JPH0352389A - Solid-state image pickup element - Google Patents

Abstract

PURPOSE:To reduce noise generated in an output signal by forming the solid- state image pickup element such that a channel potential under a reset gate electrode of the solid-state image pickup element is shallow in the source region more than in the drain region. CONSTITUTION:The channel potential under a reset gate electrode 6 of the solid-state image pickup element is shallow in the source region 2 more than in the drain region 3. Thus, the mobile direction of the charge under the electrode 6 is oriented in terms of structure toward the drain region 3, then the introduction of the charge in the source region 2 at the reset of the solid-state image pickup element is suppressed. Thus, the mixture of the charge into an output signal of the solid-state image pickup element is suppressed and noise caused in the output signal is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a solid-state image sensor, and more particularly, to a solid-state image sensor having a reset gate section for reducing noise in an output signal.

[Prior Art] FIG. 5 is a longitudinal sectional view showing a reset gate portion of a conventional solid-state image sensor.

As shown in FIG. 5, a source region 12 and a drain region 13 are selectively formed on the surface of the semiconductor substrate l1. A gate insulator [15
is formed. The reset gate electrode 16 is formed with a uniform thickness on the gate insulating film 15 on the surface of the semiconductor substrate between the source region 12 and the drain region 13, and the surface of the reset gate electrode 16 is covered with an insulating film. There is. In the MOS transistor configured in this manner, the channel region 14 is formed on the surface of the semiconductor substrate 11 directly under the reset gate electrode 16, that is, on the surface of the semiconductor substrate 11 between the source region 12 and the drain region 13. . Therefore, by applying a voltage to the reset gate electrode 1B, the channel region 14 is changed to the source region 1.
2 and the drain region 13 .

In addition, in the reset gate section of the conventional solid-state image sensor configured as described above, the source region 12 is connected to the output section of the solid-state image sensor, the drain region 13 is connected to the power supply, and the reset gate electrode 16 is connected to the source region 12 . By applying a predetermined potential to turn on the channel region 14,
Charges accumulated within the solid-state image sensor can be reset.

Thereafter, the reset is completed by turning off the channel region 14.

FIG. 6 is a schematic diagram showing the charge distribution within the reset gate upon completion of reset. As shown in FIG. 6, channel charges 14a induced in the channel region 14 in the on state are transferred to the source region 12 and F in the off state.
These charges are distributed to the drain region 13 as source charges 12a and drain charges 13a, respectively.

FIG. 7 is a schematic diagram showing the structure of a solid-state imaging device equipped with a conventional reset device together with a circuit diagram of an amplifier circuit A. As shown in FIG. 7, the surface of the P-type semiconductor substrate 20 has N
A type 0 drain region 23 and source region 24 are selectively formed. Reset gate electrode 25 is formed on the surface of semiconductor substrate 20 between source region 24 and drain region 23 with an insulating film (not shown) interposed therebetween. N-
The type channel region 22 is formed on the surface of the semiconductor substrate immediately below the reset gate electrode 25, that is, between the source region 24 and the drain region 23. It functions as a switch that electrically connects the drain region 23 and the drain region 23 . The drain region 23 is connected to the power supply VRD.

Further, an N-type region 21 is provided on the surface of the P-type semiconductor substrate 20.
is formed, and the N-type region 21 and the source region 24 are electrically connected. Gate electrodes 28 and 27 are N
The gate electrode 2 is formed on the type 1 region 21 with an insulating film (not shown) interposed therebetween. 27+7) A transfer electrode 28 is formed on the N- type fii region 2i via a niha insulation wX (not shown).

As a result, a predetermined COD is formed as a solid-state image element in the N1 type region 2I on the P type semiconductor substrate.

On the other hand, the source region 24 is connected to the input end of the amplifier circuit A. In this amplifier circuit A, a series body of MOS transistors T--TQ and a series body of MOS transistors T=I, T4 are connected in parallel between the power supply VDD and the ground GND. The gate electrode of the MOS transistor T constitutes the input terminal of the amplifier circuit A. The drain electrode of MOS} transistor TllT2 is MOS} transistor T3.
are commonly connected to the gate electrodes of the two. MOS} The gate electrodes of the transistors Ta-Ta are commonly connected to the bias terminal ■1, and this bias terminal! , is externally applied with a bias voltage, so the MOS} transistor T*
- T4 is in a semi-conducting state. The drain electrode of the MOS transistor T3IT4 is connected to the output terminal. Therefore, an input signal manually inputted from the input terminal of this amplifier circuit A is amplified by the amplifier circuit A and outputted from the output terminal.

In the solid-state imaging device configured in this way, the signal charge accumulated in the N-type region 21 is transferred to the gate electrode 28.2.
7 and the transfer electrode 28 by externally applied pulses. This signal charge is then transferred to the source region 2.
4 as a signal voltage to the amplifier circuit A, where it is amplified and output.

However, since charges remain in this type of solid-state imaging device, the charges in the solid-state imaging device are reset by applying a voltage to the reset gate electrode 25 to turn on the channel region 22.

This suppresses the occurrence of noise in the output signal of the solid-state image sensor.

[Problems to be Solved by the Invention] However, in a conventional solid-state imaging device having a reset gate section, as shown in FIG. A large amount is also introduced into 12. As a result, as shown in FIG.
There is a problem that occurs.

The present invention has been made in view of such problems, and includes:
It is an object of the present invention to provide a solid-state imaging device that suppresses noise generation and has excellent output signal sensitivity.

[Means for Solving the Problems] A solid-state imaging device according to the present invention includes a source region and a drain region formed on the surface of a semiconductor substrate, and an insulating film formed on the semiconductor substrate between the source region and the train region. In the solid-state imaging device having a reset gate electrode, the channel potential under the reset gate electrode is shallower on the source region side than on the drain region side.

[Function] In the present invention, the channel potential under the reset gate electrode of the solid-state imaging device is formed to be shallower on the source region side than on the drain region side. Therefore, the direction of movement of charges under this electrode can be structurally directed toward the drain region, so that it is possible to suppress charges from being introduced into the source region at the time of resetting the solid-state imaging device. Therefore, this charge is suppressed from being mixed into the output signal of the solid-state image sensor, and noise generated in the output signal can be reduced.

[Example] Next, an example of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a sectional view showing a reset gate portion of a solid-state image sensor according to a first embodiment of the present invention. As shown in FIG. 1, a source region 2 and a drain region 3 are selectively formed on the surface of a semiconductor substrate 1. A gate insulator M5 is formed on the entire surface of the semiconductor substrate 1, and this gate insulator [I5 is formed on the substrate surface between the source region 2 and the drain region 3 from approximately half the region on the drain region 3 side. Also, approximately half the region on the source region 2 side is formed to be thicker. The reset gate electrode 6 is a gate insulating film 5 on the substrate between the source region 2 and the drain region 3.
The surface of the reset gate electrode 6 is covered with an insulating film. Further, the channel region 4 is formed on the surface of the semiconductor substrate in a region immediately below the reset gate electrode 6, that is, on the surface of the semiconductor substrate between the source region 2 and the drain region.

In the solid-state imaging device mounting the reset gate section configured in this way, the source region 2 is connected to the output section of the solid-state imaging device, and the drain region 3 is connected to the power source.

Therefore, when charges remain in the solid-state imaging device, by applying a predetermined potential to the reset gate electrode 6 and turning on the channel region 4, the charges accumulated in the solid-state imaging device can be reset. I can do it. Thereafter, the reset is completed by turning off the channel region 4.

FIG. 2 is a schematic diagram showing the charge distribution within the reset gate section upon completion of reset of the solid-state image sensor according to the first embodiment of the present invention. As shown in FIG. 2, channel charges 4a induced in the channel region 4 in the on state are distributed to the source region 2 and the drain region 3 as source charges 2a and drain charges 3a in the off state. However, since the gate insulating film 5 on the channel region 4 is formed thicker on the source region 2 side, the channel potential under the gate electrode e is lower than that in the source region.
Since the depth becomes shallower on the second side, introduction of charge into the source region 2 is significantly suppressed.

Therefore, as shown in FIG. 3, which shows the output signal of the solid-state imaging device according to the first embodiment of the present invention, in the case of this embodiment, the noise N generated in the output signal is significantly reduced compared to the conventional one.

FIG. 4 is a sectional view showing a reset gate portion of a solid-state image sensing device according to a second embodiment of the present invention. As shown in FIG. 4, a source region 12 and a drain region 13 are selectively formed on the surface of the semiconductor substrate 11. A gate insulator M10 is formed on the surface of this semiconductor substrate 1. The reset gate electrode 9 is formed with a uniform thickness on the gate insulation gto on the surface of the semiconductor substrate 1 between the source region 2 and the drain region 3, and the surface of the reset gate electrode 9 is covered with an insulating film. ing. Channel region 8 is formed on the surface of semiconductor substrate 1 directly under reset gate electrode 9, as in the first embodiment. In this embodiment, an ion implantation layer 7 is provided on the surface of approximately half of the channel region 8 on the source region 2 side. Therefore, the conductivity type impurity concentration decreases in this portion, and the channel potential becomes shallow.

Therefore, in this embodiment as well, the same effects as in the first embodiment can be obtained. Furthermore, in the first embodiment, the potential of the channel region 4 is formed shallowly through five steps of nitride film growth, photolithography, nitride film etching, oxidation, and nitride film etching, whereas in the present embodiment, the potential of the channel region 4 is formed shallowly. Since the ion implantation layer 7 can be formed only by photolithography and ion implantation steps, manufacturing is easy.

[Effects of the Invention] As explained above, according to the present invention, the channel potential under the reset gate electrode of the solid-state imaging device is formed to be shallower on the source region side than on the drain region side. The amount of charge introduced into the source region at the time of resetting the solid-state image sensor can be reduced. Therefore, the noise generated in the output signal of the solid-state image sensor can be reduced, the sensitivity of the output signal can be improved, and flickering can be prevented even in low illuminance.

[BRIEF DESCRIPTION OF THE DRAWINGS] Fig. 1 is a sectional view showing a reset gate portion of a solid-state image sensor according to a first embodiment of the present invention, and Fig. 2 is a sectional view of a solid-state image sensor according to a first embodiment of the present invention. FIG. 3 is a waveform diagram showing the output signal of the solid-state image sensor according to the first embodiment of the present invention, and FIG. 4 is a schematic diagram showing the charge distribution in the reset gate section upon completion of reset. FIG. 5 is a cross-sectional view showing a reset gate portion of a solid-state image sensor according to an embodiment of the present invention; FIG. 5 is a cross-sectional view showing a reset gate portion of a conventional solid-state image sensor;
FIG. 7 is a schematic diagram showing the charge distribution in the reset gate section at the time of completion of reset of a conventional solid-state image sensor; FIG. FIG. 8 is a waveform diagram showing output signals of a conventional solid-state image sensor. 1.11.20; Semiconductor substrate, 2, 12, 24; Source region, 2a+ 12a: Source charge, 3, 13.23
; Drain region, 3 a. 1 3 a: Drain charge, 4, 8, 14, 22; channel region, 4a+
14a: channel charge, 5.10.15; gate insulating film, 6, θ, 18, 25; reset gate electrode, 7; ion implantation layer, 21N type one region, 2B, 27; gate electrode, 28; transfer electrode, nlN; noise, T. ,T2+T
．． ．． T4; MOS} transistor, Is; pin pin, VDDI Vl? o; Power supply, GND; Grounding

Claims (1)

[Claims] (1) In a solid-state imaging device having a source region and a drain region formed on the surface of a semiconductor substrate, and a reset gate electrode formed on the semiconductor substrate between the source region and the drain region with an insulating film interposed therebetween, the reset A solid-state imaging device characterized in that a channel potential under the gate electrode is shallower on the source region side than on the drain region side.