JPH0990331A - Liquid crystal display - Google Patents

Abstract

(57) An object of the present invention is to provide a liquid crystal display device which is provided with a polycrystalline silicon thin film transistor element having good mobility and threshold characteristics and having high reliability, and which realizes excellent display performance. A quartz substrate 1 formed so as to contain 200 ppm or more of OH groups is used as a substrate for a TFT array substrate of a liquid crystal display device. This gives p-
Hydrogenation of the active layer 2 made of Si can be effectively performed, and as a result, a liquid crystal display device having excellent display performance can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which is provided with a polycrystalline silicon thin film transistor element having good mobility and threshold characteristics and can realize excellent display performance.

[0002]

2. Description of the Related Art A liquid crystal display device is suitably used for an image display device of various information processing devices, a portable television, a wall-mounted television and the like, and is a display device which can be particularly thin and lightweight.

Among such liquid crystal display devices, a thin film transistor (hereinafter abbreviated as TFT) formed of polycrystalline silicon (hereinafter abbreviated as p-Si) is provided in the pixel portion as a switching element, while having a similar structure. A so-called drive circuit-integrated active matrix liquid crystal display device in which a liquid crystal drive circuit is formed by providing TFTs also on the periphery of a switching element array substrate is a compact and thin liquid crystal display device including a drive circuit system as a whole. Research and development have been actively conducted as a liquid crystal display device that can realize the above.

Particularly, in the p-Si TFT, the liquid crystal driving circuit can be integrally formed on the peripheral portion of the switching element array substrate as described above, and further, the TFT capable of realizing the performance corresponding to the high-definition liquid crystal display device. Since,
In particular, there are great expectations for its use as a projection type liquid crystal display device.

In such a liquid crystal display device, the TF
High-speed operating characteristics and high reliability of T are required.

That is, the mobility which is a typical characteristic of TFT is required to be 30 cm ² / vs or more. This is 1 of a-Si
It is a much higher value than the cm ² / vs level.

Further, the liquid crystal drive voltage generally needs to be about 15 V, and the reliability for continuing normal operation at that voltage is required.

A TFT having such high performance and high reliability
Is generally called a high temperature process when manufacturing a semiconductor layer that includes an active layer, which is the main part of it.
It needs to be manufactured using a process at temperatures above 00 ° C.

This is because the polycrystalline silicon (p-S) used in the active layer of the thin film transistor (TFT) is used in order to increase its mobility as an electrical characteristic of the active layer.
This is because it is necessary to form the grain size of i) to a large grain size.

Also, when thermal oxidation is used when forming the gate oxide film, a high temperature process of 800 ° C. or higher is required to obtain high reliability.

In order to cope with such a high temperature process, it is necessary to use a highly heat resistant substrate such as a quartz substrate as an electrically insulating substrate. In particular, a so-called highly heat-resistant quartz substrate has been used as a high-performance substrate in order to suppress planar distortion of the substrate due to thermal stress at high temperatures. For example, as shown in Japanese Patent Application Laid-Open No. 4-320064, a so-called high heat-resistant substrate is used, and a TFT excellent in operation characteristics and reliability as described above is provided while preventing thermal deformation of the substrate as much as possible. A method of obtaining the TFT has been studied, and a TFT has been formed thereby.

[0012]

However, when a so-called high heat-resistant substrate is used, the TF having the above-mentioned sufficient performance in terms of both the operating characteristics and the reliability.
It turns out that T cannot be obtained.

It is considered that this is because the hydrogenation treatment is indispensable for forming the p-SiTFT, but this effect cannot be sufficiently obtained for the high heat resistant substrate. .

Hydrogenation treatment means thin film transistor (TF).
T) At the stage of forming the metal wiring in the forming process, the entire TFT switch element array substrate is exposed to hydrogen plasma to p-
This is a method of hydrogenating the Si film.

Large crystal grains grow in p-Si formed at 600 ° C. or higher, and high mobility is realized by this, but dangling bonds are formed in the crystal grain boundaries and crystal grains. It contains many crystal defects including the first one. Therefore, in order to terminate these crystal defects and improve the operating characteristics of the TFT, the hydrogenation treatment is performed.

As a theoretical explanation of the phenomenon of such hydrogenation, the passage of hydrogen during hydrogenation is analyzed in detail in the following documents. That is, J. Electrochem. Soc., Vol138 (11) 3240 Uday Mitra et al. The main point will be described with reference to FIG. 4, and the route through which hydrogen diffuses to the active layer is The route C shown in FIG. 4 is the main route. Once plasma-excited hydrogen reaches an electrically insulating substrate such as a quartz substrate 501, it penetrates into the substrate and then diffuses further inside. Then, hydrogen further reaches the back surface of the p-Si layer 502 which is the active layer of the TFT, and diffuses into this p-Si. Since the dangling bond is terminated in the p-Si502 layer in which hydrogen is diffused in this way, the diffusion of hydrogen is also slowed down, and finally the dangling bond is terminated with a predetermined concentration of hydrogen to reach an equilibrium state. The hydrogenation phenomenon will also stop.

However, the high heat resistant substrate 5 as described above is used.
P- formed on the switch element array substrate using 01
Hydrogenation does not sufficiently effectively act on the semiconductor layer composed of the Si layer 502, and as a result, the operating characteristics such as mobility and threshold value of the completed TFT and reliability are insufficient. There is a problem of becoming.

However, since the effect of the hydrogenation treatment increases depending on the treatment time, the hydrogenation treatment is performed for a long time, and the operating characteristics such as the mobility and threshold value of the completed TFT are improved. It is also possible to adopt a method of making the reliability sufficient.

However, such a method is not realistic from the viewpoint of manufacturing in view of the throughput of hydrogenation treatment, and the long-time heat treatment causes an ultrafine wiring structure of the TFT. There is also a problem that various structures of the above cause deterioration at the time of manufacturing, and reliability is lost due to them.

The present invention has been made in order to solve such a problem, and is provided with a polycrystalline silicon thin film transistor element having particularly good mobility and threshold characteristics and high reliability, and has an excellent display. An object is to provide a liquid crystal display device that can realize performance.

[0021]

A liquid crystal display device according to the present invention includes a polycrystalline silicon thin film transistor element as an element forming at least one of a liquid crystal drive circuit and a pixel section switching element, and a pixel section switching element connected to the polycrystalline silicon thin film transistor element. A pixel electrode for which voltage application is controlled, a switch element array substrate formed on a first electrically insulating substrate, and a counter electrode that is arranged to face the pixel electrode with a gap therebetween. In a liquid crystal display device, which is formed on an electrically insulating substrate, and has a counter substrate which is opposed to the switch element array substrate while holding the gap, and a liquid crystal layer which is sealed and sandwiched in the gap. The first electrical insulating substrate of the switch element array substrate is made of a material having silicon and oxygen in a skeleton of a network of molecular structure, and is electrically isolated. A sexual substrate, 200 ppm of molecular structure OH groups of the material forming the electrical insulating substrate
It is characterized in that it is an electrically insulating substrate containing the above.

Alternatively, in the above liquid crystal display device,
The first electrically insulative substrate of the switch element array substrate is an electrically insulative substrate formed of a material having silicon and oxygen in the skeleton of a network of molecular structure, and the electrically insulative substrate is formed. It is characterized in that the molecular structure of the contained material is an electrically insulating substrate containing OH groups in the range of 200 ppm to 1000 ppm.

As will be described later, theoretically, the greater the ppm of the OH group in the composition of the substrate, the more effective the hydrogen treatment effect can be obtained. Therefore, there is theoretically no upper limit.

However, in practice, if the substrate contains too much OH groups, the substrate will be a TFT of a liquid crystal display device.
It becomes unsuitable for use as an array substrate. This is because the shrinkage of the substrate becomes large during the heat treatment, and especially when forming an array substrate, a pattern shift occurs, and T
It becomes impossible to form the FT array substrate.

As described above, in the case of a liquid crystal display device having a structure in which a so-called high heat-resistant substrate is used as the substrate for the switch element array substrate, it is difficult to make the characteristics of the TFT sufficient.

This is because the hydrogenation treatment is essential for forming the p-Si TFT, but it is extremely difficult to obtain a sufficient effect of this hydrogenation treatment when a high heat-resistant substrate is used. It turned out. As a result of investigating the cause of the insufficient hydrogenation treatment, it was found that there is a problem with the high heat-resistant substrate as a hydrogen pass during the hydrogenation treatment.

That is, as described in the above-mentioned documents and the like, it is known that the hydrogenation effect is determined depending on the progress of hydrogen diffusion, but at this time, hydrogen is easily diffused. It was considered that the hydrogenation effect of the substrate was good, and conversely, the hydrogenation effect of the substrate with a small hydrogen diffusion was not good.

A network formed by bonding Si and O is formed in the skeleton of the molecular arrangement structure of the quartz substrate which is the material for forming the quartz substrate.

The above Si and O are bonded to each other, and basically S
It forms a bond in the form of iO ₂ , and this bond is Si
O as a center, forms a crystalline state in which the O is located at the apex of the regular tetrahedron. And when this is crystallized to a more regular state, it becomes a crystal.

By the way, the structure at the molecular level of an electrically insulating substrate such as a normal glass substrate is generally glassy (amorphous). That is, although a Si—O network is formed, it is not as crystallized as the above-mentioned quartz. In other words, the network formed by combining Si—O has short-range order, but not as long-range as crystal. Therefore, looseness / contraction of the bond occurs.

That is, in the case of a crystal, the bond between Si and O is strictly and regularly determined, but in the glass state, there is some degree of freedom in the bond. For this reason, in the glass state, there are parts where the elements are connected at short distances where the distances are reduced, while where the elements are connected at long distances where the distances are relaxed, and further progress from loose bonds. However, unbonded hands are also mixed and bonded.

During the hydrogenation process, hydrogen advances in the network in the electrically insulating substrate such as a quartz substrate. That is, according to the above-mentioned literature, hydrogen diffuses through the substrate during the hydrogenation process, but from the microscopic point of view of the material of the substrate, its structure is silicon (Si).
It is formed by a network having skeletons of oxygen and oxygen (O), and hydrogen runs and diffuses in this network.

During the diffusion of the hydrogen, when diffusing in the skeleton of the bond which has loosened and becomes a dangling bond among the above-mentioned mixed bonds, the dangling bond is unstable and stabilized. Attempts to combine with hydrogen. Therefore, hydrogen (H) trapped in Si among the dangling bonds of the composition in the substrate is Si
Hydrogen (H) trapped by oxygen (O) forming a -H bond
Form an OH group. As a result, hydrogen that was originally desired to be injected into the active layer of the TFT is trapped by dangling bonds in the substrate in the course of its progress, and hydrogenation of p-Si in the active layer cannot be effectively performed. Conceivable. Therefore, if the substrate contains 200 ppm or more of OH groups in advance, such a problem can be solved.

In addition, an OH group of 200 p was previously formed in the substrate.
If an excessive amount such as pm or more is contained, the probability that the OH group will release hydrogen (H) during heat treatment such as hydrogenation also becomes high, so the released hydrogen can also be used. It is more suitable for the chemical treatment.

Moreover, when the number of such OH groups increases,
Each skeleton of the network formed by the combination of Si-O that forms the substrate material at the molecular level becomes wide due to loose bonds,
It is also conceivable that hydrogen will easily pass through it.

Therefore, in the present invention, by using a substrate containing 200 ppm or more of OH groups in advance as the electrically insulating substrate, hydrogenation treatment is effectively performed on p-Si of the active layer of the TFT. As a result, the electrical operating characteristics and reliability of the liquid crystal display device can be improved.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the invention of a liquid crystal display device according to the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a view showing a sectional structure of a p-Si TFT used in the liquid crystal display device according to the present invention. Less than,
The structure will be described according to the manufacturing process.

As a substrate for a TFT array substrate of a liquid crystal display device, a quartz substrate 1 formed so as to contain OH groups in an amount of 200 ppm or more, specifically 500 + 200 ppm is used in the embodiment of the present invention. The thickness of this quartz substrate 1 is 1.1 mm.

On the quartz substrate 1, a p-S film having a film thickness of 100 nm is formed.
An active layer 2 made of an i film is provided. This active layer 2
Is obtained by polycrystallizing amorphous silicon as a starting material by a solid phase growth method to form p-Si. The active layer 2 is formed into a so-called island pattern by separating the elements after forming the p-Si film.

The gate oxide film (gate insulating film) 3 is an oxide film formed by a thermal oxidation method and has a film thickness of 70 nm.

The gate electrode 4 is an electrode formed by patterning a p-Si film doped with phosphorus (P) and having a reduced resistance by etching or the like, and has a film thickness of 400 nm.

The first interlayer insulating film 5 is formed of an oxide film having a thickness of 800 nm, and the metal wiring 6 made of Al is passed through the contact hole formed in the first interlayer insulating film 5 and the active layer 2 is formed.
Is connected to the upper portion of the source region 7.

The hydrogenation treatment of p-Si of the active layer 2 is performed after forming up to this state. That is, the entire TFT array substrate formed to this state is exposed to hydrogen plasma, and hydrogen is diffused to the active layer 2.
The p-Si dangling bond is effectively terminated by the composition of the quartz substrate 1, and the electrical characteristics and reliability of the active layer 2 used as a TFT for a liquid crystal display device are effectively improved.

Then, the second interlayer insulating film 8 is formed of a laminated film of a nitride film / oxide film, a contact hole is bored therethrough, and the second interlayer insulating film 8 is connected to the upper part of the drain region 9 of the active layer 2 through the transparent hole. The electrode 10 is formed by using ITO (indium tin oxide compound) as a material, and the main part of the TFT portion of the TFT array substrate 11 as shown in FIG. 3 is formed.

Further, the TFT array substrate 11 and the counter substrate 12 on which a counter electrode (not shown) is formed are arranged so as to face each other with a gap, and the liquid crystal layer 13 is sandwiched between the two substrates. The periphery of the liquid crystal display device is sealed with a sealing material 14, and an exterior assembly 15 is applied to form a main part of the liquid crystal display device of the present invention. Further, in the case of a liquid crystal display device integrated with a drive circuit, the periphery of the quartz substrate 1 outside the display region has a structure similar to that of the TFT 17 as a switching element as shown on the right side of FIG. The TFT 18 set to a suitable specification as an element for a drive circuit,
The TFT 17 is arranged in the same process as the TFT 17.

Here, in order to confirm the effect of the present invention,
We use the TF for the liquid crystal display device according to the present invention as described above.
The following experiment was conducted using the T array substrate 11 (TFT 17 formed on the T array substrate 11).

As the substrate 101 according to the present invention, (1) O
Substrate containing 200 ppm of H group and conventional substrate 10
As (2), (2) a p-SiTFT having the above-mentioned structure is formed on a substrate containing 50 ppm of OH groups, and hydrogen is supplied to the channel region 16 of the p-SiTFT, particularly the active layer 2. Was treated.

Experiments were conducted for the hydrogenation treatment time of 1 hour and 3 hours for all the substrates.

The mobility and threshold voltage were evaluated as the finally obtained electrical operating characteristics of the TFT 17. The result is shown in FIG.

First, when the mobility was confirmed, the processing time
With the substrate 101 according to the present invention for 1 hour, 65 cm ² / vs
It is sufficiently fast, but with the substrate 102, 25 cm ² / v
It stopped at a speed of about s.

Further, when the processing time was set to 3 hours, the mobility of all the substrates was improved as an absolute value, but the substrate 101 according to the present invention of (1) was 115 cm ² / vs.
However, with the conventional substrate 102 of (2), it is 35 cm ² / vs, which is only 1.4 times at most. Therefore, the mobility of the substrate 101 and the substrate 1
Regarding the ratio with the mobility of 02, there is an even larger difference from 0.38: 1 to 0.30: 1.

That is, the longer the hydrogenation treatment time, the more
The characteristics of the substrate 101 are improved steadily along with that, but the increase rate of the substrate 102 is lower than that.

Further, the threshold voltage has a similar tendency.
After the processing time of 1 hour, the substrate 101 had a high threshold voltage of about 2.5V, while the substrate 102 had a high threshold voltage of about 8V.

When this is treated for 3 hours, the substrate 101 could be reduced to about 1V, but the substrate 102 could be reduced to only about 7V.

Considering the quartz substrate, a network is basically formed by Si and O. However, since it has a glassy structure, it is not as rigid as a crystal. Therefore, various bonds and loose bonds occur. (The state of this bond can be measured by the FTIR method or the like.) Since the highly heat-resistant substrate that is said to have few OH groups has relatively few multi-membered rings, those high heat-resistant substrates that have been used as conventional substrates can be used. It is considered that when the heat-resistant substrate was used, hydrogenation did not sufficiently work on the TFT formed thereon.

Therefore, by using a substrate having a large number of OH groups, hydrogenation can be sufficiently performed and defects at the interface between the active layer and the gate oxide film can be reduced. Therefore, T
FT mobility, threshold voltage, drain leakage current, etc.
The electrical characteristics are improved, and in the liquid crystal display device using such a TFT, the voltage writing to the pixel can be sufficiently performed, and the leak current when the TFT is off can be suppressed. Can also be reduced. Therefore, a liquid crystal display device having good display quality and high reliability can be realized.

Here, it has been generally said that a substrate having a large amount of OH groups is likely to undergo thermal deformation during heat treatment. surely,
Due to the molecular structure, thermal deformation was likely to occur when heat treatment was performed at 1000 ° C or higher. However, it was also confirmed that such thermal deformation has almost no practical problem in the case of the heat treatment process at about 900 ° C or less.

By the way, the reason why the composition of the material of the electrically insulating substrate according to the present invention is that the OH group is 200 ppm or more is that the liquid crystal display device formed by using the substrate according to the present invention is used. Although the threshold voltage of the TFT can be lowered to 1V, when the conventional substrate is used, the threshold voltage of the TFT can be reduced to about 7V at most. Therefore, the conventional substrate is used. in case of,
This is because it is not possible to comply with the drive voltage generally adopted as a preferable drive voltage for a liquid crystal display device.

That is, in the field of liquid crystal display devices, 15 V on the signal side and 6 V on the scanning side are supported as general driving voltages, and further, in recent years, further miniaturization, thinning and low power consumption of liquid crystal display devices have been achieved. To cope with this, it is necessary to further reduce the driving voltage. However, when the conventional substrate is used, the threshold value of the TFT cannot be lowered below 7V, as is clear from FIG.
There is a problem that not only the current threshold voltage of the TFT is not met, but also the drive voltage cannot be further lowered. However, it is needless to say that the threshold voltage can be adapted to the conventional 6V specification by using the substrate having a large number of OH groups as in the present invention, and further lower voltage can be sufficiently dealt with. All of the conventional problems can be solved.

[0061]

As is clear from the detailed description above, according to the present invention, a polycrystalline silicon thin film transistor element having particularly good mobility and threshold characteristics and high reliability is provided, and an excellent display is obtained. A liquid crystal display device that can realize performance can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an outline of a structure of a p-Si TFT formed on a quartz substrate.

FIG. 2 shows, among the operating characteristics of thin film transistors (TFTs) respectively formed on (1) the substrate 101 according to the present invention and (2) the conventional substrate 102, the mobility changed depending on the hydrogenation treatment time. It is a figure which shows the measurement result of and threshold voltage.

FIG. 3 is a diagram showing an outline of a liquid crystal display device of the present invention.

FIG. 4 is a diagram showing a diffusion path through which hydrogen diffuses through a substrate during a general hydrogenation process.

[Explanation of symbols]

 1 ... Quartz substrate 2 ... Active layer 3 ... Gate oxide film 4 ... Gate electrode 5 ... First interlayer insulating film 6 ... Metal wiring 7 ... Source region 8 ... Second interlayer insulating film 9 ... Drain region 10 ... Transparent electrode 11 ... TFT array substrate 12 ... Counter substrate 13 ... Liquid crystal layer 14 ... Encapsulation material 15 ... Exterior assembly

Claims (2)

[Claims] 1. A polycrystalline silicon thin film transistor element as an element forming at least one of a liquid crystal drive circuit and a pixel section switching element, and a pixel electrode connected to the pixel section switching element and having a voltage application controlled. A switch element array substrate formed on the first electrically insulating substrate, and a counter electrode facing the pixel electrode with a gap therebetween are formed on the second electrically insulating substrate, A counter substrate, which is arranged so as to face the switch element array substrate while maintaining the gap, and is sealed in the gap.
A liquid crystal display device having a sandwiched liquid crystal layer, wherein the first electrically insulating substrate of the switch element array substrate is formed of a material having silicon and oxygen in a skeleton of a molecular network. The liquid crystal display device is characterized in that the molecular structure of the material forming the electrically insulating substrate is an electrically insulating substrate containing 200 ppm or more of OH groups. 2. A polycrystalline silicon thin film transistor element arranged as an element forming at least one of a liquid crystal drive circuit and a pixel section switching element, and voltage application controlled by being connected to the pixel section switching element. The pixel electrode and the switch element array substrate formed on the first electrically insulating substrate, and the counter electrode facing the pixel electrode with a gap therebetween are formed on the second electrically insulating substrate. In the liquid crystal display device, the liquid crystal display device includes a counter substrate arranged to face the switch element array substrate while holding the gap, and a liquid crystal layer sealed and sandwiched in the gap. The first electrically insulating substrate is an electrically insulating substrate formed of a material having silicon and oxygen in a skeleton of a network of molecular structure, The liquid crystal display device, wherein the molecular structure of the material forming the substrate is electrically insulating substrate containing 200ppm or more to 1000ppm or less OH groups.