PATENT ABSTRACT
A collector system for extreme ultraviolet (EUV) radiation includes a collector mirror and a radiation-collection enhancement device (RCED) arranged adjacent an aperture member of an illuminator. The collector mirror directs EUV radiation from an EUV radiation source towards the aperture member. The RCED redirects a portion of the EUV radiation that would not otherwise pass through the aperture of the aperture member or that would not have an optimum angular distribution, to pass through the aperture and to have an improved angular distribution better suited to input specifications of an illuminator. This provides the illuminator with greater amount of useable EUV radiation than would otherwise be available from the collector mirror alone, thereby enhancing the performing of an EUV lithography system that uses such a collector system with a RCED.

PATENT DESCRIPTION
CLAIM OF PRIORITY 
     This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/341,806, filed on Apr. 5, 2010, which application is incorporated by reference herein, and is a Divisional Application of U.S. patent application Ser. No. 13/065,008, filed on Mar. 11, 2011, and which is incorporated by reference herein. 
    
    
     FIELD 
     The present disclosure relates generally to collectors for extreme ultraviolet (EUV) radiation, and in particular to EUV collector systems having enhanced EUV radiation collection capability. 
     BACKGROUND ART 
     EUV collector systems are used in EUV lithography systems to collect EUV radiation from an EUV radiation source and direct the EUV radiation to an aperture typically referred to as or associated with the intermediate focus. The radiation from the intermediate focus is then relayed by an illuminator to illuminate a reflective reticle. Radiation reflected from the illuminated reticle is then projected onto a wafer coated with a photosensitive material such as photoresist that records the reticle image. The wafer is then processed to form integrated microcircuits. 
       FIG. 1  is a schematic diagram of a generalized configuration of a collector system  10 N that uses a normal-incidence collector (NIC) mirror MN.  FIG. 2  is a schematic diagram of a generalized configuration of a collector system  10 G that uses a grazing-incident collector (GIC) mirror MG. Each collector system  10 N and  10 G has an EUV radiation source RS that emits EUV radiation  12 , a central axis A 1 , and an intermediate focus IF. Each collector system  10 N and  10 G is shown arranged adjacent an illuminator  20  that has an entrance aperture member  22  that defines an entrance aperture  24 . Entrance aperture member  22  is arranged at or near the intermediate focus IF. NIC mirror MN has a common input and output side  17 , while GIC mirror MG has an input end  16  and an output end  18 . 
     In each collector system  10 N and  10 G, an important performance metric for EUV lithography is the amount and angular distribution of EUV radiation  12  the collector mirror MN or MG can deliver to the intermediate focus IF and through the entrance aperture  24  of illuminator  20 . As mentioned above, also of importance is the angular distribution of the EUV radiation  12  delivered through entrance aperture  24  of illuminator  20 . Entrance aperture  24  is used to define the limits of the intermediate focus IF so that illuminator  20  can have the proper field size and numerical aperture for illuminating the reticle (not shown). 
     However, because neither type of collector system  10 N or  10 G can be made to perform perfectly, and because of magnification constraints on the system design, entrance aperture member  22  of illuminator  20  may also end up intercepting a substantial amount of EUV radiation  12 L, so that this intercepted EUV radiation  12 L is lost and is not utilized by the illuminator  20 , as illustrated in  FIG. 3 . 
     Also, due to design limitations or manufacturing imperfections in the collector system  10 N or  10 G, EUV radiation  12  passing through the entrance aperture  24  may not have the optimum angular distribution for use by the illuminator  20 . This lost or non-optimum EUV radiation  12 L is problematic because as much useable EUV radiation  12  as possible must be provided to illuminator  20  so that there is sufficient radiation to uniformly illuminate the reticle and adequately expose the photosensitive material (photoresist) on the wafer. 
     SUMMARY 
     The present disclosure is directed to EUV collector systems having enhanced EUV radiation collection capability. The enhanced EUV radiation collection capability is provided by a radiation-collection enhancement device (RCED) that is arranged at or adjacent an illuminator entrance aperture member that defines an entrance aperture. One RCED on either side of the illuminator entrance pupil (aperture) can be used, or two RCEDs on either side of the illuminator entrance pupil (aperture) can be used. The RCED can be configured so that EUV radiation that would otherwise not pass through the entrance aperture is redirected through the entrance aperture. In addition, by selectively configuring the inner surface of the RCED, a desired angular distribution (e.g., one that is more compatible with illumination system requirements) of the EUV radiation passing through the entrance aperture can be obtained. 
     The RCED need not be circularly symmetric and can have one or more different types of inner surfaces (e.g., polished, planar, rough, undulating, etc.) that can grazingly reflect or otherwise re-direct incident EUV radiation. Some of this redirected EUV radiation can be used to illuminate discrete detectors that may be, for example, part of an EUV lithography alignment system. A roughened inner surface, for example, may be employed in certain applications, and on some or all of the at least one inner surface, where it is advantageous to scatter the otherwise less useful EUV radiation through the entrance aperture of the illuminator, for example to homogenize the radiation distribution in the far field. The one or more inner surfaces are thus referred to herein below also as “redirecting surfaces.” 
     Some embodiments of the RCED include multiple inner surfaces, such as defined by concentric mirror shells. The RCED can be attached to the entrance aperture of the illuminator or can be spaced apart therefrom. 
     The RCED can be configured (or be exchanged out for another RCED at a semiconductor manufacturing facility) to accommodate changes in the requirements on the EUV radiation being delivered to the illuminator. The RCED can be used to reduce the collection specifications on the collector mirror, making it easier to build and/or lower the cost of the collector system. The RCED is particularly useful in mitigating adverse affects due to collector system misalignments and perturbations. The RCED can be configured so that the captured light that would otherwise be lost or be less useful because of improper angular distribution can be redirected to the illuminator while still preserving (or at least substantially preserving) the etendue of the collector-illuminator system. 
     An example RCED uses grazing-incident reflection to direct otherwise lost or less useful radiation through the entrance aperture of the illuminator. The redirecting surface of RCED can be highly polished and have a coating that maximizes the critical angle for grazing-incident reflection and enhances the collection solid angle. The coating may comprise a single layer or multilayer. Example coating materials include Ruthenium for a single-layer coating and Mo/Si for multilayer coatings. 
     Thus, an aspect of the disclosure is a collector system for collecting and directing EUV radiation from an EUV radiation source through an aperture of an aperture member. The collector system includes a collector mirror configured to collect and direct the EUV radiation toward the aperture. The collector system also includes a radiation-collection enhancement device arranged at or adjacent the aperture and configured to collect a portion of the EUV radiation that would not otherwise pass through the aperture or would pass through the aperture at less than optimum angular distribution and redirect said portion of the EUV radiation through the aperture and with an angular distribution better suited for use by the illuminator. 
     Another aspect of the disclosure is a method of collecting EUV radiation from an EUV radiation source and directing the EUV radiation through an aperture. The method includes collecting the EUV radiation from the radiation source and directing the EUV radiation to the aperture. The method also includes collecting a portion of the directed EUV radiation that would not otherwise pass through the aperture with at least one redirecting surface arranged adjacent the aperture, and redirecting said portion of EUV radiation through the aperture. 
     Another aspect of the disclosure is a method of collecting EUV radiation in EUV lithography system having an aperture member with an aperture. The method includes generating the EUV radiation with an EUV radiation source. The method also includes collecting the EUV radiation from the EUV radiation source with an EUV collector and directing the EUV radiation to the aperture. A first portion of the directed EUV radiation is directed to pass through the aperture and a second portion of EUV radiation is directed to be intercepted by the aperture member. The method further includes collecting the second portion of EUV radiation with at least one first redirecting surface arranged adjacent the aperture, and redirecting the portion of EUV radiation through the aperture so that both the first and second portions of the directed EUV radiation pass through the aperture. 
     Additional features and advantages of the disclosure are set forth in the detailed description below, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description which follows, the claims, as well as the appended drawings. The claims set forth hereinbelow constitute part of this specification and are incorporated herein directly and by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a generalized prior art NIC collector system, illustrating how some of the focused EUV radiation does not make it through the -entrance aperture; 
         FIG. 2  is a schematic diagram of a generalized prior art GIC collector system, illustrating how some of the focused EUV radiation does not make it through the entrance aperture member; 
         FIG. 3  is a close-up cross-sectional view of the entrance aperture, illustrating how a portion of the EUV radiation generally directed to the intermediate focus is blocked by the entrance aperture member; 
         FIG. 4  is a close-up cross-sectional view similar to  FIG. 3 , but that includes an example RCED and that shows how the RCED redirects EUV radiation that would otherwise be lost to pass through the entrance aperture; 
         FIG. 5A  is a cross-section view of an example multi-shell RCED; 
         FIG. 5B  is a face-on view of the multi-shell RCED of  FIG. 5A  showing the spokes of a support structure (“spider”) for the two reflective shells; 
         FIG. 6  is a schematic cross-sectional view of an example RCED that is spaced apart from the entrance aperture member and that is attached thereto via a support structure; 
         FIG. 7  is a generalized NIC collector system similar to that of  FIG. 1 , but with a RCED; 
         FIG. 8  is a more detailed schematic diagram of an example EUV NIC collector system and that includes a RCED and a LPP EUV source; 
         FIG. 9  is a generalized GIC collector system similar to that of  FIG. 2 , but with a RCED; 
         FIG. 10  is a more detailed schematic diagram of an example EUV GIC collector system and that includes a RCED and a LPP EUV source; 
         FIG. 11  is an isometric view of an example conic RCED having circular symmetry and linear walls; 
         FIG. 12  is a side cross-sectional view of an example conic RCED having circular symmetry and curved walls; 
         FIG. 13  is a cross-section view of an example RCED, where the inner wall includes a plurality of facets and has non-circular symmetry; 
         FIG. 14  is a cross-sectional view of an example RCED, where the inner wall includes a variety of different configurations such as planar, roughened, undulating and curved polished; 
         FIG. 15  is a lateral cross-sectional view of an example RCED where the inner surface includes an undulating surface; 
         FIG. 16  is similar to  FIG. 12  but includes a roughened inner surface portion adjacent the output end; 
         FIG. 17  is a schematic diagram similar to  FIG. 2  and shows an example GIC collector system  10 G with illuminator  20 , illustrating the etendue limitations associated with transferring the EUV radiation from the radiation source to the illuminator; 
         FIG. 18A  is similar to  FIG. 4  and illustrates an example embodiment where the RCED includes front and rear tapered bodies (sections) on either side of the aperture member; 
         FIG. 18B  is similar to  FIG. 18A , except that the tapered bodies are separated from the aperture member; 
         FIG. 19  is similar to  FIG. 5A  and illustrates an example RCED that includes multiple inner surfaces on either side of the aperture member; 
         FIG. 20  is similar to  FIG. 6  and illustrates another example RCED that includes a single front mirror shell and a single rear mirror shell as stood off from aperture member by respective stand-off support structures; 
         FIG. 21  is similar to  FIG. 16  and illustrates another example RCED having front and rear tapered bodies on either side of the aperture member; 
         FIG. 22  is similar to  FIG. 21  except that the RCED includes cooling channels on its outer surface; 
         FIG. 23  is a schematic diagram of an EUV lithography system that uses an EUV collector system that employs the RCED of the present disclosure. 
     
    
    
     The various elements depicted in the drawing are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawing is intended to illustrate an example embodiment of the disclosure that can be understood and appropriately carried out by those of ordinary skill in the art. 
     In the discussion below, the term “far field” is generally understood as being a substantial distance beyond the intermediate focus IF, e.g., 1 meter or greater. 
     DETAILED DESCRIPTION 
       FIG. 4  is a close-up, cross-sectional view of an entrance aperture member  22  of illuminator  20  similar to  FIG. 3 , but showing an example RCED  100  arranged along central axis A 1  and adjacent entrance aperture member  22  on the side closest to EUV radiation source RS (not show in  FIG. 4 ; see e.g.,  FIGS. 1 and 2 ). RCED  100  has a body portion  110  that includes a central aperture  114  that, along with body portion  110 , defines a tapered inner surface  120  that goes from wider at an input end  122  to narrower at an output end  124 , i.e., the taper generally narrows in the +Z direction. 
     Inner surface  120  is designed to redirect at least a portion of EUV radiation  12 L so that this EUV radiation  12 L, which would otherwise not pass through entrance aperture  24  or that would pass through the entrance aperture  24  but with a less than optimum angle for use by the illuminator  20 , passes through entrance aperture  24 . In an example embodiment, inner surface  120  is smooth and covered with a coating  121  (single-layer or multi-layer, as described below) designed to enhance the reflectivity of the inner surface  120  at EUV wavelengths and the grazing incidence angles of EUV radiation  12 L. Various forms for RCED  100  are discussed in greater detail below. In an example embodiment, RCED  100  is or includes a grazing-incidence mirror element. EUV wavelengths typically range from 10 nm to 15 nm, with an exemplary EUV wavelength being 13.5 nm. 
     In an example embodiment, inner surface  120  of RCED  100  is configured to match the numerical aperture (NA) requirements of illuminator  20 . In another example, RCED  100  can be adjusted or swapped out for a different RCED to accommodate changes (e.g., NA changes) in illuminator  20 . Generally, RCED  100  can be configured to match or otherwise accommodate particular angular distribution requirements of illuminator  20 . 
     An aspect of the disclosure includes using RCED  100  to reduce the focusing requirements on the collector mirror (MN or MG) to allow the design of an illuminator  20  having a smaller entrance aperture  24  than could reasonably be accommodated by using a collector mirror (MN or MG) alone. This aspect of the disclosure can serve to simplify the collector requirements and/or illuminator design, which in turn reduces the collector and/or illuminator cost. 
     In an example, RCED  100  is disposed adjacent entrance aperture member  22  of illuminator  20 , and can be attached to the entrance aperture member  22  or spaced apart therefrom. Attachment of RCED  100  to entrance aperture member  22  can be accomplished mechanically or magnetically so that the RCED  100  and the entrance aperture member  22  appear integrally formed, as shown in  FIG. 4 . A spaced-apart RCED  100  (discussed below in connection with  FIG. 6 ) may be preferred in some instances to achieve specific performance goals, or for ease of manufacture and assembly. In such case, a stand-off mechanism may be configured to achieve a precise separation distance. 
       FIG. 5A  is a cross-sectional view that illustrates an example RCED  100  that includes multiple inner surfaces  120 , such as formed by concentrically arranged mirror shells  103 - 1  and  103 - 2 . The concentric mirror shells  103 - 1  and  103 - 2  define two RCED apertures  114 - 1  and  114 - 2 .  FIG. 5B  is a face-on view of RCED  100  of  FIG. 5A  showing spokes  105  of a support structure (“spider”) that maintain the two mirror shells  103 - 1  and  103 - 2  in a spaced-apart and aligned configuration. Thus, RCED  100  generally includes at least one inner surface  120 , and in certain embodiments includes multiple inner surfaces  120 . 
       FIG. 6  is a schematic cross-sectional view of an example RCED  100  that is spaced apart from entrance aperture member  22  of illuminator  20 , and that is attached thereto via a stand-off support structure  113 . In  FIG. 5A  and in  FIG. 6 , the intermediate focus IF is shown as located in the plane PL defined by entrance aperture member  22  of illuminator  20 . The intermediate focus IF represents the central location of the focused EUV radiation distribution formed by the GIC collector system  10 G. 
     It is noted here that while RCED  100  redirects at least a portion of EUV radiation  12 L that otherwise would not make it through entrance aperture  24  of illuminator  20 , in some embodiments RCED  100  is configured to also redirect through the entrance aperture  24  at least some EUV radiation  12  that would in fact have made it through the entrance aperture  24  had it not been redirected (see, e.g., one of the scattered EUV radiation  12  in  FIG. 16 ). In such an embodiment, the redirection of EUV radiation  12  that would have made it through the entrance aperture  24  anyway will typically be done to change the angular distribution of the EUV radiation  12  passing through entrance aperture  24  and thereby make such EUV radiation  12  better suited to meet the angular input requirements of the illuminator  20 . In an example, the redirection of EUV radiation  12  is optimized to the angular distribution requirements of illuminator  20 . 
     The output end  124  of RCED  100  can be smaller than entrance aperture  24  and still provide improved light collection. Experiments have shown that an RCED  100  with an output end  124  having a diameter of  4  mm passes substantially the same amount of EUV radiation  12  as a 6 mm entrance aperture  24  but resulted in a better angular distribution of the EUV radiation  12  in the far field. 
     NIC Collector with RCED 
       FIG. 7  shows a generalized NIC collector system  150  similar to NIC collector system  10 N of  FIG. 1 , but with a RCED  100  arranged adjacent entrance aperture member  22  of illuminator  20 .  FIG. 8  is a more detailed schematic diagram of an example NIC collector system  150  based on the generalized NIC collector system  10 N of  FIG. 7 . 
       FIG. 7  and  FIG. 8  show the illuminator  20  acceptance angle θ that applies generally for both types of collector systems. The numerical aperture NA of illuminator  20  is given by NA=n·sin θ, where n is the refractive index of the medium, which is presumed to be a vacuum for an EUV lithography system (i.e., n=1). 
     With reference to  FIG. 8 , NIC collector system  150  includes a high-power laser source LS that generates a high-power, high-repetition-rate laser beam  11  having a focus F 11 . NIC collector system  150  also includes along a central axis A 1  a fold mirror FM and a large (e.g., ˜600 mm diameter) ellipsoidal NIC mirror MN that includes a surface S 1  with a multilayer coating  154 . The multilayer coating  154  provides good reflectivity at EUV wavelengths. NIC collector system  150  also includes a Sn source  160  that emits a stream of Sn pellets (or droplets)  162  that pass through and are irradiated by laser beam focus F 11 . 
     In the operation of NIC collector system  150 , laser beam  11  from laser source LS irradiates Sn pellets (or droplets)  162  as the pellets (or droplets) pass through the laser beam focus F 11 , thereby produce a high-power laser-produced plasma source LPP-RS. Laser-produced plasma source LPP-RS typically resides on the order of a few hundred millimeters from NIC mirror MN and emits EUV radiation  12 , as well as energetic Sn ions, particles, neutral atoms, and visible, UV and infrared (IR) radiation. The portion of the EUV radiation  12  directed toward NIC mirror MN is collected by the NIC mirror MN and directed (focused) toward entrance aperture  24  to intermediate focus IF to form intermediate radiation distribution RD. 
     As discussed above, some of the EUV radiation  12  (identified as  12 L) has a trajectory that would be blocked by entrance aperture member  22 . However, at least a portion of EUV radiation  12 L is collected by RCED  100  and redirected through entrance aperture  24  of illuminator  20 . This provides more EUV radiation  12  for forming a far-field radiation distribution RD, and thus more radiation for ultimately forming an image of the reticle at the wafer in an EUV lithography system. 
     It is noted here that the EUV radiation directed toward entrance aperture  24  by the EUV collector system is not tightly focused precisely at intermediate focus IF and does not generally form a perfectly uniform far-field radiation distribution RD. Rather, the radiation distribution RD formed by the collector system at the intermediate focus IF is somewhat ill-defined due to imperfections (aberrations) in the particular collector system used, as well as scattering effects in the collector system. Further, illuminator  20  is typically designed so that it does not require as an input a sharply focused spot or a crisply defined disk.  FIG. 12 , introduced and discussed below, shows an intermediate focus region IFR and that schematically illustrates a more realistic extent of the intermediate focus IF as caused by aberrations and scattering, and that is representative of the extent of an actual EUV radiation distribution. 
     Illuminator  20  typically is configured to receive EUV radiation that passes through entrance aperture  24  with a specified angular distribution and uniformity. The illuminator  20  serves to condense and uniformize this EUV radiation for uniformly illuminating the reflective reticle (usually to within a few percent (e.g., between 2% and 5% uniformity). Thus, RCED  100  may be designed to capture additional misdirected EUV radiation from the collector mirror and redirect it to meet the illuminator specifications, thereby enhancing illuminator performance, and in particular increasing the amount of EUV radiation that can be effectively used to illuminate the reticle in an EUV lithography system. 
     In an example embodiment, the NIC mirror MN or GIC mirror MG is formed with looser (reduced) tolerances than would otherwise be possible, and RCED  100  is used to compensate for the reduced tolerances, errors, misalignments, thermal distortions, etc. The combination of the collector mirror and RCED  100  can thus be used to meet the system tolerance at the intermediate focus plane PL for the radiation distribution RD. This approach makes it easier and likely less expensive to form the NIC or GIC mirror when such mirror is used in combination with a RCED  100 . 
     GIC Collector with RCED 
       FIG. 9  shows a generalized GIC collector system  180  similar to GIC collector system  10 G of  FIG. 2 , but with a RCED  100  arranged adjacent entrance aperture member  22 .  FIG. 10  is a more detailed schematic diagram of an example GIC collector system  180  based on the generalized GIC collector of  FIG. 9 . 
     GIC collector system  180  includes a laser source LS that generates a laser beam  11 . GIC mirror MG is shown as having GIC shells M 1  and M 2  arranged along central axis A 1 . In practice, one or more GIC shells can be used. A lens L and a fold mirror FM serve to direct laser beam  11  along central axis A 1  and through the GIC mirror MG in the −Z direction to a focus Fll on the opposite side of GIC mirror MG from laser source LS. In an example embodiment, GIC shells M 1  and M 2  include Ru coatings, which are relatively stable and can tolerate a certain amount of Sn coating. Note that fold mirror FM and laser beam  11  from laser source LS are shown located between GIC mirrors MG and the intermediate focus IF. An alternative arrangement places laser source LS and fold mirror FM between the input end  16  of GIC mirror MG and the laser beam focus F 11 . 
     A high-mass, solid, moving Sn target  182  having a surface  184  is arranged along central axis A 1  so that a portion of the surface  184  of Sn target  182  is at focus F 11 . A target driver  186  (e.g., a motor) is shown for moving Sn target  182  by way of example. The laser beam- 11  incident upon surface  184  of Sn target  182  forms laser-produced plasma source LPP-RS. Moving Sn target  182  at high speed allows for laser beam  11  to be incident upon surface  184  of Sn target  182  at a different location for each laser pulse. 
     The emitted EUV radiation  12  from laser-produced plasma source LPP-RS formed on Sn target  182  is generally in the +Z direction and travels through GIC mirror MG in the opposite direction of laser beam  11 , i.e., in the +Z direction. Some of EUV radiation  12  passes directly through RCED  100  and to intermediate focus plane PL to form radiation distribution RD, while other EUV radiation  12 L is collected by RCED  100  and directed through entrance aperture  24  by grazing-incidence reflection from reflective inner surface  120 . As with the NIC collector system  150 , this configuration provides more useful radiation (e.g. an angular distribution the better meets the illuminator specifications) passing through the intermediate focus aperture radiation for forming radiation distribution RD and thus more radiation for ultimately forming an image of the reticle at the wafer in an EUV lithography system. 
     While the example EUV radiation source has been described above as an LPP EUV radiation source, a discharge-produced plasma (DPP) EUV radiation source can also be used in connection with the embodiments of the present disclosure. 
     Example RCEDs 
     RCED  100  can have a wide range of configurations that have a generally tapered shape in the +Z direction when placed in front of (i.e. on the collector side of) the entrance aperture member  22 , and a generally shape in the -Z direction when placed behind (i.e. on the illuminator side of) the entrance aperture member  22 . 
     If the RCED  100  is intended to homogenize and otherwise improve the angular distribution of EUV radiation in the far field behind entrance aperture member  22  of illuminator  20 , then it can have a fairly complex inner surface configuration. For example, the inner surface configuration can include a precisely contoured reflecting surface or an undulating surface or even a roughened inner surface configured to uniformize and otherwise optimize the EUV radiation coming from, for example, distributed shells of a multi-shell GIC mirror MG. On the other hand, if RCED  100  is intended to distribute EUV radiation to larger angles behind entrance aperture member  22  to optionally illuminate an alignment structure beyond the field of the illuminator  20 , then the inner surface  120  of RCED  100  can be preferably configured to maximize the angles passing through entrance aperture  24  of illuminator  20 . Or, if RCED  100  is only intended to maximize the amount of EUV radiation  12  through the entrance aperture  24  of illuminator  20 , then inner surface  120  can be designed to have one or more surface configurations that achieve this goal. 
       FIG. 11  is an isometric view of an example RCED  100  that illustrates an example conic RCED  100  that has a reflective inner surface  120  with a linear taper. RCED  100  has a central axis AC. A coating  121  is shown on inner surface  120 . The linear taper can be configured to correspond (e.g., match) the NA or the angular distribution of illuminator  20 . A simple version of RCED  100  includes a polished inner surface  120  that, along with coating  121 , grazingly reflects EUV radiation  12 L. 
       FIG. 12  is a longitudinal cross-sectional view of an example RCED  100  that illustrates an example where the RCED  100  that has a reflective inner surface  120  with a curved (i.e., flared) taper. As discussed above, the curved taper can be configured to correspond (e.g., match) the NA or the required angular distribution of illuminator  20 . 
       FIG. 13  is lateral cross-sectional view of an example RCED  100  that has an inner surface  120  that is not rotationally symmetric and that has a plurality of (e.g., eight) inner surfaces  120 F- 1  through  120 F- 8 . The faceted inner surface  120 F can be, for example, linearly tapered or curved tapered. 
       FIG. 14  is similar to  FIG. 13 , and shows an example RCED  100  having a variety of inner surfaces  120 , such as one or more inner surface  120 F, an undulating or grooved inner surface  120 G, a roughened inner surface  120 R and a polished, curved inner surface  120 P. Such a multi-form inner surface  120  may be employed for specialized applications. 
       FIG. 15  is a lateral cross-sectional view of an example RCED  100  where inner surface  120  includes an undulating or grooved inner surface  120 G. Such an inner surface  120 G can serve to smooth out or otherwise optimize the far-field EUV radiation distribution RD without using scattering from a high-spatial-frequency roughened surface. 
       FIG. 16  is similar to  FIG. 12 , but includes a portion of roughened inner surface  120 R adjacent output end  124 . The portion of Roughened surface  120 R serves to provide wider scattering angles for EUV radiation  12 L than a polished inner surface  120  (e.g.,  120 P; see  FIG. 14 ), and serves to uniformize or otherwise improve (or optimize) the EUV radiation distribution RD at entrance aperture  24  of illuminator  20 . 
     Body portion  110  of RCED  100  may be formed from a metal, a ceramic, a plastic or a glass or glass-like material. In an example, body portion  110  (including inner surface  120 ) of RCED  100  is smooth and has a controlled high-spatial-frequency roughness (as understood in the art of EUV mirrors) to control scattering. However, example embodiments include cases where inner surface  120  (and optional coating  121 ) are configured with a surface roughness configured to generate a select scattering (e.g., a broad scattering) of EUV radiation collected by the RCED  100 , as discussed above in connection with  FIG. 16 . If body portion  110  of RCED  100  is made of a plastic or other material that can be cast, then it can indeed be made very inexpensively and with a high degree of surface smoothness limited only by the smoothness of the master cast. Such a plastic or other non-metal RCED substrate can be coated with a high-atomic-number material (e.g., Ruthenium) to improve or optimize the grazing incidence reflection from the inner surface  120  of RCED  100 . 
     If RCED  100  is to be subjected to a significant thermal load, then a preferred body material may be a metal. In an example embodiment, a metal body portion  110  of RCED  100  has an inner surface  120  that is polished to a desired smoothness, or is electroformed. Example metals for body portion  110  of RCED  100  include stainless steel, nickel, copper, aluminum, and like metals that can be highly polished. Another example material for body portion  110  of RCED  100  is a thermally resistant material such as ZERODUR. In an example embodiment, body portion  110  of RCED  100  is configured to support a cooling mechanism, such as cooling channels  129  (see  FIG. 4 ). 
     As discussed above, inner surface  120  may include a reflective coating  121  tailored to optimize the reflectivity of EUV radiation  12  at grazing incidence. While Ru is a preferred coating material, other high-atomic-number materials—such as Cu, Au, Pd, Sn, Pt, and Au—can also be used, as long as the specific application would not prohibit the use of such a coating. In addition, a resonant multilayer coating  121  can be used. Such a coating  121  would serve to broaden the acceptance angle and can increase the efficiency of RCED  100 . An example multilayer coating  121  includes layers of Mo and Si. 
     RCED with Front and Rear Sections 
     The amount of EUV radiation  12  that can be transferred from radiation source RS through GIC mirror MG and to illuminator  20  is limited by the overall system etendue , and in particular the design input etendue of the illumination system. However, in the case of a grazing incidence collector it is worth noting that the etendue of the individual GIC shells (M 1 , M 2 , etc.) will typically be considerably smaller than that of the illuminator  20 , and that the far-field EUV radiation distribution RD from the GIC will have gaps due to the nature of the separated shells. Thus, much of the EUV radiation  12 P (Refer to  FIG. 17 ) that would be lost can be recovered by RCED  100  without violating the etendue principle, and in particular without exceeding the etendue of the illuminator  20 . Indeed, the RCED  100  can be used to redistribute the angular distribution of the far field radiation to better match the input angular distribution specifications of the illuminator  20  without violating the optical invariant (i.e., the etendue principle). 
     As discussed above, much of the recovered EUV radiations  12 P gets directed into dark spaces on either side of the unaided far-field images formed by the GIC mirrors MG. This serves to homogenize and further optimize the far-field radiation distribution RD. 
       FIG. 17  is a schematic diagram similar to  FIG. 2  and shows an example GIC collector system  10 G with illuminator  20 . Illuminator  20  and entrance aperture member  22  define input and output acceptance angle limits  19 -I and  19 -O on the input and output sides of the entrance aperture  24 . Even for the EUV radiation  12  that passes through entrance aperture member  22 , some of this EUV radiation  12 P has an angle relative to central axis A 1  that precludes this radiation from entering and being processed by illuminator  20 . This is because the image formation process associated with GIC collector system  10 G is imperfect and is generally directed to trying to get as much EUV radiation  12  as possible from radiation source RS to illuminator  20 . 
     With reference to  FIG. 18A , in an example embodiment RCED  100  includes front and rear sections  110 F and  110 R on either side of entrance aperture member  22 .  FIG. 18B  is similar to  FIG. 18A , except that the front and rear sections  110 F and  110 R are separated from entrance aperture member  22 .  FIGS. 18A and 18B  illustrate EUV radiation  12  that passes through RCED  100  with no bounces ( 12 ), one bounce ( 12 L) and two bounces ( 12 P). 
     Note that the front and rear sections  110 F and  110 R can also be considered separate RCEDs with possibly different curvatures or patterning on the front RCED versus the rear RCED. Accordingly, so the description of these sections  110 F and  110 R as being part of one RCED  100  or as being two different RCEDs is the same, and in some instances herein, front and rear RCED sections are referred to simply as front and rear RCEDs. In an example, front and rear sections  110 F and  110 R are axially tapered in opposite directions, as shown. 
       FIG. 19  is similar to  FIG. 5A  and illustrates an example RCED  100  that includes multiple inner surfaces  120  on either side of entrance aperture member  22 , such as formed by two sets of concentrically arranged mirror shells, namely front mirror shells  103 F- 1  and  103 F- 2 , and rear mirror shells  103 R- 1  and  103 R- 2 . Each of the mirror shells  103 F- 1 ,  103 F- 2 ,  103 R- 1  and  103 R- 2  can be considered sections of RCED  100  or even a separate RCED  100 . 
     Once again, it is noted that front section  110 F may have one or more surfaces whereas rear section  110 R may have a number of surfaces different from the front section  110 F. Similarly, front section  110 F may be separated from the entrance aperture member  22  while rear section  110 R may be attached to or separated from the entrance aperture member  22 , or vice versa. 
       FIG. 20  is similar to  FIG. 6  and illustrates another example RCED  100  that includes a single front mirror shell  103 F and a single rear mirror shell  103 R as stood off from entrance aperture member  22  by respective stand-off support structures  113 F and  113 R. Front and rear mirror shells  103 F and  103 R can also be considered as separate RCED sections or as separate RCEDs with different curvatures, different stand-offs, and a different number of surfaces between the front and rear RCEDs, etc. 
       FIG. 21  is similar to  FIG. 16  and illustrates another example RCED  100  having front and rear sections  110 F and  110 R on either side of entrance aperture member  22 . Front and rear sections  110 F and  110 R have respective axial lengths LF and LR, and in an example have an axial taper, as shown.  FIG. 22  is the same as  FIG. 21  and includes cooling channels  129  arranged on each of the sections  110 F and  110 R to cool these sections by flowing a cooling fluid through the cooling channels  129 . In an example, one of the cooling channels  129  runs around the input end  122 . As discussed above, front and rear sections  110 F and  110 R can also be considered as separate RCED sections or as separate RCEDs with different curvatures, different cooling configurations, different stand-offs, and different number of surfaces between the front and rear RCEDs, etc. 
     A desirable feature in a collector system is the ability to filter out unwanted broadband infrared radiation  240  generated by the EUV radiation source RS. Thus, with reference again to  FIG. 22 , an IR filter  250  is disposed adjacent input end  122  or output end  124  of front section  110 F. Other locations for IR filter  250  are also possible. IR filter  250  is configured to filter out broadband infrared radiation  240  that may also be collected and reflected by the grazing incidence or normal incidence collector and delivered to entrance aperture member  22 . 
     In an example embodiment, IR filter  250  comprises a low-density, free-standing grating having crossed-grating lines  252  (see insets,  FIG. 22 ) and a support frame  254 . The crossed-grating lines  252  have a period smaller than the wavelength of infrared radiation  240 . If the areal density of crossed-grating lines  252  is relatively low (e.g. only 3% areal density coverage with metal crossed-grating lines  252 ) then the filtration of the infrared radiation  240  can be high while letting most (e.g. ˜97%) of the EUV radiation pass through. Where IR filter  250  has metal crossed-grating lines  252 , it can be thermally attached to the cooled RCED  100  to carry away any thermal load to which it may be subjected. 
     Thus, an aspect of the methods disclosed herein includes filtering infrared radiation  240  from the EUV radiation source RS immediately upstream or downstream of the at least one redirecting surface associated with RCED  100 . 
     An example method of making a suitable crossed-grating-based IR filter  250  is now described. To filter infrared radiation  240  while transmitting EUV radiation, the grating period needs to be less than the IR radiation wavelength. Also, since a substrate will generally absorb EUV radiation, it is preferred that the grating be freestanding, or alternatively, the supporting substrate be very thin (i.e., membranous) and be made of a material that has low absorption at 13.5 nm. For example, a half-micron thick Si membrane would reduce the EUV transmission at 13.5 nm by about a factor of 2×. If a 0.1 micron thick Si membrane were used, it would have a transmission at 13.5 nm of about 87%, which might be deemed acceptable. 
     For a linear grating in the vertical (Y) direction, polarization components of the infrared radiation  240  in the Y direction would get reflected, with some absorption in the metal of the grating depending on its conductivity. To reflect all polarization components, a crossed-grating is employed, i.e., grating lines running in both the X and Y directions. 
     All wavelengths below the period of the grating would pass thru the grating spaces. Any EUV radiation that hits the grating lines will get absorbed, while that which passes through the spaces is transmitted. If the grating lines represent only 5% of the grating area, then 5% of the EUV radiation will be absorbed, 95% will be transmitted, and substantially no infrared radiation at wavelengths longer than the grating period will be transmitted. 
     To produce a master pattern of grating lines with the appropriate period and the appropriate linewidths, a suitable substrate is selected. An example substrate is a silicon wafer or thin glass. The wafer is coated with a thin chrome layer (e.g., less than 0.1 micron thick) as an adhering layer. The thin chrome layer is then coated with a thin (e.g., about 0.1 micron) plate-able metal layer, such as gold or other suitable metal. The metal layer is then coated with photoresist of a desired thickness. A master grid pattern with the appropriate period is lithographically formed in the photoresist layer. 
     Developing the photoresist provides the negative of the grating pattern in the photoresist atop the plate-able metal layer. The photoresist layer is then plated with the same plate-able metal as the underlying plate-able metal layer. The photoresist is then washed away, e.g., using acetone. The resulting structure is now a thick metal grating atop of the approximately 0.2 micron thick chrome and plate-able metal layers supported by the substrate. 
     A support structure can be attached to the outside of the metal grating structure so that the metal structure can be free-standing inside of the support structure. An example support structure is a washer that is epoxied to the metal grating structure. The grating structure periphery can be made to be thick and free of grating lines. At this point, chrome and plate-able metal layers are removed, e.g., using a liquid or beam etch process. Next, the substrate is removed, e.g., by a liquid etch suitable for the particular substrate (e.g., HF for a glass substrate). 
     The result is a free-standing, metal crossed grating supported around its outer edge so that it can be handled and also mounted into position relative to the RCED  100 . 
     EUV Lithography System with EUV Collector and RCED 
       FIG. 23  is an example EUV lithography system (“lithography system”)  300  according to the present disclosure. Example lithography systems are disclosed, for example, in U.S. Patent Applications No. U.S. 2004/0265712A1, U.S. 2005/0016679A1 and U.S. 2005/0155624A1, which are incorporated herein by reference. 
     Lithography system  300  includes a system axis AS and an EUV radiation source RS that emits working EUV radiation  12  nominally at λ=13.5 nm. Lithography system  300  also includes along system axis AS an EUV collector mirror (NIC or GIC)  310  and a RCED  100  as described above. EUV collector mirror  310  and RCED  100  comprise a collector system  312 . Collector system  312  also optionally includes EUV radiation source RS. EUV radiation source RS may include, for example, a LPP EUV radiation source or a DPP EUV radiation source. 
     An illuminator  20  with an input end  20 A and an output end  20 B is arranged along system axis AS and adjacent and downstream of collector system  312 . Illuminator  20  includes entrance aperture member  22  with entrance aperture  24 . 
     EUV collector mirror  310  (shown configured as a GIC mirror for illustration) collects EUV radiation  12  from EUV radiation source RS located at source focus SF. The collected EUV radiation  12  is directed to entrance aperture  24 , with the intention of forming a radiation distribution RD at intermediate focus IF. RCED  100  operates as described above to enhance the EUV radiation  12  focusing process by redirecting at least a portion of EUV radiation  12 L that would otherwise not pass through entrance aperture  24  to the illuminator  20 , to pass through entrance aperture  24 . Thus, illuminator  20  receives at input end  20 A EUV radiation  12  at the intermediate focus plane PL from radiation distribution RD and outputs at output end  20 B a more uniform EUV radiation  12 ′ (i.e., condensed EUV radiation) to a reflective reticle  336 . Where lithography system  300  is a scanning type system, EUV radiation  12 ′ is typically formed as a substantially uniform line of EUV radiation at reflective reticle  336  that scans over the reflective reticle  336 . 
     It is also noted that illuminator  20  may image a portion of the EUV radiation passing through entrance aperture  24  to a region outside of the reticle patterned area (e.g., in a kerf), and that this EUV radiation (denoted  12 ′A in  FIG. 23 ) can be used for alignment purposes, e.g., by being incident upon reticle alignment marks that reside outside of the patterned area used for forming microcircuit features. In an example embodiment, EUV radiation  12 ′A is detected by a photodetector  360 , which forms electronic signals S 360  that can be processed (e.g., in a computer, not shown) to perform alignment. 
     A projection optical system  326  is arranged along (folded) system axis AS downstream of illuminator  20  and reflective reticle  336 . Projection optical system  326  has an input end  327  facing output end  20 B of illuminator  20 , and an opposite output end  328 . Reflective reticle  336  is arranged adjacent the input end  327  of projection optical system  326  and a semiconductor wafer  340  is arranged adjacent output end  328  of projection optical system  326 . Reflective reticle  336  includes a pattern (not shown) to be transferred to semiconductor wafer  340 , which includes a photosensitive coating (e.g., photoresist layer)  342 . In operation, the uniformized EUV radiation  12 ′ irradiates reflective reticle  336  and reflects therefrom, and the reticle pattern is imaged onto surface of photosensitive coating  342  of semiconductor wafer  340  by projection optical system  326 . In a lithography system  300 , the reticle image scans over the surface of photosensitive coating  342  to form the pattern over the exposure field. Scanning is typically achieved by moving reflective reticle  336  and semiconductor wafer  340  in synchrony. 
     Once the reticle pattern is imaged and recorded on semiconductor wafer  340 , the patterned semiconductor wafer  340  is then processed using standard photolithographic and semiconductor processing techniques to form integrated circuit (IC) chips. 
     Note that the components of lithography system  300  are shown lying along a common folded system axis AS in  FIG. 23  for the sake of illustration. One skilled in the art will understand that there can be more than one fold in lithography system  300 , and that there can be an offset between entrance and exit axes for the various components such as for illuminator  20  and for projection optical system  326 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the spirit and scope of the disclosure. Thus it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.