JPH04316367A - Solid-state image sensing device and manufacture thereof - Google Patents

Abstract

PURPOSE:To reduce the production of smear with a widened dynamic range. CONSTITUTION:After a p-type well 2 with lower density is formed on an n-type semiconductor substrate 1, an n-type region 3 with lower density is formed by heat-diffusing n-type impurities within that specified region. Then, p-type separation section 4, 5 is formed and diffusion-formed with heat-treatment to make the impurity distribution in the lower density n-type region 3 a bell-bottom shape. Then, a higher density n-type region 8 is formed within the lower density n-type region 3 with ion implantation method and further, heat-treated at about a temperature where the impurities are not re-diffused to activate the implanted ions.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state imaging device with less smear and a wide dynamic range, and a method of manufacturing the same.

0002

2. Description of the Related Art In recent years, CCD image pickup devices have come into wide use in the field of consumer and professional video cameras. However, conventional CCD image sensors,
In general, smears were more likely to occur and the dynamic range was narrower than with image pickup tubes.

[0003] Therefore, as a method for reducing smear in a CCD image sensor, it is necessary to prevent electrons generated when light obliquely incident on a photodiode is photoelectrically converted near the CCD channel portion to enter the CCD channel portion. A so-called bell-bottom structure has been proposed as a structure to prevent this (Japanese Patent Publication No. 2-19632).

Furthermore, with the CCD image pickup device, the image quality is inferior to that of the image pickup tube system, as the dark current generated in the photodiode makes the image look grainy and causes white spots. By coating the surface of the photodiode, which has many crystal defects and interface states, with a highly concentrated p-type region, the dark current from the surface is drastically reduced, resulting in image quality equal to or higher than that of an image pickup tube. It was resolved.

However, according to this method, in a photodiode having the same area, the low concentration n-type region on the surface is covered with the high concentration p-type region, so the volume of the low concentration n-type region is effectively reduced. The problem arises that the dynamic range becomes narrower.

On the other hand, high-precision cameras such as HDTVs
As CD imaging devices have come into use, the area per unit pixel must be further reduced, making it difficult to sufficiently widen the dynamic range.

As a countermeasure against this problem, a low concentration n-type region which is a photodiode is placed inside the substrate, and the surface portion is formed with a low concentration n-type region.
Attempts have been made to form high concentration impurity regions that do not serve as mold regions. In order to achieve this, a method is being considered in which the photodiode region is formed using highly accelerated ion implantation without reducing the effective amount of impurities in the n-type region.

FIG. 4 is a cross-sectional view of one pixel portion of a conventional CCD image sensor. A low concentration p-type well 2 is formed in an n-type semiconductor substrate 1 . Furthermore, a low concentration n-type region 3 which becomes one region of the photodiode is formed within a predetermined region of the p-type well 2. P-type isolation portions 4 and 5 are formed adjacent to this low concentration n-type region 3. The isolation section 4 electrically isolates the photodiode and the CCD channel section 7 of the pixel adjacent thereto, and the isolation section 5 electrically isolates the photodiode formed in one pixel section from the CCD channel section 7.
electrically separate. Separation section 6 connects p-type well 2 and CC
It is formed between the D channel portion 7 and separates the two. Further, a gate insulating film 9 is formed on the surface of the substrate 1, and a transfer gate electrode 10 is formed above the CCD channel section 7. A highly doped p-type region 11 is formed on the surface of the lightly doped n-type region 3. Furthermore, an insulating film 12, an aluminum light-shielding film 13, a protective film 1
4 is formed to constitute a CCD image sensor.

In a conventional CCD image pickup device in which a photodiode portion is formed by high-acceleration ion implantation, each of the low concentration n-type region 3, separation portions 4 and 5, separation portion 6, and CCD channel portion 7 is The pn junction surface 15 of the impurity layer is
The shape is shown in FIG. When an impurity layer is formed by high-acceleration ion implantation in this manner, the pn junction surface does not widen. In FIG. 5, 18 indicates a negative fixed charge, 19 indicates a positive fixed charge, and 20 indicates a charge neutral region.

When a photodiode is formed using the high acceleration ion implantation method, the lightly doped n-type region 3 does not have a bell-bottom structure as shown in the conventional example. Therefore, the depletion layer 1
6 has a bell-shaped shape as shown in FIG. 15 when the CCD image sensor is operated.

[0011]

[Problems to be Solved by the Invention] When the lightly doped n-type region 3 is formed by a highly accelerated ion implantation method, the amount of n-type impurities in the photodiode region effectively increases. Therefore, the amount of charge that can be accumulated in the low concentration n-type region increases, and the dynamic range of the CCD image sensor can be expanded. However, as shown in FIG. 5, the lateral spread of the n-type region is smaller than when the photodiode region is formed by normal high-temperature heat treatment, so the depletion layer 16 that spreads when the CCD image sensor is operated. The area becomes smaller. Therefore, when using the high acceleration ion implantation method, the obliquely incident light 1
Electrons generated by 7 easily enter the CCD channel portion 7, and smear is likely to occur.

For comparison, FIG. 6 shows the cross-sectional shape of a pn junction surface and a depletion layer in an operating state of a CCD image sensor in which a photodiode is formed by high-temperature heat treatment. The symbols used in the figure correspond to the symbols in FIG. 5.

When a photodiode is formed by high-temperature heat treatment, the low concentration n-type region 3 has a bell-bottom structure. Therefore, the depletion layer 16 has a shape as shown in FIG. 6 when the CCD image sensor is operated.

On the other hand, when impurities are implanted using high-acceleration ion implantation and a photodiode portion is formed by high-temperature heat treatment, the impurity distribution is the same as when high-acceleration ion implantation is not used. This results in impurity distribution. Therefore, the amount of charge accumulated in the low concentration n-type region 3 is small, making it impossible to expand the dynamic range of the CCD image sensor.

As described above, an object of the present invention is to provide a CCD image sensor with less smear and a larger dynamic range.

[0016]

[Means for Solving the Problems] In order to solve the above problems, the present invention forms a high concentration n-type region inside a low concentration n-type region by high-temperature heat treatment to form a photodiode having a bell-bottom structure. did. Then, a low concentration n-type region is formed by high-temperature heat treatment, and a high concentration n-type region is further formed therein by ion implantation, thereby forming a photodiode having a bell-bottom structure.

[0017]

[Operation] The depletion layer in the lightly doped n-type region of the photodiode region formed by high-temperature heat treatment spreads to almost the entire area of the n-type region when the device operates. Therefore, electrons generated by light incident obliquely from the opening of the photodiode are absorbed in the low concentration n-type region. Further, a depletion layer is formed in this low concentration n-type region and the surrounding p-type region. This depletion layer forms a potential well that becomes deeper toward the heavily doped n-type region formed at a predetermined depth in the depth direction of the photodiode. Therefore, electrons generated by obliquely incident light are collected in the high concentration n-type region of the photodiode through the potential well. Therefore, electrons generated by obliquely incident light do not enter the CCD channel portion, and the occurrence of smear is suppressed.

Furthermore, the highly doped n-type region formed deep in the photodiode has a narrow area and a shallow depth. However, since this region has a high concentration, the amount of charge that can be stored increases dramatically, and the dynamic range of the CCD image sensor is greatly expanded.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a cross-sectional view of one pixel portion of a CCD image sensor according to an embodiment of the present invention.

As shown in the figure, a low concentration p-type well 2 is formed in an n-type semiconductor substrate 1. Further, a low concentration n-type region 3, which becomes one region of the photodiode, is formed in a predetermined region of the p-type well 2, and p-type isolation portions 4, 5 are formed adjacent to the low concentration n-type region 3. is formed. The separation section 4 is a CCD of a pixel adjacent to the photodiode.
The isolation section 5 separates the CCD channel section 7 from the photodiode formed in one pixel section. Further, a separation section 6 is provided to separate the CCD channel section 7 from the p-type well 2. A highly doped n-type region 8 is formed at a predetermined depth from the surface of the lightly doped n-type region 3 . Further, a gate insulating film 9 is formed on the surface of the substrate 1, and a transfer gate electrode 1 is formed on the surface of the substrate 1.
0 is formed in a portion of the gate insulating film 9 above the CCD channel portion 7 . A highly doped p-type region 11 is formed on the surface of the lightly doped n-type region 3 . Further, an insulating film 1 is formed on the gate oxide film 9 and the transfer gate electrode 10.
2. An aluminum light-shielding film 13 and a protective film 14 are laminated to form a CCD image sensor.

Here, the low concentration n-type region 3 forming the photodiode has a bell-bottom impurity distribution.

As described above, compared to the conventional CCD image sensor, the CCD image sensor of the present invention has a high concentration n-type layer at a predetermined depth position from the surface of the low concentration n-type region 3, which is a photodiode. This is at the point where region 8 is formed.

FIG. 2 is a sectional view showing the manufacturing process of the embodiment shown in FIG. First, as shown in FIG. 2(a), a low concentration p-type well 2 is formed in an n-type semiconductor substrate 1. After this,
An n-type impurity is added to a predetermined region of the p-type well 2 at 1100°C.
The low concentration n-type region 3 is formed by thermal diffusion at a temperature of .

Next, as shown in FIG. 2(b), the p-type isolation section 4 is arranged so as to electrically isolate the photodiode from the CCD channel section 7 of the adjacent pixel. The portion 5 is connected to a photodiode formed in one pixel portion and C.
Impurities are selectively thermally diffused and formed adjacent to the low concentration n-type region 3 so as to separate the CD channel portions 7. In this way, after the formation of the low concentration n-type region 3, the isolation parts 4 and 5 are formed by diffusion through heat treatment, thereby forming the low concentration n-type region 3.
The impurity distribution in the mold region 3 becomes bell-bottom shaped. In addition, 6
, 7 are a separation section and a CCD channel section, respectively, which are formed by the same procedure as the conventional method.

Then, as shown in FIG. 2(c), a highly doped n-type region 8 is formed inside the lightly doped n-type region 3 by ion implantation.
form. That is, using a photoresist film (not shown) formed in a predetermined pattern on the substrate 1 as a mask, phosphorus ions are implanted with an acceleration energy of 320 keV. Then, the implanted ions are activated by heat treatment at a temperature of 1000° C. or lower, for example, 900° C. By performing the heat treatment at such a temperature, the impurities in the high concentration n-type region 8 are hardly diffused, the impurity distribution within this region 8 does not change much, and a sufficiently high concentration is maintained. .

Next, as shown in FIG. 2(d), after forming a gate insulating film 9 on the substrate 1, a transfer gate electrode 10 is formed so as to be located above the CCD channel section 7.
In addition, a high concentration p-type region 1 is provided on the surface of the low concentration n-type region 3.
1 is selectively formed. Then, an insulating film 12, an aluminum light shielding film 13, and a protective film 14 are sequentially laminated on the substrate 1 to complete a CCD image sensor.

[0027] In the example produced in this way,
In the operating state, the pn junction surface 15 of each impurity layer constituting the low concentration n-type region 3, the isolation parts 4 and 5, the isolation part 6, and the CCD channel part 7 has the shape shown in FIG. The reason why the pn junction surface 15 widens in this way is that in the process up to the formation of the high concentration n-type region 8 as described above, 1000
This is because the heat treatment at a temperature higher than 0.degree. C. is applied to the entire substrate, and the impurities in each impurity layer are diffused by the heat treatment. In FIG. 3, 18 indicates a negative fixed charge, 19 indicates a positive fixed charge, and 20 indicates a charge neutral region.

The depletion layer 16 has a shape as shown in the figure when the CCD image sensor is operated. This shape is C
It is wider than before the CD image sensor was operated.

In actual use, obliquely incident light 17 indicated by an arrow
enters the depletion layer 16 through the photodiode portion. At this time, the potential in the depletion layer 16 and the low concentration n-type region 3 is as follows: There is a continuous change to region 8, and highly doped n-type region 8 forms the bottom of the potential well. That is, the potential is deepest in the high concentration n-type region 8, and the potential becomes shallower radially from this region. Therefore, electrons generated by the obliquely incident light 17 in the depletion layer 16 fall into the high concentration n-type region 8, which is the deepest part of the potential of the photodiode, and C
It does not reach the CD channel section 7. Therefore, almost no smear occurs.

Electrons generated by the light 17 in the low concentration n-type region 3 similarly fall into the high concentration n-type region 8 and are accumulated therein. At this time, the total amount of accumulated charge is more than double that of conventional products, and the dynamic range of the CCD image sensor is greatly expanded.

Further, in the manufacturing method of the present invention, after the high concentration n-type region 8 is formed, heat treatment is performed at a relatively low temperature of 1000° C. or less, so that the impurity diffusion region is expanded by the heat treatment after diffusion. This prevents the potential of the photodiode from increasing. For this reason, CC
The afterimage characteristics of the D image sensor and the operating voltage of the vertical electronic shutter do not deteriorate.

The high concentration n-type region 8 may be formed before the gate insulating film 9 is formed as described above, but after the transfer gate electrode 10 is formed, ions are formed in self-alignment with respect to the transfer gate electrode 10. It may also be formed by injection.

Further, in this embodiment, the acceleration energy of the ion implantation to form the heavily doped n-type region 8 was set to 320 keV, but this acceleration energy was used to increase the depth of the heavily doped n-type region 8 to the heavily doped p-type region 11. Since the purpose is to form the p-type region 11 at the lower part of the p-type region 11, the acceleration energy may be selected according to the depth at which the high-concentration p-type region 11 is formed.

[0034]

According to the present invention, it is possible to manufacture a CCD image sensor having a photodiode that can obtain a dynamic range that is more than twice as high as that of a conventional device without deteriorating the smear characteristics.

[Brief explanation of drawings]

[Fig. 1] A cross-sectional view of one pixel of a CCD image sensor that is an embodiment of the present invention.

[Fig. 2] Cross-sectional view of the manufacturing process of one embodiment of the present invention

FIG. 3 is a cross-sectional view showing the shape of the pn junction surface and depletion layer in the operating state of one embodiment of the present invention.

[Figure 4] Cross-sectional view of one pixel of a conventional CCD image sensor

[Figure 5
] Cross-sectional view showing the shape of the pn junction surface and depletion layer in the operating state of a CCD image sensor formed by conventional high-acceleration ion implantation.

[Fig. 6] Cross-sectional view showing the shape of the pn junction surface and depletion layer in the operating state of a conventional CCD image sensor

[Explanation of symbols]

1 N-type semiconductor substrate 2 P-type well 3 Low concentration n-type region 4, 5, 6 Separation part 7 CCD channel section 8 High concentration n-type region 9 Gate insulating film 10 Transfer gate electrode 11 High concentration p-type region 12 Insulating film 13 Aluminum light shielding film 14 Protective film 15 pn junction surface 16 Depletion layer 17 Oblique incident light 18 Negative fixed charge 19 Positive fixed charge 20 Charge neutral region

Claims (2)

[Claims] 1. A semiconductor substrate of one conductivity type, a well layer of an opposite conductivity type formed in the semiconductor substrate, and a first diffusion layer forming one region of a photodiode formed in the well layer. a second diffusion layer formed on the surface of the first diffusion layer and having a conductivity type opposite to that of the first diffusion layer; and a first isolation layer formed adjacent to the first diffusion layer. a third diffusion layer forming a CCD channel section formed across the first separation section; and a second separation section separating the third diffusion layer and the well layer; The area of the first diffusion layer on the deep side of the substrate is larger than the area near the substrate surface, and a highly concentrated diffusion layer of the same conductivity type as the first diffusion layer is formed in a part of the lower part of the second diffusion layer. A solid-state imaging device comprising a diffusion layer of No. 4. 2. A step of forming a well layer of an opposite conductivity type on a semiconductor substrate of one conductivity type, and a step of forming a first diffusion layer to become a photodiode in the well layer by selectively thermally diffusing impurities. and a first diffusion layer adjacent to the first diffusion layer.
a step of forming a separation section, and a step of forming a CC adjacent to the separation section.
a step of forming a second diffusion layer that will become a D channel portion;
forming a second isolation part that separates the second diffusion layer from the well layer; selectively implanting impurity ions into a predetermined region within the first diffusion layer; and further steps for activation. A method for manufacturing a solid-state imaging device, comprising a step of heat treatment.