JPH06302796A - Contact image sensor - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a contact-type image sensor capable of effectively preventing destruction of elements due to static electricity. [Structure] A plurality of pairs of photodiodes 10 and TFTs 12 are formed in one matrix-connected block. The TFTs included in one block have a common gate electrode 18, and a light blocking layer 19 for blocking light is formed along the gate electrode. This light-shielding layer is grounded to the reference potential of the circuit, for example, ground in order to prevent static electricity from being charged. As a result, the electric charges charged in the light shielding layer immediately escape to the reference potential.
The chance of static electricity being destroyed is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact type image sensor used as a document reading device in a small facsimile, a copying machine or the like.

[0002]

2. Description of the Related Art FIG. 3 shows a circuit of a contact type image sensor in which a photodiode, which is a photoelectric conversion element, and a thin film transistor (TFT) are combined to form a sensor element, which are arranged linearly and connected by matrix wiring. is there.

In the contact type image sensor of FIG. 3, m × n photodiodes 10 _1-1 to 10 _nm and TFTs are provided in total.
Each of 12 _{1-1 to} 12 _nm is divided into n blocks, and each block includes m blocks. And the corresponding numbered photodiode and T connected in series
The FT constitutes one sensor element, and these are arranged linearly to constitute a contact image sensor for one line (hereinafter also referred to as “photodiode 10” and “TFT 12”). The drain of the TFT 12 is connected to the power supply 20 via the common power supply line 22, and the source side is connected to the cathode of the corresponding photodiode 10. The gate of the TFT 12 is common to all the TFTs included in one block. A light shielding layer is formed in the upper layer of the TFT 12 to prevent the light incident on the photodiode from being applied to the TFT.

When the light reflected from the document enters, the photodiode 10 performs photoelectric conversion and accumulates the electric charge generated by the photoelectric conversion in the junction capacitance of the pn junction. The accumulated charge is extracted as a current flowing between the source and drain of the TFT by applying a pulse voltage to the gate of the corresponding TFT 12, and the corresponding amplifiers 16 _{1 to} 16
_It is amplified by _m and becomes an image signal. The performance required for this TFT is that the gate has a pulse ON.
Sometimes, a current as large as possible flows between the source and the drain, and when the gate is off, no current flows between the source and the drain as much as possible. However, when light is incident on the TFT, a photocurrent flows even when the gate pulse is off and is off.
The current increases. The light-shielding layer of the TFT is such an OFF
It is intended to prevent an increase in current.

In the circuit of FIG. 3, the pulse applied to the gate of the TFT 12 is supplied from a dedicated integrated circuit (IC) 14, but the gate electrodes of the TFTs included in one block are commonly connected, The pulses from the integrated circuit 14 are simultaneously supplied to the gate electrodes of the TFTs in the same block. Therefore, the charges accumulated in the photodiode 10 are extracted for each same block, and each is amplified by the amplifier 16. The integrated circuit 14 sequentially performs such a pulse supply operation from the first block to the nth block, whereby an image signal for one line is obtained.

[0006]

In the contact type image sensor, a large number of semiconductor elements are formed on a single substrate, and even if one of them is broken, it cannot be used as an image sensor. Static electricity charged on the sensor is the most common cause of semiconductor element breakage. For example, if the sensor is charged with static electricity, the element such as the TFT is destroyed even if a person touches it a little, and it becomes unusable. As described above, the conventional contact image sensor has a problem that it is vulnerable to static electricity.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a contact-type image sensor capable of effectively preventing damage to an element due to static electricity.

[0008]

According to the present invention for achieving the above object, a plurality of sensor elements each having a photodiode and a thin film transistor combined are arranged on the same substrate, and the sensor elements are divided into a plurality of blocks to form a matrix. In the contact type image sensor connected by wiring, the light shielding layer of the thin film transistor formed on the upper layer of the thin film transistor of each block is connected to a reference potential.

The reference potential is, for example, the ground potential.

[0010]

According to the invention described in claim 1, the light-shielding layer formed in the upper layer of the common gate electrode of the thin film transistor is connected to the reference potential, so that the light-shielding layer is not in a floating state. Even if charged, it immediately escapes to the power source having the reference potential, so that the light-shielding layer is always kept at the reference potential. Therefore, the possibility that the TFT is electrostatically destroyed is greatly reduced.

According to the second aspect of the present invention, by the above configuration,
By setting the reference potential to the ground potential, the reference potential can be easily obtained, and even if the light shielding layer is charged, it immediately becomes the ground potential, so that the light shielding layer is always kept in the zero bias state. Therefore, the possibility that the TFT is electrostatically destroyed is greatly reduced.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a contact image sensor according to an embodiment of the present invention, and FIG. 2 is an enlarged plan view showing one photodiode and TFT of this embodiment. It should be noted that, of the constituent elements of this embodiment shown in FIGS. 1 and 2, those having the same functions as those of the constituent elements in FIG. 3 are designated by the same reference numerals, and detailed description thereof will be omitted.

The matrix wiring of the circuit of FIG. 1 is almost the same as that of the circuit of FIG. 3, and n × m photodiodes and TFs are used.
T is divided into n blocks, and each block contains m blocks. The pulse supply operation of the integrated circuit at the time of reading the original is the same, and the operation of supplying the pulse to the gates of the TFTs included in one block all at once is performed by sequentially performing the operation for all blocks.
Read the image information for a line. In addition, the amplifier 16 ₁ ~
The 16 _m has two input terminals, one of which is connected to the anode of the photodiode 10 and the other of which is grounded. Therefore, the anode side of the photodiode is in an imaginary short-circuit state and is virtually maintained at zero volt. This can effectively prevent crosstalk at the portions where the anodes intersect.

In FIG. 2, a gate electrode 18 extending in the lateral direction is a gate electrode common to all the TFTs included in one block, and 22 is a 5-volt element formed in a layer different from this gate electrode. It is a power line. A source S and a drain D are formed in a layer below the gate electrode via an insulating film, and one TFT 12 is formed by the gate G, the source S and the drain D. And T
The source of the FT 12 is directly connected to the cathode of the photodiode 10 below it.

The TFT 1 is provided above the gate electrode 18 in FIG.
A square region 38 for wire bonding is formed in the vicinity of 2. On the other hand, a bonding pad 40 to which the wiring from the integrated circuit 14 is connected is formed on the opposite side of the power supply line 22.
Although not shown in the drawing, a large number of such regions 38 and pads 40 are provided in the lateral direction along the gate electrode 18 for each TFT 12. Semiconductor (inside device)
Thus, the process of manufacturing is completed until the region 38 and the pad 40 are formed in this way, and the region 38 and the pad 40 are left unconnected. Then, after that, a large number of regions 38 provided along the gate electrode 12 and the pads 40 are successively connected by the wires 42 by a bonding machine or the like. By connecting in this manner, the gate region 3
The wire 42 connecting the pad 8 and the pad 40 intersects the power supply line 22 in the air, and a sufficient distance is maintained between the power supply line 22 and the wire 42.

In this way, the power supply line 22 and the wire 4
When 2 is crossed, the stray capacitance between the power supply line 22 and the wire 42 becomes very small, and even if a pulse for reading image information is supplied to the gate electrode 18, the power supply line 22 and the gate electrode The noise generated between and is greatly reduced, and the quality of the image when the document is read is greatly improved.

The contact type image sensor of this embodiment is different from the conventional one in that the light shielding layer 19 of the TFT is grounded to the zero bias portion of the circuit constituting the image sensor. The light-shielding layer 19 is intended to simply prevent the light incident on the photodiode from being applied to the TFT, so that the light-shielding layer has conventionally been left in a floating state. For this reason, there is no path through which electric charge escapes in this portion, and therefore the TFT 12 is often destroyed by static electricity when touched by humans. However, in the present embodiment, by grounding the light-shielding layer 19, even if the light-shielding layer 19 is charged, it immediately escapes to the reference potential of the circuit, so that the light-shielding layer 19 does not accumulate and therefore the TFT 1
The risk of electrostatic breakdown of 2 is greatly reduced. As described above, according to this embodiment, the yield of the contact image sensor can be improved, the cost of the product can be reduced, and the reliability of the contact image sensor can be improved.

The present invention is not limited to the above embodiments, but various modifications can be made within the scope of the gist thereof. For example, in the above embodiment, the case where the reference potential is zero volt has been described, but the reference potential is not limited to this.

[0019]

As described above, according to the invention described in claim 1, since the light shielding layer provided on the upper layer of the gate electrode of the TFT is connected to the reference potential, the light shielding layer is prevented from being charged with static electricity or the like. It can be effectively prevented, which greatly reduces the possibility that the TFT will be destroyed by static electricity, thus improving the product yield and
A contact image sensor with high reliability can be provided.

According to the second aspect of the present invention, in addition to the above effects, by setting the reference potential to the ground potential, the reference potential can be easily obtained, and even if the light shielding layer is charged. Since it is immediately brought to the ground potential, it is possible to provide a contact image sensor capable of always maintaining the light-shielding layer at zero bias.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a contact image sensor according to an embodiment of the present invention.

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

FIG. 3 is a circuit diagram of a conventional contact image sensor.

[Explanation of symbols]

10 _1-1 to 10 _nm photodiode 12 _{1-1 to} 12 _nm thin film transistor (TFT) 16 _{1 to} 16 _m amplifier 18 gate electrode 19 light shielding layer 20 power supply 22 power supply line 38 gate region 40 pad 42 wire

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoichi Nagatake 2-6-3 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Steel Corporation

Claims (2)

[Claims] 1. A contact type image sensor in which a plurality of sensor elements, each comprising a combination of a photodiode and a thin film transistor, are arranged on the same substrate, and the sensor elements are divided into a plurality of blocks and connected by matrix wiring. A contact-type image sensor characterized in that a light-shielding layer of a thin film transistor formed in an upper layer is connected to a reference potential. 2. The reference potential is a ground potential.
The contact image sensor described.