JPH10209412A - Solid-state imaging element and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress falling of energy density of an electromagnetic field and to increase the number of photons incident on a photo-detecting element, by specifying a minimum width of an opening-part lower end of right-shielding film on a photoelectric transfer element and the shape of an opening end part. SOLUTION: On an N-type semiconductor substrate 1, a P-type well 2 is formed and a photo-detecting element 3 is formed, then, a transfer element 4 of N-type embedded channel is formed and an element separation region 12 is formed. Then a read-out region 11 for reading out an electric charge from the photo-detecting element 3 to a transfer element array 4 is formed, with a gate insulating film 5 formed on the surface of transfer element array 4. Further, a transfer electrode 6 is formed on the gate insulating film 5. Then, an inter-layer insulating film 7 is formed on the surface of N-type semiconductor substrate 1 including the upper part of transfer electrode 6, and a light-shielding film 8 comprising tungsten, etc., is formed, with an opening 9 for allowing the photodetecting element 3 to be irradiated with light is formed. The minimum width at the lower end of the opening is 1μm or less while the end part of the opening is tapered, at least at a part, where an opening width succeedingly increases as approaching an upper end.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention aims at improving the sensitivity of a solid-state image pickup device, particularly an interline transfer type or frame interline transfer type solid-state image pickup device.

[0002]

2. Description of the Related Art A conventional interline-type solid-state image pickup device has been published in July 1987 at the National Convention of the Institute of Television Engineers of Japan.
Page. FIG. 3 shows a sectional structure of a unit pixel of the solid-state imaging device. As shown in FIG. 1, an interline solid-state imaging device includes a P-type well 2, a light-receiving element 3, a transfer element array 4, and a readout area 11 in an N-type semiconductor substrate 1.
After the element isolation region 12 is formed, a transfer electrode 6 is formed above the transfer element column 4 via a gate insulating film 5, and the N-type semiconductor substrate including the transfer electrode 6 is further formed thereon. After an interlayer insulating film 7 is formed thereon, a light-shielding film 8 is formed to suppress the occurrence of smear, and thereafter, an opening 9 for allowing light to enter the light receiving element 3 is formed. At this time, the end of the light shielding film is formed so as to be located inside the light receiving element 3 so that light does not enter the light receiving element 3.

At this time, "1995 IEDM Technical Digest, 160 pages, FIG.
(IEDM Technical Digest, pp. 160, Fig. 2, 1
995) ", the shape of the light-shielding film end is substantially perpendicular (90 degrees) to the N-type semiconductor substrate. After that, usually, a flattening film is formed, and a microlens is formed to increase the light collection rate and improve the sensitivity.

As a conventional solid-state imaging device with a microlens, as disclosed in Japanese Patent Application Laid-Open No. Hei 5-183136, light condensed by a microlens can be efficiently obtained without being blocked by a light-shielding film. For this purpose, it has been proposed to form the opening end of the light-shielding film in a tapered shape so as to spread outward.

[0005]

In the solid-state imaging device having such a structure, the sensitivity decreases in proportion to the pixel area. Here, it is assumed that the portions constituting the pixel such as the light receiving element, the transfer element and the aperture are also reduced at the same ratio as the unit pixel size. In particular, the aperture is an important part for allowing light to enter the light receiving element. When the aperture is reduced, the sensitivity decreases in proportion to the reduction of the aperture even if the light receiving element is not reduced. If the aperture is widened to improve the sensitivity, the sensitivity is improved, but smear is also increased. Therefore, it is necessary to make the aperture as wide as possible in consideration of the relationship between sensitivity and smear. However, even if the pixel area is reduced and the apertures are made as large as possible, the width between the apertures, for example, when the aperture shape is rectangular, the shorter side is the wavelength of the incident light (0.4 μm to
0.8 μm). That is, the pixel size is 3.
In the case of a 5 μm square, the opening width is about 0.8 μm. When the aperture width is substantially equal to the wavelength of the incident light, the energy of the light passing through the aperture decreases, and the sensitivity decreases to a rate equal to or more than the reduction ratio of the aperture.

When designing a fine cell in which the opening width is about the same as the wavelength of the incident light, it is necessary to suppress a decrease in the energy of light passing through the opening and a decrease in sensitivity. However, in the case of such a fine cell, in order to suppress the decrease in sensitivity, providing a microlens as in the related art and condensing incident light to increase sensitivity requires a cell size of 5 μm square or less. Becomes impossible because a microlens cannot be manufactured as designed. Therefore, some other measures need to be taken.

[0007]

As a result of intensive studies to solve the above-mentioned problems, a light-shielding film having an opening in at least a part of a plurality of photoelectric conversion elements formed on a semiconductor substrate is provided. The solid-state imaging device is characterized in that the minimum width of the lower end of the opening of the film is 1 μm or less, and the shape of the opening end has a taper in which at least a part of the opening gradually widens toward the upper end.

It is particularly preferable that the angle of the taper be in the range of 60 to 70 degrees with respect to the semiconductor substrate. Further, it is preferable that the material of the light shielding film is tungsten.

In manufacturing the solid-state imaging device of the present invention,
The opening of the light-shielding film is formed by dry etching using a mixed gas containing an isotropic etching gas such as SF ₆ and an anisotropic etching gas such as O ₂ and Cl ₂ .

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The number of electrons generated by photoelectric conversion when light is incident on a light receiving element is proportional to the number of photons incident on a semiconductor substrate. The number of photons depends on the energy of light, ie the energy of the electromagnetic field, in particular the energy of the electric field. For example, if light having a wavelength λ [m] is incident on a semiconductor substrate with energy U ₀ [W], the number of photons incident per unit time is determined by converting energy U ₀ [W] to energy hc per photon. / Λ [J]. Here, h is Planck's constant (6.6262 × 10 ⁻³⁴ [J
s]) and c are the speed of light in vacuum (2.9997 × 10 ⁸ [m
s ^-1 ]). When passing through the opening, the electric field generates a node, that is, a standing wave whose amplitude becomes zero at the end of the light-shielding film, which is the surface of all layers, due to interference in the opening. If the aperture is sufficiently wider than the wavelength of the incident light, the standing wave will create multiple antinodes so that the energy density of the electromagnetic field passing through the aperture will be constant with the reduction of the aperture. As long as the electric field passing through the opening, that is, the energy density of the light is constant, the energy of the light passing through the opening, that is, the number of photons incident on the light receiving element decreases in proportion to the reduction ratio of the opening, and as a result, the sensitivity Also decrease at the same reduction ratio. However, when the aperture width is substantially equal to or smaller than the wavelength of the incident light, only one antinode of the standing wave can be generated. It becomes smaller as the opening width decreases. For this reason,
The energy of light passing through the aperture, that is, the number of photons incident on the light-receiving element, is reduced to be equal to or greater than the aperture reduction ratio, and as a result, the sensitivity is also reduced to be equal to or greater than the aperture reduction ratio. Therefore, the present invention provides a method in which the aperture width is substantially equal to the wavelength of the incident light.
A solid-state imaging device having an opening width of μm or less, for example, a solid-state imaging device having a unit pixel size of 3.5 μm square and an opening width of about 0.8 μm is formed by tapering the end portion of the light-shielding film so that the opening is tapered. (Determined by the lower end of the taper, and the distance between the tips is defined as the opening width) is equal to or less than the wavelength of the incident light, but gradually becomes wider except at the tip, so that the number of antinodes is one or more. A standing wave can be generated, a decrease in the energy density of the electromagnetic field can be suppressed, and a sharp decrease in sensitivity can be suppressed. FIG. 4 shows a simulation result of the taper angle dependence when the opening width is 0.8 μm and 0.5 μm. Here, the thickness of the light-shielding film was 0.4 μm, and the thickness of the interlayer insulating film below the light-shielding film was 0.3 μm. It can be seen that the sensitivity is improved when the conventional open end shape is substantially perpendicular (90 degrees) to the semiconductor substrate by tapering the open end shape. In particular, the maximum effect is obtained when the taper angle is 60 to 70 degrees.

In the present invention, the configuration in which the shape of the opening end is tapered is, at first glance, similar to the configuration of the prior art.
However, in the related art, the shape of the opening end is tapered in order to avoid blocking the optical path of the light condensed by the microlens, and in the configuration in which the microlens is not provided as in the present invention, the opening end shape is tapered. It was not known at all that the rapid decrease in sensitivity could be suppressed. The present invention has been completed based on such completely new findings.

Next, a method for manufacturing the solid-state imaging device of the present invention will be described.

FIG. 1 is a sectional view showing an example of a manufacturing process of the solid-state imaging device of the present invention. First, a P-type well 2, a PNP junction photodiode (light-receiving element) 3, and a transfer element of an N-type buried channel layer are implanted into an N-type semiconductor substrate by implanting a P-type impurity such as boron and an N-type impurity such as phosphorus. A column 4, a readout region 11, and a p-type element isolation region 12 are formed. On the surface, for example, a gate insulating film 5 such as a thermal oxide film or a three-layer film (ONO film) of an oxide film-nitride film-oxide film is formed. Polycrystalline silicon is deposited by a phase growth (CVD) method or the like, and is patterned and etched to form a transfer electrode 6. Thereafter, an interlayer insulating film 7 is formed by, for example, a CVD method or the like (FIG. 1A). Next, for the purpose of smear reduction, the light-shielding film 8 is formed by, for example, sputtering using a material such as tungsten (FIG. 1B). After that, a resist 13 is applied, and the resist 13 is patterned to form the opening 9. At this time, in order to reduce smear due to oblique incidence of light, patterning is performed so that the position of the opening end 10 is inside the light receiving element region. Then, an opening 9 is formed by etching. At this time, the shape of the opening end 10 is tapered by, for example, using a combination of an isotropic etching gas and an anisotropic etching gas, and changing the gas pressure, the bias high-frequency power between the parallel plates, and the like (FIG. 1).
(C)).

[0014]

The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples.

Embodiment 1 A first embodiment of the present invention will be described with reference to FIG.

First, boron is implanted into the N-type semiconductor substrate 1 so as to have an impurity concentration of 3 × 10 ¹⁵ cm ⁻³ and a junction depth of 6 μm to form a P-type well 2. Boron is implanted so as to have an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ and a junction depth of 0.2 μm at 10 ¹⁶ cm ⁻³ and a junction depth of 1.5 μm, and a PNP junction type photodiode (light receiving element) 3 is obtained. Form. Next, phosphorus was added to 8 × 10 ¹⁶ cm ⁻³ ,
Implantation is performed to a junction depth of 1 μm to form an N-type buried channel transfer element 4. Next, boron is implanted so as to have an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ and a junction depth of 1.0 μm to form an element isolation region 12. Then, boron is implanted so as to have an impurity concentration of 3 × 10 ¹⁶ cm ⁻³ and a junction depth of 1.0 μm to form a readout region 11 for reading out charges from the light receiving element 3 to the transfer element array 4. An ONO film is formed as a gate insulating film 5 on the surface of the transfer element array 4 with a thickness such that the gate capacitance is equivalent to an oxide film having a thickness of 750 °, and phosphorus diffusion is performed on the gate insulating film 5. A transfer electrode 6 of polycrystalline silicon having a sheet resistance reduced to 20 to 30 Ω / □ is formed to a thickness of 4000 °. Next, an oxide film is deposited on the surface of the N-type semiconductor substrate 1 including the upper part of the transfer electrode 6 by a CVD method at a thickness of 2200 ° to form an interlayer insulating film 7. Next, the light-shielding film 8 is formed by sputtering, for example, using tungsten or the like as a material. At this time, the thickness of the light-shielding film 8 is set to 4000 ° so as not to transmit light (FIG. 1A).

Next, in order to form an opening 9 for allowing light to enter the light receiving element 3, the opening 9 is patterned by a photolithography step and an etching step (FIG. 1B). The opening shape may be, for example, a stripe or a square. The unit pixel size is 3.5μ
In the case of an m-square, for example, when the shape of the opening is a stripe, the position of the light-shielding film end 10 is located inside the light receiving element so that the light does not directly enter the transfer element 4 even when the light is obliquely incident. Then, the opening width becomes approximately 0.8 μm. In the etching using the resist 13 as a mask, the gas pressure is set to 250 mtorr and the high frequency power is set to 100 W in plasma etching using a mixed gas containing an isotropic etching gas SF ₆ and an anisotropic etching gas O ₂ and Cl ₂ . This is performed so that the shape of the light-shielding film end portion 10 is tapered (FIG. 1C). As described above, by making the shape of the light-shielding film end portion 10 tapered, a decrease in energy of light passing through the opening 9 can be suppressed, and a decrease in the number of photons incident on the light receiving element 3 can be suppressed.

Embodiment 2 Next, a second embodiment of the present invention will be described with reference to FIG.

The steps up to patterning the resist 13 for forming the opening 9 are the same as in the first embodiment. When etching the light shielding film 8, the plasma pressure is set to 250 mtorr and the high-frequency power is set to 100 W in the plasma etching in which the mixture ratio of the isotropic etching gas SF ₆ and the anisotropic etching gas O ₂ and Cl ₂ is 15: 5: 1. Then, the angle of the taper is set in the range of 60 to 70 degrees. At this time, the maximum effect of the present invention is obtained, and the sensitivity is improved by 40% when the opening width is 0.8 μm, as compared with the existing structure in which the shape of the light-shielding film end 10 is 90 degrees. Also,
In addition to the interline transfer type and frame transfer type solid-state imaging devices, in a solid-state imaging device in which a light-shielding film having an opening with a width of 1 μm or less is formed on a photoelectric conversion element, the shape of the light-shielding film end is tapered. Thereby, a similar effect can be obtained.

[0020]

The effect of the present invention is that the sensitivity is improved for a fine pixel having an opening width of 1 μm or less even though no microlens is provided. This is because the opening width (the opening is determined by the lower end of the taper and the distance between the tips is the opening width) by making the shape of the end portion of the light-shielding film taper so as to increase as the distance from the semiconductor substrate increases. The wavelength is the same as or less than the wavelength, but becomes gradually wider except at the tip. As a result, a standing wave with one or more antinodes is generated between the apertures, which can suppress the decrease in the energy density of the electromagnetic field. And the number of photons incident on the light receiving element increases as compared with the case where the shape of the end portion of the light shielding film is 90 degrees, and the sensitivity is improved. Especially, when the taper angle is 60
The maximum effect is obtained by setting the angle in the range of -70 degrees. At this time, the sensitivity is improved by 40% or more. This means
For example, when the pixel area is reduced from 5.0 μm square to 3.5 μm square, the dark current is reduced in proportion to the pixel area, and in addition to the above-mentioned improvement in sensitivity, the light shielding film edge before the pixel area is reduced. The ability is the same as when the shape of the part is vertical. In addition, the reduction in the chip area due to the reduction in the pixel area improves the yield, and further, the size of the camera can be reduced.

[Brief description of the drawings]

FIGS. 1A to 1C are schematic cross-sectional views illustrating an example of a manufacturing process of a solid-state imaging device of the present invention.

FIG. 2 is a schematic sectional view showing an embodiment of the second solid-state imaging device according to the present invention.

FIG. 3 is a schematic cross-sectional view showing a unit pixel of a conventional interline solid-state imaging device.

FIG. 4 is a graph showing simulation results of taper angle dependence when the opening width is 0.8 μm and 0.5 μm.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... N-type semiconductor substrate 2 ... P-type well 3 ... photoelectric conversion element 4 ... signal transmission element 5 ... gate insulating film 6 ... transfer electrode 7 ... interlayer insulating film 8 ... light shielding film 9 ... opening 10 ... light shielding film end 11 ... Readout area 12: Element isolation area 13: Resist

Claims (5)

[Claims] 1. A light-shielding film having an opening provided on at least a part of a plurality of photoelectric conversion elements formed on a semiconductor substrate, wherein a minimum width of a lower end of the opening of the light-shielding film is 1 μm or less and an opening end of the opening is formed. A solid-state imaging device characterized in that an opening width is gradually tapered toward an upper end. 2. The solid-state imaging device according to claim 1, wherein the angle of the taper at the end of the light shielding film is within a range of 60 to 70 degrees. 3. The solid-state imaging device according to claim 1, wherein the material of the light-shielding film is tungsten. 4. The solid-state imaging device according to claim 1, wherein the opening is formed by dry etching using a mixed gas containing an isotropic etching gas and an anisotropic etching gas. Production method. 5. The method according to claim 1, wherein the isotropic etching gas is SF ₆ ,
Method for manufacturing a solid-state imaging device according to claim 4, anisotropic etching gas being a O ₂ and Cl _2.