Abstract:
An illumination system and condenser for use in photolithography in the extreme ultraviolet wavelength region having a first non-imaging optic element collecting electromagnetic radiation from a source and creating a desired irradiance distribution and a second non-imaging optic element receiving the electromagnetic radiation from the first non-imaging optic element and redirecting and imaging the electromagnetic radiation. The electromagnetic radiation emanating from the second non-imaging optic element is suitable for being received by other conventional optical surfaces to provide a desired irradiance distribution with a desired angular distribution and desired shape. Facets are used to provide the desired illumination over the desired illumination field. Reflective facets may be placed on the second non-imaging optic, which can reduce the number of mirrors and increase efficiency. The condenser and illumination system are used in combination with a projection optic to project the image of a reticle or mask onto a photosensitive substrate, such as a semiconductor wafer used in the manufacture of electronic or semiconductor devices. The condenser of the present invention provides an efficient condenser in a compact package and provides desirable illumination properties for imaging relatively small feature sizes less than 0.13 microns.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a condenser and illumination systems for projecting the image of a reticle onto a photosensitive substrate as used in photolithography in semiconductor manufacturing, and more particularly to a condenser suitable for use in the extreme ultraviolet or soft X-ray wavelengths having non-imaging optics forming a desired irradiance and desired angular distribution.  
         BACKGROUND OF THE INVENTION  
         [0002]    Photolithography is often used in the manufacture of many devices and in particular, electronic or semiconductor devices. In a photolithographic process, the image of a reticle or mask is projected onto a photosensitive substrate. As the element or feature size desired to be imaged on the photosensitive substrate becomes ever smaller, technical problems often arise. One of these problems is illuminating the reticle or mask so that its image can be projected onto the photosensitive substrate. As the element or feature size of semiconductor devices become ever smaller, there is a need for photolithographic systems providing a resolution of less than 0.13 micrometers. In order to achieve the imaging of these relatively small element or feature sizes, shorter wavelengths of electromagnetic radiation must be used to project the image of a reticle or mask onto the photosensitive substrate. Accordingly, it is necessary for photolithographic systems to operate at the extreme ultraviolet wavelengths, below 157 nanometers, and into the soft X-ray wavelengths, around 1 nanometers. Additionally, projection optics having the required resolution and imaging capabilities often result in utilization of a portion of a ring field. One such projection optic system used in photolithography is disclosed in U.S. Pat. No. 5,815,310 entitled “High Numerical Aperture Ring Field Optical Reduction System” issuing to Williamson on Sep. 29, 1998, which is herein incorporated by reference in its entirety. While the projection optic system disclosed therein can achieve a working resolution of 0.03 microns, there are few illumination sources or illumination systems that can provide the required illumination properties for projecting the image of the reticle or mask onto the photosensitive substrate. An illuminating system is disclosed in U.S. Pat. No. 5,339,346 entitled “Device Fabrication Entailing Plasma-Derived X-Ray Delineation” issuing to White on Aug. 16, 1994. Therein disclosed is a condenser for use with a laser-pumped plasma source having a faceted collector lens including paired facets, symmetrically placed about an axis. Another illumination system is disclosed in U.S. Pat. No. 5,677,939 entitled “Illuminating Apparatus” issuing to Oshino on Oct. 14, 1997. Therein disclosed is an illumination system for illuminating an object in an arcuate pattern having a reflecting mirror with a parabolic-toric body of rotation in the reflection type optical integrator having a reflecting surface for effecting the critical illumination in the meridoinal direction and a reflecting surface for effecting the Kohler illumination in the sagittal direction. Another illuminating system is disclosed in U.S. Pat. No. 5,512,759 entitled “Condenser For Illuminating A Ring Field Camera With Synchrotron Emission Light” issuing to Sweatt on Apr. 30, 1996, which is herein incorporated by reference in its entirety. Therein disclosed is a condenser comprising concave and convex spherical mirrors that collect the light beams, flat mirrors that converge and direct the light beams into a real entrance pupil of a camera, and a spherical mirror for imaging the real entrance pupil through the resistive mask and into the virtual entrance pupil of the camera. Another illumination system is disclosed in U.S. Pat. No. 5,361,292 entitled “Condenser For Illuminating A Ring Field” issuing to Sweatt on Nov. 1, 1994. Therein disclosed is a condenser using a segmented aspheric mirror to collect radiation and produce a set of arcuate foci that are then translated and rotated by other mirrors so that all the arcuate regions are superposed at the mask.  
           [0003]    However, these prior illumination systems may not provide the desired illumination and are relatively complicated. Additionally, many of these systems are relatively large, having many surfaces resulting in loss of energy. Some are also difficult to align and may require adjustment.  
           [0004]    Accordingly, there is a need for an improved illumination system and condenser for use in the extreme ultraviolet that provides a desired irradiance over a predetermined field or area with a desired irradiance and angular distribution for use in photolithography.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to an illumination system comprising a condenser having non-imaging optic elements. A first non-imaging optic element is used to collect light from a source and create a desired or predetermined irradiance distribution. A second non-imaging optic element receives electromagnetic radiation from the first non-imaging optic element and redirects the electromagnetic radiation into collimated nearly spherical or flat wavefronts. Facets placed at the pupil of the illumination system shapes the electromagnetic radiation and provides uniform illumination over a desired area. The facets may be provided on the second non-imaging optic element. Additional objective optics may be utilized to further process the electromagnetic radiation or to relay the electromagnetic radiation to the desired area at a reticle or mask, the image of which is projected onto a photosensitive substrate.  
           [0006]    Accordingly, it is an object of the present invention to provide a desired irradiance over a predetermined field or area.  
           [0007]    It is a further object of the present invention to provide a predetermined angular and irradiance distribution.  
           [0008]    It is yet a further object of the present invention to increase the étendue of a source of electromagnetic radiation.  
           [0009]    It is an advantage of the present invention that it is an efficient condenser for the desired wavelength.  
           [0010]    It is a further advantage of the present invention that it is relatively compact.  
           [0011]    It is a feature of the present invention that non-imaging optic elements are used.  
           [0012]    It is another feature of the present invention that a relatively small number of reflective surfaces are utilized.  
           [0013]    It is yet a further feature of the present invention that a faceted optic element is used.  
           [0014]    These and other objects, advantages, and features will be readily apparent in view of the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 schematically illustrates the illumination system of the present invention.  
         [0016]    [0016]FIG. 2 schematically illustrates optical element  18  illustrated in FIG. 1.  
         [0017]    [0017]FIG. 3 schematically illustrates the illumination system illustrated in FIG. 1 in combination with a projection optic.  
         [0018]    [0018]FIG. 4A schematically illustrates an embodiment of the non-imaging optic element used to form a desired irradiance distribution.  
         [0019]    [0019]FIG. 4B is a plan view graphically illustrating the desired irradiance distribution formed by the non-imaging optic element illustrated in FIG. 4A.  
         [0020]    [0020]FIG. 5A schematically illustrates an embodiment of the non-imaging optic element in the shape of an axicon used to redirect the electromagnetic radiation into collimated nearly spherical or flat wave fronts and an illumination field or area having a desired shape.  
         [0021]    [0021]FIG. 5B is a plan view graphically illustrating the desired irradiance and angular distribution having a desired arcuate illumination field or area formed by the non-imaging optic element illustrated in FIG. 5A.  
         [0022]    [0022]FIG. 5C schematically illustrates an embodiment of the non-imaging optic element having a shape used to redirect the electromagnetic radiation into collimated nearly spherical or flat wave fronts and an illumination field or area having a desired shape.  
         [0023]    [0023]FIG. 5D is an elevational view graphically illustrating a non-imaging optical element having an odd asphere shaped base surface and Fresnel facets.  
         [0024]    [0024]FIG. 5E is a perspective view graphically illustrating the desired irradiance and angular distribution having a desired rectangular illumination field or area formed by a non-imaging optic element.  
         [0025]    [0025]FIG. 6 graphically illustrates parameters utilized in calculating reflective surfaces of the non-imaging optic elements.  
         [0026]    [0026]FIG. 7A is a plan view illustrating a desired irradiance and angular distribution used in photolithography, and referred to as quadrupole.  
         [0027]    [0027]FIG. 7B is a plan view illustrating a desired irradiance and angular distribution used in photolithography, and referred to as uniform.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    [0028]FIG. 1 schematically illustrates the illumination system  10  of the present invention utilizing non-imaging optics. An extreme ultraviolet radiation, EUV, source  12 , which may be a laser plasma source, a capillary discharge tube or synchrotron, provides electromagnetic radiation in the extreme ultraviolet to a non-imaging optic element  14 . The collector or first non-imaging optic element  14  collects the electromagnetic radiation from the source and creates a desired or predetermined irradiance distribution in the pupil of the illumination system  10 . The irradiance distribution may be uniform, annular, quadruple or other known or desired irradiance distribution. Various irradiance distributions have known desirable properties for imaging different patterns onto a photosensitive substrate. Coatings having a graded thickness profile may be place on the first non-imaging optic element  14  to improve reflectivity. The electromagnetic radiation reflected from the first non-imaging optic element  14  is collected by a shaper or second non-imaging optic element  18 . The pupil of the illumination system  10  is located near or adjacent the second non-imaging optic element  18 . Entering rays  16  are reflected from the second non-imaging optic element  18  and exit as emerging rays  20 . The non-imaging optic element  18  redirects and collimates the electromagnetic radiation, illustrated as rays  20 , into nearly spherical or flat wavefronts. This allows the use of conventional optical surfaces to be used in the remainder of the illumination system  10 . The second non-imaging optic element  18  may have a plurality of facets placed thereon providing uniform illumination over a desired area or illumination field. The facets are placed at the pupil, adjacent the second non-imaging optic element  18 , of the illumination system  10 . Electromagnetic radiation, illustrated by rays  20 , is reflected from the second non-imaging optic element  18  and received by a conventional optical element  22 . Electromagnetic radiation is reflected from optical element  22 , illustrated by rays  24 , and received by a second conventional optical element  26 , which in turn is reflected as rays  28  to illuminate a reticle  30 . The optical elements  22  and  26  may be conventional objective optical elements. For example, optical element  22  may correct for coma and be relatively flat. Optical element  26  may be an oblate spheroid. The optical elements  22  and  26  are preferably anamorphic aspheres and are designed to image light reflected or scattered by the second non-imaging optical element  18  onto a reticle and into the entrance pupil of projection optics. The anamorphic departures are used to maintain a constant round pupil across a reticle. Optical elements  22  and  26  may relay or image illumination to the entrance pupil of the projection optic.  
         [0029]    This illumination fills the pupil with a predetermined irradiance and angular distribution, which is referred to as pupil fill. The present invention permits the pupil fill to be modified as desired to enhance the imaging of a pattern on a reticle. Generally, desirable pupil fills are well known, or can be determined relatively easily with a test reticle. Pupil fill or angular distribution may be more precisely identified by the radiometric term radiant intensity, which is commonly expressed in Watts per steradian. Lithographers often use pupil fill to describe this quantity. However, confusion can arise because the irradiance distribution at the pupil is sometimes also called the pupil fill. It is important to realize that the radiant intensity looking back from the reticle or mask from each field point will not necessarily be the same as the irradiance in the pupil. An example is if irradiance distribution were uniform, but the right half of the reticle or mask had a radiant intensity which was the right half of a circle and the left half of the mask had a radiant intensity which was the left half of a circle.  
         [0030]    Additionally, the present invention makes possible the illumination of a desired area on a reticle or mask, such as an arcuate, rectangle, or other shaped area. The present invention also provides a desired irradiance distribution, with a uniform irradiance distribution usually desired, but may provide for a different profile or irradiance distribution if desired. The present invention also provides an illuminated region that is effectively self-luminous. Additionally, each point in the illumination region is illuminated by the same desired angular spectrum, whether it is uniform, annular, quadrupole, or other desired distribution. The present invention also provides that the exit pupil for each point in the illumination region is in the same location to prevent telecentricity errors.  
         [0031]    The first non-imaging optic element  14  collects electromagnetic radiation from the source  12 . The combination of the first and second non-imaging optic elements  14  and  18  forms the desired irradiance distribution at the illumination system  10  or condenser pupil and forms an image of the source  12 . The second non-imaging optic element  18 , functioning as a beam shaper. The beam shaper located at the pupil of the illumination system  10  or condenser scatters the incident radiation so that the image of the source fills the desired region of the reticle or mask  30 . Each point of the reticle or mask  30  receives light or electromagnetic radiation from a large number of discrete points in the pupil. The discrete points may number in the millions depending upon the size of the facets  34  on the second non-imaging optic element  18 . While the pupil is not completely filled, because the source étendue is less than that accepted by the projection optics, the pupil fill is more uniform than can be obtained using conventional segmented optics. Conventional optical elements limits the condenser to fill with only tens of points or lines in the pupil. The improved pupil fill, of the present invention, is achieved by manufacturing the second non-imaging optic element  18  with many individual facets using lithography and a gray scale mask.  
         [0032]    The illumination system or condenser is effectively a critical condenser with a non-imaging optic element  18  or beam shaper at the stop to distribute the photons in a desired fashion across the reticle or mask  30 . In the preferred embodiment, the stop is at the second non-imaging optic element  18 , which serves simultaneously as a beam shaper and to re-image the source at infinity. Combining these two functions on the second non-imaging optic element  18  reduces the number of optics in the illumination system and improves system throughput.  
         [0033]    [0033]FIG. 2 more clearly illustrates the second non-imaging optic element  18 . Preferably, facets  34  are formed on the base surface  32  of base  31 . It should be appreciated that the facets  34  have been greatly exaggerated in size and tilt for illustrative purposes. The facets  34  are shaped and tilted such that the angular spectrum reflected from the non-imaging optic element  18  forms the predetermined or desired shape or illumination field at the reticle or mask when imaged by the optical elements  22  and  26 , illustrated in FIG. 1. Facets  34  are chosen so that the angular spectrum produces an arcuate region when imaged. Diffraction from the facets will increase the étendue. Facets  34  should be small enough to provide reticle plane uniformity, and large enough to minimize edge scattering. For example, a one hundred and twenty millimeter diameter or dimension non-imaging optical element may have ten micrometer dimension facets. This provides good uniform illumination and pupil fill. The facets may be reduced in size to at least four micrometers. The facets  34  may preferably form a Fresnel surface.  
         [0034]    The beam shaper or second non-imaging optical element  18  receives electromagnetic radiation or light from the collector or first non-imaging optical element  14  and redirects it over the desired illumination field. Conceptually, this can be divided into two separate tasks: reimaging of the source  12  which is accomplished by the base surface  32  and blurring the source image over the illumination field which is accomplished by engineering a scattering surface composed of many small facets  34 .  
         [0035]    The combination of the collector or first non-imaging optical element  14  and the beam shaper or second non-imaging optical element  18  surfaces reimages the source, at finite or infinite conjugates. The shaper base surface  32  is determined by 1) solving the differential equations that define the collector or first non-imaging optical element  14 , 2) calculating the angle of incidence of the rays θ at the beam shaper surface, and 3) calculating the slope of the shaper surface necessary to redirect the incident radiation towards a finite or an infinite conjugate. The slopes of the shaper surface may be integrated to give the surface profile or the slopes may be directly integrated into the ray tracing software. The base beam shaper surface can be polished conventionally and the scattering function applied on the base surface. However, the preferred implementation is to manufacture a Fresnel surface using the same small, preferably four to ten micron square facets, that are required for the scattering.  
         [0036]    For the Fresnel surface, each base facet tilt redirects the chief ray for that facet (i.e. one that starts at the center of the source to and goes to the center of said facet) to the image of the source. The Fresnel facet tilt is added to that scattering tilt, as described below. Using a Fresnel base surface simplifies the manufacture of the scattering pixels, because the substrate is not curved, standard lithographic processing can be used.  
         [0037]    In order to illuminate the desired region at the mask, the pupil (i.e. the beam shaper surface) is divided into many small facets. Each facet directs light to a different portion of the illumination field. This engineered scattering surface allows the illumination of arbitrarily shaped regions. With sufficiently small facets, each point in the illumination field can receive light from thousands or millions of points distributed randomly over the pupil. The random distribution in the pupil ensures the angular distribution of the radiation or radiant intensity at each point on the reticle is substantially the same. The deviation of each facet from the base surface is determined by focal length of the remaining conventional optical surfaces and the paraxial imaging equation.  
         [0038]    Referring to FIGS. 1 and 2, the illumination system  10  of the present invention illuminates a reticle with an image of a predetermined illumination field or area having desired irradiance and a desired angular distribution. This illumination is desirable for photolithographic applications. Additionally, the illumination system  10  of the present invention increases the étendue of the electromagnetic radiation source by evenly distributing the available power across the illumination field or arcuate area. The étendue of an optical system is a geometrical quantity related to the cross sectional area of the source and the angular subtents collected by the aperture. Laser plasma and capillary discharge sources are generally small, less than approximately one millimeter. Synchrotrons have small angular extends, which must be taken into account in the design process. As a result, it is often necessary to increase the étendue of the source. The present invention achieves a desired illumination pattern, a desired angular distribution, and high efficiency, all in a compact package. The present invention is preferably applicable to irradiate a portion of a ring field or arcuate region of a projection optic with extreme ultraviolet electromagnetic radiation. One such projection optic system is disclosed in U.S. Pat. No. 5,815,310 entitled “High Numerical Aperture Ring Field Optical Reduction System” issuing to Williamson on Sep. 29, 1998, which is herein incorporated by reference in its entirety.  
         [0039]    Referring to FIGS. 1 and 2 illustrating the illumination system  10  of the present invention, a relatively small source  12 , such as a laser plasma source or a capillary discharge tube emits electromagnetic radiation with the desired wavelength in the extreme ultraviolet region. The collector or first non-imaging optic element collects the energy or electromagnetic radiation and forms a desired irradiance distribution provided to the shaper or second non-imaging optic  18 . The non-imaging surfaces of the non-imaging optic elements are the solution to differential equations. These differential equations are well known to those skilled in the art and are disclosed in a book entitled “Computer Aided Optical Design of Illumination and Irradiating Devices” by Kusch, and published by Aslan Publishing House, Moscow, 1993. Using equations found in this book non-imaging optic elements are readily designed for point sources with rotationally symmetric angular intensity and rotationally symmetric pupil fills. For a finite sized source and non-rotationally symmetric systems more general equations may be used. Such equations are well known to those skilled in the are and may be found in an article entitled “Formulation of a Reflector-Design Problem for a Lighting Fixture” by J. S. Schruben published in the Journal of the Optical Society of America, Vol. 62, No. 12, December 1972 and in an article entitled “Tailored Reflectors for Illumination” by D. Jenkins and R. Winston published in Applied Optics, Vol. 35, No. 10, April 1996. The reflectivity of the coatings and the angular intensity distribution of the source are both taken into account when designing the non-imaging optic surfaces. The base surface of second non-imaging optic element  18  is non-imaging and collimates electromagnetic radiation from the first non-imaging optic element  14  so as to be reflected by the second non-imaging optic element  18  in a parallel bundle. On the surface  32  of the second non-imaging optic element  18  is formed an array of facets shaped and tilted such that the angular spectrum reflected from the second non-imaging optic element  18  forms the desired shape at the reticle or mask when imaged by the remaining conventional optics  22  and  26  of the present invention. The second non-imaging optical element  18  may be fabricated using lithographic techniques.  
         [0040]    [0040]FIG. 3 illustrates the extreme ultraviolet illumination system  10 , as illustrated in FIGS. 1 and 2, in combination with an extreme ultraviolet projection optic  36 , such as that disclosed in U.S. Pat. No. 5,815,310. Illumination system  10  provides a desired irradiance and a desired angular distribution in a predetermined illumination field, such as a portion of a ring field or arc, for illuminating the reticle  30 . Reticle  30  is a reflective reticle. As a result, the electromagnetic radiation strikes the reticle  30  slightly off axis with respect to a line normal to the reticle  30 . The electromagnetic radiation is collected and reflected by a first mirror  38 , which is collected and reflected by a second mirror  40 , which is collected and reflected by a third mirror  42 , which is collected and reflected by a fourth mirror  44 , which is collected and reflected by a fifth mirror  46 , which is collected and reflected by a sixth mirror  48 , which projects the image of the reticle onto a photosensitive substrate  50 . Sufficient clearance is provided between the EUV illumination system  10  and the projection optic  36  to permit the reticle  30  and the photosensitive substrate  50  to be scanned in a parallel direction such that the illumination field projects the image of the reticle over a predetermined area of the photosensitive substrate. The reticle  30  and the photosensitive substrate  50  are positioned for parallel scanning. All of the optical elements are positioned to not interfere with any needed stages for the movement of the reticle  30  and photosensitive substrate  50 . Accordingly, the entire system, illumination system  10  and projection optic  50 , are preferably positioned entirely between the reticle  30  and the substrate  50 .  
         [0041]    [0041]FIG. 4A schematically illustrates the profile of a collector or first non-imaging optical element  214 . The collector or first non-imaging optical element  214  may be axially symmetrical. The collector or non-imaging optic element  214  has a curved reflective surface  215 . The curved reflective surface  215  is calculated or determined based upon the desired irradiance distribution at the pupil. A relatively small EUV source  212  is positioned near the origin of the non-imaging optical element  214 . The electromagnetic radiation from the source  212  is collected and reflected by the first non-imaging optical element  214 . The reflected electromagnetic radiation forms a desired irradiance distribution.  
         [0042]    [0042]FIG. 4B graphically illustrates one such irradiance distribution. An annular irradiance distribution  52  is illustrated. The annular irradiance distribution  52  is characterized by a surrounding dark field  54  with a contained annular illumination  56  and dark center  58 . While an annular irradiance distribution  52  has been illustrated, any desired irradiance distribution may be obtained depending upon the calculated profile or shape of surface  215  of the non-imaging optical element  214 . For example, top hat, quadrupole, uniform or other know desired irradiance distribution.  
         [0043]    [0043]FIG. 5A illustrates a shaper or second non-imaging optical element  218 . The second non-imaging optical element  218  is another embodiment of the second non-imaging optical element  18  illustrated in FIGS. 1 and 2. The second non-imaging optic element  218  is formed on a substrate or base  231 . The base  231  has a surface  232 . The surface  232  may have a shape of a Fresnel axicon. While FIG. 5A illustrates the second non-imaging optic element  218  as a conical section, the conical surface of an axicon may be placed on a conventional round lens for mounting. While FIG. 5A illustrates an axicon, it should be appreciated that the second non-imaging optical element  218  may have other shapes depending upon the source and desired irradiance or angular distribution. For example, if angular distribution of the source and the pupil fill is rotationally symmetric, the second non-imaging optic element has a shape of an odd-asphere. When there is no rotational symmetry, the second non-imaging optic element will have an unusual shape. The unusual shape is determined to provide the function of collimating the rays. The surface  232  has a plurality of reflective angled or tilted facets  234  thereon. The surface  232  in combination with the plurality of facets  234  shape or redirect the electromagnetic radiation collimating it and forming spherical or flat wavefronts having a uniform illumination over a desired illumination area or field. The second non-imaging optical element  218  may have a relatively large number of reflective facets  234 , with each facet  234  having a dimension of approximately ten to four micrometers. While ten to four micrometers is the most useful range of facet sizes, smaller and larger facets would also work. The angle or tilt of each facet  234  is calculated to provide an arcuate illumination area or field with uniform illumination and the desired irradiance distribution. The facets  234  may be made by etching or other photolithographic techniques. The uniform illumination has a desired irradiance distribution created by the collector or first non-imaging optical element  14  or  214  illustrated in FIGS. 1, 3, and  4 A.  
         [0044]    [0044]FIG. 5B graphically illustrates a desired illumination area or field having an arcuate shape. Additionally, graphically illustrated is an annular angular distribution  152 . The annular angular distribution  152  has a dark field  154  surrounding an annular illumination  156  and dark center  158 . While the annular angular distribution has been graphically illustrated, it will be understood by those skilled in the art that the angular distribution appears annular from any point in the illumination area or field. Therefore, the illumination area or field is uniformly illuminated with annular irradiance and angular distribution.  
         [0045]    [0045]FIG. 5C is a perspective view illustrating another embodiment of the second non-imaging optical element. In this embodiment the non-imaging optical element  318  has a base  331  with a base surface  332  formed thereon. The base surface  332  may have an unusual shape. The unusual shape is determined to provide the function of collimating the rays. Reflective facets  334  are formed on the base surface  332 . The position and tilt of the facets  334  are determined so as to provide a desired illumination over a desired illumination field.  
         [0046]    [0046]FIG. 5D is an side elevational view illustrating another embodiment of the second non-imaging optic element. In this embodiment the second non-imaging optic element  418  has a base  431  with a base surface  432  having the shape of an odd asphere. An odd asphere is a surface whose sag can be written as a function of the radial distance on the vertex plane, the locus of points (x, y, z(r)), where r=(x 2 +y 2 ) 1/2 . An axicon is a special case of an odd asphere. An odd asphere is non-imaging because it does not image points to points at any conjugates. For this reason, the first non-imaging optic element or collector may also be an odd ashpere. On base surface  432  are placed facets  434 . The facets are positioned so as to provide a desired illumination over a desired area or illumination field.  
         [0047]    [0047]FIG. 5E is a perspective view schematically illustrating a rectangular illumination field  360 . The illumination field or area  360  has a desired irradiance distribution and desired angular distribution or radiant intensity. Irradiance distribution is commonly expressed in Watts per meter squared. It is important to realize that the radiant intensity looking back from the reticle or illumination field to the pupil from each field point, will not necessarily be the same as the irradiance distribution in the pupil. The illumination field  360  is graphically illustrated with a uniform irradiance distribution  352 . Several illumination cones  357  are illustrated, but it should be appreciated that the illumination cones  357  as well as the uniform irradiance distribution  352  are across the entire illumination field  360 . Accordingly, a desired irradiance distribution may be obtained with a desired pupil fill or radiant intensity in any desired shape of illumination field.  
         [0048]    [0048]FIG. 6 graphically illustrates the parameters utilized to determine or define the surface of a non-imaging optic element. Point  512  represents a source, line  514  represents a reflector surface, and line  519  represents the illumination plane. The non-imaging optic element may be defined by differential equations which relate the angular distribution of the point source to an arbitrary output irradiance distribution. By non-imaging it is meant an optical element that does not have a finite number of foci. A system of differential equations define the surface. The solution suitable for determining the surface of the non-imaging optid element, such as that illustrated in FIG. 1 as elements  14  and  18  or  214 ,  218 ,  318 , and  418  in FIGS. 4A, 5A,  5 C, and  5 D may be obtained by referring to the following differential equations:  
                    ϕ          x       =            ±       xE        (   x   )             ρ        (   α   )       /     (   ϕ   )            sin        (   ϕ   )                                r          ϕ       =                   ϕ          x          r                   tan        (   α   )                                     
 
         [0049]    where;  
         [0050]    E(x) is irradiance;  
         [0051]    ρ is the reflectance;  
         [0052]    I(Φ) is the intensity;  
         [0053]    r is a radial distance between a source and the non-imaging optical element reflective surface;  
         [0054]    Φ is an angle between the radial distance r and a perpendicular line from an illumination plane extending through the source;  
         [0055]    θ is an angle between a perpendicular line from an illumination plane and a ray reflected from the non-imaging optical element reflective surface;  
         [0056]    x is the distance along an illumination plane from the intersection of the illumination plane and a perpendicular line extending through the source and a ray reflected from the non-imaging optical element reflective surface, and α=(Φ−θ)/2  
         [0057]    Additionally, referring to FIG. 6, x=r sin(Φ)+tan(θ)[r cos(Φ)+D] 
         [0058]    Accordingly, many desired rotationally symmetric irradiance distributions may be achieved by the fabrication of a reflective surface defined by the above differential equations. The reflective surface determined to produce the desired irradiance distribution forms the first non-imaging optic element. Accordingly, the image of the source is not formed. By non-imaging it is meant that the image of the source is not formed by the non-imaging optic element. Therefore, by solving the above differential equations, a reflector or collector can be described by an interpolating function, assuming a point source, which can accommodate any irradiation distribution as long as x(Φ) increases with Φ. As can be appreciated from the above equations, the reflectance of the EUV coatings can be utilized to create the desired irradiance distribution. The second non-imaging optic, element 18 in FIGS. 1 and 3 and elements 218, 318, and 418 in FIGS. 5A, 5C, and  5 D, is used to redirect the incident radiation such that it fills the illumination area or field when imaged by the remaining conventional optical elements.  
         [0059]    FIGS.  7 A-B illustrate various irradiance distributions desirable for use in photolithography depending upon the dominant characteristics of the intended pattern being reproduced. FIG. 7A graphically illustrates a quadrupole irradiance distribution  254 . The quadrupole irradiance distribution has a dark field  254  and four illumination fields  256 . FIG. 7B graphically illustrates a uniform irradiance distribution  352 .  
         [0060]    Accordingly, the present invention, by using a first non-imaging optic element to provide the desired irradiation distribution in combination with a second non-imaging optic element to provide nearly spherical or flat wavefronts or collimated electromagnetic radiation permits the use of conventional optical surfaces in the remainder of the illumination system. This makes possible a desired irradiance having desired angular distribution. Additionally, the use of facets placed at the pupil or stop and preferably on the second non-imaging optic element provides uniform illumination over the desired illumination field. While the facets may be transmissive, they are preferably reflective and placed on the surface of the second non-imaging optic element. Refractive or transmissive elements may unduly attenuate or absorb the EUV electromagnetic radiation. The present invention also permits a change in numerical aperture by masking the pupil or changing the first and second non-imaging optical elements. This provides a desired sigma or coherence, ratio of numerical apertures, for the system.  
         [0061]    Additionally, although the preferred embodiment has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit and scope of this invention.