Abstract:
A liquid crystal display (LCD) with slant structures is disclosed. The LCD includes a first plate, a second plate opposite to the first plate and a liquid crystal layer between the first plate and the second plate. The first plate further includes a first alignment film over the first plate and the first alignment film has an indication direction. The second plate includes a number of pixels. Each of the pixels includes the following from the bottom to the top: a pixel electrode, a reflector, a thin film layer consisting of a number of thin films, and a second alignment film. The thin films are formed by photolithography and the thin film layer and the second alignment film together form one slant structure with a single inclined side.

Description:
This application incorporates by reference Taiwan application Serial No. 090131394, filed Dec. 18, 2001. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to the liquid crystal display (LCD) with slant structures, and more particular to the liquid crystal on Silicon (LCoS) with slant structures. 
     2. Description of the Related Art 
     LCoS with higher resolution is now highly demanded and becomes more popular because first, the markets of portable equipments, such as Personal Digital Assistance (PDA) and mobile phones, and the large size monitor, such as project television, are growing rapidly and also the higher image quality is demanded. The structure of the LCoS is different from the conventional LCD by using one silicon substrate and one glass substrate as its two opposite substrates. And liquid crystal molecules are then sandwiched between the two substrates. 
     In FIG. 1, the cross section of a single pixel  100  of a reflective LCoS is shown. The reflective LCoS has an upper plate  101  and a lower plate  102 . The lower plate  102  comprises a silicon substrate  104 . By using the semiconductor manufacturing process, devices such as the control circuit (not shown) of the pixel, the pixel electrode  105 , the reflector  106  and so on are fabricated on the silicon substrate  104 . The upper plate  101  of the LCoS comprises a glass substrate  107  and a transparent electrode  108  on the glass substrate  107  The upper plate  101  and the lower plate  102  are opposite to each other and there is a gap in-between. The liquid crystal molecules are filled in the gap to form a liquid crystal layer  109 . Moreover, an alignment film  110  is formed over the upper plate  101  and another alignment film  112  is formed over the lower plate  102 . The alignment films  110  and  112  control the orientation of the liquid crystal molecules. That is to say the liquid crystal molecules of the liquid crystal layer  109  orientate along the indication direction of the alignment films  110  and  112 . On the LCoS panel, all the pixel electrodes  105  are not connected to each other and separated by slits  114 . The slit  114  are either covered by alignment film  112  or filled with an insulating material. 
     Conventionally, the indication direction of alignment films is formed by rubbing. The rubbing process is performed by using special velvet cloth to rub the alignment film in a certain direction so that the alignment films  110  and  112  can have indication direction. However, it is difficult to always precisely match the special velvet cloth and the alignment film. The deviation of the special velvet cloth during the rubbing process can cause the unevenness of the alignment films. Also, electrostatic discharge generated during the rubbing process can influence the operation of the electric devices and the orientation of the liquid crystal molecules. Furthermore, the remaining residues can scratch the alignment films  110  and  112 . 
     In addition to rubbing process, other methods such as photo-align is also available. However, it is not mature enough to be applied in large-scale. 
     Moreover, while a voltage is applied to the transparent electrode  108  of the upper plate  101  and the pixel electrode  105  of the lower plate  102 , the orientation of the liquid crystal molecules in the liquid crystal layer  109  will change and consequently the light transmissivity of the liquid crystal molecules will also change. In other words, the orientation of the liquid crystal molecules in the pixels and the brightness of the pixels can be controlled by the applied voltage. However, to show a complete image, the brightness and the applied voltage of each pixel are not equal. A transverse electric field exists at two the edge of every two adjacent pixels with unequal applied voltage. The strength of the transverse electric field and the difference of the applied voltage of two adjacent pixels are in direct proportion. As a result of the transverse electric field, the brightness of the edge of pixels may deviate from expectation and even dark bands may exist on the image. This is so-called fringe effect. Therefore, it is a hot issue to eliminate the fringe effect in order to produce better image. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an LCD with slant structures with the following advantages: 
     1. using an alternative process without the drawbacks of rubbing; and 
     2. having reduced transverse electric field and no fringe effect. 
     The invention achieves the above-identified objects by providing an LCD including a first plate, a second plate opposite to the first plate and a liquid crystal layer between the first plate and the second plate. The first plate further includes a first alignment film over the first plate and the first alignment film has an indication direction. The second plate includes a number of pixels. Each of the pixels includes the following from the bottom to the top: a pixel electrode, a reflector, a thin film layer consisting of a number of thin films, and a second alignment film. The thin films are formed by photolithography and the thin film layer and the second alignment film together form one slant structure with a single inclined side. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (Prior Art) is the cross-sectional view showing a single pixel of a reflective LCoS. 
     FIG. 2 shows a Vertical Alignment (VA) mode liquid crystal on Silicon (LCoS) panel according to a preferred embodiment of the invention. 
     FIG.  3 A and FIG. 3B show the arrangements of the liquid crystal molecules of a VA mode liquid crystal display Panel while the voltage is applied and while no voltage is applied. 
     FIG. 4 shows the cross section of an inverse Twisted Nematic (Inverse TN) mode LCoS panel according to the second referred embodiment of the invention. 
     FIG.  5 A and FIG. 5B show the arrangements of the liquid crystal molecules of an inverse TN mode liquid crystal display Panel while the voltage is applied and while no voltage is applied. 
     FIG. 6 shows a cross section of a lower plate of a LCD, having slant structures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The spirit of the invention focuses on applying the photolithography manufacturing process to form a plurality of thin films on the lower plate. These thin films together with the alignment film combine to form a thin film layer, which are configured as slant structures. By this figure, no indication direction formed by rubbing is needed to be formed on the alignment film on the lower plate, the transverse electric field is obstructed and the fringe effect is diminished. 
     Referring to FIG. 2, a Vertical Alignment (VA) mode liquid crystal on Silicon (LCoS) panel  200  according to a preferred embodiment of the invention is shown. On the lower plate  202 , two adjacent pixel electrodes  203  and  204  are separated bye slit  205  or the insulating material filled in the slit  205 . Over the pixel electrode  203  and the pixel electrode  204 , a reflector  207  is formed. In this invention, before the alignment film  209  is manufactured on the reflector  207 , a number of thin films are sequentially formed on the top surface of the lower plate  202  by semiconductor photolithography manufacturing process. The sequentially formed thin films then combine to form a thin film layer  208 . The material of each of the thin film is silicon dioxide. Taking the thin film layer  208  covering the pixel electrode  204  as an example, each thin film is arranged over the pixel electrode  204 . The lower thin film, (i.e. the thin film which is closer to the lower plate  202 ), is always longer than the upper thin film, (i.e. the thin film which is less close to the lower plate  202 ) and all of the thin films are aligned to each other at one end and are also aligned to the edge of the slit  206 . Consequently, the desired stair-like slant structure of the thin film layer  208  is formed on the pixel electrode  204 , as shown in FIG.  2 . 
     Then, an alignment film  209  is formed over the lower plate  202 , covering the pixel electrodes  203 ,  204 , the reflector  207 , the slit  205 , the slit  206  and the thin film layer  208 . One of the advantages of the slant structures of the invention is that no indication direction of the alignment film  209  is needed to be formed by rubbing. Thus, the manufacturing process is simplified. Disadvantages caused by the conventional rubbing process, such as unevenness, electrostatic discharge, and scratch of the alignment film  209  due to residues remaining on the lower plate  202 , can be avoided. Further more, the thin film layer is fabricated by the conventional photolithography process. Equipments therefore are widely applied in the semiconductor manufacturing process. Thus, a better performance can be obtained without further equipment investment. 
     Because of the stair-like thin film layer  208 , the alignment film  209  forms as at least one slanted structure, as shown in FIG.  2 . The angle of inclination of the alignment film  209  depends on the number and the thickness of the thin film, and length difference between each thin film  208 . For example, an alignment film  209  with a larger angle of inclination can be obtained by using a larger number of thick thin films. 
     An alignment film  211  with an indication direction, such as perpendicular to the major axis of the slit  205  and slit  206 , is formed on the upper plate  201 . Then, liquid crystal molecules  210 ,  212  are sandwiched between the upper plate  201  and the lower plate  202 . The liquid crystal molecules  210  sense the way the alignment film  209  are orientated and arrange perpendicular to the alignment film  209 . Also, because the slant structure of the alignment film  209 , the liquid crystal molecules  210  are not perpendicular to the lower plate  202  but with pretilt angle. The surface of the alignment film is manufactured to be smooth and uniform so that liquid crystal molecules  210  near the side of the alignment film  209  have equal pretilt angles, as shown in FIG.  2 . The degree of the pretilt angle of the liquid crystal molecules  210  is determined by the slop of the slant structure of the thin film layer  208 . 
     FIG.  3 A and FIG. 3B show the arrangements of the liquid crystal molecules of a VA mode liquid crystal display Panel while the voltage is applied and while no voltage is applied. FIG. 3A shows the passive state, while the voltage is applied and FIG. 3B shows the active state, while no voltage is applied. Spacings on the lower plate and the slant structure of the alignment film are not shown in FIG.  3 A and FIG.  3 B. The indication direction of the alignment film  304  on the upper plate is indicated by arrow. In FIG. 3A, while no voltage is applied, most of the liquid crystal molecules  300  are perpendicular to both the upper plate and the lower plate, except that few liquid crystal molecules  300  near the alignment film  302  are perpendicular to the alignment film  302  and form a pretilt angle to the lower plate. Therefore, at the passive state while the voltage is applied, the liquid crystal molecules  300  have low light transmissivity and the pixels are dark. In FIG. 3B, while enough voltage is applied, most of the liquid crystal molecules  300  will be influenced by the liquid crystal molecules near the lower plate with a pretilt angle and orientate for 90 degree to be parallel to both the upper plate and the lower plate. Therefore, the liquid crystal molecules  300  have high light transmissivity and full brightness. 
     FIG. 4 shows the cross section of an inverse Twisted Nematic (Inverse TN) mode LCoS panel  400  according to the second referred embodiment of the invention. Similar to the VA mode LCoS panel  200  according to the first preferred embodiment of the invention, by photolithography, a number of thin films  408  with different lengths are sequentially formed on the surface of the lower plate  402  and form a stair-like structure. An alignment film  409  is then formed over the thin films  408 . The alignment film  409  covering the stair-like structure thin films  408  has an indication direction but no rubbing process is needed. The advantages of the slant structure of the thin films  408  and the alignment film  409  are the same as what have been mentioned so that it is not necessary to repeat herein. 
     As to the manufacturing of the upper plate  401 , an alignment film  411  with another indication direction is needed to form on the surface of the upper plate  401 . What is different from the VA mode LCoS panel is that the indication direction of the alignment film  411  is parallel to the major axis of the slit. The direction of the indication direction of the alignment film  411  is either vertically coming out from the paper (indicated by ⊙) or vertically going in the paper (indicated by ⊕). 
     Then, liquid crystal molecules  410 ,  412  are sandwiched between the upper plate  401  and the lower plate  402 . The liquid crystal molecules near the alignment film  409  are arranged in an order perpendicular to the alignment film  409 . Because the alignment film  409  is a slant structure, the liquid crystal molecules near and perpendicular to the alignment film  409  form a pretilt angle. The degree of the pretilt angle of the liquid crystal molecules  410  is determined by the slop of the slant structure of the alignment film  409 . 
     FIG.  5 A and FIG. 5B show the arrangements of the liquid crystal molecules of an inverse TN mode liquid crystal display Panel while the voltage is applied and while no voltage is applied. Spacings on the lower plate and the slant structure of the alignment film are not shown in FIG.  5 A and FIG.  5 B. The indication direction of the alignment film  504  on the upper plate is indicated by arrow. As shown in FIG. 5A, while no voltage is applied, the majority of the liquid crystal molecules  500  are perpendicular to both of the upper plate and the lower plate, except that few liquid crystal molecules near and perpendicular to the alignment film on the lower plate. Therefore, the liquid crystal molecules  500  have low light transmissivity and full brightness. On the other hand, as shown in FIG. 5B, while an enough voltage is applied, liquid crystal molecules near the alignment film  502  on the lower plate have a pretilt angle, the liquid crystal molecules adjacent to the alignment film  504  on the upper plate orientate along the indication direction and the liquid crystal molecules in-between twist for 90 degree. Therefore, the liquid crystal molecules  500  have high light transmissivity and full brightness. 
     The formation of stair-like thin films and the alignment film with slant structure induce the pretilt angle of the liquid crystal molecules, which is one of the main advantages of the invention, regardless of the VA mode or the inverse TN mode liquid crystal display panel. However, a further advantage of the invention is the reducing of the fringe effect. Because of the barrier of the stair-like thin films and the alignment film with slant structure, the transverse electric field, which is caused by the unequal voltage between two pixels, is greatly reduced. Consequently, the liquid crystal molecules can arrange in a much evener order and the image quality of the liquid crystal display panel is highly improved. 
     The invention is not limited to be implemented in the LCoS but can be applied in any general liquid crystal display (LCD). As shown in FIG. 6, a cross section of a lower plate of a LCD, having slant structures, is shown. The main difference between the LCD and the LCoS is that the lower plate  602  of the LCD is a glass substrate and the lower plate of the LCoS is a silicon substrate. The pixel electrode  604  of each pixel is formed on the glass substrate individually and there is no slit between pixel electrodes. According to the spirit of the invention, stair-like thin films  606  are formed on the pixel electrode  604  by photolithography. The characters and the manufacturing process are similar to what have been mentioned, so they will not be repeated herein. Then, an alignment film  608  is formed over the stair-like thin films. No rubbing process is needed to form the indication direction of the alignment film  608 . The thin films  606  and the alignment film  608  then together form at least a slant structure. The advantages of the slant structure are previously stated. What is needed to be notified is that each pixel electrode  604  can include more than one slant structures as shown in FIG.  6 . 
     To sum up, the slant structures of the liquid crystal display according to the invention are manufactured by first forming a plurality of thin films with different length on the lower plate, by using photolithography, and forming an alignment film over the thin films. The alignment film and the thin films together form the slant structure. Consequently, the alignment film has an indication direction without the process of rubbing and the liquid crystal molecules near the alignment film on the lower plate have a pretilt angle. Furthermore, the slant structures can obstruct the transverse electric field between pixel electrodes and eliminate the fringe effect. 
     While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.