Patent Publication Number: US-2004041965-A1

Title: Transflector with a high gain of light efficiency for a liquid crystal display

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates generally to a liquid crystal display (LCD), and more particularly to a transflector of an LCD.  
       [0003] 2. Description of Related Art  
       [0004] LCDs are conventionally identified with two types, i.e., reflective LCD and transmissive LCD. A reflective LCD displays images by a reflector thereof to reflect the ambient light in front of the LCD that passes through the liquid crystal thereof to reach the reflector, the display performane is thus degraded when the enviroment is not bright enough for clear images. On the other hand, a transmissive LCD can overcome the problem resulted from the weak ambient light by introduction of a backlight source behind the liquid crystal. However, the backlight source consumes additional electric power and is disadvantagable to reduce the size and weight of the LCD system. A new type of LCD called transflective or partially reflective LCD provides both display modes of reflection and transmission for more flexable applications. When the enviroment is dark or not bright enough, the backlight source in a transflective LCD is turned on to provide enough illuminant, and on the other hand, turned off to save electric power when the enviroment is bright. To provide both display modes of reflection and transmission, a transflective LCD is equipped with a transflector thereof. FIG. 1 shows, for example, a typical transflector  12  on a transparent substrate  10  in a transflective LCD, which includes a reflective region  12   a  for the reflective display mode and a transmissive region  12   b  with an opening w for the transmissive display mode, respectively. The reflective region  12   a  reflects the frontlight when the LCD is operated in reflective display mode, and the transmissive region  12   b  permits the backlight to pass through the transflector  12  when the LCD is operated in transmissive display mode. However, the apparatus  12  induces a new problem of optical performane. For a transflective LCD to be oprated in either reflective display mode or transmissive display mode, part of the transflector  12  is reflective and the other is transmissive. As a result, the reflected light for the image display in reflective display mode is reduced due to the reduced reflective area and the usage of backlight in transmissive display mode is low since the backlight  14  behind the reflective region  12   a  is blocked by the reflective region  12   a  and only the backlight  16  behind the transmissive region  12   b  is provided for image disply. Therefore, the optical performane of both reflective display mode and transmissive display mode are degraded due to the reduced usage of light. Under considertion of the optical performance in reflective display mode, the transmissive region  12   b  of the transflector  12  for a transflective LCD cannot be too large, and has a typical opening ratio of about 15-40% for a transflector in practice. As a result, the backlight for the most part is wasted.  
       SUMMARY OF THE INVENTION  
       [0005] One object of the present invention is to provide a transflector with a high gain of light efficiency for a transflective LCD to improve the transmissive display mode thereof.  
       [0006] Another object of the present invention is to provide a transflector with a high gain of light efficiency for a transflective LCD to improve the reflective display mode thereof.  
       [0007] Yet another object of the present invention is to svae the electric power consumption of the backlight source for a transflective LCD in addition to better optical performane.  
       [0008] In a transflective LCD including a bottom plate and an upper plate with a liquid crystal therebetween, according to the present invention, a transflector is arranged on the bottom plate side and includes a reflective region and a transmissive region with a micro optical apparatus to gather the backlight to the transmissive region of the transflector. Thereby the gain of light efficiency for such optical arrangment is increased up to 120-400% or more, and the electric power consumption of the backlight source is also economized. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:  
     [0010]FIG. 1 shows a typical transflector for a conventional LCD;  
     [0011]FIG. 2 shows the first embodiment transflector according to the present invention;  
     [0012]FIG. 3 shows an embodiment arrangement of a TFT for the transflector of FIG. 2;  
     [0013]FIG. 4 shows another embodiment arrangement of a TFT for the transflector of FIG. 2;  
     [0014]FIG. 5 shows the second embodiment transflector according to the present invention;  
     [0015]FIG. 6 shows the third embodiment transflector according to the present invention;  
     [0016]FIG. 7 shows the fourth embodiment transflector according to the present invention;  
     [0017]FIG. 8 shows the fifth embodiment transflector according to the present invention;  
     [0018]FIG. 9 shows the sixth embodiment transflector according to the present invention;  
     [0019]FIG. 10 shows the seventh embodiment transflector according to the present invention, and FIG. 10A shows an alternative arrangement of which the micro optical apparatus to gather the backlight to the transmissive region is a film or plate attached to the LCD cell;  
     [0020]FIG. 11 shows an embodiment arrangement of a TFT for the transflector of FIG. 10;  
     [0021]FIG. 12 shows an embodiment micro lens structure according to the present invention, among which FIGS. 12A and 12B are the cross-sectional views of the micro lens in two crossover directions and FIG. 12C is the contour map of the micro lens;  
     [0022]FIG. 13 shows another embodiment micro lens structure according to the present invention, among whick FIGS. 13A and 13B are the cross-sectional views of the micro lens in two crossover directions and FIG. 13C is the contour map of the micro lens;  
     [0023]FIG. 14 shows an embodiment arrangement of the transmissive region of the transflector according to the present invention, and FIG. 14A is the contour map of the micro lens thereof;  
     [0024]FIG. 15 shows another embodiment arrangement of the transmissive region of the transflector according to the present invention, and FIG. 15A is the contour map of the micro lens thereof;  
     [0025]FIG. 16 shows an embodiment process to form the transflector according to the present invention, of which FIG. 16A is the cross-sectional view after coating a positive photoresistor  18  on a substrate  10 , FIG. 16B is the cross-sectional view after transferring a pattern of micro lens with a mask  46 , FIG. 16C is the cross-sectional view of the micro lens  18  after middle baking, FIG. 16D is the cross-sectional view of the micro lens  18  after hard baking, FIG. 16E is the cross-sectional view after deposition of an over coating  20 , FIG. 16F is the cross-sectional view after deposition of an ITO  22 , FIG. 16G is the cross-sectional view after transferring a pattern of diffusive layer with a mask  48 , FIG. 16H is the cross-sectional view after forming a diffusive layer  24 , and FIG. 16I is the cross-sectional view after deposition of a reflective layer  26  on the diffusive layer  24 ;  
     [0026]FIG. 17 shows the profile in X-direction of the micro lens  18  during the process of FIG. 16, of which FIG. 17A is the profile of the micro lens  18  before middle baking, FIG. 17B is the profile of the micro lens  18  after middle baking, and FIG. 17C is the profile of the micro lens  18  after hard baking; and  
     [0027]FIG. 18 shows the profile in Y-direction of the micro lens  18  during the process of FIG. 16, of which FIG. 18A is the profile of the micro lens  18  before middle baking, FIG. 18B is the profile of the micro lens  18  after middle baking, and FIG. 18C is the profile of the micro lens  18  after hard baking. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0028]FIG. 2 shows the first embodiment transflector according to the present invention. Between a substrate  10  and a transflector  12  is arrange a micro lens  18  on which is covered with an over coating  20 . The refractive index n L  of the micro lens  18  is 1.4-2.5, and the refractive index n C  of the over coating  20  is under the condition of |n L −n C |≧0.02. The micro lens  18  gathers the backlight  14  and  16  to be projected to the transmissive region  12   b . It is noted that the backlight  14  behind the reflective region  12   a  formerly unable to be directly projected to the transmissive region  12   b  is now used for image display and thus improves the gain of light efficiency thereof. As is well known in the art, an LCD includes an upper plate and a bottom plate inserted with a liquid crystal therebetween, of which the bottom plate has active or passive switching elements thereof, such as thin film transistors (TFT) and diodes. Hereinafter the substrate  10  refers to the bottom plate and, furthermore, above the transflector  12  there are a liquid crystal and an upper plate that are also well known in the art and not illustrated in details in the drawings. The transflector  12  can be formed by deposition of a metal and selectively etching the metal. In the apparatus of FIG. 2, the micro lens  18  has a width l and a height h of 2-10 μm in the middle of the micro lens  18  to have h/l of 0.02-0.3. The average elevation angle α from the edge to the central top surface of the micro lens  18  is 1-2.5 degrees. The micro lens  18  has an average focus length f, and the over coating  20  has a thickness t of 2-16 μm to have f/t of 0.8-1.3. As is well known in the art, a cell gap refers to the thickness of the liquid crystal layer between the upper plate and the bottom plate. In this embodiment the average cell gap d T  of the transmissive region  12   b  equals to the average cell gap d R  of the reflective region  12   a  since the transflector  12  is a flat plate on the over coating  20 .  
     [0029]FIG. 3 shows an embodiment arrangement of a TFT for the transflector of FIG. 2, in which the TFT  21  is formed on the substrate  10  and covered by the over coating  20  as same as the micro lens  18  and its drain is connected to the metal  12   a  with a connection  23 .  
     [0030]FIG. 4 shows another embodiment arrangement of a TFT for the transflector of FIG. 2. The TFT  21  is alternatively formed on the over coating  20  and its drain is connected to the metal  12   a  with a connection  23  that may be formed with the same metal layer for the transflector  12 .  
     [0031]FIG. 5 shows the second embodiment transflector according to the present invention with an inner diffusive reflector (IDR) structure. Similarly to the arrangement of FIG. 2, on the substrate  10  there is a micro lens  18  and an over coating  20 , while the transmissive region  22  is formed on the over coating  20  with a deposited indium tin oxide (ITO), indium zinc oxide (IZO), or the material with a transmissivity of more than 20% such as thin aluminum (Al), silver (Ag) and their alloy. An insulator  24  is deposited on the over coating  20 , and a reflective layer  26 , such as Al, Ag and their alloy, is deposited on the insulator  24 . To form a roughness on the surface of the reflective layer  26  for scaterring, the insulator  24  is formed with a scraggly topology. Due to the thickness of the insulator  24 , the average cell gap d T  of the transmissive region  22  is greater than the average cell gap d R  of the reflective layer  26 , and in this embodiment Δd is 0.15-3 μm where Δd=d T −d R . The average undulate angle β of the reflective layer  26  in the IDR structure is 2-20 degrees. The backlight  14  and  16  from the backlight source to the panel have a divergent angle θ M , preferably in the arrange of 0-35 degrees for better display performance.  
     [0032]FIG. 6 shows the third embodiment transflector according to the present invention with an inner diffusive reflector (IDR) structure, in which the optical arrangement is similar to that of FIG. 3 except that a plurality of micro prisms  19  are alternatively employed and may be arranged to be an array with a period. It is noted that other optical micro apparatus, such as hologram grating, may be selected for the transflector to gather the backlights even micro lens and micro prism are designed hereinwith for examplariry embodiments.  
     [0033] The fourth embodiment transflector is shown in FIG. 7, in which the IDR structure is still employed, while the transmissive region  30  and the reflective layer  32  are both formed on the insulator  28  such that the cell picth d T  and d R  between the transmissive region  30  and the reflective layer  32  are equal.  
     [0034]FIG. 8 is the embodiment transflector applied in a color LCD, in which a color pixel cell  34  has three liquid crystal cells  34   r ,  34   g  and  34   b  for red, green and blue, respectively. Each liquid crystal cell  34   r ,  34   g  or  34   b  has a respective transmissive region  22   r ,  22   g  and  22   b  formed on the over coating  20  corresponding to the micro lens  18   r ,  18   g  and  18   b  on the substrate  10 , respectively. The micro lens  18   r ,  18   g  and  18   b  are made of color filtering material so as to be also serving as the color filter of the pixel.  
     [0035]FIG. 9 is another embodiment transflector applied in a color LCD. Similarly, a pixel cell has three liquid crystal cells  34   r ,  34   g  and  34   b  with the same structure as that in FIG. 3 except that respective color filters  36   r ,  36   g  and  36   b  are formed on the transmissive regions  22   r ,  22   g  and  22   b  and above is deposited with transparent electrodes  38   r ,  38   g  and  38   b , such as ITO. The micro lens  18   r ,  18   g  and  18   b  are transparent, instead of color dependent.  
     [0036]FIG. 10 is an embodiment transflector with another micro lens arrangement. The transmissive region  22   r ,  22   g  and  22   b  of a color LCD are formed on the substrate  10  whose rear side is formed with the micro lens  18   r ,  18   g  and  18   b  and further covered with the over coating  20 . The thickness of the substrate  10  is t 0 , the height h in the middle of the micro lens  18  is 0.3-5 μm, the average elevation angle α from the edge to the central top surface of the micro lens  18  is 0.5-8 degrees, and the average focus length f of the micro lens  18  is 250-700 μm. Typically, it is called LCD cell, as referred with number  13  in FIG. 10, from the bottom plate  10  to the top plate  11  including the liquid crystal and related means therebetween, and thus the optical apparatus referred with number  15  including the micro lens  18  and over coating  20  to gather the backlight to the transmissive regions  22  is arranged outside the LCD cell  13 , which may be a film or plate attached to the LCD cell  13  as shown in FIG. 10A. As aforementioned, the optical apparatus  15  to gather the backlight to the transmissive regions may alternatively be a hologram plate including a plurality of hologram pattern as grating to gather the backlight to the transmissive regions of the LCD cell  13 .  
     [0037]FIG. 11 shows an embodiment arrangement of a TFT for the transflector of FIG. 10, in which the TFT  21  is formed on the substrate  10  and covered by the over coating  28  and its drain is connected to the metal  26  with a connection  23 .  
     [0038]FIG. 12 is the top view of a transflector in a liquid crystal cell, in which the transmissive region  40  is arranged near the center of the reflective region  42 . As the profile of FIGS. 12A and 12B sectioned with two crossover lines AA′ and BB′, the micro lens  44  is a long and narrow hill, and its contour map is shown in FIG. 12C.  
     [0039]FIG. 13 is the top view of another transflector in a liquid crystal cell, in which the transmissive region  40  deviates from the center of the reflective region  42 . As the profile of FIGS. 13A and 13B sectioned with two crossover lines AA′ and BB′, the micro lens  44  is a bias hill, and its contour map is shown in FIG. 13C.  
     [0040]FIG. 14 is the top view of yet another transflector in a liquid crystal cell, in which the transmissive region  40  includes three rectangle sub-regions  40   a ,  40   b  and  40   c . FIG. 11A shows the contour map of the micro lens  44  that comprises three micro lens sub-structures  44   a ,  44   b  and  44   c  corresponding to the three sub-regions  40   a ,  40   b  and  40   c , respectively.  
     [0041]FIG. 15 is the top view of further another transflector in a liquid crystal cell, and FIG. 15A shows the contour map of the micro lens  44 . This embodiment structure is similar to that of FIG. 14 except that three sub-regions  40   a ,  40   b  and  40   c  of the transmissive region  40  are substantially circle.  
     [0042] In other embodiments, the micro lens can be formed by stacked multi-layer materials each layer with same or different refractive index, the shape of the transmissive region can be changed, and the area ratio of transmissive region to reflective region is 5-400%. With the same type of LCD, the gain of light efficiency according to the present invention is 120-400% or more. Moreover, since the efficiency of the backlight is improved, the reflective region can be enlarged by increasing its area ratio, thereby the display performance in the reflective display mode can also be improved. If the material of the micro lens is chosen with a transmissivity more than 70% for the light wavelength of 400 nm, the LCD will have a better color performance.  
     [0043] In these embodiments, if positive type of liquid crystal (dielectric anisotropy Δε&gt;0) is selected for the LCD, preferably, An (the difference between the refractive indexes no and ne of ordinary and exta-ordinary light) is 0.05-0.095, Δnd T  is 280-460 nm, and Δnd R  is 200-320 nm, and if negative type of liquid crystal (Δε&lt;0) is selected for the LCD, then Δn is 0.06-0.12, Δnd T  is 320-480 nm, and Δnd R  is 150-360 nm.  
     [0044]FIG. 16 is an embodiment process for manufacturing the micro lens of FIG. 5. As shown in FIG. 16A, the substrate  10  is coated with a positive photoresistor  18 , for example, the MFR series, PC series or NN series of JSR company product. The steps of coating the photo-resistor  18  include rotating in 300 rpm for 3 seconds, then rotating in 800 rpm for 30 seconds, and pre-baking in 60-120° C. for 2-10 minutes. In FIG. 16B, the pattern of the micro lens is transferred to the photoresistor  18  with mask  46 , exposing in 200-600 mJ/cm 2 , developing with surfacants developer solution TMAH and rinsing with water for 60 seconds, and proceeding the post exposing in 200-600 mJ/cm 2 . The micro lens  18  thus formed is shown in FIG. 16C. After middle baking 2-15 minutes in 80-180° C. to soften the micro lens  18 , the resultant structure is shown in FIG. 16D. Further, hard baking 30-60 minutes in about 200° C. and depositing an over coating  20  on the micro lens  18 , the resultant structure is shown in FIG. 16E. In FIG. 16F, ITO  22  is deposited on the over coating  20 , and then a positive photoresistor  24  is further coated, as shown in FIG. 16G, followed with transferring and developing the pattern of diffusive layer with mask  48  to form the resultant structure showed in FIG. 16H. As shown in FIG. 161, a reflective layer  26  is selectively formed on the diffusive layer  24 .  
     [0045]FIGS. 17 and 18 are the scanned profile of the micro lens  18  formed by the process of FIG. 16 in different gradation. The widths of the micro lens  18  in this embodiment in X and Y directions are 21 and 63 μm, respectively, and the spaces between adjacant micro lens in X and Y directions are 4 and 12 μm, respectively. FIGS. 17A and 18A are the profiles of the micro lens  18  in X and Y directions, respectively, before the middle baking, FIGS. 17B and 18B are the profiles of the micro lens in X and Y directions, respectively, after the middle baking, and FIGS. 17C and 18C are the profiles of the micro lens in X and Y directions, respectively, after the hard baking.  
     [0046] The inventive transflector, method thereof, and process to form the transflector can be applied to various types of LCDs, such a-Si TFT LCD, poly-Si TFT LCD, thin film diode (TFD) LCD, and passive matrix super twisted nematic (STN) LCD.  
     [0047] From the above, it should be understood that the embodiments described, in regard to the drawings, are merely exemplary and that a person skilled in the art may make variations and modifications to the shown embodiments without departing from the spirit and scope of the present invention. All variations and modifications are intended to be included within the scope of the present invention as defined in the appended claims.