JP2888266B2 - Charge transfer device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge transfer device, and more particularly to a CCD charge transfer device provided with a charge detection section having a floating diffusion layer.

[0002]

2. Description of the Related Art FIG. 3 shows a plan configuration of an output section of a CCD charge transfer device. 4 is a transfer channel, 5 to 9 are transfer electrodes,
10 and 11 are barrier regions, 12 is an output gate electrode, 13
Is a reset electrode, 14 is a floating diffusion layer, 15 is a reset drain, and 16 is an output amplifier. The output operation will be briefly described below. The signal charge transferred through the transfer channel 4 is transferred to the floating diffusion layer 14 at a constant cycle, under the output gate electrode 12 fixed at a predetermined potential. The output amplifier 16 detects the potential of the floating diffusion layer, which changes according to the magnitude of the signal charge, to the outside. The signal charge is discharged to the reset drain 15 and set to the reference potential V _RD at the same time as the charge transfer by turning the reset electrode 13 into a conductive state.

FIG. 4 shows a cross-sectional structure taken along line AA 'of FIG. 3 in a conventional CCD charge transfer device. 1 denotes an N-type silicon substrate, 2 denotes a P-well layer, and 3 denotes an N-type buried layer. The same components as those in FIG. 3 are denoted by the same reference numerals.

[0004]

When the CCD charge transfer device shown in FIGS. 3 and 4 is applied to a horizontal shift register of a two-dimensional image sensor, the transfer channel width is set to 20 in order to secure a transfer charge amount. Generally, it is about 30 μm. On the other hand, it is desirable to reduce the capacitance of the floating diffusion layer by reducing the area as much as possible to improve the detection sensitivity, and the width thereof is designed to be about 8 μm or less. Therefore, it is necessary to narrow down the channel width near the output unit.

[0005] As shown in FIG. 4, the impurity concentrations of the P-well layer 2 and the N-type buried layer 3 are usually constant along the transfer direction.

For these reasons, the potential becomes shallower in the transfer direction due to the narrow channel effect in the narrowed-down portion of the transfer channel. There is a problem that the image is significantly deteriorated.

An object of the present invention is to provide a new charge transfer device which eliminates the above-mentioned conventional disadvantages.

[0008]

According to the present invention, in a narrowing portion of a transfer channel adjacent to a floating diffusion layer,
A diffusion layer forming a buried channel formed immediately below the plurality of transfer electrodes includes at least three regions having the same conductivity type and different impurity concentrations, and each of the regions has a boundary between the regions positioned immediately below the transfer electrode. Thus, a charge transfer device having an impurity distribution such that the potential gradually increases in the charge transfer direction can be obtained.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view taken along FIG. 3A-A 'and a schematic view showing an impurity concentration distribution of an N-type buried layer for explaining a first embodiment of a charge transfer device according to the present invention. is there. In the present embodiment, N
The mold buried layer is formed of three regions 3, 3 ', 3 "in which the impurity concentration is sequentially increased along the transfer direction. When the channel width is constant, the potential is deep along the transfer direction. This makes it possible to alleviate the potential gradient against the transfer direction due to the narrowing of the channel width and to improve the transfer efficiency.

FIG. 2 is a sectional structural view taken along FIG. 3A-A 'and a schematic view showing an impurity concentration distribution of a P-well layer for explaining a second embodiment of the charge transfer device according to the present invention. In this embodiment, P
The well layer is formed so that the impurity concentration gradually decreases along the transfer direction, and as in the first embodiment, when the channel width is constant, the potential becomes deeper along the transfer direction. Is set. Therefore, the potential gradient against the transfer direction due to the narrowing of the channel width can be reduced, and the transfer efficiency can be improved.

In the first embodiment, an example is shown in which the N-type buried layer is formed of three regions having different impurity concentrations.
It is also possible to form the region with three or more regions having different impurity concentrations or a region in which the impurity concentration changes gradually. Further, in the second embodiment, the example of the P well layer formed so that the impurity concentration gradually decreases along the transfer direction has been described, but the P well layer is formed of a plurality of regions where the impurity concentration changes stepwise. It is also possible. Furthermore, the first
The present invention can also be realized by changing the impurity concentrations of both the N-type buried layer and the P-well layer by combining the embodiment with the second embodiment.

[0012]

As described above, according to the present invention, the narrow channel effect is obtained by setting the impurity concentration distribution so that the potential becomes deeper along the transfer direction in the narrowed portion of the transfer channel adjacent to the floating diffusion layer. , The potential gradient opposite to the transfer direction can be reduced, so that the transfer efficiency can be greatly improved as compared with the related art, and an extremely excellent reproduced image can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional structural view of a CCD charge transfer device and a schematic diagram of an impurity concentration distribution of an N-type buried layer according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a CCD charge transfer device according to a second embodiment of the present invention and a schematic diagram of an impurity concentration distribution in a P-well layer.

FIG. 3 is a plan view of an output section of the CCD charge transfer device.

FIG. 4 is a sectional structural view of a CCD charge transfer device showing a conventional example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 N-type silicon substrate 2 P-well layer 3, 3 ′, 3 ″ N-type buried layer 4 transfer channel 5 to 9 transfer electrode 10, 11 barrier region 12 output gate electrode 13 reset electrode 14 floating diffusion layer 15 reset drain 16 output Amplifier

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 21/339 H01L 27/14-27/148 H01L 29/762-29/768

Claims (1)

(57) [Claims] In a narrowed portion of a transfer channel adjacent to a floating diffusion layer, a diffusion layer forming a buried channel formed immediately below a plurality of transfer electrodes is formed of at least three regions having the same conductivity type and different impurity concentrations from each other. Become
The charge transfer device according to claim 1, wherein each of the regions has an impurity distribution such that a boundary between the regions is located immediately below the transfer electrode, and the potential gradually increases in the charge transfer direction.