JP4115128B2 - Photoelectric conversion device and image forming system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photoelectric conversion apparatus and an image forming system, and more particularly to a photoelectric conversion apparatus and an image forming system related to a scanner, a video camera, a digital still camera, a radiation detection apparatus including X-rays and α rays, and the like.
[0002]
[Prior art]
In recent years, a photoelectric conversion device called a CMOS sensor using a CMOS process has attracted attention. The CMOS sensor is expected to be used particularly in the field of portable information devices because of the ease of peripheral circuit mounting and low voltage driving.
[0003]
FIG. 5 is an equivalent circuit diagram of a conventional photoelectric conversion device. As shown in FIG. 5, the unit pixel includes a photodiode 1 that is a photoelectric conversion element, an amplification MOSFET (Metal Oxide Silicon Field Effect Transistor) 2 that generates an amplification signal based on a signal generated by the photodiode 1, and an amplification MOSFET 2. A reset switch 4 that resets the input to a predetermined voltage, a selection switch 5 that controls conduction between the source electrode of the amplification MOSFET 2 and the vertical output line 7, and a transfer switch that controls conduction between the photodiode 1 and the gate electrode of the amplification MOSFET 2 3 is provided.
[0004]
FIG. 5 also shows a vertical scanning circuit 6 for controlling on / off of the amplification MOSFET 2, transfer switch 3 and reset switch 4 of each pixel, a constant current source 9 for reading a signal from each pixel, and each pixel. The vertical output line 7 for transmitting the signal read from the signal, the signal storage unit 10 for storing the signal read from each pixel, and the horizontal scanning circuit 11 for controlling the reading of the signal stored in the signal storage unit 10. And a transfer gate 8a for accumulating the noise signal of each pixel in the signal accumulation unit 10 and a transfer gate 8b for accumulating the signal on which the noise signal is superimposed in the signal accumulation unit 10.
[0005]
Control signals φRES (n), φSEL (n), and φTX (n) are applied to the gate electrodes of the reset switch 4, the selection switch 5, and the transfer switch 3 from the vertical scanning circuit 6. Here, n is a positive integer indicating an arbitrary number of rows.
[0006]
FIG. 7 is a diagram showing a unit configuration of the vertical scanning circuit 6 of FIG. Control signals φRES (n), φSEL (n), φTX (n) are obtained by ANDing the selection signals generated by the unit block in the nth row of the shift register and the external input pulses φRES ′, φSEL ′, φTX ′. Has been generated.
[0007]
FIG. 6 is a timing chart showing the operation of the photoelectric conversion device shown in FIG. When a certain row (hereinafter referred to as n rows) is selected by the vertical scanning circuit 6, first, the reset signal φRES (n) becomes low and the reset switch 4 is turned off.
[0008]
Next, when the selection signal φSEL (n) becomes high and the selection switch 5 is turned on, the source of the amplification MOSFET 2 becomes conductive with the vertical output line, and a source follower circuit is formed by the selected pixel and the constant current source 9. The
[0009]
Subsequently, φTN becomes high, and the N output corresponding to the reset state of the pixel is read out to the signal storage unit 10 via the transfer gate 8b. Thereafter, the transfer switch 3 is turned on for a certain period by the transfer pulse φTX (n), and the optical signal generated in the photoelectric conversion element 1 is transferred to the gate of the amplification MOSFET 2.
[0010]
Subsequently, φTS becomes high, and the S output corresponding to the optical signal is read out to the signal storage unit 10 via the transfer gate 8a. By executing the difference between the correlated N signal and S signal, a good optical response output with less noise can be obtained.
[0011]
[Problems to be solved by the invention]
However, the conventional technology has the following problems.
[0012]
First, the dynamic range cannot be sufficiently large. That is, the input of the source follower circuit is reset to a voltage lower than the power supply voltage supplied to the pixel by the threshold value of the reset switch, so that the input range of the source follower circuit is a voltage range below that level.
[0013]
In particular, when the substrate potential of the reset switch is the ground potential, the threshold value of the reset switch increases due to the substrate bias effect. For this reason, in the prior art, the dynamic range is limited by the input range of the source follower.
[0014]
Second, in the case of a photoelectric conversion device having a transfer switch 3 that controls conduction between the photodiode 1 in the pixel and the gate of the amplification MOSFET 2, there is a concern about the problem of afterimage. The transfer efficiency of the photocharge generated in the photodiode 1 depends on the reset potential of the gate of the amplification MOSFET 2. When the reset potential of the gate is lowered, the transfer efficiency is deteriorated, so that photocharge remains in the photodiode 1 and an afterimage is generated. In addition, since fluctuations in the residual charge amount occur, random noise increases and the S / N ratio of the image deteriorates.
[0015]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides a photoelectric conversion element that converts an optical signal into a signal charge, and an enhancement-type amplification field effect transistor that generates an amplified signal based on the signal charge converted by the photoelectric conversion element. An enhancement-type reset field-effect transistor that resets the potential of the input section of the amplification field-effect transistor, and a transfer field-effect transistor that transfers the charge of the photoelectric conversion element to the input section of the amplification field-effect transistor When, in the photoelectric conversion device having a threshold of the reset field-effect transistor, characterized in that said lower than the amplification field-effect transistor and the threshold of the transfer field effect transistor.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings.
[0017]
(Embodiment 1)
FIG. 1 is an equivalent circuit diagram of a pixel portion of the photoelectric conversion apparatus according to Embodiment 1 of the present invention. The pixel of the photoelectric conversion device of the present embodiment includes a photodiode 1 that is a photoelectric conversion element, an amplification MOSFET 2 that generates an amplification signal based on a signal generated by the photodiode 1, and a signal from the photodiode 1 to the input gate of the amplification MOSFET 2. And a transfer switch 3 for transferring charges.
[0018]
A reset switch 4 for resetting the input of the amplification MOSFET 2 and a selection switch 5 for selecting a predetermined pixel output by controlling conduction between the source electrode of the amplification MOSFET 2 and the vertical output line 7 are provided. Control gates φTX, φRES, and φSEL having a high level of VDD and a low level of GND are applied to the gates of the transfer switch 3, the reset switch 4, and the selection switch 5 to drive the switches.
[0019]
When VDD is supplied to the drain of the amplification MOSFET 2 and the selection switch 5 is turned on, a source follower circuit having the constant current source 9 as a load is formed, and an output voltage corresponding to the gate potential of the amplification MOSFET 2 is output vertically. Appears on line 7. All the MOSFETs included in the unit pixel are enhancement type MOSFETs, but only the threshold value of the reset switch 4 is set lower than the threshold values of the other MOSFETs.
[0020]
FIG. 2A is a potential diagram of a pixel when the reset switch 4 of FIG. 1 is turned on. FIG. 2B is a potential diagram of the pixel of FIG. 1 when the reset switch 4 is turned off. 2A and 2B show the potential along the path from the photodiode 1 to the power supply VDD through the reset switch 4. In FIG. 2A and FIG. 2B, the potential of the conventional technology is shown by a broken line for comparison.
[0021]
2A and 2B, the region (PD) is the potential of the photodiode 1, the region (TX) is the potential of the channel region of the transfer switch, the region (FD) is the potential of the gate electrode of the amplification MOSFET 2, The region (RES) is the potential of the channel region of the reset switch 4, and the region (VDD) is the potential of the drain electrode of the reset switch 4.
[0022]
In this embodiment, for example, in a CMOS process in which VDD is 5V, the threshold value when the substrate bias effect of the NMOSFET is not present is 0.8V, and the threshold value when the assumed substrate bias effect is influenced is 1.4V. In some cases, the threshold value of the reset switch 4 is set to 0.3 V when there is no substrate bias effect and 0.9 V when there is.
[0023]
When the reset switch 4 is turned on, as shown in FIG. 2A, the reset potential is increased by 0.5 V, and the input range of the source follower can be expanded.
[0024]
When the reset switch 4 is turned off, as shown in FIG. 2B, the reset potential is increased by 0.5 V, the source-drain voltage of the transfer switch is increased, and the potential gradient is increased. For this reason, the transfer efficiency of photocharge is improved, and the afterimage is greatly reduced.
[0025]
This is very important particularly in a photoelectric conversion device in which random noise due to fluctuations in residual charge is improved by depleting the photodiode 1 after photocharge transfer. That is, when the electric field between the source and drain of the transfer switch 3 becomes strong, transfer efficiency is improved and deeper depletion is realized.
[0026]
In this embodiment, since the reset switch 4 is an enhancement type MOSFET, even when a charge close to a saturation signal is transferred from the photodiode 1, the potential of the reset switch 4 becomes a barrier, and the charge leaks to the reset power source side. Never do. Accordingly, the dynamic range is wide even on the saturation signal side, and a photoelectric conversion device having good linearity can be provided.
[0027]
(Embodiment 2)
The photoelectric conversion device according to the second embodiment of the present invention is manufactured by using a 5V CMOS process as in the first embodiment, and the threshold values of the normal MOSFET and the reset switch are also set in the same manner as in the first embodiment. ing.
[0028]
However, in this embodiment, the high level of the control signal φRES applied to the reset switch is boosted to 6V.
[0029]
FIG. 3 is a conceptual diagram showing the inside of a vertical scanning circuit for supplying a control signal to each of the transistors 3 to 5 in FIG. In the output stage that outputs φRES to each pixel, the high level is boosted to VRESH. That is, the pulse φRES ′ having the amplitude of VDD to GND from the shift register main body is used as an original signal, the level conversion circuit 23 converts the pulse amplitude to VRESH to GND, boosts the high level to VRESH = 6V, and each pixel The reset switch 4 is supplied.
[0030]
Since the threshold value Vth2 of the reset switch 4 is 0.9V as described above, the reset potential of the pixel is reset to VDD = 5V regardless of the threshold value Vth2 of the reset switch.
[0031]
Therefore, the dynamic range is further expanded as compared with the photoelectric conversion device of the first embodiment. Further, by reducing the fixed pattern noise component caused by the threshold value Vth2 of the reset switch, the final fixed pattern noise can be reduced even when the common mode removal efficiency is finite in the subsequent difference circuit.
[0032]
Here, by setting the threshold value Vth2 of the reset switch 4 low, this effect can be obtained at a relatively low potential of the high level VRESH of the control signal φRES, and therefore, deterioration of the gate oxide film of the reset switch is avoided. Can do.
[0033]
(Embodiment 3)
FIG. 4 is a structural plan view of a unit pixel of the photoelectric conversion device according to the third embodiment of the present invention. The photoelectric conversion device of the present embodiment is devised for reducing the chip size area. On the semiconductor substrate, the photodiode 1 and a plurality of NMOS are provided in an active region 14 formed in a P-type well region (not shown, but formed on the entire surface of FIG. 4).
[0034]
The photodiode 1 is composed of a P-type well and a PN junction formed by an N-type diffusion layer injected into the active region 14. The amplification MOSFET 2, the reset switch 4, and the selection switch 5 are constituted by a source-drain region 16 formed of an N-type diffusion layer and a gate electrode formed of a polysilicon layer on the channel region.
[0035]
In the transfer switch 3, the source electrode is common to the N-type diffusion layer of the photodiode 1, and the drain is composed of a source-drain region 16 of an N-type diffusion layer similar to other NMOSs.
[0036]
Here, reference numerals 2 to 5 indicate the gate electrodes of the respective NMOSs. The vertical output line 7 and the wirings 13 and 17 are formed of a metal wiring layer above the polysilicon layer.
[0037]
Also, contact holes and through holes 15 are formed to connect the semiconductor substrate and the metal wiring layer, and the polysilicon layer and the metal wiring layer.
[0038]
For convenience of explanation, some wiring is omitted. Each NMOS channel region is implanted with a P-type diffusion layer (channel dope, hereinafter referred to as “CD”) for threshold adjustment. This CD is divided into two steps, CD1 and CD2. CD1 is injected into all NMOS channel regions, but CD2 is injected only into the channel regions of the amplification MOSFET 2 and the selection switch 5.
[0039]
A region indicated by a broken line 12 indicates a region projected by the resist at the time of CD2 ion implantation, and covers the reset switch 4 and the transfer switch 3 so that CD2 is not implanted. Finally, the threshold values of the reset switch 4 and the transfer switch 3 are lower than the threshold values of the amplification MOSFET 2 and the selection switch 5. By setting the threshold value of the reset switch 4 low, the dynamic range of the photoelectric conversion device can be expanded and the transfer efficiency can be improved.
[0040]
Further, the amplification MOSFET 2 and the drain of the reset switch 4 are a common N-type diffusion layer, and the power supply voltage SVDD is supplied by the wiring 13. Since this diffusion layer is electrically separated from the others in the P-type well, the power supply voltage can be supplied as a separate system from the power supply DVDD of the peripheral circuit.
[0041]
Here, the power supply voltage SVDD of the pixel can be easily lowered below the power supply voltage DVDD of the peripheral circuit that generates the control signal for each switch, and the reset potential of the pixel does not depend on the threshold value Vth2 of the reset switch. SVDD can be set.
[0042]
As in the first and second embodiments of the present invention, when the 5V CMOS process and threshold setting are adopted, the power supply voltage SVDD of the pixel is set to 4.1 V or less with respect to the power supply voltage DVDD of the peripheral circuit. This state can be easily achieved, and a photoelectric conversion device with less fixed pattern noise can be realized.
[0043]
Here, by setting the threshold value Vth2 low, it is not necessary to lower SVDD as compared to the conventional case where SVDD has been required to be 3.6 V or less, and the dynamic range is not compressed. Further, since it is not necessary to boost the high level of the control signal applied to the gate of the reset switch 4 as in the second embodiment, the level conversion circuit can be reduced and the chip area can be reduced.
[0044]
Further, since a voltage higher than the standard power supply voltage is not applied to the gate of the reset switch 4, deterioration of the gate oxide film can be avoided. In addition, the pixel area can be reduced by making the drains of the amplification MOSFET 2 and the reset switch 4 the same diffusion layer as in this embodiment.
[0045]
(Embodiment 4)
FIG. 8A and FIG. 8B are a schematic configuration diagram and a schematic cross-sectional view of an X-ray imaging apparatus in which the photoelectric conversion device described in any of Embodiments 1 to 3 is mounted. First, the configuration of the X-ray imaging apparatus will be described. A plurality of photoelectric conversion elements and transistors are formed in the a-Si sensor substrate 6011. The shift register SR1 and the flexible circuit board 6010 on which the detection integrated circuit IC is mounted are connected.
[0046]
The opposite side of the flexible circuit board 6010 is connected to the circuit boards PCB1 and PCB2. A plurality of the a-Si sensor substrates 6011 are bonded on the base 6012. A lead plate 6013 is mounted under the base 6012 constituting the large photoelectric conversion device in order to protect the memory 6014 in the processing circuit 6018 from X-rays.
[0047]
On the a-Si sensor substrate 6011, a phosphor 6030 for converting X-rays into visible light, for example, CsI is applied or pasted. X-rays are detected using the photoelectric conversion device described in FIG. In this embodiment, as shown in FIG. 8B, the whole is housed in a case 6020 made of carbon fiber.
[0048]
FIG. 9 shows an application example of the photoelectric conversion apparatus of this embodiment to an X-ray diagnostic system.
[0049]
X-rays 6060 generated by the X-ray tube 6050 pass through the chest 6062 of the patient or subject 6061 and enter the photoelectric conversion device 6040 having the phosphor mounted thereon. This incident X-ray includes information inside the body of the patient 6061. The phosphor emits light in response to the incidence of X-rays, and this is photoelectrically converted to obtain electrical information. This information is converted to digital, image processed by an image processor 6070, and can be observed on a display 6080 in the control room.
[0050]
Further, this information can be transferred to a remote place by a transmission means such as a telephone line 6090 and can be displayed on a display 6081 such as a doctor room in another place or stored in a storage means such as an optical disk, and a doctor at a remote place makes a diagnosis. It is also possible. It can also be recorded on the film 6110 by the film processor 6100.
[0051]
In addition, although this embodiment demonstrated the case where a photoelectric conversion apparatus was applied to an X-ray diagnostic system, radiation, such as a nondestructive inspection apparatus using radiation other than X-rays, such as alpha rays, beta rays, and gamma rays. The present invention can also be applied to an imaging system. In addition to the radiation imaging system, the present invention can also be applied to an image forming system such as a scanner, a video camera, and a digital still camera.
[0052]
【The invention's effect】
As described above, according to the present invention, a photoelectric conversion device with a wide dynamic range is possible. Moreover, according to the present invention, a photoelectric conversion device with reduced fixed pattern noise is realized. In addition, according to the present invention, a photoelectric conversion device that can obtain a good image signal with little afterimage is realized.
[Brief description of the drawings]
FIG. 1 is an equivalent circuit diagram of a pixel portion of a photoelectric conversion device according to a first embodiment of the present invention.
FIG. 2 is a potential diagram of the pixel in FIG. 1;
3 is a conceptual diagram showing the inside of a vertical scanning circuit that supplies a control signal to each of transistors 3 to 5 in FIG. 1;
FIG. 4 is a structural plan view of a unit pixel of a photoelectric conversion device according to a third embodiment of the present invention.
FIG. 5 is an equivalent circuit diagram of a conventional photoelectric conversion device.
6 is a timing chart showing an operation of the photoelectric conversion device shown in FIG.
7 is a diagram showing a unit configuration of the vertical scanning circuit 6 of FIG. 5;
FIGS. 8A and 8B are a schematic configuration diagram and a schematic cross-sectional view of an X-ray imaging apparatus according to Embodiment 4 of the present invention. FIGS.
9 is a schematic configuration diagram of an X-ray imaging system including the X-ray imaging apparatus of FIG.
[Explanation of symbols]
1 Photodiode 2 Amplification MOSFET
3 Transfer switch 4 Reset switch 5 Selection switch 6 Vertical scanning circuit 7 Vertical output line 8 Transfer gate 9 Constant current source 10 Signal storage unit 11 Horizontal scanning circuit 12 Broken line 13 indicating region shielded by resist during CD2 ion implantation 13 Wiring 14 Active region 15 Contact or through hole 16 Source / drain region 17 Wiring 21 Unit block 23 of vertical shift register Level conversion circuit

Claims (8)

A photoelectric conversion element that converts an optical signal into a signal charge;
An enhancement-type amplification field effect transistor that generates an amplification signal based on the signal charge converted by the photoelectric conversion element;
An enhancement type reset field effect transistor that resets the potential of the input of the amplification field effect transistor ;
A transfer field-effect transistor that transfers the charge of the photoelectric conversion element to the input portion of the amplification field-effect transistor ;
The photoelectric conversion device , wherein a threshold value of the reset field effect transistor is lower than threshold values of the amplification field effect transistor and the transfer field effect transistor . Further, a read field effect transistor for reading an amplified signal generated by the amplification field effect transistor is provided, and threshold values of the read field effect transistor and the amplification field effect transistor are made equal. The photoelectric conversion device according to claim 1 . 3. The photoelectric conversion device according to claim 1, wherein the transfer field effect transistor and the read field effect transistor are enhancement type field effect transistors. 4. The photoelectric conversion device according to claim 1 , wherein an equal power supply voltage is supplied to each field effect transistor . 5. 5. The method according to claim 1, further comprising setting means for making a level of a signal for turning on the field effect transistor for reset relatively larger than a level of a signal for turning on the field effect transistor for amplification. The photoelectric conversion device according to item. When the level of a signal for turning on the reset field effect transistor is Vg, the power supply voltage is VDD, and the threshold value of the reset field effect transistor is Vth2,
Vg> VDD + Vth2
The photoelectric conversion device according to claim 1, wherein: further,
VDD ≦ Vg
The photoelectric conversion device according to claim 6, wherein: An image forming system comprising the photoelectric conversion device according to claim 1 .

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