JP2003134396A - Imaging device, driving method thereof, radiation imager using the element, and radiation imaging system using the device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable high speed high sensitivity moving image photographing and wide dynamic range low noise still image photographing by using one imaging device. SOLUTION: A plurality of picture elements are provided which comprise; a photodiode PD for converting optical energy to an electric signal; MOS transistors M6, M7 which are connected with the photodiode PD and in which the electric signal outputted from the photodiode PD is inputted; and a MOS transistor M3 arranged between the photodiode PD and the MOS transistor M7 or a MOS transistor M2 and the MOS transistor M3 which are arranged between the photodiode PD and the MOS transistor M6 and between the photodiode PD and the MOS transistor M7, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup device, a method for driving the image pickup device, a radiation image pickup apparatus using the image pickup device, and a radiation image pickup system using the same. The present invention is
In particular, it is preferably used for a large area radiation (X-ray) image pickup device and system for reading an image by using high-energy radiation such as X-rays and gamma rays, an image pickup element that can be used for the same, and an image pickup element driving method. is there.

[0002]

2. Description of the Related Art Digitization is progressing in various fields of medicine. In the field of X-ray diagnosis, incident X-rays are converted into visible light by a scintillator (phosphor) and II (image intensifier), and a two-dimensional digital X-ray with high sensitivity is used to capture the visible light image captured by a TV camera. Fluoroscopes have been developed.

Further, there is a system in which a fluoroscopic (moving image) and photographing (still image) system is configured by combining the fluoroscopic system and the film photographing system.

A two-dimensional X-ray image pickup device has been developed as a still image pickup device.

As a two-dimensional X-ray image pickup device, for example, a small CCD image pickup device has been put into practical use for dentistry, and a maximum of 43 cm □ of amorphous silicon (a -Si) is used for mammography and chest imaging. A large still image pickup device has been made. An image sensor using an amorphous silicon semiconductor on a glass substrate can be easily obtained as a large plate, and there is one that realizes a large plate X-ray imaging device by laminating four panels. An example of this type of technology is US Pat. No. 5,315,10.
There is one described in No. 1.

There is also a proposal to construct a large X-ray image pickup device using a plurality of single crystal image pickup devices (silicon image pickup device, etc.). An example of this type of technology is U.S. Pat.
And US Pat. No. 6,0059,111. As a single crystal image pickup device, there are a CCD type image pickup device using silicon, a MOS type image pickup device, a CMOS type image pickup device and the like.

As described above, in the field of medical X-ray diagnosis, which is being digitized, it is expected that the still image pickup device will have higher sensitivity and the next-generation moving image pickup device (such as fluoroscopy).

An X-ray image pickup device using an image pickup device is provided with a fluorescent plate that emits light in response to X-rays emitted from an X-ray source, and an image pickup device that picks up the light emitted from the fluorescent plate. When shooting with such an X-ray imaging device, a small amount of X-rays that have little effect on the human body are emitted during positioning before the main shooting, and high-definition images are shot during the main shooting. In order to do so, emit a stronger X-ray. For this reason,
At the time of positioning, an image that is slightly inferior to the image is required, but an image with high sensitivity is required, and at the time of actual shooting, it is necessary to obtain an image with larger gradation and less noise. As a simple method, there is a method in which the output is gained by a circuit outside the sensor chip, or a method in which the output stage is gained in the sensor.
However, this method has a demerit that noise itself generated in the sensor chip is also amplified, and it is also difficult to change the dynamic range for each shooting mode.

[0009]

(First Problem) In the field of X-ray diagnosis described above, X-ray irradiation conditions greatly differ between fluoroscopy (moving image) and still image shooting. It was difficult to realize the above performance with the same imaging device.

FIG. 11 is a perspective view (moving image) in the medical field,
The relationship between the X-ray dose and the output during shooting (still image) is shown. To capture fluoroscopy (moving images), humans are continuously irradiated with X-rays, and in order to reduce the X-ray exposure dose as much as possible, the irradiation dose of X-rays is several tens to 100 minutes compared to still image shooting. Need to be 1. In addition, a maximum reading speed of 60 to 90 frames / second is required. To perform this reading, a fluoroscopic (moving image) imaging device is several tens times more sensitive and several tens of times faster than still image shooting. High speed is required. Also for still image shooting
A dynamic range close to 4 digits is required, but for fluoroscopy (video), a dynamic range of about 2 digits is sufficient.

The conventional II (image intensifier) and the X-ray image pickup device of the TV camera have high sensitivity, but the dynamic range of the TV camera is smaller than the dynamic range of film, and it is difficult to use it for photographing a still image for diagnosis. Moreover, since the resolution of II (Image Intensifier) is not sufficient for diagnostic still image shooting, it is generally used for fluoroscopy (moving images). Further, the device and system become large and the cost increases. In addition, a perspective / imaging system in which a film system is combined with an II (image intensifier) / TV camera also requires a large device and system, which increases the cost.

Although the CCD type image pickup device is a complete depletion type and has high sensitivity, it is not suitable for a large area image pickup device. Since the CCD type image pickup device is a charge transfer type, the transfer becomes a problem as the area becomes larger and the number of transfer stages increases (the number of pixels becomes higher). That is, the drive voltage is different between the drive end and the vicinity of the center, and complete transfer becomes difficult. The power consumption is expressed by CVf ² (C is the capacitance between the substrate and the well, V is the pulse amplitude, and f is the pulse frequency). The larger the area, the larger C and V, and the CMOS power consumption. It is more than 10 times larger than the image sensor. As a result, the peripheral drive circuit becomes a heat source and noise source, and high S
It will not be / N. Thus, the CCD type image pickup device has a surface that is not suitable for a large image pickup device.

In the structure of a simple large plate image pickup device using a large number of single crystal image pickup devices, a dead space is always formed at the alignment portion of each image pickup device (peripheral circuits such as shift registers, multiplexers and amplifiers, and signals from the outside). Area for connecting external terminals for power supply and power supply, protection diode against static electricity and protection circuit for protection resistance is required separately from the pixel area), and this area is a line defect (image joint). And the image quality is degraded. Therefore tapered FO
The P (fiber optic plate) is used to guide the light from the scintillator to the pixel area of the image sensor while avoiding the dead space, but it requires a very expensive tapered FOP and increases the manufacturing cost. . Further, in the tapered FOP, it becomes difficult for light from the scintillator to enter the FOP depending on the taper angle, the output light amount is reduced, the sensitivity of the image sensor is canceled, and the sensitivity of the entire apparatus deteriorates.

The manufacturing process of the amorphous silicon type large plate image pickup device is advantageous in that a large plate is obtained as compared with the CCD type image pickup device or the CMOS type image pickup device, but the glass substrate is more preferable than the single crystal silicon semiconductor substrate. The fine processing of the upper semiconductor is difficult, and as a result, the capacitance of the output signal line becomes large. This capacitance causes the largest noise (kTC noise) and limits the improvement of sensitivity. In addition, amorphous silicon does not have sufficient semiconductor characteristics for high-speed operation, making it difficult to record moving images at 30 frames / sec or more on large plates.

(Second Problem) As one method for solving the difference in the required images between the photographing at the time of positioning and the actual photographing, there is a means for switching the dynamic range and the gain in the pixel cell in the sensor. An example is shown in FIG.
Described in. PD is a photodiode, which is previously reset to the reset potential Vr via the MOS transistor M1. When shooting with high sensitivity, the accumulation period (or
The charge generated in the photodiode PD during the X-ray irradiation time is P
It is accumulated in D and the parasitic capacitance C1, read by the transfer transistor M2, and accumulated in the capacitance C3. The potential change at this time is current-amplified by the source follower amplifier composed of the transistor M6 and output. The transistor M4 is a pixel row selection switch. At this time, by designing the capacitor C3 to be small, it is possible to increase the potential change of C3 with respect to the photocharge, and it is possible to read with high sensitivity. On the other hand, in the actual photographing, by turning on the transistor M3, the capacity for receiving the photocharges transferred from the transfer transistor M2 becomes C2 + C3, and a larger dynamic range can be obtained.

Although the switching of the sensitivity and the switching of the dynamic range can be realized by the above method, the transistor of the pixel source follower (M6 in FIG. 12).
It is necessary to design a size suitable for the high-sensitivity shooting mode, and it is preferable that the size of the gate electrode is small.
However, in this case, the 1 / f noise of the MOS transistor deteriorates the image quality. Generally 1 / f of MOS transistor
It is said that noise is inversely proportional to the area of the gate electrode, and the influence of 1 / f noise becomes large when the gate size is small.

(Purpose of the present invention)
To provide a large-area thin-type radiation imaging device, especially an X-ray imaging device, which enables transparent (moving image) with high sensitivity, still image shooting with high definition and wide dynamic range, and can provide a seamless whole image. Is.

Another object of the present invention is to provide an image pickup device capable of performing gain switching and dynamic range switching.

[0019]

An image pickup device of the present invention comprises a photoelectric conversion means for converting light energy into an electric signal, and an electric signal electrically connected to the photoelectric conversion means and outputted from the photoelectric conversion means. Between the first and second amplification means to which a signal is input and the photoelectric conversion means and the first or second amplification means, or between the photoelectric conversion means and the first and second amplification means. A plurality of pixels each having switch means provided between the means and the means.

The radiation image pickup apparatus of the present invention is characterized by including an image pickup apparatus formed by arranging a plurality of the image pickup elements of the present invention, and a scintillator for converting radiation into light.

A radiation image pickup system of the present invention uses the radiation image pickup apparatus of the present invention, and a driving method of the image pickup element of the present invention relates to a method of driving the image pickup element of the present invention.

In a preferred aspect of the present invention, for example, a source follower amplifier having a different gate size is selected depending on the image capturing mode to take countermeasures against 1 / f noise. That is, each pixel has a plurality of source follower amplifiers having different gate areas, and the parasitic capacitance of the gate of the source follower amplifier is used to selectively change the capacitance value for receiving photocharge. In the case of high-sensitivity imaging (positioning imaging in the case of the X-ray sensor described above), high-sensitivity imaging becomes possible by selecting a source follower amplifier with a small gate size, and in the case of wide dynamic range imaging (described above). The main shooting with the X-ray sensor), and by selecting a source follower amplifier with a large gate size, 1 / f noise is small and shooting with a wide dynamic range is possible.

[0023]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

(First Embodiment) FIG. 1 is an equivalent circuit diagram of a pixel cell of an image sensor according to the first embodiment of the present invention.

In FIG. 1, PD is a photodiode,
C1 is a capacitance formed by parasitic capacitance or additional capacitance connected to the cathode side of the photodiode PD, and M1 is a reset transistor that resets the photodiode PD and the capacitances C1 to C3. M2 is a transistor serving as a transfer switch connected to the capacitor C1, and C
Reference numeral 2 is a capacitance for accumulating charges transferred from the photodiode PD and the capacitance C1, M4 is a selection switch, and M6 is a source follower transistor whose gate is connected to the capacitance C2. Further, M3 is a transistor serving as a transfer switch connected to the capacitor C1, C3 is a capacitor for accumulating charges transferred from the photodiode PD and the capacitor C1, and M5.
Is a selection switch, and M7 is a source follower transistor whose gate is connected to the capacitor C2. Thus, the charge accumulated in the photodiode PD and the capacitor C1 is 2
It is designed to be output via the output means of the system.

The operation of the pixel cell of FIG. 1 will be described. The potential of the photodiode PD and the capacitance C1 formed by the parasitic capacitance or the additional capacitance is reset to the reset potential Vr by the reset transistor M1. When light is incident on the photodiode PD during the accumulation period, the generated photocharge is stored in the photodiode PD and the capacitor C1. By turning on the transistor M2 serving as the transfer switch or the transistor M3 serving as the transfer switch, the potential of the capacitor C2 or the capacitor C3 changes according to the generated photocharge.

Here, the capacitances C2 and C3 are dominated by the parasitic capacitances of the gate electrodes of the transistors M6 and M7, respectively. The transistors M4 and M5 are row selection switches, and are switches for selecting a certain row of pixel cells that are two-dimensionally arrayed. Transistors M4 and M5
Is connected to the power supply V. Although not shown, the output node OUT is connected to a constant current source and forms a source follower amplifier together with the transistors M4 and M6. Transistors M5 and M7 are also transistor M
4 and M6 form a source follower amplifier with a constant current source and select transistors M2 and M4.
By selecting the transistors M3 and M5, either source follower amplifier can be selected.

In the present embodiment, it is possible to increase the capacitance C3 with respect to the capacitance C2 by increasing the gate electrode size of the transistor M7 with respect to the transistor M6. In terms of circuit operation, the parasitic capacitance is greatly influenced by the parasitic capacitance between the gate and the drain.
It is possible to adjust the capacitances C2 and C3 by changing the gate width W of M6 and M7. Also, the transistor M6,
When the gate width W and the gate length L of M7 are changed at the same ratio, the current driving capability of the transistor does not change,
Similar DC characteristics of the source follower can be obtained, which facilitates the design.

The advantage of being able to select the read capacities C2 and C3 depending on the case is as described in the prior art, and the dynamic range (D-range) depends on the photographing conditions.
e) can be selected. Further, the present invention has the following advantages.

Generally, 1 / f noise is a cause of SN deterioration of an image pickup device (CMOS sensor) formed by MOS transistors. The influence of 1 / f noise increases as the gate electrode area decreases due to the miniaturization of MOS transistors. It is said that the power density of 1 / f noise is inversely proportional to W × L of the MOS transistor. SN
When the dynamic range is widened by the conventional method to improve the image quality, and the shooting time is increased or the light amount is increased, the KTC noise and the optical shot noise are improved, but the 1 / f noise cannot be improved. . According to the present invention, in the mode of shooting with a wide dynamic range, in addition to the effect of improving SN obtained by the conventional technique, the improvement of 1 / f noise can also be obtained.

If the gate size of the transistor M6 is W = 1 μm and L = 1 μm, and the gate size of the transistor M7 is W = 10 μm and L = 10 μm, the gate-source capacitance of the transistor M7 is 10 times. That is, the capacity C3 is 10 times larger than C2, and the dynamic range is 10 times larger. Therefore, when imaging with the same amount of light,
It is possible to operate within the dynamic range up to 10 times the storage time. On the other hand, 1 / f noise can be reduced to 1/100 due to the influence of the gate electrode area (1/100).

Therefore, according to the present invention, the 1 / f noise of the source follower amplifier is improved, so that an image pickup device having a good SN can be obtained. Further, since the capacitance and the gate electrode of the source follower are commonly used, the degree of integration can be improved, and the sensitivity of the photodiode can be increased and the SN ratio can be improved. it can.

Although the MOS characteristics of the source follower transistor are NMOS in the above description and the equivalent circuit, similar effects can be obtained when the transistor is PMOS. The driving timing is not limited to the above example, and the photodiode PD, the capacitor C1, and the capacitor C2 (or the capacitor C3) are reset while the transistors M2 and M4 (or the transistors M3 and M5) are turned on in advance. However, the accumulation may be started after the reset transistor M1 is closed. In this case the transistor M
It is possible to eliminate the feedthrough caused by opening and closing the gates of 2, M3, M4 and M5.

The gate of the transistor M2 and the gate wiring of the transistor M4 can be made common to reduce the wiring. Similarly, the gate wirings of the transistor M3 and the transistor M5 can be shared.

When the present invention is applied to an X-ray image pickup device, as described in the prior art, the X-ray image pickup device has a large merit that the sensitivity and the dynamic range can be switched, and the X-ray image pickup device has a large format. Since it is easier to use than in a portable camera, the disadvantage of increasing the pixel size is small, and it is easy to configure the pixel according to the present invention. Therefore, the image pickup device according to the present invention is particularly effective when applied to an X-ray image pickup apparatus.

(Second Embodiment) A second embodiment according to the present invention will be described with reference to FIG. FIG. 2 is an equivalent circuit diagram of a pixel cell of the image sensor of the second embodiment according to the present invention.

In the present embodiment, the capacitor C1 is directly connected to the gate of the transistor M6 without passing through the transistor M2. When high sensitivity is required Transistor M4
Select. At this time, the capacity is C1 + C2. If a wide dynamic range is required, select M3 and M5. The capacity at this time is C1 + C2 + C3. The capacity ratio at this time is 10. That is, C1 + C2 + C3 = 1
It becomes 0 (C1 + C2). Therefore, C3 = 9 (C1 + C
2). An image pickup apparatus using this embodiment will be described in the fourth embodiment. Therefore, as described in detail, the junction capacitance C1 of the photodiode is made as small as possible in order to realize high sensitivity. In this embodiment, when the gate size of the transistor M6 is W = 1 μm and L = 1 μm, and the gate size of the transistor M7 is W = 9 μm and L = 9 μm, the gate-source capacitance C3 of the transistor M7 is 9 times the capacitance C2. Become. When selecting the transistor M4 and M3, M5
When is selected, the dynamic range becomes 10 times.
The effect of 1 / f noise reduction is improved to about 1/81 in the same way as in the first embodiment, and the effect of the present invention is also effective in this embodiment.

(Third Embodiment) According to the present invention, the same effect can be obtained in an image pickup device having a complete transfer type photodiode. A third embodiment according to the present invention will be described with reference to FIG. FIG. 3 is an equivalent circuit diagram of a pixel cell of an image sensor according to the third embodiment of the present invention.

In this embodiment, the photodiode PD section is completely depleted by the reset operation, and then the accumulation of photocharges is started. As a resetting method, it is possible to completely deplete the photodiode PD by turning on the transistors M1 and M8 at the same time.
After the accumulation, the generated photocharge is transferred to the capacitor C1 by the transfer switch M8. By designing the photodiode PD to be completely depleted again, it is possible to transfer all the generated photocharges to the capacitor C1, and it is possible to obtain a sensor with high sensitivity. The operations of the transistors M2 to M7 are the same as in the first embodiment. This makes it possible to select the dynamic range and obtain an image with less noise. Further, the effect of this embodiment is effective when the transistor M2 is omitted as in the second embodiment.

(Fourth Embodiment) FIG. 4 is a partial plan view of one image pickup element when the CMOS type image pickup element having the one pixel circuit of the second embodiment is used in an X-ray image pickup apparatus. Each pixel is an amplification MOS for movie mode (high sensitivity mode) as described later.
-Type transistor (M6 in Fig. 2) and the light intensity is 10
Amplification MOS with a large area in the still image mode (normal mode), which is 0 times or more and requires a wide dynamic range
Type transistor (M7 in FIG. 2). Further, each pixel has a means for switching the sensitivity according to the switching of these operation modes. As a result, an image sensor capable of capturing a wide dynamic range when capturing a still image and high sensitivity when capturing a moving image is realized. In this embodiment, as described later,
Nine tiles of this image sensor are attached to form a large-scale image sensor.

In FIG. 4, 11 is an end portion of the image pickup device, 1
2 is a boundary of pixel cells, 13 is a block of a vertical shift register as a scanning circuit, 14 is a block of a common processing circuit such as a multiplexer, a horizontal shift register, a common amplifier, 15 is an external terminal, and 16 is provided on an external terminal 15. The bumps, 17 are protective resistors, and 18 are protective diodes.

FIG. 7 shows a case where one image pickup element is taken out from the currently mainstream 8-inch wafer. By CMOS process
Create a single 136 mm square CMOS image sensor.

In a medical X-ray image pickup device, the size of a pixel (pixel cell) may be as large as about 100 μm □ to 200 μm □. The size in this embodiment is 160 μm □.

FIG. 8 shows an image pickup element portion of a 408 mm square large area X-ray moving image pickup apparatus formed by laminating nine such image pickup elements.

FIG. 9 shows a cross section taken along the line AA 'of FIG. Gd ₂ O ₂ S and Cs using europium, terbium, etc. as active substances
A scintillator 21 made of I or the like is installed on a FOP (equal magnification optical transmission means) 22. The X-rays strike the scintillator 21 and are converted into light such as visible light. The image sensor 23 detects light such as visible light. The scintillator 21 is preferably selected so that its emission wavelength matches the sensitivity of the image sensor 23. In this embodiment, the light passes through the scintillator 21.
The FOP 22 is used to prevent the X-rays from hitting the image sensor 23. However, if the scintillator 21 absorbs the X-rays sufficiently, such as using the X-rays with low energy, it is not necessary to use the FOP. The base 24 mounts the image pickup device 23, and the substrate 25 is an external processing substrate having a circuit for supplying a power source, a clock, and the like of the image pickup device 23, and for extracting and processing a signal from the image pickup device 23. The flex 26 is an electrical connection portion of each image pickup device and an external processing substrate by TAB (Tape Automated Bonding).

In this embodiment, a vertical shift register, a horizontal shift register, a multiplexer, an output amplifier, an external terminal, an electrostatic protection circuit, etc. (protection diode and protection resistor), which are conventionally arranged on the outer peripheral portion of the imaging device, are provided in the imaging device. It is arranged in the pixel area. With this configuration, the entire surface of the image sensor becomes the pixel area.
There is no dead space around the pixel that causes pixel defects.

A large plate image pickup device can be formed by arranging these image pickup elements in a tile shape so that there is substantially no gap. Here, the fact that there is substantially no gap means that there is no gap between the image pickup elements in the image formed by the nine image pickup elements. Further, with the above circuit configuration, it is possible to obtain a seamless large plate image. Furthermore, one block of the shift register that processes one line is placed so that it fits within one pitch. These blocks are arranged side by side to form a series of vertical shift register blocks and horizontal shift register blocks. These blocks extend linearly in the vertical and horizontal directions.

Input of a clock or the like of the image pickup device, power supply, and output of a signal from the image pickup device pass through a TAB flex connected to an external terminal provided at the end of the image pickup device, and between the external processing substrate arranged on the back side of the image pickup device. Done in. The thickness of the TAB flex is thin enough for the size, even if it passes through the gap between the image sensors.
No image defects occur.

FIG. 5 shows a state of a pixel cell in which the unit block (unit for selecting and driving one row) of the vertical shift register in FIG. 4 is arranged in one region (one pixel cell) together with one pixel circuit. The one-pixel circuit is the one shown in the second embodiment. The area of the unit block and the pixel circuit does not reflect the embodiment because it is a schematic diagram. Further, in the actual layout, the light except the photodiode of the pixel circuit is covered with a light shielding layer (not shown) to prevent the light from the scintillator from entering other than the photodiode. The vertical shift register shows a simple circuit composed of a static shift register and a transfer gate to generate a transfer signal ΦTX, a reset signal ΦRES, and a selection signal ΦSEL. These are driven by a signal from a clock signal line (not shown). The circuit configuration of the shift register is not limited to this, and an arbitrary circuit configuration can be adopted depending on various driving methods such as addition and thinning-out reading. However, as in the present embodiment, the functional block is arranged in one pixel cell together with the pixel circuit, the shift register is provided in the pixel region, and the image sensor of the entire pixel region is realized.

As a scanning circuit, not a shift register
An n to 2 ⁿ decoder can also be used. By connecting the output of the counter that increments sequentially to the input of the decoder, it becomes possible to perform sequential scanning in the same way as the shift register, while by inputting the address of the area where the image is to be obtained to the input of the decoder, random scanning can be performed. An image of the area can be obtained.

The common processing circuit arranged in each area (pixel cell) in the pixel area is a final signal output amplifier, a serial-parallel conversion multiplexer, and a buffer that collectively process a plurality of gate circuits and the like in common. Means a circuit that does.

FIG. 6 shows a layout (schematic diagram) of a one-pixel circuit corresponding to the second embodiment. Pixel cell size is 160 μm
□

In FIG. 6, reference numeral 31 is a photodiode as a photoelectric conversion portion, and 32 is a moving image mode (high sensitivity mode).
Corresponding first amplification MOS transistor (pixel amplifier 1),
Reference numeral 33 is a second amplification MOS transistor (pixel amplifier 2) compatible with the shooting mode (normal mode), and 34 is a switching MOS transistor (switching between the moving image mode (high sensitivity mode) and the shooting mode (normal mode, wide dynamic range mode) ( Mode selector switch), 35 is a selection MOS type transistor for selecting the photoelectric conversion unit, 36 is a reset MOS type transistor (reset switch) for discharging the charges accumulated in the photoelectric conversion unit, and 37 is an output signal line Is. First
The size of the amplification MOS type transistor of is W = 1 μm and L = 1 μm. The size of the second amplification MOS transistor is W = 10μ
m, L = 10 μm.

In the pixel portion, the signal charge generated by the photodiode is stored in the junction capacitance. The accumulated charge is converted into a charge / voltage by an amplification MOS transistor (pixel amplifier) that functions as a source follower, and is output as a voltage.

There are peculiar conditions required for the photoelectric conversion unit in the X-ray image pickup device for both fluoroscopy (moving image) and photographing (still image). The present embodiment taken to satisfy this condition will be described in detail.

(Motion Picture Mode) In a photodiode having a pn junction, when the photo-generated carrier Q _P is accumulated in the capacitance C of the photodiode and converted into a voltage, the photo-signal voltage V _P due to the photo-generated carrier is V _P = Q _P / C ・ ・ ・ ・ (1) There is reset noise generated every time the photodiode is reset. This appears as random noise. The reset noise V _N is V _N = √ (kTC) ··· (2) (k: Boltzmann constant, T: temperature (K)). The S / N ratio is V _P / V _N = Q _P · √ (1 / (kTC)) ··· (3). In order to obtain the maximum sensitivity (S / N ratio) when shooting movies, it is desirable to make the storage capacity C as small as possible.

The magnitude ΔV of the output of the source follower (pixel amplifier) can be expressed as follows.

ΔV = G · Q _P / C _MOS Here, G is the gain of the source follower, C _MOS is the parasitic capacitance of the pixel amplifier, and Q _P is the signal charge accumulated in C _{MO S.} As ΔV increases for the same signal charge Q _P , the charge /
The voltage conversion gain becomes large, which is advantageous from the viewpoint of S / N and the like. To increase ΔV for the same signal charge Q,
Since the gain G of the source follower is usually about 0.7 to 0.9 and hardly changes, the capacitance C _MOS is made as small as possible.

Movie mode (high sensitivity mode) of this embodiment
Then, the photodiode and the first amplification MOS transistor (pixel amplifier 1) are directly connected by turning off the mode changeover switch (M3 in FIG. 2). At this time, the signal storage capacitance is equal to the junction capacitance C _{PD of the} photodiode (C1 in FIG. 2) and the first capacitance.
This is a combined capacitance of the capacitance C1 (C2 in FIG. 2) parasitic on the amplification MOS type transistor (pixel amplifier 1). In order to obtain the highest sensitivity in the movie mode (high sensitivity mode), set the C _PD to the minimum capacity. In order to increase the light utilization rate, it is better to have a larger photodiode area, but if the photodiode area is made large, the capacitance C _PD tends to increase, so by making the electrode section (not shown) as small as possible,
Use a structure that does not increase C _PD . Similarly the first amplification MO
S-type transistor (pixel amplifier 1) parasitic capacitance C1 (Fig. 2
Similarly, C2) is set to the minimum capacity. The parasitic capacitance is the first amplification MOS transistor (pixel amplifier 1)
Since it depends on the area of, the area should be as small as possible.

(Still image mode) The amount of charge that can be stored in the storage capacitor is Q _P = V _P · C (1) Since the dynamic range increases in proportion to the amount of charge that can be stored, it is necessary to increase the storage capacity C as necessary to widen the dynamic range during still image shooting. In the present invention, only this large storage capacitor is not separately provided in the pixel, and a large area for still image mode is used.
Two amplification MOS type transistors (pixel amplifier 2) are provided, and the parasitic capacitance of this transistor is used as the storage capacitance.

Further, 1 / f noise (flicker noise) and thermal noise are likely to occur in the MOS type transistor, which is random noise and produces a random background image. In terms of device design, assuming that the channel length of a MOS transistor is L and the channel width is W, thermal noise is proportional to √ (L / W) and 1 / f noise is inversely proportional to L · W. To reduce noise, the channel length L should be minimized and the channel width W should be set large. 1 / f noise, which becomes large at low frequencies in a source follower as an amplifier that becomes a particularly large noise source, becomes a particular problem in the still image mode. In the structure where the first amplification MOS type transistor (pixel amplifier 1) designed to have high sensitivity in the moving image mode is shared in the still image mode and a large storage capacity for a wide dynamic range is separately provided in the pixel, The 1 / f noise of the pixel amplifier is large and adversely affects the image quality. On the other hand, 1 / f noise is not a problem when shooting high-speed movies. Therefore, a large area second amplification MOS transistor (pixel amplifier 2) for the still image mode is provided, and a mode changeover switch for selecting the second amplification MOS transistor (pixel amplifier 2) is provided. In the still image mode, a low noise pixel amplifier is used to improve the image quality, and the parasitic capacitance of this transistor is used as a storage capacitor in the still image mode to ensure a wide dynamic range.

Further, in the present embodiment, the PMOS type transistor whose intrinsic 1 / f noise is small is used as the source follower. This makes it 1 compared to NMOS type transistors.
It can be reduced to about 10/10. When the image pickup device of the present invention is used in an X-ray image pickup device, X-rays passing through the scintillator directly hit the transistor. The PMOS transistor is more preferable than the NMOS transistor because it has higher X-ray durability (increase in leak current and less variation in threshold Vth).

In the pixel circuit of this embodiment, the transfer switch is not provided, and the photodiode and the gate of the amplification transistor are directly connected to each other to form a photoelectric conversion section. In this embodiment, kTC noise is generated when the photoelectric conversion unit is reset because it is not a complete transfer, but it is known to use a double sampling circuit to remove this noise.

In the medical X-ray image pickup device, the size of the pixel may be as large as about 50 μm □ to 200 μm □, so that two amplification MOS transistors corresponding to the operation mode may be arranged in the pixel cell. A sufficiently large aperture ratio can be realized.

In this embodiment, since the shift register is arranged in the pixel area, the X-rays passing through the scintillator directly hit the shift register. The shift register circuit is used to sequentially transfer pulse signals. A static shift register is used as the shift register. In principle, the static type is relatively insensitive to X-rays,
It can be used in a place where the X-ray directly hits as in this embodiment. Therefore, if a static shift register is used, it is possible to realize an image pickup device with less reliability and less X-ray damage and errors.

Since this embodiment uses a CMOS type image pickup device as an image pickup device, it consumes less power and is suitable for a large-sized image pickup device.

The reason why the multiplexer is built in the image pickup device is to speed up the operation of the image pickup device.

A signal is taken out from the image pickup device to the outside through an external terminal, but there is a large stray capacitance around this external terminal. Therefore, the transmission characteristic of the signal can be compensated by providing the amplifier in the preceding stage of the external terminal. Further, the nine image pickup elements can be driven by a common drive pulse, so that the peripheral drive pulse generation circuit is also easy. Further, it can be seen that the common driving allows the image pickup element driving circuit to be made common, which is also excellent in mounting.

(Fifth Embodiment) FIG. 10 shows an application example of the radiation imaging apparatus according to the present invention to an X-ray diagnostic system. X-ray 60 generated by X-ray tube 6050
60 passes through the chest 6062 of the patient or subject 6061, and the scintillator 21, FOP2 as shown in FIG.
2, the radiation enters the radiation imaging device 6040 including the imaging element 23 and the external processing substrate 25. The incident X-ray contains information on the inside of the body of the patient 6061. The scintillator emits light in response to the incidence of X-rays, and the image sensor photoelectrically converts the light to obtain electrical information. This information is converted to digital, image-processed by the image processor 6070, and can be viewed on the display 6080 in the control room.

Further, this information can be transferred to a remote place by a transmission means such as a telephone line 6090, can be displayed on a display 6081 such as a doctor room at another place, or can be stored in a recording means such as an optical disc. It is also possible to diagnose. Also the film processor 6
It is also possible to record by 100 on the film 6110.

As described above, the present invention is a medical X
Although it can be applied to a line sensor, it is also effective when applied to other uses.

[0072]

According to the present invention, the following effects can be obtained. (1) High-speed / high-sensitivity moving image shooting / wide dynamic range / low-noise by providing amplification means such as amplification MOS transistors of different sizes according to the image capture mode and mode switching means (switch means) in each pixel Still images can be captured with a single image sensor. (2) By arranging multiple image sensors with the above effects, a seamless full-screen image can be provided, and high-speed / high-sensitivity moving image shooting / wide dynamic range / low-noise still image shooting can be combined. A line imaging device can be provided.

(3) In the image sensor according to the present invention, M
Each pixel has two kinds of amplification means such as OS transistors,
Gain selection and dynamic range switching can be performed by selecting either one. Also, by using them as source followers with different gate sizes, 1 / f noise can be suppressed, and when shooting high-sensitivity images by switching shooting modes, it has a wide dynamic range and low noise. You can select when you want to take a picture of.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of a pixel cell of an image sensor according to a first embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of a pixel cell of an image sensor according to a second embodiment of the present invention.

FIG. 3 is an equivalent circuit diagram of a pixel cell of an image sensor according to a third embodiment of the present invention.

FIG. 4 is a partial plan view of an image sensor according to a fourth embodiment of the present invention.

5 is a diagram showing the relationship between the pixel circuit in FIG. 4 and a unit block of a vertical shift register.

6 is a conceptual plan view showing the relationship between a photoelectric conversion unit and a pixel amplifier in the pixel cell in FIG.

FIG. 7 is a plan view showing an image sensor according to an embodiment of the present invention and a wafer serving as an original thereof.

FIG. 8 is a plan view showing a 3 × 3 array of image pickup devices and an array of scanning circuits according to an embodiment of the present invention.

FIG. 9 is a cross-sectional view showing the configuration of the image pickup device according to the embodiment of the present invention, and shows a cross section taken along the line AA ′ of FIG. 8.

FIG. 10 is a conceptual diagram showing a configuration of a radiation imaging system according to a fifth exemplary embodiment of the present invention.

FIG. 11 is a characteristic diagram showing the relationship between X-ray dose and output during fluoroscopy (moving image) and imaging (still image) in the medical field.

FIG. 12 is an equivalent circuit diagram of a pixel cell of a conventional image sensor.

[Explanation of symbols]

PD Photodiodes C1, C2, C3 Capacitances M1 to M7 Transistors 11 Edges of image sensor 12 Borders of pixel cells 13 Vertical shift register blocks 14 Common processing circuit block 15 including horizontal shift registers External terminals 16 Bumps 17 Protection resistors 18 Protection diode 21 Scintillator 22 FOP 23 Image sensor 24 Base 25 Substrate (external processing substrate) 26 Flexible substrate (TAB electrical connection) 31 Photodiode 32 First amplification MOS transistor (pixel amplifier 1) 33 Second amplification MOS type transistor (pixel amplifier 2) 34 Switching MOS type transistor (mode changeover switch) 35 Selection MOS type transistor 36 Reset MOS type transistor (reset switch) 37 Output signal line

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Noriyuki Kaifu             3-30-2 Shimomaruko, Ota-ku, Tokyo             Non non corporation F term (reference) 5C024 AX11 CX03 CX41 CX43 CY11                       GX03 GX09 GY31

Claims (17)

[Claims] 1. A photoelectric conversion means for converting light energy into an electric signal, and first and second photoelectric conversion means electrically connected to the photoelectric conversion means and inputted with the electric signal output from the photoelectric conversion means. A plurality of pixels including amplification means are provided, and between the photoelectric conversion means and the first or second amplification means, or between the photoelectric conversion means and the first and second amplification means. An image pickup device comprising a plurality of pixels each having a switch means provided therein. 2. The image pickup device according to claim 1, wherein the switch means is provided between the photoelectric conversion means and the first or second amplifying means, and in the first image capturing mode. The image pickup device is characterized in that the switch means is turned off, and the switch means is turned on in the second image capturing mode. 3. The image pickup device according to claim 1, wherein the switch means is provided between the photoelectric conversion means and the first and second amplifying means, respectively, and is provided in a first image capturing mode. At times, the switch means provided between the photoelectric conversion means and the first amplification means is turned on, and in the second image capturing mode,
An image pickup device characterized in that a switch means provided between the photoelectric conversion means and the second amplification means is turned on. 4. The image pickup device according to claim 2 or 3, wherein the first image capturing mode is a high sensitivity mode and the second image capturing mode is a normal mode. Image sensor. 5. The image pickup device according to claim 2, wherein the first image capturing mode is a moving image mode and the second image capturing mode is a still image mode. Image sensor. 6. The image pickup device according to claim 1, wherein the first and second amplifying means are insulated gate field effect transistors, and the electric signal is the insulated gate. Type field effect transistor is input to the gate electrode of the imaging device. 7. The image pickup device according to claim 6, wherein the insulated gate field effect transistor forms a source follower amplifier. 8. The image pickup device according to claim 6, wherein the area of the gate electrode of the insulated gate field effect transistor of the first amplification means is equal to the insulated gate field effect transistor of the second amplification means. An image pickup device having an area smaller than that of a gate electrode of an effect transistor. 9. The image pickup device according to claim 8, wherein the first and second amplification means have a charge storage section, and the capacitance of the charge storage section of the first amplification means is the second amplification section. An image pickup device characterized by being smaller than the capacity of the charge storage portion of the means. 10. The image sensor according to claim 1, further comprising a transfer switch unit and a capacitor between the photoelectric conversion unit and the switch unit. 11. A radiation imaging device, comprising: an imaging device formed by arranging a plurality of imaging devices according to claim 1; and a scintillator for converting radiation into light. apparatus. 12. The radiation imaging apparatus according to claim 11, further comprising an equal-magnification optical transmission unit between the scintillator and the imaging apparatus. 13. The radiation imaging apparatus according to claim 12, wherein the unity-magnification optical transmission unit includes a fiber optic plate. 14. The radiation imaging apparatus according to claim 11, a signal processing unit that processes a signal from the radiation imaging apparatus, and a signal for recording the signal from the signal processing unit. Recording means, display means for displaying the signal from the signal processing means, transmission processing means for transmitting the signal from the signal processing means, and a radiation source for generating the radiation. A radiation imaging system characterized by: 15. A photoelectric conversion means for converting light energy into an electric signal, and electrically connected to the photoelectric conversion means,
In a driving method of an image sensor including a plurality of pixels having first and second amplifying means to which the electric signal output from the photoelectric converting means is input, in the first image capturing mode, the photoelectric converting means is provided. In the image pickup device, the electric signal from the converting means is read out through the first amplifying means, and in the second image capturing mode, the electric signal from the photoelectric converting means is read out through the second amplifying means. Driving method. 16. The image pickup device driving method according to claim 15, wherein the first image capture mode is a high sensitivity mode and the second image capture mode is a normal mode. Device driving method. 17. The image pickup device driving method according to claim 15, wherein the first image capturing mode is a moving image mode, and the second image capturing mode is a still image mode. Device driving method.