JPS605105B2 - solid-state imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a solid-state imaging device comprising a photosensitive element manufactured on the same semiconductor single crystal substrate, and a switch and a scanning circuit for selectively reading out the output from the photosensitive element in time-sequential manner. It is related to.

A solid-state imaging device must have a resolution comparable to that of the imaging tube used in current television broadcasting, and for this reason, a 500 x 50 solid photoelectric conversion element and an equivalent (x, y) coordinate selection device are required. switches, and 500 stages of x-scan and y-scan circuits to open and close the switches. Therefore, they are typically made using silicon MOS integrated circuit technology, which has a high integration density. FIG. 1 shows a cross-sectional structure of a conventional image sensor using a photoconductive substance as a photosensitive element.

Here, 1 is a semiconductor substrate of the first conductivity type (for example, n-type),
2 is a MOS field effect transistor for switching (hereinafter referred to as MO
(abbreviated as ST). 3 is a gate electrode provided through an oxide film 4, and 5 and 6 are second conductivity type (for example, p type)
5 is hereinafter referred to as a drain and 6 as a source.

7 is a planar electrode in resistive contact with the drain 5, 8 is a photoconductive thin film, and 9 is a transparent electrode to which a target voltage 10 is applied.

The operation of this device will be briefly explained. The photoconductive thin film 8 changes its conductivity depending on the incident light, so that the planar electrode 7
A voltage ΔV corresponding to the incident light is supplied from the target voltage. When the switch unit 2 is turned on by the selection pulse output from the scanning circuit, the voltage ΔV appearing at the electrode 7, that is, the drain 5, is read out from the source 6 as a video signal.
By the way, photoconductive thin films have a high voltage value, usually 2.
It is 0-30V.

Therefore, it is necessary to apply a voltage higher than the threshold voltage to the thin film, and generally a voltage of 80V to 150V is applied. The changed voltage △V increases as the amount of incident light increases.
The target voltage VT approaches or becomes equal to the difference between the target voltage VT and the threshold voltage Vth. For example, if VT is 100 V and Vth is 30 V, ΔV becomes 70 V, and this voltage appears at the drain 5. On the other hand, the breakdown voltage of a MOST that can be manufactured using a normal MOS process is 20V, or at most 30V, so when the amount of light is large, voltage breakdown occurs at the drain junction, making it impossible to extract optical signals. Also, the changing voltage △V
increases in linear proportion to the amount of light, so if we try to keep the voltage ΔV low from the viewpoint of preventing voltage breakdown, it is necessary to suppress the upper limit of the amount of incident light, which narrows the range of the amount of light that can be detected. An object of the present invention is to improve the breakdown voltage of the drain junction to which high voltage is applied in the solid-state imaging device described above. The present invention will be explained in detail below using examples.

FIG. 2 shows a cross-sectional structure of a photosensitive element constituting the solid-state image sensor of the present invention. Reference numeral 11 represents an n-type semiconductor substrate, and reference numeral 12 represents a switch MOST section.

1 3 is a gate electrode provided through an oxide film 14;
Reference numerals 5 and 16 denote sources and drains formed of highly concentrated P-type impurity layers.

Reference numeral 17 denotes a resistive P-type impurity layer formed to surround the drain in the plane and depth direction.

18 is a planar electrode in contact with the drain 16, 19 is a photoconductive thin film, and 20 is a transparent electrode for applying a target voltage 21.

In general, the breakdown voltage of a PN (or NP) junction can be considered to be proportional to the sum of the widths of depletion layers generated by applied voltages in the P region near the junction, and therefore, the breakdown voltage increases as the impurity concentration decreases. The concentration of the low concentration P-type impurity layer 17 may be the same as or lower than that of the substrate, and is preferably selected from 1 to 4 to 1 to 6 (/ground) depending on the required breakdown voltage. Also, the depth of the low concentration P-type impurity layer is MO
It needs to be smaller than the channel length of ST, for example,
The channel length is loAm, and the depth of the high concentration impurity layer is 1 French m.
In this case, it is best to choose between 2 and 5 meters.

The impurity concentration is preferably about 1 to 1 to 1 to 6. As a result, the breakdown voltage can be increased to about 60 V at a concentration of 1 to 6 particles/plane, and to about 150 V at a concentration of 1 particle/plane. Therefore, not only is the operating voltage and reliability higher, but the upper limit of the amount of incident light is 4 to 4.
It can be expanded by 6 times, making it possible to use it for various applications where the contrast of incident light is large. The above manufacturing method only requires adding a P diffusion step to the conventional manufacturing process, and can be easily realized without sacrificing the integration density of the bran image elements.

Next, other embodiments will be described. FIG. 3 shows a cross-sectional structure of an image sensor having an offset gate structure MOST. 22 is an n-type semiconductor substrate, 23 is a switch MOS
This is the T section.

24 is a gate electrode provided through an oxide film 25, and 26
, 27 are sources and drains formed of highly concentrated P-type impurity layers.

28 is a low concentration P-type impurity layer that contacts a part of the drain 27 and extends to the left end of the gate electrode 24.

29 is a planar electrode in resistive contact with the drain 27; three electrodes are photoconductive thin films; 31 is a transparent electrode for applying a target voltage 32;

In this structure, due to the voltage drop caused by the resistive diffusion layer 2M, the voltage applied between the right end of the diffusion layer 28 and the substrate 22 is reduced, and as a result, voltage breakdown is less likely to occur.
However, if the resistance of the diffusion layer 28 becomes larger than necessary, the conductance decreases, so generally the diffusion layer 28
8 was formed by using ion implantation to obtain a breakdown voltage of 200 V when the length of the impurity layer was 5 m and the concentration was about 1 × 1 and 1/2. I can do that. (The driving depth is usually l.
At this time, the impurity density of the impurity layer is about 1 to 5/. ) In order to manufacture this device, it is only necessary to add a step of forming a low concentration P-type impurity layer to the conventional manufacturing process, and an image sensor with high breakdown voltage can be obtained. Therefore, it is possible to widen the operating voltage margin of the element and the range of the amount of detected light, and to improve the reliability, and the practical effects of the present invention are very large. In the above embodiment, a MOST was used as the switch connected to the photosensitive element, but it is of course possible to use a junction field effect transistor or a bipolar transistor without departing from the spirit of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a conventional solid-state image sensor, FIG. 2 is a cross-sectional view of a solid-state image sensor according to the present invention, and FIG. 3 is a cross-sectional view of a solid-state image sensor different from FIG. 2 according to the present invention. . Tsutomu 1 illustration Younger brother Z illustration Shigeru 3 illustration

Claims (1)

[Claims] 1. In a solid-state imaging device, in which an electrode pattern including MOS field effect transistors for switches that are opened and closed by a scanning circuit is arranged on a first conductivity type semiconductor substrate in a two-dimensional manner, and a photoconductive film is provided on the electrode, MOS for switch
A solid-state imaging device characterized in that a second conductivity type high concentration impurity layer in resistive contact with the electrode pattern of a field effect transistor is surrounded by a second conductivity type impurity layer having a low concentration.