Abstract:
A collector optical system for EUV and X-ray applications is disclosed, wherein the system includes a plurality of mirrors arranged in a nested configuration that is symmetric about an optical axis. The mirrors have first and second reflective surfaces that provide successive grazing incidence reflections of radiation from a radiation source. The first and second reflective surfaces have a corrective shape that compensates for high spatial frequency variations in the far field intensity distribution of the radiation.

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of priority under 35 U.S.C. §365 of International Patent Application Serial No. PCT/EP2009/000538, filed on Jan. 28, 2009, designating the United States of America, which in turn claims the benefit of priority under 35 U.S.C. §119 of European Patent Application Serial No. EP 08001535.7, filed on Jan. 28, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to reflective optical systems, and more particularly to improved collector optical systems for EUV and X-ray applications. 
     The invention has various applications in scenarios where EUV and X-ray radiation is used, but is particularly useful in lithography and imaging applications. 
     A simplified block diagram of an EUV lithography system is shown in  FIG. 1  (PRIOR ART). The ultra-violet source  102  is normally an hot plasma the emission of which is collected by the collector  104  and delivered to an illuminator  106 . The latter illuminates a mask or reticle  108  with the pattern to be transferred to the wafer  110 . The image of the mask or reticle is projected onto the wafer  110  by the projection optics box  112 . EUV lithography systems are disclosed, for example, in US2004/0265712A1, US2005/0016679A1 and US2005/0155624A1. 
     Presently, the most promising optical design for collectors  104  is based on nested Wolter I configuration, as illustrated in  FIG. 2  (PRIOR ART). Each mirror  200  is a thin shell consisting of two sections (surfaces)  202 ,  204 : the first one  202 , closer to the source  102  is a hyperboloid whereas the second  204  is an ellipsoid, both with rotational symmetry, with a focus in common. 
     The light source  102  is placed in the focus of the hyperboloid different from the common focus. The light from the source  102  is collected by the hyperbolic section  202 , reflected onto the elliptic section  204  and then concentrated to the focus of the ellipsoid, different from the common focus, and known as intermediate focus (IF)  206 . 
     From an optical point of view, the performance of the collector  102  is mainly characterized by the collection efficiency and the far field intensity distribution. The collection efficiency is the ratio between the light intensity at intermediate focus  206  and the power emitted by the source  102  into half a sphere. The collection efficiency is related to the geometry of the collector  104 , to the reflectivity of each mirror  200 , to the spatial and angular distribution of the source  102 , to the optical specifications of the illuminator. The far field intensity distribution is the 2D spatial distribution of the light intensity beyond the intermediate focus  206  at distances that depends on the illuminator design but that are normally of the same order of magnitude as the distance between the source  102  and intermediate focus  206 . 
     The collector  104  is normally used in conjunction with a hot plasma source  102 . Thus, the thermal load from UV radiation on the collector  104  is very high and a proper cooling system is required. The cooling system is positioned on the back surface of each mirror  200  in the shadow area that is present on the back side of both the elliptical section  204  and the hyperbolic section  202  (see  FIG. 2  (PRIOR ART)). 
     The purpose of the collector  104  in EUV sources is to transfer the largest possible amount of in-band power emitted from the plasma to the next optical stage, the illuminator  106 , of the lithographic tool  100  (see  FIG. 1 ), the collector efficiency being as defined hereinabove. For a given maximum collection angle on the source side, the collector efficiency is mainly determined by collected angle and by the reflectivity of the coating on the optical surface of the mirrors. 
     A problem with known systems is how to significantly increase collector efficiency. The present invention seeks to address the aforementioned and other issues. 
     A. Far Field Improvement 
     A further problem with known systems is that the degree of uniformity of far field intensity distribution for the collector is lower than it might be, thereby effecting collector efficiency. 
     The uniformity of the far field intensity distribution has a direct impact on the illumination uniformity on the mask or reticle  108 , thus affecting the quality of the lithographic process. Although the illuminator  106  is designed to partly compensate for the non-uniformity of the far field intensity distribution, there is a need for a design of the collector  104  that limits the non-uniformity of the far field intensity distribution. In particular, the high spatial frequency variation of the far field intensity distribution may be difficult to compensate. 
     The main source of the high spatial frequency variation of the far field intensity distribution is the shadowing due to the thickness of the mirrors  200 . Such thickness is limited toward small values by mechanical and thermal considerations. Indeed, enough mechanical stiffness is required to the mirror  200  for manufacturing and integration purposes. In addition, the thermal conductance of the mirror  200  must be high enough to transfer the high thermal load produced by the hot plasma to the location of the cooling system. 
     In some cases, depending on the illuminator  106  and source specifications, the shadowing due to the mirror thickness can be almost completely avoided with a suitable collector design, i.e. in some cases a collector design is possible whereby the shadowing at far field due to shell thickness is almost completely absent even if the thickness of the shells is not zero but, at the same time, realistic values (from a manufacturing and thermal point of view) are used for the mirror thickness. The use of extended sources  102  helps in reducing the non-uniformity of the far field intensity distribution due to mirror thickness. However, there are cases in which the illuminator  106  and source  102  specifications do not allow a collector design with the above property. In particular, limitations of the optical aperture at the intermediate focus  206  prevent part of the light filling the dips in the far field intensity distribution. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a collector optical system for EUV and X-ray applications, in which radiation is collected from a radiation source and directed to an image focus, comprising: a plurality of mirrors arranged in nested configuration, the or each mirror being symmetric about an optical axis extending through the radiation source and the or each mirror having at least first and second reflective surfaces, whereby, in use, radiation from the source undergoes successive grazing incidence reflections at said first and second reflective surfaces; and characterised in that one or more of said at least first and second reflective surfaces incorporates a corrective shape so as to compensate the high spatial frequency variation of the far field intensity distribution of said radiation. 
     Preferably, said corrective shape consists in a proper modification or correction of the nominal shape of each mirror such that part of the radiation is redirected in a direction that fills the dips in the far field intensity distribution. 
     Preferably, the corrective shape is mathematically described by a proper function or by its power expansion up to a given order. Preferably, the corrective shape is described by a polynomial correction. Preferably, said corrective shape is described by
 
Δ r   i ( z )= k   i ( c   1   z+c   2   z   2   +c   3   z   3 )
 
where
         Δr i (z) is the variation in the mirror radius and z is the position along the optical axis measured starting from the joint point between the hyperbolic and the elliptical section of each mirror and pointing from the intermediate focus to the source   the subscript i enumerates the mirrors from innermost to outermost mirror, and   the values for the parameters k i , c 1 , c 2 , and c 3 , are listed in Table A.2. and A.3 hereinbelow.       

     Preferably, the or each mirror, the mirror is specified by Table A.1 hereinbelow, where 1 is the innermost mirror: 
     In one embodiment, a total of 7 mirrors in nested configuration are provided. 
     Preferably, the or each mirror comprises a Wolter I mirror. Preferably, for the or each mirror: the first reflective surface, closest to the source, has a hyperbolic shape. Preferably, for the or each mirror: the second reflective surface, furthest from the source, is obtained by rotating an elliptical profile around an axis that is not an axis of symmetry of the ellipse. 
     In another embodiment, the or each mirror two of more of the mirrors each have a different geometry. 
     Preferably, the or each mirror is formed as an electroformed monolithic component, and wherein the first and second reflective surfaces are each provided on a respective one of two contiguous sections of the mirror. 
     Preferably, one or more of the mirrors has mounted thereon, for example on the rear side thereof, one or more devices for the thermal management of the mirror, for example cooling lines, Peltier cells and temperature sensors. 
     Preferably, one or more of the mirrors has mounted thereon, for example on the rear side thereof, one or more devices for the mitigation of debris from the source, for example erosion detectors, solenoids and RF sources. 
     According to another aspect of the present invention there is provided a collector optical system for EUV lithography. 
     According to another aspect of the present invention there is provided an EUV lithography system comprising: a radiation source, for example a LPP source, the collector optical system as disclosed herein; an optical condenser; and a reflective mask. 
     In another aspect of the invention, there is provided an imaging optical system for EUV or X-ray imaging. 
     In another aspect of the invention, there is provided an EUV or X-ray imaging system, comprising: the imaging optical system as disclosed herein; and an imaging device, for example a CCD array, disposed at the image focus. 
     An advantage of the invention is to compensate the high spatial frequency variation of the far field intensity distribution when this is not possible by a proper design of the Wolter I configuration. The present invention involves suitable modification or correction of the nominal shape of each mirror such that part of the light is redirected in a direction that fills the dips in the far field intensity distribution. This correction can be applied to either the elliptical or hyperbolic part of each Wolter I mirror or to both parts. The correction can be mathematically described by a proper function or by its power expansion up to a given order. In the latter case a polynomial correction is obtained. The amount of the correction must be carefully chosen case by case to optimise the far field intensity distribution. Indeed, a wrong or excessive correction can increase the depth of the dips or produce unwanted peaks at some positions of the far field intensity distribution. 
     B. Multiple Reflections 
     A problem with conventional collector designs is that, to improve the overall efficiency of grazing incidence collectors  104 , collection of radiation emitted at large angle is required. However, this also implies larger grazing incidence angles (a.k.a. grazing angle) and a consequent limitation of the collector efficiency, as the reflectivity of the coating is a decreasing function of the grazing incidence angle. Thus, for a given design, the increase of the efficiency is smaller and smaller as the collected angle increases. 
     There is a need to overcome this limitation of the collector efficiency. 
     According to one aspect of the present invention there is provided a collector optical system for EUV and X-ray applications, in which radiation is collected from a radiation source and directed to an image focus, comprising: a plurality of mirrors arranged in a nested configuration, the or each mirror being symmetric about an optical axis extending through the radiation source and the or each mirror having first and second reflective surfaces, whereby, in use, radiation from the source undergoes successive reflections at said first and second reflective surfaces; and characterised by one or more of the mirrors further comprising a third reflective surface, whereby, in use, radiation from the source undergoes successive reflections at said first, second and third reflective surfaces. 
     Preferably, for the mirrors having the third reflected surface, the first reflective surface, closest to the source, has a hyperbolic shape, the second reflective surface has a hyperbolic shape, and the third reflected surface, furthest from the source, has an ellipsoid shape. Preferably, the second hyperbolic shape has one focus in common with the first hyperbolic shape and one focus in common with the ellipsoid. Preferably, the second focus of the first hyperbolic shape is the source focus and said second focus of the ellipsoid is the intermediate focus. 
     Preferably, said plurality of mirrors comprises: a first set of two or more mirrors, the mirrors of said first set having first and second reflective surfaces; and a second set of two or more mirrors, the mirrors of said second set having first, second and third reflective surfaces. Preferably, the first set is innermost in the nested configuration and the second set is outermost in the nested configuration. 
     Preferably, for said first set, (a) the first reflective surface, closest to the source, has a hyperbolic shape, and/or (b) the second reflective surface, furthest from the source, is obtained by rotating an elliptical profile around an axis that is not an axis of symmetry of the ellipse. Preferably, for the first set, the mirrors are Wolter I or elliptical mirrors. 
     Preferably, the mirrors of said first set and or said second set are shaped such that the reflections at said first, second and third reflective surfaces are grazing incidence reflections. 
     In one embodiment, the mirrors are specified in accordance with Tables B.1 and B.2 set out hereinbelow. 
     Preferably, the or each mirror is formed as an electroformed monolithic component, and wherein the first and second reflective surfaces, and/or said first second and third reflective surfaces, are each provided on a respective one of two contiguous sections of the mirror. 
     Preferably, one or more of the mirrors has mounted thereon, for example on the rear side thereof, one or more devices for the thermal management of the mirror, for example cooling lines, Peltier cells and temperature sensors. 
     Preferably, one or more of the mirrors has mounted thereon, for example on the rear side thereof, one or more devices for the mitigation of debris from the source, for example erosion detectors, solenoids and RF sources. 
     In one embodiment of a full collector, a hybrid configuration may be used (see Section “Hybrid design” below, referring to  FIGS. 9 and 10 ) in which the innermost mirrors are Wolter I (or even elliptical) mirrors whereas the outermost are based on the new proposed configuration. Indeed, an inner (small) 3-reflections mirror would collect a very small solid angle and thus a large number of them would be required if used as innermost mirrors. 
     Thus, to overcome the limitations of conventional collectors, a new design is proposed in which the number of reflections is increased from 2 in the Wolter configuration to 3. Despite of the loss due to the additional reflection, an advantage of the use of 3 reflections is that it reduces the grazing incidence angles on all reflections and thus allows the collection of larger angles. 
     C. Hybrid Design 
     When the optical specifications of the collector  104  require low value for the minimum angle (Numerical Aperture) of the light at the Intermediate Focus, the number of mirrors in a nested Wolter I collector  104  increases rapidly, as innermost mirrors  200  collect only a small fraction of the angular emission from the source (see, e.g.,  FIG. 2 ). In such cases, the resulting optical design includes many closely spaced mirrors  200 . This situation has two major drawbacks:
         1) the final cost of the collector  104  increases with the number of mirrors  200  required to match the optical specifications;   2) since the innermost mirrors  200  are closely spaced, the available space for positioning the cooling system is very limited and may jeopardize the cooling performance.       

     According to one aspect of the present invention there is provided a plurality of mirrors arranged in a nested configuration, the or each mirror being symmetric about an optical axis extending through the radiation source and the or each mirror having at least one reflective surface, whereby, in use, radiation from the source undergoes a grazing incidence reflection at said at least one reflective surface reflective surface; and characterised by at least two of the mirrors having different geometries, and by the plurality of mirrors being optically matched. 
     Preferably, the mirrors are optically matched such that there is no, or substantially no, angular gap or obscuration between the solid angle collected by one mirror and the solid angle collected by the adjacent mirrors, and/or such that there is no, or substantially no, angular gap between one mirror and the adjacent mirror on the intermediate focus side of the collector optical system. In this context, “optically matched” means that the shells are designed such that there is no, or substantially no, angular gap or obscuration between the solid angle collected by one shell and the solid angle collected by the adjacent shells. Similarly, there is no, or substantially no, angular gap between one shell and the adjacent shells on the IF side. 
     In one embodiment, the first, innermost mirror of the plurality of mirrors has one reflective surface and the other mirrors have first and second reflective surfaces, whereby, in use, radiation from the source undergoes successive grazing incidence reflections at the first and second reflective surfaces. 
     Preferably, said different geometries include Wolter I, elliptical and combination of parabolic profiles. 
     In one embodiment, said plurality of mirrors comprises: a first set of two or more mirrors, the mirrors of said first set having a first geometry; and a second set of two or more mirrors, the mirrors of said second set having a second geometry, the first geometry and the second geometry being different. 
     In another embodiment, said plurality of mirrors comprises: a first mirror, the first mirror having a first geometry; and a set of two or more mirrors, the mirrors of said set having a second geometry, the first geometry and the second geometry being different. 
     Preferably, the first geometry is Wolter I, elliptical or combination of parabolic profiles. Preferably, the second geometry is Wolter I, elliptical or combination of parabolic profiles. 
     In one embodiment, there are 5 mirrors in the set. Preferably, the first mirror is an elliptical mirror, and the 5 mirrors in the set are Wolter I mirrors. Preferably, the mirrors are as specified in Table D.1 set out hereinbelow. 
     Preferably, the or each mirror is formed as an electroformed monolithic component, and wherein the first and second reflective surfaces are each provided on a respective one of two contiguous sections of the mirror. 
     Preferably, one or more of the mirrors has mounted thereon, for example on the rear side thereof, one or more devices for the thermal management of the mirror, for example cooling lines, Peltier cells and temperature sensors. 
     Preferably, one or more of the mirrors has mounted thereon, for example on the rear side thereof, one or more devices for the mitigation of debris from the source, for example erosion detectors, solenoids and RF sources. 
     The present invention seeks to reduce the number of mirrors, while matching the given optical specification, for example by a suitable combination of elliptical and Wolter I mirrors in the same collector. In particular embodiments, the use of one or more elliptical mirrors as innermost mirrors in the collector reduces the total number of mirrors. This is mainly because the elliptical mirror collects radiation from the source for all its length whereas a Wolter I mirror collects radiation from the source only on its hyperbolic section. 
     An advantage of the invention is that the number of shells is reduced and that it allows enough volume available for the cooling. 
     In certain embodiments, where there are not too many elliptical shells collecting relatively large emission angles, collection efficiency is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  (PRIOR ART) shows an example of a known EUV lithography system; 
         FIG. 2  (PRIOR ART) shows a ray diagram for the collector optics of the EUV lithography system of  FIG. 1 ; 
         FIG. 3  depicts an embodiment of a nested collector in accordance with one aspect of the invention; 
         FIG. 4  illustrates the far field intensity distribution simulated by ray tracing for a conventional collector and without the inclusion of the present invention; 
         FIG. 5  shows an example of the correction for a mirror in accordance with one aspect of the invention, for the collector of  FIG. 3 ; 
         FIG. 6  shows the result of the ray tracing of the collector of  FIG. 3  with the correction according to the invention; 
         FIG. 7  (PRIOR ART) illustrates a Wolter I collector consisting of 15 shells, designed to collect large angles; 
         FIG. 8  shows an embodiment of a nested collector in accordance with one aspect of the invention, and matching the same specification in terms of Numerical Aperture at intermediate focus as the collector of  FIG. 7 ; 
         FIG. 9  (PRIOR ART) illustrates an example of the optical layout of a nested grazing incidence Wolter I collector with 7; and 
         FIG. 10  shows an embodiment of a nested collector in accordance with one aspect of the invention, and matching the same optical specifications as the collector of  FIG. 9 . 
     
    
    
     In the description and drawings, like numerals are used to designate like elements. Unless indicated otherwise, any individual design features and components may be used in combination with any other design features and components disclosed herein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the illustrations of optical elements or systems herein, unless indicated otherwise, cylindrical symmetry around the optical axis is assumed; and references to an “image focus” are references to an image focus or an intermediate focus. 
     The design and construction of the collector  104  according to the invention is as set out above in relation to  FIGS. 1 and 2 , except as described hereinafter. 
     A. Far Field Improvement 
       FIG. 3  depicts an embodiment of a nested collector in accordance with one aspect of the invention. This embodiment is the same as described with reference to  FIGS. 1 and 2 , except as described below. The design here is of a 7-mirror nested Wolter I collector, and the corresponding specifications are listed in Table A.1. However, it will be appreciated by skilled persons that any suitable number of mirrors may be used. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE A.1 
               
             
             
               
                   
                   
               
               
                   
                 Hyperbola 
                 Ellipse 
                 Mirror radii [mm] 
               
             
          
           
               
                   
                   
                   
                 Radius of 
                   
                 Radius of 
                   
                 Ellipse- 
                   
               
               
                   
                   
                 Conic 
                 curvature 
                 Conic 
                 curvature 
                   
                 hyperbola 
               
               
                 Mirror # 
                 Type 
                 Constant 
                 [mm] 
                 Constant 
                 [mm] 
                 Maximum 
                 vertex 
                 Minimum 
               
               
                   
               
             
          
           
               
                 1 
                 Wolter I 
                 −1.01019385 
                 2.1365 
                 −0.99852911 
                 1.4137 
                 36.2242 
                 34.3239 
                 24.6418 
               
               
                 2 
                 Wolter I 
                 −1.01738536 
                 3.6308 
                 −0.99755671 
                 2.3494 
                 46.7105 
                 44.1731 
                 31.5202 
               
               
                 3 
                 Wolter I 
                 −1.02914859 
                 6.0526 
                 −0.99599879 
                 3.8505 
                 59.8238 
                 56.4726 
                 40.0528 
               
               
                 4 
                 Wolter I 
                 −1.04841826 
                 9.9610 
                 −0.99350491 
                 6.2582 
                 76.3193 
                 71.9129 
                 50.6512 
               
               
                 5 
                 Wolter I 
                 −1.08035217 
                 16.2846 
                 −0.98949472 
                 10.1427 
                 97.2646 
                 91.4560 
                 63.8362 
               
               
                 6 
                 Wolter I 
                 −1.13469930 
                 26.6371 
                 −0.98296834 
                 16.4983 
                 124.2702 
                 116.5224 
                 80.2740 
               
               
                 7 
                 Wolter I 
                 −1.23207212 
                 44.0419 
                 −0.97209459 
                 27.1823 
                 159.9860 
                 149.3855 
                 100.8206 
               
               
                   
               
             
          
         
       
     
     The far field intensity distribution simulated by ray tracing for a spherical extended source with a FWHM of 0.5 mm and without the inclusion of the present invention is shown in  FIG. 4 . The 6 dips  402  in the far field intensity distribution that are evident in  FIG. 4  are due to the shadowing of the mirror thickness. The shadow of the outermost mirror is of course not evident in  FIG. 4 . 
     In accordance with this embodiment of the invention, onto the nominal design of  FIG. 3  is superimposed a polynomial correction given by the following equation, for both the hyperbolic and the elliptical section  204  of each shell (mirror)  200 ,
 
Δ r   i ( z )= k   i ( c   1   z+c   2   z   2   +c   3   z   3 )
 
where Δr i (z) is the variation in the mirror radius and z is the position along the optical axis in millimeters measured starting from the joint point between the hyperbolic and the elliptical section of each mirror and pointing from the intermediate focus to the source. The subscript i in the above equation enumerates the mirrors from 1 (innermost mirror) to 7 (outermost mirror). The values for the parameters k i , c 1 , c 2 , and c 3 , are listed in Table A.2. and A.3.
 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE A.2 
               
             
             
               
                   
                   
               
               
                   
                 k 
                   
               
             
          
           
               
                 Mirror # 
                 Hyperbola 
                 Ellipse 
               
               
                   
               
             
          
           
               
                 1 
                 0.3 
                 0.2 
               
               
                 2 
                 0.3 
                 0.5 
               
               
                 3 
                 0.3 
                 0.7 
               
               
                 4 
                 0.3 
                 0.8 
               
               
                 5 
                 0.3 
                 1 
               
               
                 6 
                 0.3 
                 1 
               
               
                 7 
                 0.3 
                 1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE A.3 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Hyperbola 
                 Ellipse 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 c 1   
                 0.001 
                 0.00005 
               
               
                   
                 c 2   
                 −0.00001 
                 −0.00000035 
               
               
                   
                 c 3   
                 −0.00000001 
                 −0.0000000005 
               
               
                   
                   
               
             
          
         
       
     
     An example of the above correction for mirror #5 is shown in  FIG. 5 . The result of the ray tracing of the collector of  FIG. 3  with the corrections given in Table A.2 and Table A.3 is shown in  FIG. 6 . It can be noted that the dips in the far field distribution due to the shadowing of the mirror thickness have disappeared. 
     The present invention has been here described with reference to a Wolter I optical design for the collector configuration. However, it may be equally well applied to any optical configuration based on nested mirrors, including, in particular, pure elliptical designs. 
     B. Multiple Reflections 
       FIG. 7  (PRIOR ART) shows the design of a conventional Wolter I collector consisting of 15 shells (mirrors), designed to collect large angles. For clarity/simplicity, only the outer mirror  200  is highlighted: this has first  202  and second  204  reflective surfaces. The design parameters are listed in Table B.1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE B.1 
               
             
             
               
                   
                   
               
               
                   
                 Hyperbola 
                 Ellipse 
                 Mirror radii [mm] 
               
             
          
           
               
                   
                   
                   
                 Radius of 
                   
                 Radius of 
                   
                 Ellipse- 
                   
               
               
                   
                   
                 Conic 
                 curvature 
                 Conic 
                 curvature 
                   
                 hyperbola 
               
               
                 Mirror # 
                 Type 
                 Constant 
                 [mm] 
                 Constant 
                 [mm] 
                 Maximum 
                 vertex 
                 Minimum 
               
               
                   
               
             
          
           
               
                 1 
                 Wolter I 
                 −1.01019385 
                 2.1365 
                 −0.99852911 
                 1.4137 
                 35.3252 
                 34.3239 
                 29.2746 
               
               
                 2 
                 Wolter I 
                 −1.01380178 
                 2.8875 
                 −0.99803171 
                 1.8922 
                 40.8749 
                 39.6810 
                 33.7193 
               
               
                 3 
                 Wolter I 
                 −1.01848425 
                 3.8582 
                 −0.99739256 
                 2.5075 
                 47.0620 
                 45.6488 
                 38.6538 
               
               
                 4 
                 Wolter I 
                 −1.02454641 
                 5.1084 
                 −0.99657323 
                 3.2967 
                 53.9760 
                 52.3122 
                 44.1406 
               
               
                 5 
                 Wolter I 
                 −1.03239021 
                 6.7151 
                 −0.99552402 
                 4.3084 
                 61.7236 
                 59.7716 
                 50.2520 
               
               
                 6 
                 Wolter I 
                 −1.04255366 
                 8.7791 
                 −0.99417996 
                 5.6059 
                 70.4351 
                 68.1489 
                 57.0718 
               
               
                 7 
                 Wolter I 
                 −1.05577182 
                 11.4338 
                 −0.99245489 
                 7.2738 
                 80.2730 
                 77.5949 
                 64.6991 
               
               
                 8 
                 Wolter I 
                 −1.07307485 
                 14.8598 
                 −0.99023248 
                 9.4269 
                 91.4447 
                 88.3007 
                 73.2527 
               
               
                 9 
                 Wolter I 
                 −1.09430983 
                 19.4418 
                 −0.98755786 
                 12.0871 
                 104.1079 
                 100.5148 
                 83.1861 
               
               
                 10 
                 Wolter I 
                 −1.12243334 
                 25.3834 
                 −0.98409595 
                 15.5415 
                 118.6004 
                 114.4429 
                 94.2993 
               
               
                 11 
                 Wolter I 
                 −1.16163731 
                 33.0910 
                 −0.97941741 
                 20.1821 
                 135.4602 
                 130.5096 
                 106.6401 
               
               
                 12 
                 Wolter I 
                 −1.21697553 
                 43.3984 
                 −0.97309226 
                 26.4698 
                 155.4237 
                 149.3995 
                 120.6282 
               
               
                 13 
                 Wolter I 
                 −1.29756350 
                 57.6394 
                 −0.96442027 
                 35.1577 
                 179.5869 
                 172.0902 
                 136.7247 
               
               
                 14 
                 Wolter I 
                 −1.42211299 
                 78.1026 
                 −0.95213443 
                 47.6020 
                 209.7405 
                 200.1068 
                 155.4179 
               
               
                 15 
                 Wolter I 
                 −1.63312120 
                 109.3155 
                 −0.93389437 
                 66.3805 
                 249.0615 
                 236.0823 
                 177.3177 
               
               
                   
               
             
          
         
       
     
       FIG. 8  shows an embodiment of a collector based on the invention and matching the same specification in terms of Numerical Aperture at intermediate focus of the Wolter design of  FIG. 7 . The collector in  FIG. 8  consists of 17 mirrors. However, it will be appreciated by skilled persons that any suitable number of mirrors may be used. The first 9 inner mirrors in  FIG. 8  are Wolter I mirrors, identical to the first 9 inner mirrors of  FIG. 7 . The outer 8 mirrors are in accordance with the invention: for clarity/simplicity, only the outer mirror  200  is highlighted: this has first  202  and second  204  and third  205  reflective surfaces. The design parameters for this embodiment are provided in Tables B.2 and B.3. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE B.2 
               
             
             
               
                   
                   
               
               
                   
                 1 st  Hyperbola 
                 2 nd  Hyperbola 
                 Ellipse 
               
             
          
           
               
                   
                   
                   
                 Radius of 
                   
                 Radius of 
                   
                 Radius of 
               
               
                   
                   
                 Conic 
                 curvature 
                 Conic 
                 curvature 
                 Conic 
                 curvature 
               
               
                 Mirror # 
                 Type 
                 Constant 
                 [mm] 
                 Constant 
                 [mm] 
                 Constant 
                 [mm] 
               
               
                   
               
               
                 10 
                 3 reflections 
                 −1.33758725 
                 21.3244 
                 −1.04207680 
                 12.9908 
                 −0.98976428 
                 11.7106 
               
               
                 11 
                 3 reflections 
                 −1.46555915 
                 23.7944 
                 −1.05648598 
                 17.9347 
                 −0.98746339 
                 14.3598 
               
               
                 12 
                 3 reflections 
                 −1.63207184 
                 27.4985 
                 −1.07383470 
                 23.7013 
                 −0.98456808 
                 17.7021 
               
               
                 13 
                 3 reflections 
                 −1.81172763 
                 34.3724 
                 −1.09104852 
                 28.8721 
                 −0.98097774 
                 21.8605 
               
               
                 14 
                 3 reflections 
                 −2.03509277 
                 44.2012 
                 −1.11060710 
                 34.3523 
                 −0.97651910 
                 27.0459 
               
               
                 15 
                 3 reflections 
                 −2.46210484 
                 54.5611 
                 −1.14332818 
                 44.1901 
                 −0.97077220 
                 33.7649 
               
               
                 16 
                 3 reflections 
                 −2.93371314 
                 76.3229 
                 −1.14609546 
                 56.1803 
                 −0.96903340 
                 38.6701 
               
               
                 17 
                 3 reflections 
                 −4.17455417 
                 93.2478 
                 −1.23744029 
                 70.0556 
                 −0.95401338 
                 53.5898 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE B.3 
               
             
             
               
                   
                   
               
               
                   
                 Mirror radii [mm] 
               
             
          
           
               
                   
                   
                   
                 1 st   
                   
                   
               
               
                   
                   
                   
                 hyperbola - 
                 Ellipse - 
               
               
                   
                   
                   
                 2 nd   
                 2 nd   
               
               
                 Mirror 
                   
                   
                 hyperbola 
                 hyperbola 
               
               
                 # 
                 Type 
                 Minimum 
                 vertex 
                 vertex 
                 Maximum 
               
               
                   
               
             
          
           
               
                 10 
                 3 reflections 
                 79.4910 
                 97.7987 
                 114.2028 
                 115.3360 
               
               
                 11 
                 3 reflections 
                 84.4112 
                 108.0994 
                 126.4284 
                 127.7924 
               
               
                 12 
                 3 reflections 
                 90.2220 
                 119.6290 
                 140.3750 
                 141.9930 
               
               
                 13 
                 3 reflections 
                 99.0188 
                 132.2649 
                 156.0743 
                 157.9382 
               
               
                 14 
                 3 reflections 
                 109.4400 
                 146.1280 
                 173.7322 
                 175.8772 
               
               
                 15 
                 3 reflections 
                 117.4959 
                 161.9175 
                 194.2078 
                 196.8072 
               
               
                 16 
                 3 reflections 
                 136.7241 
                 184.9095 
                 218.5841 
                 219.7478 
               
               
                 17 
                 3 reflections 
                 136.6512 
                 198.9031 
                 245.1200 
                 249.0386 
               
               
                   
               
             
          
         
       
     
     Using a reference reflectivity profile for Ruthenium at 13.5 nm, the new design of  FIG. 8  has an efficiency of about 21% greater than the Wolter I design of  FIG. 7 . As the plasma sources used in EUV lithography have a very low efficiency, even small increases of the collector efficiency implies great benefits in term of diminished thermal power from the source. 
     C. Hybrid Design 
       FIG. 9  (PRIOR ART) shows the optical layout of a nested grazing incidence Wolter I collector with 7 mirrors, designed to illuminate the intermediate focus with radiation between 1.5° and 8°. For clarity/simplicity, only the outer mirror  200  is highlighted: this has first  202  and second  204  reflective surfaces. 
       FIG. 10  shows a design matching the same optical specifications as the collector of  FIG. 9 , but configured in accordance with the present invention. It consists of 1 elliptical mirror as the innermost mirror  207 , followed by 5 Wolter I mirrors. However, it will be appreciated by skilled persons that any suitable number of mirrors may be used. (For clarity/simplicity, only the outer mirror  200  is highlighted: this has first  202  and second  204  reflective surfaces.) It can be noted, when comparing the collector  104  of  FIG. 10  to that of  FIG. 9 , that the total number of mirrors  200  is decreased from 7 to 6, and that the shadow region on the rear side of the mirror  200  that can be used to position the cooling system is increased. Thus, manufacturing complexity is reduced and enhanced cooling is facilitated. 
     The dimensions of the embodiment shown in  FIG. 10  are listed in Table D.1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE D.1 
               
             
             
               
                   
                   
               
               
                   
                 Hyperbola 
                 Ellipse 
                 Mirror radii [mm] 
               
             
          
           
               
                   
                   
                   
                 Radius of 
                   
                 Radius of 
                   
                 Ellipse- 
                   
               
               
                   
                   
                 Conic 
                 curvature 
                 Conic 
                 curvature 
                   
                 hyperbola 
               
               
                 Mirror # 
                 Type 
                 Constant 
                 [mm] 
                 Constant 
                 [mm] 
                 Maximum 
                 vertex 
                 Minimum 
               
               
                   
               
             
          
           
               
                 1 
                 Ellipse 
                 — 
                 — 
                 −0.99165756 
                 6.2831 
                 58.3310 
                 — 
                 36.3780 
               
               
                 2 
                 Wolter I 
                 −1.03662221 
                  7.9277 
                 −0.99441040 
                 5.4394 
                 71.5153 
                 68.3835 
                 50.8913 
               
               
                 3 
                 Wolter I 
                 −1.05571905 
                 11.9520 
                 −0.99165225 
                 8.1347 
                 87.5221 
                 83.5727 
                 61.7865 
               
               
                 4 
                 Wolter I 
                 −1.08493511 
                 17.9720 
                 −0.98754639 
                 12.1609 
                 107.1304 
                 102.1277 
                 74.8727 
               
               
                 5 
                 Wolter I 
                 −1.12852641 
                 27.2703 
                 −0.98164975 
                 18.0654 
                 131.2648 
                 125.0704 
                 91.1342 
               
               
                 6 
                 Wolter I 
                 −1.19754721 
                 41.5917 
                 −0.97281004 
                 27.0270 
                 161.5243 
                 153.7090 
                 110.7959 
               
               
                   
               
             
          
         
       
     
     Alternative embodiments may involve including or combining mirrors of different geometry in different positions in the nested collector, including Wolter I mirrors, elliptical mirrors, and a combination of parabolic mirrors. 
     Encompassed by the invention are collector optics for imaging (e.g. EUV or X-ray), and imaging systems incorporating such optics; the design of such imaging optics and imaging systems is discussed in, for example, European patent application no. 06425539.1 (attorney&#39;s ref. ML00H19/P-EP).