JP2000193974A - Liquid crystal display - Google Patents

Abstract

(57) Abstract: A vertical alignment type liquid crystal display device that controls the tilt direction of liquid crystal molecules when a voltage is applied by using a bank-shaped protrusion provided on a substrate surface can improve the brightness of a panel. Provided is a structure of a liquid crystal display device. SOLUTION: A first device having an active element, a pixel electrode, and a first alignment film for aligning liquid crystal molecules in a direction perpendicular to the film surface.
A second substrate having a common electrode, and a second alignment film for aligning liquid crystal molecules in a direction perpendicular to the film surface; and a negative electrode sealed between the first substrate and the second substrate. A liquid crystal display device having a liquid crystal layer having a dielectric anisotropy of: having a protrusion for regulating the tilt direction of liquid crystal molecules between the electrode and the alignment film, and between the pixel electrode and the common electrode. When a driving voltage is applied, the voltage applied to the liquid crystal layer in the region where the protrusions are formed is controlled so as to be substantially equal to or less than the liquid crystal threshold voltage.

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 a method of manufacturing the same, and more particularly, to a vertical alignment for controlling a tilt direction of liquid crystal molecules when a voltage is applied by using a bank-like projection provided on a substrate surface. Liquid crystal display device.

[0002]

2. Description of the Related Art Conventionally, as a liquid crystal display (LCD) using an active matrix, a liquid crystal material having a positive dielectric anisotropy is horizontally disposed on a substrate surface and between the opposing substrates. A TN (Twisted Nematic) mode liquid crystal display device, which is oriented so as to be twisted by 90 degrees with a liquid crystal display, is widely used. However, the TN mode liquid crystal display device has a serious problem of poor viewing angle characteristics, and various studies have been made to improve the viewing angle characteristics.

[0003] The inventors of the present application have conducted intensive studies from this point of view, and as an alternative to the TN mode, a structure in which a liquid crystal material having negative dielectric anisotropy is vertically aligned and provided on the substrate surface, For example, an MVA (Multi-domain Verti
Cal Alignment) type liquid crystal display devices have been proposed and the viewing angle characteristics have been greatly improved (for example, see Japanese Patent Application No. 9-361384 by the same applicant).

The MVA type liquid crystal display device has a VA (Vert) for vertically aligning a liquid crystal material having a negative dielectric anisotropy.
In a liquid crystal display device of the (icically aligned) mode, a bank-like projection made of a light-transmitting resin (eg, resist) is provided on a substrate, and the direction in which liquid crystal molecules are obliquely aligned when a voltage is applied is determined within one pixel. It is intended to improve the viewing angle characteristics by restricting the angle in a plurality of directions.

Although there are various theories on the mechanism of the manifestation of the alignment control, the equipotential surface of the liquid crystal layer is generally distorted by the bank-like projections when a voltage is applied, and the liquid crystal is obliquely inverted at both edges of the bank-like projections. It is considered to be oriented in the direction (see FIG. 18A). On the other hand, when the alignment regulating forces at both edges are equal, the liquid crystal alignment on the bank-like projection is parallel to the direction in which the azimuth differs from the edge by 90 degrees or 180 degrees, that is, the direction in which the bank-like projection extends. It falls down and stabilizes.

As the thickness of the light-transmitting resin, that is, the height of the bank-like projections, increases, the transmittance in the white state increases, but the leakage decreases in the black state, so that the contrast decreases. Since the MVA type liquid crystal display device was initially developed for use in monitors, the height of the bank-like projection was set with priority given to contrast.

[0007]

In an MVA type liquid crystal display device, an alignment control structure such as a bank-like projection is formed on a display portion, and at the edge of the structure, the liquid crystal alignment is in a predetermined direction. Due to the slant, the transmittance in the white state is lower than that of the TN panel. That is, in the above-described conventional liquid crystal display device, the height of the bank-shaped protrusions is set with priority given to contrast, and thus is not optimized in terms of transmittance, and the transmittance is reduced due to the occurrence of alignment disorder. Was. Specifically, when observing the liquid crystal orientation on the bank-shaped protrusions in the MVA type liquid crystal display device, the liquid crystal fell in the opposite direction (90 degrees and 180 degrees), and a constricted domain (reverse tilt domain) was generated. Thus, the alignment disorder was generated from the starting point to the outside of the bank (see FIG. 18B), and such disorder of the liquid crystal alignment caused a decrease in transmittance.

In consideration of power saving of a panel and mounting on a notebook personal computer, improvement of the brightness of the MVA system is a major problem, and it is necessary to minimize a decrease in transmittance on a bank-like projection and an edge portion. Is desired. The purpose of the present invention is to
An object of the present invention is to provide a structure of a liquid crystal display device for improving luminance in a VA liquid crystal display device.

[0009]

An object of the present invention is to provide an active element for driving a liquid crystal, a pixel electrode to which a driving voltage is applied by the active element, and an electrode formed on the pixel electrode for applying the driving voltage. A first substrate having a first alignment film for orienting liquid crystal molecules in a direction perpendicular to the film surface when not in operation;
A second electrode having a common electrode facing the pixel electrode, and a second alignment film formed on the common electrode and aligning liquid crystal molecules in a direction perpendicular to a film surface when the driving voltage is not applied.
A liquid crystal display device having a liquid crystal layer having a negative dielectric anisotropy sealed between the first substrate and the second substrate, wherein the pixel electrode and the first And / or between the common electrode and the second alignment film, between the common electrode and the second alignment film, the pixel electrode has a projection for regulating a tilt direction of liquid crystal molecules when the driving voltage is applied. When the driving voltage is applied between the liquid crystal layer and the common electrode, the voltage applied to the liquid crystal layer in the region where the protrusion is formed is controlled to be substantially equal to or less than the liquid crystal threshold voltage. This is achieved by a liquid crystal display device characterized in that:

In the above liquid crystal display device, the voltage applied to the liquid crystal layer in a region where the protrusion is formed may be controlled by a voltage drop caused by the protrusion. In the above liquid crystal display device, the voltage applied to the liquid crystal layer in a region where the protrusion is formed may be controlled by the height and / or the dielectric constant of the protrusion.

In the above liquid crystal display device, the voltage applied to the liquid crystal layer in a region where the protrusion is formed is reduced by a voltage drop caused by the first alignment film and / or the second alignment film. You may make it control. Also,
In the above liquid crystal display device, the voltage applied to the liquid crystal layer in a region where the protrusion is formed is controlled by a film thickness and / or a dielectric constant of the first alignment film or the second alignment film. You may do so.

In the above liquid crystal display device, the first alignment film or the second alignment film may be formed such that a region where the protrusion is formed is thicker than another region. . In the above liquid crystal display device, the vertical alignment capability of the first alignment film and the second alignment film in the region where the protrusion is formed is different from the vertical alignment capability of the first alignment film and the second alignment film in another region. The liquid crystal threshold voltage of the region where the protrusions are formed may be higher than the liquid crystal threshold voltage of another region by increasing the vertical alignment ability of the alignment film.

In the above liquid crystal display device, a high resistance electrode layer may be further provided between the pixel electrode and the protrusion and / or between the common electrode and the protrusion. The voltage applied to the liquid crystal layer in a region where the protrusion is formed may be controlled by a voltage drop caused by the layer. Further, in the above liquid crystal display device,
The pixel electrode or the common electrode in the region where the protrusion is formed has a high resistance, and the voltage drop in the region of the pixel electrode or the common electrode where the resistance has been increased in the region where the protrusion is formed. The voltage applied to the liquid crystal layer may be controlled.

[0014] Further, the object is to provide an active element for driving a liquid crystal, a pixel electrode to which a driving voltage is applied by the active element, and a liquid crystal formed on the pixel electrode when the driving voltage is not applied. A first substrate having a first alignment film for orienting molecules in a direction perpendicular to the film surface; a common electrode facing the pixel electrode; and a common electrode formed on the common electrode;
A second substrate having a second alignment film for aligning liquid crystal molecules in a direction perpendicular to the film surface when the drive voltage is not applied;
A liquid crystal layer having a negative dielectric anisotropy sealed between the first substrate and the second substrate, between the pixel electrode and the first alignment film, and / or In a liquid crystal display device having a protrusion between a common electrode and the second alignment film for regulating a tilt direction of liquid crystal molecules when the driving voltage is applied, the liquid crystal display device has a structure in which the pixel electrode and the common electrode are disposed between the pixel electrode and the common electrode. When a driving voltage is applied, the liquid crystal in the region where the protrusions are formed is such that the voltage applied to the liquid crystal layer in the region where the protrusions are formed is substantially equal to or lower than the liquid crystal threshold voltage. The present invention is also achieved by a voltage control method for a liquid crystal display device, wherein the voltage applied to a layer is controlled.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Structure of MVA liquid crystal display device] The structure of an MVA liquid crystal display device will be described with reference to FIGS. FIG. 1 is a plan view of a pixel portion in the liquid crystal display device according to the present embodiment, and FIG.
It is a line sectional view.

As shown in FIG. 1 and FIG.
0, a CS electrode 12 for forming a storage capacitor;
A gate bus line 14 including a TFT gate electrode is formed. On the glass substrate 10 on which the CS electrode 12 and the gate bus line 14 are formed, a gate insulating film 16 is formed. On the gate insulating film 16, an active layer 18 constituting a channel region of the TFT is formed. On the gate insulating film 16 on which the active layer 18 is formed, a source electrode 20 connected to one side of the active layer 18 and a drain bus line 22 including a drain electrode connected to the other side of the active layer 18 are formed. Are formed. Source electrode 20
An insulating film 24 is formed on the gate insulating film 16 on which the drain bus lines 22 are formed. Insulating film 24
On the top, a pixel electrode 26 connected to the source electrode 20 is formed. On the insulating film 24 and the pixel electrode 26,
A bank-like projection 28 made of a light-transmitting material is provided in a zigzag shape. Pixel electrode 26,
On the insulating film 24 on which the bank-like projections 28 are formed, an alignment film 30 for vertically aligning liquid crystal molecules is formed.

On the other hand, on the glass substrate 40, a black matrix layer 42 is formed. On the glass substrate 40 on which the black matrix layer 42 is formed, a coloring (CF) resin layer 46 forming a color filter is formed. A common electrode 48 is formed on the colored resin layer 46. On the common electrode 48, a bank-shaped protrusion 50 made of a light-transmitting material and formed in a zigzag shape and shifted by a half pitch with respect to the bank-shaped protrusion 28 formed on the substrate 10 is formed. ing. On the common electrode on which the bank-shaped protrusions 50 are formed, an alignment film 52 for aligning liquid crystal molecules in a vertical direction is formed.

The glass substrate 10 (T
The FT substrate) and the glass substrate 40 (CF substrate) are arranged to face each other so that the alignment films 30 and 52 face each other, and a negative liquid crystal material 60 having a negative dielectric anisotropy is disposed between these substrates. Is sealed. Thus, an MVA liquid crystal display device is configured. The present invention relates to an MVA type liquid crystal display device as described above, wherein
By appropriately controlling the substantial voltage applied to the liquid crystal layer 60 on the layers 8 and 50, the transmittance of the panel is improved. That is, the voltage applied to the liquid crystal layer 60 on the bank-like projections 28 and 50 in the driving voltage range of the liquid crystal display device is controlled so as to be substantially equal to or lower than the liquid crystal threshold voltage. Thing 28, 50
It suppresses the occurrence of a constricted domain on the upper side and suppresses a decrease in transmittance due to alignment disorder.

As shown in FIG. 3, the tilt angle of the liquid crystal molecule decreases as the applied voltage increases, and the constricted domain increases with the increase in the applied voltage. That is, if the voltage applied to the liquid crystal layer can be reduced, particularly if the voltage can be lowered to a voltage lower than the liquid crystal threshold voltage, the collapse of the liquid crystal molecules can be sufficiently suppressed, and the constricted domain can be suppressed to a small value. Therefore, by controlling the device parameters of the liquid crystal display device so that such a voltage is selectively applied to the liquid crystal molecules on the bank-like projections, the transmittance of the panel can be improved.

There are various methods for selectively applying different voltages to the liquid crystal molecules on the bank-like projections. For example, there are a method for defining the height of the bank-like projections at a predetermined height, A method for regulating the thickness of the alignment film to a predetermined thickness, a method for selectively increasing the vertical alignment ability of the alignment film on the bank-like projection, and a high-resistance electrode layer interposed between the bank-like projection and the electrode There is a method for causing the above to occur.

Hereinafter, embodiments for achieving the above means will be described in detail. [First Embodiment] The liquid crystal display according to a first embodiment of the present invention will be explained with reference to FIG. FIG. 4 is a partial sectional view and a circuit diagram illustrating the structure of the liquid crystal display device according to the present embodiment.

The voltage applied to the liquid crystal layer on the bank-like projections is attenuated to some extent by the bank-like projections. However, if the height of the bank-like projections is low, the voltage attenuation width is small, and the liquid crystal falls. Becomes large, and the domain emerges strongly and induces misalignment out of the bank. On the other hand, if the height of the bank-like projections is sufficiently high, the attenuation width of the voltage becomes large, so that the tilting of the liquid crystal becomes small, and the constricted domain is suppressed to a small one, which makes it difficult to induce alignment disturbance outside the bank. When the voltage applied to the liquid crystal layer becomes lower than the liquid crystal threshold voltage, the improvement of the transmittance of the panel reaches saturation as shown in FIG. Therefore, by controlling the voltage applied to the liquid crystal layer on the bank-shaped projections to be equal to or lower than the liquid crystal threshold voltage, the constricted domain can be suppressed to a small value.

Therefore, in the present embodiment, the voltage applied to the liquid crystal layer on the bank-shaped projection is controlled to be equal to or lower than the liquid crystal threshold voltage by setting the bank-shaped projection to a predetermined height. The liquid crystal display will be described. FIG.
In the liquid crystal display device shown in FIG. 4, the region where the bank-shaped protrusions are formed has a cross-sectional structure as shown in FIG. That is, the pixel electrode 26 and the common electrode 48 are opposed to each other while sealing the liquid crystal 60, and the bank-like projection 28 is formed on the pixel electrode 26. Although a region where a bank-shaped protrusion is formed also exists on the common electrode 48, there is no difference from the case where the bank-shaped protrusion is formed on the pixel electrode 26, and thus description thereof is omitted here.

In such a structure, considering an electric circuit formed between the pixel electrode 26 and the common electrode 48, as shown in FIG. 4B, the liquid crystal layer 60 and the bank-like projection 28 are respectively formed. It can be regarded as a combination circuit combining a resistor and a capacitor. Here, R _lc is the liquid crystal layer resistance, R _r is the resistance of the bank-like projections, the C _lc capacitance of the liquid phase layer, is C _r is the capacitance of the bank-like projections.

In the circuit shown in FIG. 4B, considering the case where the resistance values of the liquid crystal layer 60 and the bank-like projections 28 are sufficiently high when AC is applied, the current flowing through the resistance circuit is very small and can be ignored. Becomes Therefore, the equivalent circuit of the sectional structure shown in FIG. 4A can be approximated to the series connection of capacitors as shown in FIG. 4C. The voltage V _lc applied to the upper liquid crystal layer is obtained by the following equation.

V_lc= ｛C_r/ (C_r+ C_lc)｝ V (1) Capacitance C_lc, C_rSets the relative dielectric constant of the liquid crystal layer to ε_lc,bank
Let the relative permittivity of_r, The vacuum permittivity is ε₀, Embankment
The area of the formation area of the origin is S, and the thickness of the liquid crystal layer is d. _lc,bank
Let d be the thickness of the projection_rThen C_lc= Ε₀ε_lcS / d_lc, C_r= Ε₀ε_rS / d_r .. (2) is substituted into equation (1).
It looks like this:

[0027] _{V lc = {ε r d lc} / (ε r d lc + ε lc d r)} V ... (3) Therefore, the voltage V _lc applied to the liquid crystal layer on the bank-like projections
Can be defined by the dielectric constant and the thickness of the liquid crystal layer and the bank-shaped protrusions. Once the dielectric constant of the liquid crystal layer and the thickness of the liquid crystal panel are determined, the voltage V _lc is approximately equal to the liquid crystal threshold voltage by the bank-shaped protrusions. It can be controlled to be equal to or less than that.

Equation (3) includes both the dielectric constant and the film thickness of the resin film, and the transmittance can be optimized by controlling one or both of them. For example, depending on the resin film for forming the bank-like projections, there is a case where the application of a thick film impairs the uniformity or makes it difficult to form a pattern, but if the thickness and the dielectric constant are controlled in combination, It is also possible to improve the transmittance without making the film thickness too large.

The dielectric constant of the bank-shaped projection is smaller than the dielectric constant of the liquid crystal for controlling the alignment. In addition, a polymer resin is often used for the bank-like projections because of the ease of pattern formation. On the other hand, the dielectric constant of the polymer resin is approximately 2.0.
As described above, the dielectric constant of the liquid crystal having a negative dielectric anisotropy in the vicinity of the liquid crystal threshold voltage is generally in the range of 3.0 to 5.0, and the dielectric constant of the polymer resin used for the bank-like projections Is 2.0
~ 5.0. The liquid crystal threshold voltage is approximately 2.
0-3.0V. Therefore, when determining the range of the film thickness d _r of the conditions are satisfied bank-like projections by substituting these values into equation (3), the thickness d _r of the bank-like projection is the liquid crystal panel thickness In the ratio of d, 0.3 ≦ d _r /d≦0.6.

As described above, according to the present embodiment, the thickness of the bank-like projection is adjusted so that the voltage applied to the liquid crystal layer on the bank-like projection is substantially equal to or less than the liquid crystal threshold voltage. Since the control is performed, the inclination of the liquid crystal on the bank-shaped protrusion can be reduced. As a result, the constricted domain can be suppressed to a small one, so that it is possible to suppress a decrease in transmittance due to the induction of alignment disorder outside the bank.

[Second Embodiment] The liquid crystal display according to a second embodiment of the present invention will be explained with reference to FIGS. The same components as those of the liquid crystal display device according to the first embodiment shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted or simplified. FIG. 5 is a partial sectional view and a circuit diagram for explaining the structure of the liquid crystal display device according to the present embodiment, and FIG. 6 is a graph showing the relationship between the coating film thickness of the alignment film and the rotation speed of the spinner.

In the present embodiment, a description will be given of a liquid crystal display device in which the voltage applied to the liquid crystal layer on the protrusions is controlled to be equal to or lower than the liquid crystal threshold voltage by an alignment film formed on the bank-like protrusions. . Since the thickness of the alignment film is usually sufficiently smaller than that of the liquid crystal layer or the bank-shaped protrusions, the voltage component attenuated by the alignment film is generally small. For this reason, in the liquid crystal display device according to the first embodiment, no consideration was given to the alignment film. However, when the thickness of the alignment film is large, the value cannot be ignored. _Can be a factor for controlling the voltage _Vlc applied to.

In this case, the area where the bank-shaped projections are formed has a sectional structure as shown in FIG. That is, the pixel electrode 26 and the common electrode 48 are arranged to face each other in a state where the liquid crystal 60 is sealed, and a bank-like protrusion 28 is formed on the pixel electrode 26, and the bank-like protrusion 28 is formed on the bank-like protrusion 28. An alignment film 30 is formed. In such a structure, considering an electric circuit formed between the pixel electrode 26 and the common electrode 48, as shown in the first embodiment, the equivalent circuit includes a capacitor formed by the liquid crystal layer 60,
A capacitor composed of the alignment film 30 and a capacitor composed of the bank-like projections 28 are connected in series (FIG. 5B). Therefore, the voltage V _lc applied to the liquid crystal layer on the alignment _{film, V lc = {ε al ε} r d lc / (ε r ε lc d al + ε al ε r d lc + ε lc ε al d r)} V ... (4) Here, ε _al is the relative dielectric constant of the alignment film, and d _al is the thickness of the alignment film.

Therefore, the voltage V _lc applied to the liquid crystal layer on the bank-like projections can be controlled also by the film thickness and the dielectric constant of the alignment film. This makes it possible to set the applied voltage _Vlc to be substantially equal to or lower than the liquid crystal threshold voltage. However, if the dielectric constant of the alignment film is different from that of the liquid crystal, a phenomenon such as image sticking is likely to occur. Therefore, it is desirable to control the transmittance by increasing the thickness of the alignment film. Also, if the thickness of the alignment film other than on the bank-like protrusions is increased, the driving voltage is shifted to a lower voltage side as a whole, leading to a decrease in transmittance. Therefore, only the alignment film on the bank-like protrusions is selectively used. It is desirable to make it thicker.

The alignment film is usually formed by spin coating, and the thickness of the alignment film can be controlled by the rotation speed of the spinner. For example, as shown in FIG.
The coating film thickness of the alignment film tends to decrease with an increase in the rotation speed of the spinner.
In order to obtain a coating film thickness of, coating may be performed at a rotation speed of about 1500 rpm.

Various methods are conceivable for selectively increasing the thickness of the alignment film. For example, an alignment film having a predetermined thickness is formed on the entire surface and then formed on the bank-like projections by lithography. The alignment film is patterned so that only the alignment film remains, and then the alignment film is applied again on the entire surface to form an alignment film in which the thickness on the bank-like projections is larger than the thickness in other regions. can do.

As described above, according to the present embodiment, the thickness of the alignment film is controlled so that the voltage applied to the liquid crystal layer on the bank-like projections is substantially equal to or lower than the liquid crystal threshold voltage. Therefore, the inclination of the liquid crystal on the bank-like projections can be reduced. As a result, the constricted domain can be suppressed to a small one, so that it is possible to suppress a decrease in transmittance due to the induction of alignment disorder outside the bank.

In the above embodiment, the case where only the alignment film is controlled has been described. However, by controlling both the alignment film and the bank-like protrusion, the voltage applied to the liquid crystal layer on the bank-like protrusion is controlled by the liquid crystal. The threshold voltage may be substantially equal to or lower than the threshold voltage. [Third Embodiment] The liquid crystal display according to a third embodiment of the present invention will be explained with reference to FIGS. The same components as those of the liquid crystal display device according to the first and second embodiments shown in FIGS. 4 to 6 are denoted by the same reference numerals, and description thereof will be omitted or simplified.

FIG. 7 is a partial sectional view for explaining the structure of the liquid crystal display device according to the present embodiment, and FIG. 8 is a graph showing a change in the liquid crystal threshold voltage with a change in the vertical alignment capability. In the present embodiment, a liquid crystal display device in which the voltage applied to the liquid crystal layer on the protrusion is controlled to be equal to or lower than the liquid crystal threshold voltage by controlling the vertical alignment ability of the alignment film.

The alignment of the liquid crystal layer depends on the vertical alignment capability of the alignment film. That is, the higher the vertical alignment capability of the alignment film, the stronger the anchoring of the liquid crystal molecules to the alignment film,
It becomes difficult to orient to voltage application. Therefore, by selectively increasing the vertical alignment ability of the alignment film on the bank-like projection, it is possible to make it difficult to selectively align the liquid crystal layer on the bank-like projection.

That is, as shown in FIG. 7, by selectively forming the alignment films 30a and 52a having enhanced vertical alignment ability in the region where the bank-like projections 28 are formed, Can be controlled so that the voltage applied to the liquid crystal layer 60 is substantially equal to or lower than the liquid crystal threshold voltage. The vertical alignment ability of the alignment film can be improved by blending a large amount of a vertical alignment component (a polyimide having a long alkyl group in a side chain) into a constituent material of the alignment film. For example, by forming an alignment film in which the vertical alignment component is increased by about 30% as compared with the conventional alignment film, the transmittance becomes approximately 1% of the saturation amount as shown in FIG.
The voltage at which the voltage becomes 0%, that is, the threshold voltage of the liquid crystal molecules was increased from about 4.1 V to about 6.0 V.

As described above, by increasing the vertical alignment ability of the alignment film on the bank-like projections, the liquid crystal is unlikely to fall down in the region where the bank-like projections are formed even when the same voltage is applied, and Only in the upper liquid crystal layer, the liquid crystal threshold voltage shifts to the higher voltage side. Therefore, the voltage applied to the liquid crystal layer on the protrusion can be controlled to be substantially equal to or higher than the liquid crystal threshold voltage without attenuating the drive voltage.

Various methods are conceivable for selectively increasing the vertical alignment capability of the alignment film. For example, an alignment film having a high vertical alignment capability is formed on the entire surface, and then a bank-like shape is formed by ordinary lithography. The alignment film is patterned so that only the alignment film on the protrusion remains, and then, an alignment film having a low vertical alignment capability is applied again on the entire surface, so that the vertical alignment capability on the bank-like projections is in other regions. It is possible to form an alignment film higher than the vertical alignment ability.

As described above, according to the present embodiment, the vertical alignment ability of the alignment film is controlled so that the voltage applied to the liquid crystal layer on the bank-like projections is substantially equal to or lower than the liquid crystal threshold voltage. Therefore, the inclination of the liquid crystal on the bank-shaped protrusion can be reduced. As a result, the constricted domain can be suppressed to a small one, so that it is possible to suppress a decrease in transmittance due to the induction of alignment disorder outside the bank.

[Fourth Embodiment] The liquid crystal display according to a fourth embodiment of the present invention will be explained with reference to FIGS. The same components as those of the liquid crystal display device according to the first to third embodiments shown in FIGS. 4 to 8 are denoted by the same reference numerals, and description thereof will be omitted or simplified. FIG. 9 is a partial sectional view and a circuit diagram for explaining the structure of the liquid crystal display device according to the present embodiment, and FIG. 10 is a graph showing the relationship between the resistance value of the ITO film and the oxygen partial pressure during film formation.

In this embodiment, a high-resistance electrode layer is provided between the bank-shaped protrusions and the electrodes, and the resistance applied to the high-resistance electrode layers is controlled to reduce the voltage applied to the liquid crystal layer on the protrusions. A liquid crystal display device controlled to have a liquid crystal threshold voltage or lower will be described. In general, the resistance of the electrode material used for the pixel electrode and the common electrode is sufficiently small, and the voltage components attenuated by these electrodes are ignored. However, by increasing the electrode resistance, the value becomes _{nonnegligible} , and can be a factor for controlling the voltage _Vlc applied to the liquid crystal layer on the bank-like projections.
In addition, if the electrode portion in the region below the bank-shaped protrusion is selectively subjected to a high resistance treatment, or if a high-resistance electrode layer is interposed between the bank-shaped protrusion and the electrode, such a case can be obtained in this manner. The voltage V _lc applied to the liquid crystal layer on the bank-like projections can be controlled by the formed high resistance layer.

In this case, the region where the bank-shaped protrusions are formed has a cross-sectional structure as shown in FIG. That is, the pixel electrode 26 and the common electrode 48 are arranged to face each other in a state where the liquid crystal 60 is sealed, and the bank-like projection 28 is formed on the pixel electrode 26, and the bank-like projection 28 and the pixel electrode 26, a high resistance electrode layer 64 is formed.

In such a structure, considering an electric circuit formed between the pixel electrode 26 and the common electrode 48, as shown in the first embodiment, the equivalent circuit is the liquid crystal layer 6.
0 and a bank 28
Is connected in series with the high resistance electrode layer 64 (FIG. 9B). Therefore, the voltage V _lc applied to the liquid crystal layer on the bank-like protrusions _{are, V lc = {ε r d} lc / (ε r d lc + ε lc d r)} (V-IR el) ... (5) Become. Note that R _el is the resistance value of the high resistance electrode layer, and I is the current.

Therefore, the voltage V _lc applied to the liquid crystal layer on the bank-like projections can be controlled by the high-resistance electrode layer, whereby the voltage V _lc applied to the liquid crystal layer on the bank-like projections can be controlled. It is possible to set _lc to be substantially equal to or lower than the liquid crystal threshold voltage. A transparent electrode material such as an ITO film is used for the pixel electrode and the common electrode. In the case of an ITO film, for example, the resistance value is controlled by controlling the gas partial pressure during film formation by sputtering. can do. That is, as shown in FIG. 9, the gas partial pressure of O ₂ gas relative to Ar gas (P _O2 /
The resistance value of the ITO film increases as the value of P _Ar increases, and the oxygen partial pressure is determined based on the resistance value obtained by equation (5), whereby the high-resistance electrode layer having a predetermined resistance value is obtained. Can be formed.

Various methods are conceivable for selectively forming the high-resistance electrode layer. For example, after a resist pattern exposing a region where a bank-like projection is to be formed is formed by ordinary lithography, By depositing a high-resistance ITO film on the entire surface by sputtering, and then removing the resist pattern together with the overlying ITO film and leaving the ITO film only in the region where the bank-like projection is to be formed, the bank-like projection is formed. It is possible to selectively form a high-resistance electrode layer only in a region in which is formed.

As described above, according to the present embodiment, the resistance value of the high-resistance electrode layer is adjusted so that the voltage applied to the liquid crystal layer on the bank-like projections is substantially equal to or lower than the liquid crystal threshold voltage. Since the control is performed, the inclination of the liquid crystal on the bank-shaped protrusion can be reduced. As a result, the constricted domain can be suppressed to a small one, so that it is possible to suppress a decrease in transmittance due to the induction of alignment disorder outside the bank.

In the above embodiment, the case where only the resistance value of the high-resistance electrode layer is controlled has been described. However, by controlling both the resistance value and the bank-like projection, the voltage applied to the liquid crystal layer on the bank-like projection is controlled. The applied voltage may be approximately equal to or lower than the liquid crystal threshold voltage. Further, as described in the second embodiment, an alignment film may be further considered.

In the above embodiment, the case where the high-resistance electrode layer is provided between the pixel electrode and the bank-shaped projection has been described. However, as shown in FIG. Is a pixel electrode 26 made of a high-resistance material.
a, when formed with the common electrode 48a (FIG. 11)
(A), the high-resistance electrode 2 is formed on a part of the pixel electrode 28 or the common electrode 48 in the area where the bank-shaped protrusion 28 is formed.
The same applies to the case of forming 6b (FIG. 11B).

When the resistance of a part of the pixel electrode or the common electrode is increased, the resistance can be increased by forming the region 26c in which the thickness of the electrode is selectively reduced (FIG. 11). (C)).

[0055]

Examples of the present invention will be described below. In addition, the characteristic evaluation of the liquid crystal display device in an Example was performed based on the evaluation panel created as follows. First, an acrylic resin r ₁ (ε _r1 = 3.2) or an acrylic resin r ₂ (ε _r2 = 2.6) is formed on the evaluation substrate so that bank-like projections are alternately arranged. did.

Next, the surface of the bank-like projection formed in this manner was modified by plasma ashing. Then
A vertical alignment film (ε _al = 4.3) was applied by a spinner, and a cell was assembled after the alignment film was cured. After cell assembly, a fluorine-based liquid crystal having negative dielectric anisotropy (ε _lc = 4.15, V _th
= 2.43) to prepare an evaluation panel having a cell thickness d = 4.0 μm.

The comparative example shown in the following examples is an evaluation panel in which a bank-like projection was prepared under the conditions conventionally used, and an acrylic resin r ₁ was used as the bank-like projection material.
A 1.4 μm-thick bank-like projection, 80 nm thick
Is formed. Using Example 1 acrylic resin r _1, a plurality of evaluation panels with different thicknesses of the bank-like protrusions created by the above procedure.

When the upper limit of the driving voltage of the liquid crystal display device is considered to be about 5 V, the voltage applied to the liquid crystal layer on the bank-like projection is substantially equal to or lower than the liquid crystal threshold voltage. the film thickness d _r1, by substituting the above values into equation _{(3), V lc = V} th ≧ {ε r1 d lc / (ε r1 d lc + ε lc d r1)} V 2.43 ≧ {(3.2 × (4-d _r1 )) / (3.2 × (4-d _r1 ) + 4.15 ×
It is represented as d _r1 ｝ × 5. Therefore, by solving this equation, the film thickness d _r1 is obtained as d _r1 ≧ 1.80 μm.

Therefore, the thickness d ₁ of the bank-like projection made of the acrylic resin r _{1 is set} to 1.4 μm, 1.6 μm, and 1.8.
An evaluation panel was prepared with a thickness of μm and 2.2 μm, and the measurement of each transmittance and the orientation observation were performed. As a result, as shown in FIG. 12, the transmittance of the evaluation panel was found to increase by increasing the thickness of the bank-like projections.
In particular, when the bank-like projections are formed with the film thickness of 1.8 μm or more obtained by the above calculation, the transmittance when the driving voltage is applied is about 6 μm as compared with the case where the film thickness is 1.4 μm. % (Tmax / Tmim = 1.06).

As a result of observation of the orientation, as shown in FIG. 13, when the thickness of the bank-like projection is small, the constricted domain on the bank-shaped projection becomes prominent, and the outside of the bank starting from the constricted domain is used. Although the orientation disorder of the
(A)), the constricted domain decreases with an increase in the thickness of the bank-like projection (FIG. 13B), and when the thickness of the bank-like projection becomes 1.8 μm or more, the constricted domain becomes small and the constricted domain It was found that the disturbance of the orientation outside the bank based on the base was suppressed (FIG. 13C).

From the above results, the voltage applied to the liquid crystal on the protrusions is equal to or lower than the liquid crystal threshold voltage by forming the bank-like protrusions with a thickness equal to or greater than the thickness of the protrusions obtained based on the equation (3). The tilt of the liquid crystal is kept small,
The constriction domain was found to be minor. [Example 2] Acrylic resin r _2, a plurality of evaluation panels with different thicknesses of the bank-like protrusions created by the above procedure.

When the upper limit of the driving voltage of the liquid crystal display device is considered to be about 5 V, the voltage applied to the liquid crystal layer on the bank-like projections is substantially equal to or lower than the liquid crystal threshold voltage. the film thickness d _r2, by substituting the above values into equation _{(3), V lc = V} th ≧ {ε r2 d lc / (ε r2 d lc + ε lc d r2)} V 2.43 ≧ {(2.6 × (4-d _r2 )) / (2.6 × (4-d _r2 ) + 4.15 ×
It is expressed as d _r2 ｝ × 5. Therefore, by solving this equation, the film thickness d _r2 is obtained as d _r2 ≧ 1.59 μm.

Therefore, the acrylic resin r_TwoEmbankment consisting of
Film thickness d_r21.6 μm to make an evaluation panel
Then, the transmittance was measured and the orientation was observed. as a result,
As shown in FIG. 14, the transmittance of the evaluation panel
System resin₁To form 1.4μm thick bank-like projections
And acrylic resin r ₁1.6μ using
Increased than when m-thick bank-shaped protrusions are formed
I understood. In particular, acrylic resin₁1.4μ using
Compared with the conventional condition of forming m-shaped bank-shaped protrusions
And the transmittance at the time of applying the driving voltage is improved by about 6%.
Was.

As a result of observing the alignment, it was found that the alignment disorder was suppressed as in the case of Example 1 shown in FIG. From the above results, the voltage applied to the liquid crystal on the protrusions becomes equal to or lower than the liquid crystal threshold voltage by forming the bank-like protrusions with the film thickness larger than the bank-like protrusion obtained based on the equation (3). It was clarified that the tilting of the liquid crystal was suppressed to a small level and the constricted domain was small.

Example 3 A plurality of evaluation panels each having a 1.4 μm-thick bank-like projection made of an acrylic resin r ₁ and having the upper surface covered with alignment films having different thicknesses were subjected to the above procedure. Created by The upper limit of the driving voltage of the liquid crystal display device is 5
When it is considered to be about V, the thickness d _{al of the} alignment film at which the voltage applied to the liquid crystal layer on the bank-like projections becomes substantially equal to or less than the liquid crystal threshold voltage is expressed by the above equation (4). V _lc = ε _al ε _r d _lc / (ε _r ε _lc d _al + ε _al ε _r d _lc +
_{ε lc ε al d r)}} V 1.43 ≧ {(4.3 × 2.6 × (2.6-d al)) / (3.2 × 4.15 × d al
+ 4.3 × 3.2 × (2.6−d _al ) + 4.15 × 4.3 × 1.4)｝ × 5 Therefore, by solving this equation, the film thickness d _al is obtained as d _al ≧ 460 nm.

Therefore, an alignment film having a thickness of about 500 nm on the bank-shaped projections and a thickness of about 80 nm in other regions is formed on the 1.4 μm-thick bank-shaped projections made of the acrylic resin r _1. Was formed, and the transmittance was measured and the orientation was observed. The alignment film having a selectively increased thickness was formed by the following procedure. First, an alignment film having a thickness of about 420 nm was formed on the substrate on which the bank-like projections were formed by applying an alignment film repeatedly at about 500 to 1000 rpm. Next, the orientation film was patterned by a usual lithography technique so that only the orientation film on the bank-like projections remained. Thereafter, an alignment film was applied again on the entire surface under the condition of 1500 rpm. Thus, an alignment film having a thickness of about 500 nm on the bank-like projections and a thickness of 80 nm in other areas was formed.

As a result of measuring the transmittance of the evaluation panel prepared as described above, as shown in FIG.
Compared with the conventional condition of 0 nm, the transmittance at the time of applying the driving voltage was improved by about 6%. Further, as a result of orientation observation, it was found that the orientation disorder was suppressed as in the case of Example 1 shown in FIG.

From the above results, by forming the alignment film with a thickness not less than the thickness obtained based on the equation (4), the voltage applied to the liquid crystal on the protrusion becomes equal to or less than the liquid crystal threshold voltage, so that the liquid crystal falls. Was kept small, and the constriction domain was found to be minor. Consisting Example 4 Acrylic resin r ₁ thickness 1.4μ
A plurality of evaluation panels having m bank-like projections and having a vertical alignment ability in the projection forming area different from that in the other areas were prepared by the above procedure.

That is, as the alignment film in the region where the bank-like projections are formed, an alignment film in which the mixing ratio of the vertical alignment component (polyimide component having a long alkyl group in the side chain) is increased by about 30% is selectively formed. did. In addition, the alignment film in which the vertical alignment ability is selectively enhanced, first, an alignment film having a high vertical alignment ability is formed on the entire surface, and then the alignment film is formed by lithography such that only the alignment film on the bank-like projections remains. After patterning, an alignment film having a low vertical alignment ability was applied again on the entire surface to form a film.

The transmittance of the evaluation panel thus formed was measured and the orientation was observed. As a result, as shown in FIG. 16, the transmittance of the evaluation panel was higher than that in the case of using the conventional alignment film by using the alignment film in which the mixing ratio of the vertical alignment component was increased by 30%. The transmittance at the time of application was improved by about 6%. Further, as a result of orientation observation, it was found that the orientation disorder was suppressed as in the case of Example 1 shown in FIG.

From the above results, by selectively increasing the vertical alignment ability of the alignment film in the region where the bank-like projections are formed, the voltage applied to the liquid crystal on the projections becomes equal to or lower than the liquid crystal threshold voltage. It was revealed that the collapse was kept small and the constricted domain was small.
Example 5 A plurality of evaluation panels having a 1.4 μm-thick bank-shaped protrusion made of an acrylic resin r ₁ and having different resistance values of electrodes under the protrusion were prepared by the above procedure.

Assuming that the upper limit of the driving voltage of the liquid crystal display device is about 5 V, the electrode resistance necessary for the voltage applied to the liquid crystal layer on the bank-like projections to be substantially equal to or less than the liquid crystal threshold voltage. R _el, by substituting the above values into equation _{(5), V lc = {} ε r d lc / (ε r d lc + ε lc d r)} (V-IR
_el ) 2.43 ≧ ｛(3.2 × 2.6) / (3.2 × 2.6) + 4.15 × 1.4｝ × (5-
IR _el ). Therefore, by solving this equation, I
R _el becomes IR _el ≧ 0.866 [V]. Here, assuming that the frequency f of the drive power supply is 30 Hz and the protrusion area is 1.0 cm ² × 0.25, the current I is I =
Since denoted 2PaifCV, obtained as _{I = I lc = 2πfC lc V} lc = 2 × 3.14 × 30 × 3.53 × 10 -10 × 2.43 = 1.62 × 10 -7 [A]. Therefore, the electrode resistance R is determined as R ≧ 5.34 × 10 ⁶ [Ω].

Therefore, the acrylic resin r₁Consisting of
The resistance value of the electrode under the 1.4 μm bank-like projection is 5 × 10
⁶Create an evaluation panel with Ω, measure transmittance and orientation
Observations were made. As a result, the transmittance of the evaluation panel was as shown in FIG.
As shown in FIG. 7, the electrode resistance was 5 × 10 ^-FiveConventional Ω
Compared to the conditions, the transmittance when the driving voltage is applied is about 6
% Had improved.

As a result of observing the orientation, it was found that the disorder of the orientation was suppressed as in the case of Example 1 shown in FIG. From the above results, by forming an electrode having a resistance value or more based on the equation (5) below the bank-shaped protrusion, the voltage applied to the liquid crystal on the protrusion becomes equal to or lower than the liquid crystal threshold voltage. Is kept small,
The constriction domain was found to be minor.

[0075]

As described above, according to the present invention, an active element for driving a liquid crystal, a pixel electrode to which a drive voltage is applied by the active element, and a drive voltage not applied are formed on the pixel electrode. A first substrate having a first alignment film for orienting liquid crystal molecules in a direction perpendicular to the film surface; a common electrode facing the pixel electrode; and a drive electrode formed on the common electrode without applying a driving voltage. A second substrate having a second alignment film for aligning liquid crystal molecules in a direction perpendicular to the film surface; and a negative dielectric anisotropy sealed between the first substrate and the second substrate. Between the liquid crystal layer and the pixel electrode and the first alignment film, and / or between the common electrode and the second alignment film, to control the tilt direction of liquid crystal molecules when a driving voltage is applied. Drive voltage was applied between the pixel electrode and the common electrode in the liquid crystal display device having In this case, the voltage applied to the liquid crystal layer in the region where the protrusions are formed is controlled so as to be substantially equal to or lower than the liquid crystal threshold voltage. Can be prevented. Further, it is possible to suppress a decrease in the transmittance of the panel due to the disorder of the liquid crystal alignment.

[Brief description of the drawings]

FIG. 1 is a plan view showing a structure of an MVA liquid crystal display device.

FIG. 2 is a schematic sectional view showing a structure of an MVA type liquid crystal display device.

FIG. 3 is a graph showing a relationship between a tilt angle of liquid crystal molecules and an applied voltage.

FIG. 4 is a partial sectional view and a circuit diagram showing a structure of the liquid crystal display according to the first embodiment of the present invention.

FIG. 5 is a partial cross-sectional view and a circuit diagram illustrating a structure of a liquid crystal display according to a second embodiment of the present invention.

FIG. 6 is a graph showing a relationship between a coating film thickness of an alignment film and a rotation speed of a spinner.

FIG. 7 is a partial cross-sectional view illustrating a structure of a liquid crystal display according to a third embodiment of the present invention.

FIG. 8 is a graph showing a change in liquid crystal threshold voltage with a change in vertical alignment ability.

FIG. 9 is a partial sectional view and a circuit diagram showing a structure of a liquid crystal display according to a fourth embodiment of the present invention.

FIG. 10 is a graph showing the relationship between the resistance value of an ITO film and the oxygen partial pressure during film formation.

FIG. 11 is a partial cross-sectional view illustrating a structure of a liquid crystal display according to a modification of the fourth embodiment.

FIG. 12 is a graph showing the relationship between the transmittance and the applied voltage in the evaluation panel of Example 1.

FIG. 13 is a diagram showing the results of observing the orientation of the evaluation panel of Example 1;

FIG. 14 is a graph showing the relationship between the transmittance and the applied voltage in the evaluation panel of Example 2.

FIG. 15 is a graph showing the relationship between the transmittance and the applied voltage in the evaluation panel of Example 3.

FIG. 16 is a graph showing the relationship between the transmittance and the applied voltage in the evaluation panel of Example 4.

FIG. 17 is a graph showing the relationship between the transmittance and the applied voltage in the evaluation panel of Example 5.

FIG. 18 is a diagram illustrating a problem of a conventional liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Glass substrate 12 ... CS electrode 14 ... Gate bus line 16 ... Gate insulating film 18 ... Active layer 20 ... Source electrode 22 ... Drain bus line 24 ... Insulating film 26 ... Pixel electrode 28 ... Bank-shaped protrusion 30 ... Alignment film 40 … Glass substrate 42… Black matrix layer 46… CF resin layer 48… Common electrode 50… Bank-like projections 52… Alignment film 60… Liquid crystal material 62… Liquid crystal molecules 64… High resistance electrode layer

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiro Koike 4-1-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa F-term in Fujitsu Limited (reference) 2H090 HA14 HC05 HC10 HD14 JA03 JC03 KA04 LA01 LA04 MA01

Claims (10)

[Claims] An active element for driving a liquid crystal, a pixel electrode to which a driving voltage is applied by the active element, and a liquid crystal molecule formed on the pixel electrode, the liquid crystal molecules being applied when the driving voltage is not applied. A first substrate having a first alignment film oriented in a direction perpendicular to the pixel electrode; a common electrode facing the pixel electrode; and a liquid crystal molecule formed on the common electrode and not applied with the driving voltage. A second substrate having a second alignment film oriented in a direction perpendicular to the film surface; and a liquid crystal having a negative dielectric anisotropy sealed between the first substrate and the second substrate. A liquid crystal display device comprising: a layer between the pixel electrode and the first alignment film; and / or
A projection between the common electrode and the second alignment film for regulating a tilt direction of liquid crystal molecules when the driving voltage is applied; and a driving voltage between the pixel electrode and the common electrode. Wherein the voltage applied to the liquid crystal layer in the region where the protrusions are formed is controlled to be substantially equal to or less than the liquid crystal threshold voltage. . 2. The liquid crystal display device according to claim 1, wherein the voltage applied to the liquid crystal layer in a region where the protrusion is formed is controlled by a voltage drop caused by the protrusion. Display device. 3. The liquid crystal display device according to claim 2, wherein the voltage applied to the liquid crystal layer in a region where the protrusion is formed is controlled by a height and / or a dielectric constant of the protrusion. A liquid crystal display device characterized by the above-mentioned. 4. The liquid crystal display device according to claim 1, wherein a voltage drop caused by the first alignment film and / or the second alignment film is applied to the liquid crystal layer in a region where the protrusion is formed. A liquid crystal display device, wherein the voltage is controlled. 5. The liquid crystal display device according to claim 4, wherein a thickness and / or a dielectric constant of the first alignment film or the second alignment film is applied to the liquid crystal layer in a region where the protrusion is formed. A liquid crystal display device, wherein the applied voltage is controlled. 6. The liquid crystal display device according to claim 4, wherein the first alignment film or the second alignment film has a thickness in a region where the protrusion is formed is larger than in other regions. A liquid crystal display device comprising: 7. The liquid crystal display device according to claim 1, wherein a vertical alignment capability of the first alignment film and the second alignment film in a region where the protrusion is formed is different from that of the first alignment film in another region. A liquid crystal threshold voltage in a region where the protrusion is formed is higher than a liquid crystal threshold voltage in another region by increasing the vertical alignment ability of the alignment film and the second alignment film. Display device. 8. The liquid crystal display device according to claim 1, further comprising a high-resistance electrode layer between the pixel electrode and the protrusion and / or between the common electrode and the protrusion. A liquid crystal display device, wherein the voltage applied to the liquid crystal layer in a region where the protrusion is formed is controlled by a voltage drop caused by a high resistance electrode layer. 9. The liquid crystal display device according to claim 1, wherein a resistance of the pixel electrode or the common electrode in a region where the protrusion is formed is increased, and the resistance of the pixel electrode or the common electrode is increased. Wherein the voltage applied to the liquid crystal layer in the region where the protrusion is formed is controlled by the voltage drop in the region. 10. An active element for driving a liquid crystal, a pixel electrode to which a driving voltage is applied by the active element, and a liquid crystal molecule formed on the pixel electrode, the liquid crystal molecules being applied when the driving voltage is not applied. A first substrate having a first alignment film oriented in a direction perpendicular to the pixel electrode; a common electrode facing the pixel electrode; and a liquid crystal molecule formed on the common electrode and not applied with the driving voltage. A second substrate having a second alignment film oriented in a direction perpendicular to the film surface; and a liquid crystal having a negative dielectric anisotropy sealed between the first substrate and the second substrate. The tilt direction of the liquid crystal molecules when the driving voltage is applied between the layer and the pixel electrode and the first alignment film and / or between the common electrode and the second alignment film. In a liquid crystal display device having a regulating protrusion, the pixel electrode and the common electrode The protrusions are formed such that, when the driving voltage is applied between the electrodes, the voltage applied to the liquid crystal layer in the region where the protrusions are formed is substantially equal to or less than the liquid crystal threshold voltage. Controlling the voltage applied to the liquid crystal layer in a specified area.