Abstract:
In general, in one aspect, the invention features a system that includes a catoptric projection objective having an optical axis and including a plurality of projection objective elements positioned between an object plane and an image plane, the object and image planes being orthogonal to the optical axis, the projection objective being configured so that during operation the projection objective directs radiation reflected at the object plane to the image plane to form an image at the image plane of an object positioned in a field at the object plane, the field having a first dimension of 8 mm or more and a second dimension of 8 mm or more, the first and second dimensions being along orthogonal directions. The system also includes an illumination system including a plurality of illumination system elements, the illumination system being configured so that during operation the illumination system directs the radiation to the field at the object plane, where a chief ray of the radiation has an angle of incidence of 10° or less at the object plane.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to illumination systems and to microlithography exposure system that use illumination systems. 
       BACKGROUND 
       [0002]    Illumination systems are widely used in microlithography to illuminate a reticle with radiation having a desired homogeneity and pupil fill. A projection objective is then used to transfer a pattern from the reticle to a substrate by forming an image of the reticle on a layer of a photosensitive material disposed on the substrate. In general, illumination systems fall into three different classes: dioptric systems; catoptric systems; and catadioptric systems. Dioptric systems use exclusively refractive elements (e.g., lens elements) to shape radiation from a source to have desired properties at an object plane of the projection objective. Catoptric systems use exclusively reflective elements (e.g., mirror elements) to shape the radiation. Catadioptric systems use both refractive and reflective elements to shape the radiation. 
       SUMMARY 
       [0003]    Microlithography exposure systems are disclosed that feature illumination systems for illuminating reflective reticles. In order to illuminate the reticle, at least the last element in the radiation path of the illumination system is positioned on the same side of the reticle as the projection objective. Accordingly, the optical design of such systems should account for the relative positioning of the last element of the illumination system relative to the elements of the projection objective. In certain embodiments, the last element of the illumination system is a grazing incidence mirror, configured relative to the projection objective to illuminate a relatively large field at the object plane with relatively low incidence angles. 
         [0004]    In general, in a first aspect, the invention features a system that includes a catoptric projection objective having an optical axis and including a plurality of projection objective elements positioned between an object plane and an image plane, the object and image planes being orthogonal to the optical axis, the projection objective being configured so that during operation the projection objective directs radiation reflected at the object plane to the image plane to form an image at the image plane of an object positioned in a field at the object plane, the field having a first dimension of 8 mm or more and a second dimension of 8 mm or more, the first and second dimensions being in orthogonal directions. The system also includes an illumination system including a plurality of illumination system elements, the illumination system being configured so that during operation the illumination system directs the radiation to the field at the object plane, where a chief ray of the radiation has an angle of incidence of 10° or less (e.g., about 9° or less, about 8° or less, about 7° or less, about 6°) at the object plane. 
         [0005]    Embodiments of the system can include one or more of the following features. For example, the chief ray can be the chief ray that intersects the object plane at a central field point. 
         [0006]    In embodiments, the projection objective includes a first projection objective element and the illumination system includes a first illumination system element, the first projection objective element being the first element in a path of the radiation from the object plane to the image plane and the first illumination system element being the last element in the illumination system in the path of the radiation prior to the object plane, where the first projection objective element can be closer to the object plane than the first illumination system element. The first illumination system element can be positioned a distance z min  or more from the object plane, where z min  is given by the equation: 
         [0000]    
       
         
           
             
               
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         [0000]    where NAO is the numerical aperture of the projection objective at the object plane, σ is the relative numerical aperture of the illumination system at the object plane, CRAO is the chief ray angle of a central field point at the object plane, dy is the dimension of the field in the direction orthogonal to the optical axis, y 0  is a distance between the central field point and the optical axis, and y min  is a minimum separation between the radiation and either the first illumination system element or the first projection system element. The first illumination system element can be positioned further from the optical axis than the field at the object plane. 
         [0007]    The field at the object plane can be rectangular-shaped or can be arc-shaped. 
         [0008]    The first illumination system element can be a mirror. The mirror can be a plane mirror or a curved mirror (e.g. a torodial mirror). The mirror can be arranged as a grazing incidence mirror. 
         [0009]    In some embodiments, the illumination system includes a field mirror including a plurality of facet mirrors and during operation the illumination system images each facet mirror to the object plane. The facet mirrors can be rectangular mirrors or arc-shaped mirrors 
         [0010]    The first dimension can be 9 mm or more (e.g., about 10 mm or more, about 12 mm or more, about 15 mm or more, about 20 mm or more, about 30 mm or more, about 40 mm or more, about 50 mm or more). The second dimension can be 9 mm or more (e.g., about 10 mm or more, about 12 mm or more, about 15 mm or more, about 20 mm or more, about 30 mm or more, about 40 mm or more, about 50 mm or more). In some embodiments, the first dimension is about 20 mm or less (e.g., about 15 mm or less, about 12 mm or less, about 10 mm or less). In certain embodiments, the second dimension is about 40 mm or less (e.g., about 30 mm or less, about 26 mm or less, about 20 mm or less, about 15 mm or less, about 12 mm or less, about 10 mm or less). The chief ray at a central field point can have an angle of incidence of 10° or less (e.g., about 9° or less, about 8° or less, about 7° or less, about 6°). The projection objective can have an image-side numerical aperture of 0.25 or more (e.g., 0.3 or more, 0.35 or more, 0.4 or more, 0.45 or more, 0.5 or more). In some embodiments, the projection objective has an object side numerical aperture of about 0.06 or more. 
         [0011]    The projection objective can be a reduction projection objective. In certain embodiments, the projection objective is a transfer projection objective. The projection objective can include an even number (e.g., 2, 4, 6, 8, or more) of curved (e.g., convex or concave) mirrors. 
         [0012]    The object can be a reticle. The object can be configured to reflect radiation from the illumination system. 
         [0013]    The system can include a source configured to produce radiation that is directed by the illumination system to the object plane. The radiation can have a wavelength that is less than 400 nm (e.g., about 248 nm or less, about 193 nm or less, about 13 nm or less). 
         [0014]    The system can be a microlithography exposure system (e.g., a scanning microlithography exposure system). 
         [0015]    In general, in another aspect, the invention features a system that includes a catoptric projection objective having an optical axis and including a plurality of projection objective elements including a first projection objective element, the projection objective being configured so that during operation the projection objective directs radiation from an object plane to an image plane to form an image at the image plane of an object positioned in a field at the object plane, the first projection objective element being the first element in a path of the radiation from the object plane to the image plane and the field having a dimension of 8 mm or more in a direction orthogonal to the optical axis. The system also includes an illumination system having a plurality of illumination system elements including a first illumination system element, the illumination system being configured so that during operation the illumination system directs the radiation to the field at the object plane, where the first illumination system element is the last illumination system element in the path of the radiation prior to the object plane. A chief ray of the radiation has an angle of incidence of 10° or less at the object plane, and the first illumination system element is on the same side of the object plane as the first projection objective element. Embodiments of the system can include one or more of the features listed above with respect to the first aspect. 
         [0016]    In general, in another aspect, the invention features a system that includes a projection objective including a plurality of projection objective elements including a first projection objective element, the projection objective being configured so that during operation the projection objective directs radiation from an object plane to an image plane to form an image at the image plane of an object positioned at the object plane, the first projection objective element being the first element in a path of the radiation from the object plane to the image plane. The system also includes an illumination system having a plurality of elements including a grazing incidence mirror, the illumination system being configured so that during operation the illumination system directs radiation to the field at the object plane, the grazing incidence mirror being the last element in the illumination system in the path of the radiation prior to the object plane. The first projection objective element is closer to the object plane than the grazing incidence mirror. Embodiments of the system can include one or more of the features listed above with respect to the first aspect. 
         [0017]    In general, in a further aspect, the invention features a system that includes a catoptric projection objective having an optical axis and including a plurality of projection objective elements positioned between an object plane and an image plane, the object and image planes being orthogonal to the optical axis and the projection objective being configured so that during operation the projection objective directs radiation reflected at the object plane to the image plane to form an image at the image plane of an object positioned in a field at the object plane, the field having a dimension of 8 mm or more in a direction orthogonal to the optical axis. The system also includes an illumination system including a plurality of illumination system elements, the illumination system being configured so that during operation the illumination system directs the radiation to the field at the object plane. A chief ray of the radiation has an angle of incidence of 10° or less at the object plane. The system is a scanning microlithography exposure system and the direction orthogonal to the optical axis is a scan direction of the scanning microlithography exposure system. Embodiments of the system can include one or more of the features listed above with respect to the first aspect. 
         [0018]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description, drawings, and claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a schematic diagram of a microlithography exposure system. 
           [0020]      FIG. 2A  is a schematic diagram of an illumination system of a microlithography exposure system. 
           [0021]      FIGS. 2B-D  are schematic diagrams showing aspects of an illumination system. 
           [0022]      FIG. 2E  are plots of different intensity profiles through sections of an illuminated object field. 
           [0023]      FIG. 3A  shows an embodiment of a microlithography exposure system. 
           [0024]      FIG. 3B  is a schematic diagram showing components of the microlithography exposure system shown in  FIG. 3A . 
           [0025]      FIG. 4  is a schematic diagram showing components of a microlithography exposure system. 
           [0026]      FIG. 5  is a diagram of an embodiment of a projection objective configured to image a reticle from an object plane to an image plane and a grazing incidence mirror for directing radiation to the reticle. 
       
    
    
       [0027]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0028]    Referring to  FIG. 1 , a microlithography exposure system  100  generally includes a light source  110 , an illumination system  120 , a projection objective  101 , and a stage  130 . A Cartesian coordinate system is shown for reference. Light source  110  produces radiation  112  at a wavelength λ which is collected by illumination system  120 . Illumination system  120  interacts with (e.g., expands and homogenizes) the radiation and directs radiation  122  to a reticle  140  positioned at an object plane  103 . Projection objective  101  directs radiation  142  reflected from reticle  140  onto a light sensitive layer (e.g., a resist) on a substrate  150  positioned at an image plane  102  of projection objective  101 , forming an image of reticle  140  at image plane  102 . Generally, microlithography exposure system  100  is configured to image a certain portion of reticle  140  positioned at a certain region of object plane  103  to image plane  102 . This region of object plane  103  is referred to as the object field and the corresponding portion at image plane  102  is referred to as the image field. The radiation on the image-side of projection objective  101  is depicted as rays  152 . As shown in  FIG. 1 , 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 . In embodiments where lithography tool  100  is a scanner, the tool includes a reticle stage that moves reticle  140  in a scan direction with respect to illumination system  120 . 
         [0029]    Projection objective  101  includes a reference axis  105  (e.g., an optical axis). In certain embodiments, such as where projection objective  101  is symmetric with respect to a meridional section, reference axis  105  is perpendicular to object plane  103  and passes through the center of the object field. In certain embodiments, axis  105  intersects both the object field and the image field of projection objective  101 . In some embodiments, both the object field and the image field of projection objective  101  are not intersected by axis  105 . Such fields are referred to as off-axis fields. 
         [0030]    In general, projection objective  101  can be designed to provide a desired magnification of the reticle image. In some embodiments, projection objective  101  is a reduction objective. In other words, the image at image plane  102  is smaller than the object being imaged (e.g., reduced 4× or more, 5× or more, 6× or more, 8× or more). In certain embodiments, projection objective  101  is a transfer objective or relay lens, where the object and image are the same size. In some embodiments, the image is larger than the object. 
         [0031]    Projection objective  101  can be designed to have a desired numerical aperture (NA) at image plane  102 . This is referred to as the image-side numerical aperture. In some embodiments, the image-side numerical aperture is about 0.1 or more (e.g., 0.2 or more, 0.3 or more, 0.4 or more). In certain embodiments, projection objective  101  is designed to have a very high image-side numerical aperture. For example, in some embodiments, the image side numerical aperture is in a range from 0.5 to 1 (e.g., about 0.5 or more, about 0.6 or more, about 0.7 or more, about 0.8 or more, about 0.9 or more). In some embodiments, the image-side NA can be greater than 1. For example, where an immersion liquid (e.g., as a liquid lens or planar film of liquid) is used between the final element in projection objective  101  and the substrate at the image plane, the image-side numerical aperture can be more than 1 (e.g., about 1.1 or more, about 1.2 or more, about 1.3 or more). 
         [0032]    Projection objective  101  also has a NA at object plane  103 , referred to as the object-side NA. In general, the object-side NA is related to the image-side NA by the magnification of projection objective  101 . Where projection objective  101  is a transfer objective or relay lens, the object and image-side NA&#39;s are the same. Where projection objective  101  is a reduction objective, the object-side NA is smaller than the image-side NA. In some embodiments, projection objective  101  can have an object-side NA of 0.0625 or more (e.g., about 0.08 or more, about 0.09 or more, about 0.1 or more, about 0.15 or more, about 0.2 or more). 
         [0033]    Illumination system  120  has a relative numerical aperture at object plane  103 . [The relative numerical aperture, s, refers to is the quotient between the numerical aperture of the illumination system (NAI) and the numerical aperture of the projection objective (NAO): σ=NAI/NAO. Both NAI and NAO are quantities measured in the object plane of the projection objective. In other words, NAO is the same as the object-side NA discussed above. In general, the value for σ is zero for perfectly coherent illumination (i.e., plane wave illumination propagating along a single direction) and greater than 2 for incoherent illumination. Values between zero and 2 typically describe partial coherent illumination which is typical for microlithography exposure system. In certain embodiments, 0.2&lt;σ&lt;1 holds. 
         [0034]    Light source  110  is selected to provide radiation at a desired operational wavelength, λ, of tool  100 . In some embodiments, 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. 
         [0035]    Typically, for projection objectives designed for operation in lithography tools, wavelength λ is in the ultraviolet portion of the electromagnetic spectrum. For example, λ 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). λ can be more than about 2 nm (e.g., about 5 nm or more, about 10 nm or more). In embodiments, λ can be about 193 nm, about 157 nm, about 13 nm, or about 11 nm. Wavelengths in the 1 nm to 100 nm range (e.g., 13 nm) are referred to as Extreme UV (“EUV”) wavelengths. Using a relatively short wavelength may be desirable because, in general, the resolution of a projection objective is approximately proportional to the wavelength. Therefore, shorter wavelengths can allow a projection objective to resolve smaller features in an image than equivalent projection objectives that use longer wavelengths. In certain embodiments, however, λ can be in non-UV portions of the electromagnetic spectrum (e.g., the visible portion). 
         [0036]    Typical light sources for wavelengths between 100 nm and 200 nm are excimer lasers, for example an ArF-Laser for 193 nm, an F 2 -Laser for 157 nm, an Ar 2 -Laser for 126 nm and a NeF-Laser for 109 nm. Since the transmission of the optical materials deteriorates with decreasing wavelength, the illumination systems can be designed with a combination of refractive and reflective components (i.e., catadioptric). For wavelengths in the EUV wavelength region, such as between 10 nm and 20 nm, lithography exposure apparatus  100  is designed as all-reflective (i.e., catoptric). Examples of EUV light sources are a Laser-Produced-Plasma-source, a Pinch-Plasma-Source, a Wiggler-Source or an Undulator-Source. 
         [0037]    Referring to  FIG. 2A , illumination system  120  includes optical components arranged to form a radiation beam with a homogeneous intensity profile and desired pupil fill. Typically, illumination system  120  includes a collector  210 , configured to collect radiation from source  110  and direct the radiation as a beam along an optical path to beam shaping optics  220 . Typically, collector  210  will produce a collimated or convergent beam. 
         [0038]    In general, the shape and intensity profile of the radiation exiting collector  210  different from a desired shape and intensity profile of the radiation at object plane  103 . For example, with reference to  FIG. 2B , a beam profile  212  between collection optics and beam shaping optics  220  is typically substantially circular in shape with an intensity profile that can vary substantially across its width. 
         [0039]    As discussed previously, the object field is the portion of object plane for which reticle  140  is imaged to image plane  102 . In general, the shape of the object field at object plane  103  is determined by projection objective  101 . Usually, the object field corresponds to a region of object plane  103  for which a reticle is imaged to image plane  102  with relatively low aberrations. Typically, the shape of the object field is dependent on the type of projection objective  101 . In stepper-type lithography tools, the object field is generally rectangular in shape. In scanner-type lithography tools, the object field is typically rectangular or arc-shaped. Catoptric projection objectives, for example, typically have an arc-shaped object field. 
         [0040]    Accordingly, beam shaping optics  220  include one or more components configured to provide a beam of radiation at objection plane  103  having a desired intensity profile across the object field and a desired pupil fill. For example, in some embodiments, beam shaping optics  220  can provide a beam having a substantially homogeneous intensity profile across the object field (e.g., the radiation intensity inside the object field varies by about ±5% or less) having the same size and shape as the object field. Other profiles are also possible as discussed below. 
         [0041]    Referring to  FIG. 2E , in general, the intensity profile of illumination across the object field can vary. Generally, it is desirable that the intensity is substantially constant in the object field, and substantially zero on either side of the field. This profile corresponds to the curve shown as  222 A. In some embodiments, the radiation intensity inside the object field varies by about ±10% or less (e.g., ±8% or less, ±5% or less). 
         [0042]    Other intensity distributions inside the object field are also possible. For example, the intensity distribution can be approximately trapezoidal (curve  222 B) or approximately Gaussian (curve  222 C). In the case of a scanning system, such a variation in radiation intensity typically can occur in the scan direction. 
         [0043]    In general, the edge of the field is determined as the location where the illumination intensity is half of I max , where I max  is the maximum illumination intensity within the field. 
         [0044]    Referring to  FIG. 2C , in catoptric systems, such as in microlithography exposure system designed for use at EUV wavelengths, an arc-shaped object field  222  is typically desired. Arc-shaped object field  222  corresponds to a segment of an annulus which is characterized by an inner radius of curvature, IR f , an outer radius of curvature, OR f , and a width, w f . Arc-shaped field  222  is also characterized by a height at the meriodonal plane of the projection objective, d y , which in this case is the difference between OR f  and IR f . For arc-shaped object field  222 , IR f  and OR f  are substantially constant across the width of the field. A Cartesian coordinate system is provided for reference in object plane  103 . Width w f  is measured along the x-axis, while height d y  is measured along the y-axis (where the y-z plane is the meridional plane of projection objective  100 ). In general, IR f  can vary as desired. In some embodiments, IR f  is about 40 mm or more (e.g., about 50 mm or more, about 60 mm or more, about 80 mm or more, about 100 mm or more, about 150 mm or more). In certain embodiments, IR f  is about 500 mm or less (e.g., about 400 mm or less, about 300 mm or less, about 250 mm or less, about 200 mm or less, about 150 mm or less, about 100 mm or less). IR f  can be in a range from about 50 mm to about 250 mm (e.g., in a range from about 100 mm to about 200 mm). 
         [0045]    Referring to  FIG. 2D , in some embodiments, object field  222  of microlithography exposure system  100  is rectangular in shape. As for the arc-shaped field, the rectangular field is characterized by width w f  and height d y . 
         [0046]    In general, for arc-shaped and/or rectangular field shapes, w f  may vary as desired. In certain embodiments, w f  can correspond to a reticle die width (or multiples of reticle die widths). For example, w f  can be selected so that the field corresponds to one, two, three or more die widths on the wafer. In some embodiments, w f  is about 20 mm or more (e.g., about 30 mm or more, about 40 mm or more, about 50 mm or more, about 60 mm or more, about 80 mm or more, about 100 mm or more, about 120 mm or more). In certain embodiments, w f  can be in a range from about 50 mm to about 250 mm (e.g., in a range from about 80 mm to about 200 mm). 
         [0047]    Height d y  can vary. In certain embodiments, it can be desirable to have a relatively large field height. For example, generally, a larger field height can be used to expose a larger image field on a substrate, reducing exposure time for a substrate and increasing throughput for the microlithography exposure system relative to apparatus with smaller field heights. In scanning system, for example, a larger field height can allow for relaxed dose control, because the exposure time for each resist point is increased. In some embodiments, dy is 4 mm or more (e.g., 5 mm or more, 6 mm or more, 8 mm or more, about 10 mm or more, about 20 mm or more, about 30 mm or more, about 40 mm or more, about 50 mm or more). In certain embodiments, dy is about 100 mm or less (e.g., about 80 mm or less, about 60 mm or less, about 50 mm or less). dy can be in a range from 5 mm to about 100 mm (e.g., in a range from 6 mm to about 60 mm, from 8 mm to about 40 mm, such as from about 10 mm to about 20 mm). 
         [0048]    As discussed above, illumination systems generally include beam shaping optics configured to provide a beam of radiation at object plane  103  having a desired intensity profile across the object field and a desired pupil fill. For example, in some embodiments, beam shaping optics  220  include a field raster plate that directs radiation from collection optics  210  to object plane  103  in a way that provides substantially homogeneous illumination of object field  222 . Moreover, beam shaping optics  220  can include one or more components configured to provide a desired fill of the exit pupil of illumination system  120 , which is located at the entrance pupil of the projection objective  101 . For example, beam shaping optics  220  can include one or more components that provide circular, annular, dipolar, or quadrupolar illumination at the entrance pupil of projection objective  101 . An appropriate pupil raster plate can be used to perform this function. 
         [0049]    In some embodiments, the illumination system includes a grazing incidence mirror that directs radiation from the pupil raster plate to the reticle. As used herein, a grazing incidence mirror refers to a mirror for which a maximum angle of incidence for a chief ray of the projection objective is more than 45°. In some embodiments, the maximum chief ray angle of incidence is about 60° or more (e.g., about 70° or more, about 75° or more, about 80° or more). A chief ray is a path of radiation through a microlithography exposure system that intersects the object plane at a point in the object field and intersects the optical axis of the projection objective at the aperture stop of projection objective. 
         [0050]    The grazing incidence mirror can be a curved mirror. For example, where the field raster elements are rectangular, a curved grazing incidence mirror can be used to form an arc-shaped object field distorting the images of the rectangular raster elements to form arc-shaped images. Examples of curved grazing incidence mirrors are shown in U.S. Pat. No. 7,186,983 B2, entitled “ILLUMINATION SYSTEM PARTICULARLY FOR MICROLITHOGRAPHY,” which issued on Mar. 6, 2007, the entire contents of which is incorporated herein by reference. 
         [0051]    Referring to  FIG. 3A , an example of such an illumination system is shown as illumination system  379 , which along with projection system  371 , form a microlithography projection exposure apparatus. Illumination system  379  directs radiation to a reticle  140  positioned at an object plane  381 . Projection objective  371  images a portion of reticle  140  illuminated by the radiation to a wafer  373  positioned at an image plane  383 . Reticle  140  is supported by a reticle stage  369  and wafer  373  is supported by a wafer stage  375 . 
         [0052]    Illumination system  379  includes a source  301 , a collector  303 , a field raster plate  309 , a pupil raster plate  315 , and mirrors  325 ,  323 , and  327 . Source  301  produces radiation that is directed by collector  303  towards field raster plate  309 . The path of the radiation is illustrated by a number of rays  340 , including a chief ray  345 . Field raster plate  309  reflects the radiation to pupil raster plate  315 . Field raster plate  309  includes a number of mirrors, each of which is imaged by illumination system  379  onto object plane  381 , overlapping at an object field. Pupil raster plate  315  also includes a number of mirrors, which are arranged to provide a desired illumination shape at each point in the field at object plane  381 . Mirrors  325  and  323  relay the radiation to mirror  327 . Mirror  327 , which is the grazing-incidence mirror, directs the radiation to object plane  381 . 
         [0053]    Projection system  371  includes a first mirror  377  and subsequent mirrors  390 ,  391 ,  392 ,  393 , and  394  arranged along an optical axis  347 . Radiation from illumination system  379  is reflected from reticle  140 , and is sent toward first mirror  377  along a path illustrated by a number of rays, including ray  345 . This radiation is then reflected by first mirror  377  and is subsequently directed to wafer  373  at image plane  383  via mirrors  390 ,  391 ,  392 ,  393 , and  394 , where an image of reticle  140  is formed. 
         [0054]    Referring also to  FIG. 3B , as discussed previously, mirror  327  directs rays  340  from toward reticle  140  positioned at object plane  381 . Mirror  327  is arranged at an angle ω with respect to optical axis  347 . Generally, ω is selected based on the direction of rays  340  prior to mirror  327  and the desired illumination angle with respect to object plane  381 . ω can be in a range from about 10° to about 80° (e.g., from about 20° to about 70°, from about 30° to about 60°, from about 40° to about 50°). 
         [0055]    Rays  340  illuminate object field  322  and the illuminated portion of reticle  140  reflects rays  340  toward first mirror  377 . Object field  322  has a height, d y . The central field point of object field  322  is located a distance y 0  from optical axis  347 . Here, the central field point refers to the location equidistant from the edges of the object field in the meridional plane of the projection objective. 
         [0056]    Chief ray  345  intersects object plane  381  at the central field point. Chief ray  345  has an incident angle at the central field point denoted by CRAO with respect to a normal  342  of object plane  381 . In general, CRAO can vary depending upon the specific design of the microlithography exposure system. In general, the microlithography exposure system is designed so that CRAO is greater than 0° so that chief ray  345  is not reflected back towards mirror  327  when the reticle  140  is positioned at object plane  381 . Typically, the microlithography exposure system is designed so that rays  340  reflected from reticle  140  are not blocked by mirror  327  prior to mirror  377 . In certain embodiments, it is desirable to have a relatively low CRAO as high values of CRAO can lead to unwanted imaging effects of the projection objective. For example, high values of CRAO can lead to shadow effects at the reticle that distort the reticle information passed into the projection objective. In certain embodiments, the microlithography exposure system can be designed so that CRAO is about 10° or less (e.g., about 9° or less, about 8° or less, about 7° or less, about 6° or less, about 5° or less). 
         [0057]    Upon reflection from reticle  140 , rays  345  pass mirror  327  before incidence on mirror  377 . The minimum separation between rays  345  before and after reflection from reticle  140 , measured at mirror  327 , is denoted by y min . Generally, the microlithography exposure system is designed so that y min  is sufficiently large that, allowing for the physical thickness of the base of mirror  327 , none of rays  340  are occluded by mirror  327 . In some embodiments, y min  is about 2 mm or more (e.g., about 4 mm or more, about 5 mm or more, about 6 mm or more, about 8 mm or more, about 10 mm or more, about 15 mm or more, about 20 mm or more). 
         [0058]    The minimum separation between mirror  327  and object plane  381  is denoted by z′. In general, z′ can vary. In certain embodiments, z′ is relatively small, which can allow for a design in which mirror  327  is relatively small. However, in such designs, CRAO tends to increase with increasing field height dy. Accordingly, utilizing a relatively small z′ can limit the dy where a relatively low value of CRAO is desired. Alternatively, in some embodiments, z′ can be selected so that dy is relatively large while and CRAO is relatively small. For example, z′ can be selected so that CRAO is 10° or less (e.g., 8° or less, 6° or less) while dy is more than 8 mm (e.g., about 10 mm or more, about 20 mm or more, about 30 mm or more, about 40 mm or more). 
         [0059]    In certain embodiments, desired values of dy and CRAO can be achieved by making z′≧z min , where z min  is given by the formula: 
         [0000]    
       
         
           
             
               
                 z 
                 min 
               
               = 
               
                 
                   dy 
                   + 
                   
                     y 
                     min 
                   
                 
                 Γ 
               
             
             , 
           
         
       
     
         [0000]    in which 
         [0000]    
       
         
           
             
               
                 
                   
                     Γ 
                     = 
                     
                       tan 
                        
                       
                         [ 
                         
                           arcsin 
                           [ 
                           
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   arctan 
                                    
                                   
                                     ( 
                                     
                                       
                                         
                                           ( 
                                           
                                             
                                               y 
                                               0 
                                             
                                             - 
                                             
                                               dy 
                                               / 
                                               2 
                                             
                                           
                                           ) 
                                         
                                         · 
                                         
                                           tan 
                                            
                                           
                                             ( 
                                             CRAO 
                                             ) 
                                           
                                         
                                       
                                       
                                         y 
                                         0 
                                       
                                     
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                             - 
                             
                               σ 
                               · 
                               NAO 
                             
                           
                           ) 
                         
                         ] 
                       
                     
                   
                   ] 
                 
                 + 
                 
                   tan 
                    
                   
                     [ 
                     
                       arcsin 
                       [ 
                       
                         
                           sin 
                            
                           
                             ( 
                             
                               arctan 
                                
                               
                                 ( 
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           y 
                                           0 
                                         
                                         + 
                                         
                                           du 
                                           / 
                                           2 
                                         
                                       
                                       ) 
                                     
                                     · 
                                     
                                       tan 
                                        
                                       
                                         ( 
                                         CRAO 
                                         ) 
                                       
                                     
                                   
                                   
                                     y 
                                     0 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                         
                         - 
                         NAO 
                       
                       ) 
                     
                     ] 
                   
                 
               
               ] 
             
             , 
           
         
       
     
         [0000]    where NAO is object-side numerical aperture of the projection objective and σ is the relative numerical aperture of the illumination system at the object plane. 
         [0060]    While in the foregoing embodiment, mirror  327  was positioned closer to object plane  381  than the first mirror  377  in projection objective  371 , other configurations are also possible. For example, in some embodiments, the grazing incidence mirror that is the last mirror in the radiation path in the illumination system is positioned further from the object plane than the first mirror in the radiation path in the projection objective. Referring to  FIG. 4 , which shows an example of such a configuration, grazing incidence mirror  427  is positioned further from object plane  481  than mirror  477 , which is the first mirror in a projection objective. As indicated in the figure, the minimum distance between mirror  427  and object plane  481  is indicated by d G  and the minimum distance between mirror  477  and object plane  481  is indicated by d M1 . Here, d G &gt;d M1 . Also shown in  FIG. 4  are rays  445 , which are directed by mirror  427  to illuminate an object field  440  at object plane  481 . 
         [0061]    Optical axis  447  of the projection objective is also shown. Mirror  477  extends a distance y M1  from optical axis  447  on the same side of optical axis  447  as mirror  427 . The minimum distance between the base of mirror  427  and optical axis  447  is y G , where y G &lt;y M1 . 
         [0062]    In embodiments where the grazing incidence mirror in the illumination system is further from the object plane than the first mirror in the projection objective, the separation, y min , between rays before and after reflection from the reticle, refers to the minimum separation of the rays at the first mirror in the projection objective, rather than the grazing incidence mirror as defined in  FIG. 3B . 
         [0063]    Configurations where the grazing incidence mirror in the illumination system is further from the object plane than the first mirror in the projection objective can include numerous benefits. For example, such configurations can satisfy the z′&gt;z min  relationship discussed above, allowing for relatively large field heights (dy) and a relatively low CRAO. Furthermore, because mirror  477  is positioned closer to object plane  481  than mirror  427 , there is no possibility that rays  445  will be occluded by mirror  427  after reflecting from the reticle at object plane  481 . Accordingly, y M1  is not constrained by the physical thickness of the base of mirror  427 , allowing for thicker bases to be used for mirror  427 . In other words, it is not necessary that y G &gt;y M1 . Accordingly, larger, more robust and stable mounts can be used for mirror  427  relative to configurations where thin mirror substrates are used because the base thickness constrains y M1 . The reduced size and space constraints on mirror  427  and its mount can also allow for additional heat shielding to be used between mirror  427  the components of the projection objective, which can reduce imaging aberrations due to heating of the projection objective by mirror  427 . 
         [0064]    Referring to  FIG. 5 , an example of a catoptric projection objective  500  and grazing incidence mirror  527  for illuminating a reticle positioned at an object plane  540 , is shown, where mirror  527  is positioned further from object plane  500  than a first mirror  577  in the radiation path in projection objective  500 . Projection objective  500  images an object field at object plane  540  to an image field at an image plane  575 . Projection objective  500  is a catoptric objective and, in addition to mirror  577 , includes mirrors  578 ,  579 ,  580 ,  581 ,  582 ,  583 , and  584 , presented in order respect to the path of radiation from object plane  540  to image plane  575 . Mirrors  577 - 584  are positioned along an optical axis, labeled OA. As shown in  FIG. 5 , each of the mirrors  577 - 584  corresponds to a segment of a rotationally symmetric surface about OA. 
         [0065]    Projection objective  500  has an image side numerical aperture of 0.35, an object side NA of 0.0875, and is a reduction objective with a magnification of 4×. The relative numerical aperture, σ, at object plane  540  of the illumination system is 0.8. The object field has a height, dy, of 40 mm and the CRAO is 6.392°. The distance, y 0 , between the central field point and the optical axis is 158 mm. 
         [0066]    Grazing incidence mirror  527  is positioned further from object plane  540  than first mirror  577 . In particular, the distance between mirror  577  and object plane  540  is 664 mm, while the minimum distance between mirror  527  and object plane  540  is 813 mm. In the meridional plane, the angle, ω, between mirror  527  and the optical axis is 58°. 
         [0067]    The minimum separation, y min , between rays  545 , before and after reflection from the reticle measured at mirror  577  is 4.1 mm. 
         [0068]    Each mirror  577 - 584  in projection objective  500  is an aspherical mirror. Aspherical mirror surfaces can be described by the equation: 
         [0000]    
       
         
           
             
               
                 P 
                  
                 
                   ( 
                   h 
                   ) 
                 
               
               = 
               
                 
                   
                     δ 
                     · 
                     h 
                     · 
                     h 
                   
                   
                     1 
                     + 
                     
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               1 
                               + 
                               CC 
                             
                             ) 
                           
                           · 
                           δ 
                           · 
                           δ 
                           · 
                           h 
                           · 
                           h 
                         
                       
                     
                   
                 
                 + 
                 
                   
                     C 
                     1 
                   
                    
                   
                     h 
                     4 
                   
                 
                 + 
                 … 
                 + 
                 
                   
                     C 
                     n 
                   
                    
                   
                     h 
                     
                       
                         2 
                          
                         n 
                       
                       + 
                       2 
                     
                   
                 
               
             
             , 
             
               δ 
               = 
               
                 1 
                 R 
               
             
           
         
       
     
         [0000]    where P(h) is a distance of the aspherical surface from a plane perpendicular to the optical axis as a function of a perpendicular distance h from the optical axis, and R is a radius of curvature of the mirror at its apex. The parameter CC is the conic constant of the aspheric surface, and parameters C 1  to C n  are aspheric constants. 
         [0069]    Design data for each mirror in projection objective  500  is shown in Tables I and II. Table I provides R values and distances between mirror surfaces as measured along the optical axis (Referred to as “Thickness”). Table II provides a conic constant and aspheric constants for each mirror. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Surface 
                 Radius 
                 Thickness 
                 Mode 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Object 
                 INFINITY 
                 664.079 
                   
               
               
                   
                 Mirror 1 
                 −2248.408 
                 −457.265 
                 REFL 
               
               
                   
                 STOP 
                 INFINITY 
                 0.000 
               
               
                   
                 Mirror 2 
                 1720.732 
                 607.265 
                 REFL 
               
               
                   
                 Mirror 3 
                 410.127 
                 −296.765 
                 REFL 
               
               
                   
                 Mirror 4 
                 915.188 
                 1385.693 
                 REFL 
               
               
                   
                 Mirror 5 
                 −1017.861 
                 −297.964 
                 REFL 
               
               
                   
                 Mirror 6 
                 −918.296 
                 354.963 
                 REFL 
               
               
                   
                 Mirror 7 
                 374.571 
                 −254.963 
                 REFL 
               
               
                   
                 Mirror 8 
                 329.021 
                 294.957 
                 REFL 
               
               
                   
                 Image 
                 INFINITY 
                 0.000 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
             
             
               
                 Surface 
                 CC 
                 C 1   
                 C 2   
                 C 3   
               
               
                   
               
               
                 Mirror 1 
                 0.000000E+00 
                 9.827635E−10 
                 −6.947687E−15 
                 7.014244E−20 
               
               
                 Mirror 2 
                 0.000000E+00 
                 −8.617847E−11 
                 −2.559048E−15 
                 2.420769E−21 
               
               
                 Mirror 3 
                 0.000000E+00 
                 −8.656531E−10 
                 5.405318E−15 
                 4.204975E−20 
               
               
                 Mirror 4 
                 0.000000E+00 
                 −5.941284E−11 
                 −1.211262E−18 
                 3.529081E−23 
               
               
                 Mirror 5 
                 0.000000E+00 
                 2.650008E−10 
                 3.827055E−16 
                 −7.598667E−22 
               
               
                 Mirror 6 
                 0.000000E+00 
                 5.266996E−09 
                 −4.453655E−14 
                 3.218187E−19 
               
               
                 Mirror 7 
                 0.000000E+00 
                 8.429248E−09 
                 8.347048E−13 
                 1.580837E−17 
               
               
                 Mirror 8 
                 0.000000E+00 
                 2.468501E−10 
                 3.310980E−15 
                 3.228099E−20 
               
               
                   
               
               
                 Surface 
                 C 4   
                 C 5   
                 C 6   
                 C 7   
               
               
                   
               
               
                 Mirror 1 
                 −4.375930E−25 
                 −3.576925E−30 
                 8.030189E−35 
                 0.000000E+00 
               
               
                 Mirror 2 
                 −3.480416E−24 
                 4.082890E−28 
                 −2.197698E−32 
                 0.000000E+00 
               
               
                 Mirror 3 
                 −1.935657E−24 
                 2.726140E−29 
                 −1.533818E−34 
                 0.000000E+00 
               
               
                 Mirror 4 
                 −3.324389E−28 
                 5.301103E−34 
                 −4.669457E−40 
                 0.000000E+00 
               
               
                 Mirror 5 
                 1.018524E−26 
                 −3.492727E−32 
                 6.035810E−38 
                 0.000000E+00 
               
               
                 Mirror 6 
                 2.835576E−24 
                 −1.042146E−28 
                 8.093551E−34 
                 0.000000E+00 
               
               
                 Mirror 7 
                 −7.363060E−21 
                 1.659178E−24 
                 −1.489458E−28 
                 0.000000E+00 
               
               
                 Mirror 8 
                 3.862187E−25 
                 −2.851222E−31 
                 1.052318E−34 
                 0.000000E+00 
               
               
                   
               
             
          
         
       
     
         [0070]    While the embodiments described above relate to catoptric optical systems, in general, the principles disclosed herein can be applied to catadioptric systems as well. For example, in some embodiments, the illumination system can be a catadioptric illumination system. In certain embodiments, catoptric or catadioptric illumination systems can be used in conjunction with catadioptric or dioptric projection objectives. 
         [0071]    As an example, a catoptric illumination system can be used to deliver radiation from a broadband light source, such as a mercury i-line source, to, for example, a dioptric projection objective. The dioptric projection objective can be designed to provide chromatic aberration reduced to a level acceptable for the application for which the system is designed (e.g., for chip packaging applications).