Abstract:
A LC cell is manufactured by the method including the steps of: rubbing a first alignment layer coating a first substrate, such that the first alignment layer has a first pretilt angle associated therewith; exposing said second alignment layer coating a first substrate to light such that said second alignment layer has at least one second pretilt angle associated therewith; and providing a liquid crystal material between said first and second substrates. The materials for the first and second alignment layers include a polyimide and a polysiloxane based material.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is related to a liquid crystal cell, more particularly to a liquid crystal cell which includes one substrate coated with a rubbed alignment layer and the other substrate coated with a photo-aligned alignment layer, and the method for fabricating that. 
     Generally, the liquid crystal cell comprises two substrates and liquid crystal formed between these substrates, the liquid crystal comprising anisotropic molecules. To provide an orderly alignment of liquid crystal in the cell for the uniform brightness and the high contrast ratio of the liquid crystal cell, conventional rubbing is carried out on alignment layers coating substrate. The rubbing is mechanical friction on the alignment layer so as to provide a pretilt of liquid crystal molecules defined by a pretilt angle and a pretilt angle direction. The pretilt angle refers to a polar angle and the pretilt angle direction refers to a azimuthal angle between the surface of alignment layer and the pretilt. 
     The pretilt of a liquid crystal molecule adjacent a first alignment layer is called a first pretilt of a first alignment layer, and the pretilt of a liquid crystal molecule adjacent a second alignment layer is called a second pretilt of a second alignment layer. Thereby, the pretilt of a liquid crystal molecule in the middle of two layers is determined by the interaction between pretilts of the first and second alignment layer. 
     The liquid crystal cell is classified a vertical aligned liquid crystal cell and a horizontal aligned liquid crystal cell depending upon the pretilt angle. The vertical aligned liquid crystal cell typically defines a liquid crystal cell having a pretilt angle of an alignment layer larger than 60°, the horizontal aligned liquid crystal cell typically refers to a liquid crystal cell having a pretilt angle of an alignment layer less than 5°. 
     There are several modes for liquid crystal cells according to relationships between a first pretilt angle direction of a first alignment layer and a second pretilt angle direction of a second alignment layer facing the first substrate. If the first pretilt angle direction is perpendicular to the second alignment direction, it is called a twisted nematic(TN) mode liquid crystal cell. If they are parallel with each other, the liquid crystal cell is called an electrically controlled birefringence(ECB) mode liquid crystal cell and a bend mode liquid crystal cell. In addition, it is called a In-Plane Switching(IPS) mode liquid crystal cell if a pretilt angle direction is shifted depending on the voltage. 
     A conventionally used liquid crystal display is mainly a twisted nematic liquid crystal display (TNLCD), in which the transmittance is dependent according to the viewing angle at each gray level. Especially, while the transmittance is symmetrical in the horizontal direction, the transmittance is asymmetrical in the vertical direction. Therefore, in the vertical direction, the range with inverted image phenomenon is occurred so that the vertical viewing angle becomes very limited. 
     To overcome said problems, a multi-domain TNLC cell such as a two-domain liquid crystal cell, and a four-domain liquid crystal cell is introduced. The multi-domain liquid crystal cell has a wider viewing angle by providing more than domains in each pixel, domains having different pretilts with respect to each other, so as to compensate the viewing angle dependence of each domain. 
     The most popular process to obtain said multi-domain liquid crystal cell is a mechanical rubbing process, as shown in FIG. 1. Rubbing is performed mechanically on an entire substrate 1 coated with alignment layer 8 such as polyimide, so that microgrooves are formed on the surface of the alignment layer 8, as shown in FIG. 1a, and FIG. 1b. To divide two domains in a pixel, in FIG. 1c and FIG. 1d, a photoresist 11 is coated over entire alignment layer 8 surface, and the photoresist 11 of one domain is removed by exposing light, reverse rubbing process is carried out on one domain as shown in FIG. 1e. The remained photoresist is removed by exposing light, then, two domains are provided on the substrate 1 as shown in FIG. 1f. In the two-domain liquid crystal cell obtained thereby, the inversion of viewing angle is compensated by the aforementioned process. 
     However, the rubbing process causes a dust particle and/or an electrostatic discharge, so the yield is reduced and/or the substrate is damaged. The manufacturing process becomes too complicated to apply in industry, because the process includes a photolithography, which is coating photoresist layer and removing a part of the layer by exposing light, for dividing domains. 
     Therefore, it is a photo-alignment method that is introduced to simplify alignment process as well as to prevent the damage of substrate. The photo-alignment is the process in which a pretilt angle direction of alignment layer is given by the irradiation of linearly polarized ultraviolet light. The alignment layer used in the photo-alignment method is mainly including PVCN(polyvinyl cinnamate). When ultraviolet light is irradiated into the photo-aligned layer coating the substrate, it causes cyclo-addition between the cinnamoyl groups of cinnamic acid side chains that belong to different photopolymers. Thereby, the direction of the photopolymer configuration i.e., the pretilt of alignment layer is aligned uniformly. 
     One example of the photo-alignment method is disclosured as a following process. The photo-alignment method comprises double exposure of linearly polarized ultraviolet light into a substrate coated with PVCN to determine a pretilt, the pretilt including an alignment direction, a pretilt angle direction and pretilt angle. First linearly polarized ultraviolet light is perpendicularly irradiated into the alignment layer coating substrate so as to determine a plurality of pretilt angle directions. Then, second linearly polarized light is obliquely irradiated into the alignment layer again, to determine a pretilt angle and a pretilt angle direction. The pretilt angle and pretilt angle direction are obtained by controlling the second oblique direction relative to the substrate coated with the alignment layer. 
     However, the photo-alignment method has problems that the process is complicated due to the double exposure and the pretilt angle is too small, for example, the obtained pretilt angles being approximately 0.15°, 0.26° and 0.30° respectively when the oblique irradiation angles are 30°, 45° and 60°. In addition, it takes long time to irradiate light into the alignment layer so total tact time is prolonged, as well, the alignment stability of photo-alignment method is weaker than that of rubbing method. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a liquid crystal cell having an alignment stability and a wider viewing angle by simplifying the process and the fabrication method thereof. 
     To achieve the object mentioned above, the method for fabricating the liquid crystal cell of the present invention comprises the following steps of: providing a first alignment layer on a first substrate; rubbing said first alignment layer such that said first alignment layer has a first pretilt angle associated therewith; providing a second alignment layer on a second substrate; exposing said second alignment layer to light such that said second alignment layer has at least one second pretilt angle associated therewith; and providing a liquid crystal material between said first and second substrates. 
     The first alignment layer comprises polyimide, but the second alignment layer comprises photopolymers, the photo-polymers including polysiloxane based materials. The pretilt angle of this invention is controlled depending upon the photo-energy of the ultraviolet light irradiating. 
     To determined the second pretilt, this invention includes double irradiation of this invention. The double irradiation comprises the steps of: irradiating polarized light in the perpendicular direction to the second alignment layer and irradiating unpolarized light in the oblique direction to the second alignment layer. In these steps, the latter step can be prior to the former. 
     Another method for determining the second pretilt in this invention is using a flowing effect. The method comprises the steps of: irradiating polarized light in the perpendicular direction to the second substrate so as to determine a second pretilt angle and two pretilt angle direction; and injecting liquid crystal materials between said first substrate and said second substrate in the perpendicular direction to the polarized direction of the light so as to select a pretilt angle and a second pretilt angle direction. 
     These and other objects and advantages of the present invention will become clear from the following description of preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a to 1f show a conventional reverse-rubbing process. 
     FIG. 2 is showing a cross-sectional view of liquid crystal cell according to this invention. 
     FIG. 3 is showing a photo-irradiation device used for photo-alignment process. 
     FIG. 4 is a graph illustrating the relationship between the photo-energy of ultraviolet light and the pretilt angle of the photo-aligned layer formed with polysiloxane based materials. 
     FIGS. 5a to 5f show process for fabricating a liquid crystal cell according to this invention. 
     FIGS. 6a to 6f show another process for fabricating a liquid crystal cell according to this invention. 
     FIGS. 7a to 7d show a cross-sectional view of a TN mode liquid crystal cell according to this invention 
     FIGS. 8a to 8d show a cross-sectional view of a ECB mode liquid crystal cell according to this invention. 
     FIGS. 9a to 9d show a cross-sectional view of a bend mode liquid crystal cell according to this invention. 
     FIGS. 10a to 10b show a cross-sectional view of a IPS mode liquid crystal cell according to this invention. 
     FIGS. 11a to 11g show one process for fabricating a multi-domain liquid crystal cell according to this invention. 
     FIGS. 12a to 12g show another process for fabricating a multi-domain liquid crystal cell according to this invention. 
     FIGS. 13a to 13f show another process for fabricating a multi-domain liquid crystal cell according to this invention. 
     FIGS. 14a to 14j show another process for fabricating a multi-domain liquid crystal cell according to this invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     FIG. 2 is a drawing showing the liquid crystal cell, and 1 and 2 indicate a first substrate and a second substrate of the liquid crystal cell, respectively. The first substrate 1 is coated with the first alignment layer 8 formed with polyimide and is rubbed so as to determine a first pretilt defined a first pretilt angle and a first pretilt angle direction. The second substrate 2 is coated with the second alignment layer 9. The material for the second alignment layer 9 includes photopolymers such as polysiloxane based materials. Polysiloxane cinnamate, one of the polysiloxane based materials has the following structural formulas: polysiloxane cinnamate I: ##STR1## Z can be selected from the group consisting of OH, CH 3  or from mixtures thereof, 
     m=10-100, 
     l=1-11, 
     L=0 or 1, 
     K=0 or 1, 
     X, X 1 , X 2 , Y═H, F, Cl, CN, CF 3 , C n  H 2n+1 , or OC n  H 2n+1 , wherein n can be from 1 to 10, or from mixtures thereof; 
     polysiloxane cinnamate II: ##STR2## Z can be selected from the group consisting of OH, CH 3  or from mixtures thereof, 
     m=10-100, 
     l=1-11, 
     L=0 or 1, 
     K=0 or 1, 
     X, X 1 , X 2 , Y═H, F, Cl, CN, CF 3 , C n  H 2n+1  or OC n  H 2n+1 , wherein n can be from 1 to 10, or from mixtures thereof. 
     Then, the second substrate 2 coated with a second alignment layer 9 is exposed to ultraviolet light by a photo-irradiation device in FIG. 3. The device includes the lamp 3 generating ultraviolet light, a lens 4 and a polarizer 5 linearly polarizing the ultraviolet light from lamp 3. To irradiate ultraviolet light into the alignment layer 9 of the substrate 2, the ultraviolet light generated from the lamp 3 is passed through the lens 4 and linearly polarized through polarizer 5, then the ultraviolet light is irradiated into the alignment layer 9 coating substrate 2. The lamp 3 is the Mercury(Hg) lamp having the wave length of 365 nm. 
     In that time, the pretilt angle formed on the alignment layer 9 can be controlled by photo-energy. When ultraviolet light is perpendicularly irradiated into the substrate 2 coated with alignment layer 9 of polysiloxane based materials, the pretilt angle of the alignment layer 9 surface is controlled in a broad range according to the photo-energy of the ultraviolet light, as shown in FIG. 4. Referring to this figure, the pretilt angle can be controlled depending upon the photo-energy irradiated into the alignment layer 9 (the wavelength of ultraviolet light is 350 nm). The pretilt angle exponentially decreases according to the photo-energy of ultraviolet light to at almost 0° at 6,000 mJ/cm. 
     The vertical aliged liquid crystal cell can be fabricated by adopting photo-energy less than 2,000 mJ/cm 2 , and the horizontal aligned liquid crystal cell can be fabricated by adopting photo-energy more than 5,000 mJ/cm 2 . 
     FIG. 5 is showing one embodiment of a process for fabricating a liquid crystal cell which comprises a first substrate, a second substrate and a liquid crystal layer injected therebetween. The first substrate 1 coated with a first alignment layer 8 is mechanically rubbed to determine a pretilt, the pretilt meaning a pretilt angle and a pretilt angle direction, as shown in FIG. 5a and FIG. 5b. 
     The polarized light is irradiated in the perpendicular direction onto the second substrate 2 coated with a second alignment layer 9 so as to determine a second pretilt angle and two pretilt angle direction facing each other as shown in FIG. 5c and FIG. 5d. To select one pretilt angle direction, nonpolarized light is irradiated in the oblique direction onto the alignment layer 9 on the substrate 2 so as to determine a second pretilt oriented with one pretilt angle direction. In FIG. 5f, attaching the first substrate 1 and the second substrate 2, liquid crystal materials are injected between two substrates 1,2 so as to align uniformly by the stable anchoring provided by the first pretilt. 
     In addition, it is also possible that the oblique-irradiation can be carried out prior to the perpendicular irradiation in this embodiment. 
     FIG. 6 shows another embodiment of a process for fabricating a liquid crystal cell which comprises a first substrate, a second substrate and a liquid crystal layer injected therebetween. The first substrate 1 coated with a first alignment layer 8 is mechanically rubbed to determine a pretilt, the pretilt meaning a pretilt angle and a pretilt angle direction, as shown in FIG. 6a and FIG. 6b. 
     The polarized light is irradiated in the perpendicular direction into the second substrate 2 coated with a second alignment layer 9 so as to determine a second pretilt angle and two pretilt angle direction facing each other as shown in FIG. 6c and FIG. 6d. To select one pretilt angle direction, this embodiment adopts the flowing effect of liquid crystal materials in which the pretilt angle direction is determined according to the flowing direction of liquid crystal material flown as shown in FIG. 6e, FIG. 6f. Attaching the first substrate 1 having uni-pretilt determined alignment layer 8 by rubbing and second substrate 2 having two pretilts oriented with two pretilt angle directions determined a photo-aligned layer 9, then liquid crystal materials are injected between two substrates. By the flowing effect of liquid crystal materials, the liquid crystal materials adjacent the second alignment layer 9 are aligned uniformly in the second pretilt angle direction by a single exposure. 
     There are several mode of liquid crystal cell depending upon the configuration between the first pretilt angle direction and the second pretilt angle direction determined by either abovementioned process. 
     FIG. 7 refers to a TN mode liquid crystal cell. FIG. 7a and FIG. 7b show a vertical aligned liquid crystal cell controlling depending upon the voltage. FIG. 7c and FIG. 7d show a horizontal aligned liquid crystal cell. 
     FIG. 8 refers to an a ECB mode liquid crystal cell. FIG. 8a and FIG. 8b show a vertical aligned liquid crystal cell controlling depending upon the voltage. FIG. 8c and FIG. 8d show a horizontal aligned liquid crystal cell. 
     FIG. 9 is referring a bend mode liquid crystal cell. FIG. 9a and FIG. 9b show a vertical aligned liquid crystal cell controlling depending upon the voltage. FIG. 9c and FIG. 9d showing a horizontal aligned liquid crystal cell. 
     FIG. 10 refers to an IPS mode liquid crystal cell in which the liquid crystal molecules are shifted in plane depending on the voltage. As further shown in FIGS. 10a and 10b, elements 7a and 7b designate a thin-film transistor (TFT) gate electrode and a counter or common electrode, respectively, on second or a lower substrate 2. In order to simplify the description herein, remaining portions of the TFT (e.g. source and drain) are not shown in FIGS. 10a and 10b. 
     This invention can be applied in a multi-domain liquid crystal cell to provide wider viewing angle. Some embodiments of process for manufacturing the multi-domain liquid crystal cell are in FIG. 11, FIG. 12, FIG. 13 and FIG. 14. 
     FIG. 11 is shows one embodiment of this invention to provide a multi-domain liquid crystal cell in which the first alignment layer 8 is provided a first pretilt by mechanically rubbing, and the second alignment layer 9 is provided two pretilts in two domains by using the light. 
     FIG. 11a and FIG. 11b show the rubbing process to provide a first pretilt on the alignment layer 8 with low pretilt angle, almost 0°. FIG. 11c-FIG. 11f are showing the process for forming two second pretilts on two domains of the second alignment layer 9 with a high photo-energy to provide low pretilt angle less than 5°. The polarized light is irradiated in the perpendicular direction onto the second alignment layer 9 so as to determined a second pretilt angle and two pretilt angle directions, as shown in FIG. 11d. To select a first pretilt angle direction for a first domain I, the nonpolarized light is irradiated in the first oblique direction to the second substrate 2 in which a second domain II is covered with the mask 10. Thereby, the 2-1th pretilt is formed on a first domain I, the 2-1th pretilt defined a second pretilt angle and a first pretilt angle direction, as shown in FIG. 11e. 
     To select a 2-2th pretilt angle direction for a second domain II, the mask 10 covering the second domain II is moved to the first domain I. The nonpolarized light is irradiated in the second oblique direction to the second substrate 2 in which a first domain I is covered with the mask 10. Thereby, the 2-2th pretilt is formed on a second domain II, as shown in FIG. 11f, the 2-2th pretilt meaning a second pretilt angle and a second pretilt angle direction. 
     Assembling the first substrate 1 and the second substrate 2, liquid crystal materials are injected between two substrates 1,2. The molecules of liquid crystal materials are arranged in the different direction between domains as shown in FIG. 11g depending the second pretilts. 
     Thereby, the viewing angle is compensated by differently aligning the liquid crystal molecules according to domains so as to get a wider viewing angle liquid crystal cell. 
     In this embodiment, the two-domain liquid crystal cell is possible to obtain multi-domain liquid crystal cell without photolithograpy. In addition, the alignment stability is provided by the first pretilt. 
     FIG. 12 shows another embodiment of this invention to provide a vertical aligned mode multi-domain liquid crystal cell in which the first alignment layer 8 is provided a first pretilt by mechanically rubbing, and the second alignment layer 9 is provided two pretilts in two domains by using the light. 
     FIG. 12a and FIG. 12b show the rubbing process to provide a first pretilt on the alignment layer 8 with high pretilt angle larger than 60°. FIG. 12c-FIG. 12f show the process for forming two second pretilts on two domains of the second alignment layer 9 with a low photo-energy to provide high pretilt angle less than 60°. The polarized light is irradiated in the perpendicular direction onto the second alignment layer 9 so as to determine a second pretilt angle and two pretilt angle directions, as shown in FIG. 12d. To select a first pretilt angle direction for a first domain I, the nonpolarized light is irradiated in the first oblique direction to the second substrate 2 in which a second domain II is covered with the mask 10. Thereby, the 2-1th pretilt is formed on a first domain I, the 2-1th pretilt defined a second pretilt angle and a first pretilt angle direction, as shown in FIG. 12e. 
     To select a 2-2th pretilt angle direction for a second domain II, the mask 10 covering the second domain II is moved to the first domain I. The nonpolarized light is irradiated in the second oblique direction to the second substrate 2 in which a first domain I is covered with the mask 10. Thereby, the 2-2th pretilt is formed on a second domain II, as shown in FIG. 12f, the 2-2th pretilt meaning a second pretilt angle and a second pretilt angle direction. 
     Assembling the first substrate 1 and the second substrate 2, liquid crystal materials are injected between two substrates 1,2. The molecules of liquid crystal materials are arranged in the different direction between domains as shown in FIG. 12g depending upon the second pretilts. This vertical algined liquid crystal cell has a bend mode in the first domain I and a ECB mode in the second domain II. 
     Thereby, the viewing angle is compensated by differently aligning the liquid crystal molecules according to domains so as to get a wider viewing angle liquid crystal cell. 
     In this embodiment, the two-domain liquid crystal cell is accomplished by low photo-energy so it is possible to obtain multi-domain liquid crystal cell without photolithograpy. In addition, the alignment stability is provided by the first pretilt. 
     FIG. 13 shows another embodiment of process for multi-domain liquid crystal cell. 
     FIG. 13a and FIG. 13b show the rubbing process to provide a 1-1th pretilt and 1-2th pretilt on a first domain I and a second domain II of the first substrate 1 with different pretilt angles, such as 1-1th pretilt angle is larger than 1-2th pretilt angle. The alignment layer for dividing domain is shown in FIG. 13a, an organic alignment layer 8A is covered with an inorganic alignment layer 8B on the first substrate 1. In the organic alignment layer 8A, pretilt angle is formed larger than in the inorganic alignment layer 8B. Thus, the 1-1th pretilt is defined as a low 1-1th pretilt angle and a first pretilt angle direction, and the 1-2th pretilt is defined as a high 1-2th pretilt angle and a first pretilt angle direction. 
     FIG. 13c-FIG. 13d show the process for two domains in the second substrate 2 with dividing two pretilts by differing two pretilt angles. The substrate 2 coated with a second alignment layer 9 is covered with a mask 10 comprising a transparent part for a first domain I and a semi-transparent part for a second domain II. The polarized light is irradiated in the perpendicular direction onto the second substrate so as to determine a high 2-1th pretilt angle, a second alignment direction and two second pretilt angle directions on a first domain I, and a low 2-2th pretilt angle and two second pretilt angle directions on a second domain II. To select a second pretilt angle direction for a first domain I and a second domain II, the nonpolarized light is irradiated in the oblique direction to the second substrate 2. Thereby, the 2-1th pretilt and 2-2th pretilt are formed on a first domain I and a second domain II, respectively, the 2-1th pretilt meaning a high 2-1 pretilt angle and a second pretilt angle direction, and the 2-1th pretilt meaning a low 2-2 pretilt angle and a second pretilt angle direction, as shown in FIG. 13e. 
     Assembling the first substrate 1 and the second substrate 2, liquid crystal materials are injected between two substrates 1,2. The molecules of liquid crystal materials are aligned in the different directions between domains as shown in FIG. 13f depending the pretilt angles. 
     Thereby, the viewing angle is compensated by differently aligning the liquid crystal molecules according to domains so as to get a wider viewing angle liquid crystal cell. 
     In this embodiment, the two-domain liquid crystal cell is possible to obtain multi-domain liquid crystal cell without photolithograpy. In addition, the alignment stability is provided by the rubbed first alignment. 
     FIG. 14 shows a process for fabricating a four-domain liquid crystal cell. A two-domain first substrate 1 is prepared by the reverse rubbing, as shown in FIG. 14a, FIG. 14b and FIG. 14c. The Four-domain second substrate 2 is prepared by changing the photo-irradiating direction, as shown in FIG. 14d-FIG. 14i. 
     Attaching two substrates 1,2, liquid crystal materials are injected between two substrates 1,2. Then, the viewing angle is compensated by differently aligning the liquid crystal molecules according to each domain, as shown in FIG. 14g so as to get a wider viewing angle liquid crystal cell. 
     This invention can be adopted to the various mode liquid crystal cells such as a TN mode, an ECB mode, a bend mode, and an IPS mode by controlling the alignment direction. 
     In this invention, it is possible to provide alignment stability by rubbed first alignment layer, and to increase the yield by aligning using light instead of rubbing so as to eliminating damages caused by rubbing process. 
     In addition, the multi-domain liquid crystal cell can be obtained by a simple process without a photolithograpy for reverse rubbing. 
     It is to be understood that the form of the present invention herein show and described is to be taken as a preferred example of the same and that various application such as the change the photo-irradiation order, may be resorted to without departing from the spirit of the present invention or the scope of the subjoined claims.