JP3421571B2 - Liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal display device which has a wide field angle and is free from persistence. SOLUTION: In the liquid crystal display device where the direction of inclination of liquid crystal molecules 31 is designated by an electric field in an oblique direction at the edge of a picture element electrode 19 and that of an orientation control window 24, a phase plate 43 is provided between an upper polarizing plate 42 and liquid crystal 40. The phase plate 43 functions to cancel the phase difference in liquid crystal 40, and the transmittance is made minimum in the state that liquid crystal molecules 31 are slightly inclined. The lower end of an applied voltage is made slightly higher than a threshold to not only quicken the response but also maximize the contrast ratio.

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 (L
The present invention relates to a liquid crystal display (CD), and particularly to a liquid crystal display device having a wide viewing angle, a fast response, and no afterimage.

[0002]

2. Description of the Related Art In recent years, LCDs have been actively developed and produced as flat panel displays. LCDs are excellent in terms of thinness and low power consumption.
It has become the mainstream in the field of equipment. In particular, an active matrix LCD in which a TFT is arranged as a switching element for controlling the rewriting timing of pixel information in each pixel is
Since it is possible to display a large-screen and high-definition moving image, it is widely used for various televisions, personal computers, and also monitors for portable computers, digital still cameras, video cameras and the like.

A TFT is a field effect transistor (FET) obtained by forming a semiconductor layer and a metal layer in a predetermined shape on an insulating substrate.
r). In the active matrix LCD, the TFT is connected to each capacitance for driving the liquid crystal formed between a pair of substrates sandwiching the liquid crystal.

FIG. 9 is an enlarged plan view of the display pixel portion of the LCD,
FIG. 10 is a sectional view taken along the line BB. On the lower transparent substrate (50), a gate electrode (51) of a TFT made of a metal such as Cr, Ta, or Mo is formed integrally with the gate line (1), and the gate electrode (51) is covered with SiNx and / or SiO2. A gate insulating film (52) made of, for example, is formed. On the gate insulating film (52), p-Si (53) which becomes an active layer of TFT is formed. p-S
i (53) is an injection stopper (54) such as SiO2 patterned on the gate electrode (51).
Is used to form a lightly doped (LD) region (LD) containing a low concentration of impurities such as phosphorus and arsenic, and a source and drain region (NS) also containing a high concentration of impurities outside the region (LD). ND) is formed. Immediately below the implantation stopper (54) is an intrinsic layer containing substantially no impurities, which is a channel region (CH). An interlayer insulating film (55) made of SiNx or the like is formed so as to cover the p-Si (53), and the interlayer insulating film (5
5) On top, a source electrode (56) made of Al, Mo, etc.
And a drain electrode (57) are formed and are connected to the source region (NS) and the drain region (ND) through contact holes formed in the interlayer insulating film (55). The drain electrode (16) is formed integrally with the drain line (2). On the entire surface that covers this TFT, S
OG (SPIN ON GLASS), BPSG (BORO-PHOSPHO SILI
CATE GLASS), flattening insulation film such as acrylic resin (58)
Are formed. IT is formed on the flattening insulating film (58).
A pixel electrode (59) for driving a liquid crystal, which is made of a transparent conductive film such as O (indium tin oxide), is formed, and a flattening insulating film (5)
It is connected to the source electrode (56) through a contact hole opened in 8). An alignment film (71) made of polyimide or the like is provided on the entire surface covering these. The alignment film (71) is a vertical alignment film having a pretilt angle of substantially 0 °. The TFT substrate is constructed as described above.

On the upper transparent substrate (60), R, G, and B color filters (61) are formed in a region corresponding to the pixel electrode (59), and a light shielding film (62) is formed in the gap. There is. A common electrode (63) made of ITO and an alignment control window (64) formed as an absent portion of ITO in a region facing the pixel electrode (59) are formed on the entire surface on this. The TFT is formed on the common electrode (63).
The same vertical alignment film (72) as that on the substrate side is provided. The counter substrate is configured as described above.

These TFT substrate (50) and counter substrate (6
Liquid crystal (80) having negative dielectric anisotropy between 0)
Is enclosed. Further, the substrate (50) and the substrate (60)
Polarizing plates (91) and (92) whose polarization axis directions (P, Q) are orthogonal to each other are provided on the outer side of. In the LCDs mentioned here, the liquid crystal molecules (81) having negative dielectric anisotropy have an initial alignment substantially in the normal direction (H) of the substrate due to the vertical alignment films (71, 72). Is controlled to be. In this case, when no voltage is applied, the light incident from the lower side passes through the lower polarizing plate (91) to become linearly polarized light, passes through the liquid crystal layer (80), and passes through the upper polarizing plate (92).
And the display is recognized as black. When a voltage is applied, the linearly polarized light that has passed through the lower polarizing plate (91) undergoes birefringence in the liquid crystal layer (80), changes to elliptically polarized light, and is transmitted by the upper polarizing plate (92). The display approaches white. This method is called a normally black (NB) mode. On the contrary, when the polarization axes (P, Q) are arranged in parallel with each other, the display becomes white when no voltage is applied and approaches black due to voltage application. ) Called.

This LCD is characterized in that firstly, an alignment control window (64) is provided in the common electrode (63), and secondly, the initial alignment of the liquid crystal (80) is the normal direction (H) without pretilt. 3), and thirdly, the flatness is enhanced by the flattening insulating film (58) which is the base of the pixel electrode (59).
With this configuration, when a voltage is applied, an electric field (82) in an oblique direction is generated at the edge of the pixel electrode (59) and the edge of the alignment control window (64), and the liquid crystal molecules (81) generate an electric field in the oblique direction (82). It preferentially tilts in the direction opposite to the tilt direction.
As a result, due to the continuity of the liquid crystal, as shown by the arrow in FIG. 9, different alignments are aligned in each region with the alignment control window (64) as a boundary, and a wide viewing angle is realized.

[0008]

However, this L
In CD, when the display changes from black to white, that is, when the voltage is applied and the liquid crystal molecules (81) are tilted from the normal direction (H) due to voltage application, the alignment is momentarily disturbed It takes time for the tilt direction to be determined by the action of the directional electric field (82) and for stable orientation. Therefore, the response becomes slower than in the case where the tilt direction of the liquid crystal is uniformly determined in advance by the pretilt. As a result, there is a problem that an afterimage is generated and the display quality is deteriorated.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and a liquid crystal display device in which liquid crystal is sandwiched between a pair of electrode substrates each having a liquid crystal driving electrode formed on the substrate. In the configuration described above, the voltage applied to drive the liquid crystal is equal to or higher than the threshold for driving the liquid crystal, and the maximum or minimum transmittance is obtained at the lower end of the applied voltage range.

As a result, the liquid crystal is always driven, so that the response is fast and the maximum contrast ratio is obtained. In particular, a polarizing plate is provided outside the pair of electrode substrates, and a retardation plate is provided between the electrode substrate and the polarizing plate on the light incident side. As a result, the relationship between the applied voltage and the transmittance is changed by the phase difference of the liquid crystal and the phase difference of the retardation plate, and the minimum or maximum transmittance is obtained at the applied voltage equal to or higher than the threshold value.

[0011]

1 is a plan view of a display pixel portion of an LCD according to an embodiment of the present invention, and FIG.
It is a sectional view taken along the line A. Lower transparent substrate (1
0), a gate electrode (11) of Cr, Ti, Ta or the like is formed, and a gate insulating film (12) is formed so as to cover the gate electrode (11). On the gate insulating film (12), the p-Si film (13) is formed in an island shape so as to pass above the gate electrode (11). In the p-Si film (13), the region directly above the gate electrode (11) is a non-doped channel region (CH), and both sides of the channel region (CH) are lightly doped with N-type impurities such as phosphorus. LD
The (lightly doped) region (LD) and the outside thereof are the source region (N) heavily doped with the same impurities.
S) and the drain region (ND), and has an LDD structure.

On the channel region (CH), an implantation stopper (14) used as a mask at the time of ion implantation when forming the LD region (LD) is left.
An interlayer insulating film (15) is formed so as to cover the p-Si film (13), and a drain electrode (16) is formed on the interlayer insulating film (15).
And the source electrode (17) are formed, and p- through the contact holes opened in the interlayer insulating film (15).
It is connected to the drain region (ND) and the source region (NS) of the Si film (13). These drain electrodes (1
6) and the source electrode (17) to cover SOG, BP
A flattening insulating film (18) such as SG or acrylic resin is formed, and ITO (indium tantalum) is formed on the flattening insulating film (18).
A pixel electrode (19) made of in oxide) is formed in a vertically long shape, and is connected to the source electrode (17) through a contact hole opened in the flattening insulating film (18).
An alignment film (35) of polyimide or the like is formed on this. This alignment film (35) is a vertical alignment film having a pretilt angle of substantially 0 °, at least within 1 °.

At a position facing the TFT substrate (10), an upper transparent substrate (20) serving as a counter substrate is disposed with a liquid crystal layer (40) interposed therebetween, and a color filter (21) is provided on the facing surface. , A light shielding film (22) is formed, and a common electrode (23) such as ITO is formed on these. In the common electrode (23), an alignment control window (24) formed as an absent portion of ITO is provided in a region facing the pixel electrode (19). Orientation control window (24)
As shown in FIG. 1, the pixel has a shape in which the central portion of a vertically long pixel is cut longitudinally and is bifurcated at an angle of about 45 ° from both ends to face the corner portion of the pixel. The same vertical alignment film (36) as that on the substrate (10) side is provided on the common electrode (23).

On the outer sides of the TFT substrate (10) and the counter substrate (20), there are provided polarizing plates (41) (42) whose polarization axis directions (P, Q) are orthogonal to each other. Further, between the counter substrate (20) and the polarizing plate (42) on the upper side, a retardation plate (43) having the structure according to the present invention is provided. The retardation plate (43) is made of polycarbonate or PVA uniaxially stretched in the plane direction, and of the refraction indices (nx, ny, nz) in the three spatial directions, the refraction indices nx and ny are different from each other. Is. Then, as shown in FIG. 1, the optical axis C having the refractive index nx is arranged so as to be orthogonal to the direction L in which the liquid crystal molecules (31) are mainly inclined. This direction L is defined by the vertically long pixel electrode (19).
Of the liquid crystal molecules (31) whose tilt direction is controlled by the edges of the liquid crystal molecules and the corresponding edges of the vertical section of the alignment control window (24). Further, the polarization axis directions (P, Q), the optical axis C, and the tilt direction L form an angle of 45 ° with each other.

For example, when the liquid crystal (30) has a positive refractive index anisotropy and the retardation plate (43) also has a positive refractive index anisotropy, the tilt direction L of the liquid crystal molecules (31) and the retardation plate ( When the optical axis C of 43) is made orthogonal to each other, as will be described later, an action of reducing the phase difference in the liquid crystal (30) occurs. As a result, the transmittance becomes minimum (maximum in normally white (NW) mode) when the liquid crystal molecules (31) are slightly inclined.
When the liquid crystal (30) and the retardation plate (43) have different polarities of refractive index anisotropy, the optical axis C of the retardation plate (43) is parallel to the tilt direction L of the liquid crystal molecules (31). By arranging them, a similar effect can be produced. As will be described later, the retardation plate (43) has an action of reducing the phase difference in the liquid crystal (30), and has a minimum transmittance (normally white (N) when the liquid crystal molecules (31) are slightly inclined.
In W) mode, set to maximum).

The LCD of the present invention is driven as follows. FIG. 3 shows the relationship between the applied voltage V and the transmittance T in the NB mode, and FIG. 4 shows the same relationship in the NW mode. 3A and 4A show the relationship between the applied voltage V and the transmittance T according to the present invention, and FIGS. 3B and 4B are comparative examples. , The same relationship as in the past. From FIG. 3A or FIG. 4A, in the present invention, the lower end Vd of the applied voltage range Vr is higher than the threshold voltage Vt driven by the liquid crystal, and the liquid crystal is always driven.

In the present invention, as shown in FIG. 2, a retardation plate (4) is provided between the upper substrate (20) and the upper polarizing plate (42).
By providing 3), the phase difference in the liquid crystal (30) is reduced in the phase difference plate (43).
Therefore, the VT characteristic curve is shown in FIG.
As a result of changing from the conventional VT characteristic curve shown in (b),
As shown in FIGS. 3A and 4A, the lower end voltage Vd
With lower voltage applied, lower transmittance Td (upper transmittance Tu)
Is equal to the minimum value Tmin (maximum value Tmax). That is, it can be said that when the lower end voltage Vd is applied, the phase difference in the liquid crystal (30) and the phase difference in the retardation plate (43) are just canceled. Therefore,
The contrast ratio Tu / Td is equal to Tmax / Tmin, which is the maximum. When no voltage is applied, the transmittance T is slightly higher (lower) than the minimum value Tmin (maximum value Tmax).

In the present invention, when the display changes from black (white) to gray or white (black), liquid crystal molecules (31)
From the state where the tilt angle has already been determined and the direction of the tilt has been determined by the diagonal electric field (32), the tilt angle is increased and further tilted. For this reason, the response becomes faster than in the case of tilting from the state of being completely oriented in the normal direction (H).

However, as shown in FIGS. 3 (b) and 4 (b), if the lower end Vd of the applied voltage range is simply increased in order to speed up the response, the lower transmittance Td (upper transmittance Tu) becomes higher (lower) than the minimum transmittance Tmin (maximum transmittance Tmax), and the contrast ratio Tu /
Td becomes smaller than the maximum contrast ratio Tmax / Tmin. On the other hand, in the present invention, FIG.
Also, as shown in FIG. 4A, even if the lower end voltage Vd is increased to speed up the response, the maximum contrast ratio Tm
ax / Tmin is obtained.

As another embodiment of the present invention, a spatial retardation plate having an optical action in the plane direction and the thickness direction can be used as the retardation plate (43). Such a spatial retardation plate can be obtained by further stacking the above-mentioned planar retardation plate with a vertical retardation plate having an optical action in the thickness direction. Since the vertical phase difference plate is a phase difference plate having nz smaller than nx and ny, by combining with the plane phase difference plate, nx, ny, and nz are spatial phase plates different from each other. As the vertical retardation plate used in this way, VAC manufactured by Sumitomo Chemical Co., Ltd. can be used. Since the spatial retardation plate acts in the direction of reducing the extraordinary light component in the liquid crystal (40) caused by the change in the viewing angle, it is possible to realize a wide viewing angle.

Further, as a spatial retardation plate, n from the beginning
It is also possible to use those integrally manufactured as having different x, ny and nz. Next, the relationship between the applied voltage V and the transmittance T when the phase difference of the phase plate (43) is changed will be described with reference to FIGS. 5 to 8. 5 shows that when the phase difference (R) is 20 μm, similarly, FIG. 6 shows that R = 30 μm,
FIG. 7 is a simulation result of obtaining the VT characteristic curve when R = 50 μm. FIG. 8 shows a comparative example, which is a simulation result of obtaining a conventional VT characteristic curve without using the retardation plate (43). Both show characteristics in the normally black mode. The phase difference R is obtained by multiplying the difference between ny and nx by the thickness of the phase difference plate (43), here 100 μm. In all the figures, the layer thickness of the liquid crystal (30) is 3.6 μm and Δn is 0.10. In each figure, R, G,
The characteristic curve about each color light of B is shown.

From these results, the following is considered. In FIGS. 5, 6 and 7, the transmittance T is minimized and the minimum voltage value, that is, the lower end voltage Vd is 2.9 V or more.
Within the range of 3.0V. On the other hand, in FIG.
The transmittance T is minimum in the range of V to 2.7 V, and the threshold voltage Vt
Is around 2.7 V, and it can be seen that the transmittance T increases when the applied voltage V exceeds this. In this case, in order to obtain the maximum contrast ratio, the lower end voltage Vd is 2.7.
It is set to about V or less. In the present invention, as shown in FIGS. 5 to 7, by providing the phase difference plate (43), the transmittance T becomes minimum and minimum at the lower end voltage Vd higher than the threshold voltage 2.9V. Such characteristics can be obtained. Further, it can be seen that the lower end voltage Vd increases as the phase difference R increases.

[0023]

In a liquid crystal display device for displaying a wide viewing angle by inclining a vertically aligned liquid crystal in a predetermined direction, a retardation plate is provided to change the applied voltage-transmittance characteristic and a threshold value driven by the liquid crystal. By controlling the transmittance to be minimum or maximum with a slightly higher voltage, the liquid crystal was always driven, and the maximum contrast ratio could be obtained. Therefore, the liquid crystal is voltage-controlled so as to be tilted at a desired angle from a state in which the liquid crystal is slightly tilted in a predetermined direction, so that the response is increased and a good display without an afterimage is possible.

[Brief description of drawings]

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

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 is an applied voltage-transmittance characteristic diagram showing a liquid crystal driving method according to the present invention.

FIG. 4 is an applied voltage-transmittance characteristic diagram showing a liquid crystal driving method according to the present invention.

FIG. 5 is an applied voltage-transmittance characteristic diagram in the liquid crystal display device according to the embodiment of the present invention.

FIG. 6 is an applied voltage-transmittance characteristic diagram in the liquid crystal display device according to the embodiment of the present invention.

FIG. 7 is an applied voltage-transmittance characteristic diagram of the liquid crystal display device according to the embodiment of the present invention.

FIG. 8 is an applied voltage-transmittance characteristic diagram of a conventional liquid crystal display device.

FIG. 9 is a plan view of a conventional liquid crystal display device.

10 is a cross-sectional view taken along the line BB of FIG.

[Explanation of symbols]

10 substrates 11 Gate electrode 13 p-Si 16 drain electrode 17 Source electrode 18 Flattening insulating film 19 pixel electrodes 20 substrates 23 Common electrode 24 Orientation control window 30 liquid crystal layer 31 liquid crystal molecules 32 Diagonal electric field 35, 36 Vertical alignment film 41,42 Polarizing plate 43 Phase plate

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/133 505 G02F 1/13363 G02F 1/1337 505 G09G 3/36

Claims (3)

(57) [Claims] 1. A liquid crystal is sandwiched between a pair of electrode substrates each having an electrode for driving a liquid crystal formed on the substrate, and an oblique electric field is generated.
Tilts liquid crystal molecules in different directions in each region
In the liquid crystal display device which causes the lower end of the voltage range to be applied to drive the liquid crystal, at least the threshold liquid crystal is driven, and, at the lower end of the range of applied voltages, maximum or minimum transmittance is obtained, this application
At the lower end of the voltage range, the liquid crystal will
The angle of inclination is as the applied voltage rises.
A liquid crystal display device characterized by being further inclined so as to increase the degree . 2. A liquid crystal driving electrode is formed on a substrate.
The liquid crystal is sandwiched between a pair of electrode substrates that form a vertical alignment film.
Therefore, the liquid crystal display device in which the initial alignment of the liquid crystal molecules is the normal direction
Position, a retardation plate is provided outside at least one of the electrode substrates.
Vignetting, the lower end of the voltage range to be applied to drive the liquid crystal, the liquid crystal
Is equal to or higher than the threshold to drive and the lower end of this applied voltage range
In, the phase difference in the liquid crystal and the phase difference plate
The phase difference is canceled out, and the maximum or minimum value of the transmittance T is obtained.
A liquid crystal display device characterized by being obtained. 3. An alignment control window, which is an absent region of the electrode, is formed on the electrode so as to obtain a predetermined edge, and a direction in which the liquid crystal tilts at the edge of the electrode and / or the edge of the alignment control window. 3. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is controlled.