Optics: measuring and testing – By light interference – Having shearing
Reexamination Certificate
2000-07-28
2001-05-15
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having shearing
C356S521000
Reexamination Certificate
active
06233056
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to phase-shifting point diffraction interferometers (PS/PDI) that is capable of a system level-flare test that can be performed in parallel with wavefront metrology.
REFERENCES
The following publications are cited in this application as superscript numbers:
1.
D. M. Williamson, “The elusive diffraction limit”, OSA Proceedings on Extreme Ultraviolet Lithography, Optical Society of America, Washington, D.C., 23, 68-76 (1994).
2.
W. Linnik, “A simple interferometer to test optical systems,” Proceedings of the Academy of Science of the USSR, 1, 210-212 (1933).
3.
R. N. Smartt and W. H. Steel, “Theory and application of point-diffraction interferometers,” Jap. J. Appl. Phys., 14, Suppl. 14-1, 351-356 (1975).
4.
G. E, Sommargren, “Phase shifting diffraction interferometry for measuring extreme ultraviolet optics,” OSA Trends in Optics and Photonics Vol. 4, Extreme Ultraviolet Lithography, G. D. Kubiak and D. R. Kania, eds. (Optical Society of America, Washington, D.C. 1996), pp. 108-112.
5.
G. E. Sommargren, “Diffraction methods raise interferometer accuracy,” Laser Focus World, 32, 61-71, (August 1996).
6.
J. E. Bjorkholm, et al, “Phase-measuring interferometry using extreme ultraviolet radiation,” J. Vac. Sci. Technol. B, 13, 2919-2922 (1995).
7.
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).
8.
H. Medecki, et al, “Phase-shifting point diffraction interferometer,” Opt. Lett., 21, 1526-1528 (1996).
9.
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).
10.
E. Tejnil, et al, “At-wavelength interferometry for EUV lithography,” J. Vac. Sci. Technol. B, 15, 2455-2461 (1997).
11.
K. A. Goldberg, et al, “Characterization of an EUV Schwarzschild objective using phase-shifting point diffraction interferometry,” in Emerging Lithographic Technologies, David E. Seeger, Editor, Proceedings of SPIE Vol. 3048, 264-270 (1997).
12.
P. Carre, “Installation et utilisation du comparateur photoelectric et interferential du bureau international des poids et mesures,” Metrologia, 2, 13-17 (1966).
13.
R. Crane, “Interference phase measurement,” Appl. Opt., 8, 538-542 (
1969).
14.
J. H. Bruning, et al, “Digital wavefront measuring interferometer for testing optical surfaces and lenses,” Appl. Opt., 13, 2693-2703 (1974).
15.
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).
16.
E. Leith, et al, “Electronic holography and speckle methods for imaging through tissue using femtosecond gated pulses,” Appl. Opt., 30, 4204-4210 (1991).
17.
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.
18.
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.
19.
R. Beguiristain, et al, “High flux undulator beam line optics for EUV interferometry and photoemission microscopy,” in High Heat Flux Engineering III, A M Khounsary, Editor, Proceedings of SPIE Vol. 2855, 159-169 (1996).
20.
D. Attwood, et al, “Undulator radiation for at-wavelength interferometry of optics for extreme-ultraviolet lithography,” Appl. Opt., 32, 7022-7031 (1993).
21.
H. Medecki, U.S. Pat. No. 5,835,217 issued Nov. 10, 1998.
22.
P. de Groot, “Derivation of algorithms for phase-shifting interferometry using the concept of a data-sampling window,” Appl. Opt., 34, 4723-4730 (1995).
23.
K. Freischlad and C. Koliopoulos, “Fourier description of digital phase-measuring interferometry,” J. Opt. Soc. Am. A, 7, 542-551 (1990).
24.
Y. Surrel, “Design algorithms for phase measurements by the use of phase stepping,” Appl. Opt., 35, 51-60 (1996).
25.
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).
26.
K. Creath, “Comparison of phase-measuring algorithms” in Surface Characterization and Testing, Proceedings of SPIE Vol. 680, 19-28 (1986).
27.
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).
28.
E. N. Leith and J. Upatnieks, “Reconstructed wavefronts and communication theory”, J. Opt. Soc. Am., 52, 1123 (1962).
29.
E. N. Leith and J. Upatnieks, “Wavefronts reconstruction with diffused illumination and three-dimensional objects,” J. Opt. Soc. Am., 54, 1295 (1964).
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 emergence of extreme ultraviolet (EUV) projection lithography has placed stringent demands on interferometric metrology systems. In order to achieve diffraction-limited performance, EUV lithographic systems require wavefront tolerances on the order of 0.02 waves rms (0.3 nm rms at a wavelength of 13.4 nm).
1
While the accuracy of interferometry is typically limited by the quality of the reference surface or wave, a class of interferometers has been developed in which extremely high quality reference waves are created by diffraction from small apertures.
2-5
EUV lithographic systems rely on wavelength-specific reflective multilayer coatings. To accurately probe phase effects in these resonant reflective structures, at-wavelength metrology is required. Various at-wavelength interferometric measurement techniques including lateral-shearing interferometry,
6
Foucault and Ronchi testing
7
have been reported. These methods, however, have yet to demonstrate the accuracy required for the development of EUV lithographic imaging systems. In order to meet the accuracy challenge, an EUV-compatible diffraction-class interferometer, the phase-shifting point diffraction interferometer (PS/PDI), was developed by Medecki et al.
8, 21
The reference wavefront accuracy of the PS/PDI has been demonstrated to be better than &lgr;
EUV
/300 (0.045 nm) within a numerical aperture of 0.082.
9
The PS/PDI is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. In the PS/PDI, as illustrated in
FIG. 1A
, the optical system
2
under test is illuminated by a spherical wave
5
that is generated by an entrance pinhole
6
in a mask
4
that is placed in the object plane of the optical system
2
. To assure the quality of the spherical-wave illumination, pinhole
6
is chosen to be smaller than the resolution limit of the optical system. Grating
8
splits the illuminating beam
5
to create the required test and reference beams
10
and
12
, respectively. A PS/PDI mask
20
is placed in the image plane of the optical system
2
to block the unwanted diffracted orders generated by the grating
8
and to spatially filter the reference beam
12
using a reference pinhole
16
. The test beam
10
, which contains the aberrations imparted by the optical system, is largely undisturbed by the image-plane mask by virtue of it passing through window
14
in the PS/PDI mask
20
that is large relative to the point-spread function (PSF) of the optical system under test. The test and reference beams propagate to th
Goldberg Kenneth Alan
Naulleau Patrick P.
Burns Doane Swecker & Mathis L.L.P.
Font Frank G.
Lee Andrew H.
The Regents of the University of California
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