TECHNICAL FIELD

The present invention relates to a liquid crystal display of a so-called IPS (In-Plane Switching) scheme in which an electric field substantially in parallel with a substrate is applied to a liquid crystal layer for operation, and to a production process thereof.

BACKGROUND ART

In general, display of a liquid crystal display is realized by applying an electric field to liquid crystal molecules in a liquid crystal layer sandwiched between a pair of substrates to change the alignment direction of the liquid crystal molecules and utilizing the resulting change in the optical property of the liquid crystal layer. Conventionally, a liquid crystal display of a so-called active drive type having a switching device such as a thin-film transistor for each pixel is represented by a TN (Twisted Nematic) display scheme in which an electrode is provided for each of a pair of substrates sandwiching a liquid crystal layer between them, the direction of an electric field applied to the liquid crystal layer is set to be substantially perpendicular to the interface between the substrates, and the optical rotatory power of the liquid crystal molecules constituting the liquid crystal layer is utilized to achieve display. In the liquid crystal display of the TN scheme, a small viewing angle is regarded as the greatest problem.

On the other hand, “Patent Document 1,” “Patent Document 2,” “Patent Document 3,” “Patent Document 4,” “Patent Document 5” and the like have disclosed an IPS scheme in which an inter-digital electrode formed on one of a pair of substrates is used to produce an electric field having a component substantially in parallel with the substrate surface to rotate liquid crystal molecules constituting a liquid crystal layer in a plane substantially in parallel with the substrate and the birefringence of the liquid crystal layer is used to realize display. The IPS scheme has advantages such as a wider viewing angle and a lower load capacity due to the in-plane switching of the liquid crystal molecules as compared with the conventional TN scheme. The IPS scheme is considered as a new and promising liquid crystal display which will replace the TN scheme and has made rapid advances in recent years. In addition, another type of the IPS scheme has been disclosed in “Patent Document 6” in which at least one of paired electrodes for applying an electric field to a liquid crystal layer is made of a transparent conductive film to improve transmittance.

The liquid crystal display of the IPS scheme (abbreviated as IPS-TFT-LCD) with favorable viewing angle characteristics (luminance contrast ratio, tone and color reversal) and bright display represents prospective technology for monitors or televisions with a large display area. In the liquid crystal display, an alignment control film provided with a liquid crystal alignment control ability is formed on the interface between a liquid crystal layer and each of a pair of substrates sandwiching the liquid crystal layer between them. However, to put IPS-TFT-LCDs for supporting large screens of 20 inches or more into practice use in the future, it is necessary to develop a new structure and process for large-size displays (large panels).

In particular, for an IPS-TFT-LCD having many stepped structures on a surface opposite to a liquid crystal layer, it is difficult to perform uniform alignment processing on an alignment control film over a large screen. A margin in performing the alignment processing on the alignment control film is significantly smaller than that of the conventional TN scheme, especially a normally open type TN scheme which is predominant at present (bright display at low voltage and dark display at high voltage). The reasons for the small margin include three points described below as (1) to (3).

(1) Stepped Structure

In the IPS-TFT-LCD, it is necessary to provide a number of elongated electrodes (which may be referred to as inter digital electrodes) having a width of approximately several microns in principle. This causes minute stepped structures to be formed therein. The height of the step depends on the thickness of the electrodes or the shapes of various films formed thereon, and typically is equal to 0.1 micron (mμ) or larger. An alignment control film (also referred to as an alignment film) made of a polymer film such as polyimide is formed in the uppermost layer of those films.

In conventional mass production technology, the alignment control film is subjected to rubbing processing to provide a liquid crystal alignment ability (initial alignment). Meanwhile, a cloth for the rubbing is formed by binding thin fibers with a thickness of approximately 10 to 30 microns. Essentially, each of the thin fibers provides shearing force in a predetermined direction for a local portion of the alignment film to perform the processing of giving the liquid crystal alignment ability. While very thin fibers of approximately several microns are present as the fibers, such very thin fibers have not been put into practical use since rigidity for providing certain frictional force is required for the rubbing. The interval between the electrodes in the IPS scheme is approximately 10 to 30 microns which is substantially the same as the diameter of the fibers, so that sufficient rubbing is not performed near the steps and misalignment tends to occur. The misalignment leads to reduced image quality such as a higher black level, an associated lower contrast ratio, and uneven luminance.

(2) Alignment Angle

In the IPS-TFT-LCD, the initial alignment direction needs to be set in principle at a certain angle or more shifted from the direction in which the electrode extends or the direction perpendicular thereto. The electrode refers to a signal wiring electrode, a common electrode in pixels, and a pixel electrode. The definition of the initial alignment direction through the rubbing requires the fibers of approximately 10 to 30 microns to rub in a predetermined angular direction as described above. However, the step of the wire such as the signal wiring electrode, the common electrode in pixels, or the pixel electrode extending in a certain direction at their ends draws the fibers toward the step from the set angle to produce misalignment, thereby reducing image quality such as a higher black level.

(3) Expression of Dark Level

One of the characteristics of the IPS-TFT-LCD is excellent expression of a dark level (black display). Thus, misalignment is easily noticeable as compared with the other schemes. In the conventionally normally open type TN scheme, the dark level is provided while a high voltage is applied. In this case, most of liquid crystal molecules align in the direction of the electric field which is one direction perpendicular to the substrate surface at a high voltage, and the dark level is provided from the relationship between the arrangement of the liquid crystal molecules and the placement of a polarizing plate. Thus, the uniformity of the dark level hardly depends on the initial alignment state at a low voltage in principle. In addition, since human eyes recognize uneven luminance as a relative ratio of luminance and make response close to a logarithmic scale, they are sensitive to variations in the dark level. From this viewpoint, the conventional normally open type TN scheme in which the liquid crystal molecules are forcedly arranged in one direction at a high voltage is advantageous in that it is not sensitive to the initial alignment state.

On the other hand, in the IPS scheme, display of a dark level is performed at a low voltage or no voltage, so that it is sensitive to disturbance of the initial alignment state. In particular, when homogeneous alignment is used in which the alignment directions of liquid crystal molecules are in parallel with each other on an upper substrate and a lower substrate, and the light transmission axis of one of polarizing plates is set in parallel with the alignment direction of the liquid crystal molecules and the light transmission axis of other polarizing plate is set orthogonally thereto (called a birefringence mode), polarized light incident on the liquid crystal layer is transmitted with almost no disturbance of lineal polarization. This is effective in providing excellent expression of the dark level.

The transmittance T in the birefringence mode is expressed by the following equation:
T=T0·sin2{2θ(E)}·sin2{(π·deff·Δn)/λ}
where T0represents a coefficient which is a numerical value determined mainly by the transmittance of the polarizing plate for use in the liquid crystal panel, θ(E) represents an angle between the alignment direction of liquid molecules (the effective optical axis of the liquid crystal layer) and the polarized light transmission axis, E represents an applied electric field intensity, deffrepresent the effective thickness of the liquid crystal layer, Δn represents the refractive index anisotropy of liquid crystal, and X represents the wavelength of light. The product of the effective thickness deffof the liquid crystal layer and the refractive index anisotropy Δn of the liquid crystal, that is, doff·Δn is called retardation. The thickness doffof the liquid crystal layer does not refer to the thickness of the whole liquid crystal layer but corresponds to the thickness of the liquid crystal layer which actually changes in the alignment direction when a voltage is applied thereto. This is because the liquid crystal molecules near the interface of the liquid crystal layer do not change in the alignment direction due to the influence of anchoring at the interface even when a voltage is applied thereto. Thus, assuming the thickness of the whole liquid crystal layer sandwiched between the substrates is dLC, the relationship deff<dLCis always found between the thicknesses dLCand deff. The difference between them can be estimated at approximately 20 nm to 40 nm, although it depends on the liquid crystal material used in the liquid crystal panel and the type of the interface in contact with the liquid crystal layer, for example the material of the alignment film.

As apparent from the above equation, the term sin2{2θ(E)} depends on the electric field intensity, and the luminance can be adjusted by changing the angle θ in accordance with the electric field intensity E. For the normally close type, polarizing plates are set to satisfy θ=0 when no voltage is applied, and it is sensitive to disturbance of the initial alignment direction.

In this manner, the uniformity of alignment is a very important factor in the IPS scheme, and problems in the currently used rubbing technique have become apparent. In general, the rubbing alignment processing includes many problems associated with the rubbing processing technique such as TFT breakage due to static electricity produced by friction, unfavorable display due to misalignment from disordered fiber ends of a rubbing cloth or dust, and the need for frequent exchanges of rubbing cloths. For the purpose of solving the problems associated with the rubbing alignment processing, a so-called “rubbing-less” alignment technique for aligning liquid crystal molecules without the rubbing has been studied and various processes thereof have been proposed. Among other things, a process has been proposed in which polarized ultraviolet rays or the like are irradiated to the surface of a polymer film to align liquid crystal molecules without the rubbing.

As an example, a process disclosed in “Non-Patent Literature Document 1” is characterized in that it does not require the conventional rubbing processing and realizes the alignment of liquid crystal molecules in a predetermined direction through irradiation of polarized light. The process is advantageous in presenting no problems such as damages on the film surface and static electricity associated with the rubbing technique and providing a simpler production process in view of industrial production. The process has attracted attention as a new liquid crystal alignment processing process without using the rubbing processing.

As a material of the liquid crystal alignment film used in the previous reports, the use of a polymer compound having a photoreactive group in the side chain of a polymer has been proposed for the need to provide photochemical sensitivity to polarized light. A representative example thereof is polyvinylcinnamate, in which case it is thought that dimerization in the side chain through light irradiation develops anisotropy in a polymer film to align the liquid crystal. Another proposal involves dispersing low-molecular dichroic azo dye in a polymer material and irradiating a film surface with polarized light to allow the alignment of liquid crystal molecules in a predetermined direction. In addition, the alignment of liquid crystal molecules achieved by irradiating a particular polyimide film with polarized ultraviolet rays or the like has been reported. In this case, it is contemplated that the light irradiation decomposes the polyimide main chain in a certain direction to develop the liquid crystal alignment.Patent Document 1: JP-B-63-21907Patent Document 2: U.S. Pat. No. 4,345,249Patent Document 3: WO91/10936Patent Document 4: JP-A-6-22739Patent Document 5: JP-A-6-160878Patent Document 6: JP-A-9-73101Patent Document 7: Japanese Patent No. 3303766Patent Document 8: JP-A-11-218765Non-Patent Document 1: W. M. Gibbons et al., Nature, 351, 49(1991)

DISCLOSURE OF THE INVENTION

In this manner, the photo-alignment process through light irradiation has been proposed and studied as the rubbing-less alignment technique for solving the problems in the rubbing alignment technique, but it has the following problems from a practical standpoint. In a polymeric material obtained by introducing a photoreactive group in the side chain of a polymer represented by polyvinylcinnamate, the heat stability of alignment is insufficient and satisfactory reliability is not ensured from a practical viewpoint. In this case, since it is thought that the side chain of the polymer corresponds to the site of the structure which develops the alignment of liquid crystal, it is difficult to say that the technique is preferable in providing more uniform alignment of liquid crystal molecules and more resistant alignment. When low-molecular dichroic dye is dispersed in a polymer, the dye itself for aligning liquid crystal is the low molecular substance, and from the viewpoint of practical use, problems remain in terms of reliability for heat and light.

In addition, in the process of irradiating particular polyimide with polarized ultraviolet rays, the polyimide itself is reliable in heat resistance or the like, but it is thought that the alignment mechanism is caused by decomposition through the light, and it is thus difficult to ensure sufficient reliability for practical use. Specifically, when the liquid crystal alignment with the polarized light irradiation is applied in the future, it is necessary not only to initially align the liquid crystal but also to develop more stable alignment from the viewpoint of reliability. In view of actual industrial application, selection of a thermally stable polymer structure is desired. From those points, the polymer material proposed conventionally for the liquid crystal alignment through light irradiation is not sufficient in the alignment property and the stability, which actually presents a significant problem in realizing the rubbing-less alignment through light irradiation.

Thus, it is an object of the present invention to provide, particularly, a large-sized liquid crystal display capable of solving the inherent problem of the small production margin in the alignment processing in the IPS-TFT-LCD described above, reducing the occurrence of defective display due to variations in the initial alignment direction, realizing the stable liquid crystal alignment, and having high image quality with a higher contrast ratio. It is another object of the present invention to provide a process of producing a high-quality and high-definition liquid crystal display with excellent mass productivity.

To achieve the abovementioned objects, the present invention provides a liquid crystal display comprising: a pair of substrates, at least one of the substrates being transparent; a liquid crystal layer disposed between the pair of substrates; a group of electrodes formed on one of the pair of substrates for applying an electric field having a component substantially in parallel with a surface of the substrate to the liquid crystal layer; a plurality of active devices connected to the group of electrodes; an alignment control film disposed between the liquid crystal layer and at least one of the pair of substrates; and optical means formed on at least one of the pair of substrates for changing the optical property of the liquid crystal layer in accordance with an alignment state of molecules of said liquid crystal layer, wherein at least one of the alignment control films is an alignment control film comprising photoreactive polyimide and/or polyamic acid provided with an alignment control ability by irradiation of substantially linearly polarized light.

The present invention is characterized in that liquid crystal molecules in the liquid crystal layer on the alignment control film have a long axis in a direction orthogonal to a polarization axis of the substantially linearly polarized light for irradiation. In particular, it is desirable that the photoreactive alignment control film is polyamic acid or polyimide comprising at least cyclobutanetetracarboxylic acid dianhydride as acid anhydride and at least aromatic diamine as diamine.

The present invention is characterized in that the cyclobutanetetracarboxylic acid dianhydride and its derivative are a compound represented by a formula [17]:


where R1, R2, R3, R4each represent a hydrogen atom, a fluorine atom, an alkyl group or alkoxyl group with a carbon number of 1 to 6.

On the other hand, the aromatic diamine compound contains at least one of compounds selected from a group of compounds consisting of ones represented by formulas [18] to [32]:


where R1, R2, R3, R4each represent a hydrogen atom, a fluorine atom, an alkyl group or alkoxyl group with a carbon number of 1 to 6, or a vinyl group {—(CH2)m—CH═CH2, m=0,1,2} or an acetyl group {—(CH2)n—C≡CH, n=0,1,2}, and in the formula [5], X represents a bond group —S—, —CO—, —NH—.

When the alignment control film is formed as a thin film having a thickness from 1 nm to 100 nm, the light transmittance is improved and the efficiency of light reaction with polarized light irradiation is effectively improved. In addition, when the liquid crystal display is produced, the voltage for driving the liquid crystal is effectively applied to the liquid crystal layer. Furthermore, when the alignment control film on the electrode is formed as a thin film having a thickness from 1 nm to 50 nm, and even from 1 nm to 30 nm, it is possible to reduce a direct current voltage component (a so-called residual DC voltage) remaining between the electrode/alignment control film/liquid crystal layer/alignment control film/electrode in each pixel of the liquid crystal display, and after-image and persistence characteristics are effectively enhanced.

In addition, the present invention is characterized in that the liquid crystal layer of the liquid crystal display has a pretilt angle equal to or smaller than one degree. In the conventional rubbing alignment technique, the end of the electrode step acts as a guide for the fibers of a rubbing cloth to draw the fibers in the direction in which the step extends, and the fibers do not extend to the corner of the step, which may prevent the alignment processing to cause misalignment. In particular, when at least one of a pixel electrode, a common electrode, and a common electrode wire is formed of a transparent electrode, the alignment state near the electrode step is easily recognized, and the present invention is effective. Especially, when the transparent electrode is formed of an ion-doped titanium oxide film or an ion-doped zinc oxide film (ZnO), the present invention effectively works. When the pixel electrode and the common electrode opposite thereto are arranged in parallel with each other and formed of zigzag bending structures, the adhesion of the liquid crystal alignment film to an underlying organic insulating film may be poor, and the conventional rubbing alignment processing may cause defective display such as stripping of the alignment film. In such a case, the present invention is effective.

The present invention is particularly effective when the common electrode and/or the pixel electrode is formed on the organic insulating film and the liquid crystal alignment film is formed on the organic insulating film and the electrodes. In addition, the present invention is characterized in that the liquid crystal molecules have substantially the same alignment control directions at two interfaces between the liquid crystal layer and the alignment control film formed on each of the paired substrates.

The present invention is characterized in that the liquid crystal alignment processing is performed by irradiating the liquid crystal alignment film with polarized light. The present invention is characterized in that the polarized light used in the alignment processing has a wavelength range from 200 to 400 nm. In addition, the present invention is more effective when polarized light with at least two wavelengths, that is, substantially linearly polarized light with a first wavelength and light with a second wavelength are used in the alignment processing.

The present invention is also characterized in that the liquid crystal alignment film has a glass transition temperature equal to or higher than 250° C. The present invention more effectively functions by applying at least one processing of heating, irradiation of infrared rays, irradiation of far infrared rays, irradiation of electron beams, and irradiation of radioactive rays when the irradiation of the liquid crystal alignment film with the polarized light is performed to provide the liquid crystal alignment ability. When the liquid crystal alignment ability is provided by irradiating the liquid crystal alignment film with the polarized light, the heating, irradiation of infrared rays, irradiation of far infrared rays, irradiation of electron beams, or irradiation of radioactive rays is performed to accelerate the provision of the liquid crystal alignment ability through the polarized light irradiation and induce cross-link reaction or the like, thereby effectively promoting and stabilizing the liquid crystal alignment ability. Especially, at least one processing of heating, irradiation of infrared rays, irradiation of far infrared rays, irradiation of electron beams, and irradiation of radioactive rays is performed to overlap in time with the irradiation of polarized light, so that the present invention more effectively functions.

The present invention effectively works by performing imidation calcination processing of the alignment control film to overlap in time with the irradiation of polarized light. In particular, when at least one processing of heating, irradiation of infrared rays, irradiation of far infrared rays, irradiation of electron beams, and irradiation of radioactive rays is performed in addition to the polarized light irradiation of the liquid crystal alignment film, it is desirable to set the temperature of the alignment control film in a range from 100 to 400° C., and more preferably, from 150 to 300° C. It is possible and effective that the processing of heating, irradiation of infrared rays, irradiation of far infrared rays also serves as the imidation calcination (firing) processing.

In the present invention, the target contrast ratio is equal to or higher than 500:1, and the target time for eliminating after-image is equal to or shorter than five minutes. The time for eliminating after-image is determined by a process defined in the following embodiments.

