JPH0425756B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a solid-state imaging device, and more particularly to a structure for compensating for a difference in dark current characteristics between an optical black section and a light receiving section.

[Technical background of the invention]

Figure 1 shows a conventional example of an interline transfer type image sensor using a charge-coupled device (CCD), where 1 is a photosensitive pixel section consisting of a pn photodiode, 2 is a vertical CCD register section, and 3 is a horizontal The CCD register section, 4 is a sensor area, and 5 is an optical black area for black level generation (hereinafter referred to as OB area), which is covered with aluminum to block light input, and is used as an area sensor. It is formed in only a few areas.

FIG. 2 shows a cross-sectional structure of a part of the OB area 5 and a part of the sensor area 4 of the image sensor. That is, 11 is, for example, a p-type silicon substrate, 12 is an n ⁺ layer provided on a part of the surface of the substrate 11 and has a conductivity type opposite to that of the substrate 11 (corresponding to the photosensitive pixel section 1), and 13 is also on the surface of the substrate. above n ⁺ layer 1
2 ⁽ corresponding to the vertical register section 2), 14 is an overflow drain section (excess charge discharge section),
15 is an overflow control gate section, 1
6 is an element isolation part (channel stopper part), 17
is a first layer polycrystalline silicon electrode, 18 is a silicon oxide (SiO ₂ ) film, 19 is a second layer polycrystalline silicon electrode, 20 is a first layer aluminum film, 21 is a protective insulating film, and 22 is a second layer polycrystalline silicon electrode. The layer is an aluminum film.

In the above image sensor, the sensor area 4
When light is incident on the silicon substrate 11, the generated signal charges are accumulated in the n ⁺ layer 12 of the photosensitive pixel section 1. At this time, if excess charge is generated, it is discarded into the overflow drain section 14 via the overflow control gate section 15. The charges accumulated in the n ⁺ layer 12 of the photosensitive pixel section 1 are transferred to the n ⁺ layer 13 of the vertical register section 2, and then transferred within the vertical register section 2 toward the horizontal register section 3 by a transfer clock pulse. Ru. The charges transferred to the horizontal register section 3 are transferred in the horizontal direction, converted into voltage by the output section 6, and read out.

Note that even if the CCD image sensor completely blocks light input, an output component (referred to as a dark output component or dark current) is observed at room temperature, and this output component increases as the temperature rises. When such a dark output component becomes large, a problem arises in that the dynamic range of the charge readout section (vertical register section 2 in this example) becomes small. Therefore,
Signal processing is performed to subtract the increase in the dark output component due to temperature rise, and in order to detect the increase in the dark output component,
An OB area 5 is provided. Therefore, O.B.
Region 5 has the same structure as sensor region 4 except for the second layer of aluminum film 22.

[Problems with background technology]

By the way, when forming the second layer aluminum film 22 as described above by vapor deposition, in order to recover damage to the substrate part, for example, nitrogen (N ₂ ) is 90% and hydrogen (H ₂ ) is 10% at 450°C. 15% atmosphere
Annealing must be performed for about 30 minutes to 30 minutes. In this case, H ₂ plays an important role and restores the damage. It should be noted that the above-mentioned annealing is performed after the patterning of the aluminum film, since aluminum cannot be completely etched before the patterning of the aluminum film is formed. Further, there is a difference in annealing effect between the OB region 5 where the aluminum film 22 is present on the surface and the sensor region 4 where the aluminum film 22 is not present. This is because inside the aluminum film 22
Since H ₂ is included, the annealing effect progresses more in the OB region 5 covered with the aluminum film 22 than in the sensor region 4. By this,
There will be a difference in the dark output component between the OB region 5 and the sensor region 4, which will cause problems in canceling out the variation in the dark output component as described above.

[Purpose of the invention]

The present invention was made in view of the above circumstances, and
There is no difference in the dark output component between the optical black area and the sensor area, and the variations due to temperature in the dark output component in both areas are equal, and the dark output component due to temperature fluctuations in the sensor area is the same. It is an object of the present invention to provide a solid-state imaging device that is capable of accurately compensating for fluctuations in , and can prevent a decrease in the dynamic range of a charge readout section.

[Summary of the invention]

That is, the present invention provides a solid-state imaging device in which a sensor region and an optical black region having the same structure but shielded from light by forming an aluminum film on the surface are formed on a semiconductor substrate of one conductivity type. The gate insulating film in the region is characterized by having a two-layer structure having hydrogen shielding properties.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

The CCD image sensor shown in FIG. 3 is different from the conventional CCD image sensor described above with reference to FIGS.
A silicon nitride (Si ₃ N ₄ ) film 31 is formed under the polycrystalline silicon electrode 17 of the layer, in other words,
The difference is that the gate insulating film in the OB region 5 has a two-layer structure of a silicon oxide film 32 and a silicon nitride film 31, and the other points are the same.
In the figure, the same parts as in FIG. 2 are given the same reference numerals.

To briefly describe the method for manufacturing the above-mentioned CCD image sensor, first, a p-type silicon substrate 11 is coated with a SiO ₂ film 32 having a thickness of, for example, about 1000 Å.
Next, after covering with a Si ₃ N ₄ film of, for example, about 1000 Å, the Si ₃ N ₄ film 31 in the OB region 5 is left, and the other Si ₃ N ₄
All films are etched off. After that, the traditional
Using the same manufacturing method as CCD image sensors,
A resist pattern is formed by photolithography,
Using this as a mask, place at predetermined positions on the surface of the substrate 11 an n ⁺ part 12 which will become a photosensitive pixel part, an n ⁺ part 13 which will become a vertical register part, an n ⁺⁺ part 14 which will become an overflow drain part, and a channel stopper part.
The p ⁺ portions 16 are sequentially formed. Next, the SiO ₂ film 3
A first layer of polycrystalline silicon electrode 17 is formed at a predetermined position on a two-layer structure film of 2 and Si ₃ N ₄ film 31, and a second layer of polycrystalline silicon electrode 17 is formed on this through a SiO ₂ film 33 as an interlayer insulating film. A layer of polycrystalline silicon electrode 19 is formed. Next, p ⁺ ions are implanted into the entire surface to form an n ^- portion 15 that will serve as an overflow control gate. Next, a SiO ₂ film 18 is formed on the entire surface by CVD (chemical vapor deposition), and a first layer of aluminum film 20 is deposited on it to form the photosensitive pixel portion.
An aperture window is provided in a portion corresponding to the n ⁺ portion 12. Next, as the protective insulating film 21, for example, PSG (phosphorus
After forming a silicate glass film and depositing a second layer of aluminum film 22 thereon, all parts other than the OB region 5 are etched off. then 450
°C, 15 minutes to 30 minutes in an atmosphere of 90% _N2 and 10% _H2
Anneal for a minute.

In the case of the CCD image sensor having the above-described structure, the diffusion coefficient of H ₂ in the Si ₃ N ₄ film 31 is much smaller than that in the SiO ₂ film 31, so the temperature at about 450°C is In this annealing, H ₂ contained in the aluminum film 22 is not diffused into the semiconductor substrate 11. Then, the annealing of the OB region 5 results in H ₂ diffused from the sensor region 4.
The annealing effect on the OB region 5 and the sensor region 4 is the same.

Therefore, the dark time output components of the OB region 5 and the sensor region 4 are the same, and they vary in the same way due to temperature fluctuations, so there is no difference between them. As a result, by subtracting the dark output component in the OB area 5 from the dark output component in the sensor area 4, it is possible to accurately compensate the dark output component in the sensor area 4 even for the temperature fluctuations mentioned above. Therefore, the dynamic range of the vertical register section does not decrease due to the fluctuation, and the characteristics, especially at high temperatures, are significantly improved.

Note that the present invention is not limited to the above embodiments, and the gate insulating film structure may include an alumina film formed on the SiO ₂ film, in short, a two-layer structure having H ₂ shielding properties. do it. Furthermore, the present invention is not limited to interline transfer type CCD image sensors, but also applies to frame transfer type CCD image sensors.
It goes without saying that it can be applied to CCD image sensors as well, but the point is that it has a photosensitive pixel part, a charge readout part, an excess charge discharge part, etc. on the surface of a semiconductor substrate of one conductivity type, and a part becomes an optical black area. It can be applied to solid-state imaging devices.

〔Effect of the invention〕

As described above, according to the solid-state imaging device of the present invention, there is no difference in the dark output component between the optical black region and the sensor region, and the variations due to temperature in the dark output component in both regions are equal. Therefore, it is possible to accurately compensate for the fluctuations in the dark output component due to temperature fluctuations in the sensor area, preventing a decrease in the dynamic range of the charge readout section, and greatly improving the readout characteristics at high temperatures. be able to.

[Brief explanation of drawings]

FIG. 1 is a front view schematically showing the configuration of a conventional image sensor, FIG. 2 is a sectional view taken along the line - in FIG. 1 and viewed in the direction of the arrow, and FIG. 1 is a sectional view showing a main part of an embodiment of an imaging device. 1...Photosensitive pixel section, 12...Vertical register section,
4...Sensor area, 5...Optical black area, 11...Semiconductor substrate, 31...Silicon nitride film, 32...Silicon oxide film.

Claims (1)

[Scope of Claims] 1. A solid-state imaging device in which a sensor region and an optical black region having the same structure but shielded from light by forming an aluminum film on the surface are formed on a semiconductor substrate of one conductivity type. A solid-state imaging device characterized in that a gate insulating film in an optical black region has a two-layer structure having hydrogen shielding properties. 2. The solid-state imaging device according to claim 1, wherein the gate insulating film is comprised of a silicon oxide film formed on a semiconductor substrate and a silicon nitride film formed thereon.