JPH03155139A - Image reader and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide a small and high-speed facsimile device of G4 specification by using a scanning circuit composed of polycrystalline silicon thin-film transistors in which the field-effect carrier mobility is 60cm<2>/V.S. CONSTITUTION:A thin-film photoelectric device 11 composed of amorphous silicon and a scanning circuit composed of polysilicon thin-film transistors are formed on a sensor substrate 1, which is used in the image reader at the data input of an imaging device such as facsimile. The carrier mobility in the active layer of the thin-film transistor is more than 60cm<2>/V.S. To form the active layer, thin polysilicon 23 is first formed on a quartz substrate 22 by low pressure CVD. The polysilicon is turned into noncrystalline silicon by implanting Si<+> ions. The noncrystalline silicon is treated for solid phase growth in a nitrogen atmosphere so that it is recrystallized to make polysilicon.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to an image reading device used in an image information input section of an image information processing device such as a facsimile, and particularly to a sensor portion thereof.

(Prior Art) Currently, a reading device used in an image information input section of an image information processing apparatus is required to be low cost, compact, and high performance. As a reading device that realized this, a technology was developed in Japanese Patent Laid-Open No. 60-22 in which the scan circuit and switching circuit were constructed using polysilicon thin film transistors, and amorphous silicon was used as the photoreceptor layer.
It is described in Publication No. 881.

In 7Q, the polysilicon film formation method for this channel part is to form a silicon thin film by low-pressure CVD at a film-forming temperature of approximately 2,000 to 3,000 degrees Celsius, and after patterning, to thermally oxidize it at 1,100 to 1,150 degrees Celsius in an oxygen atmosphere. About 1
At the same time as forming a good gate insulating film for 500 people, the first
This is a method of growing the crystal grain size of each layer of silicon thin film. It is stated that this results in good polysilicon, and that the characteristics are further improved by further performing hydrogen plasma treatment.

(Problem to be Solved by the Invention) By the way, the characteristic data of the above-mentioned reading device is described in [Materials of the 23rd Research Meeting of the 147th Committee on Amorphous Materials of the Japan Society for the Promotion of Science (Hl, 3.23) J. . In this document, the characteristics of TPT are 7aJ/V・S for electron mobility and 5cd/V・S for hole mobility, and 8pcQ for A4 size.
/am sensor reading speed is up to 2.6m/Qi
It is stated that ne scanning is possible. This reading speed is based on the G3 standard for facsimile (A4 version 8pej!/n
This is sufficient speed for a reading speed of 10 Hls/ff1ne). However, the G4 standard (A3 version 16 peffi/m) is expected to increase in demand in the future.
For a reading speed of 1 ms/l2ine),
It's not at a usable level.

This document also states that the performance of TPT was improved by hydrogen plasma treatment, that is, an electron mobility of 40a#/V-S and a hole mobility of 17Li/V.S were obtained. however,
According to experiments conducted by the present applicant, even this value is insufficient for the characteristics of a film transistor used in a reading device for the G4 standard.

Further, the reading device described in Japanese Patent Application Laid-Open No. 60-22881 has a structure in which reflected light from a document is made incident on a photoelectric conversion film through a focusing lens and a transparent insulating substrate.

Therefore, by providing a light-shielding layer between the transparent substrate and the photoelectric conversion section, a complete contact method is possible in which the illumination light enters from the back side of the transparent substrate and the reflected light from the document is directly photoelectrically converted without using a focusing lens. Therefore, there is a problem in that it is not possible to downsize the entire device.

In view of the above points, an object of the present invention is to provide a compact reading device that is capable of high-speed reading that complies with the facsimile G4 standard.

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above-mentioned problems, the reading device of the present invention has the following features: The field effect mobility of the polycrystalline silicon thin film transistor constituting the scanning circuit section is 60 aJ/V·S or more. Features.

Further, in the method for manufacturing a reading device of the present invention, the active layer of a polycrystalline silicon thin film transistor constituting a scanning circuit section is made by amorphizing the polycrystalline silicon thin film by ion implantation, and then recrystallizing it by annealing treatment. It is characterized by being made of polycrystalline silicon.

(Operation) The present applicant has considered the problem of slow reading speed, which has conventionally been considered a problem with reading devices.

This problem occurs because the scanning speed of the scanning circuit is rate-determining among the photoelectric conversion section and scanning circuit section that make up the reading device. It is determined by the performance (especially mobility) of In other words, the desired scanning speed cannot be obtained because the mobility is small. FIG. The results of a simulation to determine the relationship with mobility show that the mobility required to comply with the G3 standard is ~2aJ/
In contrast, in order to create a reading device compatible with the G4 standard, the mobility of thin film transistors must be ~60ad.
/V-3 or higher is required. Therefore, this problem can be solved by increasing the mobility of the thin film transistor to 60a#/V·S or more.

It is conceivable to create a drive circuit using a crystalline SL and provide a photoelectric conversion section made of a corpus silicon thin film thereon. However, since it is not possible to achieve a completely contact structure as described above, that is, a structure in which illumination light enters from the back side of the substrate, it is not possible to reduce the size and cost of the device, and there are also disadvantages of IC sensors such as COD. However, it is not possible to increase the size of the board, and it is not possible to differentiate the equipment.

Therefore, in the present invention, an attempt was made to improve the characteristics of thin film transistors by increasing the grain size of polycrystalline silicon. As a method, polycrystalline silicon deposited by low pressure CVD is made amorphous by ion implantation with electrically neutral ions (for example, Si + ions), and then recrystallized by high-temperature annealing in a nitrogen atmosphere. That's what I did.

The method of implanting Si+ ions to make polycrystalline Si amorphous and then annealing it to solid phase growth is mainly called SOI (Silicon on In5ulat).
or) methods that have been developed in the field (e.g., APPl
, Pbys.

Lett, 34■, 11979). By leaving a seed crystal or a layer serving as a seed crystal and growing from that portion, single crystal or polycrystalline silicon with uniform crystal orientation is grown. In contrast, in the method used in the present invention, a polycrystalline silicon thin film deposited on an insulating substrate is made amorphous over the entire region from the interface with the insulating substrate to the surface layer, and then 5
This is a method of solid phase growth at a temperature of 50 to 700°C. In this case, since there is no seed crystal or crystal layer to serve as a seed, the probability of nucleation is kept low, and once a nucleus is generated, it will collide with other nuclei and grow into a crystal of sufficient size before the nucleation stops. can be formed. Although single crystallization cannot be measured with this method for the above-mentioned reasons, it is possible to measure an increase in crystal grain size relatively easily.

When polycrystalline silicon is made amorphous by Si + ion implantation, the portion to be made amorphous has a distribution in the depth direction, so uniformity can be achieved by implanting multiple times while changing the acceleration voltage. (For example, 50keV and 120keV
The center of the distribution in the case of 700 and 1800 people respectively
It's a person. ) In other words, by performing ion implantation multiple times while changing the acceleration voltage to uniformly amorphize the deposited polycrystalline silicon layer, it is possible to increase the crystal grain size and to make the entire film amorphous. This can prevent problems such as a portion being severely damaged or an interface that is likely to become a nucleation source being unable to become sufficiently amorphous.

The grain size of a polycrystalline thin film formed by solid phase growth from a silicon thin film that has been uniformly amorphized through the above process has a large dependence on the annealing temperature. A polycrystalline silicon film with a film thickness of 2000 is 2.5×10 at an acceleration voltage of 50 keV.
ms(!l-”, 5.4X101sQ at 120keV
The relationship between annealing temperature and crystal grain size when in-phase growth is performed after 11-" ion implantation is as follows:
Average particle size 0 at 0℃, 7.1711.1.2 at 650℃
um, 1.8- at 600°C, and the lower the temperature, the larger the particle size. (See the transmission electron micrographs in Figures 7 to 9.) By growing at a lower temperature within the range where solid-phase growth occurs, nuclei generation is suppressed and the nuclei grow in contact with other nuclei. It is speculated that this is because it can grow larger before it stops. However, lowering the temperature also increases the incubation time until nuclei are generated, which also increases the time required to complete recrystallization, whereas at 700°C crystallization completes in about 2 hours 650℃, 600℃
At ℃, approximately 15 hours and 40 hours are required, respectively.

Next, regarding the problem that conventional reading devices require a focusing lens, the illumination light from the light source placed on the back side of the transparent insulating substrate focuses on a specific part of the document that is placed in close proximity to the photoelectric conversion unit. This problem can be solved by providing a light-shielding film so that the light reaches only the original, and receiving the reflected light from the original via a transparent electrode so that the light can be received on the top surface of the photoelectric conversion unit.

In the reading device with the above configuration, as a result of making a prototype of the reading device with the above configuration, the characteristics of the thin film transistor are that the electron mobility is 65aJ/V-3 and the hole mobility is 70cd/V-3.
At V.S., I was able to find a reading device that can operate in the 64 standard. At present, the electron mobility is lower than the hole mobility, and the reason is not clear, but in general, the electron mobility should be higher than the hole mobility, and if the process is optimized, then FIG. 6 shows the results of the reading device according to the present invention, which is expected to further improve not only electron mobility but also hole mobility.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

In FIG. 1, the reading device has a sensor substrate ω sandwiched between spacers (2a) and (2b) made of glass substrates, etc., and adhesively fixed to a thin film glass (a) via an adhesive layer (2). On the opposite side of the reading device (photoelectric conversion element section (1) to be described later)
On the side where 1) is not placed, a transparent EL light source substrate (2) is adhered. Also, on the other side of the reader (
On the side where a photoelectric conversion element section (11), which will be described later, is arranged, a thin film glass (a) is arranged to protect the photoelectric conversion element section (11) from sliding from the original (14). . On the EL light source board 0 on the opposite side of this reading device,
1. Thin film consisting of
n, A(1, low work function metal electrode such as Mg-Al2 0
, SiO□, etc., are sequentially formed to form an organic EL
forming a light source. Thin film glass of sensor board (a)
On the side, there is a photoelectric conversion element (11) made of an a-St thin film and a scanning circuit part (12) made of a polycrystalline silicon film transistor.
) is formed.

A document (14) is slid on the thin film glass (A) by a platen roller (13).

Next, the operation of reading the original (14) will be explained.

To read the M draft (14), the light from the organic EL light source passes through the EL light source substrate ■, the sensor substrate ■, the adhesive layer (3), and the thin film glass (
To) and through! 1 (14>) is reflected with a reflectance corresponding to the white and black of the surface, enters the photoelectric conversion unit (11), and is converted into light. In this case, the range of light irradiated onto the @ manuscript is the light shielding layer (15) in the photoelectric conversion section (11).
), and the reflected light from the screen can be read directly without using a converging lens by suppressing the combined thickness of the adhesive layer (3) and the film glass (a) to less than 1/2 of the pixel pitch. It becomes possible. Here, the light guide window formed of the light shielding layer may be formed near the photoelectric conversion section or inside the photoelectric conversion section as shown in FIG.

The equivalent circuit diagram of the photoelectric conversion section and the drive circuit section is shown in Fig. 2, and the time chart is shown in Fig. 3.
When a clock signal and a data signal are inputted from 1B) to a shift register (16) formed by a D-type flip-flop, the P-channel thin film transistors (20-a) to (20-e) are inputted one clock at a time as shown in FIG. ) is o
It becomes n state. When (20-a) to (20-a) are turned on, a charging current proportional to the amount of light irradiated so far flows through the parasitic capacitance of the reverse biased photodiode, and this is detected by the data line (21). Photoelectric conversion can be performed by In Figures 2 and 3,
Although an example of operation is shown in which the data line (21) is terminated with a resistor, by connecting an inverting amplifier using an operational amplifier to the data line (21), a minute current can be extracted in an amplified form.

Next, referring to FIGS. 4 and 5, the sensor board (1
The manufacturing method of 1) will be explained. Note that FIG. 4 is a cross-sectional view of the sensor substrate (11), and FIG. 5 is an explanatory diagram of the manufacturing process.

(First step) A polycrystalline silicon thin film (23) with a film thickness of 500 to 3,000 wafers is formed on a quartz substrate (22) at a substrate temperature of 600 to 650° C. using a low pressure CVD method. Si+ ions are implanted into the film. This ion implantation is carried out over the entire polycrystalline silicon film (23) (especially in the thickness direction).
In order to uniformly amorphize the material, the acceleration voltage is changed and the process is repeated several times. At this time, the depth of amorphization can be changed by changing the accelerating voltage, and the entire film can be uniformly amorphized. By uniformly making the entire film amorphous in this way, the probability of nucleation during the recrystallization process can be reduced and a polycrystalline film with large grain size can be obtained.

Now, when making the entire film amorphous, it is important to make the substrate interface also amorphous. The reason for this is that if the interface is not made amorphous, the probability of nucleation at the interface is high.
During the growth process, the time it takes for a nucleus to collide with another nucleus and stop growing is short, making it impossible to obtain large crystal grains. An example of this phenomenon is LP on a quartz substrate.
A case can be considered in which an a-Si film is deposited by the CVD method and then grown in a solid phase. In this case, although the film is uniformly amorphous, there are abrasive interfaces.

When this film is grown in a solid phase, nuclei grow in a columnar manner from the interface toward the film surface. However, as mentioned above, the probability of nucleation at the interface is high, so the crystal grain size only grows to about 0.5 μm regardless of the solid phase growth temperature, and the mobility only grows to about 10a&/V・S. I can't get it. However, even in this case, by making the interface amorphous by ion implantation, generation of nuclei at the interface can be prevented and crystal grains can be made larger.

Moreover, a range of about 20 to 200 keV is used as the accelerating voltage. There is an appropriate amount of dose; when the dose is less than that, the polycrystalline film is not sufficiently amorphous and the mobility of the recrystallized film is low; conversely, when the dose is too large, the mobility of the recrystallized film is low. Experimental results have shown that mobility decreases. The reason why the mobility of the recrystallized film decreases when this dose is too large is that when Si is used as the ion species to be implanted to make the film amorphous, N2 is mixed in because the mass is the same, and this is mixed into the film. It is assumed that this is because it is taken inside. When the content of N2 is high, the nucleus growth rate becomes slow and the frequency of nucleus generation becomes relatively large, so that the crystal grain size in the recrystallized film does not become large.

In fact, a polysilicon film with a film thickness of 8.4
10"as-" (50keV) +1.8 x 10
When Si is implanted in two stages at "an-" (120 keV), the crystal grain size in the recrystallized film is only about 0.5 μm, and this ion implantation condition is considered to result in too large a dose. The optimum dose is considered to be 1X10" low 2 to I voltage 120k
An appropriate value was a dose of 5.4 x 101san-'' at eV. In the above example, SL+ ions were used as the implanted ions, but the same effect can be obtained with elements that are inactive in semiconductors, such as Ge. Be expected.

When using this Ge, etc., N2
Since impurities that reduce the nucleus growth rate do not occur, there is only a lower limit as an appropriate value for the dose amount.

After the polycrystalline silicon thin film as described above is made amorphous, solid phase growth is performed in a nitrogen atmosphere. As mentioned above, the solid phase growth conditions are 40 hours at 600 °C, 15 hours at 650 °C, and 2 hours at 700 °C, so the solid phase growth is in the range of 1 to 50 hours at 60.0 to 700 °C. is appropriate. The average particle size immediately after forming by low pressure CVD method through the above process is 30-300, and the carrier mobility is 5.
The polycrystalline silicon film, which was about ci/V・S, has an average grain size of 1.5 to 3 μm and a carrier mobility of 60a+f/V−3.
It comes to exhibit the above characteristics. Further, in the thin film transistor manufactured by the above-described process, the threshold voltage can be controlled by implanting a donor or acceptor into the channel by ion implantation, similar to a crystalline silicon transistor. When it is desired to control the threshold voltage when producing a C-MOS device, etc., a step of implanting impurity ions for controlling the threshold voltage is added to the above-mentioned steps. This step may be performed either before or after solid phase growth, or after forming the gate insulating film.

Figures 10 to 13 show n-chM OS and p-c
hM.

The dependence of S threshold and mobility on channel implantation dose was shown. In either case, the threshold value can be controlled by channel implantation, and it can be seen that the change in mobility at this time is small. The characteristic that the mobility does not decrease even if channel implantation is performed to reduce this threshold value is a very advantageous characteristic in increasing the response speed.

(Second Step) After patterning the recrystallized polycrystalline silicon film described above, thermal oxidation is performed, and unevenness remains on the gate insulating film. If the gate insulating film has these irregularities, it results in an increase in leakage current and a decrease in breakdown voltage. One possible way to prevent this is to smooth the polysilicon film before oxidation.

As a smoothing method, it is effective to use downflow etching using a mixed gas of CF4 and 02.

In this case, oxyfluoride is formed in the gas phase and is deposited on all four parts, and because it is difficult to deposit on the convex parts, the polysilicon is etched only in the convex parts, and no smoothing is achieved.(Third stage) Gate electrode A polycrystalline silicon film (25) is deposited by low-pressure CVD, and patterned after phosphorus diffusion treatment to lower the resistance.Next, after masking the necessary parts with resist, phosphorus and boron are sequentially deposited. In the above-described step of ion implantation, the resistance of the active layer other than directly under the gate oxide film is reduced, and the source and drain portions (26) of the thin film transistor are formed.

(Fourth step) A first interlayer insulating layer (27) is formed and a contact hole is provided. A first metal thin film (28) is formed as this first interlayer insulating layer using a silicon oxide film, silicon nitride, etc. by a low pressure CVD method or a plasma CVD method,
A contact portion, a wiring portion, and a light shielding portion are formed by patterning. The material of the metal thin film is AQ, AQ-C.
u, A(t.Si-Cu, etc.) can be used.

(Fifth step) Next, a second interlayer insulating layer (29) is formed and contact holes are provided. As this second interlayer insulating layer (29),
A silicon oxide film, silicon nitride, or the like formed by low pressure CVD or plasma CVD is used.

Next, a second metal thin film (30) is formed, and a contact portion,
The wiring section, the individual electrodes of the photoelectric conversion section, and the common electrode are formed by patterning. For metal wiring, Cr, Cr
/kl, Ti, Mo, etc. are used. These metals.

This is because it forms an electrical ohmic contact at the interface with amorphous silicon and is thermally stable.

When using Cr/AQ, at least the upper part of the individual electrode portion on which the amorphous silicon layer is deposited is
It is better to etch 9 layers.

(Sixth Step) An amorphous silicon film (31) is deposited to a thickness of 1 to 2-2 by the plasma CVD method, and the pixel portion is left by patterning as a zero-order transparent conductive film electrode (32). T
, 0. (India Tin Oxide) film (
A film with a thickness of 800 mm) is deposited by reactive sputtering, and the pixel portion and the connection portion with the common electrode by the second metal thin film (30) are left by patterning. Through the steps described above, an amorphous silicon Schottky diode that performs photoelectric conversion is produced. In the above example, the amorphous silicon-ITO interface is used as a Schottky barrier, but in order to improve temperature characteristics and manufacturing variations, M
An IS structure is effective, and for example, a silicon carbide film can be used as the intermediate layer. In this case, after depositing an amorphous silicon film, an amorphous silicon carbide film (33
), it is possible to improve the characteristics without increasing the number of steps. FIG. 4 shows an embodiment including an amorphous silicon carbide film (33).

The sensor substrate (11) is completed by depositing a protective film and making holes for the bonding pads.

〔Effect of the invention〕

With the above configuration, the present invention enables high-speed reading,
It is possible to supply a small-sized reading device that can be supplied to an image processing device that requires a high-performance reading device such as a G4 standard facsimile.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of a reading device according to the present invention;
FIG. 2 is an equivalent circuit diagram of a drive circuit showing an embodiment of the reading device according to the present invention, FIG. 3 is a time chart diagram of input and output signals showing an embodiment of the reading device according to the present invention, and FIG. 4 is a diagram according to the present invention. 5(a) to 5(f) are process diagrams for explaining the process of manufacturing the sensor substrate shown in FIG. 4, and FIG. An explanatory diagram showing the operating speed of the reading device and the reading device shown in FIG. 1. FIG. 7 is a transmission electron micrograph showing the crystal structure of a polycrystalline silicon film annealed at 600°C. Figure 9 is a transmission electron micrograph showing the crystal structure of a film annealed at 650°C for 7 days.
Figures 10 and 11 are transmission electron micrographs showing the crystal structure of the film annealed at °C.
Characteristic diagrams showing the channel implant dependence of threshold and mobility of MOs, Figures 12 and 13 are p-chM
FIG. 2 is a characteristic diagram showing the dependence of OS threshold and mobility on channel implantation. ■...Sensor board (2-a), (2-b)...Spacer■...Adhesive layer (f)...Thin glass■...EL light source board■...Transparent electrode■ ... Hole conductive organic thin film (8) ... Electron conductive organic fluorescent thin film (9) ... Low work function metal electrode (10) ... Protective layer (11) ... Photoelectric conversion element part ( 12)...Drive circuit (polysilicon TPT) (1
3)...Platen roller (14)...Original (15)...Light shielding layer

Claims (3)

[Claims] (1) A reading device including at least a transparent insulating substrate, a photoelectric conversion section disposed on the transparent insulating substrate, and a scanning circuit section made of a polycrystalline silicon thin film transistor disposed on the transparent insulating substrate, A reading device characterized in that the field effect mobility of the polycrystalline silicon thin film transistor constituting the scanning circuit portion is 60 cm^2/V·S or more. (2) The active layer of the polycrystalline silicon thin film transistor constituting the scanning circuit section according to claim 1 is formed by a first step of making the polycrystalline silicon thin film amorphous by ion implantation, and recrystallization by an annealing treatment. A method for manufacturing a reading device, characterized in that the manufacturing method includes a second step of forming polycrystalline silicon. (3) The active layer of the polycrystalline silicon thin film transistor constituting the scanning circuit section according to claim 2 includes a first step of depositing by low pressure CVD method, and ion implantation of Si^+ ions multiple times at different acceleration voltages. A method for manufacturing a reading device, comprising: a second step of performing an annealing treatment in a nitrogen atmosphere; and a third step of performing an annealing treatment in a nitrogen atmosphere.