Abstract:
An optical system for projecting a laser-beam on a mask to illuminate the mask includes a beam homogenizing arrangement including spaced arrays of microlenses. The beam homogenizing arrangement redistributes light in the laser beam such that the intensity of light in the laser-beam on the mask is nearly uniform along a transverse axis of the laser-beam. A stop extending partially into the laser-beam between the microlens arrays provides a more uniform light-intensity on the mask along the transverse axis than can be achieved by the microlens arrays alone.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to optical systems for projecting an image of a mask on a substrate in laser material processing applications. The invention relates in particular to methods and apparatus for uniformly illuminating the mask. 
     DISCUSSION OF BACKGROUND ART 
     In laser material processing applications, such as crystallization, annealing, or nozzle drilling systems, a certain spatial distribution of laser radiation on a substrate or material being processed is often required. One well-known method of providing the spatial distribution includes illuminating an area of a mask which has a pattern of apertures therein with the laser radiation, and projecting an image of the aperture patterns on the substrate. Certain applications, particularly laser crystallization, demand a very high degree of uniformity of illumination of the mask. 
     Several arrangements have been used or proposed for providing such uniform illumination on a mask. The complexity of the arrangements is usually inversely dependent on the quality of the laser radiation delivered from the laser providing that radiation. More complex designs are required for lasers that provide beams that are multimode in at least one axis, are not symmetrical in cross-section, or have an intensity distribution that is not Gaussian in at least one axis. The effectiveness of any such arrangement, of course, can be compromised if the distribution of radiation in the beam varies with time. This can occur in gas-discharge lasers, particularly in high-pressure, pulsed gas-discharge lasers such as excimer lasers. Such variations can be random variations on a spatial scale that is a fraction of the overall dimensions of the laser-beam, and can appear as spatial modulations in a more general distribution of the radiation on the substrate. The variations can also be longer term, temporal variations that effect primarily the general distribution of the radiation on the substrate. Optical arrangements for re-distribution of radiation in a laser-beam have relied on using devices such as anamorphic optical systems, diffractive optical elements, and “beam homogenizing” devices such as microlens arrays, diffusers, and light-pipes. 
     In prior-art excimer-laser projection systems it has been possible to provide a general or intensity variation as low as between about 1% about 2% of nominal over the illuminated area using a combination of anamorphic optical elements and anamorphic microlens arrays to shape and homogenize radiation in the laser-beam. Radiation distribution at this level of uniformity often rises from a low level at edges of the illuminated area to a maximum at the center of the illuminated area. This is sometimes referred to by practitioners of the art as a “center-up” distribution. In certain demanding applications, laser crystallization in particular, an absolute intensity variation of less than 1.5% is preferred. When random and temporal variations of energy distribution are combined with the 1% and 2% general energy distribution variation of 1.5% or less is difficult to achieve consistently. Accordingly, there is a need to reduce the variation in general distribution of energy below the level that has been achieved to date in prior-art laser projection systems. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for illuminating a mask with a beam of radiation from a laser. In one aspect, the present invention comprises directing the laser beam through a plurality of optical elements located on a longitudinal axis. The optical elements are arranged to project the beam onto the mask to illuminate the mask. The configuration and arrangement of the optical elements is selected such that the intensity of radiation in the laser-radiation beam on the mask is nearly uniform in a transverse axis of the beam. Uniformity of radiation in the laser-radiation beam on the mask in the transverse axis is optimized by partially blocking at least one edge of the laser-radiation beam at a location between selected ones of the optical elements. 
     In another aspect of the invention, the edge blocking of the laser-radiation beam is accomplished by a least one stop extending partially into the laser-radiation beam at the selected location. In one preferred embodiment of the invention, the stop has a width less than the transverse-axis width of the laser-radiation beam and the stop has a rounded tip at an end thereof extending into the laser-radiation beam. In one example, the nearly uniform distribution provided by the optical elements is the above discussed “center-up” distribution having a single, central, peak value and a 2σ (two standard deviations) uniformity of about 2.08%. In one uniformity-optimization provided by the edge-blocking with the stop, the optimized distribution has two peak values having a centrally located trough value therebetween, and has a 2σ uniformity of about 1.36%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
         FIG. 1  is a three-dimensional view schematically illustrating an excimer laser projection system in accordance with the present invention including an excimer laser delivering a laser-beam having a long-axis and a short-axis perpendicular to each other, an anamorphic telescope arranged to expand and shape the laser-beam, a beam homogenizer including two pairs of cylindrical-microlens arrays for spatially redistributing energy in the expanded, shaped laser-beam, a narrow stop arranged to partially block the expanded, shaped and partially homogenized beam between two of the microlens arrays, and condensing and field lenses for focusing the shaped, homogenized beam onto a mask. 
         FIG. 2  schematically illustrates one preferred example of the beam-stop of  FIG. 1  having a rounded tip for insertion into the beam. 
         FIG. 3  is an elevation view of the projection system of  FIG. 1  seen in the short-axis of the laser-beam, and schematically illustrating a preferred positioning of the beam-stop between arrays in one pair of the microlens arrays of  FIG. 1  and illustrating further detail of the condensing optics and mask. 
         FIG. 4A  is an elevation view seen in the short-axis of the laser-beam, and schematically illustrating details of the beam-stop and the beam between the microlens arrays of  FIG. 3 . 
         FIG. 4B  is a plan view from above seen in the long-axis of the laser-beam, and schematically illustrating further details of the beam stop and the beam between the microlens arrays of  FIG. 4A . 
         FIG. 5  is a graph schematically illustrating intensity as a function of distance along the long-axis of the beam on the mask in one example of the projection system of  FIG. 1  from which the beam stop has been removed from the beam. 
         FIG. 6  is a graph schematically illustrating intensity as a function of distance along the long axis of the beam on the mask in another example of the projection system of  FIG. 1  in which the beam stop is of the form depicted in  FIG. 2 , and aligned with the propagation axis of the beam and partially inserted into the beam by an experimentally determined distance along the short-axis direction of the beam. 
         FIG. 7  is a graph schematically illustrating intensity as a function of distance along the long axis of the beam on the mask in yet another example of the projection system of  FIG. 1  in which the beam stop is of the form depicted in  FIG. 2 , and aligned with the propagation axis of the beam and partially inserted into the beam along the short axis direction of the beam beyond the distance of the example of  FIG. 6 . 
         FIGS. 8A-D  are three-dimensional views schematically illustrating alternate arrangements of two or more beam stops between the microlens arrays of  FIG. 3 . 
         FIG. 9  is a three-dimensional view schematically illustrating an arrangement of a beam stop in accordance with the present invention between microlens arrays of another pair of microlenses of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1 ,  FIGS. 2A and 2B ,  FIG. 3 , and  FIGS. 4A and 4B  schematically illustrate an embodiment 10 of an optical system in accordance with the present invention for projecting an image of a mask on a substrate. An excimer laser (not shown) delivers a beam  14  propagating along a system axis (the Z-axis in an X, Y, Z, Cartesian axis system). In an optical system such as system  10  it is usual to provide a variable attenuator (also not shown) to allow power in the beam to be varied according to the application. A description of such an attenuator is not necessary for understanding principles of the present invention. 
     Beam  14 , on leaving the excimer laser, has an elongated cross-section. In one example of an excimer laser the beam leaving the laser has a width of about 12.0 mm and a length of about 35.0 mm. The length and width of the beam define the X and Y-axes, which are often referred to by practitioners of the excimer laser art as the long-axis and short-axis respectively. 
     Turning mirrors  42  and  44  direct the beam (after having traversed any attenuator) into an anamorphic telescope  18 , here, including cylindrical lenses  46  and  48  and a spherical lens  50 . The purpose of telescope  18  is to adapt the beam to the aperture of a beam-homogenizer formed by microlens arrays  54 ,  56 ,  58  and  60 . Details of the telescope and other important system groups are depicted in  FIGS. 1 and 3 .  FIG. 1  is a three dimensional view.  FIG. 3  is a view in the plane of the short-axis of optical system  10  showing further detail of components of system  10 . In  FIG. 3 , the long-axis appearance of certain components is schematically depicted in dashed lines and designated by reference numerals having a subscript L. In  FIGS. 1 and 3 , only the general direction of propagation of beam  14  is depicted, as a single line collinear with the longitudinal optical axis (the Z-axis) of system  10 . In  FIGS. 4A and 4B , multiple lines  14  depict bounds of the beam. 
     A turning mirror  52  directs the collimated beam into the beam homogenizing arrangement  20  comprising microlens arrays  54 ,  56 ,  58 , and  60 . Microlens array  54  includes a plurality of elongated plano-convex cylindrical microlenses  55  and microlens array  56  includes a plurality of elongated plano-convex microlenses  57 . Microlens arrays  54  and  56  can be described as the “long-axis beam-homogenizer”. Preferably there are twelve microlenses in each array, however, in  FIG. 3  only four microlenses are depicted in each array for convenience of illustration. The microlenses in each array are aligned parallel to the short-axis and have positive optical power in the long-axis and zero optical power in the short-axis. The microlenses in one array are arranged as a long-axis optical relay with corresponding microlenses in the other array. Beam  14  next traverses microlens arrays  58  and  60 , forming what can be described as the “short-axis beam-homogenizer”. Microlens array  58  includes a plurality of plano-convex cylindrical microlenses  59  and microlens array  60  includes a plurality of planoconvex microlenses  61 . Again, only four microlenses are depicted in each array for convenience of illustration. The microlenses in each array are aligned parallel to the long-axis and have positive optical power in the short-axis and zero optical power in the long-axis. The microlenses in one array are arranged as a short-axis optical relay with corresponding microlenses in the other array. 
     Located between microlenses  58  and  60  is an elongated partial-shutter or beam-stop  62 , details of a preferred form of which are schematically depicted in  FIG. 2 . Interaction of the stop with the beam, and a preferred location of the stop with respect to the beam are schematically depicted in  FIGS. 4A and 4B . The purpose of stop  62  is to prevent the above discussed “center-up” intensity distribution in an image projected on the substrate by the optical system. Stop  62  preferably has a width W (see  FIG. 2 ) that is between about 5% and about 50% of the long-axis width BW of beam  14  between microlenses  58  and  60  (see  FIG. 4B ). The stop preferably has a rounded tip  62 A having a radius about equal to W/2. The stop is preferably positioned over the longitudinal axis  15  of the optical system (see again  FIG. 4B ). The stop is preferably positioned closer to microlens array  60  (the exit microlens array of the short-axis beam-homogenizer) than to microlens  58  (the entrance microlens array of the short-axis beam-homogenizer), and most preferably positioned immediately adjacent the exit microlens array. It is also possible that stop  62  be located adjacent microlens array  60 , between lens  22  and microlens array  60 . Stop  62  preferably extends into the beam in the short-axis direction for a distance between about 3% and about 35% of the short axis beam height (see  FIG. 4A ). The stop must not, however, extend across the system optical axis. The optimum extension-distance may vary from system to system but can be quickly determined experimentally for any stop dimension in the preferred range. 
     After traversing the short-axis beam-homogenizer, the collimated beam  14  traverses a spherical lens  22  having positive power and is directed by turning mirrors  66 ,  68 , and  70  to a plano-convex cylindrical lens  24  having positive power in the short-axis and zero-optical power in the long-axis. After traversing lens  24  the beam traverses another plano-convex cylindrical lens  26 . Lens  26  has positive power in the long-axis and zero-optical power in the short-axis. An effect of lenses  22 ,  24 , and  26  is project beam  14  on a mask  28  with an elongated cross-section (indicated in  FIG. 1  by dashed line  30 ) having a length between about 25 mm and 125.0 mm and a width  8  between about 3 mm and 25 mm. That portion  14 S (see  FIG. 3 ) of beam  14  passing through patterns of apertures (not shown) is directed by turning mirrors  72  and  74  to an imaging lens  32 . Imaging lens  32  focuses light  14 S as an image (not shown) of the aperture patterns in mask  28 . The long-axis distribution of light intensity on mask  28  produced by the above described optical elements (normally center-up) can be modified according to the shape and positioning of stop  62 . This modification is discussed below, beginning with reference to  FIG. 5 . 
       FIG. 5  is a graph schematically illustrating intensity as a function of distance along the long-axis of the beam on mask  28  in one example of the optical system  10  of  FIG. 1  from which stop  62  has been removed from the beam. Intensity distribution is measured between points designated by dashed lines L 5  and R 5 . It can be seen that between those lines the intensity rises steadily from each line never falling below the lowest value in the measurement range (indicated by horizontal line H 5 ) and reaching a peak value about mid-way between lines L 5  and R 5 . This is the above-described center-up distribution that stop  62  is able to modify. In this measurement, the intensity variation between the lines L 5  and R 5  is 2σ=2.08% (where σ is the standard deviation from the mean). 
       FIG. 6  is a graph schematically illustrating intensity as a function of distance along the long-axis of the beam on mask  28  in one example of the optical system  10  of  FIG. 1  including a stop  62  in accordance with the present invention. In this example, the long-axis beam width (BW) between microlens arrays  58  and  60  is about 100 mm. Stop  62  has a width W of about 15 mm with a rounded tip  62 A having a radius of about 7.5 mm. Microlens arrays  58  and  60  are axially spaced apart by about 330 mm, and stop  62  is located about 15 mm from microlens array  60 . Short-axis beam width BH at the location of stop  62  is about 25 mm. It is believed that stop  62  extends between about 3 mm and 6 mm into the beam in the short-axis direction into the beam. 
     It should be noted, in this regard, that the exact extension of the beam was not measured, and in fact, as the edge of the beam can not be precisely defined, an exact extension is equally difficult to define. An optimum extension of the stop was determined by testing various extension depths of the stop and measuring the long-axis intensity distribution of radiation at the mask level. 
     Intensity distribution is measured between points designated by dashed lines L 6  and R 6 . It can be seen that between those lines the intensity initially rises steadily from each line to a peak value close to each of the lines falling to a lower value, centrally, between the two peaks. The intensity, however, never falls below the lowest (edge) value in the range, indicated by horizontal line H 6 . In this measurement the intensity variation between the lines L 6  and R 6  is about 1.36% (2σ). 
       FIG. 7  is a graph schematically illustrating intensity as a function of distance along the long-axis of the beam on mask  28  in another example of the optical system  10  of  FIG. 1  including a stop  62  in accordance with the present invention. In this example, the dimensions of stop  62 , the spacing of the microlens arrays, the beam widths between the microlens arrays and the axial distance position of stop  62  from microlens array  60  are the same as in the example of  FIG. 6 . In this example, however, stop  62  extends deeper into the beam in the short-axis direction into the beam than in the example of  FIG. 6 . 
     Intensity distribution is measured between points designated by dashed lines L 7  and R 7 . It can be seen that between those lines the intensity initially rises steadily from each line to a peak value close to each of the lines falling to a value below the lowest (edge) value in the range, indicated by horizontal line H 7 . Further, there is significant, relatively high frequency, modulation over about one-half of the long-axis extent of the beam. This modulation has a peak-to-valley excursion comparable to the total intensity variation in the example of  FIG. 6 . In the graph of  FIG. 7 , the intensity variation between the lines L 7  and R 7  is about 7.14% (2σ). 
     In other experiments, the effect of placing a stop at other locations was investigated, for example, closer to microlens array  58  than to microlens array  60 , and at various positions between microlens arrays  54  and  56 . In each case, the effect was to produce modulation comparable to or greater than the modulation exhibited in the example of  FIG. 7 . 
     It is believed that a stop having a rounded tip, whether semicircular as in the examples described, or having some non-semicircular curvature such as elliptical, parabolic, or hyperbolic, will provide an intensity distribution having less modulation than would be produced by a tip having an angular form, however, the use of a stop having a tip of an angular form is not precluded. It is also possible that a variation of intensity less than 1.3% may be obtained by arranging two or more stops  62  in the edge of the beam. Some possible arrangements of the stops between microlens arrays  58  and  60  are schematically depicted in  FIGS. 8A ,  8 B,  8 C, and  8 D. 
     In the arrangement of  FIG. 8A  there are two stops, one thereof in an upper edge of the beam and the other in the lower edge of the beam. The stops, here, are aligned with each other, and aligned over system axis  15 . In the arrangement of  FIG. 8B  there are also two stops, but each thereof is in the upper edge of the beam, and the stops are aligned with one on either side of the system axis in the long axis direction. In the arrangement of  FIG. 8C  there are two stops in the upper edge of the beam aligned as in the arrangement of  FIG. 8B  and one stop in the lower edge of the beam aligned over the system axis as in  FIG. 8A . In the arrangement of  FIG. 8D , there is one stop in the upper edge of the beam and one stop in the lower edge of the beam. Here, the stops are aligned displaced from the system axis on opposite sides thereof. 
     It may also be possible to improve short-axis beam uniformity by inserting one or more stops into the beam between microlens arrays  54  and  56  of the long-axis beam homogenizer. An arrangement in which one stop is inserted is depicted in  FIG. 9 . Here, the stop extends partially into the beam in the long-axis direction. Those skilled in the art will recognize without further illustration or detailed description that multiple stop arrangements are also possible for improving short-axis beam uniformity. 
     It is emphasized, here, that the multiple stop arrangements described above are merely a sample of possible such arrangements that may provide improved beam uniformity. Whatever the number and alignment of the stops, however, each stop should have a width less than the long-axis beam width at the location of the stops, and should not extend into the beam across the system axis. It is also emphasized that while the present invention is described above in the context of a particular excimer-laser projection system in which the efficacy of the invention has been experimentally determined, the invention is applicable in other laser projection systems having a different arrangement of beam shaping, projection optics, or beam homogenizing optics. 
     The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.