JPH078030B2 - Driving method for solid-state imaging device - Google Patents

Description

The present invention relates to a method for driving a solid-state imaging device, and more particularly to a method for applying a driving voltage to a charge transfer electrode.

(Prior Art) Since solid-state imaging devices have excellent features such as small size, light weight, and long life, their use in video cameras and the like has been remarkably increasing. 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 BB 'is shown in FIG. 1 (b). That is, reference numerals 4 ... Represent a photoelectric conversion unit composed of a photodiode, and 20 is a charge reading unit provided along the arrangement of the photoelectric conversion units 4. The charge reading section 20 includes a charge transfer channel 3 for charge storage / transfer.
A plurality of transfer electrodes 6-9, ... Are arranged on the upper side of the insulating film 21 along the charge transfer direction. In this case, two stages of transfer electrodes (6,
7), (8, 9), ... Are provided below the transfer electrodes 6, 8, .. A shift channel 5 is formed between them. Further, in this example, the four transfer electrodes 6,
A plurality of sets of transfer electrodes, each set consisting of 7, 8 and 9, are provided. In the photoelectric conversion units 4, ..., An n-type region serving as a pixel is formed on the surface of the P-well 2 formed in the n-type semiconductor substrate 1, and photoelectrically converts input light to generate signal charges. This signal charge is accumulated. Further, the charge transfer channel 3 has an n-type region formed in the surface of the P well 2 in the charge transfer direction. Further, in the P well 2, an overflow channel 10 is formed immediately below the pixel n-type region 4 and has a shallow impurity concentration, and the n-type substrate 1 ′ below the overflow channel 10 is a vertical overflow. It is a drain.

Next, a method of driving the above solid-state imaging device will be described with reference to the drive voltage waveform shown in FIG. 5 and the in-substrate potential / charge distribution shown in FIGS. 6 and 7. That is, a predetermined positive voltage is applied to the n-type substrate 1, the driving voltage φV ₁ of the front transfer electrode 6 of a certain transfer stage in each set of transfer electrodes is 0 V, and the driving voltage φV ₂ of the subsequent transfer electrode 7 is a negative voltage. (−V _L ), the drive voltage φ of the preceding transfer electrode 8 of the next transfer stage in the charge transfer direction
-V _L as V _3, the period T _A of the state in which 0V is being applied to the driving voltage .phi.V ₄ of a subsequent stage transfer electrode 9, the potential-charge distribution in the substrate cross section shown in FIG. 1 (b) is Figure 6 As shown in. Here, 11 is the transfer channel potential, 12 is the shift channel potential, 13 is the complete depletion potential of the photodiode, 14b is the overflow channel potential, 15 is the overflow drain potential, and Q is the charge. Above period
At T _A , the shift channel potential 12 is almost 0 V, the shift channel is closed, and the signal charge Q is not transferred. Since the overflow channel potential 14b rises from the potential 14a when the potential of the n-type substrate 1 is 0V to the positive potential side, when the signal charge Q becomes excessive, it passes through the overflow channel 10 and overflow drain. It flows into 1 '.

Next, from the above-mentioned state, the transfer electrode 6 is transferred from the time t ₀ to the time t ₁
During the period T _B in which the positive voltage V _H is applied up to the pulse shape, the potential / charge distribution is as shown in FIG. 7. In this case, since the transfer channel potential 11 'and the shift channel potential 12' are higher than in the case of FIG. 6, the signal charge Q flows into the transfer channel 3 via the shift channel 5.

In this way, the signal charge Q flowing into the transfer channel 3 below the front-stage transfer electrode 6 is applied to the transfer electrodes 6 to 9, ...
20 are sequentially transferred in the charge transfer direction.

By the way, a coupling capacitance C ₁ is formed between the transfer electrode 6 and the P well 2, the P well 2 has a resistance component, and a capacitance C _j is present between the P well 2 and the n-type substrate 1. Since it exists, an equivalent circuit representing the vicinity of the photodiode is as shown in FIG. Here, R is the resistance value of the P well 2,
The point B is one end of each of the capacitances C ₁ and C _j and the resistance value R, and the photodiode full depletion potential 13 and the overflow channel potential 14b are formed on the basis of the potential at the point B. On the other hand, transfer channel potential 11 'and shift channel potential 1
2 ', strongly affected by the transfer electrode potential .phi.V _1, is controlled in proportion to substantially transfer electrode potential .phi.V _1. Also, P well 2
Has a photodiode 4 arranged in the row and column directions and a transfer channel 3 formed in the column direction, and is therefore grounded outside them. Therefore, P well 2
The resistance value R of is larger toward the center of the photodiode array.

Due to the existence of the capacitances C ₁ and C _j and the resistance value R as described above, when the drive pulse φV ₁ is applied to the transfer electrode 6 as shown in FIG. 5, the potential at the point B in FIG. , C ₁ >> C _j , it changes as shown in FIG. 9, and even immediately before the shift channel 5 is closed (immediately before time t ₁ ), the potential does not become zero but becomes positive.

On the other hand, FIG. 10 shows a state in which, when strong light is incident on the solid-state imaging device, the charges due to photoelectric conversion become excessive and the excess charges are discharged to the overflow drain 1 '.
In this state, when a drive pulse is applied to the transfer electrode 6 as shown in FIG.
Potential of the P well 2 via the coupling capacitor C ₁ between the well 2 rises between times t ₀ ~t _1. 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 overflow channel potential 14b 'and the shift channel potential 12' becomes smaller. As a result, there is a problem that the amount of charges read out to the transfer channel 3 becomes small. In this case, the larger the resistance value R of the P well 2 is, the larger the time constant for recovering the P well potential to the zero potential (ground potential) becomes, and the overflow channel potential 14b '.
Takes a long time to fall to the original potential 14b, and the amount of charge read during the drive pulse application time (t _{0 to} t ₁ ) decreases. Moreover, since the resistance value R of the P well 2 becomes larger toward the center of the photodiode array as described above, the amount of light (saturation light amount) at which the discharge of excess charge starts between the center and the periphery of the photodiode array is Different,
In addition, there is a problem that the amount of charges (saturation signal amount) read out when the excess charges are discharged is different and becomes non-uniform.

(Problems to be Solved by the Invention) According to the present invention, as described above, the coupling capacitance between the well region where the photodiode array of the solid-state imaging device is formed and the transfer electrode and the resistance component of the well region exist, and Due to the fact that the value of the resistance component varies depending on the position in the photodiode array, in the conventional transfer electrode driving method,
There is a problem in that the potential fluctuation in the well region when a drive pulse is applied becomes large and the amount of read charges becomes small, and when strong light is incident, the saturated light amount and the saturated signal amount become uneven depending on the position in the photodiode array. It is possible to prevent the potential fluctuation of the well region during driving and the reduction of the read charge amount due to the fluctuation of the potential in the well region, and even when strong light is incident, the central portion and the peripheral portion of the photodiode array are uniform. It is an object of the present invention to provide a driving method of a solid-state imaging device that can obtain a saturated light amount and a saturated signal amount.

[Structure of the Invention] (Means for Solving the Problems) The present invention relates to a plurality of photoelectric conversion units having a charge storage layer that photoelectrically converts incident light to generate signal charges and stores the signal charges. A plurality of photoelectric conversion units are provided in the semiconductor substrate, the charge reading unit having an accumulation region for reading out the signal charges accumulated in the photoelectric conversion unit, and the excess charge discharge unit discharging the excess charges generated in the photoelectric conversion unit. In the method for driving the solid-state imaging device formed in the well region of, when the charge is read by the charge reading unit,
With respect to the transfer electrode to which the read drive pulse voltage is applied, a voltage of opposite sign is applied to another transfer electrode in synchronization with the leading edge of the drive pulse voltage.

(Operation) By applying a pulse voltage of opposite sign to another transfer electrode as described above, it is possible to cancel the potential variation of the well region due to the transfer electrode to which the read drive pulse voltage is applied. The potential variation in the well region is suppressed, and the decrease in the amount of read charges from the photodiode charge storage layer can be suppressed. Further, the potential of the well region becomes small immediately before the end of the drive pulse application time. Therefore, it is hardly affected by the nonuniformity of the resistance value of the well region in the photodiode array. As a result, the saturated light amount and the saturated signal amount can be uniformly obtained in the central portion and the peripheral portion of the photodiode array.

Embodiment An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 2 shows a drive voltage φV ₁ applied to the transfer electrodes 6 to 9, ... When reading the photodiode accumulated charges in the solid-state imaging device as described above with reference to FIGS. 1 (a) and 1 (b). An example of the waveforms of ~ φ V ₄ , ... Is shown. That is, the solid-state imaging device to which the method of the present invention is applied includes a plurality of photoelectric conversion units having a charge storage layer that accumulates the signal charges while photoelectrically converting incident light to generate signal charges as described above. A plurality of photoelectric conversion units, each of which includes a charge reading unit having a storage region for reading out signal charges stored in the photoelectric conversion unit, and an excess charge discharging unit for discharging excess charges generated in the photoelectric conversion unit. The transfer electrode of the charge transfer portion is formed in the well region in the substrate and
It is provided above a part of the well region so as to face each other. Then, in the driving method according to the present example, before reading the charges from the photodiode, for example, the ground potential (0 V) is applied to the front stage transfer electrodes 6 and 8, and the negative potential −V _L is applied to the rear stage transfer electrodes 7 and 9, At the time of reading charges, for example, a positive potential is applied to the front stage transfer electrode 6.
A driving pulse φV ₁ of V _H is applied, and a voltage φV ₃ of opposite sign (in this example, −V _L potential) is applied to another front stage transfer electrode 8 in synchronization with the leading edge of the driving pulse φV ₁ . Since the negative potential −V _L is applied to the latter-stage transfer electrode 7, the potential inside the substrate under the transfer electrode 7 is low and a potential barrier in the charge transfer direction is formed.

According to the above driving method, an equivalent circuit representing the vicinity of the photodiode to be read is as shown in FIG. 3, and the potential at point B (well region near the photodiode) in FIG. 3 is the fourth. It changes as shown in the figure. Here, referring to FIGS. 1 (a) and 1 (b), in FIG.
C ₁ , C ₂ , R, and C _j will be described. That is, R is the resistance value of the well region 2 near the photodiode, and
Normally contacts the ground end at its end. Also,
C ₁ is the coupling capacitance existing between the well region 2 and the transfer electrode 6 to which the driving voltage φV ₁ is applied, and C ₂ is between the well region 2 and the transfer electrode 8 to which the driving voltage φV ₃ is applied. C _j is a junction capacitance between the well region 2 and the substrate 1, and V _OFD is a bias potential (overflow drain potential) applied to the substrate 1. In this case, C ₁ C ₂
Since there are relationship »C _j, C _j min to the driving pulse application is not little effect, and a positive potential fluctuation at the point B by the potential change and negative pulse application point B by pulses applied offset As a result, the potential at the point B hardly rises. Therefore, as shown in FIG. 7, the photodiode charge storage layer potential 13 and the overflow channel potential 14b hardly rise, and immediately before the end of application of the drive pulse (immediately before the shift channel 5 in FIG. 1b is closed). The potential at point B in () is negligibly smaller than the potential in the conventional example (see FIG. 9). Therefore, the overflow channel potential 14b
The potential difference between the shift channel potential 12 'and the shift channel potential 12' is suppressed from becoming unnecessarily small, and the amount of read charges from the photodiode storage layer is suppressed from decreasing. In addition, even if the resistance value of the well region in the photodiode array is not uniform, it is hardly affected by it.
That is, according to the driving method described above, the period shown in FIG.
The distribution of potentials and charges in the substrate corresponding to T _A and T _B is usually obtained in the same way as shown in FIGS. 6 and 7, and strong light is emitted from the light receiving surface (photodiode of the solid-state imaging device). Arrangement surface)
Even when it is incident on, the saturated light amount and the saturated signal amount can be uniformly obtained in the central portion and the peripheral portion in the photodiode array.

Incidentally, in the above-described embodiment, at the time of reading the charge, the reading drive pulse voltage of the positive voltage is applied to the transfer electrode 6 and at the same time, the negative voltage is applied to the transfer electrode 8 in synchronization with the leading edge. If the voltage is applied to any of 7, 8 and 9, the same effect as in the above embodiment can be obtained. When applying a positive read drive pulse voltage to any of the transfer electrodes 7, 8 and 9 other than the transfer electrode 6, any one of the transfer electrodes other than this (6 or If a negative voltage is applied to 7 or 8 or 9) in synchronization with the leading edge of the read drive pulse voltage, the same effect as in the above embodiment can be obtained.

[Effects of the Invention] As described above, according to the driving method of the solid-state imaging device of the present invention, it is possible to prevent the potential fluctuation of the well region during the read driving of the photodiode accumulated charges and the reduction of the read charge amount, which is strong. Even when light is incident, it is possible to obtain a uniform saturated light amount and a saturated signal amount both in the central portion and the peripheral portion of the photodiode array.

[Brief description of drawings]

FIG. 1 (a) is a plane pattern showing a part of a solid-state imaging device to which the method for driving a solid-state imaging device according to the present invention is applied, and FIG. 1 (b) is BB 'in FIG. Sectional view along the line,
FIG. 2 is a waveform diagram showing an applied voltage to the transfer electrode in FIG. 1 (a) according to one embodiment of the method of the present invention, and FIG.
FIG. 4B is a circuit diagram showing an equivalent circuit showing the vicinity of the photodiode in FIG. 4B, FIG. 4 is a diagram showing a state of potential change in the well region near the photodiode in FIG. 1B, and FIGS. FIG. 11 relates to a driving method of a conventional solid-state image pickup device, FIG. 5 is a waveform diagram showing an applied voltage to a transfer electrode in FIG. 1 (b), and FIGS. 6 and 7 are periods in FIG. T _A , T _B
1 (b) corresponding to FIG. 1 (b) showing a state of potential / charge distribution in the substrate, FIG. 8 is a circuit diagram showing an equivalent circuit representing the vicinity of the photodiode in FIG. 1 (b), and FIG. The figure is first
Figures 10 (a) and 11 (b) show changes in the potential of the well region in the vicinity of the photodiode in Fig. 5 (b), during periods T _A and T _B in Fig. 5 when strong light enters the solid-state imaging device. It is a figure which shows the mode of the electric potential and charge distribution in the corresponding substrate. 1 ... n type substrate, 2 ... P well, 3 ... transfer channel, 4 ...
Photoelectric conversion part, 5 ... Shift channel, 6-9 ... Transfer electrodes,
10 ... overflow channels, 1 '... overflow drain, C _1, C ₂ ... coupling capacitance, C _j ... junction capacitance, the resistance value of R ... well region, 20 ... charge reading unit.

Claims (1)

[Claims] 1. A plurality of photoelectric conversion units having a charge storage layer that photoelectrically converts incident light to generate signal charges and stores the signal charges, and a storage region for reading out the signal charges stored in the photoelectric conversion units. Driving a solid-state imaging device including a charge reading unit having a plurality of photoelectric conversion units and an excess charge discharging unit configured to discharge excess charges generated in the photoelectric conversion unit, wherein the plurality of photoelectric conversion units are formed in a well region in a semiconductor substrate. In the method, when reading out charges by the charge reading unit,
A driving method of a solid-state imaging device, wherein a voltage having an opposite sign is applied to another transfer electrode in synchronization with a leading edge of the drive pulse voltage with respect to a transfer electrode to which a read drive pulse voltage is applied.