Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-11-22
2001-11-06
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06312855
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to a method of a phase shift mask. More particularly, the invention relates to a three-phase phase shift mask to resolve the corner round problem occurred in an unexposed edge area caused by diffraction or scattering of light.
2. Description of the Related Art
As the integration of an integrated circuit is demanded higher and higher, the design is developed toward a direction of further shrinking the devices and the circuit. The photolithography technique plays one of the most important roles for the shrinkage. For examples, the sizes of any structures related to a metal-oxide semiconductor (MOS) such as a thin film pattern and the dopant area are basically determined by this technique. Thus, whether the integration of semiconductor industry can be further developed down to, over even under, a linewidth of 0.15 micron, is determined by the development of photolithography technique. According to the great demand, methods to enhance the resolution of photomask such as using optical proximity correction (OPC) and phase shift mask (PSM) have been proposed.
The method of optical proximity correction is to eliminate the deviation in critical dimension (CD) caused by the proximity effect. When a light beam is incident on a wafer through the pattern of a photomask, the light beam is scattered so that the area of the wafer spotted by the light is enlarged. On the other hand, the light beam may be reflected from the semiconductor substrate of the wafer to cause an interference with the incident light beam. As a result, a double exposure is caused to change the exposure degree of the wafer. The proximity effect is even more obvious when the critical dimension is close to the wavelength of the incident light.
Referring to
FIG. 1A
to
1
D, a conventional method of optical proximity correction is drawn. In
FIG. 1A
, a photomask
100
having a pattern of three rectangular masking areas are shown and denoted as
105
, while the rest area of the photomask is transparent and denoted as
110
. The substrate material of the photomask
100
is typically glass or quartz that forms the transparent area
110
. The masking areas
105
are typically made of a layer of chromium (Cr). In
FIG. 1B
, when a light beam is incident on the photomask
100
, a wafer substrate
120
underneath would have three dark regions
125
and a light regions
130
on a substrate
120
.
As shown in
FIGS. 1A and 1B
, the masking areas
105
are in rectangular shape, however, the pattern transferred onto the wafer substrate
120
becomes dark regions
125
with rounded corners and smaller dimensions. Patterns or masking areas in other region that is not shown in this figure may be distorted or deformed in other form. For example, when the mask regions of the pattern are designed close to each other, after exposure, the patterns transformed into the wafer substrate might merge with each other or deviate from the original pattern.
In general practice, to compensate the above deformation of patter, at the corners or edge of the masking areas
105
, assistant features such as serifs
150
and
155
at the corners and along the edge as shown in FIG.
1
C. the serifs
150
at the corners are added to resolve the problem of rounded corner, while the serifs
155
along the edge are added to restore the desired dimensions of the pattern. As shown in
FIG. 1D
, using this method, the fidelity of the pattern transferred from the photomask
100
to the wafer substrate
120
is very much improved. The dark areas
125
a
has a much less rounded corners, while the dimensions of these areas
125
a
are closer to those
105
on the photomask
100
.
However, when the distance between patterns is further reduced or the critical dimension of is further shrunk to lower than 0.1 microns, this method meets its bottleneck. That is, using this method for compensation or amendment of the patterns, the available spaces or areas for forming or adding the assistant feature such as serifs are too small.
SUMMARY OF THE INVENTION
The invention provides a three-phase phase shift mask. A transparent substrate is provided. A non-transparent pattern covering a first portion of the transparent substrate is formed, while a second portion of the substrate remains transparent. The three phase-shift areas are on the second portion of the substrate, these three phase-shift area are different from each other with a phase shift of 120°. At any corner of the non-transparent pattern, a proximity region around the corner is equally partitioned by the three phase-shift areas. Apart from the proximity region, the second portion of the transparent substrate comprises only phase edges between two phase-shift areas instead of three.
The three-phase phase shift mask provides an effect of eliminating deformation such as a corner rounding effect of the transferred pattern. This is because at each corner of the pattern on the photomask, an incident light is split into three light beams transmitting through these three phase shift areas. Since the phase shift of these three phase shift regions are different from each with a phase shift of 120°. As a total, the sum of these three light beams straying into the corner is zero. As a result, the diffraction, scattering or even interference at the corner pattern on the wafer is eliminated. The fidelity of the transferred pattern is thus enhanced. In addition, on other parts of the transparent photomask, edge phases are formed between two phase shift areas only. It has been mentioned that any two of the three phase shift areas are different in phase shift of 120°, therefore, even at the phase edges, the light cannot be cancelled totally. There is no worry for forming any unwanted dark area on the wafer.
The method of fabricating the three-phase phase shift mask is also provided in the invention. In the three-phase phase shift mask, three phase shift areas are formed in two etching steps. Some portions of the transparent substrate other than the non-transparent pattern formed thereon is etched twice to for the third phase shift area.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
REFERENCES:
patent: 5900338 (1999-05-01), Garza et al.
Chen Anseime
Huang I-Hsiung
Hwang Jiunn-Ren
Rosasco S.
Thomas Kayden Horstemeyer & Risley LLP
United Microelectronics Corp.
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