JPH0691239B2 - Solid-state image sensor - Google Patents

Description

The present invention relates to a solid-state image sensor.

[Conventional technology]

In recent years, solid-state imaging devices have tended to have higher density and higher resolution. In order to achieve such a high density, it is necessary to sufficiently secure the light receiving region of the element and prevent the sensitivity from decreasing.
Conventional solid-state imaging devices using charge-coupled devices include interline types and frame transfer types. Generally, the frame transfer type is advantageous for higher resolution. In addition to the above, there is a drawback that the device size becomes large and the yield decreases because a memory unit is required.

FIG. 2 is a plan view schematically showing a conventional frame transfer type solid-state imaging device. The signal charges photoelectrically converted by the image pickup unit 1 are transferred downward by the pulses supplied from the drive terminals 5 and 6 in the vertical blanking period, and are accumulated in the memory unit 2. This signal charge is transferred to the horizontal register 3 one by one in a horizontal row by a pulse applied to the drive terminals 6 and 7 in the horizontal blanking period, and further transferred to the left in the horizontal register 3 to output the output amplifier 4 to the left. A video signal is output from the output terminal 8 via the output terminal 8.

FIG. 3 shows a cross-sectional view of a unit element in the horizontal direction of the image pickup section 1 shown in FIG. In the figure, 11 is a semiconductor substrate of one conductivity type, for example, P type, 12 is an N type semiconductor layer forming a buried channel, and each channel is separated by a channel stopper 13. 14 is a gate oxide film, 15 is a transfer electrode,
16 is a surface oxide film. Incident light from the outside is emitted from the front surface side or the back surface of the element.

In general, polycrystalline silicon is wired as an electrode on the surface of the element, and when light is incident from the surface of the element, a part of incident light is absorbed by the polycrystalline silicon, resulting in a decrease in sensitivity. . For this reason, an element in which the silicon substrate is thinly cut to about 10 μm and light is irradiated from the back surface is advantageous.
However, it is difficult to controllably thin the silicon substrate to about 10 μm. Further, in the conventional frame transfer type element, the charge coupled element itself essentially serves as both the light receiving section and the shift register section. Therefore, in the case of continuously capturing an image for each field. Is equipped with a field memory unit for accumulating one imaged image and reading out a signal for each horizontal line.

[Problems to be solved by the invention]

In the conventional frame transfer type solid-state imaging device, in addition to the photoelectric conversion unit, the field memory unit 2 for temporarily storing the captured image is required, so that the device size increases and the yield decreases. There was a drawback to do.
Further, it is difficult to thin the silicon substrate with good controllability in an element that irradiates incident light from the back surface.

An object of the present invention is to solve such problems, to provide a solid-state image pickup device capable of forming an element having a small size and a high sensitivity and a high resolution with a high yield.

[Means for solving problems]

In the solid-state imaging device of the present invention, shift registers composed of a plurality of columns of charge-coupled devices are formed in parallel on one main surface of a semiconductor substrate of one conductivity type, and a plurality of semiconductors of conductivity types opposite to the semiconductor substrate are formed on the back surface. A region is formed, and a buried electrode whose surface is covered with an insulating film is formed so as to penetrate from the main surface to the back surface, and the shift register and the semiconductor region are horizontally separated by the buried electrode. The region constitutes a pixel which becomes a light receiving portion by being separated by a channel stop or the semiconductor substrate in the charge transfer direction, and each pixel constituted by the semiconductor region is arranged corresponding to one transfer stage of the shift register, The semiconductor region and the charge coupled device may be electrically coupled or separated by the embedded electrode.

[Work]

In the device according to the configuration of the present invention, a photodiode that controls photoelectric conversion is formed on the back surface of the semiconductor substrate, and a shift register by a charge-coupled device for transferring photoelectrically converted signal charges is formed on the substrate front surface side. The diode and the charge-coupled device are configured to be coupled by a gate electrode made of polycrystalline silicon formed by penetrating the substrate from the front surface side to the back surface side, and the photoelectric conversion unit and the shift register unit are spatially arranged. Since they are formed independently, the photoelectric conversion function and the signal charge transfer function can be performed independently. Further, since the photoelectric conversion unit, that is, the photodiode and the shift register unit, that is, the field memory unit are formed on both surfaces of the substrate, respectively, and are not formed in the same plane as in the conventional case, the element size It is possible to reduce.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of a unit element of a semiconductor chip showing a main part of one embodiment of the present invention. In this figure, a buried channel 12 is formed on the surface side of a P-type semiconductor substrate 11 as in the conventional case, and a shift register including charge-coupled devices is formed. On the other hand, a photodiode 21 made of an N-type semiconductor region is formed on the back surface side of the substrate, and a gate electrode 22 made of polycrystalline silicon is formed through the substrate 11 from the front surface side to the back surface side via an oxide film 23. There is. That is, the photodiode is the source of the MOS transistor and the buried channel 1
A transistor having the drain 2 and the gate electrode 22 as the gate is formed inside the substrate.

The gate electrode 22 also plays the role of element isolation between the pixels of the photodiode 21 or between the channels of the charge coupled device. The thickness of the semiconductor substrate 11 is about several tens of μm, and incident light is emitted from the back surface of the device.

Incident light is photoelectrically converted by the photodiode 21 and accumulated. When a read pulse voltage is applied to the gate electrode 22 after a certain period of time, the signal charges are transferred from the photodiode along the gate electrode 22 to the buried channel 12. By this operation, the photodiode 21 is set to the reference potential, and photoelectric conversion and signal charge accumulation are started again. on the other hand,
The charges transferred to the buried channel 12 are transferred and output in the channel in synchronization with the pulse applied to the transfer gate electrode 22. During this transfer period, photoelectric conversion is performed in the photodiode 21 and the signal charge of the next field is accumulated. That is, the shift register operation in the buried channel 12 and the photoelectric conversion operation in the photodiode 21 are
It will be done independently in different places. Furthermore, since the photodiode 21 and the shift register including the buried channel 12 are formed on both surfaces of the substrate 11, the element size does not increase.

Further, in the device of this embodiment, light is incident from the back surface and is accumulated in the photodiode 21, but in order to efficiently transfer the signal charge from the photodiode 21 to the buried channel 12, It is necessary to reduce the length of the electrode 22, that is, the thickness of the substrate 11 to several μm to several tens of μm. In this element, when the gate electrode 22 is formed, a groove of several μm to several tens of μm is formed in the silicon substrate 11,
Predetermined oxidation is performed to form an oxide film 23, and then polycrystal silicon is embedded in this groove, and the gate electrode 22 is made to have conductivity. In such an element structure, when the substrate silicon is etched from the substrate rear surface, the substrate rear surface portion of the oxide film 23 covering the gate electrode 22 serves as an etching stopper action for the substrate silicon, and the thickness of the substrate can be accurately controlled. .

〔The invention's effect〕

As described above, according to the present invention, since the photodiode can be formed on the back surface of the substrate and the shift register can be formed on the front surface side of the substrate, the element size can be reduced.
There is an effect that a device having extremely high sensitivity and high resolution can be formed with high yield.

[Brief description of drawings]

FIG. 1 is a sectional view of a unit device of a main part of an embodiment of the present invention,
2 and 3 are a schematic plan view of a conventional frame transfer type solid-state image pickup device and a sectional view of a unit device of the image pickup section. 1 ... Imaging unit, 2 ... Memory unit, 3 ... Horizontal register,
4 ... Output amplifier, 5,6,7 ... Drive terminal, 8 ... Output terminal, 11 ... P-type semiconductor substrate, 12,12 '... N-type semiconductor layer, 13,13' ... Channel stopper, 14 …… Gate oxide film, 15 …… Transfer electrode, 16 …… Surface oxide film, 21,21 ′ ……
Photodiode, 22,22 '... Gate electrode, 23,23' ...
…Oxide film.

Claims (1)

[Claims] 1. A shift register composed of a plurality of columns of charge-coupled devices is formed in parallel on one main surface of a semiconductor substrate of one conductivity type, and a plurality of semiconductor regions of opposite conductivity type to the semiconductor substrate are formed on the back surface. And a buried electrode whose surface is covered with an insulating film is formed so as to penetrate from the main surface to the back surface,
The shift register and the semiconductor region are horizontally separated by the embedded electrode, and the semiconductor region is separated by a channel stop in the charge transfer direction or by the semiconductor substrate to form a pixel serving as a light receiving portion. Each pixel constituted by is arranged corresponding to one transfer stage of the shift register, and the semiconductor region and the charge coupled device are electrically coupled or separated by the embedded electrode. Image sensor.