Abstract:
A domain-divided twisted nematic crystal cell and method of fabricating thereof. The method of the present invention comprises the steps of providing first and second substrates, forming a photo-alignment layer in each domain of the first substrate having two or more domains, wherein thicknesses of the photo-alignment layers corresponding to the domains are different from each other. A pretilt angle in each domain is formed by light irradiation on the photo-alignment layer, wherein the pretilt angles corresponding to the domains are different from each other due to the different thicknesses of the photo-alignment layer in different domains. The first and second substrates are positioned to face each other, and liquid crystal is injected between the first and second substrates.

Description:
This is a continuation of application Ser. No. 09/084, 582, filed May 27, 1998, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to a domain-divided twisted nematic liquid crystal cell and a method for fabricating thereof. 
     B. Description of the Prior Art 
     A TN LC cell has a characteristic such that the light transmittance of each gray level varies according to the viewing angle. 
     FIG. 1A is a graph showing the relationship between light transmittance of the TN LC cell and voltage. FIG. 1B is a graph showing the relationship between light transmittance and the right-and-left directional viewing angle of the TN LC cell. FIG. 1C is a graph showing the relationship between light transmittance and the up-and-down directional viewing angle of the TN LC cell. 
     As shown in FIGS. 1B and 1C, light transmittance is symmetrical with respect to the right-and-left directional viewing angle, but light transmittance is asymmetrical with respect to the up-and-down directional viewing angle. That is, there is a region in which the gray inverts in the up-an-down directional viewing angle. Therefore, this results in a problem that the viewing angle becomes narrow. This gray inversion is caused by the alignment of the TN LC cell. 
     As a method of solving this problem of narrow viewing angle, a domain-divided TN LC cell (DDTN LC cell) has been proposed. The DDTN LC cell has generally been fabricated by a rubbing method or a photo-alignment method. 
     Fabricating the DDTN LC cell by the rubbing method, however, requires complex procedures and high manufacturing cost. For example, in the rubbing method, each substrate must undergo a polyamide or a polyimide coating process twice, photo-lithography process once, and a rubbing process once. Moreover, because the rubbing method includes a photo-lithography process, it is difficult to secure pretilt angle stability of the polyamide or polyimide alignment layer and reliability of the panel. 
     The photo-alignment method too has problems, such as lengthened light irradiation time. FIGS. 2A to  2 D are section views showing the conventional photo-alignment process of DDTN LC cell. In FIG. 2A, a photo-alignment layer  12  is formed by coating photo-alignment material uniformly on a substrate  10 . And in FIG. 2B, a first pretilt angle α 1  is formed by light irradiation of the substrate. As shown in FIG. 2C, a first domain I is shielded by a mask  14 , and additional light irradiation is performed on only a second domain II. As a result, compared with first domain I, because the quantity of light irradiation is more in second domain II, as shown in FIG. 2D, second pretilt angle α 2  which is lower than first pretilt angle α 1  is formed. 
     As previously mentioned, the conventional photo-alignment method has a problem that the overall light irradiation time becomes long. This is caused by the additional step of light irradiation and the light absorption with using the mask. Furthermore, this makes the method complex and also generates dust in the process of providing and removing the mask. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a simpler and more reliable method for fabricating a domain-divided twisted nematic liquid crystal cell. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises: providing a first substrate having a first domain and a second domain; forming a first photo-alignment layer having a first thickness over the first domain and a second photo-alignment layer having a second thickness different from the first thickness over the second domain; and irradiating the first and second photo-alignment layers with a light to impart first and second pretilt angles different from each other, respectively. 
     In a further aspect, the invention comprises: a first substrate having a first domain and a second domain; and a first photo-alignment layer and a second photo-alignment layer over the first and second domains, respectively. The first photo-alignment layer has a thickness different from the thickness of the second photo-alignment layer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 C are graphs showing light transmittance characteristics of the conventional TN LC cell. 
     FIGS. 2A to  2 D are section views showing the conventional photo-alignment process of fabricating a DDTN LC cell. 
     FIGS. 3A to  3 F are section views showing a method of fabricating a DDTN LC cell according to a first embodiment of the present invention. 
     FIGS. 4A to  4 E are section views showing a method of fabricating a DDTN LC cell according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     For a given quantity of light irradiated, the pretilt angle of a photo-alignment layer depends on the thickness of the photo-alignment layer. For example, generally, the thicker the photo-alignment layer, the smaller the pretilt angle becomes. Hence, when one pixel is divided into two or more domains, and the photo-alignment layers formed over the domains have different thicknesses, and a light such as ultraviolet light is irradiated, then although the irradiating time is the same, the pretilt angle in each domain becomes different from each other. That is, the domain whose thickness of the photo-alignment layer is thick has a relatively low pretilt angle, but the domain whose thickness of the photo-alignment layer is thin has a relatively high pretilt angle. 
     FIGS. 3A to  3 F are section views showing a method of fabricating a DDTN LC cell according to a first embodiment of the present invention. 
     In FIG. 3A, a photoresist of a thickness d on a first substrate  100  is coated to form photoresist layer  102 . 
     And, in FIG. 3B, each pixel on the first substrate  100  is divided into first and second domains I and II by a mask pattern. Only first domain I is shielded by a mask  104  and the photoresist layer  102  is exposed. 
     As shown in FIG. 3C, photoresist pattern  102   a  is formed by removing only the photoresist on the second domain II. Though the FIG. 3C shows a case of using a positive photoresist, a negative photoresist may also be used. 
     In FIG. 3D, a photo-alignment layer  106  of a specific thickness d 2  is coated uniformly on the first substrate  100 . Then, on the first domain I a photo-alignment layer  106  of thickness d 1  is deposited and on the second domain II a photo-alignment layer  106  of thickness d 2  is deposited. At this time, the relation thereof is d 2 =d+d 1 . Accordingly, on the second domain II a photo-alignment layer  106  having a relatively high thickness is formed. 
     Subsequently, ultraviolet light is irradiated over the first substrate  100  to impart pretilt angles. At this time, because the thicker the photo-alignment layer is, the smaller the pretilt angle becomes, as shown in FIG. 3E, in the first domain I a relatively high pretilt angle β 1  is formed and in the second domain II a relatively low pretilt angle β 2  is formed. The light irradiation might be performed one or more times depending on the characteristics of the photo-alignment process used. 
     In FIG. 3F, the first substrate  100  and a second substrate  110  formed in a manner as shown in FIGS. 3A to  3 E are positioned as the first and second substrates to face each other. And then, the DDTN LC cell is completed by injecting LC between the first and second substrates to form an LC layer  120 . 
     As previously mentioned, while in the first domain I of the DDTN LC cell, its lower part has high pretilt angle β 1  and its upper part has low pretilt angle β 2 , in the second domain II, its lower part has low pretilt angle β 2  and its upper part has high pretilt angle β 1 . Therefore, this compensates light transmittance and a wide viewing angle is obtained. Additionally, the alignment process of the second substrate  110  might be performed by the conventional rubbing method or the conventional photo-alignment method or the photo-alignment method of the present invention. 
     FIGS. 4A to  4 E are section views showing a method of fabricating a DDTN LC cell according to a second embodiment of the present invention. 
     FIG. 4A, a photo-alignment layer  202  of specific thickness D 1  is deposited on a first substrate  200 . 
     And in FIG. 4B, each pixel on the first substrate  200  is divided into first and second domains I and II by a mask pattern. First domain I is shielded by a mask  204 , and the photo-alignment layer  202  is irradiated by a light with high intensity, for example 80 W to 90 W, thereby etching the photo-alignment layer  202  on second domain II to a depth of D. At this time, as an irradiating light, a laser may be used. 
     As a result, in FIG. 4C, on first domain I a first photo-alignment pattern  202   a  is formed relatively thick, on second domain II a second photo-alignment pattern  202   b  is formed relatively thin. That is, first photo-alignment layer pattern  202   a  has a thickness of D 1 , the second photo-alignment layer pattern  202   b  has a thickness of D 2 , and the relation relationship thereof is D 1 =D 2 +D. 
     When an ultraviolet light is irradiated on first substrate  200  for a specific time, as shown in FIG. 4D, a first pretilt angle δ 1  and a second pretilt angle δ 2  are formed in first domain I and second domain II respectively. First pretilt angle δ 1  is relatively smaller than second pretilt angle δ 2 . 
     In FIG. 4E, first substrate  200  and second substrate  210  formed in a manner as shown in FIGS. 4A to  4 D are positioned as the first and second substrates to face each other. And then, the DDTN LC cell is completed by injecting LC between the first and second substrates to form an LC layer  220 . 
     As previously mentioned, while in the first domain I of the DDTN LC cell, its lower part has low pretilt angle δ 1  and its upper part has high pretilt angle δ 2 , in the second domain II, its lower part has high pretilt angle δ 2  and its upper part has low pretilt angle δ 1 . Therefore, this compensates light transmittance and wide viewing angle is obtained. Additionally, the alignment process of the second substrate  210  might be performed by the conventional rubbing method or the conventional photo-alignment method or the photo-alignment method of the present invention. 
     Compared with the method for fabricating a DDTN LC cell by the rubbing method, because the present invention does not use a rubbing process, it avoids dust generation, which may affect the picture quality. Moreover, because it does not involve a photolithography process, it prevents the pretilt angle from being damaged and reliability of elements from falling. Also, the overall process is simplified. 
     Compared with the conventional photo-alignment process, the present invention shortens light irradiating time and does not involve a masking process during light irradiation. This avoids dust generation and simplifies the process. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method of the present invention and in construction of this multi-domain LC cell without departing from the scope or spirit of the invention. For example, the present invention is not limited to two domain LC cells, but may be applied to multi-domain LC cells having two or more domains. 
     The foregoing description assumes that the photo-alignment material used has the characteristic such that for a given quantity of light irradiated, the thicker the photo-alignment layer, the smaller the pretilt angle of the photo-alignment layer becomes. However, a photo-alignment material having an opposite characteristic may be used as well—that is the thicker the photo-alignment layer, the greater the pretilt angle becomes. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.