Abstract:
A method is disclosed for controlling a pretilt angle direction for a liquid crystal cell comprising the steps of first setting the magnitude of pretilt angle and a plurality of pretilt angle directions in an alignment layer. This first step is achieved by irradiating linearly the alignment layer with polarized or unpolarized UV light. One of the plurality of pretilt angle directions is then selected by exposing the alignment layer to UV light a second time.

Description:
This application is a continuation of U.S. patent application Ser. No. 09/457,388, filed Dec. 9, 1999, U.S. Pat. No. 6,226,006 which is a continuation of U.S. patent application Ser. No. 08/672,183, Jun. 27, 1999, now U.S. Pat. No. 6,414,737, the entire contents which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a liquid crystal cell, and particularly to a method for controlling a pretilt angle direction in liquid crystal cell in which photoreaction occur in a polymerized layer by ultraviolet irradiation. 
     Liquid crystals are the liquids consisting of anisotropic molecules. The average direction of long axes of these molecules is referred as the director of LC. The director distribution in a LC bulk is determined by its anchoring on the rigid substrates and characterized by the direction of the axes of easy orientation, corresponding to the minimum of the surface energy of an LC, pretilt angle between the axis of easy orientation and the substrate plane, and tilt angle between the director of LC and the substrate plane. 
     In order to obtain the uniform brightness and high contrast ratio of a liquid crystal display, the LC molecules must be appropriately aligned after being injected between the substrates of the cell. Not only the value of the director tilt but the direction of this tilt (i.e. direction of the axis of easy orientation) is important for normal operation of LC devices constituting double- and multi-domain structure. Such alignment is achieved by providing an alignment layer on the surface of the substrate. A rubbing process can be used for aligning liquid crystal molecules. In this rubbing process, a polyamide alignment layer is first coated on the substrate and the rubbing is performed mechanically, so that microgrooves are formed on the surface of the alignment layer. The liquid crystal molecules are thus uniformly aligned due to the intermolecular interaction between the polyamide molecules and the liquid crystal molecules. 
     In the above described rubbing process, however, defects are formed in the microgrooves which cause light scattering and random phase distortion. Also during the rubbing process, dust and electrostatic charges are produced in the alignment layer, so that the substrate is damaged and yield is decreased. 
     To solve the aforementioned problem, photo-alignment process has been recently introduced. As an example of the photo-alignment method, a method has proposed by KOBAYASHI, etc. (SID 95 DIGEST, p. 877) in which the pretilt angle direction is determined by irradiating the UV light twice into an alignment layer consisting of polyvinylcinnamate (PVCN) based polymer, as shown in FIGS. 1A and 1B. 
     In particular, as shown in FIG. 1A, when the linearly polarized UV light irradiates to alignment layer in the direction perpendicular to the surface of the substrate  16 , the alignment layer  15  becomes a photo-polymerized due to cross linking between polymer molecules. The bonding direction of the photo-polymer molecules depends on the polarization direction of the linearly polarized UV light. The liquid crystal is thus aligned according to the bonding direction of the photo-polymer molecules. 
     Then, the linearly polarized UV light whose polarization direction is perpendicular to the polarization direction of the first UV light is irradiated at an angle φ to the alignment layer  15 . The pretilt angle of the alignment layer  15  is formed in this step and the magnitude of the pretilt angle varies according to the irradiation angle of the UV light. For example, the pretilt angles are approximately 0.15°, 0.26°, or 0.30°, when the irradiation angles are 30°, 40°, or 60°, respectively. 
     In KOBAYASHI, however, the method has some drawbacks the thermostability of the tilt angle on the PVCN based materials is poor, the scope of the pretilt angle is small and does not cover the range needed for an applications, only polarized exciting light could be used. Moreover, the method requires rather complicated geometry of the irradiation and suitable for the only materials revealing the light-induced easy axis direction perpendicular to the polarization of the exciting light. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for controlling pretilt angle direction for liquid crystal cell in which control of the pretilt angle direction is easy and large pretilt angle can be obtained. 
     In order to achieve the object, the method for controlling pretilt angle for liquid crystal cell comprises the first step of irradiating UV light to an alignment layer to form pretilt angle and orient the alignment axis and second step of irradiating UV light to the alignment layer in the direction of oblique to the surface. The order of these step is reversible. 
     The first and second exposures to UV light can be performed at an angle, preferably 0-60°, or perpendicular to the substrate surface. Moreover, both polarized and unpolarized UV light can be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are views illustrating the conventional method for controlling pretilt angle for liquid crystal cell; 
     FIG. 2 is a view illustrating an UV light irradiation and birefringence measurement apparatus; 
     FIGS. 3A and 3B are views illustrating method for controlling pretilt angle direction for liquid crystal cell according to first embodiment of the present invention; 
     FIG. 4 is a graph showing pretilt angle dependent on a absorption energy of the UV light; 
     FIGS. 5A and 5B are views illustrating the method for controlling pretilt angle direction for liquid crystal cell according to second embodiment of the present invention; 
     FIGS. 6A and 6B are views showing the method for controlling pretilt angle direction according to third embodiment of the present invention; 
     FIGS. 7A and 7B are views showing the method for controlling pretilt angle direction according to fourth embodiment of the present invention; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a view illustrating ultraviolet(UV) irradiating and birefringence measuring apparatus for photo-alignment process. In this device, UV light generated from Hg lamp  11  is linearly polarized through a lens  12   a  and a polarizer  17   c,  and is directed to the alignment layer  15  coated on substrate  16 . A laser beam generated from a laser  18  is pulsed by beam by a chopped  23 , and then it is polarized by a polarized  17   a.  The pulsed beam is then transmitted to the alignment layer  15  through a compensator  25 , through a polarizer  17   b  and a lens  12   b,  and finally inputted to a digital oscilloscope  20  so that the birefringence caused by anisotropy of the alignment layer  15  can be measured. 
     FIGS. 3A and 3B illustrate first embodiment of the present invention. In this embodiment, the alignment layer includes polysiloxane based material or polyvinylfluorocinnamate (PVCN-F). In this alignment layer, pretilt angle and the direction of the easy axis are determined by one time irradiation of the UV light. The structural formulas of the polysiloxanecinnamate and PVCN-F are indicated below. Further, these materials can be used in other embodiments of the present invention. 
     PVCN-F:                           
     examples of polysiloxanecinnamate include:                           
     Z=OH, CH 3 , or a mixture of OH and CH 3    
     M=10-100 
     l=1, 3, 4, 5, 6, 7, 8, 9, or 10 
     K=0, 1, or 2 
     X, X 1 , Y=H, F, Cl, CN, CF 3 , OCF 3 , C n H 2n+1  or OC n H 2n+1  (n=1-10) 
     The polymer solution, the polysiloxanecinnamate or PVCN-F solution, is prepared using 1:1 mixture of 1,2-dichloroetane and chlorobenzene. A concentration of the solution is 20 g/l. A droplet of this solution is dropped in the center of the substrate and then coated on the whole area of the substrate by a spin-coating for 20 second at 2,000 rpm. As a result, a polymer film is deposited on the substrate. The thickness of the film, measured by Linnik interferometer, is 1,000 Å, and can be controlled by changing the concentration of the polymer solution and/or revolution speed of a spin-coating machine used for spin-coating. 
     As shown in FIG. 3A, when the linearly polarized UV light  190  is irradiated to the alignment layer  15  in the perpendicular direction of the surface of the substrate, the direction of alignment-axis is determined to be perpendicular to the polarization direction of the UV light. Further, two symmetric and bidirectional pretilt angles φ 1  are created on both sides of the alignment-axis. The size of the pretilt angle φ 1  is variable and depends on the duration of UV exposure, i.e., the amount of UV energy absorbed by alignment layer  15 . 
     FIG. 4 is a graph showing the relation between the pretilt angle and the irradiation time of the UV light, i.e., the amount of absorption energy of the UV light. As shown in the figure, the larger the absorption energy is, the smaller the pretilt angle becomes. Accordingly, the direction of the alignment axis is determined by the polarization direction of the UV light irradiation, and the size of pretilt angle is dependent upon the amount of UV absorption energy. 
     After irradiating linearly polarized UV light, another light is irradiated to the layer to the direction oblique to the surface of the alignment layer, that is, one direction of two symmetric pretilt angles is then selected by irradiating alignment layer  15  at an angle θ 1 , 0°&lt;θ 1 ≦60° with non-linearly polarized UV light, specially unpolarized UV light  200 , such that all the molecules of alignment layer  15  are oriented at one of the symmetric pretilt angle directions, as in FIG.  3 B. The alignment layer  15  preferably includes polysiloxane based material or PVCN-F. In this case, the irradiation direction of the second UV light forms acute angle with one of two symmetric pretilt angle directions. 
     FIGS. 5A and 5B illustrate a second embodiment of the present invention. In the figure, unpolarized UV light(wavelength λ=365 nm)  210  irradiates alignment layer  15  coated on the substrate  16  at an angle θ 2  to the normal direction of the surface of the substrate  16 . Although unpolarized UV light is used in this embodiment, the non-linearly polarized UV light, which means that the light is circularly or elliptically polarized, or the light does not polarized can be used either. θ 2  is preferably in the range of 0°&lt;θ 2 ≦60°. After the initial irradiation shown in FIG. 5A, the molecules of alignment layer  15  are set or oriented with a large number of one sided pretilt angle directions, while the magnitude of the pretilt angles formed by each of these directions with the surface is substantially the same. After the second irradiation shown in FIG. 5B, however, only one of the pretilt angle directions will be selected. 
     Thereafter, when linearly polarized UV light  220  irradiates the alignment layer  15  at a perpendicular angle, as shown in FIG. 5B, only the pretilt angle direction perpendicular to the polarization direction of this UV light is selected, so that the desired pretilt angle direction can be obtained. Furthermore, the size or magnitude of the pretilt angle produced in the alignment layer  15  varies depending upon the amount of UV energy absorbed, as noted above with respect to FIG.  4 . 
     FIGS. 6A and 6B illustrate a third embodiment of the present invention. As shown in FIG. 6A, unpolarized UV light  230  irradiates alignment layer  15  at an angle θ 3  to the normal direction of the surface of the substrate, where the θ 3  is in the range of 0°&lt;θ 3 ≦60°. Thereafter, linearly polarized UV light  240  irradiates alignment layer  15  again at an angle θ 4 , as shown in FIG. 6B, where the θ 4  is in the range of 0°&lt;θ 4 ≦60°. 
     The resulting orientations of the molecules of alignment layer  15  after the steps shown in FIGS. 6A and 6B is similar to that of FIGS. 5A and 5B, respectively. Namely, after the irradiation shown in FIG. 6A, the molecules of the alignment layer  15  are oriented at a large number of one sided pretilt angle directions, as in FIG.  5 A. Moreover, as in FIG. 5B, only one of these directions is selected after the second irradiation shown in FIG.  5 B. The irradiation direction of the second UV light forms acute angle with the pretilt angle directions by first irradiation. In this embodiment, non-linearly polarized UV light, including circularly polarized, elliptically polarized, and unpolarized UV light, can be used instead of unpolarized UV light. By using non-linearly polarized UV light, the method becomes simplified compared with the conventional method using linearly polarized UV light. 
     A fourth embodiment of the present invention is shown in FIGS. 7A and 7B. As shown in FIG. 7A, linearly polarized light  250  irradiates alignment layer  15  at an angle θ 5 , 0°&lt;θ 5 ≦60°, relative to the normal of the surfaces of alignment layer  15  to orient the molecules of alignment layer  15  at first and second pretilt angle directions, similar to that shown in FIG.  3 A. One of these directions is then selected by irradiating alignment layer  15  at an angle θ 6 , 0°&lt;θ 5 ≦60°, with non-linearly polarized UV light, specially unpolarized UV light  260 , such that all the molecules of alignment layer  15  are oriented at one of the first and second pretilt angle directions, as in FIG.  7 B. In this embodiment, the irradiation direction of the second UV light forms acute angle with one of the first and second pretilt angle directions. 
     In aforementioned methods according to the present invention, the size of the pretilt angle and the two symmetric pretilt angle directions are determined by an initial exposing an alignment layer including a polysiloxane based material or PVCN-F to UV light. One of these pretilt angles is then selected by a second exposure to UV light. Accordingly, the pretilt angle can be controlled easily. 
     While the invention has been described in its preferred embodiments, this should not be construed as limitation on the scope of the present invention. Accordingly, the scope of the present invention should be determined not by the embodiment illustrated, but by the appended claims and their legal equivalents.