Abstract:
A high power narrow band, high repetition rate laser light source optical improvement apparatus and methods are disclosed with a fast angularly positionable mirror having a mirror mounting frame, a reflective optic with a coefficient different from that of the mounting frame, at least one flexure mount fromed in the mounting frame that is flexible having flexure arm attached the flexture to the mounting frame. The apparatus may include a flexure force mechanism having an elongated rod. The force mechanism may pre-stress the flexure. The mirror maybe a grating which includes a substrate with metallic layer formed on the substrate, and a protective coating made of silica formed on the reflective metallic layer. The grating maybe actively tuned using an electro- or magneto-sensitive element. Oxides of the metal in the reflective layer may be removed by a hydrogen purge system exposed to deep ultraviolet radiation.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/173,190, filed Jun. 14, 2002, now U.S. Pat. No. 6,750,972, which is a continuation-in-part of Ser. No. 10/141,201 filed May 7, 2002, now U.S. Pat. No. 6,839,372, Ser. No. 10/000,991 filed Nov. 14, 2001, now U.S. Pat. No. 6,795,474, Ser. No. 09/716,041, filed Nov. 17, 2000, now U.S. Pat. No. 6,778,584, Ser. No. 09/854,097, filed May 11, 2001 now U.S. Pat. No. 6,757,316, and is related to U.S. patent application Ser. No. 09/895,664, filed on Jun. 29, 2001, entitled LINE NARROWING UNIT WITH FLEXURAL GRATING MOUNT, published on Mar. 21, 2002, Publication No. 2002/0034208, and now issued as U.S. Pat. No. 6,496,528, issued to Titus et al. on Dec. 17, 2002 and U.S. patent application Ser. No. 10/608,521, filed on Jun. 26, 2003, with inventor Rafac, et al., entitled METHOD AND APPARATUS FOR STABILIZING OPTICAL DIELECTRIC COATINGS, the disclosures of which, to the extend not incorporated herein are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to line narrowing modules used in high power very high repetition rate narrow banded gas discharge lasers, e.g., excimer and molecular fluorine gas discharge lasers and particularly optical components relating to such modules. 
     BACKGROUND OF THE INVENTION 
     Gas discharge ultraviolet lasers used as light sources for integrated circuit lithography typically are line narrowed. A preferred prior art line narrowing technique is to use a grating based line-narrowing unit along with an output coupler to form the laser resonator cavity. The gain medium within this cavity is produced by electrical discharges into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F 2  laser). 
     Such a prior art system  1  is shown in  FIG. 1 , which is extracted from Japan Patent No. 2,696,285. The system  1  shown includes output coupler (or front mirror)  4 , laser chamber  3 , chamber windows  11 , and a grating based line-narrowing unit  7 . The line-narrowing unit  7  is typically provided on a lithography laser system  1  as an easily replaceable unit and is sometimes called a “line narrowing package” or “LNP” for short. This prior art unit includes two beam-expanding prisms  27  and  29  and a grating  16  disposed in a Littrow configuration. Gratings used in these systems  1  are extremely sensitive optical devices. A typical grating  16  surface may have 10,000 grooves per inch created in an aluminum layer or layers on a thick glass substrate. These gratings  16  and techniques for fabricating them are described in U.S. Pat. No. 5,999,318, which is incorporated herein by reference. A prior art technique for avoiding distortion of the grating  16  surface is to mount the grating  16  on a metal grating mount made of a material having a small co-efficient of thermal expansion closely matched to the thermal expansion co-efficient of the grating  16  glass substrate. The gratings  16  can deteriorate rapidly under ultraviolet radiation in the presence of oxygen in standard air. For this reason, the optical components of line narrowing units  7  for lithography lasers  1  are typically purged continuously during operation with nitrogen. For example as shown in the above referenced Japanese patent from which  FIG. 1  is taken, a nitrogen purge system  40  may include, e.g., a nitrogen bottle  44  containing nitrogen under pressure may, when valve  48  is opened provide nitrogen through a nitrogen purge line  45  under flow control from, e.g., a flow control mechanism  46  into the interior of the enclosure surrounding the line narrowing package  7 . As shown in this Japanese patent, the nitrogen gas  45  is injected behind the grating  16  and purposely not flowed across the reflecting facets of the grating  16 . 
       FIG. 2  shows a prior art line-narrowing unit  50  fabricated by applicants&#39; employer, Cymer, Inc., as a part of a prior art line narrowed lithography KrF laser system incorporating such a device. The unit  50  includes, e.g., three beam expanding prisms  58 ,  60  and  62 , a tuning mirror  64  and a grating  16 . Note that the nitrogen purge from bottle  44  enters the unit on the back side of the tuning mirror  46  to avoid purge flow directly on the grating  16  face, as shown in the above referenced Japanese patent. In this system  20  the wavelength of the laser beam  6  can be controlled, e.g., in a feedback arrangement in which the wavelength of the beam  6  is measured by a beam wavelength monitor  22  and a computer controller  24  uses the wavelength information to adjust the angular position of the tuning mirror  64  to control the wavelength to a desired value. A wavefront (and thus also bandwidth) control device  70  can be used, e.g., to mechanically bend the grating  16  to make it slightly concave, for example, or slightly convex. This device  70  is described in detail in U.S. Pat. No. 5,095,492 assigned to Cymer. Use of this device  70  can permit, e.g., reduction of the bandwidth somewhat, but it can still go out of specification when the laser is run at high duty cycle. 
     For many years, designers for line narrowed lasers have believed that distortions of the laser beam could be caused by gas flow near the face of the grating  16 . Therefore, laser designers in the past have made special efforts to keep the purge nitrogen from flowing directly on the face of the grating  16 . Several examples of these efforts are described in the Japan Patent 2,696,285 referred to above. In the example shown in extracted  FIG. 1 , the purge flow is directed from N 2  gas bottle  44  toward the backside of grating  16  through purge line  45 . 
     Line narrowed ultraviolet laser light sources currently in use in the integrated circuit industry typically produce about 10 mJ per pulse at repetition rates of about 4000 Hz and duty factors (duty cycles) of about 20 percent. Increased integrated circuit production can be achieved at higher repetition rates and greater duty cycles. Applicants&#39; employer is currently selling a 4000 Hz gas discharge lithography laser, including some, e.g., in a MOPA configuration. Higher repetition rates along with higher duty cycle demands and requirements of ever narrower bandwidths, can, even in MOPA configurations, place sever constraints on optical components, e.g., the LNP  50  to be able to maintain consistent narrow bandwidths. A significant portion of the problem arises from the requirements to maintain rather strict parameter controls on the laser beams traveling through the system, e.g., including wavefront control, which can impact the performance of the LNP among other optical and lasing components of the system  2 . 
     A need exists for more reliable line narrowing packages  50  and other optical elements of the laser system  2  to accommodate such requirements as high repetition rate, high duty cycle, bandwidth stability, included not exceeding some specific bandwidth and/or not allowing the bandwidth to go below some lower limit in addition to not exceeding some upper limit, exposure dose stability, and the like for gas discharge lasers of the type disclosed in the present application. 
     It has also been known to mount the mirror  64  on a mirror mount as shown in  FIGS. 6 and 6A  using a plurality of, e.g., three, mounting balls, e.g., made of aluminum, including, e.g., two right (as shown in the figure) mounting balls  156  and one left (as shown in the figure) mounting ball  158 . The mounting balls  156 ,  158  may, e.g., be contained in cylindrical pockets, e.g., as shown more clearly in  FIG. 9  at  156   a  and  156   b  showing a version of an aspect of an embodiment of the present invention, which are formed in the mirror mount  152 . The mounting balls  156 ,  158  may cooperate with spring loaded mounting clips  168  contained respectively on a right (as shown in the figure) elevation wall  162  of the mirror mount  152  and on a left (as shown in the figure) mounting wall  164  of the mirror mount to hold the mirror  154  in place firmly, but without distorting the face of the mirror or its flatness, as will be understood by those skilled in the art. Problems have arisen, however, with such mountings, e.g., induced by thermal stressing of the mirror mount  152  and the mirror  154 , which, due to the fact that the former of which is usually made of metal, e.g., aluminum, invar or stainless steel, and the latter of glass, e.g., fused silica, having very different coefficients of thermal expansion will expand differently when heated. 
     It has been found that the prior art mounting just described can cause several problems most notably in negatively impacting the LNP  50  in performing its functions within the very stringent requirements for, e.g., wavelength and bandwidth accuracy and stability. What has been discovered to be occurring is that differing thermal expansion of the mirror  154  and the mount  154  tends to force the mirror to slide across the mounting ball surfaces, as was the original design intent. However, the glass surface of the mirror  154  has been found to intermittently and unpredictably sometimes adhere to the mounting ball surfaces or at least one of them causing intermittent and unpredictable distortions in the face of the mirror  154  with concomitant disastrous results in the maintenance of wavelength and/or bandwidth stability. An aspect of an embodiment of the present invention is for the purpose of resolving this problem and related species of problems that similarly impact the contribution of the mirror  64  in the LNP in the performance of its functions. 
     It has also been noted that as optical elements, e.g., the grating  16  is the LNP  50  are being subjected to higher fluence environments grating  16  failures have increasingly been observed. The observation is also that these failures, by in large, are a result of oxidation of the aluminum reflective layer that forms the reflecting surface of the grating. An aspect of an embodiment of the present invention is directed to a solution to this increasingly more significant problem. 
     A prior art line narrowed gas discharge laser, e.g., a KrF excimer laser or an ArF excimer laser, operating at relatively low average power, typically less than 5 W, will produce a laser beam centered, respectively at about 248 nm or 193 nm with a bandwidth of less than 0.6 pm. Such lasers can run without problems at repetition rates, up to 2000 Hz and even above that as long as average power is below 5 W. More modern lasers with, e.g., a MOPA configuration can operate at repetition rates exceeding 4000 and with even higher power. However, this, combined with duty cycle requirements, and in some cases customers have been known to operate such laser lithography light sources at near 100% duty cycle over long stretches of time, days or weeks. A typical lithography gas discharge laser has a pulse energy of 10 mJ. By way of example, such lasers can be run at 2 kHz in bursts of 200 pulses with pause between bursts of about, e.g., 0.4-0.45 seconds. Such an operation will produce an average power of: 
     
       
         
           
             
               
                 
                   
                     P 
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                         10 
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                         ⁢ 
                         
                           mJ 
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                           200 
                         
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                         ⁢ 
                         pulses 
                       
                       
                         0.5 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         sec 
                       
                     
                     = 
                     
                       4 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       W 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     When the interburst delay is decreased, e.g., with the laser running the same 200 pulse bursts with a 0.1 second interburst delay will have an average power of: 
                     P   ave     =         10   ⁢           ⁢     mJ   ·   200     ⁢           ⁢   pulses       0.2   ⁢           ⁢   sec       =     10   ⁢           ⁢   W               (   2   )               
At maximum, the laser is run in continuous mode which, at 2000 Hz and 10 mJ pulse energy, is equivalent to 20 W average power. These factors simply go up with increases in repetition rates, e.g., to and above 4000, increasing such things as the thermal loading in optical components, including particularly the line narrowing package. The increases on thermal loading only serve to make the problems of maintaining precision operation of such elements as the reflecting mirror  64  and the grating  16  more difficult at the very time when the requirements for precision in operations are becoming even more stringent.
 
       FIG. 2A-1  is a side view showing a prior art method of mounting a grating  16  to the floor of the LNP  50  enclosure. In this case, the thick glass substrate of the grating is attached to mounting plate  16 A at each of three spots with very short epoxy dots  17 A-C.  FIG. 2A-2  shows the approximate dot positions at  17 A,  17 B and  17 C. The mounting plate  16 A is screwed securely to the floor of the LNP enclosure with two screws  16 B and  16 C. In this prior art design, the mounting plate  16 A is made of invar, which has a co-efficient of thermal expansion close to zero and to that of the grating  16  glass substrate which may be, e.g., ultra low expansion glass and also close to zero in coefficient of thermal expansion. However, the LNP  50  enclosure is aluminum, which has a co-efficient of thermal expansion substantially different from both invar and the grating  16  glass substrate. As a consequence, temperature excursions in the LNP  50  produces bending stresses on the mounting plate  16 A which is screwed tightly to the bottom of the LNP  50  enclosure and the plate  16 A in turn produces bending stresses on the grating  16  through the short epoxy dots. As long as the temperature excursions are small the dots are flexible enough to prevent significant distortion; however, at high repetition rates in the range of 2000 Hz to 4000 Hz and above and high duty factors, thermal distortion in the grating  16  has been so great as to substantially adversely affect the quality of the laser beam both in terms of bandwidth and wavelength centerline stability, and the problem is only getting worse. 
     As noted in the above referenced U.S. Pat. No. 6,496,528, the cause of beam quality deterioration with increasing beam energy, the cause was not initially readily apparent, however, applicants recognized that differential thermal expansion was causing an undesirable bending of the grating  16  which was causing, e.g., the deterioration in beam quality. As a result, applicants designed several modifications in the LNP to solve this problem as discussed in the above referenced U.S. Pat. No. 6,496,528. 
     One such improvement known in the art is shown in  FIGS. 3A , B and C, which are top, side and bottom views respectively of a grating mount  16 A″ with a single H-Flex joint  121 . This mount  16   a ″ is comprised of invar which, as stated above has a very small thermal coefficient of expansion about the same as the, e.g., Ultra-Low-Expansion (“ULE”) made by Corning, or e.g., fused silia grating  16  glass substrate. The mount  16 A″ is about ½ inch thick and has a length approximately equal to the thick glass substrate of the grating  16  to be mounted on it. Holes  120  are cut into the mount  16 A″ primarily to reduce its weight. An H-Flex joint  121  is machined into the mount  16 A″ as shown in  FIG. 4A . Two dog-bone shaped holes  124  are cut into the mount  16 A″ producing an “H” shape flex joint  121  so as to leave four flex legs  126  each about 0.060 inch thick. The grating  16  is preferably mounted to the mount  16 A″ with three short epoxy dots at locations  128  about 4 mils high and 1 cm in diameter as shown in  FIG. 4A . 
     The mount  16 A″ is screwed securely to the bottom of the LNP  50  enclosure using threaded holes  130  shown in  FIG. 4C . The H-Flex joint  121  permits thermal expansion and contraction of the LNP  50  enclosure bottom without transmittal of any significant stress to the grating  16 . The joint  121  provides very little resistance to small forces in the long direction of the grating  16 . The expansion co-efficient is about 0.001 inch per pound, in this long direction, but is extremely strong and resistant to forces in any other direction. 
     A third embodiment according to the prior art can be described by reference to  FIGS. 5A , B, C, D, E and F.  FIG. 5A  is a top view,  FIG. 5B  is a side view and  FIG. 5C  is a bottom view. This mount  16 A′″ is similar to the one described above and shown in  FIG. 4A . However, this mount  16 A′″ has two H-Flex joints  134  and  133  as shown enlarged in  FIGS. 5D and 5E  respectively. The mount  16 A′″ is made of aluminum as is the LNP  50  enclosure. The mount  16 A′″ is mounted to the enclosure as described in the previous embodiment. The grating  16  is attached to the mount  16 A′″ with three short epoxy pillars as described above at the positions  132 A,  132 B and  132 C as shown in  FIGS. 5A ,  5 D and  5 E. The surface of the mount  16 A′″ at the epoxy positions (in this as well as the other embodiments) is preferably abraded with #40 grit dry blast alumina to produce a good epoxy surface. The legs of H-Flex joint  134  and H-Flex joint  133  are about 0.030 inch wide. H-Flex joint  133  permits expansion of the mount  16 A′″ relative to the grating  16  in the long direction of the grating and H-Flex joint  134  permits expansion in a short direction of the grating  16  as indicated by the drawings. The second H-Flex joint  134  permits the use of aluminum as the mount  16 A′″ material, which is less expensive and easier to machine than invar. Preferably, a four-mill shim (not shown) is used in this and the other embodiments when attaching the grating  16  to the mount to assure that the epoxy dots are the correct thickness (i.e., height). 
     Preferred designs of grating mounts  16 A- 16 A′″ of the prior art take into consideration the material used for the mount. For example, in the  FIGS. 3A-D  and in  FIGS. 4A-C  examples, invar is used for the mounts  16 A′ and  16 A″ which has a thermal co-efficient of thermal expansion similar to the thick ULE glass grating  16  substrate. Therefore, the grating  16  is attached with the three short epoxy dots at two far apart locations to the long part  100  of the mount  16 A′ with no attachment to the short part  102  of the mount  16 A′. In this example, one end of the long portion  100  of the mount  16 A′ is attached to the LNP  50  enclosure and the short portion  102  of the mount  16 A′ at the other end of the mount  16 A′ is separately attached to the LNP  50  enclosure. The H-flexure joints in the second example  16 A″ permits the mount  16  and the LNP  50  enclosure (having significantly different coefficients of thermal expansion) to expand and contract at different rates. 
     In the  FIG. 5A-E  examples, the mount  16 A′″ is securely attached at two locations to the enclosure housing. This produces no significant stresses since both the mount  16 A′″ and the enclosure housing are aluminum. The grating  16  substrate which has a coefficient of expansion substantially different from aluminum is attached to each of the three separate parts of the mount  16 A′″ which by reason of the two H-flexure joints  133 ,  134  are free to move relative to each other. Thus, if in the  FIGS. 3A-D  and  FIGS. 4A-C  examples the mount were made of aluminum then in each case the long part of the mount only should be attached to the chamber floor and the grating should be attached to both parts. Also if in  FIGS. 5A-E  example, invar were used for the mount, the grating should be attached to the long solid part and the mount should be attached to the chamber at points  132 A,  132 B and  132 C. 
     Turning now to  FIG. 6  there is shown a prior art mounting for a maximum reflecting mirror according to the prior art as discussed above. The maximum reflecting mirror, referred to as RMax  150  corresponds, e.g., to the mirror  64  in  FIG. 2 . In the past, as noted above, this mirror  150  comprised of an almost totally reflecting, i.e., RMax, multiple layer mirror, has been mounted onto a mirror mount  152 . In efforts to meet the problems created in the prior art design, applicants tried a variety of alternative materials and combinations of materials for the mounting balls  156 ,  158 , with varying degrees of partial but incomplete success. 
     Applicants have proposed in the past to carry out active control of the wavefront of a laser produced from a gas discharge, e.g., excimer or molecular fluorine, laser using, e.g., a bending screw and a stepper motor. In the past, this solution may have been too expensive for the amount of benefit to the laser performance being within required specifications. However, as noted above, the requirements are becoming exceptionally more demanding and wavefront, particularly in its impact on line narrowing, e.g., in an eschelle grating type line narrowing unit as described above, is becoming more critical to control. Currently in many lasers in the field, a grating bending screw is set at a compromise position empirically determined between a hot and cold position of the grating and then left alone, so-called set and forget. However, this solution if becoming increasingly less satisfactory and the requirements being placed on the laser systems, particularly, e.g., due to lithography light source requirements, a stepper motor active control may not even be satisfactory. An aspect of an embodiment of the present invention seeks to resolve these issues. 
     Applicants have also discovered that even with a flexure mounting, the forces exerted on the optical element in simply moving the flexure element can be sufficient to introduce discernable and unacceptable effects on such things as beam profile, wavelength, bandwidth or the like. Therefore another aspect of an embodiment of the present invention is meant to correct this problem. 
     It is also known to protect laser optics, particularly containing metal halide crystal glass, e.g., MgF 2 , using a silicon oxyfluoride coating, e.g., fluorine doped fused, i.e., amorphous, silica, as discussed e.g., in U.S. Pat. No. 6,466,365, entitled FILM COATED OPTICAL LITHOGRAPHY ELEMENTS AND METHOD OF MAKING, issued to Maier et al. on Oct. 15, 2002, for purposes of, e.g., providing a protective coating that is adequately transmissive in the VUV range of wavelengths. Such coatings serve, e.g., to prevent the introduction of impurities into the crystal structure voids from the surrounding ambient atmosphere, with resultant damaging effects to the optical performance of the optical element. Similarly, applicant assignee has used such coatings to prevent DUV damage to transmissive optical elements, as discussed, e.g., in United States Published Patent Application No. 2003/0219056 A1, entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS, with inventors Yager et al., published on Nov. 27, 2003, based on an application Ser. No. 10/384,967, filed on Mar. 8, 2003. The coating serves to provide fluorine atoms to replace fluorine atoms removed from, or perhaps displaced in the crystal structure of the fluoride glass due to high fluence of DUV radiation through the glass, e.g., in laser chamber window, thereby preventing fluence related optical damage to the glass of the optical element. Applicants herein propose yet another utilization for such protective coatings in relation to laser optical systems. 
     It has been known for some time that line narrowing module gratings are subject to failures, which have been identified as relating to oxidation of the metallic reflective coating, e.g., aluminum on the grating surface. Currently coatings on metallic reflecting surfaces exposed to DUV light, e.g., at 193 nm consist of fluoride coating materials, e.g., MgF 2 , due to the resistance of such materials to damage in the DUV range of fluence and high transmissivity, i.e., transparency, of such coatings. However, such coatings have not been successful in protecting gratings exposed to high fluence of DUV light. An aspect of an embodiment of the present invention addresses this problem. 
     It is well known that organic materials, e.g., certain polymers, cannot be used in portions of laser systems according to those employing various embodiments of the present invention. Not only do they typically not react well to UV light, but also are highly chemically reactive to laser gases, e.g., fluorine or chlorine. However, in certain applications in such laser systems in the optical path, but sealed from exposure to the reactive gases, organic materials, e.g., Teflon, may be necessary for use due to the large differences in price between such seals and the, e.g., metal c-ring seals used in contact with the reactive laser gases. An aspect of an embodiment of the present invention deals with the solution of certain problems arising from using such organic materials in seals in the optical path, e.g., external to the laser chamber. 
     SUMMARY OF THE INVENTION 
     A high power narrow band, high repetition rate laser light source optical improvement apparatus and methods are disclosed which may comprise a fast moving angularly positionable tuning mirror comprising: a mirror mounting frame comprising a first material and a relatively flat mounting surface area; a reflective optic comprising a second material having a coefficient of thermal expansion different from that of the first material of the mounting frame; at least two attachment points of attachment between the mounting frame and the reflective optic on the mounting frame surface; and, at least one flexure mount formed in the mounting frame that is flexible in a flexure axis corresponding to a longitudinal axis of thermal expansion of the mounting frame and the reflective optic, positioned at one of the at least two points of attachment. The flexure mount may comprise: a flexure body formed from the material of the mirror mounting frame and separated from the material of the mirror mounting frame to allow relative movement between the flexure and the mirror mounting frame; at least one flexure arm formed from the material of the mirror mounting frame and attached at one end to the mirror mounting frame and at the other end to the flexure. The apparatus and method may further comprise a flexure force mechanism made of the second material or a third material having a coefficient of thermal expansion that is essentially the same as that of the second material; the flexure force mechanism comprising an elongated rod; a flexure force mechanism slot generally aligned with the flexure axis and sized to snuggly fit the flexure force mechanism between a slot wall at one end of the flexure force mechanism slot and the flexure body at the other end of the flexure force mechanism slot. The force mechanism may pre-stress the flexure. The mirror may be a grating. The grating may have a metallic reflective surface and a protective coating over the reflective coating comprising a dense glassy material that is essentially non-porous to undesired contaminants exposure of which to the reflective coating is desired to be prevented, which may comprise an amorphous silica, a doped amorphous silica, which may be halide doped and the halide may be fluorine. The grating may be actively tuned using an electro- or magneto-sensitive element. Oxides of the metal in the reflective layer may be removed by a hydrogen purge. Exposed optical sealing members sensitive to DUV light may be shielded by an annular protection shield placed on the optic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first prior art line narrowed laser system; 
         FIG. 2  shows a second prior art line narrowed laser system; 
         FIGS. 2A-1  and  2 A- 2  show a prior art grating mounting technique; 
         FIGS. 3A , B, C, D and E show prior art grating mountings; 
         FIGS. 4A , B and C show prior art grating mountings; 
         FIGS. 5A , B, C, D, and E show prior art grating mountings; 
         FIGS. 6A  and B show schematically aspects of a prior art mounting for a maximum reflecting mirror RMax according to the prior art; 
         FIG. 7  shows schematically an aspect of an embodiment of the present invention relating to an RMax mounting; 
         FIG. 8  shows in more detail an enlarged plan view of an aspect of an embodiment of the present shown schematically in  FIG. 7 ; 
         FIG. 9A  shows a perspective view of an aspect of an embodiment of the present invention shown more schematically in  FIG. 7 ; 
         FIG. 9B  shows an alternative embodiment of a flexure mounting according to an aspect of an embodiment of the present invention; 
         FIG. 10  shows schematically a mechanism and method for protecting grating reflective coatings according to an aspect of an embodiment of the present invention; 
         FIG. 11  shows an isometric perspective view of a grating mount according to an aspect of an embodiment of the present invention, looking from the side to which the grating will be attached; 
         FIG. 12  shows an isometric perspective view of the grating mount of  FIG. 11  generally along the longitudinal axis of the mounting; 
         FIGS. 12A and 12B  show an alternative embodiment of the grating mount of  FIG. 12  according to aspects of an embodiment of the present invention, with  FIG. 12B  being a cross-sectional view along the lined  12 B- 12 B in  FIG. 12A ; 
         FIG. 13  shows an isometric perspective view of the grating mount of  FIGS. 11 and 12  looking from the opposite side of the mounting from where the grating will be attached; 
         FIG. 14  shows a perspective partially exploded view of an external optical assembly according to aspects of an embodiment of the present invention; 
         FIG. 15  shows another perspective partially exploded view of the external optical assembly according to aspects of an embodiment of the present invention; 
         FIG. 16  shows a cross-sectional view of the external optical assembly shown in  FIGS. 14 and 15  along the lines  16 - 16  in  FIG. 13 ; 
         FIG. 17  shows schematically an aspect of an embodiment of the present invention; 
         FIG. 18  shows schematically an aspect of an embodiment of the present invention. 
         FIG. 19  shows schematically another aspect of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to an embodiment of the present invention shown partly schematically in  FIGS. 7-9 , problems with wavefront, wavelength and bandwidth selection and wavelength and bandwidth stability, among other things, have been resolved. Turning now to  FIG. 7  there is shown schematically a mirror mount  152  containing mounting balls  156  and also a mounting ball  158  on a flexure mount assembly  170  that enables translation of the mounting ball along the principal axis of thermal expansion as the mirror  154  and mount  154  differentially thermally expand. Shown in more detail in  FIG. 8  the flexure mount assembly  170  can comprise, e.g., a flexure mount  180  which can contain a cylindrical opening  180   a , shown in  FIG. 9 , which is flexibly attached to the mount  152  by a pair of upper (as shown in the figure) and lower (as shown in the figure) flexure arms  186  and  188 . The flexure arms  186  and  188  and the flexure mount  180  itself are separated from the rest of the mount  152  by slots  182  and  184  through the material of the mount  152 . 
     Thus formed, the flexure mount  180  can easily translate under force induced by any sticking action between the mirror  154  and the mounting ball  158  in the cylindrical opening  180   a . Thus the mirror  154  is still optically firmly and snugly held in place by the spring clip  164  (not shown in  FIGS. 7-9 ) while being allowed to expand at the same time as any sticking action occurs between the mirror and any of the mounting balls  156 ,  158 . Distortion of the mirror face and the concomitant very severe undesirable effects are effectively eliminated according to this aspect of an embodiment of the present invention. 
     Turning now to  FIG. 9   a  there is shown an aspect of an embodiment of the present invention showing a flexure force bar  185 , similar to that described below in regard to  FIGS. 11-13 . The force bar  185  is contained in a force bar slot  189  and abuts at one end the mounting  152  and at the other end the flexure  180   a.    
     Turning now to  FIG. 10  there is shown schematically a grating  190 , which may comprise a substrate, which may comprise, e.g., a glass substrate  192 , e.g., an ultra-low expansion (“ULE”) glass e.g., as supplied by Corning, or, e.g., BK7 Schott glass supplied, e.g., by Valley Supply Corp of Westford Mass., which may be covered, e.g., with a thin film coating  194 , e.g., of a sufficiently reflective while optically durable coating, e.g., of a metal, e.g., of aluminum. The reflective coating  194  may then be covered with a protective coating  196 , which also may be a thin film coating of, e.g., a very low porosity, e.g., glassy, material, which is sufficiently transparent to, e.g., ultraviolet light. Applicants have found that at least one such material that is suitable is, e.g., a doped fused silica glass, e.g., a fluorine doped fused silica glass, e.g., as supplied by Corning Inc. (e.g., a silicon oxyfluoride). Applicants have shown that, particularly at 193 nm, such a material for a coating layer is highly resistant to UV damage from extended and/or high fluence exposure to UV light. Advantages of the combination include the ability to use relatively low cost thin metal reflecting coatings as the reflecting surface, while extensively reducing the oxidation and like damage to such reflective coatings, particularly under DUV light fluence, than is the case with less effective prior art protective coating layers. For example, existing coatings of, e.g., magnesium fluoride or other fluoride coating materials, which have high resistance to UV damage, are not acceptable long life coatings for reflective metallic layers, because over time they allow migration of oxidizing contaminants into the metallic layer or perhaps even contain oxidizing contaminants resulting from their manufacture, which also leech down into the reflective layer supposedly being protected. Fused silica with only a doping of fluorine has the advantage of being less so porous, because it is a much more glassy and non-porous material and, therefore, very suitable to prevent migration of undesirable contaminants, e.g., oxidants, through the coating to damage the underlying thin metal reflective coating. Applicants have found that the presence of a halide, e.g., fluorine in the, e.g., silicon oxyfluoride also make the performance better than the prior art in, e.g., reducing the formation of oxides of the metal, e.g., Al 2 O 3 , especially under DUV light fluence. It will be understood by those skilled in the art that other forms of amorphous relatively dense glassy materials may be used that have the required relatively high transparency for DUV, e.g., other forms of amorphous silicon (silica), and including, e.g., fused silica doped with other materials, e.g., other halides than fluorine. 
     Turning now to  FIGS. 11-13  there is shown an improved grating mounting  200  according to an embodiment of the present invention. The grating mounting  200  may comprise, e.g., an elongated generally rectangular body  202  including, e.g., a flexure mounting  221  formed in the body  202 . The flexure mounting  221  may be formed by cutting from the mounting body  202  a pair of dog-boned shaped slots  224  on either side of a flexure  230 , with, e.g., the respective opposite ends of the dog-bone shaped slots forming with one end of the body  202  and with an opening  225  in the mounting body  202  four flexure arms, respectively  224  and  226 . The dog-boned shaped slots are so called due to the shape left after the flexure arms  224  and  226  are formed and those skilled in the art will appreciate that they need not be dog-boned shaped, per se. The flexure mount  221  is for mounting the grating mounting body  202  to, e.g., to the floor of the enclosure of a line-narrowing module  50 . This flexure mount is designed to account for differential thermal expansion between the mount  200  and the enclosure of the line-narrowing module  50  in the longitudinal axis of the body  202 . 
     In addition the grating mount  200  may also comprise a longitudinal flexure mount  240  and a lateral flexure mount  234  for attaching the grating to the mount body  202 , as described above. The longitudinal flexure mount  250  may be formed of a flexure  250  and arms  252  formed by cutting lateral slots  242  and lateral openings  244  in the body to form the flexure  250  and arms  252 , as is known in the art. Similarly the lateral flexure  234  may be formed as is known in the art. As is also known in the art, the grating may be attached to the flexure  250  and the flexure  234  to account for differential thermal expansion of the mount body  202  with respect to the grating (not shown in  FIGS. 11-13 ). Attachment may be, e.g., with an adhesive, e.g., an epoxy at attachment points  266 ,  267  and  268 , with the attachment points  266  and  267 , respectively, on the flexure  250  and the flexure  234 . 
     Applicants have discovered that along with the beneficial results of the flexure mounting of the grating (not shown) to the grating mounting body  202  as just described certain problems manifest themselves with this arrangement as requirements being placed on the laser systems using such line narrowing units, e.g., for lithography uses, e.g., regarding does stability and control, bandwidth stability and control and wavelength stability and control, operate to place ever continuing requirements for such thins as tighter and tighter wavefront control, e.g., within the line narrowing module. Applicants have examined, e.g., wavefront instabilities, e.g., over time and with duty cycle changes influencing the fluence seen by the grating and the differential thermal expansion of the grating and the mounting and have determined that even the flexure mounting discussed above is insufficient to account for beam fluctuation due to the thermal loading on the grating and the grating mount. 
     Applicants have also determined that the cause is due to the fact that even the forces exerted on the grating by the action of the flexure mountings, particularly the longitudinal axis flexure mounting  250  in flexing are enough to cause sufficient distortion in the grating reflecting slots, longitudinally and laterally, to be seen in a failure of the laser output light to meat or remain within specifications. According to an aspect of an embodiment of the present invention, applicants have proposed a solution to this problem, as shown in  FIGS. 11-13 . 
     As can be seen from  FIGS. 11-13 , the grating mounting body may be provided with, e.g., a force rod slot  258  into which may be snuggly fit a force rod  260 , such that the force rod  260  at one end abuts a force rod detaining wall  262  and at the other end abuts the flexure  250  at a force rod engagement side  264  of the flexure  250 . The force rod  260  may be made, e.g., of an ultra low expansion (“ULE”) glass, which is the same material as the bulk of the grating itself, and thus will expand exactly the same amount or essentially exactly the same amount as the grating itself as temperature changes. In operation, therefore, the differential expansion of the grating mount  200  and the grating (not shown) attached to the grating mount  200  at attachment points  266 ,  267  and  268  will cause the grating mount  200  to try to expand the distance between mounting points  266  on the one hand and both  267  and  268  on the other hand on the grating mount  200  more than the respective points on the grating expand in the longitudinal axis of the grating mount  200 . Similarly there will be differential expansion between mounting points  267  and  268  generally orthogonally to the longitudinal axis but of a lesser degree and which is handled by flexure mount  234 . In regard to the longitudinal axis, however, the flexure mount  250  still exerts a force on the grating in absorbing the differential longitudinal axis expansion between the grating mount  200  and the grating (not shown). Therefore, the flexure mount  250  will tend to move toward the center of the longitudinal axis of the grating mount  200  and the force rod  260 , expanding at the same rate as the grating itself, will prevent that motion and thereby absorb the force from the flexure mount  250  attempting to flex. The flexure  250  and flexure arms  252  may be, e.g., also be pre-stressed with the force rod  260  in a direction towards the near end of the grating mount  200 , such that the stress that would otherwise be translated to the grating due to differential expansion under thermal stress will be further absorbed in the pre-stressed arms  252  of the flexure  250 , along with in the force rod  260 . 
     Turning now to  FIGS. 12A and 12B  there is shown another alternative embodiment of a grating mount  200  according to aspects of an embodiment of the present invention. In the embodiment of  FIGS. 12A and 12B  an additional glass expander  400  is added and placed in an expander slot  402 . The expander  400  may be made, w.f., of the same ULE glass as the rod  260  and may abut the rod  260  on one side and the body  202  of the grating mount  200  on the other side, in the longitudinal direction of the grating mount. The expander  400  may be snugly fit into the slot  402 , such that the expander as it expands in the lateral direction of the grating mount  200 , the aluminum grating mount will expand slightly more, giving the expander  400  room to expand in the lateral direction of the grating mount  200 . The expander  400  may have on it the mounting points  267 ,  268  for the grating (not shown) such that the expander  400  can take the place of the flexure mount  34  in the embodiment of  FIG. 12 . 
     Turning now to  FIGS. 14-16  there is shown a laser system optical window mounting  270  according to an embodiment of the present invention. The window mounting  270  may include, e.g., an inner window mount  272 , which may be attached to the wall of a laser chamber (now shown) by mounting bolts  280  and sealed using, e.g., a foil coated metal sealing ring  290 . 
     The window mount  270  may also include, e.g., an inner sealing ring  282 , which may be attached to the inner window mount  272  by a plurality of sealing ring attachment screws  292  and also have a foil coated metal c-ring seal  284  within a sealing groove  285  to form a seal between the inner window mount  272  and the inner sealing ring  282 . The inner sealing ring  282  may also have a foil covered metal seal ring  286  contained in a sealing ring groove  287  to provide a seal between the inner sealing ring  282  and an outer sealing ring (not shown) also held in place by the sealing ring attachment screws  292 . 
     The inner window mount may be formed with in internal beam passage  300  through which the output laser beam produced in the laser chamber (not shown) exits the chamber. Also included may be, e.g., an outer window mounting  274  having an internal beam passage  302  and a window receiving recess  312 . The outer window mount may include, e.g., mounting screws  322  to attach the outer window mounting  274  to the inner window mounting  272  with alignment pegs  287  inserted into corresponding alignment openings  288  in the outer window mounting. 
     The recess  312  is sized and shaped to contain therein when the inner window mount  272  is attached to the outer window mount  274  a window optic  304 , an o-ring seal  306  and a support foil  308 , e.g., made of relatively compliant tin to support the window optic  304  without putting undue stress on the window optic  304 . The window  304  is held in place by the inner window mount  272  and the outer window mount  274  at an angle to the beam passage  300  so as to insure almost complete transmissivity of the beam through the window  304 . The passage  302  is offset laterally to the optical path of the laser beam from the passage  300  to account for the refractive offset of the beam passing through the window. 
     Due to the angled nature of the window mounting  272  and the geometries involved some areas of the o-ring seal  306  may be exposed to UV light fluence. This can be very damaging to such o-ring, typically made from organic materials, e.g., polymers or elastomers, e.g., Teflon and can be decomposed from this exposure. This can cause seal failure over time or in a worst case introduce elastomer debris into the laser gas environment inside the chamber with potentially very serious performance degradation issues. While the o-ring seal  306  is not directly exposed to DUV, back scattering and reflection from within the window mount  272  assembly, e.g., from the surfaces of the window  304  itself, has been found to be damaging the o-ring seals  306 . 
     In order to solve this problem, applicants have placed a protective shielding coating  310 , e.g., on the annulus of the window  304 . This coating of, e.g., a thin film of aluminum deposited on the annulus of the window will shield the o-ring from the UV fluence. It will be understood that the protective shield coating need not be entirely annular in shape or even extend around the entire window circumference, but need only protect the o-ring seal  306  from the, e.g., back-scattered DUV light. 
     Turning now to  FIG. 17 , three is shown an arrangement for beam control for a laser of the type discussed in the present application. In the past beam control using, e.g., the grating, e.g., to control wavefront of the beam, has been proposed to be done with a stepper motor attached, e.g., to a large screw and/or screw and spring arrangement adapted to bend the entire grating optic or the grating mounting and optic simultaneously as shown for example in  FIG. 2A-2  and also in U.S. Pat. No. 6,496,528 referenced above.  FIG. 17  illustrates schematically the placement of an active grating mount bending mechanism  350 , e.g., a piezoelectric or other electro- and/or magneto-sensitive element, e.g., that bends or otherwise distorts itself under the influence, e.g., of an electric or magnetic field. The element  350  may have a voltage source for DC or AC voltage, which in the later case may be modulated in some desirable fashion, and a ground of common voltage connection  354 . When, e.g., in the case of a DC actuated element, e.g., a piezoelectric sheet  350  there is applied a voltage to the voltage supply terminal  352 , the piezoelectric element will, e.g., deform, e.g., by some controlled amount to, e.g., actively change the curvature of the grating (not shown in  FIG. 17 , and/or the grating mount  202 . 
     Turning now to  FIG. 18 , there is shown another embodiment of an active grating controller for a grating mount  202 , which may, e.g., incorporate several electro- or magneto-sensitive elements  360 ,  362 ,  364  and  366 , each with a respective stimulating mechanism, e.g., respectively, voltage supplies  370 ,  374 ,  378  and  382 , and ground connections, respectively  372 ,  376 ,  380  and  382 . As shown schematically in  FIG. 18 , this plurality of, e.g., electro-sensitive elements  360 ,  362 ,  364  and  366  may be variously arranged to, when selectively actuated, bend and twist the grating and/or the grating mount  202  to which the grating is attached. It will be understood that the grid of elements, e.g.,  360 ,  362 ,  364  and  366  illustratively shown by example in  FIG. 19  could comprise a larger array and connected to stimulators in a variety of polarities to control the bending or twisting of the grating and/or the grating mount as may be required and as understood by those skilled in the art of, e.g., piezoelectric actuators. 
     Turning now to  FIG. 19 , there is shown another aspect of an embodiment of the present invention, which may comprise an enclosure  393  containing, e.g., a DUV optical element, e.g., a grating  392 . The optical element, e.g., grating  392  may be exposed to a source of DUV light through an opening  393  in the enclosure  392 . The enclosure may have a purge inlet opening  394  and a purge outlet opening  396 . The interior of the enclosure  392  may be purged through the purge inlet opening  396  by a purge gas, e.g., containing hydrogen (H 2 ), e.g., mixed with a more inert gas, e.g., helium, argon, neon, xenon, and the like, with helium being preferred according to an aspect of an embodiment of the present invention. 
     Applicants have found that damage to optical elements containing, e.g., reflective metallic surfaces, e.g., the grating in a line narrowing module of, e.g., a narrow band DUV light source, is, in part, induced by DUV excited photoemission of electrons from, e.g., the metallic coating, e.g., aluminum, e.g., where φ=4.08 eV and hν=5 eV, which can, e.g., cause photo-assisted oxidation of the metallic reflective coating, even under a protective coating. It is quite easy for oxygen present in the environment to take an electron from the aluminum forming, e.g., Al 2 O 3 . Without incident DUV photons oxidation proceeds at a more normal slower rate, given the protective coating, or even without a protective coating. 
     Applicants have found that a purge of, e.g., hydrogen, e.g., mixed in a more inert gas, e.g., in about a 5% by weight percentage will tend to remove the O 2  from the oxidized surface of the optical element, e.g., a reflective grating  392 . This purge will also tend to scavenge residual O 2  within the enclosure, whether there due, e.g., to diffusion into or outgasing into the gas content of the enclosure, which may be a vacuum or may be, e.g., nitrogen. The reactions occurring may be, e.g., H═H═O→H 2 O+hν and H+H+O+hν=H 2 O. The H 2 O may then be conveniently removed through the purge gas outlet  396 . 
     It will be understood that the enclosure  392  may be a sealed optical enclosure, e.g., part of a line narrowing module, containing, e.g., a grating  392  irradiated by DUV during actual laser operation or may be a vessel used to reclaim optical elements, e.g., gratings  392  by exposure to hydrogen and DUV apart from a line narrowing module enclosure or apart from the line narrowing module being irradiated during laser operation, i.e., of the entire line narrowing module is removed for optical element reclamation at a site remote from the laser on which the grating was irradiated during laser use. 
     Many other changes and modifications may be made to the presently disclosed embodiments and aspects of embodiments of the present invention, without departing from the concept and scope of the inventions. The appended claims, therefore, should not be considered to be limited to particular preferred embodiments and the claims should be interpreted to cover those disclosed embodiments and equivalents.