Abstract:
A beam delivery unit and method of delivering a laser beam from a laser light source for excimer or molecular fluorine gas discharge laser systems in the DUV and smaller wavelengths is disclosed, which may comprise: a beam delivery enclosure defining an output laser light pulse beam delivery path from an output of a gas discharge laser to an input of a working apparatus employing the light contained in the output laser light pulse beam; a purge mechanism operatively connected to the beam delivery enclosure; an in-situ beam parameter monitor and adjustment mechanism within the enclosure, comprising a retractable beam redirecting optic; a beam analysis mechanism external to the enclosure; and, a retraction mechanism within the enclosure and operable from outside the enclosure and operative to move the retractable beam redirecting optic from a retracted position out of the beam path to an operative position in the beam path. The BDU may also include a beam attenuator unit contained within the enclosure adjustably mounted within the enclosure for positioning within the beam delivery path. The BDU may have at least two enclosure isolation mechanisms comprising a first enclosure isolation mechanism on a first side of the enclosure from the at least one optic module and a second enclosure isolation mechanism on a second side of the enclosure from the at least one optic module, each respective enclosure isolation mechanism comprising a flapper valve having a metal to metal seating mechanism and a locking pin assembly. A precision offset ratchet driver operative to manipulate actuator mechanisms in difficult to reach locations may be provided. An external kinematic alignment tool may be provided. A method of contamination control for a BDU is disclosed comprising selection of allowable materials and fabrication processes.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is a continuation of U.S. application Ser. No. 10/739,961, filed on Dec. 17, 2003, which is a continuation-in-part of United States Published Patent Application No. US/20020191654A1, entitled LASER LITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY, with inventors Klene, et al., published on Dec. 19, 2002, based upon a U.S. application Ser. No. 10/141,216, filed on May 7, 2002; and United States Published Patent Application No. US/20030043876A1, entitled LITHOGRAPHY LASER WITH BEAM DELIVERY AND BEAM POINTING CONTROL, with inventors Lublin et al., published on Mar. 6, 2003, based upon a U.S. Application Ser. No. 10/233,253 filed on Aug. 30, 2002; and United States Patent Published Application No. US/20030091087A1, entitled LITHOGRAPHY LASER SYSTEM WITH IN-PLACE ALIGNMENT TOOL, with inventors Ershov et al., published on May 15, 2003, based upon a U.S. application Ser. No. 10/255,806, filed on Sep. 25, 2002; and U.S. application Ser. No. 10/384,967, entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS, filed on Mar. 8, 2003, with Attorney Docket No. 2003-0005-01; and U.S. application Ser. No. 10/425,361, entitled LITHOGRAPHY LASER WITH BEAM DELIVERY AND BEAM POINTING CONTROL, filed on Jun. 29, 2003, with Attorney Docket No. 2003-0040-01, the disclosures of each of which is hereby incorporated by reference.  
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of gas discharge lasers and apparatus and methods for delivering the output laser light pulse beam to a manufacturing tool, e.g., a micro-lithography exposure illumination tool, and monitoring, preserving and modifying beam qualities prior to such delivery.  
       BACKGROUND OF THE INVENTION  
       [0003]     Long optics life is extremely important and extremely difficult to achieve in vacuum ultraviolet light (“VUV”) and other deep ultraviolet (“DUV”) and extreme ultraviolet (“EUV”) applications. Requirements, e.g., of &gt;12 billion pulse optic lifetimes can create an extreme need for contamination control measures, e.g., in the design of beam delivery units (“BDUs”), i.e., methods and apparatus for delivering the output laser light pulse beam to a manufacturing tool, e.g., a micro-lithography exposure illumination tool, and monitoring, preserving and modifying beam qualities prior to such delivery.  
         [0004]     Electric discharge gas lasers are well known and have been available since soon after lasers were invented in the 1960s. A high voltage discharge between two electrodes excites a laser gas to produce a gaseous gain medium. A resonance cavity containing the gain medium permits stimulated amplification of light which is then extracted from the cavity in the form of a laser beam. Many of these electric discharge gas lasers are operated in a pulse mode.  
         [0005]     Excimer lasers are a particular type of electric discharge gas laser and they have been known since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled COMPACT EXCIMER LASER. This patent has been assigned to Applicants&#39; employer, and the patent is hereby incorporated herein by reference. The excimer laser described in Patent &#39;884 is a high repetition rate pulse laser.  
         [0006]     These excimer lasers, when used for integrated circuit lithography, are typically operated in an integrated circuit fabrication line “around-the-clock” producing many thousands of valuable integrated circuits per hour; therefore, down-time can be very expensive. For this reason most of the components are organized into modules which can be replaced within a few minutes. Excimer lasers used for lithography typically must have its output beam reduced in bandwidth to a fraction of a picometer. This “line-narrowing” is typically accomplished in a line narrowing module (called a “line narrowing package” or “LNP” for KrF and ArF lasers) which forms the back of the laser&#39;s resonant cavity (A line selection unit “LSU” is used for selecting a narrow spectral band in the F 2  laser). The LNP is comprised of delicate optical elements including prisms, mirrors and a grating. Electric discharge gas lasers of the type described in Patent &#39;884 utilize an electric pulse power system to produce the electrical discharges, between the two elongated electrodes. In such prior art systems, a direct current power supply charges a capacitor bank called a “charging capacitor” or “C 0 ” to a predetermined and controlled voltage called the “charging voltage” for each pulse. The magnitude of this charging voltage may be in the range of about 500 to 1000 volts in these prior art units. After C 0  has been charged to the predetermined voltage, a solid state switch is closed allowing the electrical energy stored on C 0  to ring very quickly through a series of magnetic compression circuits and a voltage transformer to produce high voltage electrical potential in the range of about 16,000 volts (or greater) across the electrodes which produce the discharges which lasts about 20 to 50 ns.  
         [0007]     It is important not only to carefully monitor and control parameters of the laser output light pulse beam, but to deliver that light, e.g., to a manufacturing tool, often located across a room or even on a different floor in a manufacturing facility with proper light parameter characteristics which might be altered by the apparatus and method of such delivery, and therefore need monitoring and perhaps even modification as part of the deliver system and method.  
         [0008]     A BDU system, e.g., can be a purged gas tight system. Periodically module replacement may be required for various reasons. Exposure to open air is detrimental to optics life, i.e., when the purge environment is breached. It is highly desirable to be able to remove and replace an optics module in the BDU system without contaminating (exposing) other modules to open air while the module is being removed and/or replaced. It is further desirable to be able to do such repair/replacement without then having to repurge the entire BDU.  
         [0009]     During, e.g., the fabrication of a BDU, it may become necessary, e.g., to optically align the BDU, including, e.g., its internal modules and/or the input and output ports of the entire BDU with an external alignment methodology. There are occasions where the previous methods of aligning the BDU and/or the BDU modules using a prior, e.g., total station technique may not be feasible, e.g., due to space limitations, i.e., close proximity to other elements f the laser system or surrounding structures or machinery and/or small working areas, e.g., prohibiting open beam alignment.  
         [0010]     The BDU system, e.g., when operating at 193 um wavelength can have a hard time achieving optics element/module long lifetimes, e.g., due to the effects of contamination on the optics. One of the biggest problems confronted by the BDU system design can be, e.g., in-situ beam alignment. In-situ, open beam alignment, e.g., can be very detrimental to optics life.  
         [0011]     Some lasers, e.g., XLA lasers may have uneven spatial coherence, which is a problem that needs to be corrected.  
         [0012]     Long optic life can be extremely important in DUV, VUV and EUV light source and delivery systems, including e.g., excimer or molecular fluorine gas discharge lasers with associated beam delivery units. Requirements for lives exceeding 12B pulses can create extreme need for contamination prevention, detection and control measures in the design, e.g., of the BDU in order to achieve the lifetime requirements.  
       SUMMARY OF THE INVENTION  
       [0013]     A beam delivery unit and method of delivering a laser beam from a laser light source for excimer or molecular fluorine gas discharge laser systems in the DUV and smaller wavelengths is disclosed, which may comprise: a beam delivery enclosure defining an output laser light pulse beam delivery path from an output of a gas discharge laser to an input of a working apparatus employing the light contained in the output laser light pulse beam; a purge mechanism operatively connected to the beam delivery enclosure; an in-situ beam parameter monitor and adjustment mechanism within the enclosure, comprising a retractable beam redirecting optic; a beam analysis mechanism external to the enclosure; and, a retraction mechanism within the enclosure and operable from outside the enclosure and operative to move the retractable beam redirecting optic from a retracted position out of the beam path to an operative position in the beam path. The BDU may also include a beam attenuator unit contained within the enclosure adjustably mounted within the enclosure for positioning within the beam delivery path. The BDU may have at least two enclosure isolation mechanisms comprising a first enclosure isolation mechanism on a first side of the enclosure from the at least one optic module and a second enclosure isolation mechanism on a second side of the enclosure from the at least one optic module, each respective enclosure isolation mechanism comprising a flapper valve having a metal to metal seating mechanism and a locking pin assembly. A precision offset ratchet driver operative to manipulate actuator mechanisms in difficult to reach locations may be provided. An external kinematic alignment tool may be provided. A method of contamination control for a BDU is disclosed comprising selection of allowable materials and fabrication processes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  shows a perspective view of an aspect of an embodiment of the present invention;  
         [0015]      FIG. 2  shows an exploded view of the embodiment of the present invention shown in  FIG. 1 ;  
         [0016]      FIG. 3A  shows a side view of the embodiment of the present invention shown in  FIG. 1 ;  
         [0017]     FIGS.  3 B-E show cross-sectional views of the embodiment of  FIGS. 1-3A  taken along the cross-sectional lines B-B, C-C, D-D and E-E of  FIG. 3A ;  
         [0018]      FIG. 4  shows a schematic view of an exemplary beam delivery unit according to an aspect of an embodiment of the present invention;  
         [0019]      FIGS. 5-7  show external kinematic alignment tool (“EKAT”) according to an aspect of an embodiment of the present invention;  
         [0020]      FIG. 8  shows an aspect of an embodiment of the present invention wherein, e.g., an attenuator can be provided that can, e.g., be moved out of the beam path;  
         [0021]      FIG. 9  shows an aspect of an embodiment of the present invention including a beam homoginizer;  
         [0022]      FIGS. 10A  and B show a shutter unit according to an aspect of an embodiment of the present invention;  
         [0023]      FIG. 10C  shows a shutter pivot pin according to an aspect of an embodiment of the present invention;  
         [0024]      FIGS. 11A  and B show a top view and a cross-sectional side view of a tool according to an aspect of an embodiment of the present invention; 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]     An aspect of an embodiment of the present invention establishes, e.g., proven materials, design configurations, and cleaning processes that when implemented result in achieving required optic lifetimes, e.g., on the order of &gt;12 billion pulses. Optic lifetimes are measured, e.g., in terms of some % of transmission loss. Transmission loss of less than 1% over 12 billion pulses is accomplished according to aspects of an embodiment of the present invention.  
         [0026]     According to another aspect of an embodiment of the present invention there is provided, e.g., a method and apparatus for in-situ optics module replacement without open air contamination of the rest of the BDU system. The module can, e.g., be used wherever necessary to control contamination to the BDU due to potential module repair/maintenance/replacement.  
         [0027]     According to aspects of another embodiment of the present invention beam alignment, power measurements, and beam analysis can be provided without exposure to the atmosphere. Being able to, e.g., perform optical alignment or to perform beam analysis and power measurements without exposing the optics can, e.g., significantly enhance optics lifetime.  
         [0028]     Turning now to  FIG. 1 a  perspective view of an embodiment of the present invention is shown to include a beam delivery unit  20  (shown in FIG. _) beam alignment and inspection unit  22 . The alignment unit  22  may include, e.g., a beam cover  30  including a housing  32 . Mounted on the housing  32  may be, e.g., a light trap assembly  88 .  
         [0029]     Turning now to  FIG. 2  there is shown an exploded view of the beam alignment unit  22 .  FIG. 3A  shows a side view of the embodiment of the invention shown in  FIGS. 1 and 2 . FIGS.  3 B-E show a cross-sectional views of the beam alignment and inspection assembly  22  along the cross-sectional lines A-A, B-B, C-C, D-D and E-E shown in  FIG. 3A .  
         [0030]     Referring to  FIGS. 1-3E  it can be seen that the beam alignment and inspection unit  22  may have a pair of micro-switches  34  mounted on the housing  32 , e.g., by a pair of M2.5-0.45×16 mm socket head cap screws  92 , which respective micro-switch  34  can be used, e.g., to indicate that a beam delivery unit  20  beam enclosure section (not shown in  FIGS. 1-3E  connection flange (not shown) is connected to the respective side of the housing  32 . The unit  22  may also include, e.g., an external pressure 180° 5.002″ square×0.093″ high c-seal  36  providing a sealing connection between the beam cover  30  and the housing  32 . The unit  22  may also include, e.g., an indium gasket  40  having, e.g., a 49.25 mm inner diameter and a 0.265 mm thickness for providing a seal between a, e.g., 50.8 mm diameter by 9.53 mm thick flat optical window  42 , e.g., designed for 193 nm light and being made, e.g., of fused silica. The flat  42  may be held in place by a plurality of washer clips  80  connected to the cover plate  30 , e.g., by respective M3-0.5×4 mm socket head cap screws  82  and M3 split lock washers  80 . The flat window  42  may also be covered by a window cover  84 , which may be attached to the beam cover  30  by a plurality of M4-0.7×8 mm socket head cap screws. This entire assembly may be, e.g., enclosed within the light trap assembly  88  attached to the beam cover  30  by, e.g., a plurality of M6-1×20 mm socket head cap screws  136 . Also attached to the window cover  84  may be a micro-switch  34  that can serve, e.g., to provide, e.g., interlock protection, e.g., to indicate a beam open condition. For example, the microswitch  34  may be positioned such that when an adjacent fixture, e.g., a beam enclosure tube, is placed in position, e.g., where the beam opening cover  136  is in  FIG. 1 , the normally open spring loaded trip switch (not shown) that is part of the micro-switch  34  is moved to the shut position, e.g., by contacting the beam enclosure tube. If the beam enclosure tube is thereafter removed, the micro-switch can open under the spring bias, and be used in a control system, e.g., to prevent laser firing with the potential open beam condition indicated. Also mounted on the beam cover  30  may be a vacuum connection  104  having, e.g., a cover  140  and a VCR gasket retainer assembly  134 . The beam cover  30  may be attached to the housing  32  by a plurality of M6-1×20 mm socket head cap screws  60 .  
         [0031]     The housing may have a first and second beam passage opening  126  and  128 , each of which may be covered for shipment by a respective input port and output port inspection shipping cover  130 ,  132 , each of which may be sealed by, e.g., an inconel 180° 3.131 outer diameter×2.905 inner diameter  718 , tin coated external pressure c-seal  124 . In normal operation, the laser output beam transiting the beam delivery unit  20  may pass through the openings  126 ,  128 .  
         [0032]     Contained within the unit  22  may be a plurality of assemblies, e.g., a beam energy alignment actuator assembly  50 , an alignment tool inspection interface mounting assembly  52  and a beam energy alignment prism mount assembly  54 .  
         [0033]     The alignment tool interface assembly mount  52  may include, e.g., a pair of alignment tool alignment shafts  170  which may be, e.g., press fitted into openings in the assembly mount  52 . The assembly mount may be attached to the interior wall of the housing  32  by a plurality of M4-0.7×8 mm vented/plshd (polished) socket head cap screws  94 . A plurality of dowel pins  172  may be used to hold in place in openings in the mount assembly  52  a respective one of a plurality of magnets  174 .  
         [0034]     The beam energy alignment prism mount assembly  54  may include a total internal reflectance right angle prism  160 , which may be, e.g., 30 mm×30 mm excimer grade fused silica. The prism  160  may be held in place on a 45° angled prism shelf  166  by a spring clip optical mount  162 , which may be attached to the mount assembly  54  by a plurality of vented/plshd M2-0.4×4 mm socket head cap screws  162 . The spring clips, as shown in  FIG. 3E  can extend over a respective corner of the prism  160  to hold the prism  160  in place. Spherical balls  176  forming hemispherical protrusions from the mount assembly  54  may interface with the magnets  174  to ______.  
         [0035]     The beam energy alignment actuator assembly  50  may include, e.g., an actuator assembly L-bracket  180 , which may, e.g., be attached to the inner wall of the beam cover  30  by a pair of vented/plshd M4-0.7×10 mm socket head cap screws  90 . also formed as a part of the L-Bracket may be lead screw inner shelf  186  and a lead screw outer shelf  188 . Each of the inner shelf  186  and outer shelf  188  may have contained therein a respective unshielded radial ball-bearing  182  each held in place in the respective shelf  186 ,  188  by a respective bearing clamp  190 , each of which may be held in place, e.g., by a plurality of vented/plshd M3-0.5×5 mm socket head cap screws  192 . A lead screw  194  may pass through each respective ball bearing  182  and be held in place for rotational movement about the longitudinal axis of the lead screw  194  by an inner and an outer retaining ring  196 .  
         [0036]     Also attached to the actuator assembly  50  may be, e.g., a beam tanslator carrier  200  which may have a bracket portion to which is attached the prism assembly  54  by a plurality of M3-0.5 4 mm D×6 mm socket head cap screws  100  and a traveling member  202  threadedly engaging the lead screw  194 . also guiding the movement of the translator member  200  can be, e.g., a shaft  184  which also passes through an opening in the traveling member  202 .  
         [0037]     The lead screw may be attached through an opening in the beam cover  32  to an external actuator screw  64  which may be connected to one end of a helical flex coupling  204 , which may be attached at each end respectively to the external actuator screw  64  and the lead screw by a respective vented/plshd M3-0.5×16 mm socket head cap screw  206 .  
         [0038]     In operation it can be seen that the external actuator screw may be operated to move the beam translator carrier  200  and the attached prism assembly  54 , e.g., toward the inner shelf  186 , e.g., to position the prism  160  toward the right as shown in  FIG. 3C , positioning the prism in the path of the beam, the center of which is in the cross-hairs shown in  FIG. 3C . This then reflects the bean into the light trap assembly  86  through the window  42 . In the light trap assembly  86 , a first reflective surface  87  reflects the light to a second reflective surface  88 , absorping, e.g., 40% of the light intensity in the process, and the second reflective surface  88  reflects the light to an exit window  89 , absorbing, e.g., 40% of the remaining light intensity in the process and the exit window may also absorb, e.g.,  405  or the remaining light as it exits the light trap assembly  86 .  
         [0039]     It will also be understood that moving the prism  160  in the opposite direction leaves an opening for the beam path, through the opening between the alignment shafts  170  and the opening in the assembly  54 , as shown in  FIG. 3C , between the input port  126  and the output port  128 . It will also be understood that the shipping covers  130  and  132  are removed after shipping and the unit  22  is attached to adjacent sections of the beam delivery unit with the inlet port  126  and outlet port  128  open to allow the laser output beam to pass into and out of the unit  22  as desired during operation or to be deflected for measurement/alignment of the beam.  
         [0040]     Turning Now to  FIGS. 5-7 , there is shown an external kinematic alignment tool (“EKAT”)  252 , which may include EKAT-Laser diode  254 . The EKAT  252  may e.g., be mounted to the input of a 1 st  module  250 ,  250 ′ of a BDU  20 , or, e.g., to the output port  128  of an alignment and measurement unit  22  shown in  FIGS. 1-3E , or between modules as shown in  FIG. 4 . With the EKAT laser diode  254  attached, e.g., to the output side of a 1 st  module, an EKAT-alignment target  256 , can, e.g., be mounted to the input of a 2 nd  recipient module  260 ,  260 ′ of the BDU  20 , e.g., shown also in  FIG. 4 , usually the next BDU module  260  in line, and, e.g., the BDU module  260  can then be adjusted in the X, Y and Z directions until the alignment target  256  is centered to the laser beam of the EKAT-Laser diode  254 . With the EKAT laser diode  254  attached to the output side of the 1 st  module  250 , a small pinhole aperture.  
         [0041]     (not shown) can, e.g., be attached, e.g., to minimize the spot size. An EKAT-reflective mirror  270 , e.g., as shown in  FIGS. 5 and 6  can, e.g., be attached to the recipient BDU module  250 , usually the BDU module next module in line, as noted above, and can be, e.g., rotated about each axis (θX, θY, θZ) to ensure that the laser beam is reflected upon itself. The recipient module  260  can be leveled to gravity, e.g., using a precision level.  
         [0042]     Such a mounting may consist of, e.g., a mounting plate  264 , which may, e.g., be adapted to fit over a module cover plate  210 , e.g., with radial openings  266  to receive fittings on the module  250 , e.g., cover plate set screws  212 , when the mounting plate is attached to&#39;the module cover plate  210 , e.g., by spring loaded attachment screws  263 , which may, e.g., threadedly engage threaded openings  265 , e.g., on the module cover plate  210 . Guide posts  266  may, e.g., extend from one side of the mounting plate  264  and have spherical feet  267 , which may engage openings  214  in the side wall  216  of the module housing  250 , having, e.g., guide magnets that insure proper positioning of the spherical feet  267 . Separators  268  may also be provided.  
         [0043]     The mounting plate  264  may also have, e.g., a plurality of mounting posts  270  to which may be attached an instrument platform  272 . The instrument platform  272  may have a moveable portion  274  which may be attached to the instrument platform by a pivot pin  275 , and a pair of adjustment screws  276  and spring biased against the motion created by the adjustment screws  276  by springs  277 .  
         [0044]     In operation an instrument, e.g., the laser diode  252 , or a target (not shown) of a mirror  270  can be placed in the moveable portion  274  and fixed using the set screw  279 , such that, e.g., the laser diode  252  and be aimed at an adjacent module, e.g.,  260 , e.g., before the enclosure between modules is in place, e.g., to align the adjacent modules  250 ,  260  before putting the enclosure in place. This could be done with a target in the moveable portion of one module and a laser beam from the laser diode, e.g., in a position, e.g., as shown in  FIGS. 5-7  on an opposite side of a module housing  250 , and, e.g., a mirror on an adjacent module  260  housing, with, e.g., the laser diode bean traveling through the target (not shown) and reflecting off of a mirror in the moveable portion f an adjacent module  260  having an instrument platform on its respective mounting plate and the reflected beam also striking the target on the module  250 . The module  250  or  260  may be fixed and not moveable in the system and the other module may be moveable until, e.g., the reflected beam is moved to the center of the target, indicating that the two adjacent modules are aligned. Alternatively, the laser diode may be mounted on a module  250  mounting plate moveable portion  274  and the module  250  aligned to another module by aligning the beam to a target on another adjacent module.  
         [0045]     It will be understood, that in normal operation, the mounting plate  264  and the position of the moveable portion  274  on the instrument platform  272  may be established, e.g., in a perfectly normal relationship, e.g., by using a known calibration jig (not shown) before the mounting plate  264  is used on a module housing  250  for alignment with, e.g., another module housing  260 . That is, once the moveable portion  274  is properly aligned using the calibration jig and attached to a module, e.g.,  250  for alignment with another module, e.g.,  260 , it is the respective modules that are moved (normally one will be fixed in the system and not moveable) to effect the desired target alignment, giving the indication that the modules are then properly aligned. The spacers can, then, serve to protect the instrument platform from contacting other objects causing the jig established alignment of the mounting plate  264  and moveable portion  274  to get out of the established alignment during use.  
         [0046]     In some cases, e.g., with applicants&#39; assignee&#39;s MOPA laser systems, e.g., the XLA series, e.g., there can be an excess of light energy, e.g., early in the life for the system, which can, in fact, actually be detrimental to overall efficiency of the system, e.g., by causing shorter lifetimes, e.g., of certain optical elements, which, therefore, must then be replaced more often. With increasing pulse count the system can, e.g., lose efficiency in other ways, e.g., by causing decreasing transmission of certain optical elements/modules, and or, metrology units, etc. The output power of such systems could, e.g., be kept constant at some level of power output, e.g., by increasing the chamber voltage, however, at some point the voltage reaches a maximum. For this and like reasons modules will need to be replaced.  
         [0047]     Turning now to  FIG. 8  there is shown an embodiment of the present invention wherein, e.g., an attenuator  280  can be provided that can, e.g., be moved out of the beam path. While in the beam path however, the attenuator  280  could, e.g., substantially increase the efficiency of the system, e.g., by about 20% or more later in lifetime, thereby extending module lifetimes and decreasing the overall cost of operation. The attenuator can be, e.g., a beam splitter (BS)  282  located upstream of a beam expander, e.g., mounted in a beam analysis module (“BAM”)  28 , shown in  FIG. 4 . The beam splitter can, e.g., divert a portion of the beam to a beam dump  284 . This BS could consist, e.g., of a thin uncoated CaF2 flat partially reflecting mirror. The two surface Fresnel reflections of such a mirror could, e.g., divert a portion of the beam into the beam dump  284 . It will be understood that the beam splitter  282  partially reflecting mirror could be, e.g., substituted for the prism in the embodiment of  FIGS. 1-3E  and the beam light trap assembly  88  constitute the beam dump  284  of the present embodiment.  
         [0048]     The attenuation efficiency can be controlled, e.g., by the angle of incidence of the laser output bean transversing the BDU  20  on the partially reflecting mirror. According to aspects of this embodiment of the present invention, e.g., when the laser reaches its end of life, with the attenuator  280  in place, the beam splitter  282 , e.g., can be removed, e.g., during a regular field service by removing the covers and removing the entire beam splitter  282  subassembly. Alternatively the BS could be mounted as shown, e.g., in  FIGS. 1-3E  on a translator mechanism, which can be moved in and out of the beam path without breaking the seal integrity of the BDU. The technique/mechanical design used for the alignment tool, discussed above, may, e.g., be used to move the attenuator in and out of the beam. Moving an optical flat out of the beam path could produce, e.g., a small beam shift. This beam shift can be compensated for, e.g., by a translational move of the beam expander plate, which can be, e.g., located just downstream of the attenuator  280 . The dynamic range of the kinematic beam expander mount could cover the motion needed. It may be very important to maintain absolute alignment of the modules and enclosure tubes and their relationship to true horizontal and vertical, e.g., to maintain beam profiles and shapes, which may not be otherwise maintainable, even if, e.g., alignment of the BDU reflecting mirrors keeps the beam on some beam path through the BDU.  
         [0049]     One solution to poor spatial coherence can be, e.g., to reduce spatial coherence of a laser beam by mixing parts of the laser beam. Usually a laser beam has higher intensity in the center and lower intensity at the outer edge. When used properly the suggested approach according to aspects of an embodiment of the present invention can, e.g., homogenize a laser beam, e.g., to make the difference between the center and periphery smaller.  
         [0050]     Turning now to  FIG. 9  there is shown a homoginizer  300 , which may include, e.g., two prisms, e.g., two dove prisms  302   a ,  302   b , known also as beam reversion prisms, attached to each other on the bases  304   a ,  304   b . The beam  306 , one half of which,  306   a  goes through the first dove prism  302   a  and one half of which goes through the second dove prism  302   b , in going into the respective prism  302   a ,  302   b , can be, e.g., totally internal reflected (“TIR”) from the respective base  304   a ,  304 .  
         [0051]     The application of, e.g., some pressure to the environment containing the prisms  302   a ,  302   b , can, e.g., spoil or frustrate the TIR, e.g., producing an effect known as frustrated total internal reflection (“FTIR”). By varying the applied pressure it is possible to change the total internal reflection across a range of from 100% TIR to 0% (total optical contact). The common edge  310  of the two prisms  302   a ,  302   b , can, e.g., be aligned with the beam centerline axis,  312 . In case 100% TIR reflection, the two parts of the beam will be reversed, i.e., including the respective intensity distributions, e.g., as shown in  FIG. 9 . In case of 0% reflection the two parts of the beam will be exchanged between each other with no change of intensity distribution. In case of 50% reflection one part of the beam will be mixed with a reversed other part of the beam. The intensity distribution can, e.g., be homogenized and spatial coherence decreased, because of the mixing up of the different parts of the beam. The applied pressure could be applied and controlled, e.g., through the electrical field, e.g., using a piezoelectric actuator (not shown). In operation therefore, with the two prisms attached to each other and FTIR used to control the optical coupling level between two prisms it is possible, e.g., to obtain the best homogenizing, e.g., by controlling the parts of the beam that are mixed, as opposed to the beam as whole, e.g., as shown in  FIG. 9B . It will be understood that static or dynamic control is contemplated.  
         [0052]     It is also an aspect of an embodiment of the present invention to provide for modular contamination control, e.g., in order to replace/maintain, e.g., optical/metrology elements in a BDU without contaminating the entire BDU. This can be done using shutter arrangements disclosed in the co-pending prior application Ser. Nos. 10/384,967 and 10/425,361, respectively, Attorney Docket Nos. 2003-0005-01 and 2003-0040-02, filed, respectively on Mar. 8, 2003 and Apr. 29, 2003, and assigned to the common assigned of the present application, the disclosures of which are hereby incorporated by reference. Such a shutter arrangement can be as described in more detail below.  
         [0053]     A shutter unit  330 , e.g., as shown in  FIGS. 10A  and B, may be included in a BDU  20 , e.g., on opposite sides of the BDU enclosure from an optical/metrology unit. The shutter  330  may include, e.g., a shutter body  332 , which may conform generally to the shape of the adjacent BDU module, e.g., having a generally circular shape, with a plurality of openings  334 , through which, e.g., attachment bolts may extend to, e.g., attach the shutter unit  330  to a module housing and/or to a beam enclosure tube. The shutter assembly body  332  may have a shutter recess  336 . within the shutter recess  336  may be, e.g., a shutter plate  340  having, e.g., an opening  342 . The shutter opening  342  may be of any selected shape, e.g., rectilinear, e.g., generally square. The shutter opening  342  may have a reverse beveled edge  344  around its periphery.  
         [0054]     The shutter  346  may be of the same size and shape to mate with the shutter opening  342  and have a beveled edge  348  around its periphery which can serve, e.g., to seat the shutter  346  to the shutter opening  342  when the shutter  346  is shut.  
         [0055]     The shutter body  332  may have an actuator flange  350  to which may be attached or integrally formed an actuator shelf  352 . An actuator mounting plate  354  may be attached to the actuator shelf  352 . An actuator in the form of a shutter flip knob  360  may be attached, e.g., by a pair of set screws  362  to a shutter pivot pin  364 . The shutter pivot pin  364  may have, e.g., a wider diameter portion  366  and an smaller diameter portion  368 , as shown in  FIG. 10C .  
         [0056]     The wider diameter portion may extend into an opening in the flip knob  360  and be engaged by the set screws  362 . The wider diameter portion  366  may also have an opening  370  at the upper end thereof with a fitting  372 , e.g., an allen wrench fitting, for adjusting the shut position of the shutter  346  in relation to the position of the flip knob  360  in a shut position of the flip knob  360 .  
         [0057]     The shutter may have shutter plate  380  having an opening therethrough (not shown) to receive the narrower diameter portion  368  of the shutter pivot pin  364 , which may be held in place with respect to the shutter plate  380 , e.g., by a set screw  382 .  
         [0058]     The actuator mounting plate  354  may have a first locking hole (not shown) aligned with a detent  374  on the end of a spring loaded locking pin  384  when in an open position of the shutter  346 , as shown in  FIGS. 10A  and B, and in a second locking hole (not shown) when in a shut position of the shutter  346 .  
         [0059]     In operation the shutter  346  may normally be in the open position as illustrated in  FIGS. 10A  and B. when desired, e.g., to isolate an adjacent BDU module, including, e.g., a beam enclosure transit portion, e.g.,  250 ,  250 ′ shown in  FIG. 4 , an optical module, e.g.,  260  shown in  FIG. 4 , or a metrology module, e.g., BAM  28  ( 260 ′) shown in  FIG. 4 , the operator can, e.g., pull out the spring loaded locking pin  384  and move the shutter flip knob  360  ninety degrees to the other locking hole (not shown) to close the shutter  346  and seat it with the shutter plate  340 , with the beveled edges  344 ,  348  respectively on the shutter  346  and the shutter plate  340  sealing the adjacent portion of the BDU  20  while work is done on the portion isolated by the shutter(s). It will also be understood that the shutter unit  330  may be placed to have a higher pressure on the side of the shutter recess  336  in order to assist in seating the shutter  346  against the shutter plate  340 , e.g., if the BDU  20  is normally under pressure, the recess could face the pressurized portion of the BDU  20  while, e.g., maintenance is done on an adjacent module and if the BDU  20  is normally under a vacuum, vice-versa.  
         [0060]     Turning now to  FIGS. 11A  and B there is shown a top view and a cross-sectional side view of a precision offset ratchet driver  400  according to an aspect of an embodiment of the present invention. This tool has been rendered a necessity due to, e.g., placement of, e.g., allen-type screws and bolts and other like actuators, in, e.g., difficult or almost impossible places to reach within the BDU and its various modules and even interconnections between them. According to an aspect of an embodiment f the present invention the tool  400  can, e.g., enable a user to adjust, e.g., such a screw or bolt or the like in hidden or difficult to reach areas.  
         [0061]     The offset ratchet tool  400  can, e.g., be driven using a ratchet driver, e.g., an ordinary socket wrench attachment, which can, e.g., extend into a ratchet opening  404  in a rotatable ratchet spool  404 . The spool  404  may be provided with a sprocketed locking mechanism (not shown) of the socket wrench locking feature may be employed. A bit (not shown) may be, e.g., snapped into a rotatable tool chuck  408  at the end of an extension shaft of the tool  400 . The too chuck  408  may be driven, e.g., by a continuous cable (not shown) looped around the rotatable chuck  408  and the ratchet spool  404 . Tension on this cable may hold the tool  400  together, or other means may be employed to hold the spool  404  and chuck  408  on the extension shaft  406 . The cable may pass inside of a hollowed portion f the shaft  408  or in a grooved exterior wall (not shown).  
         [0062]     Applicants, according to aspects of an embodiment of the present invention, have established proven materials and design configurations as well as prohibited materials that in conjunction lead to longer optic lifetimes for, e.g., BDUs, e.g., in VUV, DUV and EUV applications. Cleaning processes are also disclosed that enhance optic lifetimes. Such lifetimes are measured, at least in part, by percentage of transmission loss over live, e.g. to  12  pulses and beyond. applicants have created a regime that will result in 1% or less loss of transmission in, e.g., the BDU optics over 12B pulses and above.  
         [0063]     applicants have determined that to achieve the above mentioned objectives for optical lifetimes in, e.g., BDUs that metals for fabrication of the BDU should be exclusively selected from the groups listed below:  
         [0064]     Bare metals, i.e., not plated or otherwise finished metals: 
        Al, Cu, Sn, In, Ni, and alloys of brass, stainless steel and invar.        
 
         [0066]     Metals for Fabrication: 
        For Enclosures:     6061-T651 al, per ASME SB-209 or SB-211, with conformance certification required;     Others than enclosures (except seals):     Al as defined above and stainless steel type UNS 302 (UNS 30200), UNS 304 (UNS 30400) and 316L (UNS 31600) per ASTM A276.     Metal finishes:     Surface finishes: 
            For all purged surfaces, including surfaces exposed to UV—must be at least 32 Ra before plating.    
            Nickel plating: 
            Use of Electro-less plating which is denominated by class, depending on thermal treatment applied subsequent to plating, from, e.g., no treatment, treatment for required hardness, non-heat treatable aluminum alloys processed to improve adhesion of the nickel and heat treatable aluminum alloys processed to improve adhesion of the nickel, and according to grade, i.e., thickness of the nickel coatings, e.g., from 0.0010-0.0015″, to 0.0005-0.0010″ to 0.0015″ minimum thickness.    
            Thin dense chrome plating: 
            Allowable in pure 99.99% form but not to be used for electrical conductivity.    
            Welding/soldering:     Welding: 
            Filler-less, fusion of base metals only.    
            Soldering: 
            Flux-less, tin and lead only.    
            Miscellaneous approved materials:     Glasses: 
            Pyrex, Fused silica, Schott filter glasses, ultra low expansion (“ULE”) glasses and Zerodur.    
            Crystals: 
            Calcium fluoride, Magnesium Fluoride, quartz and Saphire.    
            Ceramics: 
            High density (non-porous) ceramics, e.g., Alumina (99% A 2 O 3 ), Lucalux, Silicon Nitride and Lead Zirconate Titanate.    
            VUV foil, e.g., for UV shielding meeting the requirements of all foils being certified UHV foil according to the American Society for Testing and Materials (“ASTM”) B479 designation and 3.1.4 and 10.3.1 standards.     Additional materials permitted provided sully shielded from UV light:     Krytox, Teflon and Nylon, and Epoxy, Scott Weld 2216, made by 3M.        
 
         [0093]     Applicants have also discovered that required design practices include the elimination of virtual leaks and features that are difficult to clean. Blind holes, e.g., must be vented or used in conjunction with vented fastener hardware. Fasteners must be precision cleaned nickel plated stainless steel vented fasteners rated to High Vacuum equipment, e.g., as supplied by U-C Components, Inc. or the equivalent. Design features that are prone to trapping particulate contamination are to be avoided if at all possible, including, e.g., blind holes, rough surfaces, surface interfaces that can propagate liquids, rivets, etc. Particular attention must be provided to such elements as optical mounts and translation stages/mechanisms for which lubricants and other disallowed materials are not permitted.  
         [0094]     The BDU must also be free of purge volume dead zones, i.e., the elimination to the greatest extent possible of volumes where air/purge gas may become trapped and not circulated.  
         [0095]     All fabricated parts must also be cleaned for VUV applications using a precision cleaning process prior to assembly. Verification of the details of the cleaning procedure must be provided and obtained before assembly. A precision cleaning process meeting the requirements of must be used, such that, contamination (unwanted material) is minimized to prevent interference with proper operations, which is more stringent than just visually clean (absent all particulate and non-particulate matter visible to the unaided eye and/or corrected vision), and is required to any “significant surface” which means any surface of an item or product that is required to meet established cleanliness levels, and establishes maximum amounts of contamination allowable, either by area or volume or per component. Precision cleanliness levels are required, meaning that levels may be described as “quantitative,” since particulate coujts, with or without nonvolatile residue (“NVR”) or other suitable film/non-particulate method limits may be required.  
         [0096]     All fabrication process cycles must be performed under control procedures suitable for VUV opto-mechanical parts fabrication meeting the requirements of acceptable processes and materials such that cleanliness standards are met; packaging requirements to, e.g., preserve such cleanliness requirements; and maintain inspection and detection minimums to preserve such cleanliness and also tests to, e.g., measure and determine out-gassing and the like.  
         [0097]     It will be understood by those skilled in the art that many variations and modifications of the above described preferred embodiment(s) of the present invention and aspects thereof can be made without departing from the scope and intent of the appended claims and the invention should be interpreted in scope and meaning only from the appended claims and not limited to any particular preferred embodiment (s) or aspect thereof. For example, purging can be accomplished with a pressurized system, e.g., with an inert gas like N 2  or a vacuum system, or combinations thereof in different modules that may be isolated from each other by a pressure resistant interface, e.g., an optical element, and the term purged shall be interpreted to include, e.g., either such pressurized or vacuum arrangements.