JPS5992562A - Formation of buried channel - Google Patents

Abstract

PURPOSE:To contrive to optimize conditions for driving a BCCD device by a method wherein the impurity concentration of a BCCD part is selectively increased by performing As and B ion implantation into a semiconductor substrate at the same time by means of the same mask. CONSTITUTION:P<+> channel stoppers 2, the BCCD parts 3, photo diodes 4 are formed in the p type substrate 1. The BCCD parts 3 are formed by the simultaneous ion implantation of As<+> and B<+> by means of the same mask. Then, a shallow part is n, and a deep part is p<+>. That the substrate impurity concentration under the BCCD is high and p<+> means that it functions as a barrier to a smear phenomenon caused by the mixture of a signal charge, generated at depth of the substrate in an interline transfer system image pick-up element, into the BCCD by lateral diffusion. As a result, the smear can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for forming a buried channel, and in particular, to a method for forming a buried channel, which can reduce the depletion voltage of a buried layer with respect to the threshold voltage vT of a BCCD, and also to an image sensor using a BCCD. To provide a method for forming a buried channel with an impurity profile that can reduce smear in conventional configurations and their problems In interline transfer type solid-state image sensors, when it is desired to increase the dynamic range, vertical The large amount of transferred charge and the swell beam phenomenon of BCCD are mentioned.

In order to increase the amount of transferred charge, it is sufficient to increase the dose of the buried channel, but in this case the depletion voltage φmi
As n becomes higher, the driving conditions of the element are restricted. Therefore, if a low resistance substrate is used to suppress the depletion voltage φmin, the threshold voltage VT will increase. This also imposes restrictions on the driving conditions of the element.

As described above, the depletion voltage φmin, the L threshold voltage V
T is closely related to the impurity concentration of the substrate, and is constrained by each driving condition, making it particularly difficult to lower the voltage without deteriorating the characteristics of the element.

Purpose of the Invention The present invention has been made to solve the above-mentioned drawbacks, and
The purpose of this invention is to optimize the driving conditions of a BCCD device by a method of selectively increasing the substrate impurity concentration in the CCD section.

Structure of the Invention The present invention selectively increases the impurity concentration in the BCCD section 3 by simultaneously implanting As and B ions into a semiconductor substrate using the same mask.

DESCRIPTION OF EMBODIMENTS The present invention will now be described in detail with reference to the drawings. 1st
When forming the impurity concentration profile shown in the figure, P+
A combination of and B+ is also possible. But in this case,
Since the diffusion coefficients of P+ and B+ are approximately the same, a drive-in process for B+ is required to extend the diffusion length of B+ compared to the N-type diffusion layer. That is, it is necessary to perform the formation in the order of B+ ion implantation, drive-in, and then P+ ion implantation.

However, if a combination of A8+ and B+ is used as in the present invention, due to the difference in the diffusion coefficients of 88+ and B+, even if ions are implanted at the same time, As+ and B+ will differ depending on the thermal history of the subsequent process, as shown in Figure 2. This results in a concentration profile like this, and these are combined to form the effective profile shown in FIG.

Compared to the combination of the door and B+ described above, ion implantation is performed using the same mask and no drive-in is required, so the process is simpler. Furthermore, compared to the conventional method in which a single impurity (P+, Am'') is used in the buried layer, only one additional process is required for B+ ion implantation.

Next, a specific example will be described in which an interline transfer type image sensor is formed on a P-type substrate using the present invention.

(Example) FIG. 3 shows a cross-sectional view of an image sensor formed by the method of the present invention. In the figure, 1 is a P-type substrate, 2 (d,
In the P+ channel source bar-1S ij B CCD section, 4 indicates a photodiode. In the same element,
In the 8000 part 3, ions of A8+ and B+ are simultaneously implanted using the same mask, so that the shallow part is n and the deep part is p+.

As shown in Figure 3, the fact that the substrate impurity concentration under the BCCD is high and P+ means that signal charges generated deep in the substrate in the interline transfer type image sensor mix into the BCCD due to lateral diffusion. It acts as a barrier against the smear phenomenon that occurs, and as a result, the smear is reduced and 5 pages can be printed.

According to this embodiment, by changing the absolute values of the doses of As'' and B+ and their ratio, the substrate impurity concentration of the BCCD part and the impurity concentration of the buried layer can be checked without prolonging the lateral diffusion of the buried layer. As a result, the maximum transfer charge amount can be increased and the depletion voltage can be lowered.Thus, the threshold voltage vT and depletion voltage φmin can be optimized, and a high resistance substrate can be used. This enables low-current driving.

(Example 2) One structure of a solid-state imaging device includes a fourth well structure for discharging excess signal charges toward the substrate.

In the figure, 21 is an n-type substrate, 22 is a B CCD section,
23 is a photodiode section.

This requires a well concentration and a lateral diffusion length xi such that the well of the photodiode section 22 is completely depleted by back bias, and the well of the BCCD section 23 is not depleted by that voltage.

6. The method for forming wells using the present invention is described in the sixth section.
This will be explained with reference to Figures (a) and (b). First, Figure 6 (a
), a p-type well 34 with a low concentration that satisfies the above conditions for the photodiode section 33 is formed on the n-type substrate 31, and the well of the KBCCD section 32 is formed with a concentration that does not cause depletion, as shown in FIG. This can be done by increasing the dose of Envy B+ so that it becomes possible.

According to this method, the lateral extent of the well of the ECCD section 32 is at most about 2 μm, so the well can be formed in a self-aligned manner.

Furthermore, in the method of forming a well under the above conditions at the beginning of the process, the concentration of the well in the BCCD region must be increased, and the lateral diffusion length xj increases, and the lateral spread cannot be ignored. However, according to the method of this invention, the spread in the lateral direction can be suppressed to a low level, as shown in FIG. 5(b).

As described in detail, according to the method of the present invention, the depletion voltage of a BCCD can be reduced in a simple manner, and smear in an image sensor using a 7-vane or BCCD can be reduced. Highly useful.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the impurity concentration profile formed by the method of the present invention, FIG. 2 is a diagram showing the concentration profile of individual impurities of 88+ and B+, and FIG. 3 is a diagram showing the interline formed by the method of the present invention. FIG. 4 is a cross-sectional view of a conventional p-type well structure for discharging excess charge to the substrate, and FIGS. FIG. 2 is a cross-sectional view of a p-well structure for draining charge to a substrate. 1...P-type substrate, 2...Channel stopper, 3.32...BCCD section, 4, 33
・・・・・・Photodiode. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3

Claims (1)

[Claims] Method 0 of forming a buried channel characterized by selectively increasing the impurity concentration in the BCCD portion by simultaneously implanting A8 and B ions into a semiconductor substrate using the same mask.