JPS6211792B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image sensor using amorphous silicon.

In order to popularize facsimile machines in homes, it is desired that the devices be made smaller. The biggest obstacle to miniaturizing such devices is the size of the photoelectric conversion system.

Conventional facsimile photoelectric conversion devices (hereinafter referred to as image sensors) are MOS type or
Semiconductor ICs such as CCD type have been used. However, the chip size of this semiconductor image sensor is small, a few millimeters square, and in order to convert, for example, a 20 cm wide A4 document into an electrical signal, an optical system is required to reduce the document image to a few millimeters wide. , securing the optical path length (50 to 60 cm in A4 size) has been considered the key to miniaturizing the device.

An image sensor called a contact image sensor is attracting attention as a solution to this problem.
This is a large image sensor with a photoelectric conversion area that is the same size as the document width, and is used in close contact with the document, so there is no need to use a lens optical system to reduce the document image, so the device size is significantly reduced. Miniaturization is achieved. The photoelectric conversion material used in such a contact type image sensor must have excellent photoelectric conversion characteristics and be able to be formed uniformly over a large area. As such a material, arsenic
Products using selenium-tellurium amorphous semiconductors, cadmium sulfide, cadmium selenide, etc. are currently under research and development, but these materials contain pollutants such as arsenic, cadmium, and selenium, and have poor thermal stability. There were some problems, such as poor performance.

On the other hand, amorphous silicon, which has recently been attracting attention as a material for low-cost solar cells, is an excellent photoconductive material, and research has already begun on its application to not only solar cells but also to vidicons for electrophotography and imaging. This amorphous silicon is usually formed by glow discharge decomposition of monosilane or reactive sputtering of silicon;
It has the advantage of being able to easily obtain a uniform thin film over a large area, and its constituent materials are non-polluting silicon and hydrogen. When amorphous silicon is applied to a contact image sensor, the dark resistivity of amorphous silicon is relatively high at 10 ⁹ to ^{10 12} Ω-cm, and the photoconductivity is compared to cadmium sulfide. Because it is small compared to etc., it should be operated as an accumulation type. This accumulation-type operation uses a method in which charges held on the sensor surface are erased by charges generated by light during the scanning time.
Therefore, when it is dark, it is necessary to maintain the charge applied to the sensor surface during the scanning time. Therefore
In order to use amorphous silicon, which has a resistivity of 10 ⁹ to 10 ¹² Ω-cm, as an image sensor, it is necessary to create a blocking structure that prevents charge injection from the electrodes in order to further increase the apparent resistivity. desirable. To prevent charge injection from this electrode, P-type amorphous silicon is used as a blocking layer for electrons, and P-type amorphous silicon is used as a blocking layer for holes.
A transparent dielectric film such as Si ₃ N ₄ or SiO ₂ is used. However, the thickness of this transparent dielectric film is approximately 200 mm due to the need to transport carriers generated by light to the electrodes.
It must be made extremely thin, about 100 Å thick. For this reason, multiple image sensors (for A4 size documents)
A transparent dielectric blocking layer of approximately 200 Å is placed on a substrate (actually used in the arsenic-selenium-tellurium-based amorphous semiconductor image sensor mentioned above) with electrodes containing 1,700 islands of transparent conductive films. When an amorphous silicon sensor is formed, the blocking effect becomes incomplete due to the step difference in the electrodes (the step height including the thickness of the transparent conductive film and the thickness of the extraction electrode is 1500 Å or more), resulting in poor charge retention characteristics in the dark. Sometimes things got worse.

FIG. 1 is a schematic diagram when an amorphous silicon image sensor is formed on a conventional substrate, and the incompleteness of the blocking effect will be explained based on this diagram. In FIG. 1, tin oxide or indium tin oxide is formed to a thickness of 500 Å as a transparent conductive film 12 on a transparent substrate 11, chromium is formed to a thickness of 1000 Å as a common optical layer and an extraction electrode 13, and gold is formed as a contact pad 14 to a thickness of 1 μm. A sensor substrate is formed by etching the electrode group. The dimensions of this island-shaped electrode are determined by the sensor resolution, but a sensor of 8 dots/mm usually has a width of 100 μm and a gap between the island-shaped electrodes of 25 μm. On the substrate with such an island-like electrode group, a hole blocking layer 15 is made of high resistance amorphous silicon 16 doped with 200 Å of Si ₃ N ₄ and 20 ppm of boron, and a P-type layer doped with 250 ppm of boron. An amorphous silicon layer 17 is formed to a thickness of 0.3 μm. This amorphous silicon and Si ₃ N ₄ are formed by a glow discharge decomposition method in a device that maintains silane or a gas such as nitrogen or ammonia at a vacuum of 0.05 to 5 Torr, but it can also be formed by a silicon reactive sputtering method. Formable. Furthermore, it has been confirmed that the high-resistance amorphous silicon layer can function as a sensor even when an amorphous silicon layer doped with oxygen or oxygen and boron is used. In this manner, aluminum is finally formed as a common electrode 18 with a length of 20 cm and a thickness of 0.5 μm to form a contact type image sensor. At this time, the electrode level difference that reduces the blocking effect mainly exists in the thin Si ₃ N ₄ layer 15, and as is clear from the figure, the thickness is the same as that of the transparent conductive film 12 and the common optical film 13. The total thickness is 1500 Å. In addition _{, this step occurs between each individual electrode, and the gap between these electrodes is extremely narrow at 25 μm, making it difficult for glow discharge when forming Si 3 N 4 to penetrate} due to the shielding effect of the electrodes _. hard to adhere to.

The present invention aims to reduce the imperfection of such blocking effect in an amorphous silicon-contact image sensor and improve charge retention characteristics in the dark.

According to the present invention, it is possible to obtain an image sensor in which the influence of the electrode step is reduced by reducing the step of the electrode and shortening the effective length of the step portion. The structure of the sensor is a lower transparent dielectric film and 10 ¹⁰
High resistance amorphous silicon layer of Ω-cm or more, 10 ⁶ Ω-
P-type amorphous silicon with a resistance of 10 cm or more and 10 ¹¹ Ω-cm or less and a group of separated metal electrodes are formed on a transparent conductive film formed into a long and thin island shape (approximately 22 cm for an A4 size sensor). They are characterized in that they are sequentially formed on a substrate with metal electrodes formed by opening windows in the form of stripes.

Next, the present invention will be explained by examples.

However, in the embodiment of the present invention shown in FIG. 2, a chromium general film 22 with a width of 3 mm, a length of 20 cm, and a thickness of 1000 Å is formed by opening a window of 100 μm in width on a glass substrate 21 and a chromium film 22 with a width of 2 mm.
A transparent conductive film (for example, tin oxide) 23 with a length of 20 cm and a thickness of 500 Å is attached to form a sensor substrate. Approximately Si ₃ N ₄ is deposited on this substrate as a hole blocking layer 24.
200 Å, high resistance amorphous silicon layer 25 of 2.5 μm,
P-type amorphous silicon layers 26 are sequentially formed to a thickness of 0.3 μm, and finally aluminum is formed as individual electrodes 27.
The film is deposited to a thickness of 0.5 μm and etched to a width of 100 μm to form an image sensor. In the image sensor formed in this way, the film thickness is thin.
The steps formed in the Si ₃ N ₄ blocking layer 24 are 500 Å and 1000 Å, respectively, which are smaller than the 1500 Å shown in FIG. Moreover, since there is no step groove between each individual electrode as shown in FIG. 1, the blocking layer 24 has the advantage of easily adhering to the step wall, and compared to FIG. can be improved. To actually evaluate the blocking effect, we measured the signal output of the sensor in the dark, and when the accumulation time was 20msee, the output of the sensor with the structure shown in Figure 1 in the dark was
Compared to 50mV, the sensor with the structure shown in Fig. 2 according to the present invention has a small voltage of 8mV, and the S/R of the signal when irradiated with light is small.
The N ratio was improved by 6.25 times.

As described above, it is clear that the present invention is very useful for the practical application of contact type image sensors using amorphous silicon.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an amorphous silicon image sensor formed on a conventional substrate, in which 11 is a transparent glass substrate, 12 is a tin oxide conductive film, 13 is a general light and extraction electrode, and 14 is a gold plate. 15 is a contact pad, 15 is a Si ₃ N ₄ blocking layer, 16 is a high resistance amorphous silicon layer, 17 is a P-type amorphous silicon layer, 18 is a common aluminum electrode, and 19 is an incident light. FIG. 2 is a schematic diagram of an amorphous silicon image sensor according to the present invention, in which 21 is a transparent glass substrate, 22 is a chromium light film, 23 is a tin oxide transparent conductive film, and 24 is a Si ₃ N ₄ blocking film. 25 is a high-resistance amorphous silicon layer, 26 is a P-type amorphous silicon layer, and 27 is an aluminum individual electrode.

Claims (1)

[Claims] 1. A transparent dielectric film of high resistance amorphous silicon with a resistance of ¹⁰ Ω-cm or more, P of 10 ⁶ Ω-cm or more and 10 ¹¹ Ωcm or less
An amorphous silicon image sensor characterized in that it is formed by sequentially stacking amorphous silicon and a group of separated metal-friendly electrodes.