JP3086142B2 - Liquid crystal display - Google Patents

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 using a thin film transistor (hereinafter, referred to as a TFT) as a switching element.

[0002]

2. Description of the Related Art As a liquid crystal display device using the above-mentioned thin film transistor, the one shown in FIG. 5 is conventionally known. This liquid crystal display device has a configuration in which liquid crystal is sealed between a pair of opposing substrates, and one (lower) substrate has a plurality of transparent pixel electrodes on an insulating substrate 53a such as glass or quartz. 51 and TFTs 52 are formed in a matrix, and a plurality of gate lines 57 and a plurality of data lines 59 are formed.
Cross each other and are insulated and separated by an interlayer insulating film provided therebetween. The drain electrode 56 of the TFT 52
Is connected to a pixel electrode 51, a gate electrode is connected to a gate line 57 that supplies a scanning signal for turning on and off the TFT, and a source electrode 58 is connected to a data line 59 that inputs a video signal to the pixel electrode via the TFT. I have.

The other (upper) substrate opposing the one substrate includes a counter electrode 54, a black stripe (light shielding layer)
The opposing substrate 53b on which 512 is formed is bonded with a gap of several μm. Note that a portion where the black stripe 512 is not formed forms a pixel opening 513.

A liquid crystal material is sealed in a gap between the two substrates, and an alignment film 55 for aligning liquid crystal molecules is formed on the liquid crystal side of the two substrates and rubbed. Furthermore,
A polarizing plate 511 is provided on the outside of both substrates opposite to the liquid crystal.

When a display is performed using such a liquid crystal display device, it is necessary that a voltage applied to a pixel electrode at a certain scanning timing is sufficiently held at the pixel electrode until the next scanning timing. Usually, the liquid crystal capacitance between the pixel electrode and the counter electrode is not sufficient, so that a charge holding capacitor (hereinafter referred to as an auxiliary capacitance) is formed in parallel with the pixel electrode. In the liquid crystal display device of FIG.
A storage capacitor line 510 is provided separately from the pixel electrode 5.
A storage capacitor is formed in a portion where the storage capacitor 1 and the storage capacitor line 57 overlap each other, using the insulating film formed therebetween as a dielectric. For example, as disclosed in JP-A-63-70832, this auxiliary capacitance is provided without providing another auxiliary capacitance line.
The gate line may also be used as an auxiliary capacitance line, and may be formed at a portion where the gate line 57 and the pixel electrode 51 overlap.

In driving this liquid crystal display device, FIG.
As shown in (1), by sequentially inputting the scanning pulse signal 61 to the gate line and inputting the video signal 62 to the data line, the video signal is input to the pixel electrode.
As a result, the voltage 63 applied to the liquid crystal is changed according to the image signal, and the light transmittance of the pixel is changed using the electro-optical characteristics of the liquid crystal to display an image.

[0007]

The above-described TFT has a parasitic capacitance Cgd between the gate electrode and the drain electrode, so that the ON signal and the OFF signal are generated at the moment when the gate signal changes from the ON signal to the OFF signal. 6 and the potential of the pixel electrode shifts by an amount indicated by 64 in FIG. This shift amount is Cgd / (Cgd + Clc, where Clc is the capacitance value of the liquid crystal and Cs is the capacitance value of the auxiliary capacitance.
+ Cs). In this case, since the liquid crystal has dielectric anisotropy, Clc fluctuates according to the voltage applied to the liquid crystal. Therefore, the shift amount of the pixel electrode potential changes according to the video signal, and a DC (direct current) component is generated. Generally, when a DC voltage is applied to the liquid crystal, flicker occurs to degrade display quality. In addition, an afterimage, which is considered to be caused by the formation of the electric double layer at the interface between the alignment film and the liquid crystal, occurs.

It is also known that the liquid crystal capacitance changes according to the applied voltage, so that the image response characteristics deteriorate. This phenomenon is called JAPAN DISP
As described on pages 416 to 417 of LAY '89 DIGEST, TF satisfying the condition of charge preservation is satisfied.
This occurs because the voltage applied to the liquid crystal capacitance changes due to the change in the capacitance of the liquid crystal during the OFF period of T.

To solve these problems, it is effective to increase the capacitance value of the auxiliary capacitance to reduce the influence of the fluctuation of the liquid crystal capacitance. However, simply increasing the capacitance value of the auxiliary capacitance is insufficient in the charging capability of the TFT.
There is a problem that display characteristics are deteriorated, such as a decrease in image contrast. Also, when the capacitance value of the auxiliary capacitor is increased, the area for forming the auxiliary capacitor is increased, and the pixel opening (513 in FIG. 5) is reduced. Therefore, there is a problem that the aperture ratio is reduced and the image becomes dark.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art. The present invention reduces the influence of a change in the liquid crystal capacitance due to the voltage, secures a sufficient charge capacity of the TFT, and further increases the aperture ratio. It is an object of the present invention to provide a liquid crystal display device which can be used.

[0011]

A liquid crystal display device according to the present invention comprises a pixel electrode for applying a voltage to the liquid crystal, and a pixel electrode for switching the pixel electrode, provided on one of a pair of substrates sandwiching the liquid crystal. Thin film transistors are provided in a matrix, and a plurality of scanning lines for scanning the thin film transistors and a plurality of data lines for supplying video signals to the pixel electrodes through the thin film transistors cross each other and are insulated and separated. And a semiconductor layer of the thin film transistor is made of polycrystalline silicon having a mobility of 10 cm ² / V · S or more, and the pixel electrode And the scanning line or the auxiliary capacitance line are formed so as to partially face each other with an interlayer insulating film interposed therebetween, and the pixel electrode and the scanning line or the auxiliary line of the interlayer insulating film are formed. A storage capacitor is formed by filling a through hole provided in a portion facing the quantum line with a high dielectric thin film, and a capacitance value of the storage capacitor is determined by a liquid crystal formed between the pixel electrode and the counter electrode. The capacitance is set to be at least ten times the capacitance value, thereby achieving the above object.

In the liquid crystal display of the present invention, it is preferable that the high dielectric thin film has a relative dielectric constant of 20 or more.

In the liquid crystal display device of the present invention, the high dielectric thin film is made of tantalum oxide, Pb (Zr, Ti) O ₃ ,
(Pb, La) (Zr, Ti) O 3, BaTiO 3, Sr
TiO ₃ , Ba _1-x Sr _x TiO ₃ (0 <X <1), Ba ₄
Ti ₃ O ₁₂ , Bi ₂ WO ₆ , Bi ₂ WO ₃ and SrTaO
And one or more of these materials.

In the liquid crystal display device of the present invention, the high dielectric thin film is formed by forming a material having a relative dielectric constant of 20 or more at a temperature of 600 ° C. or less.

[0015]

In the present invention, the capacitance value of the auxiliary capacitance is at least 10 times (preferably at least 20 times) the capacitance value of the liquid crystal capacitance formed between the pixel electrode and the counter electrode. The effect of the change due to the voltage can be reduced. In addition, the semiconductor layer of the TFT has a mobility of 10 cm ² / V ·
Since it is made of polycrystalline silicon of S or more, even if the capacitance value of the auxiliary capacitance is increased, the charging capability of the TFT can be made sufficient.

When a material having a relative dielectric constant of 20 or more is used as the high dielectric thin film, the auxiliary capacitance value can be increased and the aperture ratio can be increased. Such materials include tantalum oxide, PZT (Pb (Zr, Ti) O ₃ ), PLZ
T ((Pb, La) (Zr, Ti) O ₃ ), BaTi
O ₃ , SrTiO ₃ , Ba _1-x Sr _x TiO ₃ (0 <X <
1), a material containing one or more of Ba ₄ Ti ₃ O ₁₂ , Bi ₂ WO ₆ , Bi ₂ WO ₃ and SrTaO can be used.

A high dielectric thin film material is filled in a through-hole formed in the interlayer insulating film by using the scanning line also as an auxiliary capacitance line, and this is used as a dielectric material in an overlapping portion of the pixel electrode and the scanning line to form an auxiliary capacitance. Or an auxiliary capacitance line may be formed separately, and an auxiliary capacitance may be formed at an overlapping portion of the auxiliary capacitance line and the pixel electrode.

When the high dielectric thin film is formed at a temperature of 600 ° C. or less, no distortion occurs even when a glass substrate is used, and a high quality liquid crystal display device can be obtained.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view showing one substrate (TFT forming side substrate) of a liquid crystal display device according to one embodiment of the present invention. This TFT formation side substrate has a plurality of transparent pixel electrodes 11 and TFTs 10 formed in a matrix on an insulating substrate, a plurality of gate lines 22 for supplying a scanning signal for turning on and off the TFT 10, and a TFT. A plurality of data lines 13 for inputting video signals to the pixel electrodes 11 cross each other and are insulated and separated by an interlayer insulating film (not shown) provided therebetween.

The semiconductor layer 21 of the TFT 10 has a mobility of 10c.
It is made of polycrystalline silicon of m ² / V · S or more, and two regions of the semiconductor layer 21 are a drain region and a source region. The drain region of the TFT 10 is connected to the pixel electrode 11 via the connection line 14 at the contact hole 12 of the interlayer insulating film, the gate electrode is branched from the gate line 22, and the source region is the contact hole 1 of the interlayer insulating film.
2 is connected to the data line 13.

The gate line 22 corresponding to the adjacent pixel electrode 11 is also used as an auxiliary capacitance line, and a superposed portion where the pixel electrode 11 and the gate line 22 face each other has
A high dielectric thin film 41 is filled in the through hole 31 of the interlayer insulating film, and an auxiliary capacitance is formed in parallel with the pixel electrode 11 using this as a dielectric. This high dielectric thin film 41 is made of a high dielectric thin film material having a relative dielectric constant of 20 or more, and the capacitance value of the auxiliary capacitance is 10 times the capacitance value of the liquid crystal capacitance formed between the pixel electrode 11 and the counter electrode. The above is desirably 20 times or more.

This substrate is bonded to the opposing substrate on which the opposing electrode and the black stripe are formed with a gap of several μm therebetween, and a liquid crystal material is sealed in the gap. An alignment film for aligning liquid crystal molecules is formed on the inside of both substrates and rubbed, and polarizing plates are provided on the outside of both substrates.

This liquid crystal display device can be manufactured, for example, as follows. First, LPCVD (Low Pressure C) using a Si ₂ H ₆ gas is formed on a glass substrate having a high strain point temperature that can withstand a heat treatment of about 600 ° C.
chemical Vapor Deposition
According to n), an amorphous silicon film is formed at a substrate temperature of about 450 ° C. In an N ₂ atmosphere at 600 ° C. for about 2
Anneal for 0 hours and perform solid phase growth to obtain a polycrystalline silicon film. Further, the polycrystalline silicon film is melted and recrystallized by excimer laser annealing, thereby improving the crystallinity of the polycrystalline silicon film and increasing the mobility. Thereafter, the polycrystalline silicon is etched to form an island-shaped semiconductor layer 21 as shown in FIG.

Next, a plasma C is formed on the semiconductor layer 21.
A SiO ₂ film serving as a gate insulating film is formed by a VD method to about 100
is formed to a thickness of nm. A polycrystalline silicon film having a thickness of about 250 nm is formed thereon, and is patterned to form a gate line 22.

An impurity element (phosphorus in the case of Nch, boron in the case of Pch) is ion-implanted into the substrate in this state under the conditions of about 1 × 10 ¹⁵ ion / cm ² and about 40 keV, and 550 is implanted. Activate impurity ions at ° C. By this step, impurity ions are implanted into the semiconductor layer 21 except immediately below the gate line 22 to form a source region and a drain region, and the semiconductor layer 21 immediately below the gate line 22 becomes a channel portion. At the same time, impurity ions are also implanted into the gate line 22 made of polycrystalline silicon, so that the resistance of the gate line 22 is reduced. If it is necessary to further reduce the resistance, a metal wiring such as aluminum may be superimposed on the gate line made of polycrystalline silicon into which the impurity ions have been implanted.

Next, an interlayer insulating film made of a SiO ₂ film having a thickness of about 500 nm is formed, and a through hole having a size required for an auxiliary capacitor is formed in the interlayer insulating film on the gate line 22 as shown in FIG. 31 is formed in a penetrating state. The size of the through hole 31 can be made different depending on the dielectric material used and the liquid crystal capacitance. The through hole 31
Need not necessarily be formed in a penetrating state, and an interlayer insulating film may remain at the bottom. That is, the interlayer insulating film also has a dielectric constant, and the dielectric constant required for the storage capacitor may be designed based on the relationship between the dielectric constant and the dielectric constant of tantalum oxide, which is a high dielectric thin film described later.

Thereafter, tantalum oxide is deposited by a sputtering method, and dry etching is performed. This allows
As shown in FIG. 4, a high dielectric thin film 41 made of tantalum oxide, which is a pattern approximately 2 μm larger in each of the vertical and horizontal directions than the through hole 31 of the interlayer insulating film, is formed of the through hole 31.
It is formed by filling inside.

Next, annealing is performed at 550 ° C. in an oxygen atmosphere to obtain a high dielectric thin film 41 made of tantalum oxide.
To reduce the leakage current. The relative permittivity of the tantalum oxide film thus formed is 20 to 25, which is higher than the relative permittivity of the conventional silicon oxide film of about 3.5 which is used as the dielectric of the auxiliary capacitance. Therefore, the value of the auxiliary capacitance can be increased even if the small-sized auxiliary capacitance is formed as described above.

Thereafter, a transparent conductive film of ITO (Ind
(Im Tin Oxide) is formed by sputtering, and is patterned by etching to form the pixel electrode 11 as shown in FIG. As a result, the gate line 22 corresponding to the pixel adjacent to the pixel electrode 11 is also used as an auxiliary capacitance line, and the high dielectric thin film 41 formed between the part where the gate line 22 and the pixel electrode 11 partially oppose each other. Is used as a dielectric to form an auxiliary capacitance.

Further, after a contact hole 12 is formed in the interlayer insulating film portion on the source region and the drain region, for example, aluminum or the like is formed and patterned to form the data line 13 and the drain region and the pixel electrode 1.
1 is formed. Further, a silicon nitride film is formed thereon, and a connection terminal portion with a peripheral driving circuit is removed by etching to form a protective film.

Thereafter, an alignment film for aligning liquid crystal molecules is formed on the substrate and on the counter substrate on which the counter electrode and the black stripe are formed, and a rubbing process is performed. A liquid crystal material is sealed in the gap between the substrates to form a liquid crystal panel. By providing polarizing plates on both sides of the liquid crystal panel, the liquid crystal display device of this embodiment is completed.

The liquid crystal display thus obtained is
Since the capacitance value of the auxiliary capacitance is as large as 10 times or more the capacitance value of the liquid crystal capacitance, the influence caused by the voltage change of the liquid crystal capacitance was small. Further, since the mobility of the semiconductor layer constituting the TFT is as high as 10 cm ² / V · S,
Even when the capacitance value of the auxiliary capacitor was increased, the charging capability of the TFT could be sufficiently ensured. Further, a high dielectric material having a relative dielectric constant of 20 or more is used as the dielectric material of the auxiliary capacitor, and the capacitance value of the auxiliary capacitor can be increased without increasing the area of the auxiliary capacitor. I was able to.

In the above embodiment, tantalum oxide is used as the dielectric of the auxiliary capacitance, but the present invention is not limited to this, and other materials may be used. In particular, it is desirable to use a high dielectric thin film material having a relative dielectric constant of 20 or more, since it is possible to increase the auxiliary capacitance value and increase the aperture ratio. For example, tantalum oxide, PZT, PLZT, BaT
iO ₃ , SrTiO ₃ , Ba _1-x Sr _x TiO ₃ (0 <X <
_{1), Ba 4 Ti 3 O} 12, Bi 2 WO 6, Bi 2 WO 3 and of SrTaO, it is possible to use a material containing at least one kind or more. As a technique for using such a material having a high dielectric constant as an auxiliary capacitor, see Japanese Patent Application Laid-Open No. 5-6316.
No. 8 discloses that a perovskite oxide is constituted by an auxiliary capacitance, but this is a means for securing a large auxiliary capacitance value with a smaller auxiliary capacitance area. Therefore, the present invention is technically different from the present invention in which the auxiliary capacitance value is made larger than before and the driving TFT is a polycrystalline silicon TFT having high mobility.

In the above embodiment, the high dielectric thin film is formed by the sputtering method, but may be formed by the vapor deposition method. When the film is formed at a temperature of 600 ° C. or lower, which is lower than the strain point of the glass substrate, no distortion occurs even when the glass substrate is used. For the same reason, the annealing treatment for reducing the leakage current is also 6
It is desirable to carry out at a temperature of 00 ° C or less.

In the step of filling the through holes with tantalum oxide, the tantalum oxide film may be formed by a sputtering method and then dry-etched after the formation of the through holes, and a resist is coated, exposed and developed on the interlayer insulating film. May be performed to form a resist pattern for forming a through hole, and then tantalum oxide may be formed on the resist pattern by a sputtering method and lifted off.
Also, in order to reduce the leakage current of tantalum oxide,
Although annealing was performed at 550 ° C. in an oxygen atmosphere, nitrogen may be contained in the tantalum oxide by flowing nitrogen into the chamber during sputtering of tantalum oxide, or tantalum may be used as a target material instead of tantalum oxide. Used, in an oxygen atmosphere after sputtering, 55
Tantalum oxide may be formed by annealing at 0 ° C.

In the above embodiment, a gate line is also used as an auxiliary capacitance line, and a high dielectric thin film material is filled into a through hole formed in an interlayer insulating film, and this is used as a dielectric to form a pixel electrode and a gate line. Although the storage capacitor is formed in the overlapping portion, a storage capacitor line may be formed separately from the gate line, and the storage capacitor may be formed in the overlapping portion using the storage capacitor line and the pixel electrode as both electrodes.

[0038]

As is apparent from the above description, according to the present invention, the capacitance value of the auxiliary capacitance is preferably at least 10 times the capacitance value of the liquid crystal capacitance formed between the pixel electrode and the counter electrode. Is 20 times or more. Therefore, the effect of the change in the liquid crystal capacitance due to the voltage can be reduced to a negligible extent, and the deterioration of the response characteristics, the occurrence of flicker and the afterimage, and the like can be reduced. In this case, the load capacitance to be charged by the TFT becomes large, but since the high mobility polycrystalline silicon having a mobility of 10 cm ² / V · S or more is used as an active layer serving as a channel portion of the TFT, the TFT is charged. Ability to be sufficient. When a high dielectric thin film material having a relative dielectric constant of 20 or more is used as the dielectric of the auxiliary capacitance, the auxiliary capacitance can be increased even if the area of the auxiliary capacitance is small. Therefore, a bright liquid crystal display device with high display quality can be provided with a high aperture ratio. Further, when the high dielectric thin film material is formed at a temperature of 600 ° C. or lower, no distortion occurs even when a glass substrate is used, and a high-quality liquid crystal display device can be obtained at low cost.

[Brief description of the drawings]

FIG. 1 is a plan view showing one substrate of a liquid crystal display device according to one embodiment of the present invention.

FIG. 2 is a plan view illustrating a manufacturing process of the liquid crystal display device of the embodiment.

FIG. 3 is a plan view illustrating a manufacturing process of the liquid crystal display device of the example.

FIG. 4 is a plan view illustrating a manufacturing process of the liquid crystal display device of the example.

FIG. 5 is a perspective view showing a conventional liquid crystal display device.

FIG. 6 is a diagram illustrating a driving method of the liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 TFT 11 Pixel electrode 12 Contact hole of interlayer insulation film 13 Data line 14 Connection line 21 Island-shaped semiconductor layer 22 Gate line 31 Through hole of interlayer insulation film 41 High dielectric thin film

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-82821 (JP, A) JP-A-5-53141 (JP, A) JP-A-6-181313 (JP, A) JP-A-6-181313 167720 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/136

Claims (4)

(57) [Claims] A pixel electrode for applying a voltage to the liquid crystal and a thin film transistor for switching the pixel electrode are provided in a matrix on one of a pair of substrates sandwiching the liquid crystal therebetween, and the thin film transistor is scanned. And a plurality of data lines for supplying a video signal to the pixel electrode via the thin film transistor are intersected with each other, and are provided so as to be insulated and separated from each other. In a liquid crystal display device provided with electrodes, the semiconductor layer of the thin film transistor has a mobility of 10 cm ² / V
S is made of polycrystalline silicon or more, and the pixel electrode and the scanning line or the auxiliary capacitance line are formed so as to partially face each other with an interlayer insulating film interposed therebetween, and the pixel electrode of the interlayer insulating film is formed. And a through-hole provided in a portion facing the scanning line or the storage capacitor line is filled with a high dielectric thin film to form a storage capacitor, and the capacitance value of the storage capacitor is determined by the pixel electrode and the counter electrode. A liquid crystal display device having a capacitance value of 10 times or more of a liquid crystal capacitance formed therebetween. 2. The liquid crystal display device according to claim 1, wherein the high dielectric thin film has a relative dielectric constant of 20 or more. 3. The method according to claim 1, wherein the high dielectric thin film is tantalum oxide, P
b (Zr, Ti) O ₃ , (Pb, La) (Zr, Ti)
O ₃ , BaTiO ₃ , SrTiO ₃ , Ba _1-x Sr _x TiO ₃
(0 <X <1), Ba ₄ Ti ₃ O ₁₂ , Bi ₂ WO ₆ , Bi _2
3. The liquid crystal display device according to claim 1, wherein the liquid crystal display device comprises one or more of WO3 and SrTaO. 4. The liquid crystal display device according to claim 1, wherein the high dielectric thin film is formed by forming a material having a relative dielectric constant of 20 or more at a temperature of 600 ° C. or less.