JPH0471342B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention relates to a 1-magnification image sensor used for reading optical information in a device such as a facsimile, and particularly to an image sensor using amorphous silicon as a photoconductive material. (Prior art and its issues) As a type of image sensor that uses amorphous silicon as a photoconductive material and reads out information by detecting resistance changes in the photoconductive material due to light irradiation, Fig. 1 The planar type shown in is known. FIG. 1 shows a cross section of one device, in which a photoconductive layer 1 made of amorphous silicon is patterned and formed on a substrate 2, and electrodes 3 and 4 are opposed to each other on the surface of the photoconductive layer 1. It is provided. 5 is a protective layer to protect this image sensor,
A transparent and insulating layer is used. Image light representing the information to be read enters from above the protective layer 5, reaches the photoconductive layer 1 between the opposing electrodes 3 and 4, and lowers its resistance value. To read this resistance value change, for example, as shown in FIG. 2, a voltage E is applied to the photoconductive layer 1 from one electrode, and the current value accompanying the resistance value change of the photoconductive layer 1 due to light irradiation is It is sufficient to convert it into a voltage using the amplifier 6 and detect it as the output voltage Vout. The image sensor shown in FIG. 1 is formed by arranging a large number of elements in a direction perpendicular to the paper surface depending on the reading width and density. For example, in the case of a one-dimensional image sensor that reads a 256 mm width at a density of 8 lines/mm, 2048 elements are arranged. By the way, in such an image sensor, the ratio of the resistance value of the photoconductive layer 1 in the dark to the resistance value during light irradiation is 10 ⁵ to 10 ⁶ in the case of sunlight, etc. ¹⁰²
~10 ³ or less, so the S/N ratio is small,
Another problem is that the dynamic range between bright and dark is small, making it difficult to read intermediate tones. Further, the photoconductive layer 1 made of amorphous silicon has a property that its resistance value decreases as the temperature rises, and when the ambient temperature rises by several tens of degrees Celsius, the resistance value in the dark decreases by about one order of magnitude. Therefore, if the temperature of the photoconductive layer 1 rises by several tens of degrees due to changes in the environment or heat generation from the facsimile lamp, the resistance value ratio in the dark and in the light irradiation will change.
There is also the problem that the S/N ratio decreases from 10 ² to 10 ³ to about 10 to 10 ² . (Objective) An object of the present invention is to increase the resistance value ratio between the dark time and the light irradiation time of an image sensor having an amorphous silicon layer as a photoconductive layer. (Structure) As an embodiment of the present invention, as shown in FIGS. 3A and 3B (the protective layer 5 is omitted in FIG. 3B), counter electrodes 3 and 4 are formed on the surface of the photoconductive layer 1. The points are the same as in Figure 1, but there is an insulating layer 10 on the back side.
An electrode 11 is further formed through the electrode 11.
is arranged so as to face at least a part of the gap 12 between the opposing electrodes 3 and 4 on the front surface, and is configured to be applied with a bias voltage. In the case of a one-dimensional image sensor as shown in FIG. 3, a plurality of sensor elements 1 as shown in FIG.
3-1, 13-2, --, one line of electrodes 11 may be formed in common. Further, in this embodiment, the electrode 11 is wider than the area of the opposing electrode gap 12 on the surface of the photoconductive layer 1, and is arranged so as to face the entire surface of the gap area. may be the same as or less than the distance L of the gap 12, and it is sufficient that it is formed so as to face at least a part of the area of the gap 12. When applying a constant bias voltage to the electrode 11 of the image sensor of the present invention configured as described above, a negative voltage Eg is applied with respect to the read power supply voltage E, as shown in FIG. This results in
The resistance value of the photoconductive layer 1 in the dark increases, and the resistance value ratio between the dark and light irradiation increases. Furthermore, when the present invention is also used to prevent a decrease in the resistance value ratio due to a rise in temperature, a voltage that increases negatively as the temperature rises is applied to the electrode 11 as shown in FIG. A bias voltage applying means 14 may be provided. Bias voltage application means 14
For example, there are those in which the output voltage of a thermocouple is amplified and applied, or in which a constant voltage is divided by a thermistor and a fixed resistor and then applied. In the embodiment shown in FIG. 3, the substrate 2 may be made of glass or ceramics, and a layer may be formed on its surface to prevent alkali ions from entering the photoconductive layer. The electrode 11 is formed by forming a film of a metal such as Ni, Cr, Mo, NiCr, Al or a metal oxide such as SnO ₂ or In ₂ O ₃ on the substrate 2 by means such as vacuum evaporation or sputtering. It is obtained by patterning using lithography. The insulating layer 10 is an insulating layer made of SiO ₂ , SiN, Al ₂ O ₃ or the like, and the electrode 11 is formed by a glow discharge decomposition method (also referred to as a plasma decomposition method or a plasma CVD method) or a low pressure CVD method, or by a plasma method. Alternatively, it can be formed by an oxidation method using an anodic oxidation method, and patterned by an ordinary photolithography method. The photoconductive layer 1 is an amorphous silicon layer,
It may contain hydrogen or halogen atoms (Cl, F, etc.). This is formed, for example, by a glow discharge decomposition method of silane (SiH ₄ ). It is known that hydrogen is contained in the glow discharge decomposition method using silane, and an appropriate doping gas such as PH ₃ , B ₂ H ₆ , SiH ₂ Cl ₂ ,
Film formation may be performed by mixing SiF ₄ or the like. This can also be obtained by patterning it using the usual photolithography method. The electrodes 3 and 4 are made of metal such as Al, NiCr, Cr, Mo, etc., and are coated on the entire surface by vacuum evaporation or sputtering, and then have an opposing gap 12 formed on the surface of the photoconductive layer 1 by ordinary photolithography. Alternatively, a lift-off method may be used, in which a photosensitive resin such as photoresist is patterned and formed in advance in areas where the electrodes 3 and 4 are not formed, and then a metal layer is applied over the entire surface. Alternatively, the photosensitive resin may be removed by removing the photosensitive resin. The protective layer 5 is transparent because image light is incident thereon, and is an insulating layer. As this protective layer 5, a SiN film formed by glow discharge decomposition of nitrogen gas or ammonia gas together with silane,
An SiO ₂ film formed by a sputtering method, or a film obtained by dissolving a resin in a solvent, applying it, drying and curing, etc. can be used. Next, the present invention will be illustrated in more detail with reference to Examples. Example 1 Using quartz glass as the substrate 2, a NiCr layer with a thickness of 0.1 μm was formed on the surface by vacuum evaporation.
Electrodes 11 were formed by patterning using the photolithography method. As the etchant at this time, an aqueous solution of ceric ammonium nitrate and perchloric acid was used. Next, a SiO ₂ layer is formed to a thickness of 0.3 μm using SiH ₄ gas and oxygen gas under a pressure of 0.5 Torr using a low-pressure CVD method, and is patterned using a photolithography method to form an insulating layer 10. was formed. At this time, fluoric acid and ammonium fluoride were used as etchants. Next, the board temperature
An amorphous silicon layer was formed to a thickness of 0.8 μm by a glow discharge decomposition method at 300° C. under a pressure of 1 Torr of SiH ₄ gas, and was patterned by photolithography to form a photoconductive layer 1. At this time, a mixed acid of hydrofluoric acid, nitric acid, and acetic acid was used as an etchant. Finally, in order to form electrodes 3 and 4, an Al layer was formed to a thickness of 0.8 μm by vacuum evaporation and patterned by photolithography. At this time, a mixed acid of phosphoric acid, nitric acid and acetic acid was used as an etchant. Note that the protective layer 5 was omitted in this example. In this embodiment, the gap 12 between the opposing electrodes 3 and 4 is
The interval L is 30 μm, the width W of the electrode 11 is 50 μm,
As shown in FIG. 3, the electrode 11 was arranged to face the entire surface of the electrode 12. A readout power supply voltage E of 10 V was applied to the image sensor prepared as described above, and the characteristics of one element were measured using a microammeter. The results are shown in the table below.

[Table] Apart from this, the electrode 11 and the insulating layer 10
The characteristics of the sensor created without using
It was almost the same as when Eg = 0V. From the above results, by applying a negative bias voltage to the electrode 11, the resistance value in the dark increases, and as a result, the resistance value ratio between the light irradiation and the dark is approximately
It can be seen that what was 10 ² has increased to 10 ⁴ or more. It has been confirmed that the effect of this bias voltage improves the resistance value ratio between light irradiation and darkness under various film formation conditions, whether or not hydrogen or halogen atoms are contained in the amorphous silicon layer. It was done. However, the optimum value of the bias voltage Eg in each case will be different. Example 2 In an image sensor prepared in the same manner as in Example 1, a bias voltage of -3V was applied to the electrode 11 at 20°C, and then, as shown in FIG. Alternatively, when the bias voltage was made to increase negatively at a gradient of about -0.3V/°C in proportion to the rise in surrounding temperature, a nearly constant resistance value ratio was obtained up to about 40°C. (Effects) As described above, according to the present invention, the resistance value ratio between light irradiation and dark time is improved, and as a result, noise is reduced or the dynamic range is expanded, making it possible to read intermediate tones. sensor can be achieved. Furthermore, if the bias voltage is made to follow temperature changes (especially increases), it is possible to achieve a highly reliable image sensor with less change (especially decrease) in the resistance value ratio between light irradiation and darkness. .

[Brief explanation of the drawing]

Figure 1 is a sectional view showing a conventional image sensor.
Fig. 2 is an electric circuit diagram of one element, Fig. 3 A and B are a sectional view and a plan view showing an embodiment of the present invention, and Fig. 4 is an electric circuit diagram of one element in an embodiment of the present invention. , the fifth is an electric circuit diagram regarding one element in another embodiment of the present invention. 1... photoconductive layer of amorphous silicon, 3,
4... Counter electrode, 10... Insulating layer, 11... Bias voltage application electrode, 12... Gap between opposing electrodes.

Claims (1)

[Scope of Claims] 1. An image sensor in which electrodes are provided facing each other on the same plane of an amorphous silicon layer, and the electrodes detect a change in the resistance value of the amorphous silicon layer corresponding to the amount of light irradiation. Between the opposing electrodes on the back surface of the assilicon layer
An image sensor characterized in that an electrode for applying a bias voltage is provided in a region facing at least a portion of the image sensor with an insulating layer interposed therebetween. 2. The image sensor according to claim 1, wherein the bias voltage is applied depending on the temperature of the amorphous silicon layer or its surroundings.