Abstract:
A catadioptric lens barrel structure for a projection exposure apparatus includes first and second lens barrels having parallel optical axes. The lens barrel structure also includes a third lens barrel transversely connecting the first lens barrel to the second lens barrel. A support structure firmly secures the plurality of lens barrels. The support structure includes a plurality of connecting rods for securing the first and second lens barrels, support platforms for individually supporting the lens barrels, and a plurality of flexures for resiliently supporting the lens barrel structure. The connecting rods are made of a material having a low coefficient of thermal expansion to minimize errors due to temperature changes, vibrational or rotational motion, and other conditions. Alternatively, the support structure may comprise a base support platform for firmly supporting base portions of the first and second lens barrels, and an upper support platform for flexibly supporting upper portions of the lens barrels. The base and upper support platforms are also made a material having a low coefficient of thermal expansion. In one embodiment, the lens barrel structure is used in semiconductor fabrication. A reticle is placed in front of an entrance end of the first lens barrel and a semiconductor wafer is placed in front of an exit end of the second lens barrel. A light beam passes through the reticle, travels through the lens barrel structure, and exposes a semiconductor wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a catadioptric lens barrel which can be attached to an optical projection unit, such as a projection exposure apparatus used to manufacture semiconductor wafers. More particularly, the present invention relates to a catadioptric lens barrel structure including its support structure, the lens barrel structure comprising a plurality of lens barrels aligned in parallel, and at least one transverse lens barrel connecting the plurality of lens barrels. 
     2. Description of the Related Art 
     A lens barrel is a tubular mechanical structure containing a series of lenses which are aligned to transfer an image from one end of the lens barrel to the other. The lens barrel may include a sturdy threaded body and durable metal retaining rings, often referred to as “sub-barrels,” to carry a series of lens holders, commonly referred to as “cells.” Another type of lens barrel is known as a catadioptric lens barrel. A catadioptric lens barrel contains not only refractive elements, such as lenses, but also reflective elements, such as mirrors. 
     Both of these lens barrel designs are used in conventional optical systems. For example, optical projection units often have a lens barrel and may be used in a projection exposure apparatus for production of semiconductor wafers. The projection exposure apparatus includes a template positioned near an entrance end of the lens barrel and a semiconductor wafer positioned near an exit end. The projection exposure apparatus shines light through the template, photographically reduces a pattern on the template, and sequentially transfers the pattern, through the lens barrel, onto a predetermined area on the semiconductor wafer. The template may be of a form such as a reticle or a mask. The semiconductor wafer is a silicon wafer with a photo-sensitive coating for making semiconductor devices, such as microprocessor or memory chips. 
     A catadioptric lens barrel system using two or more lens barrels, arranged in parallel, offers benefits not found in conventional single barrel systems. The parallelism of the lens barrels reduces the overall height of the lens barrel, but increases the overall width. Reduced height is important for many reasons, such as for ease of transport. This type of catadioptric lens barrel design, however, poses several problems. First, keeping proper alignment of a plurality of lens barrels can be difficult. Second, the focus depth of the projection system changes as temperature changes. Coefficients of thermal expansion of the lens barrels, including their internal components, and the indices of refraction of all lens materials vary with temperature. Errors related to these factors collectively alter the focusing properties of the optical system. Third, the lens barrel structure tends to be more sensitive to movement or vibration than conventional single barrel designs. 
     In light of the foregoing, there is a need for a catadioptric lens barrel structure that holds the lens barrels stabily and accurately, and minimizes distortion errors due to temperature changes, vibrational, or other external forces, so that the resulting image remains focused as it is transferred and reduced from the reticle to the semiconductor wafer. 
     SUMMARY OF THE INVENTION 
     The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims. 
     To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, one aspect of the invention is directed to a lens barrel structure. The lens barrel structure may be used, for example, in a projection exposure apparatus in semiconductor manufacturing to reduce a pattern on a reticle through the projection exposure apparatus to a semiconductor wafer. The lens barrel structure comprises a plurality of lens barrels and a plurality of support structures. Each lens barrel has an optical axis. The plurality of optical axes have a parallel orientation. The support structures are parallel to each other and perpendicular to the optical axes of the lens barrels. The support structures maintain the parallel orientation of the lens barrels. The support structures may comprise a first pair of connecting rods for supporting lower portions of the lens barrels, and a second pair of connecting rods for supporting upper portions of the lens barrels. Alternatively, the Support structure may comprise a first support plate for supporting lower portions of the lens barrels, and a second support plate for supporting upper portions of the lens barrels. The support structures are made of a material having a low coefficient of thermal expansion. 
     The present invention is also directed to a lithography system comprising a lens barrel structure such as for use in a semiconductor manufacturing. 
     The present invention is further directed to a method for making a lens barrel structure, which, in one embodiment, is used in projection exposure apparatus in a semiconductor manufacturing to reduce a pattern on a reticle through the projection exposure apparatus to a semiconductor wafer. The method comprises the steps of disposing a plurality of lens barrels in a parallel orientation, and providing a plurality of support structures for maintaining the parallel orientation of the lens barrels. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a perspective view showing a front side of the catadioptric lens barrel structure according to a first embodiment of the present invention; 
     FIG. 2 is a perspective view showing a back side of the catadioptric lens barrel structure according to the first embodiment of the present invention; 
     FIG. 3 is a schematic diagram of the catadioptric lens barrel structure consistent with the principles of the present invention; and 
     FIG. 4 is a sectional front view of the catadioptric lens barrel structure according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. The invention will be further clarified by the following examples, which are intended to be exemplary of the invention. 
     In one embodiment, a catadioptric lens barrel structure is provided with a plurality of lens barrels and a plurality of support structures. The support structures provide enhanced stability and are made of a material that reduces the effects of environmental changes on the lens barrel structure. The lens barrels run parallel to each other while the support structures maintain the parallel orientation of the lens barrels as well as minimize rotational or vibrational movement of the lens barrels. The support structures comprise a first pair of connecting rods for positioning lower portions of the lens barrels, and a second pair of connecting rods for positioning upper portions of the lens barrels. 
     In accordance with the invention, a catadioptric lens barrel structure is provided with a plurality of lens barrels. Each lens barrel has an optical axis along the length of the lens barrel. The plurality of optical axes run parallel to each other. 
     In one embodiment, as illustrated in FIG. 1, a catadioptric lens barrel structure  100  comprises a pair of lens barrels including a first lens barrel  102  and a second lens barrel  112 . First lens barrel  102  has a first optical axis  104  running along its length. Second lens barrel  112  has a second optical axis  114  running along its length. First and second optical axes  104 ,  114 , respectively, run parallel to each other and define an imaginary frontal plane  120 . First optical axis  104  and frontal plane  120  define a first optical plane  158  as a reference plane perpendicular to frontal plane  120  and passing through first optical axis  104 . Similarly, second optical axis  114  and frontal plane  120  define a second optical plane  159  as a reference plane perpendicular to frontal plane  120  and passing through second optical axis  114 . 
     Lens barrels  102  and  112  can be made of a rigid material, such as stainless steel or silicon carbide. For example, stainless steel commonly known in the industry as SS 17-7PH, SS 440 series, or SS 300 series may be used. Lens barrels  102  and  112  are quite large and heavy when used in a semiconductor production. For example, first lens barrel  102  may be as large as one meter (approximately 40 inches) in length, slightly less than a half meter (approximately 17 inches) in diameter, and weighing almost 500 kilograms (approximately 1100 pounds). 
     First and second lens barrel,  102  and  112 , respectively, may be constructed of several different types of lens barrels. A traditional lens barrel, for example, is a type of lens barrel where the barrel carries a plurality of lens holders (commercially known as “cells”), on which lenses are mounted. The plurality of lens holders are stacked in a serial orientation along the optical axis of the lens barrel. Another type of lens barrel comprises a series of sub-barrels connected end to end by fasteners. FIGS. 1 and 2 show first and second lens barrels  102  and  112  as the traditional lens barrel type. 
     Consistent with the principles of the invention, a catadioptric lens barrel structure is provided with at least one transverse lens barrel connecting one lens barrel to the next successive lens barrel. Each transverse lens barrel comprises at least two reflective elements. The reflective elements are angularly oriented. The reflective elements reflect an image of the reticle reflected off a reflective element in front of an exit end of the first lens barrel to the second lens barrel and onto a wafer in front of an exit end of the second lens barrel. 
     As shown in FIG. 1, catadioptric lens barrel structure  100  comprises, for exemplary purposes, a transverse lens barrel  122  connecting first lens barrel  102  to second lens barrel  112 . Transverse lens barrel  122  carries therein, for example, a pair of mirrors  124  (not shown in FIG. 1) and  126 . As best shown in FIG. 3, mirror  124  is located inside first lens barrel  102 . Mirrors  124  and  126  have angular orientations. Mirrors  124 ,  126 , reticle  108 , and semiconductor wafer  118  are discussed in greater detail below with respect to FIG.  3 . Transverse lens barrel  122  is preferably made of a material having a low coefficient of thermal expansion. For example, the coefficient of thermal expansion may be as low as 6 ppm/° C. or below. Table 1 below shows some examples of materials having this property. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Materials 
                 Coefficients of Thermal Expansion 
               
               
                   
               
             
             
               
                 Alumina Ceramic (Al 2 O 3 ) 
                 5.7 ppm/° C. 
               
               
                 Invar 
                 0.64-3.11 ppm/° C. 
               
               
                   
                 (depending on heat treatment) 
               
               
                 Ultra Low Expansion glass (ULE ™) 
                 0.03 ppm/° C. 
               
               
                 Zerodur 
                 0.05 ppm/° C. 
               
               
                 Silicon Carbide (SiC) 
                 0.4-5.1 ppm/° C. 
               
               
                   
                 (depending on temperature range) 
               
               
                   
               
             
          
         
       
     
     The purpose of using low expansion materials for transverse lens barrel  122  is to minimize errors related to distortion of the lens barrel structure  100  due to environmental changes. The distortion may include, for example, expansion or contraction due to temperature changes, rotational and vibrational motion, or other motion caused by other external forces. When temperature changes, either increases or decreases, each part of the lens barrel structure will expand or contract accordingly. The change in length can be mathematically determined according to this expression: 
     
       
         ΔL=α·L·ΔT 
       
     
     whereby a is the coefficient of thermal expansion of the part; L is the initial length of the part; and ΔT is the temperature difference as final temperature is subtracted from initial temperature. A coefficient of thermal expansion of a material represents a characteristic of the material in response to thermal changes, and is measured in parts per million for every centigrade of temperature gradient (ppm/° C.). By selecting materials having extremely low coefficients of thermal expansion, each part will expand or contract very minimally. 
     Also consistent with the principles of the invention, a catadioptric lens barrel structure is provided with a plurality of support structures. In a first embodiment, the support structures comprise a first pair of connecting rods positively positioning base portions of the lens barrels and a second pair of connecting rods positively positioning upper portions of the lens barrels. 
     According to the first embodiment of the invention, as shown in FIGS. 1 and 2, a catadioptric lens barrel structure is provided with a plurality of support structures  132 . Support structures  132  include a first pair of connecting rods  134  and  136 , and a second pair of connecting rods  138  and  140 . Connecting rods  134  and  138  are located on a front side of the lens barrel structure  100 . Connecting rods  136  and  140 , shown in FIG. 2, are located on a back side of the lens barrel structure  100 . First and second pairs of connecting rods  134 ,  136 ,  138 , and  140 , respectively, run parallel to each other and perpendicular to first and second optical axes  104 ,  114 , respectively. The first pair of connecting rods  134 ,  136  position base portions of first and second lens barrels  102 ,  112 , respectively. The second pair of connecting rods  138 ,  140  position upper portions of first and second lens barrels  102 ,  112 , respectively. First pair of connecting rods  134  and  136  are preferably positioned as high as possible across second lens barrel  112  to provide optimum support to the lens barrel structure  100 . 
     First and second pairs of connecting rods  134 ,  136 ,  138 , and  140  are preferably made of a material having a low coefficient of thermal expansion. For example, the coefficient of thermal expansion may be as low as 6 ppm/° C. or below. Similar to the materials for transverse lens barrel  122 , selection of materials having this property includes alumina ceramic (Al 2 O 3 ), Invar, ULE™ glass, Zerodur glass, and silicon carbide (SiC). The same reasons for selecting these materials have been discussed above with respect to Table 1. 
     Therefore, first and second pairs of connecting rods  134 ,  136 ,  138 , and  140 , respectively, along with transverse lens barrel  122 , substantially maintain the distance D between first and second optical axes  104 ,  114 , respectively, regardless of temperature changes. Consistent with the principles of the invention, distance D is a key parameter of a catadioptric lens barrel system having parallel lens barrels construction. First and second pairs of connecting rods  134 ,  136 ,  138 , and  140 , respectively, are made of the same material selected from Table 1 so that they will expand or contract, if at all, at the same rate, thus maintaining the distance D substantially constant. Therefore, first and second pairs of connecting rods  134 ,  136 ,  138 , and  140 , respectively, maintain first and second lens barrels  102  and  112 , respectively, substantially parallel, and maintain distance D substantially constant. Transverse lens barrel  122  is also made of a material selected from Table 1, and preferably, although not necessarily, of the same material as connecting rods  134 ,  136 ,  138 , and  140 . If transverse lens barrel  122  is made of a different material than the connecting rods  134 ,  136 ,  138 , and  140 , then some kind of adjustment mechanism will be needed. Such adjustment mechanism may include a set of flexures (not shown) located inside transverse lens barrel  122 . 
     Also consistent with the principles of the invention, the support structures comprise a plurality of sets of clamps and a plurality of support platforms. Each set of clamps attaches to both ends of each connecting rod to fasten the connecting rod to the plurality of lens barrels. Each support platform supports a lens barrel. 
     As shown in FIGS. 1 and 2, support structure  132  is provided with four pairs of clamps  134   a  and  134   b ,  136   a  and  136   b ,  138   a  and  138   b ,  140   a  and  140   b , and a pair of support platforms  106  and  116 . Each pair of clamps  134   a  and  134   b ,  136   a  and  136   b ,  138   a  and  138   b , or  140   a  and  140   b , attach to both ends of a connecting rod  134 ,  136 ,  138 , or  140 , respectively, to fasten the connecting rod  134 ,  136 ,  138 , or  140 , to the first and second lens barrels  102 ,  112 , respectively. First support platform  106  has an opening  107  to accommodate and to support first lens barrel  102 . Second support platform  116  has an opening  117  to accommodate and to support second lens barrel  112 . Openings  107  and  117  preferably are slightly larger than first and second lens barrels  102  and  112 , respectively, to allow clearance for the lens barrels  102  and  112  to expand and contract. Support structures  132  may further include flanges (not shown) to secure first and second lens barrels  102 ,  112 , respectively, to first and second support platforms  106 ,  116 , respectively. Clamp  134   a  fastens one end of connecting rod  134  to first support platform  106 . Clamp  136   a  fastens one end of connecting rod  136  to first support platform  106 . Clamps  134   a  and  136   a  may be formed as an integral part of support platform  106 . Similarly, clamp  134   b  fastens the other end of connecting rod  134  to second support platform  116 . Clamp  136   b  fastens the other end of connecting rod  136  to second support platform  116 . Clamps  134   b  and  136   b  may be formed as an integral part of support platform  116 . 
     Clamps  134   a  and  134   b ,  136   a  and  136   b ,  138   a  and  138   b ,  140   a  and  140   b , are configured to facilitate connecting rods  134 ,  136 ,  138 , and  140  in maintaining distance D. Clamps  138   a  and  140   a  are preferably positioned on opposing peripheries of first lens barrel  102  aligned with first optical plane  158 . Clamps  134   a  and  136   a  are preferably positioned on opposing edges of first support platform  106  also aligned with first optical plane  158 . Similarly, clamps  138   b  and  140   b  are preferably positioned on opposing peripheries of second lens barrel  112  aligned with second optical plane  159 . Clamps  134   b  and  136   b  are preferably positioned on opposing edges of second support platform  116  aligned with second optical plane  159 . 
     Further consistent with the principles of the invention, the support structures comprise a plurality of flexures attachable to lower faces of the support platforms for resiliently supporting the lens barrel structure. In one embodiment, the plurality of flexures comprise four types of flexures. A first flexure is a rigid mounting. A second flexure has one degree of freedom in a direction parallel to the connecting rods. A third flexure has one degree of freedom in a direction perpendicular to the connecting rods. A fourth flexure has two degrees of freedom in the directions parallel and perpendicular to the connecting rods. 
     FIGS. 1 and 2 show support structure  132  comprising a plurality of flexures  144 ,  146 ,  148 , and  150 . Flexures  144 ,  146 ,  148 , and  150 , are attachable to lower faces  152  and  154  of support platforms  106  and  116 , respectively, for resiliently supporting lens barrel structure  100 . Flexures  144 ,  146 ,  148 , and  150  promote flexibility in allowing lens barrel structure  100  to move when, for instance, the temperature changes, thus preventing lens barrel structure  100  from distorting or buckling. A first flexure  144  is a rigid mounting. A second flexure  146  has one degree of freedom in a direction parallel to connecting rods  134 ,  136 ,  138 , and  140 . Second flexure  146  allows lens barrel structure  100  to expand or contract in the direction parallel to connecting rods  134 ,  136 ,  138 ,  140 , as they expand or contract. A third flexure  148  has one degree of freedom in a direction perpendicular to connecting rods  134 ,  136 ,  138 , and  140 . Third flexure  148  allows lens barrel structure  100  to expand or contract in the direction perpendicular to connecting rods  134 ,  136 ,  138 , and  140 , as lens barrels  102  and  112 , and support platforms  106  and  116 , expand or contract. A fourth flexure  150  has two degrees of freedom in the directions parallel and perpendicular to connecting rods  134 ,  136 ,  138 , and  140 . Fourth flexure  150  allows lens barrel structure  100  to expand or contract in both directions parallel and perpendicular to connecting rods  134 ,  136 ,  138 , and  140 , as the whole lens barrel structure  100  expand or contract. 
     In one embodiment, first flexure  144  is preferably attached on lower surface  154  of second support platform  116 , aligned with second optical plane  159 , and located as close to a periphery of second lens barrel  112  as possible. Fourth flexure  150  is attached on lower surface  152  of first support platform  106 , aligned with first optical plane  158 , and located as close to a periphery of first lens barrel  102  as possible, and on a diagonally opposing corner from first flexure  144 . Second flexure  146  is attached on lower surface  152  of first support platform  106 , aligned with first optical plane  158 , and located on a diametrically opposing periphery of first lens barrel  102  from fourth flexure  150 . Third flexure  148  is attached on lower surface  154  of second support platform  116 , aligned with second optical plane  159 , and located on a diametrically opposing periphery of second lens barrel  112  from first flexure  144 . FIGS. 1 and 2 show first flexure  144  located in front of second lens barrel  112  along second optical plane  159 , while fourth flexure  150  is located behind first lens barrel  102  along first optical plane  158 . FIGS. 1 and 2 also show second flexure  146  located in front of first lens barrel  102  along first optical plane  158 , while third flexure  148  is located behind second lens barrel  112  along second optical plane  159 . 
     The configuration of flexures  144 ,  146 ,  148 , and  150  may be rotated around the four corners pursuant to the following three guidelines. One, first and fourth flexures,  144  and  150 , respectively, are located on diagonally opposing corners. Two, second flexure  146  is located on a logical anchor position facilitating lens barrel structure  100  to move in a direction parallel to connecting rods  134 ,  136 ,  138 , and  140 . Three, third flexure  148  is located on a logical anchor position facilitating lens barrel structure  100  to move in a direction perpendicular to connecting rods  134 ,  136 ,  138 , and  140 . 
     Therefore, when temperature changes, lens barrels  102  and  112 , as well as support platforms  106  and  116 , will expand or contract accordingly. However, connecting rods  134 ,  136 ,  138 , and  140 , will substantially maintain distance D and control rotational motion of the lens barrels  102  and  112 , while flexures  144 ,  146 ,  148 , and  150  will allow the whole lens barrel structure  100  to expand or contract, thus, preventing the lens barrel structure  100  from distorting or buckling. 
     FIG. 3 illustrates the path of exemplary light beam  128  through lens barrel structure  100  as discussed with respect to FIGS. 1 and 2. Light source  130  illuminates reticle  108 . Reticle  108  is shown as placed on an image plane  109 . Light source  130  enters first lens barrel  102 . A reflective element  110 , such as a mirror, is provided and positioned in front of an exit end  105  of first lens barrel  102  on an opposing end of reticle  108 . Mirror  110  preferably has an optical axis which is aligned with first optical axis  104 . Light beam  128  is reflected off of mirror  110 , illustrated as light beam  128 ′. Light beam  128 ′ travels back through first lens barrel  102 . Light beam  128 ′ then hits mirror  124  carried inside transverse lens barrel  122 , is reflected off of mirror  124 , as light beam  128 ″, and is reflected off of mirror  126 , as light beam  128 ′″. Light beam  128 ′″ travels through second lens barrel  112  and hits wafer  118 . Mirrors  124  and  126  are preferably oriented at 45° angle from light beams  128 ′ and  128 ″, respectively. Therefore, in this embodiment if imaginary extension lines  125  and  127  are drawn from mirrors  124  and  126 , respectively, it will be shown that mirrors  124  and  126  are perpendicular to each other. 
     In a second embodiment, the catadioptric lens barrel structure is also provided with a plurality of lens barrels such as shown in FIGS. 1 and 2. The plurality of support structures according to the second embodiment comprise a base support plate and an upper support plate, replacing the first and second pairs of connecting rods. The base support plate supports lower portions of the lens barrels, and the upper support plate supports upper portions of the lens barrels. 
     Similar to the first embodiment, a catadioptric lens barrel structure is provided with a plurality of lens barrels. Each lens barrel has an optical axis running along the length of its corresponding lens barrel. The plurality of optical axes run parallel to each other. 
     In the second embodiment illustrated in FIG. 4, a catadioptric lens barrel structure  200  comprises a pair of lens barrels including a first lens barrel  202  and a second lens barrel  212 , and a plurality of support structures  232 . Support structures  232  comprise a base support plate  234  and an upper support plate  236 , instead of the connecting rods  134 ,  136 ,  138 , and  140 , as shown in FIGS. 1 and 2. First and second lens barrels  202  and  212 , respectively, are similar to first and second lens barrels  102  and  112 , respectively, according to the first embodiment. First lens barrel  202  has a first optical axis  204  running along its length. Similarly, second lens barrel  212  has a second optical axis  214  running along its length. First and second lens barrels,  202  and  212 , respectively, are shown to be a sub-barrel type. First lens barrel  202  comprises sub-barrels  202   a ,  202   b ,  202   c , and  202   d . Each sub-barrel  202   a ,  202   b ,  202   c , or  202   d , carries a plurality of cells, each cell holding a lens. Second lens barrel  212  comprises sub-barrels  212   a ,  212   b , and  212   c . Each sub-barrel  212   a ,  212   b , or  212   c , carries a plurality of cells, each cell holding a lens. As explained with respect to the first embodiment, first and second lens barrel  202 ,  212 , respectively, can also be made of a rigid material, such as silicon carbide or stainless steel. For example, the barrel may be made of SS 17-7 PH, SS 440 series, or SS 300 series. Similar to the first embodiment as shown in FIGS. 1 and 2, lens barrel structure  200  may also comprise a transverse lens barrel (not shown) connecting first lens barrel  202  to second lens barrel  212 . The transverse lens barrel carries a pair of angularly oriented mirrors (not shown) to reflect an image of the reticle from first lens barrel  202  to second lens barrel  212  and onto the semiconductor wafer in front of an exit end of the second lens barrel  212 . 
     Further consistent with the principles of the invention, the catadioptric lens barrel structure may also be provided with a plurality of support structures. The support structures include a base support plate and an upper support plate. The base and upper support plates run parallel to each other and perpendicular to the first and second optical axes. Each support plate has a plurality of openings to accommodate and to support the corresponding plurality of lens barrels. The base support plate preferably has sufficient thickness to Support the lens barrels. The upper support plate is preferably thinner than the base support plate. The upper support plate provides flexibility and controls rotation of the lens barrels, thus, preventing distortion of the lens barrel structure. The support structures are made of a material having a low coefficient of thermal expansion. 
     In the embodiment illustrated in FIG. 4, catadioptric lens barrel structure  200  is provided with support structures  232 . Support structures  232  include a base support plate  234  and an upper support plate  236 . Base and upper support plates,  234  and  236 , respectively, run parallel to each other and perpendicular to first and second optical axes,  204  and  214 , respectively. In the embodiment shown, base support plate  234  has sufficient thickness to support base portions of first and second lens barrels,  202  and  212 , respectively. Upper support plate  236  is thinner than base support plate  234 , thus providing flexibility and control of rotation of the lens barrels. Therefore, upper support plate  236  prevents distortion of lens barrel structure  200 . Base support plate  234  has a first opening  248  and a second opening  250  to accommodate and support the corresponding first and second lens barrels,  202  and  212 , respectively. Upper support plate  236  has a first opening  252  and a second opening  254  to accommodate and support the corresponding first and second lens barrels,  202  and  212 , respectively. 
     Base support plate  234  and upper support plate  236  are preferably made of a material having a low coefficient of thermal expansion. For example, the coefficient of thermal expansion may be as low as 6 ppm/° C. or below. The transverse lens barrel is also preferably made of a material having a low coefficient of thermal expansion. For example, the coefficient of thermal expansion may be as low as 6 ppm/° C. or below. Table 1 above shows examples of materials having this property. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the catadioptric lens barrel structure, the material chosen for the present invention, and in construction of the number of lens barrels arrangement and the support structure as well as other aspects of the invention without departing from the scope or spirit of the invention. Although the support structures according to the first embodiment comprise a plurality of connecting rods, while according to the second embodiment comprise a pair of support plates, these support structures may be used in combination. For instance, the support structures may use a base support plate to support the base portions of the lens barrel, and use a pair of connecting rods to support the upper portions of the lens barrels. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.