Abstract:
An system and method that expand emitted light from a laser beam without changing spatial coherence or producing speckle. The system includes a laser source and an optical projection system having a multiplexing device. The multiplexing device expands light emitted by laser source into plural beams having light intensity about equal to each other without changing spatial coherence. The multiplexing device has a plurality of spatially separated beam splitters positioned parallel to and on a same side of a mirror. The system further includes an illuminating optical system that focuses each of the plural beams and a projection optical system that projects an image of a mask illuminated with light output from illuminating optical system onto a substrate.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a system and method for expanding a laser beam without expanding its spatial coherence.  
           [0003]    2. Background Art  
           [0004]    In lithography, or other environments (e.g., holography), expansion of an excimer laser beam or deep UV (DUV) excimer laser beam is necessary because an illumination system field is typically much bigger than the laser beam. Typically, laser beams are 10 mm×10 mm or 5 mm×20 mm, while an illumination field may be 120 mm×25 mm. Although the laser beam is described as having a rectangular or square cross-section, various cross-sections of light can be used. Generally, lithography devices use an arrangement consisting of one reflector and one partial reflector (or beam splitter) to preliminarily expand the laser beam in an optical multiplexer before expanding the preliminarily expanded beam further in other parts of the lithography tool. Unfortunately, expansion with typical optical devices (lenses, prisms) increases the spatial coherence of the laser and creates a speckle problem. Therefore, other optical devices can be used. The drawback of using the reflector/beam splitter arrangement is that it requires a complicated design of a “staircase” partial reflector, which consists of patches of coatings having a stepwise change in reflectivity based on predetermined parameters. This arrangement requires an exact match of the size and position of the laser beam and the “staircase” patch pattern. Also, a practical implementation of the “staircase” partial reflector leads to uncoated areas between the patches and the expanded beam, which results in a “zebra” pattern with dark areas cutting through bright areas of a beam cross section. Further, excimer lasers have a tendency to change the beam size and divergence over the time.  
           [0005]    Therefore, a system and method for expanding an emitted light from a laser without changing spatial coherence of the light, without producing speckle patterns, and that eliminates the requirement for the “staircase” partial reflectors is needed.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    Embodiments of the invention provide an optical system comprising a laser source and a multiplexing device. The multiplexing device has a plurality of spatially separated beam splitters positioned parallel to and on a same side of a minor. The multiplexing device expands light emitted by laser source into plural beams having light intensity substantially equal to each other without changing spatial coherence. The optical system further comprises an illuminating optical system that focuses each of the plural beams and a projection optical system that projects an image of a mask illuminated with light output from illuminating optical system onto a substrate.  
           [0007]    Other embodiments of the invention provide a light multiplexing device comprising a reflector and a plurality of spatially separated beam splitters positioned on a same side of and parallel to the reflector. The multiplexer expands light emitted by a laser source into a plurality of beams having light intensity substantially equal to each other without changing a spatial coherence of light emitted by laser.  
           [0008]    Some advantages provided by the embodiments of the invention are that a laser beam is expanded without changing its spatial coherence and without producing speckle patterns through the use of uniform partial reflectors that are much easier to manufacture and produce than “staircase” beam splitters. Another advantage is that it is less critical that the laser beam be accurately aligned with respect to beam splitters, which is critical in the previous devices.  
           [0009]    Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
       [0010]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment(s) of the invention and, together with the description, explain the purpose, advantages, and principles of the invention.  
         [0011]    [0011]FIG. 1 depicts an optical system according to an embodiment of the invention.  
         [0012]    FIGS.  2 A- 2 B depict optical multiplexer elements and light travel within a portion of the optical system in FIG. 1, according to embodiments of the present invention.  
         [0013]    [0013]FIG. 3 depicts optical multiplexer elements and light travel within a portion of the optical system in FIG. 1, according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 4 depicts optical multiplexer elements and light travel within a portion of the optical system in FIG. 1, according to an embodiment of the present invention.  
         [0015]    [0015]FIG. 5 depicts an adjustment system and multiplexer elements within a portion of the optical system in FIG. 1, according to an embodiment of the present invention.  
         [0016]    In the drawings, most like reference numbers indicate the same or substantially the same elements. Furthermore, the left-most digit(s) of the reference numbers indicate the number of the drawing in which the reference number is first used. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    A system  100  for expanding light  102  emitted from a laser  104  without changing spatial coherence of the light  102  and that substantially eliminates speckle patterns is shown in FIG. 1. The laser  104  can be an excimer or deep UV excimer laser. The light  102  is received by a multiplexer  106  in a beam conditioner  108 . The beam conditioner  108  outputs light to illumination optics  110 , which in turn transmits light through a mask or reticle  112  onto a substrate  116  via projection optics  114 . One embodiment for this system can be a lithography system, or the like. Another embodiment can be a holography system. Although expansion is performed by multiplexer  106 , multiplexer  106  can be a pre-expansion system or first expansion system that expands the light about four to six times, while further expansion can be carried out by other optics in system  100 . By using the pre-expansion system  106 , speckle and other problems related to conventional expansion system can be substantially eliminated.  
         [0018]    Turning to FIG. 2A, an embodiment of the multiplexer  106  is shown. The multiplexer  106  comprises a reflector  200  with a reflecting surface  202  that lies in a plane extending from the reflecting surface  202 . First and second beam splitters  204  and  206 , which can be  50 / 50  or any other ratio beam splitters having a multilayer dielectric coating than can produce expanded beams with about equal intensities, are located on a same side of the reflector  200  and lie in planes that are parallel to the plane extending from the reflecting surface  202 . A distance d between the reflector  200  and the first beam splitter  204  is equal to a same distance d between the first beam splitter  204  and the second beam splitter  206 . The distance d is defined by an angle α, which is an angle the light  102  intersects an axis of symmetry  208  of the first beam splitter  204 , and a width a of the beam  102  according to the following formula:  
           d=a /(2*sin α).  (1)  
         [0019]    Also, angle α, the width a of the beam  102 , and the temporal coherence length L of the laser  104 , are related according to the following formula:  
         tan α&lt; a/L.   (2)  
         [0020]    Further, the first beam splitter  204  is laterally shifted by b and the second beam splitter  206  is laterally shifted by  4   b  relatively to an edge  210  of the reflector  200 , where:  
           b=d *tan α.  (3)  
         [0021]    Ideally, angle α is much smaller than a/L. In some embodiments, a value for angle α would be chosen and the other parameters would be calculated based on the chosen value.  
         [0022]    The temporal coherence length L of the laser  104  is defined by λ 2 /Δλ, where Δλ is the spectral range of the radiation and λ is the central wavelength of the laser  104 . As an example, wavelength&#39;s used in typical excimer lasers for microlithography are 248, 193, and 157 nm. Spectral range of radiation varies depending on the design of the lithographic tool and laser. The spectral range of radiation can be as small as 1 pm and as broad as 100 pm. Thus, the range of coherence lengths L can be from 0.25 mm to 40 mm.  
         [0023]    The side of the width a used for calculations is based on which side of the laser beam  204  needs to be expanded. In one example of ranges for the different parameters a light beam can be 5 mm×20 mm. Hence, the width a is 5 mm and is expanded four times. In other embodiments, expansion of width a can be 4 to 6 times. In this example the temporal coherence length L is 20 mm, although L varies depending on spectral range, and the incident angle α is 10° (degrees). Thus, in this example, d=5 mm/2*sin 10=14.4 mm and b=14.4 mm*tan(10)=2.54 mm.  
         [0024]    In operation of the embodiment shown in FIG. 2A, the light  102  emitted by the laser  104  is received at a predetermined angle α at the first beam splitter  204  that reflects a first portion of the light  102  toward the reflector  200  and transmits a second portion of the light toward the second beam splitter  206 . The reflector  200  receives the first portion of the light  102  and reflects a third portion of the light  102  toward the second beam splitter  204 . The second portion of the light  102  is received at the second beam splitter  206 , which reflects a fourth portion of the light  102  toward the reflector  200  and transmits a fifth portion of the light  102  to produce a first output beam  212 . The third portion of the light  102  is received at the second beam splitter  206 , which reflects a sixth portion of the light  102  toward the reflector  200  and transmits a seventh portion of the light  102  to produce a second output beam  214 . The reflector  200  receives the fourth portion of the light  102  and reflects an eighth portion of the light  102  to produce a third output beam  216 . Finally, the reflector receives the seventh portion of the light  102  and reflects a ninth portion of the light  102  to produce a fourth output beam  218 . The first through fourth output beams  212 - 218  can be equal in intensity, and are about 25% the intensity of the input beam  102 . One way this can be done is using 50/50 beam splitters.  
         [0025]    As seen in FIG. 2B, another embodiment of the present invention includes the second beam splitter  206  being laterally shifted by  2   b  relatively to an edge  210  of the reflector  200  instead of the  4   b  lateral shift in FIG. 2A. Through this arrangement of moving the second beam splitter  206   2   b , the third beam of light only generates the second output  222  instead of being partially reflected and partially transmitted. Otherwise, similar to the light travel above, three output beams  220 ,  222 , and  224  with about the same intensity are produced. The intensity of the output beams  220 ,  222 , and  224  can be maintained through the use of a 66:33 beam splitter  204  and a 50:50 beam splitter  206 .  
         [0026]    With reference now to FIG. 3, another embodiment of the multiplexer  106 ′ is shown. In this embodiment, the multiplexer comprises a reflector  300  and first, second, and third beam splitters  302 ,  304 , and  306 , which can be 50/50 beam splitters. The relationship of the beam splitter parameters d, b, α, and L are as described above. In this embodiment, the first beam splitter  302  is spaced a distance d away from a plane extending from a reflecting surface  308 , the second beam splitter  304  is spaced a distance  2   d , and the third beam splitter  306  is spaced a distance  4   d . Also, the first beam splitter  302  is laterally shifted a distance b from an edge  310  of the reflector  300 , while the second beam splitter  304  is laterally shifted a distance  4   b  and the third beam splitter is laterally shifted a distance  10   b.    
         [0027]    In operation of the embodiment shown in FIG. 3, the light  102  is received at a predetermined angle α at the first beam splitter  302  that reflects a first portion of the light  102  toward the reflector  300  and transmits a second portion of the light  102  toward the second beam splitter  304 . The second beam splitter  304  reflects a third portion of the light  102  toward the reflector  300  and transmits a fourth portion of the light  102  toward a third beam splitter  306 . The first portion of the light  102  received at the reflector  300  is reflected as a fifth portion of the light  102  toward the second beam splitter  304 . The beam splitter  304  reflects a sixth portion of the light  102  toward the reflector  300  and transmits a seventh portion of the light  102  toward the third beam splitter  306 . The third portion of the light  102  is received at the reflector  300  and reflected as an eighth portion of the light  102  toward the third beam splitter  306 . The fourth portion of the light  102  is received at the third beam splitter  306  and reflected as a ninth portion of the light  102  toward the reflector  300 . The third beam splitter  306  transmits a tenth portion of the light  12  to produce a first output beam  312 .  
         [0028]    The reflector  300  receives the sixth portion of the light  102  and reflects an eleventh portion of the light  102  toward the third beam splitter  306 . The third beam splitter  306  receives the seventh portion of the light  102  and reflects a twelfth portion of the light  102  toward the reflector  300  and transmits a thirteenth portion of the light  102  to produce a second output beam  314 . The third beam splitter  306  receives the eighth portion of the light  102  and reflects a fourteenth portion of the light toward the reflector  300  and transmits a fifteenth portion of the light to produce a third output beam  316 .  
         [0029]    The ninth portion of the light  102  is received by the reflector  300  that reflects a sixteenth portion of the light  102  to produce a fourth output beam  318 . The eleventh portion of the light  102  is received at the third beam splitter  306  and reflected as a seventeenth portion of the light  102  toward the reflector  300  and transmitted as an eighteenth portion of the light  102  to produce a fifth output beam  320 . The twelfth portion of the light  102  is received at the reflector  300  and reflected as a nineteenth portion of the light  102  to produce a sixth output beam  322 . The reflector  300  receives the fourteenth portion of the light  102  and reflects a twentieth portion of the light  102  to produce a seventh output beam  324 . Finally, the reflector  300  receives the seventeenth portion of the light  102  and reflects a twenty first portion of the light  102  to produce an eighth output beam  326 . Therefore, through the arrangement shown in FIG. 3, eight output beams  312 - 326  are produced each having approximately ⅛ the total intensity as the input beam  102 .  
         [0030]    Although not shown for convenience, it is to be appreciated that other embodiments of the present invention can be generalized for 2 N  times expansion or multiplexing of the light  102  from the laser  104 . This expansion of the light  102  is also called “multiplexing”. The number of beam splitters, which can be 50/50 beam splitters or any other required for the embodiment, in each subsequent case must be equal to N. The angle α of the light beam  102  relative to the first beam splitter in a general case is defined by equation (2) above. The beam splitters are numbered starting with the closest one to a reflector: 1, 2, . . . k, . . . N. A distance of the first beam splitter from the reflector is d, where d is defined by equation (1) above. The k-th beam splitter is positioned at a distance (k−1)*d from a preceding beam splitter. Also, the first beam splitter is shifted laterally relatively to an edge of the reflector by b, where b is defined by equation (3) above. The k-th beam splitter is laterally shifted relative to the preceding beam splitter by (k−1)* 3   b.    
         [0031]    In other embodiments, the ratio of reflection to transmission in the beam splitters can be altered slightly to account for light loss within the system  100 . This is to compensate for absorption in material of the beam splitter, less than desired reflectivity, and scattering of light. Further, the beam splitters are a predetermined thickness so that the lateral shift of the beam  102  inside the beam splitter body due to refraction is minimized. In lithography applications, for example, the predetermined thickness is between 1 mm and 3 mm. However, other thickness values can be used for other implementations of the present invention without departing from the scope of the present invention.  
         [0032]    Now with reference to FIG. 4, another embodiment of the multiplexer  106 ″ is shown. This multiplexer  106 ″ generates N times expansion of the light beam  102 , as compared to 2 N  times expansion of the light beam  102  in the embodiments discussed above. The multiplexer  106 ″ comprises, in parallel, a first reflector  400 , a first beam splitter  402 , a second reflector  404 , and a second beam splitter  406 . Determination of the spacing between the elements is similar to that as described above.  
         [0033]    In operation, the light  102  is received at a predetermined angle α at a first beam splitter  402  that reflects a first portion of the light  102  toward a first reflector  400  and transmits a second portion of the light  102  toward a second beam splitter  406 . The first portion of the light  102  received at the first reflector  400  is reflected as a third portion of the light  102  toward the second beam splitter  406 . The second portion of the light  102  is received at the second beam splitter  406  and reflected as a fourth portion of the light  102  toward a second reflector  404  and transmitted as a fifth portion of the light  102  to produce a first output beam  408 . The second beam splitter  406  receives the third portion of the light  102  and reflects a sixth portion of the light  102  toward the second reflector  404  and transmits a seventh portion of the light  102  to produce a second output beam  410 . The fourth portion of the light  102  is received at the second reflector  404  and reflected as an eighth portion of the light  102  to produce a third output beam  412 . Finally, the sixth portion of the light is received at the second reflector  404  and reflected as a ninth portion of the light to produce a fourth output beam  414 . Each of said output beams  408 - 414  will have an intensity of about 25% of the incident beam  102 .  
         [0034]    Turning to FIG. 5, an adjusting system  500  for a multiplexer  106  is shown. Merely as an example, a two beam splitter multiplexer  106 , similar to that shown in FIG. 2, can be the environment for the adjusting system  500 . In this system  500 , the multiplexer  106  is secured in a housing  502  that has beam splitter securing devices  504 , a reflector securing device  506 , and a detector securing device  508  for a detector  510 . In some embodiments, detection  510  can be a sectional detector (e.g., a quad detector) that more precisely determines characteristics of a detected beam. An adjustment device  512  is coupled to the securing devices  504 ,  506 , and  508 . The adjustment device  512  is also coupled to a controller  514  that controls adjustment of the securing devices  504 ,  506 , and  508 , with three degrees of freedom as shown by the arrows, based on signals received from the detector  510 .  
         [0035]    In operation, the detector  510  generates a signal when the light  102  from the laser  104  falls outside of a non-detection area  516 , which can result either from misalignment of the laser  104  or a distorted beam  102 . The non-detection area  516  can be a width a of the light  102 . When this signal from the detector  510  is received at the controller  514 , the controller  514  sends a control signal to the adjustment device  512  to adjust the positioning of the beam splitters using the beam splitter securing devices  504 . As described above, the beam splitter securing devices  504  can adjust the beam splitters in three degrees, as is shown by the arrows. Once adjusted, the light beam  102  again transmits through only the non-detection area  516  of the detector  510 , which ensures that the multiplexer  106  will accurately produce expanded light beams. As can be appreciated, the adjusting system  500  can be modified to accommodate any number of beam splitters and reflectors.  
         [0036]    It is to be appreciated that the adjustment of the beam splitters or other elements within the multiplexer  106  can be done manually. In that embodiment, a user would be alerted, based either on a detector or through visual determination, that the light  102  is reaching areas of the multiplexer outside of a predetermined area. Then, the user would make mechanical adjustments to realign the light beam  102 .  
       CONCLUSION  
       [0037]    Example embodiments of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalence.