Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1998-06-10
2001-02-13
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06187480
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 87105067, filed Apr. 3, 1998, the full disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a photolithography process, and more particularly to a phase-shifting mask and the manufacturing method thereof.
2. Description of the Related Art
In semiconductor fabrication, photolithography is an important and indispensable technique that is used to transfer circuit layout patterns through a mask onto predefined locations on a semiconductor wafer. Many processes in semiconductor fabrications, such as etching and ion implantation, require the use of photolithography. In a photolithographic process, resolution and depth of focus (DOF) are two major checkpoints used to appraise the quality of the pattern definition. A high level of integration requires a high resolution of pattern definition since the feature size is very small. To increase the resolution, a laser source with a very short wavelength, such as a krypton (Kr) deep ultraviolet laser with a wavelength of 2480 Å (angstrom), is used as the exposure light in the photolithographic process. The use of a short-wavelength exposure light, however, will result in a shallow DOF. To allow high resolution and good DOF, one solution is to use the so-called phase-shifting mask (PSM).
Fundamentally, a PSM is formed by adding phase shifter layers to a conventional mask, which phase shifter layers cause destructive interference to the light passing through it such that the contrast and resolution of the resulting pattern definition can be increased. One benefit of the PSM is that it can increase the resolution of pattern definition without having to change the wavelength of the exposure light.
In semiconductor fabrication, line patterns are usually required to form structures such as metal interconnections, bit lines and word lines of dynamic random access memories (DRAM). Conventionally, a post-photoresist is used to execute a photolithography process. In highly integrated of devices, the line width of the line patterns is increased. The so-called alternating PSM is used to define line patterns with a high resolution.
A conventional alternating PSM photolithographic process is illustratively depicted in the following with reference to
FIG. 1
, which is a cross-sectional view of a conventional alternating PSM. A method of fabricating the conventional alternating PSM comprises the step of forming a patterned chromium film
102
as a mask on a light-penetrable substrate
100
such as a glass substrate or a quartz substrate, and the step of etching the substrate
100
to form a trench
103
. There are two areas that can cause a phase shift of 180° when incident light passes through them. These are the thickness
11
of a region
10
of the substrate
100
which is exposed by the chromium film
102
and the thickness
12
of a region
20
where the trench
13
is formed. However, the difference between the region
10
of the substrate
100
which is exposed by the chromium film
102
and the region
20
where the trench
13
is formed directly affects the phase shift caused to the incident light. Also, it is difficult to control the etching process that creates thickness
13
. As a result, the incident light passing through the region
10
of the substrate
100
, which is exposed by the chromium film
102
, and the region
20
where the trench
13
is formed cannot undergo the phase shift of 180°. This means that the alternating PSM can not achieve the required interference.
Another conventional alternating PSM photolithographic process is illustratively depicted in the following with reference to
FIGS. 2A-2C
. Referring first to
FIG. 2A
, a chromium film
202
is formed on a light-penetrable substrate
200
such as a glass substrate, a quartz substrate, or other suitable material. Next, referring to
FIG. 2B
, the chromium film
202
is defined and a chromium film
202
a
is left to cover a sheltered region S. A phase shift layer
204
is formed on the light-penetrable substrate
200
. The material of the phase shift layer
204
is, for example, SiO
x
N
y
or Mo
z
SiO
x
N
y
. Then, referring to
FIG. 2C
, the phase shift layer
204
is defined by using the light-penetrable substrate
200
as an etching stop layer and a phase shift layer
204
a
is left to cover a shifting region P.
A shift angle from the phase shift layer
204
is the key to determining either destructive interference or constructive interference to light passing through a penetrating region T and the shifting region P during exposure. One factor that influences the shift angle is the characteristics of the phase shift layer
204
. Another important factor is the thickness
205
of the phase shift layer
204
. A conventional method of fabricating the mask described above is to define a pattern of the chromium film
202
and deposit and pattern the phase shift layer
204
a
on the semi-finished substrate
200
, which has the chromium film
202
and a photoresist layer already formed thereon. It is difficult to gain a uniform thickness from coating a material on a square substrate. Furthermore, coating the phase shift layer
204
on the substrate
200
with a pattern of chromium film
202
already formed on it results in poor step-coverage. The reasons described above make the thickness
205
of the phase shift layer
204
a
non-uniform and make a difference in the phase angles of the incident light passing through regions of the mask. This results in the poor resolution of the PSM illustrated above.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to provide an alternating phase shift mask for use in photolithography in semiconductor fabrication which can avoid the poor coverage of the phase shift layer and non-uniform thicknesses, and enhance the resolution of the PSM.
The invention achieves the above-identified objects by providing an alternating phase shift mask, which comprises a phase shift layer places between a light-penetrable substrate and a sheltering layer. The sheltering layer covers a sheltered region formed on the phase shift layer. The phase shift layer also has an opening between the sheltered region to expose the light-penetrable substrate, where the exposed portion of the substrate defines a penetrating region while the portion of the phase shift layer between the sheltered region without being removed define the phase shifting region. The incident light travels through the penetrating region without changing its phase but travels through the phase shifting region with a wave phase angle shifted by 180°. When the lights from the penetrating region interferes with the light from the phase shifting region, the resolution of photolithographic process is enhanced.
The invention achieves the above-identified objects by providing a method of fabricating an alternating phase shift mask which comprises a step of forming a phase shift layer on a light-penetrable substrate; a step of forming a sheltering layer on the phase shift layer; and a step of defining the sheltering layer and the phase shift layer to form a PSM with a penetrating region, a phase shift region and a sheltered region. Because the phase shift layer can be deposited with an accurate thickness according to the characteristic of the material and to the shift angle, the invention is free from the faults and the limits of the conventional process.
REFERENCES:
patent: 5275896 (1994-01-01), Garofalo et al.
patent: 5358808 (1994-10-01), Nitayama et al.
patent: 5380608 (1995-01-01), Miyashita et al.
patent: 5478678 (1995-12-01), Yang et al.
patent: 5549995 (1996-08-01), Tanaka et al.
patent: 5605776 (1997-02-01), Isao et al.
Rosasco S.
United Microelectronics Corp.
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