Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-06-02
2001-09-18
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C250S216000, C438S069000
Reexamination Certificate
active
06291811
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a solid state image sensing device and, more particularly, to a solid state image sensing element, a process of fabrication thereof and a solid state image sensing device equipped with the solid state image sensing element.
DESCRIPTION OF THE RELATED ART
A CCD (Charge Coupled Device) type image sensing device is a typical example of the solid state image sensing device, and is described hereinbelow. However, the following description is applicable to another kind of solid state image sensing device such as a MOS (Metal-Oxide-Semiconductor) type solid state image sensing device.
FIG. 1
illustrates the first prior art slid state image sensing device. A p-type well
1
is formed in a surface portion of an n-type silicon substrate
2
, and an n-type impurity region
3
is nested in the p-type well
1
. A heavily-doped p-type impurity region
4
is formed over the n-type impurity region
3
, and the heavily-doped p-type impurity region
4
and the n-type impurity region
3
form a p-n junction serving as a photo diode.
An n-type charge transfer region
5
is further formed in the p-type well
1
, and is spaced from the photo diode, i.e., the n-type impurity region
3
and the heavily-doped p-type impurity region
4
. Though not shown in
FIG. 1
, photo diodes are arranged along the n-type charge transfer region
5
, and the photo diodes and the n-type charge transfer region
5
form in combination an image sensing line. A heavily doped p-type impurity region
6
is formed in such a manner as to of the image sensing line, and electrically isolates the photo diodes and the n-type charge transfer region
5
from adjacent image sensing lines. Thus, a large number of photo diodes are arrayed in the p-type well
1
. However, description is focused on only one of the photo diodes and the n-type charge transfer region
5
.
A read-out transistor
7
is associated with the photo diode and the n-type charge transfer region
5
. In detail, a surface portion of the p-type well
1
between the photo diode and the n-type charge transfer region
5
provides a channel region
7
a
, and the channel region
7
a
is covered with a gate oxide layer
7
b
. A charge transfer electrode
7
c
is formed on the gate oxide layer
7
b
, and is covered with a silicon oxide layer
8
. The silicon oxide layer
8
is over-lain by a photo shield layer
9
, and an opening
9
a
is formed in the photo shield layer
9
over the photo diode. For this reason, image-carrying light is incident onto the photo diode through the opening
9
a
, and the n-type charge transfer region
5
is prevented from the light.
The photo shield layer
9
is covered with a transparent insulating layer
10
, and the opening
9
a
is filled with the transparent material. A thick photo resist layer
11
is laminated on the transparent insulating layer
10
, and provides a flat upper surface
11
a
. An on-chip lens
12
is formed on the flat upper surface
11
a
, and is located over the photo diode so as to focus the image carrying light on the photo diode. The thick photo resist layer
11
is made from photo resist solution through a baking. The on-chip lens
12
is also made from a piece of photo resist. A photo resist layer is patterned into pieces of photo resist through lithographic techniques, and the piece of photo resist thermally cured at 150 degrees to 200 degrees in centigrade. Then, the piece of photo resist is shaped into a semi-spherical configuration as shown.
The second prior art solid state image sensing device is disclosed in Japanese Patent Publication of Unexamined Application (JPA) No. 2-65171, and
FIG. 2
illustrates the second prior art solid state image sensing device. A p-type well
21
is formed in a surface portion of an n-type silicon substrate
22
, and an n-type impurity region
23
is nested in the p-type well
21
. A heavily-doped p-type impurity region
24
is formed over the n-type impurity region
23
, and the heavily-doped p-type impurity region
24
and the n type impurity region
23
form a p-n junction serving as a photo diode.
An n-type charge transfer region
25
is further formed in the p-type well
21
, and is spaced from the photo diode. The photo diode and the n-type charge transfer region
25
form an image sensing line together with other photo diodes. A heavily doped p-type impurity region
26
is formed in such a manner as to surround the image sensing line, and electrically isolates the photo diodes and the n-type charge transfer region
25
from adjacent image sensing lines.
A read-out transistor
27
is associated with the photo diode and the n-type charge transfer region
25
, and comprises a channel region
27
a
, a gate oxide layer
27
b
over the channel region
27
a
and a charge transfer electrode
27
c
formed on the gate oxide layer
27
b
. The charge transfer electrode
279
is covered with a silicon oxide layer
28
, and the silicon oxide layer
28
is overlain by a photo shield layer
29
. An opening
29
a
is formed in the photo shield layer
29
over the photo diode, and allows image-carrying light to be incident onto the photo diode through the opening
29
a
. The photo shield layer
29
prevents the n-type charge transfer region
25
from the incident light. The photo shield layer
29
is topographically covered with a transparent insulating layer
30
, and the transparent insulating layer
30
forms a deep recess
30
a
. The deep recess
30
a
is located over the photo diode. The deep recess
30
a
is partially filled with silica glass, and the piece of silica glass
31
forms a curved upper surface
32
. The curved upper surface
32
forms a shallow recess nested in the deep recess
30
a
. The shallow recess is filled with silicon nitride, and the silicon nitride has a refractive index larger than the silica glass. For this reason, the piece of silicon nitride
33
serves as a lens. The upper surface of the lens
33
is planarized as shown.
The on-chip lens
12
occupies the wide area over the photo diode
3
/
4
and the n-type charge transfer region
5
, and gathers the incident light fallen thereonto. For this reason, the photo diode
3
/
4
is sensitive to the variation of the incident light. However, the first prior art solid state image sensing device encounters a problem inhigh price. As described hereinbefore, the on-ship lens
12
is formed of photo resist solidified through the baking, and, accordingly, is brittle. The brittle on-chip lens is liable to be broken during the fabrication of the first prior art solid state image sensing device, and decreases the production yield. This makes the price of the first prior art solid state image sensing device high.
Another reason for the high price is serious influences of dust. The on-chip lenses
12
project from the flat upper surface
11
a
of the photo resists layer
11
, and form valleys therebetween. If a dust particle falls into the valley, the dust particle is hardly eliminated from the valley, and makes the product defective. For this reason, the first prior art solid state image sensing device requires extremely high cleanliness, and such an extremely high clean ambience increases the production cost of the first prior art solid state image sensing device.
Yet another reason for the high price is a complicated packaging structure. The on-chip lens
12
has the exposed curved surface. If the exposed curved surface is held in contact with transparent layer which has a large refractive index, the on-ship lens
12
loses the convergent function. For this reason, the on-chip lens
12
is required to be exposed to the air, or is covered with an extremely low refractive index material layer. The manufacturer takes these requirements into account, and designs the package for the first prior art solid state image sensing device. The package is complicated, and increases the production cost.
The second prior art solid state image sensing device is less costly, because the curved surface of the lens
33
is embedded into the piece of silica glass
31
. However, the second pr
Lee John R.
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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