JP2877382B2 - Solid-state imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a solid-state imaging device.

(Prior Art) A solid-state imaging device using a semiconductor has a small size, a light weight, and a high reliability. Therefore, technical improvement is rapidly progressing as a substitute for a conventional imaging tube. A typical fixed imaging device is a CCD imaging device.

7 (a) and 7 (b) are a plan view and a sectional view taken along the line AA 'showing a main part configuration of a conventional CCD image pickup device. A photodiode 2 serving as a photosensitive portion, a signal charge transfer portion 3 for transferring signal charges photoelectrically converted by the photodiode 2 and stored, and a signal charge of the photodiode 2 are transferred to a p ^- type silicon substrate 1. A read channel section 4 (usually called a field shift gate) for reading data is formed in the section 3. In the figure, only one photodiode corresponding to one pixel is shown, but usually a plurality of the photodiodes are arranged in a matrix, and a plurality of signal charge transfer units are arranged along the photodiode array to form a two-dimensional CCD.
An imaging device is configured. The photodiode 2 is mounted on the substrate 1
And a pn junction diode in which an n-type diffusion layer 5 is formed. Signal charge transfer section 3, n-type diffusion layer 6 is formed on the surface of the substrate 1 is constituted by a buried channel type CCD transfer electrode 8 via the gate insulating film _7, 82 are formed on the . Transfer electrodes 8 ₂ is disposed extending over the substrate 1 of the read channel unit 4 reads the channel portion 4
Also serves as a transfer electrode. A p-type diffusion layer 9 is formed as a channel stopper layer on the surface of the substrate other than the photodiode 2, the signal charge transfer section 3, and the read channel section 4. The substrate surface on which the elements are formed is covered with a CVD insulating film 10.

In such a CCD imaging device, since the n-type diffusion layer 5 of the photodiode 2 is floating, the signal charges photoelectrically converted and accumulated by the photodiode 5 can be completely read out to the charge transfer unit 3. However, there is a problem that some signal charges remain in the photodiode 5, and the remaining charges appear as afterimages on the reproduced image. The number of remaining signal charges depends on the number of signal charges stored in the photodiode before reading. As the number of stored signal charges increases, the number of remaining signal charges increases, and this remaining signal leaks through the channel of the signal charge reading unit 4. Doing so causes afterimages. this is,
In order to obtain a high-definition image, the number of pixels is increased and the density is increased, and the channel length of the signal charge readout section 4 is shortened. This is because the leakage current flowing in the weakly inverted state of the signal charge reading unit 4 increases due to the short channel effect. In addition, the afterimage appears more remarkably due to the following phenomenon. This will be described with reference to the potential distribution in FIG. Although the charge transfer section 3 is depleted before reading, when signal charges are read out and accumulated in the charge transfer section 3, the potential of the channel of the transfer section 3 changes with time during reading. In particular, when the short channel effect occurs, the channel potential of the signal charge reading section 4 also changes according to that of the transfer section 3. That is, the larger the number of read signal charges, the lower the channel potential of the read unit, and thus the number of signal charges remaining in the photodiode increases. In such a case, the afterimage becomes more remarkable.

In the case of a CCD image pickup device, the short channel effect of the signal charge readout channel section 4 occurs at a channel length of about 4 μm unlike a normal MOS integrated circuit. For a CCD imager,
This is because the area of the transfer electrode is larger than that of a normal MOS integrated circuit, so that the gate insulating film is likely to be broken. To prevent this, the gate insulating film 7 needs to have a thickness of about 900 mm.

The phenomenon of solid afterimages described above appears more greatly in a stacked CCD image pickup device in which a photoelectric conversion film is stacked on a CCD substrate. In the stacked CCD imaging device, the photodiode 2 is a storage diode only for storing signal charges, and a photoelectric conversion film needs to be connected to the storage diode. For that purpose, it is necessary to increase the impurity concentration of the n-type diffusion layer 5 of the diode. This is because the amount of remaining signal charge at the time of reading becomes larger.

In order to solve such a problem, conventionally, a portion of the channel region of the signal charge transfer portion adjacent to the signal charge readout channel portion has a lower concentration of n-type diffusion than the n-type diffusion layer of the channel region of the signal charge transfer portion. It has been proposed to provide a layer (JP-A-63-311757). This is usually a MOS
It can be said that the principle of the so-called LDD structure used as a method for suppressing the short channel effect in the integrated circuit is applied to the signal charge reading section. However, this method has a problem that the number of steps is increased for forming a new diffusion layer region, and a step such as ion implantation is required, so that the gate insulating film is deteriorated.

(Problems to be Solved by the Invention) As described above, in the conventional CCD imaging device, the signal charges accumulated in the diffusion layer of the floating are not completely read out to the transfer unit, but appear as an afterimage on a reproduced image, In particular, when the number of pixels is increased and the density is increased, there is a problem that a short-channel effect is generated in the signal charge readout portion, and the afterimage phenomenon is increased.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a solid-state imaging device in which an afterimage phenomenon that is remarkably caused by such a short channel effect is effectively reduced.

[Constitution of the Invention] (Means for Solving the Problems) The present invention relates to a solid-state imaging device having a signal charge storage unit, a signal charge transfer unit, and a signal charge readout channel unit connecting the signal charge accumulation unit and the signal charge transfer unit. The gate insulating film of the channel portion and a part of the signal charge transfer portion adjacent to the channel portion is thinner than that of the remaining portion of the signal charge transfer portion.

(Operation) According to the present invention, by shortening the gate insulating film of the signal charge readout portion, the short channel effect is reduced, so that the leakage flowing through the channel of the signal charge readout portion during the signal charge readout period is sufficiently small. Become. As a result, the afterimage on the reproduced image is reduced. Since the area of the read channel portion is smaller than that of the signal charge transfer portion, a decrease in reliability caused by making the thickness of the gate insulating film in this portion smaller than that of the signal charge transfer portion does not cause much problem.

(Example) Hereinafter, an example of the present invention will be described.

FIGS. 1 (a) and 1 (b) show the main structure of a CCD image pickup apparatus according to one embodiment. 7 (a) and 7 (b) are assigned the same reference numerals as in FIGS. 7 (a) and 7 (b), and detailed description is omitted. The structure of this embodiment differs from the conventional structure in that
Is that it is thinner than the gate insulating film ₇₁ of the signal charge readout channel unit 4 of the gate insulating film 7 ₂ signal charge transfer section 3 (FIG. 1 (the hatched area in plan view of a)). For example when the film thickness of the gate insulating film ₇₁ of the signal charge transfer unit 4 and 900 Å, that of the gate insulating film 7 _{and second} signal charge readout channel unit 3 and about 600 Å.

FIGS. 2A to 2D show an example of a manufacturing process for obtaining the structure of this embodiment. First, an ultra-thin sacrificial oxide film (not shown) is formed on a p ^- type silicon substrate 1 and (a)
As shown in FIG. 7, a p-type diffusion layer 9 serving as a channel stopper is formed in a field region, an n-type diffusion layer 5 for forming a photodiode of the photosensitive section 2 and a buried channel for forming a signal charge transfer section 3 are formed. The n-type diffusion layers 6 are respectively formed by ion implantation or the like. Then after removing the sacrificial oxide film, forming a (b) as shown in, 900 Å approximately oxide film to be a gate insulating film ₇₁ of the signal charge transfer section 3 to the entire surface by thermal oxidation. Thereafter, the oxide film of the signal charge readout channel portion 4 in which a mask (not shown) such as a photoresist is patterned is selectively etched to reduce its thickness.
A gate insulating film 7 ₂ to about 600 Å. In this case, once all the etching removal of the oxide film in this region, may be formed gate oxide insulating film 7 ₂ of about 600Å anew subjected to a thermal oxidation after removing the mask. However, in that case, a thin thermal oxide film is formed in advance so that the other region finally becomes 900 °. Then as shown in (d), by repeating the deposition and patterning of two-layer polycrystalline silicon film to form the transfer electrodes _81, 82. Finally, a CVD oxide film is deposited on the entire surface.

FIGS. 3A to 3D show another example of the manufacturing process.
The process is the same as that of the previous example until the diffusion layer of each part is formed as shown in FIG. Then in this example, to form an oxide film of the preferred film thickness of, for example 600Å as the gate insulating film 7 _{and second} signal charge readout channel unit 4 as shown in (b). Then, as shown in (c), an oxidation resistant mask 21 made of, for example, a silicon nitride film is formed in the region of the signal charge readout channel portion 4 and subjected to thermal oxidation to form a gate insulating film of the charge transfer portion 3 having a thickness of about 900 °. to form a film _71. As with subsequent previous example, to form the transfer electrodes _81, 82 as shown in (d).

In CCD imaging device of this embodiment, since the gate insulating film 7 _{and second} signal charge readout channel unit 4 is thin, the short channel effect is suppressed, the channel length of the signal charge readout channel unit 4 be the same as conventional, The leakage current flowing in the weak inversion state during the signal charge readout period is smaller than in the conventional case. Therefore, the afterimage on the reproduced image due to the residual charge of the photodiode is reduced, and a high-quality image can be obtained. Since the area of the signal charge readout channel section 4 is smaller than the area of the signal charge transfer section 3, a decrease in the reliability due to the reduction in the thickness of the gate insulating film in this area hardly causes a problem.

FIGS. 4 (a) and 4 (b) show the main structure of a CCD image pickup apparatus according to another embodiment. Parts corresponding to FIGS. 1A and 1B are denoted by the same reference numerals as those in FIGS. 1A and 1B, and detailed description is omitted. In this embodiment, as shown by hatching in FIG. 4 (a), not only the signal charge readout channel portion 4 but also a thin gate insulating film 7 that partially cuts into the channel region of the signal charge transfer portion 3 therefrom. Forming _two .

According to this embodiment, the same effect as the previous embodiment can be obtained. Further, in the case of this embodiment, since the potential distribution in the substrate at the cross section of FIG. 4 (b) is as shown in FIG. 4 (c),
The signal charge in the channel of the signal charge transfer unit 3 is localized in a position away from the signal charge readout channel unit 4.
Therefore, the variation of the channel potential of the signal charge readout channel unit 4 due to the variation of the channel potential of the signal charge transfer unit 3 is reduced, and the leakage current is more effectively reduced.

The present invention can also be applied to a stacked type CCD imaging device.

FIG. 5 is a cross-sectional view showing a main part structure of an embodiment applied to a stacked CCD, corresponding to FIG. 1 (b). First
After a CCD substrate is formed in the same manner as in the drawing, a contact hole is formed on the n-type diffusion layer 5 serving as a storage diode, and a relay electrode 11 is formed. After that, CVD insulating film 1
2 is formed, and a contact hole for the relay electrode 11 is formed in this, and a current collecting electrode 13 of each pixel is formed. A photoelectric conversion film 14 of amorphous Si or the like is deposited thereon, and a transparent conductive film 15 is formed on the surface.

In the stacked CCD imaging device of this embodiment, the photoelectric conversion film 14
The signal charges (electrons) photoelectrically converted in (1) are collected by the current collecting electrode 13 and accumulated in the n-type diffusion layer 5 via the relay electrode 11. The operation of reading out the signal charge is the same as in FIG.

Is thinner than the gate insulating film 7 ₂ similarly the signal charge readout channel unit in the embodiment of Figure 1 the gate insulating film ₇₁ of the signal charge transfer section in this embodiment. This allows
As in the embodiment of FIG. 1, the afterimage due to the short channel effect can be reduced.

FIG. 6 shows a second embodiment of the stacked CCD image pickup device.
2 is a cross-sectional structure of an embodiment corresponding to the embodiment in the figure. As with the embodiment of Figure 2, it is provided in the form of biting a thin gate insulating film 7 ₂ of the signal charge readout channel portion to a part signal charge transfer section.

According to this embodiment, the same effect as the embodiment of FIG. 2 can be obtained.

The present invention is not limited to the above embodiment, but can be implemented with various modifications without departing from the spirit thereof.

[Effects of the Invention] As described above, according to the present invention, it is possible to obtain a CCD image pickup device in which afterimages caused by an increase in leakage current in a signal charge readout channel portion accompanying an increase in the number of pixels and a density are effectively reduced. Can be.

[Brief description of the drawings]

1 (a) and 1 (b) are a plan view and a sectional view taken along line AA 'of the CCD image pickup device according to one embodiment of the present invention, and FIGS. FIGS. 3 (a) to 3 (d) are cross-sectional views for explaining another example of a manufacturing process, and FIGS. 4 (a) to 4 (c) are other examples. FIG. 5 is a plan view showing a main part structure of a CCD image pickup device, a cross-sectional view taken along line AA 'thereof, and a diagram showing a potential distribution in a substrate. FIG. FIG. 6 is a cross-sectional view showing a main part structure of another embodiment similarly applied to a stacked CCD image pickup device. FIGS. 7A to 7C show main part structures of a conventional CCD image pickup device. FIG. 2 is a plan view, an AA ′ cross-sectional view thereof, and a diagram showing a potential distribution in a substrate. 1 ... p ^- -type silicon substrate, 2 ... photodiode (photosensitive section), 3 ... signal charge transfer section, 4 ... signal charge readout channel unit, 5 ... n-type diffusion layer, 6 ... n-type diffusion layer, _71, 7 ₂ gate insulating film, 8 ₁ , 8 ₂ transfer electrode, 9 p-type layer, 10 CVD insulating film, 11 relay electrode, 12 CVD insulating film, 13 current collecting electrode, 14 photoelectric conversion film , 15 ... Transparent conductive film.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 27/14-27/148 H01L 29/762-29/768

Claims (1)

(57) [Claims] 1. A semiconductor substrate comprising: a plurality of photosensitive portions; a signal charge transfer portion; and a signal charge readout channel portion for reading signal charges photoelectrically converted and stored in the respective photosensitive portions to the signal charge transfer portion. In the solid-state imaging device, the signal charge readout channel part and a part of the gate insulating film of the signal charge transfer part adjacent thereto are thinner than those of the remaining part of the signal charge transfer part. apparatus.