Abstract:
Single-layered color cholesteric liquid crystal display devices and fabrication methods thereof are provided. The color cholesteric liquid crystal display device includes a first substrate structure having a base and periodic protrusion banks. A second substrate structure is disposed opposite the first substrate structure to enclose the periodic protrusion banks, and divide a plurality of color sub-pixel channels. A plurality of color cholesteric liquid crystals are respectfully filled in each of the color sub-pixel channel, wherein the base and the periodic protrusion banks are made of continuously integral material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from a prior Taiwanese Patent Application No. 097111653, filed on Mar. 31, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to liquid crystal display (LCD) devices, and in particular to single layer color cholesteric liquid crystal display devices and fabrication methods thereof. 
     2. Description of the Related Art 
     Liquid crystal display (LCD) devices have many advantages such as a smaller size, lighter weight and lower power consumption, and are applicable in a variety of electronic and communication devices including notebook computers, personal digital assistants (PDA), mobile phones and the like due to its lighter weight, thinner profile, and portability. Conventional reflective memorable color liquid crystal display devices are widely applicable in electronic books, electronic papers, and the likes. The structures and fabrication methods of conventional cholesteric liquid crystal display devices use a tri-layered red (R), green (G), and blue (B) pixel stacked structure corresponding to various different driving methods. However, stacking tri-layered R, G, and B pixels may result in optical aberration and misalignment during fabrication. Moreover, the tri-layered R, G, and B liquid crystal layer stacked structure are so complicated that layout of electrodes is difficult to design and an LCD panel using the stacked structure has a rigid port, resulting in an intricate fabrication process and high fabrication costs. 
     Accordingly, the use of single layer color cholesteric liquid crystal display devices and fabrication methods thereof can effectively ameliorate optical aberration, simplify the fabrication process, and reduce fabrication costs. Particularly, the device is applicable to the field of color flexible LCD devices. U.S. Pat. No. 5,825,451, the entirety of which is hereby incorporated by reference, discloses a single layer color cholesteric LCD device using a combination of light decomposable/polymerizable chiral agents and a single layer cholesteric LC structure. The single layer color cholesteric LCD device is illuminated by a UV light disrupting or reducing contents of chiral agent at a single region to achieve colorizing of a single layer cholesteric LCD device. However, the single layer color cholesteric LCD device is easily affected by ambient light deteriorating display stability thereof. 
     U.S. Pat. No. 6,741,321, the entirety of which is hereby incorporated by reference, discloses an LCD device using a single LC layer and a double substrate assembly process. Different color LC materials are respectively injected into LC channels during fabrication. However, the single layer LCD assembly is not sealed enough such that overflow occurs between the adjacent LC channels resulting in color mixing and color saturation. 
       FIG. 1A  is a cross section of a conventional single layer color cholesteric LCD device. Referring to  FIG. 1A , a conventional single layer color cholesteric LCD panel  2  includes a lower substrate  6  and an upper substrate  12  opposed to each other and with a gap therebetween. An enclosed structure  8  is interposed between the lower substrate  6  and the upper substrate  12 , dividing a plurality of stripe color sub-pixel LC channels. A first electrode  4  and an alignment layer  14  are disposed on the lower substrate  6 . A second electrode  10  and an alignment layer  14  are disposed on the upper substrate  12 . The first electrode  4  and the second electrode  10  are substantially perpendicular to each other. 
       FIG. 1B  is a schematic view of the lower substrate of the conventional single layer color cholesteric LCD device of  FIG. 1A . In  FIG. 1B , the enclosed structure  8  divides a plurality of (R, G, B) stripe color sub-pixel LC channels C R , C G , and C B . Perpendicularly crossed segments  16  and  18  are disposed on one end of the LC channels C G  and C B  to seal the LC channels. After the lower substrate  6  and upper substrate  12  are assembled, the second end L 2  of the enclosed structure  8  are sealed by a sealant enclosing the LC channels C G  and C B , while the first end L 1  of the LC channels C R  remains opened to serve as an LC injection opening. Each of the LC channels C R , C G , and C B  is sequentially injected with respective color LCs and sealed. Before filling each of the single layer color LC channels, however, conventional methods require sealing of the second end of the enclosed structure. Furthermore, when separately injecting LC into the LC channels, different color LCs may overflow to adjacent LC channels and mix, thus resulting in color mixing and color saturation. 
     BRIEF SUMMARY OF THE INVENTION 
     The features and aspects of the invention are related to single layer color cholesteric liquid crystal display devices and fabrication methods thereof. A bank portion and a base portion of the first substrate are optionally designed as a continuous unified structure, creating different length liquid crystal (LC) channels. An adhesion layer is optionally formed on another opposing substrate or the bank portion. The two opposing substrate are assembled. The assembled structure is sequentially cut, injected red, green, and blue LC, and sealed to prevent color mixing and reduce color saturation. 
     Embodiments of the invention provide a color cholesteric liquid crystal display device, comprising: a first substrate with a base portion and a periodic protruding bank portion; a second substrate opposing the first substrate to create a plurality of color sub-pixel channels; and a plurality of color cholesteric liquid crystals respectively filled in each of the color sub-pixel channels, wherein the base portion and the periodic protruding bank portion are a continuous unified structure. 
     Embodiments of the invention also provide a fabrication method for a color cholesteric liquid crystal display device, comprising: providing a first substrate; forming a patterning mask layer on the first substrate; etching the first substrate to create a base portion and a periodic protruding bank portion; forming a first conductive layer on the first substrate; removing the mask layer and the first conductive layer thereon, leaving a first electrode on the base portion along a first direction; providing a second substrate with a second electrode along a second direction; assembling the first substrate and the second substrate to adhere the periodic protruding bank portion to the second substrate and divide a plurality of color sub-pixel channels; and sequentially injecting a plurality of color cholesteric liquid crystals in the corresponding color sub-pixel channels and sealing each of the color sub-pixel channels with a sealant material. 
     Embodiments of the invention further provide a fabrication method for a color cholesteric liquid crystal display device, comprising: providing a first substrate; forming a thin-film layer on the first substrate; imprinting the first substrate with a template to create a base portion and a periodic protruding bank portion; depositing a first conductive layer on the first substrate; removing the thin-film layer on the bank portion and the first conductive layer on the thin-film layer, leaving a first electrode on the base portion along a first direction; providing a second substrate with a second electrode along a second direction; assembling the first substrate and the second substrate to adhere the periodic protruding bank portion to the second substrate and divide a plurality of color sub-pixel channels; and sequentially injecting a plurality of color cholesteric liquid crystals in the corresponding color sub-pixel channels and sealing each of the color sub-pixel channels with a sealant material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1A  is a cross section of a conventional single layer color cholesteric LCD device; 
         FIG. 1B  is a schematic view of the lower substrate of the conventional single layer color cholesteric LCD device of  FIG. 1A ; 
         FIG. 2A  is a flowchart illustrating an exemplary embodiment of a fabrication method for a single layer color cholesteric LCD device according to the invention; 
         FIG. 2B  is a flowchart illustrating another embodiment of a fabrication method for a single layer color cholesteric LCD device according to the invention; 
         FIGS. 3A-3H  are schematic views of an exemplary embodiment of each fabrication method step for a single layer color cholesteric LCD device of the invention; 
         FIGS. 4A-4F  are schematic views of another exemplary embodiment of each fabrication method step for a single layer color cholesteric LCD device of the invention; 
         FIG. 5  is a plan view of an embodiment of the enclosed structure of the invention; 
         FIGS. 6A-6D  are cross sections of several embodiments of the substrate structure assembly filled with liquid crystal; 
         FIGS. 7A-7C  are schematic views of an embodiment of each step for injecting each color LC into respective LC channels of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 2A  is a flowchart illustrating an exemplary embodiment of a fabrication method for a single layer color cholesteric LCD device according to the invention. Referring to  FIG. 2A , preparation of a first substrate structure (such as a lower substrate structure) is initially performed including providing a first substrate (step S 210 ) and forming a patterning mask on the first substrate (step S 212 ). Subsequently, the first substrate is etched to create a protruding bank structure on the first substrate surface (step S 214 ). For example, the first substrate is etched to create a base portion and periodic protruding bank portions, wherein trenches are divided therebetween. A conductive layer, serving as a first electrode, is blankly formed on the first substrate (step S 216 ). The mask and part of the conductive layer on the bank portion are removed (step S 218 ), thereby leaving a first electrode along a first direction on the first substrate. 
     Subsequently, preparation of a second substrate structure (such as an upper substrate structure) is performed including providing a second substrate (step S 220 ) and forming a patterned second electrode along the second direction on the second substrate (step S 222 ). An adhesion layer is formed overlying the second substrate (step S 226 ). 
     The first and second substrate structures are assembled opposed to each other with a gap interposed therebetween (step S 230 ). The patterned bank portions and the adhesion layer are tightly combined to prevent mixtures of color LCs between adjacent LC channels from overflowing. A first color cholesteric LC is filled in a first stripe LC channel and then sealed (step S 240 ). The assembly structure is cut to expose a second stripe LC channel (step S 250 ). A second color cholesteric LC is filled in the second stripe LC channel and then sealed (step S 260 ). The assembly structure is cut to expose a third stripe LC channel (step S 270 ). A third color cholesteric LC is filled in the third stripe LC channel and then sealed (step S 280 ). After all the three color cholesteric LCs are filled and sealed, fabrication of the single layer color cholesteric LCD device is completed (step S 290 ). 
     According to another embodiment of the invention, preparation of another first substrate structure (such as a lower substrate structure) is optionally or alternatively performed including providing a first substrate (step S 310 ) and forming a thin-film layer, such as photoresist, on the first substrate (step S 312 ). The thin-film layer and the first substrate are imprinted by a template to create a protruding bank structure on the surface of the first substrate (step S 314 ). The bank structure includes a base portion and a periodic protruding bank portion. Next, a conductive layer, serving as a first electrode, is blankly formed on the first substrate (step S 316 ). The thin film layer and part of the conductive layer on the bank portion are removed (step S 318 ), thereby leaving a first electrode along a first direction on the first substrate, as shown in  FIG. 2B . Other fabrication steps (S 320 - 390 ) are substantially similar to the corresponding steps of the fabrication method of  FIG. 2A , and for simplicity, their detailed descriptions are omitted. 
     According to another embodiment of the invention, if the first substrate is adhesive, formation of the adhesion layer on the second substrate (step S 226 ) can be omitted. The first and second substrate can thus be tightly combined. 
       FIGS. 3A-3F  are schematic views of an exemplary embodiment of each fabrication method step for a single layer color cholesteric LCD device of the invention. Referring to  FIG. 3A , a first substrate  410  is provided. A patterned mask layer  420  is formed on the first substrate  410 . For example, a patterned photoresist is lithographically formed on the first substrate. Subsequently, an etching step  425  is performed. By using the patterned mask layer  420  as an etching mask, the first substrate is etched comprising a base portion  410 ′ and periodic protruding bank portions  412 . The periodic protruding bank portions  412  can be composed of trenches  415  to serve as liquid crystal channels, as shown in  FIG. 3B . The first substrate can be made of rigid substrates or flexible substrates. For example, the flexible substrates comprise polycarbonate (PC) substrates, polyethersulfone (PES) substrates, polyethylene terephthalate (PET) substrates, and polyimide (PI) substrates. Circuitry components such as thin film transistors (TFTs) and capacitors to control pixel electrodes can be optionally formed on the first substrate. 
     Next, referring to  FIG. 3C , a first conductive layer is formed on the first substrate, separately deposited on part of the conductive layer  430   a  on the base portion  410 ′ within the trenches  415  and deposited on part of the conductive layer  430   b  on the mask layer  420  over the bank portions  412 . Subsequently, the patterned mask layer  420  and the overlaid conductive layer  430   b  are removed, leaving a first electrode along a first direction on the base portion  410 ′, as shown in  FIG. 3D . 
     Referring to  FIG. 3E , a second substrate  450  with second electrodes  460  along a second direction thereon is provided. Note that the first direction and the second direction are substantially perpendicular to each other, thereby creating a passive matrix pixel array. According to an embodiment of the invention, an adhesion layer  470  can be blankly formed on the second substrate  450 . Next, the first substrate and second substrate are assembled opposing each other such that the periodic protruding bank portions  412  and the adhesion layer  470  are tightly combined. A plurality of color sub-pixel channels are thus divided by the periodic protruding bank portions  412 . A plurality of color cholesteric liquid crystals are sequentially injected into the corresponding color sub-pixel channels and each of the color sub-pixel channels are sealed with a sealant material. 
     According to another embodiment of the invention, a patterned adhesion layer  475  can be selectively formed on the second substrate  450 , as shown in  FIG. 3F . The patterned adhesion layer  475  can be made of a glue material or a solidified material. For example, the solidified material comprises a light solidified material or a thermoset material. The thickness of the adhesion layer  475  is less than the thickness (height) of the patterned enclosed structure  412 . Further, the adhesion layer is free from the areas corresponding to each of the color sub-pixel channels to reduce driving voltages and to improve contrast ratio. Note that the patterned adhesion layer can be directly formed on the patterned bank portions of the first substrate. By using precisely controlled position and inkjet techniques, the adhesion layer can be directly formed on the corresponding bank portions, as shown in  FIG. 3G . 
     According to another embodiment of the invention, if the first substrate  410  is adhesive, formation of the adhesion layer on the second substrate  450  can be omitted. The bank portions of the first substrate are tightly combined with the second substrate, as shown in  FIG. 3H . 
       FIGS. 4A-4F  are schematic views of another exemplary embodiment of each fabrication method step for a single layer color cholesteric LCD device of the invention. Referring to  FIG. 4A , a first substrate  510  is provided. A thin-film layer  520 , such as photoresist is formed on the first substrate  510 . The thin-film layer  520  and the first substrate  510  are imprinted by a template  525  to create a base portion  510 ′ and periodic protruding bank portions  512 . The periodic protruding bank portions  512  can be composed of trenches  515  to serve as liquid crystal channels, as shown in  FIG. 4B . The first substrate can be made of rigid substrates or flexible substrates. For example, the flexible substrates comprise polycarbonate (PC) substrates, polyethersulfone (PES) substrates, polyethylene terephthalate (PET) substrates, and polyimide (PI) substrates. Circuitry components such as thin film transistors (TFTs) and capacitors to control pixel electrodes can be optionally formed on the first substrate. 
     Next, referring to  FIG. 4C , a first conductive layer is formed on the first substrate, separately deposited on part of the conductive layer  530   a  on the base portion  510 ′ within the trenches  515  and deposited on part of the conductive layer  530   b  on the thin-film layer  520   b  over the bank portions  512 . Subsequently, the thin-film layer  520   b  and the overlaid conductive layer  530   b  over the bank portion  512  are removed, leaving a first electrode along a first direction on the base portion  510 ′, as shown in  FIG. 4D . 
     Referring to  FIG. 4E , a second substrate  550  with second electrodes  560  along a second direction thereon is provided. Note that the first direction and the second direction are substantially perpendicular to each other, thereby creating a passive matrix pixel array. According to an embodiment of the invention, an adhesion layer  570  can be blankly formed on the second substrate  550 . Next, the first substrate  510  and second substrate  550  are assembled opposing each other such that the periodic protruding bank portions  512  and the adhesion layer  570  are tightly combined. A plurality of color sub-pixel channels are thus divided by the periodic protruding bank portions  412 . A plurality of color cholesteric liquid crystals are sequentially injected into the corresponding color sub-pixel channels and each of the color sub-pixel channels are sealed with a sealant material. 
     According to another embodiment of the invention, a patterned adhesion layer  575  can be selectively formed on the second substrate  550 , as shown in  FIG. 4F . The patterned adhesion layer  575  can be made of a glue material or a solidified material. For example, the solidified material comprises a light solidified material or a thermoset material. The thickness of the adhesion layer  575  is less than the thickness (height) of the patterned enclosed structure  512 . Further, the adhesion layer is free from the areas corresponding to each of the color sub-pixel channels to reduce driving voltages and to improve contrast ratio. Note that the patterned adhesion layer can be directly formed on the patterned bank portions of the first substrate. By using precisely controlled position and inkjet techniques, the adhesion layer can be directly formed on the corresponding bank portions. 
     According to another embodiment of the invention, if the first substrate  510  is adhesive, formation of the adhesion layer on the second substrate  550  can be omitted. The bank portions of the first substrate are tightly combined with the second substrate. 
       FIG. 5  is a plan view of an embodiment of the enclosed structure of the invention. In  FIG. 5 , a patterned enclosed structure  620  comprises a plurality of stripe wall structures  610 . One end of each stripe wall structures  610  connects to and is perpendicular to a straight end line  622  and the other end of the stripe wall structures  610  connects to a bulk region  640 , thereby dividing a first LC channel C 1  with a first LC injection opening, a second closed LC channel C 2 , and a third closed LC channel C 3 . The length of the first LC channel C 1  exceeds that of the second LC channel C 2 , and the length of the second LC channel C 2  exceeds that of the third LC channel C 3 . The bulk region  640  can enhance adhesion between the patterned enclosed structure and the adhesion layer, thereby preventing LC overflow between adjacent LC channels during injection of the color LCs. 
       FIGS. 6A-6D  are cross sections of several embodiments of the substrate structure assembly filled with liquid crystal. Referring to  FIG. 6A , the combination of the first and second substrate structure which is filled with liquid crystal includes the first substrate  410  and second substrate  450  opposed to each other with a plurality of parallel LC channel for containing the respective color cholesteric LCs interposed therebetween. Each LC channel corresponds to sub-pixel regions  435 R,  435 G, and  435 B of the single layer color cholesteric LCD device. A first electrode  430   a  such as a pixel electrode along a first direction is disposed on the base portion  410 ′ of the first substrate, and a second electrode  460  such as a common electrode along a second direction is disposed on the second substrate  450 , and the first direction and the second direction are substantially perpendicular to each other. An adhesion layer  470  can be optionally disposed between the bank portions  412  of the first substrate and the second substrate  450  such that the adhesion layer is tightly adhered so as to prevent LC overflow between adjacent LC channels during injection of color LCs. 
     According to another embodiment of the invention, a patterned adhesion layer  475  corresponding the periodic bank portions can be optionally disposed between the bank portions  412  of the first substrate and the second substrate  450  such that the adhesion layer is tightly adhered so as to prevent LC overflow between adjacent LC channels, as shown in  FIG. 6B . Since the adhesion layer is free from the areas corresponding to each of the color sub-pixel channels, driving voltages can be thus reduced and contrast ratio can be increased. 
     According to another embodiment of the invention, if the bank portions  412  of the first substrate are adhesive, formation of the adhesion layer on the second substrate  450  can be omitted. The bank portions of the first substrate are tightly combined with the second substrate. 
     Referring to  FIG. 6C , the combination of the first and second substrate structure which is filled with liquid crystal includes the first substrate and second substrate opposed to each other with a plurality of parallel LC channel for containing respective color cholesteric LCs interposed therebetween. Each LC channel corresponds to sub-pixel regions  535 R,  535 G, and  535 B of the single layer color cholesteric LCD device. Part of remaining thin-film layer  520   a  and a first electrode  530   a  such as a pixel electrode along a first direction are disposed on the base portion  510  of the first substrate, and a second electrode  560  such as an electrode along a second direction is disposed on the second substrate  550 , and the first direction and the second direction are substantially perpendicular to each other. An adhesion layer  570  can be optionally disposed between the bank portions  512  of the first substrate and the second substrate  550  such that the adhesion layer is tightly adhered so as to prevent LC overflow between adjacent LC channels during injection of color LCs. 
     According to another embodiment of the invention, a patterned adhesion layer  575  corresponding the periodic bank portions can be optionally disposed between the bank portions  512  of the first substrate and the second substrate  550  such that the adhesion layer is tightly adhered so as to prevent LC overflow between adjacent LC channels, as shown in  FIG. 6D . Since the adhesion layer is free from the areas corresponding to each of the color sub-pixel channels, driving voltages can be thus reduced and contrast ratio can be increased. 
     According to another embodiment of the invention, if the bank portions  512  of the first substrate are adhesive, formation of the adhesion layer on the second substrate  550  can be omitted. The bank portions of the first substrate are tightly combined with the second substrate. 
       FIGS. 7A-7C  are schematic views of an embodiment of each step for injecting each color LC into respective LC channels of the invention. Referring to  FIG. 7A , a first color (e.g., red) cholestic LC  690 R is filled into a first LC channel, and the first LC channel is then sealed by a first sealant  680   a . For example, a red cholesteric CL material comprises a mixture of red dye or twisted nematic liquid crystal layer doped with chiral agent. The first sealant  680   a  can comprise a light solidified material or a thermoset material. Next, a first cutting procedure is performed such as cutting along line B-B to uncover the second stripe LC channel C 2 . The first cutting procedure can be performed by dice-cutting and laser-cutting. 
     Referring to  FIG. 7B , a second color (e.g., green) cholestic LC  690 G is filled into a second LC channel, and the second LC channel is then sealed by a second sealant  680   b . For example, a green cholesteric CL material comprises a mixture of green dye or twisted nematic liquid crystal layer doped with chiral agent. The second sealant  680   b  can comprise a light solidified material or a thermoset material. Next, a second cutting procedure is performed such as cutting along line C-C to uncover the third stripe LC channel C 3 . The second cutting procedure can be performed by dice-cutting and laser-cutting. 
     Referring to  FIG. 7C , a third color (e.g., blue) cholestic LC  690 B is filled into a third LC channel, and the third LC channel is then sealed by a third sealant  680   c . For example, a blue cholesteric CL material comprises a mixture of blue dye or twisted nematic liquid crystal layer doped with chiral agent. The third sealant  680   c  can comprise a light solidified material or a thermoset material. After all the three color cholesteric LCs are filled and sealed, fabrication of the single layer color cholesteric LCD device is completed. 
     While the invention has been described by way of example and in terms of the several embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.