JPH0675245A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To lower the drain leak current value by protecting a transparent electrode and passivating only a thin-film transistor necessitating hydrogen- passivation treatment with hydrogen. CONSTITUTION:An insulating substrate 5, a picture element part 1 including a polycrystal silicon thin-film transistor for switching a formed on the substrate 5, a driving circuit part 2 for driving the picture element part 1 formed adjacent to the picture element part 1 and including the transistor and a thin-film transistor array substrate having the protective layer consisting of silicon nitride film 3 or the plasma silicon oxide film 3 formed on the surface of the picture element part 1 and driving circuit part 2 are provided in the liq. crystal display device. At least a part of the thin-film transistor part for switching of the picture element 1 is opened, and the hydrogen concn. of the polycrystal silicon active layer at the open part 4 is higher than that of the polycrystal silicon active layer at the closed part.

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 capable of reducing a drain leak current value of a polycrystalline silicon thin film transistor used in a pixel portion.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have the great advantages of thinness, light weight, and low power consumption. Therefore, they have been actively used for display devices of OA equipment such as liquid crystal televisions, Japanese word processors and desktop personal computers. There is. At the same time, a liquid crystal display device applying a thin film transistor or a thin film transistor array using polycrystalline silicon as an active layer has been actively developed for the purpose of improving display characteristics.

Conventionally, a thin film transistor using polycrystalline silicon as an active layer has been applied to a driving circuit of a pixel switching device by integrating a switching device of a pixel part which is a display part of a liquid crystal display device and a thin film transistor. That is, a pixel portion thin film transistor for applying a voltage to liquid crystal in a pixel and an application to a driving circuit portion thin film transistor for driving the pixel portion thin film transistor.

However, in particular, the thin film transistor in the pixel portion is required to have a reduced drain leak current value as a characteristic thereof. This is because the drain leakage current is generated on the OFF side of the transistor operation, so that the normal O
N / OFF switching function is no longer fulfilled,
This is because the image quality of the liquid crystal display device is particularly affected. The cause of this drain leakage current is that the electric field is concentrated between the gate and drain of the thin film transistor,
Of the silicon bond defects in the active layer polycrystalline silicon,
This is because if there is a defect due to dangling bonds, an abnormal leak current will occur at the drain junction.

As one of the measures to reduce the drain leakage current value, there is a hydrogen passivation technique of forcibly adding hydrogen to the formed transistor from the outside. The forcibly added hydrogen acts as a terminator of dangling bonds of the active layer polycrystalline silicon and releases the carriers captured by the defects, so that the dangling bonds do not act as defects. Therefore, the drain leak current value is reduced. In particular, the effect to lower the leakage current value at a low gate voltage (V _G). Further, the hydrogen passivation technique is effective in lowering the threshold voltage, improving the mobility in the channel, improving the current driving capability, and the like.

Therefore, a liquid crystal display device in which a hydrogen passivation technique is applied to a polycrystalline silicon thin film transistor is widely used in a liquid crystal television, an office automation equipment and the like.

[0007]

However, the hydrogen passivation technique has the following three major problems.

[0008] Part 1, when excessively perform hydrogen passivation, in particular will be shifted to the negative threshold voltage in the n-type thin film transistor, the gate voltage (V _G) is "0
That is, a current flows even in V ″, resulting in a depletion type. Further, when the ratio of the negative shift of the threshold voltage increases, a punch-through current flows out and the characteristics deteriorate when the applied drain voltage is increased. It is believed that this is because the excess hydrogen in the channel acts like a donor.

Therefore, 1) the function as a circuit is lost because normal switching is not performed. 2) Even in the OFF state, a current flows, causing the circuit to generate heat and cause thermal runaway.
For these reasons, there is a problem that the thin film transistor does not exhibit normal switching characteristics, which is a fatal defect for the thin film transistor of the driving circuit.

No. 2 is ITO (Indum Tin) which is a transparent electrode when hydrogen passivation is performed without a protective film.
Oxide) deteriorates, the resistance value rises, and the transmittance decreases. It is considered that this is because the radical of hydrogen in the plasma causes the reduction of ITO.

The third problem is that the effect disappears when exposed to a high temperature process of 350 ° C. or higher after hydrogen passivation. This is because the added hydrogen is desorbed. Therefore, there is a problem that hydrogen passivation must be performed almost at the end of the manufacturing process.

Although the hydrogen passivation technique has the above-mentioned problems, it is an important technique for reducing the drain leakage current value in a liquid crystal display device using a polycrystalline silicon thin film transistor, and the hydrogen passivation technique is improved. Was desired.

The present invention has been made in order to solve such a problem, and protects a transparent electrode, and performs hydrogen passivation only on a polycrystalline silicon thin film transistor which requires hydrogen passivation, so that the drain leakage current value can be reduced. An object is to provide a liquid crystal display device that can be lowered.

[0014]

A liquid crystal display device of the present invention includes an insulating substrate, a pixel portion including a switching polycrystalline silicon thin film transistor formed on the substrate,
A drive circuit portion formed adjacent to the pixel portion and including a polycrystalline silicon thin film transistor for driving the pixel portion, and a protection formed of a plasma silicon nitride film or a plasma silicon oxide film formed on the surface of the pixel portion and the drive circuit portion. In a liquid crystal display device having at least a thin film transistor array substrate having a layer, at least a switching thin film transistor portion of a pixel portion is opened, and a hydrogen concentration of a polycrystalline silicon active layer of the opening portion is a non-opened portion of a polycrystalline silicon active layer. It is characterized by being higher than the silicon active layer.

In the thin film transistor array substrate relating to the liquid crystal display device of the present invention, an opening is provided in a part of a protective layer made of a plasma silicon nitride film or a plasma silicon oxide film formed on the surface of the pixel portion and the driving circuit portion, Hydrogen passivation is performed only on necessary thin film transistors using the plasma silicon nitride film or the plasma silicon oxide film as a mask. As a result, the hydrogen concentration of the polycrystalline silicon active layer in the opening portion becomes higher than that of the polycrystalline silicon active layer in the non-opening portion. The hydrogen concentration of the polycrystalline silicon active layer not subjected to hydrogen passivation is generally 10 ¹⁷ to 10 ¹⁹ atom / cm ³ or less, but hydrogen passivation in the thin film transistor array substrate according to the liquid crystal display device of the present invention is polycrystalline silicon. It is preferable that the hydrogen concentration of the active layer is about 10 ²⁰ to 10 ²¹ atom / cm ³ . Hydrogen concentration is 10 ²⁰ to 10 ²¹ atom /
This is because in the range of cm ³ , the drain leakage current value is reduced and abnormal switching characteristics are not exhibited.

The liquid crystal display device of the present invention is manufactured as follows. As the insulating substrate material, non-alkali glass, quartz or the like can be used. A polycrystalline silicon film is formed on this substrate by a known method. That is, first, the reduced pressure CV is applied
D, an amorphous silicon layer is deposited using a plasma CVD apparatus, and then heat treatment is performed at a temperature of about 600 ° C. to form a polycrystalline silicon layer. After that, elements are separated into a predetermined shape, a gate oxide film is formed, and a gate electrode is formed. I / O implantation for the source and drain is performed on the polycrystalline silicon layer by self-alignment using the gate electrode as a mask.
Type and n-type. After forming the first interlayer insulating film so as to cover the entire surface of the substrate after the ion implantation, a contact hole is formed at a predetermined position, and a metal wiring which makes ohmic contact with the ohmic junction is formed through this contact hole. . Further, a second interlayer insulating film is formed thereon, a contact hole is formed at a predetermined position, and then a transparent electrode to be a pixel portion is formed and processed into a predetermined shape.

A plasma silicon nitride film (p-SiN) or a plasma silicon oxide film (p-SiO) that protects the substrate surface after the above steps is formed on the entire substrate surface by the plasma CVD method. The film forming temperature is preferably 350 ° C or lower. After that, at least the switching thin film transistor portion of the pixel portion is opened by a photolithography method. The opening portion may be only the thin film transistor portion for switching of the pixel portion, but it is also preferable to further open the buffer of the output stage of the drive circuit portion and the analog switch portion of the output stage of the drive circuit portion. in this case,
In particular, the current driving capability is improved. The opening is processed into a predetermined shape by a dry etching method. Finally, hydrogen passivation is performed. Hydrogen passivation is preferably performed at 200 ° C or higher, for example, capacitive junction type p-CV.
D by exposure to hydrogen plasma at 270 ° C. Further, it is preferable that the temperature is higher than that of the plasma silicon nitride film formation. As a result, the hydrogen concentration can be changed between the opening and the non-opening. A liquid crystal display device can be obtained by stacking the thin film transistor array substrate thus manufactured on a counter substrate, injecting liquid crystal, and laminating the liquid crystal.

[0018]

Function: A plasma silicon nitride film (p-
SiN) or plasma silicon oxide film (p-SiO)
Does not permeate hydrogen even in hydrogen plasma.

It is considered that this is due to the following reason. 1) Since p-SiN and p-SiO already have a high concentration of hydrogen (about several percent), hydrogen from the outside does not diffuse there. 2) Since p-SiN and p-SiO themselves release hydrogen at 200 ° C or higher, hydrogen from hydrogen plasma does not particularly diffuse into the film. 3) At temperatures below 300 ° C, p-SiN, p-Si
The hydrogen released from O rarely diffuses through the solid. Therefore, the plasma silicon nitride film (p-SiN) or the plasma silicon oxide film (p-SiO) sufficiently acts as a mask when hydrogen passivation is performed at 200 ° C. or higher. Therefore, the required amount of hydrogen passivation can be performed only in the opening while protecting the transparent electrode and the like.

[0020]

Embodiments of the present invention will be described with reference to FIGS. FIG. 1 is a diagram of a completed state of a thin film transistor array substrate. FIG. 2 is an enlarged view of the pixel unit 1 shown in FIG. FIG. 3 is a sectional view of a thin film transistor having an opening and a non-opening.

The pixel portion 1 is formed on the substrate 5 by the thin film transistor shown in FIG. 3, the pixel capacitor shown in FIG. 2 and the transparent electrode. The drive circuit unit 2 is composed of a shift register 2a, a buffer or an analog switch 2b using the thin film transistor shown in FIG. 3 as a CMOS. Then, the final protective film is formed of the plasma silicon nitride film 3, but the opening 4 is formed at a predetermined position.

In this embodiment, only the thin film transistor of the pixel portion 1 is opened. This is because the thin film transistor of the pixel portion 1 requires a very small drain leak current, and thus hydrogen passivation is absolutely necessary. Therefore, as shown in FIG. 1, the thin film transistor array substrate is completed in a state in which the opening 4 is formed in the predetermined pixel thin film transistor in the plasma silicon nitride film 3 which is the final protective film. Then, at the end of the process, this state is exposed to hydrogen plasma. For this reason, hydrogen is added only to the thin film transistor in the opening 4, and hydrogen is not added to the other portions, which shows the passivation effect. As a result, the drain leak current is reduced only in the thin film transistor in the pixel portion, and desired characteristics can be obtained.

Further, the details of the pixel portion 1 and the opening portion 4 of this embodiment will be described with reference to FIGS. Pixel part 1
Forms a gate oxide film 7 on the active layer polycrystalline silicon 6 and forms a gate wiring polycrystalline silicon 8. After that, the source / drain diffusion portions 9 are formed in a self-aligned manner. A first interlayer insulating film 10 is formed thereon, and a metal wiring 11 is formed via a contact hole 12. Further, the second interlayer insulating film 14 is formed, and the transparent electrode 1 is formed thereon.
3 is formed. After that, a plasma silicon nitride film 3 is formed as a final protective film, and a predetermined opening 4 is formed. In this embodiment, the opening 4 is a thin film transistor portion of the pixel portion as shown in FIG. The other portion is covered with the plasma silicon nitride film 3 and serves as a mask in hydrogen plasma. Therefore, the transparent electrode 13 does not deteriorate. Further, as shown in FIG. 3, even in the thin film transistors adjacent to each other, hydrogen passivation is performed using plasma silicon nitride as a mask.

Thereafter, as shown in FIG. 4, the formed thin film transistor array is bonded to the counter substrate 16,
Liquid crystal 15 is injected, and exterior assemblies 5a, 5b, 5c
Then, the liquid crystal display device is completed.

Next, the manufacturing method of this embodiment will be described. 1) The substrate is a quartz substrate, the size is 5 inches, and the substrate thickness is 1.1.
mm is used. 2) The active layer is formed by vertical low pressure CVD using disilane (S
i ₂ H ₆ ) and amorphous-silicon film thickness T _poly = 2
000 Å deposited. After that, heat treatment is performed at a temperature of 600 ° C. to form a polycrystalline silicon film. 3) If necessary, after peeling the back surface, element isolation is performed into a predetermined shape. 4) If necessary, dummy oxidation is performed to ion-implant the active layer. After that, gate oxidation is performed with a film thickness T _ox = 500 Å, and gate poly deposition is continuously performed with a film thickness T _poly = 4000 Å. 5) Ion implantation is performed on the source and drain. Here, one of p-type and n-type is selected. 6) Form a first interlayer insulating film. 7) Form a contact hole in the first interlayer insulating film. 8) Form metal wiring. 9) A second interlayer insulating film is formed and a contact hole is formed at a predetermined place. 11) A transparent electrode is formed and processed into a predetermined shape. 11-1) A plasma silicon nitride film (p
-SiN) is formed. However, the film formation temperature at this time is
It is 170 ℃. 11-2) Pass through a photolithography process and process into a predetermined shape by a dry etching method. 11-3) Hydrogen is passivated by exposing it to hydrogen plasma at 270 ° C. by capacitive junction type p-CVD. 12) Cut out from the substrate into a predetermined shape. After that, the liquid crystal display device is obtained by stacking with a counter substrate, injecting liquid crystal, and bonding.

The characteristics of the liquid crystal display device thus obtained are shown in FIGS. This characteristic is the change in the drain current (I _D) in the case of changing the gate voltage (V _G) of the thin film transistors are used in a pixel portion (transfer characteristics). FIG. 5 shows a plasma silicon nitride film (p-
FIG. 6 shows the transfer characteristics of the drive circuit thin film transistor hydrogen-passivated using SiN) as a mask, and FIG. 6 shows the transfer characteristics of the thin film transistor hydrogen-passivated without a mask. Note that FIG.
6A and 6B show the characteristics before hydrogen passivation, and FIG. 6B shows the characteristics after hydrogen passivation. As is clear from this result, the drain leak current value is lower in FIG. 6 than in FIG. 5, and the hydrogen passivation is effective. Therefore, according to the method described above, it is possible to obtain the effect of reducing the drain leakage current of the pixel thin film transistor without significantly changing the characteristics of the thin film transistor of the driving circuit unit.

The liquid crystal display device of this embodiment has the following effects. The first effect is a reduction in the drain leak current of the thin film transistor in the opening as shown in FIG. As an example, the characteristics of the thin film transistor used in the pixel portion are shown, but it is reduced by about one digit compared with the conventional example. On the other hand, the characteristic change of the thin film transistor of the representative circuit portion in the unopened portion can hardly be confirmed. Therefore, the drain leakage current of the thin film transistor in the pixel portion can be reduced without changing the characteristics of the thin film transistor in the circuit portion. As a result, the thin film transistor in the driving circuit portion operates at high speed without the threshold voltage shifting, and the thin film transistor in the display pixel portion has a small drain leakage current, and the liquid crystal applied voltage is well held, and is a high-performance liquid crystal display. The device can be obtained.

As a second effect, p-SiN is effective as a mask, and it is possible to selectively increase the hydrogen concentration in only the pixel portion and perform hydrogen passivation.

As a third effect, since p-SiN is formed at a film forming temperature of 350 ° C. or less, hydrogen diffuses to the thin film transistor during the film formation of this plasma silicon nitride, and the effect of hydrogen passivation is exhibited. There is nothing to do. The hydrogen plasma treatment is performed at a temperature of 200 ° C. or higher, which is higher than that of the plasma silicon nitride film formation. For this reason,
Since hydrogen is released from plasma silicon nitride during processing,
Further, hydrogen from the hydrogen plasma does not diffuse. Further, high-concentration hydrogen (about several percent) is already mixed in the formed plasma silicon nitride, and hydrogen does not diffuse even when exposed to hydrogen plasma. Therefore, when using plasma silicon nitride at the above temperature setting,
It has a masking effect that hydrogen does not permeate. Further, most of the released hydrogen is released to the surface side. That is, at a temperature of about 400 ° C or lower, the diffusion coefficient of hydrogen into a solid is small and causes almost no problem.

As a fourth effect, since the transparent electrode having almost no resistance in hydrogen plasma can be completely protected, the characteristics of the transparent electrode do not change after hydrogen passivation.

The second embodiment is an example in which the opening area is expanded to the buffer or the analog switch which is the output stage of the drive circuit. This is especially because buffers and analog switches require current drivability (how much drain current can flow with a change in a certain gate voltage), and hydrogen passivation is also effective for obtaining this characteristic. As an effect, in addition to the above-described first to fourth effects, the current driving force of the output stage is improved, and high-speed operability and operation reliability are improved.

In the third embodiment, the final protective film is a plasma silicon oxide film. Since this film also contains a large amount of hydrogen in the film, it has a masking effect similar to the above. Therefore, even if this film is used as the final protective film, the same effect is obtained.

[0033]

The liquid crystal display device of the present invention is a thin film transistor having a pixel portion formed on a substrate and a protective layer made of a plasma silicon nitride film or a plasma silicon oxide film formed on the surface of a driving circuit portion. Since at least the switching thin film transistor portion of the pixel portion is opened in the array substrate, hydrogen passivation can be selectively performed only in a necessary portion.

Further, as a result, the hydrogen concentration of the polycrystalline silicon active layer in the opening portion becomes higher than that of the polycrystalline silicon active layer in the non-opening portion, so that the drain leak current of the thin film transistor can be reduced and the liquid crystal is applied. It is possible to obtain a high-performance liquid crystal display device having good voltage retention.

Further, since hydrogen passivation can be easily and selectively carried out, failures in the manufacturing process are eliminated and a liquid crystal display device having excellent reliability can be obtained.

[Brief description of drawings]

FIG. 1 is a completed state diagram of a thin film transistor array of an example.

FIG. 2 is an enlarged view of a pixel portion shown in FIG.

FIG. 3 is a cross-sectional view of a thin film transistor of an example.

FIG. 4 is a diagram showing a liquid crystal display device of an example.

FIG. 5 is a diagram showing transfer characteristics of a masked thin film transistor.

FIG. 6 is a diagram showing transfer characteristics of an unmasked thin film transistor.

[Explanation of symbols]

1 ... Pixel part, 2 ... Drive circuit part, 2a ......... Shift register, 2b ...... Buffer or analog switch, 3 ......... Plasma silicon nitride film, 4 ......... Opening part, 5 ... ...... Substrate, 5a, 5b, 5c ……. Exterior assembly, 6 ……. Active layer polycrystalline silicon, 7 ……. Gate oxide film, 8 ………… Gate wiring polycrystalline silicon, 9 ...
...... Diffusion parts of source and drain, 10 ... First interlayer insulating film, 11 ... Metal wiring, 12 ... Contact hole, 13 ... Transparent electrode, 14 ... Second interlayer insulating film, 15 ... Liquid crystal, 16 ... Counter substrate.

Claims (1)

[Claims] 1. An insulating substrate, a pixel portion including a switching polycrystalline silicon thin film transistor formed on the insulating substrate, and a polycrystalline silicon thin film formed adjacent to the pixel portion and driving the pixel portion. A liquid crystal display device comprising at least a thin film transistor array substrate having a drive circuit section including a transistor and a protective layer formed on the surface of the pixel section and the drive circuit section and formed of a plasma silicon nitride film or a plasma silicon oxide film, The polycrystalline silicon thin film transistor portion for switching of the pixel portion is opened, and the hydrogen concentration of the polycrystalline silicon active layer in the opening portion is higher than the hydrogen concentration of the polycrystalline silicon active layer in the non-opening portion. Liquid crystal display device.