JPS5888977A - Solid-state image pickup device - Google Patents

Abstract

PURPOSE:To decrease the change in residual image and spectrum characteristics with an applied voltage, by forming a part having small forbidden band width at a part of the light incident side of a photoelectric conversion film. CONSTITUTION:An incident light 11 is applied to a photoelectric conversion film 8 made of amorphous Si via a transparent potential 10. The film 8 forms a part 9 having a small fobidden band width at a part of the light incident side. Carriers formed with the light are collected on an electrode 7. In applying a reading pulse being a VCH to a gate electrode 5, since an n<-> region 3 is charged to VCH-VT (where; VT is a threshold voltage of a transistor consisting of an n<+> region 2, the region 3 and the gate electrode 5), the signal charges are transferred to th region 3. In the vertical transfer, a potential gradient is formed by applying a pulse Vf to the gate electrodes 5, 5' sequentially and the transfer is done sequentially toward the arrow 13.

Description

DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to provide a photoconductive film stacked solid-state imaging device that can obtain stable spectral characteristics required for the purpose of achieving a photoconductive film.

Solid-state imaging devices have shown rapid development along with advances in integrated circuit technology, and recently, devices with a practical number of picture elements (approximately 200,000) for single-chip color cameras have been realized. On the other hand, improvements have been made in terms of performance, and in particular, solid-state imaging devices in which a photoconductive film is logged on an St substrate with a scanning function have features such as high sensitivity and little blooming.
It is useful as a device that is comparable to an image pickup tube. Such devices include, for example, Japanese Patent Application Laid-open No. 54-103630 and Japanese Patent Application Laid-Open No. 55-39404.

In order to realize such a photoconductive film stacked solid-state imaging device, it is essential to develop a film that has an excellent photoelectric conversion function, as well as the development of an St substrate that has an excellent scanning function. In the above publication, a heterojunction film of II-Vl group compounds or an amorphous St film is used as the photoconductive film. Since the film forming process of amorphous St film is generally carried out in a low-pressure gas atmosphere, it has excellent coverage when formed on a substrate with unevenness. It had the disadvantage that the spectral characteristics changed with respect to voltage. A solid-state imaging device using only a Si substrate has extremely stable afterimages and spectral characteristics, so the above disadvantages are offset by the advantages of laminating a photoconductive film, namely increased sensitivity and reduced pluming. .

The solid-state imaging device of the present invention was developed to overcome the above-mentioned conventional drawbacks, and by forming a portion with a small forbidden band width on a part of the light incident side of the photoelectric conversion film,
The present invention provides a solid-state imaging device in which afterimages and changes in spectral characteristics due to applied voltage are significantly reduced.

FIG. 1 shows a cross-sectional view of a unit pixel of a solid-state imaging device according to an embodiment of the present invention. In this embodiment, a charge-coupled device (hereinafter referred to as COD) is used as the scanning function, but this is not limited to C0D- (Backet Brigade Device (B
Needless to say, it may be a BD (BD) or a MOS matrix type element.

In the figure, 1 is a P-type St substrate, and 2 is an n-type region forming a diode between it and the P-type Si substrate 1. 3 is n
- It is a recessed channel in the mold area. 4 is an insulating film under the read gate electrode 6. Gate electrode 6
serves both as a read operation and a transfer operation, and as described later, the read operation and the transfer operation are distinguished by the height of the pulse applied here. Reference numeral 6 denotes an insulator made of low-melting point glass or the like, and only a portion of the ♂-type region 2 is open.

7 is a collection electrode for photo-excited carriers, ¥o, Ta.

It is formed in a mosaic shape for isolation between picture elements, and is electrically connected to the ♂-type region 2. Reference numeral 8 denotes a photoelectric conversion film made of amorphous St 2 according to the present invention, in which a portion 9 with a small forbidden band width is formed in a portion on the light incident side. The portion 9 where the forbidden band width is small has a large light absorption coefficient, so not only blue light but also red light is absorbed in this portion. 10 is IT
A transparent electrode made of O (Indium Tin Oxide) and 11 are incident light. 12 is an external power supply for applying an optimum voltage to the amorphous Sis.

Next, the operation of the solid-state imaging device will be explained. Second
The figure shows a plan view of the main parts of this device. Figure 3 (,),
(b) shows the potential change of the i-type region 2 when there is a driving pulse waveform and incident light. First, carriers generated by light in the first field are collected at electrode A. When a read pulse of vcH is applied to the gate electrode 6, the i-type region 2 is charged to vCH''-vT (here, vT is the dark value of the transistor consisting of the male region 2, the i-type region 3, and the gate electrode 6). voltage), the signal charge is transferred to the i-region 3. 12 and 12' are barrier portions formed by ion implantation of phosphorus or the like in order to perform the transfer operation.For vertical transfer, the signal charges are transferred to the gate electrodes 6 and 6' sequentially. By applying the pulse Vφ, a potential gradient is formed and the voltage can be transferred sequentially in the direction shown by the arrow 13.Second
In the field, the charges collected in the i-region 2' can be read out in a similar manner.

Next, a method for manufacturing the solid-state imaging device will be described. P
P or the like is diffused into the WS i substrate 1 by thermal diffusion or the like to form an ♂ type region 2. The buried channel 3 in the square region is formed by ion implantation of P, A5, or the like. Furthermore, after forming a gate oxide film 4 on the surface of the P-type 84 substrate 1, a gate electrode 6 is formed of amorphous silicon (Poly-8T) or the like. Thereafter, the insulating layer 6 is formed, but after opening a part of the ♂-type region 2, melting 70- is performed to alleviate the step difference, so a low melting point material such as phosphosilicate glass is preferable. After forming the insulating layer 6, Al
The first electrode 7 for charge collection is formed from L, Mo, W, Ta, Cr, or the like. This first electrode is formed in a mosaic shape to perform picture element separation.

The main methods for forming the amorphous Sta are a method using glow discharge and a method using sputtering. In the glow discharge method, H2 is added to SiH4 or Si2H6 as a base. The amount of H contained in the amorphous St gold is preferably 10 to ao%. The formation of the layer 9 with a narrow forbidden band width, which is a feature of the present invention, is achieved by temporarily reducing the H2 gas flow rate or increasing the substrate temperature during the formation of the amorphous St. This can be obtained by reducing the H concentration.

The layer 9 with a narrow forbidden band width can be formed by reducing the H content, but the method is not limited to this method. For example, the forbidden band width can also be narrowed by containing Ge or the like. It is possible to achieve exactly the same effect.

When manufacturing by sputtering method, amorphous St can be obtained by sputtering using St as a target and introducing Ar and H2 as atmospheric gases, but amorphous St can be obtained by changing the mixing ratio of H2 and Ar. Since it is possible to control the H content in 8i, it is possible to create a layer with a narrow forbidden band width in a part of amorphous Si, similar to the glow discharge method. Figure 4 shows the dependence of the forbidden band width on the H content. From this figure, for example, 30
% hydrogen content layer 8 was formed to a thickness of about 1 μm, and then a 6% hydrogen content layer (layer 9 with a narrow forbidden band width) was formed to a thickness of about 0.3 μm.
Furthermore, a layer containing 30% hydrogen (amorphous 5ta) with a thickness of 0.2 μm
Once formed, amorphous St having the structure shown in FIG. 1 can be obtained.

In addition, when partially containing Ge, etc. in the sputtering method, an arbitrary amount of Go can be contained by using an alloy tough get mixed in the St target or by placing Go on a part of the St target surface. I can do it. Amorphous Si formed in this way
The element of this example can be obtained by forming a transparent electrode 1o made of indium tin oxide on O by a sputtering method or the like.

Next, the effects of the solid-state imaging device according to the embodiment of the present invention will be described. FIG. 6 shows the voltage dependence of the photocurrent of the solid-state imaging device in the embodiment of the present invention and the conventional example. In the conventional example, a layer 9 with a narrow forbidden band width is not formed in the amorphous Si 8, but the photocurrent tends to increase as the voltage increases.

Reference numerals 51 and 52 show characteristic curves in the case of the conventional example, where the characteristic curve 61 is for red light and the characteristic curve 62 is for blue light. A particular problem in the conventional example is that the rate of increase in photocurrent for red light and blue light is different. Usually, in a photoconductive film laminated solid-state imaging plate, the voltage applied to the photoconductive film has a range. This means that the spectral characteristics of amorphous St differ depending on the set voltage, and when a filter is formed on a solid-state image sensor as a single-plate color camera, color reproducibility deteriorates.

However, by providing the layer 9 with a narrow forbidden band width as in the solid-state imaging device of the embodiment of the present invention, the voltage dependence of the photocurrent becomes smaller, and the color reproducibility when used as a single-chip color camera is further improved. Ru. In FIG. 5, the characteristic curve 53.5
4 is related to an embodiment of the present invention, 63 is for red light, and 64 is for blue light.

Still another effect of the embodiments of the present invention is improvement of afterimages. Figure 6 shows the field dependence of the afterimage. The afterimage in the field is less than 6 inches in the characteristic curve 61 related to the embodiment of the present invention, but it is about 1 in the conventional case (characteristic curve 62) in which the layer 9 with a narrow bandgap width is not formed.
It is slow at 8%, 0, and residual image components remain for a long time.

The effect of forming the portion with a small forbidden band width shown above is mainly due to the large difference between the electron mobility μe and the hole mobility μh of amorphous St. That is, normally μe
is about 10-'7/V-sea and ph is about 10
-2crA/V-sea or less. Therefore, in the case of the conventional example shown in FIG. 7(a), when the narrow forbidden band width layer 9 does not exist, the excitation carrier number rear by blue light is 71.
As shown in , electrons 72 are generated near the transparent electrode 1o, and the main traveling carriers are electrons 72, but carriers excited by red light are generated near the first electrode 7 of amorphous Sts, as shown in 73. The main traveling carriers become holes 74, and the saturation characteristics of red light deteriorate.

On the other hand, in the case of the embodiment of the present invention shown in FIG. Most of the blue light is thermally absorbed, and most of the red light is also absorbed in a small part of the bandgap where the absorption coefficient is large. Therefore, carriers excited by light in the visible range, including blue light and red light, are generated in a small portion of the forbidden band, as shown in 76, and the main carriers in the film are electrons with high mobility. Therefore, electrons can easily reach the first electrode 7 even in a low electric field, improving the saturation characteristics of the photocurrent and reducing the wavelength dependence of the photocurrent.

Furthermore, since the main traveling carriers are electrons, afterimages are naturally improved.

As described above, the solid-state imaging device of the present invention is made of amorphous S with a small bandgap width formed on a part of the light incident side.
The present invention provides a solid-state imaging plate in which a photoconductive film such as T is laminated on a scanning device, and has improved color reproducibility by stabilizing afterimages and voltage dependence of spectral characteristics. Together with its high sensitivity and low pluming characteristics, its industrial significance can be said to be extremely large.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the main parts of a solid-state imaging device according to an embodiment of the present invention, FIG. 2 is a plan view of the main parts of the solid-state imaging device, and FIGS. A diagram showing drive pulses and diode potential changes, Figure 4 shows amorphous S
Figure 6 is a diagram showing the current/voltage characteristics of amorphous St in the conventional and embodiments of the present invention. The afterimage characteristic diagrams of the solid-state imaging device, FIGS. 7(a) and 7(b) are band model diagrams for explaining the operation of the conventional solid-state imaging device and the present invention, respectively. 1...P type or i substrate, 2...-n type region, 3
......n-region, 6...gate electrode, 6...
... Insulator, 7 ... Collection electrode (first electrode), 8
...Amorphous St layer, 9...Layer (portion) with a small forbidden band width, 1o...Transparent electrode. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Fig. 2 Fig. 3 WA Fig. 4
r+Pn) Fig. 5, 57 sense 11 ox (V) Fig. 6 Field ridge Fig. 7 2

Claims (1)

[Scope of Claims] (1) A solid-state imaging device comprising a semiconductor substrate having means for sequentially selectively scanning signal charges and a photoelectric conversion film laminated on the semiconductor substrate, the light incident side of the photoelectric conversion film 1. A solid-state imaging device characterized in that a forbidden band width of a part of the solid-state imaging device is smaller than a forbidden band width of another part. 2)) The solid-state imaging device according to claim 1, wherein the photoelectric conversion film is mainly made of amorphous St. (3) A solid-state imaging device according to claim 2, characterized in that a portion of the amorphous St having a small forbidden band width is formed by making the hydrogen content smaller than other portions. .