JPS6350912B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solid-state imaging device with high sensitivity and little blooming, and in particular to a photoconductive film-stacked solid-state imaging device that provides low afterimage and stable spectral characteristics required for a color camera. It provides:

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 laminated on a Si substrate with a scanning function have features such as high sensitivity and little blooming, making them comparable to image pickup tubes. It is useful as a device for obtaining Examples of such devices include Japanese Patent Application Laid-open Nos. 54-103630 and 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 a Si substrate that has an excellent scanning function. In the above publication, a heterojunction film of - group compounds or amorphous Si is used as a photoconductive film.
Membranes, etc. are used. Since the film forming process of amorphous Si 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. Solid-state imaging devices using only a Si substrate have extremely stable afterimages and spectral characteristics, so the drawbacks mentioned above are offset by the advantages of stacking photoconductive films, namely increased sensitivity and reduced blooming. .

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 example, a charge-coupled device (hereinafter referred to as this) is used as the scanning function.
This is explained using a CCD (called a CCD).
Backet Brigade, but not limited to
Needless to say, it may be a device (BBD) or a MOS matrix type element.

In the figure, 1 is a P-type Si substrate, and 2 is an n-type region forming a diode between it and the P-type Si substrate 1. 3 is an n ^- type region and is a buried channel. 4 is an insulating film under the read gate electrode 5. The gate electrode 5 serves both a reading operation and a transfer operation, and as described later, the reading operation and the transfer operation are distinguished by the height of the pulse applied thereto. Reference numeral 6 is an insulator made of low melting point glass or the like, and only a portion of the n ⁺ type region 2 is open.
7 is a collection electrode of photo-excited carriers made of Mo,
It is made of Ta, Al, etc., and is formed in a mosaic shape for isolation between picture elements, and is electrically connected to the n ⁺ type region 2. Reference numeral 8 denotes a photoelectric conversion film made of amorphous Si 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. Since the light absorption coefficient of the portion 9 with the small forbidden band width is large, not only blue light but also red light is absorbed in this portion. 10 is ITO (Indium
11 is incident light. 12 is an external power supply for applying an optimum voltage to the amorphous Si 8.

Next, the operation of the solid-state imaging device will be explained. FIG. 2 shows a plan view of the main parts of this device. FIGS. 3a and 3b show potential changes in the n ^- type region 2 when there is a drive pulse waveform and incident light.
First, carriers generated by light in the first field are collected at the electrode 7. to gate electrode 5
When a read pulse of V _CH is applied, the n ^- type region 2
is charged to V _CH −V _T (where V _T is n ⁺
threshold voltage of the transistor consisting of type region 2, n ^- type region 3 and gate electrode 5), the signal charge is
n ^- moved to area 3. Reference numerals 12 and 12' denote barrier portions formed by implanting ions of phosphorus or the like in order to perform a transfer operation. In the vertical transfer, a potential gradient is formed by sequentially applying pulses V to the gate electrodes 5 and 5', and the data can be transferred sequentially in the direction shown by the arrow 13. In the second field
The charges collected in the n ^-region 2' can be read out in a similar manner.

Next, a method for manufacturing the solid-state imaging device will be described. P and the like are diffused by thermal diffusion of the P-type Si substrate 1 to form an n ⁺ -type region 2 . The buried channel 3 in the n ^- type region is formed by ion implantation of P or _AS . Furthermore, after forming a gate oxide film 4 on the surface of the P-type Si substrate 1, a gate electrode 5 is formed of amorphous silicon (Poly-Si) or the like.
Thereafter, the insulating layer 6 is formed, but after opening a part of the n ⁺ type region 2, melt flow is performed to alleviate the step difference, so a low melting point material such as phosphorus silicate glass is preferable. After forming the insulating layer 6, a first electrode 7 for charge collection is formed from Al, 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 amorphous Si 8 include a method using glow discharge and a method using sputtering. In the glow discharge method, SiH ₄ or Si ₂ H ₆
Addition of _H2 , etc. is carried out based on this. The amount of H contained in amorphous Si is preferably 10 to 40%. 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 H ₂ gas flow rate or increasing the substrate temperature during the formation of the amorphous Si. This can be obtained by reducing the H concentration contained.

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 obtain the same effect. In addition, when manufacturing by sputtering method, amorphous Si can be obtained by sputtering with Si as a target and Ar and H ₂ introduced as atmospheric gases, but H ₂
It is possible to control the H content in amorphous Si by changing the mixing ratio of layers can be created. Figure 4 shows the dependence of the forbidden band width on the H content. From this figure, for example, 30% hydrogen content 8 is approximately 1 μm.
After that, a layer containing 5% hydrogen (layer 9 with a narrow forbidden band width) is formed to a thickness of approximately 0.3 μm, and a layer containing 30% hydrogen (amorphous Si 8) is further formed to a thickness of 0.2 μm, as shown in Fig. 1. Amorphous Si with a similar structure is obtained. In addition, when partially containing Ge etc. in the sputtering method, an alloy target mixed in the Si target or a part of the Si target surface is used.
By installing Ge, an arbitrary amount of Ge can be contained. The element of this example can be obtained by forming a transparent electrode 10 made of indium tin oxide on the amorphous Si thus formed 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. 5 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. 5
Reference numerals 1 and 52 indicate characteristic curves in the case of the conventional example, in which the characteristic curve 51 is for red light, and the characteristic curve 52 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 Si differ depending on the set voltage, which results in poor color reproducibility when a filter is formed on a solid-state image sensor in a single-chip color camera. 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, characteristic curves 53, 54 relate to embodiments of the invention, 53 for red light;
4 is for blue light.

Still another effect of the embodiments of the present invention is improvement of afterimages. FIG. 6 shows the field dependence of the afterimage. The afterimage in the field is 5% or less in the characteristic curve 61 related to the embodiment of the present invention, but it is slow at about 18% in the conventional case (characteristic curve 62) in which the layer 9 with a narrow bandgap width is not formed, and
Afterimage components remain for a long time.

The above-mentioned effect of forming the portion with a small forbidden band width is mainly due to the large difference between the electron mobility μ _e and the hole mobility μ _h of amorphous Si.
That is, normally μ _e is about 10 ⁻¹ cm ² /V·sec, but μ _h is about 10 ⁻² cm ² /V·sec or less. Therefore, in the case of the conventional example shown in FIG. 7a, the layer 9 with a narrow forbidden band width
When the blue light does not exist, carriers excited by the blue light are generated near the transparent electrode 10 as shown at 71, and the main traveling carriers are electrons 72, but carriers excited by the red light are generated near the transparent electrode 10 as shown at 73.
Since it is generated near the first electrode 7 of Si8, the main traveling carriers are holes 74, which deteriorates the saturation characteristics of red light.

On the other hand, in the case of the embodiment of the present invention shown in FIG. 7b, if a part with a small forbidden band width is formed on a part of the incident light side, the light absorption coefficient in that part increases, so that the incident light becomes blue. Most of the red light, as well as light, is absorbed in a small portion of the forbidden band width 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 75, and the main carriers traveling in the film are electrons with high mobility. , electrons can easily reach the first electrode 7 even in a low electric field, and the saturation characteristics of the photocurrent are improved.
The wavelength dependence of the photocurrent is also reduced. Furthermore, since the main traveling carrier is electronic, afterimages are naturally improved. In addition, as shown in FIG. 1, a layer above a portion 9 of the photoelectric conversion film 8 where the forbidden band width is small (the region indicated by a point approximately in the center of the film 8 in FIG. 1) has a relatively narrow forbidden band width than this portion 9. By providing the large part 7,
In particular, there is no surface recombination of the blue light excitation carrier that occurs when the portion 9 is provided in contact with the transparent electrode 10, and there is also the disadvantage that the optical reproducibility of blue light is degraded in order to improve the optical reproducibility of red light. It's something that doesn't exist.

As described above, the solid-state imaging device of the present invention is a solid-state imaging device in which a photoconductive film made of amorphous Si or the like is laminated on a scanning device with a small forbidden band width formed on a part of the light incident side. This product provides a solid-state imaging plate that improves color reproducibility by stabilizing afterimages and the voltage dependence of spectral characteristics.In addition to its high sensitivity and low blooming characteristics, its industrial significance is can be said to be extremely large.

[Brief explanation of the drawing]

FIG. 1 is a 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. 3a and 3b are drive pulses and diodes of the solid-state imaging device. A diagram showing potential changes, Figure 4 is a correlation diagram between hydrogen content and forbidden band width of amorphous Si, and Figure 5 is a diagram showing current/voltage characteristics of amorphous Si in conventional and embodiments of the present invention. , FIG. 6 is an afterimage characteristic diagram of solid-state imaging devices in the conventional and embodiments of the present invention, and FIGS. 7a and 7b are band model diagrams for explaining the operations of the conventional and inventive solid-state imaging devices, respectively. 1... P-type Si substrate, 2... n-type region, 3... n ^-
region, 5... gate electrode, 6... insulator, 7...
Collection electrode (first electrode), 8...Amorphous Si layer, 9...
...Layer (portion) with small forbidden band width, 10...Transparent electrode.

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 mainly made of amorphous Si laminated on the semiconductor substrate; A solid-state imaging device characterized in that a photoelectric conversion film mainly made of crystalline Si is provided with a portion on the light incident side of the film, the forbidden band width of which is smaller than the forbidden band width of other portions. 2. The solid-state imaging device according to claim 1, wherein the portion of the amorphous Si where the forbidden band width is small is formed by making the hydrogen content smaller than the other portion.