Abstract:
An excimer or molecular fluorine laser includes a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, and a resonator including the discharge chamber and a pair of resonator reflectors for generating an output laser beam. One of the resonator reflectors is an output coupling interferometer including a pair of opposing reflecting surfaces tuned to produce a reflectivity maximum at a selected wavelength for narrowing a linewidth of the output laser beam. One of the pair of opposing reflecting surfaces is configured such that the opposing reflecting surfaces of the interferometer have a varying optical distance therebetween over an incident beam cross-section which serves to suppress outer portions of the reflectivity maximum to reduce spectral purity. Preferably, this surface is non-planar, and may include a step, a recess or a raised or recessed curved portion of a quarter wavelength in height or depth, respectively.

Description:
PRIORITY  
       [0001]    This Application is a rule 53(b) continuation application which claims the benefit of priority to U.S. patent application Ser. No. 09/715,803, filed Nov. 17, 2000, which claims the benefit of priority to U.S. Provisional Patent Applications Nos. 60/195,169, filed Apr. 6, 2000, 60/166,854, filed Nov. 22, 1999, and 60/166,277, filed Nov. 18, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to narrow band lasers and particularly to an excimer or molecular fluorine laser having output coupling interferometer.  
           [0004]    2. Discussion of the Related Art  
           [0005]    Narrow band excimer lasers (λ193 nm, 248 nm) are applied in photolithographic applications for production of integrated circuits. Excimer laser radiation is used for making structures in the dimensional range of &lt;0.18-0.25 μm (KrF-laser radiation) or &lt;0.13-0.18 μm (ArF-laser radiation). The molecular fluorine laser emitting around 157 nm (F 2 -laser) is being developed for feature sizes &lt;0.13 μm. Achromatic imaging optics are difficult to produce for this wavelength region. For this reason radiation of narrow bandwidth is desired to control imaging errors caused by chromatic aberration. Acceptable bandwidths are typically less than 0.6 pm.  
           [0006]    Another important beam parameter is the spectral purity, or the bandwidth which contains 95% of the output pulse energy. High numerical aperture (NA) optics use &lt;1 pm bandwidth radiation. This can achieved by using of two spectral narrowing elements such as a grating and intracavity etalon or etalon output coupler.  
           [0007]    Etalon outcoupling mirrors have been used for a long time and in various different types of lasers. A simple example of a plane-plane cavity for an excimer laser can be formed by a highly reflective (HR) back-mirror and an uncoated solid etalon as an outcoupling resonator reflector.  
           [0008]    U.S. Pat. No. 5,901,163 and 5,856,991 each to Ershov relate to a resonator including an etalon output coupler for a narrow band excimer laser, as shown in FIG. 1 (which is FIG. 3 of the &#39;991 patent). The resonator consists of a line narrowing module ( 18 ) consisting of an echelle grating and a prism beam expander, and a plane-parallel air spaced etalon ( 44 ) as an outcoupling mirror.  
           [0009]    The echelle grating based line narrowing module produces a laser beam having a spatial variation in wavelength (chirp) along a beam cross section direction (direction of dispersion). FIG. 2 shows a typical spatial distribution of a laser spectrum across the beam created by the grating. The laser resonator used for generating the spectrum in FIG. 2 consists of an echelle grating, prism beam expander and a typical partially reflecting outcoupling mirror having a reflectivity of, e.g., 20-25 %.  
           [0010]    Thus, for the arrangement of FIG. 1, the line narrowing module ( 18 ) provides a spatial distribution of wavelengths at the outcoupling etalon that is approximately given by: 
           λ( x )=λ(0)+( dλ/dx ) x   (equation 1); 
           [0011]    where x is the coordinate along the short beam axis, and x=0 is the beam center. For the example depicted in FIG. 2, the “spatial chirp” is dλ/dx≈0.83 pm/mm. This value depends on the linear dispersion of the echelle grating and the laser design (i.e., the distance between the grating and outcoupling etalon, the discharge width, etc.).  
           [0012]    [0012]FIGS. 3 a ,  3   b  show two calculated spatial distributions of laser spectra for two different gratings (dλ/dx=0.83 pm/mm and 1.24 pm/mm), an airspaced plane-parallel uncoated etalon with FSR=1.6 pm as outcoupler and otherwise the same resonator designs. FIG. 3 c  shows the measured spectrum for a grating with dλ/dx=1.24 pm/mm and an outcoupler etalon with FSR=1.6 pm. The calculations are in a good agreement with the experimental findings (i.e., compare FIGS. 3 b  and  3   c ).  
           [0013]    To avoid “side modes” the following relation is fulfilled: 
           ( dλ/dx )·≦0.5  FSR   (equation 2); 
           [0014]    where b is the beam width in front of the etalon. Higher values for dλ/dx can be achieved by using more highly dispersive gratings, or bending the grating such as is disclosed in U.S. Pat. No. 5,095,492 to Sandstrom. As it is desired to produce still smaller structures on silicon substrates, it is desired to further reduce the spectral purity of excimer laser exposure beams.  
         SUMMARY OF THE INVENTION  
         [0015]    It is therefore an object of the invention to provide a narrow band excimer or molecular fluorine laser having improved spectral purity.  
           [0016]    In accordance with this object, an excimer or molecular fluorine laser is provided including a discharge chamber filled with a gas mixture, multiple electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, and a resonator including the discharge chamber and a pair of resonator reflectors for generating an output laser beam. One of the resonator reflectors is an output coupling interferometer including a pair of opposing reflecting surfaces tuned to produce a reflectivity maximum at a selected wavelength for narrowing a linewidth of the output laser beam.  
           [0017]    In a first aspect of the invention, one of the pair of opposing reflecting surfaces is configured such that the opposing reflecting surfaces of the interferometer have a varying optical distance therebetween over an incident beam cross-section which serves to suppress outer portions of the reflectivity maximum to reduce spectral purity. Preferably, this surface is non-planar, and may include a step, a recess or a raised or recessed curved portion of a quarter wavelength in height or depth, respectively.  
           [0018]    In a second aspect of the invention, the laser includes a first photodetector and a beam splitter. The beam splitter is positioned to reflect a portion of the beam reflected from the output coupling interferometer to the photodetector. The interferometer is tuned substantially to a maximum intensity of interference fringes reflecting therefrom. Preferably, a second photodetector and a second beam splitter are positioned to monitor the beam transmitted through the output coupling interferometer. Information detected at the second photodetector is used by a processor for maximizing an energy stability of the transmitted beam.  
           [0019]    In a third aspect of the invention, an etalon spectrometer is positioned to detect spectral information of the beam transmitting through the output coupling interferometer. The output coupling interferometer is tuned to produce a maximum intensity of interference fringes of the etalon spectrometer.  
           [0020]    In a fourth aspect of the invention, a position sensitive photodetector and a beam splitter are included. The beam splitter is positioned to reflect a portion of the beam reflected from the output coupling interferometer to the position sensitive photodetector. The output coupling interferometer is tuned substantially to a maximum intensity of reflection interference fringes. Preferably, a second photodetector and a second beam splitter are also included, wherein information is detected at the second photodetector of the beam transmitted through the interferometer and used by a processor for maximizing energy stability of the transmitted beam.  
           [0021]    In a fifth aspect of the invention, the output coupling interferometer is disposed within a housing. A pressure control unit controls a pressure within the housing and between the first and second opposing reflecting surfaces of the interferometer. The pressure control unit preferably included an inert gas filled bellows fluidly coupled with the housing. The interior volume of the bellows is adjustable for adjusting the pressure within the housing and between the first and second opposing reflecting surfaces of the output coupling interferometer.  
           [0022]    In a sixth aspect of the invention, a beam expander is disposed before the output coupling interferometer. The beam expander reduced the divergence of the beam incident at the interferometer, the resolution of the interferometer is improved, and the spectral purity is improved in accord with the object of the invention. The beam expander may include one or more beam expanding prisms or a lens arrangement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 schematically shows a conventional narrow band excimer laser including a grating-based line-narrowing module and an etalon output coupler.  
         [0024]    [0024]FIG. 2 shows a spatial distribution of a laser spectrum for a narrow band laser having a conventional partially reflecting output coupling mirror.  
         [0025]    [0025]FIG. 3 a  shows a first calculated spectrum of an output beam of a conventional KrF excimer laser having a grating and an etalon output coupler.  
         [0026]    [0026]FIG. 3 b  shows a second calculated spectrum of an output beam of a conventional KrF excimer laser similarly configured as that for calculating the spectrum of FIG. 3 a , except that it has a grating with higher spatial dispersion.  
         [0027]    [0027]FIG. 3 c  shows a measured spectrum of an output beam of a conventional KrF excimer laser having the grating of FIG. 3 b.    
         [0028]    [0028]FIG. 4 a  schematically shows a first output coupling interferometer in accord with a first embodiment.  
         [0029]    [0029]FIG. 4 b  schematically shows a second output coupling interferometer in accord with a second embodiment.  
         [0030]    [0030]FIG. 4 c  shows a calculated spectrum of a KrF excimer laser including the first output coupling interferometer of FIG. 4 a.    
         [0031]    [0031]FIG. 5 a  schematically shows a third output coupling interferometer in accord with a third embodiment.  
         [0032]    [0032]FIG. 5 b  schematically shows a fourth output coupling interferometer in accord with a fourth embodiment.  
         [0033]    [0033]FIG. 5 c  shows a calculated spectrum of a KrF excimer laser including the third output coupling interferometer of FIG. 5 a.    
         [0034]    [0034]FIG. 6 a  schematically shows a fifth output coupling interferometer in accord with a fifth embodiment.  
         [0035]    [0035]FIG. 6 b  schematically shows a sixth output coupling interferometer in accord with a sixth embodiment.  
         [0036]    [0036]FIG. 6 c  shows a calculated spectrum of a KrF excimer laser including the fifth output coupling interferometer of FIG. 5 a.    
         [0037]    [0037]FIG. 7 schematically shows an excimer or molecular fluorine laser resonator including an output coupling interferometer in accord with any of the first through sixth embodiments, and further including a control unit for tuning the interferometer.  
         [0038]    [0038]FIG. 8 a  schematically shows a first preferred embodiment of the control unit of FIG. 7.  
         [0039]    [0039]FIG. 8 b  schematically shows a second preferred embodiment of the control unit of FIG. 7.  
         [0040]    [0040]FIG. 9 a  schematically shows a third preferred embodiment of the control unit of FIG. 7.  
         [0041]    [0041]FIG. 9 b  schematically shows a fourth preferred embodiment of the control unit of FIG. 7.  
         [0042]    [0042]FIG. 10 a  schematically shows a fifth preferred embodiment of the control unit of FIG. 7.  
         [0043]    [0043]FIG. 10 b  schematically shows a sixth preferred embodiment of the control unit of FIG. 7.  
         [0044]    [0044]FIG. 11 shows a pressure control unit for a pressure tuned output coupling interferometer in accord with a preferred embodiment.  
         [0045]    [0045]FIG. 12 shows an excimer or molecular fluorine laser system in accord with a preferred embodiment. 
     
    
     INCORPORATION BY REFERENCE  
       [0046]    What follows is a cite list of references each of which is, in addition to those references cited above and below, and including that which is described in the related art description and in the priority section, and the above invention summary, and the abstract below, are hereby incorporated by reference into the detailed description of the preferred embodiment below, as disclosing alternative embodiments of elements or features of the preferred embodiments not otherwise set forth in detail below. A single one or a combination of two or more of these references may be consulted to obtain a variation of the preferred embodiments described in the detailed description below and within the scope of the present invention. Further patent, patent application and non-patent references are cited in the written description and are also incorporated by reference into the detailed description of the preferred embodiment with the same effect as just described with respect to the following references:  
         [0047]    German Utility Model No. 299 07 349.1;  
         [0048]    U.S. Pat. No. 5,901,163, 5,856,991, 6,028,879, 5,559,816, 4,977,563, 4,611,270, 6,061,382, 5,406,571, 5,852,627, 3,609,856, 5,095,492 3,471,800, 3,546,622, 5,440,574, and 5,479,431;  
         [0049]    Japanese patents no. 8-274399, 2-152288, 60-16479, and 62-160783;  
         [0050]    S. Marcus, Cavity Dumping and Coupling Modulation of an Etalon-Coupled CO 2  Laser, J. Appl. Phys., Vol. 63, No. 9 (September 1982);  
         [0051]    H. Lengfellner, Generation of Tunable Pulsed Microwave Radiation by Nonlinear Interaction of Nd:YAG Laser Radiation in GaP Crystals, Optics Letters, Vol. 12, No. 3 (March 1987);  
         [0052]    W. Born and E. Wolf, Principles of Optics, at p. 325, Pergamon (1970);  
         [0053]    Shaw, Excimer Laser Resonators, Physics and Technology of Laser Resonators, at pp237-245, Bristol New York (1989)  
         [0054]    Magni, Resonators with Variable Reflectivity Mirrors, in Shaw, at pp.94-105, see above;  
         [0055]    Giuri et al, Appl. Opt. 26,1143 (1997);  
         [0056]    U.S. patent applications Ser. No. 60/178,445, 09/317,527, 09/317,695, 09/130,277, 09/244,554, 09/454,803, 60/212,183, 09/657,396, 09/484,818, 09/599,130, 09/602,184, 09/629,256, 60/173,993, 60/166,967, 60/170,919, 60/200,163, 60/215,933 and 60/235,116, each of which is assigned to the same assignee as the present application.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]    The preferred embodiments below describe an output coupling interferometer designed to suppress the “side modes” of an excimer or molecular fluorine laser output beam. The preferred embodiments describe preferred outcoupling devices that serve as a combination of a spectral purity reducing optical element and a divergency reducing optical element. The effect of the output coupling interferometric devices described below can be mathematically derived by modifying calculations relating to an air-spaced optical etalon. Such an etalon comprises a pair of plane-parallel reflecting surfaces separated by an air gap (wherein inert gases typically reside in the “air” gap). The reflectivity formula for an air-spaced etalon can be found in physics textbooks like Born “Optics”: 
           R= 4 R   0 sin 2 δ/((1 −R   0 ) 2 +4 R   0 sin 2 δ)  (equation 3); 
         [0058]    where δ=(2π/λ)d, d is the spacer thickness or separation between the plates of the etalon, and R 0  is the reflectivity of the etalon mirrors.  
         [0059]    Maximum reflectivity is achieved for δ=(m±½)π or (m±½) λ/2=d 1 . Reflectivity R=0 is achieved for δ=mπ or m λ/2=d 2  (m-integer). The plate separation difference for these two cases is given by d 1 −d 2 =±λ/4. Etalon plates can also have curved reflecting surfaces as long as their separation always remains constant (see S. Marcus, J. Appl. Phys. Vol. 53, No. 9 at 6029 (September 1982). In this case, the spacing between the two etalon plates at the center of the beam cross section x=0 and across the spatial extent of the plates may be set to get R=R max . The arrangement can be modified from this etalon configuration where the spacing remains constant over the spatial extent of the plates such that the spacing is varied. In this modified interferometric configuration, at some lateral position “x” where the spacing decreases (or increases) by λ/4 (or the phase changes by ±π/2), the reflectivity R is zero.  
         [0060]    Now we can take into consideration the wavelength chirp (dλ/dx≠0). We set the maximum reflectivity at the beam center x=0 and zero reflectivity at that x-position where according to equation 1, λ(x)−λ(0)≈ FSR, or free spectral range of the etalon (measured in wavelength units). We get: 
           d ( x )/(1 +FSR/ λ(0))− d (0)=±λ(0)/4  (equation 4) 
         [0061]    or a phase change of δ(x)/(1+FSR/λ(0))−δ(0)=±π/2. Some outcoupling interferometer arrangements can be designed by which equation 4 can be fulfilled as these are described below with reference to FIGS. 4 a - 6   c , below.  
         [0062]    In preferred embodiments, the material of the plates of the etalon of the present invention is calcium fluoride, magnesium fluoride, and/or fused silica, and alternatively barium fluoride, lithium fluoride and strontium fluoride.  
         [0063]    In an alternative embodiment, a beam expander may be installed before the interferometers of any of the preferred embodiments described below to expand the beam in the x-direction. Such a beam expander may be one or more prisms or a pair of lenses. The inner surfaces of the etalon can be specially shaped using reactive ion etching (RIE), or simply ion beam etching.  
       Preferred Embodiment A  
       [0064]    Preferred embodiment A is shown at FIGS. 4 a  and  4   b . FIG. 4 a  shows an interferometer for use as an output coupling reflector for an excimer or molecular fluorine laser. The interferometer includes a pair of optical blocks  2  and  4 . Reflecting surface  6  of optical block  2  opposes a central reflecting step surface  8  of optical block  4 . Reflecting surface  6  of optical block  2  also opposes reflecting side surfaces  10  of optical block  4 . The gap spacing d between central reflecting step surface  8  and the reflecting surface  6  is set for maximum reflectivity of an the excimer or molecular fluorine laser beam having a predetermined wavelength. The gap spacing is d+λ/4 between the reflecting surface  6  and the outer reflecting surfaces  10 .  
         [0065]    Alternative arrangements may include optical block  2  having the λ/4 step and optical block  4  having a planar reflecting surface, or each of optical blocks  2  and  4  having a λ/8 step. Other such alternative arrangements would be understood by those skilled in the art whereby the optical path difference between the reflectivity maximum portions and the reflectivity zero or suppressed portion or portions is λ/4.  
         [0066]    [0066]FIG. 4 b  shows an alternative interferometer for use as an output coupling reflector for an excimer or molecular fluorine laser to that shown and described with respect to FIG. 4 a . The interferometer of FIG. 4 b  includes a pair of optical blocks  2  and  12 . Reflecting surface  6  of optical block  2  opposes a central reflecting recess surface  14  of optical block  12 . Reflecting surface  6  of optical block  2  also opposes reflecting side surfaces  14  of optical block  12 . The gap spacing d between central reflecting recess surface  14  and the reflecting surface  6  is set for maximum reflectivity of an the excimer or molecular fluorine laser beam having a predetermined wavelength. The gap spacing is d−λ/4 between the reflecting surface  6  and the outer reflecting surfaces  16 .  
         [0067]    Again, alternative arrangements may include optical block  2  having the λ/4 recess and optical block  12  having a planar reflecting surface, or each of optical blocks  2  and  12  having a λ/8 recess. Other such alternative arrangements are possible whereby the optical path difference between the reflectivity maximum portions and the reflectivity zero or suppressed portion or portions is λ/4.  
         [0068]    Thus, preferred embodiment A is characterized in that the optical path length near the beam center differs by that at the outer regions by ±λ/4. Preferably, this is achieved by forming additional coatings at the center region. the substrate of the blocks  2 ,  4  and/or  12  themselves may be formed with the step(s) or recess(es) described above. Other methods for providing step profiles are described at Giuri (see citation above) such as for use in unstable resonators.  
         [0069]    The output coupling interferometer of preferred embodiment A exemplified at FIGS. 4 a - 4   b  preferably includes external surfaces of blocks  2 ,  4  and  12  with antireflection (AR) coatings. As mentioned, many modifications within the scope of preferred embodiment A are conceivable (e.g., two inner surfaces having equal phase jumps of π/4, or otherwise adding up to phase jumps of π/2 from the outer regions).  
         [0070]    The calculated spectrum of a narrow band KrF-laser using an output coupling interferometer according preferred embodiment A is shown at FIG. 4 c . The calculations provide a rough picture of the spectrum of the output beam that could be expected. Advantageously, sidebands are suppressed at the boundaries between the central surface  8  or  14  of FIGS. 4 a  and  4   b  to the side surfaces  10  and  16 , respectively, thereby reducing the spectral purity of the beam in accordance with the object of the invention.  
         [0071]    The following calculation is described for obtaining the spectrum shown at FIG. 4 c  for the embodiments of FIGS. 4 a - 4   b:    
         [0072]    We set maximum reflectivity R max  at x=0 
               ⇒       (     m   ±     1   2       )            λ        (   0   )       2         =     d        (   0   )               (   a   )                               
 
         [0073]    and R=0, e.g., at the position x where e.g., λ(x)=λ(0)+FSR  
                     ⇒                m            λ        (   x   )       +   FSR     2         =     d        (   x   )                                      m                   λ        (   0   )         2          (     1   +     FSR     λ        (   0   )           )       =     d        (   x   )                       (   b   )                                      m                   λ        (   0   )         2     =       d        (   x   )         (     1   +     FSR     λ        (   0   )           )                                    (   c   )          -          (   a   )                   gives                 the                 requirement                                    d        (   x   )         1   +     FSR     λ        (   0   )             -     d        (   0   )         =     ±       λ        (   0   )       4                       (   c   )                               
 
       Preferred Embodiment B  
       [0074]    The preferred embodiment B is depicted at FIGS. 5 a  and  5   b . FIG. 5 a  shows an interferometer for use as an output coupling reflector for an excimer or molecular fluorine laser. The interferometer includes a pair of optical blocks  18  and  20 . Reflecting surface  22  of optical block  18  opposes a curved, raised central reflecting surface  24  of optical block  20 . The curved, raised surface  24  is preferably symmetrical about its center and is further preferably Gaussian in shape. Reflecting surface  22  of optical block  18  also opposes reflecting side surfaces  26  of optical block  20 . The gap spacing d between the center, or peak, of curved, raised central reflecting surface  24  and the reflecting surface  22  is preferably set for maximum reflectivity of an the excimer or molecular fluorine laser beam having a predetermined wavelength. The gap spacing reduces gradually over the curved surface  24  until the gap spacing becomes d+λ/4 between the reflecting surface  22  and the outer reflecting surfaces  26 .  
         [0075]    Alternative arrangements may include optical block  18  having the λ/4 curved, raised portion and optical block  20  having a planar reflecting surface, or each of optical blocks  18  and  20  having opposed λ/8 curved, raised portions. Other such alternative arrangements would be understood by those skilled in the art whereby the optical path difference between the reflectivity maximum portions at the peaks of the curved, raised portions and the reflectivity zero or suppressed portion or portions is λ/4.  
         [0076]    [0076]FIG. 5 b  shows an alternative interferometer for use as an output coupling reflector for an excimer or molecular fluorine laser to that shown and described with respect to FIG. 5 a . The interferometer of FIG. 5 b  includes a pair of optical blocks  28  and  30 . Reflecting surface  32  of optical block  28  opposes a central reflecting curved, recess surface  34  of optical block  30 . The curved, recessed surface  34  is preferably symmetrical about its center and is further preferably Gaussian-shaped. The gap spacing increases along curved recessed portion  34  until reflecting surface  32  of optical block  28  also opposes reflecting side surfaces  36  of optical block  30 . The gap spacing d between central reflecting recess surface  34  and the reflecting surface  32  is set for maximum reflectivity at its maximum depth preferably at its center of an the excimer or molecular fluorine laser beam having a predetermined wavelength. The gap spacing gradually reduces along the recess surface  34  until the gap spacing becomes d−λ/4 between the reflecting surface  32  and the outer reflecting surfaces  36 .  
         [0077]    Again, alternative arrangements may include optical block  28  having the λ/4 curved, recess portion and optical block  30  having a planar reflecting surface, or each of optical blocks  28  and  30  having a recess with maximum depth being λ/8. Other such alternative arrangements are possible whereby the optical path difference between the reflectivity maximum portions and the reflectivity zero or suppressed portion or portions is λ/4 preferably at the point of maximum depth or depths of the recess or recesses.  
         [0078]    Thus, preferred embodiment B is characterized in that the optical path length near the beam center differs by that at the outer regions by ±λ/4. The path difference gradually makes this transition between the center and outer regions of the interferometer. The output coupling interferometer of preferred embodiment B exemplified at FIGS. 5 a - 5   b  preferably includes external surfaces of blocks  18 ,  20 ,  28  and  30  with antireflection (AR) coatings. As mentioned, many modifications within the scope of preferred embodiment B are conceivable (e.g., two inner surfaces having equal phase jumps of π/4 at their maximum or minimum gap spacing, or otherwise adding up to phase jumps of π/2 at their maximum or minimum gap spacing from the outer regions).  
         [0079]    The calculated spectrum of a narrow band KrF-laser using an output coupling interferometer according to preferred embodiment B is shown at FIG. 5 c . The calculations provide a rough picture of the spectrum of the output beam that could be expected. The calculations follow those provided above for preferred embodiment A, but are modified according to the curvature of the raised or recess curved portions of the interferometer blocks. Advantageously, sidebands are suppressed as the gap spacing changes from the peak at R=maximum out to the boundaries between the central surface  24  or  34  of FIGS. 5 a  and  5   b  and the side surfaces  26  and  36 , respectively, thereby reducing the spectral purity of the beam in accordance with the object of the invention.  
       Preferred embodiment C  
       [0080]    The preferred embodiment C is depicted at FIGS. 6 a  and  6   b . FIG. 6 a  shows an interferometer for use as an output coupling reflector for an excimer or molecular fluorine laser. The interferometer includes a pair of optical blocks  38  and  40 . Reflecting surface  42  of optical block  38  opposes a curved, raised central reflecting surface  44  of optical block  40 . The curved, raised surface  44  is preferably symmetrical about its center and further is preferably cylindrically shaped. Reflecting surface  42  of optical block  38  also opposes reflecting side surfaces  46  of optical block  40 . The gap spacing d between the center, or peak, of curved, raised cylindrical reflecting surface  44  and the reflecting surface  42  is preferably set for maximum reflectivity of an the excimer or molecular fluorine laser beam having a predetermined wavelength. The gap spacing reduces gradually away from the center of the cylindrically-curved surface  44  until the gap spacing becomes d+λ/4 between the reflecting surface  42  and the outer reflecting surfaces  46 , where the reflectivity of the interferometer is substantially zero.  
         [0081]    Alternative arrangements may include optical block  38  having the λ/4 curved, raised cylindrical central portion and optical block  40  having a planar reflecting surface, or each of optical blocks  38  and  40  having opposed λ/8 curved, raised and opposed cylindrical portions. Other such alternative arrangements would be understood by those skilled in the art whereby the optical path difference between the reflectivity maximum portions at the peaks of the curved, raised portions and the reflectivity zero or suppressed portion or portions is λ/4.  
         [0082]    [0082]FIG. 6 b  shows an alternative interferometer for use as an output coupling reflector for an excimer or molecular fluorine laser to that shown and described with respect to FIG. 6 a . The interferometer of FIG. 6 b  includes a pair of optical blocks  38  and  48 . Reflecting surface  50  of optical block  38  opposes a central reflecting curved, recessed surface  52  of optical block  48 . The curved, recessed surface is preferably symmetrical about its center and further is preferably cylindrically-shaped. The gap spacing decreases away from center along cylindrically-curved recessed portion  52  until reflecting surface  50  of optical block  38  also opposes reflecting side surfaces  54  of optical block  48 . The gap spacing d between central reflecting recess surface  52  and the reflecting surface  50  is set for maximum reflectivity at its maximum depth preferably at its center of an the excimer or molecular fluorine laser beam having a predetermined wavelength. The gap spacing gradually reduces along the cylindrical recess surface  52  until the gap spacing becomes d−λ/4 between the reflecting surface  50  and the outer reflecting surfaces  54 .  
         [0083]    Again, alternative arrangements may include optical block  38  having the λ/4 curved, recessed portion and optical block  48  having a planar reflecting surface, or each of optical blocks  38  and  48  having a recess with maximum depth being λ/8. Other such alternative arrangements would be understood by those skilled in the art whereby the optical path difference between the reflectivity maximum portions and the reflectivity zero or suppressed portion or portions is λ/4 preferably at the point of maximum depth or depths of the recess or recesses.  
         [0084]    Thus, preferred embodiment C is characterized in that the optical path length near the beam center differs by that at the outer regions by ±λ/4. The path difference gradually makes this transition along a cylindrical raised or recessed surface of at least one of two optical blocks between the center and outer regions of the interferometer. The output coupling interferometer of preferred embodiment C exemplified at FIGS. 6 a - 6   b  preferably includes external surfaces of blocks  38 ,  40  and  48  with antireflection (AR) coatings. As mentioned, many modifications within the scope of preferred embodiment C are conceivable (e.g., two inner surfaces having equal phase jumps of π/4 at their maximum or minimum gap spacing, or otherwise adding up to phase jumps of π/2 at their maximum or minimum gap spacings preferably at their center from outer regions).  
         [0085]    The calculated spectrum of a narrow band KrF-laser using an output coupling interferometer according to preferred embodiment C is shown at FIG. 6 c . The calculations provide a rough picture of the spectrum of the output beam that could be expected. The calculations follow those provided above for preferred embodiment A, but are modified according to the cylindrical curvature of the raised or recess curved portions of the interferometer blocks. Advantageously, sidebands are suppressed as the gap spacing changes from the peak at R=maximum out to the boundaries between the central surface  44  or  52  of FIGS. 6 a  and  6   b  and the side surfaces  46  and  54 , respectively, thereby reducing the spectral purity of the beam in accordance with the object of the invention.  
         [0086]    Embodiment C is similar to embodiment B, but is easier for preparation. The inner surfaces are preferably uncoated and the outer surfaces are preferably antireflection (AR)-coated. At least one inner surface is cylindrically curved along the x-axis. The radius of curvature is preferably selected to coincide with the diameter of the beam profile, just as the central raised or recessed portions of any of the preferred embodiments is selected to extend just to suppress the tails of the spectral distribution of the beam without suppressing too much of the main portion of the beam which would result in greatly reduced gain. The radius of curvature r can be determined by the following estimation: 
           d ( x )= d (0)+λ/4  (equation 5) 
         [0087]    where x is nearly given (equation 1)by x≈0.7 FSR/(dλ/dx). With FSR=1.6 pm and (dλ/dx)=1.24 pm/mm we get x≈1 mm. Based on equation 5, the radius of curvature r is given by 
           r= 2× 2 /λ  (equation 6) 
         [0088]    With λ=248 nm we get a preferred radius of curvature of r=8 m.  
         [0089]    It is understood by those skilled in the art that the preferred Gaussian and cylindrical shapes described above with respect to embodiments B and C, and the step or recess of embodiment A are illustrative shapes. For example, any of a wide variety of curvatures may be used that are preferably symmetrical about their center and have a reflectivity maximum at that center portion and then curve to effect a change in the reflectivity to suppress side bands. The shape may be a series of two or more connected straight portions or a combination of one or more straight portions and one or more curved portions. For example, the center portion may be triangularly or trapezoidally-shaped, or may have a central curved portion that connects with the outer R=0 portions via straight connectors.  
         [0090]    In further aspects of the invention, a narrow band excimer laser containing an output coupling interferometer and grating-based -narrowing optics located at the rear of the laser are synchronized to maximize performance. The preferred embodiments below relate to synchronization procedures of the narrow band unit and the outcoupling etalon.  
         [0091]    [0091]FIG. 7 schematically shows an excimer or molecular fluorine laser resonator including an output coupling interferometer in accord with any of the first through sixth embodiments. In addition, the synchronization procedures described herein may be applied with systems including conventional etalon output couplers such as has been described above and/or that may be described in the references cited above and incorporated by reference into this application.  
         [0092]    The resonator shown in FIG. 7 includes a gas reservoir or discharge chamber  202  and a pair of main discharge electrodes  246 ,  248  for energizing the gases in the discharge chamber  202  by electrical discharge excitation. The chamber  202  has a windows  203  on either end and sits between a rear optics module  210  and a front optics module  212 . The rear optics module  210  shown includes a beam expander  211  and a retro-reflection grating  213 . A slit  215  is shown disposed between the chamber  202  and the front optics module  212 .  
         [0093]    The front optics module  212  includes an output coupling interferometer  60 . The interferometer  60  has a first beam splitter  62  and a second beam splitter  64  in front of and behind it along the optical path of the output beam  220 . As will be seen in the preferred embodiments that follow, one or both of the first and second beam splitters  62  and  64 , respectively, may be used. The first beam splitter  62  shown in FIG. 7 is for reflecting a portion of the beam reflected by the interferometer  60  to a first photodiode  66 . The second beam splitter  64  shown in FIG. 7 is for reflecting a portion of the beam transmitted by the interferometer  60  to a second photodiode  68 . An interferometer control unit  70  is shown in FIG. 7 for receiving signals from the first and second photodiodes  66  and  68 , respectively, and for controlling the tuning of the interferometer  60 . Further details of the preferred excimer or molecular fluorine laser resonator, and the preferred overall laser system, are provided below with reference to FIG. 12.  
         [0094]    The output coupling interferometer  60  located inside of the front optics module  212  of FIG. 7 is adjusted as controlled by the control unit  70  to reflect radiation at or very near a spectral maximum produced by the echelle grating  213 . This grating  213  is located inside the narrow band unit of the rear optics module  210  of FIG. 7. Synchronization in this sense means the interferometer  60  has its maximum of reflection at the maximum of the spectral distribution of the intracavity laser radiation produced by the narrow band optics of the rear optics module  210 . Several preferred embodiments are disclosed for tuning the interferometer  60  to match the “maximum” wavelength selected by the rear optics module  210 .  
         [0095]    There are generally three synchronization techniques that are particularly preferred:  
         [0096]    1) A first preferred technique includes tuning of the outcoupling interferometer  60  to get maximum intracavity power reflection. This entails a minimum degree of outcoupling and minimized intracavity losses.  
         [0097]    2) A second preferred technique includes tuning of the outcoupling interferometer  60  to get maximum intensity of interference fringes behind a monitor etalon disposed where the second photodiode  68  is shown in FIG. 7.  
         [0098]    3) A third preferred technique includes tuning of the outcoupling interferometer  60  and detection of the reflected (or transmitted) light by a position—sensitive detector (e.g., double- or quadrant photodiode) disposed where either of the first or second photodiodes  66  or  68 , respectively, are shown in FIG. 7.  
       Preferred Embodiment A  
       [0099]    A first preferred embodiment of the front optics module  212  for realizing the first preferred synchronization technique is schematically depicted at FIGS. 8 a  and  8   b . Referring to FIG. 8 a , the front optics module  212  receives a beam via the slit  215  from the discharge chamber  202  (not shown) and rear optics module  210  (not shown). The beam is partially reflected and partially transmitted at the output coupling interferometer  60 , which is illustratively shown as having the optics blocks  2  and  4  of FIG. 4 a.    
         [0100]    The reflected beam is partially reflected at beam splitter  62 . The remainder of the beam reflected by the interferometer  60  re-enters the discharge chamber  202  (not shown) en route to the rear optics module  210  (not shown). The beam portion reflected at the beam splitter  62  preferably passes through each of projection optics  74 , a filter  76  and an attenuator  78  before being detected at the first photodiode  66 .  
         [0101]    The transmitted beam is partially reflected at beam splitter  64 . The remainder of the beam transmitted by the interferometer  60  passes out of the resonator as the output laser beam  220  of the excimer or molecular fluorine laser system, preferably en route to an applications process such as to an imaging system for photolithography or for TFT annealing or micro-machining or other industrial process. The beam portion reflected at the beam splitter  64  preferably passes through a filter  76  and attenuator  78  before being detected at the second photodiode  68 .  
         [0102]    The control unit  70 , also referred to as a photodiode detection unit, receives signals from each of the first and second photodiodes  66  and  68 , respectively. The control unit  70  signals a piezo control unit  72  for tuning the interferometer  60  based on the signals received from the first and second photodiodes  66  and  68 , respectively. The embodiment shown at FIG. 8 b  is the same as that shown at FIG. 8 a  except that a pressure control unit  74  is used for tuning the interferometer  60 .  
         [0103]    With respect to the preferred embodiment A of the front optics module  212 , the beamsplitter  62  reflects a portion of the radiation reflected at the output coupling interferometer  60  to the first photodiode  66 . The first photodiode  66  detects the reflected power. The second photodiode  68  is used for detection of the outcoming pulse energy, which is maintained nearly constant when the laser is operated in an energy stabilized mode. The slit  215 , having its slit edges preferably adjusted parallel to the long axis of the beam profile, is disposed in the optical pathway between the discharge chamber  202  and the beamsplitter  62 . The slit  215  advantageously serves to reduce the side modes before the beam encounter the interferometer  60 . As discussed, a main difference between the embodiments shown in FIGS. 8 a  and  8   b  is that the free spectral range of the interferometer  60  of FIG. 8 a  is piezo-controlled and the free spectral range of the interferometer  60  of FIG. 8 b  is pressure-controlled. Any of the embodiments of this invention may use piezo or pressure-controlled tuning of the interferometer  60  (see also FIGS. 9 a - 9   b ,  10   a - 10   b  and  11 , e.g.).  
       Preferred Embodiment B  
       [0104]    A second preferred embodiment of the front optics module  212  for realizing the second preferred synchronization technique is schematically depicted at FIGS. 9 a  and  9   b . The second preferred embodiment includes the interferometer  60 , beam splitter  64 , control unit  70  and either the piezo or pressure tuning unit  72  or  74  as shown in FIGS. 9 a  and  9   b , respectively, each being preferably the same as already described above with respect to the first embodiment of FIGS. 8 a  and  8   b , respectively. In this second embodiment, the beam portion reflected at the beam splitter  64  preferably passes through a diffusor before encountering a monitor Fabry-Perot etalon  82 . An attenuator  84  and imaging optics  85  are disposed after the monitor etalon  82 . An array detector or camera  86  is disposed after the monitor etalon  82  to capture images  88  of its transmitted fringe spectrum. The control unit  70  receives a signal from the camera  86  used for tuning the interferometer  60 .  
         [0105]    With respect to the embodiments of FIGS. 9 a  and  9   b , a portion of the radiation transmitted by the interferometer  60  is reflected by beamsplitter  64  to the system including the monitor etalon  82  which preferably includes the Fabry Perot etalon  82 , the diffusor  80 , imaging optics  85  and, e.g., a CCD-camera  86  or photodiode array or other position sensitive image detector. The CCD-camera  86  detects the interference fringes  88  behind the Fabry Perot etalon  82 . The outcoupling interferometer  60  is tuned to get maximum intensity of the fringes.  
       Preferred Embodiment C  
       [0106]    A third preferred embodiment of the front optics module  212  for realizing the third preferred synchronization technique is schematically depicted at FIGS. 10 a  and  10   b . The optical system is similar to that of preferred embodiment A referring to FIGS. 8 a  and  8   b  (see above), and those description of same elements will not be repeated here. The first photodiode  66  of FIGS. 8 a  and  8   b  is exchanged by a position-sensitive detector  90  (e.g., a double-or quadrant photodiode). An advantage of preferred embodiment C in comparison to preferred embodiments A and B is that the double- or quadrant photodiode  90  delivers additional information about the direction of the detuning of the interferometer  60 .  
         [0107]    [0107]FIG. 11 schematically shows a preferred embodiment for a pressure tuning unit  74  for pressure tuning the output coupling interferometer  60 , for use with any of the embodiments shown in FIGS. 8 b ,  9   b  or  10   b.  The preferred pressure tuning assembly includes a stepping motor  92 , a fine spindle  94 , a bellows  96  and a gas connector  98  to an interferometer housing  100  having windows for transmitting the laser beam. The stepper motor  92  or any other motor drives an inert gas filled bellow  96 , wherein the gas is preferably nitrogen or alternatively a noble gas such as argon or helium. The bellow  96  is fluidly connected with the sealed-off housing  100  containing the interferometer  60 . As the volume in the bellows  96  is adjusted, the pressure in the bellows  96  and in the housing  100 , and particularly in the gap between the optics blocks of the interferometer  60 , changes accordingly.  
         [0108]    The object of the invention set forth above are thus met. The details of the front optics module  212  and particularly relating to the output coupling interferometer  60  of the preferred embodiments may be advantageously used to achieve an excimer or molecular fluorine laser having improved spectral purity by suppressing side bands of the laser beam for such industrial applications as microlithography, TFT annealing and micromachining, among others.  
         [0109]    Referring now to FIG. 12, the preferred excimer or molecular fluorine laser system will not be described. A gas discharge laser system, preferably a DUV or VUV laser system, such as an excimer, e.g., ArF or KrF, or molecular fluorine (F 2 ) laser system for deep ultraviolet (DUV) or vacuum ultraviolet (VUV) lithography, is schematically shown at FIG. 12. Alternative configurations for laser systems for use in such other industrial applications as TFT annealing and/or micromachining, e.g., are understood by one skilled in the art as being similar to and/or modified from the system shown in FIG. 12 to meet the requirements of that application. For this purpose, alternative DUV or VUV laser system and component configurations are described at U.S. patent applications Ser. No. 09/317,695, 09/317,526, 09/130,277, 09/244,554, 09/452,353, 09/317,527, 09/343,333, 60/122,145, 60/140,531, 60/162,735, 60/166,952, 60/171,172, 60/141,678, 60/173,993, 60/166,967, 60/147,219, 60/170,342, 60/162,735, 60/178,445, 60/166,277, 60/167,835, 60/171,919, 60/202,564, 60/204,095, 60/172,674, 09/574,921 and 60/181,156, and U.S. Pat. Nos. 6,005,880, 6,061,382, 6,020,723, 5,946,337, 6,014,206, 5,559,816, 4,611,270, 5,761,236, each of which is assigned to the same assignee as the present application, and those references set forth above, are hereby incorporated by reference.  
         [0110]    The system shown in FIG. 12 generally includes a laser chamber  202  having a pair of main discharge electrodes  46 ,  48 , e.g., as described above with respect to FIG. 7, connected with a solid-state pulser module  204 , and a gas handling module  206 . The solid-state pulser module  204  is powered by a high voltage power supply  208 . The laser chamber  202  is surrounded by optics module  210  and optics module  212 , forming a resonator. The optics module  210  is preferably controlled by an optics control module  214 , or may be alternatively directly controlled by a computer  216 , and the front optics module  212  is preferable controlled by the control unit  70  described above, which may be a part of or separate from the module  214 .  
         [0111]    The computer  216  for laser control receives various inputs and controls various operating parameters of the system. A diagnostic module  218  receives and measures one or more parameters of a split off portion of the main beam  220  via optics for deflecting a small portion of the beam toward the module  218 , such as preferably a beam splitter module  222 , as shown. The beam  220  is preferably the laser output to an imaging system (not shown) and ultimately to a workpiece (also not shown), and may be output directly to an application process. The laser control computer  216  communicates through an interface  224  with a stepper/scanner computer  226  and other control units  228 .  
       Laser Chamber  
       [0112]    The laser chamber  202  contains a laser gas mixture and includes one or more preionization electrodes (not shown here, but described above with respect to FIGS.  2 - 6 ) in addition to the pair of main discharge electrodes  46 ,  48 . Preferred main electrodes  46  and  48  are described at U.S. patent applications Ser. Nos. 09/453,670, 60/184,705 and 60/128,227, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference. Other electrode configurations are set forth at U.S. Pat. Nos. 5,729,565 and 4,860,300, each of which is assigned to the same assignee, and alternative embodiments are set forth at U.S. Pat. Nos. 4,691,322, 5,535,233 and 5,557,629, all of which are hereby incorporated by reference. Preferred preionization units are described at United States patent application of Bragin et al, serial number not yet assigned, entitled Corona Preionization Assembly for a Gas Laser, filed Oct. 19, 2000, and details and alternative configurations are additionally set forth at U.S. patent applications Ser. Nos. 60,162,845, 60/160,182, 60/127,237, 09/535,276 and 09/247,887, each of which is assigned to the same assignee as the present application, and alternative embodiments are set forth at U.S. Pat. Nos. 5,337,330, 5,818,865 and 5,991,324, all of the above patents and patent applications being hereby incorporated by reference.  
       Power Supply Circuit and Pulser Module  
       [0113]    The solid-state pulser module  204  and high voltage power supply  208  supply electrical energy in compressed electrical pulses to the preionization and main electrodes  46 ,  48  within the laser chamber  202  to energize the gas mixture. Components of the preferred pulser module and high voltage power supply may be described at U.S. patent applications Ser. Nos. 60/149,392, 60/198,058, 60/204,095, 09/432,348 and 09/390,146, and 60/204,095, and U.S. Pat. Nos. 6,005,880 and 6,020,723, each of which is assigned to the same assignee as the present application and which is hereby incorporated by reference into the present application. Other alternative pulser modules are described at U.S. Pat. Nos. 5,982,800, 5,982,795, 5,940,421, 5,914,974, 5,949,806, 5,936,988, 6,028,872 and 5,729,562, each of which is hereby incorporated by reference. A conventional pulser module may generate electrical pulses in excess of one Joule of electrical power (see the &#39; 988  patent, mentioned above).  
       Laser Resonator  
       [0114]    The laser resonator which surrounds the laser chamber  202  containing the laser gas mixture includes optics module  210  including line-narrowing optics for a line narrowed excimer or molecular fluorine laser, which may be replaced by a high reflectivity mirror or the like in a laser system wherein either line-narrowing is not desired, or if line narrowing is performed at the front optics module  212 , or a spectral filter external to the resonator is used, or if the line-narrowing optics are disposed in front of the HR mirror, for narrowing the linewidth of the output beam.  
         [0115]    The laser chamber  202  is sealed by windows transparent to the wavelengths of the emitted laser radiation  220 . The windows may be Brewster windows or may be aligned at another angle, e.g., 5°, to the optical path of the resonating beam. One of the windows may include the interferometer  60  described above which also serves to output couple the beam.  
       Extra-Resonator Features  
       [0116]    After a portion of the output beam  220  passes the outcoupler of the optics module  212 , that output portion impinges upon beam splitter module  222  which includes optics for deflecting a portion of the beam to the diagnostic module  218 , or otherwise allowing a small portion of the outcoupled beam to reach the diagnostic module  218 , while a main beam portion  220  is allowed to continue as the output beam  220  of the laser system. the diagnostic module may include the photodiode  68  and/or monitor etalon  82 , described above. Preferred optics include a beamsplitter or otherwise partially reflecting surface optic. The optics may also include a mirror or beam splitter as a second reflecting optic. More than one beam splitter and/or HR mirror(s), and/or dichroic mirror(s) may be used to direct portions of the beam to components of the diagnostic module  218 . A holographic beam sampler, transmission grating, partially transmissive reflection diffraction grating, grism, prism or other refractive, dispersive and/or transmissive optic or optics may also be used to separate a small beam portion from the main beam  220  for detection at the diagnostic module  218 , while allowing most of the main beam  220  to reach an application process directly or via an imaging system or otherwise.  
         [0117]    The output beam  220  may be transmitted at the beam splitter module  222  while a reflected beam portion is directed at the diagnostic module  218 , or the main beam  220  may be reflected, while a small portion is transmitted to the diagnostic module  218 . The portion of the outcoupled beam which continues past the beam splitter module  222  is the output beam  220  of the laser, which propagates toward an industrial or experimental application such as an imaging system and workpiece for photolithographic applications. Variations of beam splitter modules  222  particularly for a molecular fluorine laser system are set forth at U.S. patent applications Ser. Nos. 09/598,552 and 60/140,530, which are each assigned to the same assignee as the present application and are hereby incorporated by reference.  
       Beam Path Enclosure  
       [0118]    Also particularly for the molecular fluorine laser system, and for the ArF laser system, an enclosure (not shown) may seal the beam path of the beam  220  such as to keep the beam path free of photoabsorbing species. Smaller enclosures may seal the beam path between the chamber  202  and the optics modules  210  and  212  and between the beam splitter module  222 , which itself may be within the same or a separate enclosure, and the diagnostic module  218 . The preferred enclosure is described in detail in U.S. patent applications Ser. Nos. 09/343,333, 09/598,552, 09/594,892, 09/131,580 and 60/140,530, each of which is assigned to the same assignee and is hereby incorporated by reference, and U.S. Pat. Nos. 5,559,584, 5,221,823, 5,763,855, 5,811,753 and 4,616,908, all of which are hereby incorporated by reference.  
       Diagnostic Module  
       [0119]    The diagnostic module  218  preferably includes at least one energy detector. This detector measures the total energy of the beam portion that corresponds directly to the energy of the output beam  220  (see U.S. Pat. No. 4,611,270 and U.S. patent application Ser. No. 09/379,034, each of which is assigned to the same assignee and is hereby incorporated by reference). An optical configuration such as an optical attenuator, e.g., a plate or a coating, or other optics may be formed on or near the detector or beam splitter module  222  to control the intensity, spectral distribution and/or other parameters of the radiation impinging upon the detector (see U.S. patent applications Ser. No. 09/172,805, 60/172,749, 60/166,952 and 60/178,620, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference).  
         [0120]    One other component of the diagnostic module  218  is preferably a wavelength and/or bandwidth detection component such as a monitor etalon or grating spectrometer (see U.S. patent applications Ser. No. 09/416,344, 60/186,003, 60/158,808, 60/186,096, 60/186,096 and 60/186,096 and 60/202,564, each of which is assigned to the same assignee as the present application, and U.S. Pat. Nos. 4,905,243, 5,978,391, 5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394, all of the above wavelength and/or bandwidth detection and monitoring components being hereby incorporated by reference. This monitor etalon can be the same one described above with respect to FIGS. 9 a  and  9   b , or a second monitor etalon. The spectrometer may be within a temperature and pressure controlled housing such as is described in the 60/158,808 application.  
         [0121]    Other components of the diagnostic module may include a pulse shape detector or ASE detector, such as are described at U.S. patent applications Ser. No. 09/484,818 and 09/418,052, respectively, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference, such as for gas control and/or output beam energy stabilization, or to monitor the amount of amplified spontaneous emission (ASE) within the beam to ensure that the ASE remains below a predetermined level, as set forth in more detail below. There may be a beam alignment monitor, e.g., such as is described at U.S. Pat. No. 6,014,206 which is assigned to the same assignee and is hereby incorporated by reference.  
       Control Processor  
       [0122]    The processor or control computer  216  receives and processes values of some of the pulse shape, energy, ASE, energy stability, energy overshoot for burst mode operation, wavelength, spectral purity and/or bandwidth, among other input or output parameters of the laser system and output beam. The processor  216  also controls the line narrowing module to tune the wavelength and/or bandwidth or spectral purity, and controls the power supply and pulser module  204  and  208  to control preferably the moving average pulse power or energy, such that the energy dose at points on the workpiece is stabilized around a desired value. In addition, the computer  216  controls the gas handling module  206  which includes gas supply valves connected to various gas sources.  
         [0123]    Further details of the control processor  216  such as for performing burst overshoot control and controlling the gas supply unit by monitoring total input energy to the discharge, among other parameters, for determining the timing and amounts of gas replenishment actions, are described at U.S. patent application Ser. No. 60/159,525, which is assigned to the same assignee as the present application and is hereby incorporated by reference.  
       Gas Mixture  
       [0124]    The laser gas mixture is initially filled into the laser chamber  202  during new fills. The gas composition for a very stable excimer or molecular fluorine laser in accord with the preferred embodiment uses helium or neon or a mixture of helium and neon as buffer gas(es), depending on the particular laser being used. Preferred gas compositions are described at U.S. Pat. Nos. 4,393,405 and 4,977,573 and U.S. patent applications Ser. No. 09/317,526, 09/513,025, 60/124,785, 09/418,052, 60/159,525 and 60/160,126, each of which is assigned to the same assignee and is hereby incorporated by reference into the present application. The concentration of the fluorine in the gas mixture may range from 0.003% to 1.00%, and is preferably around 0.1%. An additional gas additive, such as a rare gas, such as xenon, may be added for increased energy stability and/or as an attenuator as described in the 09/513,025 application incorporated by reference above. Specifically, for the F 2 -laser, an addition of xenon and/or argon may be used. The concentration of xenon or argon in the mixture may range from 0.0001% to 0.1%. For an ArF-laser, an addition of xenon or krypton may be used also having a concentration between 0.0001% to 0.1%. For the KrF laser, an addition of xenon or argon may be used also having a concentration between 0.0001% to 0.1%.  
       Gas Replenishment, General  
       [0125]    Halogen and rare gas injections, total pressure adjustments and gas replacement procedures are performed using the gas handling module  206  preferably including a vacuum pump, a valve network and one or more gas compartments. The gas handling module  206  receives gas via gas lines connected to gas containers, tanks, canisters and/or bottles. Some preferred and alternative gas handling and/or replenishment procedures are described at U.S. Pat. Nos. 4,977,573 and 5,396,514 and U.S. patent applications Ser. No. 60/124,785, 09/418,052, 09/379,034, 60/159,525, 60/171,717, and 60/159,525, each of which is assigned to the same assignee as the present application, and U.S. Pat. Nos. 5,978,406, 6,014,398 and 6,028,880, all of which are hereby incorporated by reference. A xenon gas supply may be included either internal or external to the laser system according to the &#39;025 application, mentioned above.  
       Line-Narrowing  
       [0126]    A general description of the line-narrowing features of the preferred embodiment is provided here, followed by a listing of patent and patent applications being incorporated by reference as describing variations and features that may used with the preferred embodiments described above for providing an output beam with a high spectral purity or bandwidth (e.g., below 0.6 pm). Exemplary line-narrowing optics contained in the optics module  210  include a beam expander, an optional etalon and a diffraction grating, which produces a relatively high degree of dispersion, for a narrow band laser such as is used with a refractive or catadioptric optical lithography imaging system. As referred to above, the front optics module  212  may include line-narrowing optics (e.g., outcoupling interferometer, birefringent plate, grating, grism) as well (see the 60/166,277, 60/173,993 and 60/166,967 applications, each being assigned to the same assignee and hereby incorporated by reference).  
         [0127]    The beam expander of the above exemplary line-narrowing optics of the optics module  210 , and that of the embodiment described above in front of the output coupling interferometer  60 , preferably includes one or more prisms. The beam expander may include other beam expanding optics such as a lens assembly or a converging/diverging lens pair. The grating or a highly reflective mirror is preferably rotatable and in Littrow configuration so that the wavelengths reflected into the acceptance angle of the resonator can be selected or tuned. Alternatively, the grating, or other optic or optics, or the entire line-narrowing module may be pressure tuned, such as is set forth in the 60/178,445 and Ser. No. 09/317,527 applications, each of which is assigned to the same assignee and is hereby incorporated by reference. The grating may be used both for dispersing the beam for achieving narrow bandwidths and also preferably for retroreflecting the beam back toward the laser tube  202 . Alternatively, a highly reflective mirror may be positioned before or after the grating which receives a reflection from the grating and reflects the beam back toward the grating, such as in a Littman configuration, or the grating may be a transmission grating. One or more dispersive prisms may also be used, and more than one etalon may be used.  
         [0128]    Depending on the type and extent of line-narrowing and/or selection and tuning that is desired, and the particular laser that the line-narrowing optics are to be installed into, there are many alternative optical configurations that may be used. For this purpose, those shown in U.S. Pat. Nos. 4,399,540, 4,905,243, 5,226,050, 5,559,816, 5,659,419, 5,663,973, 5,761,236, and 5,946,337, and U.S. patent applications Ser. No. 09/317,695, 09/130,277, 09/244,554, 09/317,527, 09/073,070, 60/124,241, 60/140,532, 60/147,219 and 60/140,531, 60/147,219, 60/170,342, 60/172,749, 60/178,620, 60/173,993, 60/166,277, 60/166,967, 60/167,835, 60/170,919, 60/186,096, each of which is assigned to the same assignee as the present application, and U.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849, 5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094, 4,856,018, 5,970,082, 5,978,409, 5,999,318, 5,150,370 and 4,829,536, and German patent DE 298 22 090.3, are each hereby incorporated by reference into the present application.  
         [0129]    Optics module  212  preferably includes means for outcoupling the beam  220 , such as a partially reflective resonator reflector. The beam  220  may be otherwise outcoupled such as by an intra-resonator beam splitter or partially reflecting surface of another optical element, and the optics module  212  would in this case include a highly reflective mirror. The optics control module  214  preferably controls the optics modules  210  and  212  such as by receiving and interpreting signals from the processor  216 , and initiating realignment or reconfiguration procedures (see the &#39;241, &#39;695, &#39;277, &#39;554, and &#39;527 applications mentioned above).  
         [0130]    While exemplary drawings and specific embodiments of the present invention have been described and illustrated, it is to be understood that that the scope of the present invention is not to be limited to the particular embodiments discussed. Thus, the embodiments shall be regarded as illustrative rather than restrictive, and it should be understood that variations may be made in those embodiments by workers skilled in the arts without departing from the scope of the present invention as set forth in the claims that follow, and equivalents thereof.