Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1999-09-30
2002-10-08
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S315000, C430S324000, C438S217000, C438S238000, C438S254000
Reexamination Certificate
active
06461796
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a photolithography technique and, more particularly, to a method of manufacturing a semiconductor device, including a step of forming wells in a semiconductor substrate using the photolithography technique.
2. Description of the Background Art
A so-called lithography technique of forming or mass producing patterns of miniaturized devices or circuits by making use of materials sensitive to light or radiation, has been used in a process of manufacturing semiconductor devices, typically semiconductor integrated circuits.
A process of forming a triple well structure as one example of the process of manufacturing a semiconductor devices using the photolithography technique will be described below. The triple well structure is characterized in that an N well and a P well are provided in a surface layer of a semiconductor substrate in such a manner as to be adjacent to each other, and a bottom N well (hereinafter, referred to as “BN well”) is provided on the underside of the P well. The BN well is provided for preventing latch up caused by a pnpn thyristor circuit formed when a transistor is formed on the wells provided in the surface layer of the semiconductor substrate. In addition, the BN well may be formed not on the underside of the P well but on the underside of the N well.
FIGS. 20
to
23
are partial sectional views illustrating steps of forming the above-described triple well structure. First, as shown in
FIG. 20
, an underlying film
102
, typically a silicon oxide film is formed on a semiconductor substrate
101
and an N well
103
is formed in a surface layer of the semiconductor substrate
101
using the conventional lithography technique. Then, an acid generating chemical amplification resist film
106
of positive type is formed on the semiconductor substrate
101
.
The acid generating chemical amplification resist film
106
is made from a resist containing an acid generating agent and an alkali-soluble resin to which a solution suppressing base is introduced. In a case where the positive type acid generating chemical amplification resist film is used, when the resist film is exposed to exposure rays, acid is generated in the resist film and the solution suppressing base is decomposed by the acid functioning as a catalyst, with a result that the resist film is made alkali-soluble; while, when the resist film is not exposed to exposure rays, since acid is not generated in the resist film, the resist film is left as alkali-insoluble.
Next, as shown in
FIG. 21
, using a mask having a non-mask region and a mask region, a portion of the resist film
106
positioned under the non-mask region of the mask is irradiated with monochromatic rays, to be made alkali-soluble. Then, the alkali-soluble portion of the resist film
106
is removed by subjecting to baking treatment and development treatment, thereby forming a resist mask
106
.
At this time, there remains the resist film
106
under the mask region of the mask. Because of the effects of standing waves formed by monochromatic rays used as exposure rays as well as of such an uneven intensity distribution of the exposure rays that the intensity of the incident rays at the lower portion of the resist film is smaller than that at the upper portion of the resist film, as shown in
FIG. 21
, the side wall of the remaining portion of the resist film
106
is formed with a taper portion
106
a
, that is, a portion horizontally broadened toward the bottom. In this specification, the horizontal distance between the upper edge of the side wall and the bottom edge of the portion horizontally broadened from the side wall is hereinafter called the width of the taper. That is to say, the distance W shown in
FIG. 21
is called the width of the taper.
After formation of such a resist mask
106
, as shown in
FIG. 22
, ions of a P-type impurity such as boron are implanted in the surface layer of the substrate
101
via the resist mask
106
, to form a P well
104
, and then ions of an N-type impurity such as phosphorous are implanted in the surface layer of the substrate
101
at a higher energy using the resist mask
106
, to form a BN well
105
on the underside of the P well
104
. The resist mask
106
is then removed, to form a triple well structure shown in FIG.
23
. Here, with respect to the BN well
105
thus formed, since the taper portion
106
a
is formed on the side wall of the resist mask
106
as described above, a portion curved upwardly (called a BN extension portion
105
is formed at the end portion, positioned under the taper
106
a
, of the BN well
105
as shown in FIG.
22
.
At the above-described lithography step, as shown in
FIG. 24
, at a portion of the non-mask region where is separated a specific distance from the edge of the mask region of a mask
107
, the exposure rays are allowed to pass therethrough in all directions without any cutoff by the mask
107
. Accordingly, incident rays (for example, incident rays
108
a
and
108
b
) reach a bottom region
106
b
of the resist film
106
positioned under the above portion of the non-mask region.
On the other hand, at a portion of the non-mask region, near the edge of the mask region of the mask
107
, the exposure rays traveling from the mask region side are not allowed to pass therethrough because of cutoff by the mask
107
. Accordingly, the exposure rays traveling from the mask region side never reach a bottom region
106
c
of the resist film
106
positioned under the above portion of the non-mask region.
Further, since each incident ray radiated on the resist film
106
is absorbed in the resist film
106
, the intensity of the incident ray becomes decayed along with the advance of the incident ray in the resist film
106
. After all, the intensity of the incident ray, when it reaches the bottom of the resist film
106
, becomes quite weak. While part of incident rays reaching the bottom of the resist film
106
is reflected thereby, the intensities of the reflected rays are very weak because the incident rays are decayed at the bottom of the resist film
106
.
As is apparent from the above description, the bottom region
106
b
positioned under the portion of the non-mask region separated the specific distance from the edge of the mask region of the mask
107
receives the exposure rays in all directions although the intensities thereof are weaker than those at the upper portion of the resist film
106
, and consequently the bottom region
106
b
of the resist film
106
c
an obtain the intensities of the exposure rays which are large enough to make the resist film
106
alkali-soluble.
On the contrary, the bottom region
106
c
positioned under the portion of the non-mask region near the edge of the mask region of the mask
107
receives only the exposure rays traveling in the limited directions, the intensities of the exposure rays being, as described above, weaker than those at the upper portion of the resist film
106
, and consequently the bottom region
106
c
of the resist film
106
cannot obtain the intensities of the exposure rays which are enough to make the resist film
106
alkali-soluble.
As a result, the bottom region
106
c
of the resist film
106
positioned under the portion of the non-mask region near the edge of the mask region of the mask
107
is left as alkali-insoluble, and is not removed after development treatment. In this way, as shown in
FIG. 21
, the side wall of the resist film
106
is formed with the portion horizontally broadened toward the bottom, that is, the taper portion
106
a.
When the resist film is irradiated with monochromatic rays, standing waves occur in the resist film by interference between the incident rays and the reflected rays from the surface of the semiconductor substrate. Since portions of the resist film corresponding to the middle of the nodes (called antinodes hereunder) of the standing waves are strongly sensitized and portions of the resist film c
Chacko-Davis Daborah
Huff Mark F.
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
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