JP2006113478A - Liquid crystal display device and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device having excellent viewing field angle characteristics and display quality, and to provide its manufacturing method. <P>SOLUTION: The liquid crystal display device has a liquid crystal panel 1 having a liquid crystal layer 30 held between an alignment layer 14 covering the surface of pixel electrodes 13 and an alignment layer 23 covering the surface of a counter electrode 22, wherein a voltage for black display to be applied between the pixel electrode and the counter electrode to display a black image is shifted to a lower level with decrease in the thickness of the alignment layers 14, 23, and the voltage for black display is shifted to a higher level with decrease in pretilt angles of the alignment layers 14, 23. The alignment layers 14, 23 are formed of an alignment layer material having such characteristics that the pretilt angle of the film decreases with decrease in the film thickness so as to compensate the shift amount in the voltage for black display depending on changes in the film thickness. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a liquid crystal display device and a manufacturing method thereof, and more particularly, to a liquid crystal display device using an OCB (Optically Compensated Bend) technology capable of realizing a wide viewing angle and a high-speed response and a manufacturing method thereof.

  Liquid crystal display devices are applied to various fields by taking advantage of features such as light weight, thinness, and low power consumption.

  Currently, a twisted nematic (TN) type liquid crystal display device widely used in the market is constructed by arranging liquid crystal molecules having optically positive refractive index anisotropy by twisting approximately 90 ° between a pair of substrates. The optical rotation of the light incident on the liquid crystal layer is adjusted by controlling the twisted arrangement. Although this TN type liquid crystal display device can be manufactured relatively easily, its viewing angle is narrow and its response speed is slow, so that it is inconvenient for displaying moving images such as TV images.

  On the other hand, OCB type liquid crystal display devices are attracting attention as liquid crystal display devices capable of improving the viewing angle and response speed. In the OCB type liquid crystal display device, a liquid crystal layer having liquid crystal molecules arranged in a bend between a pair of substrates is sandwiched. This OCB type liquid crystal display device has an improved response speed as compared with the TN type liquid crystal display device, and can further optically self-compensate the influence of birefringence of light passing through the liquid crystal layer depending on the alignment state of the liquid crystal molecules. There is an advantage that the corner is wide.

When an image is displayed using such an OCB type liquid crystal display device, the birefringence is controlled and combined with a polarizing plate, for example, a black image is displayed by blocking light when a high voltage is applied, and when a low voltage is applied. It is conceivable to display a white screen through light.
Japanese Patent Laid-Open No. 7-84254

  In a liquid crystal display device, a liquid crystal layer is sandwiched between a pair of electrodes via an alignment film. The alignment film is formed by applying an alignment film material and then performing an alignment process (rubbing process) as necessary. Such an alignment film may be locally formed to be thinner than a set value due to coating unevenness due to insufficient supply of the alignment film material at the time of application or contamination of foreign matter.

  For example, when displaying a black image, a black voltage necessary to display a black image is applied between a pair of electrodes, but the alignment film is a region having a thickness less than the set value and a region having a thickness smaller than the set value. And the voltage applied to the liquid crystal layer is different. That is, in a region having a film thickness smaller than the set value, a voltage higher than the voltage that should be originally applied to the liquid crystal layer is applied. In particular, in a configuration in which a liquid crystal layer is formed of a liquid crystal material having a relatively high dielectric constant, such as an OCB type liquid crystal display device, variation in applied voltage to the liquid crystal layer due to unevenness in the film thickness of the alignment film becomes significant. . For this reason, a uniform black image cannot be displayed, resulting in display unevenness.

  The present invention has been made in view of the above-described problems, and an object thereof is to provide a liquid crystal display device excellent in viewing angle characteristics and display quality, and a method for manufacturing the same.

A liquid crystal display device according to a first aspect of the present invention provides:
A liquid crystal panel having a liquid crystal layer sandwiched between alignment films covering the surfaces of the pair of electrodes is provided, and the black to be applied between the pair of electrodes to display a black image as the alignment film becomes thinner. A liquid crystal display device in which the black voltage is shifted to a higher level as the voltage is shifted to a lower level and the pretilt angle of the alignment film is reduced.
The alignment film is formed of an alignment film material having a characteristic that the pretilt angle becomes smaller as the film thickness becomes thinner so as to offset the shift amount of the black voltage with respect to the change in film thickness.

A method of manufacturing a liquid crystal display device according to the second aspect of the present invention includes:
A liquid crystal panel having a liquid crystal layer sandwiched between alignment films covering the surfaces of the pair of electrodes is provided, and the black to be applied between the pair of electrodes to display a black image as the alignment film becomes thinner. A method of manufacturing a liquid crystal display device in which the voltage is shifted to a lower level and the black voltage is shifted to a higher level as the pretilt angle of the alignment film decreases.
The liquid crystal display device, wherein the alignment film is formed of a material having a characteristic that the pretilt angle is reduced as the film thickness is reduced so as to cancel out the shift amount of the black voltage with respect to the change in film thickness.

A step of preparing an alignment film material having a characteristic that the pretilt angle decreases as the film thickness decreases;
Based on the relationship of the pretilt angle with respect to the film thickness of the alignment film material, the pretilt angle change amount can be obtained so as to cancel the black voltage shift amount with respect to the change amount from the set value of the alignment film thickness. Determining a set value for the thickness;
Applying alignment film material with the determined setting value; and
Applying alignment treatment to the applied alignment film material;
It is provided with.

  According to the present invention, it is possible to provide a liquid crystal display device excellent in viewing angle characteristics and display quality and a method for manufacturing the same.

  A liquid crystal display device and a manufacturing method thereof according to an embodiment of the present invention will be described below with reference to the drawings. In this embodiment, a liquid crystal display device using an OCB (Optically Compensated Bend) mode method will be described as an example of the liquid crystal display device.

  As shown in FIG. 1, the OCB type liquid crystal display device includes a liquid crystal panel 1 configured by sandwiching a liquid crystal layer 30 between a pair of substrates, that is, an array substrate 10 and a counter substrate 20. The liquid crystal panel 1 is, for example, a transmission type, and is configured to be able to transmit backlight light from a backlight unit (not shown) arranged on the array substrate 10 side to the counter substrate 20 side. Further, the liquid crystal panel 1 includes an effective display unit 2 that substantially displays an image. The effective display unit 2 is composed of display pixels PX arranged in a matrix.

  The array substrate 10 is formed using an insulating substrate 11 having optical transparency such as glass. The array substrate 10 includes a switch element 12, a pixel electrode 13, an alignment film 14 and the like on one main surface of an insulating substrate 11. The switch element 12 is disposed in each display pixel PX, and includes a TFT (Thin Film Transistor), a MIM (Metal Insulated Metal), or the like. The pixel electrode 13 is disposed in each display pixel PX and is electrically connected to the switch element 12. The pixel electrode 13 is formed of a light-transmitting conductive member such as ITO (Indium Tin Oxide). The alignment film 14 is disposed so as to cover the entire main surface of the insulating substrate 11, and is formed of a light transmissive material.

  The counter substrate 20 is formed using an insulating substrate 21 having optical transparency such as glass. The counter substrate 20 includes a counter electrode 22 and an alignment film 23 on one main surface of an insulating substrate 21. The counter electrode 22 is disposed in common in all the display pixels in the effective display portion 2 and is formed of a conductive member having optical transparency such as ITO. The alignment film 23 is disposed so as to cover the entire main surface of the insulating substrate 21 and is made of a light-transmitting material.

  In the color display type liquid crystal display device, the liquid crystal panel 1 includes display pixels of a plurality of colors, for example, red (R), green (G), and blue (B) color pixels. That is, the red pixel includes a red color filter that transmits red wavelength light, the green pixel includes a green color filter that transmits green wavelength light, and the blue pixel includes a blue color filter that transmits blue wavelength light. Yes. These color filters are arranged on the main surface of the array substrate 10 or the counter substrate 20.

  The array substrate 10 and the counter substrate 20 having the above-described configuration are arranged in a state where a predetermined gap is maintained with a spacer (not shown), and are bonded together by a sealing material. The liquid crystal layer 30 is sealed in the gap between the array substrate 10 and the counter substrate 20. The liquid crystal molecules 31 included in the liquid crystal layer 30 can be selected from materials having positive dielectric anisotropy and optically positive uniaxiality. In particular, in the OCB type liquid crystal display device, the liquid crystal layer 30 is formed of a liquid crystal material having a high dielectric constant, for example, a liquid crystal material having a dielectric constant of 10 or more.

  Such an OCB type liquid crystal display device optically compensates for the retardation of the liquid crystal layer 30 including the liquid crystal molecules 31 bend-aligned as shown in FIG. 1 in a predetermined display state in which a voltage is applied to the liquid crystal layer 30. An optical compensation element 40 is provided. The optical compensation element 40 is arranged on one main surface of the liquid crystal panel 1, that is, the outer surface on the array substrate 10 side, and on the other main surface of the liquid crystal panel 1, that is, the outer surface on the counter substrate 20 side. And the second compensation element 40B.

  For example, as illustrated in FIG. 2, the first compensation element 40A includes a polarizing plate 41A and a plurality of optical elements 42A and 43A having a function as a retardation plate. Similarly, the second compensation element 40B includes a polarizing plate 41B and a plurality of optical elements 42B and 43B that function as retardation plates.

  The optical elements 42A and 42B mainly function as retardation plates having retardation (phase difference) in the thickness direction, as will be described later. The optical elements 43A and 43B function as retardation plates having retardation (phase difference) mainly in the in-plane direction, as will be described later.

  As shown in FIG. 3, the alignment films 14 and 23 have been subjected to parallel alignment processing (that is, have been rubbed in the direction indicated by the arrow A in the figure). Thereby, the orthogonal projection (liquid crystal alignment direction) of the optical axis of the liquid crystal molecules 31 is parallel to the arrow A in the figure. In a state where an image can be displayed, that is, in a state where a predetermined bias is applied, the liquid crystal molecules 31 are arranged in the cross section of the liquid crystal layer 30 defined by the arrow A as shown in FIG. A bend arrangement between 20 and 20.

  At this time, the polarizing plate 41A is arranged so that its transmission axis faces the direction indicated by the arrow B in the drawing. Further, the polarizing plate 41B is arranged so that the transmission axis thereof faces the direction indicated by the arrow C in the drawing. That is, the transmission axes of the polarizing plates 41A and 41B form an angle of 45 ° with respect to the liquid crystal alignment direction A, and are orthogonal to each other. Thus, the arrangement in which the transmission axes of the polarizing plates are orthogonal to each other is called crossed Nicol, and the birefringence amount (retardation amount) of the object between the pair of polarizing plates is effectively 0 or an integral multiple of the wavelength. Light is not transmitted and a black image is displayed. Conversely, if the birefringence amount (retardation amount) of the object between the pair of polarizing plates is effectively λ / 2 with respect to the incident light having the wavelength λ, the light is transmitted and a white image (or color image) is displayed. Is done.

  The optical elements 43A and 43B compensate for the influence of retardation of the liquid crystal layer 30 that is affected when the screen is observed from the front direction in a specific voltage application state (for example, a state where a high voltage is applied to display a black image). It has such retardation amount. That is, in the OCB type liquid crystal display device, even when a high voltage is applied to the bend-aligned liquid crystal molecules, all the liquid crystal molecules are not aligned along the normal direction of the substrate, and the retardation of the liquid crystal layer is completely It will not be zero.

  Therefore, the optical axes of the optical elements 43A and 43B are set in parallel to a direction D perpendicular to the direction in which retardation is generated in the liquid crystal layer 30, that is, the liquid crystal alignment direction (the optical axis direction when the liquid crystal molecules are orthogonally projected). The optical elements 43A and 43B have retardation in the direction D. This corresponds to “retardation plates having retardation in the in-plane direction” 43A and 43B. Here, the in-plane direction is defined by the in-plane X-direction and Y-direction, but when considering the refractive index of each optical member such as a liquid crystal layer or a retardation plate, The main refractive indexes nx, ny, and nz when each optical member is orthogonally projected in the plane are taken into consideration.

  Thereby, the retardation in the in-plane direction of the liquid crystal layer 30 is compensated, and the liquid crystal layer 30 and the optical elements 43A and 43B are combined to form a state in which the retardation amount Δn · d is effectively zero. A black image can be displayed when the screen is observed from the front. Note that Δn = ne−no (where ne is an extraordinary ray refractive index and no is an ordinary ray refractive index), and d is the thickness of the optical element.

  The optical elements 42A and 42B have optical characteristics (for example, negative uniaxiality) opposite to those of the liquid crystal molecules 31. That is, in the OCB type liquid crystal display device, when displaying a black image, since a relatively high voltage is applied to the liquid crystal layer 30, the majority of the liquid crystal molecules 31 are aligned in the electric field direction (method of the substrate). Stand up in the line direction). The liquid crystal molecules 31 are molecules having positive uniaxial optical characteristics in which the main refractive index nz in the major axis direction of the molecule is larger than the main refractive indexes nx and ny in the other direction. Here, for the sake of convenience, the major axis direction (thickness direction) of the liquid crystal molecules 31 is defined as the Z direction, and the in-plane directions perpendicular thereto are defined as the X direction and the Y direction.

  In a state where the liquid crystal molecules 31 stand up in the normal direction of the substrate, the main refractive index distribution is isotropic when the screen is observed from the front direction (that is, the in-plane main refractive index is equivalent (nx = ny)). Therefore, retardation does not occur. However, when the screen is observed from an oblique direction, the main refractive index nz in the major axis direction increases (nx, ny <nz) due to the influence of the side surface of the liquid crystal molecules 31, and retardation corresponding to the inclination direction occurs. For this reason, part of the light that has passed through the liquid crystal layer 30 is transmitted through the crossed Nicols polarizing plates 41A and 41B and becomes brighter (that is, a black image cannot be displayed).

  Therefore, the optical elements 42A and 42B are set so that the main refractive index nz in the thickness direction is relatively small and the in-plane main refractive indexes nx and ny are relatively large (nx, ny> nz). Yes. This corresponds to “retardation plates having retardation in the thickness direction” 42A and 42B. Here, the thickness direction is defined by the Z direction orthogonal to these in addition to the in-plane X direction and Y direction, and the refractive index of each optical member such as a liquid crystal layer or a retardation plate is defined. In consideration, all of the main refractive indexes nx, ny, and nz are considered three-dimensionally.

  By using such optical elements 42A and 42B in combination, the total amount of retardation in the liquid crystal layer 30 and the optical compensation element 40 can be made substantially zero. Thereby, the retardation in the thickness direction of the liquid crystal layer 30 can be canceled, and the influence of the retardation in the liquid crystal layer 30 can be compensated when the screen in a state where the black image is displayed is observed from an oblique direction. it can.

  In this way, when a relatively high voltage is applied to the liquid crystal layer to display a black image, the retardation of the liquid crystal layer generated in the in-plane direction is compensated with the “retarder having retardation in the in-plane direction”, and By compensating the retardation of the liquid crystal layer generated in the oblique direction with the “retardation plate having retardation in the thickness direction”, viewing angle characteristics and display quality can be improved in the OCB type liquid crystal display device.

  By the way, the liquid crystal panel 1 is configured by sandwiching a liquid crystal layer 30 via alignment films 14 and 23 that respectively cover the surfaces of a pair of electrodes, that is, the pixel electrode 13 and the counter electrode 22. In the case where film thickness unevenness occurs in these alignment films 14 and 23, display unevenness may occur. This principle will be described below.

  For example, when displaying a black image, a voltage necessary for displaying a black image (hereinafter referred to as a black voltage) is applied between the pixel electrode 13 and the counter electrode 22. However, the voltage actually applied to the liquid crystal layer 30 (hereinafter referred to as a liquid crystal applied voltage) is different from the black voltage applied between the pixel electrode 13 and the counter electrode 22, and the alignment films 14 and 23, and , Depending on the capacity of the liquid crystal layer 30. This can be explained using the equivalent circuit shown in FIG.

That is, the black voltage applied between the pixel electrode 13 and the counter electrode 22 is V, the liquid crystal application voltage of the liquid crystal layer 30 is V _LC , the capacitance is C _LC, and the application voltage of the alignment film 14 is V _PI1. and the capacitance as _{C PI1,} further, when the capacity is the voltage applied to the alignment film 22 and the _{V PI2} was _{C PI2,} relationship _{V LC = V / (1 +} C LC / C PI1 + C LC / C PI2) holds.

Therefore, for example, even if a voltage of 5 V is applied between the pixel electrode 13 and the counter electrode 22 as the black voltage V, the liquid crystal applied voltage V _{LC that} is actually applied to the liquid crystal layer 30 is the capacitance of the liquid crystal layer 30. It depends on C _LC , the capacitance C _PI1 of the alignment film 14, and further the capacitance C _PI2 of the alignment film 22, and consequently becomes smaller than 5V. In other words, when the capacitance C _PI1 of the alignment film 14 and the capacitance C _PI2 of the alignment film 22 change, the liquid crystal applied voltage V _LC changes. This means that the liquid crystal applied voltage _VLC changes as the film thickness along the electric field direction of the alignment films 14 and 22 changes.

In the region where the film thickness of the alignment film is smaller than the set value, the liquid crystal applied voltage is higher than in the region where the film thickness is the set value. Further, in the region where the film thickness of the alignment film is thicker than the set value, the liquid crystal applied voltage is lower than that in the region where the film thickness is the set value. In particular, in the configuration forming the liquid crystal layer 30 by the liquid crystal material having a relatively high dielectric constant, since the capacitance C _LC of the liquid crystal layer 30 is large, larger difference of the liquid crystal application voltage due to the thickness difference of the alignment film.

  That is, when a black voltage is applied between a pair of substrates, a predetermined black image is displayed by applying a predetermined liquid crystal applied voltage to the liquid crystal layer in a region where the film thickness of the alignment film is a set value. In this case, in a region where the film thickness of the alignment film is thinner than the set value, a voltage having a level higher than a predetermined liquid crystal application voltage is applied to the liquid crystal layer, so that a predetermined black image cannot be displayed. Similarly, in a region where the film thickness of the alignment film is thicker than the set value, a voltage having a level lower than a predetermined liquid crystal application voltage is applied to the liquid crystal layer, so that a predetermined black image cannot be displayed. Therefore, in a liquid crystal panel in which the film thickness of the alignment film is uneven, when a predetermined voltage (a black voltage set with the film thickness of the alignment film being a set value) is applied between the pair of substrates, display unevenness is caused. Will occur.

  That is, in a region where the thickness of the alignment film is smaller than the set value, a voltage higher than a predetermined liquid crystal applied voltage is applied to the liquid crystal layer, so that a black image necessary for displaying a predetermined black image is displayed. The voltage shifts to a lower level than the black voltage set assuming that the thickness of the alignment film is a set value. Similarly, in a region where the film thickness of the alignment film is thicker than the set value, a voltage lower than a predetermined liquid crystal application voltage is applied to the liquid crystal layer, which is necessary for displaying a predetermined black image. The black voltage shifts to a higher level than the black voltage set with the alignment film thickness being a set value.

  Such a shift of the black voltage due to the change in the thickness of the alignment film is also apparent from the measurement results as shown in FIG. In FIG. 5, the horizontal axis represents the applied voltage applied between the pair of electrodes, and the vertical axis represents the normalized transmittance of the liquid crystal panel. In this measurement, the normalized transmittance with respect to the applied voltage is measured with the film thickness as a parameter under the condition that the pretilt angle of the alignment film is constant, and the applied voltage at which the normalized transmittance is substantially zero (the bottom of the normalized transmittance). Is a black voltage. Thereby, the shift amount of the black voltage depending on the film thickness of the alignment film is grasped. As shown in FIG. 5, it can be seen that the black voltage to be applied between the pair of electrodes shifts to a lower level as the thickness of the alignment film is reduced in order to display a black image.

  On the other hand, in an OCB type liquid crystal display device or the like, the black voltage shifts depending on the pretilt angle of the alignment film. That is, in the region where the pretilt angle of the alignment film is smaller than the set value, the black necessary for aligning the liquid crystal molecules contained in the liquid crystal layer along the electric field direction between the pair of electrodes (displaying a predetermined black image). The voltage is shifted to a higher level than the black voltage set by assuming that the pretilt angle of the alignment film is a set value. Similarly, in the region where the pretilt angle of the alignment film is larger than the set value, the black voltage required to align the liquid crystal molecules contained in the liquid crystal layer along the electric field direction between the pair of electrodes is equal to the pretilt angle of the alignment film. Shifts to a lower level than the black voltage set as the set value.

  Such a shift of the black voltage due to the change in the pretilt angle of the alignment film is also apparent from the measurement results as shown in FIG. In FIG. 6, the horizontal axis represents the applied voltage applied between the pair of electrodes, and the vertical axis represents the normalized transmittance of the liquid crystal panel. In this measurement, the standardized transmittance with respect to the applied voltage is measured with the pretilt angle as a parameter under the condition that the film thickness of the alignment film is constant, and the applied voltage at which the normalized transmittance is substantially zero (the bottom of the normalized transmittance). Is a black voltage. Thereby, the shift amount of the black voltage depending on the pretilt angle of the alignment film is grasped. As shown in FIG. 6, it can be seen that the black voltage to be applied between the pair of electrodes is shifted to a higher level in order to display a black image as the pretilt angle of the alignment film decreases.

  Therefore, in this embodiment, the alignment film is formed of an alignment film material having a characteristic that the pretilt angle becomes smaller as the film thickness becomes thinner so as to offset the shift amount of the black voltage with respect to the change in film thickness. For example, the alignment film is formed of a polyamic acid-based alignment film material. For example, as shown in FIG. 7, the alignment film material has a characteristic that the pretilt angle is reduced as the film thickness is reduced. By adopting an alignment film material having such characteristics, it is possible to self-compensate for a black voltage shift due to a change in film thickness.

  That is, in the process of forming the alignment film, the setting value of the film thickness when applying the alignment film material is to grasp the shift amount of the black voltage due to the film thickness change under the condition that the pretilt angle is constant as shown in FIG. Then, after grasping the shift amount of the black voltage due to the change of the pretilt angle under the condition of the constant film thickness as shown in FIG. 6, the film as shown in FIG. It is determined based on the relationship of the pretilt angle to the thickness.

  As a result, when a predetermined black voltage is applied between a pair of electrodes, the liquid crystal applied voltage applied to the liquid crystal layer is higher in the thin portion, but the pretilt angle is reduced, so that a black image is displayed. As a result, a display equivalent to a thick part (a part where the film thickness is a set value) becomes possible. Therefore, the black voltage shift accompanying the film thickness change can be self-compensated, and an image with excellent display quality with little display unevenness can be displayed.

(Example)
Below, the manufacturing method of a liquid crystal display device is demonstrated. That is, the insulating substrate 11 on which the switch element 12 and the pixel electrode 13 are formed is prepared. On the other hand, the insulating substrate 21 on which the counter electrode 22 is formed is prepared. Subsequently, the alignment film 14 is formed on the surface of the insulating substrate 11 on the pixel electrode 13 side, and the alignment film 23 is formed on the surface of the insulating substrate 21 on the counter electrode 22 side.

  That is, as described above, the alignment film material for forming these alignment films 14 and 23 desirably has a characteristic that the pretilt angle decreases as the film thickness decreases. For example, a polyamic acid-based alignment film material is used. prepare.

  Then, based on the relationship of the pretilt angle to the film thickness of the alignment film material as shown in FIG. 7, the pretilt angle change cancels the black voltage shift amount with respect to the change amount from the set value of the alignment film thickness. The set value of the film thickness is determined so that the amount can be obtained. In determining the set value of the film thickness, the shift amount of the black voltage is grasped by using the film thickness as a parameter under the condition that the pretilt angle is constant as shown in FIG. 5, and the film as shown in FIG. Under the condition that the thickness is constant, the shift amount of the black voltage is grasped using the pretilt angle as a parameter.

  In the example shown in FIG. 5, it can be seen that the amount of shift of the black voltage when the thickness of the alignment film is changed by 20 nm is about 0.13V. Further, in the example shown in FIG. 6, it can be seen that the shift amount of the black voltage when the pretilt angle of the alignment film is changed by 2 ° is about 0.13V.

  That is, when the alignment film thickness is reduced by 20 nm relative to the set value, the black voltage shifts to about 0.13 V, but when the pretilt angle of the alignment film is reduced by 2 ° with respect to the set value. The black voltage shifts to about 0.13V. In other words, if the pretilt angle is reduced by 2 ° when the alignment film is thinned by 20 nm, the black voltage shift amount due to the change in film thickness can be offset. Further, when the thickness of the alignment film becomes 20 nm thicker than the set value, the black voltage shifts to about 0.13 V, but when the pretilt angle of the alignment film becomes 2 ° larger than the set value. The black voltage shifts to about 0.13V. That is, if the pretilt angle is increased by 2 ° when the alignment film is 20 nm thick, the black voltage shift amount due to the change in film thickness can be offset.

  In this embodiment, attention is paid to a problem that a region having a film thickness thinner than a set value is locally formed due to uneven application of alignment film material or mixing of foreign matters. For this reason, in particular, the set value is determined so as to self-compensate the black voltage shift when the thickness of the alignment film becomes thinner than the set value. That is, in the example shown in FIGS. 5 and 6, if the pretilt angle is reduced by 2 ° when the thickness of the alignment film is reduced by 20 nm, the black voltage shift amount due to the change in thickness can be offset. Become. The alignment film material prepared here has a characteristic that the pretilt angle decreases as the film thickness decreases. Therefore, based on the relationship of the pretilt angle to the film thickness shown in FIG. The design value of the film thickness is determined so that the pretilt angle is reduced by 2 ° when the thickness is reduced. Here, the set value of the film thickness is determined to be 50 nm.

  Then, the alignment film material is applied with the determined set value. That is, the alignment material is applied to the surface of the insulating substrate 11 on the pixel electrode 13 side with a thickness of 50 nm, and the alignment material is similarly applied to the surface of the insulating substrate 21 on the side of the counter electrode 22 with a thickness of 50 nm. To do. Then, an alignment process (rubbing process) is performed on the applied alignment film material. Thereby, the array substrate 10 and the counter substrate 20 each having the alignment films 14 and 23 are formed.

  Subsequently, the alignment film 14 of the array substrate 10 and the alignment film 23 of the counter substrate 20 face each other and a predetermined gap is formed between them, and then the two substrates are bonded together, and then formed between these substrates. The liquid crystal layer 30 is formed by sealing a liquid crystal material containing liquid crystal molecules 31 having a high dielectric constant in the gap. Thereby, the liquid crystal panel 1 is formed.

  Subsequently, the first compensation element 40 </ b> A is disposed on the outer surface of the array substrate 10, and the second compensation element 40 </ b> B is disposed on the outer surface of the counter substrate 20. Thus, an OCB type liquid crystal display device is configured.

  As described above, according to this embodiment, in the OCB type liquid crystal display device capable of improving the viewing angle and the response speed, the black voltage shifts to a lower level as the alignment film becomes thinner, and the alignment The black voltage shifts higher as the pretilt angle of the film decreases. The alignment film is formed using an alignment film material whose pretilt angle changes as the film thickness changes. This alignment film material has a characteristic that the pretilt angle becomes small in a thin part. For this reason, the set value of the film thickness is determined so as to obtain a pretilt angle change amount that cancels the shift amount of the black voltage with respect to the change amount from the set value of the film thickness of the alignment film. Thereby, the shift of the black voltage accompanying the change in film thickness can be self-compensated, and an image with good display quality with little display unevenness can be displayed.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the spirit of the invention in the stage of implementation. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine the component covering different embodiment suitably.

FIG. 1 is a cross-sectional view schematically showing a configuration of an OCB type liquid crystal display device as an embodiment of the present invention. FIG. 2 is a diagram schematically showing a configuration of an optical compensation element applied to the OCB type liquid crystal display device. FIG. 3 is a diagram showing the relationship between the optical axis direction and the liquid crystal alignment direction of each optical member constituting the optical compensation element shown in FIG. FIG. 4 is a diagram for explaining a generation mechanism of display unevenness accompanying a change in the thickness of the alignment film. FIG. 5 is a diagram showing an example of the relationship between the applied voltage applied between a pair of electrodes and the normalized transmittance of the liquid crystal panel when the film thickness is a parameter under the condition that the pretilt angle of the alignment film is constant. is there. FIG. 6 is a diagram showing an example of the relationship between the applied voltage applied between a pair of electrodes and the normalized transmittance of the liquid crystal panel when the pretilt angle is used as a parameter under the condition that the film thickness of the alignment film is constant. is there. FIG. 7 is a diagram illustrating an example of the relationship between the film thickness of the alignment film material and the pretilt angle.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Liquid crystal panel 10 ... Array substrate 14 ... Orientation film 20 ... Opposite substrate 23 ... Orientation film 30 ... Liquid crystal layer 31 ... Liquid crystal molecule 40 ... Optical compensation element

Claims (9)

A liquid crystal panel having a liquid crystal layer sandwiched between alignment films covering the surfaces of the pair of electrodes is provided, and the black to be applied between the pair of electrodes to display a black image as the alignment film becomes thinner. A liquid crystal display device in which the black voltage is shifted to a higher level as the voltage is shifted to a lower level and the pretilt angle of the alignment film is reduced.
The liquid crystal display device, wherein the alignment film is formed of an alignment film material having a characteristic that the pretilt angle becomes smaller as the film thickness becomes thinner so as to offset the shift amount of the black voltage with respect to the change in film thickness.   The liquid crystal display device according to claim 1, wherein the alignment film is formed of a polyamic acid-based alignment film material.   The liquid crystal display device according to claim 1, wherein the liquid crystal layer is formed of a liquid crystal material having a dielectric constant of 10 or more.   The liquid crystal display device according to claim 1, wherein the liquid crystal molecules contained in the liquid crystal layer are bend-aligned between the pair of substrates in a display state.   An optical compensation element that optically compensates for retardation of the liquid crystal layer in a predetermined display state in which a voltage is applied to the liquid crystal layer, and birefringence due to liquid crystal molecules contained in the liquid crystal layer by the voltage applied to the liquid crystal layer; The liquid crystal display device according to claim 1, wherein an image is displayed by changing the amount.   The liquid crystal display device according to claim 5, wherein the optical compensation element includes a retardation plate having retardation in a thickness direction thereof and a retardation plate having retardation in an in-plane direction thereof. A liquid crystal panel having a liquid crystal layer sandwiched between alignment films covering the surfaces of the pair of electrodes is provided, and the black to be applied between the pair of electrodes to display a black image as the alignment film becomes thinner. A method of manufacturing a liquid crystal display device in which the voltage is shifted to a lower level and the black voltage is shifted to a higher level as the pretilt angle of the alignment film decreases.
A step of preparing an alignment film material having a characteristic that the pretilt angle decreases as the film thickness decreases;
Based on the relationship of the pretilt angle with respect to the film thickness of the alignment film material, the pretilt angle change amount can be obtained so as to cancel the black voltage shift amount with respect to the change amount from the set value of the alignment film thickness. Determining a set value for the thickness;
Applying alignment film material with the determined setting value; and
Applying alignment treatment to the applied alignment film material;
A method for manufacturing a liquid crystal display device, comprising:   The method of manufacturing a liquid crystal display device according to claim 7, wherein the alignment film is formed of a polyamic acid-based alignment film material.   The step of determining the set value of the film thickness includes a step of grasping a black voltage shift amount with the film thickness as a parameter under a condition where the pretilt angle is constant, and a black value with the pretilt angle as a parameter under a condition with a constant film thickness. The method for manufacturing a liquid crystal display device according to claim 7, further comprising a step of grasping a voltage shift amount.

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