Patent Publication Number: US-2010110351-A1

Title: Transflective liquid crystal displays

Description:
PARTIES TO A JOINT RESEARCH AGREEMENT 
     At least some of the subject matter disclosed in this patent application was developed under a joint research agreement between Chi Mei Optoelectronics Corporation and the University of Central Florida. 
     BACKGROUND 
     This description relates to transflective liquid crystal displays. 
     A transflective liquid crystal display (TR-LCD) includes transmissive (T) and reflective (R) sub-pixels. In some examples, a backlight unit is used to illuminate rendered images when the display is operating in the T mode, and ambient light is used when operating in the R mode. Backlight passes the liquid crystal (LC) layer once, while the ambient light traverses the liquid crystal layer twice. 
       FIG. 1  is a cross-sectional view of an example pixel region of a transflective LCD device  8  having a dual cell gap configuration. The transflective LCD device  8  includes an upper substrate  10  (color filter substrate), a lower substrate  20  (TFT array substrate), and a liquid crystal layer  30  between the substrates  10  and  20 . A color resin layer  11  and an upper transparent electrode  12  acting as a common electrode are formed on an inner surface of the upper substrate  10 . An upper polarizer  14  and a retardation film  13  acting as a quarter-wave plate are formed on an outer surface of the upper substrate  10 . The inner surface refers to the surface that is closer to the liquid crystal layer  30 , and the outer surface refers to the surface that is farther away from the liquid crystal layer  30 . 
     An insulating layer  21 , a lower transparent electrode  22  acting as a pixel electrode, a patterned passivation layer  23 , and a reflective pixel electrode  24  are sequentially formed on a surface of the lower substrate  20 . A lower polarizer  25  is formed on another surface of the lower substrate  20 . The T sub-pixel has a first cell gap d 1  between the upper transparent electrode  13  and the lower transparent electrode  22 , whereas the R sub-pixel has a second cell gap d 2  between the upper transparent electrode  13  and the reflective pixel electrode  24 . In this example, the first cell gap d 1  is about twice as large as the second cell gap d 2  such that incident rays of light have about the same phase retardation for the transmissive and reflective modes. 
       FIG. 2  is a cross-sectional view of an example pixel region of a transflective LCD  74  that includes a TFT substrate  70 , a color filter substrate  80 , and a liquid crystal layer  90  between the substrates  70  and  80 . A common electrode  71 , a reflector  72 , and a comb-like pixel electrode  73  are formed on the TFT substrate  70 . A color filter layer  81  and an in-cell retarder  82  are formed on the color filter substrate  80 . The transflective LCD  74  incorporates a double cell gap configuration in which the transmissive and reflective regions have different cell gaps. 
     SUMMARY OF THE INVENTION 
     A single cell gap fringe field switching (FFS) based transflective LCD using a negative dielectric anisotropic liquid crystal material is provided. A homogeneous alignment is used for the transmissive regions, and a hybrid alignment configuration is used for the reflective regions. A “pixel common inversion” (PCI) electrode structure is used in which the common electrode is placed between the liquid crystal layer and the pixel electrode. 
     In general, in one aspect, a liquid crystal display includes an upper substrate; a lower substrate that is closer to a backlight unit than the upper substrate; and a liquid crystal layer between the lower and upper substrates, the liquid crystal layer including liquid crystal molecules having a negative dielectric anisotropy. The display includes pixels between the upper and lower substrates, each pixel having a transmissive region and a reflective region in which the transmissive region has a cell gap substantially the same as the cell gap of the reflective region, the transmissive region having a transparent pixel electrode, the reflective region having a reflective pixel electrode. The display includes an upper alignment layer between the upper glass substrate and the liquid crystal layer; and a lower alignment layer between the lower substrate and the liquid crystal layer, the upper and lower alignment layers oriented such that the liquid crystal molecules are homogeneously aligned in the transmissive region, and the liquid crystal molecules have a hybrid alignment in the reflective region in which liquid crystal molecules closer to the lower substrate are aligned in a direction different from the liquid crystal molecules closer to the upper substrate. The display includes a common electrode in which the common electrode and the pixel electrode are at a same side relative to the liquid crystal layer, and the orientation of the liquid crystal molecules are controlled based on fringe electric fields generated by the pixel and common electrodes when a voltage difference is applied between the pixel electrode and the common electrode. 
     Implementations can include one or more of the following features. In the transmissive region, the common electrode is between the liquid crystal layer and the transparent pixel electrode, and in the reflective region, the common electrode is between the liquid crystal layer and the reflective pixel electrode. The transparent pixel electrode and the reflective pixel electrode are electrically connected and receive a pixel voltage that corresponds to a gray level to be shown by the pixel. A first linear polarizer is coupled to the lower substrate, and a second linear polarizer is coupled to the upper substrate, in which the transmission axis of the first and second linear polarizers are substantially perpendicular to each other. In the transmissive region the alignment directions of the upper and lower alignment layers are substantially parallel to the transmission axis of one of the first and second linear polarizers, and the alignment direction of the lower alignment layer in the reflective region is at an angle in a range of 30° to 60° with respect to the transmission axis of one of the first and second linear polarizers. The common electrode and the pixel electrode include indium-tin-oxide. The common electrode in the transmissive region includes stripes, the common electrode in the reflective region includes stripes, and the stripes of the common electrode in the transmissive region are at an angle in a range of 120° to 160° relative to the stripes of the common electrode in the reflective region. 
     The common electrode in the reflective region includes stripes each having a width in a range from 1 to 3 μm and a spacing between stripes in a range from 2 to 4 μm, and the common electrode in the transmissive region includes stripes each having a width in a range from 2 to 4 μm and a spacing between stripes in a range from 4 to 6 μm. The display includes data bus lines made of conductive metals including MoW, an alloy Al—Nd, or a stacked layer of Mo/Al materials, each data bus line having a chevron shape in a pixel region and is covered and electrically shielded by a common electrode stripe from the liquid crystal layer. In the transmissive region, the common electrode includes stripes, and the surface alignment direction of the liquid crystal layer is at an angle in a range between 5° to 20° with respect to a direction perpendicular to the common electrode stripes. In the transmissive region, the surface pretilt angle on both the upper and the lower substrate is between 0° to 10° relative to the substrate surface. In the reflective region, the common electrode includes stripes, the surface alignment direction of the liquid crystal layer on the lower substrate is at an angle in a range between 5° to 20° with respect to a direction perpendicular to the common electrode stripes, the surface pre-tilt angle of the liquid crystal layer on the upper substrate is in a range between 85° to 90° relative to a surface of the upper substrate. In the reflective region, the surface pretilt angle on the lower substrate is between 0° to 10° relative to the lower substrate. A black matrix is formed on the upper substrate covering a thin film transistor area and a boundary area between the transmissive region and the reflective region. The liquid crystal display includes a barrier wall between the transmissive region and the reflective region. The barrier wall has a height substantially the same as the thickness of the liquid crystal layer and defines a cell gap of the liquid crystal layer. 
     In general, in another aspect, a display includes a pixel having a transmissive region and a reflective region. The transmissive region has a liquid crystal layer, a transparent pixel electrode, and a common electrode. The liquid crystal layer is between a first substrate and a second substrate, and includes liquid crystal molecules that are aligned substantially along a same direction when the pixel is in a dark state. The transparent pixel electrode and the common electrode are at a same side relative to the liquid crystal layer, and orientation of the liquid crystal molecules is controlled based on fringe electric fields generated by the transparent pixel electrode and common electrode when a voltage difference is applied between the transparent pixel electrode and the common electrode. The reflective region has a liquid crystal layer, a reflective pixel electrode, and a common electrode. The liquid crystal layer is between the first substrate and the second substrate, and has a hybrid alignment in which liquid crystal molecules closer to the first substrate are aligned in a direction different from the liquid crystal molecules closer to the second substrate when the pixel is in the dark state. The reflective pixel electrode and the common electrode are at the same side relative to the liquid crystal layer such that orientation of the liquid crystal molecules is controlled based on fringe electric fields generated from the reflective pixel electrode when a voltage difference is applied between the reflective pixel electrode and the common electrode. 
     Implementations can include one or more of the following features. The common electrode of the transmissive region is electrically coupled to the common electrode of the reflective region. The transparent pixel electrode of the transmissive region is electrically coupled to the reflective pixel electrode of the reflective region. The display includes a plurality of pixels in which the common electrodes of different pixels are electrically connected together. The common electrode is between the transparent pixel electrode and the liquid crystal layer. The common electrode includes stripes. 
     In general, in another aspect, a transflective liquid crystal display includes a backlight unit, an upper substrate, a lower substrate that is closer to the backlight unit relative to the upper substrate, and a liquid crystal layer between the lower and upper substrates, the liquid crystal layer including a liquid crystal material having a negative dielectric anisotropy. The display includes a first linear polarizer, a second linear polarizer having a transmission axis that is perpendicular to that of the first linear polarizer, the upper and lower substrates being between the first and second linear polarizers, and a plurality of pixels between the upper and lower substrates. Each pixel has a transmissive region and a reflective region, the liquid crystal cell gap in the transmissive region being substantially the same as the cell gap in the reflective region, the liquid crystal molecules being homogeneously aligned in the transmissive region and having a hybrid alignment in the reflective region, the alignment direction of the liquid crystal layer on upper and lower substrate surfaces in the transmissive region being substantially parallel to the transmission axis of one of the first and second linear polarizers, and the alignment direction of the liquid crystal layer on the lower glass substrate in the reflective region being aligned at an angle in a range between 30° to 60° with respect to the transmission axis of one of the first and second linear polarizers. Each pixel includes a transparent pixel electrode in the transmissive region, a reflective pixel electrode in the reflective region, and a common electrode having many stripes, in which a driving voltage is applied between the pixel electrode and the common electrode and between the reflective pixel electrode and the common electrode to reorient liquid crystal molecules to cause the pixel to show various gray levels. 
     Implementations can include one or more of the following features. Each pixel includes a barrier wall at a boundary area between the transmissive region and the reflective region to reduce light leakage, the barrier wall extending into the liquid crystal layer. The barrier wall includes color resin. The barrier wall is made of two overlapping color resin layers having two colors that are different from the pixel color. For example, the barrier wall in a red pixel includes blue and green color resin. The barrier wall includes an over coating dielectric layer. The barrier wall has a height in a range from 0.4 to 3.2 μm and a width in a range from 3 to 20 μm. The barrier wall has a height that is substantially the same as the thickness of the liquid crystal layer and defines a cell gap of the liquid crystal layer. The first and third transparent electrodes include indium-tin-oxide. The stripes of the common electrode in the transmissive region and the stripes of the common electrode in the reflective region have a chevron geometry. The chevron geometry has a chevron angle in a range between 120° to 160°. In the reflective region the stripes of the common electrode each has an electrode width in a range from 1 to 3 μm, and an electrode spacing in a range from 2 to 4 μm, and in the transmission region the electrode stripes each has an electrode width in a range from 2 to 4 μm and an electrode spacing in a range from 4 to 6 μm. The liquid crystal display includes a data bus line made of conductive metals including MoW, an alloy Al—Nd, or a stacked layer of Mo/Al materials. The liquid crystal display includes data bus lines each having a chevron shape in each pixel region. Each data bus line is covered and electrically shielded by a common electrode stripe from the liquid crystal layer. The surface alignment direction of the liquid crystal layer in the transmissive region has an angle in a range from 5° to 20° with respect to a direction that is perpendicular to the common electrode stripes. In the transmissive region, the surface pretilt angles of the liquid crystal layer on both the lower and upper substrates are in a range between 0° to 10° relative to respective substrate surfaces. A surface alignment direction of the liquid crystal layer on the lower substrate in the reflective region has an angle in a range from 5° to 20° with respect to a direction that is perpendicular to the common electrode stripes, and a surface pre-tilt angle of the liquid crystal layer on the upper substrate in the reflective region is in a range from 85° to 90°. In the reflective region, the surface pretilt angle of the liquid crystal layer on the lower substrate is in a range between 0° to 10° relative to the lower substrate surface. 
     In general, in another aspect, a method of fabricating a transflective liquid crystal display includes using a first mask to define a gate line and a gate electrode; using a second mask to define an active layer for a thin film transistor; using a third mask to define an embossing pattern for a reflective pixel electrode; using a fourth mask to define a pixel electrode; using a fifth mask to define a source electrode, a drain electrode, and a data bus line; using a sixth mask to define gate, source, and drain pad contact windows; and using a seventh mask to define a common electrode having a chevron geometry and having openings to facilitate generation of fringe fields during operation of the display. 
     Implementations can include one or more of the following features. A barrier wall is formed between the transmissive region and the reflective region at the same time that a color filter layer is formed. An eighth mask is used to define an alignment direction of an alignment layer for aligning, in a reflective region, liquid crystal molecules near a lower substrate that is closer to a backlight module that an upper substrate, the alignment direction of the alignment layer in the reflective region being at an angle in a range between 30° to 60° with respect to a transmission axis of a linear polarizer used in the display. 
     In general, in another aspect, a liquid crystal display includes pixels, each pixel including a transmissive region and a reflective region. The transmissive region has a liquid crystal layer having a homogeneous alignment, the transmissive region having an alignment layer, a common electrode, and a pixel electrode that are on a same side of the liquid crystal layer. The reflective region has a liquid crystal layer having a hybrid alignment, the reflective region having an alignment layer, a common electrode, and a reflective pixel electrode that are on a same side of the liquid crystal layer, in which the alignment layer of the reflective region has an alignment direction that is different from that of the alignment layer of the transmissive region. 
     Implementations can include one or more of the following features. The alignment layer of the transmissive region has an alignment direction that is between 30° to 60° relative to that of the alignment layer of the reflective region. The alignment directions of the upper and lower alignment layers in the transmissive region are substantially parallel to the transmission axis of a first linear polarizer or a second linear polarizer, the pixels being between the first and second linear polarizers. In the transmissive region, the common electrode is between the pixel electrode and the liquid crystal layer. In the reflective region, the common electrode is between the reflective pixel electrode and the liquid crystal layer. The common electrode includes stripes in the transmissive region and the reflective region. The common electrode stripes in the reflective region extend along a first direction, and the common electrode stripes in the transmissive region extends along a second direction that is different from the first direction. The first direction is at an angle between 20° to 60° relative to the second direction. The common electrode stripes in the reflective region have a stripe width and a stripe spacing that are different from those of the common electrode stripes in the transmissive region. The reflective and transmissive regions have common electrode stripes with stripe widths and stripe spacing that are configured to cause a voltage-transmittance curve to match a voltage-reflectance curve. When a pixel voltage corresponding to a dark state is applied between the reflective pixel electrode and the common electrode, the liquid crystal layer in the reflective region functions as a quarter wave plate. When a pixel voltage corresponding to a bright state is applied between the reflective pixel electrode and the common electrode, the liquid crystal layer in the reflective region is driven to have its effective optic axis rotated about 45° away from its initial alignment direction. In the reflective region, a lower portion of the liquid crystal layer has a surface pretilt angle between 0° to 10° relative to a lower substrate, and an upper portion of the liquid crystal layer has a surface pretilt angle between 85° to 90° relative to an upper substrate. 
     In general, in another aspect, a transflective liquid crystal display includes pixels, each pixel including a transmissive sub-pixel; a reflective sub-pixel; and a barrier wall between the transmissive sub-pixel and the reflective sub-pixel, the barrier wall extending into a liquid crystal layer of the pixel. 
     Implementations can include one or more of the following features. The barrier wall includes color resin. The color resin of the barrier wall includes a same material as that in a color filter used in the display. The barrier wall includes a dielectric layer. The barrier wall has a height that is between 10% to 90% of a cell gap of a liquid crystal layer of the pixel. The barrier wall has a width that is between 3 to 20 microns. 
     In general, in another aspect, a method of fabricating a liquid crystal display includes, in a transmissive region of a pixel, forming a pixel electrode above a first substrate; in a reflective region of the pixel, forming a reflective pixel electrode above the first substrate; forming a passivation layer above the pixel electrode and the reflective pixel electrode; forming a common electrode above the passivation layer; configuring alignment layers in the transmissive region of the pixel to cause liquid crystal molecule to be homogeneously aligned in the transmissive region; and configuring alignment layers in the reflective region of the pixel to cause liquid crystal molecules to have a hybrid alignment in the reflective region. 
     Implementations can include one or more of the following features. Configuring alignment layers in the transmissive and reflective regions includes using a first photomask to expose a first portion of an alignment layer in the transmissive region to cause the first portion of the alignment layer to have a first alignment direction, and using a second photomask to expose a second portion of the alignment layer in the reflective region to cause the second portion of the alignment layer to have a second alignment direction. The first direction is at an angle in a range between 30° to 60° relative to the second direction. Configuring alignment layers in the transmissive and reflective regions includes rubbing a portion of an alignment layer in the transmissive region to have a first alignment direction by using a first mask that exposes the portion of the alignment layer in the transmissive region and covers a portion of the alignment layer in the reflective region, and rubbing a portion of the alignment layer in the reflective region to have a second alignment direction by using a second mask that exposes the portion of the alignment layer in the reflective region and covers the portion of the alignment layer in the transmissive region. The first direction is at an angle in a range between 30° to 60° relative to the second direction. 
     Other aspects can include other combinations of the features recited above and other features, expressed as methods, apparatus, systems, program products, and in other ways. 
     Advantages of the aspects and implementations may include one or more of the following. The transflective display can be used in mobile devices, can have good sun light readability, low power consumption, thin profile, light weight, high resolution, wide viewing angle, high brightness, and low manufacturing cost. The transflective display does not need an in-cell retarder and can still achieve a good dark state. The driving voltages can be reduced by use of the pixel common inversion electrode structure. When color filter materials are used to construct barrier walls, light leakage can be reduced, resulting in a darker dark state, while not requiring additional fabrication steps. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1 and 2  are cross-sectional views of example transflective LCDs having dual cell gap configurations. 
         FIG. 3  is a cross-sectional view of an example single cell gap transflective LCD in a dark-state. 
         FIG. 4  is a cross-sectional view of an example single cell gap transflective LCD in a bright-state. 
         FIG. 5  is a flow diagram of an example process for fabricating a transflective LCD. 
         FIG. 6A  is a top view of an example sub-pixel of a transflective LCD. 
         FIG. 6B  is a top view of an example common electrode. 
         FIG. 7A  is a cross sectional view of an example TFT area along a line A-A′ in the sub-pixel of  FIG. 6 . 
         FIG. 7B  is a cross sectional view of an example data bus line area along a line B-B′ in the reflective region of  FIG. 6 . 
         FIG.7C  is a cross sectional view of an example data bus line area along a line C-C′ in the transmissive region of  FIG. 6 . 
         FIG. 7D  is a cross sectional view of an example boundary area between a transmissive region and a reflective region along a line D-D′ in the sub-pixel of  FIG. 6 . 
         FIG. 8  is a top view of an example sub-pixel of a transflective LCD having a barrier wall between the transmissive region and the reflective region. 
         FIG. 9A  is a cross sectional view of an example sub-pixel of a transflective LCD along a line E-E′ in  FIG. 8  showing a barrier wall that includes patterned green and blue layers. 
         FIG. 9B  is a cross sectional view of an example sub-pixel of a transflective LCD along the line E-E′ in  FIG. 8  showing a barrier wall that includes a patterned over coating layer. 
         FIG. 10  is a top view of an example sub-pixel structure of a transflective LCD. 
         FIG. 11A  is a graph showing example simulated V-T and V-R curves of a transflective LCD. 
         FIG. 11B  is a graph showing the V-T and V-R curves of  FIG. 11A  after normalization. 
         FIG. 11C  is a graph showing example simulated 2D images having various brightness values generated by a transflective LCD. 
         FIG. 11D  shows example simulated iso-contrast contour plots of the transmissive region of a transflective LCD. 
         FIG. 11E  shows example simulated iso-contrast contour plots of the reflective region of a transflective LCD. 
         FIG. 12A  shows example simulated liquid crystal orientations of a pixel in which a barrier wall is not used. 
         FIG. 12B  shows a corresponding image shown by the pixel of  FIG. 12A . 
         FIG. 12C  shows example simulated liquid crystal orientations of a pixel in which a barrier wall is used. 
         FIG. 12D  shows a corresponding image shown by the pixel of  FIG. 12C . 
         FIG. 13A  is a graph showing example simulated V-T and V-R curves of a transflective LCD. 
         FIG. 13B  is a cross sectional diagram of an example transflective LCD whose simulated V-T and V-R curves are shown in  FIG. 13A . 
         FIG. 13C  is a graph showing example simulated V-T and V-R curves of a transflective LCD. 
         FIG. 13D  is a cross sectional diagram of an example transflective LCD whose simulated V-T and V-R curves are shown in  FIG. 13C . 
         FIG. 14A  is a cross sectional view of an example transflective LCD. 
         FIGS. 14B and 14C  show graphs of example relationships between simulated tilt and rotational angles and cell gaps at various positions of a transflective LCD under various configurations when the reflector electrode is between the common electrode and the liquid crystal layer. 
         FIGS. 14D and 14E  show graphs of example relationships between simulated tilt and rotational angles and cell gaps at various positions of a transflective LCD under various configurations when the common electrode is between the reflector electrode and the liquid crystal layer. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a cross-sectional view of an example pixel  132  of a single cell gap fringe field switching (FFS) based transflective liquid crystal display (TR-LCD)  134 . The pixel  132  has a transmissive (T) region  136  and a reflective (R) region  138 . In the example of  FIG. 3 , the pixel  132  is operating in a dark-state (e.g., the pixel  132  is driven to show a low luminance level). The pixel  132  uses a pixel-common inversion electrode structure in which a common electrode  105  is positioned between a liquid crystal layer  120  and a pixel electrode  102  and a reflector electrode  103 . The common electrode  105  has elongated openings with stripes between the openings (see  FIG. 6 ). Fringe fields generated by the common electrode  105 , the pixel electrode  102 , and the reflector electrode  103  switch the pixel  132  to various gray scale levels. A liquid crystal material having negative dielectric anisotropy is used. Upper and lower alignment layers are configured such that the liquid crystal layer in a transmissive region  136  has a homogenous alignment, whereas the liquid crystal layer in a reflective region  138  has a hybrid alignment. This allows the display  134  to achieve a wide viewing angle without using compensation films, use low pixel driving voltages to drive the pixel  132 , and use a single gamma curve when operating in both the transmissive mode and the reflective mode. 
     The transmissive region and reflective region are sometimes referred to as transmissive sub-pixel and reflective sub-pixel, respectively. 
     Although one pixel is shown in  FIG. 3  (and FIGS.  4  and  6 - 10 ), it is understood that the display  134  includes a plurality of pixels (e.g., an array of rows and columns of pixels) that are used together to show images. The transflective LCD includes a backlight unit  130 , a lower substrate  100  that is near the backlight unit  130 , and an upper substrate  110  that is near a viewer. The liquid crystal layer  120  is between the two substrates  100  and  110 , which in turn are between a bottom linear polarizer  115   a  and a top linear polarizer  115   b.    
     In this description, a “pixel” refers to a unit that can be independently controlled to show a particular gray level. In some examples, three pixels having red, green, and blue colors (or other number of pixels and combination of colors) together form a color pixel, and each pixel is referred to as a “sub-pixel” of the color pixel. 
     A gate insulator layer  101  is formed on the lower substrate  100 . In the R region  138 , the gate insulator layer  101  is processed to have circular-shaped embossing patterns  101   a,  while the surface of the gate insulator layer  101  in the T region  136  remains smooth. In the R region  138 , a reflector electrode  103  is formed on the gate insulator layer  101  with circular-shaped embossing patterns. This causes “bumps” to form on the surface of the reflector electrode  103  so that the reflector electrode  103  can scatter ambient light in various directions, resulting in more uniform luminance at various viewing angles. The reflector electrode  103  functions both as a reflector to reflect light and as an electrode to generate electric fields to control orientations of the liquid crystal molecules. 
     In the T region  136 , a transparent pixel electrode  102  made of a transparent conductive material, such as indium tin oxide (ITO), is formed on the gate insulator layer  101 . The pixel electrode  102  is electrically connected to the reflector electrode  103 . A passivation layer  104  made of dielectric materials, such as SiOx and/or SiNx, is formed on the pixel electrode  102  and the reflector electrode  103 . 
     A transparent common electrode  105  having many elongated branches or stripes is formed on the passivation layer  104 , in which most of the stripes of the common electrode  105  overlap the pixel electrode  102  and the reflector electrode  103 . In some examples, the common electrode  105  of all the pixels in the display  134  are electrically connected to have a common voltage. The width of the stripes and spacing between the stripes of the common electrode  105  in the T region  136  may be different from those in the R region  138 , as described below. 
     A black matrix  111  is patterned on the inner surface of the upper substrate  110 . A color filter layer  112  and an over coating layer  113  are formed on the inner surface of the upper substrate  110 . A lower alignment layer  106  and an upper alignment layer  114  are printed on the inner surfaces of the lower substrate  100  and upper substrate  110 , respectively. 
     The alignment layers  106  and  114  are treated differently in the T region  136  and the R region  138  so that the liquid crystal molecules in the T region  136  have a homogenous alignment, whereas the liquid crystal molecules in the R region  138  have a hybrid alignment. 
     In the transmissive region  136 , the lower alignment layer  106  is treated (e.g., by photo-alignment method or by rubbing with mask) such that its alignment axis is at an angle of 2° to 5° with respect to the horizontal direction. The top alignment layer  114  is treated such that its alignment axis is aligned along an anti-parallel direction (i.e., parallel to but in opposite direction) with respect to the alignment direction of the lower alignment layer  106 . 
     In this description, the terms “horizontal” and “vertical” are used to describe the orientations of various components of the display in the figures. Thus, for example, the surface of the substrates  100  and  110 , and the surface of the alignment layers  106  and  114 , are described and shown in the figures as being parallel to the horizontal direction. However, the display can be used in various orientations so that what we call horizontally or vertically aligned liquid crystal molecules may not be aligned along the horizontal or vertical direction using the earth as reference. Similarly, the terms “top,” “bottom,” “upper,” “lower,” “above,” and “below” are used to describe relative positions of components of the display in the figures. The display can have other orientations so that in some circumstances, for example, what we call a lower layer may be above what we call an upper layer. 
     The cell gap in the transmissive region  136  is substantially the same as the cell gap in the reflective region  138 . Because of manufacturing tolerances, the cell gap may not be entirely uniform across the entire transmissive and reflective regions. Some features of the pixel may cause the cell gap to be smaller or larger at some locations than others. Thus, when we say that a pixel has a single cell gap structure (or that the transmissive region  136  and the reflective region  138  have the same cell gap), it does not necessarily mean that the cell gap across the entire pixel is exactly the same. In this example, small differences in cell gaps, if any, are not meant to achieve a difference in optical phase retardation between the transmissive and reflective regions in order to compensate for the phase difference between the transmitted light (which passes the liquid crystal layer once) and the reflected light (which passes the liquid crystal layer twice), as is the case in the pixel structure of  FIG. 1 . 
     In the reflective region  138 , the lower alignment layer  106  is treated (e.g., by photo-alignment or by rubbing with mask) such that its alignment axis is at an angle of about 45° (or −45°) with respect to the alignment axis of the lower alignment layer  106  in the transmissive region  136 . The top alignment layer  114  in the R region  138  is treated (e.g., by photo-alignment or by printing a vertical alignment material with mask) such that the liquid crystal molecules near the top alignment layer  114  in the R region  138  have a vertical tilt angle (i.e., the liquid crystal molecules are aligned along the vertical direction). 
     In some examples, the different surface alignment directions of the transmissive and reflective regions can be achieved by the following process. First, on the bottom array substrate, the transmissive portion of the lower alignment layer  106  is photo-aligned to have a predetermined alignment angle by using a photo-mask that exposes the transmissive portion of the lower alignment layer  106  and covers the reflective portion of the alignment layer  106 . This way, the reflection portion of the lower alignment layer  106  is shielded by the photo mask and is not exposed to the irradiation UV light or ion beam used for the photo-alignment. Another photo-mask that has an opening that exposes the reflective portion of the lower alignment layer  106  and covers the transmissive portion of the alignment layer  106  is used to photo-align the reflective portion of the alignment layer  106  at a different angle. A similar process can be used to process the upper alignment layer  114  such that the transmissive and reflective portions of the upper alignment layer  114  have different alignment angles. 
     In some examples, the different surface alignment directions of the transmissive and reflective regions can be achieved by the following process. First, on the bottom array substrate  100 , a portion of the lower alignment layer  106  in the transmissive region is rubbed (e.g., by a roller) to have a predetermined alignment direction by using a mask that exposes the portion of the alignment layer  106  in the transmissive region and covers the portion of the alignment layer  106  in the reflective region. This way, the portion of the lower alignment layer  106  in the reflective region is shielded by the mask and is not exposed to the roller. Another mask that has an opening that exposes the portion of the lower alignment layer  106  in the reflective region and covers the portion of the alignment layer  106  in the transmissive region is used when rubbing the portion of the alignment layer  106  in the reflective region at a different alignment direction. 
     On the upper substrate  110 , the upper alignment layer  114  is rubbed along an anti-parallel direction (i.e., parallel to but in opposite direction) with respect to the alignment direction of the lower alignment layer  106  in the transmissive region  136 . A mask that has an opening that exposes the portion of the upper alignment layer  114  in the reflective region and covers the portion of the alignment layer  114  in the transmissive region is used when printing the portion of the alignment layer  114  in the reflective region with a vertical alignment material. 
     In some examples, the transflective LCD  134  uses a negative dielectric liquid crystal material  120  that has a dielectric anisotropy (Δε) of −4.0. In this display, a homogeneous alignment is used for the transmissive region  136  and a hybrid alignment configuration is used in the reflective region  138 . This allows a good dark state to be achieved in the reflective region  138  without using an in-cell retarder. 
     To reduce the driving voltage, a pixel common inversion (PCI) electrode structure is used, in which the common electrode  105  is positioned between the pixel electrode  102  (or the reflector electrode  103 ) and the liquid crystal layer  120 . This configuration allows the pixel electrode  102  and the reflector electrode  103  to be driven with a lower driving voltage, as compared to placing the pixel electrode  102  between the common electrode  105  and the liquid crystal layer  120 . 
     Each pixel  132  of the transflective FFS LCD  134  is normally dark when no pixel voltage (also referred to as pixel driving voltage) is applied to the pixel electrode  102  and reflector electrode  103 , or when a pixel voltage corresponding to the lowest luminance level is applied to the electrodes  102  and  103 . To achieve minimum light leakage in the reflective region  138 , the top linear polarizer  115   b  has its transmission axis aligned at about 45° (or −45°) with respect to the alignment direction of the bottom alignment layer  106  in the reflective region  138 . In the dark state, ambient (or external) light passes the top polarizer  115   b  to form linearly polarized light. 
     The liquid crystal material and the thickness of the liquid crystal layer  120  (i.e., the cell gap) are selected so that a phase retardation of λ/4 (at, e.g., λ=550 nm) is imparted between two perpendicular polarization components of light that passes the liquid crystal layer  120  in the reflective region  136 . For the light having a wavelength λ=550 nm, the liquid crystal layer  120  functions similar to a quarter wave plate. Linearly polarized light becomes circularly polarized light after passing the liquid crystal layer  120 . After the light is reflected by the reflector electrode  103 , the light propagates through the liquid crystal layer  120  again and becomes linearly polarized light that is rotated to a direction 90° relative to the transmission axis of the top polarizer  115   b.  This causes the light to be blocked by the top polarizer  115   b,  achieving a good dark state. 
     In the transmissive region  136 , in the dark state, light generated by the backlight unit  130  passes the bottom polarizer  115   a  to form linearly polarized light. The alignment layers  106  and  114  in the transmission region  136  are processed such that the alignment axes of the lower alignment layer  106  and the upper alignment layer  114  are the same, and are either parallel to the transmission axis of the lower polarizer  115   a  or the transmission axis of the upper polarizer  115   b.  This way, the liquid crystal molecules are aligned parallel to the transmission axis of the bottom polarizer  115   a  or top linear polarizer  115   b,  so the light passes through the liquid crystal layer  120  without change of polarization state. The light is blocked by the top polarizer  115   b,  resulting in a dark state, similar to that of the reflective mode. 
       FIG. 4  is a cross-sectional view of the pixel  132  of the single cell gap FFS based transflective LCD  134  in a bright-state (e.g., when the pixel  132  is driven to show a high luminance level). To achieve a maximum transmittance in the transmissive region  136 , the configuration of the common electrode  105  in the transmissive region  136  (e.g., the width of stripes and spacing of stripes of the common electrode  105 ) and a driving voltage Vmax corresponding to the maximum luminance level are selected such that a phase retardation of λ/2 (at λ=550 nm) is imparted between two perpendicular polarization components of light passing the liquid crystal layer  120  in the transmissive region  136  when V max  is applied between the electrodes  102  and  105 . 
     When a driving voltage Vmax is applied between the electrodes  105  and  102 , fringe fields  107  generated by the electrodes  105  and  102  cause liquid crystal directors to rotate about 45°. When the linearly polarized backlight from the bottom polarizer  115   a  propagates through the liquid crystal layer  120 , its polarization direction rotates 90° to allow the backlight to pass the top polarizer  115   b,  resulting in a bright state. 
     In the reflective region  138 , when V max  is applied between the electrodes  105  and  103 , fringe fields generated by the electrodes  105  and  103  rotate the bottom liquid crystal directors about 45° (the liquid crystal molecules near the bottom substrate are rotated 45°, but the liquid crystal molecules near the upper substrate remain substantially vertical). This causes the bottom liquid crystal directors to be either parallel or perpendicular to the polarization direction of the upper polarizer. 
     If the bottom liquid crystal molecules are parallel to the polarization direction of the upper polarizer, ambient light does not change polarization when it passes the liquid crystal layer, so the reflected light pass the upper polarizer, resulting in a bright state. If the bottom liquid crystal molecules are perpendicular to the polarization direction of the upper polarizer, when linearly polarized ambient light passes the liquid crystal layer  120  once, the polarization direction of the light is still not rotated and will then be reflected by the reflector. When the light passes the liquid crystal layer  120  a second time, the polarization direction of the light is maintained to allow the reflected ambient light to pass the top polarizer  115   b,  resulting in a bright state. 
       FIG. 5  is a flow diagram of an example process  154  for fabricating the transflective high-brightness FFS-LCD  134 . A first opaque metal layer is deposited (step  140 ) on the lower glass substrate  100 . A gate patterning step is performed (step  141 ), in which a gate bus line including a gate electrode and gate pads are formed by etching the first opaque metal layer using a first photo mask. The first opaque metal layer can be, for example, MoW, an alloy Al—Nd, or a stacked layer of Mo/Al. Each layer can be deposited using a sputtering process. When the first opaque metal layer is a MoW layer, etching of the MoW layer can be performed by dry etching method using SF 6  gas or CF 4  and O 2  gases. When the first opaque metal layer is an Al—Nd alloy layer or a Mo/Al stacked layer, etching of the layers can be performed by wet etching method using an etchant including a mixture of H 3 PO 4 , CH 3 COOH, HNO 3 , and H 2 O. 
     A gate insulator layer and active area deposition is performed (step  142 ), in which a SiON layer, an amorphous silicon (a-Si) layer, and an n+ a-Si layer are successively deposited by a plasma enhanced chemical vapor deposition (PECVD) method. The active region of the thin film transistor (TFT) is patterned by etching the n+ a-Si layer and a-Si layer using a second photo mask (step  143 ). The etching of the active layers can be performed by dry etching method using SF 6 , He and HCl gases. The SiON layer functions as a gate insulator layer (e.g.,  101 ). 
     An embossing pattern (which when coated with a reflective layer provides a scattering effect with respect to incident light) is formed by coating a photo sensitive organic (PSO) layer or a photo resistor (PR) layer, exposing the PSO or PR layer to UV light using a third photo mask, and developing the PSO or PR layer with a developing etchant (step  144 ). A transparent conductive layer, for example, an indium tin oxide (ITO) layer, is deposited by a sputtering method using, e.g., Ar gas, O 2  gas and ITO target (step  145 ). 
     Patterning of the pixel electrode (e.g.,  102 ) is performed by etching the ITO layer using a fourth mask (step  146 ). The ITO layer is etched by a wet etching method using HCl, HNO 3  and H 2 O as etchant. A second opaque metal layer is deposited by a sputtering process (step  147 ). Source-drain electrodes, reflective pixel electrodes and the data bus line including data pads are patterned by etching the second opaque metal layer using a fifth mask (step  148 ). The drain electrode is electrically connected to the pixel electrode (e.g.,  102 ). The second opaque metal layer can be made of, e.g., MoW, an alloy Al—Nd, or a stacked layer of Mo/Al, which can be formed by sputtering each target material. When the second opaque metal layer is a MoW layer, the MoW layer can be etched by a dry etching method using SF 6  gas or CF 4  and O 2  gases. When the second opaque metal layer is an Al—Nd alloy layer or a Mo/Al stacked layer, the layers can be etched by a wet etching method using an etchant that includes a mixture of H 3 PO 4 , CH 3 COOH, HNO 3 , and H 2 O. 
     A passivation layer of SiNx is deposited (step  149 ) using PECVD over the resultants of previous processing steps, followed by patterning portions of the gate pads and data pads by etching the passivation layer (e.g.,  104 ) formed on the pad using a sixth mask (step  150 ). Etching the passivation layer can be performed by a dry etching method using SF 6  gas or O 2  gas. A second transparent metal layer, e.g., an ITO layer, is deposited on the passivation layer using a sputtering method (step  151 ). A common electrode (e.g.,  105 ) having a chevron shape with many stripes and a matrix of common bus lines are patterned by etching the ITO layer using a seventh mask (step  152 ). The common electrode is formed to overlap the pixel electrode with a passivation layer between the common electrode and the pixel electrode. 
       FIG. 6A  is a top view of an example pixel  190  of a transflective high-brightness FFS-LCD. An opaque gate bus line  201  extends along a row direction, and an opaque data bus line  202  extends along a column direction. The opaque gate bus line  201  and data bus line  202  can be made of metals such as MoW, alloy Al—Nd, or a stacked layer of Mo/Al, and can be formed by using a sputtering process. In some examples, the gate bus line  201  and the data bus line  202  each has a thickness of about 200˜350 nm. The data bus line  202  has a chevron shape that conforms with the chevron shape of the pixel electrode  204  and the common electrode  206 . A gate insulator layer (not shown in the figure) is deposited on the gate bus line, in which the gate insulator layer can be made of SiON and can have a thickness of 400 nm. 
     A thin film transistor (TFT)  203  is disposed near an intersection of the gate bus line  201  and the data bus line  202 . The TFT  203  includes a source electrode  208  and a drain electrode  209 . The TFT  203  functions as a switch to turn on or off driving of the pixel. In the reflective region  138 , in order to provide a scattering effect with respect to incident light, the gate insulator layer is patterned to have circle shape embossing patterns. 
     In the transmissive region  136 , a planar pixel electrode  204  is formed on the gate insulator layer. The pixel electrode  204  can be made of a transparent metal layer, such as indium-tin-oxide (ITO), and can have a thickness of about 40 nm. In the reflective region  138 , a reflective pixel electrode  205  is formed on the gate insulator layer. One side of the pixel electrode  204  is connected with the source electrode  208  to receive a pixel data voltage from the data bus line  202  when the TFT  203  is turned on. 
     A chevron shape common electrode  206  having many stripes  210  and a matrix shape common bus line  212  are formed on the passivation layer. Many stripes  210  of the common electrode  206  overlap the pixel electrode  204  in the vertical direction (but electrically insulated from each other) and one stripe  214  of the common electrode  206  overlaps the data bus line  202  in the vertical direction (but electrically insulated from each other). The vertical direction refers to the direction perpendicular to the surfaces of the substrates  100  and  110 . The common electrode  206  has an opening in an area where the TFT  203  is located. A black matrix  250  (shown in dashed lines) formed on the inner surface of the upper substrate  110  covers the TFT  203  and the boundary between the transmissive region  136  and the reflective region  138 . 
     The openings in the common electrode can have various shapes. For example,  FIG. 6B  is a top view of an example common electrode  230  that defines openings  232  in the reflective region  138  and openings  234  in the transmissive region  136 . The openings  232  and  234  are at alternating positions such that each opening  232  in the R region  138  corresponds to a stripe  236  of the common electrode in the T region  136 , and each opening  234  in the T region  136  corresponds to a stripe  238  of the common electrode in the R region  138 . The common electrode  230  has an overall chevron geometry with openings  232  and  234  that are not symmetrical with respect to a border  240  between the transmissive and reflective regions. 
       FIG. 7A  is a cross sectional view of an example TFT area along a line A-A′ in the pixel of  FIG. 6A . A gate electrode  216  is formed on a lower substrate  301 . After deposition of three layers (i.e., a gate insulator  303 , an a-Si  302 , and a n+ a-Si), the active layers (a-Si and n+ a-Si) that overlap the gate electrode  216  are patterned and formed on the gate insulator layer  303 . A planar pixel electrode  204  is formed on the gate insulator layer  303 . After deposition of an opaque metal layer, a source electrode  208  and a drain electrode  209  are formed. The source electrode  208  is connected to the pixel electrode  204 . 
     A passivation layer  304  is formed above the source electrode  208  and drain electrode  209 . A common electrode  206  having many stripes is formed on the passivation layer  304 . Many stripes of the common electrode  206  overlap the pixel electrode  204 , in which the passivation layer  304  insulates the common electrode  206  from the pixel electrode  204 . 
     A black matrix  305  that includes resin with carbon particles, a double layer of chrome and chrome oxide, or a chrome-oxidized layer, is formed on an upper substrate  306 . A color filter resin layer  307  is coated on the black matrix  305 . An over coat layer  308  is coated on the color filter resin layer  307 . 
     A lower alignment layer and an upper alignment layer (not shown in the figure) are printed on the inner surfaces of the lower substrate  301  and the upper substrate  306 , respectively. The lower substrate  301  having arrayed electrodes and the upper substrate  306  having the black matrix  305  and red-green-blue color filter patterns are disposed opposite to each other and spaced apart at a predetermined cell gap. Lower and upper polarizers (not shown in the figure) are attached to the outer surfaces of the lower  301  and upper  306  substrates, respectively. 
     Liquid crystal molecules  309  having a negative dielectric anisotropy are disposed between the substrates  301  and  306 . In the transmissive region (e.g.,  136  of  FIG. 6A ), the lower polarizer has a transmission axis parallel to an alignment axis of the lower alignment layer, and the upper polarizer has a transmission axis perpendicular to that of the lower polarizer. 
       FIG. 7B  is a cross sectional view of an example data bus line area along a line B-B′ in the reflective region  138  of the pixel of  FIG. 6A . A gate insulator layer  303  having embossing patterns  312  is formed on the lower substrate  301 . A reflector layer  310 , which is connected to a pixel electrode (not shown in the figure) in the transmissive region  136 , and a data bus line  202  are formed on the gate insulator layer  303 . 
     A passivation layer  304  is deposited above the reflector layer  310  and the data bus line  202 . A common electrode  206  having several stripes is formed and patterned on the passivation layer  304 . Many of the stripes of the common electrode  206  overlap the reflector  310 . One stripe  214  of the common electrode  206  overlaps the data bus line  202 , in which the stripe  214  and the data bus line  202  are electrically insulated from each other. An electric field generated by the data bus line  202  is shielded by the stripe  214  of the common electrode  206 . 
     A color pigment layer  307  and an over coating layer  308  are coated on the upper substrate  306 . A lower alignment layer and an upper alignment layer (not shown in the figure) are printed on the inner surfaces of the lower substrate  301  and upper substrate  306 , respectively. A liquid crystal mixture  309  having a negative dielectric anisotropy is injected into the space between the upper and lower alignment layers. 
       FIG. 7C  is a cross sectional view of an example data bus line area along a line C-C′ in the transmissive region  136  of the pixel of  FIG. 6A . A planar pixel electrode  204  and a data bus line  202  are formed on a gate insulator layer  303 . A passivation layer  304  is deposited above the pixel electrode  204  and the data bus line  202 . A common electrode  206  having many stripes  210  is formed and patterned on the passivation layer  304 . Many of the stripes  210  of the common electrode  206  overlap the pixel electrode  204 . One stripe  214  of the common electrode  206  covers the data bus line  202 , so an electric field generated by the data bus line  202  is shielded by the stripe  214 . 
     A color pigment layer  307  and an over coating layer  308  are coated on the upper substrate  306 . A lower alignment layer and an upper alignment layer (not shown in the figure) are printed on the inner surfaces of the lower substrate  301  and upper substrate  306 , respectively. A liquid crystal mixture  309  having negative dielectric anisotropy is injected into the space between the upper and lower alignment layers. 
       FIG. 7D  is a cross sectional view of an example boundary area  207  between the transmissive region  136  and the reflective region along a line D-D′ in the pixel of  FIG. 6A . The gate insulator layer  303  having embossing patterns  312  is formed patterned on the lower substrate  301 . The reflector layer  310  of the reflective region  138  and the transparent pixel electrode  204  are formed on the gate insulator layer  303 . The reflector layer  310  and the pixel electrode  204  are electrically connected. 
     A passivation layer  304  is deposited above the reflector layer  310  and the pixel electrode  204 . A common electrode  206  having many stripes  210  is formed and patterned on the passivation layer  304 . Many of the stripes  210  of the common electrode  206  overlap the reflector electrode  310  and the pixel electrode  204 . 
     A black matrix  305  is formed on the upper substrate  306 . A color pigment layer  307  and an over coating layer  308  are coated on the black matrix layer  305 . A lower alignment layer and an upper alignment layer (not shown in the figure) are printed on the inner surfaces of the lower substrate  301  and the upper substrate  306 , respectively. In the transmissive region  136 , the lower alignment layer is treated such that its alignment axis is aligned along the horizontal direction, and the top alignment layer is such that its alignment axis is parallel to but in the opposite direction with respect to the alignment direction of the lower alignment. 
     In the reflective region  138 , the lower alignment layer is photo-aligned such that its alignment axis is +45° or −45° with respect to the alignment axis of the lower alignment layer in the transmissive region  136 . The top alignment layer in the reflective region  138  is treated to have an alignment direction that is perpendicular to the top substrate surface. 
     The different surface alignment directions of the transmissive and reflective regions can be achieved by the following process. The lower alignment layer in the transmissive region  136  is photo-aligned by using a photo mask that is open only to the transmissive region  136  (so that in the reflective region  138 , the irradiation UV light or ion beam used for the photo-alignment process is shielded by the photo mask). The lower alignment layer in the reflective region  138  is photo-aligned by using another photo mask that is open only to the reflective region  138 , and the lower alignment layer in the reflective region  138  is photo-aligned to have an alignment direction that is different from the lower alignment layer in the transmissive region  136 . A similar process can be used to treat the upper alignment layer in the transmissive region  136  and the reflective region  138 . A liquid crystal mixture  309  having negative dielectric anisotropy is injected into the space between the upper and lower alignment layers. 
       FIG. 8  is a top view of an example pixel  220  of a transflective high-brightness FFS-LCD that is similar to the pixel structure  190  of  FIG. 6A  except that the pixel structure  220  has a barrier wall  401  at a boundary between the transmissive region  136  and the reflective region  138 . The barrier wall  401  is formed on the black matrix  250  to control light leakage in the transmissive region  136  in the black state. 
       FIG. 9A  is a cross sectional view of an example boundary area along a line E-E′ in the pixel  220  of  FIG. 8 , in which patterned green and blue pigment layers are used as barrier wall layers. In the example of  FIG. 9A , the pixel  220  includes a red pigment layer  307 . The structure in  FIG. 9A  is similar to that in  FIG. 7D , except that the pixel  220  has a barrier wall  501  that includes a patterned green barrier wall layer  501   a  and a blue barrier wall layer  501   b.  The gate insulator layer  303 , reflector layer  310 , pixel electrode  204 , common electrode  206 , black matrix  305 , color pigment layer  307 , over coating layer  308 , upper and lower alignment layers, and liquid crystal mixture  309  are the same for  FIGS. 7D and 9A . 
     The green barrier wall layer  501   a  and the blue barrier wall layer  501   b  are deposited and patterned at the same time that the green pigment layer and the blue pigment layer are deposited and patterned to form green and blue filters for the green and blue pixels, respectively. In the example shown in  FIG. 9A , the pixel is a red pixel. A red pigment layer  307  covering the whole pixel region (both transmissive and reflective regions) is formed on the upper substrate  306 . When a green pigment layer for a green pixel is being patterned, the green barrier wall layer  501   a  having a bar shape extending along the boundary between the transmissive region  136  and the reflective region  138  remains on the red pigment layer  307 . When a blue pigment layer for a blue pixel is being patterned, the blue barrier wall layer  501   b  having a bar shape remains on the green barrier wall layer  501   a.  An over coating layer  308  is coated over the whole area. Also, the patterned barrier wall layer  501  can function as a spacer to maintain the cell gap of the liquid crystal display. In some examples, the patterned barrier wall layer  501  has a height (H 1 ) of about 0.4˜3.2 μm and a width (W 1 ) of about 3˜20 μm. 
       FIG. 9B  is a cross sectional view of an example boundary area along a line E-E′ in the pixel  220  of  FIG. 8 , in which a patterned over coating layer  502  is used as a barrier wall layer. The gate insulator layer  303 , reflector layer  310 , pixel electrode  204 , common electrode  206 , black matrix  305 , color pigment layer  307 , over coating layer  308 , upper and lower alignment layers, and liquid crystal mixtures  309  are the same for  FIGS. 9A and 9B . The difference between the pixels of  FIGS. 9A and 9B  is that the pixel of  FIG. 9A  uses green and blue pigment layers to form a barrier wall layer, whereas the pixel of  FIG. 9B  uses a patterned over coating layer  502  to from a barrier wall layer. In the example of  FIG. 9B , an additional over coating layer  502  is used as a barrier wall layer. Also, the patterned barrier wall layer  502  can act as a spacer to maintain the cell gap of the liquid crystal display. In some examples, the over coating barrier wall layer  502  has a height (H 2 ) of about 0.4˜3.2 μm and a width (W 2 ) of about 3˜20 μm. 
       FIG. 10  shows examples of orientations of liquid crystal molecules and dimensions of the pixel structure of the transflective pixel  190  of  FIG. 6A . In the reflective region  138 , the stripes of the common electrode  206  extend in a direction at an angle β 1 =60 to 80° with respect to the gate bus line  201  (which extends in the row direction). In the transmissive region  136 , the stripes of the common electrode  206  extend in a direction at an angle β 2 =100 to 120° with respect to the gate bus line  201 . 
     In some examples, in the reflective region  138 , the stripes of the common electrode  206  each has a width w 1  (referred to as electrode width) of about 1 to 3 μm, and the spacing l 1  (referred to as electrode spacing) between the stripes of the common electrode  206  is about 2 to 4 μm. In the transmissive region  136 , the stripes of the common electrode  206  each has width w 2  of about 2 to 4 μm, and the spacing l 2  between the stripes of the common electrode  206  is about 4 to 6 μm. The electrode widths (w 1  and w 2 ) and electrode spacing (l 1  and l 2 ) are designed to achieve high light efficiency and a good matching between voltage-transmittance (V-T) and voltage-reflectance (V-R) curves. 
     Upper and lower alignment layers (not shown in the figure) are coated on the glass substrates. In the transmissive region  136 , the lower alignment layer is treated such that its alignment axis α 2  is at an angle of about 3 to 23° with respect to the x-direction (which is parallel to the row direction). The top alignment layer is treated such that its alignment axis is parallel to but in opposite direction with respect to the alignment direction of the lower alignment layer. 
     In the reflective region  138 , the lower alignment layer is photo-aligned such that its alignment axis α 1  is at an angle about +45° or −45° with respect to the alignment axis of the lower alignment layer in the transmissive region  136 , i.e., α 1 =α 2 ±45°. The top alignment layer in the reflective region  138  is treated such that it has a vertical tilt angle. 
     The common electrode stripes in the transmissive region  136  and the common electrode stripes in the reflective region  138  form a chevron shape having an angle (κ) about 120° to 160° from each other. In the transmissive region  136 , the bottom alignment layer has an angle about 12° with respect to a direction  218  that is perpendicular to the common electrode stripes in the transmissive region  136 . In the reflective region  138 , the bottom alignment layer also has an angle about 12° with respect to a direction  222  that is perpendicular to the common electrode stripes in the reflective region  138 . Such alignment directions are useful when a negative dielectric anisotropic liquid crystal is used. 
       FIG. 11A  shows examples of simulated V-R curve  601  and V-T curve  602  of the transflective pixel  190  of  FIG. 10 . The horizontal axis represents the pixel data voltage. The voltage used in this description refers to the root-mean-square voltage. In this example, the transmissive region  136  and the reflective region  138  have the same cell gap of about 3.77 μm. In the transmissive region  136 , the electrode width w 2  is equal to 3 μm and the electrode spacing l 2  is equal to 5 μm. In the reflective region  138 , the electrode width w 1  is equal to 2 μm and the electrode spacing l 1  is equal to 3 μm. The liquid crystal material used is MJ98468 from Merck, which has the following physical properties: extraordinary refractive index ne=1.5512 (at λ=589 nm), ordinary refractive index no=1.4742 (at λ=589 nm), dielectric anisotropy Δε=−4.0, rotational viscosity γ 1 =136 mPa·s, and elastic constants K 11 =13.5 pN, K 22 =7 pN and K 33 =15.1 pN. 
     Under the conditions described above, in the transmissive region  136 , Vth˜2.1 Vrms, Von ˜5.0 Vrms, and Tmax ˜80% (normalized to the maximum transmittance of two parallel linear polarizers), where Vth is the threshold voltage, and Von is the driving voltage. In this example, the maximum transmittance of two parallel linear polarizers is about 0.5. In the reflective region  138 , we find Vth˜2.0 Vrms, Von ˜4.6 Vrms, and Rmax ˜90% (normalized to the maximum reflectance of light after passing the one linear polarizer twice). Here, the maximum reflectance of light after passing the upper linear polarizer twice is about 0.5. When Tmax is about 80% and Rmax is about 90%, this means that the maximum value of transmittance or reflectance is about 0.4 or 0.45, as compared to the maximum value of 0.5. 
       FIG. 11B  shows a normalized reflectance curve  603  and a normalized transmittance curve  604 . The curves  603  and  604  match each other well for data voltages 0 to 5V. There is an almost perfect grayscale match between operating the display in the transmissive and reflective modes. This allows the driving of both transmissive and reflective modes using a single gamma curve. 
       FIG. 11C  shows example simulated 2-dimensional brightness images shown by the pixel  190  of  FIG. 10 . The parameters of the pixel  190  are the same as those used in the simulations for  FIG. 11A . In this example, the pixel  190  is configured to have 256 gray levels between its full bright state (represented by L255 gray level) and full dark state (represented by L0 gray level). When a pixel voltage that corresponds to the L0 gray level is applied to the pixel  190 , the transmissive and reflective regions  136  and  138  show dark images  603   a  and  603   b,  respectively. When a pixel voltage that corresponds to L127 gray level is applied to the pixel  190 , the transmissive and reflective regions  136  and  138  show gray images  605   a  and  605   b,  respectively. When a pixel voltage that corresponds to the L255 gray level is applied to the pixel  190 , the transmissive and reflective regions  136  and  138  show bright and uniform white images  604   a  and  604   b,  respectively. 
       FIG. 11D  shows an example simulated iso-contrast contour graph  608  of a display having pixels  190  of  FIG. 10  and operating in the transmissive mode. The graph  608  simulates the viewing angles of the display in the transmissive mode without using any compensation films. The graph  608  shows that the display can achieve a 10:1 contrast ratio in the transmissive mode without grayscale inversion within a viewing cone greater than 60°. 
       FIG. 11E  shows an example simulated iso-contrast contour graph  606  of a display having pixels  190  of  FIG. 10  and operating in the reflective mode. The graph  606  simulates the viewing angles of the display in the reflective mode without using any compensation films. The graph  606  shows that the display can achieve a 10:1 contrast ratio in the reflective mode without grayscale inversion within a viewing cone greater than 45°. These viewing angles are adequate for displays used in, e.g., mobile devices. 
       FIG. 12A  shows example simulated liquid crystal orientations of a pixel in which a barrier wall is not used. The pixel structure used for generating the simulation in  FIG. 12A  is the same as the pixel  190  in  FIG. 10 . The pixel includes a reflector  310 , a pixel electrode  204 , a common electrode  206 , and liquid crystals  309 , similar to those in  FIGS. 7A to 7D . 
     In the transmissive region  136 , the liquid crystal molecules are oriented mostly parallel to the surface of the upper and lower substrates  110  and  100 . In the reflective region  138 , the liquid crystal molecules (e.g.,  336 ) near the lower substrate  100  are mostly oriented parallel to the surface of the substrate  100 , whereas the liquid crystal molecules (e.g.,  338 ) near the upper substrate  110  are mostly oriented perpendicular to the surface of the substrate  110 . Some of the liquid crystal molecules (e.g.,  320 ) in the transmissive region  136  located near a boundary  334  of the T and R regions are influenced by the liquid crystal molecules (e.g.,  330 ) in the R region  138 . As a result, some of the liquid crystal molecules in the T region  136  near the boundary  334  tilt at angles larger than the liquid crystal molecules (e.g.,  332 ) that are located farther away from the boundary  334 , resulting in light leakage at the boundary  334 . 
       FIG. 12B  shows a simulated image shown by the pixel of  FIG. 12A  in a dark state. The pixel has a region  701  that has light leakage, which can be blocked by using a black matrix. 
       FIG. 12C  shows example simulated liquid crystal orientations of a pixel in which a barrier wall is used. The pixel structure used for generating the simulation in  FIG. 12C  is the same as the pixel  220  in  FIG. 8 . In the simulation, the pixel includes a reflector  310 , a pixel electrode  204 , a common electrode  206 , liquid crystals  309 , and a patterned barrier wall layer  501  or  502 , similar to those in  FIG. 9A  or  FIG. 9B . 
     When a barrier wall  501  or  502  is used, the influence on the liquid crystal molecules (e.g.,  320 ) in the T region  136  near the boundary  334  by the liquid crystal molecules (e.g.,  330 ) in the R region  138  is reduced. The liquid crystal molecules (e.g.,  320 ) in the T region  136  near the boundary  334  maintain substantially the same orientation as the liquid crystal molecules (e.g.,  332 ) that are located farther away from the boundary between the T and R regions. As a result, the light leakage is reduced. 
       FIG. 12D  shows a simulated image shown by the pixel of  FIG. 12C  in a dark state. The pixel has a region  702  that has light leakage, but the region  702  is smaller than the region  701  of  FIG. 12B . The black matrix used to block the region  702  can have a smaller area than the black matrix used to block the region  701 . Thus, by using the barrier wall  501  or  502 , light leakage can be reduced, and the area of the black matrix can be reduced, increasing the aperture ratio of the display. 
     In some examples, the patterned barrier  501  or  502  can have a height H equal to about 0.4˜3.2 μm and a width W equal to about 3˜20 μm. In the example used to generate the simulations of  FIGS. 12C and 12D , the barrier wall  501  or  502  has a height H equal to 1.8 μm and a width W equal to 5 μm. 
       FIG. 13A  shows an example simulated V-T curve  801  and an example simulated V-R curve  802  of a pixel  810 , whose structure is shown in  FIG. 13B . The pixel  810  includes an array substrate  811 , a common electrode  812  that is connected to a reflector electrode  813 , a pixel electrode  816 , a gate insulator layer  814  and a passivation layer  815  to reduce the parasitic capacitance between the data line and the striped pixel electrodes. The total thickness of the insulation layer  814  and the passivation layer  815  is about 400 nm. The common electrode  812  has a planar shape, whereas the pixel electrode  816  has many stripes. The portion of the pixel electrode  816  in the T region  136  is referenced as  816   a,  and the portion of the pixel electrode  816  in the R region  138  is referenced as  816   b.  The electrode width and electrode spacing of the stripes of the pixel electrode  816  in the T region  136  and the R region  138  are different. The pixel electrode  816  is positioned between the common electrode  812  (and the reflector electrode  813 ) and the liquid crystal layer. As can be seen in  FIG. 13A , the V-T curve  801  does not match the V-R curve  802  very well. 
     For the simulation of  FIG. 13A , the pixel  810  has a single cell gap of 3.77 mm, and a negative dielectric liquid crystal material (MJ98468) is used. In the T region  136 , the pixel electrode  816   a  has a width w=3 μm and an electrode spacing l=5 μm. In the R region  138 , the pixel electrode  816   b  has a width w=2 μm and an electrode spacing l=3 μm. 
       FIG. 13C  shows an example simulated V-T curve  803  and an example simulated V-R curve  804  of a pixel  320 , whose structure is shown in  FIG. 13D , in which a pixel-common inversion electrode structure is used. The common electrode  206  is positioned between the liquid crystal layer and the pixel electrode  204  (and the reflector electrode  310 ). The pixel  320  has a structure similar to the pixel shown in  FIG. 7D . 
     For the simulation of  FIG. 13C , the pixel  320  has a single cell gap of 3.77 μm, and a negative dielectric liquid crystal material (MJ98468) is used, similar to those used for the simulation of  FIG. 13A . In the T region  136 , the stripes of the common electrode  206  has a width w=3 μm and a spacing l=5 μm. In the R region  138 , the stripes of the common electrode  206  has a width w=2 μm and a spacing l=3 μm. The pixel  320  includes an array substrate  301 , a pixel electrode  204  that is connected to a reflector electrode  310 , and a common electrode  206 . An insulator layer  304  having a thickness of about 200 nm is positioned between the common electrode  206  and the pixel electrode  204  (and reflector electrode  310 ). 
     As shown in  FIG. 13A , when the pixel  810  that includes insulators  814  and  815  having a thickness of 400 nm is used, the pixel driving voltage corresponding to the bright state (highest luminance) is about 5.5V. If the negative dielectric anisotropy liquid crystal material is replaced with a positive dielectric anisotropy liquid crystal material, the pixel driving voltage that corresponds to the bright state can be about 4.6V. 
     As shown in  FIG. 13C , when the pixel  320  that includes the insulator layer  304  having a thickness of 200 nm is used, the pixel driving voltage that corresponds to the bright state is about 4.7V. The pixel-common electrode inversion electrode structure allows a thinner insulator layer  304  to be used, allowing the pixel driving voltage for the bright state to be reduced from about 5.5 V (as shown in  FIG. 13A ) to about 4.7 V (as shown in  FIG. 13C ).  FIG. 13C  also shows a good match between the curves  803  and  804 , indicating that the display will have a good match in gray scale when operating in the transmissive and reflective modes. This allows the display to be driven with single gamma curve for both transmissive and reflective modes. 
       FIG. 14A  shows a cross sectional diagram of an example reflective region  138  of a pixel  340 . Relationships between liquid crystal molecule tilt angles and rotation angles at different locations in the reflective region  138 , e.g., locations A, B, and C in  FIG. 14A  for different pixel structures are simulated. The simulation results are shown in  FIGS. 14B to 14E . The pixel  340  includes a first ITO electrode  342  and a second ITO electrode  344 . For the simulations shown in  FIGS. 14B and 14C , the first ITO electrode  342  functions as a common electrode, and the second ITO electrode  344  functions as a reflector electrode. For the simulations shown in  FIGS. 14D and 14E , the first ITO electrode  342  functions as a reflector electrode, and the second ITO electrode  344  functions as a common electrode. 
       FIG. 14B  is a graph  350  showing simulated relationships between the tilt angle of liquid crystal molecules and the cell gap for cell gaps in a range between 0 to 3 μm. Curves  352 ,  354 , and  356  represent relationships between the tilt angle and the cell gap at locations A, B, and C ( FIG. 14A ), respectively. The curves  352 ,  354 , and  356  do not match very well. 
       FIG. 14C  is a graph  360  showing simulated relationships between rotation angle of liquid crystal molecules and the cell gap for cell gaps in a range between 0 to 3 μm. Curves  362 ,  364 , and  366  represent relationships between the rotation angle and the cell gap at locations A, B, and C ( FIG. 14A ), respectively. The curves  362 ,  364 , and  366  do not match very well. 
     For the simulations in both  FIGS. 14B and 14C , the pixel  340  ( FIG. 14A ) corresponds to the reflective region  138  of the pixel  810  in  FIG. 13B . The first ITO electrode  342  functions as the common electrode  813  ( FIG. 13B ), and the second ITO electrode  344  functions as the reflector electrode  816   b  ( FIG. 13B ). The reflector electrode has many stripes, in which the electrode width w is 2 μm and the electrode spacing l is 3 μm. The cell gap is 2.77 μm, and a positive dielectric anisotropy liquid crystal material is used. The pixel includes an array substrate  811 , a common electrode  812  that is connected to a reflector electrode  813 , a pixel electrode  816   a,  a gate insulator  814 , and a passivation layer  815 . The insulation layers  814  and  815  between the pixel electrode  816   a  and the common electrodes  812  (and reflector electrode  816   b ) is 400 nm. 
       FIG. 14D  is a graph  370  showing simulated relationships between the tilt angle of liquid crystal molecules and the cell gap for cell gaps in a range between 0 to 4 μm. The curves representing relationships between the tilt angle and the cell gap at locations A, B, and C ( FIG. 14A ) match well for cell gaps in a range from about 1.8 μm to 4 μm. 
       FIG. 14E  is a graph  380  showing simulated relationships between rotation angle of liquid crystal molecules and the cell gap for cell gaps in a range between 0 to 4 μm. The curves represent relationships between the rotation angle and the cell gap at locations A, B, and C ( FIG. 14A ) match well for cell gaps in a range from about 1.8 μm to 3.8 μm. 
     For the simulations in both  FIGS. 14D and 14E , the pixel  340  ( FIG. 14A ) corresponds to the reflective region  138  of the pixel  320  in  FIG. 13D . The first ITO electrode  342  functions as the reflector electrode  310  ( FIG. 13D ), and the second ITO electrode  344  functions as the common electrode  206 . The common electrode has many stripes, in which the electrode width w is 2 mm and the electrode spacing l is 3 mm. The cell gap is 3.77 μm, and a negative dielectric anisotropy liquid crystal material is used. The pixel includes an array substrate  301 , a pixel electrode  204  that is connected to the reflector electrode  310 , the common electrode  206 , and an insulator layer  304 . The insulator layer  304  has a thickness of 200 nm. 
     Comparing the simulation results in  FIGS. 14B and 14C  with those shown in  FIGS. 14D and 14E  indicates that using the pixel-common inversion electrode structure (shown in  FIG. 13D ) and a negative dielectric liquid crystal material results in the same (or almost the same) tilt angles and rotation angles for the whole pixel area (e.g., locations A, B, and C in  FIG. 14A ) for cell gaps ranging from about 1.8 μm to 3.8 μm. A pixel using the pixel-common inversion electrode structure and a negative dielectric anisotropy liquid crystal material can achieve a uniform high reflectance in the reflective region  138 . 
     The transflective LCD using pixel structures shown in  FIGS. 3 ,  4 ,  6 - 10 , and  13 D can have one or more of the following advantages.
         A single cell gap can be used for the transmissive region  136  and the reflective region  138  of the transflective pixel. Manufacturing processes can be simplified, and the display can have a higher contrast performance.   A wide viewing angle can be achieved. The fringe field switching liquid crystal display using the pixel-common inversion electrode structure can achieved a 10:1 contrast ratio without grayscale inversion within a viewing cone greater than 60° in the transmissive mode, and over within a viewing cone greater than 45° in the reflective mode, both without using any compensation films. The viewing angles are adequate for various applications, such as for use in mobile devices.   A high transmittance and a high reflectance can be achieved. By using a barrier wall (e.g.,  501  of  FIG. 9A  or  502  of  FIG. 9B ), the display can use a black matrix with a small area so each pixel can have a high aperture ratio. A maximum transmittance of about 80% and a maximum reflectance of about 90% can be achieved. The display can have a good match between V-T and V-R characteristics so that the pixels can be driven using a single gamma curve. The matching between the V-T and V-R characteristics can be achieved by using, e.g., a negative dielectric anisotropy liquid crystal material, a pixel-common inversion electrode structure, a common electrode with stripes, and a hybrid aligned nematic cell configuration in the reflective region.   A low pixel driving voltage can be used. By using the pixel-common inversion electrode structure, a thin insulator layer can be used between the common electrode and the pixel electrode (and the reflector electrode), so that a low pixel voltage can be used to drive the pixel. For example, in the bright state, the operation voltage can be about 5.0 Vrms in the transmissive mode and about 4.6 Vrms in the reflective mode.   It is not necessary to use compensation layers or in-cell retarders to achieve a good viewing angle. This simplifies the fabrication process for producing the display.       

     Other embodiments are within the scope of the following claims. Additional layers can be used in the displays described above. The components of the displays, such as the liquid crystal layer, the polarization films, and the alignment layers, can use materials and have parameters different from those described above. The common electrode  105  does not necessarily have to be connected to a ground reference voltage. When the display is operating in the transmissive mode in which the backlight unit  130  is turned on, some ambient light may be reflected by the reflective pixel electrode, so the display can operate in both the transmissive and reflective modes at the same time. The electrode widths and electrode spacing can be different from those described above. The geometry of the common electrode can be different from those shown in  FIGS. 6 ,  8 , and  10 . For example, the openings and the stripes in the common electrode can have varying widths, can be curved, and can have various shapes. In the example of  FIG. 9A , the pigment layer  307  is a red pigment layer, and the barrier wall  501  includes overlapping layers of blue and green pigment layers. The pigment layer  307  can be a green pigment layer, in which the barrier wall  501  includes overlapping layers of red and blue pigment layers. The pigment layer  307  can also be a blue pigment layer, in which the barrier wall  501  includes overlapping layers of red and green pigment layers. 
     The orientations of the liquid crystal molecules described above refer to the directions of directors of the liquid crystal molecules. The molecules do not necessarily all point to the same direction all the time. The molecules may tend to point more in one direction (represented by the director) over time than other directions. For example, when we say the liquid crystal molecules are aligned along a particular direction, we mean that the average direction of the directors of the liquid crystal molecules is generally aligned along the particular direction, but the individual molecules may point to different directions. When we say the liquid crystal molecules in the transmissive region has a homogeneous alignment, we mean that the average direction of the directors of the liquid crystal molecules in the transmissive region is generally aligned along the same direction, but the individual molecules may point to different directions.