JPH0695571B2 - Photoelectric conversion device - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a photoelectric conversion device used in a solid-state imaging device or the like.

[Description of Prior Art] Conventionally, in a photoelectric conversion device having a PN junction, an insulating film such as SiO ₂ is formed on the surface of the semiconductor facing the junction to form a planar structure, thereby stabilizing the junction surface and making the reverse direction. We are trying to improve the characteristics. Further, when it is desired to improve the photosensitivity, an element forming an N-type impurity diffusion layer on a P-type substrate is used because a photocurrent is obtained by electrons having a long lifetime as minority carriers.

FIG. 3 (a) shows a sectional view of such an element. A manufacturing method for obtaining this device will be briefly described here. Si is formed on the main surface of the P-type silicon substrate 1 which is the first conductive substrate.
An O ₂ film 2 is formed, and a P ⁺ diffusion layer 3 for electrode extraction is formed on the side opposite to the main surface. Next, the SiO ₂ film 2 formed on the main surface
A window hole is partially provided in the N-type impurity diffusion layer 4 for forming the PN junction, and the known technique (photoetching, diffusion method, etc.) is used.
Formed by.

In this case, in order to improve the photoelectric characteristics, it is necessary that the N-type impurity diffusion layer has a high concentration, the amount of impurities is drastically reduced from the surface toward the junction, and the junction is formed shallow.

Further, in the element having the above-mentioned structure, since a low-concentration P-type silicon substrate is usually used, immediately below the insulating film 2 such as SiO ₂ ,
That is, a so-called channel 5, which electrically exhibits the same negative polarity as the N-type impurity diffusion layer 4, is formed on the main surface of the P-type silicon substrate 1. When this inversion layer is formed, a leak current flows through the inversion layer and the breakdown voltage decreases. In the figure, 6 indicates a back electrode.

In order to reduce this leakage current, as shown in FIG. 3B, a P-type high-concentration impurity layer of the same conductivity type as that of the P-type substrate 1 is used as a channel stopper 7 to surround the PN junction and more than the PN junction. It can be solved by forming deeply, but
The manufacturing process for this purpose requires photo-etching for oxide film formation, high-concentration diffusion, etc., and heat treatment at high temperature is particularly necessary, so crystal defects easily occur, carrier recombination increases, and PN junction occurs. The leak current increases.

Further, if an N-type high-concentration impurity layer is formed in order to separate pixels, the light receiving area occupied by the entire chip is significantly reduced and the sensitivity is lowered. Furthermore, as the area per pixel decreases, the rate of reduction of the light receiving area increases more and more. This is because the channel stop and pixel separation regions are provided almost uniformly around the pixels.

Further, as the area of the light receiving portion decreases, another problem arises. This means that the capacitance of the PN junction is reduced by reducing the area of the light receiving portion, that is, the area of the PN junction.
The reduction in the capacitance of the PN junction causes a problem that the amount of accumulated light is reduced in a device that uses light accumulation in a solid-state imaging device or the like. Therefore, it is necessary to consider a light receiving structure that increases the light receiving unit capacitance even if the pixel area decreases.

Furthermore, when the device having the structure shown in FIGS. 3 (a) and 3 (b) is used as a photoelectric conversion device in the spectral range including the ultraviolet frequency, it causes an increase in leakage current of the PN junction photodiode. This kind of leakage current increases when the PN junction photodiode is irradiated with ultraviolet rays. It is known that this increase occurs mainly at the edge of the diffusion region where the depletion region intersects the silicon surface.

Therefore, if this leak current is not reduced by devising the structure of the photoelectric conversion device, it is used as a detector of an analyzer, for example, and is detected by an increase in leak current when continuously exposed to ultraviolet light. The life of the vessel is shortened. In addition, there are drawbacks such that the use of the detector is restricted under operating conditions where the intensity of ultraviolet rays is low, which lowers the sensitivity of the analyzer.

[Object of the Invention] It is an object of the present invention to provide a structure of a photoelectric conversion device in which leakage current is reduced and characteristics are improved.

[Summary of the Invention] Therefore, the photoelectric conversion device of the first invention of the present application includes a plurality of diffusion regions formed on a substrate to form a PN junction between the substrate and the periphery of each of the diffusion regions. A groove deeper than the PN junction formed so as to surround it, an insulating film formed on the substrate surface and the groove, and a transparent conductive film formed on the insulating film so as to partially overlap the pre-diffusion region, It is characterized in that it is formed on the transparent conductive film and is formed of a conductive film which is opaque to ultraviolet rays and which is connected to a reference potential.

The photoelectric conversion device according to the second invention of the present application includes a plurality of diffusion regions formed on a substrate to form a PN junction with the substrate, and the diffusion regions formed around the respective diffusion regions. A groove deeper than a PN junction, an insulating film formed on the substrate surface and the groove, and formed on the insulating film so as to partially overlap with the diffusion region, and connected to a reference potential to ultraviolet rays. On the other hand, it is formed of an opaque conductive film.

Then, with the above respective configurations, the pixel separation characteristic is improved,
The pn junction leakage current due to the irradiation of ultraviolet rays having a large light receiving unit capacitance is blocked, and the dark current due to the charge accumulated on the insulating film is suppressed.

[Examples of the Invention] Examples of the present invention will be described below by taking the case of using silicon, which is most commonly used as a semiconductor, as a substrate.

1 (a) and 1 (b) are a plan view and a sectional view of an apparatus showing an embodiment of the present invention.

In FIG. 1, a semiconductor substrate having a first conductor on the back surface electrode 10 such as P ⁺ having an impurity concentration of 10 ¹⁸ to 10 ¹⁹ cm ⁻³ is used ^.
A P-type silicon layer 12 having an impurity concentration of about 10 ¹⁵ cm ⁻³ is formed on the type silicon substrate 11 by epitaxial growth. As the main surface to form a SiO ₂ film 13, the SiO ₂ film 13 of the portion forming the groove is etched away surrounding the light receiving PN junction photodiode. After that, a groove is formed in the silicon epitaxial layer 12 by dry etching such as RIE (reactive ion etching) or plasma etching. At this time, it is preferable to use it together with wet etching in order to reduce damage to the silicon substrate due to dry etching. The groove depth is at least PN
It may be deeper than the junction depth and may reach the P ⁺ type silicon substrate 11. Usually, it is good to set the diffusion depth of the PN junction to twice or more.

Next, an SiO ₂ film is formed on the surface of the P-type silicon layer 12 and the grooved surface.
Form 13. At this time, since the SiO ₂ film 13 formed on the surface of the P-type silicon layer 2 also serves as an antireflection film, it is desirable that its thickness be different from that of the SiO ₂ film 13 formed on the grooved surface. . Next, a window hole is provided in the SiO ₂ film 13 by photoetching, and an N ⁺ -type impurity diffusion layer 14 is formed by an ion implantation technique, a thermal diffusion technique, or the like for forming a PN junction which will be a light receiving portion. Due to photoelectric characteristics, the N ⁺ type diffusion layer 14 needs to be diffused in a high concentration with a thickness of about 0.1 to 1.5 μm near the surface of the silicon substrate.

Next, a low resistance polysilicon layer 15 is formed by a CVD technique or the like so as to fill the gap in the grooved portion. At this time, the polysilicon layer 15 is also formed on the epitaxial layer 12 with the SiO ₂ film 13 interposed, and the polysilicon layer 15 sufficiently overlaps the N ⁺ region 14 of the PN junction to form the N ⁺ region.
SiO ₂ film 13 used as an insulating film
A desired capacity is formed by sandwiching. Next, a PSG film 16 or the like is formed as an interlayer insulating film by a CVD technique, and then the PSG film 16 is removed to form a window hole on the polysilicon layer 15 and an electrode extraction portion of the N ⁺ diffusion region 14. Then wiring Al
A film 17 and a light-shielding Al film 18 are formed.

The optical shield Al film 18 is N ⁺
At least 100Å (Angstrom) in diffusion area 14
Must overlap. This is because ultraviolet rays generally penetrate to a depth of about 100 Å, while the diffusion area has a depth of about 0.1 to 1.5 μm, so it is necessary to shield only the depletion area that extends within 100 Å of the surface of the photodiode array. Is.

In addition, the reason why aluminum (Al) is used as the shield layer in the above-described embodiments is that Al does not transmit ultraviolet rays and is widely used in the semiconductor manufacturing technology.

The photoelectric conversion device with the structure created in this way prevents the surface breakdown because it completely mechanically cuts the inversion layer around the PN junction, and it is far from the PN junction generated by long wavelength, that is, Carriers deep from the main surface do not flow into the adjacent photodiode portion, and the separation between pixels is significantly improved.

Further, since the light shielding Al film 18 covers the boundary 19 of the PN junction photodiode, increase in leak current due to ultraviolet rays is prevented. Furthermore, by grounding the Al film 18 for light shield, a capacitance is formed between the N ⁺ diffusion layer 14 and the polysilicon layer 15, and this capacitance enters in parallel with the junction capacitance, so that the capacitance of the light receiving portion increases. Becomes Furthermore, Al film for light shield
When 18 is grounded, each light receiving part is surrounded by the ground potential, so that it becomes extremely strong against noise.

FIG. 2 shows another embodiment of the present invention. FIG. 2 (a) is a plan view thereof and FIG. 2 (b) is a sectional view thereof.
In the figure, the same reference numerals as in FIG. 1 indicate the same or corresponding parts,
The difference from the structure shown in FIG. 1 is that instead of using the polysilicon layer 15 for filling the grooved portion, it is used for the light shield.
The point is that the Al film 18 itself also serves to fill the grooved portion. Other points are almost the same as those in FIG. 1, and there is no significant difference in characteristics. However, the usage pattern is slightly different. That is,
The device shown in FIG. 1 shows that the peripheral circuit can be formed on the same chip and the same process can be performed, for example, when a polysilicon gate MOSFET is adopted as the scanning circuit of the photodiode array. Therefore, the wiring Al film 17 of FIG. 1A is connected to the scanning circuit at this time. On the other hand, the device shown in FIG. 2 is used as a simple photodiode array in which the two-layer wiring of the Al film 17, the scanning circuit, etc. are not on the same chip.

In addition, when there is no need to consider deterioration due to ultraviolet rays, such as when receiving light that does not contain ultraviolet rays, it is for light shielding
It is not necessary to use the Al film 18, and the Al film 18 for light shield of the embodiment shown in FIG. 1 can be omitted to further improve the light utilization efficiency.

Further, instead of the polysilicon layer 15, another transparent conductive film (for example, ITO film, SnO ₂ film, etc.) may be used, and this structure may be applied to a normal solid-state imaging device. It will be apparent to those skilled in the art that the conductivity types of the parts may be exactly opposite.

[Effects of the Invention] As described above, according to the present invention, the leakage current of the PN junction is reduced, the breakdown voltage is improved, the separation between pixels is improved, the leakage current is prevented from increasing due to ultraviolet rays, and the capacitance of the light receiving portion is prevented from being reduced by reducing the pixel area. Especially, the performance is remarkably improved in the light receiving portion layout in which the light receiving portion is long and thin. Further, in terms of manufacturing, the conventional method for manufacturing a photoelectric conversion device, particularly a solid-state imaging device can be applied as it is, except for the step of grooving, so that an extremely excellent photoelectric conversion device that can be easily manufactured can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a photoelectric conversion device according to an embodiment of the present invention. FIG. 1A is a plan view thereof, FIG. 1B is a sectional view thereof, and FIG. 3A and 3B are configuration diagrams of a photoelectric conversion device according to an embodiment, FIG. 1A is a plan view thereof, FIG. 1B is a sectional view thereof, and FIG. 3A is a sectional view of a conventional photoelectric conversion device. FIG. 1B is a sectional view for improving the characteristics of the photoelectric conversion device. 1,12 ... P substrate or epitaxially grown P layer, 2,
13 …… SiO ₂ film, 3 …… P ⁺ diffusion region, 4,14 …… n ⁺ diffusion region, 5 …… Channel layer (inversion layer), 6,10 …… Back electrode,
7 ... Channel stopper, 11 ... P ⁺ substrate, 15 ... Polysilicon film, 16 ... Interlayer insulation film, 17 ... Al film for wiring, 18 ...
… Al film for light shield, 19 …… Boundary of PN junction.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Nishizawa 1-6-16 Yonegabukuro, Sendai City, Miyagi Prefecture (56) Reference JP-A-56-36143 (JP, A)

Claims (2)

[Claims] 1. A plurality of diffusion regions formed on a substrate to form a PN junction with the substrate, and a groove deeper than the PN junction formed surrounding each of the diffusion regions. An insulating film formed on the substrate surface and the groove, a transparent conductive film formed on the insulating film so as to partially overlap the diffusion region from the groove, and formed on the transparent conductive film. And a conductive film which is opaque to ultraviolet rays and which is connected to a reference potential. 2. A plurality of diffusion regions formed on a substrate to form a PN junction with the substrate, and a groove deeper than the PN junction formed surrounding each of the diffusion regions. An insulating film formed on the substrate surface and the groove, formed on the insulating film so as to partially overlap the diffusion region from the groove portion and the groove portion, and connected to a reference potential,
A photoelectric conversion device comprising a conductive film opaque to ultraviolet rays.