JPH06261183A - Image sensor - Google Patents

Abstract

PURPOSE:To incline potential in a monitor picture element, to accelerate the movement of a signal charge, and to reduce an after image by widening the width of the monitor picture element advancing from the terminal of the monitor picture element to an output part. CONSTITUTION:The monitor picture element 24 is branched in turn advancing to both terminals, for example, so as to be branched to two parts in the neighborhood of a point b-2, and furthermore, to two parts in the neighborhood of a point b-1, respectively, and to narrow picture element width. The width of the picture element on a part between the terminal of the monitor picture element 24 and the point b-1 is formed larger than that on a part between the point b-1 and the point b-2. Furthermore, the width on a part between the point b-2 and the output part side is formed larger than that on a part between the point b-1 and the point b-2. Therefore, assuming the picture element width on a part between the terminal of the monitor picture element and the point b-1 as 3mum, the one on a part between the point b-1 and the point b-2 as 6mum, and the one on a part between the point b-2 and the output part side as 12mum, steps can be formed in the potential advancing from the terminal of the monitor picture element to the output part side, and the movement of a gradient signal charge can be accelerated.

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, it relates to reduction of afterimage of monitor output for monitoring subject brightness.

[0002]

2. Description of the Related Art An image sensor for converting a subject image into an image signal has recently been used as a sensor for autofocus of a camera. By the way, in order to focus accurately, regardless of the brightness of the subject,
It is necessary to obtain an image signal of a constant size. That is, when the brightness of the subject is low, it is necessary to lengthen the integration time of the photoelectric conversion, and conversely, when the brightness of the subject is high, it is necessary to shorten the integration time of the photoelectric conversion so that the image signal is not saturated. It was Therefore, a monitor pixel for monitoring the subject brightness is provided on the chip.

A conventional monitor pixel will be described below 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 low level. Next, after a predetermined accumulation time, the transfer gate 11 is turned on by setting ΦTG to the high level, the signal charges flow into the transfer unit 10, and are sequentially transferred by Φ1 and Φ2.

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

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

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

The signal charge generated in the monitor pixel 34 according to the incident light flows under the gate 15 to which the constant voltage V _BA is applied and flows into the charge-voltage converter 16. The potential change of the charge-voltage converter 16 corresponding to the amount of inflowing charges is taken out from the output terminal M _OUT through the output buffer 19.

[0009]

In the image sensor, the monitor pixel has a size of about 200 .mu.m in order to monitor the incident light amount in the related pixel area, which is usually about 100 pixels. Therefore, the signal charge generated at both ends of the monitor pixel moves over 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 charges are usually moved by thermal diffusion, so that it takes a long time to flow into the output section 100, which causes a residual image. It was.

[0011]

In the image sensor of the present invention, the pixel width of the monitor pixel becomes wider toward the output section.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.
Note that the description of the same configuration as the conventional one is omitted.

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

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

Depending on the amount of incident light, the signal charge generated in the monitor pixel 14 is applied to the gate 15 to which a constant voltage V ^BA is applied.
It flows under and flows into the charge-voltage converter 16. The potential change of the charge-voltage converter 16 corresponding to the amount of inflowing charges is taken out from the output terminal M _OUT through the output buffer 19.

At this time, since the width of the pixel on the A point side from the a-2 point becomes narrower toward the A point side, the potential gradually becomes shallower due to the so-called narrow channel effect. The narrow channel effect remarkably appears when the pixel width is 6 μm or less, and therefore the potential has a gradient on the point A side from the point a-1 where the pixel width is 6 μm. In this portion, the signal charges move by thermal diffusion + drift, so that the movement is faster and the afterimage can be reduced unlike the conventional movement only by 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 the second embodiment of the present invention. The monitor pixel 24 of the present invention is branched into two in the vicinity of the point b-2, and is further branched into two in the vicinity of the point b-1.
The pixel width is narrowed.

4 (a) and 4 (b) are a sectional view and a potential diagram taken along the line BB 'in FIG. The width of the pixel is wider in the modulus from the point b-1 to the point b-2 than from the edge from the edge of the monitor pixel to the point b-1. Further, the area from the point b-2 to the output side is wider than the area from the point b-1 to the point b-2. As described above, the narrow channel effect is prominently exhibited when the pixel width is 9 μm or less. Therefore, for example, the pixel width is 3 μm and B-1 is from the edge of the monitor pixel to the point b-1.
If the pixel width from the point to 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 portion side, and the potential gradient signal charge moves faster at this step portion, and 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, the present invention widens the width of the monitor pixel from the end of the monitor pixel toward the output portion to give a gradient to the potential in the monitor pixel,
This has the effect of accelerating the movement speed of the signal charges and reducing the afterimage.

[Brief description of drawings]

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

FIG. 2 is a sectional view taken along the line AA ′ shown in FIG. 1 and a potential diagram.

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 the line BB ′ shown in FIG.

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

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

[Explanation of symbols]

 10 Transfer Section 11 Transfer Gate 12 Storage Section 13 Photosensitive Pixel 14, 24, 34 Monitor Pixel 15 Gate 16 Electrode Voltage Converter 17 Reset Gate 18 Reset Drain 19 Output Buffer 100 Output Section

Claims (1)

[Claims] 1. A photosensitive pixel that generates a signal charge according to the amount of incident light from a subject, a monitor pixel for monitoring the brightness of the subject, and an output signal according to the amount of signal charge generated in the monitor pixel. An image sensor having an output section for generating the image sensor, wherein the pixel width of the monitor pixel becomes wider toward the output section.