Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2001-05-11
2002-12-10
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S232000, C257S233000
Reexamination Certificate
active
06492668
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a solid imaging element, i.e. to a solid imaging device, and further to a method of producing the same.
2. Description of the Background Art
FIG. 13
is a view illustrating a circuit construction of a solid imaging device. Unit pixels or unit cells C are arranged in a matrix form, and each unit cell C is connected to a vertical shift register and a horizontal shift register.
Each unit cell C has a photodiode PD, a transfer switch M
1
, a reset switch M
2
, an amplifier M
3
, and a selection switch M
4
. The transfer switch M
1
, the reset switch M
2
, the amplifier M
3
, and the selection switch M
4
are each formed of a MOS transistor. The photodiode PD serves to perform a function of converting incident light into an electric signal. The signal charge Q obtained by the photodiode PD is transferred by the transfer switch M
1
to a diffusion layer FD (Floating Diffusion) for signal charge conversion. Assuming the capacitance of the FD to be C, the signal charge is converted into a voltage defined by V=Q/C.
FIG. 14
illustrates a pattern layout of each element.
FIG. 15
illustrates a cross-sectional view taken along the line XV—XV of FIG.
14
.
Referring to
FIG. 15
, an isolation oxide film
2
is formed in a principal surface of a semiconductor substrate
1
by the LOCOS (local oxidation of silicon) method. A predetermined impurity is implanted into each of the active regions of the semiconductor substrate
1
surrounded and exposed by the isolation oxide film
2
. The active regions include at least a region serving as the PD (hereafter referred to as a “PD region”) and a region serving as the FD (hereafter referred to as a “FD region”). The PD region and the FD region are in an adjacent positional relationship to sandwich a gate electrode
8
formed above the principal surface of the semiconductor substrate
1
. The side wall of the gate electrode
8
is covered with a side wall spacer.
The inside of the semiconductor substrate
1
contains a stress damage layer
31
generated by action of a mechanical load at the time of forming the isolation oxide film
2
, an etching damage layer
32
generated by etching for forming the side wall spacer, and an ion implantation damage layer
33
generated by ion implantation into the active regions. When a part &Dgr;Q of the electric charge Q is lost as a leakage current through the defects of these damage layers, the sensitivity of the solid imaging device decreases to deteriorate the pixel characteristics.
Therefore, an object of the present invention is to provide a solid imaging device in which the damage layers are reduced as much as possible.
SUMMARY OF THE INVENTION
In order to achieve the aforesaid object, a solid imaging device according to one aspect of the present invention includes a semiconductor substrate having a principal surface, a photodiode including a first diffusion layer formed on the principal surface, and a MOS (Metal Oxide Semiconductor) transistor including a second diffusion layer and a third diffusion layer formed on the principal surface as source/drain regions. The second diffusion layer serves to perform a function of converting a signal charge, which is determined by the photodiode, into a signal voltage. The second diffusion layer has an impurity concentration less than or equal to {fraction (1/10)} of an impurity concentration of the third diffusion layer
By adopting the aforesaid construction, a high concentration diffusion layer is not formed in the FD region serving as the second diffusion layer, thereby restraining the formation of the ion implantation damage layer in the semiconductor substrate at the FD region. Therefore, the leakage current caused by the defects of the damage layers can be reduced. Also, since the concentration of the diffusion layer itself decreases, the electric field at the PN junction interface is alleviated to reduce the leakage current.
In the aforesaid invention, an upper side of the second diffusion layer is preferably covered with an implantation shielding layer for shielding against ion implantation. By adopting this construction, a side wall spacer is not formed on the FD region serving as the second diffusion layer, and a high concentration diffusion layer is not formed in the FD region, either. Therefore, the etching damage layer generated at the time of forming the side wall spacer and the ion implantation damage layer generated at the time of forming the high concentration diffusion layer are not generated in the FD region. Thus, the generation of damage layers can be restrained, thereby reducing the leakage current.
In the aforesaid invention, it is preferable that the MOS transistor includes a side wall spacer formed by etching, an etching protective layer intervenes between the second diffusion layer and the side wall spacer and between the third diffusion layer and the side wall spacer for preventing generation of damages to the principal surface by the etching, and an upper side of the second diffusion layer and an upper side of the third diffusion layer are covered with the etching protective layer. By adopting this construction, generation of an etching damage layer in the FD region serving as the second diffusion layer can be avoided. Since the etching damage layer as well as the ion implantation damage layer generated at the time of forming the high concentration diffusion layer can be reduced, the total amount of damage layers can be reduced, thereby leading to decrease in the leakage current.
In order to achieve the aforesaid object of the present invention, a solid imaging device according to another aspect of the present invention includes a semiconductor substrate having a principal surface, a photodiode including a first diffusion layer formed on the principal surface, and a MOS transistor including a second diffusion layer and a third diffusion layer formed on the principal surface as source/drain regions. The MOS transistor includes a side wall spacer formed by etching. An etching protective layer intervenes between the second diffusion layer and the side wall spacer and between the third diffusion layer and the side wall spacer for preventing generation of damages to the principal surface by the etching. An upper side of the second diffusion layer and an upper side of the third diffusion layer are covered with the etching protective layer. By adopting this construction, generation of an etching damage layer in the FD region serving as the second diffusion layer can be avoided. As a result of this, the total amount of damage layers can be reduced, thereby leading to decrease in the leakage current.
In order to achieve the aforesaid object, a solid imaging device according to still another aspect of the present invention includes a semiconductor substrate having a principal surface, a photodiode including a first diffusion layer formed on the principal surface, and a MOS transistor including a second diffusion layer and a third diffusion layer formed on the principal surface as source/drain regions. The second diffusion layer serves to perform a function of converting a signal charge, which is determined by the photodiode, into a signal voltage. The MOS transistor includes an isolation oxide film formed on the principal surface, a gate electrode formed on an upper side of the isolation oxide film, and a side wall spacer that covers a side part of the gate electrode. The second diffusion layer includes a first part in which a first impurity implantation has been carried out and a second part in which a second impurity implantation has been carried out. The second part extends in a range that includes and goes beyond the first part, as viewed from above.
By adopting this construction, the additional implantation diffusion layer serving as the second part is diffused to go beyond the range of the low concentration diffusion layer and the high concentration diffusion layer serving as the first part, as viewed from above. Therefo
Mitsubishi Denki & Kabushiki Kaisha
Nelms David
Nguyen Thinh
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