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Technology Paper
The Future of Pellicles
 The
Future of Pellicles |
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The introduction of 157 nm
for next-generation optical lithography has created
a need for new pellicle materials optimized for this
wavelength. Currently, there are four strategies being
considered.
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Soft Pellicle |
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First, new fluorocarbon-based polymers
have to be developed that are transparent, damage-resistant,
and possess mechanical properties to enable their preparation
in very thin pellicle form. Initial results from the
Massachusetts Institute of Technology's Lincoln Labs
demonstrated that commercial fluoropolymers used for
pellicles at 248 and 193nm wavelengths, such as Teflon
AFR and CytopR, rapidly burst under irradiation with
157 nm light because they lack sufficient mechanical
integrity. Consequently, an extensive program was initiated
to develop and screen novel fluoropolymer candidates
with the desired properties for 157 nm lithography.
Although some polymers did show promising transmission,
their lifetime is still insufficient due to photochemical
darkening. Research is still needed to solve this fundamental
problem.
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| Hard
Pellicle |
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A hard pellicle is simply a thin, quartz glass on
a frame. Fluorinated fused silica has a sufficient lifetime
for the 157 nm process. Although thickness control and
thickness uniformity is a challenge, a good parallelism,
i.e. thickness uniformity, has been achieved. However,
even thin fused silica is several hundred times thicker
than a soft film pellicle. With a typical thickness
of 800 µm a hard pellicle would act as an additional
optical element and impact the imaging and overlay performances.
Developers have achieved good optical homogeneity and
surface finish for hard pellicles, but improvements
to the mounting frames are required to keep the pellicle
bending low in order to avoid significant optical distortion.
A circular pellicle with a circular photomask should
be used to minimize any distortion. |
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| Removable
Pellicle or Cover |
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The third strategy is to use the soft pellicle as
a photomask cover only during transportation and storage.
The pellicle would then be removed before exposure and
remounted after exposure. There are three mounting options
being considered - adhesive, magnetic, and a modified
reticle carrier. Although the process can be easily
proved in a research line, the long term contamination
control and inspection will still be a challenge and
the process will have to be proven in the very costly
production line. |
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| No
Pellicle |
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The last approach is a pellicle-less
solution. However, after 20 years of using pellicles
the "no pellicle" proposal will be a challenge
because the feature size of IC is much smaller than
before - 0.1 £gm vs. 4 £gm. In addition,
the photomask and reticle is easier to get contaminated
because the feature size is now much smaller. A reliable,
constant inspection feedback system is therefore necessary
for the success of this method.
At this time of writing, it seems that the hard pellicle
will produce enough life time and contamination free
protection for 157 nm reticle. The challenge of production
of a clean, defect free hard pellicle and mounting without
distortion or repetitive distortion will still have
some challenges to overcome.
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1978
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Vincent Shea and Walter J.
Wojcik, U.S. Patent 4,131,363, |
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1983
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Ray Winn, U.S. Patent 4,378,953,
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1985
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Ray Winn, U.S. Patent 4,536,240,
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1992
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Pei-Yang Yan, Michael S. Yeung,
Henry T. Gaw, "Printability of Pellicle
Defects in DUV 0.5 µm Lithography."
Proc. SPIE Vol.1604, p.106-117, |
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1988
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Yung-Tsai Yen, U.S. Patent
4,759,990, |
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1986
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Chris Yen and C.B. Wang, "Potential
Particle Problem from an Adhesive",
MLI Technical Publication, |
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1989
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Kasunori Imamura, U.S. Patent
4,833,051, |
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1981
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8Ronald S. Hershel, "Pellicle
Protection of Integrated Circuit Masks,"
SPIE, Vol. 275, Semiconductor Micro Lithography,
VI |
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1994
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Robert W. Murphy and Rick
Boyd, "The Effect of Pressure Differentials
on Pelliclized Photomasks," Proc. SPIE
Vol. 2322, p. 187-201, |
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1995
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Naofumi Inoue, Hiroaki Nakagawa,
Masahiro Kondou, Masanori Kitajima, "Pellicle
vs. Influence of Clean Room Environments,"
Proc. SPIE Vol. 2512, p. 60-73, |
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Yung-Tsai Yen, U. S. Patent
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2002
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Roger H. French, Rober C.
Wheland , Weiming Qiu, M. F. Lemon, Gregory
S. Blackman, Xun Zhang, Joe Gordon, Vladimir
Liberman, A. Grenville, Roderick R. Kunz,
Mordechai Rothschild, "157-nm Pellicles:
Polymer Design for Transparency and Lifetime,
" Proc. SPIE Vol. 4691, p. 576-583,
). |
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2002
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Emily Y. Shu, Fu-Chang Lo,
Florence O. Eschbach, Eric P. Cotte, Roxann
L. Engelstad, Edward G. Lovell, Kaname Okada
and Shinya Kikugawa, "Hard Pellicle
Study for 157-nm Lithography," Proc.
SPIE Vol. 4754, p. 557-568, |
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2002
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14 Kaname Okada, K. Ootsuka,
I. Ishikawa, Yoshiaki Ikuta, H. Kojima,
T. Kawahara, T. Minematsu, H. Mishiro, Shinya
Kikugawa and Y. Sasuga, "Development
of Hard Pellicle for 157 nm," Proc.
SPIE Vol. 4754, p. 569-577, |
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2001
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Andy Ma, Arun Ramamoorthy,
Barry Lieberman, C.B. Wang, Q.R. Bih, Kevin
Duong, Corbin Imai, "Removable Pellicle",
Sematech Pellicle Risk Assessment Workshop,
Sept. 27, |
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optical requirement |
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BACUS 2002 - reticle defect |
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BACUS 2003 - HAZE |
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