JPH0248875A - Driving method for solid-state image pickup device - Google Patents

Abstract

PURPOSE:To suppress the reduction in the readout charge from a photodiode charge storage layer by applying a pulse voltage of an opposite polarity to other transfer electrode than a transfer electrode receiving a readout drive pulse voltage. CONSTITUTION:In an equivalent circuit representing the vicinity of the photodiode, coupling capacitors C1, C2 and a junction capacitor Cj have a relation of C1approx.=C2>>Cj.Thus, the capacitive component Cj gives almost no effect onto the application of a drive pulse and the potential fluctuation at a point B due to the positive pulse application and the potential fluctuation at a point B due to the negative pulse application are cancelled. Thus, the photodiode charge storage layer potential and the overflow channel potential are not almost increased. Then just before the end of drive pulse application, that is, just before the shift channel 5 is closed, the potential at the point B is negligibly small. Thus, the undesired reduction in the potential difference between the overflow channel potential and the shift channel potential is suppressed. That is, the reduction in the readout charge from the photodiode storage layer is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention (Field of Industrial Application) The present invention relates to a method for driving a solid-state imaging device, and particularly to a method for applying a driving voltage to a charge transfer electrode.

(Prior Art) Solid-state imaging devices have excellent features such as small size, light weight, and long life, so their use in video cameras and the like is increasing significantly. A plan view of a part of this type of solid-state imaging device is shown in FIG. 1(a), and a cross section taken along the line B-B' is shown in FIG. 1(b). That is, 4... are photoelectric conversion units each made of a photodiode, and 20 is a charge readout unit provided along the arrangement of the photoelectric conversion units 4.... The charge readout section 20 is formed by arranging a plurality of transfer electrodes 6 to 9 degrees along the charge transfer direction on the charge transfer channel 3 for charge storage and transfer with an insulating film 21 interposed therebetween. ing. In this case, two stages of transfer electrodes (6, y), (8s9)t... are provided corresponding to one photoelectric conversion section, and the previous stage of the two stages of transfer electrodes is the transfer electrode 6.
,8. A shift channel 5 is formed between the photoelectric conversion section 4 and the charge transfer channel 3 below. Further, in this example, a plurality of sets of transfer electrodes each including the four transfer electrodes 6.7 and 8.9 are provided. The photoelectric conversion section 4... is formed in the NFI semiconductor substrate 1.
An n-type region serving as a pixel is formed on the surface of the well 2, which photoelectrically converts input light to generate signal charges and accumulates the signal charges. Further, the charge transfer channel 3
An n-type region is formed on the surface of the P-well 2 in the charge transfer direction. Further, in the P well 2, a shallow overflow channel 10 with a low impurity concentration is formed directly below the n-type region 4 for the pixel, and the n-type substrate 1' below the overflow channel 10 forms a vertical overflow. It is a drain.

Next, a method for driving the solid-state imaging device will be described with reference to the drive voltage waveform shown in FIG. 5 and the potential/charge distribution within the substrate shown in FIGS. 6 and 7.

That is, a predetermined positive voltage is applied to the n-type substrate 1, and the drive voltage φ of the previous transfer electrode 6 of a certain transfer stage in each set of transfer electrodes is
Ov as v1, drive voltage φV of the subsequent transfer electrode 7
, a negative voltage (-vL), a drive voltage φV of the previous transfer electrode 8 of the next transfer stage in the charge transfer direction, -■1, and a period TA in which OY is applied as the drive voltage φv4 of the subsequent transfer electrode 9. In this case, the potential/charge distribution in the cross section of the substrate shown in FIG. 1(b) is as shown in FIG. 6. Here, 11 is the transfer channel potential, 12 is the shift channel potential, 13 is the full depletion potential of the photodiode, and J
4b is overflow channel potential, 15 is over 7
0 - drain potential and Q is charge. The above period T
At A, the shift channel potential 12 is approximately OV, the shift channel is closed, and the signal charge Q is not transferred. Moreover, since the overflow channel potential J4b has increased from the potential 14& when the potential of the n-type substrate 1 is OV to the positive potential side, when the signal charge Q becomes excessive, -f Yanel 10fg flows into overflow drain J'.

Next, from the above-described state, during a period T during which a pulsed positive voltage vH is applied to the front-stage transfer electrode 6 from time 1o to time t,1, the potential/charge distribution becomes as shown in FIG. In this case, since the transfer channel potential 11' and the shift channel potential 12' are each higher than in the case of FIG. 46, the signal charge Q flows into the transfer channel 3 via the shift channel 5.

In this way, the signal charge Q that has flowed into the transfer channel 3 under the front-stage transfer electrode 6 charges the charge readout section 20 by applying a predetermined pattern of driving pulses to each transfer electrode 6 to 9°. They are transferred sequentially in the transfer direction.

By the way, a coupling capacitance C1 is formed between the transfer electrode 6 and the P-well 2, and the P-well 2 has a resistance component, and a capacitance, tcj, is formed between the P-well 2 and the n-board substrate 1. Therefore, the equivalent circuit representing the vicinity of the photodiode is as shown in FIG.

Here, R is the resistance value of P well 2, and point B is the capacitance C1 above.
, Cj, and the resistance value R at each end, the photodiode complete depletion potential 13 and the overflow channel potential 14.
b is formed based on the potential at point B. On the other hand, the transfer channel potential 11' and the shift channel potential 12' are
It is strongly influenced by the transfer electrode potential φ■1, and is controlled in proportion to the transfer electrode potential φv1. In addition, P-well 2 is
Since there are photodiodes 4 arranged in rows and nine columns and transfer channels 3 formed in columns, they are grounded outside. Therefore, the resistance value R of the P-well 2 increases toward the center of the photodiode array.

Due to the existence of the capacitance C1, C, and the resistance value R as described above, the drive node φV is applied to the transfer electrode 6 as shown in FIG.
, the potential at point B in FIG. 8 is normally c
t ) cj, it changes as shown in FIG. 9, and even just before the shift channel 5 closes (just before time t1), it does not become zero potential but becomes a positive potential.

On the other hand, when strong light is incident on the solid-state imaging device, FIG. 10 shows a state in which the charge generated by photoelectric conversion becomes excessive and the excess charge is discharged to the overflow drain l'l/C. In this state, when a driving pulse is applied to the transfer electrode 6 as shown in FIG. 5, the coupling capacitance between the transfer electrode 6 and the P well 2 C! The potential of the P well 2 rises between time t0 and time t. Along with this, as shown in FIG. 11, the photodiode full depletion potential 13' and the overflow channel potential 14b' rise, and the potential difference between the over 70-channel potential 14b' and the shift channel potential 12' becomes smaller. As a result, there is a problem that the amount of charge read out to the transfer channel 3 decreases. In this case, the larger the resistance value R of the P-well 2, the longer the time constant for the P-well potential to recover to zero potential (ground potential), and the longer the over 70-channel potential 14b' returns to the original potential 14b. It takes a long time to descend, and the application time (t
o~1+) The amount of charge read out becomes smaller. Moreover, as mentioned above, the resistance value R of the P-well 2 increases toward the center of the photodiode array, so that the amount of light (saturated light amount) at which excess charge begins to be discharged at the center and periphery of the photodiode array increases. There is a problem that the difference υ and the amount of charge read out when excess charge is discharged (saturation signal amount) are non-uniform.

(Problems to be Solved by the Invention) The present invention has 1. As mentioned above, there is a coupling capacitance between the well region where the photodiode array of the solid-state imaging device is formed and the transfer electrode, and a resistance component of the well region, and the value of the resistance component depends on the position in the photodiode array. Due to these differences, in conventional transfer electrode driving methods, potential fluctuations in the well region become large when driving pulses are applied, resulting in a small amount of readout charge, and when strong light is incident, This was developed to solve the problem that the saturation light amount and saturation signal amount are uneven depending on the position.It prevents potential fluctuations in the well area during driving and the accompanying decrease in readout charge amount, and allows strong light to enter. An object of the present invention is to provide a method for driving a solid-state imaging device that can obtain a uniform saturated light amount and saturated signal amount both at the center and at the periphery of a photodiode array even when

[Structure of the Invention (Means for Solving the Problems) The present invention provides a plurality of photoelectric conversion sections having a charge storage layer that photoelectrically converts incident light to generate signal charges and accumulates the signal charges; A charge readout section having an accumulation region for reading signal charges accumulated in the photoelectric conversion section, and an excess charge discharge section for discharging excess charge generated in the photoelectric conversion section, wherein the plurality of photoelectric conversion sections are arranged in a semiconductor substrate. In the method of driving a solid-state imaging device formed in a well region of the invention, when the charge readout unit reads out the charge, the leading edge of the drive pulse voltage is applied to a transfer electrode other than the transfer electrode to which the readout drive pulse voltage is applied. The feature is that a voltage of opposite sign is applied in synchronization with .

(Function) As mentioned above, 7J with the opposite sign also applies to another transfer electrode? By applying a pulse voltage, it is possible to offset potential fluctuations in the well region due to the transfer electrode to which the readout drive pulse voltage is applied, so potential fluctuations in the well region when the drive pulse is applied are suppressed, and the photodiode charge storage layer It is possible to suppress a decrease in the readout charge amount. Further, the potential of the well region immediately before the end of the drive pulse application time becomes small.

Therefore, it is hardly affected by non-uniformity in the resistance value of the well region within the photodiode array. As a result, the amount of saturated light and the amount of saturated signal can be uniformly obtained at the center and the periphery of the photodiode array.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows transfer electrodes 6 to 9 used when reading out photodiode accumulated charges in a solid-state imaging device as described above with reference to FIGS. 1(a) and 1(b). An example of the waveform of the driving voltages φv1 to φv4 applied to... is shown. That is, the solid-state imaging device to which the method of the present invention is implemented includes a plurality of photoelectric conversion sections that photoelectrically convert incident light to generate signal charges as described above, and have a charge storage layer that accumulates the signal charges. , a charge readout section having an accumulation region for reading signal charges accumulated in the photoelectric conversion section, and an excess charge discharge section for discharging excess charge generated in the photoelectric conversion section, wherein the plurality of photoelectric conversion sections are semiconductors. The charge transfer section is formed in a well region in the substrate, and transfer electrodes of the charge transfer section are provided facing each other above a part of the P well region. Then, in the driving method according to the present example, before reading charges from the photodiode, for example, the front transfer electrode 6.8 is set to a ground potential (OV), and the rear transfer electrode 7, 9 is set to a ground potential (OV).
A negative potential -vL is applied to , and at the time of charge readout, for example, a drive pulse φv1 of a positive potential VH is applied to the front-stage transfer electrode θ, and at the same time, a drive pulse φv1 of a positive potential VH is applied to another front-stage transfer electrode 8 in synchronization with the leading edge of the drive pulse φv1. Reverse sign (-VL potential in this example)
A voltage φvs is applied. Note that, since a negative potential -vL is applied to the subsequent transfer electrode 7, the potential in the substrate under the transfer electrode 7 becomes low, and a potential barrier is formed in the charge transfer direction.

According to the above driving method, the equivalent circuit surrounding the photodiode to be read out is as shown in FIG. 3, and the potential at point B (well region near the photodiode) in FIG. It changes as shown in FIG. Here, C, * C1t R, and Ct in the above Fig. 3 will be explained with reference to Figs. 1 (a) and (b). In other words, R is the resistance value of the well region 2 near the photodiode. sb,
The well region 2 normally contacts the ground end at its end. Further, C1 is the well region 2 and the drive voltage φV,
The coupling capacitance, C, that exists between the transfer electrode 6 to which C is applied
2 is the coupling capacitance existing between the well region 2 and the transfer electrode 8 to which the drive voltage φV is applied, Ct is the junction capacitance between the well region 2 and the substrate 1, and VOFD is the coupling capacitance between the well region 2 and the substrate 1.
is the bias potential (overflow drain potential) applied to the In this case, since there is a relationship of C1U02>Cj, the C5 component has almost no effect on the application of the drive pulse, and the potential fluctuation at point B due to the application of the positive At pulse and the change in the potential at point B due to the application of the negative At pulse Since the potential fluctuations at point B are canceled out, the potential at point B hardly increases. For this reason, the seventh
As shown in the figure, the potential of the 7-diode charge storage layer 1
3. The overflow channel potential 14b hardly rises, and the potential at point B just before the end of application of the drive noyrals (just before the shift channel 5 in FIG. 1 closes) is the same as the potential in the conventional example (see FIG. 9). ) is negligibly small compared to Therefore, the potential difference between the overflow channel potential 14b and the shift channel potential 12' is suppressed from becoming unnecessarily small, and the amount of charge read from the photodiode accumulation layer is suppressed from decreasing. Furthermore, even if the resistance value of the well region in the photodiode array is non-uniform and rarely occurs, it is hardly affected by this.

That is, according to the above driving method, the period T shown in FIG.
The distribution of potential and charge within the substrate corresponding to A and TB is as follows:
Normally, the results are similar to those shown in Figures 6 and 7, and even when strong light is incident on the light receiving surface (photodiode array surface) of the solid-state imaging device, the central part of the 7t diode array The saturated light amount and the saturated signal amount can be uniformly obtained in the peripheral area.

In the above embodiment, when reading the charge, a positive readout driving voltage was applied to the transfer electrode 6, and at the same time, a negative voltage was applied to the transfer electrode 8 in synchronization with the leading edge. If the voltage is applied to any of the transfer electrodes 7, 8, and 9, the same effect as in the above embodiment can be obtained. Furthermore, when applying a positive readout drive pulse voltage to any of the transfer electrodes 7, 8.9 other than the transfer electrode 6,
If a negative voltage is applied to any one of the other transfer electrodes (6, 7, 8, or 9) in synchronization with the leading edge of the readout drive pulse voltage, the same effect as in the above embodiment can be obtained. It will be done.

[Effects of the Invention] As described above, according to the method for driving a solid-state imaging device of the present invention, it is possible to prevent potential fluctuations in the well region and the accompanying decrease in the readout charge amount when driving the readout of the photodiode accumulated charge, and to prevent a strong Even when light is incident, a uniform saturated light amount and saturated signal amount can be obtained both at the center and at the periphery of the photodiode array.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 (&) shows a planar nonaturn showing a part of a solid-state imaging device to which the solid-state imaging device driving method of the present invention is implemented.
Fig. 1 fb) is a sectional view taken along the line B-B' of Fig. 1 (,), and Fig. 2 is a cross-sectional view of Fig. 1 (al
Figure 3 is a circuit diagram showing the voltage applied to the transfer electrode in Figure 1 (b), Figure 4 is a circuit diagram showing the circuit near the photodiode in Figure 1 (b) Figure 5 showing the state of potential change in the nearby well region.
11 relate to a conventional driving method for a solid-state imaging device, FIG. 5 is a waveform diagram showing the voltage applied to the transfer electrode in FIG. 1(b), and FIGS. Period T in the diagram
A, T, K are diagrams showing the potential and charge distribution inside the substrate in the corresponding Figure 1 fbl, and Figure 8 is a circuit diagram showing some circuits, such as the vicinity of the photodiode in Figure 1 (b). ,
FIG. 9 is a diagram showing potential changes in the well region near the photodiode in FIG. 1 (bl), and FIGS. 10 and 11 are diagrams showing the period TA in FIG. # It is a diagram showing the state of potential charge distribution in the substrate corresponding to T. 1... N-type substrate, 2... P well, 3... Transfer channel, 4... Photoelectric conversion unit, 5 ...Shift channel, 6-9...Transfer electrode, 510...Overflow channel, 1'...Overflow drain, C,,
C,...Coupling capacitance, C5...Junction capacitance, Le...Resistance value of well region. 2Q...Charge reading section. Applicant's Representative Patent Attorney Takehiko Suzue 1 Figure 2 Figure 3 Figure 1 Figure 10 Figure 11

Claims (1)

[Claims] A plurality of photoelectric conversion units that photoelectrically convert incident light to generate signal charges and have a charge storage layer that accumulates the signal charges, and a charge readout unit that has an accumulation region that reads out the signal charges accumulated in the photoelectric conversion units. and an excess charge discharging section for discharging excess charge generated in the photoelectric conversion section, and the plurality of photoelectric conversion sections are formed in a well region in a semiconductor substrate. Solid-state imaging characterized by applying a voltage of opposite sign to a transfer electrode to which a read drive pulse voltage is applied in synchronization with the leading edge of the drive pulse voltage, when reading out electric charges by a readout section. How to drive the device.