Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-03-30
2001-04-03
Epps, Georgia (Department: 2873)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C348S302000, C348S294000, C348S308000, C348S335000, C348S340000
Reexamination Certificate
active
06211509
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a solid-state image sensor and, more particularly, to a MOS-type solid-state image sensor including pixel units each having a signal amplification function.
MOS-type solid-state image sensors have recently received a great deal of attention because of the following advantages. A MOS-type solid-state image sensor allows a reduction in size, can be driven by a single power supply, and allows all components such as an image sensing section and peripheral circuits to be integrated into one integrated circuit chip by a MOS process.
The MOS-type solid-state image sensor suffers the problem of leakage of electrons into adjacent pixel units upon incidence of long-wavelength light and the problem of expansion of the depletion layer of a photodiode with a reduction in pixel unit size. These problems will be described below.
Leakage of Electrons into Adjacent Pixel Unit upon Incidence of Long-wavelength Light
FIG. 4
is a schematic sectional view of a structure of a conventional MOS-type solid-state image sensor which corresponds to two pixel units.
As shown in
FIG. 4
, this solid-state image sensor is formed by using a p-type Si substrate
41
on which pixel units arranged in a matrix format, a signal scanning circuit, and the like are formed. Each pixel unit includes a photoelectric conversion portion
42
for photoelectrically converting image-sensing light and an amplification transistor
43
for extracting a signal obtained by the photoelectric conversion portion
42
.
The photoelectric conversion portion
42
has an n-type diffusion layer
45
which forms a photodiode together with the p-type substrate
41
. The photoelectric conversion portion
42
also has an n-type diffusion layer
91
. A signal read gate electrode
46
is formed between the diffusion layers
45
and
91
through a gate insulating film.
The amplification transistor
43
includes a pair of n-type diffusion layers
93
and
95
and a gate electrode
94
formed between the n-type diffusion layers
93
and
95
through a gate insulating film. The gate electrode
94
is connected to the signal read gate electrode
46
though an interconnection
92
. The diffusion layer
95
is connected to a vertical signal line
47
.
On the resultant structure, a light-shielding film
48
and a focusing lens
49
are formed through an insulating film
96
. The focusing lens
49
is a microlens formed in each cell to form an optical image on the photodiode
45
of a corresponding cell. A light-shielding film
48
is formed to optically isolate adjacent cells.
In addition, on the p-type substrate
41
, a field oxide film
44
a
,
44
b
, i.e., a silicon oxide film as an insulator, is formed. The film serves as part of each element isolation region. The portion
44
a
of the field oxide film surrounds each pixel unit to isolate the pixel units from each other. The portion
44
b
of the field oxide film extends in each pixel unit to isolate the photoelectric conversion portion
42
from the amplification transistor
43
.
A shallow diffusion layer
44
-
1
serving as part of each element isolation region is formed under the field oxide film
44
a
,
44
b
. The diffusion layer
44
-
1
is a p-type layer having a higher carrier impurity concentration (lower resistance) than the substrate
41
.
The following problem is posed in the structure shown in FIG.
4
.
Long-wavelength light, e.g., red light, incident on the substrate
41
enters a deep portion of the silicon substrate
41
. The light is then photoelectrically converted into electrons corresponding to electron/photon energy in the deep portion. The signals (electrons) generated in the deep portion of the substrate move within the substrate, and many of the signals become image signals in the corresponding pixel units, but some of them leak and diffuse into adjacent pixel units. In a color image sensor, since object light must be decomposed into R (red), G (green), and B (blue) light components when received, pixel units are arranged as R, G, and B pixel units. Since these pixel units are arranged to be adjacent to each other, leakage/diffusion of signals into adjacent pixel units causes color mixture and blooming.
Expansion of Depletion Layer of Photodiode with Reduction in Pixel Unit Size
The apparatus shown in
FIG. 4
is a MOS-type solid-state image sensor including pixel units each having a signal amplification function. The MOS-type solid-state image sensor having the amplification function can cope with an increase in the number of pixel units and is suitable for a reduction in pixel unit size due to a reduction in image size.
This solid-state image sensor has a structure in which a photodiode of a photoelectric conversion portion and an amplification transistor are arranged side by side in each pixel unit on a single substrate. The potential of a signal charge storage portion is modulated with a signal charge generated by photoelectric conversion in the photoelectric conversion portion, and the amplification transistor in each pixel unit is modulated with the potential. With this operation, an amplification function is imparted to each pixel unit.
In the MOS-type solid-state image sensor having the amplification function, however, with an increase in the number of pixel units, the area of the photoelectric portion of each pixel unit decreases. As a result, the output from each photoelectric conversion portion reduces.
To solve this problem of the reduction in photodetection output (image signal output), the carrier impurity concentration of the semiconductor substrate may be decreased to expand the depletion layer of the photodiode of each photoelectric conversion portion. The decrease in the impurity concentration of the semiconductor substrate increases the diffusion current in the substrate.
In the MOS-type solid-state image sensor in
FIG. 4
, each photodiode is made up of a lightly doped p-type semiconductor substrate and an n-type semiconductor layer formed in the substrate. In this case, the amount of leakage current in each photodiode during a dark period increases depending on the depth at which the depletion layer of the photodiode extends in the semiconductor substrate. As a result, the dynamic range decreases.
In addition, when a pixel unit is irradiated with strong light to generate a large amount of carriers (electrons), especially when the amount of carriers generated exceeds the capacity of the photodiode, the carriers overflow the photodiode. In this case, the carriers leak into the photodiodes of the adjacent pixel units, resulting in considerable deterioration in image quality (blooming).
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a MOS-type solid-state image sensor which can reduce the frequency of the occurrence of color mixture and blooming which are caused by leaking signals generated in a deep portion of a semiconductor substrate owing to the penetration of long-wavelength light.
is another object of the present invention to provide a MOS-type solid-state image sensor which can reduce the influences of diffusion currents from a semiconductor substrate where each pixel unit is reduced in size and the carrier impurity concentration of the semiconductor substrate is decreased. With this sensor, leakage current in each photodiode during a dark period can be reduced, and the dynamic range can be increased. In addition, blooming and color mixture can be suppressed.
According to a first aspect of the present invention, there is provided a solid-state image sensor comprising:
a plurality of pixel units arranged on a semiconductor substrate layer of a first conductivity type in a matrix format, each of the pixel units having a photoelectric conversion portion for photoelectrically converting image-sensing light and a signal extraction portion including a field-effect transistor for extracting a signal from the photoelectric conversion portion;
a scanning circuit connected to the signal extraction portions to sequentially read and transfer the signals obtained by
Ihara Hisanori
Inoue Ikuko
Nakamura Nobuo
Nozaki Hidetoshi
Yamaguchi Tetsuya
Epps Georgia
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Spector David N.
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