JPH0513470A - Charge coupled device - Google Patents

Abstract

PURPOSE:To enable a charge coupled device to be enhanced in fringe electrical field and transfer efficiency by a method wherein a low impurity concentration region is provided at least at the center under a charge transfer region. CONSTITUTION:A P-type well 2 is formed inside an N-type semiconductor substrate 1, and an N-type charge transfer region 3 is formed inside the P-type well 2. An N<->-type region 8 lower than the charge transfer region 3 in impurity concentration is formed between the P-type well 2 and the charge transfer region 3, and a transfer electrode 7 is formed on the charge transfer region 3 through the intermediary of a gate insulating film 6. By this setup, a depletion layer under the charge transfer region is made to extend enough toward the inside of the substrate, whereby an intense fringe electric field can be induced. Therefore, by this constitution, transfer efficiency of a charge coupled device can be enhanced without deteriorating in charge transfer performance per unit area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge coupled device used in a solid-state image pickup device and the like, and more particularly to a buried channel type charge coupled device.

[0002]

2. Description of the Related Art FIG. 5 (a) is a sectional view of a solid-state image pickup device using a conventional charge-coupled device, and FIG. 5 (b) is an impurity profile taken along the line ZZ '. In the figure, 1 is an n-type semiconductor substrate, 2 is a p-type well formed on an n-type semiconductor substrate, 3 is an n-type charge transfer region formed in the surface region of the p-type well, and 4 is the same. n-type photoelectric conversion region formed in the surface region of the p-type well, 5
Is a p ⁺ type region formed between these regions to separate the charge transfer region 3 and the photoelectric conversion region 4, 6 is a gate insulating film, and 7 is a transfer electrode made of polycrystalline silicon.

Although only one transfer electrode 7 is shown in the figure, in order to actually form a charge-coupled device, a large number of transfer electrodes are arranged on the charge transfer region 3 extending perpendicularly to the paper surface. .

The area of the charge-coupled device has been gradually reduced as the number of pixels and the size of the solid-state image sensor are increased. However, even if the size is reduced, it is often impossible to reduce the maximum transferable charge amount. Therefore, conventionally, by making the junction depth of the charge transfer region 3 shallow,
We have tried to improve the charge transfer capability.

[0005]

In the conventional charge-coupled device described above, the decrease in the charge transfer capability due to the miniaturization has been covered by making the junction depth with the p-type well in the charge transfer region shallow. For example, when an ultra-high pixel count and an ultra-miniaturization are performed in a solid-state image sensor compatible with HDTV, the junction depth of the charge transfer region becomes extremely shallow, and the fringe electric field strength occupying an important position for charge transfer is reduced, resulting in a transfer. There is a drawback that efficiency deterioration is increased.

[0006]

A charge-coupled device according to the present invention comprises a first-conductivity-type semiconductor layer, a second-conductivity-type charge transfer region formed in a surface region of the semiconductor layer, and the charge-transfer device. A low-concentration second conductivity type semiconductor region having an impurity concentration lower than that of the charge transfer region or a low-concentration first conductivity type semiconductor region having an impurity concentration lower than that of the semiconductor layer, which is formed over the entire lower surface of the region or in the central portion of the lower surface; A plurality of transfer electrodes formed on the charge transfer region via an insulating film.

[0007]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1A is a sectional view of a solid-state image sensor using the first embodiment of the present invention, and FIG. 1B is an impurity profile of the XX 'line section. In the figure, 1 is an n-type semiconductor substrate, 2 is a p-type well formed on an n-type semiconductor substrate, 3 is an n-type charge transfer region formed in the surface region of the p-type well, and 4 is the same. The n-type photoelectric conversion regions 5 formed in the surface region of the p-type well are
A p ⁺ type region formed between these regions to separate the charge transfer region 3 and the photoelectric conversion region 4, 6 is a gate insulating film, 7 is a transfer electrode made of polycrystalline silicon, and 8 is a charge transfer region 3 Is an n ⁻ -type region having a lower impurity concentration than this region formed on the entire lower surface of.

Since the charge-coupled device of this embodiment has the above-mentioned structure, the charge-coupled device is downsized, and the depth of the junction with the p-type well 2 in the charge transfer region 3 is made extremely shallow. Also in this case, since the depletion layer under the transfer electrode can be sufficiently extended toward the inside of the substrate during charge transfer, a strong fringe electric field that plays an important role in charge transfer can be generated.

Here, as a comparison of the impurity concentrations, in the conventional example, as shown in FIG. 5B, the impurity concentration of the p-type well is 1 × 10 ¹⁶ cm ⁻³ , and the impurity of the charge transfer region 3 is While the concentration was 8 × 10 ¹⁶ cm ⁻³ , in this example p
The well 3 has an impurity concentration of 1 × 10 ¹⁶ cm ⁻³ , the charge transfer region has an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ , and the n ⁻ type region 8 has an impurity concentration of 5 × 10 ¹⁵ cm ⁻³ . . Here, assuming that the gate insulating film 6 has the same film thickness, channel length, and channel width, in this embodiment, the charge transfer capacity per unit area is almost the same as that of the conventional example, but the charge transfer electrode Since the fringe electric field is increased, the transfer efficiency is improved.

FIG. 2A is a sectional view of a solid-state image pickup device using the second embodiment of the present invention, and FIG. 2B is an impurity profile of the section taken along the line YY '. In FIG. 2A, the same parts as those of the first embodiment shown in FIG. 1A are designated by the same reference numerals, and a duplicate description will be omitted.

The difference between this embodiment and the first embodiment is that
The p ⁻ type region 9 is formed on the lower surface of the charge transfer region 3 in place of the n ⁻ type region 8 of the first embodiment. In the present embodiment, with the above configuration, the depletion layer under the transfer electrode can be sufficiently extended toward the inside of the substrate during charge transfer, so that the same effect as the previous embodiment can be expected.

FIG. 3 and FIG. 4 are sectional views showing the third and fourth embodiments of the present invention, respectively. The difference between the third and fourth embodiments and the first or second embodiment is that the n ⁻ type region provided on the entire lower surface of the charge transfer region 3 in the first or second embodiment. 8 or p ^- type region 9 is the third,
In the fourth embodiment, it is provided only in the central portion of the charge transfer region. In the third and fourth embodiments, with the above configuration, the depletion layer at the center of the transfer electrode where the fringe electric field is weakest is sufficiently extended toward the inside of the substrate to enhance the fringe electric field here. Since it can be achieved, the same effects as those of the first and second embodiments can be expected.

[0013]

As described above, according to the present invention, in the buried channel type charge coupled device, the low impurity concentration region is provided at least in the central portion under the charge transfer region, so that the device can be miniaturized. Then, even when the junction depth of the charge transfer region is made shallow to secure the charge transfer capability, the depletion layer under the transfer electrode can be sufficiently extended toward the inside of the substrate. Therefore, according to the present invention, the fringe electric field that plays an important role in charge transfer can be strengthened without weakening the charge transfer capacity per unit area, and the transfer efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of the present invention and its X-
The impurity profile figure of a X'line cross section.

FIG. 2 is a sectional view showing the second embodiment of the present invention and its Y- section.
The impurity profile figure of a Y'line cross section.

FIG. 3 is a sectional view showing a third embodiment of the present invention.

FIG. 4 is a sectional view showing a fourth embodiment of the present invention.

FIG. 5 is a cross-sectional view of a conventional example and an impurity profile diagram of the cross section along the line ZZ ′.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... n-type semiconductor substrate 2 ... p-type well 3 ... charge transfer region 4 ... photoelectric conversion region 5 ... p ⁺ type region 6 serving as an element isolation region 6 ... gate insulating film 7 ... transfer electrode 8 ... n ⁻ type region 9 ... p ^- type region

Claims (1)

Claim: What is claimed is: 1. A second conductivity type charge transfer region provided in a surface region of a first conductivity type semiconductor layer, and an insulating film provided on the charge transfer region. In a charge coupled device including a plurality of charge transfer electrodes, at least a central portion below the charge transfer region has a second conductivity type low impurity concentration region having an impurity concentration lower than that of the charge transfer region or the semiconductor layer. A charge-coupled device comprising a first conductivity type low impurity concentration region having an impurity concentration lower than that of an impurity concentration.