JP2003142677A - Solid-state imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable photographing with a high sensitivity without causing blooming. SOLUTION: When a subject is dark, a video signal output by a solid-state imaging device 4 is weak and the maximum value of the video signal is low, which makes a peak detection circuit 8 generate a lower substrate voltage Vsub and apply it to a semiconductor substrate of the solid-state imaging device 4. When the substrate voltage Vsub is low, an overflow barrier in each photo diode of the solid-state imaging device 4 is formed at a deeper place. Therefore, a signal charge generated by photoelectric transfer of light of long wavelengths to a relatively deep place of the semiconductor substrate is accumulated in a potential well and is effectively used, which increases a photo detection sensitivity of the solid-state imaging device 4. When the subject is bright, on the other hand, a high voltage is applied to the semiconductor substrate of the solid-state imaging device 4. Therefore, an overflow barrier is formed in a shallow place, which reduces a charge accumulation capacity of each photo diode and prevents blooming.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state image pickup device, and more particularly to a technique for increasing the sensitivity of a solid-state image pickup device.

[0002]

2. Description of the Related Art Solid-state image pickup devices are widely used as image pickup means for digital still cameras and video cameras.
In general, this type of solid-state image sensor has a large number of photodiodes arranged in a matrix on the surface of a semiconductor substrate, and a vertical charge transfer register is formed for each column of the photodiodes to generate signals received by the photodiodes. Transfer the charge column by column of the photodiode, and
The horizontal charge transfer register provided on one end side of the vertical charge transfer register transfers the signal charges for each row of the photodiodes.

In such a solid-state image sensor, an important factor that determines the sensitivity is the photoelectric conversion efficiency of the photodiode. The light incident on the photodiode is photoelectrically converted by the photodiode, and as a result, the photodiode generates a signal charge. At this time, since light having a short wavelength is greatly attenuated in the photodiode, it is photoelectrically converted at a relatively shallow portion of the photodiode. On the other hand, since light with a long wavelength has little attenuation, it reaches a relatively deep portion of the photodiode and is photoelectrically converted even in a deep portion.

FIG. 5 is a graph showing changes in the potential of the photodiode in the depth direction. In the figure, the vertical axis represents the potential and the horizontal axis represents the depth from the surface of the semiconductor substrate. As described above, the photoelectric conversion occurs at various depths of the semiconductor substrate, but the signal charges generated at a position shallower than the overflow barrier 102 shown in FIG.
The signal charges accumulated in 04 are taken into the vertical charge transfer register and transferred. On the other hand, the signal charge generated at a position deeper than the overflow barrier 102 is
It is not accumulated in the potential well 104 and is discarded on the semiconductor substrate side.

Therefore, in order to be able to utilize the signal charge generated at a deep portion in the photodiode, the photodiode may be manufactured so that the overflow barrier 102 is formed at the deepest position. In such a photodiode, light reaching a deep portion is also photoelectrically converted, so that the photoelectric conversion efficiency is increased as a whole and the sensitivity of the photodiode is improved.

[0006]

However, when the photodiode is formed so that the overflow barrier 102 is located at a deep position, the range of the potential well 104 in the depth direction is widened and the signal charge of the photodiode is increased. The storage capacity also increases. Therefore, when the subject is bright, a large amount of signal charge is accumulated in the photodiode, and blooming is likely to occur in the captured image.

The present invention has been made to solve such a problem, and an object thereof is to provide a solid-state image pickup device capable of high-sensitivity image pickup without causing blooming.

[0008]

In order to achieve the above-mentioned object, the present invention includes an n-type semiconductor substrate having a plurality of photodiodes arranged on the surface thereof, and the semiconductor substrate has a p-type semiconductor substrate on the front surface side.
A solid-state imaging device in which a type well region is formed, and the photodiode includes an n-type region formed in an upper layer portion of the p-type well region, and a signal charge generated by the photodiode receiving light. A level detection circuit that detects the magnitude of the electric signal obtained by the method, and a substrate voltage control circuit that controls the voltage applied to the semiconductor substrate based on the detection result of the magnitude of the electric signal by the level detection circuit. It is characterized by having.

In the solid-state image pickup device of the present invention, the level detection circuit detects the magnitude of the electric signal obtained from the signal charge generated by the photodiode receiving light, and the substrate voltage control circuit uses the level detection circuit. The voltage applied to the semiconductor substrate is controlled based on the detection result of the signal magnitude.
Then, for example, when the subject is dark, the electric signal is weak and the magnitude of the electric signal detected by the level detection circuit is small, whereby the substrate voltage control circuit can apply a lower substrate voltage to the semiconductor substrate. . As a result, the overflow barrier in each photodiode of the semiconductor substrate is formed at a deeper portion, and the signal charge generated by photoelectric conversion of light on the long wavelength side that reaches a relatively deep portion of the semiconductor substrate also enters the potential well. It can be accumulated and effectively used, and the photodetection sensitivity of the photodiode and thus the solid-state imaging device is improved. On the other hand, when the subject is bright, the substrate voltage control circuit can apply a high voltage to the semiconductor substrate based on the detection result of the level detection circuit, and as a result, the overflow barrier is formed in a shallow location, so The charge storage capacity of the diode is reduced, and blooming can be prevented from occurring.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an example of a solid-state image pickup device according to the present invention, FIG. 2 is a schematic plan view showing a solid-state image pickup element constituting the solid-state image pickup device of an embodiment, and FIG. 3 is taken along line AA ′ in FIG. FIG. 4 is a partial cross-sectional side view, and FIG. 4 is a graph showing a potential change in the photodiode constituting the solid-state image sensor of FIG.

The solid-state image pickup device 2 of this embodiment is shown in FIG.
As shown in FIG. 3, the solid-state image sensor 4 having the CCD structure, the sample hold circuit 6 (S / H), and the peak detection circuit 8 are included. First, the solid-state image sensor 4 that constitutes the solid-state image sensor 2 will be described in detail. Solid-state image sensor 4
As shown in FIG. 2, it is composed of a large number of photodiodes 12, a vertical charge transfer register 14, a horizontal charge transfer register 16 and the like provided on the semiconductor substrate 10.

The photodiodes 12 are arranged in a matrix, and the vertical charge transfer registers 14 are provided along the columns of the photodiodes 12 for each column of the photodiodes 12. The horizontal charge transfer register 16 is provided on one end side of the vertical charge transfer register 14 so as to be orthogonal to the vertical charge transfer register 14, that is, in the row direction of the photodiodes 12, and has a charge-voltage conversion at its output end. Floating diffusion part 18 (FD
Section 18) is provided. The video signal generated by the FD unit 18 is output to the outside of the solid-state image sensor 4 through the output circuit 20 with low impedance.

On the charge transfer paths of the vertical charge transfer register 14 and the horizontal charge transfer register 16, transfer electrodes (not shown in FIG. 2) made of pairs of transfer electrodes and storage electrodes are provided in the charge transfer direction in each transfer register. The signal charges are transferred on the transfer register by applying transfer pulses to these electrodes.

The signal charges received by the photodiodes 12 are generated by applying a read voltage to the transfer electrodes of the vertical charge transfer registers 14 so that the vertical charge transfer registers 14 are simultaneously supplied to each column of the photodiodes 12. Read out. Thereafter, the vertical charge transfer register 14 is driven by, for example, four-phase transfer pulses applied to the transfer electrodes thereof, so that the signal charges for one column of the photodiodes 12 read out to the vertical charge transfer register 14 are read. Are sequentially transferred to the horizontal charge transfer register 16, and are supplied to the horizontal charge transfer register 16 for each signal charge of one row of the photodiode 12.

The signal charges for one row of the photodiodes 12 supplied to the horizontal charge transfer register 16 pass through the horizontal charge transfer register 16 when the horizontal charge transfer register 16 is driven by, for example, a two-phase transfer pulse. F
The data are sequentially transferred to the D section 18, and output from the end of the horizontal charge transfer register 16 to the FD section 18. The signal charge is converted into a voltage in the FD section 18, and is output from the output section as a video signal to the outside with low impedance.

Next, the sectional structure around the photodiode 12 will be described in detail. The solid-state imaging device 4 is formed of an n-type semiconductor substrate 10, and a p-type well region 22 is formed on the front surface side of the semiconductor substrate 10 as shown in FIG. The photodiode 12 and the vertical charge transfer path 24 of the vertical charge transfer register 14 are formed in the upper layer portion of the p-type well region 22.

The photodiode 12 includes an n-type region 26, and the n-type region 26 and the p-type well region 22 in contact therewith form a pn junction as a photodiode. The photodiode 12 also has a p + region 28 containing a high concentration of p-type impurities on its surface for reducing noise.

The vertical charge transfer register 14 is provided adjacent to the photodiode 12, and the vertical charge transfer path 24 is provided in the n-type region 2 formed on the surface of the semiconductor substrate 10.
6 and a p-type region 30, which extend in the transfer direction of the signal charges in the vertical charge transfer register 14, that is, in the direction orthogonal to the paper surface of FIG. n-type region 26
A transfer electrode 34 made of, for example, polysilicon is formed on the above via an insulating film 32 formed on the entire surface of the semiconductor substrate 10, and the transfer electrode 34 is covered with a light shielding film 36 via the insulating film 33. There is.

A region between the photodiode 12 and the n-type region 26 is a signal charge read gate portion 38, and the transfer electrode 34 is provided in the read gate portion 3 as shown in FIG.
When the signal charges are read from the photodiode 12, the read voltage applied to the transfer electrode 34 is applied to the read gate unit 38 when the signal charges are read out from the photodiode 12, and the signal charges accumulated in the photodiode 12 are perpendicular to each other. The structure moves to the charge transfer path 24. It should be noted that while the charge is being accumulated in the photodiode 12, the transfer electrode 34
Is at ground potential.

An n-type region 26 forming the vertical charge transfer path 24
A channel stop region 40 containing a high-concentration p-type impurity is formed on the side opposite to the read gate 38. This channel stop region is the semiconductor substrate 1
It is connected to a wiring (not shown) formed on the surface 0, and is always at the ground potential.

FIG. 4 shows the outline of the potential change along the line BB 'in FIG. 2, where the vertical axis represents the potential and the horizontal axis represents the depth from the surface of the semiconductor substrate.
The semiconductor substrate 10 has a substrate voltage Vsub of, for example, 1
A voltage of about 0V is applied by the peak detection circuit 8,
At that time, as indicated by the solid line 42 in FIG.
A potential well 44 is formed in the upper layer portion of 0, more specifically, in the n-type region 26 of the photodiode 12, and an overflow barrier 46 is formed following the potential well 44. The signal charge received by the photodiode 12 and generated is accumulated in the potential well 44, and then read out to the vertical charge transfer register 14 through the read gate section 38.

Next, the sample hold circuit 6 and the peak detection circuit 8 will be described. The sample hold circuit 6 samples and holds the video signal output from the output circuit 20 of the solid-state image sensor 4 in synchronization with the drive pulse of the horizontal charge transfer register 16. The output signal of the sample hold circuit 6 is processed by the signal processing circuit 48,
The signal level is adjusted, the sync signal is inserted, and the like, which is output as the television signal 50.

The peak detection circuit 8 operates in synchronization with a frame sync pulse generated in a cycle corresponding to one screen image, and the maximum value of the output signal of the sample hold circuit 6 is obtained in each cycle of the frame sync pulse. Is generated and a substrate voltage Vsub is generated based on the detected maximum value, and is applied to the main body portion of the semiconductor substrate 10 of the solid-state imaging device 4 for one cycle of the next frame synchronization pulse. Here, the peak detection circuit 8
Outputs a lower voltage as the maximum value of the output signal of the sample hold circuit 6 is smaller. The semiconductor substrate 10 also has
A shutter pulse 54 having a voltage sufficiently higher than a normal substrate voltage is applied via the capacitor 52. As a result, the electric charge accumulated in each photodiode 12 is discarded to the main body side of the semiconductor substrate 10, and the electronic shutter operates.

Next, the operation of the solid-state image pickup device 2 thus constructed will be described. When the light from the subject enters the solid-state image sensor 4, each photodiode 12 of the solid-state image sensor 4 photoelectrically converts the light to generate and accumulate a signal charge. The signal charge generated by each photodiode 12 is sequentially transferred by the vertical charge transfer register 14 and the horizontal charge transfer register 16, converted into a voltage by the FD section 18, and then output from the output circuit 20 as a video signal.

The sample and hold circuit 6 samples and holds this video signal in synchronization with the drive pulse of the horizontal charge transfer register 16. The peak detection circuit 8 operates as the level detection circuit according to the present invention, detects the maximum value of the output signal of the sample hold circuit 6 for each cycle of the frame synchronization pulse, and further detects the substrate voltage according to the present invention. It operates as a control circuit to generate a substrate voltage Vsub based on the detected maximum value and apply it to the semiconductor substrate 10 of the solid-state imaging device 4.

Here, when the subject is dark and each photodiode 12 does not generate sufficient signal charge, the video signal output from the output circuit 20 is weak, and therefore its maximum value is also small, so the peak detection circuit. 8 generates a lower substrate voltage Vsub and applies it to the semiconductor substrate 10 of the solid-state imaging device 4. Then, when the substrate voltage Vsub becomes low, the potential in the photodiode 12 becomes as shown in FIG.
, As shown by the dotted line 56,
An overflow barrier 58 is formed at a position deeper than the normal overflow barrier 46. Therefore, the potential well 44 has a wider range in the depth direction, and the signal charge generated at a deeper portion of the photodiode 12 is also accumulated in the potential well 44. Therefore, signal charges that reach a relatively deep portion of the semiconductor substrate 10, for example, generated by photoelectric conversion of light having a wavelength of about 500 nm to 600 nm can be accumulated in the potential well 44 and can be effectively used. The light detection sensitivity of the solid-state image pickup device 4 is improved, and good photographing can be performed even when the subject is dark.

On the other hand, when the subject is bright and each photodiode 12 generates a sufficient signal charge, the video signal output from the output circuit 20 is strong and therefore its maximum value is large, so that the peak detection circuit 8 is provided. Generates a higher substrate voltage Vsub and applies it to the semiconductor substrate 10 of the solid-state imaging device 4. Then, when the substrate voltage Vsub becomes higher, the potential in the photodiode 12 becomes as shown in FIG.
In addition, the overflow barrier 46 is formed at a shallower position, for example, as shown by the solid line 42. Therefore, in this case, the photodiode 12
Only the signal charges generated at the relatively shallow portion of the above are accumulated in the potential well 44, and even if the light having a long wavelength reaches the relatively deep portion of the semiconductor substrate 10 and is photoelectrically converted, it is generated. Signal charge is potential well 4
4 is stored in the main body side of the semiconductor substrate 10 without being accumulated. Therefore, when the subject is bright, a large amount of signal charge is not accumulated in the photodiode 12, and blooming is prevented. That is, the solid-state imaging device 2 according to the present embodiment can perform high-sensitivity shooting without blooming.

In this embodiment, the maximum value of the output signal of the sample and hold circuit 6 is detected to detect the substrate voltage Vs.
Although ub is controlled, for example, 1 of the frame synchronization pulse of the output signal of the sample hold circuit 6 is used.
The structure may be such that the average level in the cycle is detected and the substrate voltage is controlled based on the result. Even in that case, the substrate voltage is appropriately controlled according to the brightness of the subject to obtain the same effect. You can

When a p-type semiconductor substrate is used, it is technically easy to form the n-type region 26 of the photodiode 12 deeply. Therefore, the photoelectric conversion result of long-wavelength light can be effectively used. It is possible to improve the sensitivity.
However, a solid-state image sensor using a p-type semiconductor substrate has
There is a serious drawback in that a shutter pulse cannot be applied to a semiconductor substrate to perform an electronic shutter. Therefore, the technique of increasing the sensitivity in the solid-state image pickup device using the n-type semiconductor substrate as in the present invention is effective.

[0030]

As described above, in the solid-state image pickup device of the present invention, the level detection circuit detects the magnitude of the electric signal obtained from the signal charge generated by the photodiode receiving light, and the substrate voltage control circuit The voltage applied to the semiconductor substrate is controlled based on the detection result of the magnitude of the electric signal by the same level detection circuit. Then, for example, when the subject is dark, the electric signal is weak and the magnitude of the electric signal detected by the level detection circuit is small, whereby the substrate voltage control circuit can apply a lower substrate voltage to the semiconductor substrate. . As a result, the overflow barrier in each photodiode of the semiconductor substrate is formed at a deeper portion, and the signal charge generated by photoelectric conversion of light on the long wavelength side that reaches a relatively deep portion of the semiconductor substrate also enters the potential well. It can be accumulated and effectively used, and the photodetection sensitivity of the photodiode and thus the solid-state imaging device is improved. On the other hand, when the subject is bright, the substrate voltage control circuit can apply a high voltage to the semiconductor substrate based on the detection result of the level detection circuit, and as a result, the overflow barrier is formed in a shallow location, so The charge storage capacity of the diode is reduced, and blooming can be prevented from occurring.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a solid-state imaging device according to the present invention.

FIG. 2 is a schematic plan view showing a solid-state image sensor that constitutes the solid-state image sensor according to the embodiment.

3 is a partial cross-sectional side view taken along the line AA ′ in FIG.

FIG. 4 is a graph showing a change in potential in a photodiode constituting the solid-state image sensor of FIG.

FIG. 5 is a graph showing changes in the potential of the photodiode in the depth direction.

[Explanation of symbols]

2 ... Solid-state imaging device, 4 ... Solid-state imaging device, 6 ... Sample hold circuit, 8 ... Peak detection circuit, 10 ... Semiconductor substrate, 12 ... Photodiode, 14 ... Vertical charge transfer register, 16 ... … Horizontal charge transfer register,
18: Floating diffusion part (FD
Part), 20 ... Output circuit, 22 ... P-type well region, 2
4 ... Vertical charge transfer path.

Continued front page    F-term (reference) 4M118 AA01 AA05 AB01 BA10 CA04                       CA18 DB06 DB08 DD09 FA06                       FA13 FA26 FA35                 5C024 CX12 CX41 EX03 GX03 GX06                       GZ05 HX13 HX20

Claims (6)

[Claims] 1. A p-type well region is formed on the front surface side of the semiconductor substrate including an n-type semiconductor substrate in which a plurality of photodiodes are arranged on a surface portion, and the photodiode is an upper layer portion of the p-type well region. A solid-state image pickup device including an n-type region formed in the above, wherein a level detection circuit for detecting a magnitude of an electric signal obtained from a signal charge received by the photodiode and having the same level. A substrate voltage control circuit that controls a voltage applied to the semiconductor substrate based on a detection result of the magnitude of the electric signal by the detection circuit. 2. The solid-state imaging device according to claim 1, wherein the substrate voltage control circuit applies a lower voltage to the semiconductor substrate as the magnitude of the electric signal detected by the level detection circuit is smaller. 3. The photodiode has a p + region formed on the surface of the semiconductor substrate and containing a high concentration of p-type impurities, and the n-type region is formed below the p + region. The solid-state imaging device according to claim 1. 4. The level detection circuit uses the largest magnitude of the electric signal among the electric signals generated from the signal charge of each photodiode as a detection result. Solid-state imaging device. 5. The level detection circuit uses the average magnitude of the electric signal generated from the signal charge of each photodiode as a detection result.
The solid-state imaging device described. 6. The photodiodes are arranged in a matrix on the semiconductor substrate, and the semiconductor substrate has a vertical charge transfer register formed adjacent to the photodiode column for each column of the photodiode. , A horizontal charge transfer register formed at one end of the vertical charge transfer register, and a charge-voltage converter, each vertical charge transfer register receiving the signal charge generated by a corresponding photodiode array. Are transferred to the horizontal charge transfer register, the horizontal charge transfer register transfers the signal charges transferred by the vertical charge transfer register for each row of the photodiodes, and the charge-voltage conversion unit transfers the signal charges. Receives the signal charge transferred by the horizontal charge transfer register from the horizontal charge transfer register 2. The solid-state image pickup device according to claim 1, wherein the level detecting means detects the magnitude of the electric signal by converting the voltage into an electric signal.