Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2000-12-08
2003-08-05
Abraham, Fetsum (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S226000, C257S215000, C257S221000, C257S233000
Reexamination Certificate
active
06603144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid-state imaging device and method, and more particularly to a solid-state imaging device having an overflow drain (hereinafter referred to as OFD) structure for draining off an excess electrical charge generated in a photodiode.
2. Related Art
A first example of prior art is the horizontal overflow drain structure shown in
FIG. 8
to
FIG. 10
, of which
FIG. 8
is a plan view of the structure,
FIG. 9
is a cross-sectional view of the structure along the cutting line Z-Z′ of FIG.
8
and the potential distribution in the operating condition, and
FIG. 10
is a cross-sectional view along the cutting line U-U′ of FIG.
8
and the potential distribution in the operating condition.
In the first prior art example of
FIG. 9
, the difference with respect to the present invention is the existence of a shutter gate
34
over the gate oxide film
48
, the shutter gate
34
is a gate of a surface channel transistor in the P well
42
. The other elements of the first layer polysilicon gate
31
, the readout gate
33
, the P
+
channel stopper
43
, the charge transfer N well
44
, and the interlayer insulation film
51
, are the same as in the structure of the present invention. Control of OFD operation is done by applying a voltage to the shutter gate
34
, so as to drain off electrons
49
in the photodiode
45
through a surface channel.
A second example of prior art is the vertical-type overflow drain structure shown in
FIG. 11
to
FIG. 13
, of which
FIG. 11
is a plan view thereof, FIG.
12
(
a
) is a cross-sectional view along the cutting line V-V′ in
FIG. 11
, FIG.
12
(
b
) is a potential distribution diagram in the operating condition, and
FIG. 13
is a potential distribution diagram in the operating condition along the cutting line W-W′ of
FIG. 12
, in which the upper line is the potential when electrons
79
are accumulated in the photodiode N well
76
, and the lower line is the potential when the electrons
79
accumulated in the photodiode N well
76
are read out to the charge transfer N well
74
by applying a voltage to the readout gate
63
so as to raise the surface potential of the P well
72
.
In a vertical-type overflow drain structure, photodiode N well
76
on the opposite side from the charge transfer N well
74
is in contact with the P
+
channel stopper
73
, and the electrons
79
accumulated in the photodiode N well
76
are controlled by the voltage applied to the N-type substrate
71
. In this case, in order that electrons from parts other than the photodiode N well
76
are not pulled out to the N-type substrate
71
, a second P well
80
is provided. The other elements of a first layer polysilicon gate
61
, a readout gate
63
, a charge transfer N well
74
, and an interlayer insulation film
81
, are the same as the structure of the present invention.
In a solid-state imaging device of the prior art as described above, however, there are the following problems.
First, in the first prior art example, in order to increase the red sensitivity of the part made up of the photodiode N well
46
and photodiode cap layer
37
of the photodiode part, it is necessary to form an N-type region as far as a deep position (1 to 2 &mgr;m) in the substrate, so that there was a tendency for variations to occur in the potential of the photodiode part (potential B in
FIG. 9
) during operation. For this reason, by considering the manufacturing margin in the impurity concentration in the photodiode N well
46
, it was necessary to make the shutter gate voltage of the OFD part high.
In the second example of prior art, because it is necessary to create a second P well
72
at a deep position (3 to 4 &mgr;m) within the substrate, there was a tendency for variations to occur in that position. For this reason, there was a tendency for variations to occur in the substrate applied potential for pulling electrons
79
away from the photodiode N well
76
. Additionally, it was necessary to apply a high voltage (15 V or higher) to the substrate.
Accordingly, it is an object of the present invention to provide a solid-state imaging device with a vertical-type overflow drain structure, which reduces the variations in the amount of accumulated charge in the photodiode part, and that enables the drain off of electrons accumulated in the photodiode with a low control voltage and having good repeatability.
SUMMARY OF THE INVENTION
To achieve the above-noted object, the present invention has the following basic technical constitution.
Specifically, a first aspect of the present invention is a solid-state imaging device comprising; an opto-electrical conversion well of a first conductivity type formed on a substrate, a separation layer of a second conductivity type to separate the opto-electrical conversion well of the first conductivity type so as to form a plurality of photodiodes, a cap layer of the second conductivity type formed on a surface of the opto-electrical conversion well, a charge drain control layer of the second conductivity type formed within the opto-electrical conversion well of the first conductivity type, a photodiode well and a charge drain well of the photodiode formed by providing the charge drain control layer, formed within the opto-electrical conversion well of the first conductivity type.
In a second aspect of the present invention, the cap layer, the separation layer, and the charge drain control layer have depths that increase in this sequence.
In a third aspect of the present invention, the cap layer, the separation layer, and the charge drain control layer have impurity concentrations that increase in this sequence.
In a fourth aspect of the present invention, a width W
1
of the separation layer is greater than a width W
2
of the charge drain control layer.
In a fifth aspect of the present invention, a width of the charge drain control layer is formed to be greater than 1 &mgr;m so as to obtain an overflow drain operation mode device.
In a sixth aspect of the present invention, a width of the charge drain control layer is formed to be at least 1 &mgr;m so as to obtain an shutter operation mode device.
A method of the present invention is a method of solid-state imaging device comprising an opto-electrical conversion well of a first conductivity type formed on a substrate, a separation layer of a second conductivity type to separate the opto-electrical conversion well of the first conductivity type so as to form a plurality of photodiodes, a cap layer of the second conductivity type formed on a surface of the opto-electrical conversion well, a charge drain control layer of the second conductivity type formed within the opto-electrical conversion well of the first conductivity type, a photodiode well and a charge drain well of the photodiode formed by providing the charge drain control layer, formed within the opto-electrical conversion well of the first conductivity type, wherein the method comprising the steps of; a first step of forming the separation layer by ion implantation, a second step of forming the cap layer by ion implantation, and a third step of forming the charge drain control layer by ion implantation over the separation layer.
REFERENCES:
patent: 4268845 (1981-05-01), Koike et al.
patent: 2001/0004116 (2001-06-01), Tsunai
patent: 57-15477 (1982-01-01), None
patent: 60-31111 (1985-07-01), None
patent: 5-335546 (1993-12-01), None
patent: 6-89998 (1994-03-01), None
Abraham Fetsum
Choate Hall & Stewart
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