Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2000-04-11
2001-12-25
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S249000
Reexamination Certificate
active
06333525
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a charge transfer apparatus which reduces the detection capacity of a signal charge detector and enhances the detection sensitivity. Particularly, there is provided a signal charge detector which suppresses the threshold value dispersion of an output electrode and a reset electrode and has stable characteristics.
2. Description of the Related Art
A charge transfer apparatus which accumulates and transfers information such as incident light and electric signal in the form of electric charges and extracts the information as a voltage signal has been broadly used for applications such as a solid image pickup apparatus in recent years. The solid image pickup apparatus is constituted of a photoelectric converter for converting light to an electric signal (signal charge), a charge transfer section for transferring the converted signal charge, and a signal charge detector (signal output section) for detecting the signal charge transferred from the charge transfer section. A charge coupled device (hereinafter abbreviated as CCD) is used as the charge transfer section. To detect the signal charge, a floating diffusion type signal charge detector is usually used.
FIG. 1
is a sectional view showing an interline transfer system CCD type solid image pickup apparatus disclosed in Japanese Patent Application Laid-Open No. 218104/1993.
Formed on a P-type semiconductor substrate
1
are an N-type impurity diffusion layer
2
as an embedded channel region, a device separating P-type impurity diffusion layer
3
, a device separating oxide film
4
a
, and a gate insulation film
5
. A first gate electrode
6
formed of polycrystalline silicon as a transfer electrode, and a second gate electrode
8
formed of polycrystalline silicon are formed on the gate insulation film
5
. An insulation film
7
is formed on the surface of the first gate electrode
6
, and a potential barrier P-type diffusion layer
9
is formed on the semiconductor substrate under the second gate electrode
8
. Each first gate electrode of the charge transfer section is electrically connected to the second gate electrode on the left side. Clocks &phgr;1, &phgr;2 are alternately applied to the connection part of the first and second gate electrodes.
A fixed output gate voltage V
2
is applied to an output gate electrode
10
. An N-type floating diffusion layer
11
as the embedded channel region for detecting a transfer charge amount is formed in the same process as that of the N-type impurity diffusion layer
2
. The potential change of the N-type floating diffusion layer
11
is detected by an output transistor
14
. A reset drain voltage V
1
is applied to an N-type drain diffusion layer
12
. The potential of the N-type floating diffusion layer
11
is periodically set to the reset drain voltage V
1
by a reset gate electrode
13
.
The operation of this charge transfer apparatus will be described. First, by applying a reset pulse to the reset gate electrode
13
, the potential of the N-type floating diffusion layer
11
is reset to the reset drain voltage V
1
. In this case, the clock &phgr;1 has a high potential, the clock &phgr;2 has a low potential, and the electric charge is accumulated under the first gate electrode to which &phgr;1 is applied. Subsequently, when the clock &phgr;1 is set to the low potential, and the clock &phgr;2 is set to the high potential, the electric charge under the final gate electrode (first gate electrode) with the clock &phgr;1 applied thereto flows into the N-type floating diffusion layer
11
through a channel under the output gate electrode
10
. As a result, a potential change is generated in the N-type floating diffusion layer
11
, and the potential change is detected by the output transistor
14
.
When the charge amount of the signal charge transferred to the N-type floating diffusion layer
11
is Q, and the capacity of the N-type floating diffusion layer
11
is C, the potential change &Dgr;V of the N-type floating diffusion layer
11
before and after the flow-in of the electric charge is as follows:
&Dgr;V=Q/C.
The magnitude of the output signal to the signal charge amount Q increases as the capacity C of the floating diffusion layer
11
decreases. Specifically, as the capacity C of the floating diffusion layer
11
decreases, the detection sensitivity defined by the output voltage to the constant signal charge amount Q rises.
Therefore, in order to enhance the detection sensitivity, in the charge transfer apparatus disclosed in the Japanese Patent Application Laid-Open No. 218104/1993, an insulation film
4
b
on the surface of the N-type floating diffusion layer
11
is formed in the same process as that of the device separating insulation film
4
a
. By disposing the insulation film
4
b
thicker than the gate insulation film
5
in this manner, the capacity of the floating diffusion layer is decreased, and the detection sensitivity of the signal charge is enhanced.
However, the reduction of the capacity C is insufficient in this charge transfer apparatus.
Moreover, in the charge transfer apparatus, the insulation film
4
b
on the surface of the N-type floating diffusion layer
11
is formed in the same process as that of the device separating insulation film
4
a
. In this case, however, three mask formation processes are necessary for the ion injection for forming the P-type impurity diffusion layer
3
under the device separating insulation film
4
a
, the ion injection for forming the N-type floating diffusion layer
11
, and the formation of the insulation films
4
a
,
4
b
on the P-type impurity diffusion layer
3
and N-type floating diffusion layer
11
. It is difficult to match the masks, and there is a problem that the apparatus provided with a desired performance cannot be formed.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above-described problem, and to provide a charge transfer apparatus with an enhanced detection sensitivity. Further object of the present invention is to provide a method of manufacturing a charge transfer apparatus provided with stable characteristics, in which the dispersions of the respective threshold values of an output electrode and a reset electrode of the charge transfer apparatus are suppressed.
A charge transfer apparatus according to this invention is featured mainly by providing a gate insulation film of said charge transfer electrode that comprises a multilayered film of a first silicon oxide film, a silicon nitride film, and a second silicon oxide film, and at least one of the gate insulation films of said output electrode and said reset electrode that is formed of a third silicon oxide film.
REFERENCES:
patent: 4087832 (1978-05-01), Jambotkar
patent: 5345099 (1994-09-01), Yamada
patent: 6018170 (2000-01-01), Hatano et al.
patent: 5-218104 (1993-08-01), None
NEC Corporation
Ngo Ngan V.
Young & Thompson
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