JPS6134263B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solid-state image sensor, and particularly to a solid-state image sensor having high sensor resolution and a large blooming suppressing effect.

For example, solid-state image sensors, typified by charge transfer devices, have been rapidly and widely used in recent years. In this type of solid-state image sensor, characteristics regarding its resolution and blooming suppression effect are particularly important. Among these, the main cause of resolution deterioration is due to the phenomenon that, for example, when a solid-state image sensor receives light, carriers generated inside the semiconductor substrate due to the light transmitted through the photosensitive pixels accumulate in adjacent photosensitive pixels due to diffusion. be. Blooming can also occur in the same manner as described above, for example, when excessive light input causes excess carrier that exceeds the storage capacity of a photosensitive pixel to diffuse through the semiconductor substrate and accumulate in adjacent photosensitive pixels, or directly flows into the signal detection section. It is based on the phenomenon of
Therefore, it is known to provide a solid-state image sensor with a function to prevent carrier diffusion, in order to suppress the deterioration of resolution and the occurrence of blooming. FIG. 1 shows an example of a conventional solid-state image sensor having this function. What is shown in FIG. 1 is a schematic cross-sectional view of a charge transfer type one-dimensional image sensor. This image sensor consists of a semiconductor layer 2 of a conductivity type opposite to that of the semiconductor substrate 1 on a semiconductor substrate 1 of one conductivity type, and a photosensitive pixel array 3 consisting of an island-shaped semiconductor region of the same conductivity type as the substrate 1 (in FIG. Although only the semiconductor region ₃₁ is shown, in reality, a large number of photosensitive pixels are arranged in the vertical direction of the paper) and a charge transfer type shift register for reading out the signal charges accumulated in this array 3. 6 (first
(Only the transfer electrode 5 provided through the insulating film 7 is shown in the figure); a shift gate 4 for transferring the signal charges accumulated in the array 3 to the charge transfer type shift register 6; A light shield film 9 provided to cover an area other than the photosensitive pixel portion and a semiconductor layer 2 provided to electrically isolate the photosensitive pixel array 3 and the charge transfer type shift register 6 are of the same electrical type. It is composed of a channel stop region 8 containing a high concentration of impurities.

Next, the operation of the solid-state image sensor shown in FIG. 1 will be explained. For convenience of explanation, the conductivity type of this semiconductor substrate 1 will be described as n-type. A zero or positive DC voltage V _BG is applied to the substrate 1 with the semiconductor layer 2 as a reference. This is operated in the same way as a normal charge transfer type one-dimensional image sensor. That is, after carriers generated by incident light are accumulated as signal charges in the photosensitive pixel array 3 for a predetermined period of time, the shift gate 4 is opened, the signal charges are transferred in parallel to the charge transfer type shift register 6, and then the shift gate 4 is closed. A clock pulse is applied to the charge transfer type shift register 6, and the signal charge is sequentially detected as a signal from the output section.

In the case shown in FIG. 1, the carrier generated in the semiconductor substrate 1 due to the long wavelength component of the incident light is the potential barrier (approximately V _B
(has a height of _G ), it cannot move toward the photosensitive pixel array 3. Therefore, the resolution will be improved for the reasons mentioned above. Furthermore, once carriers generated due to excessive input light enter the semiconductor substrate 1 due to diffusion, they cannot return to the photosensitive pixel array 3 again. Therefore, for the above-mentioned reason, the blooming suppression effect is also improved.

However, the conventional image sensor shown in FIG. 1 also has the following drawbacks. This will be explained in detail with reference to FIG. Figure 2a is a part of the cross-sectional view taken along the plane A-A' in Figure 1, Figure 2b is its impurity concentration distribution diagram, Figure 2c
is the electrostatic potential distribution diagram. The image sensor shown in FIG. 1 includes a semiconductor layer having a substantially uniform acceptor concentration N _A1 formed by epitaxial growth on a semiconductor substrate 1 having a substantially uniform donor concentration N _D1 ,
Furthermore, for example, ion implantation and thermal diffusion can be used to
It is manufactured by a step of forming an island-shaped semiconductor region ₃₁ having a distribution of donor concentration N _D2 as shown in FIG. b. Therefore, the impurity distribution becomes as shown in FIG. 2b. In Figure 2b, point x ₂ and N _D2 = N _A1
The point x ₁ becomes the pn junction surfaces 10 and 11.

In the operating state of the image sensor, the pn junctions 10 and 11 are reverse biased, and depletion layers are formed from point x ₃ to point x ₄ and from point x ₅ to point x ₆ , respectively. The region between point x ₄ and point x ₅ satisfies the charge neutrality condition. Therefore, the potential distribution is as shown in FIG. 2c. In FIG. 2c, the potential is positive toward the bottom. That is, the electric field is zero in the charge neutral region. Therefore points x ₄
Focusing on the carriers generated in the area between and point _x5 , some of them flow into the semiconductor substrate 1 due to diffusion and this carrier does not cause characteristic deterioration, but the other part accumulates in adjacent pixels due to diffusion. However, there are drawbacks such as resolution deterioration and blooming suppression effect.

The present invention has been made in view of the above circumstances, and its purpose is to provide a solid-state image sensor with improved resolution and blooming suppression effect.

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 3 shows a sectional view of an embodiment in which the present invention is applied to a charge transfer type one-dimensional image sensor.
The image sensor shown in FIG.
and a photosensitive pixel array 23 consisting of an island-shaped semiconductor region of the same conductivity type as the substrate 21 (in FIG. 3, only the photosensitive pixels ₂₃₁ of the island-shaped semiconductor region are shown).
A large number of photosensitive pixels are arranged in the vertical direction of the paper. ), a charge transfer type shift register 26 for reading out the signal charges accumulated in this array 23 (only the transfer electrode 25 provided through an insulating film 27 is shown in FIG. 3), and the array 23 a shift gate 24 for transferring signal charges accumulated in the shift register 26, and a light shield film 29 provided to cover an area other than the photosensitive pixel portion.
and a channel stop region 28 containing a high concentration of impurities of the same conductivity type as the semiconductor layer 22 provided for electrically isolating the array 23 and the shift register 26. However, since the feature of the present invention lies in the impurity concentration distribution of the semiconductor layer 22, this will be explained in more detail with reference to FIG. Figure 4a is Figure 3B
-B' plane, FIG. 4b is a diagram for explaining the impurity distribution in FIG. 4a, and FIG. 4c is a diagram for explaining the potential distribution in FIG. 4a. As is clear from FIG. 4b, the present invention is characterized in that the impurity concentration of the semiconductor layer 22 has a distribution that decreases in the direction from the surface side toward the semiconductor substrate 21 side.

Next, an example of a specific method for realizing the impurity distribution as described above will be described. For convenience of explanation, the conductivity type of the semiconductor substrate 21 will be assumed to be n-type below, but it goes without saying that it may be p-type. For example, after introducing acceptors into the main surface of a substantially uniform semiconductor substrate 21 with a donor concentration N _D3 of 10 ¹⁴ to 5×10 ¹⁵ cm ^-3 by ion implantation, the N _A2 of FIG. 4b is introduced by thermal diffusion. The distribution is as shown in .
For example, the value of N _A2 is 5×10 ¹⁴ ~ near the surface.
10 ¹⁶ cm ⁻³ , and the position of the pn junction 30 is desirably at a depth of several μm to 10-odd μm from the surface. Subsequently, donors are introduced by ion implantation, and a distribution as shown by N _D4 in FIG. 4b is obtained by thermal diffusion. N
The value of _D4 near the surface is 10 ¹⁵ to 10 ¹⁷ cm ⁻³ , and the distance to the pn junction 31 is preferably about 0.1 to 3 μm.

Next, the operation of the device shown in FIG. 3 will be explained. A zero or positive DC voltage V _BG is applied to the semiconductor substrate 21 with the semiconductor layer 22 as a reference, and the image sensor is operated in the same manner as a normal charge transfer type one-dimensional image sensor. That is, after carriers generated by incident light are accumulated as signal charges in the photosensitive pixel array 23 for a predetermined period of time, the shift gate 24 is opened, the signal charges are transferred in parallel to the charge transfer type shift register 6, and then the shift gate 24 is closed. It operates by applying a clock pulse to the shift register 26 and sequentially detecting the signal charge as a signal from the output section.

In the operating state of the image sensor shown in FIG. 3, a depletion layer is formed near the pn junctions 30 and 31. That is, as shown in FIG. 4c, the depletion regions are from point x ₉ to point x ₁₀ and from point x ₁₁ to point x ₁₂ , respectively. This potential distribution is as shown in FIG. 4c. In FIG. 4c, the positive potential increases toward the bottom. In the region between points x ₁₀ and x ₁₁ , a drift electric field is generated due to the diffusion potential due to the gradient of impurity degree. Therefore, carriers generated within this region flow out to the semiconductor substrate 21 due to this drift electric field.

Since the image sensor of the embodiment of the present invention shown in FIGS. 3 and 4 is constructed as described above, it has the following advantages compared to the conventional image sensor shown in FIGS. 1 and 2. . The first is that all carriers generated in the area on the surface side from point _x10 are accumulated in the photosensitive pixel ₂₃₁ , and all carriers generated in the area on the substrate 21 side from point _x10 flow to the substrate 21 and are transferred to adjacent pixels. The resolution is improved because the diffusion of the images is eliminated. The second is that the blooming suppression effect increases when excessive light is irradiated. For example, when signal charges accumulate to a saturation amount in the photosensitive pixel ₂₃₁ formed of an island-shaped semiconductor region in response to excessive light input, the excess carriers flow to the substrate 21 due to the drift electric field, and the carriers no longer flow into adjacent pixels. Therefore, blooming will not occur.

Although one embodiment of the present invention has been described above, it goes without saying that the present invention is not limited to this one embodiment and can be modified and changed in various ways. For example, as shown in FIGS. 5a and 5b, N _A3 and N _A4 in FIG. It is also possible to have an acceptor distribution as shown below. Increasing the concentration gradient in this way has the advantage of further improving resolution and blooming suppression effect.

Further, in the embodiment shown in FIG. 3, a photodiode is used as the photosensitive pixel, but the present invention is not limited thereto. For example, as shown in FIG. Needless to say, the electrode 33 structure may also be used. In addition, in FIGS. 5a, b and 6, the same parts as those shown in FIGS. 3 and 4 a, b are given the same reference numerals, and explanations thereof are omitted.

Furthermore, although the above embodiments have been described with respect to a charge transfer type one-dimensional image sensor, the present invention is not limited to the above-mentioned embodiments, but can be similarly applied to, for example, a two-dimensional image sensor. The signal detection part of the sensor
It goes without saying that the present invention can also be applied to circuits composed of XY address type switch circuits.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view of the main parts of a conventional solid-state image sensor, Figures 2a, b, and c are explanatory diagrams of the impurity concentration distribution and potential distribution of the solid-state image sensor shown in Figure 1, and Figure 3 is FIGS. 4a, b, and c are schematic cross-sectional views of essential parts of a solid-state image sensor according to an embodiment of the present invention; FIG. FIG. 6 is a schematic sectional view of a main part of a solid-state image sensor according to still another embodiment of the present invention. 1, 21... Semiconductor substrate, 2, 22... Semiconductor layer, 3, 23... Photosensitive pixel array, 4, 24... Shift gate, 5, 25... Transfer electrode, 6, 26...
...Charge transfer type shift register, 7, 27... Insulating film, 8, 28... Channel stop region, 9, 2
9...Light shield film, 10, 11, 30, 31...
...pn junction, 33...transparent electrode.

Claims (1)

[Scope of Claims] 1. A semiconductor substrate of one conductivity type having a substantially uniform impurity concentration; and a conductivity type opposite to that of the semiconductor substrate formed in the main surface of the semiconductor substrate, and the impurity concentration is from the surface to the semiconductor substrate. a semiconductor layer having a concentration gradient that decreases in the direction of the semiconductor substrate; an array of photosensitive pixels that generate signal charges in response to incident light incident on the semiconductor layer and accumulate the signal charges for a predetermined time; and the photosensitive layer. A solid-state image comprising: signal detection means for reading signal charges accumulated in an array of pixels; and means for applying a voltage of zero or reverse bias to the semiconductor layer to the semiconductor substrate. sensor.