JPS6065565A - Solid-state image sensor - Google Patents

Abstract

PURPOSE:To prevent the generation of a local dark curent and the irregularity due to the dark current between photosensitive elements by superposing and providing the same conductive type diffused layer as a substrate on an electron storage layer. CONSTITUTION:A transparent electrode 21 is formed through an insulating film 2 on an N type diffused region, and a negative voltage s applied. Carrier of negative charge is generated in a semiconductor by a light (radiation) incident from a window of a light shielding film 7. Since the negative voltage is applid to the electrode 21, the negative electrode near the surface of the substrate 1 is remotely disposed, and a layer having many holes, i.e., an inverting layer 22 is formed instead as a depletion preventive layer on the surface of the substrate of a photosensitive element region A. Accordingly, the photoelectrically converter negative charge is stored in a charge storage region 20 surrounded by the P-N junction to the substrate 1 and the layer 22. In other words, the carrier which is photoelectrically converted does not exist near the boundary between the substrate 1 and the film 2, and is not affected by the influence of the surface level in the boundary.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a solid-state imaging device incorporating a charge-coupled device.1. [Technical background of the invention and problems thereof] In recent years, solid-state imaging devices incorporating a charge-coupled device have been developed. devices have become widely used. FIG. 1 shows a schematic configuration of such a solid-state image sensor. In this figure, C is a unit cell region constituting one pixel, and this unit cell region is roughly divided into a photosensitive element region A having a photoelectric conversion function and a charge transfer region B. The figure shows I'f, which has a MOS type photosensitive element region, and has a two-layer structure on the charge transfer region B side, which is placed on a p-type semiconductor substrate 1 with an insulating film 2 such as a silicon oxide film interposed therebetween. A charge transfer electrode 3 is formed, and an n-type charge transfer channel 4 is provided in the surface region of the substrate 1 directly under the charge transfer electrode 3. A p-type barrier region 5 is provided at an adjacent portion of the charge transfer channel 4 to separate the circuits so that the charge in the charge transfer channel 4 does not mix with the charge in an adjacent unit cell. . Further, a light shield film 7 made of, for example, an aluminum film is provided on the insulating film 2 to prevent light from entering areas other than the photosensitive element area A, and an opening is provided in the light shield film 7 in the photosensitive element area A. A transparent electrode 8 is provided under the window portion. The charge transfer region BK between the transparent electrode 8 and the charge transfer electrode 3 is provided with a charge readout electrode 6 for transferring the charge temporarily stored in the photosensitive element region A to the charge transfer channel 4. There is.

In such an element, without applying a voltage to the transparent electrode 8, a depletion layer is formed in the substrate region under the transparent electrode θ, and charges photoelectrically converted by incident light are temporarily transferred to this depletion layer. accumulate.

Then, the accumulated charge is detected by electrically operating the readout electrode 6 and the transfer electrode 3.

By the way, in the solid-state image sensor shown in FIG.
Insulating film 2 in photosensitive element area A and semiconductor substrate 1 in contact with it
A depletion layer 10 is formed on the semiconductor substrate 1 side at the interface with the substrate 1, but at such an interface on the surface of the substrate 1, it is easily influenced by the surface state of the interface, and a local dark current tends to flow. Due to this local dark current, there was a large variation in dark current between each unit cell, and in some cases, problems such as white spots were observed in the image.

In the device shown in FIG. 2, the photosensitive element area A consists of a photodiode having a PN junction. That is, a P-N junction is formed between the substrate 1 and the diffusion layer 9 containing many impurities of the opposite conductivity type to the substrate 1, and the charges photoelectrically converted by the incident light are transferred to this P-N junction. The energy is accumulated in the diffusion layer 9 due to the energy gradient of the depletion layer 10 formed in the depletion layer 10 . The accumulated charge is detected by electrically operating the readout electrode 6 and the transfer electrode 3. In this case, since it is not necessary to apply an electric field to the photosensitive element region A to form the depletion layer 10 for charge storage, there is no need to form the transparent electrode 8, unlike the MOS type element shown in FIG. do not have.

However, even in an element in which a photodiode having such a strong P-N junction is formed in the photosensitive element area A, if the concentration of the diffusion layer 9 is low, the diffusion layer 9 is connected to the substrate 1 and the insulating film on the substrate 1. Depletion occurs up to the interface with 2. Therefore, the above MO
As in the case of the S-type element, local dark current was generated due to the influence of the surface state at the interface.

Furthermore, when the impurity concentration of the diffusion layer 9 is high, the spread of the depletion layer is small, so although the vicinity of the interface in the entire photosensitive element region A is not depleted, the P- Since the N junction surface extends to the surface of the substrate 10, the depletion layer portion near the broken line 1 in the figure out of the depletion layer 10 formed along this P-N junction surface is also located near the surface of the interface. Affected by position. Therefore, in this case as well, variations in dark current could not be avoided. An object of the present invention is to provide a solid-state imaging device in which variations due to dark current are prevented.

[Summary of the invention]

That is, in the solid-state imaging device according to the present invention, a charge transfer channel, a charge transfer electrode, and a charge readout electrode are provided in a unit cell in order to prevent depletion at the interface between the semiconductor substrate in the photosensitive element region and the insulating film in contact therewith. In addition to forming a predetermined charge transfer portion, a charge storage region having an opposite shape to the substrate is formed on the semiconductor substrate adjacent to the charge transfer portion as a photosensitive element portion, and a surface region of the semiconductor substrate above the charge storage region is formed. Transfer carriers are provided with a depletion prevention layer in which a large number of carriers having the opposite shape exist.

This depletion prevention layer may be formed, for example, on the charge storage region via an insulating film, by a transparent electrode to which a voltage is applied in a direction that excludes the transferred carriers, or so as to cover the charge storage region. It is formed by providing an impurity layer having the opposite shape to the charge storage region of the octopus.

[Embodiments of the invention]

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

FIG. 3 is a diagram showing an example of a solid-state image sensor having a MOS-type photosensitive element area A. The structure of the charge transfer area B is the same as the conventional one, so the same components as in FIG. The explanation will be omitted.

In FIG. 3, a diffusion region having a conductivity type opposite to that of the semiconductor substrate 1 is formed as a charge storage region 20 in the semiconductor substrate 1 in the photosensitive element region A. Then, an insulating film 2 is placed on this N-type diffusion region.
A transparent electrode 21 made of polysilicon, for example, is formed through the transparent electrode 21, and a negative voltage is applied to this transparent electrode.

In an element having such a configuration, negative charge carriers are generated within the semiconductor due to light (radiation) incident through the window portion of the light shield film 7. Here, since a negative voltage is applied to the transparent electrode 21 as described above, the negative charges near the surface of the substrate 1 are moved away, and the layer having a large number of holes, that is, the inversion layer 22, acts as a depletion prevention layer instead. It is formed on the substrate surface of this photosensitive element area A.

Therefore, the photoelectrically converted negative charge is transferred to the P-N between the substrate 1
The charge is accumulated in a charge accumulation region 20 yen surrounded by the junction surface and the inversion layer 22 . That is, photoelectrically converted carriers no longer exist near the interface between the substrate 1 and the insulating film 2, and are not affected by the surface states at the interface.

FIG. 4 shows a second embodiment of the present invention, in which a P expansion diffusion layer 23 having a shallower diffusion depth than N20 of the opposite conductivity type to the photosensitive confectionery region AiC and the substrate 1 is formed as a depletion prevention layer. Here, this P swelling diffusion layer 23 is connected to the charge storage region 2θ
It is formed on the charge storage region 20 over an area slightly wider than this storage region 20 with sufficient impurity concentration so that the PN junction surface between the substrate 1 and the substrate 1 does not reach the top of the substrate 1.

In such an element, when the photosensitive element area A is viewed in a direction perpendicular to the substrate 1, the areas are joined in the order of PNP. Considering the junction of these regions in terms of the energy level based on the negative charge, the N region is lower than the P region, so the N
The negative charges photoelectrically converted in the shaped charge storage region 20 remain within the charge storage region 20 . In addition, the negative charges generated in the P type substrate and the P expansion diffusion layer 23 also move to the depletion layer of the P-N junction by diffusion and enter the depletion layer, where the energy level is low in the N shadow region. Accumulated within the charge storage region 20 .

Therefore, in this case as well, negative charges serving as transfer carriers are accumulated in the charge storage region 20 inside the substrate 1, and the substrate 1 and the insulating film 2 are
It is almost unaffected by the interface with the

Incidentally, in the above embodiment, a case has been described in which an element is formed using a P-type substrate 1, but it is of course possible to use an N-type substrate and reverse the conductivity type of each region and the direction of the voltage applied to the transparent electrode. .

〔Effect of the invention〕

As described above, in the solid-state image sensor according to the present invention, a diffusion layer of the opposite conductivity type to that of the substrate is formed in the photosensitive element region as a charge storage region, and an insulating film is placed on the upper surface of this diffusion layer to form a diffusion layer of the same conductivity type as the transfer carrier. By applying a voltage (voltage in a direction that excludes transferred carriers), or by providing a diffusion layer of the same conductivity type as the substrate, which is wider than the diffusion layer and has a shallower diffusion depth, over the charge storage layer, A depletion prevention layer in which a large number of carriers having a shape opposite to that of the transfer carriers exists is formed in a surface region of a charge storage region having a shape opposite to that of the substrate.

As a result, a depletion layer is not formed near the interface between the semiconductor substrate and the insulating film in the photosensitive element region, and carriers photoelectrically converted by incident light (radiation) are transferred to the surface level (5u
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It is possible to provide a solid-state imaging device that is less susceptible to effects such as ide charge and suppresses local dark current due to the influence of the interface and variations in dark current between photosensitive elements.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the cross-sectional structure of a conventional solid-state imaging probe, respectively. FIG. 3 is a diagram showing a cross-sectional structure of a solid-state imaging probe according to an embodiment of the present invention. FIG. 4 is a diagram showing the cross-sectional structure of a solid-state imaging probe according to an embodiment of the present invention. FIG. 3 is a diagram showing a cross-sectional structure of a solid-state imaging probe according to another embodiment. DESCRIPTION OF SYMBOLS 1... Semiconductor substrate, 2... Insulating film, 3... Charge transfer electrode, 4... Charge transfer channel, 5... Barrier region, 6... Charge readout electrode, 7... Light shield membrane,
10... Depletion layer, 20... Charge storage region, 2...
・Transparent electrode, 22... Inversion layer (depletion prevention layer), 23
... P swelling diffusion layer (depletion prevention layer), A... photosensitive element region, B... charge transfer region, C... unit cell.

Claims (3)

[Claims] (1) A photosensitive element section that photoelectrically transforms incident light to generate transfer carriers, and a charge transfer section that transfers the transfer carriers and includes a charge transfer channel, a charge transfer electrode, and a charge readout electrode are provided on the same semiconductor substrate. In the solid-state imaging device, the photosensitive element portion has 1. It is characterized by comprising a charge storage region formed in the substrate and having a shape opposite to that of the substrate, and a depletion prevention layer containing a large number of carriers having a shape opposite to that of the transfer carriers in an upper region of the charge storage region. A solid-state image sensor. (2) A patent claim characterized in that the depletion prevention layer is formed by a transparent electrode provided on the charge storage region via an insulating film and to which a voltage is applied in a direction to exclude the transferred carriers. The solid-state image sensor according to the range 1 above. (3) The depletion prevention layer is formed of an impurity layer containing a large number of impurities having a shape opposite to that of the charge storage region so as to cover the charge storage region. The solid-state imaging device described.