JP2001244449A - Ccd image sensor and method of using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the accuracy of a CCD image sensor having a light- quantity detector. SOLUTION: A signal charge flowing through a light-quantity monitor 39 is returned to a re-storage part 32 via a return gate 40 to use as sensor data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CCD image sensor for obtaining image information from each pixel by transferring a photoelectrically converted signal to a CCD and a method of using the same.

[0002]

2. Description of the Related Art FIG.
It is a block diagram of one pixel of the CCD image sensor described in 0-171973. The components and functions of the pixel 20 will be described. For example, light is received and photoelectrically converted by a photoelectric conversion unit 21 of a photodiode. The photoelectrically converted signal charges are charged to the charge storage unit 2 by opening the barrier gate 24.
2 for a fixed time. The accumulated signal charges are transferred to the CCD register 23 of the charge transfer section when the transfer gate 25 is opened. The transferred signal charges are further transferred by a CCD shift register to FDA (Floating Diffusion Amplifier) in order from the end pixel, where they are converted to voltage signals and output as sensor data.

The CCD image sensor shown in FIG. 7 is provided with a light amount monitoring unit 29 for detecting the amount of light irradiated on the sensor, and converts most of the signal charges photoelectrically converted by the photoelectric conversion unit 21 into sensor data. Flow into the charge accumulating unit 22, but a part of the light amount is monitored to detect the light amount.
Flow into 9 The amount of signal charge that has flowed into the light amount monitoring unit 29 is determined by FDA (Floating Diffusi
on Amplifier). Therefore, a voltage proportional to the signal applied to the sensor is obtained.

The amount of inflow into the light quantity monitor 29 can be adjusted by the position of a stopper 28 for separating the charge accumulation section 22 and the light quantity monitor 29. The signal charge flowing into the light quantity monitor 29 detects the light quantity and is discharged to the ICG drain 27 via the integration clear gate 26. FIG. 8 is a diagram showing an overall configuration of a CCD image sensor including the pixels of FIG. CCD image sensor 10
Is a one-chip type line sensor used for distance measurement, for example. A plurality of line sensors 11 are formed in the line sensor section 10a. In each line sensor 11, a large number of pixels having a configuration described later are formed in parallel.

[0005] The monitor circuit 12 monitors the amount of light received by each line sensor 11 and controls the integration time of the signal charge. C
The CD control circuit 13 controls the transfer of the stored signal charges. The FDA 14 converts the signal charge transferred from the line sensor 11 into a voltage signal and outputs it.
Here, for convenience, the monitor circuit 12 and the FDA 1
4 are shown one by one, but actually both are provided for each line sensor 11.

[0006]

In the CCD image sensor shown in FIG. 7, a part of the signal charge photoelectrically converted by the photoelectric conversion unit 21 flows to the light amount monitor unit 29 for detecting the light amount. It is inevitable that the amount of signal charges that can be stored in the storage 22 will be reduced accordingly. Generally, the larger the amount of signal charges handled, the better the S / N (ratio between the ratio signal and the noise). Therefore, for high accuracy,
It is desirable to handle as much signal charge as possible.

In view of this problem, an object of the present invention is to provide a CCD image sensor provided with a light amount monitor, which has a large S / N ratio and can realize high accuracy, and a method of using the same.

[0008]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a CCD image sensor in which a charge accumulation portion and a light amount monitor portion separated by a reverse conductivity type region are provided in one conductivity type region of an image sensor. The sensor is provided with means for using the charge used for the light quantity measurement again as the charge for sensor output.

Conventionally, the charge storage unit and the light amount monitoring unit are
It was completely separated by the reverse conductivity type stopper. And
The signal charge photoelectrically converted by the photoelectric conversion unit was used separately for sensor data and light quantity measurement, and the charge used for light quantity measurement was finally discharged to the ICG drain. Therefore, it was impossible to use the charge used for the light quantity measurement for sensor data.

In the present invention, by providing means for using the electric charge used for the light quantity measurement again as the electric charge for sensor output, it becomes possible to return the electric charge to the accumulated charge section again after the light quantity measurement is completed. . Thereafter, as in the prior art, the signal charges are transferred to the CCD register section, sequentially transferred to the output stage, and output as sensor data. In this way, it is possible to efficiently use the signal charges converted by the photoelectric conversion unit, and the charges for sensor output increase, so that the S / N ratio can be increased and a more accurate sensor can be realized. it can.

As a means for reusing the charge used for the light quantity measurement as the charge for the sensor output, a part of a reverse conductivity type region for separating the charge accumulation section and the light quantity monitor section is omitted, and an insulating film is interposed thereon. Thus, a return gate electrode is provided. A return gate electrode is arranged on a stopper arranged to separate the charge accumulation unit and the light amount monitoring unit,
By controlling the potential of the return gate electrode,
The charge storage unit and the light amount monitoring unit can be communicated.

In particular, it is important that the return gate electrode partially overlaps the barrier gate electrode of the charge storage section. If the return gate electrode and the gate electrode of the charge storage unit are separated from each other, a potential peak occurs between them, so that all the charges in the light amount monitoring unit cannot be smoothly transferred to the charge storage unit.

As a method of using the CCD image sensor as described above, the potential of the light quantity monitor is made to match that of the charge storage. If the potential of the light quantity monitor section is different from that of the charge accumulation section, the following problem occurs. If the potential of the light amount monitoring unit is lower than that of the charge storage unit, when the potential of the return gate electrode is controlled, the charge cannot be transferred from the light amount monitoring unit to the charge storage unit. I'll call you.

Conversely, if the potential of the light quantity monitor is higher than that of the charge storage, the charge can be transferred from the light quantity monitor to the charge storage, but the flow of the charge from the photoelectric converter to the light quantity monitor will be limited. And the sensitivity of the light amount monitor section is reduced.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. A specific configuration of the pixel 30 which is a component of the line sensor according to the embodiment will be described. FIG. 1 is a perspective plan view showing a specific configuration of the pixel 30. FIG. 2 is a cross-sectional view taken along line AA of FIG.
(C) is sectional drawing which followed the BB line CC line and DD line of FIG. 1, respectively. Although FIG. 1 shows only one pixel,
A sensor array can be configured by arranging a plurality of sensors in parallel.

The pixel 30 has a length of several hundred μm and a width of several tens μm.
m, for example, a photoelectric conversion unit (hereinafter referred to as PD) 31, a charge storage unit (hereinafter referred to as ST) 32, a CCD register 33, a barrier gate (hereinafter referred to as BG) 34, a transfer gate (Hereinafter T
G) 35, an integral clear gate (hereinafter ICG) 36, and an integral clear gate drain (hereinafter IC)
GD) 37, a light amount monitoring area 39, and a return gate (hereinafter referred to as RG) 40.

The pixel 30 is formed by doping a crystalline silicon substrate with donor ions, for example, phosphorous ions at 150 keV for 2.3 to 3.0.
2.5 × 10 ¹² cm ^-2 doping and heat treatment
It is formed as an n-type layer (FIG. 2). The PD stopper 38 that separates the ST 32 from the light amount monitor 39 is provided with a doping of acceptor ions, for example, boron ions in the n-type layer.
It is formed by doping at 2.9 × 10 ¹³ cm ⁻² at 0 keV and performing heat treatment (FIG. 3A). The PD stopper 38 prevents depletion at the junction between ST32 and the light amount monitor 39. A part of the PD stopper 38 for separating the ST32 from the FDA region was removed, and an RG 40 for returning charges to the ST32 was provided.

In the ICGD 37, doping of the n-type layer with a high concentration of donor ions, for example, arsenic ion is performed at 80 keV.
It is formed by doping 5 × 10 ¹⁶ cm ⁻² and performing heat treatment. The light amount monitoring unit 39 formed with some space between the PD stopper 38 and the IC stopper 37 is formed by doping a high concentration of donor ions into the n-type layer, similarly to the ICGD 37 [FIG. a)]. Here, the light amount monitor 39 functions as an FDA.

The RG 40 is formed as a return gate electrode E6 made of a phosphorus-doped polycrystalline silicon film via an insulating film (not shown) on the surface of the n-type region where the PD stopper 38 is not provided [FIG. 3 (b)]. B
G34, TG35 and ICG36 are also composed of similar insulating films and electrodes. The gate electrodes E1 and CC made of a phosphorus-doped polycrystalline silicon film are provided on the surfaces of the ST32 and the CCD register 33 via an insulating film (not shown).
D gate electrodes E2, E3, E4, and E5 are formed (FIGS. 3A, 3B, and 3C). A constant voltage is always applied to the gate electrode E1.

The ratio between the amount of signal charge flowing into ST22 and the amount of signal charge flowing into light amount monitor 39 is determined by the position where PD stopper 38 is formed. That is, the PD stopper 3
8, the area of ST32 and the light amount monitor 39
Is determined, thereby determining the ratio of the inflow amount of the signal charge. The FDA sensitivity of the light amount monitor 39 is n
⁺ Depends on the area of the ion implantation region. Therefore, the sensitivity of the FDA on the CCD register 33 side and the sensitivity of the light amount monitor 39 are
If designed in accordance with the flow rate of the signal charge in both applications, a monitor output equivalent to the sensor output can be obtained. For example, when the flow rate ratio of the signal charge between the ST 32 and the light amount monitoring unit 39 is 10: 1, the sensitivity ratio between the FDA on the CCD sensor side and the FDA of the light amount monitoring unit 39 is 1: 1.
If it is set to 0, both outputs will be the same.

Unnecessary charges in the ST 32 and the light amount monitor 39 are stored in the ICGD3 before the integration operation, as in the conventional configuration.
7 must be discharged. For this reason, the light amount monitor 39 is formed so as to be in contact with both the BG 34 and the integral ICG 36. Next, the pixel 30 having such a configuration will be described.
Will be described. FIG. 4B is a potential diagram of each part for explaining the operation, and FIG. 5 is a flow chart of charges for explaining the operation.

Upon receiving light from the subject, the PD 31 performs photoelectric conversion of the light. When the BG 34 is opened, the photoelectrically converted signal charges move to ST32 and are accumulated for a certain period of time. At this time, part of the signal charge moves to the light amount monitor 39. The signal charge stored in ST32 is transferred to the TG by the CCD control circuit 13 in FIG.
When 35 is opened, it is transferred to the CCD register 33, further transferred from the CCD register 33 to the FDA 14, where it is converted into a signal voltage and output.

The operation of accumulating signal charges in ST32 for a certain period of time, the so-called integration operation, and the discharge of unnecessary charges before this integration operation are controlled by opening and closing BG 34 and ICG 36. Specifically, for example, before the integration operation, the BG34
And a voltage of 3 V is applied to the ICG. As a result, the potential below the ICG 36 decreases. Also,
Irrespective of the operation, a voltage of 3 V is applied to ST32, a voltage of 3 V is applied to the light quantity monitoring unit 39, and a voltage of 7 V is applied to the ICGD 37, and the potential therebelow drops.

Since the potential under ST32 also coincides with the potential under ICG36, a potential gradient is created in the pixel, and unnecessary charges in ST32 become ICGD.
It is discharged to 37. At the same time, unnecessary charges of the light amount monitor 39 are also discharged to the ICGD 37. RG is 0V. FIG. 4B is a potential diagram of each part showing this situation.

After a certain period of time, all unnecessary charges in ST32 are discharged and become empty, and the light amount monitor 39 also discharges unnecessary charges, and the potential drops. However, ICG
There is no drop below the potential below 36 and that potential matches the potential below ICG 36. Thus, the reset before the integration operation is completed. Then I
Assuming that the voltage applied to the CG 36 is 0 V, the integration operation starts. The potential under ICG36 rises and ST3
2 and the flow of charges from the light amount monitoring unit 39 to the ICGD 37 stops, and the signal charges from the PD 31 are stored in ST32 and the light amount monitoring unit 39.

When the signal charge from the PD 21 flows into the light quantity monitor 39, the potential of the light quantity monitor 39 rises according to the amount of the signal charge. In this way, the voltage signal is converted into a voltage signal proportional to the amount of light and sent to the monitor circuit 12 in FIG. As will be described later, the monitor circuit 12 integrates the voltage signal from the light amount monitor unit 39, and when the integrated value becomes equal to or more than a predetermined value, issues an integration end instruction, sets the applied voltage of the BG 34 to 0V, The flow of charges from the PD 31 to the ST 32 and the light amount monitoring unit 39 stops. This makes ST
The integration operation of the 32 signal charges ends.

After the end of the integration operation, 5 V, C
When 5V is also applied to the CD gate E2, the potential under the TG 35 and the potential under the CCD gate E2 are lower than the potentials under ST32 and RG40.
Transferred to D register 33. At this time, RG40 is 3V
Is applied, the potential below the RG 40 drops and the gate is in an open state, so that the signal charges accumulated in the light quantity monitor 39 are also transferred from the TG 35 to the CCD register 33 immediately via ST32.

At this time, the RG electrode E6 of the RG 40
It is important that it partially overlaps the gate electrode E1 of T32. This is because if they are separated from each other, a mountain of potential is formed at a portion between them, and the charge of the light amount monitor unit 39 is prevented from flowing to ST32. The signal charge transferred under the CCD gate transfers the signal charge from the CCD register 23 to the FDA 14. Move under the CCD gate. The data is sequentially transferred to the output unit and output as sensor data. The signal charge used for light amount detection by FDA and S
Since the signal charge accumulated in T is transferred to the transfer unit (CCD), the S / N ratio is increased.

FIG. 5 is a flow chart of electric charges showing the above operation in an easy-to-understand manner. Of the ten charges generated by the photoelectric conversion unit 31, two charges flowing to the light amount monitoring unit 39 are:
It shows a state in which it flows through the RG 40 to ST32, further flows to the CCD register unit 33, and finally outputs ten charges as sensor data. In this way, the charge that could not be used conventionally can be used, and the S / N ratio can be improved. For example, if the signal charge amount doubles, the variation in sensor output becomes (1
/ 2) It can be reduced to ^1/2, and the accuracy is improved.

For comparison, FIG. 4A shows a potential diagram of each portion of the conventional CCD image sensor shown in FIG. 7, and FIG. 6 shows a flow diagram of electric charges. In the conventional CCD image sensor, since no RG is provided, the photoelectric conversion unit 21 is not provided.
Of the ten charges generated in the above, two signal charges flowing into the light amount monitoring unit 29 flow into the ICGD 27 and cannot be used as signal charges. Eventually, only eight charges
It shows a state that is not used as sensor data.

However, if the unnecessary charge discharging time before the integration operation is
Since 5 V was applied to the CG, the potential of the light quantity monitor 29 after the discharge period was lower than the potential of ST22. Therefore, if an RG is temporarily set and the RG
Not only is it impossible to return the charge from the light quantity monitoring unit 29 to ST22 even if 3V is applied to the device, but there is also a possibility that the charge flows from ST22 to the light quantity monitoring unit 29 and becomes invalid.

In the present invention, the voltage applied to the ICG 36 is set to 3 V in order to surely return the electric charge of the light quantity monitor 39 to ST32. This is to make the potential of the light quantity monitor unit 39 after discharge coincide with the potential of ST32 for the reason described in the previous section. However, conventional 5V
, The amplitude range of the FDA of the light amount monitoring unit 39 is narrowed. Therefore, in some cases, it is necessary to amplify the signal with an amplifier such as an amplifier.

[0033]

As described above, according to the present invention, for example, by removing a part of a stopper for separating a charge storage portion and a light amount monitor portion and arranging a return gate, the conventional system After the measurement, the electric charge of the light amount monitoring unit discharged to the integration clear drain and discarded, so to speak, is returned to the electric charge accumulating unit, and can be used as sensor data.

In an imaging device such as a CCD sensor,
The greater the amount of signal charges to be handled, the higher the accuracy of the device, so that a CCD image sensor with significantly improved accuracy compared to the conventional method can be realized.

[Brief description of the drawings]

FIG. 1 is a plan view showing the structure of a CCD image sensor according to the present invention;

FIG. 2 is a sectional view taken along the line AA of FIG. 1;

FIG. 3A is a cross-sectional view taken along line BB of FIG. 1;
1B is a cross-sectional view taken along the line CC of FIG. 1, and FIG.
Sectional view along line DD

4A is a potential diagram of a conventional CCD image sensor, and FIG. 4B is a potential diagram of a CCD image sensor of the present invention.

FIG. 5 is a flow chart of charges of the CCD image sensor of the present invention.

FIG. 6 is a flow chart of signal charges of a conventional CCD image sensor,

FIG. 7 is a plan view showing the structure of a conventional CCD image sensor.

FIG. 8 is a plan view showing the structure of a line sensor.

[Explanation of symbols]

 Reference Signs List 10 CCD image sensor 10a Line sensor unit 11 Line sensor 12 Monitor circuit 13 CCD control circuit 14 FDA 20, 30 Pixel 21, 31 Photoelectric conversion unit (PD) 22, 32 Charge storage unit (ST) 23, 33 CCD register unit 24, 34 Barrier gate (BG) 25, 35 Transfer gate (TG) 26, 36 Integral clear gate (ICG) 27, 37 Integral clear gate drain (ICGD) 28, 38 Stopper 29, 39 Light intensity monitor 40 Return gate (RG) E1 Storage section gate electrode E2 to E5 CCD gate electrode E6 Return gate electrode

Claims (4)

[Claims] In a CCD image sensor provided with a charge accumulation section and a light quantity monitor section separated by an opposite conductivity type area in one conductivity type area of an image sensor, the charge used for the light quantity monitor is again output to the sensor output. A CCD image sensor characterized by being used as an electric charge of a CCD. 2. The device according to claim 1, wherein a part of the reverse conductivity type region for separating the charge accumulating portion and the light amount monitoring portion is omitted, and a return gate electrode is provided thereon via an insulating film. CCD image sensor. 3. The CCD image sensor according to claim 2, wherein the return gate electrode partially overlaps the barrier gate electrode of the charge storage section. 4. A method of using a CCD image sensor in which a light quantity monitor is disposed in a signal charge storage section, wherein the potential of the light quantity monitor section is made to match that of the charge storage section.