JPS58168274A - Solid state image pickup device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solid-state imaging device, and particularly to a type of solid-state imaging device in which photoconductive films are laminated.

With the spread of home video tape recorders, the demand for video cameras is rapidly increasing, but solid-state imaging devices are
Because it has many features such as small size, light weight, and low power consumption,
It is considered the favorite camera for home video tape recorders. In addition to the features of the solid-state imaging device described above, the present invention provides a solid-state imaging device with high sensitivity and good blooming resistance.

A solid-state imaging device that has a structure in which a photoconductive film is stacked on a semiconductor substrate with a signal scanning function (hereinafter referred to as a stacked solid-state imaging device) has a higher light utilization rate than a solid-state imaging device that is made only of ordinary 81 semiconductors. Because of its large size, it has high sensitivity, and since most of the incident light is absorbed by the photoconductive film, it has the characteristic that smearing phenomenon, which causes image deterioration, is less likely to occur. However, whereas the photoconductive film for a normal image pickup tube target is formed on a flat face plate, in the case of a stacked solid-state imaging device, it is formed on a semiconductor circuit board with steps. There are many restrictions on the photoconductive film.

The basic structure of the stacked solid-state imaging device will be explained below with reference to the drawings, and the drawbacks of the conventional structure will be described.

In Figure 1, a COD (charge coupled device) is used in the signal scanning circuit.
FIG. 2 is a cross-sectional view of one pixel of a stacked solid-state imaging device using the structure shown in FIG. Also, Figure 3 shows
2 is an equivalent circuit diagram of one pixel of the solid-state imaging device having the configuration shown in FIG. 1. FIG.

In these figures, an n+ type region 11 is formed on a P type 8i substrate 10 to serve as a diode region. Reference numeral 12 denotes a charge transfer stage for vertically reading out the signal charges transferred from the diode region 11, and is driven by the gate electrode 16 via the insulating layer 14. PoIJ8 with a film thickness of about 5000 people
The gate electrode 16 formed by i acts as a transfer electrode to transfer signal charges in the vertical direction, and also acts as a gate electrode to read the signal charges accumulated in the diode region 11 into the charge transfer stage 12. There is.

17 is a first electrode for collecting signal charges generated in the photoconductive film 19 by the incident light 21;
1 and is electrically connected. The first electrode 17 and the gate electrode 16 are insulated by a low melting point glass layer 16 such as P2O. No. 18 is a ZnS film with a film thickness of about 3000.
The buffer layer is made of Zn with a thickness of approximately 700A.
xCd+-xTe(In) Alleviates the step difference in the base of the formed photoconductive film 19 and provides a good base,
It also has the function of blocking injection of holes. 20 is a transparent electrode formed on the surface of the photoconductor 19, and the incident light 2
1 is incident from the transparent electrode 2o side.

In the solid-state imaging device having the above configuration, the COD is driven by two-phase driving. In this case, the largest step difference for the photoconductive film 19 is the cross section indicated by M-B in FIG.
There is a step difference of 1.0 to 1.2 μm. Further, the material of the first electrode 17 is Mo.

Metals such as Ta and W are used, but in order to prevent incident light from reaching the Si substrate and causing a smearing phenomenon, it is necessary to have sufficient light-shielding properties, and the thickness is at least 1000 Å. In order to make each picture element independent, it is necessary to separate the first electrode 17 into a mosaic pattern, and a step also occurs in the separating section 22 of FIG. These two types of steps increase the dark current of the photoconductive film 19,
Significantly lowers pressure resistance.

The thickness of the buffer layer 18 necessary for alleviating the above two steps will be described below. When 1,000 electrodes are formed in a mosaic pattern on a flat substrate, and only 1,000 steps exist, it is necessary to provide a buffer layer with a thickness of about 3,000 Å or more. Shows characteristics equivalent to . However, in an actual solid-state imaging device, in addition to the step of the first electrode, there is a difference of approximately 1. Q~ It has a level difference of 1.2 μm or more.

This step is approximately 1. It is relaxed by a glass layer with a low melting point higher than Qμ theory. At this time, the factor that affects the breakdown voltage of the photoconductive film is the maximum inclination angle.As shown in FIG. 4, when the maximum inclination angle becomes 36° or more, the breakdown voltage of the photoconductive film begins to decrease. It is desirable for solid-state imaging devices to be more compact, that is, to reduce the chip size, but the reduction in chip size does not necessarily mean that the thickness of the wiring material can be reduced, and the maximum angle of inclination of the stepped portion is This results in an increase in - As an example, in the case of a stacked solid-state imaging device having the configuration shown in FIG. 1 and a size to which an inch-long optical system is applied, the maximum inclination angle is 40 to 46 degrees. In order to prevent this inclination angle from affecting the breakdown voltage of the photoconductive film, at least about 5000 buffer layers 18 are required. When Zn5e is used as the buffer layer 18, since Zn5e has a high resistance, increasing the film thickness in this way causes a large increase in the operating voltage of the photoconductive film 19, and the withstand voltage (mainly This is not desirable for the breakdown voltage of the Si diode. In the above example, a stacked solid-state imaging device in which the signal scanning circuit is COD was described, but the signal scanning circuit is B B
D (Bucket Brigade device
) A similar problem arises in the case of a laminated monolithic imager. In addition, MOS switches are configured in a matrix, and M
A similar problem occurs in the case of a stacked solid-state imaging device having an OS address function.

Another disadvantage of using Zn5e as the buffer layer 18 is the problem of the difference between the maximum amount of signal charge generated in the photoconductive film 19 and the maximum amount of charge handled by the charge transfer stage 12.

This problem will be explained using the equivalent circuit diagram of one picture element in FIG. In FIG. 3, 31 indicates the first electrode 17 and the transparent electrode 20 (indicated by 20 squares in FIG. 3).

It is the capacitance of the photoconductive film 19 and the buffer layer 18 which are sandwiched between the gate electrode (151L), and its size is expressed by a symbol. 32 is the capacitance of the diode formed between the P-type Si substrate 10 and the n+ type region 11, and its size is expressed as Os. 33 is a capacitance that is parasitically generated between the first electrode 17 and the gate electrode 16, and its size is expressed as Op. Further, 34 is referred to as a node, and its potential is hereinafter referred to as a node potential.

Here, the node potential is equal to the potential of the first electrode 17. At this time, the charge Q crystal ax that can be accumulated per picture element is Δ
When V, it can be expressed as follows. Here, C = Cm + Cp
+ Os represents the total capacitance, whereas the maximum handling charge amount of 1 bucket + 4 buckets of the charge transfer stage 12 is Qm.
axq(x)Qmax, it causes an afterimage phenomenon and an overflow phenomenon at the transfer stage. The latter overflow phenomenon is a so-called pluming phenomenon. In the case of the solid-state imaging device in the example described above, QWi&X/Q
max > 8.

On the other hand, even when amorphous silicon is used as the photoconductive film 19, when Zn5e is used as the buffer layer 18, the above-described problems of increased operating voltage and charge lance will occur.

In addition, when the buffer layer 18 is not used, it is necessary to increase the voltage applied to the photoconductive film in order to ensure resolution, which is also undesirable for the charge balance and the breakdown voltage of the semiconductor circuit board. .

In order to prevent these phenomena, it is necessary to reduce the total capacity (-0 to 10 seconds+Cp) or lower the operating voltage of the photoconductive film 19. In particular, reducing the latter operating voltage is important in order to reduce the burden on the scanning circuit.

The present invention greatly improves the breakdown voltage problems of photoconductive films, afterimages, and blooming phenomena that have hindered the practical use of stacked solid-state imaging devices, as described above, and also reduces burn-out and resolution deterioration. An object of the present invention is to provide a solid-state imaging device in which a very small number of photoconductive films are stacked.

As mentioned above, a thick buffer layer is necessary to alleviate the level difference, but when Zn8e is used as the buffer layer as in conventional solid-state imaging devices, it increases the operating voltage of the photoconductive film. . The above problem can be solved by using a low-resistance material as the buffer layer, but at this time, it is also necessary to have a resistance higher than a certain level in order to prevent the signal charge from flowing to adjacent pixels and ensure resolution. It is.

The present inventors used Zn8 as a buffer layer for the above purpose.
Ca5e, 06B (D has been considered, but Z
nS has higher resistance than Zn5e, edge, 06
It was found that the resistance of No. 13 was too low, causing a problem in resolution. However, solid solution of ZnS and O40 or Zn
As a result of investigating the control of resistance using a solid solution of 8e and Ca5e, it was found that a very good buffer layer could be obtained.

As a measure of the resolution, the time constant τ, which is expressed as the product of the capacitance and resistance of the photoconductive film, is suitable. ZnxCd buffer layer
The photoconductive film was formed to a thickness of 5000 mm using
When 000 nyC4+-yTe(Xn) members are formed, the time constant τ becomes a formula from several hundred mB60 for X-0.5 or more, and one frame period (signal integration period: 3 s)
, :amsec), so there is no problem. However, below X=0.4, τ is several tens of myr to several m5i
r, and in this case, the buffer layer is 30oO~600o
There is no problem if there are 0 people.

Next, the solid-state imaging device of the present invention will be described based on examples. FIG. 6 is a sectional view of essential parts of a stacked solid-state imaging device according to an embodiment of the present invention in which the signal scanning circuit is a COD, except for the configuration of the imaging device and the composition and thickness of the buffer layer. are the same. Therefore, the numbers other than the buffer layer 2B are given the same numbers as in FIG. 1, and the explanation of the method of forming up to the first electrode 17 is omitted.

FIG. 6 Niote, ZnxC1+-xs as buffer layer 28
Formed on the groove 17 and PSG 16 having a thickness of 100 degrees. The formation method may be vacuum evaporation, CvD or sputtering, but in this example, Zn8 and Ca5e or Zn8 and C
dS mixed at a predetermined ratio and hot pressed X=o,
6 to 0.75 to form the buffer layer 28.

For this purpose, Znse and C, dse or ZnS and 048 may be simultaneously deposited in separate crucibles. Thereafter, ZnyCdl-yTe(In) is continuously deposited to a thickness of about 0.7 μm and subjected to vacuum heat treatment to form a photoconductive film 19. The stacked solid-state imaging device of this embodiment is constructed by forming about 1000 ITO films as the transparent conductive film 2o by sputtering.

Figure 6 and the following table compare the case where Zno, ys Cjda, and 2sse were used as the buffer layer with the case where 3000 people were using conventional Zn5e.

FIG. 6 illustrates the signal output and resolution versus the applied voltage to the photoconductive film (the potential between the first electrode 17 and the transparent electrode 20). The dependence of the signal output on the voltage applied to the photoconductive film is shown in the case of the conventional case where ZnS with a thickness of about 3000 mm is used as the buffer layer (curve M-1 in Figure 6) and in the case of this example (curve in the same figure). As is clear from the comparison with MU-2), conventionally the voltage applied to the photoconductive film was required to be 8 V or more, whereas in this embodiment, about 3 V is sufficient. Furthermore, there is almost no deterioration in resolution, as shown by curve B-2 in FIG. On the other hand, the following table is an example of a comparison between the withstand voltage (in this case, the voltage at which white spots occur) and the capacitance of the photoconductive film. By increasing the thickness of the buffer layer, the withstand voltage is greatly improved. In addition, as the operating voltage decreases, the capacitance also decreases, which is very advantageous in terms of image retention and the withstand voltage of the scanning circuit.

Furthermore, another major effect is that Zn is added to the photoconductive film.
When using yCd1-yTe(In) and using Zn8e as a buffer layer, burn-in becomes noticeable when the operating voltage is 8V or less, but when using ZnxOd+-xse or ZnxCd+-)C8 buffer layer, No burning occurs at all. The reasons for this are thought to be as follows. After the photoconductive film is deposited, the temperature is 600-550°C for 6-10
It is heat-treated for several minutes, but at this time, the luster that is the component element evaporates to some extent. As a result, when Zn8e is used as a buffer layer, many CD vacancies exist in ZnyCdl-yTe(In) and form acceptor levels. This level acts as a carrier 〇 trap. The trapped carriers form space charges and change the electric field of the photoconductive film. As a result, the carrier range μτX decreases, and the signal charge decreases in the portion where the light was incident, resulting in burn-in.

On the other hand, ZnxCA + -xse or Zn as a buffer layer
When lCl+ -xs is used, Cd in the buffer layer is Zn
It diffuses into yCd1-yTe(In) and prevents the formation of Ca vacancies. Therefore, it is considered that burn-in does not occur.

Another embodiment of the stacked solid-state imaging device of the present invention will be described again in the sixth embodiment.
This will be explained using figures.

The buffer layer 28/fi on the first electrode 17 and the insulator 16 is formed in the same manner as in the previous embodiment. The above buffer layer 28
Amorphous Si is deposited on top as a photoconductive film 19 in a thickness of about 1 μn.
Form. Thereafter, an ITO film 1000^ is formed as a transparent electrode 20 by a sputtering method. In this embodiment, amorphous Si is formed by sputtering or glow discharge. These thin film formation methods have better coverage and higher voltage resistance than vacuum evaporation methods.
The buffer layer 28 formed of Cd1-xs does not need to have any thickness in the previous embodiment, and in this embodiment, it is made of ZnXC3e.
i I −XSe! = 0.5 composition ratio, thickness 300
It is formed with 0m. FIG. 7 shows the dependence of the signal output and resolution on the voltage applied to the photoconductive film of the stacked solid-state imaging device having the above-described structure and the conventional solid-state imaging device without the buffer layer 28. Since the resistance of the buffer layer 28 is sufficiently lower than that of an amorphous layer, the dependence of the signal output on the voltage applied to the photoconductive film 1 does not depend on the presence or absence of the buffer layer 28 (Fig. ,0-2). In contrast, when there is no buffer layer 28, the resolution rapidly decreases as the voltage applied to the photoconductive film decreases as shown in curve D-1 of FIG. 7, but as shown in curve D-2. In this embodiment, as long as the signal output is saturated, there is almost no deterioration in resolution.

In the above two embodiments, the composition X of the buffer layer is decreased from the first electrode toward the photoconductive film by controlling the composition during deposition. Therefore, the resistance between adjacent first electrodes is high, which is more advantageous in terms of resolution.

Further, in the above embodiment, an example was shown in which a COD was used as the scanning circuit, but it goes without saying that the same effect can be obtained with a BBD or MOS address type.

Furthermore, similar effects can be obtained by using other photoconductive films such as 5b8s.

As explained above, the present invention relates to the improvement of a solid-state imaging device having a configuration in which a Si scanning device and a photoconductive film are combined.
This prevents the operating voltage from decreasing and the resolution from deteriorating.

Reducing the operating voltage also reduces brightening and image retention. In addition, in the photoconductive film, which conventionally suffered from a lot of burning, almost no burning occurs. Therefore, its industrial significance is extremely high.

[Brief explanation of the drawing]

Figure 1 is a sectional view of the main parts of a conventional solid-state imaging device, Figure 2 is a plan view of the main parts of the same solid-state imaging device, Figure 3 is an equivalent circuit diagram of one pixel of the same solid-state imaging device, ° Figure 4 6 is a diagram showing the relationship between the withstand voltage of the photoconductive film and the maximum inclination angle of the stepped portion in a photoconductive film stacked solid-state imaging device, FIG. FIG. 6 is a diagram comparing the characteristics of a solid-state imaging device according to an embodiment of the present invention and a conventional solid-state imaging device, and FIG. 7 is a diagram showing a comparison between a solid-state imaging device according to another embodiment of the present invention and a conventional solid-state imaging device. FIG. 10...P type S1 substrate, 11...♂ type region, 12...charge transfer stage, 14...
Insulating layer, 16...Gate electrode, 16...
・Low melting point glass layer, 17...First electrode, 18.
...Buffer layer, 19...Photoconductor, 2-0
...Transparent electrode, 21...Incoming light. Fig. 3 Fig. 4 Fig. 1 shows the angle of fire a.

Claims (1)

[Scope of Claims] (1) A semiconductor substrate having a diode region and a circuit element for scanning signal charges accumulated in the diode region, and a semiconductor substrate having an opening formed in a part of the diode region. an insulating film formed, a first electrode formed on the insulating film for each unit pixel so that a portion thereof is in contact with the diode region through the opening, and the first electrode and the insulating film. A solid-state imaging device comprising: a buffer layer formed of a compound semiconductor containing at least C6 as a component element; and a transparent electrode formed on the buffer layer with a photoconductive film interposed therebetween. (2) The buffer layer is ZnxCdl-x 5e(0(J(1)
A solid-state imaging device according to claim 1, characterized in that: (3) Buffer layer is ZnxCd+-x 8 (0<X<1)
A solid-state imaging device according to claim 1, characterized in that: (4) A solid-state imaging device according to claim 1, 2, or 3, wherein the photoconductive film is ZnyOdl-yTe (In) or amorphous silicon. (6) A solid-state imaging device according to claim 1, 2, or 3, wherein the buffer layer has a thickness of 300 μm. (6) The solid-state imaging device according to claim 2, wherein the composition ratio X of the buffer layer decreases from the first electrode toward the photoconductive film.