Patent Abstract:
A collector that includes a laser produced plasma (LPP) extreme ultra violet (EUV) light source and a first optical path from the source to a mirror. The mirror is the first mirror that light emitted from the source and traveling along the first optical path impinges upon. The collector also includes a second optical path from the source to another mirror. The other mirror is the first mirror that light emitted from the source and raveling along the second path impinges upon. The mirror and the other mirror are oriented relative to the source such that light from the source traveling along the first optical path travels in a direction opposite to light traveling from the source along the second optical path. A collector having a discharge extreme ultra violet (EUV) light source.

Full Description:
BACKGROUND 
   In the field of electronics, conductive and/or insulating features are formed on a substrate through photo-lithographic techniques. Essentially, an optical image that represents one or more patterns to be formed onto the substrate is directed onto a layer of photo resist that has been coated onto the substrate. A projection camera projects the optical image onto the photo resist layer from light that has been patterned in accordance with a mask. 
   In general, a primary measure of an electronic device&#39;s sophistication is its smallest feature size. The smallest feature size of an electronic device is largely determined by the sophistication of the lithography techniques and/or equipment employed in the device&#39;s manufacture. In particular, the shorter the wavelength of the light that is processed by the photo-lithographic equipment&#39;s projection camera optics, the smaller the smallest achievable feature size becomes. 
   Thus, in general, the smaller the wavelength of the light that is processed by the projection camera&#39;s optics, the more sophisticated the projection camera is deemed to be. Presently, considerable work is being done in the development of photo-lithographic equipment that processes light in the Extreme Ultra Violet (EUV) spectra (a range approximately from 10 to 14 nm). Part of the challenge in designing EUV photo-lithographic equipment is designing that portion of the equipment that “pre-conditions” the EUV light prior to illuminating the mask and the entrance pupil of the projection camera. 
     FIG. 1  shows a simplistic depiction of the cross section of the “shape” of light as it is reflected from the mask at a “ring field” projection camera. According to the depiction of  FIG. 1 , the light travels substantially along the z axis through arc  101 . According to one EUV approach, the arc  101  of the EUV light has a radius R between 116 mm and 124 mm over an angle θ of approximately 30°. Moreover, at least for EUV light, the illumination of the light over the arc  101  is supposed to be highly uniform (e,g., on the order of only 1% variation across the arc  101 ). 
   A condenser is used to form light into the appropriate shape and uniformity at the projection camera entry pupil. The condenser can usually be viewed as containing two components: 1) a collector; and, 2) an illumination system. The collector is designed to collect photons from a light source. The illumination system crafts the light from the collector into the appropriate shape for illuminating the mask (arc field) and illuminating the entrance pupil of the projection camera. 
   An exemplary condenser originally described in U.S. Pat. No. 6,195,201 B1 (hereinafter, “Koch et. al.”) is shown in  FIG. 2 . The collector  201  includes a light source  203  and a collection mirror  204 . The collection mirror  204  directs the light it collects into the illumination system  202 . The illumination system  202  includes a pair of faceted mirrors  205 ,  206 . The faceted mirrors  205 ,  206  effectively break down the light from the collector  201  into a plurality of beams that are recombined by relaying mirrors  207 ,  208  so as to form light of the proper shape and uniformity at the mask plane  209  of the projection camera. 
   A problem with EUV condensers is their expense. The cost of an EUV condenser is largely a function of the amount of photon energy that its light source emits. That is, the more photon energy that a light source emits, the more expensive the condenser. 

   
     DRAWINGS 
     The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which like references indicate similar elements and in which: 
       FIG. 1  shows EUV light shaped to enter a mask plane of a projection camera; 
       FIG. 2  (prior art) shows a condenser that processes light for entry to a projection camera; 
       FIG. 3  shows a first embodiment of a collector for an LPP EUV source; 
       FIG. 4  shows a second embodiment of a collector for an LPP EUV source; 
       FIG. 5  shows a third embodiment of a collector for an LPP EUV source; 
       FIG. 6  shows a fourth embodiment of a collector for an LPP EUV source; 
       FIG. 7  shows a first embodiment of a collector for a discharge source; 
       FIGS. 8   a ,  8   b  show a second embodiment of a collector for a discharge source; 
       FIG. 9  shows a third embodiment of a collector for a discharge source; and, 
       FIG. 10  shows that a faceted collector mirror can eliminate a mirror in an illumination system; 
       FIG. 11  shows a reflective mask lithography system. 
   

   DETAILED DESCRIPTION 
   In order to reduce the cost of an EUV condenser, more efficient collectors should be designed. By designing collectors that are capable of directing more photon energy from the light source into the illumination system, the amount of light energy needed from the source can be reduced; which, in turn, should lower the cost of the condenser as a whole because less expensive EUV sources can be used. 
   Two types of EUV light sources that are presently in common use are Laser Produced Plasma (LPP) sources and discharge sources.  FIGS. 3  though  6  show designs for efficient EUV collectors that include an LPP source; and,  FIGS. 7 through 9  show designs for efficient EUV collectors that include a discharge source. A discussion of these designs immediately follows. 
   Collector with LPP EUV Source 
     FIGS. 3 through 6  show designs for efficient EUV collectors that include an LPP source. According to the designs of  FIGS. 3 through 6 , efficiency is improved over prior art LPP sourced EUV collectors through then collection of light over, approximately, a sphere that surrounds the LPP source. Prior art LPP sourced EUV collectors (such as the source  203  of Koch et al. shown in  FIG. 2 ) are believed to only collect light over, approximately, no more than a hemisphere resulting in less collected photon energy than the designs observed in  FIGS. 3 through 6 . 
   Another feature of the collector designs of  FIGS. 3 through 6  that prior art LPP sourced EUV collectors are not known to exhibit is that they each collect light from the source that travels from the source in opposite directions. Both the spherical nature of the collection range and the collection of light traveling from the source in opposite directions is apparent from an analysis of each of the drawings observed in  FIGS. 3 through 6 . 
   Specifically, note that each of the collector designs of  FIGS. 3 and 4  have two mirror stages whose reflecting surfaces face one another. That is, for example the collector design of  FIG. 3  has a first mirror  301  whose reflective surface  303  faces the reflective surface  304  of a second mirror  302 . Similarly, the collector design of  FIG. 4  has a first mirror  401  whose reflective surface  403  faces the reflective surface  404  of a second mirror  402 . Each of the mirror pairs  301 ,  302  and  401 ,  402  represent the first highly reflective surface that light from the LPP EUV source  305 ,  405  impinges upon. 
   Referring to  FIG. 3 , light from the source  305  is drawn radiating in four different arcs  306 ,  307 ,  308 ,  309 . Note that, in demonstrating the approximately spherical collection range of the collector, arcs  306  and  308  correspond to oppositely traveling light from the LPP source  305  and arcs  307  and  309  correspond to oppositely traveling light from the LPP source  305 . Also, again demonstrating the spherical collection range of the collector, the pair of applicable coordinate axis shown in  FIG. 3  indicate that the design is symmetrical about the z axis. 
   According to the design of  FIG. 3 , light propagates from the source  305  and reflects off of mirrors  301  and  302 . Light that reflects off of mirror  302  reflects into grazing incidence mirror  310 . Light that reflects off of mirror  301  reflects back onto and off of mirror  302  and then into grazing incidence mirror  310 . From grazing mirror  310  the collected light is directed toward the illumination system of the condenser. 
   The near grazing incidence angle of light (e.g., less than or equal to as 15° when measured against the reflective surface of the mirror  310 ) as it passes into grazing mirror  310  permits a high collection angle for each of mirrors  301  and  302  (e.g., in a range of 75° to 90°). The grazing incidence mirror  310  also conditions the illumination beam for the downstream mirrors of the illumination system. Also, related embodiments may only collect over approximately a hemisphere rather than a sphere (e.g., just mirror  302  is employed and not mirror  301 ). 
   In an embodiment, in order to ensure efficient reflectivity off of mirrors  301 ,  302 , the angle of incidence at each of mirrors  301 ,  302  for non reflected light emanating from the source  305  is “normal” or “near normal” (e.g., less than or equal to 15° when measured against a ray that is normal to the reflecting surface of the mirror) across most, if not all, of the surface area of mirrors  301 ,  302 ). Graded reflective coatings on the mirror surfaces may permit more severe angles of incidence. 
   In an embodiment, the reflecting surface  303  of mirror  301  is approximately elliptical and the reflecting surface  304  of mirror  302  is approximately spherical. Mirror  302  may also be larger than mirror  301 . In other or same embodiments, the collection angle for both mirrors  301 ,  302  ranges from 25° to 90°. Each of mirrors  301  and  302  may be annular to make room for the source  305  and any other fixtures. In the alternative, the surfaces may be biconic as used in lens optimization software design tools with the purpose of elongating the source image. 
   The optical design of  FIG. 4  is similar to that of  FIG. 3 , except that a third mirror  406  is inserted between mirrors  401 ,  402  so as to eliminate the grazing incidence mirror  310 . That is, light propagates from the source  405  and reflects off of mirrors  401  and  402 . Light that reflects off of mirror  402  reflects off of mirror  406 . Light that reflects off of mirror  401  reflects back onto and off of mirror  402  and then off of mirror  406 . From mirror  406  the collected light is directed toward the illumination system of the condenser. 
   Again, in an embodiment, in order to ensure efficient reflectivity off of mirrors  401 ,  402 , the angle of incidence at each of mirrors  401 ,  402  for non reflected light emanating from the source  405  is “normal” or “near normal” (e.g., less than or equal to 15° when measured against a ray that is normal to the reflecting surface of the mirror) across most, if not all, of the surface area of mirrors  401 ,  402 . Also, again, graded reflective coatings on the mirror surfaces may permit more severe angles of incidence. 
   In an embodiment, the reflecting surface of mirror  401  is approximately elliptical and the reflecting surface of mirror  402  is approximately spherical. Mirror  402  may also be larger than mirror  401 . In other or same embodiments, the collection angle for both mirrors  401 ,  402  ranges from 45° to 85°. Each of mirrors  401  and  402  may be annular to make room for the source  405  and any other fixtures. 
     FIG. 5  shows another collector embodiment for an LPP EUV source. Like the designs of  FIGS. 3 and 4 , the collector design of  FIG. 5  is capable of an approximately spherical collection range. Here, light traveling from the source will impinge upon each of reflecting elements (e.g., mirrors)  550 ,  551 ,  552  and  553 . Reflecting element  554  receives light from each of reflecting elements  552  and  553 . Reflecting element  552  receives light from reflecting element  551  and reflecting element  553  receives light from reflecting element  550 . Reflecting element  554  forms output light  556 . Reflecting elements  550 ,  551 ,  552  and  553  can be elliptical or nearly elliptical, spherical or nearly spherical, conical or nearly conical or biconical or nearly biconical. 
     FIG. 6  shows another collector embodiment for an LPP EUV source. Again, the collector design of  FIG. 6  can collect light over an approximately spherical (rather than hemispherical) collection range. The light paths associated with the collector of  FIG. 6  are most easily understood in reference to axis  612  and  613 . Specifically, axis  612  and  613  can together be viewed as: 1) breaking down a first reflecting element into regions  602 ,  604  and  606 ; and 2) breaking down a second reflecting element into regions  603 ,  605 ,  607 . 
   Light that impinges upon regions  602  and  603  directly from source  601  form reflected beams  613  and  614 , respectively. These beams focus to focus point  610 . Light that impinges upon regions  604  and  605  directly from source  601  form reflected beams that pass through focus point  611  and continue forward to form reflected beams  615  and  616 . Reflected beam  615  impinges upon reflecting surface  608  and converges after its reflection at focal point  610 . Similarly, reflected beam  616  impinges upon reflecting surface  609  and converges after its reflection at focal point  610 . 
   Note also a degree of stability against movement of the source  601  is likely to result from the perspective of image  610  because a number of light beams that experience an odd number of reflections in reaching source  610  will be compensated for by a number of light beams that experience an even number of reflections in reaching source  610 . 
   Light that impinges upon region  606  directly from the source  601  reflects back to regions  603  and  605 . The light that reflects to region  603  behaves as described above for region  603 , and, the light that reflects to region  605  behaves as described above for region  605 . Similarly, light that impinges upon region  607  directly from the source  601  reflects back to regions  602  and  604 . The light that reflects to region  602  behaves as described above for region  602 , and, the light that reflects to region  604  behaves as described above for region  604 . Note that the diagram in  FIG. 6  is a cross section of the overall collector. Here, it is expected that the embodiments may be constructed where this cross section is preserved over a plurality if not all angles of view. 
   According to at least one implementation, regions  602  and  603  are part of the same annular reflective component. In combination, regions  604  and  605  may also be formed from a same, second annular reflective component that is coupled next to the annular component that forms regions  602  and  603 . Alternatively, regions  604  and  605  may be formed with different reflective components with respect to one another; and/or, may be formed from the same reflective component that forms regions  602  and  603  (either as a whole or respectively). Regions  606  and  607  may be part of the same reflective component that regions  604  and  605  are formed with (either as a whole or respectively); or, may be formed with different components from those that form regions  604  and  605 . Regions  606  and  607  may also be formed from the same annular reflective component or may be separate with respect to one another. 
   Collector with Discharge EUV Source 
   Known prior art collectors that collect EUV energy from a discharge source collect the EUV light at high “grazing” angles of incidence. Grazing angles of incidence can have poor collection efficiency given that they only collect at a collection angle no more than 45°. As such, in order to enhance the efficiency of a discharge source collector, a “normal” or “near-normal” angle of incidence is used at the collector&#39;s reflective surfaces.  FIGS. 7 through 9  show designs for efficient EUV collectors that include a discharge source. A discussion of each immediately follows. 
   The design of  FIG. 7  is similar to that of  FIG. 4  except that mirror  401  is removed. Here, discharge EUV sources generally emit more light energy than LPP sources. As such, the collection optics need not approximately surround the source as was discussed with respect to the collector designs of  FIGS. 3 through 6 . Moreover, discharge sources tend to be larger in size than LPP sources; and, as a consequence, surrounding the source with collection optics may not be practicable. 
   According to the design of  FIG. 7 , light from a discharge source  701  is reflected at near normal incidence (e.g. at or less than 15° when measured against a ray that is normal to the reflecting surface of mirror  702 ) off of mirror  702  onto mirror  703 ; which, in turn, reflects the light toward the illumination system of the condenser. In an embodiment, the collection angle of mirror  702  ranges from 45° to 85°. Also, as depicted by the coordinate axis, the collector is symmetrical about the z axis. Mirrors  702  and  703  may be annular to make room for the source  701  and any other fixtures. 
     FIG. 8   a  shows a top view and  FIG. 8   b  shows a side view of another collector design for a discharge source. According to the design of  FIGS. 8   a  and  8   b , light from discharge source  801  is reflected at near normal incidence (e.g. at or less than  150  when measured against a ray that is normal to the reflecting surface of mirror  802 ) off of a first mirror  802  toward a second mirror  803  from which it is reflected at near normal incidence toward the illumination system. Referring to the top view depiction in  FIG. 8   a , the first mirror  802  is tilted so as to direct its reflected light past the source  801  on its way toward mirror  803  without being obscured by the source  801  (i.e., the source is not in its way). 
   Here, because the side view of  FIG. 8   b  shows a continuous collection angle from about +75° to −75°, reflected light from mirror  802  needs to be directed off the side of the source  801  (as shown in  FIG. 8   a ) in order to be directed past the source  801 . Moreover, because of the continuous collection angles through their middle, mirrors  802  and  803  may be non annular (i.e., there does not exist a need to make room for the source  801  or other fixtures through the middle of the mirrors  802 ,  803 ). 
   The approach of  FIGS. 8   a  and  8   b  show the first mirror  802  being smaller than the second mirror  803 .  FIG. 9  shows a top view of an alternative design to that of  FIGS. 8   a  and  8   b  where the first mirror  902  is larger than the second mirror  903 . Here,  FIG. 9  can be directly compared against  FIG. 8   a . Again, mirrors  902 ,  903  have a continuous collection angle through their middle. As such, reflected light from mirror  902  needs to be directed off the side of the source  901  in order to be directed past the source  901 . Moreover, because of their continuous collection angles, mirrors  902 ,  903  may be non annular. 
   In both the designs of  FIGS. 8   a,b  and  9 , light is directed past the source  801 ,  901  by the first mirror  802 ,  902  to allow for a wider total collection angle at the first mirror  802 ,  702 . 
   Faceted Collector Mirrors 
   Koch et al. (discussed in the background) reveals that a faceted mirror can be used in the collector. The reflective surface of a faceted mirror is made of smaller discrete reflective surfaces that are positioned to break an incident beam into a plurality of smaller beams.  FIG. 10  shows a faceted mirror having arc shaped discrete surfaces. In alternate approaches the discrete surfaces may be square, hexagonal or some other tilted surface. 
   Presently, it has been realized that the use of faceted mirrors in the collector can be used to reduce the number of optical components in the illumination system; and, moreover, the use of faceted mirrors can be used to compensate for variations in the source&#39;s illumination properties.  FIG. 10  demonstrates the former and further discussion of  FIG. 3  demonstrates the later. 
     FIG. 10  can be compared directly with  FIG. 2 . Recall that  FIG. 2  shows a condenser system taught by Koch. Although Koch discloses that the collector mirror  204  can be faceted, Koch does not teach that the use of the faceted collector mirror can result in the elimination of optical components within the illumination system. Comparing  FIGS. 2 and 10 , note that faceted mirror  206  has effectively been eliminated from the illumination system in the condenser design of  FIG. 10 . That is, condenser  1001  is similar to the condenser design shown in  FIG. 4   a  of the present application and the illumination system  1002  includes a faceted mirror  1005  and relaying mirrors  1007 ,  1008 . 
   Recall that the original purpose of the illumination system is to effectively break down the light from the collector into a plurality of beams in order to form light of the proper shape and uniformity at the mask plane and also to properly fill the entrance pupil of the projection camera. With one or more of the mirrors  1010 ,  1011 ,  1012  in the collector  1001  being faceted, the illumination system  1002  receives light from the collector  1001  already broken down into a plurality of beams. 
   As such, one of the faceted mirrors in the illumination system (notably mirror  206 ) can be eliminated. The elimination of the reflecting mirror improves the collection efficiency of the condenser as a whole because the light will experience one less reflection and reflections are less than 100% efficient (i.e., a reflection involves some light loss, so with each reflection along the optical channel the amount of light that is lost through the channel increases). 
   Referring back to  FIG. 3 , if mirrors  301  and/or  302  are faceted, they assist in the breaking down the light from the source  305  into a plurality of beams. However, because light that impinges upon mirror  301  directly from the source will experience one more reflection than the light that impinges upon mirror  302  directly from the source, there can be an opposite image magnification imposed as between the light that reflects off of mirror  302  directly from the source  305  and the light that reflects off of mirror  302  from mirror  301 . 
   As a consequence it is possible to stabilize (in terms of position) the source image  311  created by the collector. That is, because of the opposite magnification (e.g., “positive” and “negative”) from the different beams of light, should the source  305  “move”, the beams that are magnified positively will move in one direction while beams that are magnified negatively will move in the opposite direction. As such, the position of the source image  311  should remain somewhat fixed as a consequence of the built-in compensation. Similar compensation techniques can be achieved with discharge source collectors having one or more faceted mirrors. 
   For any of the mirrors described above, the materials that could be used to form their respective reflective surfaces may include: Gold, Aluminum, Platinum, Chromium, Nickel, Molybdenum, Silicon, Beryllium, Palladium, Tungsten, Ruthenium, Rhodium, Lithium. 
   A reflective mask lithography system  1100  is shown in  FIG. 11 . According to the design of the reflective mask lithography system of  FIG. 11 , a source and collection optics (such as any of those described above)  1101  directs light to a reflective mask  1102  that is held in place by some type of mechanical fixture  1105 . Reflected light from the mask is directed into a projection camera  1103  that projects the reflected light onto a wafer  1104  that is being processed. The wafer  1104  is typically coated with some kind of photo resist. Depending on the type of photo resist (i.e., positive or negative), the light that is projected onto the photo resist will either be hardened or weakened so that specific features may be formed on the wafer. 
   In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Technology Classification (CPC): 6