JPH0526347B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a photoelectric conversion device.

Conventional solid-state image sensors are broadly divided into line sensors and area sensors.
2. Area sensors are used for reading facsimiles and the like, and area sensors are used for video cameras. With the recent development of information processing equipment, there is a growing demand for inexpensive and high-performance devices and equipment. In particular, this demand is increasing as devices become more widespread from office use to personal and home use. For example, facsimile machines for home use that cost less than 200,000 yen are being introduced into the market. In a facsimile, the system consists of a readout section, a recording section, and a communication system, but the recording section has been developed with thermal heads, etc., and the communication system has been developed using LSI.
With the development of technology, applications have become considerably low-cost, but the read-out section has a complex optical system and the sensor itself is expensive, resulting in an overall high cost. Therefore, there is a need for a technology for manufacturing this lead-out section at low cost and with high performance. If it becomes possible to reduce the cost of this part, it will be possible to realize more advanced equipment as a versatile machine with intelligent functions by organically combining it with facsimile machines, copy machines, and printers. In order to achieve this readout section, lower costs, and higher performance, an image sensor with a simple optical system is required. For this reason, in recent years, close-contact sensors have been proposed that bring the image sensor into close contact with the object to be read (for example,
Publication No. 138969, Japanese Unexamined Patent Publication No. 114292/1983. ).

However, in reality, they have had drawbacks such as insufficient characteristics, poor reliability, and external processing that is too complex to be cost-effective.

Furthermore, Japanese Patent Application Laid-Open No. 57-72370 proposes a configuration that binarizes and outputs pixel signals (Figure 8), but even in this case, pixel signals are transferred using analog signals. However, it had the disadvantage that the signal-to-noise ratio deteriorated.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a contact type image sensor that has high performance, is sufficiently reliable, and furthermore enables cost reduction.

Furthermore, another object of the present invention is to provide a contact type image sensor having a binarization output function and an excellent signal-to-noise ratio.

FIG. 1 is a block diagram of a line sensor used in the present invention. N bits of elements 8 are arranged in a line, and one element consists of a scan circuit 1, a switching circuit 2, and a photosensitive cell section 3. The scan circuit 1 is basically a shift register, and is input to the gate 5 of the switching transistor 4 of the switching circuit 2.
Control ON-OFF. The basic operation is to read out the discharge amount of the charges stored in the photosensitive cell section 3 to the output line V0 by turning on the switching transistor ₄ , which corresponds to the amount of irradiated light. The N-bit cells are sequentially read out by the scan circuit and appear on output line _V0 as serial data for each cell. As a result, the amount of light irradiated to each cell is converted into an amount of electricity in proportion to the amount of light. A feature of the present invention is that all elements, including transistors, are formed of thin films.

FIG. 2 is a specific circuit diagram of FIG. 1. Since the scan circuit 1 is basically a shift register, for example, 12 is a D flip-flop, and N
Bits are connected in cascade. As shown in FIGS. 2 and 3, a scan data input signal D _IN is applied to the scan data input terminal 11 (D _IN ), and a scan clock input signal is applied to the scan clock input terminal 10 (CL _IN ).
CL _IN is given. Since the switching circuit 2 is a switching transistor, for example, 13 is a thin film transistor, which is provided for N bits. The photosensitive cell section 3 includes, for example, a photoelectric conversion element 14
and a capacitor 15.

Figure 3 shows the operating waveforms of each part of this circuit. When the outputs Q ₁ to QN of the shift register array are sequentially output, the charging current changes to the output line as the switching transistors are sequentially selected. It comes out. Since this peak value corresponds to the amount of light in each cell, by passing the signal through a low-pass filter or a peak hold circuit, a signal level proportional to the amount of light can be obtained.

Figure 4 shows a specific implementation example of the switching transistor and photosensitive cell section of the present invention, and A and B are shown in FIG.
A cross section is shown. A source 34,
A channel 33 and drain 32 region are formed. Thereafter, a gate film 35 for gate insulation is formed by thermal oxidation or CVD, and a gate electrode material such as polycrystalline silicon is deposited and patterned to form a gate 36. Then, P as source/drain electrodes 32, 33 by ion implantation method.
Create a type or N-type area. After that, an interlayer insulating film, such as a silicon oxide film 41, is formed by the CVD method, and contact holes 37 and 43 are opened to form output lines.
An Al wiring layer and an Al layer 39 of the lower electrode of the photosensitive layer are formed.

38 is an Al wiring layer which becomes an output line. The entire photoreceptor layer 4 is made of amorphous silicon or the like.
0 is deposited by a plasma CVD method, and a transparent electrode layer 42 which becomes the upper electrode of the photoreceptor is formed thereon. The photoreceptor layer 40 has a dark current of 1 nA or less when not irradiated with light, and is set to several nA/1x when exposed to light. The advantage of this method is that both the photoreceptor and the capacitor are formed. When amorphous silicon is used as the photoreceptor layer 40, the dark current is very small and the photocurrent is large, making it suitable for this optical reading. FIG. 11 shows a typical example of the photosensitive characteristics of this amorphous silicon film, which is characterized in that it can be used at illuminances of 11× (1 lux) or less. As shown in Figure 4, the feature of the vertical type conductive type photoreceptor layer (perpendicular to the film) is that there is no need to etch off the photoreceptor layer and the upper electrode, and it is easy to simply deposit the film. .

FIG. 5 shows another example of the method of the present invention. This uses a horizontal (horizontal film) conductive type photoreceptor layer. A is a CD cross section of B, and will be explained according to the formation process. A silicon thin film for forming a transistor and a capacitor is formed on a substrate 51 by the CVD method, and after patterning, a gate oxide film 55 is formed. Thereafter, an N or P type layer is formed on the lower electrode portion 54 of the charge storage capacitor by ion implantation. After that, a gate electrode 56 of polycrystalline silicon or the like and an upper electrode 57 of the capacitor are formed, and then ion implantation is performed one more time to form an N or P type source region 52 and a negative region channel region 5.
3. A switching transistor section consisting of the drain region 611 and the gate electrode 56, and a capacitor consisting of the lower electrode 54, the upper electrode 62 and the insulating film 55 are formed. After that, an interlayer insulating film 58 is deposited, contact holes 60, 61, and 62 are opened, and an Al wiring 63 that becomes an output line and a photoreceptor layer 5 are formed.
form 9. The photoreceptor layer is a semiconductor material sensitive to light such as CdS or amorphous silicon.
It is placed in parallel with the capacitor. As a result, when no light is irradiated, the photoreceptor layer 59 has a very high resistance and does not discharge the charge accumulated in the capacitor, but when light is irradiated, the charge in the capacitor is discharged. When the switching transistor is turned on, a charging current is generated, and as a result, the amount of light is converted into the amount of electricity. The feature of the method shown in FIG. 5 is that by using the photoreceptor film as a horizontal conductor, upper and lower electrodes are not required, and that it can be used even if the film has many pinholes.

Another embodiment of the present invention uses a transistor as it is as a photoreceptor, and is characterized by the simplest structure. FIG. 6 is a circuit diagram of this system, in which a transistor 66 operates as a photoreceptor. FIG. 12 shows the optical characteristics of this transistor, and the photocurrent value can be controlled by the gate voltage VG. FIG. 6 shows a state where V _G =0 as the simplest usage example. Here, 65 is a light-shielded thin film transistor, 67 is the gate electrode of the transistor 66 that operates as a photoreceptor, V ₀ is the output line, V _SS is the common potential, and Q _N is each output of the shift register array, which is the gate of the thin film transistor 65. Output to the electrode.

Figure 7 is an example of Figure 6, where A is the EF of B.
It is a cross section. A first silicon thin film for forming a transistor is formed on the substrate 70 and then patterned, and a gate insulating film 7 is formed on it by thermal oxidation or the like.
After that, gate electrodes 76 and 77 are formed, and a source region 71, a channel portion 72, a drain 73, a photoreceptor channel 74, and a fixed electrode 75 of the transistor are formed by N-type or P-type ion implantation. Thereafter, an interlayer insulating film 79 is formed, contact holes 83, 84, and 85 are opened, and then an output line 80 made of an Al layer, a light shielding layer 81, and a fixed potential line 85 are formed. In this way, the photoreceptor area is a channel 74 of a transistor, and the capacitor uses the parasitic capacitance between the gate electrode 77 and the drain area 73 as is.

FIG. 13 shows another specific example used in the present invention,
Binary detection, that is, two levels of white or black level are output. Parallel-in/serial-out shift register 131, comparator 132, reset gate 133, photosensitive cell 1 as a data transfer circuit
Consists of 34. FIG. 14 shows a timing chart for the operation of this method, and FIG. 15 shows waveforms when incident light is converted into electrical binary levels. The operation of this circuit will be described with reference to FIGS. 13 and 15. The reset signal R turns on the reset gate transistor 137 to charge the capacitor 139 in the photosensitive cell section. After that, when light is irradiated until the next reset signal is input, the resistance of the photoreceptor 138 decreases and the accumulated charges are discharged. Furthermore, if no light is irradiated, the charged potential is maintained without being discharged. Comparator 1
36 converts and outputs the binary output _W1 . Comparator output just before the next reset signal is input
W ₁ is written to the shift register 135 by write signals W and E, and then by clock signal CL.
Output as serial data to VD output. I ₁ ~
I ₄ is the terminal voltage of each photosensitive cell, and W ₂ to W ₄ are each a binary output. The feature of this method is that the photosensitive cell is converted into binary data in a straight line, so the signal transfer is strictly a digital signal, and the S/N is
will be greatly improved. Furthermore, since the photometry timing of the photosensitive cell and the data transfer timing are determined completely independently, it is possible to extract an optical signal at any timing as an electrical signal at any time.

The scan circuit used in the present invention is required to have a certain degree of high speed. For example, the number of elements
1000 and the read cycle is 1 msec,
The scan speed is 1MHz. For this reason, the scan circuit requires a shift register that can operate at high speed and a transistor that constitutes the shift register.
FIG. 8 is an example of a scan circuit having a C-MOS configuration, and shows one element. Here, φ is the positive phase clock input, inverted φ is the negative phase clock input,
D is scan data input, and Q is scan data output. P-channel thin film transistor (P-TFT)
90 to 93 and N channel thin film transistor (N-
TFT) 94-97. Figure 9 shows an example of the structure of this CMOS-TFT.
After forming a first silicon thin film 101 on top, a gate oxide film 102 is formed, and then a gate electrode 103 is formed.
form. After this, P channel transistor 10
When boron ions are implanted into 4 and phosphorus or arsenic ions are implanted into N-channel transistor 105, respective transistors are formed. In this way, in the case of TFT,
Compared to image sensors using conventional single-crystal wafers, simply adding one ion implantation process creates a monochannel device (N-MOS or P-MOS).
A major feature is that CMOS can be created from MOS). One reason for this is that even if the channel region is P type, N
This is because the molds also commonly use an intrinsic region that does not contain impurities.

The transistor (TFT) used in the present invention is required to be fast both in a scan circuit and as a switching transistor, that is, it is necessary to improve the characteristics of the transistor. As an example of the process for forming the transistor section used in the present invention, good transistor characteristics can be obtained by using a thermal oxide film as the gate insulating film. A silicon thin film containing no impurities, which will constitute the first layer channel part and source/drain, is formed to a thickness of approximately 2000 to 5000 Å using a low pressure CVD method at a deposition temperature of 570°C, and after patterning, a silicon thin film containing no impurities is deposited at a deposition temperature of 1100°C to 1150°C. Thermal oxidation is performed in an O ₂ atmosphere to form a good gate insulating film with a thickness of approximately 1500 Å, and at the same time, the grains of the first silicon thin film are grown to become a good polycrystalline film. After this form a gate electrode of N ⁺ doped polycrystalline silicon,
Then, using the gate electrode as a mask, P ions were applied 1×
When implanted with a doping amount of 10 ¹⁵ /cm ² , only the channel remains as a negative region. After this, when H ₂ plasma treatment is performed, the characteristics are further improved. When amorphous silicon is used as the photoreceptor film in the systems shown in FIGS. 4 and 5, when hydrogen-based plasma CVD is used, the TFT is automatically subjected to H ₂ plasma treatment at the same time. It is also possible to separately perform the method shown in FIG. Figure 10 shows an example of the characteristics of an N-TFT obtained through such a process, and the channel carrier mobility is approximately 80 cm ² /V sec,
It has good characteristics, about 1/5 of that of single crystal silicon.
A scan circuit constructed using this transistor operates at approximately 2 to 5 MHz, providing sufficient high speed. Furthermore, the switching speed of the switching transistor is 100 nsec or less.

As described above, the present invention constitutes a scan circuit and a switching circuit using thin film transistors, and further uses a thin film photoreceptor as a photoreceptor layer, and has the following advantages.

(1) Since it is constructed on an insulating substrate through a simple process, there are no size restrictions like single crystal silicon, making it possible to create a close-contact sensor with a length of 10 cm to 30 cm, resulting in lower costs.

(2) By incorporating a scanning circuit and a switching circuit, only 10 external wiring lines are required, significantly reducing implementation costs. In addition, since the output line is placed on an insulator, stray capacitance is extremely small, the amplitude of the output signal can be maintained up to the power supply voltage used, the S/N ratio is greatly improved, and there is no need for a complicated subsequent amplifier. This ensures a high signal level and a clear printed image.

(3) Adoption of polycrystalline silicon TFTs as transistors improves switching speed and greatly improves reliability and stability. or
Since it is easy to convert into CMOS, good values for operating speed and power consumption can be obtained when applied to scan circuits. Furthermore, the process is simple and costs can be easily reduced.

(4) Since the photoreceptor layer is made thinner, variations in photoreceptivity due to lifetime distribution, as in single-crystal silicon, are suppressed, and the sensitivity distribution in the sensor's line direction is significantly reduced.

As described above, the photoelectric conversion device of the present invention is provided with a plurality of binary pixel signal detection means each consisting of a photoelectric conversion element, a reset switch, and a comparator, the reset timing of the plurality of reset switches is common, and the reset timing of the plurality of reset switches is common. The output of the binarized pixel signal detection means is input in parallel to the shift register, and
The binarized pixel signal which is the output from the digitized pixel signal detection means is output from the shift register in time series, and the parallel input timing of the shift register and the reset timing are set arbitrarily. , has the following unique effects.

(a) Pixel signals are transferred as digital signals, and the signal-to-noise ratio does not deteriorate after being binarized by the comparator of each pixel.

(b) Since the photometric timing of the photoelectric conversion element (from the reset timing to the parallel input timing) and the data transfer timing (the parallel input timing) are determined independently, photoelectric conversion can be performed at any photometric timing. .

(c) It is possible to provide a contact type image sensor that has high performance, is sufficiently reliable, and furthermore enables cost reduction.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a solid-state image sensor used in the present invention, FIG. 2 is a specific circuit thereof, and FIG. 3 shows its operating waveforms. Also the 13th
The figure shows another example of the block diagram of the present invention.
Figure 15 shows its operating waveform. Figure 4, Figure 5,
FIG. 7 shows a specific structural example of the present invention, and FIG. 6 is a circuit diagram of FIG. FIG. 8 shows an example of a scan circuit, and FIG. 9 shows an example of the structure of CMOSTFT. Figure 10 shows an example of the characteristics of the N-TFT used in the present invention, Figure 11 shows the optical characteristics of the photoreceptor layer, and Figure 12 shows the characteristics of the N-TFT used in the present invention.
These are the optical characteristics when using TFT as a photoreceptor.

Claims (1)

[Claims] 1 A plurality of binary pixel signal detection means each consisting of a photoelectric conversion element, a reset switch, and a comparator are provided, the reset timing of the plurality of reset switches is common, and the output of the plurality of binary pixel signal detection means is shifted. The binarized pixel signal that is input in parallel to the register and output from the binarized pixel signal detection means is outputted from the shift register in time series, and the parallel input timing and the reset timing of the shift register are set arbitrarily. A photoelectric conversion device characterized by: