JPH0327683A - Drive method for solid-state image pickup device - Google Patents

Abstract

PURPOSE:To reduce after image considerably by applying a pulse with a polarity expanding a depletion layer of a photoelectric conversion section to a conductive light shielding film in the case of reading the electric charge through the application of a read pulse to a read gate electrode. CONSTITUTION:At light shielding film application clock phiS having a negative polarity pulse generated synchronously with a period when a readout clock phiV applied to a read gate electrode is at a high level voltage VH is applied to a conductive light shielding film shielding the light to a vertical transfer register. The signal charge stored in a photo diode region 3 at a time to is read to a vertical transfer register area 4 when the voltage vH is applied to the readout gate electrode at a time t1 and a pulse phiS is applied to the light shielding film at the same time. Thus, the potential of the region 3 is changed as shown in solid lines from the substantial potential shown in dotted lines, thereby promoting the readout of the charge. Thus, the charge left in the region 3 at a time t2 is very small..

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for driving a charge-coupled device type solid-state image sensor, and particularly to a method for driving a solid-state image sensor that reduces the afterimage phenomenon. [Conventional technology A coin-interline transfer type solid-state image sensor has a cell structure as shown in FIG. In Figure 6, 1 is p
type semiconductor substrate, 2 is a p+ type channel stop, 3 is
constitutes a photodiode together with the semiconductor substrate l, n
4 is a photodiode region which is a type diffusion region; 4 is a vertical transfer register region which receives signal charges accumulated in the photodiode region 3 and serves as a charge transfer region; 5 is an oxide film covering the surface of the semiconductor substrate; denotes a vertical transfer register 1IX8ii which also serves as a readout gate electrode, 7 denotes a light-shielding film made of a conductive material such as AJ that shields parts other than the photodiode region 3, and 8 denotes a photodiode region 3 and a vertical transfer register region 4. This is the readout gate region that is located between the two and controls the readout of signal charges. The conventional driving method for this solid-state image sensor is to apply a readout clock φV shown in FIG. 7 to the vertical transfer register electrode 6. The potential diagrams at each point in time of this read operation are shown in FIGS.
Shown in c). At time t2o, the photodiode region 3 is in a charge accumulation state as shown in FIG.
When it becomes H, the charge is read out from the photodiode region 3 to the vertical transfer register region 4 via the readout gate region 8? It will be done. FIG. 8(c) shows a state in which the read operation is completed at time t22. [Problems to be Solved by the Invention] In the conventional driving method described above, at time t2■, the signal charges accumulated in the photodiode region 3 until then are read out to the vertical transfer register region 4; When most of the charges are read out, the electric field from the readout gate region 8 to the photodiode region 3 becomes very weak, and the charge is not completely read out in a finite time. Figure (c)
As shown in the figure, charges are left behind in the photodiode region 3. Therefore, this remaining charge will be observed as an afterimage in the next readout. As a countermeasure against this afterimage, there is a method of completely depleting the photodiode region by lowering the impurity concentration of the diffusion layer constituting the photodiode region 3, and applying a potential difference between the photodiode region 3 and the readout gate 8 during readout. A method can be considered in which the voltage of the read clock pulse (VH in FIG. 7) is increased to produce an effect similar to the method described above. However, reducing the impurity concentration in the photodiode region reduces the maximum charge that can be stored in that region. Currently, in solid-state imaging devices, there is a trend to increase the number of pixels in order to improve resolution, and the area of the photodiode is gradually decreasing, which is reducing the amount of charge that can be stored in the photodiode. Therefore, the former method of reducing the amount of charge that can be stored in the photodiode by 1N cannot be adopted. In addition, with the recent miniaturization of solid-state image sensors, the withstand voltage of solid-state image sensors tends to decrease further, so it is necessary to keep the amplitude of the driving clock low. Therefore, adopting the latter method is contrary to the above-mentioned technical trend.

[Means for Solving the Problems] The present invention includes a photoelectric conversion section, a transfer register section for transferring photoelectric conversion charges generated in the photoelectric conversion section, and a transfer register section for reading charges from the photoelectric conversion section to the transfer register section. A method for driving a solid-state image sensor having a readout gate section and a conductive light-shielding film that blocks light from a portion other than a photoelectric conversion section formed on a semiconductor substrate via an insulating film, the method comprising: an electrode of the readout gate section; The present invention is characterized in that when a readout pulse is applied to perform a charge readout operation, a pulse with a polarity that expands a depletion layer of the photoelectric conversion section is applied to the conductive light-shielding film. [Example] Next, an example of the present invention will be described with reference to the drawings. FIG. 1 is a waveform diagram of a driving clock used in an embodiment of the present invention. In this example, vertical transfer register t8i! A conductive light shielding film that shields the vertical transfer register from the light shielding film applying clock φS having a negative polarity pulse generated in synchronization with the period in which the high level voltage VH is applied to the readout clock φV applied to the readout gate electrode which also serves as a readout gate electrode. Apply voltage to the membrane. In FIGS. 2(a) to 2(c), t of FIG. 1 is shown. ．． tI. Photo die at each time point of t2? The potential and charge transfer state of the readout gate region, readout gate region, and vertical transfer register region are shown. The signal charges accumulated in the photodiode region 3 at time t as shown in FIG. 2(a) are read out to the vertical transfer register region 4 when a voltage of ■■ is applied to the readout gate electrode at t1. At the same time, a pulse of negative polarity is applied to the light-shielding film, so an electric field is instantaneously applied to the photodiode surface due to the coupling capacitance formed by the interlayer between the light-shielding film and the potential of the photodiode region. Figure 2 (b
), the original potential shown by the dotted line changes as shown by the solid line, and the charge readout operation is accelerated. Therefore, as shown in FIG. 2(C), the amount of charge left behind in region 3 at time t2 is extremely small. As described above, according to the present invention, while keeping the pulse amplitude value of the read clock φ■ low, it is possible to achieve the same effect as when the pulse amplitude value is made sufficiently high. This point will be further explained with reference to Figure 3. FIG. 3 shows the amount of energy required to completely deplete the photodiode region when the light-shielding film application clock φS is applied to the light-shielding film according to this embodiment in a solid-state image pickup device compatible with a 1/2-inch optical system. This is a graph showing how the voltage applied to the vertical transfer register electrode changes in relation to the amplitude of the clock φS. As shown in the figure, for example, when a pulse with an amplitude of 5 cm is applied to the light shielding film, a voltage slightly lower by 2 cm is applied to the transfer register electrode than when the light shielding film is fixed at the ground potential. The photodiode region can be depleted by applying . Therefore, according to the present invention, the amplitude of the read clock can be suppressed to a low level, so that the problem of withstand voltage between vertical transfer registers in a high-density solid-state image sensor can be alleviated. FIG. 4 is a waveform diagram of a driving clock used in another embodiment of the present invention. The difference between this embodiment and the previous embodiments is as shown in FIG.
Read the timing of the rise of the pulse of the light-shielding film application clock φS, delay the timing of the rise of the pulse of the clock φV, and read the timing of its fall? This is faster than the lock φ■. In this example,
FIG. 5 shows the potential and charge transfer state of each part at times tlo to tl4 shown in FIG. 4. At time tlo, the signal charge is M8 in the photodiode region 3 as shown in FIG. The portion is transferred to vertical transfer register area 4. At time t1■, the light shielding film application clock φS
When becomes negative, the potential of the photodiode region 3 changes from the broken line to the value shown by the solid line as shown in FIG. 5(c), and the remaining signal charges are read out. Subsequently, at time t+s, when the light shielding film application clock φS reaches the Ot level, the potential of the photodiode region also returns to the normal state, as shown in FIG. 5(d). At time tl4, the read operation ends as shown in FIG. 5(e).

As explained above, this embodiment can also greatly reduce the residual charge that causes the afterimage phenomenon. However, in this embodiment, the phase of φS is set to be different from φV, so the pulse Compared to the case where the voltage is applied, fluctuations in the potential of the well can be suppressed to a low level, interference between pulses can be reduced, and the accumulated charge in the photodiode can be extracted stably. [Effects of the Invention] As explained above, the present invention provides the following advantages:
Since a pulse with a polarity that expands the depletion layer of the photoelectric conversion section is applied to the conductive light-shielding film on the charge transfer region, according to the present invention, it is possible to almost completely eliminate unread signal charges. Afterimages can be significantly reduced.
Furthermore, according to the present invention, the impurity concentration in the photodiode region, which is the photoelectric conversion section, can be kept high, so that the signal charge that can be stored does not decrease, and the dynamic range can be widened. Furthermore, since the pulse amplitude of the read clock required to completely deplete the photodiode region can be suppressed to a low level, dielectric breakdown does not occur between the transfer register electrodes or the like.

[Brief explanation of drawings]

1 and 4 are waveform diagrams of driving clocks used in the embodiment of the present invention, FIGS. 2, 3, and 5 are operation explanatory diagrams of the embodiment of the present invention, and FIG. 6 7 is a sectional view of a solid-state image sensor, FIG. 7 is a waveform diagram of a drive pulse used in a conventional example, and FIG. 8 is an explanatory diagram of its operation. DESCRIPTION OF SYMBOLS 1...p-type semiconductor substrate, 2...channel stop, 3...photodiode region,
4... Vertical transfer register area, 5... Oxide film, 6... Vertical transfer register electrode, 7... Light shielding film, 8... Read gate area.

Claims (2)

[Claims] (1) A plurality of photoelectric conversion parts of a second conductivity type formed in a first conductivity type semiconductor region, and a plurality of photoelectric conversion parts formed in the first conductivity type semiconductor region and receiving signal charges from the photoelectric conversion part and receiving the signal. a charge transfer region of a second conductivity type serving as a charge transfer region; a readout gate region disposed between the charge transfer region and the photoelectric conversion section; and an insulating film on the charge transfer region and the readout gate region. A method for driving a solid-state imaging device, comprising: a charge transfer electrode formed on the charge transfer electrode via an insulating film; and a conductive light-shielding film having an opening on the photoelectric conversion section formed on the charge transfer electrode via an insulating film. When applying a readout pulse to the charge transfer electrode to read out signal charges from the photoelectric conversion section to the charge transfer region, the conductive light-shielding film is formed between the first conductivity type semiconductor region and the photoelectric conversion section. A method for driving a solid-state imaging device, comprising applying a pulse having a polarity in a direction that expands a depletion layer between the two. (2) The solid state according to claim 1, wherein the phase of the pulse applied to the conductive light-shielding film is delayed at the time of rising and advanced at the time of fall with respect to the readout pulse applied to the charge transfer electrode. How to drive the image sensor.