JPH0135546B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a solid-state imaging device used as a linear sensor, a two-dimensional area sensor, etc., and particularly to a photosensitive region thereof.

[Technical background of the invention]

FIG. 1 shows a portion of a conventional solid-state area sensor incorporating, for example, a charge-coupled device (CCD) (the unit cell area is denoted by A). That is, for example, a two-layer charge transfer electrode 3 is formed on a p-type semiconductor substrate 1 with an insulating film 2 interposed therebetween, and an n-type charge transfer electrode having a conductivity type opposite to that of the substrate is formed in the substrate surface area under this electrode 3. A channel 4 is provided. and,
A part of the substrate surface area adjacent to the charge transfer channel 4 has a p ⁺ type barrier region for separating the charges in the charge transfer channel 4 from mixing with charges in other adjacent unit cells. 5 is provided. Further, a photosensitive pixel 6 made of an n-type impurity layer having a conductivity type opposite to that of the substrate is provided in the substrate surface region near the charge transfer channel 4.
A photodiode having a pn junction is formed between the substrate 1 and the p-n junction, and charges generated by photoelectric conversion of incident light from the direction of the substrate surface are transferred to the p-n junction.
The energy gradient of the empty layer formed at the n-junction is stored in the pixel 6 as a signal carrier. Note that a light shielding film 7 is provided on the insulating film 2 so that light does not enter areas other than the photosensitive area B including the photosensitive pixel 6 and above it. The charge accumulated in the pixel 6 is read out to the charge transfer channel 4 by applying a driving pulse to the charge transfer electrode 3,
Further, the signal is transferred within this channel 4 in a predetermined direction. In this case, the charge transfer electrode 3 has both the function of reading out the accumulated charge of the pixel 6 to the charge transfer channel 4 and the function of transferring the charge through the channel 4.

[Problems with background technology]

By the way, since the pn junction surface of the photosensitive pixel 6 mentioned above extends to the interface between the substrate 1 and the insulating film 3, a depletion layer is generated along this pn junction surface up to the interface. When the interface is depleted in this way, it becomes susceptible to the effects of surface states at the interface, fixed surface charges in the insulating film 2 near the interface, and the like. This effect causes dark current and variations in dark current between the photosensitive regions of each unit cell, so depletion at the interface is a major factor in deteriorating images output by solid-state imaging devices. It is becoming.

[Purpose of the invention]

The present invention was made in view of the above circumstances, and
It is possible to prevent depletion at the interface between the semiconductor substrate and the insulating film, and to eliminate various defects associated with this depletion.
The present invention provides a solid-state imaging device that can improve the quality of output images.

[Summary of the invention]

That is, the present invention provides a photosensitive pixel comprising an impurity layer of a conductivity type opposite to that of the semiconductor layer that accumulates signal carriers generated by photoelectric conversion of incident light on the surface of the semiconductor layer, and a photosensitive pixel that receives the accumulated carriers of the photosensitive pixel. A solid-state imaging device is provided with a carrier transfer channel that performs carrier transfer using a transparent electrode provided on the photosensitive pixel via an insulating film and to which a voltage having a polarity that excludes the signal carrier is applied. It is characterized by the fact that

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows a part of the solid-state area sensor, and compared to the conventional example described above with reference to FIG.
A transparent electrode 21 is formed in the insulating film 2 in the photosensitive area B, facing the photosensitive pixel 6, and this transparent electrode 2
A constant voltage (negative voltage, −V in this example) having the same polarity as the signal carrier (negative charge in this example) accumulated in the photosensitive pixel 6 is applied to 1 via a wiring (not shown). Since the other points are the same as the conventional example, the same reference numerals as in FIG. 1 are used, and the explanation thereof will be omitted.

In the solid-state area sensor described above, negative charge carriers are generated within the semiconductor due to light incident from the opening of the light shield film 7 through the insulating film 2 and the transparent electrode 21 . Here, since a negative voltage is applied to the transparent electrode 21, negative charges near the substrate surface are avoided, and instead an inversion layer (depletion prevention layer) having holes (shown in the figure) is formed. 6' is formed on the surface of the substrate 1 in the photosensitive area.
As a result, the signal carrier (negative charge, shown in the figure) generated by the photoelectric conversion is transferred to the substrate.
It is accumulated in the pixel 6 surrounded by the pn junction plane between the pixels and the inversion layer 6'.

Therefore, according to the solid area sensor,
There is no signal carrier near the interface between the substrate 1 and the insulating film 2, and it is no longer affected by surface states at the interface, so dark current and dark between each unit cell pixel due to this effect are reduced. Variations in current no longer occur, and high-quality output images can be obtained.

Incidentally, it is desirable in actual use to reduce the voltage applied to the transparent electrode 21 (to a value close to the ground potential), and for this purpose, for example, as shown in FIG. It is sufficient to form a depletion prevention layer 31 made of an impurity layer having a shallow diffusion depth and having a conductivity type opposite to that of the signal carrier (p-type in this example). In addition, in FIG. 3, the same parts as in FIG. 2 are given the same reference numerals.

In the solid-state area sensor having the structure shown in FIG. 3 as described above, when the photosensitive region B is viewed in a direction perpendicular to the substrate 1, the regions are in contact with each other in the order of p-type, n-type and p-type. Considering the junction of these regions in terms of the energy level based on negative charge, the energy level of the n region is lower than that of the p region, so the signal carrier (negative charge) generated by photoelectric conversion is transferred to the n-type photosensitive layer. It is accumulated in pixel 6. That is, the depletion prevention layer 31 made of a p-type impurity layer on the surface of the substrate in the photosensitive region B serves to keep signal carriers away from the interface, and also serves to raise the energy level higher than that of the n region.
Therefore, as the energy level of the substrate surface in the photosensitive area B becomes higher than the energy level of the photosensitive pixel area, the transparent electrode 2
The negative voltage applied to 1 only needs to be small.

Note that the depletion prevention layer 31 does not have to be a p-type impurity layer, but may be formed as an n-type impurity layer that has the same conductivity type as the photosensitive pixel 6 and has a lower impurity concentration than that of the photosensitive pixel 6. In this case, the manufacturing process involves forming a two-layer transfer electrode 3 of polysilicon, for example, and then shallowly implanting impurity ions of a conductivity type opposite to that of the signal carrier into the surface of the pixel 6 to form a depletion prevention layer. , a transparent electrode 21 may be formed above this prevention layer so as to cover it.

Furthermore, although each of the above embodiments describes a solid-state area sensor using a p-type substrate, when an n-type substrate is used, the other semiconductor regions are made of the opposite conductivity type from the above embodiments, and the transparent electrode is Just apply a voltage. Further, the semiconductor substrate in each of the above embodiments may be a well layer formed in the semiconductor substrate with an impurity of a conductivity type opposite to that of the substrate, and in short, it may be a semiconductor layer of a predetermined conductivity type.

〔Effect of the invention〕

As described above, according to the solid-state imaging device of the present invention, a transparent electrode is provided above the photosensitive pixel formed on the surface of the semiconductor layer with an insulating film interposed therebetween, and the amount of light accumulated in the photosensitive pixel is By applying a voltage of the same polarity as the signal carrier in order to exclude the signal carrier from the interface between the semiconductor layer and the insulating film, a carrier of a conductivity type opposite to that of the signal carrier exists in the interface region of the photosensitive pixel surface. A depletion prevention layer is formed. Therefore, a depletion layer is not formed near the interface, and signal carriers are less susceptible to the effects of interface states, fixed surface charges, etc., and dark current due to the influence of the interface and between each unit cell photosensitive area are reduced. It is possible to suppress variations in dark current in the image forming apparatus, and it is possible to improve the quality of the output image.
Furthermore, by forming a depletion prevention layer made of an impurity layer containing a predetermined impurity on the surface of the pixel, the voltage applied to the transparent electrode can be reduced, and practical restrictions can be reduced. .

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a part of a conventional solid-state area sensor, FIG. 2 is a sectional view showing a part of a solid-state area sensor according to an embodiment of the present invention, and FIG. 3 is a sectional view showing a part of a solid-state area sensor according to an embodiment of the present invention. FIG. DESCRIPTION OF SYMBOLS 1... Semiconductor substrate, 2... Insulating film, 4... Transfer channel, 6... Photosensitive pixel, 6'... Inversion layer, 2
1...Transparent electrode, 31...Depletion prevention layer.

Claims (1)

[Scope of Claims] 1. A photosensitive pixel consisting of an impurity layer of a conductivity type opposite to that of the semiconductor layer that accumulates signal carriers generated by photoelectric conversion of incident light on the surface of the semiconductor layer, and the accumulated carriers of this photosensitive pixel. A solid-state imaging device is provided with a transfer channel for receiving signals and transferring carriers, the solid-state imaging device including a transparent electrode provided on the photosensitive pixel via an insulating film and to which a voltage having a polarity that rejects the signal carrier is applied. A solid-state imaging device characterized by: 2. The solid-state imaging device according to claim 1, further comprising an impurity layer containing an impurity of a conductivity type opposite to that of the signal carrier on the surface of the photosensitive pixel.