Abstract:
A pixel and its process for an in-plane switching liquid crystal display achieves more spreading angles and smooth performance for scattering of light for image display therewith by using a reflector having a reflective surface with roughness in nanometer scale for light scattering and contrast improvement. As a result, the reflectivity doesn&#39;t change enormously with the viewing angle and excellent anti-glare effect is obtained. Moreover, the roughness of the reflective surface is formed by the crystallization and the characteristic of the material thereof owns, and thus no additional mask is required.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to an in-plane switching (IPS) liquid crystal display (LCD), and more particularly, to a pixel and its process for an IPS LCD using a reflector having a nanometer scale roughness surface.  
       BACKGROUND OF THE INVENTION  
       [0002]     Typically, twisted nematic (TN) mode or super twisted nematic (STN) mode is employed in LCDs. Unfortunately, TN mode and STN mode LCDs are disadvantageous for their narrow viewing angles. IPS LCD is therefore proposed for wider viewing angle. In addition to the wider viewing angle, an IPS LCD has the advantages of no compensator and fast response, and furthermore, there is one mask less than the TN mode LCD for its manufacture. However, the pixel electrode and the counter electrode of an IPS LCD are made of opaque metal and thus have smooth surfaces, resulting in mirror reflection and lower contrast. Even a rough surface made of organic material such as resin can be formed underlying the pixel electrode and the counter electrode to improve this weakness, introduction of additional organic material requires more masks in the manufacture process and thus increases process complexity and cost. Furthermore, the thermal durability of the organic material is not good for it is up to only about 250° C. On the other hand, the large difference between the higher and the lower levels of the rough surface as of 0.5-1.5 um forces the LCD to have its cell gaps being enormously varied, resulting in the reflection efficiency lowered from ideally 100% to 60-85%.  
         [0003]     Therefore, it is desired an IPS LCD with improved rough surface for the reflector thereof and decreased number of mask in its manufacture process.  
       SUMMARY OF THE INVENTION  
       [0004]     An object of the present invention is to propose a pixel and its manufacture process for an IPS LCD with a reflector having a reflective surface with roughness in a nanometer scale.  
         [0005]     Another object of the present invention is to propose a pixel and its manufacture process for an IPS LCD with decreased number of mask for the reflector formation.  
         [0006]     In a pixel of an IPS LCD, according to the present invention, there are included a first structure on a substrate with a reflective surface having a nanometer scale roughness for light scattering and contrast enhancement, a second structure for switch device formation over a first part of the first structure, an LC layer over the second structure and a second part of the first structure, and a third structure above the LC layer, of which the second part of the first structure includes a reflector having the nanometer scale roughness surface, and the third structure is an optical stack. In addition, positive or negative LC may be used for the LC layer. The proposed rough surface is formed from the crystallization and the characteristic of an insulator, and due to the first part and the reflector in the second part of the first structure manufactured at a same step, only four masks are required for manufacture of an LCD, which is half of the eight or nine mask process used for a conventional scattering transreflective LCD and thereby decreasing the cost enormously. Furthermore, since the roughness of the reflective surface of the reflector is in a nanometer scale, it is achieved wider spreading angles and smooth performance for the light scattering therewith, which means the reflectivity does not vary enormously with the viewing angle and an excellent anti-glare effect is obtained additionally. On the other hand, due to the reduced difference of the height on the reflective surface of nanometer scale roughness even smaller than that of the conventional interlayer diffusion reflector (IDR), the variation between the cell gaps is lowered, and the reflection efficiency is kept at the best condition. The reflector is formed by inorganic film process and thus it endures higher temperature than that of a usual organic material.  
         [0007]     Alternatively, an embodied bottom plate comprises a substrate, a thin film transistor (TFT) on the substrate, a plurality of reflectors, a passivation covered over the reflectors, and a metal on the passivation and connected to one of the reflectors through the passivation. Each of the reflectors includes a micro scattering layer having a nanometer scale roughness surface, and a reflective layer on the micro scattering layer and conformal to the nanometer scale roughness surface, and the reflective layer is formed with the same metal layer as that of the gate electrode of the TFT. The micro scattering layer includes a conductor of ITO and an insulator thereon. The nanometer scale roughness surface is formed from the material property of the insulator, and the reflective layer is formed of metal with high reflectivity. Since the gate of the TFT and the reflective layer in the alternative embodiments are not formed at the same step, one more mask is required additionally. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The above 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:  
         [0009]      FIG. 1  shows an embodiment pixel for an IPS LCD of the present invention;  
         [0010]      FIG. 2  is the top view of the pixel shown in  FIG. 1 ;  
         [0011]      FIG. 3A  shows the rough surface of a conventional reflector;  
         [0012]      FIG. 3B  shows the rough surface of the reflector of the present invention;  
         [0013]      FIGS. 4-8  show a process for the structure formation of the IPS TFT shown in  FIG. 1 ;  
         [0014]      FIGS. 9-13  show a process for the structure formation of another IPS TFT;  
         [0015]      FIG. 14  shows another embodiment for the bottom plate for the pixel shown in  FIG. 1 ; and  
         [0016]      FIG. 15  shows a further embodiment for the bottom plate for the pixel shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  shows the first embodiment of the present invention by a pixel structure  100  of an IPS LCD that is a transreflective LCD, which comprises a first structure on a substrate  102 , having a nanometer scale roughness surface thereof and including a first part with a gate electrode  110  and a second part with reflectors  112  and  114 , a passivation  116  covered over the first structure, a second structure on the passivation  116  lying upon the gate electrode  110 . The second structure includes a drain electrode  124 , a source electrode  126  and a channel formed in an amorphous silicon semiconductor layer  118  between the drain electrode  124  and the source electrode  126 . The gate electrode  110  and the second structure form a switch device, i.e., a TFT  122 , and the drain electrode  124  is connected to the reflector  114  through a contact hole  120  in the passivation  116 . When voltages are applied to the TFT  122 , corresponding transverse electric field E is built up between the reflectors  112  and  114  to twist LC  132  above the second part. Another passivation  128  is covered over the second structure, a bottom polarizer is arranged behind the substrate  102 , LC  130  is arranged between the passivation  128  and a third structure. In this embodiment, the LC  130  and  132  are selected of negative type, even though either positive type or negative type LC are available for the present invention, preferably with birefringence  Δn  ranged between 0.05-0.14 and phase retardation  Δn×d  of 50-410 nm. Moreover, the TFT  122  is NMOS.  
         [0018]     The third structure includes a color filter  138 , a scattering film between the color filter  138  and the LC  130 , a black matrix  136  made of black resin instead of Cr metal arranged in front of the color filter  138 , a compensator  140  on the color filter  138 , a top polarizer  142  on the compensator  140 . The structure of the gate electrode  110  and the reflectors  112  and  114  includes a conductor  104  such as ITO and IZO, an insulator  106  such as silicon nitride (SiN x ) over the conductor  104 , and a reflective layer  108  made of high reflective metal such as aluminum, silver and aluminum alloy over the insulator  106 . When the insulator  106  is formed on the conductor layer  104 , due to crystallization and the characteristic of the material, the surface of the insulator  106  becomes of roughness in nanometer scale. Thus, there&#39;s no need to apply resin thereon to form a rough surface as in conventional process. Furthermore, since the reflectors  112  and  114  are manufactured at the same step for the gate electrode  110 , the pixel structure  100  of an LCD can be manufactured by four mask process, which is only half of the 8 or 9 mask process used for conventional scattering transreflective LCDs and thus reduces the cost enormously.  
         [0019]     The rough surfaces of the reflectors  112  and  114  have smaller waviness difference and waviness period, so there&#39;s wider scattering angle and smooth effect, which means that the reflectivity does not vary with the viewing angle enormously and excellent anti-glare effect is obtained. As a result, the reflection efficiency can be kept at the best condition. In addition, since inorganic film process is used, the inventive reflector can endure higher temperature than conventional organic reflector.  
         [0020]      FIG. 2  is the top view of the LCD pixel structure  100  shown in  FIG. 1 , in which the source electrode  126  is connected to bus  127 .  
         [0021]      FIG. 3  is for comparison between the rough surfaces of the reflector of the present invention and the conventional reflector. In  FIG. 3A , the rough surface of a conventional reflector has waviness difference H of 0.5-1.5 um and waviness period L of 5-20 um. In contrast, the rough surface of the reflector of the present invention shown in  FIG. 3B  has the difference in height H′ of 5-50 nm and the waviness period L′ of under 20 nm. Since the difference in height of the rough surface of the ultra micro reflector (UMR) is smaller, the gap variation of the LC is reduced, thereby keeping the reflection efficiency at the best condition, and making the scattering angle wider and more uniform simultaneously.  
         [0022]      FIGS. 4-8  show the top views and cross-sectional views of the LCD pixel structure  100  during its manufacture process. As shown in  FIG. 4 , a conductor  104  made of ITO is first deposited on the substrate  102 , and an insulator  106  composed of SiN x  is then deposited on the conductor  104 . When the insulator  106  is formed on the conductor  104 , a nanometer scale roughness is formed on the surface of the insulator  106  due to the crystallization and the characteristic of the material itself, as shown in  FIG. 3B . In follow, a metal with high reflectivity, such as aluminum, silver and aluminum alloy, is deposited on the insulator  106  to form the reflective layer  108 . The reflective layer  108  is conformal to the rough surface of the insulator  104  so as to have the roughness in nanometer scale. Then, the reflective layer  108 , the insulator  106  and the conductor  104  are etched to form the electrodes  110 ,  112  and  114  that have the roughness in nanometer scale on their surfaces.  
         [0023]     Referring to  FIG. 5 , a passivation  116  is formed to cover on the electrodes  110 ,  112  and  114 , and an amorphous semiconductor layer  118  is formed on the top surface of the electrode  110 . Then, as shown in  FIG. 6 , the passivation  116  on the electrode  114  is etched until the electrode  114  is exposed so as to form a contact hole  120 .  
         [0024]     Furthermore, a second metal is deposited on the passivation  116 , followed by selective etch to form the TFT  122 , as shown in  FIG. 7 . The gate electrode of the TFT  122  is the electrode  110 , and the second metal after selectively etched becomes the drain electrode  124  and the source electrode  126 . The drain electrode  124  is connected to the reflector  114  through the contact hole  120 , and the source electrode  126  is connected to the bus  127 . Finally, a second passivation  128  is deposited to cover on the TFT  122 , as shown in  FIG. 8 . In  FIG. 8 , the area ratio of the transparent region  144  to the reflective region formed by the reflector  112  and  114  is in the range of 10-400%.  
         [0025]      FIGS. 9-13  show a manufacture process for another IPS TFT structure  200 , which is similar to that shown in  FIGS. 4-8 , only that the reflectors  202  and  204  within the TFT structure  200  are bent.  
         [0026]      FIG. 14  shows another embodiment for the bottom plate for the pixel shown in  FIG. 1 . A bottom plate  300  comprises a substrate  302 , a TFT  304  on the substrate  302 , an insulator  306  on the substrate  302 , reflectors  308  and  310  on the insulator  306 , a passivation  312  covered on the reflectors  308  and  310 , a metal  314  on the passivation  312  and connected to the reflector  310  through the passivation  312 , and another passivation  316  covered on the metal  314 . The TFT  304  herewith is PMOS, and the metal  314  and the drain electrode  3042  of the TFT  304  are made of the same layer of metal. The reflectors  308  and  310  both include a micro scattering layer formed with an ITO  318  and an insulator  320  that has a rough surface with roughness in nanometer scale, and a reflective layer  322  on the insulator  320  conformal to the rough surface of the insulator  320  to have the nanometer roughness. The reflective layer  322  and the gate  3044  of the TFT  304  are made of the same layer of metal. In addition to the material in the foregoing description, the insulator  320  may be made of amorphous silicon or poly-silicon. Alternatively, the micro scattering layer may be formed by a layer of crystalline seeds and an insulator formed by high temperature sintering crystallization. In this embodiment, the gate electrode  3044  of the transistor  304  and the reflectors  308  and  310  are not made at a same step, and thus one more mask than the first embodiment is needed.  
         [0027]      FIG. 15  shows a further embodiment for the bottom plate for the pixel shown in  FIG. 1 . Similar to the bottom plate  300 , the structure of bottom plate  400  also comprises a substrate  302 , a TFT  402 , an insulator  306 , reflectors  308  and  310 , a passivation  312 , a metal  314 , and another passivation  316 . However, the TFT  402  is a CMOS.  
         [0028]     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.