Dual-domain lateral shearing interferometer

Optics: measuring and testing – By light interference – For dimensional measurement

Reexamination Certificate

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C356S521000

Reexamination Certificate

active

06707560

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to lateral shearing interferometers (LSI) for making highly accurate measurements of wavefront aberrations. The invention overcomes the inaccuracies associated with conventional implementations of grating-based LSIs.
References
The following publications are cited in this application as superscript numbers:
1. G. E. Sommargren, “Diffraction methods raise interferometer accuracy,” Laser Focus World, 32, 61-71, (8/96).
2. A. K. Ray-Chaudhuri, et al, “Alignment of a multilayer-coated imaging system using extreme ultraviolet Foucault and Ronchi interferometric testing,” J. Vac Sci Technol. B, 13, 3089-3093 (1995).
3. H. Medecki, et al, “Phase-shifting point diffraction interferometer,” Opt. Lett., 21, 1526-1528 (1996).
4. P. Naulleau et al, “Characterization of the accuracy of EUV phase-shifting point diffraction interferometry,” in
Emerging Lithographic Technologies II,
Yuli Vladimirski, Editor, Proceedings of SPIE Vol. 3331, 114-123, (1998).
5. P. Carre, “Installation et utilisation du comparateur photoelectric et interferential du bureau international des poids et mesures,” Metrologia, 2, 13-17 (1966).
6. R. Crane, “Interference phase measurement,” Appl. Opt., 8, 538-542 (1969).
7. J. H. Bruning, et al, “Digital wavefront measuring interferometer for testing optical surfaces and lenses,” Appl. Opt., 13, 2693-2703 (1974).
8. M. Takeda, et al, “Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry,” J. Opt. Soc. Am., 72, 156-160 (1982).
9. E. Leith, et al, “Electronic holography and speckle methods for imaging through tissue using femtosecond gated pulses,” Appl. Opt., 30, 4204-4210 (1991).
10. K. A. Goldberg, et al, “A 3-D numerical study of pinhole diffraction to predict the accuracy of EUV point diffraction interferometry,” OSA Trends in Optics and Photonics Vol. 4, Extreme Ultraviolet Lithography, G. D. Kubiac and D. R. Kania, eds, (Optical Society of America, Washington, D.C. 1996), pp. 133-137.
11. D. A. Tichenor, et al, “Development and characterization of a 10× Schwarzschild system for SXPL,” in
OSA Proceedings on Soft X-Ray Projection Lithography,
Vol. 18, A. M. Hawryluk and R. H. Stulen, eds., (Optical Society of America, Washington, D.C., 1993), pp. 79-82.
12. P. de Groot, “Derivation of algorithms for phase-shifting interferometry using the concept of a data-sampling window,” Appl. Opt., 34, 4723-4730 (1995).
13. K. Freischlad and C. Koliopoulos, “Fourier description of digital phase-measuring interferometry,” J. Opt. Soc. Am. A, 7, 542-551 (1990).
14. Y. Surrel, “Design algorithms for phase measurements by the use of phase stepping,” Appl. Opt., 35, 51-60 (1996).
15. J. Tome and H. Stahl, “Phase-measuring interferometry: applications and techniques,” in
Optical Testing and Metrology II,
Proceedings of SPIE Vol. 954, 71-77 (1988).
16. K. Creath, “Comparison of phase-measuring algorithms” in
Surface Characterization and Testing,
Proceedings of SPIE Vol. 680, 19-28 (1986).
17. H. Stahl, “Review of phase-measuring interferometry,” in
Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection,
Proceedings of SPIE Vol. 1332, 71-77 (1990).
18. J. E. Bjorkholm, et al., “Phase-measuring interferometry using extreme ultraviolet radiation,” J. Vac. Sci. & Technol. B, 13, 2919-2922 (1995).
19. P. Naulleau, et al., “The EUV phase-shifting point diffraction interferometer: a sub-angstrom reference-wave accuracy wave front metrology tool,” Appl. Opt., 38, 7252-7263 (1999).
20. K. A. Goldberg, “Extreme Ultraviolet Interferometry,” Ph.D. dissertation (University of California, Berkeley, 1997).
21. D. Attwood, et al., “Tunable coherent radiation in the soft X-ray and extreme ultraviolet spectral regions,” IEEE J. Quantum Electron., 35, 709-720 (1999).
22. V. Ronchi, “Forty years of history of a grating interferometer,” Appl. Opt., 3, 437-451 (1964).
23. A. Lohmann and O. Bryngdahl, “A lateral wavefront shearing interferometer with variable shear,” Appl. Opt., 6, 1934-1937 (1967).
24. S. Yokozeki and T. Suzuki, “Shearing interferometer using the grating as the beam splitter,” Appl. Opt., 10, 1575-1580 (1971).
25. J. C. Wyant, “Double frequency grating lateral shear interferometer,” Appl. Opt., 12, 2057-2060 (1973).
26. H. O. Bartlett and Yajun Li, “Lau interferometry with cross gratings,” Optics Comm., 48, 1-6 (1983).
27. J. C. Wyant, “Use of an ac heterodyne lateral shear interferometer with real-time wavefront correction systems,” Appl. Opt., 14, 2622-2626 (1975).
28. J. Schwider, “Single sideband Ronchi test,” Appl. Opt., 20, 2635-2642 (1981).
29. J. Braat and A. Janssen, “Improved Ronchi test with extended source,” J. Opt. Soc. Am. A, 16, 131-140 (1999).
30. P. Naulleau and K. A. Goldberg, “Dual-domain point diffraction interferometer,” Appl. Opt, 38, 3523-3533 (1999).
31. D. Malacara,
Optical Shop Testing
, (John Wiley & Sons, New York, 1992), pp. 346-348.
32. D. W. Sweeney, et al., “EUV optical design for a 100 nm CD m imaging system,” in
Emerging Lithographic Technologies II,
Y. Vladimirsky, ed., Proc. SPIE Vol. 3331, 2-10 (1998).
33. M. P. Rimmer, “Method for evaluating lateral shearing interferograms,” Appl. Opt., 13, 623-629 (1974).
All of the above publications are herein incorporated by reference in their entirety to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
The recent development of extreme ultraviolet (EUV) optics for use in next-generation lithography systems has led to several advancements in EUV interferometry.
2,3,18
With a demonstrated reference-wavefront accuracy of better than &lgr;
EUV
/350 (0.04 nm at &lgr;
EUV
=13.4 nm)
19
, the phase-shifling point diffraction interferometer (PS/PDI)
3,19,20
is believed to be the most accurate EUV interferometer available. Although the PS/PDI has been proven to have high-accuracy, broad applicability of the PS/PDI is severely limited by its small dynamic range and the fact that it must be used with a highly coherent EUV source such as undulator radiation.
An alternative to the PS/PDI, with relaxed coherence requirements, is the lateral shearing interferometer (LSI).
22-29
The Ronchi interferometer
22
is perhaps the simplest realization of the LSI. Although not yet fully characterized for accuracy at EUV, this type of interferometer has previously been used for at-wavelength characterization of EUV optics.
2, 18
More recently, a carrier-frequency (off-axis) implementation of the Ronchi interferometer has been used in the characterization of an EUV Schwarzschild objective. Direct comparison of this carrier-frequency LSI to the PS/PDI has demonstrated a rms measurement agreement of ~&lgr;
EUV
/70. However, the development of next-generation EUV lithography systems requires interferometry with accuracy preferably far exceeding &lgr;
EUV
/100.
A problem with the conventional Ronchi interferometer is that it produces many interfering beams causing confusion in the data analysis and limiting the accuracy of the device. Another problem with this simple interferometer, limiting its accuracy, is that it is susceptible to noise added by high-frequency components in the test-optic wavefront.
Various methods have been described to overcome the limitation caused by multiple interfering beams. One particularly simple and elegant solution is the single-sideband LSI.
28
In operation of the single-sideband LSI, an illumination beam is spatially filtered by a pinhole in the object plane, thus illuminating the test optic with a nearly spherical wave. A grating beamsplitter is placed in front of the image plane, where the illuminating beam is focused. Two of the diffracted orders propagate through a single large window in an image-plane mask with the remainder of the diffracted orders being blocked by the mask. Typically the two orders are chosen to be the zero order and either the +1 or the −1 order. The two beams propagate to the detector where they over

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