Abstract:
A counter-rotating anamorphic prism pair assembly with variable spacing allows the simultaneous adjustment of a prism pair using an adjustment member to circularize in cross section a range of elliptical laser beam cross sections. A first prism rotates and translates towards or away from an incident laser beam while a second prism simultaneously rotates towards or away from the laser beam in a fixed counter-rotating relationship with the first prism. The degree of rotation and translation is determined by a mechanical linkage connecting the two prisms.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to optics, and more particularly to conversion of a laser beam with an elliptical cross section to a beam with a circular cross section. 
     2. Description of Related Art 
     Elliptically shaped (in cross section) light beams output from a laser can be converted into a more desirable circular beam by using a pair of prisms. The output beam from a laser is often in the cross-sectional shape of an ellipse; however, the elliptical shape does not lend itself to optimal performance of associated systems, thereby giving rise to various techniques for converting the elliptical beam into a circular one. Such beam conversion is useful, e.g., in laser beam scanning lithography where a group of parallel laser beams are modulated and scanned over a photosensitive medium to form an image on the medium. Applications are, for instance, in the semiconductor industry for lithography for integrated circuits. 
     These methods of converting such beam cross sections usually involve transmitting a laser beam through a pair of prisms and then rotating and translating the prisms in relation to each other until the desired cross section was achieved. An incident laser beam is applied to the prism pair, and then an iterative process begins of manipulating the prisms relative to each other. This not only increases post adjustment alignment time for downstream optics, but this also increases the complexity of downstream assemblies due to significant angular and transverse displacement of the output beam relative to the input beam. Therefore, there is a need to be able to quickly adjust an anamorphic prism pair to change the ellipticity of an input laser beam while minimizing angular and transverse beam displacements resulting from the adjustments. 
     SUMMARY 
     In accordance with the invention, the above problem is overcome by linking an anamorphic pair of prisms, where a first prism simultaneously rotates counter to a second prism, by mechanical linkages. An anamorphic assembly is an optical system providing two different magnifications along two perpendicular axes such as the present assembly where prisms convert an elliptically cross sectioned laser beam into a circularly shaped cross sectioned beam. The prisms are linked such that the first prism translates and rotates simultaneously towards or away from the axis of an input laser beam. Meanwhile, the second prism rotates towards or away from the axis of the input laser beam in a counter-rotating relationship with the first prism. These movements are effected by a single slide adjustment member which translates a slide upon which both prisms are attached. 
     A first prism mount, upon which the first prism is attached, has a distal end which is attached to the base and a proximal end which is attached to the slide. A second prism mount, upon which the second prism is attached, also has a distal end and a proximal end; however, this distal end is attached to the slide and the proximal end is attached to the base. This arrangement allows the simultaneous adjustment of both prisms using a single slide adjustment member while maintaining the circularity of an output beam cross section over a range of elliptical input beam cross sections. This arrangement also allows for minimizing the angular and transverse displacement of the output beam relative to the input beam. 
     Furthermore, there is an associated method of simultaneously adjusting a prism pair where a laser beam is input into the entrant face of the first prism, then the slide adjustment member is adjusted. This adjusting rotates and translates the first prism towards or away from the laser beam and rotates the second prism simultaneously towards or away from the laser beam in a fixed counter-rotating manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a view of the present counter-rotating anamorphic prism assembly. 
     FIG. 2 shows a view of the FIG. 1 counter-rotating anamorphic prism assembly in the negative extreme position. 
     FIG. 3 shows a view of the counter-rotating anamorphic prism assembly of FIG. 1 in the positive extreme position. 
     FIG. 4 shows a view of the angular adjustment member and the first distal pivot of FIG.  1 . 
     FIG. 5 shows a view of the anamorphic prism assembly, the transverse displacement of the laser beam, the angles of rotation of the prisms, and the angles of incidence and refraction of the laser beam through the assembly of FIG.  1 . 
     FIG. 6 shows a view of a prism and the incident area. 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an optical apparatus in accordance with this invention. The adjustable prism apparatus  2  has a base  4  which allows removal of an associated cover (not shown) and also allows easy access to the internal assembly contained within and which will be discussed in detail below. On base  4  is mounted a slide  6  which moves translationally over a specified range. Prism apparatus  2  further includes a first prism mount  8  and a second prism mount  10  mounted to base  4  and slide  6 . 
     Slide  6  translates on base  4  by slide adjustment member  16 , which is, e.g., an adjustment screw anchored to base  4  in this embodiment. Slide adjustment member  16  has a ball end facing towards slide  6  and can be translated towards or away from slide  6  by rotating it about its own axis. Slide  6  is maintained in contact with slide adjustment member  16  at contact point  50  by a small spring force (e.g., 4 lb. force, nominal) acting on slide  6  (the spring is not shown); slide  6  is not rigidly fastened to slide adjustment member  16  thus allowing the rotation of member  16  about its axis. 
     First prism mount  8  has two pivots located respectively at both its ends. First distal pivot  30  is located at the top of first prism mount  8 , called first distal end  26 , and first proximal pivot  32  is located at the lower end of first prism mount  8 , called first proximal end  28 . Furthermore, first distal pivot  30  is in contact with angular adjustment member  18 , which is described in further detail below for FIG.  4 . The first distal and proximal pivots  30 ,  32  are collinearly aligned in first prism mount  8 , but this does not preclude noncollinear arrangements in other embodiments. First distal pivot  30  is secured onto base  4  near angular adjustment member  18 , located at the top of base  4 , and first proximal pivot  32  is secured onto slide  6 . Also, first distal pivot  30  is held within a slotted channel in first prism mount  8  (slot is not shown) to allow the translation of slide  6 . Second prism mount  10  also has two pivots located at both ends. Second distal pivot  38  is located near the top of prism mount  10  in second distal end  34  and second proximal pivot  40  is located near the bottom of prism mount  10  in second proximal end  36 . Second distal and proximal pivots  38 ,  40  are also aligned collinearly, but again this does not preclude a noncollinear arrangement in other embodiments. In an opposite arrangement from first prism mount  8 , second distal pivot  38  is secured onto slide  6  and second proximal pivot  40  is secured onto base  4 . Second distal pivot  38  is also held within a slotted channel on second prism mount  10  (slot is not shown) to allow the translation of slide  6 . All four pivots  30 ,  32 ,  38 ,  40  are fastened to either base  4  or slide  6  with, e.g., dowels (not shown) which allows the rotation of the prism mounts  8 ,  10  about their respective pivots. First distal pivot  30  is further attached to angular adjustment member  18  which is used to correct output laser beam  24  for manufacturing error in apparatus  2  or in input laser beam  20  angular errors. Operation of angular adjustment member  18  is described in greater detail below. 
     First prism  12  is mounted onto first prism mount  8  at first proximal end  28  with an adhesive in such a way that a first entrant face  42  of first prism  12  is substantially coplanar with first proximal pivot  32 . Second prism  14  is also mounted onto second proximal end  36  in such a way that a second entrant face  46  is substantially coplanar with second proximal pivot  40 . It is not necessary that first and second entrant faces  42 ,  46 , respectively, and first and second proximal pivots  32 ,  40 , respectively, are exactly coplanar. Both entrant faces  42 ,  46  and both proximal pivots  32 ,  40  may be non-coplanar but substantially close. Both prisms  12 ,  14  are of fused silica having an index of refraction of n=1.504 in one particular embodiment. 
     FIG. 2 shows the translation towards the negative extreme position of slide  6  through a distance, d, relative to base  4 . As the input laser beam  20  (from a conventional laser source) is illuminated through first entrant face  42 , slide adjustment member  16  is rotated about its axis to translate slide  6  parallel and towards input laser beam  20 . This causes first prism mount  8  to rotate about first distal pivot  30  as first proximal end  28  rotates towards input laser beam  20 . Simultaneously, second distal pivot  38  is linearly translated through distance, d, towards input laser beam  20 . This in turn causes second prism mount  10  to rotate about second proximal pivot  40 . The simultaneous rotations of first and second prism mounts  8 ,  10  result in the rotation and translation of first prism  12  and the rotation of second prism  14  in a counter-rotating manner. 
     Additionally, because first prism  12  and second  14  are mounted such that input laser beam  20  is incident upon entrant faces  42 ,  46  and perpendicularly to the axis of first and second proximal pivots  32 ,  40 , respectively, a sweep of beams may also be applied in another embodiment. Such a sweep of beams is preferably illuminated upon first entrant face  42  such that the beams are coplanar with each other and this plane is parallel with the axis of first and second proximal pivots  32 ,  40 , respectively. The true circularity of the resulting output laser beam  24  can be monitored with a change coupled device (CCD) camera (camera not shown) or any commercially available beam monitoring device. Such a camera can be utilized with a beam splitter and placed downstream of prism apparatus  2 . In another embodiment, another type of conventional camera, utilized with a beam splitter, may be placed either upstream or downstream of prism apparatus  2 , but it is preferable to locate a camera downstream to monitor the circularity of the cross section of output laser beam  24 . 
     FIG. 3 shows the translation towards the extreme positive position of slide  6  through a distance, d, relative to base  4 . Again, as input laser beam  20  is illuminated through first entrant face  42 , slide adjustment member  16  is rotated about its axis to translate slide  6  parallel and away from input laser beam  20 . This causes first prism mount  8  to rotate about first distal pivot  30  as first proximal end  28  rotates away from input laser beam  20 . Simultaneously, second distal pivot  38  is linearly translated through distance, d, away from input laser beam  20 . Again, this causes second prism mount  10  to rotate about second proximal pivot  40  and the simultaneous rotations of first and second prism mounts  8 ,  10  further results in the rotation and translation of first prism  12  and the rotation of second prism  14  in a counter-rotating manner opposite from the direction as shown in FIG.  2 . 
     FIG. 4 shows angular adjustment member  18  of FIG. 1 which is used to correct output laser beam  24  angle errors. Angular adjustment member  18  is shown as an adjustment screw which is screwed into base  4  in this embodiment. After slide  6  and prisms  12 ,  14  have been adjusted to circularize the cross-sectional area of input laser beam  20  (as discussed above for FIGS.  2  and  3 ), the axis of output laser beam  24  might deviate from the axis of input laser beam  20  due either to manufacturing errors in the mechanical linkages and prisms  12 ,  14  or in input laser beam  20  angular errors. Therefore, in order to keep the axis of output laser beam  24  substantially parallel with the axis of input laser beam  20 , correction of output laser beam  24  angle is effected by rotating angular adjustment member  18  about its own axis. This rotation translates first distal pivot  30  in a parallel direction either towards or away from input laser beam  20  and this translation adjusts the angle of incidence for input laser beam  20  with first prism  12  to effect a beam correction. 
     FIG. 5 shows geometrically the relationship between first prism  12  and second prism  14 . The input laser beam  20  enters first entrant face  42  at angle A 1 , which is the angle of incidence of input laser beam  20  at first entrant face  42 . As input laser beam  20  passes through first prism  12 , it defines angle B 1 , which is the angle of refraction of input laser beam  20  at first entrant face  42 . Input laser beam  20  again refracts as it passes first refractant face  44  defining angle A 2 , which is the angle of incidence of input laser beam  20  at first refractant face  44 , and angle B 2 , which is the angle of refraction of input laser beam  20  at first refractant face  44 . Input laser beam  20  is designated intermediate refracted laser beam  22  as it passes from first prism  12  to second prism  14 . This intermediate refracted beam  22  then enters second prism  14  defining angle A 3 , which is the angle of incidence of intermediate refracted laser beam  22  at second entrant face  46 , and angle B 3 , which is the angle of refraction of intermediate refracted laser beam  22  at second entrant face  46 . The angles of first prism  12  and second prism  14  are discussed in greater detail below. Finally, as intermediate beam  22  passes second refractant face  48 , it defines angle A 4 , which is the angle of incidence of intermediate refracted laser beam  22  at second refractant face  48 , and angle B 4 , which is the angle of refraction of intermediate refracted laser beam  22  at second refractant face  48 . The initial input laser beam  20  enters first prism  12  and finally emerges from second prism  14  as output laser beam  24 . The linear distance between where input laser beam  20  enters first entrant face  42  and where output laser beam  24  exits second refractant face  48  is the transverse displacement, t. Transverse displacement, t, is ideally held constant over the range of motion by prism apparatus  2 . Furthermore, the angular difference between input laser beam  20  and output laser beam  24  is preferably minimized by prism apparatus  2  in maintaining an angular error of approximately 7.5 arc-min at the negative extreme in FIG.  2  and an angular error of approximately 6.0 arc-min at the positive extreme in FIG.  3 . 
     FIG. 6 shows the dimensions of first and second prisms  12 ,  14 , respectively. Both prisms  12 ,  14 , are defined by a prism height PH and a prism width PW which are preferably equal in one embodiment. Prisms  12 ,  14  are further defined by a prism length PL and an apex angle α. Furthermore, the preferable area upon which input laser beam  20  is incident upon first and second prism  12 ,  14 , is bordered by outside frame OF. The prism material for this embodiment is fused silica. However, this does not preclude the use of other materials suitable for the use of prisms. The aforementioned dimensions for one embodiment are shown in the following table. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Dimension 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 α (degrees) 
                 15.37° 
               
             
          
           
               
                   
                 PH 
                 25.40 
                 mm 
               
               
                   
                 PW 
                 25.40 
                 mm 
               
               
                   
                 PL 
                 12.23 
                 mm 
               
               
                   
                 OF 
                 3.00 
                 mm 
               
               
                   
                   
               
             
          
         
       
     
     The wavelength of input laser beam  20 , which is to be circularized, is 257.25 nm in one particular embodiment. Because prism apparatus  2  circularizes elliptical laser beams with a variable transverse and lateral radius, apparatus  2  operates over a range of laser beam cross sections. Apparatus  2  is such that first and second prism mounts  8 ,  10 , respectively, are in a nominal position when the transverse radius of the input laser beam  20  measures 0.435 mm and the lateral radius measures 0.223 mm. Slide adjustment member  16  may then be adjusted to translate slide  6  to the negative extreme position shown in FIG. 2 to accommodate a laser beam with a minimum transverse radius of 0.348 mm and to the positive extreme position shown in FIG. 3 to accommodate a maximum transverse radius of 0.522 mm, where both beams have a lateral radius of 0.223 mm. These varying transverse radii may be summarized by a scale factor in relation to the nominal radius of 0.435 mm, as shown in the following table. (These dimensions, of course, are only illustrative of one embodiment.) 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                 Transverse Radius 
                 Lateral Radius 
                 Scale 
                 Slide 
               
               
                 (mm) 
                 (mm) 
                 Factor 
                 Position 
               
               
                   
               
             
             
               
                 0.348 
                 0.223 
                 0.8 
                 Negative extreme 
               
               
                 0.435 
                 0.223 
                 1.0 
                 Nominal 
               
               
                 0.522 
                 0.223 
                 1.2 
                 Positive extreme 
               
               
                   
               
             
          
         
       
     
     The absolute value of change in rotation of PI, which is the angle between first entrant face  42  and a plane perpendicular to an axis of input laser beam  20 , is related to the absolute value of translational distance, d, which slide  6  travels by the following: 
     
       
         Δθ first prism =tan −1 ( d/L   1 )  
       
     
     where L 1  is the length from first distal pivot  30  to first proximal pivot  32 . Likewise P 2 , which is the angle between second entrant face  46  and the plane perpendicular to an axis of input laser beam  20 , is also related to the absolute value of translational distance, d, by the following: 
     
       
         Δθ second prism =tan −1 ( d/L   2 )  
       
     
     where L 2  is the length from second distal pivot  38  to second proximal pivot  40 . A third value, L 3 , is the distance from first proximal pivot  32  to second proximal pivot  40 . All three values, L 1 , L 2 , and L 3 , are chosen to minimize the angular displacement and changes in transverse displacement, t, of output laser beam  24  over the above range of scale factors. The changes in rotation P 1 , P 2  of first and second prisms  12 ,  14 , respectively, are such that the changes occur in a counter-rotating manner as discussed above. 
     The relationship between the laser beam radii (scale factor), the transverse displacement, t, between input laser beam  20  and output laser beam  24 , first prism  12  and second prism  14  orientation, and the angles of incidence and refraction from first prism  12  and second prism  14  (in degrees) is summarized in the following table. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Value by Scale Factor 
               
             
          
           
               
                   
                 Dimension 
                 0.8 
                 1 
                 1.2 
               
               
                   
                   
               
             
          
           
               
                   
                 t (mm) 
                 0.2520619 
                 0.2666618 
                 0.2789777 
               
               
                   
                 P1 
                 22.800 
                 28.600 
                 31.700 
               
               
                   
                 P2 
                 33.491 
                 41.700 
                 46.098 
               
               
                   
                 A1 
                 22.800 
                 28.600 
                 31.700 
               
               
                   
                 A2 
                 30.304 
                 33.933 
                 35.824 
               
               
                   
                 A3 
                 22.307 
                 28.594 
                 31.513 
               
               
                   
                 A4 
                 29.991 
                 33.929 
                 35.711 
               
               
                   
                 B1 
                 14.934 
                 18.563 
                 20.454 
               
               
                   
                 B2 
                 49.354 
                 57.076 
                 61.655 
               
               
                   
                 B3 
                 14.621 
                 18.559 
                 20.341 
               
               
                   
                 B4 
                 48.733 
                 57.067 
                 61.366 
               
               
                   
                   
               
             
          
         
       
     
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. In particular, even though much of the preceding discussion is of a prism material of fused silica and a particular laser beam wavelength of 257.25 nm, alternative embodiments of this invention include various other prism materials and laser beam wavelengths. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.