Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-12-28
2003-03-04
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06528216
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase shift mask and a fabrication method thereof which are used in a semiconductor light exposing process.
2. Description of the Background Art
As the integration density of semiconductor devices has increased, a distance between elements of a transistor in devices, (e.g., a capacitor, a wire, etc.) has decreased. Therefore, a technique is required for forming a fine pattern needed for semiconductor device fabrication.
Generally, a light exposing mask used for a light exposure process for forming a photoresist film pattern is formed by coating a light shield film such as a chromium film, an aluminum film, etc. on an upper surface of a quartz substrate. A part of the light shield film is then etched using an ion beam etching method, etc. to form a light exposure pattern. However, when a light exposure mask which uses the above-described light shield pattern is used, it is impossible to form a fine pattern having a visibility limit which is slightly lower than that of a stepper light source.
Recently, phase shift masks have been studied and used for forming fine patterns. Phase shift masks are formed with a semi-transparent phase shift region and a transparent light transmitting region. The light transmitting region and phase shift region have different refractive indexes and transmittance with respect to an incident wave. When the light wave which is transmitted by the phase shift region reaches the wafer, the wave has a phase difference. Therefore, the phase shift mask has information concerning a light intensity and information concerning the phase, where the light exposure mask which uses the conventional light exposure film pattern only has information concerning the light intensity.
The wave which is transmitted by the phase shift region and the wave which is transmitted by the light transmitting region interfere due to their phase difference. As a result, the photoresist film pattern formed using the phase shift mask has visibility superior to the photoresist film pattern formed based on the light exposure mask, so that it facilitates forming a fine pattern.
When the pattern formed on the mask is arranged based on an anisotropic pitch, (i.e., when the size of each light transmitting region formed on the mask is significantly larger than the wavelength of the exposing light) a phase having a shape similar to the shape of the light transmitting region pattern is formed on an upper surface of the wafer. When the size of the light transmitting region pattern has feature size similar to the wavelength of the exposing light, a circular-shaped phase is formed irrespective of the shape of the light transmitting region pattern.
FIG. 1
illustrates a conventional phase shift mask. The phase shift mask includes a phase shift region
1
and a light transmitting region
2
. As shown therein, a square-shaped light transmitting region
1
is arranged with an “X” pitch in a first direction and is arranged at a “Y” pitch in a second direction. The phase shift region
1
is positioned among the thusly arranged light transmitting regions
2
. The phase shift region
1
is formed of the same material over its entire area and has the same thickness, thereby obtaining the same transmittance.
FIG. 2A
is a cross-sectional view taken along line II-II′ of the phase shift mask in FIG.
1
.
FIG. 2B
illustrates an amplitude and phase which are obtained when the wave
5
, which is transmitted by each light transmitting region
2
of the phase shift mask of
FIG. 2A
, and the wave
6
, which is transmitted by each phase shift region
1
, reach the wafer. As shown therein, the wave
5
does not overlap itself and has a 180° phase difference with respect to the wave
6
. In addition, the amplitude of the wave
5
is larger than the amplitude of the wave
6
.
Since the wave
5
, which is transmitted by the light transmitting region
2
of
FIG. 2A
, and the wave
6
, which is transmitted by the phase shift region
1
, have a 180° phase difference, the waves symmetrically interfere. As a result, as shown in
FIG. 2C
, a wave
7
is formed. The wave
7
forms an image having a large visibility on an upper surface of the wafer.
FIG. 3A
is a cross-sectional view taken along line III-III′ of the phase shift mask in FIG.
1
. As shown therein, the light transmitting region
2
is formed at a “Y” pitch, and the value of “Y” is smaller than the value of “X”.
FIG. 3B
illustrates an amplitude and phase which are obtained when the wave
10
, transmitted by each light transmitting region
2
of the phase shift mask of
FIG. 3A
, and the wave
11
, transmitted each phase shift region
1
, reach the wafer. As shown therein, the waves
10
overlap and have a 180° phase difference with respect to the waves
11
. The overlapping of the waves
10
increases as the value of “Y” of the arrangement pitch of the light transmitting region
2
decreases. The amplitude of the wave
10
is larger than the amplitude of the wave
11
.
Since the phase shift regions of the conventional phase shift mask all have the same transmittance, the amplitude of the wave
11
is the same as the amplitude of the wave
6
, both of which are transmitted by the phase shift region
1
.
Since the wave
10
and the wave
11
of
FIG. 3B
have a phase difference of 180°, the waves symmetrically interfere. The overlapping waves
10
and the waves
11
combine to form a wave
12
having a phase and amplitude as shown in FIG.
3
C. The wave
12
formed by the above-described interference and overlapping phenomena forms a certain image on the wafer which has a decreased visibility. As the arrangement pitch “Y” of the light transmitting region
2
is decreased, the size of the overlap which is formed by the waves
10
increases, and the visibility of the image formed on the wafer decreases.
FIG. 4
illustrates an image formed on the wafer during a light exposing process using the phase shift mask as shown in FIG.
1
. As shown in
FIG. 1
, the phase shift mask includes a square light transmitting region
2
having a pitch of “X” in a first direction and a pitch of “Y” in a second direction. As shown in
FIG. 4
, an elliptical image
15
is formed by such a mask on the upper surface of the wafer.
When the size of each light transmitting region has a value similar to that of the exposing light's wavelength, a circular image is not formed. Rather, an elliptical shape image
15
is formed, because when the pitches “X” and “Y” are different, the visibility of the image formed on the wafer is different in each direction. Therefore, the shape of the image formed on the wafer is expanded in the direction that the arrangement pitch between the light transmitting regions is narrow and overlap of transmitted waves occurs.
Therefore, as the arrangement pitch of the light transmitting region
2
is decreased, the shape of the image formed on the wafer changes. As the integration of the semiconductor device is increased, such distorting effect is increased.
In particular, the above-described shape variation is increased when the arrangement pitch between the light transmitting region patterns is below two times of the exposing light's wavelength. When the wavelength of a light source is smaller than that of the arrangement pitch between the light transmitting region patterns, it is possible to overcome the above-described problems. However, using such a light source creates problems.
When the image formed on the wafer is varied, errors in the fabrication process increase, thereby decreasing the yield, and the reliability of the semiconductor device is decreased.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a phase shift mask and a fabrication method thereof that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
In accordance with the purpose of the invention, as embodied and broadly described, in one aspect the invention includes a phase shift mask, including: a plurali
Birch & Stewart Kolasch & Birch, LLP
Hynix / Semiconductor Inc.
Rosasco S.
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