Abstract:
Providing a high peak power short pulse duration gas discharge laser output pulse comprises a pulse stretcher a laser output pulse optical delay initiating optic diverting a portion of the output laser pulse into an optical delay having an optical delay path and comprising a plurality of confocal resonators in series aligned to deliver an output of the optical delay to the laser output pulse optical delay initiating optic. The plurality of confocal resonators comprises four confocal resonators comprising a twelve pass four mirror arrangement. An apparatus and method may comprise a plurality, e.g., two pulse stretchers in series and may include spatial coherency metrology.

Description:
RELATED APPLICATIONS 
   The present application is a continuation-in-part of U.S. patent application Ser. No. 10/712,545 filed on Nov. 13, 2003, now U.S. Pat. No. 6,928,093 entitled LONG DELAY AND HIGH TiS PULSE STRETCHER and related to U.S. Published Patent Application No. 2002/0154671A1, published on Oct. 24, 2002 with inventors Knowles et al., entitled LINE SELECTED F2 TWO CHAMBER LASER SYSTEM, based upon an application Ser. No. 10/056,619, filed on Jan. 23, 2002, and of U.S. Published Patent Application No. 2003/0138019A1, published on Jul. 24, 2003, with inventors Rylov et al., entitled TWO CHAMBER F2 LASER SYSTEM WITH F2 PRESSURE BASED LINE SELECTION, based on an application Ser. No. 10/243,102, filed on Sep. 13, 2002, and to U.S. Pat. Nos. 6,067,311, entitled EXCIMER LASER WITH PULSE MULTIPLIER, issued to Morton et al. on May 23, 2000, based upon an application Ser. No. 09/148,514, filed on Sep. 4, 1998, and U.S. Pat. No. 6,314,119, entitled EXCIMER LASER WITH PULSE AND BEAM MULTIPLIER, issued to Morton on Nov. 6, 2001, based upon an application Ser. No. 09/183,860, filed on Oct. 30, 1998, and U.S. Pat. No. 6,535,531, entitled GAS DISCHARGE LASER WITH PULSE MULTIPLIER, issued to Smith et al. on Mar. 18, 2003, based on application Ser. No. 10/006,913, filed on Nov. 29, 2001, and U.S. Pat. No. 6,625,191, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP RATE GAS DISCHARGE LASER SYSTEM, issued to Knowles et al. on Sep. 23, 2003, based upon an application Ser. No. 10/012,002 filed on Nov. 30, 2001, and U.S. Pat. No. 6,690,704, entitled CONTROL SYSTEM FOR A TWO CHAMBER GAS DISCHARGE LASER, issued to Fallon et al. on Feb. 10, 2004, based on an application Ser. No. 10/210,761, filed on Jul. 31, 2002, and U.S. Pat. No. 6,693,939, entitled LASER LITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY, issued to Kiene et al. on Feb. 17, 2004, based on an application Ser. No. 10/141,216, filed on May 7, 2002, and U.S. Pat. No. 6,704,339 entitled LITHOGRAPHY LASER WITH BEAM DELIVERY AND BEAM POINTING CONTROL, issued to Lublin et al. on Mar. 9, 2004, based on an application Ser. No. 10/233,253, filed on Aug. 30, 2002, and U.S. Pat. No. 6,704,340, entitled LITHOGRAPHY LASER SYSTEM WITH IN-PLACE ALIGNMENT TOOL, issued to Ershov et al. on Mar. 9, 2004, based on an application Ser. No. 10/255,806, filed on Sep. 25, 2002, the disclosures of all of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention elates to high peak power short pulse duration gas discharge laser systems, e.g., excimer or other fluorine gas discharge lasers, e.g., molecular fluorine gas discharge laser systems, utilized, e.g., in applications such as integrated circuit lithography, wherein downstream optics may be severely damages or suffer shortened lifetimes due to optical fluence damage mechanisms driven more by peak power than total integrated power of the light passing through the downstream optical elements. 
   BACKGROUND OF THE INVENTION 
   It is also known that spatial coherence is a factor in laser light sources, e.g., excimer and other gas discharge laser light sources, e.g., molecular fluorine laser light sources properly performing according to specification in, e.g., the use of such light, e.g., in the DUV range, for, e.g., exposure of photoresists on integrated circuit wafers. The makers of such integrated circuit lithography tools are demanding tighter and tighter specifications for spatial coherence. Some lasers manufactured by applicants&#39; assignee, e.g., XLA lasers, are borderline passing tests for spatial coherence according to current practice and may become even more susceptible to being out of specification in the future as specifications are defined more tightly. In the past the metrology used to define spectral coherence was to, e.g., find a point with maximum interference fringe contrast in the laser beam and use that as a measure of spatial coherence. However this data point does not represent the property of coherence of the whole beam. Applicants have, therefore, determined that a better means of measuring beam spatial coherence is needed and propose such a method in the present application. 
   SUMMARY OF THE INVENTION 
   An apparatus and method for providing a high peak power short pulse duration gas discharge laser output pulse is disclosed which may comprise a pulse stretcher which may comprise a laser output pulse optical delay initiating optic diverting a portion of the output laser pulse into an optical delay having an optical delay path and comprising; a plurality of confocal resonators in series aligned to deliver an output of the optical delay to the laser output pulse optical delay initiating optic. The plurality of confocal resonators comprises four confocal resonators comprising a twelve pass four mirror arrangement. Each of the plurality of confocal resonators may comprise a first concave spherical mirror having a radius of curvature and a second concave spherical mirror having the same radius of curvature and separated by the radius of curvature. The pulse stretcher may comprise a first confocal resonator cell which may comprise: a first concave spherical mirror having a radius of curvature receiving an input beam from the laser output pulse optical delay initiating optic comprising the portion of the output laser pulse at a first point on a face of the first concave spherical mirror and generating a first reflected beam; a second concave spherical mirror having the same radius of curvature and separated from the first concave spherical mirror by the radius of curvature and receiving the first reflected beam at a first point on a face of the second concave spherical mirror and generating a second reflected beam incident on a second point on the face of the first concave spherical mirror, the second reflected beam being reflected by the first concave spherical mirror from the second point on the first mirror to form an output beam from the first confocal resonator cell; and, a second confocal resonator cell receiving the output beam of the first confocal resonator cell as an input beam of the second confocal resonator cell. The apparatus and method may form part of a beam delivery unit and may be part of an integrated circuit lithography lights source or an integrated circuit lithography tool. The apparatus and method may comprise a plurality, e.g., two pulse stretchers in series and may include spatial coherency metrology. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a partially schematic cross-sectional view of a pulse stretcher according to aspects of an embodiment of the present invention; 
       FIG. 2  shows a partially schematic perspective view of the pulse stretcher according to  FIG. 1 ; 
       FIGS. 3-5  show aspects of the operation according to an embodiment of the present invention showing, e.g., the tilt tolerance aspect of an embodiment of the present invention; 
       FIG. 6  illustrates partially schematically in cross section, e.g., the tilt tolerance of the pulse stretcher according to  FIGS. 1-2 . 
       FIG. 7  shows a measurement of the two dimensional spatial coherence of an output laser pulse passed through two pulse stretchers in series according to aspects of an embodiment of the present invention; 
       FIG. 8  shows a measurement of the two dimensional spatial coherence of an output laser pulse passed through a single pulse stretcher according to aspects of an embodiment of the present invention; 
       FIG. 9  shows a measurement of the two dimensional spatial coherence of an output laser pulse without any pulse stretching according to an aspect of an embodiment of the present invention; and 
       FIG. 10  shows a two dimensional measurement of the intensity distribution of an output laser pulse according to an aspect of an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   According to aspects of an embodiment of the present invention applicant has designed an optical pulse stretcher for a laser light source, e.g., a gas discharge laser light source, e.g., a KrF or ArF or molecular fluorine gas discharge laser, e.g., for use in integrated circuitry lithography illumination, which has a long optical delay, but is constrained to have a practical physical length, e.g., less than about 8 feet, e.g., in order to be mounted on existing laser frames or contained within a beam delivery unit and fit, e.g., in a fabrication facility clean room sub-floor room. According to aspects of an embodiment of the present invention, the pulse stretcher may be, e.g., a multi-passing system with a minimum number of optics, e.g., four, consistent with proper operation. This, in addition, e.g., minimizes the number of adjustments necessary to align a system, and according to aspects of an embodiment of the present invention the system is designed to allow for a considerable amount of misalignment over systems of the prior art. According to an aspect of an embodiment of the present invention the pulse stretcher comprises, e.g., a unique optical design that produces 12 passes with only 4 mirrors. Such a pulse stretcher is capable of, e.g., an optical pulse stretching having, e.g., an 80 ns delay from a physical length of about 2 meters and a total of 4 mirrors. According to aspects of an embodiment of the present invention also, the pulse stretcher disclosed, e.g., does not suffer the focusing problems of, e.g., a Herriott Cell nor the re-entry and symmetry problems of, e.g., a White Cell. 
   What is so remarkable about aspects of an embodiment of the present invention, in addition to its space efficiency is its stability. The design is so stable that it may require no adjustments for alignment. According to aspects of an embodiment of the present invention stability can be derived, e.g., from the fact that the design is essentially 4 confocal resonators, having, e.g., the re-entry characteristic of a confocal resonator. According, e.g., the beam will retrace its path no matter what the angle orientation exists between the two mirrors forming, e.g., the respective confocal resonator, as long as the beam intercepts the respective next mirror in the respective confocal resonator. This concept can be most easily identified by examining one section of the layout as shown in  FIGS. 3-6 . Turning first to  FIGS. 1 and 2 , however, there is shown a pulse stretcher  18  according to aspects of an embodiment of the present invention. 
   The pulse stretcher  18  may comprise, e.g., four focusing mirrors, e.g., concave spherical mirrors  20 ,  21 ,  22 ,  23 , which may be, e.g., 10 cm in diameter, e.g., for handling adequately a beam size of e.g., 1.2 cm×1.2 cm. Each of the mirrors  20 ,  21 ,  22  and  23  is separated by a radius of curvature of the spherical mirror preceding it in a respective confocal resonator cell and may have, e.g., a radius of curvature of, e.g., about 1.6-2 meters. In operation, e.g., the beam  1  can enter the delay path formed by the mirrors  20 ,  21 ,  22 ,  23  through a beam splitter (not shown in  FIGS. 1 and 2  for clarity reasons) and be incident at a first point  1  on the mirror  20 . From point  1  on the mirror the reflected beam  2  is incident on point  2  on mirror  21 , and from there, the reflected beam  3  returns to mirror  20  at point three. From point  3  on mirror  20 , the reflected beam  1   a  is incident on point  4  on mirror  22  and from there the reflected beam  2   a  is incident on point  5  on mirror  23  and the reflected beam from point  5  on mirror  23  is returned to mirror  22  as reflected beam  3   a  incident on point  6  on mirror  22 . 
   A third confocal resonator cell is then set up as the beam reflected from point  6  on mirror  22 , beam  1   b  reflected to point  7  on mirror  20  and from there is reflected as beam  2   b  incident on point  8  on mirror  21  and then returned to mirror  20  at point  9  on mirror  20  as beam  3   b . The reflected beam from point  9  on mirror  20 , beam,  1   c  is incident on point  10  on mirror  22  and reflected from there as beam  2   c  to point  11  on mirror  23  and from there, reflected beam  3   c  is incident on point  12  on mirror  22  which is aligned to return reflected beam  1 ′ to the beam splitter (not shown in  FIGS. 1 and 2 ). 
   Turning now to  FIGS. 3-6  it can be seen that no matter what the angle orientation of the mirrors  20 ,  21 ,  22 ,  23  in a respective confocal resonance cell, the beam will always come back to the same point  12  on mirror  22 .  FIGS. 3-6  illustrate the effect within a single confocal resonance cell of misalignment from perfect alignment, e.g., as illustrated in  FIGS. 1-2 . Because of, e.g., this property, the 12 pass design  18  will always be aligned as long as the mirrors, e.g., mirrors  20 ,  21 ,  22 ,  23  are positioned well enough to redirect the beam from a first mirror in a respective confocal resonator cell to the correct opposing mirror. Therefore, the angular allowance of the system is driven be the size of the mirrors and the size of the beam. This also means, e.g., that the design is almost completely immune to, e.g., initial misalignment or, e.g., vibration problems that cause relative movements between the mirrors, e.g., mirrors  20 ,  21 ,  22  and  23 , provided that the variations are small enough as to not to redirect the beam off the respective opposing mirror. 
   Turning now to  FIG. 3  there is shown, e.g., a first of the confocal resonance cells according to  FIGS. 1 and 2 , showing, e.g., beams  1 ,  2  and  3  in a first confocal resonance cell as shown, e.g., in  FIGS. 1 and 2 , e.g., with the mirrors  20  and  21  aligned so that, e.g., the full extent of mirror  20  is used to separate points  1  and  3  and showing the reflection from point  2  on mirror  21  returning to point  3  on mirror  20 , from which it is reflected to point  4  on mirror  22  (not shown in  FIGS. 3-5 ). 
   Turning to  FIG. 4  three is shown according to an aspect of an embodiment of the present invention the effect of, e.g., a small misalignment of mirror  20 , e.g., a 1.5° tilt, such that, e.g., the point  2  on mirror  21  to which beam  2  travels from point  1  on mirror  20 , due to the misalignment is displaces almost completely across the face of the mirror  21 , but remaining on the face of the mirror  21 . As can be seen, the respective beam  3 , incident an point  3  of mirror  20  is also reflected to a point  3  that is displaced across the face of the mirror  20  from that shown e.g., in  FIGS. 1-3 , but as can also be seen, the beam  1   a  reflecting from point  3  on mirror  20  to point  4  on mirror  22  remains incident on a point  4  on mirror  22  that is essentially the same as illustrated in  FIGS. 1-3 , despite the misalignment of mirror  20 . 
   Turning to  FIG. 5 , three is illustrated schematically, e.g., the effect of a misalignment of mirror  21  according to aspects of an embodiment of the present invention wherein the beam  2  is incident on mirror  21  at a point  2  displaces across the face of mirror  21 , also displacing the point  3  on mirror, similarly to  FIG. 4 , but with the beam  1   a  reflected from point  3  on mirror  20  in  FIG. 5  again returning to the proper point  4  on mirror  22  (not shown in  FIG. 5 .) 
     FIGS. 4 and 5  illustrate, e.g., that despite misalignment of mirror  20  with respect to mirror  21 , which can include misalignment of both from the perfect alignment, illustrated schematically in  FIGS. 1-3 , the beam reflects back upon itself and so long as it remains within the confines of the surface of the mirror  20  (the first mirror of the respective confocal resonator) the exit beam from the respective confocal resonator will arrive at the proper place on the next mirror in sequence, e.g., mirror  22  (not shown in  FIGS. 4 and 5 ). 
   Turning now to  FIG. 6  there is shown schematically the operation of the entire pulse stretcher according to an aspect of an embodiment of the present invention with, e.g., a slight tilt in a mirror  20 ,  21 ,  22  or  23 , e.g., mirror  21 .  FIG. 6  shows that despite the misalignment the last beam  1 ′ remains perfectly aligned with the beam splitter (not shown) output of the delay path for the pulse stretcher  18  according to aspects of an embodiment of the present invention. 
   In operation a single pulse stretcher of the type described according to aspects of an embodiment of the present invention may stretch a typical excimer or other fluorine gas discharge laser, e.g., a molecular fluorine gas discharge laser, having a pulse duration of the output laser pulse of on the order of about 40 ns having, e.g., a T IS  of on the order of about, e.g., 8 ns, to a pulse having several peaks not greater than, e.g., about 40% of the input peak power to the pulse stretcher  18  according to aspects of an embodiment of the present invention, and having, e .g., a T IS  of on the order of about 45 ns. 
   It will also be understood, that increasing the radius of curvature of the mirrors  20 ,  21 ,  22  and  23  can increase by the achievable pulse stretching and T IS , at the expense of some increase in overall length of the pulse stretcher  18  according to aspects of an embodiment of the present invention and also larger mirror size and, therefore, a larger housing footprint transversely of the overall pulse stretcher length. According to another aspect of an embodiment of the present invention, a method of scanning the laser beam and calculating weighted average of the spatial coherence is proposed, e.g., for measuring more accurately the spatial coherence of an output laser beam pulse as is pertinent to proper performance of the output laser beam pulse in properly serving the function of, e.g., an integrated circuit lithography tool light source, e.g., a DUV light source. Implementation of this method revealed interesting aspects of laser output light pulse beam profiles, e.g., in regard to spatial coherence, e.g., for XLA beam spatial coherence profiles. Applicants have discovered that an aspect of using, e.g., a beam stretcher according to aspects of an embodiment of the present invention can provide very beneficial output laser pulse beam spatial coherency properties. It is most desirable to limit spatial coherency. 
   Utilizing, e.g., two pairs of pin holes, and an X-Y automated scanning setup (not shown) along with imaging optics (not shown) and a photo-diode array (“PDA”), and along with computer control to, e.g., acquire and analyze the data, applicants have reviewed the spatial coherency in two dimensions of a beam that has not been passed through a pulse stretcher, a so-called Optical Pulse Stretcher (“OpuS”) provided along with certain of applicants&#39; assignee&#39;s products, e.g., XLA series products. This scanning means of estimation of output laser pulse coherence produced data illustrated, e.g., in  FIGS. 7-9 , showing respectively the information regarding two dimensional beam coherency for, respectively an unstretched pulse, i.e., a pulse not passed through applicants&#39; assignee&#39;s OpuS ( FIG. 7 ), a pulse passed through a single stage pulse stretcher, e.g., applicants&#39; assignee&#39;s OpuS, and an output laser pulse beam passed through a two stage Opus. Applicants&#39; assignee&#39;s OpuS in addition to stretching the pulses to improve, e.g., T IS , performs certain, e.g., optical flipping and rotating and the like, of the output laser pulse beam, with results indicated illustratively in  FIGS. 7-9 . 
   
     
       
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE I 
             
             
                 
                 
             
             
                 
                 
                 
               XLA two 
             
             
                 
                 
               XLA one 
               OpuS 
             
             
                 
                 
               2 × OpuS 
               4 × OPuS 
             
             
                 
               XLA no OPuS 
               (XLA100) 
               (XLA105 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Peak contrast 
               0.58 
               0.48 
               0.30 
             
             
                 
               Weighted 
               0.37 
               0.22 
               0.11 
             
             
                 
               Average 
             
             
                 
                 
             
           
        
       
     
   
   As shown in  FIG. 7 , and listed in Table I, the output laser pulse has a peak contrast of about, e.g., 0.3, and a weighted average overall of about, e.g., 0.11.  FIG. 7  shows that the horizontal and vertical coherency is low, with, e.g., most of the beam being in regions  52  (0-0.125) as indicated in the bar graph to the right of the illustration or region  54  ((0.125-0.250), with some small portions of the beam in region  50  (0.250-0.375), and some further still smaller portions in other ranges, which are due to boundary effects of the measurement setup. These measurements were taken with a 2×OpuS pulse stretcher and a 4× Opus pulse stretcher in place in the beam path. 
   Turning to  FIG. 8  there is shown an illustration of the beam becoming more coherent, particularly as measured in the x-axis, including much more of the beam in range  50  (0.250-0.375) and also including still further areas in range  56  (0.375-0.500). These measurements were taken with only a 2× OpuS in place in the beam path. 
   As shown in  FIG. 9 , the beam is even more coherent when both pulse stretchers are out of the beam path, now including a more definite distribution of more or less equal areas in the ranges  50 - 54  and distributed more or less symmetrically about the vertical centerline axis of the beam along the x-axis and further now including a significant portion in range  58  (0.500-0.625) with some small portions of the beam in the ranges  70  (0.625-0.750),  72  (0.750-0.875) and  74  (0.875-1.000). Coherency is being measured through the diffraction fringes set up by the beam passing through the pin holes across the beam profile, with the more coherent light in the laser beam resulting in more fringes and more contrast. 
   For the beam of  FIG. 8 , as indicated in Table I, the maximum contract increased to 0.48 and the overall weighted average increased to 0.22 and for  FIG. 9 , the maximum contract increased to 0.58 and the overall weighted average to 0.37. This amounts to, e.g., an almost one half increase in the maximum contrast and an almost two thirds decrease in overall weighted average. 
   As can be seen from the above, the pulse stretcher has not only the beneficial results of increasing pulse length and decreasing peak pulse intensity, resulting in higher T IS  but also is a very efficient reducer of spatial coherence in the output laser light beam. 
   Turning now to  FIG. 10 , there is shown a beam intensity profile in two dimensions, including, e.g., intensities ranging from 10-308.8 arbitrary units of scale, in region  100 , generally around the periphery of the beam profile to 2101-2400 arbitrary units of scale (region  114 ) generally at the center of the beam profile, with regions  102  (308.8-607.5),  104  (607.5-906.3),  106  (906.3-1205),  108  (1205-1504),  108  (1504-1803),  112  (1803-2101) and  114  (2101-2400) generally from the periphery to the center of the beam profile. 
   It will be understood by those skilled in the art that many changes and modifications may be made to the present invention and aspects of the present invention without departing from the scope and content of the appended claims and that the appended claims should not be limited in scope or content to the particular aspects of preferred embodiments disclosed in the present application.