U.S. patent application number 10/341159 was filed with the patent office on 2003-06-05 for photomask and pattern forming method used in a thermal flow process and semiconductor integrated circuit fabricated using the thermal flow process.
Invention is credited to Ishida, Shinji, Iwasaki, Haruo, Yoshii, Tsuyoshi.
Application Number | 20030104289 10/341159 |
Document ID | / |
Family ID | 18531675 |
Filed Date | 2003-06-05 |
United States Patent
Application |
20030104289 |
Kind Code |
A1 |
Iwasaki, Haruo ; et
al. |
June 5, 2003 |
Photomask and pattern forming method used in a thermal flow process
and semiconductor integrated circuit fabricated using the thermal
flow process
Abstract
The invention relates to a photomask for use in a thermal flow
process in which: a photomask is prepared in which a plurality of
exposure openings are formed; a resist is applied to the surface of
a layer of a semiconductor integrated circuit that is to undergo
processing; this resist is patterned by an exposure process through
the photomask to form a plurality of openings in the resist that
correspond to each of the exposure openings; and the patterned
resist is then heated to cause each of the openings to shrink;
wherein at least a portion of exposure openings among the plurality
of exposure openings are formed in shapes that compensate for
anisotropic deformation that occurs in the openings when the
patterned resist is heated to cause each of the openings to shrink.
Since the openings that are formed in the resist are provided in
advance with shapes that compensate for the deformation that occurs
when the openings shrink, these openings attain the proper shape
after undergoing shrinking and deformation.
Inventors: |
Iwasaki, Haruo; (Tokyo,
JP) ; Ishida, Shinji; (Tokyo, JP) ; Yoshii,
Tsuyoshi; (Tokyo, JP) |
Correspondence
Address: |
Hayes Soloway PC
175 Canal Street
Manchester
NH
03101
US
|
Family ID: |
18531675 |
Appl. No.: |
10/341159 |
Filed: |
January 13, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10341159 |
Jan 13, 2003 |
|
|
|
09757841 |
Jan 10, 2001 |
|
|
|
Current U.S.
Class: |
430/5 ;
257/E21.257; 430/311; 430/322 |
Current CPC
Class: |
H01L 21/31144 20130101;
G03F 1/00 20130101; G03F 7/40 20130101 |
Class at
Publication: |
430/5 ; 430/311;
430/322 |
International
Class: |
G03F 009/00; G03F
007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2000 |
JP |
2000-002582 |
Claims
What is claimed is:
1. A photomask for use in a thermal flow process in which: a
photomask is prepared in which a plurality of exposure openings are
formed; a resist is applied to the surface of a layer of a
semiconductor integrated circuit that is to undergo processing;
said resist is patterned by an exposure process through said
photomask to form a plurality of openings in said resist that
correspond to each of said exposure openings; and said resist that
has been patterned is heated to cause each of said openings to
shrink; wherein at least a portion of exposure openings among said
plurality of exposure openings are formed in shapes so that said
openings are caused to become corresponding desired shapes due to
anisotropic deformation that occurs when each of said openings is
caused to shrink by heating said resist on which the patterning has
been carried out.
2. A photomask according to claim 1 wherein at least a portion of
exposure openings among said plurality of exposure openings are
formed in shapes that are elongated in a direction that is
approximately orthogonal to the direction toward other said
exposure openings that are close.
3. A photomask according to claim 2 wherein at least a portion of
exposure openings among said plurality of exposure openings are
enlarged in a direction that is approximately orthogonal to the
direction toward said other exposure openings that are close.
4. A photomask according to claim 3 wherein the degree of
enlargement of said exposure openings becomes smaller as the
distance among said other exposure openings that are close becomes
larger.
5. A photomask according to claim 2 wherein at least a portion of
exposure openings among said plurality of exposure openings are
arranged in lines that are close together, and each of said
exposure openings that are arranged in these lines is enlarged in a
direction that is approximately orthogonal to the direction of said
arrangement.
6. A photomask according to claim 3 wherein said exposure openings
are formed in rectangular shapes in which the direction of
enlargement is the direction in which the long sides extend.
7. A photomask according to claim 2 wherein each of said exposure
openings is enlarged in substantially all directions, and at least
a portion of exposure openings among said plurality of exposure
openings are formed such that the degree of enlargement is smaller
in the direction toward other exposure openings that are close than
other directions.
8. A photomask according to claim 7 at least a portion of exposure
openings among said plurality of exposure openings are formed as
rectangles in which the short sides extend in the direction toward
said other exposure openings that are close and the long sides
extend a direction that is approximately orthogonal to this
direction.
9. A method of forming patterns for use in a thermal flow process
in which: a resist is applied to the surface of a layer of a
semiconductor integrated circuit that is to undergo processing;
said resist is patterned to form a plurality of openings in said
resist; and said resist that has been patterned is heated to cause
each of said openings to shrink; wherein at least a portion of
exposure openings among said plurality of exposure openings are
formed in shapes so that said openings are caused to become
corresponding desired shapes due to anisotropic deformation that
occurs in said openings when said resist that has been patterned is
heated to cause said openings to shrink.
10. A method of forming patterns according claim 9 wherein at least
a portion of openings of said plurality of openings are formed in
shapes that are elongated in a direction that is approximately
orthogonal to the direction toward other said openings that are
close.
11. A method of forming patterns according to claim 10 wherein at
least a portion of openings among said plurality of openings are
enlarged in a direction that is approximately orthogonal to the
direction toward other said openings that are close.
12. A method of forming patterns according to claim 11 wherein the
degree of enlargement of said openings is in inverse proportion to
the distance to said other exposure openings that are close.
13. A method of forming patterns according to claim 10 wherein,
when at least a portion of openings among said plurality of
openings are arranged in lines that are close together, each of
said openings that are arranged in these lines is enlarged in a
direction that is approximately orthogonal to the direction of said
arrangement.
14. A method of forming patterns according to claim 11 wherein said
openings are formed in rectangular shapes in which the direction of
enlargement is the direction in which the long sides extend.
15. A method of forming patterns according to claim 10 wherein at
least a portion of openings among said plurality of openings are
enlarged in substantially all directions such that the degree of
enlargement is smaller in the direction toward other openings that
are close than other directions.
16. A method of forming patterns according to claim 15 wherein at
least a portion of openings among said plurality of openings are
formed as rectangles in which the short sides extend in the
direction toward said other openings that are close and the long
sides extend in a direction that is approximately orthogonal to
this direction.
17. A method of forming patterns for use in a thermal flow process
in which: a photomask is prepared in which a plurality of exposure
openings are formed; a resist is applied to the surface of a layer
of a semiconductor integrated circuit that is to undergo
processing; said resist is patterned by an exposure process through
said photomask to form a plurality of openings in said resist that
correspond to each of said exposure openings; and said resist that
has been patterned is heated to cause each of said openings to
shrink; wherein said method of forming patterns uses the photomask
according to claim 1 in said exposure process.
18. A semiconductor integrated circuit in which a prescribed
portion having fine planar shapes is treated by a prescribed
process through openings in a resist that have been formed by a
method of forming patterns according to claim 9.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a photomask used in a
thermal flow process, a method of forming patterns used in a
thermal flow process, and a semiconductor integrated circuit in
which a portion having fine planar shapes is treated by a
prescribed process through openings in a resist.
[0003] 2. Description of the Related Art
[0004] In recent years, fine-patterned semiconductor integrated
circuits constructed by using thin-film techniques are being used
for a variety of purposes, and these constructions are increasing
in fineness with each year. As an example, photolithography is one
technique for achieving fine patterning of the layers of a
semiconductor integrated circuit.
[0005] In a case of forming through-holes in the insulating film of
a semiconductor integrated circuit, a resist is applied to the
surface of the insulating film that is to undergo processing, and
the resist is then exposed using a photomask in which a plurality
of exposure openings are formed. The resist is then developed to
form openings at the exposed portions, and this resist is used as a
mask to etch the insulating film through the openings.
[0006] This type of photolithography is used not only for the
formation of through-holes described above but for various other
purposes such as introducing impurities into a semiconductor
substrate and patterning wiring lines. In this type of
photolithography, a photomask is formed in which the pattern that
is to be exposed is enlarged in all directions, following which the
exposure process is carried out with this photomask using reducing
optics to expose a pattern of the desired dimensions on the
resist.
[0007] In this technique, a pattern that is finer than a prescribed
dimension cannot be exposed due to the limits of optical
resolution. However, there is now demand to reduce resist openings
below the exposure limit dimensions, and the thermal flow process
has been developed as a means of realizing such a reduction.
[0008] Referring now to FIG. 1A-FIG. 2C, one example of the thermal
flow process of the prior art is next described.
[0009] As shown in FIG. 1A, a DRAM (Dynamic Random Access Memory)
that is in the process of fabrication is first prepared as
semiconductor integrated circuit 100, which is the object of
processing. In semiconductor integrated circuit 100 that is here
taken as an example, gate oxide film 102 is formed on the surface
of semiconductor substrate 101, and gate electrodes 103 and 104 of
the transistor elements that will serve as memory cells are formed
in a prescribed pattern on the surface of this gate oxide film 102.
Gate oxide film 102 is partitioned by element isolation regions 105
according to the positions of memory cells, and the space around
gate electrodes 103 and 104 is filled with interlayer dielectric
film 106, which is a prescribed layer.
[0010] In semiconductor integrated circuit 100 which is taken as an
example here, contact hole 107 of a bit contact is formed from the
surface of interlayer dielectric film 106 to the surface of gate
oxide film 102 at a position between the pair of gate electrodes
103 and 104, as shown in FIG. 2C. Photomask 111, in which is formed
exposure opening 110 that corresponds to this contact hole 107, is
therefore prepared as shown in FIG. 1c.
[0011] The structure of this photomask 111 is such that shield film
113 is formed on the underside of transparent base member 112 and
exposure opening 110 is formed by partially removing this shield
film 113. This exposure opening 110 is formed at position that
corresponds to contact hole 107, and its dimensions in all
directions are greater than the dimensions of contact hole 107.
[0012] Resist 115 is then applied to the surface of interlayer
dielectric film 106, which is a prescribed layer of semiconductor
integrated circuit 100, to form a prescribed film thickness as
shown in FIG. 1B, and the above-described photomask 111 is arranged
parallel to and confronting the surface of resist 115 at a
prescribed distance from the surface of resist 115.
[0013] In this configuration, resist 115 is exposed to light by
exposure device (not shown in the figure) through exposure opening
110 of photomask 111, and as shown in FIG. 2A, this resist 115 is
then developed to form opening 116 that corresponds to exposure
opening 110. In the photolithographic technique of the prior art, a
contact hole is formed in interlayer dielectric film 106 of
semiconductor integrated circuit 100 through this opening 116 in
resist 115.
[0014] However, since it is impossible to form contact hole 107 of
a diameter that is still smaller than the dimension limited by
exposure resolution, resist 115 that has been patterned as
described hereinabove is heated and softened in a thermal flow
process to shrink opening 116 as shown in FIG. 2B.
[0015] Since opening 116 of resist 115 thus attains a diameter that
is smaller than the exposure limit dimension, an extremely small
diameter contact hole 107 can be formed from the surface of
interlayer dielectric film 106 to the surface of gate oxide film
102 by etching interlayer dielectric film 106 of semiconductor
integrated circuit 100 through opening 116 in resist 115.
[0016] Exposure opening 110 of photomask 111, which is used in the
exposure process in the above-described thermal flow process, is
therefore formed at dimensions that approach the limit dimensions
of the exposure process and in a shape that is an enlargement in
all directions of opening 116 that has been shrunk by heating
resist 115.
[0017] When the dimensions of exposure opening 110 approach the
exposure limit dimensions, the shape of an exposure beam that
passes through exposure opening 110 is deformed by such factors as
diffraction. The shape of the exposure of opening 116 in resist 115
is therefore roughly oval in shape even though exposure opening 110
is square, and the shape of opening 116 following the thermal flow
process becomes approximately circular.
[0018] Since no problem is raised if the plan shape of contact hole
107 that-is formed at the exposure limit dimensions as described
above is substantially circular, exposure opening 110 of photomask
111 is typically formed as a square in order to simplify design and
fabrication. As a result, in a case in which the exposure dimension
of opening 116 in resist 115 is set to a circle of diameter "a",
exposure opening 110 of photomask 111 is formed as a square having
sides of length "a".
[0019] In the interest of simplifying the explanation here, a case
is described in which the process of exposing resist 115 using
photomask 111 is carried out in equal proportions, but in a case in
which the exposure process is performed in the above-described
exposure limit dimensions, the pattern of openings of photomask 111
is typically exposed on resist 115 in a form that is reduced by
reduction optics.
[0020] After forming opening 116 in resist 115 by an exposure
process using photomask 111 in the above-described thermal flow
process, this resist 115 is heated to shrink opening 116, whereby a
process can be performed on interlayer dielectric film 106 at
dimensions that are smaller than the exposure limit dimension.
[0021] Nevertheless, when resist 115 is heated and softened to
shrink opening 116 that was formed by the exposure process as
described hereinabove, opening 116 deforms as it shrinks due to the
surface tension of this resist 115. It has been confirmed by the
inventors of this invention that this deformation occurs in
accordance with the positional relationships between the plurality
of openings 116. Specifically, when shrinking a plurality of
openings 116 by heating resist 115, the degree of shrinkage at each
of openings 116 that are close to each other is smaller in the
direction between openings 116 while the degree of shrinking is
greater in the direction orthogonal to this direction.
[0022] In some types of high-integration DRAM referred to as
"{fraction (1/4)} pitch DRAM," for example, a plurality of contact
holes 107 are arranged linearly in a direction that is inclined
45.degree. from the directions of arrangement of the bit lines and
word lines. Photomask 111 for forming such a plurality of contact
holes 107 has a shape in which a plurality of square exposure
openings 110 are arranged in a line in a 45.degree. direction, as
shown in FIG. 3A.
[0023] When resist 115 is exposed using this type of photomask 111,
a plurality of round openings 116 is thus arranged in a line in a
45.degree. direction, as shown in FIG. 3B. When this resist 115 is
heated and openings 116 are caused to shrink, however, the degree
of shrinkage in the direction of arrangement is small, while the
degree of shrinkage in the direction orthogonal to this direction
is great, and, as shown in FIG. 3C, each of openings 116 therefore
forms an oval that is elongated in the direction of arrangement of
openings 116.
[0024] In a thermal flow process of the prior art, the formation of
openings 116 of a desired shape in desired positions is problematic
due to deformation according to the positional relationship between
the plurality of openings 116, as described in the foregoing
explanation, and the proper realization of prescribed fine
processing on semiconductor integrated circuit 100 is therefore
also problematic.
SUMMARY OF THE INVENTION
[0025] It is an object of the present invention to provide a
photomask that can properly realize prescribed fine processing on a
semiconductor integrated circuit in a thermal flow process; a
pattern forming method that can properly realize prescribed fine
processing on a semiconductor integrated circuit in a thermal flow
process; and a semiconductor integrated circuit in which prescribed
fine processing is properly realized.
[0026] According to one aspect of the present invention, a
photomask is used in a thermal flow process in which: a photomask
is prepared in which a plurality of exposure openings are formed; a
resist is applied to the surface of the layer of a semiconductor
integrated circuit that is to undergo processing; the resist is
patterned by an exposure process through the photomask to form a
plurality of openings in the resist that correspond to each of the
exposure openings; and the resist in which the patterning has been
carried out is heated to cause each of the openings to shrink;
wherein at least a portion of the exposure openings among the
plurality of exposure openings are formed in a shape that
compensates for the anisotropic deformation that occurs in the
openings when each of the openings is caused to shrink by heating
the patterned resist. In the thermal flow process that uses the
photomask of the present invention, when the resist that is applied
to the surface of a layer of a semiconductor integrated circuit
that is to undergo processing is patterned by an exposure process
by means of the photomask and a plurality of openings are formed in
the resist that correspond to the plurality of exposure openings
that are formed in the photomask, these openings are formed in a
shape that compensates for the anisotropic deformation that occurs
when the resist is heated to cause each of the openings to shrink.
When the resist that has been thus patterned is heated and the
openings are caused to shrink, these openings are anisotropically
deformed as they shrink. However, since each of the openings has
been formed in advance in a shape that compensates for this
anisotropic deformation, the openings attain the proper shape after
shrinkage and deformation.
[0027] At least a portion of exposure openings among the plurality
of exposure openings may be formed in a shape that is elongated in
a direction that is approximately orthogonal to the direction
toward other exposure openings that are close. At least a portion
of exposure openings among the plurality of exposure openings may
also be enlarged in a direction that is approximately orthogonal to
the direction toward other exposure openings that are close. The
degree of enlargement of said exposure openings becomes smaller as
the distance among said other exposure openings that are close
becomes larger. When the plurality of openings that have been
formed in this way are caused to shrink by heating the resist, the
plurality of openings that neighbor each other attain the proper
shape upon shrinking because the degree of shrinkage is smaller in
the direction toward other openings while the degree of shrinkage
is greater in the direction orthogonal to this direction due to
such factors as the surface tension of the resist.
[0028] At least a portion of the exposure openings among the
plurality of exposure openings are arranged in lines that are close
together, and each of the exposure openings that are arranged in
these lines may be enlarged in the direction that is approximately
orthogonal to the direction of this arrangement.
[0029] The exposure openings may be formed in a rectangular shape
in which the direction of enlargement is the direction in which the
long sides extend.
[0030] Each of the exposure openings may be enlarged in
substantially all directions, and at least a portion of the
exposure openings among the plurality of exposure openings may be
formed such that the degree of enlargement is smaller in the
direction toward other exposure openings than other directions that
are close.
[0031] At least a portion of the exposure openings among the
plurality of exposure openings may be formed as rectangles in which
the short sides extend in the direction toward other exposure
openings that are close and the long sides extend in a direction
that is approximately orthogonal to this direction.
[0032] The term "enlargement of the openings in the resist" in this
invention assumes the dimensions before the openings are caused to
shrink by the thermal flow process with respect to the final
desired dimension of the openings that have been caused to shrink
by the thermal flow process, and thus indicates that the dimensions
of exposure of the resist are made greater.
[0033] For example, if the diameter of round openings that are
caused to shrink by the thermal flow process is "a" and the
openings are caused to shrink to "1/b" by the thermal flow process,
the openings before being caused to shrink by the thermal flow
process are circles having a diameter of "a.times.b," but in the
present invention, the exposure dimension of the resist openings is
made "a.times.b" or greater in the direction of enlargement.
[0034] In addition, the term "enlargement of exposure openings in
the photomask" in the present invention means that, when forming
openings of a desired dimension in the resist, the exposure
openings are made larger than dimensions that are designed based
merely on these openings. For example, in a case in which the
diameter of circular openings that are caused to shrink by the
thermal flow process as described hereinabove is "a" and the
openings are caused to shrink to "1/b" by the thermal flow process,
the openings before being caused to shrink by the thermal flow
process are circles of diameter "a.times.b." If the exposure optics
are equal power, square exposure openings measuring "a.times.b" on
each side would be formed in the photomask, but in the present
invention, the exposure openings that are formed in the photomask
are rectangles in which the short sides are "a.times.b" in length
and the long sides are longer than "a.times.b."
[0035] Furthermore, "approximately all directions" in the present
invention means substantially all directions involved in the
formation of the exposure openings and includes 360.degree. of the
two-dimensional directions that are parallel to the surface of the
photomask, the four directions to the left and right and forward
and rear that are parallel to the surface of the photomask, and the
two directions that are parallel to the four sides of the exposure
openings that are formed in a rectangular shape.
[0036] The pattern forming method according to another aspect of
the present invention is a pattern forming method used in a thermal
flow process in which: a resist is applied to a surface of the
layers of a semiconductor integrated circuit that is to undergo
processing; the resist is patterned to form a plurality of openings
in the resist; and the resist that has been patterned is heated to
cause each of the openings to shrink; wherein at least a portion of
exposure openings among said plurality of exposure openings are
formed in shapes so that said openings are caused to become
corresponding desired shapes due to anisotropic deformation that
occurs in said openings when said resist that has been patterned is
heated to cause said openings to shrink.
[0037] The pattern forming method according to another aspect of
the present invention is a pattern forming method used in a thermal
flow process in which: a photomask is prepared in which a plurality
of exposure openings are formed; a resist is applied to a surface
of the layers of a semiconductor integrated circuit that is to
undergo processing; this resist is patterned by an exposure process
through the photomask to form a plurality of openings in the resist
that correspond to the exposure openings; and the patterned resist
is heated to cause each of the openings to shrink; wherein the
photomask of the present invention is used during the exposure
process.
[0038] According to another aspect of the present invention, a
prescribed portion of a semiconductor integrated circuit having
fine planar shapes is treated by a prescribed process through
openings in a resist that have been formed by the method of forming
patterns of the above-described invention.
[0039] The above and other objects, features, and advantages of the
present invention will become apparent from the following
description with reference to the accompanying drawings which
illustrate examples of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIGS. 1A-1C and FIGS. 2A-2C are schematic vertical section
frontal views showing a semiconductor integrated circuit that is to
undergo processing for explaining an example of a thermal flow
process of the prior art;
[0041] FIG. 3A is a plan view showing an example of a photomask of
the prior art;
[0042] FIG. 3B is a plan view showing a resist in which openings
have been formed by exposure using the photomask shown in FIG.
3A;
[0043] FIG. 3C is a plan view showing the state of openings that
have been caused to shrink by heating the resist shown in FIG.
3B;
[0044] FIG. 4A is a plan view showing one embodiment of the
photomask according to the present invention;
[0045] FIG. 4B is a plan view showing a resist in which openings
have been formed by exposure using the photomask shown in FIG.
4A;
[0046] FIG. 4C is a plan view showing the state of openings that
have been caused to shrink by heating the resist shown in FIG.
4B;
[0047] FIG. 5 is a characteristics chart showing the degree of
deformation of the openings caused by heating of the resist;
[0048] FIG. 6A is a plan view showing the first modification of the
photomask according to the present invention;
[0049] FIG. 6B is a plan view showing the resist in which openings
have been formed by exposure using the photomask shown in FIG.
6A;
[0050] FIG. 6C is a plan view showing the state of openings that
have been caused to shrink by heating the resist shown in FIG.
6B;
[0051] FIG. 7 is a plan view showing an actual example of the
dimensions of each part of a photomask according to the present
invention;
[0052] FIG. 8A is a plan view showing a modification of the
exposure pattern formed on the resist;
[0053] FIG. 8B is a plan view showing the pattern of openings that
are formed on the photomask of the prior art; and
[0054] FIG. 8C is a plan view showing the pattern of openings that
are formed on a photomask of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0055] An embodiment according to the present invention will be
described below with reference to FIGS. 4A-4C and FIG. 5.
Components of this embodiment that are identical to components of
the above-described example of the prior art are identified by the
same term, and detailed explanation is omitted.
[0056] As shown in FIGS. 4A-4C, photomask 200 of this embodiment is
also used to pattern resist 201 in a thermal flow process, and a
plurality of exposure openings 202 are formed corresponding to the
processing positions of a semiconductor integrated circuit (not
shown in the figure) that is to undergo processing.
[0057] In more detail, in the pattern forming method of this
embodiment, for example, nine openings 203 arranged in three rows
and three columns are formed in resist 201, which is applied to the
surface of semiconductor integrated circuit, as shown in FIG. 4C.
The distances between these openings 203 is relatively small in the
direction from front to back (vertical direction in the figure) and
relatively large in the direction from left to right.
[0058] In the present embodiment as well, nine exposure openings
202 are formed on photomask 200 in three rows and three columns
that are close to each other in the front and rear directions but
distant from each other toward the right and left, as shown in FIG.
4A, but these exposure openings 202 are formed in a shape that
compensates for the anisotropic deformation that occurs in openings
203 when resist 201 is heated to cause openings 203 to shrink.
[0059] In other words, in photomask 200 of this embodiment, each of
the plurality of exposure openings 202 that are close to each other
in the front and rear directions but distant from each other toward
the left and right is formed in a rectangular shape, which is a
square that has been enlarged toward the right and left. As a
result, in photomask 200 of this embodiment, each of the plurality
of exposure openings 202 that are close to each other toward the
front and rear and arranged in lines is enlarged toward the left
and right, which are directions orthogonal to the direction of
arrangement. As a result, these exposure openings 202 are formed as
rectangles having long sides that extend in the left and right
directions, which are the directions of enlargement, and the sides
that extend in the front and rear directions toward the other
nearby exposure openings 202 are therefore the directions in which
the short sides of these rectangles extend.
[0060] In the pattern forming method of this embodiment, however,
each of the plurality of exposure openings 202 of photomask 200 is
actually enlarged in almost all directions compared to the
dimensions of opening 203 for the exposure process in resist 201.
However, the degree of enlargement of these exposure openings 202
is small in the front and rear directions that extend toward other
closely neighboring exposure openings 202, and exposure openings
202 are thus formed as shapes that are expanded toward the left and
right.
[0061] Exposure openings 202, which are arranged both toward the
front and rear and toward the right and left of photomask 200 as
described in the foregoing explanation, are also close to each
other toward the right and left, although not a's close as toward
the front and rear, and exposure openings 202 are therefore also
enlarged toward the front and rear, which is the direction
orthogonal to the right and left. As described above, however, the
degree of enlargement of exposure openings 202 is great toward the
left and right and small toward the front and rear, the degree of
enlargement in each direction being inversely proportional to the
distance to a neighboring opening in that direction.
[0062] A brief explanation is next presented regarding a thermal
flow process that uses photomask 200 of this embodiment in the
above-described construction.
[0063] First, resist 201 is applied to the surface of the
semiconductor integrated circuit that is to undergo processing, and
this resist 201 is then patterned by means of an exposure process
by photomask 200.
[0064] Then, as shown in FIG. 4B, a plurality of openings 203
corresponding to the plurality of exposure openings 202 of
photomask 200 are formed in resist 201. When this resist 201 is
heated and each of openings 203 is caused to shrink, openings 203
of resist 201 attain a small diameter that is less than the
exposure limit dimensions, whereby a desired process can be
performed in a fine area of a semiconductor integrated circuit.
[0065] When resist 201 is heated and each of openings 203 is caused
to shrink as described hereinabove, however, anisotropic
deformation occurs according to the positional relationship between
these openings 203. The deformation of these openings 203 is
believed to arise due to such factors as the surface tension of
resist 201. When openings 203 are caused to shrink by heating
resist 201, the degree of shrinkage in openings 203 that are close
to each other is small in the directions toward the other openings
while the degree of shrinking is great in the direction that is
orthogonal to this direction, as shown in FIG. 5.
[0066] However, since exposure openings 202 in photomask 200 of
this embodiment are formed in shapes that compensate for the
anisotropic deformation of openings 203 as shown in FIG. 4A,
openings 203 that are formed in resist 201 by an exposure process
that uses this photomask 200 are formed in an oval shape that is
enlarged in the direction that is substantially orthogonal to the
direction toward other openings 203 that are close, as shown in
FIG. 4B. When the plurality of openings 203 that are thus formed
are caused to shrink by heating resist 201, openings 203 assume a
substantially circular shape as shown in FIG. 4C due to the
occurrence of anisotropic deformation according to the positional
relation between the openings.
[0067] As a result, fine openings 203 can be formed in a proper
shape in resist 201 in a thermal flow process that uses photomask
200 of this embodiment, and appropriate processing can thus be
realized in a fine area of semiconductor integrated circuit. In
photomask 200 of this embodiment, moreover, each of exposure
openings 202 is formed as a rectangle in which the right and left
directions, which are the directions of chief enlargement, are the
directions in which the long sides extend, and the design and
fabrication of of the photomask is thus facilitated.
[0068] The present invention is not limited to the above-described
embodiment, and various modifications are possible in the scope or
spirit of the invention. For example, in the above-described
embodiment, an example was described in which openings 203 of a
desired shape were formed on resist 201 by a photolithographic
technique using photomask 200, but these openings 203 may also be
formed on resist 201 by a direct writing technique that does not
use photomask 200.
[0069] Although a case was described in the above-described
embodiment in which openings 203 that were formed on resist 201
were arranged in lines extending toward the front and rear,
openings 203 may also arranged linearly in a direction that is at
an angle, as in the previously described example of {fraction
(1/4)}-pitch DRAM of the prior art simply adapting the
above-described photomask 200 to this type of arrangement, however,
means that the exposure openings that were originally square must
be enlarged in a direction that extends at an angle. The exposure
openings must therefore be enlarged to form a parallelogram or
diamond shape, and this complicates the design and fabrication of
the photomask.
[0070] When this becomes a problem, a plurality of exposure
openings 211 of photomask 210 are formed in rectangular shapes in
which the long sides extend in an oblique direction that is
orthogonal to direction of arrangement of these openings 211.
[0071] In this case, a plurality of openings 212 are formed as
inclined oval shapes in resist 201 as shown in FIG. 6B, and these
oval openings 212 are enlarged in directions that are orthogonal to
the direction in which the openings are close to each other,
whereby these openings 212 become proper circles when caused to
shrink by heating resist 201.
[0072] The inventors of the present invention actually produced
photomask 210 on an experimental basis in which openings 212 in
resist 201 were arranged in a 45.degree. direction as described in
the foregoing explanation. In this case, the average diameter of
openings 212 in resist 201 that were caused to shrink by heating
resist 201 was set to 0.15 .mu.m, and the pitch toward the front
and rear as well as to the right and left of the plurality of
openings 212 that were arranged in a 45.degree. direction was set
to 0.3 .mu.m.
[0073] Taking into consideration the data of FIG. 5, exposure
openings 211 of photomask 210 were formed in rectangular shapes
with short sides of 0.23 .mu.m and long sides of 0.4 .mu.m as shown
in FIG. 7, and it was confirmed that openings 212 were finally
formed having substantially the above-described dimensions.
[0074] In the above-described embodiment, an example was described
in which regularly arranged openings 203 were formed in resist 201,
but the present invention may also be adapted to a case in which
irregularly arranged openings 203 are formed in resist 201. It is
also obvious that the conditions of deformation for openings of
identical shape that are arranged in a line at uniform intervals as
described hereinabove will differ for openings at the two ends and
for openings in the central area.
[0075] However, the inventors designed photomask 222 in which all
of openings 221 can be formed in proper shape assuming three
openings 221 having a diameter of 0.2 .mu.m are arranged in resist
201 in a line at a pitch of 0.35 .mu.m as shown in FIG. 8A.
[0076] In a photomask of the prior art in such a case, three square
exposure openings measuring 0.2 .mu.m on each side are arranged in
a line at a pitch of 0.35 .mu.m as shown in FIG. 8B. In contrast,
it was confirmed that in photomask 222 of this invention, central
exposure openings 223 are preferably formed as rectangles measuring
0.24.times.0.30 .mu.m and exposure openings 223 at the two ends are
preferably formed as rectangles measuring 0.27.times.0.30 .mu.m, as
shown in FIG. 8C. In other words, in a case in which a plurality of
exposure openings 223 are arranged in a line, the degree of
enlargement in the direction of arrangement of exposure openings
223 at both ends is preferably greater than the degree of
enlargement of exposure openings 223 in the central area.
[0077] Among the above-described embodiments, a case was described
in which exposure openings 202 of photomask 200 were enlarged in
substantially all directions, i.e., toward the front, rear, left
and right, with the degree of this enlargement for the front-rear
directions differing from that for the left-right directions.
However, it is also possible for exposure openings 202 to be
enlarged in only specific directions.
[0078] While preferred embodiments of the present invention have
been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the following claims.
* * * * *