Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Insulative material deposited upon semiconductive substrate
Reexamination Certificate
2001-04-05
2003-04-08
Cuneo, Kamand (Department: 2829)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Insulative material deposited upon semiconductive substrate
C438S780000, C430S313000, C430S314000
Reexamination Certificate
active
06544903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resist pattern forming method in a lithography technique and a semiconductor device manufacturing method using the same.
2. Description of the Related Art
In general, a technique for forming a pattern based on a circuit design is referred to as lithography technique in a semiconductor manufacturing process. Requirements of the lithography technique in the semiconductor manufacturing process include resolution (minimum line width which can be formed on a substrate), focal depth (range in which a clear image can be obtained in the back and front of the focal surface), alignment precision, etching resistance, throughput and low costs.
As well known, a short-wavelength light source is used, the NA (Numerical Aperture) of a projecting lens is increased or the like as a method for improving resolution. However, since the increase of the NA value reduces the focal depth, the NA value needs to be set such that a practical focal depth can be ensured.
The basic procedure of the lithography technique comprises the steps of coating a substrate with a resist (resist coating), exposing the resist in a pattern corresponding to a circuit design (exposure) and developing the exposed resist to form a resist pattern on the substrate (development). A conventional resist pattern forming method will be described below with reference to
FIGS. 1A-1D
.
FIGS. 1A-1D
are sectional process drawings of a conventional resist pattern forming method (conventional example 1).
A resist pattern having a line width of 110 nm is formed by this conventional resist pattern forming method.
First, a substrate
1
is coated with a resist
2
having a film thickness of 500 nm (FIG.
1
A). This is a positive resist.
Subsequently, the resist
2
is irradiated with KrF light
3
by a stepper (not shown; a kind of aligner) through a photomask
14
to expose a mask pattern (FIG.
1
B). At this time, the optical conditions are a numerical aperture NA of 0.68 and a coherence factor &sgr; of 0.75. The wavelength of the KrF light is 248 nm. The exposure amount is 35.0 mJ/cm
2
. By this exposure, an acid generating agent is photodecomposed in an exposed portion of the resist
2
, thereby generating an acid. As a photomask
14
, a mask obtained by forming a metallic thin film
14
a
having an aperture pattern on a glass substrate
14
b
is used.
Subsequently, heat treatment is performed at 105° C. for 90 seconds (FIG.
1
C). By this heat treatment, a protective group in the exposed portion of the resist
2
is reacted by the acid catalyst elimination reaction, thereby increasing hydrophilicity of the exposed portion of the resist
2
. As a result, a developer soluble portion
12
a
which can be dissolved by a developer is formed.
Then, development is performed by using a TMAH (Tetramethyl Ammonium Hydroxide) aqueous solution having a concentration of 2.38% at a liquid temperature of 23° C. for a development time of 60 seconds (FIG.
1
D). By this development, the developer soluble portion
12
a
is dissolved and removed and an unexposed portion remains. Thus, a resist pattern
12
b
having a line width of 110 nm is obtained.
Subsequently, when the above-described mask pattern is exposed with KrF light by using a lens having an NA of 0.68 and a of 0.75, an experiment shown in the above
FIGS. 1A-1D
is carried out as described above by moving a wafer stage of the aligner relatively to the lens to check the focal depth with which a developed resist pattern having a line width design dimension of 110 nm±10% can be obtained.
As a result, a CD (critical dimension)-Focus curve shown in
FIG. 2
is obtained. Two broken lines in parallel to the horizontal axis shown in
FIG. 2
are lines each showing a line width of 110 nm±10%. As shown in
FIG. 2
, in this optical system, a focal depth with which a line having a design dimension of 110 nm can be formed with an error of 10% is very shallow and substantially 0 &mgr;m. The above optical system currently has the most advanced resolution in practice. However, the result of the above-described experiment showed that almost no focal depth could be obtained even though this system is used.
In the above conventional example 1, a case where a resist pattern is formed on a wafer having a planar surface across the whole is assumed. Normally, however, steps are present in some regions on the surface in an actual semiconductor device. Such an example will be described below.
FIGS. 3A-3D
are sectional process drawings of another conventional resist pattern forming method (conventional example 2). A substrate
11
has a step of 0.2 &mgr;m in
FIGS. 3A-3D
. During a semiconductor manufacturing process, steps of this size are spontaneously generated by a circuit pattern laminated on a semiconductor wafer. The method of conventional example 2 is employed on this substrate
11
under the same conditions as in the above conventional example 1. In conventional example 2 as well, a resist pattern having a line width of 110 nm is formed.
In conventional example 2, the substrate
11
is first coated with a resist
2
having a film thickness of 500 nm (
FIG. 3A
) as in the case of conventional example 1.
Subsequently, the resist
2
is irradiated with KrF light
3
by a stepper (not shown) through a photomask
14
to expose a mask pattern (FIG.
3
B). At this time, the focus is adjusted to the upper level portion A of the step. Therefore, since the focal depth is even less than 0.1 &mgr;m as described above, a pattern having designed dimensions can not be obtained on the lower level portion B of the step.
Subsequently, heat treatment is performed at a temperature of 105° C. for 90 seconds (FIG.
3
C).
Then, development is performed by using a TMAH aqueous solution having a concentration of 2.38% at a liquid temperature of 23° C. for a development time of 60 seconds (FIG.
3
D). By this development, a developer soluble portion
12
a
is dissolved and removed and an unexposed portion remains. Thus, a resist pattern
12
b
having a line width of 110 nm is obtained on the upper level portion A of the step. On the other hand, since the lower level portion B is not within the focal depth in the exposure, the pattern precision is degraded. After the developer soluble portion
12
c
is dissolved and removed, a resist pattern having a line width of 110 nm cannot be obtained. A resist pattern
12
d
having a deformed pattern remains.
As described above, in conventional resist pattern forming methods, when a resist pattern having the same line width as the resolution of the aligner is to be formed, a sufficiently practical focal depth cannot be obtained, thereby resulting in difficulty to form a resist pattern having a desired line width. Therefore, these methods are not practical.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a resist pattern forming method by which a resist pattern having a finer line width can be formed in high precision with a sufficiently practical focal depth than a line width with which a sufficient focal depth can be conventionally obtained so that contradicting problems between the resolution and the focal depth are solved.
A resist pattern forming method according to a first aspect of the present invention has the steps of forming a resist pattern having a line width greater than a desired line width by exposure and development, and reducing the line width by exposing the whole surface of said resist pattern and developing the resist pattern.
According to the resist pattern forming method of this aspect of the present invention, a resist pattern having a line width greater than a desired line width has only to be exposed in a process of exposing a resist pattern. Therefore, practical resolution can be ensured even with a finer line width than a line width with which a sufficient focal depth can be obtained.
That is, an advantage is that a desired resist pattern finer than a line width with which a sufficient focal depth can be obtained can be formed in
Cuneo Kamand
Kilday Lisa
NEC Corporation
Young & Thompson
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