KR100595876B1 - Method for fabricating photodiode of image sensor - Google Patents

Abstract

The present invention provides a process for forming a photodiode having a curved junction in order to have a high light sensitivity for short wavelength light and to increase the depletion region, and to easily apply the curved junction to a practical image sensor manufacturing process. In the photodiode forming method of the present invention, in the photodiode forming method, a hemispherical shape in which the photodiode is formed on the first conductive semiconductor layer is locally opened in a number of places, but the convex shape is convex. A first step of forming a mask pattern having a; A second step of forming a second implantation ion implantation layer under the surface of the semiconductor layer by ion implantation; Removing the mask pattern; And a fourth step of diffusing impurities in the ion implantation layer by a thermal process.

Photodiode, depletion layer, short wavelength, light sensitivity, photoresist, doped oxide film

Description

Method for fabricating photodiode of image sensor             

1 is a circuit diagram of a conventional image sensor unit pixel;

2 is a plan view in which a light sensing region and a transfer gate are laid out according to a conventional technique;

3 is a cross-sectional view taken along line AA ′ of FIG. 2;

4A through 4D are cross-sectional views illustrating a process for manufacturing a pinned photodiode according to a preferred embodiment of the present invention.

5 is a plan view of an exemplary ion implantation mask for forming a P ⁰ ion implantation region.

* Explanation of symbols for main parts of the drawings

401: P ⁺ silicon substrate 402: P- epi layer

403: field insulating film 404: gate electrode

405: first photoresist pattern 406: N ^- ion implantation region

407: second photoresist pattern 408: P ⁰ ion implantation region

408a: P ⁰ diffusion region 406a: N ^- diffusion region

410: PN junction

The present invention relates to a pinned photodiode applied to a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor and a manufacturing method thereof.

In a CCD or CMOS image sensor, a photodiode is an introduction part that converts incident light according to each wavelength into an electrical signal. Ideally, all incident light has a quantum efficiency of 1 at all wavelengths. Because it is a case of collecting, efforts are being made to achieve this.

1 is a circuit diagram of a unit pixel of a conventional image sensor, and includes one photodiode (PD) and four NMOS transistors, and four transistors include a transfer gate (Tx), a reset gate (Rx), and a drive gate. (MD), and the select gate Sx. In addition to the unit pixel, a load transistor is formed to read an output signal.

FIG. 2 is a plan view of a conventional light sensing region including a photodiode, a field region, and a transfer gate Tx, and FIG. 3 is a cross-sectional view taken along line AA ′ of FIG. Is a photodiode of a P / N / P type pinned photodiode. Referring to FIG. 3, the P / N / P type pinned photodiode has an epitaxially grown P- epitaxial layer 22 formed on the P ⁺ substrate 21, and the N ^- diffusion inside the P- epitaxial layer 22 is formed. The region 24 is formed, and the P ⁰ diffusion region 25 is formed above the N ⁻ diffusion region 24 and below the surface of the P- epi layer 22. Therefore, if a reverse bias is applied between the N ^- diffusion region 24 and the P region (P ⁰ diffusion region, P-epi layer) of the pinned photodiode, the impurity concentrations of the N ^- diffusion region 24 and the P region may be properly adjusted. when N ^- diffusion region 13 is fully depleted (fully depletion) presented as N ^- diffusion region (24) P- epitaxial layer 22 is present in the lower part and the N ^- diffusion region (24) P ⁰ diffusion present in the upper As the depletion region extends to the region 25, more depletion layer expansion occurs to the P- epi layer 22 having a relatively low dopant concentration.

In an image sensor having such a photodiode, electrons stored in the photodiode are taken out of the photodiode to obtain an electrical output signal (voltage or current). The maximum output signal is directly proportional to the number of electrons that can be taken out of the photodiode. Therefore, to increase the output signal, the number of electrons generated and stored in the photodiode by light must be increased. In order to eliminate the influence of hot electrons and use only electrons generated by irradiation of light as electrical signals, the generation and storage of electrons must be performed in a depletion region, thus generating a large amount of electrons in the photodiode. In order to increase the area of the depletion layer.

As described above, electrons generated in the depletion layer of the pinned photodiode are converted into an electrical signal (voltage or current), and the dopant concentration of the surface layer (P ⁰ diffusion region) is formed so that the depletion layer can be formed from the surface to the depth. Is ion implanted to be much higher than the dopant concentration of the underlying layers (N ^- diffusion region and P- epi layer).

On the other hand, in the case of visible light, the shorter the wavelength, the smaller the penetration depth from the silicon surface and the longer the longer wavelength, the longer the penetration depth. The starting point of the pip layer should not be too far from the surface and at the same time the depletion layer region should be deep enough to the surface, so that the implantation depth of the surface layer (P ⁰ diffusion region) is not too deep. Care should be taken to ensure that the dopant concentration is sufficiently high compared to the dopant concentration of the opposite conductivity type present at the bottom. As a result, it is very difficult in the conventional pinned photodiode structure to satisfy the increase in photosensitivity and depletion region for short wavelength light at the same time.

In order to improve this problem, the present applicant has developed a photodiode of PN junction or PNP, NPN multi-junction photodiode under the name 'photodiode of photo sensor and manufacturing method thereof' on December 22, 1999. There has been proposed a technique of forming a curved portion where the type impurities contact each other (Patent Application No. 98-57306). In other words, we proposed a technique to improve the capacitance and short-wavelength light sensitivity of the photodiode by forming a curved portion of the surface layer and the lower layer by using an ion implantation mask that is locally opened at the time of ion implantation for forming the surface layer of the photodiode. have.

In the present invention, to provide a specific process technology for obtaining the above-described curved junction. In other words, an object of the present invention is to form a photodiode having a curved junction in order to have a high light sensitivity for short wavelength light and to increase a depletion region, and to easily apply the curved junction to a practical image sensor manufacturing process. It is to provide a process method.

In the photodiode forming method of the present invention for achieving the above object, in the photodiode forming method, a hemispherical shape in which the photodiode is formed on the first conductive semiconductor layer to be locally opened in a number of places, but the convex shape is convex. A first step of forming a mask pattern having a; A second step of forming a second implantation ion implantation layer under the surface of the semiconductor layer by ion implantation; Removing the mask pattern; And a fourth step of diffusing impurities in the ion implantation layer by thermal processing.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention. do.

4A through 4D are cross-sectional views illustrating a process of manufacturing a pinned photodiode according to an exemplary embodiment of the present invention.

First, referring to FIG. 4A, a wafer having a low concentration P- epi layer 402 grown on a P ⁺ silicon substrate 401 is prepared, and then a field insulating layer 403 and a gate electrode 404 of a transfer transistor are formed. . Subsequently, a first photoresist pattern 405 which is an ion implantation mask pattern in which the active region where the photodiode is to be formed is opened is formed, and then high energy ion implantation is performed in the P-epi layer 402 of the photodiode active region. N ^- ion implantation region 406 is formed.

Next, referring to FIG. 4B, the ion implantation mask pattern 405 is removed to form a second photoresist pattern 407 that is an ion implantation mask pattern in which a plurality of active regions where photodiodes are to be formed are locally opened.

Next, as illustrated in FIG. 4C, the second photoresist pattern is flowed by baking at a temperature of 150 ° C. to 200 ° C. FIG. Then, a second photoresist pattern 407a having a hemispherical shape is formed. By forming the second photoresist pattern 407a in a semispherical shape as described above, the present invention can form a curved junction more easily than in the prior art.

In other words, if the hemispherical shape does not have a hemispherical shape, the difference between the ion implantation depths is excessively large between the portion with and without the photoresist pattern 407a, and thus it is difficult to obtain a curved junction even after the subsequent thermal process.

Subsequently, low energy ion implantation is performed to form the P ⁰ ion implantation region 408 formed locally and below the surface of the P- epi layer 402 of the photodiode active region.

A plan view of an exemplary ion implantation mask 407 for forming a P ⁰ ion implantation region 408 form a, P ⁰ ion implanted region 408, as shown in a bar, Fig. 5 which is shown in Figure 5 The ion implantation mask 407 may employ a mesh-like lattice mask.

Subsequently, FIG. 4D shows that the dopants of the P ⁰ ion implantation region 408 and the N ⁻ ion implantation regions 406 are diffused by a subsequent thermal process, resulting in a final P ⁰ diffusion region 408a and an N ⁻ diffusion region 406a. This is a completed state.

Referring to FIG. 4D, the pinned photodiode can be seen that the PN junction area is larger than the conventional one because the PN junction 410 is curved near the silicon surface, thereby increasing the capacitance of the photodiode. In addition, because the PN junction 410 is curved, a depletion layer is formed very close to the silicon surface even though the P ⁰ diffusion region 408a has relatively low concentration of impurities. Therefore, the light sensitivity of short wavelength light such as blue light can be increased.

As a result, the photodiode according to the manufacturing process of the present invention may have a high light sensitivity for short wavelength light and increase the depletion region.

On the other hand, the characteristics of the image sensor transfer gate can be changed by the amount and distribution of impurities in the P ⁰ diffusion region and the N ^- diffusion region forming the photodiode, the present invention, between the lattice type ion implantation mask and the transfer gate The dopant concentration of the P ⁰ diffusion region and the N ^- diffusion region in the adjacent region of the transfer gate can be controlled by the alignment and misalignment of the transfer gate, thereby controlling the characteristics of the transfer gate. For example, when the impurity concentration of the P ⁰ diffusion region in the edge region of the transfer gate is low, the impurity concentration diffused into the channel region of the transfer gate will be small, thereby lowering the barrier potential of the transfer gate. This can increase the amount of charge transferred from the photodiode to the sensing node (the other junction of the transfer gate). That is, the charge transport efficiency can be increased.

Although the present embodiment shows a configuration using a P-epi layer, the present invention can be applied even when a PNP photodiode is formed directly on a P-type silicon substrate without the P-epi layer, and also a PNP junction structure. The present invention can also be applied to a photodiode having a PN junction structure other than the pinned photodiode of.

In addition, a P ⁰ ion implantation mask may be a doped oxide film, i.e., PSG, BSG, BPSG, etc., rather than a photoresist, and the doped oxide film is formed by a mask and an etching process, and then flows by a thermal process. Since the ion implantation mask pattern can be formed in a hemispherical shape. Thus, it is possible to form curved joints more easily.

As such, although the technical idea of the present invention has been described in detail according to the above-described preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

The photodiode according to the present invention has high photocharge generating efficiency for short wavelength light and has a large capacitance, thereby greatly improving the characteristics and reliability of the image sensor to which the photodiode is applied.

Claims (6)

In the photodiode forming method, A first step of forming a mask pattern having a hemispherical shape in which a plurality of regions where a photodiode is to be formed is locally opened on the first conductive semiconductor layer; A second step of forming a second implantation ion implantation layer under the surface of the semiconductor layer by ion implantation; Removing the mask pattern; And A fourth step of diffusing impurities in the ion implantation layer by a thermal process Photodiode forming method comprising a. The method of claim 1, The first step, Forming a photoresist pattern in which a plurality of regions where photodiodes are to be formed are locally opened on the semiconductor layer; And And baking the photoresist pattern at a temperature of 150 to 200 ° C. to form a convex hemispherical shape. The method of claim 1, And the mask pattern is a doped oxide film. In the image sensor manufacturing method having a pinned photodiode, A first step of forming a first mask pattern on which a region in which a photodiode is to be formed is entirely opened on a first conductive semiconductor layer; Performing a ion implantation to form a first ion implantation layer of a second conductivity type in the semiconductor layer; A third step of removing the first mask pattern; A fourth step of locally opening a plurality of regions where the photodiode is to be formed, and forming a second mask pattern having a hemispherical shape having a convex shape; A fifth step of forming a second ion implantation layer of a first conductivity type under the surface of the semiconductor layer by ion implantation; A sixth step of removing the second mask pattern; And A seventh step of diffusing impurities in the first and second ion implantation layers by a thermal process; Image sensor manufacturing method comprising a. The method of claim 4, wherein The fourth step, Forming a photoresist pattern in which the number of regions where the photodiode is to be formed is locally opened; And Baking at a temperature of 150 to 200 ° C. to form the convex hemispherical shape by flowing the photoresist pattern. The method of claim 4, wherein The mask pattern is an image sensor manufacturing method characterized in that the doped oxide film.

Images