Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-06-29
2003-01-07
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S292000, C257S461000, C257S462000
Reexamination Certificate
active
06504193
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-185848, filed Jun. 30, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to solid-state image device which is used in CCD (Charge Coupled Device) and CMOS image sensors.
A cause of noise included in signals transmitted from a solid-state image device is the surface recombination phenomenon observed on the surface of the photodiode.
FIG. 8
is a schematic diagram illustrating the structure of a CMOS image sensor of conventional art used in a solid-state image device. The CMOS image sensor has in principle a photodiode
107
and a gate electrode
105
. The photodiode
107
receives photons, converts them into electric signals, and accumulates electric charge. The gate electrode
105
is used to read out the charge accumulated in the photodiode
107
.
In this CMOS image sensor, a P-well region
102
is formed on a P-type semiconductor substrate
101
, for example. An element isolation film
103
is selectively formed on the surface of this P-well region
102
. within the region confined by the element isolation film
103
, a gate oxide film
104
is formed on the surface of the P-well region
102
. On the gate oxide film
104
and almost in the center of the region confined by the element isolation film
103
, the gate electrode
105
is formed. In the channel region in the surface of the P-well region
102
, impurity is introduced to provide an impurity layer
106
. The impurity layer
106
controls the threshold voltage of a MOS transistor containing the gate electrode
105
.
In the P-well region
102
between the element isolation film
103
and the gate electrode
105
, a photodiode layer
107
is formed. The photodiode layer
107
is formed by implanting an N-type impurity into the P-well region
102
. T he photodiode layer
107
is formed in self-alignment with the readout gate
105
. o n the surface of the photodiode layer
107
, a surface shield layer (P
+
)
108
is formed. The surface shield layer
108
is formed by ion implanting a high concentration of a P-type impurity into the P-well region
102
. This surface shield layer
108
works to prevent depletion of the surface of the photodiode layer
107
and formed in self-alignment with the gate electrode
105
.
In the P-well region
102
to the opposite side of the surface shield layer
108
beyond the gate electrode
105
, a drain region
109
is formed. When a voltage is applied to the gate electrode
105
, the charge read out from the photodiode layer
107
is transferred to the drain region
109
. The drain region
109
is formed in self-alignment with the readout gate
105
. Part of the gate oxide film
104
, located on the drain region
109
, is removed to form a drain electrode
110
there. The charge transferred to the drain region
109
is supplied to a detector (not shown) through the drain electrode
110
.
In the conventional CMOS image sensor having a photodiode layer
107
and a gate electrode
105
, surface recombination (generation of dark current) in the photodiode layer
107
is prevented by forming a surface shield layer
108
on the surface of the photodiode layer
107
.
This type of CMOS image sensors, however, have a surface shield layer
108
that covers all the surface of the photodiode layer
107
. As a result, a region
111
, which works as a potential barrier, is formed near the gate electrode
105
in the surface shield layer
108
when a voltage is applied to the gate electrode
105
to read out the charge accumulated in the photodiode layer
107
. This region
111
extends above the photodiode layer
107
toward the side surface of the channel region which is formed under the gate electrode
105
. Then it becomes hard to read out charge because of this potential barrier
111
when the charge is read out from the photodiode layer
107
. This phenomenon poses a problem that the gate electrode
105
requires a high voltage for reading out charge from the photodiode layer
107
.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a solid-state image device and method of manufacturing the same that can prevent the generation of dark current and transfer charge in the charge accumulation layer with a low voltage.
According to a first aspect of the present invention, there is provided a solid-state image device comprising: a semiconductor layer of a first conductivity type; a charge accumulation layer of a second conductivity type formed in the semiconductor layer, the charge accumulation layer performing photoelectric conversion and accumulating charge; a gate electrode formed on an insulator film on the semiconductor layer and reading out charge in the charge accumulation layer, the gate electrode being positioned above part of the charge accumulation layer; a drain region of the second conductivity type formed in the surface of the semiconductor layer corresponding to one side of the gate electrode, to which charge read out from the charge accumulation layer is transferred by the gate electrode; a punch-through stopper region of the first second conductivity type formed between the drain region and the charge accumulation layer; and a shield layer of the first conductivity type formed on the surface of the semiconductor layer corresponding to another side of the gate electrode, the shield layer contacting a surface of the charge accumulation layer.
According to a second aspect of the present invention there is provided a solid-state image device comprising: a semiconductor layer of a first conductivity type; a gate electrode formed on an insulator film on the semiconductor layer; a charge accumulation layer of a second conductivity type formed to one side of the gate electrode, the charge accumulation layer performing photoelectric conversion and accumulating charge; a drain region of the second conductivity type formed in the surface of the semiconductor layer to another side of the gate electrode, to which charge read out from the charge accumulation layer is transferred via the gate electrode; and a shield layer of the first conductivity type formed in the surface of the semiconductor layer on the charge accumulation layer, wherein the position of the charge accumulation layer is shifted from the surface shield layer toward the gate electrode.
According to a third aspect of the present invention there is provided a method of manufacturing a solid-state image device comprising the steps of: forming a gate insulator film on the surface of a semiconductor layer of a first conductivity type; forming a charge accumulation layer by implanting a second conductivity type impurity into the semiconductor layer, the charge accumulation layer performing photoelectric conversion and accumulating charge; forming an impurity layer by implanting the first conductivity type impurity into the semiconductor layer to control the threshold voltage; forming a gate electrode on the gate insulator film, the position of the gate electrode being shifted toward one end of the charge accumulation layer; forming a punch-through stopper region of the first conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the punch-through stopper region confining one end of the charge accumulation layer; forming a shield layer of the first conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the shield layer contacting another end of the charge accumulation layer; and forming a drain region of the second conductivity type in the semiconductor layer, in self-alignment with the gate electrode, the drain region contacting the punch-through stopper region.
According to a fourth aspect of the present invention there is provided a method of manufacturing a solid-state image device comprising the steps of: forming a gate insulator film on the surface of a first conductivity type semiconductor layer; forming
Iida Yoshinori
Ishiwata Hiroaki
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