Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2000-04-03
2001-04-10
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S053000, C355S071000, C359S030000, C359S265000, C359S273000, C359S582000, C359S299000, C430S024000, C430S311000, C430S314000
Reexamination Certificate
active
06215546
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement of photolithography processes, and more particularly to a method of optical correction for improving the pattern shrinkage caused by scattering of the light.
2. Description of the Related Art
Photolithography processes include an exposure step in which a desired pattern is transferred onto a photoresist coated on a substrate by use of a photomask. The photoresist is removed selectively depending on the intensity of the light imaged thereon. When the optical path of light passing through the photomask deviates from the desired optical path, the pattern size obtained on the photoresist does not correspond to the size of the pattern on the photomask. This is known as the proximity effect.
The proximity effect can be partially compensated by amending the patterns on the photomask. For example, if it is known that an image formed on a photoresist layer is narrower than the pattern on the photomask, the pattern on the photomask may be designed wider than the desired size of the image. Data sets of photomasks used in a photolithography process, including amendments, can be stored as a database in a computer and accessed by different users.
In addition, pattern shrinkage caused by scattering of the light during the exposure step is also a problem, especially when the size of devices is tiny, for example, 0.5 microns or even less than 0.25 microns. Referring to
FIG. 1
, a top view of the pattern shrinkage after photolithography processes is shown. In the left side of
FIG. 1
, the length of pattern
10
on a photomask is b, and the distance between the edges of the two patterns
10
is a. The patterns transferred onto a photoresist layer by photolithography are shown in the right side of
FIG. 1
, which is marked
12
. Because of the effect of scattering of the light at the opaque edges of the photomask during exposure, the length of patterns on the photoresist layer is shortened from b to b′, i.e. b>b′.
Methods have been developed to solve the problems caused by scattering of light. For example, serifs or hammerheads are added in the edges of the patterns on photomasks so that the areas of the patterns are larger than before. Thus, the exposed area on photoresist is decreased, and the pattern shrinkage phenomenon caused by scattering of the light is therefore reduced. The ‘serif’ represents a small block added on the center or the edges of the patterns, while the ‘hammerhead’ represents a connection (fusion) of two serifs.
Referring to
FIG. 2
, a top view of a corrected pattern on a photomask after photolithography processes is shown. First, patterns
10
are formed on a photomask (not shown). In addition, serifs
15
or hammerheads
17
are added to the edges of the patterns
10
as shown in FIG.
2
. The patterns are transferred onto a photoresist layer (not shown) by photolithography processes so that an accurate patterns
12
are formed. As described above, the serifs
15
or hammerheads
17
are added to the edges of the patterns
10
on a photomask so that the areas of the patterns on the photomask are larger than before. Thus, the exposed area on photoresist is decreased, and the pattern shrinkage phenomenon caused by scattering of the light is therefore reduced. Accordingly, the length b of the pattern on the photomask equals to the length b′ of the pattern on the photoresist layer, i.e. b=b′.
However, the formation of these photomasks as described above is complicated because the additional serifs or the hammerheads must be formed individually by electron beams. If there are many serifs or hammerheads needed in a photomask, the time or the money for fabricating a photomask is increased. Furthermore, it is observed from results of experiments that when the space between two patterns is close to a multiple of the wavelength, a standing wave effect can occur. This makes the patterns formed on the photoresist wavy. Further, the use of serifs or hammerheads does not entirely resolve the scattering of the light.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a method of optical correction for improving the pattern shrinkage caused by scattering of the light in which patterns transferred in photolithography processes are more precise, and the complexity of the optical correction is simplified so that the efficiency of forming photomasks is higher, and the cost is decreased.
Another object of the present invention is to provide a method of optical correction for improving the pattern shrinkage caused by scattering of the light in which the standing wave effect is prevented, and the pattern transfer is more accurate.
To achieve the objects described above, the present invention discloses a method of optical correction, wherein the patterns on photomasks are corrected by providing aid patterns. Therefore serifs or hammerheads are not necessary, and the processes and cost can also be decreased. According to the present invention, a chrome aid block is provided between the edges of the patterns. It is noted that the size of the chrome aid block is between ⅓ to ½ wavelength of light used in the process. Therefore, the pattern shrinkage caused by scattering of the light during exposure can be reduced, and there is no additional block formed on the photoresist layer. In addition, the standing wave effect can be prevented so that the pattern transfer is more accurate.
The present invention provides a method of optical correction for improving the pattern shrinkage caused by scattering of the light, comprising the steps of: providing a plurality of patterns on a photomask, wherein the plurality of patterns have at least two adjacent patterns, and a gap a between the edges of the two adjacent patterns is less than or equal to quintuple the wavelength &lgr; of the light used during exposure (a≦5&lgr;); and providing a chrome aid block between the edges of the two adjacent patterns, wherein the location and the size of the chrome aid block correspond to the equations:
{
1
3
λ
≤
x
=
y
≤
1
2
λ
d
+
1
2
y
=
1
2
a
,
wherein the symbol d represents the distance between the edges of the chrome aid block and the pattern, and the symbols x and y represent the lengths of the block perpendicular and parallel to the gap a, respectively. Thereby, the scattering of the light and standing wave effect are reduced so that the pattern shrinkage can be prevented, and the pattern transfer is effective.
The present invention also provides a method of optical correction for improving the pattern shrinkage caused by scattering of the light, comprising the steps of: providing a plurality of patterns on a photomask, wherein the plurality of patterns have at least two adjacent patterns, and a gap a between the edges of the two adjacent patterns is greater than quintuple the wavelength &lgr; of the light used during exposure (a>5&lgr;); and providing chrome aid blocks adjacent to the edges of the two adjacent patterns, wherein the location and the size of the chrome aid blocks correspond to the equations:
{
1
3
λ
≤
x
=
y
≤
1
2
λ
1
4
λ
<
d
<
2
λ
,
wherein the symbol d represents the distance between the edges of the chrome aid block and the adjacent pattern, and the symbols x and y represent the lengths of the block perpendicular and parallel to the gap a, respectively. Thereby, the scattering of the light and standing wave effect is reduced so that the pattern shrinkage can be prevented, and the pattern transfer is effective.
REFERENCES:
patent: 5308991 (1994-05-01), Kaplan
patent: 5324600 (1994-06-01), Jinbo et al.
patent: 5546225 (1996-08-01), Shiraishi
patent: 5789124 (1998-08-01), Todd
patent: 5955221 (1999-09-01), Sanders et al.
patent: 6140020 (2000-10-01), Cummings
Chen Quentin
Chu Ronfu
Hsu Chungwei
Lin Jengping
Adams Russell
Brown Khaled
Darby & Darby
Nanya Technology Corporation
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