Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-03-06
2001-09-25
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06294295
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to rim type attenuating phase shifting masks used for forming contact holes and more specifically to the use of different transmittance phase shifting materials for pattern regions having different duty ratios.
(2) Description of the Related Art
Attenuating phase shifting masks find wide usage in the printing of contact holes on a layer of photoresist formed on an integrated circuit wafer. In those patterns which have contact holes on dense sub patterns and contact holes on more isolated sub patterns it is a problem to optimize the mask for the different sub pattern regions. At lower sigma settings the depth of focus is greater for the isolated pattern than for the more dense pattern, however side lobe effect problems become more significant. At higher sigma settings the depth of focus is smaller for the isolated pattern than for the more dense pattern, however side lobe effect problems are decreased. The value of sigma is a measure of the degree of coherence of the light used to project the mask pattern. A smaller the value of sigma indicates a greater coherence of light. For completely coherent light the value of sigma is zero. For incoherent light the value of sigma is 1.0. The value of sigma is variable in the exposure system and depends on the process settings. Projection systems typically use light having a value of sigma of about 0.6.
The depth of focus can be increased by increasing the transmittance of the attenuating phase shifting material but this causes side lobe problems in the dense portions of the mask. Trade-offs are necessary between the depth of focus of the different pattern regions; problems due to side lobe effect; and the degree of coherence of the light used to project the mask pattern.
U.S. Pat. No. 5,725,973 to Han et al. describes a photomask for suppressing proximity effect in a mask pattern. The proximity effect is suppressed by forming an optical transmittance control film pattern in the transmission area between opaque mask patterns.
U.S. Pat. No. 5,786,114 to Hashimoto describes a phase shift mask having phase shift portions of both Levenson type and Halftone type. A method of controlled oxidation of the halftone material to increase and stabilize the transmittance of the halftone material is described.
U.S. Pat. No. 5,882,827 to Nakao describes a phase shift mask of both Levenson type and Halftone type.
U.S. Pat. No. 5,849,439 to Mitsui describes a method of controlling the transmittance and thickness of phase shifting material using a reverse sputtering process.
U.S. patent application Ser. No. 09/473,027 (TSMC-99-118), Filed Dec. 28, 1999, entitled “USING DIFFERENT TRANSMITTANCE WITH ATTENUATE PHASE SHIFT MASK (APSM) TO COMPENSATE ADI CRITICAL DIMENSION PROXIMITY”, and assigned to the same Assignee describes the use of different transmittance light absorbing materials for forming line/space patterns and contact hole patterns having different duty ratios.
SUMMARY OF THE INVENTION
FIG. 1
shows a top view of a part of a mask used to form contact holes in an integrated circuit wafer. The mask has a pattern of contact holes
112
forming a regular array with the contact holes
112
having a width
114
and a distance
116
between the outer perimeter of adjacent contact holes
112
. An important parameter of the mask is the duty ratio of the holes which is equal to the distance
116
between the outer perimeter of adjacent contact holes
112
divided by the width
114
of the contact holes
112
.
FIGS. 2
shows a top view and
FIG. 3
a cross section view of a conventional rim type phase shifting mask, using attenuating phase shifting material, used to form contact holes in both a dense pattern and a more isolated pattern. The mask shown in
FIGS. 2 and 3
has contact holes
118
on a dense pattern with a low duty ratio and contact holes
120
on a more isolated pattern having a larger duty ratio. As can be seen in the cross section view shown in
FIG. 3
, the mask is a rim type mask formed on a transparent mask substrate
122
, such as quartz, having a rim of attenuating phase shifting material
124
surrounding both the holes
118
on a dense pattern and the holes
120
on a more isolated pattern. Opaque material, such as chrome
126
, is formed over the mask except those regions of the mask occupied by the transparent holes
118
on a dense pattern, the transparent holes
120
on a more isolated pattern, and the attenuating phase shifting rims surrounding the holes,
118
and
120
.
The mask shown in
FIGS. 2 and 3
is used with a photolithographic projection system, shown schematically in FIG.
24
. The projection system has an illumination system
40
, a mask holder
44
, an objective lens
48
, and a wafer holder
50
. A mask
46
is placed in the mask holder
44
and a wafer
52
is placed on the wafer holder. Light
42
from the illumination system
40
is used to project the pattern in the mask
46
onto the wafer
52
. The sigma of the light
42
from the illumination system
40
can be varied by the illumination system. The value of sigma is a measure of the degree of coherence of the light
42
from the illumination system. For completely coherent light the value of sigma is zero. For incoherent light the value of sigma is 1.0. Projection systems typically use light having a value of sigma of about 0.6.
Problems can be encountered in using this system to transfer a mask pattern, such as that shown in
FIGS. 2 and 3
, to a substrate. At lower sigma settings for the light
42
from the illumination system
40
, see
FIG. 24
, the depth of focus is greater for the isolated pattern than for the more dense pattern, however side lobe effect problems become more significant. At higher sigma settings for the light
42
from the illumination system
40
the depth of focus is smaller for the isolated pattern than for the more dense pattern, however side lobe effect problems are decreased. The depth of focus can be increased by increasing the transmittance of the attenuating phase shifting material
124
, see
FIG. 3
, but higher transmittance attenuating phase shifting material increases the problem of side lobe effect for the part of the pattern having dense holes
118
. The side lobe effect for the part of the pattern having dense holes
118
can be reduced by increasing the value of sigma for the light
42
from the illumination system
40
but this will also reduce the depth of focus for the part of the pattern having isolated holes
120
.
It is a principle objective of this invention to provide a mask for forming contact holes having both a dense hole pattern and a more isolated hole pattern which avoids the problem of side lobe effect and has a good depth of focus for both the dense hole pattern and isolated hole pattern.
It is another principle objective of this invention to provide a method of forming a mask for forming contact holes having both a dense hole pattern and a more isolated hole pattern which avoids the problem of side lobe effect and has a good depth of focus for both the dense hole pattern and isolated hole pattern.
It is another principle objective of this invention to provide a method of forming contact holes in an integrated circuit wafer, wherein the contact holes have both a dense hole pattern and a more isolated hole pattern, which avoids the problem of side lobe effect and has a good depth of focus for both the dense hole pattern and isolated hole pattern.
These objectives are achieved with a rim type contact hole mask using attenuating phase shifting material having a first transmittance for the rims around the holes in the dense hole pattern region and a second transmittance, greater than the first transmittance, for the rims around the holes in the isolated hole pattern region. The transmittance of the attenuating phase shifting material is adjusted by chemical treatment of the attenuating phase shifting material. In this chemical treatment of the attenuating phase shifting material a first chemical treatment changes both the transmitt
Chou Wei-Zen
Lin Chia-Hui
Tzu San-De
Ackerman Stephen B.
Prescott Larry J.
Rosasco S.
Saile George O.
Taiwan Semiconductor Manufacturing Company
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