Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2000-02-03
2003-06-03
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S313000, C430S317000, C430S394000, C430S396000
Reexamination Certificate
active
06573027
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device and more particularly to a method of pattern formation using a phase-shift mask.
2. Description of the Related Art
For the purpose of achieving a higher integration in a semiconductor device, formation of a minuter and more densely spaced pattern in the step of photolithography has been being sought after. Accordingly, while the exposure method with reduced projection is, in general, currently used for the exposure in the step of photolithography, the phase-shift method is employed therein so as to raise the limit of resolution further and bring about formation of a still minuter and more densely spaced pattern.
The basic principle of the phase-shift method is briefly described below, taking the case of forming the line and space (L/S) patterns by the Levenson-type phase-shift method. In this case, there is utilized an alternating phase-shift mask in which lines are formed of a light-shielding material on a transparent substrate and phase shifters are placed every other opening sections (space sections). The light passing through an opening section of this mask travels through a lens and produces an image on a resist film lying on a wafer. If the distance between two neighboring opening sections becomes considerably short, when the normal mask without a phase shifter is used, the diffracted lights traveling from these two neighboring opening sections are equal in phase so that, through their mutual interference, the images of these two neighboring opening sections cannot separate from each other. In contrast with this, with a phase-shift mask, phase difference between two lights from each neighboring opening thereof is 180 degrees and so, the two lights interfere each other, diffracted lights are destroyed and, consequently, the images of these two neighboring opening sections separate from each other.
The phase-shift method of this sort is used for the formation of repetition patterns such as the L/S patterns, solitary patterns, random patterns and the likes and applied to the fabrication of various semiconductor devices including bit lines of a DRAM (Dynamic Random Access Memory) and gate patterns of a CMOS (Complementary Metal-Oxide-Semiconductor). For example, in Proc. SPIE (Proceedings of Society of Photo-optical Instrumentation Engineering), Vol. 3051, pp. 342-351 (1997), there is reported a study in which the phase-shift method is applied to obtain 0.16 &mgr;m CMOS gate patterns.
The general method to form minute gate patterns by the phase-shift method is described below.
In this method, two exposures are made using two different masks shown in FIGS.
2
(
a
) and (
b
) separately for respective exposures and, with a composite image obtained from the exposures through these two masks, gate patterns are formed on a wafer. FIG.
2
(
a
) is a view showing the layout of a phase-shift mask (mask A) in which light-shielding sections for lines
22
and phase shifter sections
23
are formed on a transparent substrate
21
. The patterns seen in the light-shielding sections for the lines
22
are the very patterns required to be formed. FIG.
2
(
b
) is a view showing the layout of the other mask (mask B) in which a light-shielding section for protection
24
is formed in order to protect, against the second exposure, portions of a positive resist that are placed in the position of the gate patterns required to be formed, after the first exposure with the mask A is performed.
FIG. 3
presents patterns which are transcribed on the positive resist lying on a wafer, using the aforementioned mask A, mask B or both. FIG.
3
(
a
) shows transcription patterns obtained by the first exposure with the mask A. Ring-shaped patterns are therein transcribed, which indicates patterns other than those formed by the light-shielding sections for the lines
22
are also transcribed. The sections of each ring-shaped patterns are formed by the edge of each phase shifter section. This results from a fact that the amplitude of the light is weakened at the edge sections around the phase shifter sections
23
and, thus, the light is shielded in substance. FIG.
3
(
b
) shows the positioning relation between patterns transcribed by the first exposure with the mask A and patterns transcribed by the second exposure with the mask B. At the time of the second exposure, the mask B must be aligned in such a way that the portions of the resist placed in the position of the gate patterns required to be formed are well protected by the light-shielding section for protection
24
. FIG.
3
(
c
) showed resist patterns obtained, after the second exposure following the first exposure is made and then a subsequent development is carried out. Since irradiation is applied, at the second exposure, to the superfluous part of the patterns that are transcribed at the first exposure by shielding the light, the resist in that part is removed by the development, and thereby the prescribed resist patterns are formed.
Next, a method in which resist patterns are formed using two different masks described above and then the gate patterns are formed on a silicon substrate is described.
FIG. 4
is two sets of schematic cross-sectional views illustrating the steps of this formation method.
FIGS. 4
(
a
1
)-(
a
4
) is cross-sectional views taken along the line A—A′ of FIG.
3
(
b
) and
FIGS. 4
(
b
1
)-(
b
4
) are cross-sectional views taken along the line B—B′ of FIG.
3
(
b
).
First, upon a silicon substrate
101
, a gate oxide film
102
is formed and thereon a polysilicon film
103
is formed. Further, over that, a positive photoresist film
105
is formed. A first exposure is then applied to this silicon substrate through the mask A (
106
) (
FIGS. 4
(
a
1
), (
b
1
)). Regions of the resist corresponding to the edge sections of the phase shifter sections
23
and the light-shielding sections for the lines
22
in the mask A are not irradiated so that ring-shaped patterns of the unexposed regions shown in FIG.
3
(
a
) are transcribed.
Next, the mask B (
107
) is aligned so as to make its transcription patterns take the positioning relation shown in FIG.
3
(
b
) and, then, through this mask B, a second exposure is made (
FIGS. 4
(
a
2
), (
b
2
)). By this second exposure, the part of patterns of the unexposed regions caused by the edge sections of the phase shifter sections
23
in the mask A is also irradiated.
Subsequently, by performing a development, resist patterns
109
taking the shape shown in FIG.
3
(
c
) are formed (
FIGS. 4
(
a
3
), (
b
3
)). Using these resist patterns as a mask, the polysilicon film
103
is then etched and thereafter the resist
109
which becomes redundant is removed, and thereby gate patterns
110
corresponding to the resist patterns
109
are formed (
FIGS. 4
(
a
4
), (
b
4
)).
The conventional method described above, however, has the following problem. Namely, as the spacing of the patterns narrows, the thinning of the patterns becomes conspicuous. If the final dimensions of the patterns very much shift from the designed values thereof because of that, various problems such as a decrease in the yield, a lowering of the reliability, the deterioration of element characteristics and the like are brought about.
The reason why such a thinning of patterns takes place is described, taking the case of the manufacturing method described above. In FIG.
3
(
b
), if the spacing of the patterns or the distance between the lines (W
1
) narrows, the margin (W
2
) between the mask A and the mask B becomes small. When the margin (W
2
) becomes considerably small like this, the unexposed resist sections
108
that should be protected become liable to be affected by stray light. Consequently, the width (W
3
) of the resist patterns formed after performing the development is reduced, or in other words, the thinning of the patterns takes place.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a manufacturing method of a semiconductor d
Baxter Janet
Katten Muchin Zavis & Rosenman
NEC Corporation
Walke Amanda C.
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