Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-03-11
2001-12-04
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06326106
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to measurement of overlay between the patterns of lower and upper layers in manufacturing a semiconductor device.
2. Description of the Related Art
In manufacturing a semiconductor device, processes of forming a film to be etched, forming a photoresist, transferring a pattern from a photomask to the photoresist by photolithography, transferring the pattern from the photoresist to the film to be etched by etching, and removing the photoresist are repeated a plurality of number of times to form a circuit pattern having a plurality of layers or the like. Hence, an overlay shift between the photomasks is an important factor in determining the degree of integration of the semiconductor device.
To cope with this overlay shift, an overlay measuring pattern is formed in, e.g., a scribing region of a photomask, where neither circuit pattern nor the like necessary for the manufacture of a semiconductor device is formed, and the overlay measuring pattern is measured by an overlay measuring apparatus of an optical pattern edge detection scheme, thereby measuring the overlay accuracy between the photomasks.
FIGS. 1A and 1B
show a popular related art of an overlay measuring pattern which is called a box pattern. As shown in
FIG. 1A
, this related art comprises an overlay measuring pattern
11
called a main scale and an overlay measuring pattern
12
called a sub scale. The overlay measuring patterns
11
and
12
are formed on photomasks for forming the patterns of lower and upper layers, respectively.
The overlay measuring pattern
11
has a rectangular portion
13
as an opening in the pattern formed from chromium or the like. The overlay measuring pattern
12
has a rectangular portion
14
as a pattern formed from chromium or the like. The overlay measuring patterns
11
and
12
have characters such as “No. 1” and “No. 2” to identify the target transfer films on which the patterns are transferred, as shown in FIG.
1
A. These characters are also transferred upon transferring the patterns.
The definitions of main and sub scales are relative. The overlay measuring pattern
11
may be used as the sub scale, and the overlay measuring pattern
12
may be used as the main scale. The rectangular portion
13
may be a pattern formed from chromium or the like, and the rectangular portion
14
may be an opening in the pattern formed from chromium or the like. Alternatively, both of the rectangular portions
13
and
14
may be patterns formed from chromium or the like, or both of the rectangular portions
13
and
14
may be openings.
The overlay measuring patterns
11
and
12
are formed on the photomasks such that the centers of the rectangular portions
13
and
14
match when the photomasks are accurately overlaid, as shown in FIG.
1
B. Hence, when the overlay measuring patterns
11
and
12
are overlaid, the difference between a distance x
1
from a left side X
1
of the rectangular portion
13
to the left side X
1
of the rectangular portion
14
and a distance x
2
from a right side X
2
of the rectangular portion
13
to the right side X
2
of the rectangular portion
14
, (x
1
−x
2
)/2, represents the overlay shift in the X direction, i.e., in the horizontal direction.
Similarly, when the overlay measuring patterns
11
and
12
are overlaid, the difference between a distance y
1
from a lower side Y
1
of the rectangular portion
13
to the lower side Y
1
of the rectangular portion
14
and a distance y
2
from an upper side Y
2
of the rectangular portion
13
to the upper side Y
2
of the rectangular portion
14
, (y
1
−y
2
)/2, represents the overlay shift in the Y direction, i.e., in the vertical direction.
When patterns are to be transferred to three or more target transfer films, two or more overlay measuring patterns
11
as main scales are consecutively formed on the target transfer film as the first layer, as shown in FIG.
2
. Sub scales corresponding to the second and subsequent layers are overlaid on the respective overlay measuring patterns
11
. Using the overlay measuring pattern
11
transferred to the target transfer film as the first layer as a reference, the overlay accuracy of the photomasks of the second and subsequent layers is measured.
A photoresist is exposed and developed, and then, heated to a temperature within the range of about 90° C. to 150° C. by a hot plate or oven for drying and hardening. The upper portion of the photoresist defining the opening end largely thermally shrinks due to surface tension, so the photoresist has a tapered sectional shape. Since the taper amount depends on the volume of a photoresist, the taper amount of a photoresist applied to the same thickness is proportional to its area.
As shown in
FIG. 3A
, of areas S
1
to S
4
between the sides of the rectangular portion
13
of the overlay measuring pattern
11
transferred to a positive photoresist
15
and the edges of the overlay measuring pattern
11
, the area S
2
of a portion in contact with the right side of the rectangular portion
13
and the area S
4
of a portion in contact with the left side may be equal. However, since characters “No. 1” need be written at a portion in contact with the upper side of the rectangular portion
13
, the area S
1
of the portion in contact with the upper side is larger than the area S
3
of a portion in contact with the lower side.
For this reason, even when taper amounts &Dgr;x
1
and &Dgr;x
2
of the photoresist
15
have a relation &Dgr;x
1
=&Dgr;x
2
in an X-direction section, as shown in
FIG. 3B
, taper amounts &Dgr;y
1
and &Dgr;y
2
of the photoresist
15
have a relation &Dgr;y
1
<&Dgr;y
2
in a Y-direction section, as shown in FIG.
3
C.
For example, when two overlay measuring patterns
11
are consecutively formed in the X direction, as shown in
FIG. 4A
, the taper amounts &Dgr;y
1
and &Dgr;y
2
of the photoresist
15
have a relation &Dgr;y
1
<&Dgr;y
2
in a Y-direction section, as shown in
FIG. 4C
, and at the same time, the taper amounts &Dgr;x
1
and &Dgr;x
2
of the photoresist
15
also have a relation &Dgr;x
1
<&Dgr;x
2
in an X-direction, as shown in FIG.
4
B.
When the taper amount of the sectional shape of the photoresist
15
on a film
16
to be etched is nonuniform, as shown
FIG. 5A
, the retreat amount of the etched film
16
increases at the largely tapered portion of the photoresist
15
, as shown in FIG.
5
B. For this reason, as shown in
FIG. 5C
, the overlay measuring pattern
11
is transferred to the etched film
16
while the positions of sides of the rectangular portion
13
shift from the original transfer positions.
Consequently, when the next film
17
to be etched is formed on the etched film
16
, and the rectangular portion
14
of the overlay measuring pattern
12
is transferred to a photoresist
18
on the film
17
to be etched, as shown in
FIG. 6A
, a measurement error occurs as if the rectangular portion
14
shifted in both the X and Y directions, although the rectangular portion
14
is accurately formed at the original position, as can be seen from FIG.
6
B.
Reportedly, when a 1.2-&mgr;m thick photoresist is heated at 120° C. for 90 sec, the above-described measurement error reaches 0.1 &mgr;m. Hence, erroneous information may be fed back to the next lot, or the photoresist
18
is reformed, resulting a decrease in yield.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an overlay measuring pattern, a photomask, and an overlay measuring method and apparatus capable of accurately measuring overlay shift between an overlay measuring pattern used for transfer to a lower target transfer film and an overlay measuring pattern used for transfer to an upper target transfer film.
An overlay measuring pattern according to the present invention has band-shaped portions separated from opposite sides of a rectangular portion by an equal distance and parallel to these sides of the rectangular portion. Hence, the areas between sides and band-shaped porti
Huff Mark F.
Kananen Ronald P.
Mohamedulla Saleha R.
Sony Corporation
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