(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`(19) World Intellectual Property
`Organization
`International Bureau
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`(43) International Publication Date
`14 September 2017 (14.09.2017)
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`WIPO!IPCT
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`\a
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`(10) International Publication Number
`WO 2017/153152 Al
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`GD)
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`International Patent Classification:
`GO03F 7/20 (2006.01)
`G02B 7/18 (2006.01)
`GO03F 1/24 (2012.01)
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`QD
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`International Application Number:
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`PCT/EP2017/053643
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`(22)
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`International Filing Date:
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`17 February 2017 (17.02.2017)
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`(25)
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`(26)
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`(30)
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`(71)
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`(72)
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`(74)
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`(81)
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`Filing Language:
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`Publication Language:
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`English
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`English
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`Priority Data:
`16158898.3
`
`7 March 2016 (07.03.2016)
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`EP
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`Applicant: ASML NETHERLANDSB.V. [NL/NL]; P.O.
`Box 324, 5500 AH Veldhoven (NL).
`
`Inventors: VAN BERKEL, Koos; P.O. Box 324, 5500
`AH Veldhoven (NL). KOEVOETS, Adrianus, Hendrik;
`P.O. Box 324, 5500 AH Veldhoven (NL).
`
`Agent: FILIP, Diana; PO Box 324, 5500 AH Veldhoven
`(NL).
`
`Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`
`AO, AT, AU, AZ, BA, BB, BG, BH, BN, BR, BW,BY,
`BZ, CA, CH, CL, CN, CO, CR, CU, CZ, DE, DJ, DK, DM,
`DO, DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM,GT,
`IIN, IIR, ITU, ID, IL, IN, IR, IS, JP, KE, KG, KI, KN,
`KP, KR, KW, KZ, LA, LC, LK, LR, LS, LU, LY, MA,
`MD, ME, MG, MK, MN, Mw, MX, MY, MZ, NA, NG,
`NL, NO, NZ, OM, PA, PE, PG, PH, PL, PT, QA, RO, RS,
`RU, RW, SA, SC, SD, SE, SG, SK, SL, SM, ST, SV, SY,
`TH, TJ, TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN,
`ZA, ZM, ZW.
`
`(84)
`
`Designated States (unless otherwise indicated, for every
`kind of regional protection available): ARIPO (BW, GH,
`GM, KE, LR, LS, MW, MZ, NA, RW, SD, SL, ST, SZ,
`TZ, UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU,
`TJ, TM), European (AL, AT, BE, BG, CH, CY, CZ, DE,
`DK,EE, ES, FI, FR, GB, GR, HR, HU,IE, IS, IT, LT, LU,
`LV, MC, MK, MT, NL, NO, PL, PT, RO, RS, SF, ST, SK,
`SM, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ,
`GW, KM, ML, MR,NE, SN, TD, TG).
`Published:
`
`with international search report (Art. 21(3))
`
`(54) Title: MULTILAYER REFLECTOR, METHOD OF MANUFACTURING A MULTILAYER REFLECTOR AND LITHO-
`GRAPHIC APPARATUS
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`Fig. 3
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`BA
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`(57) Abstract: A reflector for EUV radiation, the reflector comprising a reflector substrate and a reflective surface, the reflector sub-
`strate having a plurality of coolant channels formed therein, the coolant channels being substantially straight, substantially parallel to
`each other and substantially parallel to the reflective surface and configured so that coolant flows in parallel through the coolant
`channels and in contact with the reflector substrate.
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`WoO2017/153152A1|IMTINNMIUMIIMTANNANAAAA
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`Multilayer Reflector, Method of Manufacturing a Multilayer
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`Reflector and Lithographic Apparatus
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`[0001] CROSS-REFERENCE TO RELATED APPLICATIONS
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`This application claims priority of [UP application 16158898.3 which was filed on 7 March
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`2016 and whichis incorporated herein in its entirety by reference.
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`TIELD
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`[0002] The present invention relates to multilayer reflectors for EUV or X-ray radiation, to methods
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`10
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`of making such multi-layer reflectors and to lithographic apparatus using such multi-layer reflectors.
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`BACKGROUND
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`[0003]Alithographic apparatus is a machine constructed to apply a desired pattern onto a substrate.
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`A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A
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`lithographic apparatus may for cxample project a pattern from a patterning device (c.g. a mask) onto a
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`layer of radiation-sensitive material (resist) provided on a substrate.
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`[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a
`substrate determines the minimumsize of features which can be formedonthat substrate. A
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`lithographic apparatus which uses EUVradiation, being electromagnetic radiation having a
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`wavelength within the range 5-20 nm, may be used to form smaller features on a substrate than a
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`conventional lithographic apparatus (which may for example use electromagnetic radiation with a
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`wavelength of 193 nm).
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`[0005] Collecting EUVradiation into a beam, directing it onto a patterning device (e.g. a mask) and
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`projecting the patterned beam onto a substrate is difficult because it is not possible to make a
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`refractive optical element for EUV radiation. Therefore these functions have to be performed using
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`reflectors (i.e. mirrors). Even constructing a reflector for EUV radiation is difficult. The best
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`available normal incidencereflector for EUV radiation is a multi-layer reflector (also known as a
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`distributed Bragg reflector) which comprises a large numberof layers whichalternate between a
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`relatively high refractive index layer and a relatively low refractive index layer. Each period,
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`consisting of a high refractive index layer and a low refractive index layer, has a thickness equal to
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`half the wavelength (4/2) of the radiation to be reflected so that there is constructive interference
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`between the radiation reflected at the high to low refractive index boundarics. Such a multilayer
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`reflector still does not achieve a particularly high reflectivity and a substantial proportion of the
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`incident radiation is absorbed by the multilayer reflector.
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`[0006] The absorbedradiation, including infra-red radiation also emitted by the radiation source,
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`can cause the temperature of the multilayer reflector to rise. Known multilayer reflectors are formed
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`on substrates made of materials having a very low coefficient of thermal expansivity, for example
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`ULE™, However, in some cases the cross-section of the beam whenincident on a reflector may be
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`small enough that localized heating of the reflector causes undesirable deformation of the surface
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`figure of the reflector. Such deformation can cause imaging errors and the constant desire to image
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`ever smaller features means that the amount of deformation that can be tolcrated will only reduce.
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`SUMMARY
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`[0007]
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`It is an aim ofthe invention to provide an improved multilayer reflector.
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`[0008] According to the present invention, there is provided a reflector for EUV radiation, the
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`reflector comprising a reflector substrate and a reflective surface, the reflector substrate having a
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`plurality of coolant channels formed therein, the coolant channels being substantially straight,
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`substantially parallel to each other and substantially parallel to the reflective surface and configured
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`so that coolant flows in parallel through the coolant channels and in contact. with the reflector
`substrate.
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`[0009] According to the present invention, there is provided a lithographic apparatus arranged to
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`project a pattern from a patterning device onto a substrate, the apparatus comprising at least one
`reflector as described above.
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`[0010] According to the present invention, there is provided a method comprising projecting a
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`patterned beam of radiation onto a substrate, wherein the patterned beam is directed or patterned using
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`at least one reflector as described above while coolant is conducted through the coolant channels.
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`[0011] According to the present invention, there is provided a method of manufacturing a reflector
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`for a projection system of a lithographic apparatus using EUVradiation, the reflector comprising a
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`reflector substrate and a reflective surface, the substrate having a plurality of coolant channels
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`embeddedtherein, the coolant channels being substantially parallel to the reflective surface, the
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`method comprising polishing the reflective surface while a pressurised fluid is provided to the coolant
`channels.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`[0012] Embodiments of the invention will now be described, by way of example only, with
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`reference to the accompanying schematic drawings, in which:
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`Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation
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`source according to an embodimentof the invention;
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`Figure 2 depicts in cross-section a multilayer reflector;
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`Figure 3 depicts in perspective view a multilayer reflector illustrating the areas of incidence
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`oo way
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`of a projection beam;
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`Figure 4 depicts in plan a reflector according to an embodiment of the invention;
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`Figure 5 depicts in cross-section a part of a reflector according to an embodiment of the
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`invention; and
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`Tigure 6 depicts in cross-section a part of a reflector according to another embodimentof the
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`invention.
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`DETAILED DESCRIPTION
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`[0013] Figure | showsa lithographic system including a multilayer reflector according to one
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`embodiment of the invention. The lithographic system comprises a radiation source SO and a
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`lithographic apparatus LA. The radiation source SOis configured to generate an extremeultraviolet
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`(EUV)radiation beam B. The lithographic apparatus LA compriscs an illumination system IL, a
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`support structure MT configured to support a patterning device MA(e.g. a mask), a projection system
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`PS and a substrate table WT configured to support a substrate W. The illumination system IL is
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`configured to condition the radiation beam B beforeit is incident upon the patterning device MA.
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`The projection system is configured to project the radiation beam B (now patterned by the mask MA)
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`onto the substrate W. The substrate W mayinclude previously formed patterns. Wherethis is the
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`case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously
`formed on the substrate W.
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`[0014] The radiation source SO, illumination system IL, and projection system PS may all be
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`constructed and arranged such that they can be isolated from the external environment. A gas ata
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`pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A
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`vacuum may be providedin illumination system IL and/or the projection system PS. A small amount
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`of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be providedin the
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`illumination system IL and/or the projection system PS.
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`[0015] The radiation source SO shown in Figure | is of a type which maybe referred to as a laser
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`produced plasma (LPP) source). A laser 1, which may for example be a CO,laser, is arranged to
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`deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3.
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`Althoughtin is referred to in the following description, any suitable fucl may be used. The fucl may
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`for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may
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`comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a
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`plasma formation region 4. The laser beam2 is incident uponthe tin at the plasma formation region
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`4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4.
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`Radiation, including EUVradiation, is emitted from the plasma 7 during de-excitation and
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`recombination of ions of the plasma.
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`[0016]
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`‘he EUV radiation is collected and focused by a near normal incidence radiation collector 5
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`(sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may
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`have a multilayer structure (described further below) whichis arranged to reflect EUV radiation(e.g.
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`EUVradiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical
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`configuration, having twoellipse focal points. A first focal point may be at the plasma formation
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`region 4, and a second focal point may be at an intermediate focus 6, as discussed below.
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`[0017] The laser 1 may be separate from the radiation source SO. Wherethis is the case, the laser
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`beam 2 may be passed from the laser | to the radiation source SO with the aid of a beam delivery
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`system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander,
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`and/or other optics. The laser | and the radiation source SO may together be considered to be a
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`radiation system.
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`[0018] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam
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`B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual
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`radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused
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`maybe referred to as the intermediate focus. The radiation source SO is arranged such that the
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`intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation
`source.
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`[0019] The radiation beam B passes from the radiation source SO into the illumination system IL,
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`whichis configured to condition the radiation beam. The illumination system IL mayinclude a
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`facetted field mirror device 10 and a facetted pupil mirror device 11.
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`‘The faceted field mirror device
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`10 and faceted pupil mirror device 11 together provide the radiation beam B witha desiredcross-
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`sectional shape and a desired angular distribution. The radiation beam B passes from the illumination
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`system IL andis incident upon the patterning device MA held by the support structure MT. The
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`patterning device MAreflects and patterns the radiation beam B. The illumination system IL may
`include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and
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`faceted pupil mirror device 11. The faceted field mirror device 10, faceted pupil mirror device 11 and
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`other reflectors of the illumination system may have a multilayer structure as described further below.
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`[0020]
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`‘Following reflection from the patterning device MAthe patterned radiation beam B enters
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`the projection system PS. The patterning device may include a reflector having a multilayer structure
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`as described further below. The projection system comprises a plurality of mirrors which are
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`configured to project the radiation beam B onto a substrate W held by the substrate table WT. The
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`projection system PS may apply a reduction factor to the radiation beam, forming an image with
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`features that are smaller than corresponding features on the patterning device MA. A reduction factor
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`of 4 may for example be applied. Although the projection system PS has two mirrors in Figure 1, the
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`projection system may include any numberof mirrors(e.g. six mirrors). The mirrors, and any other
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`reflectors of the projection system PS, may have a multilayer structure as described further below.
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`[0021]
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`‘Lhe radiation source SO shownin Figure 1 may include components which are not
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`illustrated. For example, a spectral filter may be providedin the radiation source. The spectral filter
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`may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths
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`of radiation such as infrared radiation. Alternative radiations sources, such as free electron lasers, can
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`also be used.
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`[0022]
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`[igure 2 depicts a multilayer reflector 30 according to an embodimentof the present
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`invention. The multilayer reflector 30 comprises a plurality of alternating high refractive index layers
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`32 (somctimes referred to as spaccr layers) and low refractive index layers 34 (sometimes referred to
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`as refracting layers). A pair of adjacent layers is referred to herein as a period. The thickness of a
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`period is approximately equal to half the wavelength (A/2) of the radiation that is desired to be
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`reflected, e.g. 6.9 nmto reflect EUV radiation at 13.5 nm. There may be between 60 and 100 periods,
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`e.g. about 80. The multilayer reflector functions as a distributed Bragg reflector with constructive
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`interference betweenthe radiation reflected at the boundarics between high refractive index laycrs and
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`low refractive index layers. The multilayer reflector may be formed on a reflector substrate 38 and
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`may be provided with a capping layer 36. Capping layer 36 can be formed of various known
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`materials and helps to protect the multilayer reflector from chemical and physical damage. Tn an
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`embodimentof the invention the low refractive index layers are Mo and the high refractive index
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`layers are Si but other combinations of materials are possible.
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`[0023] Figure 3 depicts reflector 13, that is a reflector (e.g., the first) in projection system PS after
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`the patterning device MA,in a perspective view. Reflector 13 (or, 14) may be a multilayer reflector
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`of the type described above with reference to Figure 2. As can be seen, the projection beamB is
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`incident on reflector in two localized areas BA corresponding to the illuminator pupil. Reflector 13
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`may havea reflectivity of about 70% or less. Therefore reflector 13 absorbs a significant amount of
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`energy from the projection beam whenthe lithographic apparatus is operating. The energyis
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`absorbed by reflector 13 in the areas BA on whichthe projection beam is incident. ‘Wherefore the
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`reflector experiences a non-uniform temperature rise. Although the reflector substrate 38 may be
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`made of a material having a very low coefficient of thermal expansion, such asa titania silicate glass
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`(specifically ULE™manufactured by Corning Incorporated), a non-uniform temperature rise in the
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`reflector substrate 38 can lead to a significant deformation of the surface figure of reflector 13. Even
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`though the deformation of the surface figure of reflector 13 may be very small in absolute terms, duc
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`to the extreme precision required to manufacture devices with small feature sizes, such deformation
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`can lead to imaging errors. The continual desire to increase throughput, by increasing beam power,
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`will lead to an increase in the temperature rise, whilst the continual desire to image smaller features
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`makes any deformation of the surface figure more problematic.
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`[0024] Existing reflectors in the projection systems of EUV lithographic apparatus are cooled
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`passively, i.e. by radiation, conduction and convection. However none of these modes of cooling
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`allows a high rate of heat transfer. In particular, the reflectors are generally in a high vacuum or a low
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`pressure of Hydrogenso that heat transfer by convection is minimal. Active cooling of reflectors has
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`been avoided because of the risk of introducing vibrations in the reflector which could easily be more
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`problematic than the distortion caused by the localized heatrise.
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`[0025] An embodimentof the invention is a reflector for a projection system of a lithographic
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`apparatus using EUVradiation, the reflector comprising a reflector substrate and a reflective surface,
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`the substrate having a plurality of coolant channels embeddedtherein, the coolant channels being
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`substantially parallel to the reflective surface.
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`[0026]
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`Byproviding a plurality of straight coolant channels parallel to each other and parallel to the
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`reflective surface, a coolant can be circulated throughthe reflector substrate to control or reduce the
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`localized temperature rise without generating problematic vibrations in the reflector. As the coolant
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`fluid flows in parallel through the plurality of coolant channcls and in direct contact with the reflector
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`substrate, the thermal conductivity between the reflector substrate and the coolant fluid is enhanced.
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`The heat transfer capacity of the coolant system is increasedsoit is possible to reduce the mass flow
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`(kilograms per second) and average velocily (meters per second) to reduce pressure variations induced
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`by inertia and friction, respectively. Therefore vibrations are reduced.
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`[0027]
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`In an embodiment, each coolant channel has a substantially constant cross-section. By
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`making the coolant channels with a constant cross-section, it is possible to further reduce the
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`generation of vibrations in the reflector.
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`[0028]
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`In an embodiment, each coolant channel is spaced apart fromthe reflective surface by a
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`distance in the range of from 5 to 10 times the diameter of the coolant channel. By providing a
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`spacing in this range it is possible to ensure that the pressure variations of the coolant in the coolant
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`channels does not cause a problematic deformation of the reflective surface.
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`[0029]
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`In an cmbodiment, the distance between the centres of adjacent coolant channelsis in the
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`range of from 5 to 10 times the diameter of the coolant channels. By spacing the coolant channels in
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`this wayit is possible to ensure a desirably uniform temperature profile at the reflective surface.
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`[0030]
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`In an embodiment, the coolant channels have substantially the same cross-section. By
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`making the coolant channels have a substantially uniform cross-section, it is possible to reduce
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`turbulence in the flow of coolant and so further reduce the generation of vibrations in the reflector.
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`[0031]
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`In an embodiment, there are from 10 to 100, e.g. from 20 to 60, coolant channels. By
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`providing this numberof coolant channels it is possible to ensure a desirably uniform temperature
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`profile at the reflective surface.
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`[0032]
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`In an embodiment,the reflector substrate comprisesa first reflector substrate part joined to a
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`secondreflector substrate part, the second reflector substrate part having a different composition from
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`the first reflector substrate part. In this way it is possible to use a material with a very low coefficient
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`of thermal expansion for the parts of the reflector substrate that have most effect on the surface figure
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`of the reflective surface and another material having other desirable properties or lower cost for the
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`remaining parts of the reflector substrate.
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`[0033]
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`In an embodiment, the reflector substrate is formed ofa titania silicate glass. A titania
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`silicate glass can be optimised to have a low or zero coefficient of thermal expansion at a desired
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`temperature or temperature range. In a twopart reflector substrate, different grades of glass can be
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`used for the different parts.
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`[0034]
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`In an embodimentthe reflector also comprises a coolant supply system connected to the
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`coolant channels for supplying a coolant comprising water and/or carbon dioxide. Water and carbon
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`dioxide can provide sufficient heat transfer capacity at reasonable mass flow rates.
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`[0035]
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`In an embodiment, the coolant supply system is configured to supply water at a pressure in
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`the range of from 0.001 to 10 bar, for example 2 bar. Such a pressure range can provide sufficient
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`heat transfer capacity without causing undue vibrations and/or deformation of the surface figure.
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`[0036]
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`In an embodimentthe coolant supply system is configured to supply liquid carbon dioxide at
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`a pressure in the range of from 20 bar to 100 bar, desirably 50 bar to 70 bar. Such a pressure range
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`can provide sufficient heat transfer capacity without causing undue vibrations and/or deformation of
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`the surface figure.
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`[0037]
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`In an embodiment,the reflector is the first reflector after the patterning device. In many
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`configurations of lithography apparatus, the first reflector after the patterning means experiences the
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`highest heat load in use and therefore is where the invention provides the mostbenefit. ‘The present
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`invention can also provide significant advantage for other reflectors and in particular for reflectors
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`wherethe effect of surface deformation on the projected imageis highest.
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`[0038] A maskfor usein a lithographic apparatus, the mask comprising at least one reflector as
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`described above and a patterned absorberlayer.
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`[0039] An cmbodiment of the invention is a method comprising projecting a patterned beam of
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`radiation onto a substrate, wherein the patterned beam is directed or patterned using at least one
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`reflector as described above while coolant is conducted through the coolant channels.
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`[0040] An embodimentof the invention is a method of manufacturing a reflector for a projection
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`system of a lithographic apparatus using EUV radiation, the reflector comprising a reflector substrate
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`and a reflective surface, the substrate having a plurality of coolant channels embedded therein, the
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`coolant channels being substantially parallel to the reflective surface, the method comprising
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`polishing the reflective surface while a pressurised fluid or gas(air is practical) is provided to the
`coolant channels.
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`[0041] By polishing the reflective surface to its final surface figure whilst the coolant channels are
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`pressurized to their working pressure, it is possible to pre-compensate for any deformation of the
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`surface figure due to the static component of the pressure in the coolant channels.
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`[0042] Figure 4 illustrates the arrangements of coolant channels in a reflector 13 of an embodiment.
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`A plurality of coolant channels 41 are connected to input manifold 40 and output manifold 42. The
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`plurality of coolant channels 41 are straight, parallel to each other and parallel to the reflective surface
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`to minimise turbulence. They are configured to form a plurality of parallel coolant circuits to
`maximise thermal transfer between the reflector substrate and the coolant fluid. The coolant channels
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`are formed directly in the material of the reflector substrate so as to maximiseheattransfer; there 1s no
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`lining to the channels. In an embodiment, the plurality of coolant channels 41 comprises a first subset
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`of coolant channels that arc connected to a first input manifold anda first output manifold and a
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`second subset of coolant channels that are connected to a second input manifold and a second output
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`manifold. In such embodiment, the manifolds may be arranged in such mannerthat a flow of coolant
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`through the first subset of coolant channels flows in a opposite direction as a flow of coolant through
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`the second subset of coolant channels. In such an arrangement, the first input manifold and the
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`second output manifold may be arranged adjacent cach other on one side of the reflector 13, whereas
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`the first output manifold and the second input manifold are arranged adjacent each other on an
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`opposite side of the reflector 13. By arranging an opposite flow in the subsets of coolant channels, a
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`more homogeneous temperature profile of the reflector may be obtained.
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`[0043] Coolant fluid is supplied from coolant supply unit 43 to input manifold 40 and removed
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`from output manifold 42 by coolant recovery unit 44. In an embodiment input manifold 40 and
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`output manifold 42 are omitted and the coolant channels 41 are separately connected to the coolant
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`supply unit 43 and coolant recovery unit 44. Coolant fluid recovered by coolant recovery unit 44 can
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`be recirculated to coolant supply unit 43. Coolant supply unit 43 can include a temperature
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`conditioning system to ensure that the coolant fluid supplied to the input manifold 41 is at a desired
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`temperature and pressure. The coolant supply unit and coolant recovery unit are an example of a
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`coolant supply system. In the embodiment whereby the plurality of coolant channels 41 comprises a
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`first subset of coolant channels that are connected to a first input manifold and a first output manifold
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`and a secondsubset of coolant channels that are connected to a second input manifold and a second
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`output manifold, the first and second input manifolds may be supplied from a commoncoolant supply
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`unit, e.g. coolant supply unit 43. In such an arrangement, due to the flow in opposite directions
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`through the first subset of coolant channels and the second subset of coolant channels, effects of
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`pressure variations in the coolant supply may, at Icast partially, be cancelled out. In an embodiment,
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`the number of coolant channels in the first subset is substantially the sameas in the second subset.
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`[0044] Reflector 13 is the first reflector in the projection system after the patterning means. This
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`reflector experiences the highest heat load of the reflectors of the projections system. Dueto the
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`illumination pupil the heat load is concentrated on reflector 13. Therefore reflector 13 may obtain the
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`most benefit from in the invention. Other reflectors in the projection system may also obtain
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`significant benefit from the invention, e.g. small reflectors, reflectors where the beam cross-section is
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`small and/or reflectors having a high sensitivity to surface figure errors.
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`[0045] The coolant fluid may be water, which is advantageousas it has a high thermal capacity and
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`so arelatively low mass flow rate can provide a large heat transfer capacity. If the coolant fluid is
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`water, the coolant supply system may be integrated with a coolant supply system used for temperature
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`conditioning of other parts of the lithographic apparatus, e.g. the substrate table.
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`[0046] The coolant fluid may be carbon dioxide. Carbon dioxide is advantageousas it can be
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`supplied as a liquid (under pressure) so that it evaporates within the coolant channels in the regions of
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`higher temperature. The latent heat of cvaporation therefore increascs the heat transfer capacity of the
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`coolant fluid. For a given heat load, the required mass flow can be much lower than with water,
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`therewith reducing flow-induced vibrations.
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`[0047]
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`Asillustrated in Figure 5, coolant channels 41 are embeddedin the reflector substrate 38.
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`Depending on the material of reflector substrate 38, coolant channels may be formed simply by
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`creating voids of the appropriate shape andlocation in reflector substrate 38.
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`[0048] Coolant channels 41 are located a distance d1 below thereflective surface (e.g. multilayer
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`30) of reflector 13 and are spaced apart by a distance d2. Distance d1 may be greater than or equalto
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`5 times the diameter of the coolant channels in order to prevent the pressure of coolant fluidin the
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`coolant channels 41 causing a deformation of the reflective surface. Distance d1 may be less than or
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`equal to 30 times the diameter of the coolant channels in order to ensure a sufficient heat transfer from
`the reflective surface to the coolant fluid in the coolant channels. In an embodiment distance d1 is
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`equal to about 7.5 times the diameter of the coolant channels 41.
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`[0049] Distance d2 may bein the range of from3 to 30 times the diameter of the coolant channels
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`41. If distance d2 is too large, the cooling of the reflective surface may not be sufficiently uniform. If
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`distance d2 is too small, the reflector substrate may be weakened or the number of coolant channels
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`may becometo large. The optimum distance between coolant channels may depend on the distance
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`di. If the coolant channels are close to the reflective surface then a close spacing may be desirable to
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`achieve uniform cooling.
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`[0050] The cross-section of the coolant channels can be any convenient shape, e.g. square or round.
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`It is desirable that the aspect ratio of the cross-section of the coolant channels 41 not be too high,e.g.
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`less than 4. If the cross-section of the coolant channels is not circular, the diameter of the coolant
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`channels should be taken to be the largest dimension of the cross-section. The diameter may be
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`greater than or equal to 0.5 mm,desirably greater than or equal to 1 mm. If the diameter is small the
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`flow resistance may be high requiring a higher pressure difference to achieve a sufficient mass flow
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`rate and potentially causing vibrations or deformationof the reflective surface. The diameter may be
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`less than or equal to 5 mm, desirably less than or equal to 3 mm. If the diameter is too large,
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`deformation of the reflective surface due to the pressure of the coolant fluid may be too large and/or it
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`may bedifficult to achieve uniform cooling. In an embodimentthe diameter of the coolant channels
`is 2 mm.
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`[0051] The numberof coolant channels is dependent on the size of the reflector 13 and the distance
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`d2 between coolant channels. In an embodiment, there may be between 10 and 100 coolant channels,
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`desirably between 20 and 60. In an embodimentthere are about 40 coolant channels.
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`[0052] Desirably the coolant channels are straight to reduce flow resistance and make manufacture
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`casy. Desirably the coolant channels have a constant cross-section along their lengths to reduce flow
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`resistance and make manufacture easy.
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`[0053]
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`In an embodiment having 40 coolant channels which have a square cross-section of side 2
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`mm, a volume flow rate of water of 0.1 slm can be achieved with a pressure drop of about 20 Pa.
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`Such a flow is found not to induce undue vibrations or pressure deformation of the surface figure. A
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`reduction in temperature at the reflective surface of