WO2012013751A1 - Euv exposure apparatus - Google Patents

Euv exposure apparatus Download PDF

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Publication number
WO2012013751A1
WO2012013751A1 PCT/EP2011/063002 EP2011063002W WO2012013751A1 WO 2012013751 A1 WO2012013751 A1 WO 2012013751A1 EP 2011063002 W EP2011063002 W EP 2011063002W WO 2012013751 A1 WO2012013751 A1 WO 2012013751A1
Authority
WO
WIPO (PCT)
Prior art keywords
temperature
projection lens
mirror
optical element
zero cross
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/EP2011/063002
Other languages
English (en)
French (fr)
Inventor
Norman Baer
Ulrich Loering
Oliver Natt
Gero Wittich
Timo Laufer
Peter Kuerz
Guido Limbach
Stefan Hembacher
Holger Walter
Yim-Bun-Patrick Kwan
Markus Hauf
Franz-Josef Stickel
Jan Van Schoot
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Carl Zeiss SMT GmbH
ASML Netherlands BV
Original Assignee
Carl Zeiss SMT GmbH
ASML Netherlands BV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Carl Zeiss SMT GmbH, ASML Netherlands BV filed Critical Carl Zeiss SMT GmbH
Publication of WO2012013751A1 publication Critical patent/WO2012013751A1/en
Priority to US13/754,720 priority Critical patent/US9316929B2/en
Anticipated expiration legal-status Critical
Priority to US15/070,872 priority patent/US9746778B2/en
Priority to US15/650,533 priority patent/US10031423B2/en
Priority to US16/013,687 priority patent/US10317802B2/en
Priority to US16/392,220 priority patent/US10684551B2/en
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70058—Mask illumination systems
    • G03F7/7015—Details of optical elements
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/0025—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration
    • G02B27/0037—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration with diffracting elements
    • G02B27/0043—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration with diffracting elements in projection exposure systems, e.g. microlithographic systems
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/18—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical projection, e.g. combination of mirror and condenser and objective
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/08—Mirrors
    • G02B5/0816—Multilayer mirrors, i.e. having two or more reflecting layers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/08—Mirrors
    • G02B5/0891—Ultraviolet [UV] mirrors
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B7/00—Mountings, adjusting means, or light-tight connections, for optical elements
    • G02B7/18—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors
    • G02B7/181—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors with means for compensating for changes in temperature or for controlling the temperature; thermal stabilisation
    • G02B7/1815—Mountings, adjusting means, or light-tight connections, for optical elements for prisms; for mirrors with means for compensating for changes in temperature or for controlling the temperature; thermal stabilisation with cooling or heating systems
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70058—Mask illumination systems
    • G03F7/702—Reflective illumination, i.e. reflective optical elements other than folding mirrors, e.g. extreme ultraviolet [EUV] illumination systems
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70216—Mask projection systems
    • G03F7/70233—Optical aspects of catoptric systems, i.e. comprising only reflective elements, e.g. extreme ultraviolet [EUV] projection systems
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/70216—Mask projection systems
    • G03F7/70316—Details of optical elements, e.g. of Bragg reflectors, extreme ultraviolet [EUV] multilayer or bilayer mirrors or diffractive optical elements
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
    • G03F7/70808—Construction details, e.g. housing, load-lock, seals or windows for passing light in or out of apparatus
    • G03F7/70825—Mounting of individual elements, e.g. mounts, holders or supports
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
    • G03F7/70858—Environment aspects, e.g. pressure of beam-path gas, temperature
    • G03F7/70883—Environment aspects, e.g. pressure of beam-path gas, temperature of optical system
    • G03F7/70891—Temperature
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/70—Microphotolithographic exposure; Apparatus therefor
    • G03F7/708—Construction of apparatus, e.g. environment aspects, hygiene aspects or materials
    • G03F7/7095—Materials, e.g. materials for housing, stage or other support having particular properties, e.g. weight, strength, conductivity, thermal expansion coefficient
    • G03F7/70958—Optical materials or coatings, e.g. with particular transmittance, reflectance or anti-reflection properties
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/06—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diffraction, refraction or reflection, e.g. monochromators
    • G21K1/062—Devices having a multilayer structure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
    • G02B17/02—Catoptric systems, e.g. image erecting and reversing system
    • G02B17/06—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror
    • G02B17/0647—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror using more than three curved mirrors
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B3/00—Simple or compound lenses
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/08—Mirrors
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03B—APPARATUS OR ARRANGEMENTS FOR TAKING PHOTOGRAPHS OR FOR PROJECTING OR VIEWING THEM; APPARATUS OR ARRANGEMENTS EMPLOYING ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ACCESSORIES THEREFOR
    • G03B27/00—Photographic printing apparatus
    • G03B27/32—Projection printing apparatus, e.g. enlarger, copying camera
    • G03B27/52—Details
    • G03B27/54—Lamp housings; Illuminating means
    • G03B27/542—Lamp housings; Illuminating means for copying cameras, reflex exposure lighting
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K2201/00—Arrangements for handling radiation or particles
    • G21K2201/06—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements
    • G21K2201/061—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements characterised by a multilayer structure
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K2201/00—Arrangements for handling radiation or particles
    • G21K2201/06—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements
    • G21K2201/064—Arrangements for handling radiation or particles using diffractive, refractive or reflecting elements having a curved surface

Definitions

  • the present invention relates to a reflective optical element for reflecting light in a wavelength range less than 50 nm in an EUV-lithographic projection exposure system. Further, it relates to an EUV projection lens of an EUV-lithographic projection exposure system and to a method to configure an EUV projection lens.
  • the wavelengths for the projection light may be even 5 nm or less.
  • the projection lenses of the optical microlithographic projection systems comprise only reflective optical elements like mirrors and/or diffractive structures like reflective diffractive structures.
  • Projection systems which are working at a wavelength of less than about 50 nm are known as EUV (Extreme Ultra Violet) lithographic projection exposure systems.
  • a simplified EUV lithographic projection exposure system 100 is schematically shown in Fig. 1.
  • the system comprises an EUV light source 1, producing EUV light with a significant energy density in the extreme ultraviolet or EUV spectral region, especially in the wavelength range less than 50 nm, preferably in a range between 5 nm and 15 nm.
  • Discharged-produced or laser-produced plasma light sources are used as EUV light sources, making use of e.g. xenon, tin or lithium plasma which generates the extreme ultraviolet light.
  • Such sources irradiate unpolarized light under about 4 ⁇ solid angle.
  • Other sources generate a spatially more directed and a more polarized beam of extreme ultraviolet light like e.g. synchrotron radiation sources.
  • a collector mirror 2 may be used to collect the EUV light of the light source 1 to increase the energy density or irradiance of the EUV radiation and form an illumination beam 3.
  • the illumination beam 3 illuminates via an illumination system 10 a structured object M.
  • the structured object M is e.g. a reflective mask, comprising reflective and non- reflective or at least minor reflective regions to form at least one structure on it.
  • the structured object comprises or consists of a plurality of mirrors which are arranged about side by side in at least one dimension to form a mirror arrangement like a mirror array.
  • the mirrors of the mirror array are adjustable around at least one axis to adjust the incidence angle of the illumination beam 3 which is irradiated on the respective mirror.
  • the terms reflective, minor reflective and non-reflective relates to the reflectivity of EUV light of the illumination beam 3. Due to the very short wavelength of the EUV light, the reflective surfaces are usually coated if the angle of incidence for the EUV light is less than about 45°.
  • the coatings preferably comprise a multilayer of predetermined layer materials with predetermined layer thicknesses. Such mirrors are usually used for incidence angles less or far less than 45" down to about 0°. For such mirrors a reflectivity of more than 60% can be achieved due to a constructive interference of the reflected EUV light which is partially reflected at the various material boundaries of the individual layers of the multilayer.
  • a further advantage of such multilayer-coated reflective mirrors or surfaces is their property to work as a spectral filter, to make e.g. an illumination and/or projection beam of the EUV lithographic projection system more monochromatic.
  • coated mirrors are sometimes also designated as normal incidence mirrors.
  • the reflective surface comprises a metal or a metal layer like Ruthenium, or if the reflective surface consists of a metal or a metal layer, comprising e.g. Ruthenium.
  • the reflectivity can be increased up to 60% and more without the necessity of a multilayer as mentioned above.
  • these mirrors are also called grazing incidence mirrors.
  • EUV lithographic projection exposure systems often use plasma light sources.
  • the collector mirror 2 can be a grazing incidence mirror as described e.g. in WO 2002/065482 A2 or US 2004/0130809 Al.
  • the structured object M reflects parts of the illumination beam 3 into a light path which forms a projection beam 4.
  • the structured object M structures the illumination beam 3 after being reflected on it, depending on the structure on the mask M.
  • This projection beam 4 is carrying the information of the structure of the structured object and is irradiated into a projection lens 20 such that at least two diffraction orders of the structure or the structures of the structured object M pass the projection lens 20 and form a kind of an image of the structure or the structures of the structured object M on a substrate W.
  • the substrate W e.g. a wafer, comprising a semiconductor material like silicon, is arranged on a substrate stage WS which is also called wafer stage.
  • the projection beam also comprises information about the illumination condition of how the structured object M is illuminated regarding angular, polarization and intensity (or radiation power per unit area) in an object point OP of the structured object , and of how these parameters are distributed over the illuminated surface of the structured object M.
  • the kind of illumination is expressed by the term "setting". This means a predefined angular and/or polarization and/or intensity distribution with which an object point OP on the structured object is illuminated, and how these distributions depend on the spatial position on the structured object M.
  • the setting also influences the optical resolution of the projection process which is done by the projection lens 20. In general, the optical resolution can be increased if the setting is adapted to the shape of the structure on the structured object M.
  • the projection lens 20 schematically shows four mirrors 21, 22, 23 and 24 as reflective optical elements to form a kind of an image of the structure of the structured object on the wafer W.
  • EUV projection lenses 20 typically comprise 4 to 8 mirrors.
  • projection lenses with only two mirrors may also be used. These mirrors are made with highest precision regarding surface figure (or regarding their geometrical form) and surface roughness.
  • Each deviation from the desired specification results in a degradation of the image quality on the substrate or wafer W.
  • the specification is such that a deviation from the surface figure (the required or specified dimensions of the shape of the surface) of less than one tenth of the used projection wavelength is required.
  • the surface figures of the mirrors 21, 22, 23and 24 must be made with a precision of even better than 1 nm, for some mirrors the precision requirements are even a factor of 5 to 20 higher, ending up at precision ranges of much smaller than one atom layer, which means better than 0.1 nm.
  • the optical aberration of the projection lens 20 must be smaller than 1 nm, even smaller than 0.1 nm or smaller than 50 pm (picometer) of RMS value.
  • the projection beam 4 is generated by the illumination beam 3 and the diffracting properties of the structured object M.
  • One necessary condition to form an image on the substrate W is that the diffracted wave fronts, which are coming from an object point OP, interfere in the image point IP on the substrate or wafer W. To get a good image quality the interfering wave fronts must have a relative phase shift of far less than one wavelength of the projection beam light.
  • the light path of the light passing one object point OP on the structured object M can vary within the projection lens 20 in such a way that light bundles of the projection beam 4 are reflected by the mirrors 21, 22, 23, 24 of the projection lens 20 at different surface areas with different sizes. This variation depends on the illumination settings and the position of the mirrors 21, 22, 23, 24 within the projection lens 20. To make sure that the image quality is achieved under all illumination settings it is necessary that the above- mentioned surface figure is achieved with the mentioned high mechanical precision.
  • the position and orientation of these mirrors 21, 22, 23, 24 relative to each other, relative to the structured object M and relative to the substrate W must be in the same range of accuracy. This means that position and orientation of these objects (mirrors 21, 22, 23, 24, structured object M and substrate W) must be adjusted also in the nanometer range or even below.
  • a metrology is necessary to allow the manufacturing of such precise mirror surfaces, the assembling of the projection lens of the EUV lithographic projection system, the integration of the assembled projection lens into the projection system, and to allow any in-situ monitoring and control of the system during the operation of the system.
  • one further problem is the absorption of the projection beam 4 by the mirrors 21, 22, 23, 24. This absorption which could be in a range of up to 30% heats the mirrors. Depending on the absorbed heat each mirror may be deformed due to thermal expansion of the mirror.
  • One method to reduce such thermal effects during the projection step, when a certain amount of the projection beam 4 is absorbed, is to use a temperature control system to keep the very high mechanical precision data as mentioned above, especially for the surface figure of the mirrors.
  • Another or an additional method is to use as a mirror material or as a support structure for the mirror a low thermal expansion material with such a small coefficient of thermal expansion (CTE) like 5 ppb/K (or less) to reduce deformations of the mirror, if the temperature changes e.g. due to partial absorption of the projection beam 4,
  • CTE coefficient of thermal expansion
  • EUV lithographic projection exposure systems like shown in Fig. 1 are usually operated under vacuum conditions.
  • the projection lens 20 and/or the illumination system 10 are operated under reduced pressure or vacuum.
  • the reduced pressure or vacuum conditions significantly reduce the technical solutions for the above mentioned problems regarding the deformation of mirrors and their active or passive position control.
  • temperature control systems for controlling a temperature of components inside the EUV lithographic projection exposure system are quite often limited to certain technical solutions which are not essentially based on thermal convection principles.
  • a projection lens of an EUV lithographic projection exposure system in accordance to this invention comprises mirrors and a supports structure to support or suspend the mirrors. In the vicinity of a mirror the support structure is at a reference temperature. Preferably the reference temperature is the same for all support structure parts which are close to the mirrors, and the mirrors are also at this reference temperature if they are not subjected to any radiation like EUV light. Further, the projection lens comprises a heater and temperature control system to control the temperature of at least one heatable mirror.
  • the at least one heatable mirror comprises a body which comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a temperature Tok (called zero cross temperature). The zero cross temperature is selected such that it is higher than the reference temperature.
  • the heatable mirror is heated and controlled to a temperature selected from a group of determined temperatures before the formation of the temperature distribution which is caused by the exposure power of the EUV light.
  • a temperature selected from a group of determined temperatures before the formation of the temperature distribution which is caused by the exposure power of the EUV light.
  • the warm up time (if the mirror is exposed with the exposure power of the EUV light) of the heatable mirror is significantly reduced.
  • the determined temperatures are selected such that the average temperature of the mirror after the warm up is close to or at the zero cross temperature.
  • the at least one heatable mirror has a temperature at or close to the zero cross temperature.
  • Preferably more than one mirror of the projection lens is heatable.
  • a further projection lens, called first projection lens, of this invention comprises at least two mirrors with mirror bodies wherein each body comprise a material with a temperature dependent coefficient of thermal expansion which is zero at respective zero cross temperatures T 0m , T 0n .
  • the absolute vaiue of the difference between these zero cross temperatures T 0m , T 0n is more than 6K and the lens is designed to be exposed with an exposure power of more than 8W of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • the selection of materials which widely differ regarding their respective zero cross temperatures advantageously offer the possibility to make projection lenses for higher exposure powers without the drawback that the thermal induced aberration of the mirrors is increasing. This offers the possibility to manufacture EUV projection lenses with minimal aberration and high exposure power.
  • the materials of the mirror bodies or substrates are selected according to a rule which is also part of this invention. At least one material is selected in such a way that its zero cross temperature is above the largest temperature value which the mirror can get under operation. With an additional heater the mirror is warmed up to the zero cross temperature or close to it which results in minimal thermal aberration.
  • the advantage of the second projection lens is that the lens is very insensitive regarding the selected exposure power, any manufacturing tolerances of the zero cross temperature, any selected illumination settings and any mask patterns.
  • a further projection lens called third projection lens, according to this invention for at least two mirrors one material for the mirror bodies is selected.
  • This material has a temperature dependent coefficient of thermal expansion which is zero at least two zero cross temperatures T 1 omn and ⁇ 2 omn ⁇
  • the projection lens is designed such that one of the two mirrors is operated at or close to the first zero cross temperature T 1 omn and the other mirror is operated at or close to the second zero cross temperature ⁇ 2 omn , if the projection lens is subjected to the EUV exposure power which is necessary for the projection process in mass production.
  • the advantage of the third projection lens is that the lens needs fewer different substrate materials for its mirrors.
  • the present invention relates to a method to configure a EUV projection lens. Especially the method focuses on the selection of suitable materials for the mirror bodies such that the thermal aberration of the lens can be minimized.
  • Further aspects of the invention are to provide various heating means or tempering means with which the mirrors in an EUV projection lens can be heated or cooled.
  • These means also comprise mirror bodies with characteristic features like coatings or shapes which improve the temperature control of the respective mirror during its operation in the projection lens.
  • mirrors of the EUV projection lens shall also comprise any reflective elements like reflective diffractive structures, since the teachings of the present invention are not limited to mirrors.
  • Figure 1 schematically shows a simplified EUV lithographic projection exposure system with an illumination system and an EUV projection lens which comprises four mirrors.
  • Figure 2 a linear expansion coefficient (CTE) for various materials as a function of temperature. The materials have a zero cross temperature for which this coefficient is zero, (from Critical Reviews of Optical Science and Technology, Vol. CR43, p 183, article from S. F. Jacobs "Variable invariables - dimensional instability with time and temperature"; ISBN 0-8194-0953-7; 1992)
  • CTE linear expansion coefficient
  • Figure 2b Linear expansion coefficient for various Zerodur ® materials as a function of temperature with various zero cross temperatures where this coefficient is zero.
  • Figure 3a schematically shows a side view of a reflective optical element like a mirror at two different temperatures.
  • Figure 3b schematically shows a deformation of a mirror surface due to temperature variations along the x-axis of the mirror.
  • Figure 3c schematically shows a relative change of a length ⁇ L/L 0 of a body with a length L 0 as a function of temperature in units of ppb for Zerodur ® and ULE ® .
  • Figure 3d show a temperature distribution on the 4 th mirror of the EUV projection lens of
  • Figure 3e shows the temperature profile of the mirror of Fig. 3d along the x-direction trough the temperature maximum. The temperature difference to the reference temperature is given.
  • Figure 3f shows the temperature profile of the mirror of Fig. 3d along the y-direction trough the temperature maximum. The temperature difference to the reference temperature is given.
  • Figure 3g shows values of the Zernike-coefficients for the mirror M 4 of Fig. 3d to Fig. 3f in relative units.
  • Figure 3h show a temperature distribution on the 6 th mirror of the EUV projection lens of
  • Figure 3i shows the temperature profile of the mirror of Fig. 3h along the x-direction trough the temperature maxima. The temperature difference to the reference temperature is given.
  • Figure 3k shows the temperature profile of the mirror of Fig. 3h along the y-direction trough the minimum temperature between the maxima in the x-direction. The temperature difference to the reference temperature is given.
  • Figure 31 shows values of the Zemike-coefficients for the mirror M 6 of Fig. 3h to 3k in relative units.
  • Figure 4 schematically shows a mirror mounting assembly with a mirror as used in a projection lens of an EUV-lithographic projection exposure system as described in e.g. in WO 2005/026801 A2.
  • Figure 5 schematically shows the mirror mounting assembly of Fig. 4 with a heater for heating the mirror and a temperature control system for controlling the temperature of the mirror.
  • Figure 6 schematically shows an EUV projection lens which comprises 6 mirrors, a heater and a temperature or aberration control system for the sixth mirror.
  • Figure 7 shows an example for a warming-up of the six mirrors of the projection lens shown in Fig. 6, if such a lens is exposed with an EUV projection beam which has a power of 16 W after the reticle M.
  • Figure 8 shows the aberration for each mirror of the projection lens of Fig. 6 expressed by the Zernike-coefficient Z5.
  • Z5 is given by its RMS (root mean square) value as a function of the zero cross temperature when each mirror of the projection lens is in the steady state as described in Fig. 7.
  • Figure 9 shows the optimized zero cross temperature as a function of the EUV light power
  • Figure 10 schematically shows a side view of a reflective optical element like a mirror of an EUV projection lens as shown in Fig. 4 and Fig. 6. Further, a heater and a cooler are arranged nearby the mirror which is supported by supporting elements.
  • Figure 11 schematically shows a side view of a reflective optical element as shown in Fig.
  • the temperature of the reflective optical element or mirror M ko 421 is controlled by a pressure control of the pressure within the direct surrounding area or volume of the reflective element or mirror.
  • Figure 12a shows the transmission of Zerodur ® for a transmission path of 5 mm and of
  • Figure 12b schematically shows a reflective optical element or a mirror 421 which is coated with a coating C on almost its entire surface to make the mirror heatable with IR radiation.
  • Figure 12c schematically shows a reflective optical element or a mirror 421 which is at least partly coated with a coating C2 on its surface to make the mirror heatable.
  • the coating C2 is selected such that it will have a certain electrical resistance.
  • Figure 12d schematically shows a reflective optical element or a mirror M k , 421 which is at least partly heated by a multi-zone wire grid 1050 which is arranged nearby the reflective surface MS k .
  • Figure 13 schematically shows several embodiments of heaters or tempering means 300 in relation with a reflective element or mirror M k 421.
  • any thermal expansion must be minimized.
  • the actual surface data regarding the surface shape of a mirror and its actual position data must be within the specified range relative to a required or specified dimension of the shape of the surface of the mirror and relative to its required or specified position.
  • the optical aberration is smaller than l nm, even smaller than 0.1 nm or smaller than 50 pm (picometer) of RMS value.
  • RMS root-mean-square
  • the body MB k of at least one mirror or reflective optical element k of the plurality of reflective optical elements M 1 of the projection lens is made of a material or comprises a material with a temperature dependent coefficient of thermal expansion (CTE) which is zero at a temperature T 0k . This temperature T 0k is called zero cross temperature.
  • CTE temperature dependent coefficient of thermal expansion
  • Fig. 2a and 2b Examples of such materials are shown in Fig. 2a and 2b.
  • the application of such materials in EUV lithographic projection exposure systems is e.g. described in WO 2004/015477 Al.
  • the operation temperature of the lithographic projection exposure system 100, or the operation temperature of the reflective optical elements of such a system there are different materials available which have at least one zero cross temperature T 0 .
  • the usual operation temperature of the reflective optical elements is in the range of about 0°C up to about 100'C, preferably in the range of about 10°C to about 60°C, better in a range of about 20°C to about 40°C, the number of available materials is very restricted.
  • Fig. 2a Superinvar (an alloy of Fe-Ni-Co) or ULE ® (a registered trademark of Corning Inc.) are suitable materials, or according to Fig. 2b a material made of a Zerodur ® (a registered trademark of Schott AG) type may be chosen which is made of a glass ceramic.
  • ULE ® is a Titania silicate glass, which is a vitreous mixture of Si0 2 and Ti0 2 .
  • CTE(T) al (T-T 0 ), see e.g. Fig. 2a and Fig. 2b.
  • the constant or lope al is positive. This means that the material shrinks with increasing temperature if the temperature T of the material (made of ULE ® ) is below T 0 , and the material expands if its temperature T is above T 0 .
  • the constant or slope al is negative in the range of room temperature, as it is shown in Fig. 2b. This means that the material expands with increasing temperature if the temperature T is below T 0 , and the material shrinks if the temperature is above To.
  • ULE ® and Zerodur ® have a CTE-value in the range of ppb/K (10- /K) down to zero.
  • a body of e.g. about 100 mm thickness which is made of a material with a CTE-value of about 1 ppb/K expands or shrinks of about 0.1 nm if the temperature changes by one Kelvin ( K).
  • K Kelvin
  • thermal or temperature variations influence the aberration or image defects of an optical imaging system.
  • an overall change or homogenous change in temperature e.g. by a temperature offset ⁇ of the spatial temperature distribution which changes the temperature from T(x,y,z) to T(x,y,z)+ ⁇ (x, y and z designate the spatial coordinates)
  • refractive lens elements which are used in VUV lithography. This is because the refractive power changes overall in the lens element and the changes of the surface figures of the refractive lens is negligible due to the almost constant coefficient of thermal expansion (CTE) of the refractive material.
  • CTE coefficient of thermal expansion
  • Fig. 2a and Fig. 2b which comprise a coefficient of thermal expansion CTE(T) which strongly depends on the temperature T, even a homogeneous change in temperature as mentioned has a strong influence onto the aberration values of the optical system.
  • CTE(T) coefficient of thermal expansion
  • Fig. 3a and Fig. 3b on the basis of the inventors' analyses.
  • Fig. 3a schematically shows a side view of a reflective optical element like a mirror M k with a body MBk and a reflective surface MSk.
  • the mirror M has a spatial temperature distribution T(x,y,z). If the mirror body MBk comprises material with a zero cross temperature then usually the temperature distribution T(x,y,z) is different from the zero cross temperature. If the temperature distribution is changed by a constant value ⁇ , then, if e.g. the temperature is raised closer to the zero cross temperature, the mirror will expand to the mirror M k * with body MB k * , if e.g.
  • the surface figure of the surface MS k does not change its shape, but the surface MS k * of the expanded mirror M k * changes its position. It is shifted e.g. into the z- direction by a value ⁇ as shown.
  • This change in position of the reflective surface can be easily corrected by a translational movement of the heated mirror M k * . In this case the image defects or the optical aberration is almost unchanged.
  • T Ref which is called reference temperature
  • the mirror is locally heated in a surface area from xl to x2 along the x-dimension of the mirror body MBk, as shown by the temperature profile or temperature distribution ⁇ ( ⁇ ) which is added to the reference temperature T Ref , having a maximum temperature AT max .
  • the reference temperature T Ref means that at this temperature the mirror has reference shapes, also its reflective surface MSk has a reference surface figure. Any deviation due to thermal expansion is related to these reference shapes and the surface figure.
  • the interval [xl, x2] is small compared to the dimension of the mirror body in the x- direction. If the mirror body is heated up from the reference temperature T Ref by the temperature profile AT(x), and if it is made of a material with a temperature dependent coefficient of thermal expansion which is zero at a temperature T 0 , like Zerodur ® or ULE ® as shown in Fig. 2a and Fig. 2b, and if the maximum temperature of the mirror body, given as T Ref + ⁇ T max , , as an example is below the zero cross temperature T 0 and above the reference temperature T Ref due to the heating, then the reflective surface of the mirror between the coordinates xl and x2 will shrink compared with the surface at the reference temperature.
  • Fig. 3b the change in surface figure is schematically shown by MS k * which can be compared with the reference surface figure MS* at the reference temperature T Ref .
  • Fig. 3c schematically shows the relative change of a length of the body MB k AL/LQ with a length L 0 (like the one mentioned in connection with Fig. 3b) as a function of temperature in units of ppb (parts per billion, meaning in values of 10 -9 ).
  • the zero cross temperature Tok is, as an example, set to 25°C which is 3K higher that the reference temperature.
  • Curve 301 schematically shows the relative length changes for ULE ®
  • the mirror body is made of Zerodur ® , see curve 302,then the mirror body will expand relative to the body at the reference temperature, as can be seen from Fig. 3c. If the mirror surface has a temperature profile similar like shown in Fig. 3b then (in a first approximation) the mirror surface regions between LI and xl are not changed due to thermal expansion. Also the region between x2 and L2, since there the temperature is at the reference temperature or close to it. In the region between xl and x2 the mirror body will expand or shrink because the temperature is different from the reference temperature. Whether expansion or shrinkage occurs in this region depends on the material and on the relative position of the absolute temperature profile ⁇ ( ⁇ ) +T Ref and the zero cross temperature T ok to each other.
  • ⁇ ( ⁇ ) +T Ref about equal to the zero cross temperatureT 0k then there is no expansion or shrinkage of the interval [xl, x2].
  • T av + T Ref To k the surface figure deformation is minimized and on the scale of the interval [xl, x2] it can be neglected.
  • the absolute average temperature is below the zero cross temperature but within the reference temperature T Ref and a temperature given by 2T 0 -T Ref , resulting in a surface deformation MS k * for ULE ® and a surface deformation MS k " for Zerodur ® for the mirror body B k of Fig. 3a.
  • T 2T 0 - T Ref
  • the mirror has the same dimensions and surface figures as at the reference temperature T Ref which results in no additional aberration or image errors caused by the heating.
  • the operation temperature of the mirror and the zero cross temperature shall be selected such that the absolute temperature profile AT(x,y,z) + Ti 1 ⁇ 2 f which differs from the reference temperature T Ref (x,y,z) of the mirror or reflective optical element should be symmetric about the zero cross temperature T 0 .
  • this means as AT av + T R «.f (J ⁇ ( ⁇ ) dx
  • x2 x i)/(x2-xl) + TR* To-
  • an average temperature for each dimension is separately calculated. In this case the optimization to the zero cross temperature can be done to an individual average temperature or to two individual average temperatures, which are related to the dimension or to the dimensions which are most relevant for the aberration.
  • the expected absolute average temperatures are homogenously raised to the zero cross temperature of the mirror material by e.g. homogenously heating the mirror body in at least one dimension with a heating means. Since the last method offers more flexibility it will be discussed below, this method is preferred.
  • Fig. 3d to 3f show a more realistic temperature distribution of an EUV mirror as used in the projection lens 20 (Fig. 1).
  • a mirror M K with its dimensions is shown from the top.
  • a EUV-projection lens with 6 mirrors was used, as it is shown in Fig, 6.
  • the results for mirror 24 (see Fig. 6) or Iv are shown in Fig. 3d to 3f which is the fourth mirror after the reticle, counted in the direction of the projection beam.
  • the EUV-lens with the 6 mirrors is shown and explained below.
  • the temperature distribution is shown with a grey shading pattern.
  • Fig. 3e shows the temperature difference to the reference temperature as a temperature profile ⁇ ( ⁇ ) of the mirror along the x-direction trough the temperature maximum AT max .
  • Fig. 3f shows the temperature difference to the reference temperature as a temperature profile AT(y) of the mirror along the y-direction through the maximum temperature ATmax.
  • the inhomogeneous temperature profile of the mirror results in deformations of the mirror surface as explained above. These deformations result in aberrations which cause image errors or defects. This is shown in Fig. 3g in which the values of the Zernike-coefficients for the mirror M are given. These coefficients are a quantity or measure for the deviation of an ideal spherical wavefront in the image plane which generate an image point IP (see Fig. 1).
  • the mirror M 4 in this example has a mirror body made of ULE ® with a zero cross temperature of 25°C.
  • the mirror M is additionally homogenously heated by 2°C to be at 24°C before the EUV light is switched on then the aberration after switching on the EUV light results in the right bars which are significantly lower than the left ones. This means that the aberration of the EUV lens can be significantly reduced by the homogenous heating of at least one lens mirror.
  • Fig. 3h to 31 show a further realistic temperature distribution of an EUV mirror as used in the projection lens 20 (Fig. 1).
  • the mirror Me of the EUV projection lens of Fig. 6 is shown similar to Fig, 3d.
  • the EUV-projection lens with the 6 mirrors was used as for Fig. 3d to 3g and as shown in Fig. 6.
  • the results are shown in Fig. 3h to 31 for mirror 26 (see Fig. 6) or Me, meaning the sixth mirror after the reticle, counted in the direction of the projection beam.
  • the inhomogeneous temperature profile of the mirror results again in deformations of the mirror surface.
  • the values for the Zernike-coefficients for this mirror are given in Fig. 31.
  • the left value (bar) again is the value without additional heating of the mirror.
  • the right value is the value with an additional heating of the mirror.
  • the mirror Me also has a mirror body made of ULE ® with a zero cross temperature of 25°C.
  • the mirror M 6 is additionally homogenously heated up by 3.8'C from the reference temperature T Ref to be at 25.8°C before the EUV light is switched on, then the aberration, after switching on the EUV light, results in the right bars which are significantly lower than the left ones. This means again that the aberration of the EUV lens can be significantly reduced by the homogenous heating of at least one lens mirror of the projection lens.
  • the projection lens 20 as described above with reference to Fig. 1 or Fig. 6 comprises mirrors 21, 22, 23, 24 (25, 26) or in general a plurality of reflective optical elements Mi , each comprising a body MBi and a reflective surface MS
  • the EUV light is reflected from the reticle M after its illumination by an illumination system 10 of an EUV-lithographic projection exposure system 100.
  • the projection lens 20 comprises a support structure for passively or actively supporting the reflective optical elements Mj (e.g. mirrors 21, 22, 23, 24).
  • the temperature of the support structure or of at least a part of the support structure is at a reference temperature T Ref .
  • Fig. 4 schematically shows a mirror mounting assembly 400 for the mirror 421, representing one reflective optical element k from the plurality of the reflective optical elements Mi which are used in an EUV-lithographic projection exposure system 100 (Fig. 1), and as described e.g. in WO 2005/026801 A2.
  • the mirror 421 comprises a mirror body MBk made of Zerodur ® or ULE ® , or made of a material comprising e.g. one of the materials Zerodur ® or ULE ® , or made of a material having at least one zero cross temperature.
  • the mirror 421 also comprises a reflective surface 450 with an optional multilayer of predetermined layer materials with predetermined layer thicknesses to improve the reflectivity of the projection beam 4 (Fig. 1) of EUV light.
  • the mirror body MBk is supported by a support element 480.
  • the mirror 421 is supported or suspended by three mounting or linking points 451, 452, 453.
  • the mirror body MBk is connected with linking elements 471, 472, 473 with a bipod structure 461, 462, 463 to achieve a kinematic mount which holds the mirror 421 such that almost no parasitic forces and/or moments are transferred from the support element 480 to the mirror.
  • a bipod structure may comprise an actuation device.
  • the support element 480 is fixed at a housing structure 481 of the projection lens 20.
  • the housing structure is sometimes also called projection optical box or POB.
  • a support structure with a defined or controlled reference temperature TR ⁇ one of the following elements is preferably selected: The linking points 451, 452, 453, the linking elements 471, 472, 473, the bipod structures 461, 462, 463, the support element 480 or the housing structure 481.
  • the selected support structure depends among other things also on the temperature control system which is used in the EUV lithographic exposure system. For this reason in Fig. 4 all these elements are designated with T Ref which does not necessarily mean that all these elements have the same temperature during the operation of the projection system 100.
  • the reference temperature T ⁇ f is the temperature of the selected support element which supports the reflective optical element 421 in an operation mode of the EUV lithographic exposure system in which the EUV light is switched off, or in an operation mode in which the power of the EUV light is less than about 10% of the exposure power which is usually used for the projection of the object field on a reticle onto an the image field on the substrate in an mass production operation mode of the EUV lithographic exposure system.
  • This reference temperature T Ref is usually selected close to the room temperature of the clean room in which the projection lens 20 is operated, meaning that the reference temperature is in the range of about 20*C to 24°C, preferably 22*C.
  • an additional temperature control system controls the reference temperature T Ref such that this temperature is constant also during the operation of the projection lens 20.
  • T Ref the temperature of the housing structure 481 and/or the support element 480 (Fig. 4), being e.g. at 22*C as it was chosen in Figures 3.
  • the temperatures are selected such that the zero cross temperaturTe 0k Is higher than the reference temperature.
  • T Ref the reference temperature
  • the projection lens 20 comprises a heater 300 to heat at least one optical element Mk, and it comprises a temperature control system 200 to control the temperature of the at least one optical element M k to a temperature T k as shown in Fig. 5 in which like references designate the same or similar parts as in Fig. 4.
  • This distribution has an average temperature AT av as defined above.
  • the at last one heated optical element M k may be connected to a support structure in the same way as shown in Fig. 4.
  • This temperature Tk preferably is achieved before the formation of the temperature distribution ⁇ ( ⁇ , ⁇ , ⁇ ) on the optical element M k , whereas the temperature distribution results from the heating when the optical element is subjected to the EUV light during the operation mode of the projection system.
  • the advantage of the invention is that the zero cross temperature can be selected quite free, since the application of the heater 300 allows that the mirror can be operated at the favorable zero cross temperature during the projection process.
  • AT av is small like about IK than T k can also be the zero cross temperature T 0 .
  • the temperature T k is controlled by the temperature control system 200 to such a temperature with the heater 300 (see Fig. 5).
  • the heater heats the mirror M k homogenously to such a constant temperature value.
  • the temperature T k is the operation temperature of the mirror M k in an operation mode of the EUV lithographic exposure system in which the power of the EUV light off.
  • the temperature of the reflective optical element or mirror M k is increasing by the average temperature AT av to or close to the zero cross temperature
  • the temperature T k is already controlled to its value before the EUV light is switched on.
  • the heater is controlled such that the thermal energy to which the mirror Mk is subjected is constant. This means that if e.g. the mirror absorbs some other energy, e.g. some EUV light, then the heater reduces its heating power such that the total input of thermal energy onto the mirror is constant in time. This results in that the average temperature of the mirror is also constant or almost constant in time.
  • the temperature T k may be controlled at the reflective surface MS k or at the mirror body MB k .
  • the accuracy of the temperature control system or the accuracy with which the temperature T k is controllable will also influence the aberration or imaging errors. If the accuracy of the temperature of the body MB k of the reflective optical element M k is within ⁇ IK due to the temperature control and other system variations then the relative change of the length is normally less than about 10 ppb as it can be schematically seen by reference numeral 303 in Fig. 3c. However, this assumes that the operation temperature can be chosen between about T Ref + (T ok - T Ref )/2 and T Ref + 3 ® (T 0k - T Ref )/2. As it can be seen from Fig.
  • the zero cross temperature of the material for the mirror body MBk is higher than according to any design calculations or simulations which are used for the design of the projection lens. In these calculations any fluctuations of the surrounding temperature, variations in design assumptions for the lens design, the EUV light source and also the variations of reticle reflections may be considered which results in the calculation of the maximum or average temperature of the mirror Mk when the EUV projection lens is designed. If the zero cross temperature is selected several Kelvin higher than the calculated values then the EUV lens can be operated under all conditions at the preferred temperature close to the zero cross temperature at which the aberration is minimized. This operation temperature of the mirror may be achieved by a homogenous and controlled heating with the heater 300 and the controller 200. For the heater 300 various embodiments can be used.
  • heating elements formed out as metal plates and arranged closed to the mirror, preferably close to the backside of the mirror. Such plates are electrically heated.
  • electrical heating elements are in direct contact with the mirror body.
  • a further alternative or additional heater comprises an infrared source with which the mirror or reflective element is illuminated.
  • the temperature control system 200 may comprise a sensor which directly measures the mirror temperature Tk at one or several positions, in further embodiments of the invention the temperature Tk of the at least one optical element M k which is controlled by the temperature control system can be a temperature ⁇ * ⁇ of the reflective surface MS* (Fig. 5) of the optical element k or a temperature ⁇ ⁇ _ ® of the body MB k of the optical element Mk ⁇
  • Alternative Tk can be a temperature given by a function of the temperature T MB k of the body MB k and/or the temperature of the reflective surface M5 k .
  • the temperature of the reflective surface T M sk can be an average surface temperature. Such averaging can e.g.
  • the temperature of the body TMBk can be an average temperature of a plurality of temperatures measured at a plurality of spatial positions at the body MBk.
  • the spatial positions or a subset of the spatial positions at which the temperature of the mirror body is measured is arranged close to the reflective surface.
  • a temperature value or control parameter can be calculated from the measurements of the temperature or the temperatures of the reflective surface and/or the body.
  • the temperature Tk for the control system is selected from a spatial temperature distribution of the body MBk or the reflective surface ⁇ .
  • the optical aberration can be determined and the temperature controller 200 controls the mirror temperature such that the aberration becomes a minimum.
  • the temperature Tk not necessarily explicitly has to be determined.
  • a model based controller can be used the control the temperature of the mirror or the heat of the heater which is transferred to the mirror. The model may consider parameters like the power of EUV light to which the lens is exposed, illumination settings with which a reticle is illuminated, the structures on the reticle and the optical aberration of the lens.
  • the EUV light of the illuminated reticle comprises a spatial distribution of angular, polarization and intensity in accordance with an illumination setting.
  • Tk may be the temperature T MBk of the body MB k of the optical element Mk, the temperature given by a function of the temperature T MS k of the body MBk and the temperature T SI C of the reflective surface MSk or the temperature selected from a spatial temperature distribution of the body MBk or the reflective surface TMSI C , before the power of the EUV light of the projection beam 4 is switched on.
  • the temperature T k of the optical element Mi is controlled to its value by heating the heater with a first heating power at a time before the projection lens is subjected to the exposure power of the EUV light.
  • the heating power of the heater is less than the first heating power. This is to prevent a warming up of the optical element Mk .
  • a second heater for the heating of a second optical element M 2 k to a temperature T3 ⁇ 4 is used, wherein the body MB 2 k of the second optical element M 2 k also comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a temperature ⁇ 0 ⁇ higher than the reference temperature.
  • the temperature T 2 k of the second optical element ⁇ is selected according to the same relations as described above for the operation temperature Tk of the first heated mirror Mk .
  • T 2 k T
  • the optical element or mirror M ⁇ is, or preferably is heated to such a temperature 1 ⁇ 2 before the power of the EUV light of the projection beam 4 is switched on.
  • the material of the body MB& may be the same or different from the material of the body MBk of the first optical element.
  • the material has a different zero cross temperature than for the first reflective element M k may be selected such that the CTE(T)-function (see e.g. Fig. 2b) has different algebraic signs for the slopes at the respective zero cross temperatures. This requires that the material comprises at least two zero cross temperatures To and T02 , as shown in Fig.
  • Fig. 6 shows an additional schematic arrangement of an EUV-projection lens which comprise 6 mirrors as reflective optical elements Mi to M 6 which are respectively designated with reference numerals 21, 22, 23, 24, 25 and 26.
  • like references designate the same or similar parts as in Fig. 1.
  • the mirrors are numbered as they are arranged in the downstream direction of the projection beam 4 which goes from the reticle M to the wafer W. If such a EUV projection lens 20 as shown e.g. in Fig. 1 or Fig. 6 is exposed with EUV light to project an object point OP to an image point IP on the wafer W then the individual mirrors are warmed up.
  • these mirrors are at the reference temperature T Ref of e.g.
  • Fig. 7 shows an example for a warming of the six mirror of the projection lens 20 of Fig. 6, if such a lens is exposed with a projection beam 4 which has a power of 16 W after the reticle M.
  • a light source power of more than lkW is necessary for the light source 1 (see Fig. 1).
  • the shown diagram is a simulation calculation in which the reticle is illuminated with a dipole illumination setting which gives a temperature distribution on the last mirror M 6 similar as shown in Fig. 3h. The dipole can be seen on the mirror, since this mirror is arranged nearby or at a pupil plane of the projection lens 20.
  • Fig. 7 shows an example for a warming of the six mirror of the projection lens 20 of Fig. 6, if such a lens is exposed with a projection beam 4 which has a power of 16 W after the reticle M.
  • a light source power of more than lkW is necessary for the light source 1 (see Fig. 1).
  • the shown diagram is a simulation calculation in which the
  • the average temperature AT av which is the respective lower curve, is shown as a function of time.
  • the average temperatures are calculated as defined above.
  • the maximum temperature AT max of the temperature distribution ⁇ ( ⁇ , ⁇ , ⁇ ) is shown as a function of time which is represented by the respective upper curve for each mirror. It can be seen that the thermal load for the mirrors M 5 , M 3 and M 2 is such that these mirrors warm up of about 4.5K, 4.0K and 3.5K respectively. Mi warms up of about 2K and M Ms warm up of about 0.5 . Further, it can be seen that the mirrors need about 150 minutes or more to reach an almost constant steady state temperature.
  • the lens 20 may be optimized for the steady state condition. This means that the zero cross temperature for the mirrors is selected such that it is the steady state temperature or almost the steady state temperature, meaning T 0
  • the mirror is fabricated regarding its surface figure at a temperature of about 22'C (or in general at or close to a reference temperature) then this surface figure remains almost the same even if the mirror is homogenously heated to the such selected zero cross temperature as it was explained in connection with Fig. 3a. Further, if the mirror has a spatial temperature distribution T(x,y,z) due to the absorbed power of the projection beam 4 then the aberration generated by this absorption is also minimized as explained in Fig. 3b and 3c. Practically the suggested optimization (by material selection) of the zero cross temperatures has the following drawbacks:
  • the steady state temperature depends on the power of the light source, on the illuminated reticle and the used illumination setting with which the reticle is illuminated.
  • Fig. 8 shows the aberration in form of the Zernike-coefficient Z5.
  • Z5 its RMS (root mean square) value is given for each of the 6 mirrors of the lens which is shown in Fig. 6.
  • Z5 is shown as a function of the zero cross temperature if each mirror of the projection lens is in the steady state as described in Fig. 7.
  • the bars for each mirror show how the aberration increases if the zero cross temperature is selected higher or lower.
  • the calculations were done in IK steps for zero cross temperatures from 18 e C (left most bar) to 34 e C (right most bar).
  • the aberration values for various zero cross temperatures are shown e.g. by the shown arrows with the respective zero cross temperature data.
  • the best zero cross temperature in accordance to Fig. 8 should be between 26°C and 27°C which is in good agreement that lowest aberration should be achieved if the steady state temperature (25.5°C) corresponds to the zero cross temperature.
  • Fig. 8 clearly shows that if the steady state temperature of a mirror doses not match with the zero cross temperature within about ⁇ 1K then the aberration will increase.
  • Fig. 9 shows the optimized zero cross temperature as a function of the EUV light power P after the reticle M to which the EUV lens 20 is subjected during projection. The calculation is done for settings which produce the largest aberrations. It can be seen that the optimized zero cross temperature 1 ⁇ 2 for all mirrors increase in good approximation linearly with power P. The calculation was done up to 16W and ULE ® was chosen as mirror material for all 6 mirrors as for the calculations of Fig. 7 and Fig. 8. Further, it can be seen that the variation of the zero cross temperatures within one EUV lens also increases with power, if the manufactured lens 20 shall be optimized regarding optical aberration. This means that different materials have to be used for the production of the mirrors, having different zero cross temperatures with a broad variation.
  • the inventors realize for the first time that for higher exposure powers the zero cross temperatures within a projection lens must differ more than 6K, even more than 8K, if the EUV lens is optimized to the EUV light power and aberration. For this reason preferably in a further embodiment of the above described embodiments of the inventive projection lens the absolute value of the difference between the zero cross temperatures ok, To2k of the heatable optical element and the second heatable optical element is selected such that it is more than 6K, expresses as abs(T 0 k - 0 2k)> 6K.
  • the optical aberration (caused by thermal effects due to absorption of EUV light at the optical elements) can be reduced even at higher EUV light powers.
  • the zero cross temperature Tok of the at least one heatable optical element Mk is higher than the maximum of the highest reference temperature TR ⁇ and the average or maximum temperature AT av + TR* or T max + T Ref , based on the respective spatial temperature distribution AT(x,y,z), expressed as Tok > max(T Ref , AT av + T Ref ) or >T 0 m k ax(T Ref , AT ma + T Ref ).
  • the teaching of the inventive embodiments described above is independent of the number of mirrors. It can be applied e.g. to a projection lens 20 as shown in Fig. 1, having 4 mirrors 21, 22, 23 and 24, or to a projection lens 20 as shown in Fig. 6, comprising 6 mirrors 21, 22, 23, 24, 25 and 26.
  • the heater 300 for heating the mirror or optical element k advantageously comprises heating elements selected from the group consisting of I light emitting diodes, Peltier elements, optical fibers, light guide rods and IR lasers, as will be described in more detail below with the description of Fig. 13. Further, such heating elements are optionally arranged in one dimension or in two dimensions at predefined spatial coordinates, forming a grid structure.
  • these embodiments comprise an optical arrangement to configure the IR radiation
  • the optical arrangement comprises arrangement elements selected from the group consisting of collimator, focusing lens, adjustable lens, mirror and diffractive optical element, wherein the arrangement elements may be tiltable around at least on axis. Examples of such optical arrangements are shown in Fig. 13.
  • the at least one optical element Mk comprises a modification in or on the mirror body ⁇ 3 ⁇ 4.
  • the modification is selected from the group consisting of recess, blind hole, defined surface roughness, diffractive structure, spherical protrusion, spherical recess and surface curvature.
  • the modification which is described in more detail below in connection with Fig. 13, advantageously is used to guide e.g. IR radiation to heat the optical element locally.
  • the present invention also relates to a further, projection lens, in the following called first projection lens, which is designed to be exposed with an exposure power of more than 8W of EUV light with a wavelength in a wavelength range of less than 50 nm, preferably with a wavelength of 13 nm.
  • first projection lens which is designed to be exposed with an exposure power of more than 8W of EUV light with a wavelength in a wavelength range of less than 50 nm, preferably with a wavelength of 13 nm.
  • the calculations for Fig. 7, Fig. 8 and Fig. 9 were done with 13 nm and for a lens 20 with 6 mirrors, as shown in Fig. 6.
  • a lens comprises at least two reflective optical elements i like mirrors.
  • Each mirror or reflective optical element comprises a body MB) and a reflective surface MS
  • the bodies MB m , MB n of at least two reflective optical elements comprise a material with a temperature dependent coefficient of thermal expansion which is zero at respective zero cross temperatures Tom, Ton , wherein the absolute value of the difference between the zero cross temperatures T 0m , T 0n in this first projection lens is more than 6K, expresses as abs(Tom - T 0n )> 6K. This is e.g. the case for mirror M 5 and mirrors M or Ms if the lens is subjected an exposure power of 8W or more, as can be seen from Fig. 8.
  • the lens can comprise four or six reflective optical elements Mj or mirrors (as e.g. shown in Fig. 1 and in Fig. 6) and preferably the projection lens is designed to be exposed with an exposure power of more than 10W.
  • the absolute value of the difference between the zero cross temperatures T 0m , T 0n is more than 8 , expresses as absfTom - T 0n ⁇ > 8K.
  • Fig. 9 clearly shows this for the mirrors M 5 and M 4 or Me.
  • the zero cross temperatures of these mirrors differ by more than 8 K if the exposure power goes above 10 W.
  • the first projection lens and its previous embodiments in a third embodiment may comprise a support structure for passively or actively supporting the reflective optical elements Mi, as it was described in connection with Fig. 4 and Fig. 5.
  • the temperature of at least a part of the support structure is at a reference temperature T Ref , which is e.g. selected as 22"C.
  • the lens in accordance with an embodiment of the first lens comprises a heater 300 for heating at least one of the mirror bodies MB context , MB m which comprise the materials with the different zero cross temperatures.
  • a temperature control system 200 controls the temperature of the at least one heated mirror body MB n , B m to a temperature Tk.
  • the heater 300 is made such that the body can be homogenously heated. This means that the body MB adj , MB m is homogenously heated in at least one dimension of the body.
  • Various types of heaters 300 will be described below in this description.
  • the EUV light with which the reflective surfaces MS m and MSlie are exposed is reflected by the illuminated reticle and comprises a spatial distribution of angular, polarization and intensity parameters in accordance with an illumination setting.
  • at least one zero cross temperatures Tom, T 0n is selected higher than the maximum of the highest reference temperature T Ref and the respective average or maximum temperature AT mav + T re f or AT mm ax + T re f , A nav + T accomplishrf or ATnmax + T re f , based on the respective spatial temperature distribution AT rn (x,y,z), AT diligent(x,y,z), expressed as Tom > max(T Re » AT mav + T ref ), T 0m > max ⁇ T Ref , &J mmM + 3 ⁇ 4 ) or T 0n > max(T Ref ,
  • the reference temperature is constant and is chosen as the temperature of the support element 480 or the housing structure 481 (see Fig. 4 and 5). Often the backside of the mirror also has this temperature even if the lens is subjected to the exposure power of the EUV light.
  • the advantage of this selection of the zero cross temperature is that with the usage of the heater 300 the respective mirror always can be brought to the zero cross temperature by heating. This is because usually, without the application of the heater 300, the mirror only heats to an average temperature as explained with Fig. 7. If the zero cross temperature is adapted to this temperature then the optical aberration becomes minimal, as it was described in connection with Fig. 3a and Fig. 3b. However, if the zero cross temperature is selected higher than the absolute average temperature TM+ AT MAV or + AT nav then by heating the mirror with the additional heater 300 it can be heated to or close to the selected zero cross temperature to minimize aberration.
  • the manufacturing tolerances of the zero cross temperature is taken into account.
  • the materials of the first projection lens according to this invention which comprises materials with the zero cross temperatures T 0m , T 0n , vary regarding their real values of the zero cross temperature due to the manufacturing process, resulting in manufacturing tolerances ATom, ⁇ ⁇ , then the respective real values are within the temperature intervals Tom ⁇ Atom and T 0n ⁇ ⁇ ⁇ .
  • At least one zero cross temperatures T 0m is selected higher than the maximum of the highest reference temperature T Reif and the respective absolute average or maximum temperature AT coordinate, av + Tuef or AT mmax + T Ref , ⁇ ⁇ + T Ref or AT nmax + T , based on the respective spatial temperature distribution AT m (x,y,z), AT n (x,y,z), but in addition, these values are increased by the absolute value of the respective manufacturing tolerances ⁇ » ⁇ , expressed as Tom > max(T Ref , AT m consult + T Ref ) + 1 ⁇ 0 ⁇ , !
  • T 0m > max ⁇ , T mmw + T ⁇ t) + 1 ATom I or Ton > max(T Ref , ⁇ ⁇ + Tiurf) + 1 ⁇ I , Ton > maxfr Ref , ATNase rnax + T Ref ) + 1 ⁇ 0 ⁇ I .
  • Such a material selection regarding the zero cross material has the big advantage that the specification regarding the zero cross temperature can be relaxed, because the heater 300 guaranties that the respective beatable reflective optical element with the selected zero cross temperature can be heated to or close to the zero cross temperature by additional heating and such the aberration can be minimized. Preferably but not necessarily, the heating is done such that the mirror is homogenously heated. This simplification regarding the material selection results in enormous cost savings.
  • the temperature of the heated mirror bodies or the heated mirror body MB n , MB m is controlled to values of the temperature Tk which are within an interval of ⁇ 5K, better ⁇ 2K centered around the respective zero cross temperature T 0m , T 0n to minimize any thermal induced optical aberration.
  • the temperature Tk is selected from the values of the above 3 rd to 6 th embodiment of the first projection lens.
  • the heating power of the heater is selected lower than the first heating power, as mentioned in the 7 th embodiment, during the time of exposure of the projection lens 20 with the exposure power of the EUV light.
  • the temperature control system 200 controls the temperature T k such that the heating power of the heater 300, which heats the at least one body MB n , MB m of the reflective optical elements M n , M m , and the exposure power of the EUV light which is absorbed by the at least one heated body MB context, MB m is constant or is almost constant in time.
  • the first lens and it various embodiments essentially make use of materials for the reflective optical elements or mirrors which differ significantly regarding its zeros cross temperature.
  • the present invention of the first lens and its various embodiments are not limited to EUV projection lenses with 4 or 6 reflective optical elements in form of mirrors, as described in Fig. 4 and Fig. 6. It is assumed that the number of mirrors of an EUV projection lens of a lithographic projection exposure apparatus increases to more than 6 if the EUV power of the EUV exposure light also increases. The EUV light power will depend on the EUV light sources which will be available in future. Generally the number of mirrors may vary in accordance to the requirements, and the principles of the teaching in connection with the first projection lens may be applied to each mirror or to each reflective optical element, especially if used in an EUV lithographic projection exposure apparatus.
  • the heater 300 for heating the mirror or optical element Mk advantageously comprises heating elements selected from the group consisting of IR light emitting diodes, Peltier elements, optical fibers, light guide rods and IR lasers, as will be described in more detail below with the description of Fig. 13.
  • heating elements are optionally arranged in one dimension or in two dimensions at predefined spatial coordinates, forming a grid structure.
  • these embodiments comprise an optical arrangement to configure the IR radiation, the optical arrangement comprises arrangement elements selected from the group consisting of collimator, focusing lens, adjustable lens, mirror and diffractive optical element, wherein the arrangement elements may be tiltable around at least on axis. Examples of such optical arrangements are shown in Fig. 13.
  • the at least one optical element Mk comprises a modification in or on the mirror body MBk.
  • the modification is selected from the group consisting of recess, blind hole, defined surface roughness, diffractive structure, spherical protrusion, spherical recess and surface curvature.
  • the modification which is described in more detail below in connection with Fig. 13, advantageously is used to guide e.g. IR radiation to heat the optical element locally.
  • An alternative embodiment to the first lens is a second projection lens for an EUV lithographic exposure system which is described below.
  • a first embodiment of the second projection lens 20 in accordance with this invention comprises at least two reflective optical elements M
  • Each such element comprises a body MB
  • the EUV light is reflected from the reticle after illumination by an illumination system of an EUV-lithographic projection exposure system.
  • the bodies MB m , MB n of at least two reflective optical elements of the lens 20 comprise a material with a temperature dependent coefficient of thermal expansion which is zero at respective zero cross temperatures Tom, T 0n , as e.g. shown in Fig. 2a and Fig. 2b.
  • a material with a temperature dependent coefficient of thermal expansion which is zero at respective zero cross temperatures Tom, T 0n , as e.g. shown in Fig. 2a and Fig. 2b.
  • such materials are Zerodur ® or ULE ® , but also Superinvar.
  • the second lens comprises a support structure for passively or actively supporting the reflective optical elements M
  • the temperature of at least a part of the support structure is at a reference temperature T Ref , for example at 22°C as it is in the embodiments of Fig. 3 and Fig.6.
  • the second projection lens comprises a heater 300 for heating at least one of the bodies MB n , MB m of the reflective optical elements with the zero cross temperatures. Also the second lens comprises a temperature control system 200 for controlling the temperature of the at least one heated body MB n , MB m to a temperature Ticm, # T kn .
  • the EUV-light usually comprises a determined spatial distribution of intensity, angular and polarization parameters which is defined by an illumination setting.
  • T 0n is selected higher than the maximum of the highest reference temperature TRef and the respective absolute average or maximum temperature AT mav + TRef or ATmmax + Ref , AT nav + T Ref or ATTM* ® + Ref , based on the respective spatial temperature distribution AT m (x,y,z), AT n (x,y,z), expressed as T 0rn > max(T Ref; AT mav + T ref ), Tom > maxf ⁇ , ATmmax + TRef ) or T 0n > maxf Ref, AT nav + T R ef ), Ton > max(T Ref , AT nmax + J M ).
  • the second projection lens selects the material for the mirrors or reflective optical elements on the basis of the average or maximum temperature which a mirror may achieve if it is illuminated by the projection beam. Due to the application of a heater, the zero cross temperature can be reached independent of whether the projection lens is operating with low or high EUV light power of the projection beam. Preferably the maximum or average temperatures on the beatable mirror bodies are determined for the maximum power to which the projection lens may be subjected.
  • the same can be applied as described in connection with the first projection lens and the description in connection with Fig. 4 and Fig. 5.
  • a 2 nd embodiment of the second projection lens considers that the materials with the zero cross temperatures Tom T 0n may vary regarding the values of the zero cross temperatures due to manufacturing.
  • the absolute value of the difference between the zero cross temperatures Tom* T 0n is more than 6K, expresses as abs(T 0m - T 0n ) > 6K.
  • the second projection lens is designed to be exposed with an exposure power of more than 8W of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • the wavelength is in a wavelength interval selected from the group consisting of [12 nm, 14 nm], [12.5 nm, 13.5 nm], [5 nm, 15 nm], [15 nm, 25 nm] and [25 nm, 50 nm].
  • a selection of the wavelength also could be done for the other embodiments of the lenses of the EUV lithographic projection exposure systems which are described in the present specification.
  • the second projection lens at least four or six reflective optical elements M
  • the projection lens is designed to be exposed with an exposure power of more than 10W of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • the number of mirrors increase.
  • For a power of 16W usually 6 mirrors are used as shown in Fig. 6.
  • the absolute value of the difference between the zero cross temperatures ToTM, T 0n is more than 8 K in this embodiment, expresses as abs(Tom - T 0n )> 8K.
  • a 5 th embodiment of the second lens in accordance to this invention controls the temperature T k of the at least one heated body MB n , MB m within an interval of ⁇ 5 , better ⁇ 2K centered around the respective zero cross temperature T 0m , T 0n to minimize any thermal induced optical aberration.
  • the temperature T k of the at least one heated body MB n , MB m is controlled to its value by heating the heater 300 with a first heating power. This is similar to the 7 th embodiment of the first projection lens and reduces the warm-up phase of the EUV-projection system. Further, in an 7 th embodiment of the second projection lens the heating power of the 6 th embodiment is less than the first heating power when the projection lens is exposed to the exposure power of the EUV light.
  • the temperature control system 200 controls the temperature T k such that the heating power of the heater 300, which heats the at least one body MB n , MB m , and the exposure power of the EUV light which is absorbed by the at least one heated body is constant in time.
  • Constant in time in this context means that the total power of heat to which the mirror is subjected (e.g. the sum of the heating power of any heater and the absorbed light like EUV light) is only varying within an interval of the total power selected from the group consisting of [0%. 20%], [0%. 10%], [0%. 596] and [0%.
  • the EUV source provides EUV light continuously in time, like a synchrotron radiation source.
  • the total power as mentioned above is determined by an average over a number of pulses.
  • the number of pulses is selected such that it is within an interval selected from the group of intervals consisting of [1, 5], [1, 10], [1, 20], [1, 50] and [1, 100] pulses.
  • the bodies MB consult and MB m with the respective zero cross temperatures T 0m , T 0n are made of the same material.
  • the lens comprises 4 or 6 reflective optical elements in form of mirrors.
  • the number of mirrors also of the second lens may vary in accordance to the requirements and the principles of the teaching in connection with the first and second projection lens may be applied to each mirror or each reflective optical element, especially if used in an EUV lithographic projection exposure apparatus.
  • the heater 300 for heating the mirror or optical element Mk advantageously comprises heating elements selected from the group consisting of IR light emitting diodes, Peltier elements, optical fibers, light guide rods and IR lasers, as will be described in more detail below with the description of Fig. 13.
  • heating elements are optionally arranged in one dimension or in two dimensions at predefined spatial coordinates, forming a grid structure.
  • these embodiments comprise an optical arrangement to configure the IR radiation, the optical arrangement comprises arrangement elements selected from the group consisting of collimator, focusing lens, adjustable lens, mirror and diffractive optical element, wherein the arrangement elements may be tiltable around at least on axis. Examples of such optical arrangements are shown in Fig. 13.
  • the at least one optical element Mk comprises a modification in or on the mirror body MB k .
  • the modification is selected from the group consisting of recess, blind hole, defined surface roughness, diffractive structure, spherical protrusion, spherical recess and surface curvature.
  • the modification which is described in more detail below in connection with Fig. 13, advantageously is used to guide e.g. IR radiation to heat the optical element locally.
  • a third projection lens is described as a part of this invention.
  • This third projection lens makes use of the knowledge form Fig. 9 which show that the zero cross temperatures of different mirrors become more different with increasing power of EUV light, if the lens is designed such that the optical aberration is reduced to a minimum.
  • the mirrors M and Me should have a zero cross temperature with at least about IK above the reference temperature if the lens is subjected to about 16W of EUV light.
  • mirror M 5 should have a zero cross temperature T ⁇ B of about 34"C, or even higher, which is about 12K or more above the reference temperature of 22"C.
  • the heaters of these embodiments can heat the mirror, e.g. homogenously to the respective zero cross temperatures to minimize the optical aberration. So if the EUV light power will still increase, which is always aspired, then the difference in the zero cross temperatures will also increase and become more than the UK as it is for the lens which is described in Fig.6 with the values of Fig. 9. Probably this difference will even double, especially if higher EUV light power is used in future lenses, so that the difference for the coldest and the hottest mirror will be about 20K, or even more.
  • a glass ceramic like Zerodur ® may be used such that for the coldest and the hottest mirror the same material is used.
  • the colder mirror of the both is used or operated at or close to a first zero cross temperature T
  • the hotter mirror is used or operated at or close to the second zero cross temperature T 2 o which is higher than the first one.
  • This means that e.g. mirror M m and mirror M n is made of the same material with the two mentioned zero cross temperatures, designated as T ⁇ mn and T 7 omn-
  • Al glass ceramic materials with two zero cross temperatures in a temperature range of about 0°C to about 100°C exist which have a difference in its zero cross temperatures T 2 0 - fo of about 20K.
  • the usage of only one material for two mirrors will also reduce the costs of the projection lens.
  • the third projection lens comprises again at least two reflective optical elements Mi , each with a body MBj and a reflective surface MS; for projecting an object field on a reticle onto an image field on a substrate if the projection lens is exposed with an exposure power of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • EUV light is reflected from the reticle when it is illuminated by an illumination system of an EUV-lithographic projection exposure system.
  • the bodies MB m , MB n of at least two reflective optical elements comprise a material with a temperature dependent coefficient of thermal expansion which is zero at at least two zero cross temperatures T 1 ⁇ and T z 0m consult .
  • the third lens additionally comprises a support structure for passively or actively supporting the reflective optical elements M
  • the third lens comprises at least two tempering means, preferably heaters, for independently heating or cooling the at least two bodies MB n , MB m of the two reflective optical elements.
  • the third projection lens comprises a temperature control system for independently controlling the temperature of the at least two heated or cooled bodies MB n , MB m to respective temperatures T kn and a T km .
  • At least one of the two zero cross temperatures T 1 0m n and T 2 om n is higher than the reference temperature T Ref , since the reference temperature is usually in the range of 22°C.
  • the temperatures ⁇ 3 ⁇ 4 convec of the body MB n preferably is within an interval of ⁇ 5K, better ⁇ 2K centered around the first zero cross temperatures T ⁇ n
  • the temperatures Tkm of the body MB m preferably is within an interval of ⁇ 5K, better ⁇ 2K centered around the second zero cross temperatures T 2 omn ⁇
  • the temperatures Tkn and Tkm of the bodies of the reflective optical elements are controlled as close to the respective zero cross temperatures T ⁇ mn and T 2 om n as possible. It is mentioned here that in general the reference temperature T Ref may be set individual for each mirror.
  • the reference temperature T R(! f is such that always the reference temperature to the related mirror or reflective optical element is to be understood.
  • the third projection lens according to this invention may also have a cooler for the cooling of the mirror with the lowest zero cross temperature T ⁇ nm and a heater for the heating of the mirror with the higher zero cross temperature T ® *. .
  • This has the advantage that the lowest zero cross temperature not necessarily must be above the reference temperature of 22"C.
  • a cooler a material can be used in which T ⁇ m is at about 15 e C, which is used e.g. for mirror M 4 or M& if referred to Fig. 6, and in which T 2 0mn is at about 35°C, which is used e.g. for the mirrors M 3 or s .
  • T 2 0mn is at about 35°C, which is used e.g. for the mirrors M 3 or s .
  • This selection of the mirrors results essentially from Fig. 9.
  • Such a material is e.g.
  • At least one of the two zero cross temperatures T ⁇ TM and T 2 0 mn is higher than the maximum of the highest reference temperature T Ke f and the respective absolute average or maximum temperature AT MAV + TR* or AT mrnax + T BE ⁇ , AT + TRet or ATnmax + T Ref , based on the respective spatial temperature distribution AT m (x,y,z), AT diligent(x,y,z), expressed as T 1 0m n > max(T Ref/ AT mav + TRef ), T l 0mn > max(TRef # ATmmax + TRef ) ⁇ f ⁇ mn > max(T Ref , AT nav + T Ref ), T 2 omn > max(T R ef, AT NMA
  • the third projection lens comprising a material in which the zero cross temperatures T 1 ⁇ and To,TM vary regarding their real values due to manufacturing which results in manufacturing tolerances AT 1 0m n , AT 2 0mn such that the respective real values are within the intervals T ⁇ )mn ⁇ AT M and T Z om n ⁇ ATomn- Due to this tolerances advantageously at least one zero cross temperatures T ⁇ n and T 2 omn is higher than the maximum of the highest reference temperature T Ref and the respective absolute average or maximum temperature AT MAV + T Ref or AT mmax + TRef , AT NAV + T Ke f or AT NMAX + T Ref , based on the respective spatial temperature distribution AT m (x,y,z), AT n (x,y,z), which is additionally increased by the absolute value of the respective manufacturing tolerances ⁇ 1 ⁇ , AT 2 o mnf expressed as T ⁇ TM > max ⁇ , ATmav + TR, ® ) +
  • the selection of the zero cross temperature can done by Ton > max(TRef , T a «moni + T re f) or T 0rn > max(T Ref , T av m +T re f), whereby T av m and T av n are the respective average temperatures of mirrors M m and M consult as described above e.g. in connection with Fig. 7 or Fig. 3.
  • the third lens is more also robust, e.g. regarding changes in exposure powers to higher values.
  • the absolute maximum temperatures instead of the absolute maximum temperatures also the absolute average temperature may be sufficient, but in this case the third lens will be less robust against a raise of temperature of the mirrors which may result from higher exposure power.
  • the third projection lens in a 4 th embodiment is controlled such that at a time before the projection lens is subjected to the exposure power of the EUV light the temperature Tim or T km of the at least one heated body MB context, MB m is controlled to its value by heating the heater with a first heating power.
  • the control is such that during the time of the exposure of the third projection lens with the exposure power of the EUV light the heating power of the heater with the initial first heating power is less than the first heating power.
  • the temperature control system controls at least one of the temperatures and Tim such that the heating power of the respective heater and the exposure power of the EUV light absorbed by the at least one temperature controlled body MB m or MB makeup is constant in time.
  • the lens is designed to be exposed with an exposure power of more than 8 W of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • Fig. 9 shows that for higher EUV light power to which the lens is exposed, the third embodiment can be used due materials which is available today, as are already mentioned.
  • the absolute value of the difference between the zero cross temperatures T ⁇ n , T 2 0m n is chosen more than 6K, expresses as abs(T x omn - 2 omn) > 6K.
  • the projection lens is designed to be exposed with an exposure power of more than 8 W of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • Figure 9 shows an example of such a lens.
  • reflective optical elements are mirrors.
  • the embodiments of the third projection lens, as described above, may be operated such that, e.g.
  • At least one of the temperature controlled bodies MB n , MB m is controlled to its value by cooling with the respective tempering means with a first cooling power.
  • the first cooling power can be increased by the tempering means at a time when the projection lens is subjected to the EUV exposure light.
  • the cooling power describes the heat transfer per time from the temperature controlled body to the tempering means.
  • tempering means as e.g. the heater 300 for heating the mirror or optical element Mk, advantageously comprises heating elements selected from the group consisting of IR light emitting diodes, Peltier elements, optical fibers, light guide rods and IR lasers, as will be described in more detail below with the description of Fig. 13.
  • heating elements are optionally arranged in one dimension or in two dimensions at predefined spatial coordinates, forming a grid structure.
  • these embodiments comprise an optical arrangement to configure the IR radiation, the optical arrangement comprises arrangement elements selected from the group consisting of collimator, focusing lens, adjustable lens, mirror and diffractive optical element, wherein the arrangement elements may be tiltable around at least on axis. Examples of such optical arrangements are shown in Fig. 13.
  • the at least one optical element Mk comprises a modification in or on the mirror body MB k .
  • the modification is selected from the group consisting of recess, blind hole, defined surface roughness, diffractive structure, spherical protrusion, spherical recess and surface curvature.
  • the modification which is described in more detail below in connection with Fig. 13, advantageously is used to guide e.g. IR radiation to heat the optical element locally.
  • the present invention is also related to a method to configure a projection lens of an EUV lithographic projection exposure system.
  • the following method is based on the teaching of the previous embodiments of the invention.
  • the configuration method comprises the following steps:
  • a first step the number of reflective optical elements M
  • the substrate material for each reflective optical element i with its shape and size is selected, wherein the material is selected by consideration of thermal expansion.
  • At least on substrate material is selected from a group of materials which have a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperature T 0m .
  • This material is preferably used for mirrors with a high thermal load.
  • the maximum thermal load for each reflective optical element i is determined which is expected during the projection of the object field onto the image field under various conditions of the EUV projection exposure system, like EUV light power, transmission or reflection of the reticle, various illuminations settings and various pattern structures on the reticle.
  • the spatial temperature T(x,y,z) for each reflective optical element Mi based on the 5 th step, considering its thermal load is determined together with its maximum and its average temperature values T
  • a seventh step the at least one material with the zero cross temperature Tom >s selected based on the zero cross temperature T 0m .
  • the selection is done such that the maximum temperature or the average temperature T m max and T m av of the temperature T(x,y,z) of the optical element M m related to the at least one substrate material is lower than the zero cross temperature T 0m , expressed as T in maK ⁇ T 0m or T m m ⁇ Tom .
  • the mirrors and the lens are formed with the selected materials.
  • the configuration method comprises the additional steps in which the at least one material is selected such that the zero cross temperatures ⁇ ⁇ is higher than the maximum temperature or the average temperature T max and T av increased by an absolute value of respective manufacturing tolerances ⁇ ⁇ , to manufacture a material with the zero cross temperature T 0m , expressed as T 0m > T m max + 1 ⁇ 0 ⁇ , I or Tom > max(T Ref , T mmax )+ 1 AT 0m I .
  • T 0m > T m max + 1 ⁇ 0 ⁇ , I or Tom > max(T Ref , T mmax )+ 1 AT 0m I .
  • the configuration method may comprise that the one material with the zero cross temperature To m is selected such that it comprises a second zero cross temperature T 2 om such that the absolute value of the difference between the zero cross temperature and the second zero cross temperature is less than 40 K, expressed as abs (Ton, - T 2 0m ) ⁇ 40 K.
  • This has the advantage that such a material is applicable for two mirrors of the EUV projection lens, whereas the two mirrors have different thermal load which result in very different zero cross temperatures to minimize optical aberration, as described in connection with Fig. 9.
  • the configuration method may comprise that the material with the second zero cross temperature is used as substrate material for at least one additional reflective optical element, as it was just explained.
  • the configuration method may comprise a step in which a heater 300 and a temperature control system 200 is selected such that the at least one material with the zero cross temperature is heatable to its zero cross temperature.
  • heater 300 In the following various embodiments of the heater 300 are described. Further, also heating methods and special bodies for the reflective optical element M k are shown, which allow the heating of the reflective element or mirror in such a way that the thermal induced optical aberrations become minimal. These further aspects are also treated a parts of the present invention.
  • Fig. 10 schematically shows a side view a reflective optical element Mi, like a mirror 421 of an EUV projection lens, as e.g. shown in Fig. 4 and Fig. 5.
  • the optical element comprises a reflective surface MS k which could be a mirror surface 450 or also a diffractive surface.
  • the reflective surface can comprise diffractive structures to form a diffractive optical element for the incident radiation.
  • the incident radiation 1000 can be a EUV light projection beam 4 (see Fig. 1), or it can be light of another wavelength like e.g. infrared light.
  • support means are schematically shown like such ones which are described in connection with Fig. 4 and Fig.
  • support means are e.g. linking points 451, linking elements 471 or a bipod structure 461 as described in Fig. 4.
  • a support element 480 and a part of the housing structure 481 is shown.
  • a heater or in general a first tempering element 300 is shown. Parts and some embodiments of such a heater or first tempering element 300 are already described together with Fig. 5. The difference between a heater and a tempering element is that the heater only can heat in a controlled way, whereas the tempering element is able to heat and cool in a controlled way. Heating in this context means that thermal energy is transferred from the heater or tempering element to is surrounding.
  • the embodiment as shown in Fig. 10 comprises a cooler or second tempering element 350.
  • the cooler 350 or second tempering element is preferably arranged between the heater or first tempering element 300 and the support element 480 and/or the housing structure 481 (not shown) and /or the support means 451, 461, 471 (not shown, see Fig. 5). If the heater or first tempering element 300 is used to influence the temperature distribution of the reflective optical element M then there is a risk that also the temperature of other elements in the surrounding of the reflective element Mk is changed.
  • Such an element can be e.g. the support element 480, the housing structure 481 or the support means 451, 461, 471.
  • some of these elements have to be at a well controlled constant temperature to keep the reflective optical element or the mirror Mk, 421 in its position. As mentioned above, this position has to be constant within a nanometer or even sub-nanometer range. For this reason these elements are designated as temperature sensitive elements. Any heating and/or cooling of the surrounding of the reflective optical element or the mirror Mk, 421 to influence its temperature or temperature distribution, e.g. with the heater or first tempering element 300, may change this position. For this reason the cooler or second tempering element 350 is made such that it absorbs any thermal effects which are caused by the heater or first tempering element 300.
  • the temperature of the cooler or second tempering element is kept constant, resulting in a constant temperature of the temperature sensitive element such that the position of the respective optical element Mk is constant on the nanometer or even sub-nanometer level.
  • the temperature variation of these temperature sensitive elements can be reduced even if e.g. the heater or first tempering element changes its temperature to influence the temperature or the temperature distribution of the reflective optical element or mirror Mk , 421, to achieve the advantages as described in connection with the above described projection lenses of the present invention.
  • the present invention also relates to a fourth projection lens of an EUV-lithographic projection exposure system which comprises a plurality of reflective optical elements M
  • Each reflective element Mi comprises a body MB) and a reflective surface MS f to project an object field on a reticle onto an image field on a substrate if the projection lens is exposed with an exposure power of EUV light with a wavelength in a wavelength range of less than 50 nm which is reflected from the reticle while the reticle is illuminated by an illumination system of the EUV-lithographic projection exposure system.
  • the lens further comprises support means which comprises a temperature sensitive element selected from the group consisting of linking points 451, bipod structures 461, linking elements 471, a support element 480 and a housing structure 481, for passively or actively supporting at least one optical element Mk. Further, the temperature sensitive element is controlled to a constant temperature or to a predefined temperature.
  • the lens further comprises a first tempering element 300 for heating and/or cooling the at least one optical element Mk to a temperature T*, and a second tempering element 350 for tempering the temperature sensitive element to the constant temperature or to the predefined temperature. Further, the second tempering element 350 is spatially arranged between the temperature sensitive element and the first tempering element 300.
  • the heated or cooled reflective optical element Mk comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperatureT 0k which is different from the constant or predefined temperature of the temperature sensitive element.
  • the temperature of the temperature sensitive element in the fourth projection lens is at a reference temperature T ftef .
  • the reference temperature T Ref has the same meaning as for the other projection lenses of this invention or as are described herein.
  • the reference temperature T Ref 22"C, or at the temperature of the clean room in which the lithographic projection exposure apparatus is operated for mass production.
  • the zero cross temperatures T ok in this embodiment is higher than the maximum of the reference temperature T Ref and the respective average or maximum temperature added by the reference temperature, resulting in ⁇ + T ref or AT kmax + T1 ⁇ 2f based on the spatial temperature distribution ATk(x,y,z), which is expressed as Tok > max(T Ref , ⁇ + 3 ⁇ 4 ) or T 0 > k ma (TRef, ⁇ supervise, ⁇ + T ⁇ f ).
  • the material with the zero cross temperaturTe 0k varies regarding its real value of the zero cross temperature due to manufacturing processes. This results in a manufacturing tolerance ⁇ 1 ⁇ 2 such that the real value is within the interval T 0k ⁇ ⁇ 1 ⁇ 2.
  • the zero cross temperaturesT 0k is selected higher than the maximum of the reference temperature T Rcf and the respective average or maximum temperature (based on the spatial temperature distribution AT k (x,y,z)) added by the reference temperature, resulting in ATk av + T RC f or ⁇ ⁇ 9 ⁇ + T Ref , the maximum is increased by the absolute value of the manufacturing tolerance AT 0 k, expressed as T 0k > max(T Ref , AT kav + TR*) + 1 ⁇ 1 ⁇ 2 I , T 0k > maxfl , AT km ax + TR ⁇ ) + 1 ⁇ ® I .
  • the zero cross temperature T ok of the reflective optical element M k is lower than the reference temperature T Ref > or is at least 6 higher than the reference temperature.
  • the reflective optical element M k is cooled by the first tempering mans to a temperature close to this zero cross temperature.
  • This feature of the fourth projection lens can be used in the third projection lens of the present invention for the cooling of the mirror with the lowest zero cross temperature as it was described above.
  • this feature of the fourth projection lens can be used in a projection lens in accordance with the first projection lens in which the zero cross temperatures of two reflective elements differ at least by 6 K and one of the two reflective elements has a zero cross temperature close to or at the reference temperature.
  • a projection lens is designed for an EUV exposure power of more than 8 W (see also Fig. 9 with description).
  • the reflective optical element Mk is (in at least one spatial dimension) homogeneously subjected to a heat transfer caused by the first tempering element 300.
  • a homogeneous heat transfer homogeneously heats or cools the optical element Mk which results in a constant temperature distribution of the reflective optical element Mk along the at least one dimension, if only the influence of the first tempering element 300 is considered.
  • This variant of the fourth projection lens can be used in the first, second and third projection lens to e.g. homogeneously heat up the reflective optical element to or close to its zero cross temperature, as it was described in connection with these projection lenses, and as it was schematically described with Fig. 3a to 31 and Fig. 5.
  • the optical element Mk is connected to an actuator for its translational movement. This is to minimize optical aberration of the projection lens or the optical element for the case that the optical element is homogeneously heated or cooled to a temperature different from the reference temperature T Ref , as is was described e.g. in connection with Fig. 3a and Fig. 3b.
  • the first tempering element 300 is located on a side of the body MB of the at least one optical element M k which is opposite to its reflective surface MS k . This is a similar arrangement of the first tempering element 300 as it is described in Fig. 5 for the heater 300.
  • the first tempering element 300 comprises a Peltier element or a radiation source, emitting a radiation comprising a wavelength to which the body MB of the at least one optical element Mk is semitransparent.
  • the second tempering element 350 is located on a side of the first tempering element 300 which is opposite to the body MB k of the at least one optical element Mk.
  • the tempering element 300 may be a Peltier element or may comprise a Peltier element as already mentioned.
  • the Peltier element may heat or cool the surrounding around its surface side 362 which is oriented into the direction of the reflective element or mirror M k , 421.
  • the Peltier element extends parallel or approximately parallel to the surface of the reflective element Mk, 421 which is opposite of the reflective surface MSk, 450. If the Peltier element has at least such a size so that the reflective surface or even the reflective element can be covered from one side then a homogenous heating and/or cooling of the reflective element can be done which results in all the advantages as described herein (e.g. in connection with Fig. 3a).
  • the Peltier element is preferably coupled to the cooler 350, or in general to the second tempering element by e.g. coupling one of its surfaces 361 to one of the surfaces 352 of the cooler 350 or the second tempering element.
  • the cooler 350 or in general the second tempering element may have a kind of a sandwich structure such that the surface 352 which is in contact with the first tempering or Peltier element 300 is separated from the surface 351 which is oriented into the direction of the temperature sensitive elements, like the support element 480, the housing structure 481 or the support means 451, 461, 471.
  • the first tempering element 300 may comprise several Peltier elements.
  • the Peltier elements are arranges side by side in at least one dimension, to form an array-like arrangement.
  • each Peltier element of such an arrangement is controllable by a control system like e.g. the temperature control system as described in Fig. 5.
  • a controllable temperature profile on the side which is directed to the reflective element or mirror can be adjusted. This will influence the temperature distribution within the reflective element or mirror Mk, 421 in a controlled way.
  • a heater or tempering element can be used for the temperature control of the reflective element or mirror Mk, 421 in accordance to the teaching of this invention in such a way that the thermal induced optical aberration or the imaging errors are minimized.
  • the usage of the described Peltier element 300 or an arrangement of Peltier elements as a tempering means in the third projection lens according to this invention will have the advantage that the mirror with the low zero cross temperature T ⁇ n (according to the third projection lens of this invention) can be cooled down to this temperature or even lower, which is important if the zero cross temperature is below the reference temperature Tfef. Further, the mirror with the high zero cross temperature T 2 om n (or a zero cross temperature higher than the reference temperature T Ref ) can be heated up to this or the respective operation temperatures as described above in connection with one of the projection lenses according to this invention.
  • the projection lens comprises a pressure control system for the control of the pressure ⁇ within a surrounding of the at least one reflective optical element Mk.
  • a pressure control system will be described below in connection with Fig. 11.
  • the pressure control is based on a parameter selected from the group consisting of: The temperature of the reflective optical element Mk , the time, a parameter which directly or indirectly influences the temperature of the optical element Mk, an illumination setting, a change of the reticle, thermally or mechanically induced optical aberration data of the optical element Mk or the projection lens and an output parameter from a model.
  • the model input comprises data selected from the group consisting of: The temperature of the reflective optical element Mk , the time, a parameter which directly or indirectly influences the temperature of the optical element M k , an illumination setting, thermally or mechanically induced optical aberration data of the optical element M k or the projection lens and a change of the reticle.
  • the pressure control system preferably comprises a gas inlet and/or a gas outlet nearby the at least one optical element Mk.
  • the fourth projection lens according to this invention preferably is operated such that the temperature Tk of the optical element M k is controlled to its value by heating the first tempering element with a first heating power at a time before the projection lens is subjected to the exposure power of the EUV light.
  • the heating power of the first tempering element is less than the first heating power.
  • the temperature Tjt of the at least one optical element IV which is heated and/or cooled by the first tempering element 300 is the zero cross temperatureT 0k .
  • Fig. 11 schematically shows a further embodiment of the present invention similar as shown in Fig. 10 why like elements are designated with the same reference numerals.
  • the elements not described and its functions are as the ones described in Fig. 10.
  • the temperature of the reflective optical element or mirror M k , 421 is controlled by pressure control of the pressure within the surrounding of the reflective element or mirror Mk , 421, designated as ⁇ . In this case the pressure can be controlled e.g.
  • the control system which controls the pressure may comprise a model which uses input data like the mentioned function variables to optionally adjust the model and to provide output parameters like the pressure or any parameter which directly or indirectly influence the pressure in the mentioned surrounding.
  • the fifth projection lens is a projection lens of an EUV- lithographic projection exposure system.
  • the lens comprises a plurality of reflective optical elements M ; , each comprises a body MBj and a reflective surface MS] for projecting an object field on a reticle onto an image field on a substrate if the projection lens is exposed with an exposure power of EUV light with a wavelength in a wavelength range of less than 50 nm, when the light is reflected from the reticle while illuminated by an illumination system of the EUV-lithographic projection exposure system.
  • the lens further comprises support means for passively or actively supporting at least one optical element M k , a pressure control system (as described in connection with the embodiment of Fig.
  • control for the control of the pressure Ap within a surrounding of the at least one reflective optical element M k , wherein the control is based on a parameter selected from the group consisting of: The temperature of the reflective optical element Mk , the time, a parameter which directly or indirectly influence the temperature of the optical element Mb an illumination setting, a change of the reticle, thermally or mechanically induced optical aberration data of the optical element Mk or the projection lens and an output parameter from a model.
  • the model is using input data selected from the group consisting : The temperature of the reflective optical element Mk , the time, a parameter which directly or indirectly influence the temperature of the optical element Mk, an illumination setting, thermally or mechanically induced optical aberration data of the optical element M k or the projection lens and a change of the reticle.
  • the support means comprising a temperature sensitive element,(as described in connection with Fig. 10) selected from the group consisting of linking points 451, bipod structures 461, linking elements 471, support element 480 and housing structure 481, for passively or actively supporting the at least one optical element Mk, wherein the temperature sensitive element is controlled to a constant or predefined temperature by a cooler 350.
  • the cooler 350 is spatially arranged between the temperature sensitive element and the at least one optical element M k .
  • the pressure ⁇ within the surrounding of the at least one optical element Mk is in the range of 0.1 Pa to 10 Pa, more preferably in the range between 1 Pa and 5 Pa.
  • the distance between the at least one optical element Mk and the cooler 350 preferably Is in the range of 1 mm to 10 mm, more preferably in the range between 3 mm and 5 mm.
  • the pressure control system comprises a gas inlet and/or a gas outlet nearby the at least one reflective optical element M k . With the gas inlet and/or gas outlet (not shown in Fig. 11) the pressure within the surrounding of the at least one optical element k can be adjusted.
  • the distance of the cooler 350 between the at least one reflective optical element M k and the cooler 350 is adjustable to control a heat transfer between these elements.
  • the at least one reflective optical element M k also comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperaturTe 0k .
  • this zero cross temperature is different from the constant or predefined temperature of the temperature sensitive element, whose temperature is preferably but not necessarily at a reference temperature T Ref , as it was already mentioned in connection with the fourth projection lens of this invention which was described in connection with Fig. 10.
  • the fifth projection lens advantageously offers the possibility that the pressure ⁇ (which is controlled by the control system) is such that the temperature of the at least one reflective optical element M k is at its zero cross temperature.
  • the surrounding of the reflective element or mirror Mk , 421 comprise a gas with a pressure in the range of about 1 to 5 Pa (Pascal), preferably about 3.5 Pa.
  • the cooler 350 is usually shielding the temperature sensitive element such that any temperature change of the mirror 421 will have no influence on them, as already described with Fig. 10.
  • the cooler 350 is spatially arranged between the temperature sensitive element and the at least one optical element k -
  • the distance between the at least one optical element supported by the support means or the mirror 421 and the cooler 350 is about 3 to 5 mm.
  • the gas pressure as mentioned.
  • As a gas usually hydrogen is used.
  • the thermal resistance of the gas will change by about ⁇ 25% .
  • the mirror can be controlled regarding its absolute temperature and regarding its temperature distribution as a function of time.
  • the mirror 421 is at a lower temperature as it is in a steady state. In this case the pressure is reduced so that the mirror heats up faster to its steady state temperature.
  • the illumination setting and/or the reticle is changed, most probable also the power will change to which the EUV projection lens is subjected as it was described above.
  • the mirror 421 will heat up to a higher or to a lower steady state temperature.
  • the pressure is increased to reduce the thermal resistance between the cooler 350 and the mirror 421.
  • the pressure will be reduced to increase the thermal resistance of the gas surrounding to bring the mirror to its best steady state temperature which is closest to the zero cross temperature (this was explained with Fig. 7).
  • Applying the regulation of the gas pressure in the surrounding of the mirror 421 will influence the steady stated temperature.
  • the steady state temperature should be the zero cross temperature to minimize the optical aberration.
  • also small variations in the zero cross temperature of a certain mirror M k e.g.
  • the pressure control system comprises a gas inlet and a gas outlet nearby the at least one reflective optical element Mk which is supported by the support means.
  • the distance of the cooler surface 352, or in general the distance between the cooler 350 and the mirror 421 or the reflective optical element k may be adjusted to control the heat transfer between these elements.
  • Fig. 12a shows the transmission of Zerodur ® for a transmission path of 5 mm and 25 mm. It shows that Zerodur ® has an about constant absorption coefficient for light with a wavelength in the range of 700 nm up to about 2 ⁇ (herein further designated as infrared or I radiation or IR light). The attenuation length in Zerodur ® for IR light with a wavelength in this range is about 50 mm to about 100 mm. This results from Fig. 12a. Fig. 3d and Fig. 3h also show typical mirror sizes of mirrors used in an EUV projection lens. The diameters are typically in the range of 200 mm to 600 mm, and the thickness is up to 100 mm.
  • IR light passes a distance of 50 mm to about 100 mm in Zerodur" material then 63% of its energy is absorbed along its path. So the attenuation length fits well to the size of the mirrors of an EUV lens such that the mirror, if made of Zerodur ® or if comprising Zerodur ® , can be heated by the mentioned IR light. This is because the IR light deposits its energy over a distance of about 50 mm up to even 100 mm and more. Such a distance can be warmed up simultaneously in a more or less homogenous way.
  • a mirror body MB k of a mirror M k as e.g. shown in Fig. 5, Fig. 10 or Fig.
  • FIG. 12a schematically shows a further embodiment of the present invention in which a reflective optical element or a mirror Mk, 421 is coated with a coating C on or on almost its entire surface to make the mirror heatable with IR radiation.
  • the coating C differs from the reflective surface MS*, 450 which may comprise also a coating in the form of a multilayer to reflect the EUV light.
  • the coating C is selected such that the infrared radiation IR is reflected.
  • a coating material a very thin metal layer can be used. Other coatings which reflect IR radiation can also be used. Further, the coating and the wavelength of the IR radiation can be adapted such that its reflection is optimized.
  • the coating C has the function that the IR radiation cannot leave the mirror body Bk and that mirror is heated more homogenous due to the various reflections which are shown in Fig. 12b as 1002. With this method the I radiation almost instantaneously and totally deposits its energy within the mirror body MB k . To get the IR radiation into the mirror body MB k at least one surface area of it is not coated or the coating is partly transparent for the IR radiation.
  • Such surface areas are designated by 1004 and 1006 and are called IR windows.
  • the IR window or a plurality of IR windows 1004, 1006 are positioned on a suitable position on the mirror which considers the attenuation length of the IR radiation within the mirror body material, the geometrical form and the size of the mirror. Also any limitations regarding construction space is considered. However, since the attenuation length is comparatively large, or in other words in the range of the size of the mirror, the position of the IR window is not so critical. Especially, the position can be on the side of the mirror body MB k on which the reflective surface MS*, 450 of the reflective element or mirror is arranged, as shown in the example of Fig. 12b.
  • the construction space in front of the mirror can be used to couple the IR radiation into the reflective element or mirror. Most other heater or tempering means cannot be used in this space since they will block or shadow the EUV projection beam 1000.
  • the IR window surrounds the reflective surface MSk, 450, as schematically shown in Fig. 12b.
  • the IR window comprises a certain surface roughness or a diffractive structure to scatter or distribute the IR radiation within the mirror body.
  • the IR radiation may at least partly share the same construction space as the EUV projection beam 1000, as is shown in Fig. 12b for the right IR beam.
  • IR radiation source any source like Lasers, IR- diodes, filaments or lamps etc. can be used.
  • a suitable optical arrangement can be used to configure the IR radiation such that it can be coupled into the reflective element or into the mirror Mk, 421 through the IR windows.
  • Such an optical arrangement may comprise lenses and mirrors or in general refractive, diffractive and/or reflective optical elements.
  • such an optical arrangement may also comprise quartz fibers or optical fibers (in general light guides) e.g. to transport the IR radiation from the IR source to the IR windows 1004, 1006. In this case then the IR source can be positioned even outside the EUV projection lens or outside the EUV lithographic projection system.
  • the heater or tempering element or tempering means which comprises the IR source and an optional optical arrangement to configure the IR light
  • the reflective element or mirror of Fig. 12b can be used together with the tempering elements or tempering means e.g. the heater 300 of the embodiment of Fig. 10 and/or with the cooler 350 as described there, or as described in the embodiment of Fig. 11.
  • the pressure control as described in Fig. 11 can be used together with the embodiments of Fig. 12b.
  • these embodiments can advantageously be used in an EUV projection lens, especially in one as described above in accordance to this invention, preferably in the first, second and third projection lens according to this invention, having the advantage that the thermal induced optical aberration is minimized.
  • the first embodiment of the sixth projection lens for an EUV-lithographic projection exposure system comprises a plurality of reflective optical elements M
  • Each reflective optical element M k comprises a body MB
  • the at least one reflective optical element k comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperaturTe 0k ⁇
  • the body B k of the optical element M k is semitransparent to an IR radiation
  • the at least one reflective optical element k with its body MB k comprises a coating C on or on almost its entire surface of the body MB k .
  • the coating C reflects IR radiation inside the body MB k .
  • a second embodiment of the sixth projection lens comprises an IR light source or comprises an optical arrangement to form an IR light path.
  • the IR light source or the IR light path couples IR radiation into the body MB k which comprises the coating C.
  • the IR light source or the optical arrangement comprises an element selected from the group consisting of: Laser, IR-diode, filament, lens, mirror, refractive element, diffractive element, reflective element, light guide and optical fiber.
  • the body MB k preferably comprises a surface area 1004, 1006 not coated with the coating C, or comprises a surface area 1004, 1006 with a coating being semitransparent to the IR radiation, for coupling in the IR radiation into the body MB k .
  • the coating C comprises a metal.
  • the surface area 1004, 1006 is arranged on the side of the body MB k which comprises the reflective surface MS k .
  • the surface area 1004, 1006 surrounds the reflective surface MS k .
  • the surface area 1004, 1006 may comprise a surface roughness to scatter the IR radiation into the body, or the surface area may comprise a diffractive structure to distribute the IR radiation within the body MB k by diffraction.
  • the construction space nearby the reflective surface MSk is illuminated by the IR radiation and the EUV light.
  • the IR source is arranged outside the projection lens or outside the EUV-lithographic projection exposure system.
  • the optical element M k optionally is connected to an actuator for its translational movement.
  • the sixth projection lens may comprise embodiments which comprises a cooler 350 located on a side of the body MB k of the at least one optical element M k which is opposite to its reflective surface MS k . Further, the distance of the cooler 350 to the at least one reflective optical element M k preferably is adjustable for the control of the heat transfer between these elements.
  • the sixth projection lens (and its embodiments) comprises a pressure control system for the control of the pressure ⁇ within a surrounding of the at least one reflective optical element Mk, Further, the mentioned pressure control system may comprise a gas inlet and/or a gas outlet nearby the at least one optical element Mk.
  • ⁇ with EUV light (Tk ⁇ x,y,z)-T Re ») of the body MBk with the temperature Tk(x,y,z) relative to a reference temperature T Ref .
  • the temperature distribution comprises an average and maximum temperature ATk av and AT km ⁇ ,.
  • the zero cross temperature T 0 k is higher than the maximum of the reference temperature T Ref and the respective average or maximum temperature based on the spatial temperature distribution ATk(x,y,z) added by the reference temperature ATkav + T ref or ATkmax + TRef, expressed as T ok > maxf!-, ® .,, AT tav + TRef ) or Tok > maxO , AT kmax + TR ⁇ ).
  • the zero cross temperatures T 0 k is selected higher than the maximum of the reference temperature T Ref and the respective average or maximum temperature based on the spatial temperature distribution AT k (x,y,z) added by the reference temperature AT kav + T Ref or ATkmax + T Ref lake wherein the maximum is increased by the absolute value of the manufacturing tolerance AToic expressed as 3 ⁇ 4 > max ⁇ , ATka V + T Ref ) +1 ⁇ 1 ⁇ 2! , T ok > maxp AT k max + Tnef) + l ATokl .
  • the temperature Tk of the optical element Mk is controlled to its value by irradiating the optical element MBk with a first IR radiation power at a time before the projection lens is subjected to the exposure power of the EUV light.
  • the IR radiation power is less than the first IR radiation power during the time when the projection lens is subjected to the exposure power of the EUV light.
  • the present invention relates to a mirror (as shown in Fig. 12) which comprises a body MBk and a reflective surface MSk, and material with a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperatureT 0k .
  • the body MB k of the optical element M k is semitransparent to an IR radiation
  • the body MBk comprises a coating C on or on almost its entire surface of the body MBk, wherein the coating C reflects IR radiation inside the body MBk
  • the mirror is adapted for a projection lens of an EUV-lithographic projection exposure system for projecting an object field on a reticle onto an image field on a substrate if the projection lens is exposed with an exposure power of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • the mirror body MBk comprises a surface area 1004, 1006 which is not coated with the coating C, or it comprises a surface area 1004, 1006 with a coating which is semitransparent to the IR radiation, both for coupling in an IR radiation into the body MB k .
  • the coating comprises a metal.
  • the surface area 1004, 1006 is arranged on the side of the mirror body MBk on which the reflective surface MSk is arranged.
  • the surface area 1004, 1006 surrounds the reflective surface MSk.
  • the surface area 1004, 1006 comprises a surface roughness to scatter an IR radiation into the body, or the surface area comprises a diffractive structure to distribute an IR radiation within the body MBk by diffraction.
  • Fig. 12c schematically shows a further embodiment of the present invention in which a reflective optical element or a mirror Mk, 421 is at least partly coated with a coating C2 on its surface to make the mirror heatable.
  • the coating C2 differ from the reflective surface MSk, 450 which may comprise also a coating in the form of a multilayer coating to reflect the EUV light 1000.
  • the coating C2 is selected such that it will have a certain electrical resistance why it is called resistive coating C2. If in the resistive coating C2 an electrical current is generated then energy is produced which is transferred as a thermal energy to the reflective element or mirror Mk which heats up. In this way the temperature of the reflective element or mirror Mb 421 can be controlled.
  • the heater or tempering means 300 comprises the resistive coating C2 and a voltage source VS which is connected to the coating C2.
  • the resistive coating C2 may cover the surface of the body Mk- Preferably there is no resistive coating C2 between the reflective surface MSk, 450 and the body MB k .
  • the resistive coating C2 only a part or several parts of the surface of the body are coated with the resistive coating C2. These parts are selected such that a homogenous temperature distribution in the body and/or on the reflective surface can be achieved. This will minimize the aberration of the reflective element or of the mirror as in the embodiments described above.
  • the electrical current in the resistive coating C2 can be generated by an electrical voltage.
  • a voltage source VS is connected to the resistive coating C2 of the optical element Mk, 421. This can be done by cable 1008.
  • the voltage source VS is mounted on the body as shown by numeral 1010. If the voltage source is mounted on the body MBk there is also no mechanical connection of the heater or tempering means 300, which is formed by the coating C2, to a support structure as it is if a cable or wires 1008 are used.
  • the heater or tempering means 300 which is formed by the coating C2
  • the reflective element or mirror of Fig. 12c can be used together with the tempering means or heater 300 of the embodiment of Fig. 10 and/or with the cooler 350 as described.
  • the pressure control as described in connection with Fig. 11 can be used together with the embodiments of Fig. 12c.
  • the embodiments of Fig. 12b and 12c may be combined, e.g. by using the coating C for the reflection of IR light and as a resistive coating C2.
  • the IR reflection coating C (of Fig. 12b) may be covered with a resistive coating C2 which is heated by a voltage source VR as described in Fig. 12c.
  • the mirror according to the present invention in connection with Fig. 12c comprises a body Bi t and a reflective surface MSk,. Further, the mirror comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperaturTe 0k , and the body MBk is at least partly coated with a resistive coating C2, wherein the resistive coating C2 has an electrical resistance suitable to heat the body by electrical resistive heating. Further, the mirror is adapted for a projection lens of an EUV- lithographic projection exposure system for projecting an object field on a reticle onto an image field on a substrate if the projection lens is exposed with an exposure power of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • the resistive heating is between 0,01 W and 1 W.
  • the coating C2 is connected to a voltage source VS.
  • the voltage source preferably is attached to the mirror body MBk, or the voltage source is electrically connected to the mirror body MBk by a wire 1008.
  • the coating C2 of the mirror covers the mirror body MBk except in the area of the reflective surface MSk.
  • the body MBk of the optical element Mk is semitransparent to an IR radiation and the resistive coating C2 is coated on a reflective coating C coated on or on almost the entire surface of the body MBk,.
  • the reflective coating C reflects IR radiation inside the body MB ⁇
  • the mirror body MBk comprises a surface area 1004, 1006 which is not coated with the coatings C and C2, or it comprises a surface area 1004, 1006 with a coating which is semitransparent to the IR radiation.
  • These embodiments of the mirror have the advantage that an IR radiation can be coupled into the mirror body MB k at these surface areas 1004, 1006.
  • the surface area 1004, 1006 is preferably arranged on the side of the body MB k with the reflective surface MS*, and optionally the surface area 1004, 1006 surrounds the reflective surface S
  • the surface area 1004, 1006 comprises a surface roughness to scatter an IR radiation into the body, or the surface area comprises a diffractive structure to distribute an IR radiation within the body MBk by diffraction.
  • the resistive coating C2 preferably comprises a metal and also preferably the reflective and the resistive coating C, C2 comprise the same metal.
  • the present invention also relates to a seventh projection lens to make use of the advantages as described in connection with Fig. 12c.
  • the seventh projection lens of an EUV-lithographic projection exposure system comprises a plurality of reflective optical elements Mi which comprises a body MB
  • the EUV light is reflected from the reticle while the reticle is illuminated by an illumination system of the EUV-lithographic projection exposure system.
  • ⁇ comprises the features of the above described embodiments of the mirror of the present invention in connection with Fig.
  • the optical element M k is optionally connected to an actuator for its translational movement.
  • the seventh projection lens comprises a cooler 350 which is located on a side of the body MB k of the at least one optical element Mk, wherein the side is opposite to its reflective surface MS k .
  • a distance of the cooler 350 to the at least one reflective optical element M k is adjustable for the control of the heat transfer between these elements.
  • a further embodiment of the seventh projection lens according to this invention comprises a pressure control system for the control of the pressure Ap within a surrounding of the at least one reflective optical element Mk, which comprises the features of the above described embodiments of the mirror of the present invention in connection with Fig. 12c.
  • the pressure control system optionally comprises a gas inlet and/or a gas outlet nearby the at least one optical element M .
  • the projection lens heats up without heating the resistive coating C2 which is on the body MBk, if the reflective surface MSk is exposed with EUV light.
  • the zero cross temperatureT 0k of the mirror body MBk is selected higher than the maximum of the reference temperature T Ref and the respective average or maximum temperature based on the spatial temperature distribution ATk(x,y,z) added by the reference temperature ATkav + T ref or A kmax + TR*, expressed as >T 0 m k axf ⁇ , AT tav + T Ref ) or T ok > max(T Ref , ATkmax + T Ref ). Further also in the seventh projection lens any manufacturing tolerances of the zero cross temperature may be considered.
  • the material with the zero cross temperatureT 0k varies regarding its real value of the zero cross temperature due to manufacturing, which results in a manufacturing tolerance ATok such that the real value is within the intervalT 0k ⁇ ATok,.
  • the zero cross temperatures 1 ⁇ 2 is selected higher than the maximum of the reference temperature T Ref and the respective average or maximum temperature based on the spatial temperature distribution ATk(x,y,z) added by the reference temperature (ATkav + T Ref or ATk max + T Ref ).
  • This maximum is additionally increased by the absolute value of the manufacturing tolerance AToio expressed as T 0k > max(T Ref , ATkav + TR*) + 1 ⁇ 1 ⁇ 2 I , > mT 0 a k x(TRef, ATkmax + TRef) + 1 ATok I .
  • the temperature Tk of the optical element Mk is controlled to its value by heating the resistive coating C2 with a first electrical power of the voltage source at a time before the projection lens is subjected to the exposure power of the EUV light. Further, during the time when subjecting the projection lens to the exposure power of the EUV light the electrical power for heating the resistive coating C2 preferably is less than the first electrical power.
  • Fig. 12d schematically shows a further embodiment of the present invention in which a reflective optical element or a mirror Mk, 421 is at least partly heated by a multi-zone wire grid 1050 which is arranged nearby the reflective surface MSk .
  • a reflective optical element or a mirror Mk, 421 is at least partly heated by a multi-zone wire grid 1050 which is arranged nearby the reflective surface MSk .
  • Such a reflective element or mirror can be manufactured with a process which comprises the following essential steps:
  • ⁇ is chosen as used e.g. in at least one of the embodiments described in this description.
  • at least one zero cross temperature of the body material is 30°C or above.
  • the wire grid 1050 is produced on the body material, e.g. by forming a thin restive coating on the surface of the body material MBk by an electro-plating process.
  • the resistive coating may be or may comprise Invar.
  • Various pattern structures 1051 are etched into the resistive coating which forms the structure of the multi-zone wire grid 1050. This is done by application of known lithographic methods.
  • the various pattern structures 1051 are electrically connected to form a multiplexable integrated circuit.
  • the pattern structures 1051 which forms the electrical circuit are covered by an insulating layer 1052 with a low CTE, like quartz, such that the integrated circuit is fully covered.
  • the surface of the insulating layer 1052 is polished to the surface figure accuracy which is required for the reflective element or mirror Mk.
  • the polished insulating layer 1052 is covered with a multi-layer reflective coating 1054, whereby an optional compaction layer 1053 can be between the insulating layer 1052 and the reflective coating 1054 which forms the reflective surface MS ⁇
  • the area which is covered by of the multi-zone wire grid is about 50%, even up to 95% of the area of the reflective surface MS b as schematically shown in Fig. 12d. Even a higher coverage is preferred. This has the advantage that the heat which is generated along the wires of the wire grid is deposited on a large area of the body material.
  • the reflective optical element with the wire grid has the advantage that it can be heated without any additional heater 300 as mentioned in the various embodiments above.
  • the optical element can be heated up to its zero cross temperature before the EUV projection lens is exposed with the EUV projection beam.
  • the main advantage of the wire grid is that it can control the temperature very close at the reflective surface MS k . if the EUV projection beam has a non-uniform intensity distribution on the reflective surface and as a consequence is heating this surface in a non-uniform way than the wire grid will be controlled e.g.
  • control system 200 such to reduce the heating power at locations with large EUV intensity or large EUV absorption, and it will increase the heating power at locations with low or zero EUV absorption.
  • control system 200 With such a regulation an almost constant temperature profile across the reflective surface of the mirror or reflective optical element can be achieved which results in almost no thermal induced aberration.
  • the wire grid can also be used to measure the temperature with a spatial resolution which is defined by the grid structure.
  • the electrical resistance of the wires is measured which is related to the temperature.
  • the optical element with the wire grid can be used with other heating and tempering concepts as described herein.
  • the optical element with the wire grid can be combined with a heater and cooler as described with Fig. 10.
  • the wire grid can be used to just control very small temperature variations nearby the reflective surface MS k .
  • the heater 300 of the embodiment as shown in Fig. 10 may be controlled or partially controlled by the wire grid resistance values.
  • An additional advantage of such a combined system is that the temperature control system can be simpler, because no feed-forward control is necessary. This is because the surface temperature of the reflective surface can be adjusted very quick with the wire grid heater if this heater just controls a small temperature range around the zero cross temperature of the mirror material and if other heaters or heating concepts heat the mirror close to this zero cross temperature.
  • the present invention also relates to a eight projection lens and to a mirror with the described wire grid 1050 used therein.
  • the present invention relates to a mirror with a wire grid
  • the mirror comprises a body MB and a reflective surface MS k .
  • the body MB k comprises a material with a temperature dependent coefficient of thermal expansion which is zero at a zero cross temperature 1 ⁇ 4, wherein at least a part of the surface of the body MBk comprises a wire grid 1050 for electrically resistive heating of the body MBk -
  • the mirror is preferably adapted for usage in a projection lens of an EUV-lithographic projection exposure system for projecting an object field on a reticle onto an image field on a substrate if the projection lens is exposed with an exposure power of EUV light with a wavelength in a wavelength range of less than 50 nm.
  • the wire grid 1050 of the mirror is formed by a resistive coating with a coating material and the coating material is selected from the group consisting of: metal, semiconductor material, a material comprising carbon and invar.
  • the wire grid 1050 of the mirror comprises a pattern structure 1051 with at least N > 1 electrical circuits.
  • the pattern structure 1051 comprises at least N+l electrical connectors to connect the N electrical circuits to an electrical power source to subject the N electrical circuits with an electrical power.
  • the N electrical circuits can be independently controlled from each other regarding its electrical power to which they are subjected.
  • the heating power of the wire grid 1050 is between 0,01 W and 5 W, preferably between 0,01 W and 1 W.
  • the mirror with the wire grid 1050 is connected to at least one electrical power source.
  • the electrical power source may be attached to the mirror body MB k or the electrical power source is electrically connecting the mirror body MBk e.g. by a wire or by wires.
  • the electrical power source comprises at least two voltage and/or current sources.
  • the electrical power source may comprise a multiplexer circuit. With the multiplexer circuit the N electrical circuits of the wire grid are sequentially subjected with electrical power from the power source.
  • the mirror with the wire grid in accordance with the present invention comprises an insulating layer 1052, wherein the pattern structure 1051 of the wire grid 1050 is covered by the insulating layer 1052 The insulating layer 1052 comprises a low CTE material. Further, the insulating layer 1052 is polished such to having surface figure data within an accuracy of ⁇ 3 nm RMS as required for the reflective surface MS* of the mirror.
  • the reflective surface MS* of the mirror with the wire grid 1050 comprises a multilayer stack 1054 which is arranged above the insulating layer 1052, or which is arranged above a compaction layer 1053.
  • the compaction layer 1053 is arranged above the insulating layer 1052.
  • the wire grid 1050 of the mirror covers more than 50% of the area of the reflective surface MS*.
  • the above described mirror with the wire grid 1050 can be modified such that the mirror comprises also features of the mirrors as described in connection with Fig. 12b and Fig. 12c.
  • the body MBk of the mirror is at least partly coated with a resistive coating C2 except in the area of the reflective surface MS*.
  • the resistive coating C2 has an electrical resistance suitable to heat the mirror body MBk by electrical resistive heating.
  • the body MBk of the optical element M k is semitransparent to an IR radiation. If so, advantageously the body MBk of the mirror comprising a reflective coating C coated on or on almost the entire surface of the body MBk, wherein the reflective coating C reflects IR radiation inside the body MBk.
  • a resistive coating C2 is on the reflective coating C.
  • the mirror can be heated by IR radiation (due to the coating C) and/or by resistive heating due to the coating C2, wherein the area close to the reflective surface MS k is headed with the wire grid 1051.
  • the mirror advantageously comprises a surface area 1004, 1006 which is not coated with the reflective coating C and the resistive coating C2, or it comprises a surface area 1004, 1006 with a coating which is semitransparent to the IR radiation.
  • This surface area 1004, 1006 advantageously can be used for the coupling in of an IR radiation into the body MB ® , wherein the IR radiation contributes to the heating of the mirror.
  • the mirror with the reflective coating C or the resistive coating C2 comprises a metal in the coating layer, preferably the metal is the same for the reflective and the resistive coating C, C2, if both coatings are applied.
  • the surface area 1004, 1006, for coupling in an IR radiation into the body MB ® is arranged on the side of the body MB k with the reflective surface MSk , or on a the surface area 1004, 1006 which surrounds the reflective surface MSk.
  • Such a surface area 1004, 1006 advantageously may comprise a surface roughness to scatter the IR radiation into the body, or the surface area 1004, 1006 may comprises a diffractive structure to distribute the IR radiation within the body MBk by diffraction.
  • the present invention also relates to a eighth projection lens to make use of the advantages as described in connection with Fig. 12d and the advantages of the mirror which comprises a wire grid 1050 and its various embodiments as discussed above.
  • the eighth projection lens of an EUV-lithographic projection exposure system comprises a plurality of reflective optical elements M
  • Each reflective optical elements Mi comprises a body MB
  • the EUV light is reflected from the reticle while the reticle is illuminated by an illumination system of the EUV-lithographic projection exposure system.
  • the eighth projection lens comprises at least one reflective optical element M k which comprises the features of the mirror comprising the wire grid 1050 in accordance to one of the above described embodiments of this mirror.
  • the lens comprises a cooler 350 or a first tempering element 300 which is located on a side of the body MB k of the at least one optical element M k which is opposite to its reflective surface M$k.
  • a distance of the cooler 350 or the first tempering element 300 to the at least one reflective optical element Mic is adjustable for the control of the heat transfer between these elements.
  • the lens comprises a pressure control system for the control of the pressure ⁇ within a surrounding of the at least one reflective optical element M b
  • the pressure control system comprises a gas inlet and/or a gas outlet nearby the at least one optical element M k .
  • the zero cross temperatures T ok is selected higher than the maximum of the reference temperature T Ke f and the respective average or maximum temperature based on the spatial temperature distribution AT k (x,y,z) added by the reference temperature (AT , + T Ref or ATkmax + Tuef)* and the maximum is further increased by the absolute value of the manufacturing tolerance ⁇ (3 ⁇ 4, expressed as T 0k > max(T Ref , ⁇ , ⁇ + T ⁇ ) + 1 AT ok I , T ok > max ⁇ T Ref , AT kmax + ⁇ ) + 1 AT ok I .
  • the temperature T k of the optical element M k is controlled to its value by heating the wire grid 1050 with a first electrical power of the electrical power source at a time before the projection lens is subjected to the exposure power of the EUV light.
  • the electrical power for heating the wire grid 1050 is selected less than the first electrical power.
  • the eighth projection lens may comprise a controller for controlling the electrical power to which the wire grid 1050 is subjected, or for determining a resistance value of the wire grid 1050, or a resistance value of at least one of the electrical circuits of the pattern structure 1051.
  • the controller controls the temperature of the first tempering element 300 or the cooler 350. Further optionally the controller controls a distance of the first tempering element 350 or the cooler 300 to the at least one reflective optical element Mb or the controller controls the pressure ⁇ within the surrounding of the optical element M k .
  • control is based on a parameter selected from the group consisting of: temperature, resistance value of the wire grid 1050 or the resistance value of at least one electrical circuit of the pattern structure 1051 of the wire grid 1050, time, a parameter which directly or indirectly influence the temperature of the optical element Mk, an illumination setting, a change of the exposure power of the EUV light, a change of the reticle, thermally or mechanically induced optical aberration data of the optical element M or the projection lens and an output parameter from a model.
  • the controller preferably controls the temperature of the reflective surface MS ⁇ or the body MB of the mirror M k to the zero cross temperature To k .
  • the zero cross temperatureT 0k is selected lower than a reference temperature TRef, or it is selected such that it is at least 6 K higher than a reference temperature TRef.
  • Fig. 13 schematically shows several further embodiments of heaters or tempering means 300 in relation with a reflective element or mirror 421 in accordance with the present invention. Also with these embodiments the temperature of the reflective element or mirror Mi t , 421 can be controlled to an operation temperature, averaged temperature or zero cross temperature T av k, T 0k according to the above teachings, especially to reduce any optical aberration caused by thermal effects.
  • the various embodiments of the heater or tempering means 300 which will be described below can be used alone or in any combination to control the mentioned temperatures or the aberration of the reflective optical element or mirror.
  • the mirror e.g. 421 (like the ones shown in Fig. 4 and Fig. 5) or a reflective element M* is mounted to a support element 480 which is connected to a housing structure 481.
  • the mounting elements and support means which support or suspend the mirror or the reflective optical element are not shown. These can be made as e.g. described in Fig. 4 with the reference numerals 451, 461, 471.
  • a cooler 350 can be used as an option to protect the support element 480 and/or the housing structure 481 (which can be temperature sensitive elements).
  • the pressure ⁇ of the surrounding of the mirror 421 or the reflective optical element may be controlled to adjust the thermal resistance of the gas, as described with Fig.
  • the body MB ® of the reflective element or mirror Mb 421 comprises a material with a temperature dependent CTE(T) which comprises at least one zero cross temperature T 0 , like e.g. Zerodur ® or ULE" , and if this material is transparent to light of a certain wavelength like IR light, then the body may be heated with such light as it was described in connection with Fig, 12a and Fig. 12b.
  • This kind of heating has the advantage that the heat is not only deposited on the surface of the body MB k but it is also deposited inside the body.
  • LEDs 1302 may be used as sources for IR light.
  • the usage of LEDs have the advantage that they easily can be arranged about side by side in at least one dimension, to form an array-like arrangement as it was already mention in the context of the Peltier elements of Fig. 10.
  • a suitable optical arrangement can be used to configure the IR radiation which is emitted from the LEDs in such a way that it can be coupled into the reflective element or the mirror M
  • the optical arrangement 1304 is such that the IR light from LED is collected and made about parallel. This has the advantage that a defined area of the surface of the body MB k is illuminated with IR light.
  • this surface area can be adjusted in diameter, depending on the number of LEDs which are in operation. If such LEDs with such an optical arrangement are forming a grid to illuminate the body MB k in a grid- or matrix-like form.
  • the grid-like form has the advantage that IR energy is deposited as a function of the grid-coordinates which result in a much defined spatial temperature distribution T(x,y) as a function of x and y.
  • the optical arrangement 1306 of the IR source e.g. the lenses in front of the LEDs, may be adjustable to generate a focus length and/or vary the focus length 1307. With a focused IR light the body can be heated at a very local point inside.
  • the spatial temperature distribution inside the body can be adjusted in x-, y- and z-direction.
  • the direction of the IR light can be also adjusted by tilting the IR beam around at least one axis, preferably around the x- and the y-axis, then IR light of several IR beams can be focused in a narrow region within the body MBk.
  • This advantageously can be used to homogenize a temperature distribution T(x,y,z) of the body which results from e.g. a certain reticle or illumination setting.
  • the focus length 1309 is adjusted by the optical arrangement 1308 in such a way that it is very close to the reflective surface MS k .
  • an absorption layer (not shown) may be arranged below the reflective surface which absorbs the IR light. In this case the energy of the IR light is deposited very close to the reflective surface MS k .
  • optical fibers 1310 may be used to transfer the IR light from the source (e.g. IR LEDs or an IR Laser) to close to the surface of the body MB k .
  • the IR light of one IR source can be distributed also in a grid-like manner.
  • a further alternative is to use a light guiding rod 1314 to transfer the IR light from the IR source to the body.
  • an IR light source 1312 e.g. IR LEDs, IR-Laser
  • a mirror 1313 which reflects the IR light onto the surface of the body MB k .
  • the mirror can be tilted around at least one axis, better about two axes like the x- and the y-axis.
  • the IR light can be distributed over the body. This can be done e.g. by a scanning or by a raster process.
  • the body MB k of the reflective element or the mirror itself may comprise recesses 1315 in which the IR light is coupled in.
  • the surfaces or surface areas in which the IR light is coupled into the body may be or may comprise prepared surface areas with e.g. a defined surface roughness or a diffractive structure on it.
  • the recesses may comprise surface forms like spheres to improve the incidence of the IR radiation into the body.
  • protrusions or projections of the body material can be chosen, which all may comprise a surface curvature, to achieve a determined spatial distribution of the IR radiation within the body Mk.
  • a reflective coating on the mirror body may be used to increase the absorbed IR power.
  • the heater 300 of the embodiments in accordance of Fig. 13 can be integrated into the cooler 350.
  • the wavelength of the IR radiation can be controlled to control the amount of absorption within a defined thickness of the body material.
  • the wavelength may vary from about 400 nm to about 800 nm. As it can be seen from Fig. 12a, in this wavelength range the absorption strongly depends on the used wavelength.
  • the heater or tempering means 300 are suitable to control the operation temperature of the reflective element or mirror inside a projection lens in such a way that the thermal induced aberration is minimized in the steady state of the lens and during the warm-up time of the mirror. This time can be significantly reduced due to the instantaneous heating effect of the I R heating.
  • the IR heating of the body MBk of the k-th reflective optical element or the mirror M k of an EUV projection lens as shown in connection with Fig. 12b and Fig. 13 as an example, it is possible to heat this body to the at least one of its zero cross temperatures very fast.
  • the spatial temperature distribution within the body and such along its reflective surface MS can be controlled in at least two dimensions, preferably in all three spatial dimensions. This means that the temperature is controllable as a function of the x-, y- and z-coordinate of the body MB k .
  • Fig. 6 With such heaters as designated with 630 in Fig.
  • an EUV projection lens 20 can comprise at least one reflective element or mirror Mk , like mirror 26, which comprises a body MBk with comprises a material with a temperature dependent CTE(T) which has at least one zero cross temperatureT 0k .
  • This zero cross temperature 0 k is selected such that it is higher than the expected average temperature AT avk + T Ref of the element or mirror k if the projection lens 20 is exposed with the maximum EUV exposure power, coming from the reticle M.
  • the zero cross temperature is even selected higher that the respective maximum temperature of all the expected temperature distributions T(x,y,z) which may occur during the exposure with the EUV exposure power (see for example Fig, 7).
  • a control unit 620 (Fig. 6 ⁇ controls the temperature of this element Mk (like mirror 26) preferably such that the thermal induced aberration or imaging errors 622 of the projection lens (or the element or mirror M k ) becomes to a minimum during the exposure of the lens 20 with the EUV exposure beam 4 which propagates from the reticle M to the wafer W.
  • the control unit 620 controls the temperature such that the spatial temperature distribution 621 on the reflective element or mirror M k , 26 becomes homogenous, preferably that this temperature achieves the zero cross temperature T 0k ⁇
  • the body MB* and the reflective surface for the EUV light MS k is manufactured such that the surface shape or figure achieves the specified accuracy at the zero cross temperature T 0k .
  • the reflective element or mirror M is hated with the mentioned IR heaters 630, or other heaters as already have been described herein, to the zero cross temperature.
  • this heating is done before the projection lens 20 is subjected to the EUV exposure power.
  • the reflective optical element or mirror M k is heated in addition by the absorbed EUV light.
  • the heater 630 then is controlled by the control unit 620 such that the spatial temperature distribution of the reflective element or mirror M k remains almost in the same condition as before the EUV light is switched on.
  • this condition is a constant temperature of the element or mirror M k , 26.
  • the aberration 622 can be used as a control parameter, and then the heater 630 is controlled such that the aberration 622 is minimized.
  • an infrared camera can be used, since the temperature differences due to the absorption of EUV light are in the range of about 1 K up to about 20 K, depending on the mirror 21, 22, 23, 24, 25, 26 of the lens 20 (see. e.g. Fig. 3), the used reticle or mask, the power of the EUV light and the illumination setting. Due to this relatively high temperature difference the sensitivity requirements of the IR camera are not too high.
  • ⁇ or the mirror 26 anyone of the mirrors 21, 22, 23, 24, 25, 26 within the lens 20
  • a parameter related to the deformation may be used, to control the heater 630, especially a heater 630, 300 as described herein.
  • an interferometer and/or an encoder system can be used to determine any deformation of the optical element or mirror.
  • the temperature data (especially the temperature distribution data) and/or the deformation data which result from measurements as described above are used for the calculation of an expected aberration of the optical element or mirror or the lens 20.
  • the lens 20 and/or the optical element M k or the mirror 21, 22, 23, 24, 25, 26 is controlled by at least one heater 630, 300 in such a way that the aberration will be reduced.
  • a model is applied why the aberration of the lens 20 is minimized by model based control.
  • the model for the control of the lens 20 may use additional input parameters which are measured in addition to the temperature and/or deformation data of a given preselected mirror or optical element. Such input parameters are e.g.
  • the used reticle or mask the power of the EUV light, the illumination setting, a gas pressure nearby the optical element or mirror or inside the lens 20, the time for which the lens 20 is exposed by EUV light or any change in position (like e.g. z-position) of a reflective optical element like a mirror 21, 22, 23, 24, 25, or 26 of lens 20.
  • the model may control any heater 630, 300, any position of a reflective optical element like a mirror 21, 22, 23, 24, 25, or 26, the power of the EUV projection light, the illumination setting with which the reticle is illuminated, the gas pressure nearby the optical element or mirror or the gas pressure inside the lens 20 or any actuator which deforms the reflective surface of a mirror 21, 22, 23, 24, 25 or 26 of the lens 20 in a controlled way to reduce the optical aberration.
  • the output parameter or the output parameters are correlated to the input parameter or the input parameters by the model transfer-function.
  • the input parameters of the model may be used for model calibration such that the model is adjusted regarding its transfer-function, meaning that the transfer-function itself depend on the values of the input parameters and/or the output parameters.
  • the temperature data of at least one optical element M k/ like a mirror 21, 22, 23, 24, 25, 26 of the lens 20 are accumulated in time. This results in a parameter which represents a measure of the thermal load of the optical element or mirror, which could be even with spatial resolution.
  • a thermal load parameter can be used for maintenance control of the lens 20 or the EUV lithographic exposure system.
  • any degradation of the mirror due to EUV light like a degradation of the reflective multilayer stack or any compaction effects of the used substrate material, forming the mirror body, may be estimated.
  • the illuminated masks or the used illumination settings may be selected such that the spatially resolved thermal load of the mirror 21, 22, 23, 24, 25, 26 results in a long life time of the projection lens 20, to reduce maintenance costs.

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US13/754,720 US9316929B2 (en) 2010-07-30 2013-01-30 EUV exposure apparatus with reflective elements having reduced influence of temperature variation
US15/070,872 US9746778B2 (en) 2010-07-30 2016-03-15 EUV exposure apparatus with reflective elements having reduced influence of temperature variation
US15/650,533 US10031423B2 (en) 2010-07-30 2017-07-14 EUV exposure apparatus with reflective elements having reduced influence of temperature variation
US16/013,687 US10317802B2 (en) 2010-07-30 2018-06-20 EUV exposure apparatus with reflective elements having reduced influence of temperature variation
US16/392,220 US10684551B2 (en) 2010-07-30 2019-04-23 EUV exposure apparatus with reflective elements having reduced influence of temperature variation

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DE102013203338A1 (de) * 2013-02-28 2014-08-28 Carl Zeiss Smt Gmbh Modellbasierte Steuerung einer optischen Abbildungseinrichtung

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