Abstract:
A catadioptric imaging system for micro-lithographic projection features a high numerical aperture objective where most of the focusing power is produced by reflection and refraction angles are limited to avoid additional aberration. A field correcting optic is appended to a Mangin mirror in an immersive configuration for raising the numerical aperture. The optical connection between the Mangin mirror and the field correcting optic is arranged to control refraction angles by limiting angles of incidence or refractive index differences. A radially symmetric polarizing effect is achieved in a pupil to improve image contrast.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to imaging systems of the type in which a reflective surface provides substantial focusing power and more particularly relates to catadioptric reducing systems that exploit a combination of reflection and refraction for focusing purposes. The invention has particular applicability to Newtonian objectives for microlithographic imaging at high numerical apertures using deep ultraviolet light. The invention also contemplates issues of polarization management for imaging systems.  
         [0003]     2. Description of Related Art  
         [0004]     Newtonian design forms in which focusing power is attained largely by reflection have been incorporated into microlithographic instruments to accurately project images while limiting chromatic aberrations. The chromatic advantages of Newtonian design forms over all-refractive imaging systems are particularly evident for imaging with ultraviolet light where transmissive material choices are more limited.  
         [0005]     Accompanying a trend toward the use of shorter wavelengths of ultraviolet light, higher numerical apertures are also sought to achieve higher resolution by microlithographic instruments. The high refraction angles required of known refractive objectives contribute to a number of aberrations including chromatic aberrations that are difficult to correct with the limited material choices for transmitting deep ultraviolet light (i.e., less than 200 nanometers wavelength). Certain anisotropic properties, such as intrinsic birefringence, also become evident in materials at the shorter wavelengths further complicating refractive solutions.  
         [0006]     Although reflective optics are largely chromatically insensitive, a number of other aberrations accompany their use including spherical aberration and field curvature. Accordingly, refractive optics have been used in combination with reflective optics, balancing the strengths and weaknesses of each other in catadioptric forms of Newtonian designs. Examples of such catadioptric forms are disclosed in co-assigned U.S. Pat. No. 5,650,877 entitled “Imaging System for Deep Ultraviolet Lithography”, which is hereby incorporated by reference.  
         [0007]     Higher resolution requires either higher numerical apertures or shorter wavelengths for a given numerical aperture, and most beneficially, both. Each places demands on the imaging system. The shorter wavelengths (particularly 157-nanometer light) further limit material choices for transmission and amplify anisotropic characteristics that interfere with the uniform propagation of light. The higher numerical apertures require the severe bending of light rays that introduce a host of aberrations and further exacerbate material deficiencies.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     My invention contemplates imaging systems, particularly for microlithographic projection instruments, capable of high resolution imaging with low aberrations. The imaging systems in one or more embodiments achieve the higher resolution capabilities by accommodating wavelengths of deep ultraviolet light (i.e., less than 200 nanometers in wavelength) at high numerical apertures (i.e., 0.85 or greater). Aberrations are limited by design improvements that exploit the focusing power of reflective optics together with the corrective properties of refractive optics at low refraction angles. Polarization management is used to improve image contrast.  
         [0009]     One example of the invention as a catadioptric imaging system is based on a double-reflecting Mangin mirror. The Mangin mirror has an optically transmissive body, a partially reflective surface at a first side of the optically transmissive body, a concave reflective surface at a second side of the optically transmissive body, an aperture within the concave reflective surface, and a concave refractive surface at the second side of the optically transmissive body within the aperture of the concave reflective surface. The concave reflective surface and the concave refractive surface at the second side of the optically transmissive body have nominal centers of curvature located in opposite directions along a common optical axis for reducing refraction angles at which imaging light exits the optically transmissive body through the aperture. A field-correcting lens having a convex refractive surface adjacent to the concave refractive surface corrects aberrations in an image field.  
         [0010]     The partially reflecting surface is preferably a refractive surface that transmits light to the concave reflective surface. The concave reflective surface reflects the transmitted light on a converging path back toward the partially reflecting surface, and the partially reflecting surface reflects the returning light on a further converging path through the concave refractive surface toward a focal point of the Mangin mirror. The convex refractive surface of the field-correcting lens has a nominal center of curvature that departs from the nominal center of curvature of the concave refractive surface for influencing a correction in the image field.  
         [0011]     The field-correcting lens preferably includes a lens body having first and second sides located along the optical axis. The convex refractive surface is at a first side of the lens body and another refractive surface is at a second side of the lens body. The refractive surface at the second side of the lens body preferably forms an immersion interface shaped for refracting the light into a liquid optical medium adjacent to the image plane. Overall, the field-correcting lens can have a net positive power, and the immersion interface can have a nominally planar form. Alternatively, the immersion interface can have a concave form for reducing refraction between the field-correcting lens and the liquid medium.  
         [0012]     To minimize the introduction of aberrations while correcting others, refraction is preferably limited at the concave refractive surface of the Mangin mirror by limiting angles of incidence presented by the converging beam incident upon the concave refractive surface. Refraction can be similarly limited at the convex refractive surface of the field-correcting lens by limiting curvature differences between the convex refractive surface of the field correcting lens and the concave refractive surface of the Mangin mirror. Refraction can be limited at the immersion surface by limiting a difference between an index of refraction of the lens body and an index of refraction of the liquid optical medium.  
         [0013]     The partially reflective surface at the first side of the Mangin mirror&#39;s transmissive body preferably has a nominally planar form oriented substantially normal to the common optical axis of the concave reflective surface and the concave refractive surface at the second side of the optically transmissive body. A number of advantages are associated with this form. The planar surface reflects marginal rays at higher angles of convergence toward the aperture than concave surfaces that are ordinarily used in Mangin mirrors described in the prior art. The higher convergence angles support higher numerical aperture imaging with less reliance on the focusing power of refractive optics. The angles of incidence and reflection are equal with respect to the optical axis so that rays traverse the Mangin mirror&#39;s body at substantially equal angles in opposite directions to rotationally balance angularly sensitive asymmetries, such as angularly dependent birefringence. The planar form of the partially reflective surface also provides manufacturing advantages for the Mangin mirror by simplifying de-wedging requirements for orienting the concave reflective surface, the concave refractive surface, and the partially reflective surface with respect to each other along the common optical axis.  
         [0014]     Another example of a catadioptric imaging system in accordance with my invention includes a primary focusing optic having a refractive body, a reflective surface at one side of the focusing optic&#39;s refractive body, a refractive surface at an opposite side of the focusing optic&#39;s refractive body, and an aperture within the reflective surface. The refractive surface is partially transmissive for transmitting light to the reflective surface, the reflective surface has a concave form for reflecting the transmitted light on a converging path back toward the refractive surface, and the refractive surface is partially reflective for reflecting the returning light on a further converging path through the aperture toward an image plane of the imaging system. Adjacent to the aperture of the primary focusing optic is a bilateral immersion optic having a refractive body. A first immersive surface at one side of the immersion optic&#39;s refractive body is shaped for exposure to a liquid optical medium between the bilateral immersion optic and the aperture of the primary focusing optic. A second immersive surface at an opposite side of the immersion optic&#39;s refractive body is shaped for exposure to a liquid optical medium between the bilateral immersion optic and the image plane.  
         [0015]     A peripheral seal can be used for confining the liquid optical medium between the bilateral immersion optic and the primary focusing optic. The bilateral immersion optic can also shaped to accommodate a refractive index of the liquid optical medium exposed to the first immersive surface different from a refractive index of the liquid optical medium exposed to the second immersive surface. The refractive index of the liquid optical medium exposed to the second immersive surface can be higher than the refractive index of the liquid optical medium exposed to the first immersive surface for optimizing numerical aperture.  
         [0016]     The first immersive surface of the bilateral immersion optic can have a convex form, and the second immersive surface of the bilateral immersion optic can have a planar form. Refraction is preferably limited at the aperture of the primary focusing optic by limiting a difference between an index of refraction of the focusing optic&#39;s refractive body and an index of refraction of the liquid optical medium exposed to the first immersive surface of the bilateral immersion optic. Similarly, refraction is preferably limited at the first immersive surface of the bilateral immersion optic by limiting a difference between an index of refraction of the bilateral immersion optic&#39;s refractive body and the index of refraction of the liquid optical medium exposed to the first immersive surface. Refraction is also similarly limited at the second immersive surface of the bilateral immersion optic by limiting a difference between the index of refraction of the bilateral immersion optic&#39;s refractive body and an index of refraction of the liquid optical medium exposed to the second immersive surface.  
         [0017]     The invention also includes among its embodiments, a double-reflecting Mangin mirror of the type including an optically transmissive body having first and second sides, a first refractive surface at the first side of the optically transmissive body; a reflective surface at the second side of the optically transmissive body, and an aperture within the reflective surface. The Mangin mirror also includes a second refractive surface at the second side of the optically transmissive body within the aperture. The reflective surface at the second side of the optically transmissive body has a first surface form. The second refractive surface at the second side of the optically transmissive body has a second surface form that is different from the first surface form, and the first refractive surface on the first side of the optically transmissive body has a third surface form that is different from at least one of the first and second surface forms.  
         [0018]     The reflective surface at the second side of the optically transmissive body is preferably a concave reflective surface. The second refractive surface at the second side of the optically transmissive body is preferably a concave refractive surface. As such, the reflective surface and the second refractive surface can share a common optical axis and have nominal centers of curvature located in opposite directions along the common optical axis for reducing refraction angles at which imaging light exits the optically transmissive body through the aperture.  
         [0019]     The first refractive surface is preferably partially transmissive for transmitting light to the reflective surface. The reflective surface preferably has a concave form for reflecting the transmitted light on a converging path back toward the first refractive surface, and the first refractive surface is preferably partially reflective for reflecting the returning light on a further converging path through the second refractive surface toward a focal point. A nominal center of curvature of the concave refractive surface preferably departs from the focal point of the Mangin mirror to avoid retroreflecting light from the concave refractive surface.  
         [0020]     The first refractive surface preferably has a planar or convex (non-concave) surface form for optimizing the angles of convergence for the light reflected on a further converging path. A reflective coating can enhance the partial reflectivity of the first refractive surface. Preferably, the coating provides higher reflectivity in the vicinity of the light reflected from the first refractive surface and lower reflectivity in the vicinity of the light transmitted through the first refractive surface.  
         [0021]     An example of the invention as Newtonian-imaging system includes a partially reflective interface between a first optical medium and a second optical medium, a concave reflector adjacent to the second optical medium, and an aperture formed in the concave reflector. An at least partially transmissive interface is located within the aperture between the second optical medium and a third optical medium. An immersion lens incorporating a fourth optical medium is located between the third optical medium and a fifth optical medium. The fifth optical medium is a liquid optical medium for optically connecting the immersion lens to an image plane.  
         [0022]     A first optical pathway connects the first optical medium to the concave reflector through the partially reflective interface and the second optical medium. A second optical pathway connects the concave reflector to the partially reflective interface through the second optical medium. A third optical pathway connects the partially reflective interface to the image plane through the second optical medium, the at least partially transmissive interface, the third optical medium, the fourth optical medium of the immersion lens, and the fifth optical medium.  
         [0023]     The immersion optic can have a first refractive surface adjacent to the third optical medium and a second refractive surface adjacent to the fifth optical medium. The first refractive surface is preferably a convex refractive surface. The third optical medium can also be a liquid optical medium. However, the second optical medium is preferably a solid optical medium and the at least partially transmissive interface is formed between a refractive surface in the solid optical medium and the third optical medium.  
         [0024]     Yet another example of my invention is embodied in a catadioptric imaging system for deep ultraviolet light having a Newtonian form. A combined reflective and refractive optic includes a refractive body, a reflective surface at one side of the refractive body, a refractive surface at an opposite side of the refractive body, and an aperture within the reflective surface along an optical axis. The refractive surface is partially transmissive for transmitting light to the reflective surface. The reflective surface has a concave form for reflecting the transmitted light on a converging path back toward the refractive surface. The refractive surface is also partially reflective for reflecting the returning light on a further converging path through the aperture toward a focal point located along the optical axis. The refractive surface has a non-concave form so that the light approaching the refractive surface from the reflective surface is inclined to the optical axis through a first angle that is less than or equal to a second angle at which the light reflected by the refractive surface is inclined to the optical axis.  
         [0025]     The second angle, by being at least as great as the first, provides for optimizing the reflecting focusing power of the combined reflective and refractive optic to achieve a desired numerical aperture while minimizing the residual requirements for focusing power through refraction. For example, the refractive surface can have a convex form so that the second angle is greater than the first angle or a nominally planar form so that the first and second angles are substantially equal. In the case of the latter where the refractive body is made of a cubic crystalline material, the substantially planar form of the refractive surface can be used to radially balance birefringence effects of the crystalline material.  
         [0026]     The refractive surface can also be treated to influence other optical properties including intensity and polarization distributions between marginal and paraxial rays. For example, a partially reflective treatment can be applied to the refractive surface so that reflectivity of the refractive surface varies as a function of radial distance from the optical axis to compensate for variations in reflectivity as a function of variations in angles of incidence at with the light approaches the refractive surface. For purposes of polarization management, the surface forms of the reflective and refractive surfaces can be related so that rays of the light approaching the refractive surface from the reflective surface strike the surface at angles of incidence that reflect one polarization direction substantially more than another orthogonal polarization direction resulting in a radially symmetric polarization pattern. The remaining symmetrically polarized light is capable of forming higher contrast images.  
         [0027]     In fact, the invention can be specifically arranged to reduce the polarization components that can adversely affect image contrast. An immersive imaging system so arranged can include an assembly of imaging optics optically connecting an object plane with an image plane along an optical reference axis. A liquid optical medium couples an adjacent one of the imaging optics to the image plane. A pupil is located along the optical reference axis between the object plane and the image plane at an intersection of overlapping bundles of rays extending between the object points in the object plane and image points in the image plane. An angularly sensitive polarizer located within a domain of the pupil polarizes the overlapping bundles of rays within the pupil with a radial (e.g., polar orthogonal) symmetry that reduces, for example, TM polarization components of the rays parallel to axial planes of incidence at the image plane without substantially reducing TE polarization components of the rays perpendicular to the axial planes of incidence at the image plane. An angularly sensitive polarizer could also be used to reduce TE polarization components with respect to TM polarization components with similar polar orthogonal symmetry.  
         [0028]     Preferably, the symmetry is radial, and the angular sensitivity of the polarizer is apparent from both the polar coordinate angle (typically “θ”) about the reference axis (the basis for radial symmetry) and the spherical coordinate angle (typically “φ”) inclined to the reference axis (which affects the magnitude of the polarization effect. For example, while the polarization effect can vary with the inclination angle “φ” to the reference axis, the effect associated with the inclination angle “φ” can be the same throughout the full range of polar angles “θ” around the reference axis. A polar orthogonal symmetry can be achieved by favoring TE polarization components oriented in either a radial or preferably azimuthal polarization pattern.  
         [0029]     The imaging system is preferably a telecentric imaging system, particularly for purposes of microlithographic projection, in which chief rays that intersect at the pupil extend substantially parallel to the reference axis at the image plane (e.g., resist). The polarizing optic can be located adjacent to an aperture stop of the telecentric system.  
         [0030]     The TM polarization components of the rays are subject to variations in inclination to the reference axis complementary to the inclination of the rays through angle “φ” to the reference axis. The TE polarization components of the rays remain orthogonal to the reference axis despite the inclination of the rays to the reference axis. The angularly sensitive polarizer preferably reduces the TM polarization components of the rays as a function of the inclination of the rays to the reference axis at the image plane. As such, the TM polarization components of rays that are more inclined to the reference axis are reduced more than the TM polarization components of the rays that are less inclined to the reference axis. Thus, the polarization effect is most pronounced for the marginal rays, which are inclined the most to the reference axis.  
         [0031]     The polarizer can be a partially reflective surface within the imaging system. For example, the partially reflective surface can be a surface of a Mangin mirror also having a transmissive optical body, a concave reflective surface on one side of the transmissive optical body, and an aperture within the reflective surface. The partially reflective surface is located on another side of the transmissive optical body. The overlapping bundles of rays reflect from the concave reflective surface on a converging path toward the partially reflective surface and reflect from the partially reflective surface on a converging path through the aperture. The partially reflective surface can be a planar surface so that the inclination of the rays to a normal of the planar surface matches the inclination of the rays to the reference axis or a convex surface so that the inclination of the rays to a normal of the planar surface is less than the inclination of the rays to the reference axis. Preferably, the overlapping bundles of rays include marginal rays, and the marginal rays approach the partially reflective surface from the concave reflective surface at an angle of incidence at which the TE polarization components are reflected substantially more that the TM polarization components. The incidence angle of the marginal rays is preferably less than the angle of total internal reflection (TIR).  
         [0032]     An imaging system with radial polarization symmetry not confined to an immersive system can also be configured within an assembly of imaging optics optically connecting an object plane with an image plane along an optical reference axis. A pupil is located along the optical reference axis between the object plane and the image plane at an intersection of overlapping bundles of rays extending between the object points in the object plane and image points in the image plane. The imaging optics include a refractive interface located within a domain of the pupil and arranged as an angularly sensitive polarizer that polarizes the overlapping bundles of rays within the pupil with a radial symmetry that differentially affects TM polarization components of the rays parallel to axial planes of incidence at the image plane with respect to TE polarization components of the rays perpendicular to the axial planes of incidence at the image plane. The refractive interface is shaped in cooperation with the other imaging optics to limit incidence angles of the overlapping bundles of rays at the refractive interface to angles that are less that those required for total internal reflection at the refractive interface.  
         [0033]     The differential effect of the angularly sensitive polarizer preferably favors reflecting one of the polarization components over the other of the polarization components. For example, the polarizer can reflect one of the polarization components to the exclusion of the other of the polarization components as a function of the inclination of the rays with respect to a normal of the refractive interface at the point of incidence.  
         [0034]     The angularly sensitive polarizer preferably polarizes the overlapping bundles of rays within the pupil with a radial symmetry that reduces TM polarization components of the rays parallel to axial planes of incidence at the image plane without substantially reducing TE polarization components of the rays perpendicular to the axial planes of incidence at the image plane. The TM polarization components of the rays are subject to variation in inclination to the reference axis complementary to the inclination of the rays to the reference axis at the image plane. The TE polarization components of the rays remain orthogonal to the reference axis despite the inclination of the rays to the reference axis at the image plane. As such, the polarizer preferably reduces the TM polarization components of the rays as a function of the inclination of the rays to the reference axis at the image plane.  
         [0035]     The refractive interface can be formed as a non-concave refractive surface so that the inclination of imaging rays with respect to a normal of the non-concave refractive surface is no greater than the inclination of the same imaging rays with respect to the reference axis. For example, the refractive interface can be formed as a convex refractive surface so that the inclination of imaging rays with respect to a normal of the non-concave refractive surface is less than the inclination of the same imaging rays with respect to the reference axis. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0036]      FIG. 1  is a diagram of a catadioptric imaging system for microlithographic projection with an immersion optic adjacent to an image plane.  
         [0037]      FIG. 2  is a similar diagram of a catadioptric imaging system with a bilateral immersion optic adjacent to an image plane.  
         [0038]      FIG. 3  is an enlarged diagram of a Mangin mirror and single-immersion field correcting optic of the imaging system of  FIG. 1 .  
         [0039]      FIG. 4  is an enlarged diagram of a Mangin mirror and double-immersion field correcting optic of the imaging system of  FIG. 2 .  
         [0040]      FIG. 5  is a similar diagram of a catadioptric imaging system with an immersion optic adjacent to an image plane.  
         [0041]      FIG. 6  is an enlarged diagram of a Mangin mirror and single-immersion field correcting optic of the imaging system of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]     A catadioptric imaging system  10  for microlithographic projection is shown in  FIG. 1  in an arrangement for imaging a reticle  12  (object) onto a resist  14  (image). An enlarged view of a double-reflecting Mangin mirror  20 , which provides substantial focusing power for the imaging system  10 , is shown in  FIG. 3 . An assembly  18  of largely refractive components is arranged for filling the aperture of the double-reflecting Mangin mirror  20  while assisting in the correction of aberrations. Throughout the assembly  18 , refraction angles are minimized to avoid the introduction of unnecessary aberrations. A field correcting optic  22  operating in an immersion mode works in conjunction with the Mangin mirror  20  to produce a final image of the reticle  12  on a focal plane of the resist  14 .  
         [0043]     The focusing power of the double-reflecting Mangin mirror  20 , which is positioned at or near a pupil of the imaging system  10 , is apparent from the depicted paths of marginal rays  24   a  and  24   b  through the Mangin mirror  20 . The marginal rays  24   a  and  24   b  enter the Mangin mirror  20  through a partially reflective surface  26 , which is formed on a front side of an optically transmissive body  28 . The marginal rays  24   a  and  24   b  propagate through the optically transmissive body  28  and are reflected by a concave reflective surface  30  on the back side of the optically transmissive body  28 . After reflecting, the marginal rays  24   a  and  24   b  propagate along a converging path through the optically transmissive body  28  a second time to the partially reflective surface  26 . The marginal rays  24   a  and  24   b  re-encounter the partially reflective surface  26  at a higher angle incidence than the angle of incidence at which they were first transmitted through the partially reflective surface  26 . At the higher angles of incidence, each of the marginal rays  24   a  and  24  includes a substantial component that reflects from the partially reflective surface  26  for propagating along a further converging path through the optically transmissive body a third time toward an aperture  32  formed in the concave reflective surface  30 .  
         [0044]     The optically transmissive body  28  is preferably a refractive body made of an optical material that is transmissive at deep UV wavelengths of light (i.e., less than 200 nanometers), such as fused silica or calcium fluoride. The partially reflective surface  26  is preferably a refractive surface formed on the front side of the refractive body. Coatings or other treatments can be applied to the refractive surface, such as coatings regulating matters of transmission, reflection, or polarization. In addition to the varying angles of incidence between the transmissive and reflective encounters of the marginal rays  24   a  and  24   b , the rays (of which the marginal rays  24   a  and  24   b  are examples) that transmit through the partially reflective surface  26  are spaced farther from an optical axis  34  of the Mangin mirror  20  than the rays that are reflected from the partially reflective surface  26 . This affords a further opportunity to differentiate between the rays that are transmitted and the rays that are reflected at partially reflective surface  26  by treating the partially reflective surface  26  as a function of radial distance from the optical axis  34 . The treatments, which can include reflective or anti-reflective coatings, can be applied to reinforce transmission in radially defined areas of the partially reflective surface  26  primarily responsible for transmitting the rays toward the concave reflective surface  30  and to reinforce reflection in radially defined areas of the partially reflective surface  26  primarily responsible for reflecting the rays toward the aperture  32 .  
         [0045]     A concave refractive surface  36  is formed in the optically transmissive body  28  within the aperture  32 . The concave refractive surface  36 , which has a nominal center of curvature located along the optical axis  34  in a direction opposite to the location of a nominal center of curvature of the concave reflective surface  30 , limits refraction angles (e.g., to less than 10 degrees) through which the marginal rays  24   a  and  24   b  are bent upon exiting the Mangin mirror element  20 . For a similar purpose, the field optic  22 , which also has an optically transmissive body  38  (e.g., a lens body), includes a convex refractive surface  40  adjacent to the concave refractive surface  36  of the Mangin mirror. The marginal rays  24   a  and  24   b  approach both the concave surface  36  on the back side of the Mangin mirror  20  and the convex surface  40  on a front side of the field optic  22  at limited angles of incidence to reduce the amount of refraction occurring at the two surfaces  36  and  40  (e.g., to less than 10 degrees). In addition, the convex refractive surface  40  of the field element  32  fits closely together with the concave refractive surface  36  of the Mangin element  20  to limit the distance through which refraction is effective for introducing aberration between the surfaces  36  and  40 . However, the centers of curvature of the adjacent concave and convex surfaces  36  and  40  depart slightly from each other to provide a corrective focusing function.  
         [0046]     The back side of the field correcting optic  22  has a immersion surface  42  that is exposed to a liquid optical medium  44 , such as de-gassed water, for forming an immersion interface with the photo resist  14 . The immersion surface  42  preferably has a planar form (making the field optic  22  a plano-convex lens) adjacent to the resist  14 . Although the marginal rays  24   a  and  24   b  approach the immersion surface  42  at high angles of incidence, refraction is limited (e.g., to less than 10 degrees) at the surface  42  because of the liquid optical medium  44  into which the rays  24   a  and  24   b  enter has an index of refraction much closer to the index of refraction of the focusing optic&#39;s transmissive body  38  than air. The higher index of refraction of the liquid optical medium  44  also supports a significantly higher numerical aperture at which the marginal rays  24   a  and  24   b  come to focus on the resist  14 . The immersion surface  42  can also take a concave form to further limit refraction between the field correcting optic  22  and the liquid optical medium  44 , but with some sacrifice of field of view.  
         [0047]     As now apparent, the majority of the focusing power of the imaging system  10  is provided by the reflections of the Mangin mirror  20 . Increasing the curvature of the concave reflective surface  30  can increase the focusing power. However, increasing the curvature of the concave reflective surface  30  can also limit the aperture size at which the Mangin mirror  20  is effective for doubly reflecting light. Normally, the refractive surfaces of Mangin mirrors are concave to reduce the effects of spherical aberration. However, the partially reflective surface  26  of the Mangin mirror  20  is preferably non-concave, including nearly planar, so that the marginal rays  24   a  and  24   b  reflected from the partially reflective surface  26  are inclined to the optical axis  34  by converging angles “β” that are at least as great as the converging angles “α” at which the marginal rays  24   a  and  24   b  are reflected from the concave reflective surface  30 .  
         [0048]     As shown in the illustrated embodiment, the partially reflective surface  26  is a planar surface oriented normal to the optical axis  34 . Accordingly, the angle “α” at which the rays  24   a  and  24   b  are first converged by the concave reflective surface  30  is preserved by reflection from the partially reflective surface  26  so that the two reflection angles “α” and “β” are equal. Even larger “β” angles are possible by forming the partially reflective surface  26  as a convex surface. However, the planar form of the partially reflective surface  26  has significant manufacturing advantages, such as providing for more easily aligning the partially reflective surface  26  with both the concave reflective surface  30  and the concave refractive surface  34  of the Mangin mirror  20 .  
         [0049]     A table follows, listing the fabrication data in millimeters for making the illustrated catadioptric imaging system  10  operating at a 193.3 nanometer reference wavelength over a range of 192.6 to 194 nanometers with a numerical aperture of 1.2 and a 15× reducing power.  
                                                                                                                                                                                                                                                                                                                                             Element   Radius of Curvature       Aperture Diameter                Number   Front   Back   Thickness   Front   Back   Material                    Object 12   INF   7.0000                51   43.3356   37.2553   6.6881   11.6500   11.5829   Silica       Space           333.3049       52   1024.2604   −137.0424   8.0616   65.7105   66.0127   CaF 2         Space           96.2258       53   87.4466   40.8464   7.5129   53.5077   49.9088   Silica       Space           20.0310       54   −43.8045   −54.6894   5.0000   51.2325   55.5014   Silica       Space           8.5625       55   296.4268   −118.5802   10.4736   64.9622   66.0260   CaF 2         Space           0.0200       56   99.6257   56.2463   17.7410   67.2065   63.6415   CaF 2         Space           10.3943       57   266.5824   −60.6273   15.0404   64.8690   65.6625   CaF 2         Space           7.3373       58   −53.6837   −153.2293   5.000   63.6834   67.5688   Silica       Space           21.3538            APERTURE STOP 59           74.2546                Space           0.0200                   28 (26-30)   INF   −82.8138   18.9632   74.2605   76.0852   Silica            30   −82.8138       76.0852   REFL            28 (30-26)   −82.8138   INF   −18.9632   76.0852   76.0852   Silica            26   INF       53.4919   REFL            28 (26-36)   INF   11.0239   17.5713   53.4919   76.0852   Silica       Space           0.6823            38 (40-42)   6.6465   INF   2.9229   7.3577   2.5339   CaF 2         44   INF   INF   0.6000   2.5339   0.7070   H 2 O            IMAGE 14   INF       0.7070                  
 
         [0050]     Other first order data for defining the imaging system follows where FFL is measured from the first surface and BFL is measured from the last surface.  
                                                                     INFINITE CONJUGATES                EFL   53.4911           BFL   −2.9659           FFL   551.6155           F/NO   0.4219            AT USED CONJUGATES                REDUCTION   0.0667           FINITE F/NO   0.4167           OBJECT DIST   7.0000           TOTAL TRACK   601.5438           IMAGE DIST   0.6000           OAL   593.9438           PARAXIAL IMAGE HT   0.3535           PARAXIAL IMAGE DIST   0.6001           SEMI-FIELD ANGLE   0.5551           ENTR PUPIL DIAMETER   88.2731           ENTR PUPIL DISTANCE   542.9388           EXIT PUPIL DIAMETER   378.8727           EXIT PUPIL DISTANCE   226.6206                      
 
         [0051]     Another catadioptric imaging system  70  in accordance with the invention is shown in  FIG. 2 . adapted for use with a double-reflecting Mangin mirror  80  shown in  FIG. 4 . Similar to the preceding catadioptric imaging system  10 , the catadioptric imaging system  70  has a Newtonian form for projecting an image of the reticle  12  (object) onto the resist  14  (image) at a high reduction ratio. Most of the focusing power is provided by the double-reflecting Mangin mirror  80 . A refractive optical assembly  78  fills the aperture of the double-reflecting Mangin mirror  80  while compensating for various aberrations that arise within the imaging system  70 . The double-refracting Mangin mirror  80  is located at or near the pupil of a telecentric imaging system in image space. As shown in  FIG. 4 , a field correcting optic  82  is arranged as a bilateral immersion optic between two liquid optical mediums  86  and  88 . The liquid optical medium  86  connects the double-reflecting Mangin mirror  80  to the field optic  82 , and the liquid optical medium  88  connects the field optic  82  to the resist  14  in an image plane of the imaging system  70 .  
         [0052]     Similar to the preceding embodiment, marginal rays  84   a  and  84   b , which are representative of a range of rays brought to focus by the imaging system  70 , enter a refractive body  98  (a transmissive optical body) of the Mangin mirror  80  through a refractive surface  96 , which is adapted to be both partially transmissive and partially reflective. The marginal rays  84   a  and  84   b  approach the refractive surface  96  at near normal incidence and for this reason (among others) tend to transmit thorough the refractive surface  96 . After traversing the refractive body  98 , the marginal rays  84   a  and  84   b  reflect from a concave reflective surface  100  formed on the back side of the refractive body  98 . The reflected marginal rays  84   a  and  84   b  are directed along a converging path through the refractive body  98  a second time, inclined to an optical axis  94  of the Mangin mirror  80  through an angle “α”. Each of the converging marginal rays  84   a  and  84   b  includes a significant component that reflects from the refractive surface  96  on a further converging path through the refractive body  98  a third time, inclined to an optical axis  94  of the Mangin mirror  80  through an angle “β”. The refractive surface  96  is shaped (e.g., has a non-concave and, preferably, nearly planar form) so that the marginal rays  84   a  and  84   b  reflect from the refractive surface  96  at an angle “β” that is equal to or greater than the angle “α” at which the marginal rays  84   a  and  84   b  reflect from the concave reflective surface  100 .  
         [0053]     The reflectivity of the refractive surface  96  results at least in part from (a) a difference between the refractive index of the refractive body  98  and its adjacent air medium and (b) the angles of incidence at which the converging rays approach the refractive surface  96 . In addition, coatings can be applied to more efficiently govern the transmission and reflection of light across the refractive surface  96 . For example, a coating can be applied to the refractive surface  96  that further differentiates on the basis of incidence angle or position on the refractive surface  96  for more efficiently transmitting and reflecting the range of rays that are brought to focus by the imaging system  70 . For example, reflectivity of the coating can be increased in an annular region  95  of the refractive surface  96  at which the imaging rays are primarily intended to be reflected and decreased in an annular region  97  of the refractive surface  96  at which the imaging rays are primarily intended to be transmitted.  
         [0054]     The converging marginal rays  84   a  and  84   b  exit the Mangin mirror  80  through a convex refractive surface  106  within an aperture  102  of the concave reflective surface  100 . The convex refractive surface  106  has a center of curvature at least approximately matching that of the concave reflective surface  100  so that both surfaces  100  and  106  occupy different portions of an uninterrupted shape (e.g., spherical shape) on the back of the refractive body  98 . Although the converging radial rays approach the convex refractive surface  106  at relatively high angles of incidence, refraction is limited (e.g., to less than 10 degrees) by filling a space between the Mangin mirror  80  and the field correcting optic  82  with the liquid optical medium  86 . The liquid optical medium  86 , which can be degassed water, reduces refractive index differences across the convex refractive surface  106 .  
         [0055]     The field correcting optic  82  has a refractive body  108  together with a convex refractive surface  110  adjacent to the convex refractive surface  106  of the Mangin mirror  80  and a planar refractive surface  112  adjacent to the resist  14 . As such, the field correcting optic  82  can be referred to as a piano-convex optic having positive focusing power. The field correcting optic  82  can also be referred to as a bilateral immersion optic because the convex refractive surface  110  is an immersion surface exposed to the liquid optical medium  86  and the planar refractive surface  112  is an immersion surface exposed to the liquid optical medium  88 .  
         [0056]     The elevated refractive indices of the liquid optical mediums  86  and  88  limit the amount of refraction (e.g., to less than 10 degrees) across the refractive surfaces  110  and  112  of the field optic  82 . Although the marginal rays  84   a  and  84   b  approach the respective refractive surfaces  110  and  112  at substantial angles of incidence, refraction is restricted by the limited differences in the refractive indices of the optical mediums  86  and  88  on opposite sides of the refractive surfaces  110  and  112 . While degassed water is preferred for both liquid optical mediums  86  and  88 , a different optical liquid could be used for one or both. For example, the liquid optical medium  86  can be confined between the Mangin mirror  80  and the field optic  82  by a peripheral seal  114  independently of the liquid optical medium  88 . Within the same overall design of the imaging system  70 , the two liquid optical mediums  86  and  88  can be varied to accommodate different resists  14  and to make fine optical adjustments.  
         [0057]     A table follows, listing the fabrication data in millimeters for making the illustrated catadioptric imaging system  70  operating similarly to the preceding embodiment at a 193 nanometer wavelength with a numerical aperture of 1.2 and a 15× reducing power.  
                                                                                                                                                                                                                                                                                                               Element   Radius of Curvature       Aperture Diameter                Number   Front   Back   Thickness   Front   Back   Material                    Obejct 12   INF   85.8698                 61   65.7102   51.6419   20.5066   23.0980   22.3354   Silica       Space           245.4068        62   435.1054   −150.4084   9.0483   69.8704   70.1300   CaF 2         Space           102.1563        63   116.2850   43.2535   6.6875   55.8903   52.7145   Silica       Space           20.6876        64   −47.7576   −71 .4321   12.0956   54.3401   64.5469   Silica       Space           7.0370        65   134.9633   −101.5776   17.8787   78.4571   79.2839   CaF 2         Space           0.0200        66   79.3961   56.4347   4.3609   77.2784   73.6386   Silica       Space           9.9511        67   113.8151   −132.3919   14.8674   74.1772   73.8734   CaF 2         Space           6.8027        68   −73.9770   −198.0131   5.0848   72.7498   74.3303   Silica       Space           11.3173            APERTURE STOP 69           74.3728           Space           0.6827             98(96-100)   −1043.7160   −89.2413   19.7738   74.3736   74.9846   Silica            100   −89.2413       74.9846   REFL             98(100-96)   −89.2413   −1043.716   −19.7738   74.9846   74.9846   Silica             96   −1043.7160       48.5489   REFL             98(96-106)   −1043.7160   −89.2413   19.7738   48.5489   74.9846   Silica        86   −89.2413   4.0537   0.2000   10.0000   4.9516   H 2 O        82   4.0537   INF   1.8436   4.9516   2.5397   Silica        88   INF   INF   0.6000   2.5397   10.0000   H 2 O            IMAGE 14   INF       10.0000                  
 
         [0058]     Although the partially reflective surface  96  is listed as having a negative radius of curvature indicative of a concave surface on the front side of the refractive body  98 , the size of the radius of curvature is such that the surface  96  is still considered nominally planar. Also, while the reflective surface  100  and the refractive surface  106  share the same negative curvature, the reflective surface  100  is regarded as concave and the refractive surface  106  is regarded as convex in accordance with conventional optical designations.  
         [0059]     Other first order data for defining the imaging system follows where FFL is measured from the first surface and BFL is measured from the last surface.  
                                                                     INFINITE CONJUGATES                EFL   62.0784           BFL   −3.5384           FFL   562.7456           F/NO   0.4284            AT USED CONJUGATES                REDUCTION   0.0667           FINITE F/NO   0.4167           OBJECT DIST   85.8698           TOTAL TRACK   602.8784           IMAGE DIST   0.6000           OAL   516.4086           PARAXIAL IMAGE HT   0.3535           PARAXIAL IMAGE DIST   0.6001           SEMI-FIELD ANGLE   0.4835           ENTR PUPIL DIAMETER   100.9403           ENTR PUPIL DISTANCE   542.9926           EXIT PUPIL DIAMETER   220.9644           EXIT PUPIL DISTANCE   132.3550                      
 
         [0060]     The spaces between elements of both depicted imaging systems  10  and  70  are generally air and preferably nitrogen, while the elements themselves are generally glass and preferably fused silica or calcium fluoride. One or more liquid mediums, preferably degassed water, are also envisioned on one or both sides of the final element  22  or  82 . Generally, the designs favor achieving a high numeral aperture including numerical apertures substantially greater than one (e.g., 1.2) while minimizing the amount of refraction at interfaces throughout the designs.  
         [0061]     In addition, the designs favor minimizing the inclination of marginal rays with respect to the optical axis  34  or  94  throughout most of the refractive optics  18  or  78  to minimize the influence of birefringence within the cubic crystalline materials (e.g., calcium fluoride). Most of the inclination occurs within the double-reflecting Mangin mirror  20  or  80  as a result of reflection. The refractive body  28  or  98  of the Mangin mirror  20  or  80  can be fused silica to minimize the effects of birefringence. However, even if a cubic crystalline material such as calcium fluoride is used in the refractive body  28  or  98  of the Mangin mirror  20  or  80 , the multiple reflections have a clocking effect that tend to more uniformly distribute the birefringence effects. In fact, the clocking effects can be optimized by arranging the partially reflective surface  26  or  96  to approach a planer form so that the rays pass in opposite directions through the refractive body  28  or  98  at approximately equal angles with respect to the optical axis  34  or  94  (corresponding to the orientation of one of the crystal axes).  
         [0062]     Generally, a central obscuration is required to prevent paraxial rays from passing directly through the refractive body  28  or  98  between the refractive surface a 26  or  96  at the front side of the Mangin mirror  20  or  80  and the refractive surface  36  or  106  within the aperture  32  or  102  at the backside of the Mangin mirror. The obscuration assures that the Mangin mirror  20  or  80  has doubly reflected any of the light passing through the aperture  32  or  102 .  
         [0063]     The partially reflective surface  26  or  96  can also be arranged to have an angularly dependent radially symmetric polarizing effect on the light reflected on the further converging path to the aperture  32  or  102 . At angles of incidence approaching but less than an angle of total internal reflection (e.g., in the vicinity of the Brewster angle), one of two orthogonal polarization directions (i.e., the polarization direction TM in the plane of incidence) is not reflected. The transmitted TM polarized light is lost. Since the Brewster angle is referenced in the plane of incidence and the incident rays are bent within a locus of axial planes intersecting the optical axis  34  or  94 , the rays incident upon the reflective surface  26  or  96  at or near the Brewster angle are polarized with radial symmetry, apparent as an azimuthal distribution of polarized light.  
         [0064]     Normally, such polarization losses are avoided if possible to make imaging systems more efficient. However, the invention provides for exploiting the resulting polarization symmetry to enhance imaging at high numerical apertures, such as found in the immersive optical systems  10  and  70 . Randomly polarized light approaching the image plane from high angles of incidence includes a component of polarization along the optical axis that does not fully participate in image formation and has the effect of reducing overall contrast. It is the polarization direction (i.e., TM) in the plane of incidence to the resist  14  that is least desirable for image formation at the higher angles of incidence. The electric field vector of the TM polarization direction is inclined complementary to the optical axis  34  or  94  in accordance with the angle of incidence measured against a normal to the resist  14 . Interference effects of the TM polarization are diminished by differences between the inclination angles of the electric field vectors within the same plane of incidence. For example, electric field vectors relatively inclined by 90 degrees within the same plane of incidence do not interfere at all. (A negative contrast is possible at higher incidence angle differences.) The problem does not affect the TE polarization direction, whose electric field vectors remain parallel to the plane of incidence throughout the entire range of incidence angles.  
         [0065]     Further control over the incidence angles at the partially reflective surface  26  or  96  can be exploited to produce radial polarization symmetry for producing sharper imaging. Since the partially reflective surface  26  or  96  is located adjacent to a pupil of the imaging system  10 , the marginal rays  24   a  and  24   b  or  84   a  and  84   b  that approach the partially reflective surface  26  or  96  at the highest angles of incidence also approach the resist  14  at the highest angles of incidence. Accordingly, by adjusting the shape, treatment (e.g., coatings) or refractive index difference across the partially reflective surface  26  or  96 , the Brewster angle can be matched to a selected range of high incidence angles for the discarding the undesirable TM polarization within the selected range. The exact location of the Brewster angle (or a comparable angle) among the full span of incidence angles at which the converging beam strikes the partially reflective surface  26  or  96  can be set to favor transmissions of the TM polarization direction at the higher end of the span.  
         [0066]     However, the higher incidence angles should remain less than the angle of total internal reflection to preserve the desired polarizing effect. The higher incidence angles can be reduced at the partially reflective surface  26  or  96  within the bounds of total internal reflection while maintaining the desired convergence angles “β” by increasing the convexity of the partially reflective surface  26  or  96 . The converging beam that reflects from the partially reflective surface  26  or  96  is radially polarized, particularly at the higher inclination angles, favoring a polar orthogonal (e.g., azimuthal) polarization pattern capable of imaging with fuller contrast.  
         [0067]     The same Brewster angle effects at the surface of the resist  14  tend to couple the undesired TM polarization more efficiently than the desired TE polarization. However, the reflectivity of the TM polarized light is reduced by the liquid optical medium  44  or  88 , which decreases the refractive index difference across the surface of the resist  14 . For example, for resists having a refractive index of 1.8, reflectivity of the TM polarization inclined by 57 degrees can be reduced from 24 percent to 8 percent by the presence of the liquid optical medium  44  or  48  having a refractive index of approximately 1.4 (e.g., water).  
         [0068]     The polarizing effect of the partially reflective surface  26  or  96  preferably takes place within the domain of a pupil, such as adjacent to the aperture  59  or  69 , where bundles of light, e.g.,  17  and  19  of imaging system  10  or  117  and  119  of imaging system  70  emanating from the object points  13  and  15 , overlap each other. Within the region of overlap, the bundles of light  17  and  19  or  117  and  119  are treated collectively so that each retains the radially symmetric polarization pattern (e.g., an azimuthal polarization pattern) upon approaching their unique image points in the image plane (the resist  14 ). Locating a radially symmetric polarizer within a similar domain can make a similar polarizing effect conjugate to the aperture stop  59  or  69 .  
         [0069]     A double-telecentric catadioptric imaging system  130  together with another double-reflecting Mangin mirror  140  is shown in  FIGS. 5 and 6 . Similar to the two preceding embodiments, the imaging system  130  projects an image of the reticle (object) onto the resist  14  (image) at a high reduction ratio. However, the imaging system  130  is telecentric in both object space and image space.  
         [0070]     Within the telecentric object space, chief rays  141  and  143  of light bundles  137  and  139  emanating from the representative object points  13  and  15  extend parallel to a common optical axis  144 . Within the pupil space centered at an aperture stop  159 , the chief rays  141  and  143  intersect the optical axis  144  and the light bundles  137  and  139  overlap each other. Within telecentric image space, the chief rays  141  and  143 , were they not blocked by a stop in advance of the Mangin mirror  140 , would return parallel to the optical axis  144  for forming corresponding image points in the resist  14 . The light bundles  137  and  139 , which overlap within the pupil space, can be collectively treated within the pupil space for having the same or similar effect on the formation of the corresponding image points in the resist  14 .  
         [0071]     The Mangin mirror  140  provides most of the focusing power, and a field correcting optic  142  associated with the Mangin mirror  140  has an immersive interface with the resist  14  for limiting refraction and increasing the numerical aperture. A refractive optical assembly  138 , which fills the aperture  159  in advance of the Mangin mirror  140 , is telecentric in form for object space, and the Mangin mirror  140  together with the field correcting optic  142  is telecentric in form for image space, each sharing a common pupil in the vicinity of the aperture  159 .  
         [0072]     As shown in  FIG. 6 , overlapping marginal rays  134   a  and  134   b  enter the Mangin mirror  140  by passing through a convex refractive surface  146  on an optically transmissive body  148 . The marginal rays  134   a  and  134   b  propagate through the optical transmissive body  148  and reflect from a concave reflective surface  150  on converging paths inclined through angle “α” with respect to the optical axis  144 . The convex refractive surface  146  also functions as a partially reflecting surface by reflecting the returning marginal rays  134   a  and  134   b  on a further converging path inclined through angle “β” with respect to the optical axis  144  toward an aperture  152  formed in the convex refractive surface  146 .  
         [0073]     In the two preceding embodiments in which the partially reflective surfaces  26  and  96  planar or nearly planar, the angles “α” and “β” are approximately equal to each other and to the angle of incidence at which the marginal rays strike the surface, measured with respect to a surface normal at the point of incidence. However, within the Mangin mirror  140 , the angle “β” is significantly larger than the angle “α” because at the point of incidence, a surface normal  147  is inclined to the optical axis  144  due to the convexity of the convex refractive surface  146 . The actual angle of incidence “μ” is equal to the average of the angles “α” and “β”.  
         [0074]     It is the angle of incidence “μ” that in part controls the reflective properties of convex refractive surface  146 , while the angle “β” is in part responsible for the focusing power of the Mangin mirror  140 . By controlling the convexity of the convex refractive surface  146 , reflective properties of the convex refractive surface  146  can be adjusted independently of the focusing power of the Mangin mirror  140 . This independent adjustment is particularly significant for exploiting the angular polarization sensitivity of the convex refractive surface  146 . For example, the angle of incidence “μ” of the marginal rays  134   a  and  134   b  can be set less than the angle of total internal reflection so that the polarization characteristics of the marginal rays  134  remain differentially affected by the encounter of the marginal rays with the convex refractive surface.  
         [0075]     For example, the angle of incidence “μ” can be set in the vicinity of the Brewster angle so than the convex refractive surface  146  favors reflection of the TE component of polarization over the TM component of polarization, which is lost by transmission. The differential effect is preferably most pronounced at higher angles of incidence. Since the convex refractive surface  146  is within a domain of the pupil at which the bundles of rays  137  and  139  substantially overlap, the polarization effect is similar within each of the bundles of rays  137  and  139 . Thus, the TM polarization components associated with the higher incident angle rays at the image plane (the resist  14 ) are similarly minimized with a radial symmetry within each of the bundles  137  and  139  that matches the radial symmetry of the convex refractive surface  146  around the optical axis  144 .  
         [0076]     Although the differential polarizing effect of the convex refractive surface  146  is largely a function of the angle of incidence “μ” and the refractive index difference across the convex refractive surface  146 , the polarizing effect can be further controlled by applying polarizing coatings or other treatments to the convex refractive surface. The relatively high angles of incidence at the convex refractive surface  146  make the surface especially suitable for achieving the desired angularly sensitive and radially symmetric polarizing effect. However, a different surface or optical construction could also be used within the domain of the pupil to achieve a desired radially symmetric polarizing effect on the overlapping bundles of rays  137  and  139 .  
         [0077]     Similar to the embodiment of  FIG. 3 , a concave refractive surface  156  is formed within the aperture  152  for limiting the angles of incidence at which the converging rays approach the aperture  152 . The field correcting optic  152  has a convex refractive surface  160  on one side of an optically transmissive body  158  and a substantially planar surface  162  on an opposite side. The concave refractive surface  156  of the Mangin mirror  140  and the convex refractive surface  160  of the field correcting optic  152  depart in form to produce a field correcting effect through limited angles of refraction. The planar surface  162  is an immersion surface in contact with a liquid optical medium  154  for coupling the field correcting optic  152  to the resist  154  at a high numerical aperture but limited angles of refraction.  
         [0078]     A table listing fabrication data in millimeters for the catadioptric imaging system  130  at a reference wavelength of 193.3 nanometers within a spectral of 192.6-194.0 nanometers follows.  
                                                                                                                                                                                                                                                                                                 Element   Radius of Curvature       Aperture Diameter                Number   Front   Back   Thickness   Front   Back   Material                    Object       INF   8.8120                   171   43.8766   32.2866   20.9205   12.0911   12.1413   Silica                   148.3546       172   −1715.9101   A(1)   5.9311   44.9003   45.4512   CaF 2                     262.5214       173   241.5870   47.0285   4.5000   47.3665   46.7355   Silica                   18.8190       174   −38.4333   −57.1318   5.0020   49.0497   55.4000   Silica                   0.1482       175   375.3203   −63.6290   19.8929   62.8751   67.2032   CaF 2                     12.6018       176   106.4111   57.2145   19.9818   70.8697   67.1712   CaF 2                     9.1360       177   138.1296   −82.3658   15.4741   68.6578   69.3138   CaF 2                     4.5986       178   −66.3868   A(2)   5.0000   68.8332   72.1529   Silica                   11.9800            APERTURE STOP 179           76.0465                       0.0200            (146-150)   283.3503   −118.5485   23.0274   76.7809   78.0418   CaF 2              150   −118.5485       78.0418   REFL            (150-146)   −118.5485   283.3503   −23.0274   78.0418   78.0418   CaF 2              146   283.3503       56.3647   REFL            (146-156)   283.3503   14.6951   22.0074   56.3647   78.0418   CaF 2                     0.0238       (160-162)   9.0476   INF   3.5207   8.7659   3.0056   Silica       154   INF   INF   0.7539   3.0056   0.7070   H 2 O            IMAGE 14       INF       0.7070                  
 
         [0079]     As with the other tables, a positive radius indicates a center of curvature to the right, and a negative radius indicates a center of curvature to the left. Thickness is the axial distance between adjacent surfaces, and the image diameter is paraxial value rather than a ray traced value.  
         [0080]     The optical elements  172  and  178  have aspheric back surfaces designated A( 1 ) and A( 2 ). These surfaces are defined by a set of coefficients reproduced in the table below in accordance with the following equation:  
       Z   =           (   CURV   )     ⁢     Y   2         1   +       (     1   -       (     1   +   K     )     ⁢       (   CURV   )     2     ⁢     Y   2         )       1   /   2           +       (   A   )     ⁢     Y   4       +       (   B   )     ⁢     Y   6       +       (   C   )     ⁢     Y   8       +       (   D   )     ⁢     Y   10             
 
                                                       SURFACE   CURV   K   A   B   C   D                   A(1)   −0.00971830   0.000000   −2.87514E−08   −6.97112E−12   7.24275E−15   −5.39628E−18       A(2)   −0.00608055   0.000000     2.78960E−08   −2.57837E−12   1.98478E−16     4.73140E−18                  
 
         [0081]     Other first order data for defining the imaging system follows where FFL is measured from the first surface and BFL is measured from the last surface.  
                                                                     INFINITE CONJUGATES                EFL   −1458.4794           BFL   97.9869           FFL   −15239.5785           F/NO   −0.0603            AT USED CONJUGATES                REDUCTION   0.0667           FINITE F/NO   0.4167           OBJECT DIST   8.8120           TOTAL TRACK   600.0000           IMAGE DIST   0.7539           OAL   590.4341           PARAXIAL IMAGE HT   0.3535           PARAXIAL IMAGE DIST   0.7540           SEMI-FIELD ANGLE   0.0029           ENTR PUPIL DIAMETER   16845.5552           ENTR PUPIL DISTANCE   104938.4560           EXIT PUPIL DIAMETER   142.3294           EXIT PUPIL DISTANCE   85.6641                      
 
         [0082]     Although described with respect to a limited number of embodiments, the invention, the descriptions and explanations of the invention render the invention applicable in a number of different ways for such purposes as high numerical aperture imaging, reducing aberrations, accommodating of deep UV wavelengths, and enhancing image contrast.