JP3597663B2 - Solid-state imaging device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a solid-state imaging device, and more particularly to an amplification-type solid-state imaging device having an amplification function and a method of manufacturing the same.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, various techniques related to a so-called amplification type solid-state imaging device having an amplification function inside a pixel have been proposed. Such an amplification type solid-state imaging device is expected to be suitable for reducing the pixel size by increasing the number of pixels or reducing the image size.
[0003]
Here, FIG. 6 shows and describes a schematic configuration of the solid-state imaging device according to the above-described related art. As shown in the figure, this solid-state imaging device has a large number of pixels, and has a configuration in which a photoelectric conversion element and a transistor are juxtaposed on the same substrate. Then, the potential of the signal charge accumulating portion is modulated by the signal charge generated by the photoelectric conversion by the photoelectric conversion element, and the amplification transistor inside the pixel is modulated by the potential to provide an amplification function inside the pixel.
[0004]
[Problems to be solved by the invention]
However, when the number of pixels is increased as in the above-described amplification type solid-state imaging device, the area of the photoelectric conversion element corresponding to one pixel is reduced, so that the output from the photoelectric conversion element is reduced. As a method of improving the decrease in output due to the reduction ratio of the pixel area, it is conceivable to reduce the impurity concentration of the substrate in order to widen the depletion layer of the photodiode. This causes a problem such as an increase in diffusion current.
[0005]
That is, in the solid-state imaging device according to the related art, as shown in FIG. 7, an n-type semiconductor layer 202 and an n-type semiconductor layer 203 are formed on a low-concentration p-type semiconductor layer 201. A silicon oxide film 204 and a gate electrode 205 are deposited at predetermined positions. In such a configuration, the diffusion current is absorbed at a depth at which the depletion layer 206 of the photodiode reaches the depth direction of the substrate. Therefore, a diffusion current is collected in the photodiode, and a leakage current of the photodiode in a dark state is also increased. As a result, a dynamic range is reduced.
[0006]
SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to reduce the influence of diffusion current from a semiconductor substrate, to reduce the dark leakage current of a photodiode, and to increase the dynamic range of a solid-state imaging device. An object of the present invention is to provide an apparatus and a method of manufacturing the same.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a solid-state imaging device having a photoelectric conversion element that converts light into electric charge and a reset transistor for transferring electric charge of the photoelectric conversion element, which is formed on a semiconductor substrate. A first semiconductor layer, a second semiconductor layer formed on the surface of the first semiconductor layer, a third semiconductor layer formed on the surface of the first semiconductor layer, and the first semiconductor layer. A transfer electrode layer deposited on a surface of a semiconductor layer with an oxide film interposed therebetween, wherein the photoelectric conversion element is formed by the first and second semiconductor layers, and the first to third semiconductor layers and the the transfer electrode layer forming the reset transistor, and forming the third semiconductor layer corresponding to the reset drain of the transistor to be embedded in a groove formed on a surface of the first semiconductor layer, said first 3 semi-conductors The depth of the groove layer is embedded, characterized in that deeper than the substrate depth direction of the depth of the second semiconductor layer.
[0008]
That is, according to the solid-state imaging device of the present invention, the diffusion current corresponds to the drain of the transistor at a position deeper than the depth at which the depletion layer around the photodiode serving as the photoelectric conversion element reaches the depth direction of the substrate. The third semiconductor layer has an action that can be absorbed. Further, if the tip of the third semiconductor layer is sharp, the effect of the electric field is synergistic, and a greater effect is brought about.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view of a solid-state imaging device according to an embodiment, which will be described.
As shown in the figure, in the present solid-state imaging device, an n-type semiconductor layer 2 and an n-type semiconductor layer 3 are formed on the surface of a low-concentration p-type semiconductor layer 1, and a silicon oxide film 4 is formed in a predetermined region. , The gate electrode 5 is deposited. Then, a photodiode is formed by the p-type semiconductor layer 1 and the n-type semiconductor layer 2, and a reset transistor is formed by the p-type semiconductor layer 1, the n-type semiconductor layer 2, the n-type semiconductor layer 3, and the gate electrode 5. ing.
[0010]
As described above, the solid-state imaging device has a structure in which the reset transistor is provided near the photodiode. In the case of the present embodiment, the depletion layer 6 of the photodiode extends in the low-concentration p-type semiconductor layer 1, which contributes to the improvement of the sensitivity of the solid-state imaging device. Note that the same effect can be obtained even if this photodiode is formed of a low-concentration n-type semiconductor layer and a p-type semiconductor layer.
[0011]
Further, in this embodiment, the high-concentration n-type semiconductor layer 3 is deepened to a position deeper than the depth of the depletion layer 6 of the photodiode in order to reduce the leak current of the photodiode and improve the pixel separation performance. Has formed. When the reset transistor is a p-type MOS transistor, the high-concentration n-type semiconductor layer 3 can exhibit a sufficient imaging function even if the reset transistor is a p-type MOS transistor.
[0012]
According to such a configuration, as shown in FIG. 2, at a position deeper than the depth at which the depletion layer 6 of the photodiode reaches the depth direction of the substrate, the diffusion current 7 is reduced to n corresponding to the drain of the reset transistor. Can be absorbed by the semiconductor layer 3. In FIG. 2, the flow of the diffusion current 7 is schematically indicated by an arrow. However, according to the present invention, the rate at which the diffusion current 7 gathers in the photodiode is reduced, and the dynamic range of the solid-state imaging device is improved. If the tip of the drain groove is sharp, the effect of the electric field is synergistic, and the above-mentioned effect is of course increased.
[0013]
As described above, the solid-state imaging device according to the embodiment characterized in that the diffusion current is collected at the drain of the reset transistor has been described. For example, the photodiode and the reset transistor may be laid out as shown in FIG.
[0014]
That is, as shown in the figure, in this solid-state imaging device, a p-layer (element isolation region) 20 and n-layers (photodiodes) 15a to 15c are formed in a surface layer of a p-type silicon semiconductor substrate. Then, an n-layer is formed in a region where the read transistors 18a to 18c and the reset transistors 16a to 16c are formed. Thereafter, the surface is covered with an oxide film, and an electrode wiring material is further deposited.
[0015]
Then, in order to form the read transistors 18a to 18c, the read / reset lines 13a to 13c, the gates of the reset transistors 16a to 16c, and the reset lines, the electrode wiring material is processed into a desired shape and the wiring between the read lines and the reset transistors is formed. To form At this time, since the read line and the reset line are also used, the read / reset lines 13a to 13c can be formed by patterning at a time. Thereafter, after forming an element portion of the solid-state imaging device, a wiring for reading out a signal current (signal line 1) and a wiring for discharging a signal (drain line 2) are wired.
[0016]
In such a configuration, the positional relationship between the reset transistor and the photodiode as shown in FIG. 1 is not established. However, if the drain of at least one reset transistor is deeper than the depletion layer of the photodiode, the above-described configuration may be used. The effect of the invention can be obtained similarly. Generally, the diffusion length is on the order of tens of microns, so that a diffusion current flows from the order of tens of microns to the photodiode in the depth direction of the substrate. Considering that this is several times to several tens times the size of the current pixel, it is clear that there is no problem even if the position of the photodiode and the drain that absorbs the diffusion current are separated on the substrate surface. is there.
[0017]
The configuration of the unit cell of the solid-state imaging device of FIG. 3 is shown in FIG.
FIG. 4 shows a configuration of three unit cells, and assumes a solid-state imaging device having one pixel and one unit cell. Although only three unit cells are shown in this figure, the solid-state imaging device is assumed to have such unit cells arranged two-dimensionally.
[0018]
As shown in the figure, the unit cell 22 includes a photodiode 15a for converting light into electric charge, a read transistor 18a for reading out the electric charge accumulated in the photodiode 15a to a detection unit 23a, and an address capacitor for selecting a line for reading out signal electric charge. 19a, an amplification transistor 14a for amplifying the detection signal of the photodiode and outputting the amplified signal to the signal line 1, and a reset transistor 16a for resetting the charge accumulated in the detection unit 23a.
[0019]
Hereinafter, an example of a method of creating the imaging apparatus according to the embodiment shown in FIG. 1 will be described with reference to FIG. Here, a description will be given mainly of a method of forming a drain that absorbs a diffusion current, which is a feature of the present invention.
[0020]
First, a resist 8 is formed in the low-concentration p-type semiconductor layer 1 in a pattern that is opened only at a portion to be a drain of a reset transistor. Thereafter, the portion serving as the drain is etched using, for example, a reactive ion etching (RIE; Reactive Ion Etching) method to form a trench (see FIG. 6A).
[0021]
Next, the resist 8 is removed using, for example, an etcher. Thereafter, n-type polysilicon 9 is formed on the trench under conditions of good coverage. As this method, a method in which amorphous silicon is used, and reheating is performed later to obtain polysilicon may be used (see FIG. 6B).
[0022]
Thereafter, the surface is polished using, for example, a method such as CMP for flattening or an etching technique, and the n-type semiconductor layer 3 is formed only in the trench portion (see FIG. 6C). After that, the silicon oxide film 4 and the gate electrode 5 are deposited on predetermined regions of the p-type semiconductor layer 1 (see FIG. 6D).
[0023]
Through such a series of steps, the n-type semiconductor layer 3 corresponding to the drain absorbing the diffusion current, which is a feature of the present invention, can be formed.
Although the embodiment of the present invention has been described above, the present invention is not limited to this, and it is needless to say that various improvements and modifications can be made without departing from the gist of the present invention. For example, in the above embodiment, the trench technique has been described as an example of a method for forming a drain deep in a substrate. However, the present invention is not limited to this. Of course, it is also possible to employ ion implantation. Further, when the tip of the n-type semiconductor layer is formed into a sharp shape by two-stage or three-stage striking, the effect of the present invention can be further enhanced.
[0024]
【The invention's effect】
As described above in detail, according to the present invention, there is provided a solid-state imaging device which reduces the influence of a diffusion current from a semiconductor substrate, has a small leakage current when a photodiode is dark, and has a large dynamic range, and a method of manufacturing the same. be able to.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a solid-state imaging device according to an embodiment of the present invention.
FIG. 2 is a diagram for explaining a flow of a diffusion current in the device of FIG.
FIG. 3 is a diagram illustrating an example of a layout of a photodiode and a reset transistor of the solid-state imaging device according to the embodiment;
FIG. 4 is a diagram showing a circuit configuration corresponding to FIG. 3;
FIG. 5 is a diagram illustrating an example of a method for forming a drain of the solid-state imaging device according to the embodiment;
FIG. 6 is a diagram illustrating an example of a layout of a solid-state imaging device according to the related art.
FIG. 7 is a diagram for explaining a flow of a diffusion current in a solid-state imaging device according to the related art.
[Explanation of symbols]
Reference Signs List 1 low-concentration p-type semiconductor layer 2 n-type semiconductor layer 3 n-type semiconductor layer 4 silicon oxide film 5 gate electrode 6 depletion layer 7 diffusion current

Claims (3)

In a solid-state imaging device having a photoelectric conversion element that converts light into electric charge and a reset transistor for transferring electric charge of the photoelectric conversion element,
A first semiconductor layer formed on a semiconductor substrate;
A second semiconductor layer formed on a surface of the first semiconductor layer;
A third semiconductor layer formed on the surface of the first semiconductor layer;
A transfer electrode layer deposited on the surface of the first semiconductor layer with an oxide film interposed therebetween;
Has,
The photoelectric conversion element is formed by the first and second semiconductor layers, the reset transistor is formed by the first to third semiconductor layers and the transfer electrode layer, and the reset transistor corresponding to a reset drain of the reset transistor is formed. The third semiconductor layer is formed so as to be embedded in a groove formed on the surface of the first semiconductor layer, and the depth of the groove in which the third semiconductor layer is embedded is equal to the depth of the second semiconductor layer. A solid-state imaging device characterized by being deeper than the depth in the substrate depth direction . The solid-state imaging device according to claim 1, wherein a tip of the groove in which the third semiconductor layer is embedded has a sharp shape. The depth of the groove in which the third semiconductor layer is embedded is deeper than a depth of a depletion layer formed by the second semiconductor layer in a depth direction of the substrate. Solid-state imaging device.

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