JP2877656B2 - Method for manufacturing solid-state imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a solid-state imaging device having a transfer electrode having a multilayer structure.

[0002]

2. Description of the Related Art In a frame transfer type CCD solid-state image pickup device, an image pickup section which receives light from a subject accumulates information charges generated in response to irradiated light and simultaneously stores information charges accumulated for a predetermined period of time. Is transferred to the storage unit. For this reason, a transfer electrode for transferring and driving information charges is also provided in the light receiving region of light.
The sides of the transfer electrode overlap each other, and continuity of the potential action of the transfer path is ensured to prevent a decrease in charge transfer efficiency.

FIG. 5 shows a frame transfer type CCD.
FIG. 6 is a plan view showing an imaging unit of the solid-state imaging device.
-It is an X-ray sectional view. This drawing shows a vertical overflow drain structure in which excess charge is absorbed by the substrate. On one surface of an N-type silicon substrate 1, a P-type diffusion layer 2 serving as an element region is formed. In the diffusion layer 2, a plurality of isolations including a high-concentration P-type region and a thick oxide film (LOCOS) are formed. Regions 3 are formed parallel to each other. In the channel region 4 sandwiched between these isolation regions 3, an N-type impurity is diffused in the surface portion, and a buried channel layer 5 is provided. The first-layer transfer electrodes 7 are arranged in parallel with each other so as to intersect the channel region 4 with the oxide film 6 interposed therebetween. It is arranged to cover. A fixed potential is applied to each of the transfer electrodes 7 and 8 during the accumulation period, whereby a light receiving pixel is set with the four transfer electrodes 7 and 8 as one unit. After a lapse of a predetermined light receiving period, a four-phase clock pulse is applied to each of the transfer electrodes 7 and 8, and the information charges stored in each light receiving pixel are transferred to the storage section along the channel region 4. Is done. When an excessive information charge is generated in the channel region 4, the excess charge is absorbed by the silicon substrate 1 beyond the potential barrier between the silicon substrate 1 and the diffusion layer 2. .

[0004]

In the light receiving portion of the CCD solid-state imaging device as described above, since information charges are obtained by the photoelectric effect of light incident on the channel region 4, the thickness of the transfer electrodes 7 and 8 is small. In order to increase the efficiency of light incidence by reducing the thickness of the light source, a measure has been considered. In particular, if each part is miniaturized in response to high resolution, the area of the light receiving pixel becomes small, and there is a problem in improving the light receiving sensitivity by improving the incident efficiency.

However, when the thickness of the transfer electrodes 7 and 8 is reduced, when forming an interlayer insulating film for insulating between the transfer electrode 7 of the first layer and the transfer electrode 8 of the second layer, the first layer This causes a problem that the side portion of the transfer electrode 7 is easily lifted. That is, since the interlayer insulating film is formed by thermal oxidation of the surface of the transfer electrode 7 made of polycrystalline silicon, the oxide film between the silicon substrate 1 and the transfer electrode 7 during the thermal oxidation process is formed. 7 grows from the side surface of the transfer electrode 7
When the film thickness is small, the side portions of the transfer electrode 7 rise. When the side portion of the transfer electrode 7 is lifted up, the remaining portion of the lifted portion may be left in the lifted portion in a later etching step, which may cause a current leak. In addition, the effective gate length of the transfer electrode 7 becomes short, and desired characteristics cannot be obtained. In addition, protrusions are formed on the transfer electrodes 7 and 8 at floating portions, and an electric field is concentrated on the protrusions to cause transfer failure. Become.

SUMMARY OF THE INVENTION It is an object of the present invention to improve the efficiency of light incidence on an image pickup unit while preventing deterioration in reliability and characteristics.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is characterized in that a semiconductor substrate provided with a channel region serving as a charge transfer path is provided on a semiconductor substrate. In a method of manufacturing a solid-state imaging device in which a plurality of transfer electrodes intersecting the channel region are arranged in a multilayer, a first conductive film and a first insulating film are sequentially formed on the semiconductor substrate via a gate insulating film. And the first step
Forming a resist mask having a shape extending in a direction crossing the channel region of the semiconductor substrate on the insulating film, and excluding the region covered by the resist mask, the first conductive film and the first insulating film; Forming a first transfer electrode by removing the film, and removing the resist mask,
Removing the gate insulating film exposed in the region from which the first conductive film and the first insulating film have been removed; and forming a second insulating film on the semiconductor substrate so as to cover the first transfer electrode. Forming a second transfer electrode by removing the second conductive film at least except for a gap between the first transfer electrodes, and forming a second transfer electrode.
It is to provide.

[0008]

According to the present invention, the oxide film between the first transfer electrode and the semiconductor substrate is formed by forming the second insulating film by CVD or the like after oxidizing the surface of the first transfer electrode thinly. Of the first transfer electrode is prevented from rising.

[0009]

1 to 4 are sectional views showing steps of a method for manufacturing a solid-state imaging device according to the present invention. These figures show the same parts as in FIG. First, as shown in FIG. 1, a diffusion layer 11 is formed by implanting a P-type impurity such as boron ions into one surface of an N-type silicon substrate 10, and a high-concentration P is formed in the diffusion layer 11 as an isolation region. A plurality of mold regions (not shown) are formed in parallel. An N-type impurity such as phosphorus ions is implanted into the channel region sandwiched between these isolation regions to form a buried channel layer 12. The above implantation step is performed using a resist mask having a desired shape obtained by a known photolithography technique. Then, a silicon oxide film 13 serving as a gate insulating film is formed on the silicon substrate 10 on which the isolation region and the channel region are formed.
Is formed to a thickness of 100 nm by thermal oxidation. Further, a polycrystalline silicon film 14 serving as a transfer electrode and a silicon oxide film 15 serving as an interlayer insulating film are sequentially formed to have a thickness of 400 nm and a thickness of 200 nm by a CVD method.

Next, as shown in FIG. 2, a resist mask 16 patterned in a predetermined shape is formed on the silicon oxide film 15, and etching is performed in accordance with the resist mask 16 to form a transfer electrode 17 of the first layer. Form. The transfer electrodes 17 extend in a direction intersecting with the channel region provided in the diffusion layer 11, and are formed so as to be parallel to each other at regular intervals. Further, after removing the resist mask 16, the silicon oxide film 13 is etched by RIE, and the silicon substrate 10 is removed.
Expose the surface. During this etching, the silicon oxide film 15 remaining on the transfer electrode 17 is removed.
Is a protective film. As described above, if the silicon oxide film 13 is removed after the resist mask 16 is removed so that the surface of the silicon substrate 10 is exposed, impurities generated by the removal of the resist mask 16 can be removed.
Can be prevented from adhering to the surface.

Subsequently, the surface of the silicon substrate 10 exposed to the side surface of the transfer electrode 17 and the gap between the transfer electrodes 17 is thinly thermally oxidized to newly form a silicon oxide film 18 serving as a gate insulating film to a thickness of 10 nm. . Therefore, as shown in FIG. 3, another silicon oxide film 19 is formed to have a thickness of 150 nm by the CVD method so as to cover the silicon oxide film 18. This silicon oxide film 19 is formed by TEOS (Tetra
A reduced pressure CVD method using ethyl orthosilicate) is preferable. TEOS is an alcohol-like liquid at room temperature, which is decomposed by heating and grows silicon oxide according to the reaction formula: Si (OC ₂ H ₅ ) ₄ → SiO ₂ + 4C ₂ H ₄ + 2H ₂ O.
A silicon oxide film grown using S has good step coverage and is suitable for an interlayer insulating film.

After forming the silicon oxide film 19, as shown in FIG. 4, a polycrystalline silicon film 20 serving as a transfer electrode is formed on the silicon oxide film 19 to a thickness of 400 nm by a CVD method. Then, a portion of the polycrystalline silicon film 20 which overlaps with the transfer electrode 17 is removed by a well-known etching process, and a second-layer transfer electrode 2 covering a gap between the transfer electrodes 17 is formed.
Form one. This transfer electrode 21 extends in a direction crossing the channel region, like the transfer electrode 17 of the first layer.
It is arranged so that the side portion overlaps with the adjacent transfer electrode 17. Further, on these transfer electrodes 21, wirings made of aluminum or the like are provided via transfer films 17 and 21 via an insulating film.
Are formed so as to be connected to the ends of the transfer electrodes 17 and 21 when the information charges are accumulated and transferred.
Is supplied with a predetermined potential.

Here, since the second-layer transfer electrode 21 is disposed on the silicon substrate 10 via the silicon oxide film 18 formed by thermal oxidation, the second-layer transfer electrode 2
The interface level below 1 is close to the interface level below the transfer electrode 17 of the first layer. That is, since the silicon oxide films 13 and 18 that are in contact with the silicon substrate 10 under the transfer electrodes 17 and 21 are both formed by thermal oxidation, the interface states of the silicon / silicon oxide interface are formed at the respective portions. Become approximately equal. Therefore, compared to the case where only the silicon oxide film formed by the CVD method is used as the gate insulating film, the interface state of the channel region becomes more uniform, and deterioration of transfer efficiency can be prevented.

In the above embodiment, the vertical overflow drain structure in which the P-type diffusion layer 11 is formed on the N-type silicon substrate 10 is exemplified. A horizontal overflow drain structure provided with an overflow drain can be similarly implemented.

[0015]

According to the present invention, the floating of the transfer electrode of the first layer can be prevented, so that the remaining etching is less likely to occur when the transfer electrode of the second layer is formed. A desired characteristic can be obtained with each electrode by preventing a change in the effective gate length of the electrode.
Further, since the interface state under the transfer electrode of the first layer and the interface state under the transfer electrode of the second layer can be set substantially equal, the same characteristics can be obtained in each transfer electrode, and the transfer efficiency is deteriorated. Can be prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a first manufacturing step of the solid-state imaging device of the present invention.

FIG. 2 is a cross-sectional view illustrating a second manufacturing process of the solid-state imaging device of the present invention.

FIG. 3 is a cross-sectional view showing a third manufacturing step of the solid-state imaging device of the present invention.

FIG. 4 is a cross-sectional view showing a fourth manufacturing step of the solid-state imaging device of the present invention.

FIG. 5 is a plan view showing an imaging unit of a conventional solid-state imaging device.

FIG. 6 is a sectional view taken along line XX of FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 10 Silicon substrate 2, 11 Diffusion layer 3 Isolation region 4 Channel region 5, 12 Buried channel layer 6, 13, 15, 18, 19 Silicon oxide film 7, 8, 17, 21 Transfer electrode 14, 20 Polycrystalline silicon film 16 Resist mask

Claims (2)

(57) [Claims] 1. A method for manufacturing a solid-state imaging device in which a plurality of transfer electrodes intersecting the channel region are arranged in a multilayer on a semiconductor substrate provided with a channel region serving as a charge transfer path. First by thermal oxidation
Forming a first conductive film and a first insulating film in order after forming the gate insulating film; and forming a first conductive film and a first insulating film on the first insulating film so as to extend in a direction intersecting a channel region of the semiconductor substrate. Forming a first transfer electrode by forming a resist mask and removing the first conductive film and the first insulating film except for a region covered by the resist mask; after removal, on exposed to the first conductive film and the first insulating film is removed regions
Removing the serial first gate insulating film, the first gate
Table of the semiconductor substrate exposed after removing the gate insulating film
After forming a second gate insulating film on the surface by thermal oxidation, a second insulating film and a second conductive film are sequentially formed on the semiconductor substrate by a vapor deposition method so as to cover the first transfer electrode. Manufacturing a solid-state imaging device, comprising: forming a second transfer electrode by removing the second conductive film except for at least a gap between the first transfer electrodes. Method. 2. The second gate insulating film according to claim 2, wherein:
The method for manufacturing a solid-state imaging device according to claim 1, wherein the film thickness is smaller than that of the insulating film .