Abstract:
An optical compensation structure and its fabricating process are disclosed. The optical compensation structure comprises an upper polarizer film, a transparent substrate, a first retarder film (C+ plate), and a second retarder film (A-plate). The upper polarizer film provides polarization function and possesses a top surface and a bottom surface. The transparent substrate is directly laminated onto the top surface of upper polarizer film. The first retarder film is coated with a bonding layer made of crosslinking agent on one side and the bonding layer is directly laminated onto the bottom surface of upper polarizer film. The second retarder film binds to the side of first retarder film away from the upper polarizer film. The optical compensation structure is coated with the bonding layer to address the drawback of prior art where the upper polarizer film and the first retarder film are not closely adhered to each other, thereby allowing the use of one less substrate and offering a thinner compensation structure. When applied to liquid crystal display (LCD), the optical compensation structure improves the contrast and color shift problems of LCD at oblique viewing angles.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical compensation structure and its fabricating process, in particular an optical compensation structure suitable for in-plane switching liquid crystal display (IPS LCD), which improves the contrast and color shift problems of IPS LCD at oblique viewing angles by directly coating a bonding layer made of crosslinking agent on the liquid crystal retarder film (C+ plate) to bind to the upper polarizer film. 
         [0003]    2. Description of the Prior Art 
         [0004]    Liquid crystal display (LCD) is now used by all kinds of electronic devices, such as television, computer, mobile handset, and personal digital assistant (PDA). Due to its characteristics of fast response and high contrast of direct viewing angle, thin-film transistor LCD (TFT-LCD) has become the mainstream LCD technology. 
         [0005]      FIG. 1A  depicts the sectional view of a conventional LCD  10 , which typically comprises a liquid crystal cell  11  and two polarizers  12 ,  13  disposed respectively on each surface of liquid crystal cell  11 . The liquid crystal cell  11  is composed of a glass substrate and a plurality of liquid crystal molecules adhered to both surfaces of the glass substrate. Polarizer  12  (or  13 ) is made of a polarizer film  123  (or  133 ) sandwiched between two transparent substrates  121 ,  122  (or  131 ,  132 ) that provides compensation for polarization. 
         [0006]    LCD  10  adopting in-plane switching (IPS) technology claims wide viewing range for oblique angles without the use of optical compensatory sheet. That is, it offers relatively high contrast at 45° and 135° angles. But actual observation at an oblique angle finds that the completely black screen of conventional IPS LCD  10  shows yellowish or reddish hues and the contrast is not totally satisfactory.  FIG. 1B  and  FIG. 1C  show respectively the color distribution and contrast curve of viewing angles of a conventional IPS LCD under completely dark screen. It is clear that at 45° or 135° viewing angle, serious color shift occurs. The color shift problem (in particular red hue) plus the less than satisfactory contrast performance at oblique viewing angles seriously affects the display quality of IPS LCD 10 . 
         [0007]    Later on LCDs are added with a retarder film to enhance the visual effect of oblique angles.  FIG. 2A  shows the flow process of adding a retarder film to a conventional LCD upper polarizer.  FIG. 2B  depicts the sectional view of a LCD added with a retarder film. An independent structure of first phase retarder  64  (step  691 ) is formed by coating on a transparent TAC substrate  641  in sequence an alignment layer  642  and liquid crystal material  643 . In addition, an independent structure of polarizer is formed by laminating a polarizer film  62  onto another TAC substrate  611 . Next, the polarizer film  62  is adhered to substrate  641  (step  692 ), and a second phase retarder  65  coated with a layer of pressure sensitive adhesive (PSA)  631  is adhered to the first phase retarder  64  through the PSA  631 . As such, a conventional polarizer  60  with optical compensation effect is formed (step  694 ). Such polarizer  60  with optical compensation effect can be used in the liquid crystal cell  11  as shown in  FIG. 1  to constitute a liquid crystal display. For example, U.S. Pat. No. 6,717,642 discloses a technology of improving the viewing angle and display quality of LCD by adding a retarder film. 
         [0008]    In the process for polarizer  60  described above, phase retarder  64  is not directly formed on polarizer film  62  but laminated onto it through substrate  641 . Although substrate  641  provides adequate structural strength and rigidity, the resulting multi-layer polarizer  60  increases the thickness of LCD and affects adversely its transparency and optic characteristics, hence leaving room for improvement. 
       SUMMARY OF INVENTION 
       [0009]    The primary object of the present invention is to provide an optical compensation structure and its fabrication process, characterized in which a bonding layer made of crosslinking agent is directly coated on the liquid crystal (C+) retarder film in substitution of a transparent substrate to bind to the upper polarizer film so as to make the optical compensation structure thinner. 
         [0010]    Another object of the present invention is to provide a LCD with optical compensation structure, which has improved oblique angle contrast and color shift through the combination of liquid crystal retarder film (C+ plate) and uniaxial stretch film (A-plate). 
         [0011]    To achieve the aforesaid objects, the present invention provides an optical compensation structure and its fabrication process. The optical compensation structure comprises: an upper polarizer film, a transparent substrate, a first retarder film (C+ plate), and a second retarder film (A-plate). The upper polarizer film provides the polarization function and possesses a top surface and a bottom surface. The transparent substrate is directly laminated onto the top surface of upper polarizer film. The first retarder film is coated with a bonding layer made of crosslinking agent on one side and the bonding layer is directly laminated onto the bottom surface of upper polarizer film. The second retarder film binds to the side of first retarder film away from the upper polarizer film. The first retarder film satisfies the condition of nx=ny&lt;nz; the second retarder film satisfies the condition of nx&gt;ny=nz; the first retarder film and the second retarder film combined further satisfy the conditions of: 
         [0000]      0.1 nm&lt; Ro ( a )+ Ro ( b )&lt;220 nm; 
         [0000]      −270 nm&lt; Rth ( a )+ Rth ( b )&lt;60 nm; and 
         [0000]      −300 nm&lt; Rth ( a )&lt;−10 nm; 
         [0012]    where nx denotes the refractive index along x-axis of surface; ny denotes the refractive index along y-axis of surface; nz is thicknesswise refractive index along z-axis; Ro(a) and Rth(a) are respectively the in-plane retardation (Ro) and out-of-plane retardation (Rth) of first retarder film; Ro(b) and Rth(b) are respectively the Ro and Rth of second retarder film; and Ro=(nx−ny)*d; Rth={(nx+ny)/2−nz}*d; and d is film thickness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The details of the present invention will be more readily understood from a detailed description of the preferred embodiments taken in conjunction with the following figures. 
           [0014]      FIG. 1A  shows the sectional view of a conventional LCD. 
           [0015]      FIG. 1B  shows the color distribution curve of conventional IPS LCD under completely dark screen. 
           [0016]      FIG. 1C  shows the contrast curve under the viewing range of a conventional IPS LCD. 
           [0017]      FIG. 2A  shows the flow process of adding a retarder film to a conventional LCD upper polarizer. 
           [0018]      FIG. 2B  shows the sectional view of a conventional LCD added with optical compensation structure. 
           [0019]      FIG. 3  shows the sectional view of an optical compensation structure according to a first preferred embodiment of the present invention. 
           [0020]      FIG. 4A  shows the process flow for the first preferred embodiment of optical compensation structure in  FIG. 3 . 
           [0021]      FIG. 4B  shows a working diagram of the process for the first preferred embodiment of optical compensation structure in  FIG. 3 . 
           [0022]      FIG. 5  shows the sectional view of an optical compensation structure according to a second preferred embodiment of the present invention. 
           [0023]      FIG. 6A  shows the process flow for the second preferred embodiment of optical compensation structure in  FIG. 5 . 
           [0024]      FIG. 6B  shows a working diagram of the process for the second preferred embodiment of optical compensation structure in  FIG. 5 . 
           [0025]      FIG. 7  shows the sectional view of a LCD device with optical compensation structure according to a first embodiment of the invention. 
           [0026]      FIG. 8  shows the contrast curve under the viewing range of a LCD device with optical compensation structure according to a first preferred embodiment of the invention. 
           [0027]      FIG. 9  shows the sectional view of a LCD with optical compensation structure according to a second preferred embodiment of the invention. 
           [0028]      FIG. 10  shows the contrast curve under the viewing range of a LCD with optical compensation structure according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The main principle for the optical compensation structure of the invention is to coat a bonding layer made of crosslinking agent on liquid crystal retarder film (C+ plate) in substitution of a transparent substrate to allow direct adhesion to the upper polarizer film. As such, the optical compensation structure is made thinner and the oblique angle contrast and color shift problems of IPS LCD are improved. 
         [0030]      FIG. 3 ,  FIG. 4A  and  FIG. 4B  show respectively the sectional view of an optical compensation structure according to the first preferred embodiment of the invention, the process flow for the optical compensation structure, and the working diagram of the process. The optical compensation structure  22  according to the invention comprises: a transparent substrate  221 , a first retarder film  241 , a second retarder film  242 , and an upper polarizer film  223 . The transparent substrate  221  is directly laminated onto the top surface of upper polarizer film  223 . The first retarder film  241  is coated with a bonding layer  243  on one side and directly laminated onto the bottom surface of upper polarizer film  223  through the bonding layer  243 . The second retarder film  242  binds to the side of first retarder film  241  away from the upper polarizer film  223 . 
         [0031]    The transparent substrate  221  is preferably made of thermoplastic resin commonly used in the industry and preferably having excellent mechanical strength, moisture penetrability, transparency, thermal stability and optic characteristics. Examples of this kind of transparent substrate  221  include cellulose resin, such as triacetyl cellulose (TAC) and propionyl cellulose, and transparent resin, such as polyamide, polycarbonate, polyester, polystyrene, polyacrylate, and norbornene-based polymer. In consideration of the optic characteristics and durability (heat, moisture, etc.) of the polarizer, triacetyl cellulose (TAC) that has been surface treated with alkaline and saponified is the preferred choice. The Ro of TAC available on the market ranges between 0 and 5 nm, while its Rth ranges between 35 and 55 nm. The upper polarizer film  233  is a polyvinyl alcohol (PVA) film prepared by stretching the PVA film after it is absorbed with iodine or dichromatic substance, such as dichromatic dye, and having specific polarizing effect. 
         [0032]    The first retarder film  241  is made by coating an alignment layer and liquid crystal material on a transparent polymer film and orienting the liquid crystal material in specific direction such that the first retarder film  241  satisfies the condition of nx=ny&lt;nz. First retarder film  241  made according to the aforesaid condition is commonly referred to as C+ plate in the industry, where nx denotes the refractive index along x-axis of surface; ny denotes the refractive index along y-axis of surface; nz is thicknesswise refractive index along z-axis. The second retarder film  242  is made by soaking a transparent polymer film in dyes and then stretched in specific direction (i.e. uniaxial stretch) such that the second retarder film  242  satisfies the condition of nx&gt;ny=nz, which is commonly referred to as A-plate in the industry. In the prior art, the first retarder film  241  (C+ plate) cannot be directly laminated onto the upper polarizer film  223  (PVA film) because of the poor adhesion between them. The present invention allows direct adhesion of C+ plate to the PVA film via a bonding layer  243 . In a preferred embodiment of the invention, the bonding layer  243  is a crosslinking agent (or a coupling agent or a primer) that allows direct adhesion between the first retarder film  241  (C+ plate) and upper polarizer film  223 . In comparison with prior art, the present invention uses one less substrate and hence offers a thinner structure. 
         [0033]    As shown in  FIG. 4A  and  FIG. 4B , the process for fabricating the optical compensation structure as shown in  FIG. 3  comprises the following steps: 
         [0034]    Step  31 : Providing a second retarder film  242  (A-plate), which has a first surface  2421  and a second surface  2422  opposing each other. 
         [0035]    Step  32 : Coating on the first surface  2421  in sequence an alignment layer  2411  and a liquid crystal layer  2412 . The combination of alignment layer  2411  and liquid crystal layer  2412  forms essentially a first retarder film  241  (C+ plate) on the second retarder film  242 . In this preferred embodiment, the first retarder film  241  (C+ plate) is formed directly on the second retarder film  242  (A-plate) without any medium present between the two films. In another embodiment according to a different process, a layer of pressure sensitive adhesive (PSA) is added between the first retarder film  241  and the second retarder film  242  to laminate the first retarder film  241  onto the second retarder film  242 . 
         [0036]    Step  33 : Coating a bonding layer  243  to the liquid crystal layer  2412  of first retarder film  241 . The bonding layer  243  is made of crosslinking agent (or coupling agent or primer). In addition, an adhesive layer  222  (called hydrogel layer) is provided between an upper polarizer film (PVA)  223  and a transparent substrate  221  (TAC) to laminate the upper polarizer film  223  onto the transparent substrate  221 . 
         [0037]    Step  34 : By binding the bonding layer  243  to the upper polarizer film  223 , the upper polarizer film  223  is directly laminated onto the liquid crystal layer  2412  of first retarder film  241  through the bonding layer  243  to constitute the optical compensation structure  22 . 
         [0038]      FIG. 5 ,  FIG. 6A  and  FIG. 6B  show respectively the sectional view of an optical compensation structure according to a second preferred embodiment of the invention, the process flow for the optical compensation structure, and the working diagram of the process. The only difference between the optical compensation structure  22   a  shown in  FIG. 5  and the first preferred embodiment just described is the presence of an adhesive layer  222  (called hydrogel layer) on respectively the top surface and the bottom surface of upper polarizer film  223 . As such, the top surface adheres to the transparent substrate  221  through adhesive layer  222 , and the bottom surface adheres to the bonding layer  243  through adhesive layer  222 . The addition of an adhesive layer  222  in the second preferred embodiment is reflected in a different fabrication process. As shown in  FIG. 6A  and  FIG. 6B , the process for fabricating the optical compensation structure as shown in  FIG. 5  comprises the following steps: 
         [0039]    Step  51 : Providing a second retarder film  242  and an upper polarizer film  223 . The second retarder film  242  has a first surface  2421  and a second surface  2422  opposing each other. The upper polarizer film  223  has a top surface  2231  and a bottom surface  2232 . 
         [0040]    Step  52 : Coating on the first surface  2421  in sequence an alignment layer  2411  and a liquid crystal layer  2412 . The combination of alignment layer  2411  and liquid crystal layer  2412  forms essentially a first retarder film  241  on the second retarder film  242 . In this preferred embodiment, the first retarder film  241  is formed directly on the second retarder film  242  without any medium present between the two films. 
         [0041]    Step  53 : Coating a bonding layer  243  to the first retarder film  241  and drying it. 
         [0042]    Step  54 : Coating an adhesive layer  222  to the top surface  2231  and bottom surface  2232  of upper polarizer film  223  respectively through which the top surface  2231  is adhered to a transparent substrate  221  and the bottom surface  2232  is adhered to the bonding layer  243 . Consequently, the upper polarizer film  223  is disposed on the first retarder film  241  through the bonding layer  243  and the adhesive layer  222  to constitute the optical compensation structure  22   a.    
         [0043]    As shown in  FIG. 7  which is the sectional view of a first embodiment of liquid crystal display (LCD) device having an optical compensation structure disclosed in the invention, the LCD device  20  comprises: a liquid crystal cell  21 , an upper polarizer  22 , and a lower polarizer  23 . The upper polarizer  22  is the optical compensation structure  22 ,  22   a  depicted in the embodiments described above, which will be referred to as “upper polarizer  22 ” to facilitate the description of the LCD device  20 . 
         [0044]    In the first embodiment of LCD device  20 , the liquid crystal cell  21  is preferably an in-plane switching (IPS) liquid crystal cell  21 , which has serious red shift at an oblique viewing angle (45° and 135°). The liquid crystal cell  21  can also be a TN (twisted nematic) or MVA (multi-domain vertical alignment) liquid crystal cell. The liquid crystal cell  21  consists of a glass substrate and a plurality of liquid crystal molecules distributed over the glass substrate, and is defined with a liquid crystal orientation  211  based on the arrangement of liquid crystal molecules. In this embodiment, the liquid crystal orientation  211  is horizontal as shown by the arrows in  FIG. 7 . In light that liquid crystal cell  21  is a prior art and not a main feature of the invention, its detailed composition and functions will not be elaborated. 
         [0045]    The lower polarizer  23  is disposed on a lower side of liquid crystal cell  21 . In this embodiment, the lower polarizer  23  comprises: two transparent substrates  231 ,  232  and a lower polarizer film  233  (PVA film) sandwiched therebetween. The lower polarizer  23  can be defined with an extension direction  234  according to the elongation direction of its lower polarizer film  233  in the fabrication process. In this embodiment, the extension direction  234  of the lower polarizer  23  is the same as the liquid crystal orientation  211  of liquid crystal cell  21 . Since the transparent substrates  231 ,  232  and lower polarizer film  233  also belong to prior art, their detailed compositions and effects will not be elaborated. 
         [0046]    In the first embodiment of the LCD device  20 , the lower polarizer  23  further comprises a third retarder film  235  disposed between transparent substrate  231  and liquid crystal cell  21 . The third retarder film  235  satisfies the condition of nx&gt;ny=nz, which is commonly referred to as A-plate in the industry, where nx denotes the refractive index along x-axis of film surface; ny denotes the refractive index along y-axis of film surface; nz is thicknesswise refractive index along z-axis. The third retarder film  235  is defined with a direction of maximum refractivity  236  which is the same as the liquid crystal orientation  211 . The third retarder film  235  can be directly laminated to the side of lower polarizer film  233  closer to the liquid crystal cell in substitution of a transparent substrate  231 . 
         [0047]    The upper polarizer  22  is disposed on the upper side of liquid crystal cell  21 . The elements of upper polarizer  22  identical to the ones shown in  FIG. 3  will not be elaborated. Only the extension direction  224  of the upper polarizer film  223  in the LCD device  20  is defined as the direction that pierces the diagram as shown in  FIG. 7 , which is therefore perpendicular to the liquid crystal orientation  211  of liquid crystal cell  21 . Thus the extension direction  234  of lower polarizer film  233  is also perpendicular to the extension direction  224  of polarizer film  223  of upper polarizer  22 . The second retarder film  242  is defined with a direction of maximum refractivity  244  which is perpendicular to the liquid crystal orientation  211 . 
         [0048]    In this embodiment, the combination of first retarder film  241  and second retarder film  242  satisfies the following optical conditions: 
         [0000]      0.1 nm&lt; Ro ( a )+ Ro ( b )&lt;220 nm; 
         [0000]      −270 nm&lt; Rth ( a )+ Rth ( b )&lt;60 nm; and 
         [0000]      −300 nm&lt; Rth ( a )&lt;−10 mn; 
         [0049]    where Ro(a) and Rth(a) are respectively the in-plane retardation (Ro) and out-of-plane retardation (Rth) of first retarder film; Ro(b) and Rth(b) are respectively the Ro and Rth of second retarder film; and Ro=(nx−ny)*d; Rth={(nx+ny)/2−nz}*d; and d is film thickness. 
         [0050]      FIG. 8  shows the contrast curve under the viewing range of a LCD with optical compensation structure according to a first preferred embodiment of the invention. As shown, by building a first retarder film  241  and a second retarder film  242  that satisfy the aforementioned optical conditions in the upper polarizer  22 , the oblique angle contrast and color shift problems of IPS LCD are improved. Also by directly coating a bonding layer  243  on the first retarder film  241  to bind to the upper polarizer film  223  in substitution of a transparent substrate, the resulting polarizer is made thinner as compared to prior art where retarder film is separately fabricated and adhered to the polarizer. 
         [0051]      FIG. 9  and  FIG. 10  show respectively the sectional view and the contrast curve under the viewing range of a LCD with optical compensation structure according to a second embodiment of the invention. The only difference between the second embodiment of LCD device  20   a  shown in  FIG. 9  and the first embodiment is that the lower polarizer  23   a  of the former comprises: a transparent film  237 , a transparent substrate  232 , and a lower polarizer film  233  sandwiched therebetween. The transparent film  237  is a TAC plate with low retardation and satisfying the following optical conditions: 
         [0000]      0 nm&lt; Ro ( c )&lt;5 nm; and 
         [0000]      0 nm&lt; Rth ( c )&lt;5 nm; 
         [0052]    where Ro(c) and Rth(c) are respectively the in-plane retardation (Ro) and out-of-plane retardation (Rth) of transparent film  237 ; and Ro=(nx−ny)*d; Rth={(nx+ny)/2−nz}*d; and d is film thickness. Such optical compensation structure also improves the oblique angle contrast and color shift problems of IPS LCD. 
         [0053]    Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, that above disclosure should be construed as limited only by the metes and bounds of the appended claims.