JPS6220750B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a solid-state imaging device in which a photoelectric conversion element is integrated on a semiconductor substrate.

[Background of the invention]

FIG. 1a shows the basic structure of a solid-state imaging device. 1 is a horizontal scanning circuit that selects the x position, and is generally a MOS shift register or CCD.
A type (charge transfer type) shift register is utilized.
2 is a vertical scanning circuit that selects the y position, and since it has a larger load capacity than the horizontal circuit, it is generally
A MOS shift register is used. This example shows an example of a MOS type image sensor, which is a leading carrier of solid-state image sensors, but CCD, which is another carrier, is illustrated here.
In the case of a type image sensor, a CCD shift register corresponding to a pixel column is provided as a vertical scanning circuit. The two horizontal and vertical scanning circuits sequentially scan the photoelectric conversion elements 3 containing switch elements, and output signals from the individual photoelectric conversion elements two-dimensionally arranged as video through the vertical output line 4 and the horizontal switch 5. Take it out on line 6. However, in the horizontal scanning circuit
When using a CCD shift register, the CCD register itself becomes an output line, so the above video output line 6
becomes unnecessary (not shown). Since the signal from each photoelectric conversion element corresponds to the optical image projected thereon, a video signal can be extracted by the above operation.

This type of solid-state imaging device aims to improve resolution,
In order to improve image quality, a large number of photoelectric conversion elements and scanning unit circuits are required. Therefore, they are generally manufactured using MOS-LSI technology, which has a relatively high integration density and allows the photoelectric conversion element and the switch element to be manufactured in an integrated structure as shown in FIG. 1b. 7 is a switch MOS transistor (hereinafter abbreviated as MOST) equipped with a gate 8 that is opened and closed by a vertical scanning circuit, 9 is an impurity layer for forming a photodiode, and 10 is a switch connected to the vertical output line 4.
This is the drain of MOST7. Further, 11 is a Si semiconductor substrate on which these elements are integrated, and 12 is an insulating oxide film (generally a silicon oxide film is used).

This type of solid-state image sensor is characterized by a photoelectric conversion element.
A MOS switch source can be used, and a MOS shift register or CCD can be used as a scanning circuit.
It is relatively easy to integrate, as a type shift register can be used. Therefore, the number of photodiodes that can be integrated is relatively large, but for the reasons explained below, the degree of integration of photodiodes is at most about 200 x 200, and when considering the yield,
It is limited to about 100 x 100. That is, the resolution is low and the fields of application are severely limited.

The charge generated by the incident light accumulates in the capacitance of the photodiode, but the signal-to-noise ratio (hereinafter referred to as
In order to obtain an S/N ratio (referred to as S/N ratio), a predetermined signal amount, that is, a predetermined diode capacitance is required. The diode capacitance is proportional to its occupied area; in the case of a commonly used silicon substrate with a specific resistance of 5 to 10 Ωcm, the junction capacitance is 1 pF per 100 μm square, and the predetermined signal charge amount is 0.2 to 0.5 pc (picocoulomb;
110 ^-12 coulombs), a diode of 50 to 70 μm square is required. Therefore, if we consider a 500 x 500 element with the same resolution as current television broadcasting, the overall size would be 2.5 to 3.5 cm square, which is the largest size in current large-scale integrated circuits.
Considering the size of ~6 mm square, there are of course difficulties in manufacturing, and there are extremely difficult problems in manufacturing the device, such as the need to connect the photomasks used for manufacturing.

[Purpose of the invention]

It is an object of the present invention to increase the capacity of a photodiode without increasing the diode size, and
The purpose is to prevent blooming between picture elements.

[Summary of the invention]

The present invention provides a semiconductor substrate having a higher concentration than a first conductivity type semiconductor substrate so as to contact a part of the upper surface of a second conductivity type region forming a photodiode and extend into an adjacent picture element region. 1 conductivity type region is formed.

[Examples of the Invention] Hereinafter, the present invention will be described in detail with reference to Examples.

FIG. 2 is a diagram showing the structure (a) of the photodiode of the present invention, which is a constituent unit of a solid-state image sensing device, and its planar pattern. 13 is a semiconductor substrate of a first conductivity type; 14 is a switch MOST which is opened and closed by a vertical scanning circuit; and a gate electrode 16 provided through an insulating oxide film 15 and a gate electrode 16 of a conductivity type different from that of the substrate to form a photodiode. The photodiode 17-1 is made of a second conductivity type impurity layer, and the drain 17-2 is connected to the signal output line 18. Reference numeral 19 denotes an impurity layer of the same first conductivity type as the substrate, which is provided in contact with a part of the upper surface of the second conductivity type impurity layer. Here, the impurity concentration of the impurity layer 19 is preferably selected to be at least one order of magnitude higher than that of the second conductivity type impurity layer 17-1, and specifically, it is preferably selected to be 10 ¹⁷ particles/cm ³ or more. FIG. 1B is a diagram showing a layout pattern of the photodiode section, where 20 corresponds to the second conductivity type impurity layer 17-1, and 21 corresponds to the impurity layer 19 made of the first conductivity type impurity. Here, the concentration of the second conductivity type impurity layer 17-1 is set so that the concentration of the impurity layer 19 is not reversed.
Choose a lower concentration (10 ¹⁶ to 10 ¹⁸ pieces/cm ³ ). The storage capacitance of the photodiode according to the present invention corresponds to the junction capacitance of the diode, and when the impurity concentration of the second conductivity type impurity layer is sufficiently lower than the concentration of the impurity layer 19, the junction capacitance corresponds to the junction capacitance of the diode. It increases in proportion to the 1/2 power of the impurity concentration in the impurity layer. Second
If the junction area formed between the conductive impurity layer and the substrate, that is, the area of the layout pattern 20, is S', the junction capacitance of the pattern 20 is C', and the overlapping area of the patterns 20 and 21 is S'', The storage capacitance C of the photodiode of the present invention is given by the following equation.

C=C′+S″/S′・C′・√″′ (1) In equation (1), N′ and N″ are the substrate and second
From equation (1), which is the impurity concentration of the conductive type impurity layer,
Rate of increase in storage capacity of the photodiode of the present invention C/
C′ is given by the following equation.

C/C'=1+S''/S'√'''(2) Therefore, in the photodiode of the present invention, the storage capacity of the photodiode can be increased by appropriately selecting the concentration and overlapping area of the second conductivity type impurity layer. A predetermined value can be selected. Using a substrate with the most commonly used impurity concentration N'10 ¹⁵ pieces/ ^cm3 ,
When the concentration N″ of the second conductivity type impurity layer is selected to be 10 ¹⁷ particles/cm ³ and further set to S″S′, the storage capacity of the photodiode of the present invention is 11 times that of the conventional photodiode. This will result in an increase in

Furthermore, the capacitance can be increased by increasing the concentration of the impurity layer, but since a voltage of 0 to 5 V is usually applied to a photodiode,
Requires junction breakdown voltage of approximately 5V. Due to this restriction, it is preferable to suppress the concentration of the impurity layer 19 to at most 10 ¹⁸ particles/cm ³ .

The photodiode of the present invention can be easily manufactured by adding one step for forming an impurity layer of the same type as the substrate to the manufacturing process of a conventional solid-state image sensor, as shown in FIG. First, a semiconductor substrate 13 with a first conductivity type impurity concentration of about 10 ¹⁵ impurities/cm ³ is prepared.
A silicon insulating oxide film 15-1 having a thickness of about 1 μm is formed thereon, and a portion of the oxide film corresponding to a position where a MOS switch is to be formed is removed by photoetching. On top of this is a silicon oxide film 15- for the gate.
2 is formed to a thickness of about 0.1 μm by, for example, a thermal oxidation method,
Next, polycrystalline silicon 8- with a thickness of about 0.3 to 0.5 μm
1 is formed by the CVD method (abbreviation for Chemical Vapor Deposition) (FIG. 3a).

Polycrystalline silicon is removed except for the gate region by photoetching to form a gate electrode 8-2. Using this gate electrode as a mask, the oxide film 15-2 except under the gate electrode is removed, and a second conductivity type impurity (for example, boron atoms) is implanted into the substrate by thermal diffusion or ion implantation. An impurity layer 17-1 and a drain 17-2 are formed. The impurity concentration of the second conductivity type impurity layer and the drain may be selected to be approximately 10 ¹⁷ impurities/cm ³ . At this time, the second conductivity type impurity is also injected into the polycrystalline silicon for the gate to increase the conductivity of the gate electrode.
(Figure b).

An insulating oxide film 15-3, for example, a glass film containing phosphorus atoms, is formed by a CVD method, and the impurity layer 19 is
The oxide film in the area corresponding to the area is removed by photoetching. Next, impurities of the first conductivity type (for example, phosphorus atoms) are implanted into the second conductivity type impurity layer and a part of the substrate by thermal diffusion or ion implantation to form the impurity layer 19 having a concentration of 10 ¹⁸ atoms/cm ³ .
(Figure c).

Finally, an insulating oxide film 15-4, for example a glass film containing phosphorus atoms, is deposited by CVD method, and the drain 1
The oxide film on 7-2 is removed by photoetching. Through this etching hole, impurities of the second conductivity type (for example, boron atoms) are again implanted into the drain region by thermal diffusion or ion implantation to increase the impurity concentration in the drain region to 10 ¹⁹ to 10 ²⁰ atoms/cm ³ . A conductive material (for example, aluminum) is vapor-deposited and photo-etched to form a signal output wiring 18.

FIG. 4 shows an embodiment of a structural unit of a solid-state image sensing device designed to prevent blooming according to the present invention. 22 is an insulating oxide film for electrically isolating between picture elements, for example, the LOCOS method ( Local Oxidation of Silicon
A silicon oxide film 23 manufactured using a silicon oxide film 23 is a switch MOST that is opened and closed by a vertical scanning circuit, and a gate electrode 24 provided through an insulating oxide film 22 and a conductivity type different from that of the substrate form a photodiode. It is constituted by a second conductivity type impurity layer 25-1 and a drain 25-2 connected to the signal output line 27. Reference numeral 26 denotes an impurity layer of the same first conductivity type as that of the substrate, which is provided so as to contact a part of the upper surface of the second conductivity type impurity layer and extend into the adjacent picture element region. In this structure, like the one shown in FIG. 2, the storage capacity can be increased by one order of magnitude or more compared to the conventional photodiode. Furthermore, since an impurity layer 26 having the same type as the substrate and having a higher concentration than the substrate is provided between picture element regions arranged one-dimensionally or two-dimensionally, optical signal charges generated in one photodiode are transferred to other photodiodes. It can prevent it from spreading. That is, it is possible to prevent a decrease in resolution and blooming development (a phenomenon in which the storage capacitance becomes saturated when the incident light energy is strong and excess charge flows into a nearby diode). Furthermore, by selecting an appropriate impurity concentration in the impurity layer 26 so that the depletion layer formed by the drain reaches the insulating oxide film 22, the parasitic junction capacitance in the drain region can be reduced. That is, the parasitic capacitance of the signal output line 27 can be reduced, and the photodiode of this structure has the secondary advantage that the signal output can be increased and the output response time can be reduced. In order to obtain this effect, the depletion layer created by the drain must be expanded by the impurity layer 2.
The layer thickness of 6 or more is sufficient, and the layer thickness of 26 is 0.3 to 1.0.
If μm is selected and a voltage of approximately 5V is applied to the signal output line, the impurity concentration of the impurity layer will be
10 ^{- 17} pieces/cm ³ should be selected.

In the above embodiments, a combination of a MOS field effect transistor and a photodiode was considered as the constituent unit of the image sensor, but a combination with a bipolar transistor or a junction field effect transistor may be used without departing from the spirit of the present invention. can be considered.

〔Effect of the invention〕

According to the present invention, the signal storage capacity can be increased by one order of magnitude or more compared to conventional diodes.
Therefore, the area occupied by the photodiode can be reduced to 1/10 or less without reducing the signal-to-noise ratio. That is, the chip size of the image sensor can be reduced to about 1/10, and the present invention can significantly improve the degree of integration of picture elements and the manufacturing yield. Furthermore, there is an effect that the occurrence of the blaming phenomenon can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of a conventional solid-state imaging device and the structure of a photodiode as a component, and FIG. 2 is a diagram showing the structure and planar pattern of a photodiode as a component of a solid-state imaging device of the present invention. Third
The figure shows a manufacturing process for manufacturing a photodiode having the structure shown in FIG. 2, and FIG. 4 is an embodiment of a photodiode which is a constituent unit of a solid-state image sensing device of the present invention.

Claims (1)

[Scope of Claims] 1. On the surface of a semiconductor substrate of a first conductivity type, a second conductivity type region for forming a light receiving element and a second conductivity type region for forming a readout means for reading out charges accumulated in the light receiving element. The picture element area consisting of the type area is 1
In a solid-state imaging device provided dimensionally or two-dimensionally, a portion for contacting a part of the upper surface of the second conductivity type region forming the light-receiving element and forming the readout means in an adjacent pixel region. A solid-state imaging device characterized in that a first conductivity type region having a higher concentration than the semiconductor substrate is formed so as to extend to the second conductivity type region. 2. A solid-state imaging device according to claim 1, characterized in that the concentration of the first conductivity type region is approximately 10 ¹⁷ /cm ³ or more.