Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Reexamination Certificate
2000-12-21
2003-02-11
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
C430S005000, C430S394000, C430S312000, C430S322000, C430S396000
Reexamination Certificate
active
06517982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an exposure method for use in photolithography and a mask for use in photolithography. More particularly, the invention relates to a phase-shift mask and an exposure method using the phase-shift mask.
2. Description of the Related Art
To form patterns of semiconductor elements, a photolithography technique is commonly employed. A pattern of a mask is transferred to a photosensitive resin layer provided on a semiconductor substrate by the photolithography technique. The photosensitive resin is also known as “resist.” A resist is classified into two type, i.e., negative type and positive type. The negative-type resist is of the type; any part of which that has been exposed to the light applied through a mask will remain on the semiconductor substrate. The positive-type resist is of the type; any part of which that has been exposed to the light applied through a mask will be removed from the semiconductor substrate.
In recent years, it has been demanded that an image be formed on a resist layer in higher resolution to provide fine patterns of semiconductor elements. Fine semiconductor element patterns increase integration density of a semiconductor integrated circuit.
To enhance the resolution of an image formed on a resist, a phase shift exposure method is proposed in 1982. In the phase shift exposure method, the phase difference between light beams applied is utilized to improve the resolution of the image focused on a resist layer. The principle of the phase shift exposure will be described, with reference to
FIGS. 1A
to
1
D and
FIGS. 2A
to
2
D.
In the ordinary exposure, the light applied perpendicularly to a mask
106
passes through the transparent regions
150
and
151
of the mask as illustrated in FIG.
1
A. Chromium mask patterns
121
are provided on the mask
106
. The mask
106
has transparent regions
150
and
151
. The light beams passing through the transparent regions
150
and
151
have the same phase. The light beams emanate from the transparent regions
150
and
151
and pass through the projection lens of a reducing projection exposure apparatus. The two beams are then focused on the surface of a resist layer, which is on an image-forming surface.
The distance between the transparent regions
150
and
151
cannot be reduced to an infinitesimal value, for the following reason. If the distance is extremely short, the two beams passing the regions
150
and
151
overlaps at the image-forming surface as indicated by the broken lines in FIG.
1
C. The light beams, which have a same phase, intensify each other at the image-forming surface. As a result, the light-intensity distribution on the surface of the resist has one peak as illustrated in solid line in FIG.
1
D. Consequently, the chromium mask patterns
121
are not correctly transferred to the resist layer. Thus, the interval between the transparent regions
150
and
151
cannot be decreased over a certain limit. The limit R of resolution for any image formed on a resist is given as follows:
R=K
1
×&lgr;/NA
(1)
where K
1
is the constant that depends on the properties of the photosensitive resin, &lgr; is the wavelength of the light applied to the mask
106
, and NA is the numerical aperture of the projection lens that is incorporated in the reducing projection exposure apparatus. Here, the limit R is known as “Reyleigh resolution”.
In the phase shift exposure, light is applied to a resist layer through a phase shift mask
107
as is illustrated in FIG.
2
A. The phase shift mask
107
has transparent regions
152
and
153
. The region
153
is provided with a phase shifter
120
, while the region
152
has no phase shifters. The light beam passing through the transparent region
153
is delayed as it passes through the phase shifter
120
. Hence, the light beam passing through the transparent region
153
differs in phase from the light beam passing through the transparent region
152
. The thickness D that the phase shifter should have to impart a phase difference of 180° to the light beams is given as follows:
D=&lgr;/{
2×(
n−
1)} (2)
where &lgr; is the wavelength of the light applied to the phase shift mask
107
, and n is the refractive index of the phase shifter
120
. If the two light beams emanating from the transparent region
152
and the transparent region
153
, respectively, have a phase difference of 180°, their parts overlapping at the image-forming surface will cancel out each other. As a result, as shown in
FIG. 2C
, the intensity of light is nil at one part of the surface of the resist layer. It follows that the light-intensity distribution on the resist has two peaks as shown in FIG.
2
D. The chromium patterns
121
can therefore be transferred to the resist with high accuracy. Thus, the use of the phase shift mask
107
can enhance the resolution of an image focused on the surface of a resist.
Also, the phase shift exposure technique can increase the depth of focus (DOF). The term “depth of focus” means the range of distance over which the focus may be displaced without causing troubles. The reason is discussed comparing the ordinary exposure technique and the phase exposure technique in the following.
In the ordinary exposure using no phase shifters, the more the image-forming surface deviates from the focal plane, the more the two beam emanating from the transparent regions
150
and
151
overlap each other at the image-forming surface. This means that the resolution will sharply decrease if the image-forming surface of the resist deviates from the focal plane.
In the phase shift exposure, the two adjacent beams emanating from the transparent regions
152
and
153
have a phase difference of 180°. Their overlapping parts cancel out each other at the image-forming surface of the resist layer. The intensity of light is therefore zero at one part of the image-forming surface. Hence, even if defocusing occurs, that is, even if the focus deviates from the image-forming surface, the dimensional precision of the pattern, transferred to the resist, will be hardly influenced. Thus, the depth of focus can be increased in the phase shift exposure.
The phase shift exposure technique, however, cannot successfully apply to two-dimensional random patterns. The layout pattern of a semiconductor integrated circuit includes regular patterns and random patterns. Each regular pattern extends in one direction only, whereas each random pattern randomly extends first in one direction, and then in another direction. Here, examples of regular patterns are the bit lines and word lines of a DRAM (Dynamic Random Access Memory). Examples of random patterns are the wires of logic circuits. The phase shift masks are designed in accordance with the basic rule that a phase difference of 180° is imparted to two beams that have passed through two adjacent transparent regions. This basic rule can be easily applied to the regular patterns, but not to two-dimensional random patterns.
FIG. 3A
is a plan view of a phase shift mask
108
that may be used to form two-dimensional random patterns by means of the conventional phase shift exposure. The phase-shift mask
108
is designed to transfer a pattern on a positive-type resist. The mask
108
has a shield region
111
, a transparent region
113
and a transparent region
114
. The shield region
111
is identical in shape to the pattern that is to be transferred to a resist. Phase shifters
120
are provided on the transparent region
113
. The beam passing through the transparent region
113
is out of phase with respect to the beam passing through the transparent region
114
. In other words, the phase of the beam differs by 180° from that of the beam passing through the transparent region
114
.
FIG. 3C
is a sectional view of the Levenson-type mask
108
, taken along line C—C in FIG.
3
A. As
FIG. 3C
shows, the mask
108
is composed of a glass substrate
122
. A chromium film
121
is provided on the shiel
Mohamedulla Saleha R.
NEC Corporation
Young Christopher G.
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