US20070242799A1 - Illumination system - Google Patents
Illumination system Download PDFInfo
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- US20070242799A1 US20070242799A1 US11/820,375 US82037507A US2007242799A1 US 20070242799 A1 US20070242799 A1 US 20070242799A1 US 82037507 A US82037507 A US 82037507A US 2007242799 A1 US2007242799 A1 US 2007242799A1
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- mirror
- illumination system
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- light path
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- 238000005286 illumination Methods 0.000 title claims abstract description 64
- 238000003384 imaging method Methods 0.000 claims description 13
- 230000003287 optical effect Effects 0.000 claims description 11
- 238000000034 method Methods 0.000 claims description 5
- 238000009304 pastoral farming Methods 0.000 claims description 5
- 210000001747 pupil Anatomy 0.000 claims description 4
- 238000004377 microelectronic Methods 0.000 claims description 2
- 238000011144 upstream manufacturing Methods 0.000 claims 12
- 230000005855 radiation Effects 0.000 description 6
- 238000001816 cooling Methods 0.000 description 5
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 2
- 201000009310 astigmatism Diseases 0.000 description 2
- 238000009795 derivation Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 2
- 229910052737 gold Inorganic materials 0.000 description 2
- 239000010931 gold Substances 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 229910052763 palladium Inorganic materials 0.000 description 2
- 229910052703 rhodium Inorganic materials 0.000 description 2
- 239000010948 rhodium Substances 0.000 description 2
- MHOVAHRLVXNVSD-UHFFFAOYSA-N rhodium atom Chemical compound [Rh] MHOVAHRLVXNVSD-UHFFFAOYSA-N 0.000 description 2
- 229910052707 ruthenium Inorganic materials 0.000 description 2
- 238000000926 separation method Methods 0.000 description 2
- 230000005540 biological transmission Effects 0.000 description 1
- 230000000903 blocking effect Effects 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000004907 flux Effects 0.000 description 1
- 239000011888 foil Substances 0.000 description 1
- 230000008642 heat stress Effects 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000002156 mixing Methods 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
- 230000005469 synchrotron radiation Effects 0.000 description 1
Images
Classifications
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70216—Mask projection systems
- G03F7/70233—Optical aspects of catoptric systems, i.e. comprising only reflective elements, e.g. extreme ultraviolet [EUV] projection systems
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
- G02B19/0004—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed
- G02B19/0019—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed having reflective surfaces only (e.g. louvre systems, systems with multiple planar reflectors)
- G02B19/0023—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed having reflective surfaces only (e.g. louvre systems, systems with multiple planar reflectors) at least one surface having optical power
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
- G02B19/0033—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use
- G02B19/0047—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use for use with a light source
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
- G02B19/0033—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use
- G02B19/0095—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use for use with ultraviolet radiation
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/08—Mirrors
- G02B5/09—Multifaceted or polygonal mirrors, e.g. polygonal scanning mirrors; Fresnel mirrors
-
- 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
- G03F7/70083—Non-homogeneous intensity distribution in the mask plane
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70058—Mask illumination systems
- G03F7/7015—Details of optical elements
- G03F7/70158—Diffractive optical elements
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70058—Mask illumination systems
- G03F7/702—Reflective illumination, i.e. reflective optical elements other than folding mirrors, e.g. extreme ultraviolet [EUV] illumination systems
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
- G03F7/7055—Exposure light control in all parts of the microlithographic apparatus, e.g. pulse length control or light interruption
- G03F7/70575—Wavelength control, e.g. control of bandwidth, multiple wavelength, selection of wavelength or matching of optical components to wavelength
-
- G—PHYSICS
- G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
- G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
- G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
- G21K1/06—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diffraction, refraction or reflection, e.g. monochromators
Definitions
- the invention concerns an illumination system for wavelengths ⁇ 100 nm, wherein the illumination system has an object plane and a field plane.
- illumination systems ⁇ 100 nm the problem exists that light sources of such illumination systems emit radiation that can lead to an undesired exposure of the light-sensitive object in the wafer plane.
- optical components of the exposure system such as, for example, the multilayer mirror, can be heated up in this way.
- transmission filters are used in illumination systems for wavelengths ⁇ 1 00 nm.
- Such filters have the disadvantage of high light losses. In addition, they can be disrupted very easily by heat stress.
- the object of the invention is to provide an illumination system for wavelengths ⁇ 100 nm, particularly in the EUV range, in which the above-named disadvantages can be avoided.
- an illumination system that has at least one grating element and at least one physical diaphragm in a diaphragm plane.
- the physical diaphragm is situated in the beam path from the object plane to the field plane after the grating element.
- Grating elements for example, reflection gratings, particularly echelette gratings, which are also known as blazed gratings, have been known for a long time from monochromator construction for synchrotron radiation sources. For these elements good experiences, particularly with very high fluxes, were made.
- a grating element can be used in the beam path from the object plane to the image plane for spectral filtering in an illumination system for wavelengths ⁇ 100 nm, if the individual diffraction orders and the wavelengths are clearly separated from one another.
- the convergent beam bundle has a focus with a limited diameter.
- the optical element is curved concave in a meridional plane.
- the meridional plane of the optic element is defined as the plane which is perpendicular to the carrier surface of the grating element and to the grating lines.
- the optical element can be curved convex in the sagittal plane, which is perpendicular to the carrier surface and the meridional plane, and contains the centre of the grating element.
- the refractive power in the meridional direction is greater than in the sagittal direction, i.e., the element is concave, e.g., in the meridional direction and planar in the sagittal direction, or it is planar in the meridional direction and convex in the sagittal direction, or it is formed concave in the meridional direction and convex in the sagittal direction.
- the refractive power in the sagittal direction is greater than in the meridional direction, i.e., the element is concave in the sagittal direction and planar in the meridional direction, or it is planar in the sagittal direction and convex in the meridional direction, or it is formed concave in the sagittal direction and convex in the meridional direction.
- the order which is diffracted to the surface normal line is denoted the internal order and assigned positive numbers, while the order which is diffracted away from the surface normal line is designated the external order and is assigned negative numbers.
- the stigmatic imaging is achieved by a variation of the distance between the grating lines.
- the at-least one physical diaphragm according to the invention essentially serves for the purpose that light with wavelengths far above 100 nm does not enter into the illumination system. This can be achieved particularly by blocking the zeroth diffraction order. Due to the one physical diaphragm, all diffraction orders are preferably blocked except for a so called used order.
- the used order for example, can be the 1 st order.
- the rays have wavelengths in the range of 7 to 26 nm after the physical diaphragm, due to the combination of grating and physical diaphragm.
- the grating element is preferably designed as a blazed grating, which is optimized to a maximal efficiency in a pregiven diffraction order.
- Blazed gratings are known, for example, from the Lexikon der Optik [Optics Lexicon], edited by Heinz Haferkorn, VEB Bibliographic Institute, Leipzig, 1990, pp. 48 to 49. They are characterized by an approximately triangular groove profile.
- a part of the undesired radiation can be filtered out by additional diaphragms in the illumination system.
- the invention also provides a projection exposure system with such an illumination system as well as a method for the production of microelectronic components.
- FIG. 1 shows the arrangement of a grating element in the beam path of the collector unit of an illumination system
- FIG. 2 shows the grating element and the physical diaphragm of an illumination system
- FIG. 3 shows a blazed grating
- FIG. 4 shows the maximal possible diffraction efficiency for grating elements designed as blazed gratings
- FIG. 5 shows an EUV projection exposure system with an illumination system according to the invention.
- FIG. 1 shows an illumination system with a grating element 1 as well as a physical diaphragm in the diaphragm plane.
- the light of light source 3 is collected by a collecting component, collector 5 , in the illumination system shown.
- the collector 5 in this example is an ellipsoid-shaped mirror, which produces an image of light source 3 .
- the physical diaphragm has a circular opening, which is situated in the focal plane of the desired diffraction order, here the ⁇ 1. order 16 .
- the diaphragms 7 . 1 , 7 . 2 may also be cooled, but this is not shown.
- grating element 1 can be cooled, for example, by a cooling on the back side.
- the device 8 for back-side cooling of grating element 1 is preferably a liquid cooling device with inlet 10 .
- the grating in an illumination system with collector according to FIG. 1 is designed as a planar grating with the same grating period, then an astigmatic imaging of the light source results.
- a stigmatic imaging of light source 3 into the plane of the physical diaphragm 7 . 3 is necessary for a diffraction order, which is not the zeroth diffraction order. It can be attempted by various methods to correct the astigmatism, for example, by introducing additional refractive power in the meridional plane or sagittal plane or by variation of the distance between the grating lines.
- Nk ⁇ sin ⁇ +sin ⁇ (1)
- N is the number of lines
- k is the diffraction order
- ⁇ is the wavelength
- ⁇ is the angle of incidence
- ⁇ is the diffraction angle (relative to the surface normal of the carrier surface and referred to the chief ray CR before or CR after ).
- the nomenclature which is used in the following derivation is oriented to the “Lexikon der Optik [Optics Lexicon] in two volumes, edited by H. Paul, Heidelberg, Berlin, Spektrum Academic Publishers, 1999, Vol. 1, A-L, pp. 77-80.
- FIG. 2 Reference is made to FIG. 2 for the explanation of the following derivation.
- the convergent radiation of a light source (not shown) through grating element 1 is spectrally split and selected at intermediate focus 19 by physical diaphragm 7 . 3 .
- An NA of 0.12 is achieved at intermediate focus 19 .
- a convergent incident light bundle 100 is shown in FIG. 2 . This is diffracted at grating 1 .
- the beam bundle 12 which is diffracted in the 0 th diffraction order is shown along with the beam bundle 14 which is diffracted in the first diffraction order.
- the beam bundle diffracted in the 0 th diffraction order has a focal point 112 and the beam bundle diffracted in the first order has a focal point 114 .
- the image widths s before and s after are defined by the respective focal points 112 , 114 and the striking point 102 of the chief ray CR before of the incident beam bundle 100 on grating 1 .
- s before designates the distance of the striking point 102 from the focal point 112 of the beam bundle diffracted in the zeroth diffraction order
- s after denotes the distance of striking point 102 from focal point 114 of the beam bundle diffracted in the first order.
- Angle ⁇ designates the angle of incidence of the chief ray CR before of the incident beam bundle with respect to the surface normal line of the carrier surface of grating 1 and ⁇ denotes the diffraction angle of the chief ray CR after of the beam bundle 14 diffracted in the 1 st order, relative to the surface normal line of the carrier surface.
- the planes 106 , 108 are designated, which stand at striking point 102 on grating 1 perpendicular to the chief ray CR before of the striking beam bundle 100 and perpendicular to the chief ray CR after of the beam bundle 14 diffracted in the 1 st diffraction order.
- the cross section of the beam bundle is modified in planes 106 , 108 , which stand perpendicular to the chief ray CR before of the incident beam bundle or the chief ray CR after of the diffracted beam pencil. Due to the above-named conservation of balance, the divergence must be changed recriprocally. This means that if a grating is operated in the internal arrangement (
- ), then the beam becomes larger by cos ( ⁇ )/cos ( ⁇ ) and the divergence becomes smaller by the same factor. By this, the distance up to the focus or focal point is lengthened by the square quadratic factor. This factor is denoted below as the fixed focus constant c ff : c ff cos ( ⁇ )/cos ( ⁇ ) (2)
- the grating acts only in the meridional or dispersive direction.
- the grating is selected as sagittal concave.
- the grating line distance may also be varied.
- the radius must be selected such that an image width of s before c ff 2 is obtained from the image width S before in the 0 th order.
- the first condition is decisive for the effectiveness of the grating element.
- a formula for estimating the separation of the beam pencil of the different diffraction orders from one another can be derived as follows with reference to FIG. 2 .
- the difference between, e.g., ⁇ x 0 and ⁇ d 0 /2 yields, e.g., the distance of the edge rays of the diffracted beam bundle from the focal point of the beam bundle of the 0 th order.
- this distance should correspond to at least half the diameter of the beam bundle in the focal point, which is denoted ⁇ x f ; a sufficient separation of the beam bundle of the 0 th diffraction order from the beam bundles of other diffraction orders is then achieved.
- the grating element with sagittal convex curvature which is characterized by a grating efficiency of 56% can be constructed, which is characterized by the values given below in Table 1.
- Table 1 Characteristic values of a grating element with convex transverse curvature Wavelength 13.4 nm Photon energy 92.5 eV Number of lines 1600 l/mm Diffraction order 1 fixed focus constant, c ff 1.2 angle of incidence ⁇ of the 72.360 degrees chief ray CR before Diffraction angle ⁇ of the chief ⁇ 68.676 degrees ray CR after diffracted in the 1 st order Blazed depth 20.1 nm Grating-focus distance 432 mm Sagittal radius 654.555 mm NA (after grating) 0.12 Grating length 237 mm Material used Ru Microroughness 0.5 nm (rms) Grating efficiency 56 %
- the grating element is preferably configured as a blazed grating.
- FIG. 3 A blazed grating with approximately triangular groove profile is shown in FIG. 3 .
- Reference number 11 designates the ray e.g., the chief ray of a beam bundle, striking the grating element 1 designed as a blazed grating; 12 denotes the ray, e.g. the chief ray of a beam bundle, reflected at the grating in the 0 th order and 16 denotes the ray, e.g., the chief ray of a beam bundle, diffracted in the ⁇ 1 st order.
- the blazed depth B is a function of angles of incidence and reflection. Thus, it is advantageous if the blazed depth is changed as a function of the position on the grating in order to obtain a maximal diffraction efficiency with a convergent beam pencil.
- the greatest efficiency of 0.7 can be achieved with ruthenium.
- a coating of palladium or rhodium has better long-time properties, but has an efficiency ⁇ (1) of only 0.67, which is 3% poorer.
- the EUV projection exposure system comprises a light source 3 , a collecting optical component, a so-called collector 5 , which is formed as a nested collector.
- Collector 5 images the light source 3 lying in the object plane of the illumination system in a secondary light source 4 in or in the vicinity of a diaphragm plane 7 . 3 .
- Light source 3 which can be, for example, a laser plasma source or a plasma discharge source, is arranged in the object plane of the illumination system.
- Additional diaphragms 7 . 1 , 7 . 2 are arranged between grating element 1 and the physical diaphragm 7 . 3 in order to block the light of undesired wavelengths, particularly wavelengths longer than 30 mm.
- the focus of the 1 st order comes to lie in the plane of diaphragm 7 . 3 , i.e., light source 3 is imaged nearly stigmatic in the plane of diaphragm 7 . 3 by the collector and the grating spectral filter in the 1 st diffraction order.
- the imaging in all other diffraction orders is not stigmatic.
- the illumination system of the projection system comprises an optical system 20 for forming and illuminating field plane 22 with a ring-shaped field.
- the optical system comprises two faceted mirrors 29 . 1 , 29 . 2 as well as two imaging mirrors 30 . 1 , 30 . 2 and a field-forming grazing-incidence mirror 32 as the mixing unit for homogeneous illumination of the field.
- Additional diaphragms 7 . 4 , 7 . 5 , 7 . 6 , 7 . 7 are arranged in optical system 20 for suppressing stray light.
- the first faceted mirror 29 . 1 the so-called field-faceted mirror, produces a plurality of secondary light sources in or in the vicinity of the plane of the second faceted mirror 29 . 2 , the so-called pupil-faceted mirror.
- the subsequent imaging optics image the pupil-faceted mirror 29 . 2 in the exit pupil of the illumination system, which comes to lie in the entrance pupil of the projection objective 26 .
- the angle of inclination of the individual facets of the first and second faceted mirrors 29 . 1 , 29 . 2 are designed in such a way that the images of the individual field facets of the first faceted mirror 29 .
- the segment of the ring field is formed by means of the field-forming grazing-incidence mirror 32 operated under grazing incidence.
- a double-faceted illumination system is disclosed, for example, in U.S. Patent US-B-6,198,739, imaging and field-forming components in PCT/EP/00/07258.
- the disclosure contents of these documents is incorporated to the full extent in the present Application.
- the pattern-bearing mask which is also designated as the reticle, is arranged in field plane 22 .
- the mask is imaged by means of a projection objective 26 in the image plane 28 of field plane 22 .
- the projection objective 26 is a 6-mirror projection objective, such as disclosed, for example, in U.S. application No. 60/255214, filed on Dec. 13, 2000, in the U.S. Patent Office for the Applicant or DE-A-10037870, the disclosure content of which is fully incorporated into the present application.
- the object to be exposed for example, a wafer, is arranged in image plane 28 .
- the replica technique is considered, for example, as a possible manufacturing method for a grating element according to the invention.
- the invention gives for the first time an illumination system, with which undesired wavelengths can be selected directly after the light-source unit and which represents an alternative to filter foils, which are problematic, particularly with respect to the heat load.
Abstract
There is provided an illumination system. The illumination system includes (a) a mirror, (b) a diaphragm in a light path downstream of the mirror, and (c) a field plane in the light path, downstream of the diaphragm.
Description
- 1. Field of the Invention
- The invention concerns an illumination system for wavelengths ≦100 nm, wherein the illumination system has an object plane and a field plane. In illumination systems ≦100 nm, the problem exists that light sources of such illumination systems emit radiation that can lead to an undesired exposure of the light-sensitive object in the wafer plane. Furthermore optical components of the exposure system, such as, for example, the multilayer mirror, can be heated up in this way.
- In order to filter out the undesired radiation, transmission filters are used in illumination systems for wavelengths ≦1 00 nm. Such filters have the disadvantage of high light losses. In addition, they can be disrupted very easily by heat stress.
- The object of the invention is to provide an illumination system for wavelengths ≦100 nm, particularly in the EUV range, in which the above-named disadvantages can be avoided.
- According to the invention, this object is solved by an illumination system that has at least one grating element and at least one physical diaphragm in a diaphragm plane. The physical diaphragm is situated in the beam path from the object plane to the field plane after the grating element.
- 2. Description of the Prior Art
- Grating elements, for example, reflection gratings, particularly echelette gratings, which are also known as blazed gratings, have been known for a long time from monochromator construction for synchrotron radiation sources. For these elements good experiences, particularly with very high fluxes, were made.
- With respect to the use of diffraction gratings in monochromators, reference is made to the following publications, whose disclosure content is incorporated to the full extent in the present Application:
-
- H. Petersen, C. Jung, C. Hellwig, W. B. Peatman, W. Gudat: “Review of plane grating focusing for soft x-ray monochromators”, Rev. Sci. Instrum. 66(1), January 1996.
- M. V. R. K. Murty: “Use of convergent and divergent illumination with plane gratings”, Journal of the Optical Society of America, Vol. 52, No. 7, July 1962, pp. 768-773.
- T. Oshio, E. Ishiguro, R. Iwanaga: “A theory of new astigmatism and coma-free spectrometer”, Nuclear Instruments and Methods 208 (1993) 297-301.
- The inventors have now recognized that a grating element can be used in the beam path from the object plane to the image plane for spectral filtering in an illumination system for wavelengths ≦100 nm, if the individual diffraction orders and the wavelengths are clearly separated from one another.
- This is most simple for a grating element within a convergent beam bundle. The convergent beam bundle has a focus with a limited diameter.
- In order to obtain a stigmatic imaging of an object into the plane of the physical diaphragm with the aid of a grating element situated in a convergent beam path, in a first embodiment of the invention the optical element is curved concave in a meridional plane. The meridional plane of the optic element is defined as the plane which is perpendicular to the carrier surface of the grating element and to the grating lines.
- Alternatively or additionally to this, the optical element can be curved convex in the sagittal plane, which is perpendicular to the carrier surface and the meridional plane, and contains the centre of the grating element.
- If an internal diffraction order (k=1, 2, 3) is used, the refractive power in the meridional direction is greater than in the sagittal direction, i.e., the element is concave, e.g., in the meridional direction and planar in the sagittal direction, or it is planar in the meridional direction and convex in the sagittal direction, or it is formed concave in the meridional direction and convex in the sagittal direction.
- If an external diffraction order (k=−1, −2, −3) is used, the refractive power in the sagittal direction is greater than in the meridional direction, i.e., the element is concave in the sagittal direction and planar in the meridional direction, or it is planar in the sagittal direction and convex in the meridional direction, or it is formed concave in the sagittal direction and convex in the meridional direction.
- In the present application, the order which is diffracted to the surface normal line is denoted the internal order and assigned positive numbers, while the order which is diffracted away from the surface normal line is designated the external order and is assigned negative numbers.
- In another embodiment of the invention, the stigmatic imaging is achieved by a variation of the distance between the grating lines.
- The at-least one physical diaphragm according to the invention essentially serves for the purpose that light with wavelengths far above 100 nm does not enter into the illumination system. This can be achieved particularly by blocking the zeroth diffraction order. Due to the one physical diaphragm, all diffraction orders are preferably blocked except for a so called used order. The used order, for example, can be the 1st order.
- It is particularly preferred if the rays have wavelengths in the range of 7 to 26 nm after the physical diaphragm, due to the combination of grating and physical diaphragm.
- The grating element is preferably designed as a blazed grating, which is optimized to a maximal efficiency in a pregiven diffraction order. Blazed gratings are known, for example, from the Lexikon der Optik [Optics Lexicon], edited by Heinz Haferkorn, VEB Bibliographic Institute, Leipzig, 1990, pp. 48 to 49. They are characterized by an approximately triangular groove profile.
- In order to avoid too high of a heat load on the physical diaphragm in the diaphragm plane, a part of the undesired radiation can be filtered out by additional diaphragms in the illumination system.
- In addition to the illumination system, the invention also provides a projection exposure system with such an illumination system as well as a method for the production of microelectronic components.
- An example of the invention will be described below on the basis of the figures.
- Here:
-
FIG. 1 shows the arrangement of a grating element in the beam path of the collector unit of an illumination system, -
FIG. 2 shows the grating element and the physical diaphragm of an illumination system, -
FIG. 3 shows a blazed grating, -
FIG. 4 shows the maximal possible diffraction efficiency for grating elements designed as blazed gratings, and -
FIG. 5 shows an EUV projection exposure system with an illumination system according to the invention. -
FIG. 1 shows an illumination system with agrating element 1 as well as a physical diaphragm in the diaphragm plane. The light oflight source 3 is collected by a collecting component,collector 5, in the illumination system shown. Thecollector 5 in this example is an ellipsoid-shaped mirror, which produces an image oflight source 3. The convergent light bundle with an aperture of approximately NA=0.1 behindcollector 5 is deflected viagrating element 1 in grazing incidence in such a way that the intermediate image of the light source comes to lie in or in the vicinity of the diaphragm plane of the physical diaphragm 7.3. - Due to the several partial diaphragms 7.1, 7.2, arranged in front of physical diaphragm 7.3, undesired radiation can be filtered out beforehand, in order to reduce the heat load on physical diaphragm 7.3. The physical diaphragm has a circular opening, which is situated in the focal plane of the desired diffraction order, here the −1.
order 16. The diaphragms 7.1, 7.2 may also be cooled, but this is not shown. In addition,grating element 1 can be cooled, for example, by a cooling on the back side. Thedevice 8 for back-side cooling ofgrating element 1 is preferably a liquid cooling device with inlet 10.1 and outlet 10.2. Due to gratingelement 1 and physical diaphragm 7.3, the 0th order that encompasses all wavelengths of the light source can be completely blocked out in the illumination system according to the invention. In addition, all of the other orders except for the −1st order are blocked. - If the grating in an illumination system with collector according to
FIG. 1 is designed as a planar grating with the same grating period, then an astigmatic imaging of the light source results. In order to be able to well separate the diffraction orders from one another, a stigmatic imaging oflight source 3 into the plane of the physical diaphragm 7.3 is necessary for a diffraction order, which is not the zeroth diffraction order. It can be attempted by various methods to correct the astigmatism, for example, by introducing additional refractive power in the meridional plane or sagittal plane or by variation of the distance between the grating lines. - How this is derived will be given in the following.
- The starting point for subsequent considerations is the grating equation for a parallel beam bundle:
Nk·λ=sin α+sin β (1)
wherein N is the number of lines, k is the diffraction order, λ is the wavelength, α is the angle of incidence and β is the diffraction angle (relative to the surface normal of the carrier surface and referred to the chief ray CRbefore or CRafter). The nomenclature which is used in the following derivation is oriented to the “Lexikon der Optik [Optics Lexicon] in two volumes, edited by H. Paul, Heidelberg, Berlin, Spektrum Academic Publishers, 1999, Vol. 1, A-L, pp. 77-80. - Reference is made to
FIG. 2 for the explanation of the following derivation. - In the case shown in
FIG. 2 , the convergent radiation of a light source (not shown) throughgrating element 1 is spectrally split and selected atintermediate focus 19 by physical diaphragm 7.3. An NA of 0.12 is achieved atintermediate focus 19. A convergent incidentlight bundle 100 is shown inFIG. 2 . This is diffracted at grating 1. Thebeam bundle 12 which is diffracted in the 0th diffraction order is shown along with thebeam bundle 14 which is diffracted in the first diffraction order. The beam bundle diffracted in the 0th diffraction order has afocal point 112 and the beam bundle diffracted in the first order has afocal point 114. The image widths sbefore and safter are defined by the respectivefocal points striking point 102 of the chief ray CRbefore of theincident beam bundle 100 on grating 1. Here, sbefore designates the distance of thestriking point 102 from thefocal point 112 of the beam bundle diffracted in the zeroth diffraction order and safter denotes the distance ofstriking point 102 fromfocal point 114 of the beam bundle diffracted in the first order. Angle α designates the angle of incidence of the chief ray CRbefore of the incident beam bundle with respect to the surface normal line of the carrier surface of grating 1 and β denotes the diffraction angle of the chief ray CRafter of thebeam bundle 14 diffracted in the 1st order, relative to the surface normal line of the carrier surface. In addition, theplanes striking point 102 on grating 1 perpendicular to the chief ray CRbefore of thestriking beam bundle 100 and perpendicular to the chief ray CRafter of thebeam bundle 14 diffracted in the 1st diffraction order. - If one now considers in the convergent beam path of an illumination system a reflection grating, which is placed as shown in
FIG. 2 in front of theintermediate focus 19, which coincides in the present example of embodiment withfocal point 114 of the beam bundle diffracted in the 1st order, then the optical effect of the grating must be observed. This can be derived from the conservation of the phase-space volume or the light value or the Eténdue. Since the diffraction angle β for orders that are not equal to the zeroth order is not equal to the angle of incidence α, the cross section of the beam bundle is modified inplanes
c ff=cos (β)/cos (α) (2) - The following results for the bundle cross-section at the grating:
dafter=dbeforecff (3)
or for the numerical aperture NA
NA after =NA before/c ff (4)
wherein dafter denotes the bundle cross-section of the diffractedbeam bundle 14 in theplane 108 and dbefore denotes the bundle cross-section of theincident beam bundle 100 inplane 106, NAafter denotes the numerical aperture of the diffractedbeam bundle 14 and NAbefore denotes the numerical aperture of the incident beam bundle. - The following results for the image width s as previously defined, calculated starting at the grating:
safter=sbeforecff 2 (5) - Care has to be taken that the grating acts only in the meridional or dispersive direction. In order to obtain a stigmatic imaging it is advantageous to introduce an additional optical effect, e.g., in the sagittal direction.
- This can be achieved, for example, for the case when an internal diffraction order (k=1, 2, 3) is used, by a convex curvature in the sagittal direction.
- For the case when an external diffraction order (k=−1, −2, −3) is used, it is advantageous that the grating is selected as sagittal concave.
- Alternatively to a curved grating, the grating line distance may also be varied.
- For the case of a sagittal convex curvature, the radius must be selected such that an image width of sbeforecff 2 is obtained from the image width Sbefore in the 0th order. The sagittal focal distance fs can be calculated by means of the imaging equation:
f s =s before/(1/c ff 2−1) (6) - Finally, the sagittal radius results together with the angle of diffraction:
R s =f s(cos α+cos β) (7) - It will be estimated in the following on an example of embodiment how a
grating element 1 must be constructed that the following conditions are fulfilled: -
- the beam bundles of the 0th and 1st order or −1st order are separated, i.e., at the focal point of one beam pencil of one diffraction order, there is no overlap of this beam bundle by a beam bundle of another diffraction order;
- the utilization wavelength used must be separate from the unwanted wavelengths;
- the distance to the intermediate focus must be small, so the grating does not become too large;
- the diffraction geometry must be optimized for best diffraction efficiency;
- the astigmatism, which produces a defocusing effect for the internal order and a focusing effect for the external order, should remain small.
- In particular, the first condition is decisive for the effectiveness of the grating element. A formula for estimating the separation of the beam pencil of the different diffraction orders from one another can be derived as follows with reference to
FIG. 2 . The distance Δx0 between the chief rays of the beam bundles of different diffraction orders at the focal point of the 0 order from the diffraction angles is:
Δx 0 =s before sin (α+β) (8)
and the distance Δx1 between the chief rays of the beam bundles of different diffraction orders at the focal point of the diffraction order e.g. the 1st or −1st diffraction order is:
Δx 1 =s before c ff 2 sin (α+β) (9) - Since the respective other beam bundle is not focused, which means that it has an extension, it is necessary for estimating whether the beam pencils do not overlap in the focal point, to estimate the extension of the other beam bundle. This can be estimated by the divergence or the numerical aperture. For the extension of the beam bundle of the 0th order at the focal point of the diffraction order, the following results:
Δd 1=2NA c ff |s before c ff 2 −s before| (10)
and for the extension of the beam bundle of the 1st order or of the −1st order at the focal point of the 1st order or the −1st order:
Δd 0=2NA|s before c ff 2 −s before| (11) - The difference between, e.g., Δx0 and Δd0/2 yields, e.g., the distance of the edge rays of the diffracted beam bundle from the focal point of the beam bundle of the 0th order. In order to prevent an overlap of different beam bundles, this distance should correspond to at least half the diameter of the beam bundle in the focal point, which is denoted Δxf; a sufficient separation of the beam bundle of the 0th diffraction order from the beam bundles of other diffraction orders is then achieved.
- The following is thus applied:
s before sin (α+β)−NA|s before c ff 2 −s before |>Δx f (12)
or
s before c ff 2 sin (α+β)−NA c ff |s before c ff 2 −s before |c ff >Δx f (13) - With the above-given considerations and formulas, the grating element with sagittal convex curvature, which is characterized by a grating efficiency of 56% can be constructed, which is characterized by the values given below in Table 1.
TABLE 1 Characteristic values of a grating element with convex transverse curvature Wavelength 13.4 nm Photon energy 92.5 eV Number of lines 1600 l/ mm Diffraction order 1 fixed focus constant, cff 1.2 angle of incidence α of the 72.360 degrees chief ray CRbefore Diffraction angle β of the chief −68.676 degrees ray CRafter diffracted in the 1st order Blazed depth 20.1 nm Grating-focus distance 432 mm Sagittal radius 654.555 mm NA (after grating) 0.12 Grating length 237 mm Material used Ru Microroughness 0.5 nm (rms) Grating efficiency 56 % - With the
grating element 1 according to the embodiment in Table 1 in combination with a diaphragm, wavelengths above approximately 18 nm and below 8 nm can be almost completely filtered out. The heat load on the mirror of a projection system can be clearly reduced in this way. - In order to obtain a
grating element 1 with optimal diffraction efficiency, the grating element is preferably configured as a blazed grating. - A blazed grating with approximately triangular groove profile is shown in
FIG. 3 .Reference number 11 designates the ray e.g., the chief ray of a beam bundle, striking thegrating element 1 designed as a blazed grating; 12 denotes the ray, e.g. the chief ray of a beam bundle, reflected at the grating in the 0th order and 16 denotes the ray, e.g., the chief ray of a beam bundle, diffracted in the −1st order. The blazed depth B is a function of angles of incidence and reflection. Thus, it is advantageous if the blazed depth is changed as a function of the position on the grating in order to obtain a maximal diffraction efficiency with a convergent beam pencil. - If one uses such a grating element, whose local blazed depth B changes with position on the grating, then a maximal efficiency is obtained according to
FIG. 4 . AsFIG. 4 shows, the diffraction efficiency (1) depends on the material used. - In
FIG. 4 ,reference number 200 denotes the diffraction efficiency η(1) for a wavelength of λ=13.5 nm for ruthenium,reference number 202 for palladium,reference number 204 for rhodium andreference number 206 for gold. - As can be seen from
FIG. 4 , the greatest efficiency of 0.7 can be achieved with ruthenium. A coating of palladium or rhodium has better long-time properties, but has an efficiency η(1) of only 0.67, which is 3% poorer. Gold is usually used for the synchrotron grating, but has a clearly poorer efficiency than the above-named materials at λ=13.5 nm, as can be seen fromcurve 206. - An EUV projection exposure system with a
grating element 1 according to the invention is shown inFIG. 5 . The EUV projection exposure system comprises alight source 3, a collecting optical component, a so-calledcollector 5, which is formed as a nested collector.Collector 5 images thelight source 3 lying in the object plane of the illumination system in a secondary light source 4 in or in the vicinity of a diaphragm plane 7.3. -
Light source 3, which can be, for example, a laser plasma source or a plasma discharge source, is arranged in the object plane of the illumination system. The image of the primary light source, which is also designated as the secondary light source, comes to lie in the image plane 7.3 of the illumination system. - Additional diaphragms 7.1, 7.2 are arranged between
grating element 1 and the physical diaphragm 7.3 in order to block the light of undesired wavelengths, particularly wavelengths longer than 30 mm. According to the invention, the focus of the 1st order comes to lie in the plane of diaphragm 7.3, i.e.,light source 3 is imaged nearly stigmatic in the plane of diaphragm 7.3 by the collector and the grating spectral filter in the 1st diffraction order. The imaging in all other diffraction orders is not stigmatic. - In addition, the illumination system of the projection system comprises an
optical system 20 for forming and illuminatingfield plane 22 with a ring-shaped field. The optical system comprises two faceted mirrors 29.1, 29.2 as well as two imaging mirrors 30.1, 30.2 and a field-forming grazing-incidence mirror 32 as the mixing unit for homogeneous illumination of the field. Additional diaphragms 7.4, 7.5, 7.6, 7.7 are arranged inoptical system 20 for suppressing stray light. - The first faceted mirror 29.1, the so-called field-faceted mirror, produces a plurality of secondary light sources in or in the vicinity of the plane of the second faceted mirror 29.2, the so-called pupil-faceted mirror. The subsequent imaging optics image the pupil-faceted mirror 29.2 in the exit pupil of the illumination system, which comes to lie in the entrance pupil of the
projection objective 26. The angle of inclination of the individual facets of the first and second faceted mirrors 29.1, 29.2 are designed in such a way that the images of the individual field facets of the first faceted mirror 29.1 overlap in thefield plane 22 of the illumination system and thus an extensively homogenized illumination of a pattern-bearing mask, which comes to lie in thefield plane 22, is possible. The segment of the ring field is formed by means of the field-forming grazing-incidence mirror 32 operated under grazing incidence. - A double-faceted illumination system is disclosed, for example, in U.S. Patent US-B-6,198,739, imaging and field-forming components in PCT/EP/00/07258. The disclosure contents of these documents is incorporated to the full extent in the present Application.
- The pattern-bearing mask, which is also designated as the reticle, is arranged in
field plane 22. The mask is imaged by means of aprojection objective 26 in theimage plane 28 offield plane 22. Theprojection objective 26 is a 6-mirror projection objective, such as disclosed, for example, in U.S. application No. 60/255214, filed on Dec. 13, 2000, in the U.S. Patent Office for the Applicant or DE-A-10037870, the disclosure content of which is fully incorporated into the present application. The object to be exposed, for example, a wafer, is arranged inimage plane 28. - The replica technique is considered, for example, as a possible manufacturing method for a grating element according to the invention.
- The invention gives for the first time an illumination system, with which undesired wavelengths can be selected directly after the light-source unit and which represents an alternative to filter foils, which are problematic, particularly with respect to the heat load.
-
- 1 grating element
- 3 light source
- 5 collector
- 7 .1, 7.2, 7.3
- 7.4, 7.5, 7.6
- 7.7 diaphragms
- 8 cooling device
- 10.1,10.2 inlet and outlet of the cooling device
- 11 incident radiation
- 12 0th order of the wavelength used
- 14 1st order of the wavelength used
- 16 −1st order of the wavelength used
- 20 optical system
- 22 field plane
- 26 projection objective
- 28 image plane of the field plane
- 29.1, 29.2 faceted mirrors
- 30.1, 30.2 imaging mirrors
- 32 field-forming mirror
- 34 exit pupil of the illumination system
- 100 convergent incident beam bundle
- 102 striking point of the chief ray CRbefore on grating 1
- 106 plane, which is perpendicular to the chief ray CRbefore
- 106 plane, which is perpendicular to the chief ray CRafter
- 112 focal point of the beam pencil diffracted in the 0th order
- 114 focal point of the beam pencil diffracted in the 1st order
- 200, 202
- 204, 206 diffraction efficiency η(1) for different materials
Claims (24)
1-17. (canceled)
18. An illumination system comprising:
a mirror;
a diaphragm in a light path downstream of said mirror; and
a field plane in said light path, downstream of said diaphragm.
19. The illumination system of claim 18 , wherein said mirror is a facetted mirror having a plurality of facets.
20. The illumination system of claim 19 , further comprising a normal incidence mirror in said light path, downstream of said facetted mirror and upstream of said field plane.
21. The illumination system of claim 20 , wherein said normal incidence mirror is situated downstream of said diaphragm.
22. The illumination system of claim 20 , further comprising a grazing incidence mirror situated in said light path, downstream of said normal incidence mirror and upstream of said field plane.
23. The illumination system of claim 22 , wherein said diaphragm is situated in said light path, downstream of said normal incidence mirror and upstream of said grazing incidence mirror.
24. The illumination system of claim 19 , further comprising a first normal incidence mirror and a second normal incidence mirror, both of which are in said light path, downstream of said facetted mirror and upstream of said field plane.
25. The illumination system of claim 24 , wherein said diaphragm is situated downstream of said first normal incidence mirror and upstream of said second normal incidence mirror.
26. The illumination system of claim 19 , further comprising a grazing incidence mirror situated in said light path, downstream of said facetted mirror and upstream of said field plane.
27. The illumination system of claim 19 , wherein said plurality of facets includes a first facet and a second facet that are imaged into said field plane as a first image and a second image that at least partially overlap one another.
28. The illumination system of claim 19 ,
wherein said facetted mirror is a first facetted mirror, and said plurality of facets is a first plurality of facets, and
wherein said illumination system further comprises a second facetted mirror having a second plurality of facets, situated in said light path downstream of said first facetted mirror and upstream of said diaphragm.
29. The illumination system of claim 28 , wherein said second plurality of facets includes a first facet and a second facet that are imaged into an exit pupil of said illumination system.
30. The illumination system of claim 19 ,
wherein said facetted mirror is a first mirror,
wherein said plurality of facets includes a first facet and a second facet, and
wherein said illumination system further comprises a second mirror in said light path, downstream of said first mirror and upstream of said field plane, that images said first and second facets into said field plane.
31. The illumination system of claim 18 ,
wherein said mirror is a first mirror, and
wherein said illumination system further comprises a second mirror in said light path, downstream of said first mirror and upstream of said field plane.
32. The illumination system of claim 31 , wherein said second mirror is a facetted mirror having a plurality of facets.
33. The illumination system of claim 31 , wherein said diaphragm is situated downstream of said second mirror.
34. The illumination system of claim 18 , further comprising a light source that emits light into said light path, upstream of said mirror, having a wavelength of less than or equal to about 100 nm.
35. The illumination system of claim 34 ,
wherein said diaphragm is a first diaphragm, and
wherein said illumination system further comprises a second diaphragm situated in said light path downstream of said light source and upstream of said mirror.
36. The illumination system of claim 18 , wherein said diaphragm suppresses stray light.
37. The illumination system of claim 18 , wherein said diaphragm partially surrounds said light path, and partially suppresses stray light.
38. A projection exposure system comprising:
(a) an illumination system for illuminating a pattern-bearing mask, wherein said illumination system includes:
a mirror;
a diaphragm in a light path downstream of said mirror; and
a field plane in said light path, downstream of said diaphragm, for accommodating the pattern-bearing mask;
(b) a holder for holding a light-sensitive object; and
(c) a projection objective for imaging the pattern-bearing mask onto the light-sensitive object.
39. A method, comprising:
producing a microelectronic component, wherein said producing includes employing a projection exposure system having:
(a) an illumination system for illuminating a pattern-bearing mask, wherein said illumination system includes:
a mirror;
a diaphragm in a light path downstream of said mirror; and
a field plane in said light path, downstream of said diaphragm, for accommodating the pattern-bearing mask;
(b) a holder for holding a light-sensitive object; and
(c) a projection objective for imaging the pattern-bearing mask onto the light- sensitive object.
40. A method comprising:
situating a diaphragm in a light path in an illumination system, downstream of a facetted optical element and upstream of a field plane,
wherein said diaphragm suppresses stray light in said illumination system.
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US09/305,017 US6198793B1 (en) | 1998-05-05 | 1999-05-04 | Illumination system particularly for EUV lithography |
US09/679,718 US6438199B1 (en) | 1998-05-05 | 2000-09-29 | Illumination system particularly for microlithography |
DE10127298 | 2001-06-06 | ||
DE10127298 | 2001-06-06 | ||
US09/961,819 US7248667B2 (en) | 1999-05-04 | 2001-09-24 | Illumination system with a grating element |
US11/820,375 US20070242799A1 (en) | 1999-05-04 | 2007-06-19 | Illumination system |
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Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2012159968A2 (en) | 2011-05-23 | 2012-11-29 | Carl Zeiss Smt Gmbh | Illumination optical unit |
DE102011076297A1 (en) | 2011-05-23 | 2012-11-29 | Carl Zeiss Smt Gmbh | cover |
US9632422B2 (en) | 2011-05-23 | 2017-04-25 | Carl Zeiss Smt Gmbh | Illumination optical unit |
Also Published As
Publication number | Publication date |
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US7248667B2 (en) | 2007-07-24 |
US20020186811A1 (en) | 2002-12-12 |
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