intersect, e.g., alone the
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`desired trajectory path at some point 80 along the path before the desired ignition
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`site 28.
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`The target delivery control system 90, in response to a signal from the
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`system controller 60 may, e.g., modify the release point of the target droplets 94 as
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`released by the target delivery mechanism 92 to correct for errors in the target
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`droplets arriving at the desired ignition site 28.
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`An EUV light source detector 100 at or near the intermediate focus 40 may
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`also provide feedback to the system controller 60 that can be, e.g., indicative of the
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`errors in such things as the timing and focus of the laser pulses to properly intercept
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`the target droplets in the right place and time for effective and eff1cientLPP EUV
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`light production.
`5
`Turning now to FIG. 2 there is shown schematically further details of a
`controller system 60 and the associated monitoring and control systems, 62, 64 and
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`66 as shown in FIG. 1. The controller may receive, e.g., a plurality of position
`signal 134, 136 a trajectory signal 136 fiom the target position detection feedback
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`system, e.g., correlated to a system clock signal provided by a system clock 116 to
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`the system components over a clock bus 115. The controller 60 may have a pre-
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`arrival tracking and timing system 110 which can,'e.g., compute the actual position
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`of the target at some point in system time and a target trajectory computation system
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`112, which can, e.g., compute the actual trajectory of a target drop at some system
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`time, and an irradiation site temporal and spatial error computation system 114, that
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`can, e.g., compute a temporal and a spatial error signal compared to some desired
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`point in space and time for ignition to occur.
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`The controller 60 may then, e.g., provide the temporal error signal 140 to the
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`firing control system 64 and the spatial error signal 138 to the laser beam positioning
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`system 66. The firing control system may compute and provide to a resonance
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`charger portion 118 of the oscillator laser 44 magnetic reactor-switched pulse
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`compression and timing circuit 50 a resonant charger initiation signal 122 and may
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`provide, e.g., to a resonance charger portion 120 of the PA magnetic reactor-
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`switched pulse compression and timing circuit 52 a resonant charger initiation
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`signal, which may both be the same signal, and may provide to a compression circuit
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`portion 126 of the oscillator laser 44 magnetic reactor-switched pulse compression
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`and timing circuit 50 a trigger signal 130 and to a compression circuit portion 128 of
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`the amplifier laser system 48 magnetic reactor-switched pulse compression and
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`timing circuit 52 a trigger signal 132, which may not be the same signal and may be
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`computed in part from the temporal error signal 140 and fiom inputs from the light
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`V out detection apparatus 54 and 56, respectively for the oscillator laser system and the
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`amplifier laser system. The Pa could also possibly be a CW or C0; laser.
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`The spatial error signal may be provided to the laser beam position and
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`direction control system 66, which may provide, e.g., a firing point signal and a line
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`of sight signal to the laser bean positioner which may, e.g., position the laser to
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`change the focus point for the ignition site 28 by changing either or both of the
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`position of the output of the laser system amplifier laser 48 at time of fire and the
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`aiming direction of the laser output beam.
`
`In order to improve the total conversion efficiency (“TCE”), including the
`drive laser conversion efficiency (“DLCE”) relating to the conversion of drive laser
`light pulse energy into EUV photon energy and also the electrical conversion
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`efficiency (“ECE”) in converting electrical energy producing the drive laser pulses
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`to EUV light energy, and also to reduce the drive laser overall costs, as well as EUV
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`system costs, according to aspects of an embodiment of the present invention,
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`applicants propose to provide for the generation of both a drive laser pre-pulse and a
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`drive laser main pulse from the same C0; laser. This can also have a positive
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`impact on laser light focusing optics lifetimes and drive laser light input window
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`lifetime.
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`Applicants have recently determined through much investigation,
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`experimentation and analysis that the use of a C02 drive laser for LPP EUV can
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`have certain very beneficial results, e.g., in the case of a Sn-based EUV LPP plasma
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`source material. By way of example a relatively high DLCE and ECE and thus also
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`TCE number can be reached for conversion of electrical energy and also drive laser
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`light energy into EUV. However, drivel lasers such as CO2 drive lasers suffer from
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`a rather significant inability to properly focus such drive lasers as opposed to, e.g.,
`solid state lasers like Nd:YAG lasers or excimer lasers such as XeF or XeCl lasers.
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`The CO2 laser output pulse light at 10.6 urn radiation is difficult to focus tightly at
`the required dimensions.‘
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`A typical size of'a plasma formation material target droplet 94 may be on the
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`order of from 10-100 microns, depending on the material of the plasma source and
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`also perhaps the drive laser type, with smaller generally being better, e.g., from a
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`debris generation and consequent debris management point of view. With currently
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`proposed focusing schemes, e.g., as illustrated schematically and not to scale in FIG.
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`3, e.g., utilizing a focusing lens 160 a drive laser beam 152 of diameter DD (e.g.,
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`about 50m) and focal distance LL (, e.g., about 50cm, to focus l0.6micron
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`wavelength radiation into, e.g., even the largest end of the droplet range, e.g., at
`about 100microns, the divergence of a laser should be less than 2* 104 radian. This
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`value is less than diffraction limit of l.22*10.6*10‘5/50*10'3=2.6*10“ (e.g., for an
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`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.
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`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
`counterproductive, since it would then require a large central opening in a EUV
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`collector 30, reducing the EUV collection angle. The larger opening also results in
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`limiting the effect of the debris mitigation offered by the drive laser delivery
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`enclosure 150, as that is explained in more detail in one or more of the above
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`referenced co-pending applications. This decrease in effectiveness, among other
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`things can result in a decrease in the laser input window lifetime.
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`According to aspects of an embodiment of the present invention applicants
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`propose an improved method and apparatus for the input of drive laser radiation as
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`illustrated schematically and not to scale in FIG.’s 4 and 5. For, e.g., a C02 laser it
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`is proposed to use internal reflecting optics with high NA and also, e.g., using
`deposited plasmainitiation source material, e.g., Sn as a reflecting surface(s). The
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`focusing scheme may comprise, e.g., two reflecting mirrors 170, 180. Mirror 170
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`may, e.g., be a flat or curved mirror made, e.g., of molybdenum. The final focusing
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`mirror 180 can, e.g., focuses CO2 radiation in a C02 drive laser input beam 172,
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`redirected by the redirecting mirror 170 into the focusing mirror 180 to form a
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`focused beam 176 intersecting the target droplets 92 at the desired plasma initiation‘
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`site 28.
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`The focal distance of mirror 180 may be significantly less than 50 cm, e.g.,
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`5cm but not limited by this number. Such a short focal distance mirror 180 can, e.g.,
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`allow for the focus of the CO2 radiation on, e.g., 100 micron or less droplets, and
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`particularly less than 50 um and down to even about 10 um.
`‘Applicants also propose to use heating, e.g., with heaters 194, e.g., a Mo-
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`ribbon heater, which can be placed behindthe mirror 180’ according to aspects of an
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`embodiment illustrated schematically and not to scale in FIG. 5. Heating to above
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`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. under
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`the name LB462, and may be encased in a stainless steel casing to protect it from the
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`environment of the plasma generation chamber 2-6, and a similar motor 190_for the
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`mirror 170’, can be employed. Reflection of the laser radiation will be, e.g., from a
`thin film of the plasma source material, e.g., Sn, coating the mirrors 170, 180, due to
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`deposition from the LPP debris. Rotation can be used if necessary to create smooth
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`surface of the molten plasma source material, e.g., Sn. This thin film of liquid Sn
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`can form a self-healing reflective surface for the mirror 170, 180. Thus, plasma
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`source material deposition, e.g., Sn deposition on the mirror 170, 180 can be utilized
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`as a plus instead of a negative were the focusing optics in the form of one or more
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`lenses. The requirements for rouglmess (lambda/10) for 10.6um radiation can be
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`easily achieved. The mirrors 170, 180 can be steered and/or positioned with the
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`motors 192, 192.
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`Reflectivity of the liquid Sn can be estimated from Drude’s formula which
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`gives a good agreement with experimental results for the wavelengths exceeding
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`5 pm.
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`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
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`reflectivity for 10.6 um about 98.5%. For Sn the reflectivity estimate is 96%.
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`Heating of, e.g., the mirror 180’ of FIG. 5 above required melting point may
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`also be performed with an external heater (not shown) installed behind the rotating
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`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
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`plasma generation site 28.
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`As shown schematically in FIG’s 4 and 5, the laser radiation 172 may be
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`delivered into the chamber through a side port and therefore not require an overly
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`large aperture in the central portion of the collector 30. For example with
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`approximately the same size central aperture as is effective for certain wavelengths,
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`e.g., in the excimer laser DUV ranges, but ineffective for a focusing lens for
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`wavelengths such as CO2, the focusing mirror arrangement according to aspects of
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`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
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`‘delivery enclosure 300 are not in direct line of view of plasma initiation site and
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`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
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`flow 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 reaching the
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`window 202. Therefore, even if the focusing of the LPP drive laser light as
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`illustrated according to aspects of the embodiment of FIG. 5, e.g., at the distal end of
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`the drive laser delivery enclosure 200, needs to be relatively larger, e.g., for a C02
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`drive laser, the indirect angle of the debris flight path from the irradiation site 28 to
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`the distal end of the enclosure 200 allows for larger or no apertures at the distal end,
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`whereas the enlargement or removal of the apertures at the distal end of the
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`enclosure 150 illustrated in the embodiment of FIG. 3 could significantly impact the
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`ability of the enclosure 150 to keep debris from, e.g., the lens 160 (which could also
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`in some embodiments serve as the chamber window or be substituted for by a
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`chamber window). Thus, where debris management is a critical factor, the
`arrangement ofFIG.’s 4 and 5 may be utilized to keep the drive laser input enclosure
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`off of the optical axis of the focused LPP drive laser beam 152, 176 to the irradiation
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`site 28.
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`‘
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`According to aspects of an embodiment ofthe present invention, for
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`example, the laser beam 172 may be focused by external lens and form a converging
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`beam 204 with the open orifice of the drive laser input enclosure cone 200 located
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`close to the focal point. For direct focusing scheme when external lens, e.g., lens
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`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
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`plasma initiation site 28, for intersection with the droplet target 94 at about the focal
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`point of the lens 160. This can subject the distal end to a significant thermal load,
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`with essentially all of the drive laser power being absorbed by the target in the
`formation ofthe plasma and being released in or about the plasma. For the
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`suggested optical arrangement according to aspects of an embodiment of the present
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`invention with intermediate focus the cone tip can be approached to the focal point
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`(‘at distance of few millimeters) and output orifice of the cone can be very small.
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`This allows us to increase significantly the gas pressure in the gas cone, and reduce
`signi