Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-01-25
2004-06-22
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S394000
Reexamination Certificate
active
06753116
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to photolithography.
Photolithography is widely used to form patterns on semiconductor wafers during fabrication of integrated circuits. A wafer
110
(
FIG. 1
) is coated with a photoresist layer
120
. Resist
120
is irradiated from a light source
130
. A mask or reticle
140
is placed between source
130
and resist
120
. Mask
140
carries a pattern consisting of opaque and clear features. This pattern defines which areas of resist
120
are exposed to the light from source
130
. After the exposure, the resist
120
is developed so that some of the resist is removed to uncover the underlying substrate
110
. If the resist is “positive”, then the resist is removed where it was exposed to the light. If the resist is “negative”, the resist is removed where it was not exposed. In either case, the remaining resist and the exposed (uncovered) areas of substrate
110
reproduce the pattern on mask
140
. The wafer is then processed as desired (e.g. the exposed areas of substrate
110
can be etched, implanted with dopant, etc.).
The resist pattern on wafer
110
is not always a faithful reproduction of the mask. In
FIG. 2
, an opaque feature
210
M on mask
140
is a narrow line having two ends
220
M. Line
210
M should ideally be printed (reproduced) in resist
120
as line
210
R, with ends
220
R. In fact, the resist underlying the line
210
M near the ends
210
R gets overexposed. As a result, the line
210
R is shortened and the line ends are smoothened in the resist pattern, as shown at
240
.
The resist pattern can be corrected with hammerheads
310
(FIG.
3
). The hammerheads are opaque regions appended at line ends
210
M to reduce the resist exposure near the line ends. However, if the line ends
220
M are close to other opaque features, the hammerheads can be difficult to form on the mask.
SUMMARY
The invention is defined by the appended claims which are incorporated into this section in their entirety. The rest of this section summarizes some features of the invention.
Some embodiments of the present invention provide alternative techniques to reduce overexposure of the resist. In some embodiments, the resist is exposed twice. One exposure is through a mask like in the prior art, for example, in
FIG. 2
or
3
, but the light dose is smaller. The other exposure is conducted through a different (“modified”) mask which covers some or all of the resist at the line ends. The total energy delivered to the resist at the line ends is smaller as a result.
Other features and embodiments are described below.
REFERENCES:
patent: 5631112 (1997-05-01), Tsai et al.
patent: 5663893 (1997-09-01), Wampler et al.
patent: 5863712 (1999-01-01), Von Bunau et al.
patent: 6280887 (2001-08-01), Lu
patent: 6301008 (2001-10-01), Ziger et al.
patent: 6311319 (2001-10-01), Tu et al.
patent: 6566019 (2003-05-01), Kling et al.
Handbook of Microlithography, Micromachining, and Microfabrication; vol. 1: Microlithography, Editor P. Rai-Choudhury; (1997), p. 79.
MacPherson Kwok & Chen & Heid LLP
Mosel Vitelic Inc.
Rosasco S.
Shenker Michael
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