Abstract:
A method of manufacturing a multi-domain liquid crystal display device having a pixel includes the steps of forming an alignment film on at least one of a first and second substrate; covering the alignment film with a mask, there being included a first surface having a plurality of light-transmitting portion and light-shielding portions and a second surface having light-shielding portions corresponding to the light-transmitting portions; radiating light from an upper portion of the mask; and assembling the first and second substrates.

Description:
This application is a Continuation of application Ser. No. 09/686,815 filed on Oct. 12, 2000, now U.S. Pat. No. 6,479,218 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a multi-domain liquid crystal cell, and more particularly, to a method for manufacturing a multi-domain liquid crystal cell in which a liquid crystal cell of a multi-domain liquid crystal cell having a wider viewing angle is obtained by a simple process. 
     2. Discussion of the Related Art 
     Recently, a liquid crystal display (LCD) mainly used for portable televisions or notebook computers requires a large sized screen, so as to be used for a wall type television or a monitor. A twisted-nematic (TN) liquid crystal cell is generally used as an LCD. The TN liquid crystal cell has different optical transmittivity characteristics at each gray level depending on viewing angles. For this reason, a large area of the TN liquid crystal cell is limited. That is, optical transmittivity is substantially symmetrical in view of a viewing angle in left and right direction while optical transmittivity is asymmetrical in view of up and down direction. Accordingly, an image inversion area exists in the viewing angle in up and down direction. As a result, there is a problem that the viewing angle becomes narrow. To solve such a problem, there is suggested a multi-domain liquid crystal cell in which a compensation effect of a viewing angle is obtained by varying a main viewing angle in each pixel. To obtain the multi-domain liquid crystal cell, a reverse rubbing process will be described with reference to FIG.  1 . 
     As shown in FIG. 1 a , an entire substrate lion which a polyimide  12  is deposited is processed by rubbing. Thus, a mono-domain is formed as shown in FIG. 1 b . As shown in FIG. 1 c , one domain is blocked by a photoresist  13 . Rubbing is then performed in a direction opposite to the rubbing direction of FIG. 1 a . As shown in FIG. 1 d , a domain which is not blocked by the photoresist  13  is processed by reverse rubbing. As shown in FIG. 1 e , if the photoresist  13  is removed, a substrate divided into two domains having opposite pretilt angles can be obtained. 
     However, the liquid crystal cell manufactured by the above rubbing process has problems in that dust or static electricity occurs during the rubbing process, thereby reducing yield or damaging the liquid crystal cell. 
     In another related art, to solve such problems, photo-alignment methods based on UV rays have been suggested instead of rubbing One photo-alignment method will be described with reference to FIG.  2 . As shown in FIG. 2 a , a substrate  21  on which an alignment film  22  is deposited is periodically shielded by a mask  23  having a light-transmitting portion  25  and a light-shielding portion  24 . When light (solid line arrow in tilt direction on a top of the drawing) is irradiated at a tilt at an angle of θ, a first pretilt is determined in a portion  26  where light is transmitted. As shown in FIG. 2 b , the mask  23  is rearranged to shield light in the portion  26 . Then, when light (solid line arrow in tilt direction on a top of the drawing) is irradiated at a tilt at an angle of  31  θ, a second pretilt is determined in a portion  27  where light is shielded in FIG. 2 a . Thus, as shown in FIG. 2 c , a first substrate of two domains having different pretilts can be obtained. Also, as shown in FIG. 2 d , a second substrate of two domains can be obtained by the above alignment method and lower and upper substrates are assembled with each other. 
     Furthermore, as shown in FIG. 3, areas Q and R are light-shielded, and the photo-alignment methods having an angle θ of irradiation in FIGS. 2 a  and  2   b  are sequentially applied to areas O and P as shown in FIGS. 3 a  and  3   b . The areas O and P are then light-shielded, and the photo-alignment methods having an angle θ of irradiation in FIGS. 2 a  and  2   b  are sequentially applied to the areas Q and R as shown in FIGS. 3 c  and  3   d . Thus, light irradiation of total four times is performed on the substrate, thereby obtaining a substrate having four domains. 
     As described above, the first substrate and the second substrate in which four domains are formed are bonded to face each other and then the liquid crystal is injected thereto, so that a four-domain liquid crystal cell can be obtained. 
     In still another related art, as shown in FIG. 4 a , a semi-transparent portion  43  of a mask is arranged in some area of a substrate  41  on which an alignment film  42  is deposited, and then light irradiation is performed. Thus, the irradiated light is absorbed in the alignment film  42  on the substrate  41  in an aperture portion. However, some of the irradiated light is only absorbed in the alignment film  42  in an area of the alignment film  42  of the substrate  41  corresponding to the semi-transparent portion  43  of the mask. Polysiloxane based materials used as the alignment film  42  are characterized in that the size of the pretilt angle becomes small as absorbing light energy increases. Accordingly, the size of the pretilt angle formed in the alignment film can easily be controlled. Based on this characteristic, a substrate having different pretilt angles and divided pixels is manufactured, and a sectional view of the substrate is shown in FIG. 4 b . As shown in FIG. 4 c , a liquid crystal cell is manufactured in such a manner that upper and lower substrates are bonded to each other by applying the substrate of FIG. 4 b . In this structure, alignment direction of each domain is identical in each substrate but the size of the pretilt angle is different. Accordingly, a multi-domain is formed to improve a viewing angle. Also, in case that the divided pixels are applied, a four domain liquid crystal cell can be obtained as shown in FIG.  5 . In this case, areas III and IV are light-shielded and the photo-alignment method of FIG. 4 a  is applied to areas I and II (see FIGS. 5 a  to  5   d ). Subsequently, the areas I and II are light-shielded, and in the areas III and IV, a four-domain substrate is obtained by varying polarization direction of the irradiated light in the photo-alignment method of FIG. 4 a  (see FIGS. 5 e  to  5   h ). After the first and second substrates in which four domains are formed are bonded to each other by the above method, the liquid crystal is injected into the substrates so as to obtain a four-domain liquid crystal cell. 
     However, the methods for manufacturing a liquid crystal cell through the photo-alignment methods have several problems in controlling alignment direction of the multi-domain to realize a wider viewing angle. 
     The first method requires light irradiation of eight times (vertical irradiation of four times and tilt irradiation of four times) into the upper and lower substrates to form a multi-domain divided into two pixel areas, and mask bonding process of four times. Moreover, to form a multi-domain having four domains, the process steps increase two times. Thus, in addition to light irradiation and mask bonding processes of several times, a gap between the masks and the substrate should additionally be controlled. These process steps are unattractive in view of the mass production. The second method requires light irradiation of four times (vertical irradiation of two times and tilt irradiation of two times) into the upper and lower substrates and mask bonding process of two times when forming a multi-domain having two domains. In the second method, the light irradiation and the mask bonding process steps have been reduced but error may occur in arranging a number of masks, thereby reducing the productivity. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method for manufacturing a multi-domain liquid crystal cell that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a method for manufacturing a multi-domain liquid crystal cell in which a photo mask required for photo-alignment is improved to perform tilt irradiation having two different directions by irradiation of one time, so that alignment division of a unit pixel can be realized and a multi-domain liquid crystal cell can be obtained by a simple process. 
     Another object of the present invention is to provide a method for manufacturing a multi-domain liquid crystal cell in which the number of masks is reduced to reduce error that may occur in arranging the masks due to control of a gap between the masks and the substrate. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for manufacturing a multi-domain liquid crystal display device having a pixel comprising the steps of: forming an alignment film on at least one of first and second substrates; covering the alignment film with a mask, the mask including a first surface having a plurality of light-transmitting portions and light-shielding portions and a second surface having light-shielding portions corresponding to the light-transmitting portions of the second surface; irradiating light from an upper portion of the mask; and assembling the first and second substrates. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 shows a process for manufacturing a two-domain liquid crystal cell according to a related art rubbing method; 
     FIG. 2 shows a process for manufacturing a two-domain liquid crystal cell according to a related art photo-alignment method; 
     FIG. 3 shows a process for manufacturing a four-domain liquid crystal cell according to a related art photo-alignment method; 
     FIG. 4 shows a process for manufacturing a two-domain liquid crystal cell according to a related art another photo-alignment method; 
     FIG. 5 shows a process for manufacturing a four-domain liquid crystal cell according to a related art another photo-alignment method; 
     FIG. 6 shows a process for manufacturing a two-domain liquid crystal cell according to a photo-alignment method using a mask of the present invention; and 
     FIG. 7 shows a process for manufacturing a four-domain liquid crystal cell according to a photo-alignment method using a mask of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 6 shows a process for manufacturing a two-domain liquid crystal cell in which a pixel is alignment divided by UV irradiation of one time using a mask. Referring to FIG. 6, an alignment film  62  is deposited on a substrate  61 . A plurality of first and second light-transmitting portions  64   a  and  64   b  and light-shielding portions  63  are arranged at constant intervals on a first surface  65  of the substrate  61  on which the alignment film is deposited. A mask  67  is arranged on a second surface  66  in such a manner that the light-shielding portion is arranged in a position corresponding to the light-transmitting portion of the first surface  65  and the light-transmitting portion is arranged in a position corresponding to the light-shielding portion. Then, UV irradiation is performed using the mask  67  as shown in a tilt solid arrow on a top of FIG.  6 . As shown in FIG. 6, the light irradiated to the first light-transmitting portions  64   a  is obliquely irradiated at an angle of θ in a direction of arrow x so that the light is absorbed in a first domain of the alignment film  62  on the substrate  61 . The light irradiated to the second light-transmitting portions  64   b  is tilt irradiated at an angle of −θ in a direction of arrow y so that the light is absorbed in a second domain of the alignment film  62  on the substrate  61 . Accordingly, two different tilt irradiation steps are performed by irradiation of one time to alignment-divide a unit pixel, so that a liquid crystal cell having simply divided domains can be manufactured. At this time, since the size of the pixel is defined as shown in FIG. 5, the following conditions should be satisfied. 
     p 1 (mask pattern period)=p 2 (pixel pitch) 
     w(mask pattern width)=p 1 /2 
     d(distance between the first surface and the second surface of the mask)=w×tan θ (θ is an angle of irradiation) 
     g(distance between the second surface of the mask and the alignment film)=d/2 
     In the above conditions, the mask pattern period is an arrangement period of the light-transmitting portion or the light-shielding portion of the mask, the pixel pitch is an arrangement period of the pixel, and the mask pattern width is a width of the light-transmitting portion or light-shielding portion. 
     The liquid crystal cell can be manufactured by applying the photo-alignment method of FIG. 6 to the upper and lower substrates. 
     In the above pixel division method, the irradiation angle can be controlled variously by varying the distance between the first surface and the second surface based on w×tan θ. If the irradiation angle is varied, the size of the pretilt angle in each domain of the substrate is varied. Accordingly, the viewing angle between neighboring domains is compensated, so that the multi-domain liquid crystal cell can be obtained by the simple process. 
     FIG. 7 shows a method for forming a four-domain liquid crystal cell by applying the above method. That is, referring to FIG. 7 a , a first area A filled with dots on the substrate on which the alignment film is deposited is covered with the above mask while a second area B filled with oblique lines is covered with an opaque mask. In this case, as shown in FIG. 6, the first light irradiation of UV (solid line arrow on a top in the drawing) is performed at an angle of θ to alignment-divide one pixel. Thus, a pretilt is formed by the first light irradiation as shown in FIG. 7 a . In FIG. 7 a , the left arrow of the first area A denotes a pretilt direction generated by UV in direction y passed through the second light-transmitting portion  64   b , i.e., UV irradiated at an angle of −θ, while the right arrow of the first area A denotes a pretilt direction generated by UV in direction x passed through the first light-transmitting portion  64   a , i.e., UV irradiated at an angle of θ. A pretilt is not formed in the second area B of the alignment film corresponding to the opaque portion having optical transmittivity of 0%. However, two domains having different pretilt directions are formed in the area covered with the mask. Next, as shown in FIG. 7 b , the first area A in which the pretilt is determined is covered with the opaque mask while the second area A is covered with the mask  67  having the distance d′ between the first surface  65  and the second surface  66 . In this case, since the irradiation angle is varied to θ due to the variation from d to d′ of the distance between the first surface  65  and the second surface  66 , the second light irradiation (solid arrow on a top of the drawing) can be performed in the second area B to have a pretilt angle different from that of the first area A. Thus, a pretilt is formed by the second light irradiation as shown in FIG. 7 b . In FIG. 7 b , the left arrow of the second area B denotes a pretilt direction generated by UV in direction y passed through the second light-transmitting portion  64   b , i.e., UV irradiated at an angle of −θ, while the right arrow of the second area B denotes a pretilt direction generated by UV in direction x passed through the first light-transmitting portion  64   a , i.e., UV irradiated at an angle of θ. Since the second light irradiation is blocked by the mask in the first area A of the alignment film corresponding to the opaque portion having optical transmittivity of 0%, the pretilt formed by the first irradiation remains in the same manner as FIG. 7 a . In case of the varied distance d′ between the first surface  65  and the second surface  66 , two domains having pretilt directions different from the first area A are formed in the second area B covered with the mask, so that the pixel is divided into four (see FIG. 7 c ). 
     If the four-domain substrate obtained by the above method is applied to the upper and lower substrates, the main liquid crystal cell of FIG. 4 can be formed. 
     As aforementioned, the method for manufacturing a multi-domain liquid crystal cell has the following advantages. 
     In the present invention, the light-transmitting portions and the light-shielding portions are arranged on the first surface at constant intervals, and the photo-alignment is performed using the mask which is formed in such a manner that the light-shielding portions are arranged in a position corresponding to the light-transmitting portion of the first surface and the light-transmitting portions are arranged in a position corresponding to the light-shielding portion. Accordingly, alignment division of the unit pixel can be realized by irradiation of one time and the number of the manufacturing process steps can be reduced. Furthermore, since alignment division of the pixel is realized by one mask, the steps of arranging a number of masks are reduced. Thus, error that may occur in arranging the masks is reduced, so that reliability of the alignment is improved, thereby improving the productivity and lowering the production cost in case of mass production. 
     The foregoing embodiments are merely exemplary and are not to be constructed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.