JP2000049323A - Mos-type image sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a MOS-type image sensor wherein the smear generated by leakage current can be prevented. SOLUTION: This MOS-type image sensor is so structured that when electric charges in photodiodes which constitute pixels of the image sensor increase, leakage current are easily allowed to flow rather toward a reset switch than to a switch directly connected to the photodiodes and acting as a part of a reading means. So, the effective gate voltage of the reset switch in an turned off state is set higher than that of the other MOSFETs in an off state. With this method, leakage current is allowed to flow to a reset power supply through the reset switch, and the smear caused by wraparound leakage current can be prevented. More specifically, the threshold voltage of the reset switch is set lower than those of the other MOSFETs. For lower threshold voltage, for example, a gate oxide film is formed thin or ions are implanted to adjust the threshold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a MOS image sensor, and more particularly to a technique for reducing smear.

[0002]

2. Description of the Related Art As a conventional MOS image sensor, for example, there is one disclosed in Japanese Patent Application Laid-Open No. Hei 4-281681. FIG. 10 is a circuit diagram showing a schematic configuration of the conventional example. In FIG. 10, each pixel includes a photodiode D for converting incident light into electric charge, and a reset switch R for resetting the photodiode D to a constant potential.
And a frame shift switch F for sampling the photoelectric charge of the photodiode D, and a capacitor C for holding the sampled charge.
Many such pixels are arranged in the X-axis and Y-axis directions. B is a reset power supply. Further, a read circuit XY including a read switch Y for reading the Y axis and a read switch X for reading the X axis is provided.

The reset switch R, the frame shift switch F, the read switch Y for reading the Y-axis, and the read switch X for reading the X-axis are MOSFE.
T, and the threshold voltages of these MOSFETs are generally equal. And these MOS
In order to turn off the FET, a voltage at which the effective gate voltage becomes the lowest, that is, VSS (for example, 0 V) for an N-channel MOSFET is applied to a P-channel MOSFET.
If it is T, VDD (for example, 5 V) is applied to each gate.

In such a conventional MOS image sensor, when some pixels (for example, one pixel) are irradiated with intense light, the amount of electric charge accumulated in the photodiode in that part increases, and the potential decreases. FIG. 11 is a diagram for explaining the above phenomenon, and is a cross-sectional view substantially corresponding to one pixel in FIG. In FIG. 11, 10 and 11
Is a pixel, 1 is a P-type substrate (or P-well region),
2 is an N-type region serving as a drain of the reset switch R;
Is a gate of the reset switch R, 4 is a source of the reset switch R, an N-type region serving as an N pole of the photodiode D and a source of the frame shift switch F, 5 is a gate of the frame shift switch F, and 6 is a gate of the frame shift switch F. This is an N-type region serving as a drain. The PN junction formed by the P-type substrate 1 and the N-type region 4 forms a photodiode D. In FIG. 11, the readout switch Y in FIG. 10 is not shown, but the same applies when the readout switch Y is shown and the frame shift switch F is omitted. As shown, when the charge of the photodiode D increases due to light irradiation, the potential V
s <0 increases to the negative side according to the light intensity.

As described above, in the configuration shown in FIG. 11, the source of the reset switch R and the source of the frame shift switch F are connected to the N-type region of the photodiode D.
When s increases to the negative side, the effective gate-source voltage (= gate voltage-threshold voltage-source voltage) when each of the switches is turned off increases. As a result, a leak current flowing through the reset switch R and the frame shift switch F increases.

A problem does not arise because the leak current passing through the reset switch R is absorbed by the reset power source B. However, the leak current passing through the frame shift switch F passes from the pixel 10 through the readout switch Y and the vertical signal line, and further passes through the readout switch Y of a pixel (eg, the pixel 11) with weak irradiation light connected to the same vertical signal line. Then, the photodiode D of the pixel 11 is charged and becomes a false signal. The same applies to a configuration in which the readout switch Y is directly connected to the photodiode D without using the frame shift switch F.

[0007]

As described above, the conventional M
The OS-type image sensor has a problem in that a leak current flows from a pixel to which strong irradiation light is input to a pixel to which weak irradiation light is input, which becomes a false signal and generates smear.

The present invention has been made in order to solve the problems of the prior art as described above, and an object of the present invention is to provide a MOS image sensor capable of preventing the occurrence of smear due to a leak current.

[0009]

Means for Solving the Problems In order to achieve the above object, the present invention is configured as described in the claims. That is, when the charge of the photodiode increases, a leak current flows more easily to the reset switch than to the switch (the frame shift switch F or the read switch Y) which is directly connected to the photodiode and is a part of the reading means. With such a configuration, the leakage current does not flow to other pixels via the switch that is a part of the reading unit,
Therefore, smear due to a false signal can be eliminated.

In order to make the leak current more easily flow to the reset switch than to the switch which is a part of the reading means as described above,
The effective gate voltage when the reset switch is off is set to be larger than the effective gate voltage when the switch that is directly connected to the photodiode and is part of the reading means when the reset switch is off. The effective gate voltage is a value obtained by subtracting the threshold voltage of the MOSFET from the applied gate voltage (effective gate voltage = applied gate voltage−threshold voltage). The "large value" in this case means a magnitude in an absolute value, which is a positive value in the case of an Nch MOSFET and a large value in a negative value in the case of a Pch MOSFET. With this configuration, a leak current flows toward the reset switch, so that smear due to sneak can be prevented.

According to the second aspect of the present invention, the effective gate voltage applied to the gate of the first MOSFET (corresponding to a reset switch) when the first MOSFET (corresponding to a reset switch) is off is changed to the second MOSFET.
To make the value larger than the effective gate voltage applied to the gate when the FET (corresponding to a switch directly connected to the photodiode and being a part of the reading means) is turned off, the threshold voltage of the first MOSFET is set. From the second
Is set smaller than the threshold voltage of the MOSFFT.
To reduce the threshold voltage as described above, for example, there are methods of reducing the thickness of the gate oxide film or implanting ions for adjusting the threshold.

In the invention of claim 3, the effective gate voltage applied to the gate when the first MOSFET is off is higher than the effective gate voltage applied to the gate when the second MOSFET is off. In order to obtain a value, the gate voltage applied to the gate when the first MOSFET is off is set to a value higher than the gate voltage applied to the gate when the second MOSFET is off.

Further, in the invention according to claim 4,
The effective gate voltage applied to its gate when the first MOSFET off, and larger than the forward voltage V _F of the photodiode serving as a photoelectric conversion unit, and a second MOSF
The effective gate voltage applied to the gate when the ET is off is determined by the forward voltage V of the photodiode serving as the photoelectric conversion unit.
It is set smaller than _F. With this configuration, it is possible to prevent the leakage current from flowing to the substrate through the photodiode and diffusing the substrate, and arriving at another pixel to become a false signal.

Further, in the invention according to claim 5,
In an image sensor provided with a signal amplifying means for amplifying a photoelectric charge accumulated in a photoelectric conversion unit, an effective gate voltage applied to a gate of the MOSFET serving as a reset switch when the MOSFET is turned off is changed to a value of a photodiode serving as a photoelectric conversion unit. it is obtained by a larger value than the forward voltage V _F. With this configuration, even in an image sensor including the signal amplifying unit, it is possible to prevent a leak current from flowing to the substrate through D and diffusing the substrate, and arriving at another pixel to become a false signal. .

[0015]

According to the present invention, the leak current is made to flow more easily to the reset switch than to the switch which is directly connected to the photodiode and is a part of the reading means. An effect is obtained in that it does not go around to another pixel via a switch which is a part of the reading means, and therefore smear due to a false signal can be eliminated. Further, according to the fourth and fifth aspects of the invention, it is possible to prevent a leak current from flowing to the substrate through the photodiode, further diffusing the substrate, and arriving at another pixel to become a false signal. Is obtained.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIG. 1 is a diagram showing a first embodiment of the present invention, (A) is schematic cross-sectional view, (B) is the leakage current I _D it is a characteristic diagram showing a relationship between a gate voltage V _G. The circuit is shown in FIG.
Is the same as

In FIG. 1A, 10 and 11 are pixels, 1 is a P-type substrate (or a P-well region), 2 is an N-type region serving as a drain of a reset switch R, and 3 is a gate of the reset switch R. Reference numeral 4 denotes an N-type region serving as a source of the reset switch R, an N-pole of the photodiode D and a source of the frame shift switch F, 5 represents a gate of the frame shift switch F, and 6 represents an N-type region serving as a drain of the frame shift switch F. It is. The PN junction formed by the P-type substrate 1 and the N-type region 4 forms a photodiode D. Although the readout switch Y is not shown in FIG. 1, the same applies to the case where the readout switch Y is shown and the frame shift switch F is omitted. Although the substrates of the pixels 10 and 11 are shown separately for convenience of illustration, they are actually formed on the same substrate. The same applies to the following drawings. Further, although the above-described respective switches have been described as being configured with NchMOSFETs, they can be similarly configured using PchMOSFETs.

As described above, the schematic structure of each pixel is the same as that of the conventional example shown in FIG. 11, but in the present embodiment, the gate oxide film (portion under the gate 3) of the reset switch R is formed. The difference is that the film thickness is formed smaller than the gate oxide film of other MOSFETs. With this configuration, if other conditions are the same, the threshold voltage of the reset switch R is smaller than that of the other (Nch MOSFET
Is smaller at a positive value, and smaller at a negative value in the case of a Pch MOSFET). Therefore, for the same applied gate voltage, the effective gate voltage (applied gate voltage-threshold voltage)
Is large (a positive value is large in the case of an NchMOSFET, and large in a negative value in the case of a PchMOSFET). As a result, as shown in FIG. 1B, the leakage current of the reset switch R becomes larger than that of the frame shift switch F for the same applied gate voltage. FIG.
In (B), a solid line indicates a leak current flowing through the reset switch R, and a broken line indicates a leak current flowing through the frame shift switch F. V _tR is a threshold value of the reset switch R, V _tF is a threshold value of the frame shift switch F, and Vs is a potential of the photodiode D. Since Vs is a negative value, it is indicated by -Vs in FIG.

Therefore, when the charge of the photodiode D increases, the charge leaks through the reset switch R, and the charge does not flow to the frame shift switch F. In this way, a false signal due to a wraparound can be eliminated, and smear can be prevented.

Hereinafter, the reason for occurrence of smear will be described in detail. As described in the description of the related art, when the charge of the photodiode D increases due to light irradiation, the potential Vs <0 decreases according to the light intensity. Since the source of the reset switch R and the source of the frame shift switch F are connected to the N-type region of the photodiode D, the effective gate-source voltage (= gate voltage− (Threshold voltage−source voltage) increases. As a result, a leak current flowing through the reset switch R and the frame shift switch F increases. This state is shown in FIG. In addition,
2 (A) is schematic cross-sectional view, (B) is a characteristic diagram showing the relationship between the leakage current I _D and the gate voltage V _G, in (B), V _t is the reset switch R and frame shift switch F The threshold voltage Vs is the potential of the photodiode D (represented by -Vs).

The leak current passing through the reset switch R is absorbed by the reset power source B, so that no problem occurs. However, the leak current passing through the frame shift switch F passes from the pixel 10 through the readout switch Y and the vertical signal line, and further passes through the readout switch Y of a pixel (eg, the pixel 11) with weak irradiation light connected to the same vertical signal line. Then, the photodiode D of the pixel 11 is charged and becomes a false signal. This state is shown in FIG. As a result, for example, FIG.
Smear as shown in (1) occurs, and the image quality is impaired. FIG. 4 is an image obtained by imaging a white line indicating a lane on a road, and shows that smear has occurred in a part of the white line. The same applies to a configuration in which the readout switch Y is directly connected to the photodiode D without using the frame shift switch F.

In the present embodiment, the threshold voltage of the reset switch R is lowered by forming the gate oxide film of the reset switch R thinner than the other MOSFETs, and the effective gate voltage ( (Applied gate voltage−threshold voltage) is increased, thereby causing the charge of the photodiode D to leak to the reset switch R side, and preventing the charge from flowing to other pixels via the frame shift switch F to prevent smear. It was done.

(Second Embodiment) FIG. 5 is a schematic sectional view showing a second embodiment of the present invention. 5, the same reference numerals as those in FIG. 1 denote the same components. In this embodiment, instead of reducing the gate oxide film thickness of the reset switch R, the active impurity concentration on the substrate surface in the region where the reset switch R is formed is reduced by implanting ions for adjusting the threshold (pｐ). , Thereby reducing the threshold value. If the threshold value of the reset switch R becomes smaller, the same operation and effect as those of the first embodiment can be obtained. Generally, the manufacturing process is simpler by adjusting the threshold value by ion implantation than by forming two types of gate oxide film thicknesses. Also,
The same effect can be obtained by increasing the threshold value of the frame shift switch F (or the readout switch Y) by ion implantation instead of decreasing the threshold value of the reset switch R.

(Third Embodiment) FIG. 6 is a schematic sectional view showing a third embodiment of the present invention. 6, the same reference numerals as those in FIG. 1 denote the same components. In this embodiment, as a method of making the leak current to the reset switch R larger than that of the frame shift switch F, a voltage that is several hundred mV higher than the gate voltage applied when the frame shift switch F is turned off is applied to the reset switch R. It is applied. Generally, the effective gate voltage is 60 to 1
When the current rises by 00 mV, the leakage current rises by one digit, and several hundred mV
The difference in the degree causes a difference of several orders in the leakage current. Even with this configuration, the effective gate voltage when the reset switch R is off can be made higher than that of the frame shift switch F, so that the same operation and effect as those of the first and second embodiments can be obtained. Note that the above “several hundred mV” is Nc
This is a case of an hMOSFET, and a negative value in the case of a PchMOSFET.

In the embodiment shown in FIG. 6, it is necessary to prepare another power supply externally or to make it inside. But,
In this case, there is no need to adjust the threshold value, so that the manufacturing process can be simplified. In the embodiments described above, the switches (for example, the read switches X and Y) that are not directly connected to the photodiode D may have any effective gate voltage.

(Fourth Embodiment) FIG. 7 shows a fourth embodiment of the present invention.
FIG. 3 is a circuit diagram showing an embodiment of the present invention. In the embodiments described so far, the configuration using the frame shift switch F and the capacitor C for temporarily holding the photocharge has been described as an example. However, as shown in FIG. The present invention can be applied to a configuration not using C, and the same effect can be obtained. In this case, the switch directly connected to the photodiode D
It becomes the read switch Y.

(Fifth Embodiment) FIG. 8 shows a fifth embodiment of the present invention.
It is a diagram showing an embodiment of the present invention, (A) is a schematic sectional view,
(B) is a characteristic diagram showing a relationship between a gate voltage V _G and the leakage current I _D. 8, the same reference numerals as those in FIG. 1 denote the same components.

In the above embodiments, the false signal wrapping around through the frame shift switch F, the read switch Y and the vertical signal line has been described.
Reset switch R and the forward voltage threshold voltage of the photodiode D frame shift switch _F (V F: 0.
If it is larger than about 7 V), the following false signal wraparound occurs. That is, the incidence of light causes the potential of the photodiode D to drop, and when a voltage equal to or higher than the forward voltage is applied to the photodiode D, the photodiode D is forward-biased and the photocharge leaks to the substrate 1. The photocharge entering the substrate 1 diffuses into the surrounding pixels, enters the photodiode D of the surrounding pixels, and becomes a false signal. In Figure 8 in order to prevent this, the reset switch effective gate voltage larger than the forward voltage V _F of R, further effective gate voltage forward voltage V of the frame shift switch F
It is set smaller than _F. As a result, the leak current flows through the reset switch R, and smear can be eliminated.

This embodiment can be used in combination with the first to fourth embodiments. That is, in the first to fourth embodiments, when the reset switch R is turned off, the effective gate voltage applied to the gate is
The present embodiment does not simply set a value higher than the effective gate voltage applied to the gate when the switch (the frame shift switch F or the read switch Y) which is directly connected to the photoelectric conversion unit and constitutes the reading means is off. of the effective gate voltage of the reset switch R is set larger than the forward voltage V _F of the photodiode D as forms, smaller than the forward voltage V _F of the photodiode D the effective gate voltage, such as frame shift switch F By setting, it is possible to prevent the wraparound via the substrate 1.

(Sixth Embodiment) FIG. 9 shows a sixth embodiment of the present invention.
It is a diagram showing an embodiment of the present invention, (A) is a schematic sectional view,
(B) is a characteristic diagram showing a relationship between a gate voltage V _G and the leakage current I _D. In FIG. 9, N-type region 7, gate 8 and N-type region 9 form an amplifying MOSFET, and N-type region 9, gate 5 and N-type region 6 form a frame shift switch F (or readout switch Y). are doing. In addition, the same reference numerals as those in FIG.

In the embodiments described above, an example has been described in which the photodiode D serving as the photoelectric conversion unit is directly connected to the switch (the frame shift switch F or the read switch Y) of the reading unit. The present invention can be applied even when D is connected to the reading unit via the amplifying means. In FIG. 9, a MOSFET composed of an N-type region 7, a gate 8 and an N-type region 9 is used as amplifying means.
, And the photodiode D is connected to the gate 8.

In this case, the potential of the photodiode D decreases due to the irradiation of strong light, and when a voltage higher than the forward voltage is applied to the photodiode D, the photodiode D is forward-biased and the photocharge leaks to the substrate 1. The photocharge entering the substrate 1 diffuses into the surrounding pixels, enters the photodiode D of the surrounding pixels, and becomes a false signal. In Figure 9 in order to prevent this, through the reset switch R before the effective gate voltage of the reset switch R is set larger than the forward voltage V _F of the photodiode D, this by the light charge is injected into the substrate It is configured to leak to the reset power supply B. Therefore, even in this case, smear can be prevented in the same manner as in the example described above.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention,
(A) is schematic cross-sectional view, (B) is a characteristic diagram showing the relationship between the leakage current I _D and the gate voltage V _G.

[Figure 2] is a diagram showing the increase in leakage current which is a cause of smear, (A) is schematic cross-sectional view, (B) is a characteristic diagram showing the relationship between the leakage current I _D and the gate voltage V _G.

FIG. 3 is a schematic cross-sectional view showing a charge wraparound causing smear.

FIG. 4 is a diagram showing an example of smear.

FIG. 5 is a schematic sectional view illustrating a second embodiment of the present invention.

FIG. 6 is a schematic sectional view showing a third embodiment of the present invention.

FIG. 7 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 8 is a diagram showing a fifth embodiment of the present invention;
(A) is schematic cross-sectional view, (B) is a characteristic diagram showing the relationship between the leakage current I _D and the gate voltage V _G.

FIG. 9 is a diagram showing a sixth embodiment of the present invention;
(A) is schematic cross-sectional view, (B) is a characteristic diagram showing the relationship between the leakage current I _D and the gate voltage V _G.

FIG. 10 is a circuit diagram of a conventional example.

FIG. 11 is a schematic sectional view of a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... P-type board | substrate (or P well area) 2 ... N-type area | region used as the drain of reset switch R 3 ... Gate of reset switch R 4 ... Source of reset switch R, photodiode D
N-type region serving as a source of the frame shift switch F and an N-type region 5 serving as a source of the frame shift switch F 6 N-type region serving as a drain of the frame shift switch F 7 N-type region 8 Gate 9 N-type region 10 , 11 ... Pixel D ... Photodiode F ... Frame shift switch Y ... Readout switch X ... Readout switch XY ... Readout circuit

Claims (5)

[Claims] A photoelectric conversion unit comprising a photodiode;
A reset switch for resetting the photoelectric conversion unit to a predetermined potential; a reading unit including one or more switches for reading photoelectric charges accumulated in the photoelectric conversion unit;
In the image sensor including a plurality of pixels having at least one of the reset switch and the switch constituting the readout unit, at least a switch directly connected to the photoelectric conversion unit is made of a MOSFET and serves as the reset switch. The effective gate voltage applied to the gate when the first MOSFET is turned off is applied to the gate when the second MOSFET, which is directly connected to the photoelectric conversion unit and serves as a switch constituting the reading means, is turned off. A MOS type image sensor characterized in that the value is larger than the effective gate voltage. 2. The threshold voltage of the first MOSFET is set smaller than the threshold voltage of the second MOSFFT, so that the effective gate voltage applied to the gate of the first MOSFET when the first MOSFET is off is reduced. The second MOS
2. The MOS image sensor according to claim 1, wherein the value is larger than an effective gate voltage applied to the gate when the FET is off. 3. A gate voltage applied to the gate of the first MOSFET when the first MOSFET is turned off, and
The effective gate voltage applied to the gate when the first MOSFET is turned off is applied to the gate when the second MOSFET is turned off by setting the gate voltage to be larger than the gate voltage applied to the gate when the second MOSFET is turned off. 2. The MOS image sensor according to claim 1, wherein the value is larger than the effective gate voltage. The wherein the effective gate voltage applied to its gate when off of the first MOSFET, the photodiode serving as the photoelectric converter is larger than the forward voltage V _F, and the second MOSFET the effective gate voltage applied to its gate during off, MOS according to any one of claims 1 to 3, characterized in that it has less than the forward voltage V _F of the photodiode serving as the photoelectric converter Type image sensor. 5. A photoelectric conversion unit comprising a photodiode,
A reset switch for resetting the photoelectric conversion unit to a predetermined potential; a signal amplifying unit for amplifying a photoelectric charge accumulated in the photoelectric conversion unit; and one for reading out the amplified charge via the signal amplifying unit. In the image sensor including a plurality of pixels having at least the readout means including the above switch, the reset switch is made of a MOSFET,
MOS type image sensor, characterized in that the effective gate voltage applied to its gate when off the MOSFET serving as the reset switch, and to a larger value than the forward voltage V _F of the photodiode serving as the photoelectric converter.