JP3087415B2 - Image sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor used for an input section of a facsimile or the like, and in particular, a photo diode and a blocking diode are connected in series so that their polarities are opposite to each other. The present invention relates to an improvement in an image sensor configured by arranging a plurality of light receiving elements in a line.

[0002]

2. Description of the Related Art Conventionally, as an image sensor of this type, for example, as shown in JP-A-3-209767, a photo diode and a blocking diode have opposite polarities. A plurality of light receiving elements connected in series as described above are arranged in a line shape, and are already known. FIG. 7 is a circuit diagram showing a state where an equivalent circuit for one line of the image sensor and a shift register as a driving circuit are connected.
Hereinafter, the operation will be schematically described with reference to FIG. In this image sensor, a plurality of light receiving elements D each including a photodiode PD and a blocking diode BD are arranged in a line, and an anode of the blocking diode BD is arranged.
The shift register circuit SR is connected to the gate side. In such a configuration, when the shift register circuit SR starts a scanning operation of sequentially applying a signal to each photodiode PD, the photodiode PD is biased in the reverse direction by this applied voltage to charge the charge. When scanning by the shift register circuit SR is completed,
The charge of the photodiode PD is distributed to the capacitance of the photodiode PD and the capacitance of the blocking diode BD. Then, when light is irradiated to the photodiode PD and the blocking diode BD during one round of scanning, charges corresponding to the amount of irradiation of the light are discharged from the photodiode PD. Next, when a reset signal (read pulse) is applied from the shift register circuit SR to each blocking diode BD, each photodiode PD is recharged with a charge corresponding to the above-mentioned discharge amount. Since the current due to this recharging flows through the loading resistor R, this current causes the output terminal T
The voltage generated at out is detected as a read signal.

[0003]

SUMMARY OF THE INVENTION The conventional image described above is used.
In the di-sensor, the forward current of the blocking diode BD is not so large, and the photodiode PD
Of the shift register circuit SR
The pulse width of the pulse cannot be reduced so much that high-speed driving cannot be performed, and the photodiode PD is not sufficiently charged. For example, in the case of a light-receiving element that is not irradiated with light, a reset signal is generated. After the voltage is applied, the charging current does not flow normally and no voltage is generated at the output terminal, but the charging is not sufficiently performed by the forward current of the blocking diode BD during the first charging. In addition, there is a problem that a charging current corresponding to the shortage flows after the reset signal is applied, resulting in a so-called afterimage phenomenon.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image sensor which can obtain a large forward current and can be driven at high speed.

[0005]

In order to solve the above-mentioned problems, an image sensor according to the present invention comprises a first photodiode having one end connected to a reading circuit and a circuit having one end generating a drive signal. An array is formed by arranging a plurality of light receiving elements, which are connected in series with a second photodiode to be connected so that their polarities are opposite to each other, in a line on a substrate. In the image sensor, the first and second photodiodes are a non-doped layer made of hydrogenated amorphous silicon into which impurities are not implanted, and a second layer made of hydrogenated amorphous silicon into which impurities are implanted. And a first doping layer made of hydrogenated amorphous silicon into which impurities have been implanted, in order from the light-receiving side. 1M ohm cm The first doping layer has a volume resistivity of 1 K ohm cm or less and the surface side opposite to the second doping layer has an ohmic resistance. It is formed by forming a junction.

[0006]

The photoelectric conversion portion of the photodiode has a first doping layer having a relatively small volume resistivity, that is, a relatively large amount of impurities injected, and a first doping layer having a relatively large volume resistivity, that is, having a relatively large volume resistivity. Since the second doping layer having a relatively small injection amount is provided, the mobility of electrons that increases with an increase in the impurity injection amount and the electron lifetime that decreases with an increase in the impurity injection amount are reduced. At the compromise, the overall improvement in electron mobility as compared to the prior art results in an increase in the forward current of the photodiode.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the display device according to the present invention will be described below with reference to FIGS. Here, FIG.
Is an AA of the image sensor according to the present invention shown in FIG.
FIG. 2 is a plan view showing one embodiment of the image sensor according to the present invention, and FIG. 3 is a longitudinal sectional view for explaining a manufacturing process of the image sensor of the present embodiment. The image sensor according to the present embodiment has a light receiving element 3 including a first photodiode 1 and a second photodiode 2 as a blocking diode connected in series so that the polarities thereof are opposite to each other. Are arranged in a line on the transparent substrate 4 (see FIG. 2).

The first photodiode 1 and the second photodiode 2 in this embodiment have the same structure, and a common metal such as chromium (Cr) is provided on the transparent substrate 4 side. An electrode 5 is provided. And this metal electrode 5
The first doping layers 6a, 6b, the second doping layers 7a,
7b, non-doped layers 8a and 8b, and transparent electrodes 9a and 9b made of indium tin oxide or the like are sequentially laminated, and an insulating layer made of an insulating material such as polyimide is formed so as to cover these transparent electrodes 9a and 9b. 10 are laminated. Then, the first doping layers 6a and 6b and the second
The photoelectric conversion units 15a and 15b are respectively constituted by the gate layers 7a and 7b and the non-doped layers 8a and 8b.

The first doping layers 6a and 6b and the second doping layer
Although both of the gate layers 7a and 7b are common in that they are made of a hydrogenated amorphous silicon (a-Si: H) layer, the hydrogenated amorphous silicon layer forming the second dope layers 7a and 7b of this embodiment is common. Has a volume resistivity of 1 M ohm cm or more, whereas the first doping layers 6a, 6a have a hydrogenated amorphous material forming the first doping layers 6a, 6b. The volume resistivity of the silicon layer is 1K-
Is set to less than cm. Incidentally, the volume resistivity of the hydrogenated amorphous silicon layer forming the second dope layers 7a and 7b is most preferably about 10 M ohm cm. The volume resistivity of the hydrogenated amorphous silicon layer forming 6b is as follows:
About 100 ohm cm is preferred.

The insulating layer 10 includes transparent electrodes 9a, 9
contact holes 11a and 11b
And a common electrode 12 made of aluminum or the like provided on the insulating layer 10 through the contact hole 11a is connected to the transparent electrode 9a.
The lead wiring 13 made of aluminum or the like provided on the upper portion of the transparent electrode 9b is connected to the transparent electrode 9b via the contact hole 11b. The anode of the second photodiode 2
7 (transparent electrode 9b side) and a read circuit (for example, FIG.
2 (see FIG. 2) via the contact holes 11c formed in the insulating layer 10 in the same manner as the common electrode 12 and the lead-out wiring 13 described above. It is connected to the lead wiring 13 (see FIG. 2).

Next, a manufacturing process of the image sensor will be described with reference to FIG. First, a metal film 5 'made of chromium or the like is deposited on a transparent substrate 4 made of glass or the like with a thickness of, for example, about 700 angstroms by a vapor deposition method or a sputtering method. 14 are formed (see FIG. 3A).

Next, the first dopant is formed by the P-CVD method.
The a-Si: H film 6 serving as the gate layers 6a and 6b is formed to a thickness of about 1000 angstroms and the second doping layers 7a and 7b are formed.
An a-Si: H film 7 to be formed and a non-doped a-Si: H film 8 to be non-doped layers 8a and 8b are sequentially deposited on the entire surface (see FIG. 3B). Thereafter, a transparent conductive film made of indium tin oxide to be the transparent electrodes 9a and 9b is deposited to a thickness of about 800 angstroms by sputtering, and the transparent electrodes 9a and 9b are formed by photolithography (FIG. 3).
(C)). Here, the thicknesses of the second doping layers 7a and 7b and the non-doping layers 8a and 8b are set so that the sum of the thicknesses of both layers is about 0.3 to 2 μm. The thickness of the doped layers 7a and 7b is made larger than the thickness of the non-doped layers 8a and 8b. Also, the first doping layers 6a, 6a
b, that is, an a-Si: H film 6 into which impurities are implanted.
Is manufactured using a gas obtained by doping a phosphine (PH ₃ ) gas in a concentration of 0.1 to 1% in a 100% silasin (SiH ₄ ) gas. At this time, the doping amount of the phosphine gas is preferably about 1%. In this case, the volume resistivity of the completed first doping layers 6a and 6b is about 100 ohm cm. That has been confirmed. Further, the second doping layers 7a and 7b, that is, the a-Si: H film 7 into which the impurities are implanted are made of 100% silane (SiH ₄ ).
The gas is prepared by using a gas obtained by doping a phosphine (PH ₃ ) gas at a concentration of 1 ppm or less. The doping amount of the phosphine gas is preferably about 0.1 ppm. In this case, the volume resistivity of the completed first doping layers 6a and 6b is about 10 M ohm cm. Has been confirmed.

The conditions for depositing the first doping layers 6a and 6b are as follows.
The temperature is preferably set to 0 ° C. Further, the first doping layer 6
a-Si: H film 6 forming a and 6b, second dope layer 7
The a-Si: H film 7 and the non-doped layers 8a,
The a-Si: H film 8 forming the layer 8b is deposited with two or more reaction layers in succession, but when the reaction layer is a single layer, once the vacuum is broken, BHF (buffer-dehydrofluoric acid) is used. After removing the oxide film on the first doping layers 6a and 6b, an a-Si: H film 7 forming the second doping layers 7a and 7b is deposited, and then the second The oxide film on the doped layers 7a and 7b is removed, and a film 8 of a-Si: H forming the non-doped layers 8a and 8b is formed.

After completion of the above-described film deposition, the shapes of the photoelectric conversion units 15a and 15b are adjusted by dry etching (see FIG. 3C). In addition, dry etching is CF
4, using a gas such as SF6. Next, roll coat of polyimide (preferably "PIX-1400" and "PIX-8803" manufactured by Hitachi Chemical Co., Ltd., "Photonice" manufactured by Toray Industries, Inc.) is used. Alternatively, the insulating layer 10 is completed by spin coating with a thickness of about 1 μm (see FIG. 3E). Finally, a metal material such as aluminum is deposited to a thickness of about 1 μm by a vapor deposition method or a sputtering method, and then the common electrode 12 and the lead wiring 13 are formed by a photolithography method.

FIG. 5 shows the forward current characteristic of the image sensor of the present embodiment, and FIG. 6 shows the reverse current characteristic of the image sensor of the present embodiment. The electrical characteristics of the image sensor according to this embodiment will be described. First, in FIG. 5, the forward current characteristic curve of the image sensor of this embodiment is shown by a solid line, and the second doping layers 7a and 7b of this embodiment are eliminated, and the photoelectric conversion unit 15 is formed.
The characteristic curves of the image sensor having the first and second doping layers 6a and 6b and the non-doping layers 8a and 8a are shown by two-dot chain lines, respectively. Comparing the two characteristic curves, the forward current characteristic of the image sensor of the present embodiment is such that the image sensor does not have the second doping layers 7a and 7b, that is, a-Si doped with impurities: It can be seen that it is surely larger than the forward current characteristic of the image sensor of the conventional configuration having one H layer.

FIG. 6 shows the reverse current characteristic of the image sensor of this embodiment by a solid line, and shows a state where the non-doped layers 8a and 8b are removed from the image sensor having the structure shown in FIG. Are shown by the two-dot chain line. In the same figure, the uppermost solid curve (the upper side of the paper in the figure) is a reverse current in a state in which the image sensor of this embodiment is irradiated with white light of 100 lux (in FIG. 5, described as "PHOTO"). The characteristics are shown, and substantially constant characteristics are obtained regardless of the magnitude of the applied voltage. The solid line characteristic curve shown relatively low in the figure shows the reverse current characteristic when the image sensor of this embodiment is not irradiated with light (denoted as "DARK" in FIG. 6). The reverse current is considerably smaller than the reverse current characteristic in the DARK state of the image sensor having no non-doped layers 8a and 8b shown by the two-dot chain line in FIG. I understand that there is. That is, due to this difference in characteristics, the non-doped layers 8a and 8b
It can be seen that it plays a role of suppressing the reverse current at the time of ARK.

In the above embodiment, the photoelectric conversion unit 15
By providing the second doping layers 7a and 7b in addition to the first doping layers 6a and 6b having a relatively large amount of impurity implantation into the layers a and 15b, the speed increases as the amount of impurity implantation increases. A compromise between the tendency of electron mobility and the lifetime of electrons which tends to decrease with an increase in the amount of impurity implanted can be obtained. As a result, the second photodiode 2 can be obtained. Has the effect that a larger forward current value can be ensured than in the prior art.

Further, by providing the non-doped layers 8a and 8b between the second doped layers 7a and 7b and the transparent electrodes 9a and 9b, the second impurity with a small amount of impurities is injected. The image sensor can suppress an increase in the reverse current due to the provision of the doped layers 7a and 7b.

FIG. 4 is a cross-sectional view of another embodiment. The contents will be described below with reference to FIG. The same components as those in the embodiment described with reference to FIGS. 1 and 2 are denoted by the same reference numerals, and description thereof will be omitted. Hereinafter, different points will be mainly described. In this embodiment, the dimensions of the metal electrode 16 and the first dope layers 17a and 17b in the longitudinal axis direction (the horizontal direction in FIG. 4) are set shorter than those of the image sensor described with reference to FIG. Is different. That is, in the image sensor described with reference to FIG. 1, the metal electrode 5 and the first doping layers 6a and 6b are formed by the common electrode 12 and the lead-out line 1.
The first photodiode 1 or the second photodiode 2 is extended to the lower part of the third photodiode 3. Therefore, a portion located below the common electrode 12 or a portion below the lead-out wiring 13 blocks light from entering from above (upper side in FIG. 1), and this portion is used as a photodiode. It does not fulfill the role of a capacitor, but rather acts as an additional capacitor.
Since the amount of charge stored in the node itself is small,
There is a tendency that a large output cannot be obtained as a disensor.

On the other hand, in another embodiment shown in FIG. 4, the metal electrode 16 and the first doping layers 17a, 1
7b is arranged so as to be located between the common electrode 12 and the extraction wiring 13. Therefore, the first and second photodiodes 1 and 2 do not have a region that is shielded from light by the common electrode 12 or the lead-out wiring 13 as described in FIG. 1, and therefore, the image described in FIG. -Since the additional capacitance is smaller than that of the image sensor, the image sensor has good output characteristics.

In this embodiment, the photoelectric conversion unit 15
Since the configurations of a and 15b are basically the same as those of the image sensor described with reference to FIG. 1, the same operation and effect as those of the image sensor described above can be obtained.

Incidentally, the image in any of the above embodiments is described.
The electrical operation of the di-sensor is the same as that of the conventional image sensor.
Although the detailed description is omitted here since there is basically no difference from the di-sensor, the first photodiode 1 in the present embodiment is a PD of FIG. These correspond to the BDs in FIG.

[0023]

According to the present invention, the photoelectric conversion unit is provided with a first doping layer having a relatively small volume resistivity and a second doping layer having a relatively large volume resistivity. As a result, it is possible to obtain a compromise between the electron injection amount, that is, the electron velocity that tends to increase as the volume resistivity increases, and the electron lifetime that tends to decrease as the impurity injection amount increases. As a result, the forward current value of the photodiode can be increased as compared with the conventional case.
In addition, since the forward current is larger than in the conventional case, the width of the drive pulse can be reduced, so that higher-speed driving can be performed as compared with the related art. Further, by providing a non-doped layer in which impurities are not implanted on the electrode side of the second doped layer of the photoelectric conversion unit, a reverse current can be suppressed, so that a forward current can be suppressed. This has the effect of improving the electrical characteristics of the photodiode as the current increases.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of an image sensor according to the present invention, and is a sectional view taken along line AA of FIG.

FIG. 2 is a plan view showing one embodiment of an image sensor according to the present invention.

FIG. 3 is a longitudinal sectional view for explaining a manufacturing process of the image sensor according to the present invention.

FIG. 4 is a longitudinal sectional view showing another embodiment of the image sensor according to the present invention.

FIG. 5 is a characteristic diagram for explaining reverse current characteristics of the image sensor according to the present invention and a conventional image sensor.

FIG. 6 is a characteristic diagram for explaining a reverse current characteristic of the image sensor according to the present invention.

FIG. 7 is a conventional circuit diagram for explaining the operation of the image sensor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... 1st photodiode, 2 ... 2nd photodiode, 3 ... Light receiving element, 5 ... Metal electrode, 6a, 6b
... a first dope layer, 7a, 7b ... a second dope layer,
8a, 8b: non-doped layer, 9a, 9b: transparent electrode, 1
5a, 15b: photoelectric conversion unit, 16: metal electrode, 17
a, 17b... first doped layer

──────────────────────────────────────────────────の Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 31/10-31/119 H01L 27/14-27/148

Claims (1)

(57) [Claims] 1. A first photodiode having one end connected to a reading circuit and a second photodiode having one end connected to a circuit for generating a drive signal have opposite polarities. In the image sensor in which a plurality of light receiving elements connected in series as described above are arranged in a line on a substrate to form an array, the first and second photodiodes contain impurities. A non-doped layer of hydrogenated amorphous silicon not implanted, a second doped layer of hydrogenated amorphous silicon doped with impurities, and a first doped layer of hydrogenated amorphous silicon doped with impurities. And a photoelectric conversion portion formed by laminating a doping layer in order from the light receiving side. The second doping layer has a volume resistivity of 1 M ohm cm or more. Layer is 1K
An image sensor having a volume resistivity of less than or equal to cm and an ohmic junction formed on the surface side opposite to the second doping layer.