Multiple photolithographic exposures with different clear...

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Reexamination Certificate

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C430S394000, C430S396000

Reexamination Certificate

active

06777168

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to photolithography.
Photolithography is widely used to form patterns on semiconductor wafers in fabrication of integrated circuits. A wafer
110
(
FIG. 1
) is coated with a photoresist layer
120
. Photoresist
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
has a concave corner
220
M. Feature
210
M should ideally be printed (reproduced) in resist
120
as feature
210
R, with a corner
220
R. In fact, the resist region
230
in the corner's cavity gets underexposed. As a result, the corner is smoothened in the resist pattern, as shown by line
240
. See U.S. Pat. No. 6,280,887 issued Aug. 28, 2001 to Lu.
The resist pattern can be corrected with a serif
310
(FIG.
3
). The serif is a region cut out in opaque feature
210
M to increase the exposure of region
230
. However, if the feature
210
M is narrow, i.e. the dimensions D
1
, D
2
are small, the serif 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 underexposure of the resist. In some embodiments, the resist is exposed twice. One exposure is through a mask like in the prior art, for example, as in
FIG. 2
or
3
. The other exposure is conducted through a different (“modified”) mask which exposes resist regions which correspond to opaque regions of the first mask. This “modified” exposure is not conducted with a sufficient light energy dose to create a resist pattern on the wafer. For example, in the case of the positive resist, the modified exposure dose is insufficient to cause the resist to be removed during the developing process. However, the modified exposure increases the total energy dose delivered to regions such as
230
. A more faithful pattern reproduction results in some cases.
The modified and non-modified exposures can be performed in any order.
In some embodiments, the modified exposure does not use a mask. The entire resist surface is exposed.
Other features and embodiments are described below.


REFERENCES:
patent: 5407785 (1995-04-01), Leroux
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.
Handbook of Microlithography, Micromachining, and Microfabrication; vol. 1: Microlithography, Editor P. Rai-Choudhury; (1997), p. 79.

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