JP2604251B2 - Solid-state imaging device - Google Patents

Description

The present invention relates to a solid-state imaging device having an npn-structure imaging unit.

(B) Conventional technology When strong light is incident on the imaging section of a solid-state imaging device and a large amount of electric charge is generated, excessive electric charge is diffused to the periphery thereof, and in many cycles without being able to be completely transferred in a subsequent transfer cycle. It is transferred across and produces image defects called afterimages and blooming.

FIGS. 3A and 3B illustrate a vertical overflow structure for extracting such excess charge and suppressing blooming, and FIGS. 3A and 3B schematically illustrate an impurity distribution of the vertical overflow structure. Referring to A), such an impurity distribution of the npn structure can be obtained by implanting boron at an energy of several tens to one hundred and several tens keV into an n-type _Si substrate at 1500 ° C.
This is obtained by forming a p-well by performing a heat treatment at a temperature of about a long time and then forming an n-type impurity region for a photodiode or a transfer channel.

Solid-state imaging device斯Ru structure exhibits a potential profile as shown in FIG. 3 (B), by photons in the n-type S _i crystal electron - the hole pairs occurs, holes in the substrate are majority carriers relief, the charge is minority carrier relative to S _i (electrons) are collected in the potential well shown by a-b-c in FIG. An optical image is detected by transferring and outputting the charge. Also,
Excess charge overflows from the point c to the n-type _Si substrate, and blooming is suppressed.

(C) Problems to be Solved by the Invention The sectional area of the potential well indicated by abc in FIG. 3B (which is a volume in an actual solid-state imaging device) is the light sensitivity of the imaging unit of the solid-state imaging device. give. Therefore, in order to increase the sensitivity of the solid-state imaging device, the potential c must be formed deep in the substrate, that is, at a larger x.

However, the impurity distribution by the conventional ion implantation method is similar to the distribution by the diffusion method, and the impurity concentration is high on the substrate surface and the impurity concentration tends to decrease inside the substrate. In order to perform high-concentration ion implantation and long-time heat treatment, the potential profile becomes as shown by the broken line in FIG. 3B, and the potential of the potential peak c also increases. There was a disadvantage that the suppression of blooming was insufficient.

An object of the present invention is to solve the above-mentioned problems in the prior art, and an object of the present invention is to provide a solid-state imaging device capable of obtaining sufficient light sensitivity while suppressing blooming.

(D) issues means described above for solving the problems, the n-type S _i substrate photodiodes in a p-well formed in the surface layer, an n-type region such as the buried channel is formed the solid-state imaging device comprising an npn structure in it is solved by the solid-state imaging device of the present invention, characterized in that the p-well has an inverted distribution p-well in which the impurity concentration increases toward the interior S _i substrate.

(E) Operation An n-type region such as a photodiode or a buried channel is formed in a p-well formed in the surface layer of the n-type _Si substrate to form an np
In a solid-state imaging device having an n-structure,
_{By using} an inversely distributed p-well in which the impurity concentration increases toward the inside of the substrate, the cross-sectional area of the potential well can be increased without increasing the peak of the potential, and thus sufficient light can be suppressed while suppressing blooming. A solid-state imaging device with high sensitivity can be realized.

(F) Embodiment FIG. 1 shows a cross-sectional structure of an imaging section of a solid-state imaging device according to the present invention. 1 MeV or more, preferably on the surface layer of the n-type _Si substrate (1)
Inject polon with high energy of 3MeV or more, at 1000 ℃,
A p-well (2) heat-treated for about 1 hour, a buried channel (3) formed in the p-well (2), an n-type region such as a photodiode (4), or a channel stop (5) serving as a p ⁺⁺ region. ), yet it is shown polysilicon gates (6) or the like is formed through a S _i O _2.

Here, the energy at the time of boron ion implantation is set to 1 MeV or more, preferably 3 MeV or more. When the impurity is implanted at an energy of 1 MeV or less, the impurity concentration of the substrate surface layer is compared with that by the diffusion method. , While
This is because experimental results have been obtained in which the impurity distribution when boron is implanted at 1 MeV or more, particularly 3 MeV or more, has an inverse distribution such that the impurity concentration increases in the depth direction. Further, the low-temperature heat treatment is performed for a relatively short time in order not to impair such an inverse distribution. Although high-energy ion implantation is said to leave defects even after annealing, its critical dose is about 10 ¹⁴ cm ^-2 , and it has been confirmed that there is no problem with a peak impurity concentration of about 10 ¹⁸ cm ^-3. ing.

FIGS. 2A and 2B show the impurity distribution and the potential profile of the solid-state imaging device formed by the above process along the line AA in FIG.

FIG. 2 (A) shows that the P well has an increased impurity concentration in the depth direction, as described above. The potential profile is as shown by the solid line in FIG. 2 (B), and the depth x of the potential peak is larger than that of the conventional potential peak shown by the broken line, so that the light sensitivity is improved. Nevertheless, it is understood that since the potential of the mountain does not change, the blooming suppression function does not decrease.

The solid-state imaging device of the present invention as described above the effect of the (g) invention, n-type S _i photodiode in a p-well formed in the substrate surface layer, an npn structure is n-type region such as the buried channel is formed in the solid-state imaging device including, by the p-well it has an inverted distribution p-well in which the impurity concentration increases toward the interior S _i substrate, without increasing the potential hill, is possible to increase the cross-sectional area of the potential well As a result, a solid-state image sensor capable of obtaining sufficient light sensitivity while suppressing blooming can be realized.

[Brief description of the drawings]

FIG. 1 is a view for explaining a sectional structure of an embodiment of the present invention, and FIG.
FIGS. 3A and 3B are diagrams for explaining an impurity distribution and a potential profile along the line AA in FIG.
FIGS. 7A and 7B are diagrams for explaining the impurity distribution and the potential profile of the conventional example. (1) n-type _Si substrate, (2) p-well, (3)
... buried channel, (4)-... photodiode,
(5) ... channel stop, (6) ... polysilicon gate.

Claims (1)

(57) [Claims] 1. A solid-state imaging device having an npn structure in which a photodiode serving as an n-type region and a buried channel are formed in a p-well formed in a surface layer of an n-type Si substrate, wherein a gradient of impurity distribution in the p-well is provided. Is formed more gradually on the surface side of the Si substrate from the peak concentration portion than on the deeper side from the peak concentration portion.