JP3178148B2 - Image sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor,
In particular, the present invention relates to reduction of an afterimage of a monitor output for monitoring the luminance of a subject.

[0002]

2. Description of the Related Art In recent years, an image sensor for converting a subject image into an image signal has been used as a sensor for autofocus of a camera. By the way, in order to perform focusing accurately, regardless of the brightness of the subject,
It is necessary to obtain an image signal of a certain size. That is, when the brightness of the subject is low, the integration time of the photoelectric conversion needs to be increased, and when the brightness of the subject is high, the integration time of the photoelectric conversion needs to be short so that the image signal is not saturated. Was. Therefore, a monitor pixel for monitoring the luminance of the subject is provided on the chip.

Hereinafter, a conventional monitor pixel will be described with reference to the drawings.

FIG. 5 is a block diagram of a conventional image sensor. The signal charge generated according to the amount of light incident on the photosensitive pixel 13 is stored in the gate F of the storage unit 12 to which the constant voltage V _ST is applied. At this time, ΦTG is at a low level. Next, after a predetermined accumulation time, ΦTG is set to a high level to turn on the transfer gate 11, and the signal charge flows into the transfer unit 10 and is sequentially transferred by Φ1 and Φ2.

[0005] A monitor pixel 34 is provided to monitor the amount of light incident on the photosensitive pixel 13, that is, the luminance of the subject. Further, an output unit 100 for converting the signal charge generated in the monitor pixel 34 into an output signal includes the gate 15, the charge-voltage converter 16, the reset gate 17, and the reset drain 18.

FIGS. 6A and 6B are a sectional view and a potential diagram taken along the line CC 'of FIG.

A P-type diffusion layer 2 is formed on the surface of an N-type semiconductor substrate 1, and an N-type diffusion layer 4 for forming a monitor pixel and a charge-to-voltage converter is formed to suppress generation of dark current from the substrate surface. ing.

[0008] The signal charges generated in the monitor pixels 34 in response to the incident light flow into the charge-to-voltage converter 16 through the lower portion of the gate 15 to which the constant voltage _VBA is applied. A change in the potential of the charge-voltage converter 16 corresponding to the amount of charge that has flowed in is extracted from the output terminal M _OUT through the output buffer 19.

[0009]

In an image sensor, a monitor pixel generally has a size of about 200 μm in order to monitor the amount of incident light in a related pixel area of about 100 pixels. Therefore, the signal charges generated at both ends of the monitor pixel move a distance of about 100 μm.

In this case, in the conventional image sensor, since there is no potential gradient in the monitor pixel, the signal charge usually moves by thermal diffusion, so that the time required for the signal charge to flow into the output unit 100 becomes longer, and the cause of the afterimage is increased. Had become.

[0011]

According to the image sensor of the present invention, the pixel width of the monitor pixel increases toward the output portion.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.
The description of the same components as in the related art is omitted.

FIG. 1 is a block diagram of one embodiment of the present invention. A monitor pixel 14 for monitoring the amount of light incident on the photosensitive pixel 13, that is, the luminance of the subject is provided. The monitor pixel 14 of the present embodiment moves toward both ends.
The pixel width is narrow. Other than this monitor pixel, the configuration is the same as that of the conventional one.

FIGS. 2A and 2B are a sectional view and a potential diagram taken along line AA 'in FIG. A P-type diffusion layer 2 is formed on an N-type semiconductor substrate 1, and an N-type diffusion layer 3 for forming a monitor pixel and a charge-voltage converter is formed. A P ⁺ -type diffusion layer 4 is formed on the surface of the monitor pixel 14 to suppress generation of dark current from the substrate surface.

According to the amount of incident light, the signal charge generated in the monitor pixel 14 is converted into a gate 15 to which a constant voltage ^VBA is applied.
It flows into the charge-voltage converter 16 through below. A change in the potential of the charge-voltage converter 16 corresponding to the amount of charge that has flowed in is extracted from the output terminal M _OUT through the output buffer 19.

At this time, since the width of the pixel becomes narrower toward the point A from the point a-2 toward the point A, the potential is gradually reduced due to the so-called narrow channel effect. Since the narrow channel effect appears remarkably when the pixel width is 6 μm or less, a potential gradient is formed on the point A side from the point a-1 at which the pixel width becomes 6 μm. In this portion, the signal charges move by thermal diffusion and drift, so that the movement is faster and the afterimage can be reduced, unlike the conventional movement of only thermal diffusion.

Next, a second embodiment of the present invention will be described with reference to the drawings.

FIG. 3 is a block diagram of a second embodiment of the present invention. For example, the monitor pixel 24 of the present invention branches toward both ends, such as branching into two near the point b-2 and further branching into two near the point b-1.
The pixel width is narrowed.

FIGS. 4A and 4B are a sectional view and a potential diagram taken along the line BB 'of FIG. The width of the pixel is wider between points b-1 and b-2 than between point b-1 and the end of the monitor pixel. Further, the distance from the point b-2 to the output unit side is wider than that between the point b-1 and the point b-2. As described above, the narrow channel effect is remarkably exhibited at a pixel width of 9 μm or less. Therefore, for example, the pixel width is 3 μm and B-1 between the end of the monitor pixel and the point b-1.
If the pixel width between the point and the point b-2 is 6 μm, and the pixel width from the point b-2 to the output side is 12 μm,
A step is formed in the potential from the end of the monitor pixel toward the output section, and the movement of the potential gradient signal charge at the step is increased, and the afterimage can be reduced.

In this embodiment, since both ends of the pixel are not narrowed, a constant sensitivity can be maintained over the entire monitor area.

[0021]

As described above, according to the present invention, by increasing the width of the monitor pixel from the end of the monitor pixel toward the output portion, the potential in the monitor pixel is graded,
This has the effect of increasing the speed of the movement of the signal charge and reducing the afterimage.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is a sectional view and a potential diagram taken along line AA 'shown in FIG.

FIG. 3 is a block diagram of a second embodiment of the present invention.

FIG. 4 is a sectional view and a potential diagram taken along line BB ′ shown in FIG. 3;

FIG. 5 is a block diagram of a conventional example.

FIG. 6 is a cross-sectional view taken along line CC ′ and a potential diagram shown in FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Transfer part 11 Transfer gate 12 Storage part 13 Photosensitive pixel 14, 24, 34 Monitor pixel 15 Gate 16 Electrode voltage conversion part 17 Reset gate 18 Reset drain 19 Output buffer 100 Output part

Claims (1)

(57) [Claims] 1. A plurality of photosensitive pixels arranged to generate signal charges according to the amount of incident light from a subject, monitor pixels for monitoring the luminance of the subject, and signal charge amounts generated by the monitor pixels the output signal have a output unit for generating in response to the monitor pixels, the photosensitive image
It is elongated in the element array direction and extends over all photosensitive pixels
In the image sensor having a size larger than the photosensitive pixel , a pixel width of the monitor pixel increases toward the output unit.