Abstract:
A system for microlithography comprises an illumination source; an illumination optical system including, in order from an objective side, (a) a first diffractive optical element that receives illumination from the illumination source, (b) a zoom lens, (c) a second diffractive optical element, (d) a condenser lens, (e) a relay lens, and (f) a reticle, and a projection optical system for imaging the reticle onto a substrate, wherein the system for microlithography provides a zoomable numerical aperture.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of commonly assigned U.S. patent application Ser. No. 10/166,062, filed Jun. 11, 2002, now U.S. Pat. No. 6,813,003, entitled ADVANCED ILLUMINATION SYSTEM FOR USE IN MICROLITHOGRAPHY, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to microlithography, and more particularly, to illumination systems for microlithographic equipment that have high numerical apertures. 
   2. Related Art 
   Photolithography (also called microlithography) is used for manufacturing of semiconductor devices. Photolithography uses electromagnetic radiation, such as ultraviolet (UV), deep UV or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to implement photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist-covered semiconductor wafer. The illumination system illuminates a region of the reticle with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle circuit pattern onto the wafer. 
   As semiconductor device manufacturing technology advances, there are ever increasing demands on each component of the photolithography system used to manufacture the semiconductor device. This includes the illumination system used to illuminate the reticle. For example, there is a need to illuminate the reticle with an illumination field having uniform irradiance. In step-and-scan photolithography, there is also a need to vary a size of the illumination field so that the size of the illumination field can be tailored to different applications and semiconductor die dimensions. 
   Some illumination systems include an array or diffractive scattering optical element positioned before the reticle. The scattering optical element produces a desired angular light distribution that is subsequently imaged or relayed to the reticle. 
   Additionally, commonly-used die dimensions are 26×5 mm, 17×5 mm, and 11×5 mm. Thus, a standard zoom lens needs to accommodate variation in the size of the illumination field. However, a particular problem arises in the field of microlithography, where different features that are required to be formed on the semiconductor substrate require variable partial coherence on the part of the exposure optics. Specifically, partial coherence (σ), which in microlithography is commonly defined as the ratio of a numerical aperture of the illumination optics and a numerical aperture of the projection system, needs to vary depending on the nature of the feature being formed on the semiconductor substrate, e.g., the σ for trench formation may be different from the σ for line formation. 
   Accordingly, a need exists for a simple microlithographic system that can vary the partial coherence parameter over a large range, while simultaneously being able to accommodate different field sizes. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a microlithographic system that has variable partial coherence and field size. 
   One advantage of the present invention is being able to provide a microlithographic system with continuously adjustable partial coherence and discretely adjustable field size. 
   Another advantage of the present invention is being able to provide a microlithographic system where both partial coherence and field size can vary continuously. 
   Another advantage of the present invention is being able to provide a microlithographic system that can achieve the above objectives with the use of simple optics. 
   Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided a system for microlithography comprising an illumination source; an illumination optical system including, in order from an objective side, (a) a first diffractive optical element that receives illumination from the illumination source, (b) a zoom lens, (c) a second diffractive optical element, (d) a condenser lens, (e) a relay lens, and (f) a reticle, and a projection optical system for imaging the reticle onto a substrate, wherein the system for microlithography provides a zoomable numerical aperture. 
   In another aspect of the present invention there is provided a system for microlithography comprising an illumination source, an illumination optical system that receives illumination from the illumination source, and a projection optical system that receives illumination from the illumination system, wherein a ratio of a numerical aperture of the illumination system and a numerical aperture of the projection optical system is continuously variable while a field size is discretely variable. 
   In another aspect of the present invention there is provided an illumination system for microlithography comprising, in order from an objective side a first diffractive optical element, a zoom lens, a second diffractive optical element having a rectangular numerical aperture, a condenser lens, and a relay lens. 
   In another aspect of the present invention there is provided a system for microlithography comprising an illumination system including, in order from an objective side, (a) a zoom lens having a first diffractive optical element on a first side, and a second diffractive optical element on a second side, (b) a condenser lens, and (c) a relay lens, and a projection optical system, wherein a ratio of a numerical aperture of the illumination system and a numerical aperture of the projection optical system is continuously variable. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a schematic illustration of one embodiment of the present invention; 
       FIG. 2  is another illustration of the embodiment of  FIG. 1 , showing the lens arrangement; 
       FIG. 3  is a schematic illustration of another embodiment of the present invention; 
       FIGS. 4A–4C  are a ray trace diagrams illustrating a condenser lens used in an embodiment of the present invention; 
       FIGS. 5A–5B  are a ray trace diagrams illustrating a relay lens used in an embodiment of the present invention; 
       FIGS. 6A–6B  are a ray trace diagrams illustrating a zoom lens used in an embodiment of the present invention; 
       FIG. 7  illustrates an overall design of the illumination system, such as that shown in  FIG. 1 ; 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
   In recent years, photolithography used in semiconductor device fabrication has been moving to gradually shorter wavelengths, as device features shrink in size. With feature sizes shrinking to sub-micron- and sub-0.1μ range, semiconductor manufacturers have had to shift to the use of ultraviolet light, and in some cases to soft X-ray lithography (or deep UV). For example, excimer lasers, which emit light in the 248, 193 and 157 nm range, are increasingly used in semiconductor device fabrication. The illumination source in modem microlithographic equipment, as noted above, is typically a visible light laser, an excimer laser, or possibly a, soft X-ray source. (The terms “light” and “illumination” will be used interchangeably hereafter to refer to any electromagnetic radiation used for photoresist exposure.) The use of these wavelengths presents a particular challenge to the designer of semiconductor manufacturing equipment, and especially the optics used to focus and shape the beams from the excimer lasers. In the present invention, fused silica glass is preferred for 248 and 193 nm sources, while 157 nm sources typically require optical elements made of calcium fluoride or barium fluoride to effectively focus and shape the beam. 
   The embodiments described utilize both refractive and reflective optical elements. It will be understood by one of ordinary skill in the art, however, that the use of reflective surfaces is frequently dictated by engineering and design concerns, rather than fundamental principles of the invention. It is therefore understood that in the description that follows, the use of reflective (folding) optical elements is needed strictly due to engineering design choices, and their use is not required in order to practice the invention. 
     FIG. 1  illustrates a basic configuration of one preferred embodiment of the present invention. It will be appreciated that in the figures that follow, where appropriate, the dimensions are in millimeters. 
   As may be seen in  FIG. 1 , this embodiment of the present invention includes a diffractive optical element  101  (DOE 1 ), which is illuminated by an illumination source (not shown). 
   The first diffractive optical element  101  may be any element commonly used to produce diffraction, such as 2-D array of spherical microlenses, a Fresnel lens, a diffraction grating, etc. 
   From a system perspective, as illustrated in  FIG. 1 , the numerical aperture of the beam after the first diffractive optical element  101  is approximately 0.065. 
   As may be further seen from  FIG. 1 , after passing through the first diffractive optical element  101 , the beam then illuminates a zoom lens  102 . In the this embodiment, the zoom lens  102  is a 5× zoom spherical lens, with a focal length of 221.5–1107.7 mm. The diameter of the beam at this point is 180 mm. The zoom lens  102  is further illustrated in  FIG. 6 . It will be appreciated by one of ordinary skill in the art that the zoom lens  102  can use more or fewer elements, as required. One (six element design) is illustrated by the following prescription (a CODE V output): 
   
     
       
             
             
             
             
             
           
             
             
           
             
             
             
             
             
           
             
             
           
             
             
             
             
             
           
             
             
           
             
             
             
             
             
           
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
             
           
             
             
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
                 
               RDY 
               THI 
               GLA 
             
             
                 
                 
             
             
                 
               &gt;OBJ: 
               INFINITY 
               INFINITY 
             
             
                 
               STO: 
               INFINITY 
               8.000000 
             
             
                 
                2: 
               −25.24705 
               5.000000 
               ‘CaF2’ 
             
             
                 
                3: 
               55.68759 
               16.548834 
             
             
                 
                4: 
               −48.92714 
               25.342815 
               ‘CaF2’ 
             
           
        
         
             
                 
               ASP: 
             
             
                 
               K: 1.779039   KC: 0 
             
             
                 
               IC: YES   CUF: 0.000000   CCF: 100 
             
             
                 
               A: 0.146865E−05   B: 0.705843E−08   C: −.823569E−11 
             
             
                 
               D: 0.127469E−13 
             
             
                 
               AC: 0   BC: 0   CC: 0   DC: 0 
             
           
        
         
             
                 
                5: 
               −36.47260 
               194.914260 
                 
             
             
                 
                6: 
               170.18706 
               28.207990 
               ‘CaF2’ 
             
             
                 
                7: 
               510.72551 
               17.527333 
             
             
                 
                8: 
               141.82233 
               51.966932 
               ‘CaF2’ 
             
             
                 
                9: 
               −277.74471 
               12.376464 
             
           
        
         
             
                 
               ASP: 
             
             
                 
               K: −3.017335   KC: 0 
             
             
                 
               IC: YES   CUF: 0.000000   CCF: 100 
             
             
                 
               A: 0.913504E−07   B: −.173047E−11   C: −.291669E−15 
             
             
                 
               D: 0.148478E−19 
             
             
                 
               AC: 0   BC: 0   CC: 0   DC: 0 
             
           
        
         
             
                 
               10: 
               −297.59579 
               10.000000 
               ‘CaF2’ 
             
             
                 
               11: 
               143.26243 
               1101.010134 
             
             
                 
               12: 
               −352.19780 
               11.373314 
               ‘CaF2’ 
             
             
                 
               13: 
               −154.19122 
               187.731924 
             
           
        
         
             
                 
               ASP: 
             
             
                 
               K: −500.000000   KC: 0 
             
             
                 
               IC: YES   CUF: 0.000000   CCF: 100 
             
             
                 
               A: −.125463E−05   B: 0.451681E−09   C: −.724157E−13 
             
             
                 
               D: 0.418162E−17 
             
             
                 
               AC: 0   BC: 0   CC: 0   DC: 0 
             
             
                 
               IMG: INFINITY   0.000000 
             
             
                 
               SPECIFICATION DATA 
             
             
                 
               EPD 27.66000 
             
             
                 
               DIM MM 
             
             
                 
               WL 157.63 
             
           
        
         
             
                 
               XAN 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               YAN 
               0.00000 
               1.85600 
               3.71900 
             
             
                 
               WTF 
               3.00000 
               2.00000 
               2.00000 
             
             
                 
               VUY 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VLY 
               0.00000 
               0.00000 
               0.00000 
             
           
        
         
             
                 
               REFRACTIVE INDICES 
                 
             
             
                 
               GLASS CODE 
               157.63 
             
             
                 
               ‘CaF2’ 
               1.558739 
             
             
                 
                 
             
           
        
         
             
               ZOOM DATA 
             
           
        
         
             
                 
                 
               POS 1 
               POS 2 
               POS 3 
             
             
                 
                 
             
             
                 
               VUY F1 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VLY F1 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VUY F2 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VLY F2 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VUX F1 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VLX F1 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VUX F2 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               VLX F2 
               0.00000 
               0.00000 
               0.00000 
             
             
                 
               THI S5 
               194.91426 
               1.00000 
               1.00000 
             
             
                 
               THC S5 
               0 
               0 
               0 
             
             
                 
               THI S7 
               17.52733 
               86.68062 
               1.45028 
             
             
                 
               THC S7 
               0 
               0 
               0 
             
             
                 
               THI S9 
               12.37646 
               137.13744 
               222.36778 
             
             
                 
               THC S9 
               0 
               0 
               0 
             
             
                 
                 
             
             
                 
                 
               POS 1 
               POS 2 
               POS 3 
             
             
                 
                 
             
           
        
         
             
               INFINITE CONJUGATES 
             
           
        
         
             
                 
               EFL 
               221.5400 
               664.6200 
               1107.7000 
             
             
                 
               BFL 
               164.6663 
               35.0875 
               11.1078 
             
             
                 
               FFL 
               115.3771 
               610.2350 
               1583.8486 
             
             
                 
               FNO 
               8.0094 
               24.0282 
               40.0470 
             
             
                 
               IMG DIS 
               187.7319 
               187.7319 
               187.7319 
             
             
                 
               OAL 
               1482.2681 
               1482.2681 
               1482.2681 
             
           
        
         
             
               PARAXIAL IMAGE 
             
           
        
         
             
                 
               NT 
               14.4001 
               43.2004 
               72.0006 
             
             
                 
               ANG 
               3.7190 
               3.7190 
               3.7190 
             
           
        
         
             
               ENTRANCE PUPIL 
             
           
        
         
             
                 
               DIA 
               27.6600 
               27.6600 
               27.6600 
             
             
                 
               THI 
               0.0000 
               0.0000 
               0.0000 
             
           
        
         
             
               EXIT PUPIL 
             
           
        
         
             
                 
               DIA 
               53.1110 
               30.1251 
               19.3446 
             
             
                 
               THI 
               590.0538 
               758.9393 
               785.8026 
             
             
                 
               STO DIA 
               27.6600 
               27.6600 
               27.6600 
             
             
                 
                 
             
           
        
       
     
   
   As further illustrated in  FIG. 1 , a fold (mirror)  103  may be used in this embodiment to manage and reduce overall tool size by folding the optical path. As noted above, the use of a mirror  103  is optional, and is generally dictated by engineering/design choices. 
   After reflecting off the fold mirror  103 , the beam then illuminates an axicon  104  (working diameter of 170 mm). After passing through the axicon  104 , the beam has a rectangular numerical aperture of 0.046–0.009 in the Y dimension, and 0.053–0.011 in the X dimension. 
   After passing through the axicon  104 , the beam then passes through the second diffractive element (DOE 2 )  105 . The second diffractive element  105  is preferably a binary diffractive array. One example is a array of cylindrical micro-lenses. The specification for the second diffractive optical element  105  may be as follows:
     Coherence length in mm, X&amp;Y:   248 nm temporal—no specs. spatial 0.35×0.15   193 nm temporal −3, spatial 0.6×0.085   X &amp; Y beam divergence, mrad   248 nm+/−3.5 ×+/−3.5   193 nm+/−1 ×+/−1.75   Beam size (nm), X &amp; Y; 6×16; 20×20; 12×32   

   After passing through the second diffractive array  105 , the numerical aperture of the beam is approximately 0.165×0.04. 
   The beam then passes through a spherical condenser lens  106 . A condenser lens  106  usable in this embodiment can have the following characteristics: 
   
     
       
             
             
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
                 
               RDY 
               THI 
               GLA 
             
             
                 
               &gt;OBJ: 
               INFINITY 
               INFINITY 
             
             
                 
               STO: 
               INFINITY 
               75.000000 
             
             
                 
               2: 
               323.84000 
               5.000000 
               ‘CaF2’ 
             
             
                 
               3: 
               INFINITY 
               491.500000 
             
             
                 
               4: 
               −145.94000 
               5.000000 
               ‘CaF2’ 
             
             
                 
               5: 
               106.10000 
               278.500000 
             
             
                 
               6: 
               −2090.20000 
               15.000000 
               ‘CaF2’ 
             
             
                 
               7: 
               −196.34000 
               50.000000 
             
             
                 
               IMG: 
               INFINITY 
               0.000000 
             
             
                 
                 
             
           
        
       
     
   
   In this embodiment, the condenser lens  106  has a focal length of 340 mm (generally, it is expected that the condenser lens  106  will have a focal length of 300–400 mm), and the illuminated diameter is 150–30 mm. 
   After passing through the spherical condenser lens, the beam has a zoomable circular numerical aperture of 0.2125–0.043. The beam then encounters a delimiter  107  (i.e., a stop), such that the illuminated field of 112×24 mm becomes 108×22 mm. The delimiter  107  is optically conjugate with a reticle  109 , through the use of a relay lens  108  (for example, a 1× relay, or a 3×–4× relay). For design purposes, a fold  110  may be placed within the relay  108 . A stop  111  is placed in the center of the relay lens  108 , for a telecentric illumination system. 
   The relay lens  108  is used to conjugate a plane of a delimiter  107  with a plane of a reticle  109 . An example of a 1× relay lens  108  prescription is shown below (here, a 10-element design): 
   
     
       
             
             
             
             
           
             
           
             
             
             
             
           
             
           
             
             
             
             
           
             
           
             
             
             
             
           
             
           
             
             
             
             
           
         
             
                 
             
           
           
             
                 
               RDY 
               THI 
               GLA 
             
             
               &gt;OBJ: 
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               1: 
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               ‘NCaF2’ 
             
           
        
         
             
               ASP: 
             
           
        
         
             
               K: −0.916442 
                 
                 
                 
             
             
               IC: YES 
               CUF: 0.000000 
             
             
               A: 0.000000E+00 
               B: 0.000000E+00 
               C: 0.000000E+00 
               D: 
             
             
                 
                 
                 
               0.000000E+00 
             
             
               2: 
               297.03762 
               280.000000 
             
             
               3: 
               607.71047 
               32.530979 
               ‘NCaF2’ 
             
             
               4: 
               −296.65731 
               1.000000 
             
           
        
         
             
               CON: 
             
           
        
         
             
               K: −2.313366 
                 
                 
                 
             
             
               5: 
               172.28333 
               33.841572 
               ‘NCaF2’ 
             
             
               6: 
               4765.41367 
               1.000000 
               AIR 
             
             
               7: 
               129.90270 
               40.919042 
               ‘NCaF2’ 
             
             
               8: 
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               9: 
               −306.34576 
               8.000000 
               ‘NCaF2’ 
             
             
               10: 
               162.90100 
               15.103930 
             
             
               STO: 
               INFINITY 
               15.104002 
             
             
               12: 
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               8.000000 
               ‘NCaF2’ 
             
             
               13: 
               306.34576 
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               14: 
               −103.26821 
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               ‘NCaF2’ 
             
             
               15: 
               −129.90270 
               1.000000 
             
             
               16: 
               −4765.41367 
               33.841572 
               ‘NCaF2’ 
             
             
               17: 
               −172.28333 
               1.000000 
             
             
               18: 
               296.65731 
               32.530979 
               ‘NCaF2’ 
             
           
        
         
             
               CON: 
             
           
        
         
             
               K: −2.313366 
                 
                 
                 
             
             
               19: 
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               280.000000 
             
             
               20: 
               −297.03762 
               15.000000 
               ‘NCaF2’ 
             
             
               21: 
               −169.24669 
               73.362171 
             
           
        
         
             
               ASP: 
             
           
        
         
             
               K: −0.916442 
                 
                 
                 
             
             
               IC: YES 
               CUF: 0.000000 
             
             
               A: 0.000000E+00 
               B: 0.000000E+00 
               C: 0.000000E+00 
               D: 
             
             
                 
                 
                 
               0.000000E+00 
             
             
               IMG: 
               INFINITY 
               0.000000 
               AIR 
             
             
               XDE: 0.000000 
               YDE: 0.000000 
               ZDE: 0.000000 
               DAR 
             
             
               ADE: 0.000000 
               BDE: 0.000000 
               CDE: 0.000000 
             
             
                 
             
           
        
       
     
   
   A projection optical system (not shown) images the reticle down onto the semiconductor wafer (typically reducing image size by 4×, to 26×5 mm, 17×5 mm, or 11×5 mm). 
   It will be appreciated by one of ordinary skill in the art that the use of the axicon  104  in such a system improves the system&#39;s optical properties, but the invention may work without it. It will also be appreciated by one of ordinary skill in the art that the positions of the axicon  104  and the second diffractive element  105  can be reversed (i.e., the axicon  104  may be downstream from the second diffractive element  104 ), although it is believed at the present time that the arrangement shown in  FIG. 1  is preferred. 
     FIG. 2  illustrates in greater detail the arrangement of the optical elements of the illumination system. In particular,  FIG. 2  shows the zoom lens  102  (shown as a 5-element design) and its constituent elements  102   a ,  102   b ,  102   c ,  102   d  and  102   e .  FIG. 2  further shows the constituent elements of the condenser lens  106  (shown here as a four-element lens), and the 1× relay  108  (shown here as an 8-element design). It further illustrates the position of the λ/4 plate, and the reticle (mask)  109 , which is optically conjugate with the plane of the delimiter  107  through the relay lens  108 . 
     FIG. 7  is another illustration of the embodiment of  FIG. 1 , showing additional elements commonly found in a real-life microlithography system. All the optical elements illustrated in  FIG. 1  are shown in  FIG. 7 , using the same reference numerals. In addition,  FIG. 7  also shows a changer unit  701  for the second diffractive optical element  105 . It is anticipated that in order to achieve different field sizes, different diffractive optical elements, having different numerical apertures, may need to be used. Accordingly, the changer unit  701  illustrated in  FIG. 7  can be used for that purpose. It will also be appreciated that a similar changer unit may be used for the first diffractive optical element  101 , if necessary. 
     FIG. 7  also illustrates the dynamic adjustable slit  702 , which is part of the delimiter  107  assembly. The adjustable slit  702  is further described in U.S. Pat. No. 5,966,202, which is incorporated by reference herein. Together with the field framing assembly  704 , they are used to ensure that the proper beam size exists at the delimeter plane, which is optically conjugate with the reticle plane. 
     FIG. 7  also illustrates the cleanup aperture assembly  703 , which is used as a telecentric stop at the center of the relay lens. (See also U.S. Pat. No. 6,307,619, which is incorporated by reference herein). 
     FIG. 7  also illustrates the position of the λ/4 plate  112 , above plane of the reticle  108  and below the last optical element (lens) of the relay lens  108 . 
   Although the preferred embodiments of the present invention describe a system used for exposure of discrete field sizes (26×5 mm, 17×5 mm, and 11×5 mm), it is expected that the system can be made to have a continuously variable field size. This could be accomplished by the addition of other diffractive optical elements in the optical path, similar to the second diffractive optical element  105 . By the addition of one or two such elements, (e.g., additional binary diffractive arrays, or cylindrical microlens arrays), which may be placed between the condenser lens and the second diffractive optical element, and by adjusting its position along the optical axis, it is possible to achieve a microlithographic system that has both a continuously variable partial coherence, and a continuously variable field size at the wafer. 
   The use of a projection optical system (not illustrated in the figures) is well-known in the art, and is typically a 4× lens that reduces the reticle image down onto the wafer. 
   The description of another embodiment below, and the corresponding figures, use the same reference numerals to designate the same elements as in the embodiment of  FIG. 1 .  FIG. 3  illustrates the basic configuration of another preferred embodiment of the present invention. As may be seen in  FIG. 3 , this embodiment of the present invention includes a diffractive optical element  101 , which is illuminated by an illumination source (not shown). 
   The first diffractive optical element (DOE  1 )  101  may be any refractive or reflective element commonly used to produce diffraction, such as an array of spherical microlenses, a Fresnel lens, a diffraction grating, etc. The numerical aperture of the beam after the first diffractive optical element  101  is approximately 0.065 (circular). 
   As may be further seen from  102 , after passing through DOE 1   101 , light then illuminates a zoom lens  102 . In this embodiment, the zoom lens  102  is a 5× zoom spherical lens, with a focal length of 196–982 mm. The diameter of the beam at this point is 135 mm. In this embodiment, the zoom lens  102  is a five-element lens. 
   After passing though the zoom lens  102  and reflecting off a fold mirror  103 , the beam then illuminates an axicon  104 . After passing through the axicon  104 , the beam has a rectangular numerical aperture of 0.46–0.009 in the Y dimension, and 0.053–0.011 in the X dimension. 
   After passing through the axicon  104 , the beam then passes through the second diffractive element (DOE 2 )  105  (beam diameter 135 mm). The second diffractive element  105  is preferably a binary diffractive array. One example is a array of cylindrical micro-lenses. After passing through the second diffractive array  105 , the numerical aperture of the beam becomes 0.2×0.04. 
   The beam then passes through a condenser lens  106 . In this embodiment, the condenser lens  106  has a focal length of 300 mm, and the illuminated diameter is 120–25 mm. 
   After passing through the spherical condenser lens, the beam has a zoomable circular numerical aperture of 0.2125–0.043. The beam then encounters a delimiter  107  (i.e., a stop), such that the illuminated field of 120×24 mm becomes 108×20 mm. The delimiter  107  is optically conjugate with a reticle  111 , through the use of a relay lens  108 . The relay lens  108  is used to conjugate the plane of the delimiter  107  with the plane of the reticle. For design purposes, a fold  110  may be placed within the relay lens  108 . A stop  109  is placed in the center of the relay lens, for a telecentric illumination system. 
   A projection optical system (not shown) images the reticle  111  down onto the semiconductor wafer (typically reducing image size by 4×). 
   It will be appreciated by one of ordinary skill in the art that a relay lens is not always necessary to practice the invention, since the optical planes of the reticle and the delimiter are conjugate with each other. However, in most practical systems, a relay lens is used in order to ensure proper size of the field at the reticle plane, due to mechanical constraints. 
   Additionally, it will be appreciated that the field size may also be made continuous through the use of additional second diffractive elements, similar in nature to the second diffractive element  105  described above. Alternatively, a more complex zoom lens, or the use of a second zoom lens, may be used to achieve the same purpose. 
   Further, it will be appreciated that the present invention allows for the use of an even lower partial coherence σ, e.g., 0.001, if needed. A more complex zoom lens (or multiple zoom lenses) would be needed to achieve this. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.