US20090052073A1 - Microlithography projection optical system and method for manufacturing a device - Google Patents

Microlithography projection optical system and method for manufacturing a device Download PDF

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Publication number
US20090052073A1
US20090052073A1 US12/233,384 US23338408A US2009052073A1 US 20090052073 A1 US20090052073 A1 US 20090052073A1 US 23338408 A US23338408 A US 23338408A US 2009052073 A1 US2009052073 A1 US 2009052073A1
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Prior art keywords
mirror
less
image
projection objective
mirrors
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US12/233,384
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Hans-Juergen Mann
Wilhelm Ulrich
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Carl Zeiss SMT GmbH
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Carl Zeiss SMT GmbH
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Priority claimed from PCT/EP2006/008869 external-priority patent/WO2007031271A1/en
Application filed by Carl Zeiss SMT GmbH filed Critical Carl Zeiss SMT GmbH
Priority to US12/233,384 priority Critical patent/US20090052073A1/en
Assigned to CARL ZEISS SMT AG reassignment CARL ZEISS SMT AG ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: MANN, HANS-JUERGEN, ULRICH, WILHELM
Publication of US20090052073A1 publication Critical patent/US20090052073A1/en
Priority to US12/702,040 priority patent/US20100134907A1/en
Priority to US14/298,185 priority patent/US20140327898A1/en
Abandoned legal-status Critical Current

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    • 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
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B17/00—Systems with reflecting surfaces, with or without refracting 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
    • G03F7/70216—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
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
    • G02B17/08—Catadioptric 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
    • 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
    • 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
    • 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
    • G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
    • G03F1/62—Pellicles, e.g. pellicle assemblies, e.g. having membrane on support frame; Preparation thereof

Definitions

  • This disclosure relates to a microlithography projection optical system, such as a projection objective, a microlithographic tool including such an optical system, a method for microlithographic production of microstructured components using such a microlithographic tool and a microstructured component produced by such a method.
  • Projection objectives are widely used in microlithography to transfer a pattern from a reticle to a substrate by forming an image of the reticle on a layer of a photosensitive material disposed on the substrate.
  • projection objectives fall into three different classes: dioptric objectives; catoptric objectives; and catadioptric objectives.
  • Dioptric objectives use refractive elements (e.g., lens elements) to image light from an object plane to an image plane.
  • Catoptric objectives use reflective elements (e.g., mirror elements) to image light from an object plane to an image plane.
  • Catadioptric objectives use both refractive and reflective elements to image light from an object plane to an image plane.
  • the disclosure provides an optical system that can be used as projection objective in a microlithography projection exposure apparatus and that can provide enhanced performance with respect to its us in projection objective metrology and testing.
  • the disclosure features a catoptric microlithography projection optical system that includes a plurality of reflective elements configured to image radiation from an object field in an object plane of the system to an image field in an image plane of the system.
  • the system has an object-image shift of about 75 mm or less, and the image field has a size of at least 1 mm ⁇ 1 mm.
  • the disclosure features a microlithography tool including an illumination system and a catoptric microlithography projection optical system as described in the preceding paragraph.
  • the disclosure features a method of producing microstructured components that includes using the microlithography tool described in the preceding paragraph.
  • the disclosure provides an optical system with which metrology and testing can be easily implemented despite rotations of the optical system about a rotation axis.
  • optical systems e.g., high NA optical systems
  • the object-image shift in particular may be 50 mm or less (e.g., 25 mm or less).
  • the optical system can have zero object-image shift.
  • the image field size of at least 1 mm ⁇ 1 mm allows a high throughput with respect to substrates which are illuminated via the projection optical system.
  • the plurality of elements of the optical system may include four or more reflective elements.
  • the optical system may include six or more reflective elements.
  • the projection objective can be a catoptric projection objective.
  • the image plane of the optical system may be parallel to the object plane.
  • the optical system may have a field at the image plane having a minimum radius of curvature of 300 mm.
  • the optical system may have an entrance pupil which is located more than 2.8 m (e.g., more than 10 m) from the object plane. In general, in an optical system with freeform surfaces, an exactly defined pupil plane does not exist.
  • the term pupil plane is used to characterize a region perpendicular to the light being guided in the optical system where an intensity distribution corresponds to an illumination angle distribution in the object plane.
  • the object plane of the optical system may be positioned between the plurality of elements and an entrance pupil of the optical system. Alternatively, the optical system may have an entrance pupil located at infinity.
  • the imaged radiation may be reflected from an object positioned at the object plane.
  • the object positioned at the object plane may be a reticle that is imaged by the plurality of elements to the image plane.
  • the optical system may have a demagnification of 4 ⁇ .
  • a plurality of elements may be arranged to image the radiation to an intermediate image plane between the object plane and the image plane.
  • a field stop may be positioned at or near the intermediate image plane.
  • the plurality of elements may include five elements and the intermediate image plane may be located between a fourth element and a fifth element along the path of the radiation from the object plane to the image plane.
  • the object and image planes may be separated by a distance L of about 1 m or more.
  • the optical path length of the radiation from the object plane to the image plane may be about 2 L or more (e.g., about 3 L or more, about 4 L or more).
  • the plurality of elements may include at least one pair of adjacent elements in the path of the radiation, where the pair of adjacent elements is separated by about 0.5 L or more. In certain embodiments, none of the plurality of elements causes an obscuration of the exit pupil at the image plane.
  • the plurality of elements may include four or more elements having free boards of about 25 mm or less and/or free boards of about 5 mm or more.
  • the plurality of elements may include a first mirror and a second mirror, the first and second mirrors having a minimum distance from the object plane of d 1 and d 2 respectively, where d 1 /d 2 is about two or more.
  • the plurality of elements may include a first element in the path of the radiation from the object plane to the image plane, where the first element has positive optical power.
  • the optical system may include an aperture stop positioned between the object plane and the image plane.
  • the plurality of elements may include three elements and the aperture stop may be positioned between the second and third elements in the path of the radiation from the object plane to the image plane.
  • the aperture stop may be positioned at the second element or at the third element or at some other position in the projection lens, e.g. between the first and the second element.
  • the radiation may pass through the aperture stop once or twice.
  • a radiation source which is used with the optical system may be a laser radiation source having a wavelength of about 300 nm or less (e.g., about 200 nm or less, about 100 nm or less).
  • At least one of the elements is a reflective element having a rotationally asymmetric surface positioned in a path of the radiation, wherein the rotationally asymmetric surface deviates from a best-fit rotationally symmetric surface by at least ⁇ at one or more locations.
  • a rotationally asymmetric surface is referred to as a freeform surface.
  • freeform surfaces do not have an axis of rotational symmetry. Freeform surfaces according to the pre-sent disclosure differ from known aspheric rotational symmetric mirror surfaces for EUV projection objectives in that such known aspheric mirror surfaces are described via a mathematical Taylor expansion, i.e.
  • a freeform surface may be a surface that is mirror symmetric to a meridional plane of the optical system.
  • the best-fit rotationally asymmetric surface deviates by about 0.1 ⁇ or less from a surface corresponding to the equation:
  • z is the sag of the surface parallel to an axis
  • c is the vertex curvature
  • k is the conical constant
  • C j is the coefficient of the monomial x m y n
  • ⁇ is an integer.
  • the rotationally asymmetric surface deviates from the best-fit rotationally symmetric surface by about 10 ⁇ or more at the one or more locations. In certain embodiments, the rotationally asymmetric surface deviates from the best-fit rotationally symmetric surface by about 20 nm or more at the one or more locations.
  • a deviation of the type described in the preceding paragraph can provide for a proper reduction of the objective's distortion below the limit which is reachable using rotationally symmetric optical surfaces.
  • the rotationally asymmetric surface may deviate from the best-fit rotationally symmetric surface by about 100 ⁇ or more at the one or more locations.
  • the rotationally asymmetric surface may deviate from the best-fit rotationally symmetric surface by about 50 nm or more (e.g., about 100 nm or more, about 500 nm or more, about 1000 nm or more) at the one or more locations.
  • the plurality of reflective elements define a meridional plane, and the elements are mirror symmetric with respect to the meridional plane. In such embodiments, for example, restrictions on producing a freeform optical surface may be reduced.
  • having two reflective elements with freeform optical surfaces can lead to the possibility of a better aberration minimization while also allowing the possibility of meeting certain desired aberration minimization properties with less complicated to manufactured freeforms.
  • the optical system also may have, for example, three, four, five or six freeform reflective elements.
  • An optical system having no more than two reflective elements with a positive chief ray angle magnification can exhibit relatively low incident ray angles on the mirrors, thus causing lower aberrations at the outset.
  • the plurality of elements includes no more than one reflective element that has a positive chief ray angle magnification.
  • This can be in particular advantageous for an optical system having divergent chief rays at the object plane and at least one intermediate image.
  • Optical systems can be designed where it is sufficient to have only one reflective element having a positive chief ray angle magnification which serves to a redirection of the chief rays towards a central image field axis.
  • the optical system can help provide high resolution.
  • the image-side numerical aperture may be, for example, 0.25 or more (e.g., 0.28 or more, 0.3 or more, 0.35 or more, 0.4 or more).
  • the optical system can be efficiently used in a microlithography projection apparatus.
  • the optical system can have a rectangular field at the image plane with, for example, a minimum dimension of about 2 mm.
  • the rectangular field may have a first dimension of about 1 mm or more and a second dimension of about 1 mm or more where the first and second dimensions are orthogonal.
  • the second dimension may be about 10 mm (e.g., about 20 mm or more).
  • the projection quality may be limited by only diffraction, i.e. by the wavelength of the projection light.
  • An optical system with such low distortion in particular can be optimized for use, for example, with EUV light sources in the range between 10 and 30 nm.
  • the disclosure provides a higher degree of flexibility with respect to the design of the optical system and neighbouring components.
  • the chief rays may be at an angle of about 30 or more (e.g., 5° or more, 7° or more) with respect to the object plane normal at the object plane.
  • chief ray relations can be such that they lead to specific design and/or aberration minimization advantages.
  • diverging chief rays can be such that they give the possibility to control the distribution of illumination angles in the object plane by controlling an intensity distribution in the illumination optics in front of the projection objective with a low number of optical components.
  • the object plane is positioned between the plurality of elements and an entrance pupil of the optical system. This may not be possible using an optical system with convergent (negative) chief ray angles as this would involve additional components to give access to a manipulation plane to control the distribution of illumination angles via an intensity distribution in this manipulation plane.
  • Convergent chief rays can have the advantage that a good aberration control is possible and that smaller mirror sizes can be utilized to achieve a desired low aberration amount.
  • Maximum angles of incidence of the chief ray can be such that they help to avoid high aberrations at the outset.
  • the maximum angle of incidence on a surface of each of the elements may be, for example, less than 18° (e.g., less than 15°).
  • a telecentric optical system can allow an object, such as a phase shift mask, to be imaged in the object plane having height variations.
  • a telecentric optical system can tolerate height variations of a substrate arranged in the image plane.
  • the optical system can lead to a very high resolution.
  • the ratio ⁇ /NA may be about 60 or less (e.g., 50 or less).
  • the optical system can have a radiation source and an illumination system that exploit advantageously the aberration minimization by use of metrology and testing, as aberrations and distortions in the range of the wavelength of such a radiation source are possible.
  • the wavelength is in a range from about 10 nm to about 15 nm.
  • the optical system can allow for illumination in the image field without illumination angle gaps.
  • optical systems described herein can be used in a microlithographic tool and can offer the features disclosed herein. Such a tool can be used to make components.
  • embodiments can include one or more of the following advantages.
  • a catoptric projection objective is telecentric at the image plane. This can provide for constant or nearly constant image magnification over a range of image-side working distances.
  • catoptric projection objectives have extremely high resolution.
  • projection objectives can have the capability of resolving structures smaller than about 50 nm.
  • High resolution can be achieved in projection objectives that have a high image-side numerical aperture that are designed for operation at short wavelengths (e.g., about 10 nm to about 30 nm).
  • projection objectives can provide images with low aberrations.
  • projection objectives are corrected for wavefront error of about 30 m ⁇ or less.
  • projection objectives are corrected for distortion below values of about 2 nm or less.
  • catoptric objectives have a high numerical aperture and provide imaging with low image distortion, low wavefront error, and telecentricity at the image plane over a relatively large image field. These features can be achieved by use of one or more freeform mirrors.
  • projection objective metrology can be easily implemented despite rotations of the projection objective about a rotation axis.
  • embodiments of projection objectives e.g., high NA projection objectives
  • metrology can be repeatable performed in the same field position without having to relocate that field position.
  • a catoptric projection objection has no field dependent pupil obscuration or no central pupil obscuration.
  • projection objectives can be adapted for operation at a variety of different wavelengths, including visible and ultraviolet (UV) wavelengths. In certain embodiments, projection objectives can be adapted for operation at Extreme UV (EUV) wavelengths. Furthermore, in some embodiments, projection objectives can be adapted for use at more than one wavelength, or over a range of wavelengths.
  • UV visible and ultraviolet
  • EUV Extreme UV
  • catoptric projection objectives can be used in lithography tools (e.g., lithography scanners) and can provide relatively low overscan.
  • Low overscan is accomplished, for example, by using projection objectives with rectangular image fields.
  • the image can be aligned so that an edge of the rectangular field is parallel to the leading edge of the die sites, allowing one to avoid scanning the leading edge of the die sites beyond the edge of the image field in order to scan the site corners, as is typically the case when rectangular or square die sites are scanned relative to arcuate fields.
  • the disclosure provides lithography tools with relatively high throughput.
  • lithography tools can have relatively low overscan are more efficient than comparable systems that have larger overscan. Accordingly, these low overscan systems can provide higher wafer throughput than the comparable systems.
  • catoptric projection objectives are provided that have low or no field dependence of shading effects.
  • catoptric projection objectives can have their entrance pupil located far from the object plane (e.g., at infinity) providing uniform illumination angles of the chief rays on the object field. This can reduce or avoid field dependent shading effects that occurs where chief ray angles vary across the object field.
  • projection objectives can have relatively small values of chief ray incident angles and/or small variations of incident angles for rays in the meridional section for each mirror in the projection objective, resulting in an increased average reflectivity of each mirror.
  • projection objective can include features that allow a reduction in the complexity of the illumination system.
  • the location of the entrance pupil of projection objectives may be in front of the object plane.
  • chief rays starting at different field points are divergent with respect to each other. This can make the entrance pupil of the projection objective/exit pupil of the illumination system accessible without using a telescope in the illumination system to relay the illumination system's exit pupil to the location of the projection objective's entrance pupil.
  • FIG. 1 is a schematic view of a microlithography tool.
  • FIG. 2A is a schematic view showing a portion of the microlithography tool shown in FIG. 1 .
  • FIG. 2B is a cross-sectional view of a rotationally asymmetric surface and a corresponding rotationally symmetric reference surface.
  • FIG. 3 is a cross-sectional view of an embodiment of a projection objective shown in meridional section.
  • FIG. 4 is a cross-sectional view of a portion of a mirror from a projection objective shown in meridional section.
  • FIG. 5A is a schematic view of a ray path at a mirror having a positive chief ray angle magnification.
  • FIG. 5B is a schematic view of a ray path at a mirror having a negative chief ray angle magnification.
  • FIG. 6A is a view of a mirror's footprint.
  • FIG. 6B is a cross-section view of the mirror shown in FIG. 6A .
  • FIG. 7A is a plan view of an embodiment of a ring segment field.
  • FIG. 7B is a plan view of a ring segment field relative to a pair of wafer die sites.
  • FIG. 7C is a plan view of a rectangular field relative to a pair of wafer die sites.
  • FIGS. 8A-8D are schematic views of the projection objective of the embodiment of microlithography tool shown in FIG. 1 .
  • FIG. 9 is a cross-sectional view of a portion of a projection objective shown in meridional section.
  • FIG. 10 is a cross-sectional view of a projection objective shown in meridional section.
  • FIG. 11 is a cross-sectional view of a projection objective shown in meridional section.
  • FIG. 12 is a cross-sectional view of a projection objective shown in meridional section.
  • FIG. 13 is a cross-sectional view of a projection objective shown in meridional section.
  • FIG. 14 is a cross-sectional view of an optical system that includes the projection objective shown in FIG. 13 .
  • FIG. 15 is a cross-sectional view of a projection objective shown in meridional section.
  • FIG. 16A is an x-y vector plot showing calculated distortion as a function of position in the image field for the projection objective shown in FIG. 15 .
  • FIG. 16B is an x-y vector plot showing calculated chief ray angle as a function of position in the image field for the projection objective shown in FIG. 15 .
  • FIG. 17 is a cross-sectional view of a projection objective shown in meridional section.
  • FIG. 18 is a cross-sectional view of projection objective shown in meridional section.
  • a microlithography tool 100 generally includes a light source 110 , an illumination system 120 , a projection objective 101 , and a stage 130 .
  • a Cartesian co-ordinate system is shown for reference.
  • Light source 110 produces radiation at a wavelength ⁇ and directs a beam 112 of the radiation to illumination system 120 .
  • Illumination system 120 interacts with (e.g., expands and homogenizes) the radiation and directs a beam 122 of the radiation to a reticle 140 positioned at an object plane 103 .
  • Projection objective 101 images radiation 142 reflected from reticle 140 onto a surface of a substrate 150 positioned at an image plane 102 .
  • the radiation on the image-side of projection objective 101 is depicted as rays 152 .
  • the rays are illustrative only and not intended to be accurately depict the path of the radiation with respect to reticle 140 , for example.
  • Substrate 150 is supported by stage 130 , which moves substrate 150 relative to projection objective 101 so that projection objective 101 images reticle 140 to different portions of substrate 150 .
  • Projection objective 101 includes a reference axis 105 .
  • reference axis 105 is perpendicular to object plane 103 and lies inside the meridional section.
  • Light source 110 is selected to provide radiation at a desired operational wavelength, ⁇ , of tool 100 .
  • light source 110 is a laser light source, such as a KrF laser (e.g., having a wavelength of about 248 nm) or an ArF laser (e.g., having a wavelength of about 193 nm).
  • Non-laser light sources that can be used include light-emitting diodes (LEDs), such as LEDs that emit radiation in the blue or UV portions of the electromagnetic spectrum, e.g., about 365 nm, about 280 nm or about 227 nm.
  • LEDs light-emitting diodes
  • wavelength ⁇ is in the ultraviolet portion, the deep ultraviolet portion or the extreme ultraviolet portion of the electromagnetic spectrum.
  • ⁇ can be about 400 nm or less (e.g., about 300 nm or less, about 200 nm or less, about 100 nm or less, about 50 nm or less, about 30 nm or less).
  • ⁇ can be more than about 2 nm (e.g., about 5 nm or more, about 10 nm or more).
  • ⁇ can be about 193 nm, about 157 nm, about 13 nm, or about 11 nm.
  • ⁇ can be in non-UV portions of the electromagnetic spectrum (e.g., the visible portion).
  • Illumination system 120 includes optical components arranged to form a collimated radiation beam with a homogeneous intensity profile. Illumination system 120 typically also includes beam steering optics to direct beam 122 to reticle 140 . In some embodiments, illumination system 120 also include components to provide a desired polarization profile for the radiation beam.
  • L is separated from image plane 102 by a distance L, which is also referred to as the lengthwise dimension, or tracklength, of projection objective 101 . In general, this distance depends on the specific design of projection objective 101 and the wavelength of operation of tool 100 .
  • L is in a range from about 1 m to about 3 m (e.g., in a range from about 1.5 m to about 2.5 m). In certain embodiments, L is less than 2 m, such as about 1.9 m or less (e.g., about 1.8 m or less, about 1.7 m or less, about 1.6 m or less, about 1.5 m or less).
  • L can be more than about 0.2 m or more (e.g., about 0.3 m or more, about 0.4 m or more, about 0.5 m or more, about 0.6 m or more, about 0.7 m or more, about 0.8 m or more, about 0.9 m or more, about 1 m or more).
  • the ratio of the optical path length of imaged radiation to the tracklength varies depending on the specific design of projection objective 101 .
  • the ratio of this optical path length to tracklength can be relatively high.
  • the ratio of this optical path length to tracklength can be about two or more (e.g., about 2.5 or more, about three or more, about 3.5 or more, about four or more, about 4.5 or more, about five or more).
  • Projection objective 101 has a magnification ratio, which refers to the ratio of the dimensions of the field at object plane 103 to the corresponding dimensions of the field at image plane 102 .
  • magnification ratio refers to the ratio of the dimensions of the field at object plane 103 to the corresponding dimensions of the field at image plane 102 .
  • projection objectives used in lithography tools are reduction projection objectives, meaning they reduce the dimensions of, or demagnify, the image.
  • projection objective 101 can produce a field at image plane 102 whose dimensions are reduced by about 2 ⁇ or more (e.g., about 3 ⁇ or more, about 4 ⁇ or more, about 5 ⁇ or more, about 6 ⁇ or more, about 7 ⁇ or more, about 8 ⁇ or more, about 9 ⁇ or more, about 10 ⁇ or more) compared to the dimensions at object plane 103 .
  • projection objective 101 can have a demagnification of about 2 ⁇ or more, (e.g., about 3 ⁇ or more, about 4 ⁇ or more, about 5 ⁇ or more, about 6 ⁇ or more, about 7 ⁇ or more, about 8 ⁇ or more, about 9 ⁇ or more, about 10 ⁇ or more). More generally, however, projection objectives can be designed to provide a magnified image or an image the same size as the object.
  • rays 152 define a cone of light paths that form the reticle image at image plane 102 .
  • the angle of the cone of rays is related to the image-side numerical aperture (NA) of projection objective 101 .
  • Image-side NA can be expressed as
  • n o refers to the refractive index of the immersing medium adjacent the surface of substrate 150 (e.g., air, nitrogen, water, or evacuated environment)
  • ⁇ max is the half-angle of the maximum cone of image forming rays from projection objective 101 .
  • projection objective 101 can have an image side NA of about 0.1 or more (e.g., about 0.15 or more, about 0.2 or more, about 0.25 or more, about 0.28 or more, about 0.3 or more, about 0.35 or more).
  • projection objective 101 has a relatively high image-side NA.
  • projection objective 101 has an image-side NA of more than 0.4 (e.g., about 0.45 or more, about 0.5 or more, about 0.55 or more, about 0.6 or more).
  • the resolution of projection objective 101 varies depending on wavelength ⁇ and the image-side NA. Without wishing to be bound by theory, the resolution of a projection objective can be determined based on the wavelength and image-side NA based on the formula,
  • R is the minimum dimension that can be printed and k is a dimensionless constant called the process factor.
  • k varies depending on various factors associated with the radiation (e.g., the polarization properties), the illumination properties (e.g., partial coherence, annular illumination, dipole settings, quadrupole settings, etc.) and the resist material.
  • k is in a range from about 0.4 to about 0.8, but can also be below 0.4 and higher than 0.8 for certain applications.
  • Projection objective 101 is also nominally telecentric at the image plane.
  • the chief rays can deviate by about 0.5° or less (e.g., about 0.4° or less, about 0.3° or less, about 0.2° or less, about 0.1° or less, about 0.05° or less, 0.01° or less, 0.001° or less) from being parallel to each other at the image plane over the exposed field.
  • projection objective 101 can provide substantially constant magnification over a range of image-size working distances.
  • the chief rays are nominally orthogonal to image plane 102 .
  • a non flat topography of the wafer surface or defocusing of the image plane does not lead necessarily to distortion or shading effects in the image plane.
  • projection objective 101 has a relatively high resolution (i.e., the value of R can be relatively small).
  • R can be about 150 nm or less (e.g., about 130 nm or less, about 100 nm or less, about 75 nm or less, about 50 nm or less, about 40 nm or less, about 35 nm or less, about 32 nm or less, about 30 nm or less, about 28 nm or less, about 25 nm or less, about 22 nm or less, about 20 nm or less, about 18 nm or less, about 17 nm or less, about 16 nm or less, about 15 nm or less, about 14 nm or less, about 13 nm or less, about 12 nm or less, about 11 nm or less, such as about 10 nm).
  • the quality of images formed by projection objective 101 can be quantified in a variety of different ways. For example, images can be characterized based on the measured or calculated departures of the image from idealized conditions associated with Gaussian optics. These departures are generally known as aberrations.
  • One metric used to quantify the deviation of a wavefront from the ideal or desired shape is the root-mean-square wavefront error (W rms ). W rms is defined in the “Handbook of Optics,” Vol. I, 2 nd Ed., edited by Michael Bass (McGraw-Hill, Inc., 1995), at page 35.3, which is incorporated herein by reference.
  • projection objective 101 can have a relatively small W rms for images at image plane 102 .
  • projection objective 101 can have a W rms of about 0.1 ⁇ or less (e.g., about 0.07 ⁇ or less, about 0.06 ⁇ or less, about 0.05 ⁇ or less, about 0.045 ⁇ or less, about 0.04 ⁇ or less, about 0.035 ⁇ or less, about 0.03 ⁇ or less, about 0.025 ⁇ or less, about 0.02 ⁇ or less, about 0.015 ⁇ or less, about 0.01 ⁇ or less, such as about 0.005 ⁇ ).
  • field curvature refers to the peak-to-valley distance for the field point dependent position of the focal plane.
  • projection objective 101 can have a relatively small field curvature for images at image plane 102 .
  • projection objective 101 can have an image-side field curvature of about 50 nm or less (e.g., about 30 nm or less, about 20 nm or less, about 15 nm or less, about 12 nm or less, 10 nm or less).
  • Distortion refers to the maximum absolute value of the field point dependent deviation from the ideal image point position in the image plane.
  • projection objective 101 can have a relatively small maximum distortion.
  • projection objective 101 can have a maximum distortion of about 50 nm or less, (e.g.
  • about 40 nm or less about 30 nm or less, about 20 nm or less, about 15 nm or less, about 12 nm or less, 10 nm or less, 9 nm or less, 8 nm or less, 7 nm or less, 6 nm or less, 5 nm or less, 4 nm or less, 3 nm or less, 2 nm or less, such as 1 nm).
  • distortion can vary by a relatively small amount across the image field.
  • distortion can vary by about 5 nm or less (e.g., about 4 nm or less, about 3 nm or less, about 2 nm or less, about 1 nm or less) across the image field.
  • projection objective 101 includes a number of mirrors arranged to direct radiation reflected from reticle 140 to substrate 150 in a way that forms an image of reticle 140 on the surface of substrate 150 .
  • Specific designs of projection objectives are described below. More generally, however, the number, size, and structure of the mirrors generally depends on the desired optical properties of projection objective 101 and the physical constraints of tool 100 .
  • the number of mirrors in projection objective 101 may vary.
  • the number of mirrors is related to various performance trade-offs associated with the optical performance characteristics of the objective, such as the desired throughput (e.g., the intensity of radiation from the object that forms the image at image plane 102 ), the desired image-side NA and related image resolution, and the desired maximum pupil obscuration.
  • projection objective 101 has at least four mirrors (e.g., five or more mirrors, six or more mirrors, seven or more mirrors, eight or more mirrors, nine or more mirrors, ten or more mirrors, eleven or more mirrors, twelve or more mirrors).
  • objective 101 will typically have an even number of mirrors (e.g., four mirrors, six mirrors, eight mirrors, ten mirrors).
  • an odd number of mirrors can be used where all the mirrors of the projection objective are positioned between the object plane and image plane.
  • a projection objective can include an odd number of mirrors where all the mirrors are positioned between the object plane and image plane.
  • At least one of the mirrors in projection objective 101 has a freeform surface.
  • freeform mirror surfaces do not have an axis of rotational symmetry.
  • a freeform surface deviates from a best fit rotationally symmetric surface (e.g., a spherical or aspherical surface).
  • Rotationally-symmetric reference surfaces can be determined for a freeform mirror surface as follows. First, one obtains information that characterizes the freeform mirror surface under consideration. In embodiments where optical data of the mirror is known, this information includes determining the basic radius of the mirror (e.g.
  • the information characterizing the mirror can be obtained from a surface figure measurement of the mirror surface (e.g. obtained using an interferometer).
  • a surface figure measurement can provide a function z′(x′, y′) describing the mirror's surface, where z′ is the sag of the mirror surface along the z′-axis for different (x′, y′) coordinates, as illustrated in FIG. 2B .
  • the initial step also includes determining the footprint for the mirror, which refers to an area of the mirror surface that is actually used to reflect image-forming radiation in the objective. The footprint can be determined by tracing rays through the objective using a ray tracing program and extracting the mirror area contacted by the rays.
  • a local coordinate system for the surface is established for which decentration and tilt of the surface is zero. Setting the tilt and decentration of the surface gives a well defined starting point for an optimization algorithm to determine the reference surface and also define an axis, z′, along which the sag differences between the mirror surface and the reference surface can be determined.
  • the z′-axis is determined based on the conic constant, k, and basic radius, 1/c.
  • the z′-axis is the symmetry axis for the rotationally symmetric part of the rotationally asymmetric surface.
  • the z′-axis corresponds to the metrology axis (e.g. the interferometers optical axis).
  • FIG. 2B illustrates this for a two-dimensional section of a rotationally asymmetric mirror 201 , where the local coordinate system is denoted by the x′, y′ and z′ axes.
  • the boundaries for the footprint of the rotationally asymmetric mirror 201 are shown as x min and x max for the cross-section shown in FIG. 2B .
  • the initial reference surface is then established with respect to the coordinate system.
  • the initial reference surface has zero tilt and zero decentration.
  • the initial reference surface is either a spherical surface or a rotationally symmetric aspherical surface.
  • the initial reference surface is established by one designating a rotationally symmetric surface that approximates the rotationally asymmetric mirror surface.
  • the initial reference surface represents a starting point for an optimization algorithm.
  • the rotationally symmetric reference surface is a spherical surface
  • the parameters include the location of the center of the sphere within the local coordinate system, the radius, of the reference surface.
  • decentering of the sphere center from the coordinate system origin is shown by coordinates x c and z c (decentration along the y′-axis by an amount y c is not shown in FIG. 2B ).
  • the radius of the spherical surface is designated as R.
  • the parameters R, x c , y c and z c are optimized to provide a minimal value for the local distances, d i , based on the equation:
  • the parameters can include decentration and tilt of the reference surface, base radius, conical constant and aspherical coefficients. These parameters can be determined based on the equation
  • z ′ c ′ ⁇ h 2 1 + 1 - ( 1 + k ′ ) ⁇ c ′2 ⁇ h 2 + ⁇ j ⁇ A j ′ ⁇ h 2 ⁇ j ,
  • h 2 x′ 2 +y′ 2
  • A′ j are coefficients characterizing the deviation of the rotationally-symmetric reference surface from a conic surface.
  • the number of aspherical coefficients, A′ j used to fit the reference surface to the mirror surface can vary depending on the computational power of the system being used to calculate the surface, the time available, and the desired level of accuracy.
  • the reference surface can be calculated using aspherical coefficients up to third order.
  • coefficients higher than third order e.g., fourth order, sixth order are used.
  • fitting can be performed using a variety of optimization algorithms.
  • a least squares fitting algorithm such as a damped least squares fitting algorithm
  • Damped least squares fitting algorithms may be performed using commercially-available optical design software, such as Code V or ZEMAX (available from Optima Research, Ltd., Stansted, United Kingdom) for example.
  • the local distance between additional points on the mirror surface can be determined and visualized. Additional characteristics of the rotationally-symmetric reference surface can be determined. For example, a maximum deviation of the rotationally-symmetric reference surface from the rotationally-asymmetric mirror surface can be determined.
  • a freeform surface can, for example, have a maximum deviation from a best fit sphere of about ⁇ or more (e.g., about 10 ⁇ or more, about 20 ⁇ or more, about 50 ⁇ or more, about 100 ⁇ or more, about 150 ⁇ or more, about 200 ⁇ or more, about 500 ⁇ or more, about 1,000 ⁇ or more, about 10,000 ⁇ or more, about 50,000 ⁇ or more).
  • a freeform surface can have, for example, a maximum deviation from a best fit rotationally symmetric asphere of about ⁇ or more (e.g., about 5 ⁇ or more, about 10 ⁇ or more, about 20 ⁇ or more, about 50 ⁇ or more, about 100 ⁇ or more, about 200 ⁇ or more, about 500 ⁇ or more, about 1,000 ⁇ or more, about 10,000 ⁇ or more).
  • a freeform surface can have a maximum deviation from a best fit rotationally symmetric asphere of about 1,000 ⁇ or less (e.g., about 900 ⁇ or less, about 800 ⁇ or less, about 700 ⁇ or less, about 600 ⁇ or less, about 500 ⁇ or less).
  • freeform surfaces have a maximum deviation from a best fit sphere by 10 nm or more (e.g., about 100 nm or more, about 500 nm or more, about 1 ⁇ m or more, about 5 ⁇ m or more, about 10 ⁇ m or more, about 50 ⁇ m or more, about 100 ⁇ m or more, about 200 ⁇ m or more, about 500 ⁇ m or more, about 1,000 ⁇ m, about 2,000 ⁇ m or more, about 3,000 ⁇ m or more).
  • 10 nm or more e.g., about 100 nm or more, about 500 nm or more, about 1 ⁇ m or more, about 5 ⁇ m or more, about 10 ⁇ m or more, about 50 ⁇ m or more, about 100 ⁇ m or more, about 200 ⁇ m or more, about 500 ⁇ m or more, about 1,000 ⁇ m, about 2,000 ⁇ m or more, about 3,000 ⁇ m or more.
  • Freeform surfaces can have a maximum deviation from a best fit sphere by about 10 mm or less (e.g., about 5 mm or less, about 3 mm or less, about 2 mm or less, about 1 mm or less, about 500 ⁇ m or less).
  • Freeform surfaces can have a maximum deviation from a best fit rotationally symmetric asphere by 10 nm or more (e.g., about 100 nm or more, about 500 nm or more, about 1 ⁇ m or more, about 5 ⁇ m or more, about 10 ⁇ m or more, about 50 ⁇ m or more, about 100 ⁇ m or more, about 200 ⁇ m or more, about 500 ⁇ m or more, about 1,000 ⁇ m).
  • Freeform surfaces can have a maximum deviation from a best fit rotationally symmetric asphere by about 10 mm or less (e.g., about 5 mm or less, about 3 mm or less, about 2 mm or less, about 1 mm or less, about 500 ⁇ m or less).
  • the curvature of the mirror surfaces is characterized by a first and second mean principal curvature, which are determined at the point on each mirror surface that reflects the chief ray of the central field point.
  • First and second principal curvatures are calculated as described in Handbook of Mathematics by I. N. Bronstein, et al., 4 th Ed. (Springer, 2004), p. 567.
  • the first principal curvature for a mirror can be different from the second principal curvature for that mirror.
  • the absolute value of the difference between the first and second principal curvatures can be about 10 ⁇ 8 or more (e.g., 10 ⁇ 7 or more, 5 ⁇ 10 ⁇ 7 or more, about 10 ⁇ 6 or more, about 5 ⁇ 10 ⁇ 6 or more, about 10 ⁇ 5 or more, about 5 ⁇ 10 ⁇ 5 or more, about 10 ⁇ 4 or more, about 5 ⁇ 10 ⁇ 4 or more, about 10 ⁇ 3 or more).
  • the first and/or second principal curvatures can be positive or negative.
  • the first and/or second principal curvatures for a mirror surface can be relatively small.
  • the absolute value of the first principal curvature for one or more mirrors in projection objective 101 is about 10 ⁇ 2 or less (e.g., about 5 ⁇ 10 ⁇ 3 or less, about 3 ⁇ 10 ⁇ 3 or less, about 2 ⁇ 10 ⁇ 3 or less, about 10 ⁇ 3 or less).
  • the absolute value of the sum of the first principal curvatures for the mirrors in projective objective 101 can be about 10 ⁇ 3 or less (e.g., about 5 ⁇ 10 ⁇ 4 or less, about 3 ⁇ 10 ⁇ 4 , about 2 ⁇ 10 ⁇ 4 or less, about 10 ⁇ 4 or less, 5 ⁇ 10 ⁇ 5 or less, 10 ⁇ 5 or less).
  • the absolute value of the second principal curvature for one or more mirrors in projection objective 101 is about 10 ⁇ 2 or less (e.g., about 5 ⁇ 10 ⁇ 3 or less, about 3 ⁇ 10 ⁇ 3 or less, about 2 ⁇ 10 ⁇ 3 or less, about 10 ⁇ 3 or less).
  • the absolute value of the sum of the second principal curvatures for the mirrors in projective objective 101 can be about 10 ⁇ 3 or less (e.g., about 5 ⁇ 10 ⁇ 4 or less, about 3 ⁇ 10 ⁇ 4 , about 2 ⁇ 10 ⁇ 4 or less, about 10 ⁇ 4 or less, 5 ⁇ 10 ⁇ 5 or less, 10 ⁇ 5 or less).
  • the sum of the first and second principal curvatures of the mirrors in projection objective 101 can be relatively small.
  • the absolute value of the sum of the first and second principal curvatures of the mirrors can be about 10 ⁇ 3 or less (e.g., about 5 ⁇ 10 ⁇ 4 or less, about 3 ⁇ 10 ⁇ 4 , about 2 ⁇ 10 ⁇ 4 or less, about 10 ⁇ 4 or less, 5 ⁇ 10 ⁇ 5 or less, 10 ⁇ 5 or less).
  • freeform mirror surfaces can be described mathematically by the equation:
  • Z is the sag of the surface parallel to a Z-axis (which may or may not be parallel to the reference axis 105 in projection objective 101 , i.e. in general is decentered and tilted to the reference axis 105 in projection objective 101 ),
  • c is a constant corresponding to the vertex curvature
  • k is a conic constant
  • C j is the coefficient of the monomial X m Y n .
  • the values of c, k, and C j are determined based on the desired optical properties of the mirror with respect to projection objective 101 . Further, the order of the monomial, m+n, can vary as desired.
  • m+n is 10 or more (e.g., 15 or more, 20 or more).
  • the parameters for the freeform mirror equation can be determined using commercially-available optical design software.
  • m+n is less than 10 (e.g., 9 or less, 8 or less, 7 or less, 6 or less, 5 or less, 4 or less, 3 or less).
  • freeform surfaces can be described mathematically using equations other than those presented above.
  • freeform surfaces can be described mathematically using Zernike polynomials (such as presented in the manual for CODE V®, commercially available from Optical Research Associates, Pasadena, Calif.) or using two-dimensional spline surfaces.
  • Zernike polynomials such as presented in the manual for CODE V®, commercially available from Optical Research Associates, Pasadena, Calif.
  • two-dimensional spline surfaces are Bezier splines or non-uniform rational Bezier splines (NURBS).
  • Two-dimensional spline surfaces can be described, for example, by a grid of points in an x-y plane and corresponding z-values or slopes and these points.
  • the complete surface is obtained by a specific interpolation between the grid points using, e.g., polynomials or functions that have certain properties with respect to continuity or differentiability (e.g., analytical functions).
  • the number and position of freeform mirrors in projection objective 101 can vary.
  • Embodiments include projection objectives with two or more freeform mirrors (e.g., three or more freeform mirrors, four or more freeform mirrors, five or more freeform mirrors, six or more freeform mirrors).
  • Projection objective 101 generally includes one or more mirrors with positive optical power.
  • the reflective portion of the mirror has a concave surface and is referred to as a concave mirror.
  • Projection objective 101 can include two or more (e.g., three or more, four or more, five or more, six or more) concave mirrors.
  • Projection objective 101 can also include one or more mirrors that have negative optical power. This means that one or more of the mirrors has a reflective portion with a convex surface (referred to as a convex mirror).
  • projection objective 101 includes two or more (e.g., three or more, four or more, five or more, six or more) convex mirrors.
  • projection objective 300 includes six freeform mirrors 310 , 320 , 330 , 340 , 350 , and 360 .
  • Data for projection objective 300 is presented in Table 1A and Table 1B below.
  • Table 1A presents optical data
  • Table 1B presents freeform constants for each of the mirror surfaces.
  • mirror 1 corresponds to mirror 310
  • mirror 2 corresponds to mirror 320
  • mirror 3 corresponds to mirror 330
  • mirror 4 corresponds to mirror 340
  • mirror 5 corresponds to mirror 350
  • mirror 6 corresponds to mirror 360 .
  • “Thickness” in Table 1A and subsequent tables refers to the distance between adjacent elements in the radiation path.
  • the monomial coefficients, C j for the freeform mirrors, along with the amount the mirror is decentered and rotated (or tilted) from an initial projection objective design, are provided in Table 1B.
  • R the radius
  • Decenter is given in mm and rotation is given in degrees.
  • Units for the monomial coefficients are mm ⁇ j+1 .
  • Nradius is a unitless scaling factor (see, for example, the manual for CODE V®).
  • projection objective 300 is shown in meridional section.
  • the meridional plane is a symmetry plane for projection objective 300 . Symmetry about the meridional plane is as the mirrors are decentered only with respect to the y-axis and tilted about the x-axis. Further, the coefficients for the freeform mirrors having an odd degree in the x-coordinate (e.g., x, x 3 , x 5 , etc.) are zero.
  • Projection objective 300 is configured for operation with 13.5 nm radiation and has an image-side NA of 0.35 and a tracklength of 1,500 mm.
  • the optical path length of imaged radiation is 3.833 mm. Accordingly, the ratio of optical path length to tracklength is approximately 2.56.
  • Projection objective has a demagnification of 4 ⁇ , a maximum distortion of less than 100 nm, W rms of 0.035 ⁇ , and a field curvature of 28 nm. Additional characteristics of projection objective 300 are presented in the discussion of projection objective 101 that follows.
  • the first mirror in the radiation path from object plane 103 , mirror 310 has positive optical power.
  • Mirrors 320 , 340 , and 360 are also P mirrors.
  • Mirrors 330 and 350 have (N) negative optical power.
  • the sequence of mirrors in the radiation path in projection objective 300 is thus PPNPNP.
  • the maximum deviation of the freeform surfaces from a best fit sphere for each mirror is as follows: 154 ⁇ m for mirror 310 ; 43 ⁇ m for mirror 320 , 240 ⁇ m for mirror 330 ; 1.110 ⁇ m for mirror 340 ; 440 ⁇ m for mirror 350 ; and 712 ⁇ m for mirror 360 .
  • the maximum deviation of the freeform surfaces from a best fit rotationally symmetric asphere is 47 ⁇ m for mirror 310 ; 33 ⁇ m for mirror 320 , 96 ⁇ m for mirror 330 ; 35 ⁇ m for mirror 340 ; 152 ⁇ m for mirror 350 ; and 180 ⁇ m for mirror 360 .
  • the first and second principal curvature for mirror 310 are 9.51 ⁇ 10 ⁇ 4 and 9.30 ⁇ 10 ⁇ 4 respectively.
  • Respective first and second principal curvatures for the other mirrors in projection objective 300 are as follows: 2.76 ⁇ 10 ⁇ 5 and 1.56 ⁇ 10 ⁇ 5 for mirror 320 ; ⁇ 2.38 ⁇ 10 ⁇ 3 and ⁇ 2.17 ⁇ 10 ⁇ 3 for mirror 330 ; 1.79 ⁇ 10 ⁇ 3 and 1.75 ⁇ 10 ⁇ 3 for mirror 340 ; ⁇ 2.64 ⁇ 10 ⁇ 3 and ⁇ 2.10 ⁇ 10 ⁇ 3 for mirror 350 ; and 1.93 ⁇ 10 ⁇ 3 and 1.91 ⁇ 10 ⁇ 3 for mirror 360 .
  • the sum of the first principal curvature for projection objective 300 is ⁇ 3.19 ⁇ 10 ⁇ 4 .
  • the sum of the second principal curvature is 3.29 ⁇ 10 ⁇ 4 .
  • the sum of the first and second principal curvatures is 9.97 ⁇ 10 ⁇ 6 and the inverse sum of the first and second principal curvatures is 1.00 ⁇ 10
  • the arrangement of mirrors in projection objective 101 images radiation from object plane 103 to one or more intermediate-image planes.
  • projection objective 300 images radiation from object plane 103 to an intermediate image at a location 305 near mirror 360 .
  • embodiments that have one or more intermediate images also include two or more pupil planes. In some embodiments, at least one of these pupil planes is physically accessible for the purposes of placing an aperture stop substantially at that pupil plane. An aperture stop is used to define the size of the projection objective's aperture.
  • Coma at an intermediate image in projection objective 101 can be relatively large.
  • Coma can be quantified by the distance between the chief ray and the upper and lower rays at the point where the upper and lower rays cross. In some embodiments, this distance can be about 1 mm or more (e.g., about 2 mm or more, about 3 mm or more, about 4 mm or more, about 5 mm or more, about 6 mm or more, such as about 7 mm).
  • Coma at an intermediate image in projection objective can be relatively small. In some embodiments, the distance can be about 1 mm or less (e.g., about 0.1 mm or less, 0.01 mm or less).
  • mirrors in projection objective 101 are formed so that they reflect a substantial amount of radiation of wavelength ⁇ normally-incident thereon or incident thereon over a certain range of incident angles.
  • Mirrors can be formed, for example, so that they reflect about 50% or more (e.g., about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, 98% or more) of normally incident radiation at ⁇ .
  • the mirrors include a multilayer stack of films of different materials arranged to substantially reflect normally incident radiation at ⁇ .
  • Each film in the stack can have an optical thickness of about ⁇ /4.
  • Multilayer stacks can include about 20 or more (e.g., about 30 or more, about 40 or more, about 50 or more) films.
  • the materials used to form the multilayer stacks are selected based on operational wavelength ⁇ .
  • multiple alternating films of molybdenum and silicon or molybdenum and beryllium can be used to form mirrors for reflecting radiation in the 10 nm to 30 nm range (e.g., for ⁇ of about 13 nm or about 11 nm, respectively).
  • the mirrors are made of quartz glass coated with a single layer of aluminum and overcoated with one or more layers of dielectric materials, such as layers formed from MgF 2 , LaF 2 , or, Al 2 O 3 .
  • Mirrors formed from aluminum with dielectric coatings can be used, for example, for radiation having a wavelength of about 193 nm.
  • the percentage of radiation at ⁇ reflected by a mirror varies as a function of the angle of incidence of the radiation on the mirror surface. Because imaged radiation propagates through a catoptric projection objective along a number of different paths, the angle of incidence of the radiation on each mirror can vary. This effect is illustrated with reference to FIG. 4 , which shows a portion of a mirror 400 , in meridional section, that includes a concave reflective surface 401 . Imaged radiation is incident on surface 401 along a number of different paths, including the paths shown by rays 410 , 420 , and 430 . Rays 410 , 420 , and 430 are incident on portions of surface 401 where the surface normal is different.
  • the direction of surface normal at these portions is shown by lines 411 , 421 , and 431 , corresponding to rays 410 , 420 , and 430 , respectively.
  • Rays 410 , 420 , and 430 are incident on surface 401 at angles ⁇ 410 , ⁇ 420 , and ⁇ 430 , respectively.
  • angles ⁇ 410 , ⁇ 420 , and ⁇ 430 may vary.
  • the incident angles of imaged radiation can be characterized in a variety of ways.
  • One characterization is the maximum angle of incidence of meridional rays on each mirror in a meridional section of projection objective 101 .
  • Meridional rays refer to rays lying in the meridional section.
  • ⁇ max can vary for different mirrors in projection objective 101 .
  • the maximum value of ⁇ max for all the mirrors in projection objective 101 is about 75° or less (e.g., about 70° or less, about 65° or less, about 60° or less, about 55° or less, about 50° or less, about 45° or less).
  • ⁇ max can be more than about 5° (e.g., about 100 or more, about 20° or more).
  • the maximum value of ⁇ max can be relatively low.
  • the maximum value of ⁇ max can be about 40° or less (e.g., about 35° or less, about 30° or less, about 25° or less, about 20° or less, about 15° or less, about 13° or less, about 100 or less).
  • ⁇ max for mirror 310 is 8.22°
  • ⁇ max for mirror 320 is 10.38°
  • ⁇ max for mirror 330 is 22.35°
  • ⁇ max for mirror 340 is 7.49°
  • ⁇ max for mirror 350 is 24.58°
  • ⁇ max for mirror 360 is 6.15°.
  • the ratio of the maximum value of ⁇ max (in degrees) to image-side NA can be about 100 or less (e.g., about 80 or less, about 70 or less, 68 or less, about 60 or less, about 50 or less, about 40 or less, about 30 or less).
  • ⁇ CR Another characterization is the angle of incidence of the chief ray corresponding to the central field point on each mirror in a meridional section of projection objective 101 .
  • This angle is referred to as ⁇ CR .
  • ⁇ CR can vary.
  • mirror 310 has ⁇ CR of 6.59°
  • mirror 320 has ⁇ CR of 7.93°
  • mirror 330 has ⁇ CR of 20.00°
  • mirror 340 has ⁇ CR of 7.13°
  • mirror 350 has ⁇ CR of 13.06°
  • mirror 360 has ⁇ CR of 5.02°.
  • the maximum value of ⁇ CR , ⁇ CR (max) in projection objective 101 can be relatively low.
  • ⁇ CR (max) can be about 35° or less (e.g., about 30° or less, about 25° or less, about 20° or less, about 15° or less, about 13° or less, about 10° or less, about 8° or less, about 5° or less).
  • ⁇ CR (max) which is ⁇ CR for mirror 330 , is 20.00°.
  • the ratio of the maximum value of ⁇ CR (max) (in degrees) to image-side NA can be about 100 or less (e.g., about 80 or less, about 70 or less, 68 or less, about 60 or less, about 50 or less, about 40 or less, about 30 or less).
  • Each mirror in projection objective 101 can also be characterized by the range of angles of incidence, ⁇ , of rays for a meridional section of projection objective 101 .
  • ⁇ corresponds to the difference between ⁇ max and ⁇ min , where ⁇ min is the minimum angle of incidence of rays on each mirror in a meridional section of projection objective 101 .
  • ⁇ may vary for each mirror in projection objective 101 .
  • ⁇ can be relatively small.
  • ⁇ can be about 20° or less (e.g., about 15° or less, about 12° or less, about 10° or less, about 8° or less, about 5° or less, about 3° or less, 2° or less).
  • ⁇ can be relatively large.
  • ⁇ for some mirrors can be about 20° or more (e.g., about 25° or more, about 30° or more, about 35° or more, about 40° or more).
  • ⁇ max for mirror 310 is 3.34°
  • ⁇ max for mirror 320 is 4.92°
  • ⁇ max for mirror 330 is 5.18°
  • ⁇ max for mirror 340 is 0.98°
  • ⁇ max for mirror 350 is 24.07°
  • ⁇ max for mirror 360 is 2.77°.
  • the maximum value for ⁇ , ⁇ max , for all the mirrors in projection objective 101 can be relatively small.
  • ⁇ max can be about 25° or less (e.g., about 20° or less, about 15° or less, about 12° or less, about 10° or less, about 9° or less, about 8° or less, about 7° or less, about 6° or less, about 5° or less, such as 3°).
  • ⁇ max is 24.07°.
  • Another way to characterize the radiation path in projection objective 101 is by the chief ray magnification at each mirror, which refers to the quotient of the tangent of the angle between the chief ray (e.g. in the meridional section) and reference axis 105 before and after reflection from each mirror.
  • the chief ray magnification at each mirror refers to the quotient of the tangent of the angle between the chief ray (e.g. in the meridional section) and reference axis 105 before and after reflection from each mirror.
  • mirror 520 has a negative chief ray angle magnification.
  • the chief ray magnification is given by tan( ⁇ )/tan( ⁇ ).
  • having multiple mirrors with positive chief ray angle magnification can correspond to relatively large incident angles on one or more mirrors in the projection objective. Accordingly, projection objectives having only one mirror with positive chief ray angle magnification can also exhibit relatively low incident ray angles on the mirrors.
  • mirrors 310 , 320 , 330 and 350 have negative chief ray angle magnifications
  • mirror 340 has positive chief ray angle magnification.
  • projection objective 101 can vary depending on the specific design of the projection objective. Relatively large distances between adjacent mirrors (with respect to the path of the radiation) can correspond to relatively low incident ray angles on the mirrors.
  • projection objective 101 can include at least one pair of adjacent mirrors that separated by more than 50% of the projection objective tracklength. For example, in projection objective 300 , mirrors 340 and 350 are separated by more than 50% of the projection objective's track length.
  • having a large relative distance, d op-1 , between the object plane and the first mirror in the radiation path compared to the distance, d op-2 , between the object plane and the second mirror in the radiation path can also correspond to relatively low incident ray angles on the mirrors.
  • d op-1 /d op-2 is about 2 or more (e.g., about 2.5 or more, about 3 or more, about 3.5 or more, about 4 or more, about 4.5 or more, about 5 or more) can also have relatively low incident ray angles.
  • d op-1 /d op-2 is 2.38.
  • the footprint size and shape of the mirrors in projection objective 101 can vary.
  • the footprint shape refers to the shape of the mirror projected onto the x-y plane of the local coordinate system of the surface.
  • the footprint of the mirrors can be circular, oval, polygonal (e.g., rectangular, square, hexagonal), or irregular in shape.
  • the footprint is symmetric with respect to the meridional plane of projection objective 101 .
  • mirrors can have a footprint with a maximum dimension that is about 1,500 mm or less (e.g., about 1,400 nm or less, about 1,300 mm or less, about 1,200 mm or less, about 1,100 mm or less, about 1,000 mm or less, about 900 mm or less, about 800 mm or less, about 700 mm or less, about 600 mm or less, about 500 mm or less, about 400 mm or less, about 300 mm or less, about 200 mm or less, about 100 mm or less.)
  • Mirrors may have footprint that has a maximum dimension that is more than about 10 mm (e.g., about 20 mm or more, about 50 mm or more).
  • M x can be about 1,500 mm or less (e.g., about 1,400 nm or less, about 1,300 mm or less, about 1,200 mm or less, about 1,100 mm or less, about 1,000 mm or less, about 900 mm or less, about 800 mm or less, about 700 mm or less, about 600 mm or less, about 500 mm or less, about 400 mm or less, about 300 mm or less, about 200 mm or less, about 100 mm or less).
  • M x can be more than about 10 mm (e.g., about 20 mm or more, about 50 mm or more).
  • Mirror 600 is symmetric with respect to meridian 601 .
  • Mirror 600 has a dimension M y along meridian 601 .
  • M y is smaller than M x , however, more generally, M y can be smaller, the same size, or larger than M x .
  • M y is in a range from about 0.1 M x to about M x (e.g., about 0.2 M x or more, about 0.3 M x or more, about 0.4 M x or more, about 0.5 M x or more, about 0.6 M x or more, about 0.7 M x or more about 0.8 M x or more, about 0.9 M x or more).
  • M y can be about 1.1 M x or more (e.g., about 1.5 M x or more), such as in a range from about 2 M x to about 10 M x .
  • M y can be about 1,000 mm or less (e.g., about 900 mm or less, about 800 mm or less, about 700 mm or less, about 600 mm or less, about 500 mm or less, about 400 mm or less, about 300 mm or less, about 200 mm or less, about 100 mm or less).
  • M y can be more than about 10 mm (e.g., about 20 mm or more, about 50 mm or more).
  • M x and M y for mirror 310 are 303 mm and 139 mm, respectively; M x and M y for mirror 320 are 187 mm and 105 mm, respectively; M x and M y for mirror 330 are 114 mm and 62 mm, respectively; M x and M y for mirror 340 are 299 mm and 118 mm, respectively; M x and M y for mirror 350 are 99 mm and 71 mm, respectively; and M x and M y for mirror 360 are 358 mm and 332 mm, respectively.
  • the base of the mirrors may extend beyond the mirror surface (i.e., the portion of the mirror that reflects imaged radiation) in one or more directions.
  • a mirror's base can extend about 10 mm or more (e.g., about 20 mm or more, about 30 mm or more, about 40 mm or more, about 50 mm or more) beyond the optically active surface in the x- and/or y-directions.
  • Mirror base extension can facilitate mounting the mirror in projection objective 101 by providing surfaces that are not optically active that can be attached to mounting apparatus.
  • mirror base extensions should not be in a direction that occludes the radiation path in projection objective 101 .
  • the distance between the edge of a mirror and the radiation path as it passes the mirror is related to a parameter referred to as the “freeboard,” which is the minimum distance between the rays closest to a mirror's edge and the rays nearest the mirror's edge that are reflected by the mirror.
  • projection objective 101 can include one or more mirrors with freeboards of about 20 mm or more (e.g., about 25 mm or more, about 30 mm or more, about 35 mm or more, about 40 mm or more, about 45 mm or more, about 50 mm or more).
  • projection objective 101 can include one or more mirrors with freeboards of about 15 mm or less (e.g., about 12 mm or less, about 10 mm or less, about 8 mm or less, about 5 mm or less). In certain embodiments, projection objective 101 includes one or more mirrors having a freeboard between 5 mm and 25 mm. For example, in projection objective 300 , mirrors 310 , 320 , 330 , 350 , and 360 have freeboards between 5 mm and 25 mm.
  • the thickness of the mirrors in projection objective 101 may vary.
  • Mirror thickness refers to the dimension of the mirror along the z-axis. Mirrors should generally have sufficient thickness to facilitate mounting within the projection objective.
  • the thickness of mirror 600 can be characterized by a maximum thickness, T max , and a minimum thickness, T min . Typically, the difference between T max and T min will depend on the curvature of the mirror surface and the structure of the mirror's base.
  • T max is about 200 mm or less (e.g., about 150 mm or less, about 100 mm or less, about 80 mm or less, about 60 mm or less, about 50 mm or less, about 40 mm or less, about 30 mm or less, about 20 mm or less).
  • T min is about 1 mm or more (e.g., about 2 mm or more, about 5 mm or more, about 10 mm or more, about 20 mm or more, about 50 mm or more, about 100 mm or more).
  • the maximum dimension of any mirror in projection objective is about 1,500 mm or less (e.g., about 1,400 nm or less, about 1,300 mm or less, about 1,200 mm or less, about 1,100 mm or less, about 1,000 mm or less, about 900 mm or less, about 800 mm or less, about 700 mm or less, about 600 mm or less, about 500 mm or less, such as about 300 mm).
  • the maximum dimension of any mirror in projection objective is about 10 mm or more (e.g., about 20 mm or more, about 30 mm or more, about 40 mm or more, about 50 mm or more, about 75 mm or more, about 100 mm or more).
  • the shape of the field of projection objective 101 can vary.
  • the field has an arcuate shape, such as the shape of a segment of a ring.
  • a ring-segment field 700 can be characterized by an x-dimension, d x , a y-dimension, d y , and a radial dimension, d r .
  • d x and d y correspond to the dimension of the field along the x-direction and y-direction, respectively.
  • d r corresponds to the ring radius, as measured from an axis 705 to the inner boundary of field 700 .
  • Ring-segment field 700 is symmetric with respect the y-z plane and indicated by line 710 .
  • the sizes of d x , d y , and d r vary depending on the design of projection objective 101 .
  • d y is smaller than d x .
  • the relative sizes of field dimensions d x , d y , and d r at object plane 103 and image plane 102 vary depending on the magnification or demagnification of projection objective 101 .
  • d x is relatively large at image plane 102 .
  • d x at image plane 102 can be more than 1 mm (e.g., about 3 mm or more, about 4 mm or more, about 5 mm or more, about 6 mm or more, about 7 mm or more, about 8 mm or more, about 9 mm or more, about 10 mm or more, about 11 mm or more, about 12 mm or more, about 13 mm or more, about 14 mm or more, about 15 mm or more, about 18 mm or more, about 20 mm or more, about 25 mm or more).
  • d x can be about 100 mm or less (e.g., about 50 mm or less, about 30 mm or less).
  • d y at image plane 102 can be in a range from about 0.5 mm to about 5 mm (e.g., about 1 mm, about 2 mm, about 3 mm, about 4 mm).
  • d r at image plane 102 is about 10 mm or more.
  • d r can be, for example, about 15 mm or more (e.g., about 20 mm or more, about 25 mm or more, about 30 mm or more) at image plane 102 .
  • d r can be extremely large (e.g., about 1 m or more, about 5 m or more, about 10 m or more).
  • the field is rectangular in shape and d r is infinite.
  • Projection objective 300 for example, has a rectangular field. Specifically, projection objective 300 has a rectangular field with a y-dimension of 2 mm and an x-dimension of 26 mm.
  • projection objective 101 can have a maximum field dimension of more than 1 mm (e.g., about 3 mm or more, about 4 mm or more, about 5 mm or more, about 6 mm or more, about 7 mm or more, about 8 mm or more, about 9 mm or more, about 10 mm or more, about 11 mm or more, about 12 mm or more, about 13 mm or more, about 14 mm or more, about 15 mm or more, about 18 mm or more, about 20 mm or more, about 25 mm or more) at image plane 102 .
  • projection objective has a maximum field dimension of no more than about 100 mm (e.g., about 50 mm or less, about 30 mm or less).
  • the image field shape can correspond (e.g., in one or more dimensions) to the shape of die sites on a wafer that is exposed using projection objective 101 .
  • the image field can be shaped to reduce overscan when exposing the wafer. Overscan refers to the need to scan the image field beyond the edge of a die site to expose the entire site. Generally, this occurs where the shape of the image field does not conform to the shape of die site.
  • Overscan can be characterized by the ratio (e.g., expressed as a percentage) of the maximum distance between the leading edge of the image field and the trailing edge of the die site at the position where the corners at the trailing edge of the die site are exposed.
  • overscan corresponds to the ratio of d os to d y , where d os is the distance between the leading edge of image field 700 and the trailing edge of die sites 720 at the position where corners 721 and 722 are exposed.
  • projection objective can have relatively low overscan.
  • projection objective can have an overscan of about 5% or less (e.g., about 4% or less, about 3% or less, about 2% or less, about 1% or less, about 0.5% or less, 0.1% or less).
  • projection objective 101 can be used with zero overscan.
  • scanning can be achieved with zero overscan.
  • projection objective 101 introduces an object-image shift, d ois , that varies depending on the specific design of the projection objective.
  • the object-image shift refers to the distance of a point in the image field from the corresponding point in the object field, as measured in the x-y plane.
  • the object-image shift can be calculated using the formula:
  • h o refers to the distance in the x-y plane of the central field point in the object field from the optical axis and M is the projection objective magnification ratio.
  • M is the projection objective magnification ratio.
  • d o is 90 mm.
  • Projection objective 101 has a relatively small object-image shift.
  • projection objective has zero object-image shift.
  • Projection objectives having relatively small object image shifts can be have a relatively slim optical design.
  • projection objective 101 can be rotated about the axis intersecting the central field points in the object and image fields without the central field point translating with respect to, e.g., stage 130 .
  • metrology tools e.g., detection optical systems, such as those disclosed in U.S. Pat. No. 6,240,158 B1
  • zero object-image shift can facilitate easier metrology and testing of projective objective 101 where the projection objective is subject to rotations during the course of operation.
  • FIG. 8A shows a view of the object field and the image field of a projection of objective having a large object-image shift d ois .
  • a central image field point C I moves from a position C IA to a position C IB .
  • the central field points C O , C I escape a detection region and therefore escape a measurement due to rotation about the rotation axis 105 . This is not desirable.
  • FIG. 8B shows the case, where the object-image shift d ois of the projection objective 101 is zero.
  • rotation axis 105 intersects the object plane 103 and the image plane 102 at the central field points C O , C I .
  • the central field points C O , C I stay within a measurement region. In that case, testing and metrology is facilitated.
  • FIGS. 8C and 8D depict a dependency of the object-image shift on the numerical aperture of the system which often is observed when comparing different types of projection objectives.
  • FIG. 8C shows schematically a projection objective 101 having a relatively small numerical aperture. In that case, most designs of projection objectives exhibit a relatively small distance h 0 between an axis through the central field point C O and the optical axis OA.
  • FIG. 8D shows the case in a projection objective 101 with high numerical aperture.
  • the projection objective often exhibits a large distance h 0 between the axis through the central object field point C O and the optical axis OA.
  • d ois is larger than in the projection objective of FIG. 8C .
  • projection objective 101 has a d ois of about 80 mm or less (e.g., about 60 mm or less, about 50 mm or less, about 40 mm or less, about 30 mm or less, about 20 mm or less, about 15 mm or less, about 12 mm or less, about 10 mm or less, about 8 mm or less, about 5 mm or less, about 4 mm or less, about 3 mm or less, about 2 mm or less, about 1 mm or less).
  • Projection objective 300 for example, has a d ois of 57 mm.
  • Embodiments of projection objective 101 can have a relatively large image-side free working distance.
  • the image-side free working distance refers to the shortest distance between image plane 102 and the mirror surface of the mirror closest to image plane 102 that reflects imaged radiation. This is illustrated in FIG. 9 , which shows a mirror 810 as the closest mirror to image plane 102 . Radiation reflects from surface 811 of mirror 810 .
  • the image-side free working distance is denoted d w .
  • d w is about 25 mm or more (e.g., about 30 mm or more, about 35 mm or more, about 40 mm or more, about 45 mm or more, about 50 mm or more about 55 mm or more, about 60 mm or more, about 65 mm or more). In certain embodiments, d w is about 200 mm or less (e.g., about 150 mm or less, about 100 mm or less, about 50 mm or less).
  • Projection objective 300 for example, has an image-side free working distance of approximately 45 mm. A relatively large working distance may be desirable because it can allow the surface of substrate 150 to be positioned at image plane 102 without contacting the side of mirror 810 facing image plane 102 .
  • the object-side free working distance refers to the shortest distance between object plane 103 and the mirror surface of the reflective side of the mirror in projection objective 101 closest to object plane 103 that reflects imaged radiation.
  • projection objective 101 has a relatively large object-side free working distance.
  • projection objective 101 can have an object-side free working distance of about 50 mm or more (e.g., about 100 mm or more, about 150 mm or more, about 200 mm or more, about 250 mm or more, about 300 mm or more, about 350 mm or more, about 400 mm or more, about 450 mm or more, about 500 mm or more, about 550 mm or more, about 600 mm or more, about 650 mm or more, about 700 mm or more, about 750 mm or more, about 800 mm or more, about 850 mm or more, about 900 mm or more, about 950 mm or more, about 1,000 mm or more).
  • object-side free working distance of about 50 mm or more (e.g., about 100 mm or more, about 150 mm or more, about 200 mm or more, about 250 mm or more, about 300 mm or more, about 350 mm or more, about 400 mm or more, about 450 mm or more, about 500 mm or more, about 550
  • the object-side free working distance is no more than about 2,000 mm (e.g., about 1,500 mm or less, about 1,200 mm or less, about 1,000 mm or less).
  • Projection objective 300 has an object-side free working distance of approximately 300 mm.
  • a relatively large object-side free working distance may be advantageous in embodiments where access to the space between projection objective 101 and object plane 103 is desired.
  • reticle 140 is a reflective reticle, it is desirable to illuminate the reticle from the side that faces objective 101 . Therefore, there should be sufficient space between projection objective 101 and object plane 103 to allow the reticle to be illuminated by illumination system 120 at a desired illumination angle.
  • a larger object-side free working distance allows flexibility in design of the rest of tool, for example, by providing sufficient space to mount other components (e.g. an uniformity filter) between projection objective 101 and the support structure for reticle 140 .
  • projection objective 101 can be designed so that chief rays either converge, diverge, or are substantially parallel to each other at reticle 140 .
  • the location of the entrance pupil of projection objective 101 with respect to object plane 103 can vary.
  • the entrance pupil is located on the image plane side of object plane 103 .
  • object plane 103 is located between the entrance pupil and image plane 102 .
  • the distance between object plane 103 and the entrance pupil can vary.
  • the entrance pupil is located about 1 m or more (e.g., about 2 m or more, about 3 m or more, about 4 m or more, about 5 m or more, about 8 m or more, about 10 m or more) from object plane 103 (measured along an axis perpendicular to object plane 103 ). In some embodiments, the entrance pupil is located at infinity with respect to object plane 103 . This corresponds to where the chief rays are parallel to each other at reticle 140 . For projection objective 300 , the incident angle of the chief ray at the central field point at object plane 103 is 7° and the maximum variation of the chief ray angle form the central field point chief ray is 0.82°. The entrance pupil is located 1,000 mm from object plane 103 on the opposite side of object plane 103 from image plane 102 .
  • Illumination system 120 may be arranged so that the exit pupil of the illumination system is positioned substantially at the entrance pupil of projection objective 101 .
  • illumination system 120 includes a telescope subsystem which projects the illumination system's exit pupil to the location of the entrance pupil of projection objective 101 .
  • the exit pupil of illumination system 120 is positioned at the entrance pupil of projection objective 101 without using a telescope in the illumination system.
  • the exit pupil of illumination system 120 may coincide with the projection objective's entrance pupil without using a telescope in the illumination system.
  • projection objective 101 can be designed using commercially available optical design software like ZEMAX, OSLO, or Code V
  • a design is started by specifying an initial projection objective design (e.g., arrangement of mirrors) along with parameters such as the radiation wavelength, field size and numerical aperture, for example.
  • the code then optimizes the design for specified optical performance criteria, such as, for example, wavefront error, distortion, telecentricity, and field curvature.
  • the initial projection objective is designated by rotationally symmetric mirrors (e.g., spherical or aspherical mirrors) that are centered on an optical axis.
  • Each mirror is then decentered from the optical axis to a position where the mirror satisfies some pre-established criterion.
  • each mirror can be decentered from the optical axis by and amount which minimizes the chief ray angle of incidence across the mirror for particular field.
  • mirrors can be decentered by about 5 mm or more (e.g., about 10 mm or more, about 20 mm or more, about 30 mm or more, about 50 mm or more).
  • mirrors are decentered by about 200 mm or less (e.g., about 180 mm or less, about 150 mm or less, about 120 mm or less, about 100 mm or less).
  • each mirror can be tilted to a position where the mirror satisfies some pre-established criterion.
  • the tilt refers to the orientation of each mirrors symmetry axis with respect to the optical axis of the initial configuration of the projection objective.
  • Mirrors can be titled by about 1° or more (e.g., about 2° or more, about 3° or more, about 4° or more, about 5° or more). In some embodiments, mirrors are tilted by about 20° or less (e.g., about 15° or less, about 12° or less, about 10° or less).
  • a freeform shape for each mirror can be determined to optimize the projection objective design for specified optical performance criteria.
  • projection objective 101 can include one or more other components, such as one or more aperture stops.
  • shape of the aperture stop can vary. Examples of aperture stops include circular aperture stops, elliptical aperture stops, and/or polygonal aperture stops. Apertures stops can also be positioned so that the image radiation makes a double pass or a single pass through the aperture stop. Aperture stops can be interchanged in projection objective 101 and/or may have an adjustable aperture.
  • projection objective 101 includes a field stop.
  • the field stop can be positioned at or near the intermediate image.
  • Embodiments can include baffles (e.g., to shield the wafer from stray radiation).
  • projection objective 101 can include components (e.g., interferometers) for monitoring changes in the position of mirrors within the projection objective. This information can be used to adjust the mirrors to correct for any relative movement between the mirrors. Mirror adjustment can be automated. Examples of systems for monitoring/adjusting mirror position are disclosed in U.S. Pat. No. 6,549,270 B1.
  • an embodiment of a projection objective 1000 includes six mirrors 1010 , 1020 , 1030 , 1040 , 1050 , and 1060 , and has an image-side numerical aperture of 0.35 and an operating wavelength of 13.5 nm.
  • Mirrors 1010 , 1020 , 1030 , 1040 , 1050 , and 1060 are all freeform mirrors.
  • Projection objective 1000 images radiation from object plane 103 to image plane 102 with a demagnification ratio of 4 ⁇ .
  • the tracklength of projection objective 1000 is 1497 mm and the optical path length of imaged radiation is 4760 mm. Accordingly, the ratio of the optical path length to tracklength is approximately 3.18.
  • Projection objective 1000 has an aperture stop positioned close to mirror 1020 .
  • the entrance pupil of projection objective 1000 is located 1,000 mm from object plane 103 with object plane positioned between the entrance pupil and the mirrors. Due to the reflective reticle positioned at object plane 103 , illumination optics can be positioned at location 1070 , corresponding to the entrance pupil.
  • the chief ray angle of the central field point at object plane 103 is 7°.
  • the maximum variation of chief ray angles at object plane 103 is 0.82°.
  • Projection objective 1000 has a rectangular field.
  • the image-side field width, d x is 26 mm.
  • the image-side field length, d y is 2 mm.
  • Projection objective 1000 has an object-image shift of 13 mm.
  • the performance of projection objective 1000 includes an image-side W rms of 0.021 ⁇ . Distortion is less than 10 nm, and image-side field curvature is 19 nm.
  • Projection objective 1000 provides an intermediate image between mirrors 1040 and 1050 . Coma at the intermediate image is relatively large. In particular, the distance between the chief ray and the upper and lower rays at the location where the upper and lower rays cross is 7 mm.
  • mirror 1010 has positive optical power
  • mirror 1020 has negative optical power
  • mirror 1030 has positive optical power
  • mirror 1040 has positive optical power
  • mirror 1050 has negative optical power
  • mirror 1060 has positive optical power.
  • the dimension of the footprint of each mirror is as follows: 323 mm ⁇ 152 mm for mirror 1010 ; 107 mm ⁇ 59 mm for mirror 1020 ; 297 mm ⁇ 261 mm for mirror 1030 ; 277 mm ⁇ 194 mm for mirror 1040 ; 99 mm ⁇ 72 mm for mirror 1050 ; and 268 mm ⁇ 243 mm for mirror 1060 .
  • the maximum deviation of mirror 1010 from a best fit sphere is 475 ⁇ m.
  • Maximum deviation from best fit spheres of mirrors 1020 , 1030 , 1040 , 1050 , and 1060 are 1.234 ⁇ m, 995 ⁇ m, 1.414 ⁇ m, 170 ⁇ m, and 416 ⁇ m, respectively.
  • the maximum deviation of each mirror from a best fit asphere is 236 ⁇ m, 102 ⁇ m, 102 ⁇ m, 148 ⁇ m, 54 ⁇ m, and 372 ⁇ m for mirrors 1010 , 1020 , 1030 , 1040 , 1050 , and 1060 , respectively.
  • the first and second principal curvature for mirror 1010 are 1.16 ⁇ 10 ⁇ 3 and 1.05 ⁇ 10 ⁇ 3 respectively.
  • Respective first and second principal curvatures for the other mirrors in projection objective 1000 are as follows: ⁇ 3.02 ⁇ 10 ⁇ 3 and 1.13 ⁇ 10 ⁇ 3 for mirror 1020 ; 5.97 ⁇ 10 ⁇ 4 and 4.96 ⁇ 10 ⁇ 4 for mirror 1030 ; 5.50 ⁇ 10 ⁇ 4 and 3.63 ⁇ 10 ⁇ 4 for mirror 1040 ; ⁇ 2.24 ⁇ 10 ⁇ 3 and ⁇ 2.04 ⁇ 10 ⁇ 3 for mirror 1050 ; and 2.57 ⁇ 10 ⁇ 3 and 2.48 ⁇ 10 ⁇ 3 for mirror 1060 .
  • the sum of the first principal curvature for projection objective 1000 is ⁇ 3.78 ⁇ 10 ⁇ 4 .
  • the sum of the second principal curvature is 1.22 ⁇ 10 ⁇ 3 .
  • the sum of the first and second principal curvatures is 8.45 ⁇ 10 ⁇ 4 and the inverse sum of the first and second principal curvatures is 1.18 ⁇ 10
  • the chief ray angle of incidence for the central field point is 3.40°, 9.86°, 6.48°, 10.13°, 13.66°, and 7.00° for mirrors 1010 , 1020 , 1030 , 1040 , 1050 , and 1060 , respectively.
  • the maximum angle of incidence, ⁇ max , on each mirror for the meridional section is 3.94°, 10.42°, 7.45°, 14.34°, 24.28°, and 8.61° for mirrors 1010 , 1020 , 1030 , 1040 , 1050 , and 1060 , respectively.
  • ⁇ for mirrors 1010 , 1020 , 1030 , 1040 , 1050 , and 1060 are 1.13°, 2.74°, 3.42°, 9.96°, 23.69°, and 3.95°, respectively.
  • Mirrors 1010 , 1020 , 1030 , 1050 , and 1060 have freeboards that are more than 5 mm and less than 25 mm.
  • Mirror 1030 has positive chief ray angle magnification while mirrors 1040 and 1050 have negative chief ray angle magnification.
  • the image-side free working distance of projection objective 1000 is 45 mm.
  • the object-side free working distance is 252 mm.
  • d op-1 /d op-2 is 3.14. Furthermore, adjacent mirror pairs 1020 and 1030 , 1030 and 1040 , and 1040 and 1050 are separated by more than 50% of the projection objective's tracklength. Further, the distance between mirror 1010 and object plane 103 is more than 50% of the projection objective's tracklength.
  • Table 2A and Table 2B Data for projection objective 1000 is presented in Table 2A and Table 2B below.
  • the parameters and units for the parameters for Table 2A and 2B and subsequent tables are the same as the corresponding parameters and units presented in Table 1A and 1B above.
  • Table 2A presents optical data
  • Table 2B presents freeform constants for each of the mirror surfaces.
  • the mirror designations correlate as follows: mirror 1 (M 1 ) corresponds to mirror 1010 ; mirror 2 (M 2 ) corresponds to mirror 1020 ; mirror 3 (M 3 ) corresponds to mirror 1030 ; mirror 4 (M 4 ) corresponds to mirror 1040 ; mirror 5 (M 5 ) corresponds to mirror 1050 ; and mirror 6 (M 6 ) corresponds to mirror 1060 .
  • an embodiment of a projection objective 1000 includes six mirrors 1110 , 1120 , 1130 , 1140 , 1150 , and 1160 , and has an image-side numerical aperture of 0.35 and an operating wavelength of 13.5 nm.
  • Mirrors 1110 , 1120 , 1130 , 1140 , 1150 , and 1160 are all freeform mirrors.
  • Projection objective 1100 images radiation from object plane 103 to image plane 102 with a demagnification ratio of 4 ⁇ .
  • the tracklength of projection objective 1100 is 2000 mm and the optical path length of imaged radiation is 5337 mm. Accordingly, the ratio of the optical path length to tracklength is approximately 2.67.
  • Projection objective 1100 has an aperture stop 1106 positioned at mirror 1120 .
  • the entrance pupil of projection objective 1100 is located at infinity with object plane positioned between the entrance pupil and the mirrors.
  • the chief ray angle of the central field point at object plane 103 is 7°.
  • the maximum variation of chief ray angles at object plane 103 is less than 0.06°.
  • Projection objective 1100 has a rectangular field.
  • the image-side field width, d x is 26 mm.
  • the image-side field length, d y is 2 mm.
  • Projection objective 1100 has an object-image shift of 31 mm.
  • projection objective 1100 includes an image-side W rms of 0.025 ⁇ . Image-side field curvature is 10 nm. Projection objective 1100 provides an intermediate image between mirrors 1140 and 1150 .
  • mirror 1110 has positive optical power
  • mirror 1120 has positive optical power
  • mirror 1130 has negative optical power
  • mirror 1140 has positive optical power
  • mirror 1150 has negative optical power
  • mirror 1160 has positive optical power
  • the dimension of the footprint of each mirror is as follows: 291 mm ⁇ 195 mm for mirror 1110 ; 159 mm ⁇ 152 mm for mirror 1120 ; 157 mm ⁇ 53 mm for mirror 1130 ; 295 mm ⁇ 66 mm for mirror 1140 ; 105 mm ⁇ 86 mm for mirror 1150 ; and 345 mm ⁇ 318 mm for mirror 1160 .
  • the chief ray angle of incidence for the central field point is 4.38°, 4.03°, 18.37°, 7.74, 12.64°, and 5.17° for mirrors 1110 , 1120 , 1130 , 1140 , 1150 , and 1160 , respectively.
  • the maximum angle of incidence, ⁇ max , on each mirror for the meridional section is 6.48°, 6.44°, 20.05°, 9.12°, 21.76°, and 7.22° for mirrors 1110 , 1120 , 1130 , 1140 , 1150 , and 1160 , respectively.
  • ⁇ for mirrors 1110 , 1120 , 1130 , 1140 , 1150 , and 1160 are 4.27°, 4.92°, 4.09°, 3.12°, 19.37°, and 4.61°, respectively.
  • Mirrors 1110 , 1150 , and 1160 have freeboards that are more than 5 mm and less than 25 mm.
  • Mirror 1140 has positive chief ray angle magnification while mirrors 1110 , 1120 , 1130 , and 1150 have negative chief ray angle magnification.
  • the image-side free working distance of projection objective 1100 is 25 mm.
  • the object-side free working distance is 163 mm.
  • d op-1 /d op-2 is 6.57. Furthermore, adjacent mirror pair 1040 and 1050 are separated by more than 50% of the projection objective's tracklength. Further, the distance between mirror 1110 and object plane 103 is more than 50% of the projection objective's tracklength.
  • Table 3A presents optical data
  • Table 3B presents aspherical constants for each of the mirror surfaces.
  • the mirror designations correlate as follows: mirror 1 (M 1 ) corresponds to mirror 1110 ; mirror 2 (M 2 ) corresponds to mirror 1120 ; mirror 3 (M 3 ) corresponds to mirror 1130 ; mirror 4 (M 4 ) corresponds to mirror 1140 ; mirror 5 (M 5 ) corresponds to mirror 1150 ; and mirror 6 (M 6 ) corresponds to mirror 1160 .
  • an embodiment of a projection objective 1200 includes six mirrors 1210 , 1220 , 1230 , 1240 , 1250 , and 1260 , and has an image-side numerical aperture of 0.35 and an operating wavelength of 13.5 nm.
  • Mirrors 1210 , 1220 , 1230 , 1240 , 1250 , and 1260 are all freeform mirrors.
  • Projection objective 1200 images radiation from object plane 103 to image plane 102 with a demagnification ratio of 4 ⁇ .
  • a reference axis 1205 orthogonal to object plane 103 and image plane 102 intersects corresponding field points in the object and image fields.
  • the tracklength of projection objective 1200 is 1385 mm and the optical path length of imaged radiation is 4162 mm. Accordingly, the ratio of the optical path length to tracklength is approximately 3.01.
  • Projection objective 1200 has an aperture stop positioned at mirror 1220 .
  • the entrance pupil of projection objective 1200 is at infinity with object plane positioned between the entrance pupil and the mirrors.
  • the chief ray angle of the central field point at object plane 103 is 7°.
  • the maximum variation of chief ray angles at object plane 103 is less than 0.06°.
  • Projection objective 1200 has a rectangular field.
  • the image-side field width, d x is 26 mm.
  • the image-side field length, d y is 2 mm.
  • Projection objective 1200 has zero object-image shift.
  • Projection objective 1200 provides an intermediate image between mirrors 1240 and 1250 .
  • mirror 1210 has positive optical power
  • mirror 1220 has negative optical power
  • mirror 1230 has positive optical power
  • mirror 1240 has positive optical power
  • mirror 1250 has negative optical power
  • mirror 1260 has positive optical power.
  • the dimension of the footprint of each mirror is as follows: 250 mm ⁇ 153 mm for mirror 1210 ; 70 mm ⁇ 69 mm for mirror 1020 ; 328 mm ⁇ 153 mm for mirror 1230 ; 325 mm ⁇ 112 mm for mirror 1240 ; 87 mm ⁇ 75 mm for mirror 1250 ; and 269 mm ⁇ 238 mm for mirror 1260 .
  • the chief ray angle of incidence for the central field point is 6.13°, 10.61°, 8.65°, 8.26°, 14.22°, and 5.23° for mirrors 1210 , 1220 , 1230 , 1240 , 1250 , and 1260 , respectively.
  • the maximum angle of incidence, ⁇ max , on each mirror for the meridional section is 6.53°, 11.63°, 8.91°, 11.39°, 24.26°, and 7.44° for mirrors 1210 , 1220 , 1230 , 1240 , 1250 , and 1260 , respectively.
  • ⁇ for mirrors 1210 , 1220 , 1230 , 1240 , 1250 , and 1260 are 1.07°, 3.64°, 1.74°, 7.44°, 21.70°, and 4.51°, respectively.
  • Mirrors 1210 , 1220 , 1250 , and 1260 have freeboards that are more than 5 mm and less than 25 mm.
  • Mirror 1240 has positive chief ray angle magnification while mirrors 1210 , 1220 , 1230 , and 1250 have negative chief ray angle magnification.
  • the image-side free working distance of projection objective 1200 is 40 mm.
  • the object-side free working distance is 439 mm.
  • d op-1 /d op-2 is 1.91. Furthermore, adjacent mirror pair 1240 and 1250 are separated by more than 50% of the projection objective's tracklength. Further, the distance between mirror 1210 and object plane 103 is more than 50% of the projection objective's tracklength.
  • Table 4A presents optical data
  • Table 4B presents aspherical constants for each of the mirror surfaces.
  • the mirror designations correlate as follows: mirror 1 (M 1 ) corresponds to mirror 1210 ; mirror 2 (M 2 ) corresponds to mirror 1220 ; mirror 3 (M 3 ) corresponds to mirror 1230 ; mirror 4 (M 4 ) corresponds to mirror 1240 ; mirror 5 (M 5 ) corresponds to mirror 1250 ; and mirror 6 (M 6 ) corresponds to mirror 1260 .
  • an embodiment of a projection objective 1400 includes six mirrors 1410 , 1420 , 1430 , 1440 , 1450 , and 1460 , and has an image-side numerical aperture of 0.40 and an operating wavelength of 13.5 nm.
  • Mirrors 1410 , 1420 , 1430 , 1440 , 1450 , and 1460 are all freeform mirrors.
  • Projection objective 1400 images radiation from object plane 103 to image plane 102 with a demagnification ratio of 4 ⁇ .
  • the tracklength of projection objective 1400 is 1498 mm and the optical path length of imaged radiation is 3931 mm. Accordingly, the ratio of the optical path length to tracklength is approximately 2.62.
  • Projection objective 1400 has an aperture stop positioned between mirrors 1420 and 1430 .
  • the entrance pupil of projection objective 1400 is located 1,000 mm from object plane 103 with object plane positioned between the entrance pupil and the mirrors.
  • the chief ray angle of the central field point at object plane 103 is 7°.
  • the maximum variation of chief ray angles at object plane 103 is 0.82°.
  • Projection objective 1400 has a rectangular field.
  • the image-side field width, d x is 26 mm.
  • the image-side field length, d y is 2 mm.
  • Projection objective 1000 has an object-image shift of 38 mm.
  • projection objective 1000 includes an image-side W rms of 0.083 ⁇ . Distortion is approximately 100 nm, and image-side field curvature is 36 nm. Projection objective 1400 provides an intermediate image between mirrors 1440 and 1450 .
  • mirror 1410 has positive optical power
  • mirror 1420 has positive optical power
  • mirror 1430 has negative optical power
  • mirror 1440 has positive optical power
  • mirror 1050 has negative optical power
  • mirror 1460 has positive optical power.
  • the dimension of the footprint of each mirror is as follows: 326 mm ⁇ 159 mm for mirror 1410 ; 210 mm ⁇ 123 mm for mirror 1420 ; 120 mm ⁇ 66 mm for mirror 1430 ; 312 mm ⁇ 119 mm for mirror 1440 ; 112 mm ⁇ 83 mm for mirror 1450 ; and 405 mm ⁇ 379 mm for mirror 1460 .
  • the chief ray angle of incidence for the central field point is 6.70°, 8.08°, 20.41°, 6.68°, 14.52°, and 5.67° for mirrors 1410 , 1420 , 1430 , 1440 , 1450 , and 1460 , respectively.
  • the maximum angle of incidence, ⁇ max , on each mirror for the meridional section is 8.61°, 10.91°, 21.98°, 7.41°, 27.19°, and 6.86° for mirrors 1410 , 1420 , 1430 , 1440 , 1450 , and 1460 , respectively.
  • ⁇ for mirrors 1410 , 1420 , 1430 , 1440 , 1450 , and 1460 are 3.92°, 5.69°, 3.82°, 1.79°, 26.83°, and 3.20°, respectively.
  • Mirrors 1410 , 1420 , 1430 , 1450 , and 1460 have freeboards that are more than 5 mm and less than 25 mm.
  • Mirror 1440 has positive chief ray angle magnification while mirrors 1410 , 1420 , 1430 , and 1450 have negative chief ray angle magnification.
  • the image-side free working distance of projection objective 1400 is 45 mm.
  • the object-side free working distance is 291 mm.
  • d op-1 /d op-2 is 2.47. Furthermore, adjacent mirror pair 1440 and 1450 is separated by more than 50% of the projection objective's tracklength.
  • Table 6A presents optical data
  • Table 6B presents aspherical constants for each of the mirror surfaces.
  • the mirror designations correlate as follows: mirror 1 (M 1 ) corresponds to mirror 1010 ; mirror 2 (M 2 ) corresponds to mirror 1020 ; mirror 3 (M 3 ) corresponds to mirror 1030 ; mirror 4 (M 4 ) corresponds to mirror 1040 ; mirror 5 (M 5 ) corresponds to mirror 1050 ; and mirror 6 (M 6 ) corresponds to mirror 1060 .
  • projection objective 1400 can be used in an optical system 1401 that includes a light source 1405 and illumination optics including a collector unit 1415 , a spectral purity filter 1425 , a field facet mirror 1435 and a pupil facet mirror 1445 .
  • Light source 1405 is an EUV light source configured to provide radiation at 13.5 nm to the projection objective.
  • Collector unit 1415 gathers radiation from source 1405 and directs the radiation towards spectral purity filter 1415 which filters incident radiation at wavelengths other than 13.5 nm and directs the radiation at 13.5 nm towards field facet mirror 1435 .
  • field facet mirror illuminates a reflective reticle positioned at object plane 103 with radiation at 13.5 nm.
  • the radiation is provided so that the chief rays diverge from the reticle.
  • the radiation is provided to the reticle in this way without the use of additional components, such as a grazing incidence mirror.
  • an embodiment of a projection objective 1500 includes six mirrors 1510 , 1520 , 1530 , 1540 , 1550 , and 1560 , and has an image-side numerical aperture of 0.40 and an operating wavelength of 13.5 nm.
  • Mirrors 1510 , 1520 , 1530 , 1540 , 1550 , and 1460 are all freeform mirrors.
  • Projection objective 1500 images radiation from object plane 103 to image plane 102 with a demagnification ratio of 4 ⁇ .
  • the tracklength of projection objective 1500 is 1499 mm and the optical path length of imaged radiation is 4762 mm. Accordingly, the ratio of the optical path length to tracklength is approximately 3.18.
  • Projection objective 1500 has an aperture stop positioned close to mirror 1520 .
  • the entrance pupil of projection objective 1500 is located 1,000 mm from object plane 103 with object plane positioned between the entrance pupil and the mirrors. Due to the reflective reticle positioned at object plane 103 , illumination optics can be positioned at location 1501 , corresponding to the entrance pupil.
  • the chief ray angle of the central field point at object plane 103 is 7°.
  • the maximum variation of chief ray angles at object plane 103 is 0.82°.
  • Projection objective 1500 has a rectangular field.
  • the image-side field width, d x is 26 mm.
  • the image-side field length, d y is 2 mm.
  • Projection objective 1500 has an object-image shift of 7 mm.
  • the performance of projection objective 1500 includes an image-side W rms of 0.040 ⁇ . Referring also to FIG. 16A , distortion is less than about 3 nm across the image field. Image-side field curvature is 35 nm. Projection objective 1500 provides an intermediate image between mirrors 1540 and 1550 . Referring to FIG. 16B , the chief rays are orthogonal to image plane 102 to within about 0.001 rad (0.06°) at the image field.
  • mirror 1510 has positive optical power
  • mirror 1520 has negative optical power
  • mirror 1530 has positive optical power
  • mirror 1540 has positive optical power
  • mirror 1550 has negative optical power
  • mirror 1560 has positive optical power.
  • the dimension of the footprint of each mirror is as follows: 253 mm ⁇ 162 mm for mirror 1510 ; 105 mm ⁇ 66 mm for mirror 1520 ; 227 mm ⁇ 301 mm for mirror 1530 ; 182 mm ⁇ 220 mm for mirror 1540 ; 111 mm ⁇ 85 mm for mirror 1550 ; and 289 mm ⁇ 275 mm for mirror 1560 .
  • the chief ray angle of incidence for the central field point is 3.96°, 12.21°, 7.51°, 11.98°, 15.82°, and 8.08° for mirrors 1510 , 1520 , 1530 , 1540 , 1550 , and 1560 , respectively.
  • the maximum angle of incidence, ⁇ max , on each mirror for the meridional section is 4.47°, 12.81°, 8.55°, 16.91°, 27.68°, and 9.96° for mirrors 1510 , 1520 , 1530 , 1540 , 1550 , and 1560 , respectively.
  • ⁇ for mirrors 1510 , 1520 , 1530 , 1540 , 1550 , and 1560 are 1.10°, 3.61°, 4.19°, 12.12°, 27.17°, and 4.79°, respectively.
  • Mirrors 1510 , 1520 , 1540 , 1550 , and 1560 have freeboards that are more than mm and less than 25 mm.
  • Mirror 1530 has positive chief ray angle magnification while mirrors 1510 , 1520 , 1540 , and 1550 have negative chief ray angle magnification.
  • the image-side free working distance of projection objective 1500 is 45 mm.
  • the object-side free working distance is 260 mm.
  • d op-1 /d op-2 is 3.05. Furthermore, adjacent mirror pairs 1520 and 1530 , 1530 and 1540 , and 1540 and 1550 are separated by more than 50% of the projection objective's tracklength. Further, the distance between mirror 1510 and object plane 103 is more than 50% of the projection objective's tracklength.
  • Table 7A presents optical data
  • Table 7B presents aspherical constants for each of the mirror surfaces.
  • the mirror designations correlate as follows: mirror 1 (M 1 ) corresponds to mirror 1510 ; mirror 2 (M 2 ) corresponds to mirror 1520 ; mirror 3 (M 3 ) corresponds to mirror 1530 ; mirror 4 (M 4 ) corresponds to mirror 1540 ; mirror 5 (M 5 ) corresponds to mirror 1550 ; and mirror 6 (M 6 ) corresponds to mirror 1560 .
  • FIG. 17 shows another embodiment of the disclosure having six mirrors being all designed as freeform surfaces. Depicted is the object plane 3000 , the image plane 3002 , a first mirror M 1 , a second mirror M 2 , a third mirror M 3 , a fourth mirror M 4 , a fifth mirror M 5 and a sixth mirror M 6 .
  • This projection objective has an image side numerical aperture of 0.40.
  • the field shape is rectangular with a width of 26 mm and a height of 2 mm.
  • the operating wavelength is 13.5 nm.
  • the sequence of optical power of mirrors is PNPNNP. This optical system has one intermediate image between mirrors M 4 and M 5 .
  • the entrance pupil of this projection objective is located 1000 mm from object plane 3000 , with the object plane 3000 positioned between the entrance pupil and the mirrors.
  • a pupil plane is positioned between mirrors M 2 and M 3 .
  • the tracklength is 1736 mm.
  • the object image shift is 65 mm.
  • the optical path length is 4827 mm.
  • the performance of this projection objective includes an image-side W rms of 0.037 ⁇ . Distortion is smaller than 12 nm. Image-side field curvature is 25 nm.
  • the chief ray angle of the central field point at the object is 7°.
  • the maximum variation of chief ray angles at object plane 3000 is 0.82.
  • the dimension of the footprint of each mirror, given as M x ⁇ M y is as follows: 323 mm ⁇ 215 mm for mirror M 1 ; 131 mm ⁇ 102 mm for mirror M 2 ; 267 mm ⁇ 183 mm for mirror M 3 ; 70 mm ⁇ 52 mm for mirror M 4 ; 124 mm ⁇ 109 mm for mirror M 5 ; 447 mm ⁇ 433 mm for mirror M 6 .
  • the chief ray angle of incidence for the central field point for the mirrors M 1 to M 6 is 4.06°; 11.34°; 12.20°; 31.60°; 12.27° and 7.64°.
  • the maximum angles of incidence in meridional section for the mirrors M 1 to M 6 is 4.96°; 12.38°, 16.54°, 41.24°; 29.42° and 9.25°.
  • the bandwidth of angle of incidence in meridional section for mirrors M 1 to M 6 is 1.08°; 2.71°; 9.83°; 22.72°; 29.13° and 4.28°.
  • Mirrors M 2 and M 4 have freeboards that are more than 5 mm and less than 25 mm.
  • Mirror M 3 has positive chief ray angle magnification while mirrors M 1 , M 2 , M 4 and M 5 have negative chief ray angle magnification.
  • the image-side free working distance of this projection objective is 45 mm.
  • the object-side free working distance is 400 mm.
  • d OP-1 /d OP-2 is 2.67. Further, reticle and mirror M 1 as well as mirrors M 2 and M 3 are separated by more than 50% of the projection objective tracklength.
  • Tables 8A, 8B Data for the projection objective of FIG. 17 is presented in Tables 8A, 8B below.
  • Table 8A presents optical data
  • Table 8B presents aspherical constants for each of the mirror surfaces.
  • the projection objective of FIG. 17 differs from the embodiments of FIGS. 3 , 10 , 11 , 12 , 13 and 14 mainly in the shape of mirror M 4 .
  • mirror M 4 of the embodiment of FIG. 17 is convex.
  • FIG. 18 shows another embodiment of the disclosure having six mirrors being all designed as freeform surfaces. Depicted is the object plane 3000 , the image plane 3002 , a first mirror M 1 , a second mirror M 2 , a third mirror M 3 , a fourth mirror M 4 , a fifth mirror M 5 and a sixth mirror M 6 .
  • This projection objective has an image side numerical aperture of 0.35.
  • the field shape is rectangular with a width of 26 mm and a height of 2 mm.
  • the operating wavelength is 13.5 nm.
  • the sequence of optical power of mirrors is PPNPNP.
  • This optical system has one intermediate image between mirrors M 4 and M 5 .
  • the entrance pupil of this projection objective is located on the image plane side of the object plane 3000 in a distance of 1749 mm.
  • An aperture stop is positioned on mirror M 2 .
  • the tracklength is 1700 mm.
  • the object image shift is 41 mm.
  • the optical path length is 4156 mm.
  • the performance of this projection objective includes an image-side W rms of 0.020 ⁇ . Distortion is smaller than 1.1 nm. Image-side field curvature is 17 nm.
  • the chief ray angle of the central field point at the object is 6°.
  • the maximum variation of chief ray angles at object plane 3000 is 0.58.
  • the dimension of the footprint of each mirror, given as M x ⁇ M y is as follows: 169 mm ⁇ 148 mm for mirror M 1 ; 159 mm ⁇ 136 mm for mirror M 2 ; 120 mm ⁇ 61 mm for mirror M 3 ; 265 mm ⁇ 118 mm for mirror M 4 ; 101 mm ⁇ 77 mm for mirror M 5 ; 345 mm ⁇ 329 mm for mirror M 6 .
  • the chief ray angle of incidence for the central field point for the mirrors M 1 to M 6 is 8.11°; 9.49°; 21.03°; 8.01°; 13.67°; 5.03°.
  • the maximum angle of incidence in meridional section for the mirrors M 1 to M 6 is 10.31′; 12.06°; 21.56°; 8.45°; 24.59°; 6.36°.
  • the bandwidth of angle of incidence in meridional section for mirrors M 1 to M 6 is 4.56°; 5.34°; 1.85°; 1.23°; 22.98°; 3.16°.
  • Mirror M 4 has positive chief ray angle magnification while mirrors M 1 , M 2 , M 3 and M 5 have negative chief ray angle magnification.
  • the image-side free working distance of this projection objective is 45 mm.
  • the object-side free working distance is 441 mm.
  • d OP-1 /d OP-2 is 1.89. Further, mirrors M 4 and M 5 are separated by more than 50% of the projection objective tracklength.
  • Table 9A presents optical data
  • table 9B presents aspherical constants for each of the mirror surfaces.
  • the projection objective of FIG. 18 has chief rays converging to each other while starting from the object plane 3000 .

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