Laser resonator for improving narrow band emission of an excimer laser

An apparatus and method are provided for bandwidth narrowing of an excimer laser to .DELTA..lambda..apprxeq.6 pm or less with high spectral purity and minimized output power loss. Output stability with respect to pulse energy, beam pointing, beam size and beam output location is also provided. The excimer laser includes an active laser medium for generating a spectral beam at an original wavelength, means for selecting and narrowing the broadband output spectrum of the excimer laser, a resonator having at least one highly reflecting surface, and an output coupler. Means for adapting a divergence of the resonating band within the resonator is further included in the apparatus of the invention. The divergence adapting causes the spectral purity to improve by between 20% and 50% and the output power to reduce by less than 10%. A method according to the invention includes selecting and aligning the divergence adapting means.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a resonator designed for narrow-linewidth
 emission, and particularly to a resonator for an excimer laser having
 optical components for improving spectral purity, reducing spectral
 bandwidth and optimizing output power for emitting a high resolution
 photolithographic beam.
 2. Discussion of the Related Art
 To increase the capacities and operation speeds of integrated circuits,
 manufacturers are inclined to design smaller internal structures for
 devices and other components of these chips. The reduction in size of a
 structure produced on a silicon wafer is limited by the ability to
 optically resolve the structure. This resolution ability depends directly
 upon the photolithographical source radiation and optics used.
 Excimer lasers emitting pulsed UV-radiation are becoming increasingly
 important instruments in specialized material processing. The term
 "excimer" was first utilized as an abbreviation for "excited dimer",
 meaning two or more identical atoms comprising a molecule which only
 exists in an excited state, such as He.sub.2 and Xe.sub.2. Today, the term
 "excimer" has a broader meaning in the laser world and encompasses such
 rare gas halides as XeCl (308 nm), KrF (248 nm), ArF (193 nm), KrCl (222
 nm), and XeF (351 nm). Several mercury-halides are also used as active
 gases in excimer lasers, such as HgBr. Even N.sub.2, N.sub.2.sup.+,
 CO.sub.2 and F.sub.2 (157 nm) may be used as active media within excimer
 laser discharge chambers. As is apparent, many excimer lasers radiate at
 ultraviolet wavelengths making them desirable for use as lithography
 tools. The KrF-excimer laser emitting around 248 nm and the ArF-excimer
 laser emitting around 193 nm are rapidly becoming the light sources of
 choice for photolithographic processing of integrated circuit devices
 (IC's). The F.sub.2 -laser is also being developed for such usage and
 emits light around 157 nm.
 To produce smaller feature sizes on IC chips, stepper and scanner machines
 are using expensive large scale submicron projection objectives for
 imaging a reticle onto a wafer surface with high diffracting-limited
 precision. The objectives operate at deep ultraviolet (DUV) wavelengths,
 such as the emission wavelengths of excimer lasers. For example, the
 KrF-excimer laser emitting around 248 nm is currently being used as a DUV
 radiation source. To reach greater resolution limits, the large field
 objective lenses are designed and optimized in view of various possible
 and discovered imaging errors. The design optimization of the objectives
 is, however, inadequate to meet the precision demands of sub-quarter
 micron lithographic technology.
 One way to improve the resolvability of structures on IC chips is to use
 more nearly monochromatic source radiation, i.e., radiation having a
 reduced bandwidth, .DELTA..lambda.. Other strategies include using shorter
 absolute wavelength, .lambda., radiation such as that emitted around 193
 nm and 157 nm by ArF- and F.sub.2 -lasers, respectively, and increasing
 the numerical aperture (NA) of the projection lens.
 The smallest structure resolvable on an IC chip depends on the "critical
 dimension" (CD) of the photolithography equipment being used:
 ##EQU1##
 NA is a measure of the acceptance angle of the projection lens, .lambda. is
 the wavelength of the source radiation, and K.sub.1 is a constant around
 approximately 0.6-0.8. Simply increasing the numerical aperture NA to
 reduce the critical dimension CD simultaneously reduces the depth of focus
 DOF of the projection lens by the second power of NA:
 ##EQU2##
 K.sub.2 is a constant around approximately 0.8-1.0. This complicates wafer
 adjustment and adds further strain on the demand for improved chromatic
 correction of the projection lenses. Additionally, increasing the
 numerical aperture NA to reduce the critical dimension CD for achieving
 smaller structures requires a decrease in the bandwidth .DELTA..lambda. of
 laser emission according to:
 ##EQU3##
 K.sub.3 is a constant dependent on parameters associated with the
 projection lens(es). Each of the above assumes that such other laser
 parameters as repetition rate, stability, and output power remain
 constant.
 Some techniques are known for selecting and for narrowing laser emission
 bandwidths including using optically dispersive elements such as etalons,
 gratings and prisms, as well as modified resonator arrangements. See U.S.
 Pat. No. 5,095,492 to Sandstrom (disclosing a dispersive grating having a
 concave radius of curvature); U.S. Pat. No. 5,559,816 to Basting et al.
 (disclosing a technique using the polarization properties of light); U.S.
 Pat. No. 5,150,370 to Furuya et al. (disclosing a fabry-perot etalon
 within the laser resonator); U.S. Pat. No. 5,404,366, U.S. Pat. No.
 5,596,596 and E.U. Patent Pub. No. 0 472 727, each to Wakabayashi et al.
 (disclosing a concave outcoupler and a fixed aperture within the laser
 resonator); U.S. Pat. No. 4,829,536 to Kajiyama et al. (disclosing
 angularly offset etalons).
 Using this available knowledge, the bandwidth of laser emission, e.g.,
 which is naturally around 500 pm for a KrF-excimer laser, can be reduced
 to .DELTA..lambda..apprxeq.0.8 pm, sufficient to meet the demands of
 current projection lenses (NA.apprxeq.0.53) for producing quarter micron
 ship structures. Further improvements in projection objectives
 (NA.apprxeq.0.8) combined with a further reduction in laser emission
 bandwidths (.DELTA..lambda..apprxeq.0.4-0.6 pm) are expected to reduce the
 critical dimension CD using KrF-excimer laser sources down to
 CD.apprxeq.0.18 microns. See J. Mulkens et al., Step and Scan Technology
 for the 193 nm Era, Third International Symposium on 193 nm Lithography,
 Onuma, Japan (Jun. 29-Jul. 2, 1997).
 The drawback to this significant bandwidth and CD reduction is a
 correspondingly significant reduction in available laser output power.
 Narrow band efficiencies of twenty to forty percent of broadband output
 power are typical. There is thus a need for efficient spectral narrowing
 methods which minimize power loss.
 FIG. 1 shows a conventional excimer laser arrangement. A laser tube 1
 contains a laser active medium (not shown) for emitting a characteristic
 wavelength upon excitation pumping of the laser active medium. A
 wavelength selection and narrowing assembly 2 includes a dispersive
 grating 3 and at least one expanding and/or dispersive prism 4. The
 grating 3 also serves to reflect substantially all of the laser light
 incident upon it at a wavelength dependent angle. A narrow band of the
 light dispersed once through the prism 4 and incident upon the grating 3
 is reflected off of the grating 3 and back along the optical path of the
 arrangement, while all other wavelengths are reflected away from the
 optical path. The arrangement is completed with an output coupling mirror
 5 which reflects a portion of the resonating band and allows the rest to
 continue unreflected ultimately defining the output beam of the system.
 The excimer laser arrangement of FIG. 2 includes all of the elements of
 FIG. 1 except the output coupling mirror 5, and further includes a beam
 splitter 6 and a highly reflective mirror 8. The beam splitter 6 serves as
 an output coupler reflecting the narrow band laser emission 9 from the
 optical path of the resonating beam. A highly reflective mirror 8 is used
 instead of the partially reflecting output coupling mirror 5 of the
 arrangement of FIG. 1.
 SUMMARY OF THE INVENTION
 The present invention sets forth an apparatus and method for bandwidth
 narrowing of an excimer laser to .DELTA..lambda..apprxeq.0.6 pm or less
 with high spectral purity and minimized output power loss. Additional
 and/or modified optical elements within the laser resonator are used.
 Output stability with respect to pulse energy, beam pointing, beam size
 and beam output location are also improvements of the present invention.
 An apparatus according to the present invention is an excimer laser
 including an active laser medium for generating a broadband spectral beam
 at an original wavelength, means for narrowing the wavelength and/or
 selecting a spectral line of the generated broadband spectral beam, a
 resonator and a means for outcoupling the resonating band. Means for
 adapting or matching the divergence of the intracavity rays is further
 included in the apparatus according to the present invention for
 optimizing the combination of output power, spectral purity and bandwidth
 of the output beam of the excimer laser. A method according to the present
 invention includes selecting and aligning the divergence adapting or
 matching means such that the combination of output power, spectral purity
 and bandwidth of the output beam of the excimer laser is optimized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Spectral laser emissions propagate with a full-angle of beam divergence
 after n round trips within the resonator of an excimer laser approximately
 according to:
 ##EQU4##
 a is the aperture radius (or its geometrical equivalent corresponding to
 the geometry of the aperture), L is the length of the resonator, and n is
 the number of round trips an average photon emitted from the laser active
 medium traverses within the resonator before outcoupling through, e.g.,
 the outcoupling mirror 5 of the arrangement of FIG. 1, or the beam
 splitter 6A of the arrangement of FIG. 2. See S. Kawata, I. Hikima, Y.
 Ichihara, and S. Watanabe, Spatial Coherence of KrF Excimer Lasers, Appl.
 Opt., vol. 31, page 387 (1992). When dispersive elements are used for
 wavelength narrowing, the angular divergence corresponds to a finite
 bandwidth:
EQU .theta..sub.0.about..DELTA..lambda..sub.0.
 Different parts of the lateral laser beam comprise different spectral
 portions of the spectrally narrowed beam. See R. Sandstrom, Measurements
 of Beam Characteristics Relevant to DUV Microlithography on a KrF Excimer
 Laser, SPIE: Microlithography III, vol. 1264, 505, 511 (1990) (showing in
 FIG. 8 the variation in spectral content of a previously horizontally
 dispersed beam as a vertical slit mask selects out portions of the beam,
 scanning horizontally from beam center to the right). The side wings, or
 outer wavelengths, of the spectral band of the excimer laser contain a
 significant amount of energy which cannot be ignored due to its effect of
 diminishing the spectral purity and output power of the output beam. See
 Sandstrom (above), at 507-11. These side wings also contribute to the
 width of the band.
 Furthermore, spectral narrowing generally occurs in any laser resonator
 according to the spectral narrowing effect:
 ##EQU5##
 n is the number of round trips, .DELTA..lambda..sub.0 is the bandwidth
 narrowed by optical components within the resonator, and
 .DELTA..lambda..sub.L is the laser emission bandwidth. Effective spectral
 narrowing of a laser beam typically requires a large number n of round
 trips. Since an excimer laser usually has a few round trips, the natural
 spectral narrowing associated with other types of lasers is not achieved.
 Further, in an excimer laser, the side wings are amplified at the expense
 of the center of the band, since they are within the divergence/acceptance
 angle of the beam during the few round trips traversed by the beam within
 the resonator.
 At a given dispersive power, the degree of divergence compensation has to
 be carefully adapted to the number of round trips, since they, together
 with the resulting bandwidth, spectral purity and output power of the
 emitted beam, are interdependent. Spectral purity is a measure of the
 spectral energy distribution within a narrow central region around the
 line center, e.g., within a 2 pm limit. The spectral purity may also be
 defined as the energy within a specified wavelength interval divided by
 the total energy.
 FIG. 3 shows an arrangement of a resonator of an excimer laser in
 accordance with a first embodiment of the present invention. The
 arrangement includes an active laser medium 1 for emitting light having a
 characteristic wavelength. The arrangement further includes wavelength
 selecting and narrowing optics 2 comprising a grating 3 and at least one
 prism 4. The grating 3 serves as one of the two reflecting resonator
 surfaces of the first embodiment. The grating 3 reflects substantially all
 light incident upon it, each wavelength at a different angle. The other
 reflecting surface is a convex-curved, preferably cylindrical, surface of
 an output coupling mirror 15. The output coupler 15 preferably transmits a
 portion of the light incident upon it and reflects the rest.
 Alternatively, the outermost radial portion of the resonating band simply
 misses the output coupling mirror 15, which has a smaller radius than the
 resonating beam at that point. In either event or in another conventional
 way, the portion of the resonating band that exits the resonator along the
 predetermined optical path after encountering the output coupling mirror
 15 defines the emitted output beam 16 of the laser. The resonator may thus
 be operated as an unstable resonator, or alternatively, additional optics
 may be used to stabilize the resonator.
 As mentioned above, the lateral laser beam comprises a spectrum of
 wavelengths, ordered according to wavelength, as an effect of traversing
 the wavelength selection and narrowing optics 2. The optics of the
 arrangement of the first embodiment are aligned such that the center of
 the resonating band strikes approximately at the center of the mirror 15.
 That center portion is reflected back along the optical path of the
 resonator.
 Other wavelengths above and below the center of the band, are reflected off
 the mirror 15 at angles away from the optical path wherein these angles
 are enhanced due to the convex nature of the mirror 15. Wavelengths that
 are sufficiently removed from the center of the band are reflected at such
 a high angle that they no longer are accepted by apertures within the
 resonator, such as those surrounding the laser active medium 1. These
 wavelengths will not be part of any subsequently emitted output beam 16.
 Since a smaller geometrical region of the resonating beam will be accepted
 by natural apertures of the resonator, the resonating band of the first
 embodiment comprises a narrower range of wavelengths than a resonating
 band of a conventional resonator.
 The resonator of the first embodiment is an unstable resonator if a light
 ray initially propagating parallel to the optical axis of the laser cavity
 could not be reflected back between the two mirrored surfaces 3 and 15
 indefinitely without escaping from between the mirrors 3 and 15, other
 than by outcoupling. That is, the angle a ray of the resonating beam makes
 with the optical axis will increase with the number of round trips the ray
 makes within the resonator. If the grating 3 is flat and no additional
 focusing optics are provided in the arrangement of the first embodiment,
 then the first embodiment will include an unstable resonator by virtue of
 the convex output coupler 15.
 The angle of a light ray incident upon the output coupler 15 over which
 this light reflected from the outcoupling mirror 15 is dispersed out of
 the resonating beam is the acceptance angle of the beam. The smaller the
 radius of curvature of the convex outcoupling mirror 15, the smaller the
 acceptance angle of the beam. Consequently, the smaller the radius of
 curvature of the outcoupling mirror 15, the narrower the band of
 wavelengths that the ultimate emitted beam 16 will comprise. The radius of
 curvature is preferably constant over the surface of the mirror 15, but
 may change with distance from the center, or along a diameter. The focal
 length of the mirror 15 is preferably in the range of several meters, as
 are most curved components of the present invention.
 The narrowing of the beam 16 is not achieved without a price. Generally,
 with all else being the same, the narrower the bandwidth of the beam, the
 weaker the output power of that beam 16. At some point, the beam 16 can be
 so narrowed that its output power is not sufficient to perform adequate
 lithography. However, the radius of curvature of the convex mirror 15 can
 be selected appropriately, such that the output power of the beam 16 is
 sufficient, while the desired bandwidth narrowing is achieved.
 Thus, a first advantage of the first embodiment over a conventional
 arrangement having a, e.g., flat outcoupling mirror 5, such as that of
 FIG. 1, is that the spectral purity of the beam 16 of the first embodiment
 is enhanced over the prior art. A second advantage is that the bandwidth
 of the beam may be more greatly narrowed than a prior art beam while
 maintaining adequate output power, since the side wings of the resonating
 beam are dispersed out by the outcoupling mirror 16 and do not absorb
 power being amplified. Available power is then focusable on amplifying a
 more useful narrow central portion of the emission band 16. In a preferred
 embodiment, the radius of curvature of the outcoupling mirror 15 is
 adjustable to optimize the combination of output power, bandwidth and
 spectral purity of the emitted beam 16.
 The present invention is capable of improving spectral purity by between
 20% and 50%, or more while power loss is kept to less than 10%. Power loss
 may be defined as the difference between the power of a state of the art
 laser and the power of a laser according to the present invention, divided
 by the power of a state of the art laser. Typical line narrowing
 efficiency is between 0.2 and 0.4, and particularly is around 0.3 for
 lasers used in lithography.
 FIG. 4 shows an arrangement of a resonator of an excimer laser in
 accordance with a second embodiment of the present invention. The second
 embodiment has an active laser medium 11, wavelength selection and
 narrowing optics 12 including a grating 3, which preferably serves also as
 one of two reflecting surfaces of the resonator, and at least one prism
 14, and an output coupler 25. A highly reflective mirror may alternatively
 perform the reflective function of the grating 3. The output coupler 25
 may be similar to the conventional output coupler 5 of FIG. 1, or it may
 be similar to the output coupler 15 having a reflecting surface with a
 convex radius of curvature of FIG. 3, or may be another operable output
 coupler 25.
 The laser active medium 11 is contained within a housing having a first
 optical window 17A and a second optical window 17B to facilitate entrance
 and exit of the resonating beam. The windows 17A and 17B each comprise one
 or more conventionally UV transparent materials such as crystalline
 quartz, CaF.sub.2 and/or MgF.sub.2, for example.
 At least one surface of at least one, and preferably both, windows 17a and
 17B is curved. Both surfaces of one or both windows 17A and 17B may be
 curved, but preferably only the outer surfaces are curved as shown in FIG.
 4. The effective total radius of curvature of all curved surfaces of the
 windows 17A and 17B is selected to match or adapt the divergence of the
 resonating beam and optimize the combination of laser output power,
 bandwidth and spectral purity. By matching or adapting the divergence of
 the resonating beam, the angles of light rays of the resonating beam
 relative to the optical axis are changed to minimize the angle of the
 light rays relative to the optical axis. The divergence is matched or
 adapted in the present invention to optimize spectral purity and
 bandwidth. Divergence adapting is provided in the present invention by
 focusing elements and/or by the cutting of rays with an angle relative to
 the optical axis greater than a specified angle by one or more apertures.
 The prism 14 of the second embodiment has a first curved surface 18A and a
 second curved surface 18B through which the resonating beam enters and
 exits the prism 14. Alternatively, only one surface 18A or 18B may be
 curved. Another arrangement of the wavelength narrowing and selection
 optics 12 is possible wherein the resonating beam enters and exits the
 prism through the same curved surface. The curvature of each surface 18A
 and 18B is preferably convex. Alternatively, one may be concave or the
 radius of curvature may change with position on one or both surfaces 18A
 and 18B.
 A first advantage of the second embodiment over a conventional arrangement
 such as that shown in FIG. 1 is that the radius of curvature of each of
 the surfaces 17A, 17B, 18A and 18B may be selected to match the divergence
 of the laser beam, and optimize output power, bandwidth and spectral
 purity. A second advantage is that the surfaces 18A and 18B of the prism
 14 and/or the surfaces 17A and 17B of the housing containing the active
 laser medium 11 may be used to expand or narrow the resonating beam,
 depending on what is needed in the arrangement considering the properties
 and alignment of the other optics in the arrangement.
 FIG. 5A shows an arrangement of a resonator of an excimer laser in
 accordance with a third embodiment of the present invention. The third
 embodiment includes an active laser medium 21, wavelength selection and
 narrowing optics 22 including a grating 3, which preferably serves also as
 one of two reflecting surfaces of the resonator, and at least one prism
 24, and an output coupler 35. A highly reflective mirror may alternatively
 perform the reflective function of the grating 3. The output coupler 35
 may be similar to the conventional output coupler 5 of FIG. 1, or it may
 be similar to the output coupler 15 having a reflecting surface with a
 convex radius of curvature of FIG. 3, or may be another operable output
 coupler 35. An output beam 16 is transmitted past the output coupler 35.
 The prism 24 and housing of the active laser medium 21 may be configured
 as in the either of the first or the second embodiments of FIGS. 3 and 4,
 respectively, or otherwise conventionally.
 The third embodiment also includes an aperture 19 located within the
 resonator arrangement of FIG. 5A. The aperture 19 is preferably located
 near the grating 3 as shown in FIG. 5A, but may be located at various
 locations along the optical path of the resonating beam. More than one
 aperture may be placed along the optical path of the resonating band. The
 aperture 19 is preferably adjustable to optimize the combination of the
 output power, the bandwidth and the spectral purity of the output beam 16.
 The aperture 19 is blocking highly divergent beams, i.e., beams having a
 large angle relative to the optical axis of the resonator, primarily to
 improve spectral purity. When the aperture 19 is located close to the
 grating 3, the output power is not significantly affected by the presence
 of the aperture 19. An advantage of the third embodiment is that the
 spectral purity, bandwidth and output power of the output beam 16 are
 optimized over those of a conventional arrangement such as that described
 in FIG. 1.
 FIG. 5B shows an arrangement of a resonator of an excimer laser in
 accordance with a fourth embodiment of the present invention. The fourth
 embodiment of FIG. 5B includes all of the elements of the third embodiment
 of FIG. 5A. Additionally, the fourth embodiment includes a beam splitter
 6C after the output coupler 35 which transmits an output beam 26 and
 reflects a portion of the output of the output coupler 35. The reflected
 portion is received by a high resolution spectrometer 29 for determining
 the wavelength and waveform characteristics of the output beam 26. A
 second beam splitter 6D is inserted to direct a portion of the output beam
 26 toward an energy detector 28. The outputs of each of the detector 28
 and the spectrometer 29 are received by a computer 30 and processed. The
 computer then determines how the optics of the arrangement should be
 modified to optimize the laser output 26 with regard to the combination of
 output power, bandwidth and spectral purity. The optics may then be
 manually or automatically adjusted in accordance with the computer's
 instructions/suggestions. Particularly with respect to the fourth
 embodiment, the aperture size may be modified. Generally, the detector 28,
 the high resolution spectrometer 29 and the computer 30 may be used with
 any of the embodiments of the present invention to help achieve the task
 of optimizing the combination of the output power, the bandwidth and the
 spectral purity of the output beam, e.g., 26. Alternatively with respect
 to the fourth embodiment, a feed back circuit may be used for real time
 monitoring of the bandwidth, output power and spectral purity of the
 output beam 26 and adjustment of the aperture 19.
 FIG. 6 shows an arrangement of a resonator of an excimer laser in
 accordance with a fifth embodiment of the present invention. The fifth
 embodiment preferably includes wavelength selection and narrowing optics
 32 including the prism 4 of the first embodiment of FIG. 3, and the
 housing for the laser active material 1 of the first embodiment of FIG. 3.
 Alternatively, one or both of these elements 1, 4 may be substituted by
 another element disclosed in one or more other embodiments of the present
 invention, e.g., the second embodiment. Additionally, the fifth embodiment
 includes a curved grating 13 and a curved output coupling mirror 45. The
 two curved optical surfaces together form an unstable resonator
 configuration. The curvature of each element 13, 45 may be convex or
 concave, but preferably the output coupling mirror 45 is convex like the
 output coupler 15 of the first embodiment and the grating is concave, like
 the grating disclosed as element 40 in U.S. Pat. No. 5,095,492 to
 Sandstrom. Preferably, the combination of the curvatures of the output
 coupler 45 and the grating 13 cause the resonator of the fifth embodiment
 to be unstable to match the divergence for optimizing the combination of
 the output power, the bandwidth and the spectral purity of the output beam
 36. In addition, the radius of curvature of either the grating 13 or the
 output coupler 45, or both, may be adjustable. The resonator of the fifth
 embodiment may be, and preferably is, an unstable resonator, such as that
 described with respect to the first embodiment.
 FIG. 7 shows a peak embodying the spectral distribution of the output beam
 10 of FIG. 1. The output beam 10 is determined to have a bandwidth of 1.1
 pm, calculated as the full-width at half-maximum (FWHM) of the peak of
 FIG. 8 embodying the spectral distribution of the output beam 10.
 FIG. 8 shows a peak embodying the spectral distribution of the output beam
 16 of FIG. 5A, wherein the optical elements of the arrangement of the
 third embodiment included those included in the arrangement of FIG. 1 and
 an aperture 19 in front of the grating 3. The aperture 19 used in
 obtaining the spectrum of FIG. 8 reduced the bandwidth from 1.1 to 0.5 pm
 by geometrically halving the divergent output beam in front of the grating
 3.
 FIG. 9 shows a peak embodying the spectral distribution of the output beam
 46 of another arrangement. An additional optical element 27 used to obtain
 the spectrum of FIG. 9 was a cylindrical lens, having a focal length of
 preferably several meters, placed between the housing for the laser active
 material 21 and the prism 24. The prism 24 used was a prism expander such
 as that described with respect to the second embodiment of FIG. 4.
 FIG. 10 shows a peak embodying the spectral distribution of the output beam
 16 of the first embodiment of FIG. 3. The bandwidth was reduced from 1.1
 to 0.5 pm by using the convex-curved output coupler 15 instead of the
 conventional output coupler 5 of FIG. 1.
 An advantage of all of the above embodiments and improvements is that the
 bandwidth of the output beam of the excimer laser system to be used in
 microlithographic applications is reduced, while the overall laser
 efficiency is influenced only slightly. The reason is that only the
 central or principal part of the resonating beam traverses the main
 amplification region of the laser active medium after it has encountered
 one of the improved or additional optical elements of the present
 invention. Moreover, an improvement in spectral purity and stabilization
 of the beam location, pointing and exit positions is observed when one of
 the embodiments or improvements of the present invention is used over that
 of a conventional arrangement such as that shown in FIG. 1. The
 combination of output power, bandwidth and spectral purity is optimized by
 using or combining one or more embodiments of the present invention by
 decreasing an acceptance angle of the resonating beam and/or matching or
 adapting the divergence of the resonating beam.