JPH0221783A - Drive method for solid-state image pickup element - Google Patents

Abstract

PURPOSE:To reduce a highlight after-image and to expand the dynamic range by setting a potential under a transfer electrode of a vertical transfer register not in common use with a readout gate electrode higher than the potential set to a photodetector. CONSTITUTION:A charge stored in a photodiode 5 at storage is read to a CCD register 6 under the 2nd layer transfer electrode while a readout gate region 4 is set at readout. In this case, the setting potential of the photodiode 5 is set lower than the potential of a CCD register 7 under the 1st layer transfer electrode, then the charge of the photodiode 5 is all able to be moved to the CCD register 6. Thus, a clear video image without highlight after-image is obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an interline type COD solid-state image sensor,
In particular, it relates to methods of improving its properties.

[Conventional technology]

A two-dimensional COD image sensor (solid-state image sensor) consists of multiple light-receiving elements, vertical and horizontal transfer registers that transfer the charges photoelectrically converted and accumulated by the light-receiving elements, and a readout gate that reads the charges from the light-receiving elements to the transfer registers. It is provided on a substrate, and in recent years, advances in technology have led to higher density and smaller size. However, there are limits due to optical format limitations and resolution, and currently it belongs to the category of large chip area among LSIs. On the other hand, there is a reduced-type overflow drain technology that aims to improve the shortcomings peculiar to solid-state imaging devices such as blooming and smearing, and to facilitate higher density. CCDs have large chip areas and high driving voltages to make elements.
In this case, the shaking of the well due to the driving pulse cannot be ignored.

For this reason, the set voltage of the photodiode, which is a light-receiving element, differs in two dimensions, resulting in shading of the saturated output, which limits the dynamic range of the output.

In order to alleviate such drawbacks, conventional methods have been used to reduce the shaking of the well. For example, there are methods in which multiple vertical transfer register electrodes are provided for one photodiode and one of these electrodes forms a readout gate to reduce the coupling capacitance to the well, or for the same purpose, upper layer transfer Forming a readout gate with an electrode is being practiced. In the case shown in FIG. 3(a), two vertical transfer register electrodes 2.3 are provided for one photodiode 5, and a second layer electrode (part of the second layer transfer electrode 3 (readout gate) is provided. (on the region) also serves as a readout gate electrode.)3 is used for reading. Also,
In this example, the coupling capacitance is further reduced by increasing the area of the first layer transfer electrode 2 and narrowing the second layer transfer electrode 3.

FIG. 3(b) shows the state of potential and charge movement at various parts during charge accumulation and charge readout in the seven diodes along line AA' in FIG. 3(a). The charge accumulated in the photodiodes during storage is transferred to the COD register 6 under the second transmission and transfer electrode when the readout gate region 4 is ONL during readout.
load is read out. Since the potential of the photodiode during readout is higher than the potential of the CCD register 7 under the first layer transfer electrode, the amount of charge Qpo accumulated in the photodiode 5
If there is less charge, the charge will be transferred to the COD register 6 under the second layer transfer electrode.
However, when the amount of charge Qpo accumulated in the photodiode increases, the area of the COD register 6 under the second layer transfer electrode is small, so that charge remains in the photodiode as shown in FIG. 3(b). .

[Problem to be solved by the invention]

When strong light is incident on a photodiode as described above, there is a problem in that unread charges remain on the photodiode itself and are observed as a highlight afterimage.

The present invention aims to reduce the above-mentioned highlight afterimage and expand the dynamic range.

[Means to solve the problem]

The driving method of the present invention includes, on the same semiconductor substrate, a photodetector, a vertical transfer register that transfers the charges photoelectrically converted and accumulated by the photodetector, and a horizontal transfer register and a readout gate that reads the charges from the photodetector to the vertical transfer register. In a solid-state imaging device in which one element of the light receiving element is formed from a part of the plurality of transfer electrodes of the vertical transfer register, when reading charges from the light receiving element to the vertical transfer register, the The configuration is such that the potential under the transfer electrode of the vertical transfer register, which also serves as the readout gate electrode, is set higher than the potential set to the light receiving element.

〔Example〕

Next, the present invention will be explained with reference to the drawings. FIG. 1 shows an embodiment of the invention.

FIG. 1(a) is exactly the same as FIG. 3(a). However, in the present invention, as shown in FIG. 1(b), by setting the set potential of the photodiode 5 lower than the potential of the COD register 7 under the first layer transfer electrode, all the charges in the photodiode 5 are transferred to the CCD register during readout. It is possible to do so. A possible method for satisfying such conditions is to provide driving pulses or conditions to the process, and these conditions are determined in consideration of the balance of characteristics of the solid-state imaging device. This situation will be explained using FIG. 2. Figure 2 (a) shows a graph of potential potential versus gate voltage for a CCD register with readout gate area 4. Point b shows the potential when the gate voltage of the CCD register is 0, and point a shows the potential at the readout gate. It shows the gate voltage when the potential of region 4 is the same potential as point b. The waveform at the bottom of the graph shows the voltage actually applied to the gate, and when the photodiode is storing data, that is, when the CCD register is in the transfer state, it is VM.
A voltage is applied between Vr and Vr, and the read gate region is
While the FF state is maintained, when reading, V
A voltage of H is applied, and the photodiode is set to the potential at point a, that is, point b, in the case of FIG. 2(a).

From this, it can be seen that in order to realize the conditions shown in FIG. 1(b), it is sufficient to set VH to a potential lower than point a in FIG. 2. However, if the maximum amount of charge accumulated in the photodiode is Qpo, then this is Qpo÷CpoX
(b/a) VH (CPD has a capacitance of 7 photodiodes), and making VH extremely smaller than point a will lead to a decrease in the maximum amount of charge accumulated in the photodiode, resulting in a dynamic image that does not cause highlight afterimages. It is necessary to set it in consideration of the range. In other words, if the capacitance of the CCD register under the first layer transfer electrode is C1, and the capacitance of the CCD register under the second layer transfer electrode is C2, VH was lowered from point a to point e as shown in Figure 2(b). The maximum amount of charge that can be received by the CCD register (first
Maximum value of charge in the shaded area during readout in figure (b)) Qcc
Since o is given as Qcco = Ct Recognize. Furthermore, due to the operation of the vertical overflow drain, Qpo remains constant regardless of VG above a certain amount of light. If this point is QOF, then the gate voltage corresponding to Qop, that is, (Q op/
When Cpo ) (a/b) is lower than 0 point, this highlight afterimage does not occur due to the overflow drain operation, so (Cop/Cpo) (a/b ) >
Only in case c, it is necessary to adjust VH according to the method described above. Therefore, it is necessary to change the condition settings depending on the control characteristics of the overflow train.

〔Effect of the invention〕

As explained above, in order to reduce well fluctuation, a plurality of transfer electrodes of a vertical transfer register are made to correspond to a light receiving element, and a readout gate electrode is formed from a part of this solid-state image sensor. Also, by setting the potential of the light receiving element lower than the potential of the transfer electrode which also serves as the readout gate electrode, a clear image without highlight afterimages can be obtained.

[Brief explanation of the drawing]

Figures 1(a) and (b) are schematic diagrams of the present invention, Figure 2(a)
, (b) are potential potential graphs for explaining the operation of FIG. 1, and FIGS. 3(a) and 3(b) are schematic diagrams of the conventional example. 2... First layer transfer electrode, 3... Second layer transfer electrode, 4
... Readout gate region, 5... Photodiode, 6... COD register under second layer transfer electrode, 7...
COD register under the first layer transfer electrode.

Claims (1)

[Claims] A light receiving element, a vertical transfer register for transferring charges photoelectrically converted and accumulated by the light receiving element, and a horizontal transfer register and a readout gate for reading charges from the light receiving element to the vertical transfer register are provided on the same semiconductor substrate, and the light receiving element A plurality of transfer electrodes of the vertical transfer register correspond to one element, and the readout gate electrode is formed from a part of the plurality of transfer electrodes of the vertical transfer register. A solid state characterized in that, when reading charges from an element to the vertical transfer register, a potential under a transfer electrode of the vertical transfer register that serves as the readout gate electrode is set higher than a potential set to the light receiving element. How to drive the image sensor.