JPH0377711B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling photoelectric conversion characteristics of a solid-state imaging device.

Generally, in order to increase the dynamic range of an imaging device, a method of changing the gamma characteristic of photoelectric conversion is carried out using signal processing, optical elements, or the like.

Conventional charge transfer devices, especially charge-coupled devices (CCDs)
For a method of controlling photoelectric conversion characteristics of an imaging device using
IEEE International Solid State Circuits Conference (IEEE
International Solid-State Circuits
Digest of Technical Papers (Digest of Technical Papers)
Pages 38 to 39 (Method for Varying)
As seen in Gamma in Charge-Coupled Imagers), the bias voltage of the charge storage electrode that stores optical information in the CCD imager is increased from the initial potential of the charge storage period to the electrode potential at the end of the charge storage period. A method has been proposed to extend the honey cleanse. However, in this conventional method, the maximum amount of accumulated charge during the accumulation period is determined by the storage electrode potential, and if a charge greater than the maximum amount of accumulated charge is generated, that charge is swept out into the substrate. However, this swept-out excess charge diffuses within the substrate and is absorbed into potential wells near the saturated picture elements. This is a pluming phenomenon known to those skilled in the art and is undesirable for imaging devices. When the potential of the storage electrode is increased near the end of the storage period, the already saturated potential well accumulates charge again. Therefore, when observing an imaging screen using a conventional imaging method, there is a drawback that in an incident image with high light intensity, an image of a portion of strong light appears on top of a pluming phenomenon (not completely saturated), resulting in an unsightly imaging screen. Ta.

An object of the present invention is to provide a novel method for controlling photoelectric conversion characteristics of a solid-state imaging device that eliminates such drawbacks.

According to the present invention, in the junction region formed on the principal surface of a semiconductor substrate and having a conductivity type opposite to that of the substrate, the first region has a shallow junction depth and the second region has a deep junction depth. forming a photoelectric conversion element group on the main surface of the first area; forming a device for reading out signals from the photoelectric conversion element group on the main surface of the second area; and a solid that absorbs excess charge into the semiconductor substrate that is greater than or equal to the maximum amount of charge that can be accumulated in each potential well of the photoelectric conversion element group by applying a reverse bias voltage between the second region and the semiconductor substrate. In the imaging device, there is provided a photoelectric conversion control method for a solid-state imaging device, characterized in that the height of the barrier potential of the first region at the beginning of the charge accumulation period is lower than the barrier potential at the end of the charge accumulation period. It will be done. More specifically, the photoelectric conversion characteristics are controlled by lowering the reverse bias voltage at the end of the charge accumulation period than at the beginning of the charge accumulation period, and the reverse bias voltage is lowered in stages multiple times during the charge accumulation period. The photoelectric conversion characteristics of the solid-state imaging device are controlled by changing it linearly or decreasing it linearly from an arbitrary time.

Next, a method for controlling the photoelectric conversion characteristics of a solid-state imaging device according to the present invention will be described in detail with reference to the drawings. In order to simplify the explanation, an N-channel semiconductor device will be described here.

FIG. 1 is a plan view of a solid-state imaging device called an interline transfer method proposed in Japanese Patent Application No. 55-130517 by the present applicant, in which multiple columns of vertical shift registers are driven by the same charge transfer electrode group. 10
, a photoelectric conversion section 11 adjacent to one side of each vertical shift register and electrically isolated from each other, a transfer gate electrode 12 for controlling signal charge transfer between the vertical shift register and the photoelectric conversion section, A charge transfer horizontal shift register 13 electrically coupled to one end of the shift register and a device 14 for detecting signal charges are provided at one end of the horizontal shift register.

Figure 2 shows - in the imaging device shown in Figure 1.
It schematically shows a cross section on a line,
On the main surface of the N-type semiconductor substrate 15, this substrate 15 and p
Two P-type regions 22 and 23 forming a -n junction and having different junction depths are formed. Further, on the upper surfaces of the P-type regions 22 and 23, a charge transfer electrode 17 of the vertical shift register is provided via an insulating layer 16, a transfer gate electrode 18 for controlling the signal charge transfer from the photoelectric conversion section to the vertical shift register, the regions 22,
A photoelectric conversion section is formed of a layer 19 of a conductivity type different from 23, and the photoelectric conversion section is formed between an adjacent vertical shift register and a channel stop region 20 having an impurity layer higher in impurity concentration than the regions 22 and 23, for example. They are separated. In addition, parts other than the photoelectric conversion part are shielded from light by, for example, a metal layer 21.

An imaging device using such an interline transfer method transfers signal charges accumulated in the photoelectric conversion unit 11 according to the amount of incident light to the transfer gate 12, for example.
through the corresponding vertical shift register 1
Transfer to 0. After transferring the signal charge to the vertical shift register, the transfer gate is closed, and the photoelectric conversion unit 11 accumulates the signal charge for the next cycle.
On the other hand, the signal charges transferred to the vertical shift register 10 are transferred in parallel in the vertical direction, and transferred to the horizontal shift register 13 for each horizontal line of each vertical shift register. The charge sent to the horizontal shift register is transferred in the horizontal direction while a signal is transferred from the next vertical shift register, and is taken out as a signal from the charge detection section 14 to the outside.

FIG. 3 is a diagram for explaining in more detail the operation of the photoelectric conversion unit 19, which is most closely related to the present invention.
It shows the potential distribution on the - line of the photoelectric conversion section shown in FIG. 2, that is, in the depth direction of the photoelectric conversion section.
In FIG. 3, the horizontal axis represents distance in the depth direction, and the vertical axis represents potential. Now, the potential of the channel stop region 20 shown in FIG. 2 is the reference potential (in this case, 0 volts).
shall be. The N-type photoelectric conversion section 19 is set at a potential of V TG -V _T , where the potential of the transfer gate 18 is V _TG and the threshold voltage of the transfer gate _{is V T .} Further, from the low voltage V ₁ shown by the curve 31, the reverse bias voltage applied to the P-type region 22 and the substrate 15 is
When a higher reverse bias voltage V ₂ is applied, the P-type region 22 is completely depleted as shown by a curve 32. When the photoelectric conversion region 19 is irradiated with light and signal charges are accumulated, the potential of the photoelectric conversion region 19 decreases from curve 32 to curve 33, and finally reaches curve 3.
4, the junction between the photoelectric conversion section 19 and the P-type region 22 is in the forward direction, and any more charges generated in the photoelectric conversion section 19 are transferred to the semiconductor substrate 15 via the P-type region 22.
flows into. In other words, the photoelectric conversion section 19 and the P
By applying a reverse bias voltage to the substrate semiconductor and the P-type region 22 so that the junction potential of the type region 22 becomes high, excess charges generated in the photoelectric conversion section 19 can be completely swept out to the substrate semiconductor, resulting in high brightness. It is possible to completely suppress the pluming phenomenon when photographing a subject. Furthermore, the amount of signal charge generated and accumulated in the photoelectric conversion section 19 is proportional to the amount of incident light when the accumulation time is constant. That is, it is also known that gamma is 1.

Next, when the reverse bias voltage applied to the P-type region 22 and the substrate 15 is changed from the high voltage V ₂ shown by the curve 34 to the low bias voltage V ₃ , the potential distribution in the depth direction of the photoelectric conversion section becomes as shown by the curve 35. Become. That is, in the state of curve 35, more signal charges can be accumulated in the photoelectric conversion unit 19 than in the state of curve 34. This shows that by appropriately changing the reverse bias voltage applied to the P-type region 22 and the substrate 15, the maximum amount of charge accumulated in the photoelectric conversion section 19 can be arbitrarily controlled. However, even in the state of curve 35, in order to suppress the blooming phenomenon, the junction potential between the photoelectric conversion section 19 and the P-type region 22 is set higher than the surface potential of all the regions surrounding the photoelectric conversion section 19 The reverse bias voltage must be selected.

Next, a method of controlling the photoelectric conversion characteristics using the above-described function that can arbitrarily control the maximum amount of charge accumulated in the photoelectric conversion section will be described. First, FIG. 4 is a diagram showing changes in the reverse bias voltage to be applied between the P-type region 22 and the substrate 15. At the beginning of the charge accumulation period, the voltage _V2 shown in FIG. 3 is maintained. At any time t ₁ during the accumulation period, a voltage V ₃ lower than the voltage V ₂ as shown by the solid line in the same figure.
Changes to FIG. 5 is a diagram showing the relationship between the amount of charge accumulated in the photoelectric conversion unit 19 and the accumulation time when the reverse bias voltage shown in FIG. 4 is applied, and corresponds to each of the reverse bias voltages V ₂ and V ₃ . The maximum amount of charge that can be accumulated is Q ₂ and Q ₃ . Also curve 5
0, 51, and 52 each indicate a time change in the amount of charge accumulated according to a different amount of incident light, and each slope corresponds to the amount of incident light.
In other words, the larger the slope, the larger the amount of incident light. In the figure, for light irradiation below the amount of incident light shown by curve 50, the amount of incident light and the amount of accumulated charge are proportional. That is, gamma is 1. However, the amount of accumulated charge with respect to the amount of incident light between curves 50 and 51 is once saturated at the maximum amount of charge _Q2 , and then accumulation starts again after time _t1 , and reaches the maximum amount of charge Q at the end time of the accumulation period _t2 . The amount of charge is ₃ or less. This indicates that the ratio of the amount of accumulated charge to the amount of incident light is compressed compared to the case where the amount of incident light is less than or equal to the amount of incident light shown by curve 50. next,
When the amount of light irradiated is greater than the amount of incident light shown by curve 51, for example, the amount of accumulated charge for curve 52 is always saturated at the maximum amount of charge _Q3 . The relationship between the amount of incident light and the amount of accumulated charge described above can be summarized as shown by the solid line in FIG. For comparison, conventional photoelectric conversion characteristics are shown here by broken lines. As shown in the figure, according to the photoelectric conversion control method according to the present invention, since there is a region 60 in which the amount of accumulated charge is compressed with respect to the amount of incident light, it is possible to expand the range of the amount of incident light that can be imaged. In other words, even if the contrast ratio of the copy is very large, the output video amplitude can be suppressed to a specified value, so there is no need to perform white compression, white clipping, etc. in the video signal processing circuit installed after the solid-state imaging device. . In FIG. 4, even if the reverse bias voltage is lowered linearly as shown by the broken line between the time _t1 of the accumulation period and the end time _t2 of the accumulation period, the same photoelectric conversion characteristics as described above will be obtained. can get.

FIG. 7 is a diagram for explaining another embodiment of the present invention, in which the reverse bias voltage applied between the P-type region 22 and the substrate 15 is shown by solid lines at times t ₁ and t ₂ during the accumulation period. The slope of the linear voltage change is 2 times as shown by the dashed line.
It has been changed several times. According to the explanation of FIG. 5, the relationship between the amount of accumulated charge and the amount of incident light in this case is as shown in FIG. 8 at three bending points a, b, and c.
This makes it possible to further expand the range of incident light that can be imaged.

As explained above, according to the present invention, in a solid-state imaging device equipped with a function of absorbing excess charge into a semiconductor substrate using a reverse bias voltage that exceeds the maximum amount of charge that can be accumulated in a photoelectric conversion element, a By setting the bias voltage higher than the voltage at the end of the accumulation period, it is possible to arbitrarily control the photoelectric conversion characteristics of the imaging device without the blooming phenomenon.

Note that in driving the imaging device according to the present invention,
In order to reduce noise in the reproduced image, it is preferable to change the reverse bias voltage as shown in FIG. 4 or 7 during the horizontal planking period.

Further, in this embodiment, a two-dimensional interline transfer type imaging device has been described, but it is obvious that the present invention can also be applied to other types of two-dimensional imaging devices or one-dimensional solid-state imaging devices.

Further, in the embodiment, an N-channel semiconductor device has been described, but it goes without saying that the present invention can be applied to a P-channel semiconductor device by reversing the conductivity type of each region.

[Brief explanation of drawings]

FIG. 1 is a plan view of an imaging device using a charge transfer device, and FIG. 2 is a sectional view along the line shown in FIG. 1.
Figure 3 is a schematic diagram of the potential distribution on the line shown in Figure 2, Figures 4 and 7 are diagrams showing time changes in the reverse bias voltage used in the present invention, and Figure 5 is a diagram showing the accumulation time and amount of accumulated charge. FIGS. 6 and 8, which are diagrams for explaining the operation of the present invention showing relationships, are examples of photoelectric conversion characteristics of an imaging device obtained according to the present invention. In the figure, 10 is a vertical shift register, 1
1 and 19 are photoelectric conversion elements, 12 and 18 are transfer gate electrodes, 13 is a horizontal shift register, 1
4 is a signal charge detection device, 15 is a semiconductor substrate, 16
17 is an insulating layer, 17 is a charge transfer electrode of the vertical shift register, 20 is a channel stop region, 21 is a metal layer, 22 is a region with a conductivity type opposite to that of the substrate and has a shallow junction, and 23 is a junction with a conductivity type opposite to that of the substrate. indicates a deep area.

Claims (1)

[Scope of Claims] 1. A junction region formed on the main surface of a semiconductor substrate and having a conductivity type opposite to that of the substrate, including a first region with a shallow junction depth and a second region with a deep junction depth. A photoelectric conversion element group is formed on the main surface of the first area, a device for reading out signals from the photoelectric conversion element group is formed on the main surface of the second area, and a photoelectric conversion element group is formed on the main surface of the second area. By applying a reverse bias voltage between the region and the second region and the semiconductor substrate, excess charge exceeding the maximum amount of charge that can be accumulated in each potential well of the photoelectric conversion element group is absorbed into the semiconductor substrate. A photoelectric conversion control method for a solid-state imaging device, characterized in that the height of the barrier potential of the first region at the beginning of a charge accumulation period is lower than the barrier potential at the end of the charge accumulation period. 2. Claim 1, characterized in that the reverse bias voltage applied between the first region and the second region and the semiconductor substrate is lower at the end of the charge accumulation period than at the beginning of the charge accumulation period. A photoelectric conversion control method for a solid-state imaging device as described above. 3. A photoelectric conversion control method for a solid-state imaging device according to claim 2, characterized in that the reverse bias voltage is changed stepwise multiple times so as to be lowered sequentially during a charge accumulation period. 4. The photoelectric conversion control method for a solid-state imaging device according to claim 2, characterized in that the reverse bias voltage is lowered linearly from an arbitrary time during the charge accumulation period.