JP7360248B2 - Radiation resistant image sensor and radiation resistant imaging device - Google Patents

Description

The present invention relates to a radiation-resistant image sensor and a radiation-resistant imaging device.

In nuclear power plants and radiation utilization facilities, a plurality of imaging systems equipped with image sensors are installed in order to monitor the inside of the plants and facilities. Imaging systems equipped with image sensors are also used for internal investigations and other purposes for decommissioning. Image sensors included in these imaging systems are equipped with circuits including semiconductor elements.

Among the elements constituting a circuit, semiconductor elements are generally considered to be sensitive to radiation, and when a semiconductor element is irradiated with radiation, electrons and holes are generated in the oxide film of the semiconductor element.
Compared to a conductor, the mobility of electrons in an oxide film is low, so electrons and holes do not recombine and increase in accordance with the amount of radiation irradiated. Electrons in the oxide film have higher mobility than holes, so they escape from the oxide film, but holes have lower mobility, so they accumulate in the oxide film as positive charges.

If the radiation dose rate is low, the effect will be small; however, in a harsh environment such as in the case of a nuclear plant accident, the radiation dose rate will also be high, and the effect will be significant. That is, in the case of an image sensor, the image flickers, and eventually the image cannot be output.
For this reason, image sensors used in environments where radiation is irradiated are subject to countermeasures such as shielding with lead or the like or providing a distance from the radiation source.

International Publication No. 2018/110093

Patent Document 1 describes a technology for making an operational amplifier by constructing a semiconductor element with silicon carbide (hereinafter referred to as "SiC"), which has better radiation resistance than silicon (hereinafter referred to as "Si"). There is a description.

As described in Patent Document 1, radiation resistance is improved by configuring the semiconductor element with SiC.
However, since a photodiode included in an image sensor made of SiC can only accumulate ultraviolet light, it has been impossible to image visible light with an image sensor made of SiC.

An object of the present invention is to provide an image sensor with excellent radiation resistance and an imaging device using the image sensor.

In order to solve the above problems, for example, the configurations described in the claims are adopted.
The present application includes multiple means for solving the above-mentioned problems, but one example is a radiation-resistant image sensor that is placed in a nuclear power plant or a radiation utilization facility to take images, and is formed on a silicon substrate and is A photoelectric conversion unit that includes a photodiode that converts light into a signal in each pixel, a bipolar transistor that amplifies the signal in each pixel obtained by the photodiode, and a capacitor that stores the signal in each pixel that has been amplified by the photodiode. A MOSFET is formed on a substrate made of a material with a wider bandgap than silicon, and the signals accumulated in the capacitor for each pixel are reset by a reset pulse and selectively taken out by a select pulse, and each pixel taken out by the MOSFET. The radiation-resistant image sensor is provided with a signal amplifying section having an operational amplifier that amplifies the signal.

According to the present invention, an image sensor with excellent radiation resistance can be obtained, and imaging can be appropriately performed in a high radiation environment.
Problems, configurations, and effects other than those described above will be made clear by the following description of the embodiments.

1 is a block diagram showing a configuration example of a radiation-resistant imaging device according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of arrangement of a radiation-resistant imaging device according to an embodiment of the present invention; FIG. FIG. 1 is a circuit diagram showing the configuration of an image sensor section according to an embodiment of the present invention. FIG. 3 is a characteristic diagram showing an example of radiation effects on a bipolar transistor according to an embodiment of the present invention. 1 is a diagram illustrating an application example (Example 1: mobile robot example) of a radiation-resistant imaging device according to an embodiment of the present invention; FIG. FIG. 3 is a diagram showing an application example (Example 2: example of a nuclear reactor vessel) of the radiation-resistant imaging device according to an embodiment of the present invention. FIG. 7 is a diagram illustrating an application example (Example 3: example of a proton beam therapy device) of a radiation-resistant imaging device according to an embodiment of the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, one embodiment of the present invention (hereinafter referred to as "this example") will be described in detail with reference to the accompanying drawings.

[1. Device configuration]
FIG. 1 shows the overall configuration of the radiation-resistant imaging device of this example.
The radiation-resistant imaging device of this example images an imaging target 1 with an image sensor section 100. A signal obtained by imaging with the image sensor unit 100 is sent to a signal processing unit 210 of the control device 200, and the signal processing unit 210 generates an image signal in a predetermined format. The image signal generated by control device 200 is transmitted to monitor 300. The monitor 300 displays an image based on the supplied image signal.
Note that the control device 200 includes a power supply circuit 220, and the image sensor unit 100 operates with DC power supplied from the power supply circuit 220.

The image sensor section 100 includes a lens 101, a photoelectric conversion section 110, and a signal amplification section 120.
The lens 101 guides image light of the imaging target 1 to the photoelectric conversion unit 110. Lens 101 is preferably made of radiation-resistant glass, such as lead-containing glass, rather than ordinary quartz glass.

The photoelectric conversion section 110 of the image sensor section 100 is composed of an integrated circuit formed on a Si (silicon) substrate. The photoelectric conversion unit 110 photoelectrically converts incident light obtained through the lens 101 to obtain a signal for each pixel. The pixel-by-pixel signal obtained by the photoelectric conversion section 110 is supplied to the signal amplification section 120.

The signal amplifying section 120 of the image sensor section 100 is composed of an integrated circuit formed on a substrate made of a material with a wider bandgap than Si. Here, the material having a wider band gap than Si is preferably a material having a band gap at least as large as that of SiC (silicon carbide), or a material having a wider band gap. Materials other than SiC that have a band gap comparable to or larger than SiC include, for example, GaN and diamond. A circuit made of a material with a band gap equal to or greater than SiC has better radiation resistance than a circuit made of a Si substrate.
When using SiC as the signal amplifying section 120, for example, SiC called 4H-SiC is applicable. Here, "4H" indicates the type of crystal polymorphism.

The signal amplification section 120 amplifies the signal obtained from the photoelectric conversion section 110. The signal amplified by the signal amplification section 120 is transmitted to the signal processing section 210 of the control device 200.
The signal processing section 210 performs signal processing on the signal transmitted from the signal amplification section 120 into an image signal of a predetermined format. The image signal obtained by the signal amplification section 120 is transmitted to the monitor 300.

FIG. 2 shows an example of the arrangement of the radiation-resistant imaging device of this example.
In the radiation-resistant imaging device of this example, an image sensor unit 100 is placed in a high-dose field 10 and images an object 1 to be imaged in the high-dose field 10 .
The control device 200 is placed in the low dose field 20, and the image sensor section 100 and the control device 200 are connected by a cable 91. Therefore, the monitor 300 (FIG. 1) connected to the control device 200 can display an image of the imaging target 1 in the high-dose field 10, making it possible to perform image sensing in the high-dose field.

[2. Configuration of image sensor section]
FIG. 3 shows a circuit configuration of the image sensor section 100.
The circuit shown in FIG. 3 shows the configuration of one pixel of the image sensor section 100.
The image sensor section 100 of this example includes a photoelectric conversion section 110 formed on a Si substrate, and a signal amplification section 120 formed on a substrate made of a material having a wider band gap than Si (for example, SiC).

A photodiode 111, which is a light receiving element, is arranged in the photoelectric conversion unit 110, and the photodiode 111 converts incident light (at least light in the visible light band) into a signal.
A capacitor 114 is connected to the photodiode 111 via a bipolar transistor 112 , and an output signal from the photodiode 111 is supplied to the capacitor 114 via the bipolar transistor 112 .

Here, as the bipolar transistor 112, a PNP type transistor is used. Therefore, as shown in FIG. 3, the emitter is connected to the photodiode 111, the base is supplied with the power available at the terminal 113, and the collector is connected to the capacitor 114. With this connection, a signal supplied from the photodiode 111 is amplified by the bipolar transistor 112 and supplied to the capacitor 114.

The signal (charge) accumulated in the capacitor 114 of the photoelectric conversion section 110 is read out by the signal amplification section 120. That is, while the signal accumulated in the capacitor 114 is reset by the reset MOSFET (MOS field effect transistor) 121, the signal accumulated in the capacitor 114 is read out to the operational amplifier 125 via the amplifier MOSFET 122 and the select MOSFET 123. It will be done.

A power supply V _DD is supplied to each MOSFET 121 , 122 , 123 . Resetting in MOSFET 121 is performed in synchronization with a reset pulse supplied to terminal 127, and pixel selection in MOSFET 123 is performed in synchronization with a select pulse supplied to terminal 124.
The operational amplifier 125 amplifies the supplied signal, and the amplified signal (pixel signal) is obtained at the output terminal 126. The pixel signal obtained at the output terminal 126 is then supplied to the signal processing section 210 (FIG. 1) of the control device 200. Note that the operational amplifier 125 is also composed of a MOSFET.
The power source obtained at the terminal 113 of the photoelectric conversion unit 110 and the power source V _DD obtained at the MOSFETs 121, 122, and 123 are supplied from the power source circuit 220 (FIG. 1) of the control device 200. Power for driving the operational amplifier 125 is also supplied from the power supply circuit 220.

The select pulse supplied to the terminal 124 and the reset pulse supplied to the terminal 127 are generated, for example, within the signal amplification section 120 within the image sensor section 100. Alternatively, a select pulse or a reset pulse generated by the signal processing section 210 in the control device 200 may be supplied to the image sensor section 100.

Note that the circuit configuration shown in FIG. 3 shows the configuration of one pixel, and in reality, the image sensor section 100 has a number of photodiodes 111 connected to the photodiodes 111 corresponding to the number of pixels. It has a circuit of However, some circuits such as the operational amplifier 125 may be shared by a plurality of pixels.

[3. Characteristics of image sensor]
The photoelectric conversion unit 110 of this example includes a bipolar transistor 112 as an amplification element, and the bipolar transistor has the effect of being more resistant to radiation than a MOSFET.
FIG. 4 is a characteristic diagram showing an example of radiation effects on a bipolar transistor. In FIG. 4, the vertical axis represents the DC current amplification factor (h _FE ) of the bipolar transistor, and the horizontal axis represents the integrated dose. FIG. 4 shows the change characteristic α of the direct current amplification factor (h _FE ) depending on the integrated dose.

As shown in Figure 4, in the case of a bipolar transistor, as the integrated dose increases, the DC current amplification factor (h _FE ) changes due to fluctuations in the base-emitter voltage, but this fluctuation does not affect the operation of the transistor. Not within acceptable values. Specifically, the variation characteristic α of the integrated dose shown in FIG. 4 converges at an integrated dose of 10 to 30 kGy, and the DC current amplification factor (h _FE ) converges to a value of about 120 from the initial value. This state where the direct current amplification factor (h _FE ) is about 120 is a value within the allowable range for operation as a transistor, and the integrated dose is within this allowable value up to at least about 100 kGy, and the photoelectric conversion unit 110 No fluctuation occurs to the extent that the signal is no longer output. Therefore, the photoelectric conversion unit 110 including the bipolar transistor 112 has excellent radiation resistance.

By configuring the signal amplification section 120 that amplifies the signal from the photoelectric conversion section 110 using a material with a wide bandgap such as SiC, the signal amplification section 120 can also have excellent radiation resistance. . When a circuit such as an operational amplifier is constructed of a material with a wide bandgap such as SiC, it has excellent radiation resistance, as described in Patent Document 1 described in the section [Problem to be Solved by the Invention]. It is.

In the case of conventional image sensors, MOSFETs are used as amplification elements, but MOSFETs constructed on Si substrates are more sensitive to radiation than bipolar transistors, and radiation irradiation can cause an increase in leakage current and a shift in threshold voltage. There is a problem that happens. Generally, in the case of a MOSFET configured on a Si substrate, there is a risk of failure at about 10 kGy.

Furthermore, since bipolar transistors can handle larger currents than MOSFETs, the maximum signal amplification factor is greater than that of MOSFETs.
Here, in the radiation-resistant imaging device of this example, the operational amplifier 125 is constructed on a substrate made of a material with a wide band gap, such as SiC, to improve radiation-resistant performance. However, if the operational amplifier 125 is configured on a substrate made of a material with a wide bandgap such as SiC, a problem arises in that the maximum amplification factor is reduced by one to two orders of magnitude compared to an operational amplifier configured on a Si substrate.

However, in the radiation-resistant imaging device of this example, since the bipolar transistor 112 of the photoelectric conversion unit 110 can perform amplification with a high amplification factor, the decrease in the amplification factor of the operational amplifier 125 can be compensated for by the bipolar transistor 112. . Therefore, the image sensor unit 100 can output pixel signals that are not affected by radiation.
Therefore, the image sensor unit 100 included in the radiation-resistant imaging device of this example has excellent radiation resistance and can output high-quality image signals.
Note that the radiation resistance of the image sensor unit 100 can be further improved by using radiation resistant glass for the lens 101 (FIG. 1) as well.

[4. Application example of radiation-resistant imaging device (Example 1: Mobile robot example)]
FIG. 5 shows an application example (Example 1) of the radiation-resistant imaging device of this example.
This example shows an example in which an image sensor is mounted on a mobile robot.
The system shown in FIG. 5 includes a mobile robot 400 placed in a high-dose field 10. The mobile robot 400 is capable of autonomous travel under commands from the control device 200 installed in the low-dose field 20. Mobile robot 400 and control device 200 are connected by cable 92.
Here, the image sensor section 100 is attached to the mobile robot 400, and the image sensor section 100 is connected to the control device 200 with a cable 91.

With the configuration shown in FIG. 5, the image sensor section 100 of the mobile robot 400 can image the surroundings of the mobile robot 400 in the high dose field 10, and the monitor 300 connected to the control device 200 displays Work in the dose field can be visualized and displayed for a long time. Therefore, for example, it is possible to improve the efficiency and reduce costs of decommissioning work at a nuclear power plant.

[5. Application example of radiation-resistant imaging device (Example 2: Reactor containment vessel example)]
FIG. 6 shows an application example (Example 2) of the radiation-resistant imaging device of this example.
In this example, an example is shown in which the image sensor section 100 is installed for monitoring the inside of a reactor containment vessel (PCV) of a nuclear power plant.
In the system shown in FIG. 6, an image sensor section 100 is arranged in a PCV (reactor containment vessel) 10a of a nuclear power plant, which is a high-dose field. A penetration 2a for passing the cable 91 is arranged in the outer wall 2 of the reactor containment vessel, and a control device 200 and a monitor 300 are arranged in the low-dose field 20 outside the PCV 10a.

With the configuration shown in FIG. 6, the inside of the PCV can be constantly imaged and monitored by the image sensor unit 100, and the safety of the nuclear power plant can be improved and periodic inspections can be made more efficient.

[6. Application example of radiation-resistant imaging device (Example 3: Example of proton beam therapy device)]
FIG. 7 shows an application example (Example 3) of the radiation-resistant imaging device of this example.
This example shows an example in which the image sensor section 100 is installed in a proton beam therapy apparatus.
In the system shown in FIG. 7, an image sensor unit 100 is arranged in a treatment room 10b which is a high-dose field, and a control device 200 and a monitor 300 are arranged in a low-dose field 20.
Then, the cable 91 is pulled out to the low-dose field 20 outside the treatment room outer wall 3, and the image sensor unit 100 is connected to the control device 200 of the low-dose field 20 by this cable 91.
The treatment room 10b is equipped with a proton beam therapy device 4 that performs treatment by irradiating a patient 6 on a treatment table 5 with proton beams, and an image sensor unit is installed near the proton beam therapy device 4 in the treatment room 10b. 100 are placed.

With the configuration shown in FIG. 7, the state of treatment with the proton beam therapy apparatus can be monitored using high-quality images on the monitor 300, and excellent treatment can be performed. Current imaging devices used to monitor proton beam therapy systems have a problem in that the images flicker due to the effects of radiation, making them difficult to see. However, the system in this example provides images with higher quality than before. Be able to monitor. Therefore, the condition of a patient undergoing treatment in a high-dose field can be constantly monitored with high image quality. In this way, according to the system of this example, it is possible to visualize the treatment and the presence or absence of abnormalities in the patient, thereby enabling safer treatment.

[7. Modified example]
The present invention is not limited to the embodiments described above, and includes various modifications.
For example, the circuit configuration of the image sensor section 100 shown in FIG. 3 using transistors, MOSFETs, etc. is merely an example, and other circuit configurations as an image sensor may be applied.

Furthermore, in the embodiment described above, the signal read from the photodiode 111 is amplified by the bipolar transistor 112 arranged in the photoelectric conversion section 110 of the image sensor section 100. On the other hand, the signal may be stored in the capacitor 114 without being amplified in the photoelectric conversion unit 110, and the signal may be amplified only in the signal amplification unit 120 at the subsequent stage.
Further, in the above-described embodiment, a photodiode is used as a photoelectric conversion element that converts incident light into a signal on a pixel-by-pixel basis, but other photoelectric conversion elements may be used.
Further, the application examples shown in FIGS. 5 to 7 are also preferred examples, and the present invention can be applied to various imaging applications in other high-dose fields.

1... Imaging target, 2... Reactor containment vessel outer wall, 2a... Penetration, 3... Treatment room outer wall, 4... Proton beam therapy device, 5... Treatment table, 6... Patient, 10, 10a, 10b... High dose field, 20 ...Low dose field, 91, 92... Cable, 100... Image sensor unit, 101... Lens, 110... Photoelectric conversion unit, 111... Photodiode, 112... Bipolar transistor, 113... Terminal, 114... Capacitor, 120... Signal amplification unit , 121... MOSFET (for reset), 122... MOSFET (for amplifier), 121... MOSFET (for selection), 124... terminal, 125... operational amplifier, 126... output terminal, 200... control device, 210... signal processing section, 220 ...power supply circuit, 300...monitor, 400...mobile robot

Claims (6)

A radiation-resistant image sensor that is placed in a nuclear power plant or radiation utilization facility to take images,
A photodiode formed on a silicon substrate converts incident light into a signal for each pixel, a bipolar transistor that amplifies the signal for each pixel obtained by the photodiode, and a signal for each pixel amplified by the photodiode. a photoelectric conversion unit having a capacitor for accumulating;
A MOSFET is formed on a substrate made of a material with a wider band gap than silicon, and the signals accumulated in the capacitor for each pixel are reset by a reset pulse and selectively taken out by a select pulse, and each pixel taken out by the MOSFET. A radiation-resistant image sensor, comprising: a signal amplifying section having an operational amplifier that amplifies a signal of the radiation-resistant image sensor. The radiation-resistant image sensor according to claim 1, wherein the material with a wide bandgap is a material with a bandgap as wide as or wider than silicon carbide. In a radiation-resistant imaging device that has an image sensor and a signal processing unit that obtains an image signal in a predetermined format based on the signal obtained by the image sensor, and that is placed in a nuclear power plant or a radiation utilization facility and performs imaging. ,
The image sensor is
A photodiode formed on a silicon substrate converts incident light into a signal for each pixel, a bipolar transistor that amplifies the signal for each pixel obtained by the photodiode, and a signal for each pixel amplified by the photodiode. a photoelectric conversion unit having a capacitor for accumulating;
A MOSFET is formed on a substrate made of a material with a wider band gap than silicon, and the signals accumulated in the capacitor for each pixel are reset by a reset pulse and selectively taken out by a select pulse, and each pixel taken out by the MOSFET. a signal amplification section having an operational amplifier that amplifies the signal of the signal;
The signal processing section obtains the image signal from the signal amplified by the signal amplification section. The radiation-resistant imaging device. The radiation-resistant imaging device according to claim 3, further comprising a lens made of radiation-resistant glass that guides incident light to the image sensor. The radiation-resistant imaging device according to claim 3 or 4, wherein the image sensor is placed on a mobile robot placed in the nuclear power plant . The radiation-resistant imaging device according to claim 3 or 4, wherein the image sensor is arranged at a location where a proton beam therapy device as the radiation utilization facility is installed.

Images