Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1997-09-18
2001-08-28
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S291000, C257S292000, C257S232000, C438S073000
Reexamination Certificate
active
06281533
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an amplifier type solid state imaging apparatus formed on a silicon substrate and, more particularly, it relates to a solid state imaging device wherein each unit cell has a photodiode processed for device separation by means of a silicon oxide film formed by oxidizing the silicon substrate. The present invention also relates to a solid state imaging apparatus wherein the semiconductor substrate of the apparatus has for each unit cell a region located at a position deeper than the depletion layer region operating as a photodiode, in which region the impurity concentration of the semiconductor slowly increases as a function of the depth in the substrate and part of the signal charges generated in the semiconductor substrate are collected by a signal storage to provide a high dynamic range. The present invention further provides a method of manufacturing such a solid state imaging apparatus and a video system realized by using such a solid state imaging apparatus.
Solid state imaging apparatuses comprising an amplifier type sensor have been developed in recent years. Such apparatuses are featured by detecting optical signals by means of a photoelectric converter/storage and amplifying them in the vicinity of the photoelectric converter/storage.
An amplifier type MOS sensor typically comprises in each unit pixels or unit cell thereof a photodiode and amplifying means including an amplifier transistor for amplifying the signal charges photoelectrically converted and collected by the photodiode in the silicon substrate.
FIG. 1
of the accompanying drawings schematically illustrates in cross section part of a unit cell of a known amplifier type MOS sensor. As seen from
FIG. 1
, an n-type layer region
12
that constitutes a photodiode with a silicon substrate (p-type layer region)
10
is formed in an oxide film for device separation in a self-aligning manner. A device separating region
16
arranged on a p
+
-layer
14
is a silicon oxide film formed by oxidizing part of the silicon substrate
10
, which silicon oxide film is normally referred to as LOCOS (LOCal Oxidation of Silicon). Reference numerals
18
and
20
in
FIG. 1
denote respectively a contact region and a wiring layer connected to the contact region
18
, whereas reference numerals
22
and
24
denote respectively the gate of a read-out transistor and a planarizing layer.
The silicon substrate
10
is apt to become defective at and near the corresponding end of the LOCOS region
16
due to the stress generated during the local oxidation. The defect, if any, by turn gives rise to an electric current that appear as leak current of the photodiode.
Now, this problem will be discussed by referring to
FIG. 2
of the accompanying drawing.
FIG. 2
is an enlarged cross sectional view showing the boundary of the photodiode and the LOCOS region of FIG.
1
. As shown in
FIG. 2
, a depletion region
26
is formed around the n-type layer region
12
and a depleted region with a large number of defects (multi-defect region)
28
is formed in a lower boundary area of the LOCOS region
16
located adjacent to the n-type layer region
12
. Thus, a large number of electron/hole pairs will be generated by heat via the defect levels in the silicon band gap in the multi-defect regions. Then, electrons can transfer into the photodiode to appear as leak current of the photodiode, which leak current can reduce the sensitivity or the S/N ratio of the solid state imaging apparatus.
Thus, since a photodiode and a photodiode are formed in a self-aligning manner in known solid state imaging apparatus, they are accompanied by the problem of leak current on the part of the photodiode generated due to the defect at and near the corresponding end of the LOCOS region
16
.
BRIEF SUMMARY OF THE INVENTION
Therefore, the object of the present invention is to provide a solid state imaging apparatus that can prevent any leak current from appearing in the photodiode and is resistive against degradation in the sensitivity, a method of manufacturing such a solid state imaging apparatus, and a video system using such a solid state imaging apparatus.
According to a first aspect of the invention, the above object is achieved by providing a solid state imaging apparatus comprising for each unit cell a substrate of a first conductivity type, at least a first impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate for a photodiode for forming a photoelectric conversion region, a device separation region for the device separation of the photodiode, the device separation region having a second impurity layer formed in a lower area thereof, and means for amplifying the signal charges collected by the photodiode, wherein the first impurity layer is separated from the device separation region by a predetermined distance.
According to a second aspect of the invention, there is provided a solid state imaging apparatus comprising for each unit cell a substrate of a first conductivity type, at least a first impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate for a photodiode for forming a photoelectric conversion region, a device separation region for the device separation of the photodiode, the device separation region having a second impurity layer formed in a lower area thereof, and means for amplifying the signal charges collected by the photodiode, wherein the first impurity layer is located adjacent to the second impurity layer and the apparatus further comprises for each unit cell a third impurity layer located adjacent to the second impurity layer and formed continuously at least in part with the surface area of the first impurity region, the third impurity layer having an impurity concentration greater than the second impurity layer.
According to a third aspect of the invention, there is provided a solid state imaging apparatus comprising for each unit cell a substrate of a first conductivity type, at least a first impurity layer of a second conductivity type different from the first conductivity type formed in a surface area of the substrate for a photodiode for forming a photoelectric conversion region, a device separation region for the device separation of the photodiode, the device separation region having a second impurity layer formed in a lower area thereof, and means for amplifying the signal charges collected by the photodiode, wherein the second impurity layer contains a third impurity layer of the first conductivity type located on the side of the first impurity layer and has an impurity concentration greater than the second impurity layer and the first impurity layer is located adjacent to the third impurity layer.
According to a fourth aspect of the invention, there is provided a method of manufacturing a solid state imaging apparatus comprising a first step of forming for each unit cell a first impurity layer in a surface area of a substrate of a first conductivity type by implanting ions of an impurity of a first conductivity type into the substrate to an impurity concentration level higher than the substrate, using a silicon nitride film formed on the substrate as a mask, a second step of forming a device separation region on the first impurity layer, a third step of forming a second impurity layer of a second conductivity type in a surface area of the substrate and a third impurity layer of the second conductivity type in another surface area of the substrate separated from the device separation region by a predetermined distance by implanting ions of an impurity of the second conductivity type different from the first conductivity type, using an electrode formed on the substrate and a resist layer formed on the substrate and the device separation region as a mask and a fourth step of forming a wiring layer on the second impurity layer after removing the resist layer.
According to a fifth aspect of the invention, there is provided a m
Mabuchi Keiji
Miyagawa Ryohei
Oba Eiji
Sasaki Michio
Tanaka Nagataka
Kabushiki Kaisha Toshiba
Mintel William
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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