Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
1999-02-19
2001-06-26
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S494010, C359S489040, C359S900000, C356S033000, C356S364000, C356S365000
Reexamination Certificate
active
06252712
ABSTRACT:
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.
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V. M. Kozenkov, et al., “Multifinction Light-Polarization Converter”, Sov. Tech. Phys. Lett. 6(1), pp. 47-48, Jan. 1980.*
S. B. Papernyi, et al., “Formation of a Smooth Trnasverse Distribution of Intensity in a Light Beam by Phase-Rotating Plate”, Sov. J. Qantum Electron. 8((9), pp. 1165-1166, Sep. 1978.*
European Search Report Dec. 5, 1999.
Furter Gerhard
Gerhard Michael
Kaiser Winfried
Schuster Karl-Heinz
Wagner Christian
Carl-Zeiss-Stiftung
Jr. John Juba
Spyrou Cassandra
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