Method and apparatus for characterization of optical systems
Characterization of an optical system is quickly and easily obtained in a single acquisition step by obtaining image data within a volume of image space. A reticle and image plane are positioned obliquely with respect to each other such that a reticle having a plurality of feature sets thereon, including periodic patterns or gratings, is imaged in a volume of space, including the depth of focus. Metrology tools are used to analyze the detected or recorded image in the volume of space through the depth of focus in a single step or exposure to determine the imaging characteristics of an optical system. Focus, field curvature, astigmatism, spherical, coma, and/or focal plane deviations may be determined. The present invention is particularly applicable to semiconductor manufacturing and photolithographic techniques used therein, and is able to quickly characterize an optical system in a single exposure with dramatically increased data quality and continuous coverage of the full parameter space.
1. A method of characterizing an optical system comprising:
projecting an image of a reticle, positioned in a reticle plane and having a plurality of periodic pattern features thereon, with the optical system;
detecting the image of the plurality of periodic pattern features in a plane oblique to the reticle plane; and
analyzing the image of the plurality of periodic pattern features to obtain information characterizing the optical system.
2. A method of characterizing an optical system as in claim 1 wherein:
the plurality of periodic pattern features comprises a grating.
3. A method of characterizing an optical system as in claim 1 wherein:
the plurality of periodic pattern features are a plurality of gratings.
4. A method of characterizing an optical system as in claim 3 wherein:
the plurality of gratings comprises basket weaves, vertical lines, horizontal lines, and tilted lines.
5. A method of characterizing an optical system as in claim 4 wherein:
a central portion is formed of repeating vertical lines, horizontal lines, and tilted lines.
6. A method of characterizing an optical system as in claim 5 , wherein:
the central portion is bounded by basket weaves.
7. A method of characterizing an optical system as in claim 1 wherein:
the image is recorded on a photosensitive substrate.
8. A method of characterizing an optical system having an optical axis comprising:
projecting an image of a reticle having a plurality of periodic pattern features therein through the optical system;
detecting the image of the reticle simultaneously at different locations and in a direction coaxial with the optical axis; and
analyzing the image to obtain characterization of the optical system.
9. A method of characterizing an optical system as in claim 8 wherein:
the plurality of periodic pattern features comprise a plurality of rows of vertical, horizontal, and tilted lines.
10. A method of characterizing an optical system as in claim 8 where:
the step of analyzing comprises using interferometry.
11. A method of extracting optical parameters from an optical system having an optical axis comprising the steps of:
illuminating periodic patterns in an object plane of an object space of the optical system, the object plane extending over a range of depths through the object space;
imaging the periodic patterns via the optical system;
intercepting and recording the image of the periodic patterns in an image volume of the optical system; and
analyzing a recorded image of the periodic patterns formed within the image volume,
whereby optical system parameters are extracted.
12. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the recorded image is tilted within the image volume.
13. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the object plane is tilted with respect to the optical axis,
whereby a continuum of object positions as a function of field position is generated.
14. A method of extracting optical parameters from an optical system as in claim 13 wherein:
the recorded image is tilted with respect to the optical axis.
15. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the object plane and the recorded image are tilted orthogonally with respect to each other,
whereby a continuum of object positions in one axis and focus positions in another orthogonal axis is generated.
16. A method of extracting optical parameters from an optical system as in claim 12 wherein:
an envelope of feature resolution through focus is extracted.
17. A method of extracting optical parameters from an optical system as in claim 12 wherein:
astigmatism of the optical system is extracted as a function of periodic-pattern orientation.
18. A method of extracting optical parameters from an optical system as in claim 12 wherein:
coma of the optical system is extracted as a second order distortion signature versus focus mapped across the field.
19. A method of extracting optical parameters from an optical system as in claim 12 wherein:
spherical aberration of the optical system is extracted as a function of best focus difference between line sizes of the periodic pattern versus field position.
20. A method of extracting optical parameters from an optical system as in claim 12 wherein:
optimum reticle or object position is extracted as a function of field position of minimum spherical aberration as seen by minimum best focus difference between line sizes.
21. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the recorded image is analyzed using a dark field microscope.
22. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the recorded image is analyzed using white light.
23. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the recorded image is analyzed using a laser microscopic interferometer.
24. A method of extracting optical parameters from an optical system as in claim 11 wherein:
the recorded image is analyzed in a single exposure using a large aperture interferometer.
25. A method of extracting optical parameters from an optical system as in claim 11 wherein the act of analyzing further comprises:
calculating best focus position.
26. A method of extracting optical parameters from an optical system as in claim 11 wherein the act of analyzing further comprises:
calculating spherical aberrations.
27. An apparatus for characterizing an optical system comprising:
an optical system;
illumination means for projecting an image of a reticle having a plurality of periodic pattern features thereon within a volume of image space;
means for detecting the image at different locations comprising different depths of focus within the volume of image space;
means for analyzing the image and determining optical system imaging characteristics.
28. An apparatus for characterizing an optical system as in claim 27 wherein:
said means for analyzing the image and determining optical system imaging characteristics comprises analyzing interference patterns created by the image.