JPH0325974A - Solid state image sensor - Google Patents

Abstract

PURPOSE:To eliminate remaining charge and to prevent generation of an after image by providing a shallowest region of a first conductivity type well of a solid state image sensor having a vertical overflow drain structure shiftingly from the center of a photoelectric converter to a charge transfer unit side to be provided. CONSTITUTION:A P-type well 2 is formed on a N-type semiconductor substrate 1, and a N-type region 3 for forming a photoelectric converter and a N-type region 4 for forming charge transfer means are formed therein. The shallowest part of the well is formed not at the center of the converter but a part near the charge transfer means. Since the part of highest depleting potential of the converter is disposed near the charge transfer unit, the potential of the converter of the region near the charge transfer unit is pulled by a fringe electric field of a reading pulse when a pulse voltage for reading a signal charge is applied to a polysilicon electrode. Thus, all the signal charge can be read to the charge transfer unit, and no remaining charge is generated in the converter.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a solid-state image sensor, and more particularly to a structure for preventing afterimages in a solid-state image sensor having a vertical overflow drain structure.

[Conventional technology]

In solid-state imaging devices, there is a limit to the amount of signal charge that can be stored in the photoelectric conversion element, so when a bright spot with high illuminance is incident, the signal charge generated by photoelectric conversion overflows from the photoelectric conversion element and causes damage to the adjacent photoelectric conversion element. There is a pluming phenomenon that flows into the photoelectric conversion element and the charge transfer section, causing image distortion. The vertical overflow drain structure is designed to prevent this blooming phenomenon. The excess signal charge of the conversion element is extracted to the semiconductor substrate. FIG. 5 18l is a vertical cross-sectional view of a cell portion of a solid-state imaging device having a conventional vertical overflow drain structure.

A P-type well 2 is formed on an N-type semiconductor substrate 1, in which an N-type region 3 of a photoelectric conversion element and an N-type region 4 of a charge transfer means are formed.
is formed. Channel stop area 5 between each cell
separated by In addition, a polysilicon electrode 7 for applying a pulse voltage for reading and transferring charges through the insulating film 6 is formed on the button. A membrane 8 is provided.

By applying a reverse bias voltage between the P-type well 2 and the N-type semiconductor substrate 1 and bringing the N-type region 3 forming the photoelectric conversion element and the N-type semiconductor substrate 1 into a bunch-through state, excess charges are transferred to the N-type semiconductor substrate. Remove it to 1. To prevent such blooming, even if the button is pressed when a reverse bias voltage is applied to the semiconductor substrate, the N-type region of the charge transfer means is within the operating range of the charge transfer means so that charge transfer can be performed normally. It must be electrically isolated from the semiconductor substrate. For this reason, as shown in FIG. 5 (al), the P-type well is formed so that there is a shallow portion directly below the charge conversion element in the area where the charge transfer means is formed.

Conventionally, the P-type well is formed at its shallowest depth at the center of the photoelectric conversion crystal. Figure 5 (bl shows the channel potential in a depleted state with no signal charge in the conventional example in Figure 5 (al); It is getting high.

[Problem to be solved by the invention]

In the above-mentioned conventional solid-state image sensor having the vertical overflow drain structure, when signal charges are read out from the photoelectric conversion element to the charge transfer means, the depletion potential in the central part of the 9IF conversion element is high, so that the signal charges in this part are There is a problem in that the image cannot be read out and a residual image phenomenon occurs. FIG. 6 shows the movement of channel potential and signal charges when reading out signal charges applied to a solid-state imaging element having a conventional vertical overflow drain structure.

By applying a read pulse voltage to the signal charge 9Vi readout electrode generated and accumulated by the photoelectric conversion shown in Fig. 6 (al), most of the signal charge is read out to the charge transfer section as shown in Fig. 6 (bl). However, the charge at Ichito is not read out and remains as a residual charge 10 in the center of the charge conversion section where the depletion potential is high.If the amount of incident light is constant, the amount of this residual charge is in a steady state and remains constant. However, when the amount of incident light, that is, the accumulated signal charge l changes, the amount of residual charge changes, resulting in an afterimage phenomenon.

[Means to solve the problem]

The solid-state imaging probe of the present invention has a vertical overflow drain structure including a second conductive type semiconductor substrate in which a first conductive T-type well is formed which is shallower directly below the photoelectric conversion S than directly below the charge transfer. The solid-state imaging device has residual charge prevention means for charge readout, which is provided by shifting the shallowest region of the first conductivity type well from the center of the -Xt conversion section toward the charge transfer section. .

〔Example〕

Next, the present invention will be explained with reference to the drawings. FIG. 1 is a vertical sectional view of a cell portion of a solid-state image sensing device according to a first embodiment of the present invention.

A P-type well 2 is formed on an N-type semiconductor substrate 1, and an N-type region 3 serving as a +t conversion element and an N-type region 4 serving as a charge transfer means are formed in this well. Each cell is separated by a channel stop region 5. A polysilicon electrode 7 is formed to apply a pulse voltage 710 for charge readout and transfer through an insulator 11!6, and a polysilicon electrode 7 is formed to prevent the charge from entering the cell area other than the charge conversion element region. In order to prevent this, Runno 8 is provided.

The P-type well is also used when applying a reverse bias voltage gold to the N-type semiconductor substrate to punch-through the N-type region forming the photoelectric conversion element and the N-type semiconductor substrate to prevent the blooming phenomenon. is within the operating range of the element
The P-type well is shaped so that it has a shallower area than the area where the charge transfer means is formed under the charge conversion element so as to be electrically isolated from the P-type semiconductor substrate. The shallowest part is formed not at the center of the charge conversion element but near the charge transfer means.

FIG. 1 (bJ shows the channel potential in a depleted state with no signal charge in the first embodiment of FIG. 1 (al).

Light tf'! ! A high potential of the I8 element occurs in a shallow region of the P-type well near the charge transfer means.

FIG. 2 shows the channel potential and the movement of signal charges when reading signal charges according to this embodiment.

The signal charge 9 generated and accumulated by the photoelectric conversion shown in FIG. 2 (a) is read out to the charge transfer section as shown in FIG. 2 (b+) by applying a read pulse voltage to the read electrode. In the invention, as shown in the potential in Figure 2, the highest depletion potential of the charge conversion section is close to the charge transfer section, so when a pulse voltage for reading out signal charges is applied to the polysilicon electrode, a fringe of the readout pulse voltage occurs. By pulling the electric potential of the nine-electron K converter in the area close to the charge transfer section by the electric field, all signal charges can be read out to the charge transfer section, and no residual charge is generated in the photoelectric conversion section.

By eliminating the residual charge that conventionally caused the occurrence of afterimages, it is possible to prevent the occurrence of afterimages.

FIG. 3 briefly shows an example of the method for manufacturing the solid-state image sensing device of this embodiment using a vertical cross-sectional view of a cell portion.

As shown in FIG. 3 (al), ions are implanted onto the N-type semiconductor substrate 1 using the resist pattern 12 as a mask to selectively introduce P-type impurities (13). As shown in FIG. 3 (bl), P-type wells 2 with different junction depths are selectively formed.

After the insulating film 6 is formed 7c, an N-type region 3 forming a photoelectric conversion element and an H-type axial region 41I forming a charge transfer means are formed in self-alignment with the pattern of the insulating film 6.

Here, the pattern of the insulation exchanger 6 is such that the center of the resist pattern 12 at the time of ion implantation that forms the P-type well is located at the center of the resist pattern 12! It is formed so as to be shifted from the center of the N-type region 3 forming the conversion element in a direction closer to the N-type region 4 of the charge transfer means.

Thereafter, as shown in FIG. 1(a), a polysilicon electrode 7 and a switch 8 are formed to form a solid-state imaging element. For example, if the width of the N-type region of a photoelectric conversion element is 5 μm, the width of the selective oxidation pattern and the P-type well is 2 μm.
The effect is most pronounced when the amount is reduced.

FIG. 4(a) is a klvfr plane view of a cell portion of a solid-state image sensing device according to a second embodiment of the present invention.

In this embodiment, a buried photodiode is used as a photoelectric conversion element, and a P-type well 2 is formed on an N-type semiconductor substrate 1. The N-type region 4 is rounded. Each cell is separated by a channel stop region 5, and connected to the channel stop region 1'l71 on the surface of the photoelectric conversion element.
11 is formed. Further, a polysilicon electrode 7 and an R-shaped electrode 8 are formed with an insulating film 6 interposed therebetween. In this embodiment, the effect of preventing image retention by forming the shallowest part of the P-type well not at the center of the photoelectric conversion element but near the charge transfer means is the same as in the first embodiment.

In the examples described so far, we have simply used an N-type semiconductor substrate, but an N-type epitaxial/I
The effect is the same even if you use a grown one.

[Effects of the Invention] As explained above, the solid-state imaging device having the vertical O/C-flow drain structure of the present invention can form a 1g1 conductivity type well that is shallower directly below the 9T conversion section than directly below the charge transfer S. By shifting the shallowest region of the first conductivity type well from the center of the charge conversion section toward the charge transfer m side, the region of the photoelectric conversion section with the highest depletion potential can be The electric potential formed near the charge transfer section is pulled by the fringe electric field of the readout pulse voltage when reading signal charges, thereby eliminating the residual charge left behind in the region of the photoelectric conversion section with the highest depletion potential, and causing an afterimage. It has the effect of preventing the phenomenon from occurring.

[Brief explanation of drawings]

FIG. 1 (aJ is the first solid-state image sensor of the first embodiment of the present invention.
FIG. 3 is a channel potential diagram showing the channel potential and movement of signal charges during signal charge readout in the embodiment.
FIG. 4 (ta+) is a vertical cross-sectional view for explaining an example of the manufacturing method of the solid-state image sensor according to the second embodiment of the present invention, and FIG. 4 (bJ is Figure 4 (al
Channel potential diagram during depletion of the solid-state imaging device, Figure 5 (
aJ is a vertical cross-sectional view of a conventional solid-state image sensor, FIG. 5(b) is a channel potential diagram during depletion of the solid-state image sensor shown in FIG. 5ja), and FIG. 6 is a signal applied to the conventional example shown in FIG. FIG. 7 is a channel potential diagram showing the movement of channel potential and signal charges when reading out charges. 1... N-type semiconductor substrate, 2... P-type well, 3...
・N-type region of photoelectric conversion element, 4...N of charge transfer means
Mold region, 5... Channel stop region, 6... Insulation a, 7... Polysilicon electrode, 8... Continuation film, 9...
...Signal charge, 10...Residual charge, 11...R layer on the surface of the photoelectric conversion element, l2...Resist pattern, 13
...P-type impurity.

Claims (1)

[Claims] In a solid-state imaging device having a vertical overflow drain structure including a second conductivity type semiconductor substrate in which a first conductivity type well is formed which is shallower directly below the photoelectric conversion unit than directly below the charge transfer unit, the first conductivity 1. A solid-state image pickup device, comprising residual charge prevention means for reading charge, which is provided by shifting the shallowest region of the mold well from the center of the photoelectric conversion section toward the charge transfer section.