Abstract:
An optical system with at least one optical element that causes a disturbance of the distribution of polarization over the cross section of a light beam wherein at least one birefringent optical element is provided, with a thickness which varies irregularly over the cross section, such that the disturbance of the distribution of polarization is at least partially compensated.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical system that is illuminated with light of defined polarization (for example, linearly polarized), in which a locally varying disturbance of the polarization occurs, due among other things to the degree of reflection at the inclined optical boundary surfaces, dependent on the direction of polarization, and to stress birefringence in lenses. After a following analyzer, local variations in brightness appear. 
     2. Discussion of Relevant Art 
     A system in which this effect is of importance is projection illumination equipment for microlithography, with a linearly polarized excimer laser in the DUV and a catadioptric reducing objective with a polarization beam splitter, as is known, for example, from German Patent DE 196 16 922 (U.S. Ser. No. 08/845,384 filed Apr. 15, 1997), the said patent being fully incorporated herein by reference. 
     Compensators are known for the continuously variable rotation of the direction of polarization, e.g. in the form of the Soleil-Babinet compensator, but of course only with uniform action over the whole light beam cross section. 
     Phase correction plates for wavefront correction in precision optics are known for use in microlithographic projection illumination equipment, and are for example machined with a “nanosphere” fine profile by ion beam etching. 
     Birefringent crystals of magnesium fluoride and of quartz are suitable for the production of polarization optical elements with transmission in the deep ultraviolet (DUV) at, e.g., 193 nm. Besides this, elements with stress birefringence are also known, e.g. according to German Patent DE 196 37 563. 
     SUMMARY OF THE INVENTION 
     The invention has as its object to provide an arrangement with which local disturbances of the state of polarization of light in an optical system can be compensated. In particular, disturbances of homogeneity, which result from polarization disturbances, are to be compensated in optical systems with polarizing beam splitters, such as in catadioptric reducing objectives of the type mentioned hereinabove. In addition, a method of production is provided. 
     The object is attained by means of an optical system according to the invention with at least one birefringent element with irregularly varying thickness as the polarization compensator. The optical system comprises at least one optical element that causes a disturbance of the distribution of polarization over the cross section of a light beam and at least one birefringent optical element having a thickness that varies irregularly over said cross section, such that the disturbance of the distribution of polarization is at least partially compensated. 
     An embodiment is particularly preferred in which pairwise birefringent elements are provided with principal axes rotated with respect to each other, preferably through 45°. Polarization disturbances which are actually arbitrary as regards their orientation can thereby be compensated. 
     Likewise, an arrangement of compensation plates of isotropic material, in an advantageous embodiment of the invention, optimizes the correction. The method of production according to the invention comprises a process for the production of an optical system, comprising finish-mounting and adjusting a partial system, making polarization optical precision measurements of said partial system, locally differently eroding at least a first birefringent optical element corresponding to the polarization optical measurement data, and inserting said first birefringent optical element into the beam path of the optical system behind the partial system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described in more detail with reference to the accompanying drawing, in which: 
     FIG. 1 shows schematically a projection illumination equipment with birefringent polarization correction elements. 
     FIG. 2 shows schematically two polarization correction elements with different main axis directions. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The core of the projection illumination equipment according to FIG. 1 is a catadioptric reduction objective  1  which images a mask (reticle)  2  onto a wafer  3 , on a reduced scale. The mask  2  and also the wafer  3  are installed on holding and positioning systems  21 ,  31  which enable the precise positioning and the step and repeat sequence or the step and scan sequence to be carried out. 
     The mask  2  is suitably illuminated by means of a laser  41 , in particular a linearly polarized DUV excimer laser, with a subsequent illumination optics  42 . 
     The reduction objective  1  consists firstly in a known manner of lens groups  11 - 14 , the polarized beam splitter cube  15 , the deflecting mirror  17 , the concave mirror  16 , and a quarter wave plate  22 . 
     A disturbance of the state of polarization with a direction and magnitude which varies locally over the cross section results in the beam path due to manifold influences, stress birefringence among them. The radiation distribution at the polarizing beam splitter is thus inhomogeneous, and the image is distorted when no countermeasures are taken. 
     According to the invention, birefringent optical elements  21   a,    21   b  (at least one) are provided in the beam path  10  behind the lens groups  11 ,  12 , are irregular in thickness over the cross section, and at least partially compensate the local disturbances of the linear polarization in the beam path  10 . 
     For the optimum inclusion and correction of all directions of polarization, two elements  21   a,    21   b  are required, with their principal axes MCA mutually rotated, optimally by 45°, as shown in FIG.  2 . 
     The quarter wave plate  22  is, in the example, also corrected in its thickness and compensates the polarization errors arising in the further beam path, and possibly residual errors. It is to be noted that the light beam  10  passes through it twice. 
     Plates  23   a,    23   b  of isotropic material (quartz glass) have optically negative shapes to the correction plates  21   a,    21   b.  The optical path over the whole cross section of the light beam  10  is thereby again equal, so that the wavefront is not disturbed by the combination of the elements  21   a,    21   b  and plates  23   a,    23   b.    
     The free form surfaces of the elements  21   a,    21   b  and the plates  23   a,    23   b  can be produced by ion beam processing. The thickness modulation of the elements  21   a,    21   b  or plates  23   a,    23   b  thereby moves by values of up to about 3 μm. 
     To prevent reflections at the boundary surfaces, the elements  21   a,    21   b  and plates  23   a,    23   b  are united together and with the beam splitter cube  15  without any air gaps, as far as possible. This takes place by wringing or by cementing, the latter of course not in the DUV wavelength region. 
     For the same reason, the plates  23   a,    23   b  are of isotropic material and are preferably embodied integrally united as a single element. 
     It is advantageous both as regards production and as regards the exact fitting shape and jointing without gaps, however, if the plates  23   a,    23   b  are produced by the coating of the processed surfaces of the polarization compensator elements  21   a,    21   b  with isotropic material, e.g., the CVD deposition of quartz glass, which in conclusion is optically polished. The residual disturbance of the wavefront resulting from the different indices of refraction can furthermore still be corrected by ion beam or atomic beam processing with smaller height differences. 
     The quarter wave plate  22  constituted as a polarization correction element is shown only by way of example; here also, combination with an isotropic (amorphous) compensator and constructional unification with neighboring elements, particularly by wringing to the beam splitter cube  15 , is appropriate and advantageous. 
     The shape of the correction surfaces of the elements  21   a,    21   b,    22  and of the plates  23   a,    23   b  is preferably established individually for each objective. For this purpose, the quality of test illuminations can be evaluated, or the local polarization or the wavefront in the cross section of the light beam  10  can be determined by means of scanning probes.