JP2002151673A - Solid-state image pickup element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the difference between the horizontal resolution and vertical resolution of a back surface irradiation type solid-state image pickup element by improving the vertical resolution. SOLUTION: This back surface irradiation type solid-state image pickup element is provided with transfer electrodes 8 on the surface of a semiconductor substrate opposite to the light irradiating-side surface of the substrate. This image pickup element is constituted to collect signal charges 31 below the transfer electrodes 8 at each unit picture element by forming potential distributions 30 in the semiconductor substrate below the electrodes 8 by impressing required voltages upon the electrodes 8.

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, and more particularly to a back-illuminated solid-state imaging device in which a transfer electrode is provided on a surface of a semiconductor substrate opposite to a light irradiation side.

[0002]

2. Description of the Related Art Conventionally, for example, as a CCD solid-state imaging device, an interline transfer (IT) system, a frame interline transfer (FIT) system, or a frame transfer (F
T) type CCD solid-state imaging devices are known. These CCD solid-state imaging devices are configured such that a light receiving sensor, a transfer electrode, and the like are formed on a front surface side of a semiconductor substrate, and light is irradiated from the front surface side. However, the interline transfer method,
In the CCD solid-state imaging device of the frame interline transfer system, since the light receiving sensors are arranged in a matrix, the boundaries between pixels are clear but the aperture ratio for light reception is low. Further, in the CCD solid-state image pickup device of the frame transfer type, since the light receiving sensor and the vertical transfer register are also used, the aperture ratio is high, but light on the short wavelength side is absorbed by the transfer electrode, and the sensitivity is reduced.

On the other hand, an image pickup element including a transfer electrode is formed on one surface of a semiconductor substrate so as to increase the aperture ratio for receiving light and prevent light from being absorbed by a surface electrode such as a transfer electrode. There is known a so-called back-illuminated CCD solid-state imaging device which is capable of capturing an image by making image light incident thereon. This back-illuminated CCD solid-state imaging device
Usually, a frame transfer (FT) method is adopted.

[0004]

However, in the conventional back-illuminated solid-state imaging device, a channel stop region extending in the vertical direction is provided so as to partition the pixels in the horizontal direction. Although the boundaries between pixels in the direction (horizontal direction) are clear, the boundaries between pixels in the vertical direction (vertical direction) are not clear (in other words, since the boundaries are formed as vertical transfer registers, the pixel separation regions in the vertical direction). Cannot be formed), which causes a problem that the vertical resolution and the horizontal resolution are different.

The present invention has been made in view of the above, and has as its object to provide a back-illuminated solid-state imaging device with improved vertical resolution without deteriorating features of high aperture ratio and high sensitivity.

[0006]

A solid-state imaging device according to the present invention is a back-illuminated solid-state imaging device in which a transfer electrode is provided on a surface of a semiconductor substrate opposite to a light irradiation side. A required voltage is applied to the transfer electrode for each unit pixel to form a potential distribution in the semiconductor substrate under the transfer electrode, and signal charges are collected under the transfer electrode.

In the solid-state imaging device according to the present invention, when receiving light, a required voltage is applied to the transfer electrode of each unit pixel to form a potential distribution in the semiconductor substrate below the transfer electrode. Since signal charges are collected in a potential well having a deep potential distribution formed under the transfer electrode, the signal charges of each pixel are clearly separated in the vertical direction and accumulated under the transfer electrode of each pixel. This makes the boundaries between pixels in the vertical direction clear.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the solid-state imaging device of the present invention will be described below with reference to the drawings.

1 to 6 show an embodiment of the present invention. In this example, the frame transfer (FT) type back-illuminated C
This is a case where the present invention is applied to a CD solid-state imaging device. As shown in FIG. 1, the CCD solid-state imaging device 1 according to the present embodiment has a light receiving sensor function of performing photoelectric conversion and accumulation of signal charges and a charge transfer function on one surface of a semiconductor substrate, that is, the surface side. An imaging region 3 formed with a vertical transfer register 2 also having a CCD structure, a storage region 4 for temporarily storing signal charges in the imaging region 3, and a horizontal transfer register 5 connected to the storage region 4
And an output circuit 6 connected to the end of the horizontal transfer register 5
Having.

On the other surface of the semiconductor substrate, that is, on the back surface side,
As shown in FIG. 2, a channel stop region 7 that divides the imaging region 3 and the accumulation region 4 into a plurality of regions in the horizontal direction according to the number of pixels is formed over the imaging region 3 and the accumulation region 4. Image light is incident from the back side of the semiconductor substrate.

The imaging region 3 is divided by a channel stop region 7 in which a plurality of band-shaped vertical transfer electrodes 8 extending in the horizontal direction via an insulating film are arranged in a vertical direction on a region serving as a light receiving sensor and a transfer channel of the semiconductor substrate. And a plurality of vertical transfer registers 2 are formed. On both sides of the vertical transfer register 2 also serving as a light receiving sensor, charge sweeping means 9 for sweeping out excess charges is formed.

As in the case of the imaging region 3, the storage region 4 has a plurality of band-shaped vertical transfer electrodes 11 extending in the horizontal direction via an insulating film on a region serving as a transfer channel of the semiconductor substrate. A plurality of vertical transfer registers 12 are formed so as to be divided by the area 7. The horizontal transfer register 5 is configured by arranging a plurality of horizontal transfer electrodes (not shown) on a region serving as a transfer channel via an insulating film. Storage area 4 and horizontal transfer register 5
Is shielded from light.

The vertical transfer electrodes 8 in the imaging area 3
Transfer clock pulse, for example, four-phase, three-phase, two-phase drive
In this example, a three-phase drive transfer clock such as a transmission clock pulse
Pulse ΦV₁~ ΦV_ThreeIs applied. ΦV₁~ ΦV_ThreeBut
For three electrodes to be applied and in the channel stop region 7
The divided area is one pixel (so-called unit pixel) 10.
You. The same three-phase drive is applied to the vertical transfer electrodes 11 of the storage region 4.
Transfer clock pulse ΦM ₁~ ΦM_ThreeIs applied. water
The flat transfer register 5 has, for example, a two-phase drive transfer clock.
Pulse ΦH₁And ΦH_TwoIs applied.

FIG. 3 is a cross-sectional structure taken along the line YY in the imaging region 3 of FIG. 1, and FIG.
The sectional structures on the line are shown respectively. In this embodiment, the first conductivity type semiconductor substrate, for example, a first conductivity type region serving as a light receiving sensor and a transfer channel on one surface (front surface side) of the n ⁻ semiconductor substrate 21. A semiconductor region 22 is formed, and on the n-type semiconductor region 22 via an insulating film 23,
For example, a vertical transfer electrode 8 having a two-layer structure made of polycrystalline silicon is formed, and the vertical transfer register 2 is formed. A second conductivity type region, in this example, ap ⁻ semiconductor region 24 is formed on the other surface (back surface side) of the n ⁻ semiconductor substrate 22 (see FIG. 3).

A channel stop region 7 made of, for example, a p-type semiconductor region is formed in the p ⁻ semiconductor region 24 on the back surface side. The channel stop region 7 forms a plurality of vertical transfer registers 2 corresponding to the number of pixels in the horizontal direction. In the n-type semiconductor region 22 on the front surface side, the charge sweeping means 9 is formed at a portion corresponding to the channel stop region 7. The charge sweeping means 9 includes a p-type overflow control gate [OFCG｝ region (so-called barrier region) 26 and an n-type overflow drain [OFD].
It is formed by the region 27 (see FIG. 4).

In the present embodiment, especially when the image light L is received from the back side of the solid-state imaging device 1,
A required voltage (positive voltage since the signal charge is an electron in this example) is applied to the transfer electrode 8 to form a potential distribution 30 as shown in FIG. The signal charges 31 generated by photoelectric conversion are collected and accumulated under the transfer electrode 8 to which the voltage is applied. At this time, a part of the transfer electrodes in the unit pixels 10 adjacent in the vertical direction, in this example, 3
Of the two transfer electrodes 8, a voltage is applied to only the middle transfer electrode to deepen the potential immediately below the transfer electrode 8 and to form a potential distribution that makes the potential just below the transfer electrodes on both sides shallow. The signal charges 31 are collected in a deep potential well (see FIGS. 5 and 6).

The transfer electrode to which a required voltage is applied at the time of light reception may be any of a plurality of transfer electrodes constituting a unit pixel, provided that no voltage is applied to a transfer electrode in contact with at least one adjacent pixel. . The transfer electrode to which this voltage is not applied contributes to pixel separation.

Next, the operation of the back-illuminated solid-state imaging device 1 according to the present embodiment will be described. The image light L is incident from the back surface side on which the p ⁻ semiconductor region 24 is formed. At the time of this light reception, a required voltage is applied to a required transfer electrode of a plurality of transfer electrodes constituting each unit pixel, for example, a middle transfer electrode 8 in FIG. 5, and a deep voltage is applied only under the applied transfer electrode. Form a potential. Signal charges 31 corresponding to each pixel 10 generated by the photoelectric conversion are collected and accumulated under the corresponding voltage-applied transfer electrode 8 (FIG. 5, FIG.
See FIG. 6).

When an intense image light is received, surplus charges (electrons in this example) are discharged to the charge sweeping means 9, that is, to the overflow drain region 27 through the overflow control gate region 26.

After a predetermined light receiving period, a three-phase transfer clock pulse ΦV ₁ applied to the vertical transfer electrode 8 in the imaging region 3
～FaiV _3, the transfer clock pulses ΦM ₁ ~ΦM ₃ of three phases applied to the vertical transfer electrode 11 of the storage region 4, signal charges of each pixel, high-speed transfer (so-called frameshift from the imaging region 3 to the storage area 4 ) And is stored once.
Thereafter, the signal charges in the accumulation region 4 are transferred to the horizontal transfer register 5 line by line. Then, the signal charge is transferred in the horizontal transfer register 5, is converted into a charge voltage, and is output through the output circuit 6.

The solid-state imaging device 1 according to the embodiment described above.
According to this method, the light-receiving aperture ratio can be set to 100% or nearly 100% because of the back-illuminated type, and high sensitivity can be achieved. When light is received, a voltage is applied only to a part of the transfer electrodes 8 in the unit pixel 10 to collect signal charges in a potential well formed immediately below the transfer electrodes 8. , That is, the boundaries between pixels in the vertical direction are clear, and the vertical resolution can be improved. Since the pixels adjacent in the horizontal direction are separated by the channel stop region 7, the boundary between the pixels in the horizontal direction becomes clear.
Therefore, the vertical resolution can be improved and the difference between the horizontal resolution and the vertical resolution can be eliminated while taking advantage of the feature of increasing the sensitivity by the high aperture ratio.

In the above example, the case where electrons are used for signal charges has been described. However, the present invention can be applied to a case where holes are used as signal charges. At this time,
At the time of light reception, a negative voltage is applied to some transfer electrodes in the unit pixel. The conductivity type of the semiconductor substrate 25 is a conductivity type opposite to that shown in FIGS. As the configuration of the semiconductor substrate 25, various configurations can be employed as long as a back-illuminated solid-state imaging device can be configured.

[0023]

According to the solid-state imaging device of the present invention, the vertical resolution is improved and the difference between the horizontal resolution and the vertical resolution is improved while maintaining high sensitivity due to the high aperture ratio, which is a feature of the backside illumination type. Can be eliminated. Therefore, high-quality imaging can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a solid-state imaging device according to the present invention as viewed from the front side.

FIG. 2 is a configuration diagram showing one embodiment of a solid-state imaging device according to the present invention as viewed from the back surface side.

FIG. 3 is a cross-sectional view taken along the line YY in the imaging region of FIG. 1;

FIG. 4 is a cross-sectional view taken along line XX in the imaging region of FIG. 1;

FIG. 5 is an operation explanatory diagram of one embodiment of the solid-state imaging device of the present invention.

FIG. 6 is a plan view of a main part used for describing an operation of the embodiment of the solid-state imaging device according to the present invention;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... CCD solid-state imaging device, 2, 12 ... Vertical transfer register, 3 ... Imaging area, 4 ... Storage area, 5 ...
..Horizontal transfer register, 6 ... output circuit, 7 ... channel stop region, 8, 11 ... vertical transfer electrode, 9
... Charge sweeping means, 10 ... Unit pixel, 21.
.. n ^- semiconductor substrate, 22... N-type semiconductor region, 23
... insulating film, 24 ... p-type channel stop region,
25: semiconductor substrate, 26: overflow control gate region, overflow drain region, 3
0: potential distribution, 31: signal charge

Claims (2)

[Claims] 1. A back-illuminated solid-state imaging device having a transfer electrode provided on a surface of a semiconductor substrate opposite to a light irradiation side, wherein a required voltage is applied to the transfer electrode for each unit pixel when receiving light. A solid-state imaging device, wherein a potential distribution is formed in the semiconductor substrate below the transfer electrode to collect signal charges under the transfer electrode. 2. During light reception, a voltage different from that of another transfer electrode is applied to a required transfer electrode among a plurality of transfer electrodes constituting a unit pixel, and signal charges are collected under the required transfer electrode. The solid-state imaging device according to claim 1, wherein: