Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-10-12
2001-11-20
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S077000
Reexamination Certificate
active
06320648
ABSTRACT:
TECHNICAL FIELD
The present invention relates, generally, to pupil plane filters, and more particularly to the use of pupil plane filters to enhance optical imaging for both lithography and microscopy.
BACKGROUND OF THE INVENTION
The continuing goal of reducing critical dimensions (CD) in semiconductor manufacturing is putting increasing pressure on optical lithography. For further information, See “Methods and Apparatus for Integrating Optical and Interferometric Lithography to Produce Complex Patterns”, Ser. No. 09/273,399, filed on Mar. 22, 1999 with inventors Brueck, et al and U.S. Provisional Application No. 60/111,340 filed on Dec. 7, 1998 entitled “Arbitrary Lithographic Patterns” with inventors S. R. J. Brueck, Xiaolan Chen, Andrew Frauenglass and Saleem H. Zaidi. The entire contents of the foregoing are incorporated herein by reference.
The diffraction-limited resolution of optical lithography is usually expressed in a simplified form by the Rayleigh resolution criteria, viz.
CD
=
κ
1
λ
NA
(
1
)
Where &lgr; is the optical wavelength of the exposure, NA is the lens numerical aperture and &kgr;
1
is related to process latitude. Throughout the development of optical lithography for semiconductor manufacturing, NAs have increased to about 0.65 and wavelengths have decreased to about 248 nm, and significant effort is being placed on a further reduction of wavelengths to 193 nm and possibly 157 nm. However, both of these trends are facing fundamental limits. For example, aberrations increase in proportion to a high power of the NA, thereby making further increases problematic. Moreover, available optical materials typically limit the wavelength for a transmissive optical system. Thus, attention has turned to reductions in &kgr;
1
.
The Rayleigh resolution criteria of Equation 1 demonstrates the frequency space constraints of an imaging optical system. Particularly, a lens is a low pass filter [J. Goodman, Introduction to Fourier Optics, 2nd edition, (McGraw Hill, New York, 1996)] with a bandwidth of ƒ
opt
=NA/&lgr; which results in Eq. 1. The traditional Rayleigh criterion of an intensity dip of only about 20% between two resolvable peaks results in a minimum &kgr;
1
of 0.61. Using partially coherent illumination and high contrast photoresists, a satisfactory process latitude can be achieved at a &kgr;
1
of 0.61 which gives a minimum CD~&lgr; for modern, high-NA lenses. However, it is not possible to decrease
1
further in conventional imaging without losing the image entirely.
The limitations on decreasing &kgr;
1
, along with the limitations on decreasing the other factors of Equation 1, have led to the exploration of a number of resolution-enhancement techniques (RET). Moreover, the high cost of enabling post-optical lithography technologies such as: x-ray; e-beam; ion-beam and extreme-ultraviolet lithography provides an incentive to extend optical lithography as far as possible. The RETs currently include, for example, optical proximity correction (OPC), off-axis illumination (OAI), phase-shift masks (PSM) and imaging-interferometric lithography (IIL). Each of these RETs improves the image quality and allows printing of lithographic features at lower &kgr;
1
. While much of the discussion and analysis of these techniques has centered on the real-space (image) implications, there is, of course, a corresponding view that emphasizes the spatial frequency content of the image. As discussed below, one embodiment of the present invention preferably takes advantage of the spatial frequency effects of some of these techniques.
For a recent review of RET, see, for example, M. D. Levenson. “Wavefront engineering from 500 mn to 100 nm CD.” Proc. SPIE 3048. 2-13 (1997). L. W. Liebmann, B. Grenon, M. Lavin, S. Schomody, and T. Zell, “Optical proximity correction a first look at manufacturability,” Proc. SPIE 2322, 229-238 (1994)). For further information on OPC, see, for example, P. D. Robertson, F. W. Wise, A. N. Nasr, and A. R. Neureuther, “Proximity effects and influence of nonuniform illumination in projection lithography,” Proc. SPIE 334, 14 (1982), L. W. Liebmann, B. Grenon, M. Lavin, S. Schomody. and T. Zell, “Optical proximity correction, a first look at manufacturing,” SPIE 2322. 14th Annual BACUS Symposium on Photo-mask Technology and Management, 229-238(1994); J. F. Chen, T. Laidig, K. E. Wampler, R. Caidwell, “Full-chip optical proximity correction with depth of focus enhancement”, Microlithography World, Summer, 5-13(1997). For further information on OAI, see, for example, M. Noguchi, M. Muraki, Y. Iwasaki, and N. Magome. “Sub-half micron lithography system with phase-shifting effect,” Proc. SPIE 1674, 92-104 (1992). K. Tournai, H. Tanabe, H. Nozue, and K. Kasama. “Resolution improvement with annular illumination,” Proc. SPIE 1674, 753-764 (1992); K. Kamon, T. Miyamoto, Y. Myoi, H. Nagata, M. Tanaka and K. Hone, “Photolithography system using annular illumination,” Jpn. J. Appi. Phys., Vol. 30, No. 1 1B. 3021-3029(1991); N. Shiraishi, S Hirukawa, Y. Takeuchi and N. Magome, “New imaging technique for 64 M-DRAM,” SPIE 1674, Optical/Laser Microlithography V, 741-752(1992). For further information on PSM, see, for example, M.D. Levenson, N. S. Viswanathan, and R. A. Simpson, “Improving resolution in photolithography with a phase shifting mask,” IEEE Trans. Electron Devices. ED-29, 1828-1836(1982), M. D. Levenson, D. G Goodman, S. Lindsey, P. Bayer and H. Santini, “The phase shifting mask II: imaging simulations and submicron resist exposures,” IEEE Trans. Electron Devices ED-31, 753-763 (1984), H. Watanabe, H. Takenaka, Y. Todokoro, and M. lnoue, “Sub-quarter-micron gate pattern fabrication using a transparent phase shifting mask,” J. Vac. Sci. Technol. B9, 3172 (1991)), H. Jinbo and Y. Yamashita, “Improvement of phase-shifter edge line mask method,” Jpn. J. Appl. Phys. Vol. 30, 2998-3003(1991). For further information on IIL, see, for example, X. Chen and S. R. J. Brueck, “Inaging interferometric lithography for arbitrary patterns,” Proc. SPIE 3331, 214-22y (1998). X. Chen and S. R. J. Brueck, “Imaging interferometric lithography—a wavelength division multiplex approach to extending optical lithography,” J. Vac. Sci Technol. The entire contents of all of the foregoing references are incorporated herein by reference.
More particularly, the following sets forth a spatial frequency analysis of optical lithography RETs. With respect to Fourier optics, the basic goal is typically to increase the high-frequency content of the image. For a free-space optical transmission medium, the highest attainable spatial frequency is ƒ
IL
=2/&lgr;, corresponding to counterpropagating plane waves at grazing incidence to the wafer. For dense features, the resulting CD is &lgr;/4. This limit is relatively easy to approach experimentally by interferometric lithography using a small number of plane waves resulting in a repetitive pattern such as a 1D grating or a 2D dot array. See, for example, X. Chen, Z. Zhang, S. R. J. Brueck, R. A. Carpio and J. S. Petersen, “Process Development for 180-nm Structures using Interferometric Lithography and I-Line Photoresist,” Proc. SPIE 3048, 309-318 (1997), which is incorporated herein by reference. Useful patterns contain a very large number of spatial frequencies, wherein corners and edges often include frequencies beyond this limiting value even for dense patterns. While imaging systems record many spatial frequencies at once, imaging systems often only record the spatial frequencies within the ~ƒ
opt
bandwidth of the optical system. However, RETs allow extension of this bandwidth towards 2ƒ
opt
(OPC, OAI, PSM) and to the limiting value of ƒ
IL
(IIL).
For coherent illumination (see, for example, J. Goodman “Introduction to Fourier Optics” 2nd edition, (McGraw-Hill, New York, 1996)), the optical system passes all of the (electric field) frequency components of the diffraction with a unity coherent modulation transfer function (MTF) up to a frequency of ƒ
opt
, e.g.
T
coh
(&fnof
Brueck Steven R. J.
Chen Xiaolan
Adams Russell
Nguyen Hung Henry
Snell & Wilmer L.L.P.
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