Abstract:
A method and apparatus are provided for temporally stretching and smoothing of the pulses of an output beam of excimer and lithography lasers. The method and apparatus are based upon providing an optical delay line or circuit having a plurality of optical reflectors and a plurality of beam recombiners or splitters so arranged as to divide the pulse into numerous portions which vary in their travel time through the circuit. As a result, the energy of the incident pulse is greatly stretched and smoothed.

Description:
RELATED APPLICATION 
     This application claims the priority benefit under 35 U.S.C. 119(e) of U.S. Application No. 60/130,392, filed Apr. 19, 1999, and which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for stretching and smoothing the laser pulses of a ArF and F 2 -excimer lasers by using optical delay lines. The optical delay lines allow the shaping and tuning of the temporal beam profile of the laser pulses. More particularly, the present invention provides optical delay lines which provide multiple parallel beam paths and a plurality of locations for output coupling. 
     BACKGROUND OF THE INVENTION 
     Deep UV excimer lasers with wavelengths below 200 nm and high pulse energies of several mJ are applied in the photolithography resist exposure process used in the production of integrated circuits. For photolithography imaging very expensive and large optical lenses are used. The shorter the wavelength, the greater is the problem of photoinduced damage of the employed optical components. Color center formation and compaction effects are examples of such damage. The occurrence of optical material damage depends nonlinearly on the peak intensity of the applied laser pulses. If it is possible to stretch the pulse by a factor of 2 or greater, thereby reducing the peak intensity of the pulse, the lifetime of optical components can be increased by almost one order of magnitude. In other words, the longer the duration of the light pulse and the lower the peak beam intensity, the longer the lifetime of all optical components in the microlithography setup. 
     Today, the only light source in the deep UV spectral range with sufficient power for industrial applications are excimer lasers. Due to the highly complex electrical discharge process needed for the generation of light emitting excimer states, the laser pulse duration has an upper limit of a few tens of nanoseconds. At present, pulse durations of about 30 ns can be achieved; in the future pulse durations of about 50 ns should be possible by appropriate modification of electrical discharge circuits. The disadvantages of pulse stretching methods using electrical rather than optical methods are that other important laser beam parameters such as pulse-to-pulse stability or pointing stability are strongly influenced as well. Furthermore, electrical pulse stretching can be optimized usually for one special type of laser only, whereas optical pulse stretching is completely independent of the given laser configuration. 
     The Koybashi et al. patent (JP 6-214187) discloses a prior art device which functioned as an optical delay line or circuit for a pulsed laser beam. FIG. 1 is an example of this prior art delay circuit. A laser beam ( 1 ) exciting the laser encounters a flat partially reflective mirror ( 2 ) which splits the incident laser beam into a portion ( 1 A) which is transmitted as an output beam and into a portion ( 1 B) which is to be delayed. The portion to be delayed ( 1 B) is reflected perpendicularly toward a circuit of four flat reflective mirrors ( 3 ) whose reflective surfaces are so arranged as to return the reflected portion of the incident light beam to the partially reflective mirror ( 2 ). These mirrors comprise an optical delay circuit. The circuit delays the transmission of incident light by providing a longer path for light conduction. Upon reencountering the partially reflective mirror ( 2 ), the delayed portion is split once again. However, this time it is the reflected light portion which exits the optical delay device. This exiting light is delayed by an amount of time required to travel through the light pathway defined by the four reflective mirrors. The delayed portion which is transmitted through the partially reflective mirror is iteratively subjected to the above process. This extra and variable time required to traverse the circuit stretches the pulse out temporally. Light transmitted through the partially reflective mirror ( 2 ) was subject to refraction (not shown). One limitation of this prior art device was therefore the effect of refraction on the optical spread of the laser beam. 
     The Kobayshi et al. patent introduced several improvements to the above system which were focused upon stabilizing the optical axis of a processed laser beam of no specified wavelength and without apparent respect to any particular industrial application. One improvement was the use of an optical fiber, rather than a system of mirrors, to delay the transmission of a portion of an incident light beam. However, while this approach eliminates the problems due to refraction and the need to keep reflective surfaces tightly aligned, an important limitation was the substantial transmission losses of light energy by passage through the optical fiber. Such losses especially limit the applicabiity of this design to UV and DUV laser light. 
     Another improvement introduced by Kobayashi et al. was to add a compensation plate to partially correct for the effect of refraction on output beam divergence. Kobayashi also evacuated the optical delay circuit to eliminate the absorption of the light by air and the influence of air on the alignment of the reflective surfaces. FIG. 2 shows an optical delay circuit which operated within an evacuated chamber (not shown) and employed prisms ( 4 ) as the internal reflective means. A compensation plate ( 5 ) was introduced to correct the effects of refraction on the light beam as it passed through the partially reflective plate ( 2 ). One limitation of this compensation plate arrangement was that light beams which underwent multiple refractions upon passage through multiple loops of the optical delay circuit, were corrected for only a single refraction event, and not the additional refractions involved in their multiple or iterative passages through the circuit. In another improvement, Kobayashi et al. addressed this limitation by substituting a cubic beam splitter (not shown) for the partially reflective mirror ( 2 ). The cubic beam splitter presented a perpendicular face to each incident laser beam to avoid refraction of the incident beams and the effects of refraction on the optical axis. 
     The design of the above optical delay lines limits their ability to stretch out a laser pulse and smooth the temporal profile of the optical pulse. For instance, where the laser light is half-reflected and half-transmitted upon each passage through the beam splitter, one-half the initial beam exits relatively unimpeded, one-quarter exits impeded by one circuit of the delay path, and only one quarter of the incident light is impeded by more than one passage through the optical delay circuit. A very small proportion of the incident light therefore is delayed by iterated passages through the optical delay circuit; and as much as half is emitted unaffected by the optical delay methods. Thus, the peak light intensity is not as broadly distributed and reduced as necessary to satisfactorily reduce the damage associated with the peak light intensity. 
     There is thus a need for a novel designs of optical delay devices for stretching and smoothing the laser pulses of ArF- and F 2 -excimer laser for microlithography applications where typical pulse durations fall in the nanosecond range (typically 20 to 50 ns). 
     SUMMARY OF INVENTION 
     The object of the invention is to stretch the laser pulse of ArF- and F 2 -excimer lasers using various improved designs of optical delay lines, including, if necessary, imaging optics. To provide stable optics, the delay line comprises a minimum of optical components with high reflecting coatings and uses as much as possible total internal reflection for beam direction changes. Furthermore, by using delay lines with multiple points of output coupling and multiple, parallel beam pathways, it is possible to achieve an even smoother shape and greater stretching of the temporal pulse profile. By this method it is possible to greatly smooth the temporal pulse envelope in order to avoid peaks with high intensity. 
     By using optical pulse stretching, the lifetime of optical components, especially for microlithography applications, can be increased to up to one order of magnitude. 
     By this invention the already known principle of optical pulse stretching by employing optical delay lines is further developed for deep UV (DUV) ArF- and F 2 -excimer lasers with a pulse length of a few tens of nanoseconds. The invention contains methods to achieve very high pulse energy transmission even in the deep UV. Using this technique, stretching factors &gt;2 can be easily achieved. 
     The instant invention relates to a method for stretching of laser pulses of ArF- and F 2 -excimer laser for microlithography applications with typical pulse durations in the nanosecond range (typically 20 to 50 ns). 
     This special layout of delay lines can also be used for applications in the visible and infrared spectral range (e.g., with CO 2  or Nd-YAG laser) for any application where pulse stretching is required in order to fit the industrial process conditions. 
     One could also use other beam splitters or beam combiners, for example methods, which make use of different polarizations of the direct beam and the delayed beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art optical delay line or circuit. 
     FIG. 2 is a schematic diagram of a prior art optical delay circuit which partially corrects for the effects of refraction on the pulse. 
     FIG. 3 is a schematic diagram of an optical delay line having a plurality of optical reflectors and a plurality of beam recombiners to stretch a laser pulse while correcting for the effects of refraction. 
     FIG. 4 shows a schematic diagram of an optical delay line employing prisms to stretch laser beam. 
     FIG. 5 shows a schematic diagram of an optical delay line allowing a cascade of laser pulse portions within the circuit. 
     FIG. 6 shows a schematic diagram of an optical delay circuit using imaging optics to correct for divergence of the pulse. 
     FIG. 7 is a graph depicting the optical stretching provided by a 6 nanosecond (ns) roundtrip in a delay circuit. 
     FIG. 8 is a graph depicting the optical stretching achieved by a 2-cascade delay line with a 6 ns delay in each line. 
     FIG. 9 is a graph depicting the optical stretching achieved by a 2-cascade delay line with a 12 ns delay in each line. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The object of this invention is to stretch the emitted laser pulse of ArF- and F 2 -excimer lasers by using various types of optical delay lines and, if necessary, imaging optics. Furthermore, using delay lines it is possible to shape and to tune the temporal beam profile. In order to achieve this there are various possible setups of optical delay lines. 
     In the deep UV spectral range it is difficult to achieve high transmission for the radiation that passes through a delay line due to high absorption and scatter losses on dielectrically coated mirrors. Therefore, it is necessary to use a method that employs a minimum of high reflection coatings. Especially for industrial applications a delay line setup with a minimum of adjustment sensitivity has to be chosen. Several possible configurations are depicted in FIGS. 3-6. 
     In FIGS. 3 and 4, a second partially reflective mirror ( 6 ) is provided which provides a second point for diversion of a portion of the laser beam into the delay circuit and for output coupling of a portion of the beam traveling the delay circuit. The device works thusly. As in FIG. 3, the laser beam ( 1 ) is split by a partial reflector ( 2 ) (preferably of 50% reflectivity) into a transmitted portion ( 1 A) and a reflected portion ( 1 B). The reflected portion ( 1 B) of the incident laser beam is thereby reflected into the optical delay circuit defined by mirrors ( 3 ). The optical delay circuit of FIG. 4 differs from that in FIG. 3 only in that the optical delay circuit of FIG. 4 is defined by prisms using total internal reflection ( 4 ). In both FIGS. 3 and 4, this reflected portion ( 1 B) afterwards encounters a second partial reflector ( 6 ) (second beam recombiner) (preferably of 50% reflectivity) wherein the beam is split again into a portion which is reflected so as to recombine with the incident beam stream ( 1 A) which was transmitted without reflection at both the first ( 2 ) and second ( 6 ) partially reflectors. The portion of the reflected beam which was transmitted through the partial reflector ( 6 ) continues its progress through the optical delay circuit until it once again encounters the first partial reflector ( 2 ). At that point, it is split into two portions once again. One portion recombines with the portion ( 1 A) incident laser beam which is transmitted by the first partial reflector ( 2 ) and a portion which now begins to repeat the process of traveling through the optical delay circuit, and so on. 
     The portion of the incident laser beam which was originally transmitted through the first partial reflector ( 2 ) proceeds to the second partial reflector ( 6 ) where it is again split into two beams. A portion is transmitted through the second partial reflector ( 6 ) and exits the optical delay device without having progressed through the optical delay elements of the optical delay circuit. Another portion is reflected by the partial reflector ( 6 ) and is reflected into the optical delay circuit and progresses through the circuit from that point as above. 
     After each round trip or partial round trip, a portion of the delayed laser pulse is coupled out and the remaining light travels another partial or full round trip through the delay line. By this method a greatly improved prolongation of laser pulses is obtained. The length of laser pulses after having passed the delay line depends on the optical path length inside the delay line. Assuming that 50% reflection/transmission is achieved at each partial reflecting mirror, this configuration represents a substantial improvement over the prior art. The addition of a second partial reflector in the optical delay device allows only 25% of the initially incident light to avoid the delay circuit. Further, by providing for light paths which are one-half the total circuit length, the arrangement allows for a smoother temporal stretching of the laser pulse. By this method, the maximum peak intensity can be substantially reduced and pulse stretching of more than a factor of 2 can easily be achieved. 
     Another advantage of this arrangement is that all light portions propagate only in the direction of the delay line output, no light is referenced back. Therefore, high transmission values of the delay line are achievable. Further, placement of a second partially reflective mirror ( 6 ) in the optic delay circuit corrects for the refraction associated with each passage through the first partially reflective mirror ( 2 ). 
     To achieve high transmission of the delay line in the deep UV spectral range the whole system is tightened and flushed with nitrogen or any other non-reacting gas that does not show any light absorption in the spectral range between 150 and 200 nm. Alternatively, when it is desired to reduce gas absorption of light energy, the whole delay line can also be evacuated. 
     There are two additional methods to achieve high prolongation factors (&gt;2). One method is to realize a long delay line with either large geometrical dimensions or many optical components for beam folding. The disadvantage with both methods stems from the great difficulty in achieving a high optical stability for the whole system. On the other hand, using cascaded delay lines (i.e., delay lines where a light portion undergoes direction reversals and multiply encounters an optical element prior to completing a full circuit) or multiple passes through a single delay line as in FIG. 5 can allow for very large extension values (&gt;2 or 3) without a corresponding increase in geometry. An advantage of cascaded delay lines is that each single delay line of a cascaded set-up has small geometrical dimensions and the whole delay setup is modular, i.e., the user can combine as much delay lines as it is necessary for his application. The disadvantage is, that in special cases quite a lot of optical components are necessary. 
     As indicated by the light paths in FIG. 5, portions of the incident light beam which are reflected at any of the four encounters with a beam splitter are reflected into the optical delay circuit. An incident light beam must first be transmitted through a first beam splitter ( 15 ), a second beam splitter ( 16 ), and then after being reflected by a prism ( 14 ) be retransmitted a second time through both the second ( 16 ) and first ( 15 ) beam splitters. Thus, an incident light beam has four opportunities to be reflected into the optical delay circuit before exiting the device. Assuming 50% reflection/transmission at each optical reflector/beam splitter, approximately only {fraction (1/16)} th  of the incident light beam passes through without some delay due to diversion into the optical delay circuit. 
     Each additional interaction with a beam splitter greatly increases the likelihood that some portions of the incident beam which were reflected into the optical delay circuit will be retained in the circuit for a greater travel distance, and therefore a longer period of time. For example, the portion of an incident beam which was initially reflected at the first beam splitter, will travel from the first minor prism ( 11 ) to the major prism ( 13 ) to a second minor prism ( 12 ) and then to the second beam splitter ( 16 ). If a portion of this beam is to directly proceed from this point to the exit point from the optical delay device, it must first be reflected by the second beam splitter ( 16 ), whereupon it is reflected by the return prism ( 14 ) to the second beam splitter ( 16 ) again where upon it must be transmitted through both the second beam splitter ( 16 ) and the first beam splitter ( 15 ). Only one-eight of the initially reflected beam (or one-sixteenth of the incident light beam) will therefore follow this most direct route to exit the optical delay circuit. 
     To achieve the same beam properties of the laser beam after each round trip imaging optics can be introduced into the delay line. FIG. 6 illustrates the use of optical lenses to correct for beam distortions involved in the transit of light through the optical delay circuits. The imaging optics may be coated with anti-reflective materials. The imaging optics can also be finely adjustable to shape and tune the temporal profile of said pulse. Thus, any beam distortions due to the high divergence of excimer laser radiation can be corrected. 
     The Time Integral Square:        TIS   =           [     ∫       I        (   t   )               t         ]     2       ∫         I   2          (   t   )               t                                   I        (   t   )       -     Intensity                 of                 the                 light                 t   -   time                                    
     provides a measure of how much a laser pulse has been stretched out over time. This measure is therefore useful in evaluating the performance of optical delay systems. FIG. 7 shows the initial 30 ns of an unstretched excimer laser pulse and the stretched pulse after having passed a delay line with a round trip delay of 6 ns. In this case, the Time Integral Square could be increased by a factor of only 1.63. FIG. 8 show the pulse stretching after 2 cascaded delay lines with 6 ns delay time in each delay line. The TIS is now prolonged by a factor of 2.14. In FIG. 9 the original and the stretched pulse after 2 cascaded delay lines with 12 ns delay time in each delay line is shown. The TIS was prolonged by a factor of 2.76. The substantial TIS factors which are obtained with these methods are capable of substantially prolonging the life of optical materials, such as those used in microlithography, whose photodegradation is especially aggravated by the peak intensity of a light pulse. 
     Those skilled in the art will appreciate that the just disclosed preferred embodiments are subject to numerous adaptations and modifications without departing from the scope and spirit of the invention. For instance, dielectric mirrors may be used to direct or split the incident laser light. The reflectivity of the mirrors of the optical circuit may be varied from 5% to 95%. The length of the delay lines can be finely adjusted to smooth out the intensity spikes. The optical elements may be made of a variety of suitable materials including calcium flouride (CaF 2 ), fused silica, sapphire, barium flouride (BaF 2 ) and magnesium fluoride (MgF 2 ). 
     Therefore, it is to be understood that, within the scope and spirit of the invention, the invention may be practiced other than as specifically described above. The scope of the invention is thus not limited by the particular embodiments described above. Instead, the scope of the present invention is understood to encompass the claims that follow, and structural and functional equivalents thereof.