Patent Publication Number: US-11024652-B2

Title: Flexible display device and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2014-0100372, filed on Aug. 5, 2014, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The embodiments of the invention relate to a flexible display device and a method of manufacturing the same. 
     2. Description of the Related Art 
     With the development of information society, various types of requirements for a display device for displaying an image are increasing and, recently, various display devices, such as a Liquid Crystal Display (LCD), a Plasma Display Panel (PDP), and an Organic Light Emitting Diode Display Device (OLED), are being used. 
     Recently, conventional glass substrates, which lack flexibility, have rapidly been replaced by flexible display devices or folding display devices, which use flexible materials (e.g. plastic) so that, even when bent like paper, they can maintain their display performance. 
     Such a flexible display device has a problem in that, since it is repeatedly bent and folded by an external force, the adhesiveness of its metal pattern (e.g. wiring), electric conductivity, uniformity of the surface, durability, reliability and the like may degrade. 
     SUMMARY OF THE INVENTION 
     An aspect of the embodiments of the invention is to provide a display device which improves the adhesiveness of its conductor (e.g. wiring), electric conductivity, surface uniformity, durability, and realiability and which reduces manufacturing costs. 
     In accordance with an aspect of the embodiments of the invention, there is provided a flexible display device including: a substrate having multiple signal lines arranged on the substrate; a transistor disposed on the substrate, the transistor including a gate electrode, a source electrode, and a drain electrode; and a second electrode disposed to correspond to a first electrode connected to the source electrode or the drain electrode of the transistor. 
     In the flexible display device, at least one of the multiple signal lines, the gate electrode, the source electrode, the drain electrode, and the second electrode, which are formed on the substrate, may be implemented as a conductor including a metal nanowire structure and a polymer substance, the metal nanowire structure being disposed in the polymer substance. 
     In accordance with another aspect of the embodiments of the invention, there is provided a method of manufacturing a flexible display device, the method including: disposing a polymer substance having adhesiveness on a substrate or an insulation film positioned on the substrate; disposing multiple metal nanowires onto the polymer substance; and forming a conductor including a metal nanowire structure and the polymer substance by cross-connecting the multiple metal nanowires to each other so that the metal nanowire structure is formed into the polymer substance. 
     In accordance with another aspect of the embodiments of the invention, there is provided a flexible display device including: a substrate; and a conductor positioned on the substrate, the conductor including a metal nanowire structure and a polymer substance, the metal nanowire structure being disposed in the polymer substance. 
     The conductor may be at least one of a source electrode, a drain electrode, and a gate electrode of a transistor disposed on the substrate, and a signal line and a common electrode disposed on the substrate. The polymer substance may be made of the same substance as the substrate. 
     The embodiments of the invention are advantageous in that it improves the strength of attachment of the conductor (e.g. wiring), electric conductivity, surface uniformity, durability, and realiability and reduces the manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the embodiments of the invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view illustrating a part of a conductor formed on a flexible display device according to embodiments of the invention; 
         FIG. 2  is a schematic top view of a flexible OLED according to an embodiment of the invention; 
         FIG. 3  is a sectional view taken along A-A′ of the flexible OLED according to an embodiment of the invention of  FIG. 2 ; 
         FIG. 4  is a sectional view taken along B-B′ of the flexible OLED according to an embodiment of the invention of  FIG. 2 ; 
         FIG. 5A  to  FIG. 5E  schematically illustrate a method of manufacturing a conductor formed on a flexible display device according to another embodiment of the invention; 
         FIG. 6  illustrates schematic top views of a process for forming a metal nanowire structure of a flexible display device according to another embodiment of the invention; and 
         FIG. 7A  is a graph illustrating a relationship between resistance and a bending cycle in connection with a flexible display device according to another embodiment of the invention, and  FIG. 7B  is a table corresponding to the graph of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, example embodiments of the invention will be described in detail with reference to the accompanying drawings. In the following description, the same elements will be designated by the same reference numerals throughout even they are shown in different drawings. Further, in the following description of embodiments of the invention, a detailed description of known functions and configurations incorporated herein will be omitted when it is deemed that redundant detailed descriptions obscures the subject matter of the embodiments of the invention. 
     In addition, terms, such as first, second, A, B, (a), (b) or the like may be used herein when describing components of the embodiments of the invention. These terms are merely used to distinguish one structural element from other structural elements, and a property, an order, a sequence and the like of a corresponding structural element are not limited by the term. It should be noted that if it is described in the specification that one component is “connected,” “coupled” or “joined” to another component, a third component may be “connected,” “coupled,” and “joined” between the first and second components, although the first component may be directly connected, coupled or joined to the second component. Likewise, when it is described that a certain element is formed “on” or “under” another element, it should be understood that the certain element may be formed either directly or indirectly via a still another element on or under another element. 
       FIG. 1  is a schematic sectional and top view illustrating a part of a conductor formed on a flexible display device according to embodiments of the invention. 
       FIG. 1  illustrates an x-direction section view and a y-direction top view of a part of a conductor  180  and, referring to  FIG. 1 , the conductor  180  may include a polymer substance  182  having adhesiveness and a metal nanowire structure  184  inserted into the polymer substance  182 . The metal nanowire structure  184  includes multiple metal nanowires cross-connected to each other. The metal nanowire structure  184  may be a silver nanowire structure, but is not limited thereto. 
     The metal nanowires generally refer to conductive nano-sized structures, at least one of which may have a dimension (i.e. width or diameter) of less than 500 nm, e.g. less than 100 nm or 50 nm, but the dimension is not limited thereto. 
     The nanostructures may be made of any conductive material. For example, the metal nanowires may be made of a metallic material including elemental metal (e.g. transition metals) or a metal compound (e.g. metal oxide). The metallic material may be a bimetal material including at least two types of metals or a metal alloy. Suitable metals include silver, gold, copper, nickel, plating silver, platinum, and palladium, but are not limited thereto. It is to be noted that, although the description is made in connection with silver nanowires, any substance can be used. 
     As illustrated in  FIG. 1 , the metal nanowire structure  184  is inserted into the polymer substance  182 . In addition, the metal nanowire structure  184  is configured so that multiple metal nanowires are electrically connected to a different metal nanowire at at least one point. 
     This secures fatigue fracture reliability, i.e. the conductor  180 , when included in a flexible display device, can endure repeated bending or folding. The cross connection structure of the metal nanowire structure  184  reduces the ratio of occurrence of cracks, under situations of repeated bending and folding, and secures excellent durability and reliability. 
     In addition, the coupling structure or interaction between the metal nanowire structure  184  and the polymer substance  182  improves the surface uniformity of the metal nanowire structure  184 , advantageously making it unnecessary to stack a flattening layer for surface flattening. As a result, the thickness of the panel of the display device  200  as a whole can be reduced. 
     When the metal nanowire structure  184  is a silver nanowire structure, high electric conductivity of silver (Ag) secures a high level of conduction property. In addition, junction of multiple silver nanowires can minimize loss of conductivity even when any part of the nanowires is broken in the process of bending or folding. 
     The metal nanowire structure  184  may have a non-linear type that follows no rule. This is because coupling is made between metal nanowires irregularly in the process of bonding the nanowires. This will be described later in more detail. 
     Meanwhile, the polymer substance  182  may be made of a substance based on plastic having adhesiveness, such as polyimide, and may be, for example, a polyimide-based compound formed through reaction between at least one compound selected from the group consisting of ODA(4,4′-oxydianiline), BDSA(4,4′-diaminodiphenyl-2,2′-disulfonic acid), HFBAPP(2,2′-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane), and AHHFP(2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane) and at least one compound selected from the group consisting of BTDA(3,3′-4,4′-benzophenonetetracarboxylic dianhydride), TMA(trimellitic anhydride), and ODPA(4,4′-oxydiphthalic anhydride); and the polymer substance  182  may have a hydrophilic function group capable of improving adhesiveness through hydrogen bonding, but is not limited thereto. 
     The polyimide-based polymer substance  182  is advantageous in that it has adhesiveness and strong resistance to heat enough to prevent deformation or alteration during a heat-applied process, and provides the display device with flexibility. 
     Meanwhile, the polymer substance  182  may be made of the same substance as the substrate, which is the lower layer of the polymer substance  182 , or as the insulation film, thereby improving adhesiveness with respect to the substrate or the insulation film. This can further improve the durability and reliability of the flexible display device. 
     In the instance of the conductor  180  applied to the flexible display device, the polymer substance  182  needs to have a low level of hardness, in order to cope with bending or folding. 
     The above description will now be summarized: in order to provide the flexible display device with flexibility, the conductor  180  inside the flexible display device is implemented by inserting a metal nanowire structure into a polymer substance having adhesiveness. 
     One of a source electrode, a drain electrode, and a gate electrode of a transistor formed on the substrate, a signal line formed on the substrate, a common electrode (e.g. common voltage electrode of a LCD, a cathode electrode of an OLED), and the like may be formed as the above-mentioned conductor  180 . 
     In connection with the flexible display device, furthermore, when a touch screen panel (TSP) is attached to the display panel in add-on type or when a touch screen panel is embedded in the display panel in on-cell or in-cell type, the touch electrode, which may be formed on the touch screen panel or on the display panel incorporating the touch screen panel to be elongated in the transverse or longitudinal direction or to be a large block, may be formed as a conductor  180  of the above-mentioned structure, in order to provide the flexible display device with flexibility. 
     Besides this, a conductor  180  of the above-mentioned structure may be applied to any conductive pattern inside the flexible display device, in order to increase flexibility. 
     Hereinafter, a flexible OLED, which includes a conductor described with reference to  FIG. 1 , will be described. It is to be noted that such an assumption is solely for convenience of description, and embodiments of the invention can be applied not only to OLEDs, but also to other flexible display devices such as flexible LCDs. 
       FIG. 2  is a schematic top view of a flexible OLED according to an embodiment of the invention. 
     Referring to  FIG. 2 , the OLED  200  may include a substrate having multiple signal lines  210 ,  212 ,  214 ,  216 ,  218  arranged thereon; transistors T 21 , T 22 , T 23  formed on the substrate, the transistors including gate electrodes  214 ′,  216 ,  228 , source electrodes  210 ′,  212 ′,  226 , and drain electrodes  222 ,  225 ; and a second electrode formed to correspond to a first electrode  230  connected to the source electrodes  210 ′,  212 ′,  226  or the drain electrodes  222 ,  225  of the transistors T 21 , T 22 , T 23 . 
     The flexible OLED  200  may adopt a top emission scheme or a bottom emission scheme. 
     Specifically, the substrate is composed of multiple pixel areas PAs, each of which is composed of an emission area EA and a circuit area CA. 
     Multiple pixel areas PAs exist, and each circuit area CA includes three transistors T 21 , T 22 , T 23 , first to fifth lines  210 ,  212 ,  214 ,  216 ,  218 , a storage capacitor Cstg, and the like. The emission area EA of each pixel area PA may include a first line  210 , a second line  212 , a pixel electrode  230 , and a bank overlapping with the peripheral area of the emission area EA. 
     The first transistor T 21  may be a switching transistor, and includes a first source electrode  210 ′, a first drain electrode  222 , a first semiconductor layer  211 , and a first gate  214 ′; one end of the first transistor T 21  is connected to the storage capacitor Cstg, and the other end thereof is connected to the first line  210 . 
     The second transistor T 22  may be a driving transistor, and includes a second source electrode  212 ′, a second drain electrode  225 , and a second gate  228 ; one end of the second transistor T 22  is connected to the second line  212 , and the other end thereof is connected to the first transistor T 21 . 
     The third transistor T 23  may be a sensing transistor, and includes a third source electrode  226 , a third drain electrode  225 , and a third gate  216 ; the third drain electrode  225  is connected to a node between the second transistor T 22  and the pixel electrode  230 ; and the third source electrode  226  is connected to the fifth line  218 . 
     The first line  210  may be a data line, and the second line  212  may be a voltage line (VDD line) for supplying high-voltage power. The third line  214  may be a first scan line, the fourth line  216  may be a second scan line, and the fifth line  218  may be a reference voltage line, but their configurations are not limited thereto. 
     The OLED  200  includes a pixel electrode  260 , a common electrode, and at least one organic layer in an emission area EA, which is defined by intersection between the first line  210  and the third line  220 , and emits light in response to a current supplied from the first transistor T 21  formed on the substrate. 
     In this instance, at least one of the multiple signal lines  210 ,  212 ,  214 ,  216 ,  218 , the gate electrodes  214 ′,  216 ,  228 , the source electrodes  210 ′,  212 ′,  226 , the drain electrodes  222 ,  225 , and the second electrode  236  may be a conductor  180 , which has a metal nanowire structure  184  inserted into a polymer substance  182  having adhesiveness. The second electrode  236 , on the other hand, may be made of a conductor  180  when the flexible OLED  200  adopts the bottom emission scheme. In the instance of the bottom emission scheme, emitted light is reflected by the second electrode  236  and is discharged in the direction of the first electrode  230 . 
     The metal nanowire structure  184  may include multiple metal nanowires cross-connected to each other. The metal nanowire structure  184  may be a silver nanowire structure, but is not limited thereto. 
     In connection with the flexible OLED  200 , the metal nanowire structure  184  and the polymer substance  182  integrally constitute the conductor  180 . The polymer substance  182  may be a substance based on plastic having strong adhesiveness, such as polyimide. 
     Such a conductor  180  is advantageous in that it has strong adhesiveness and its surface is uniform, making it unnecessary to form a separate flattening layer. 
     At least one parts of the metal nanowire structure  184  intersect with each other and make an electric junction between them, and the metal nanowire structure  184  and the polymer substance  182  are firmly coupled to each other as an integral unit. Therefore, when the flexible display device  200  is repeatedly folded or bent, the conductor  180  can play the role of improving durability and reliability. This will be described later in more detail. 
     Meanwhile, the conductor  180  may include a photosensitive substance, which may be a photo acryl-based substance, for example, but is not limited thereto. Such a photosensitive substance has a process-related advantage in that, when the conductor  180  is patterned, the number of processes is reduced, thereby decreasing the manufacturing costs. This will be described later in connection with  FIG. 5A  to  FIG. 5E . 
       FIG. 3  is a sectional view taken along A-A′ of the flexible OLED  200  of  FIG. 2 . 
     Referring to  FIG. 3 , the OLED  200  may include a first line  210  and a second line  212  formed on a substrate  202 ; a first insulation film  229  formed on the first line  210  and the second line  212 ; a first electrode  230  formed on the first insulation film  229 ; a bank  232  formed along the periphery of the first electrode  230 ; an organic layer  234  formed on a part of the first electrode  230  exposed by the bank  232 ; and a second electrode  236  formed to cover the organic layer  234  and the bank  232 . A protection layer  238  may be formed on the second electrode  236  to protect the organic layer  234  from moisture and oxygen. 
     In this instance, at least one of the first line  210 , the second line  212 , and the second electrode  236  may be the above-mentioned conductor  180  including the the metal nanowire structure  184  and the polymer substance  182 . Each metal nanowire structure  184  may be a silver nanowire structure, but is not limited thereto, and each may include different metals. The metal nanowire structure  184  is inserted into and firmly coupled to a polymer substance  182  having adhesiveness, and may also be firmly coupled to the substrate  202 , the organic layer  234 , and the bank  232 , which are lower layers, by the polymer substance  182 . 
     In this instance, the polymer substance  182  of the signal lines  210 ,  212  may be made of the same substance as the substrate  202 . In other words, the first line  210  and the second line  212  may be made of the same substance as the substrate  202 , thereby improving adhesiveness. 
     Specifically, the substrate  202  of the OLED  200  may be made of a substance based on plastic, such as polyimide, and may be, for example, a polyimide-based compound formed through reaction between at least one compound selected from the group consisting of ODA(4,4′-oxydianiline), BDSA(4,4′-diaminodiphenyl-2,2′-disulfonic acid), HFBAPP(2,2′-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane), and AHHFP(2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane) and at least one compound selected from the group consisting of BTDA(3,3′-4,4′-benzophenonetetracarboxylic dianhydride), TMA(trimellitic anhydride), and ODPA(4,4′-oxydiphthalic anhydride); and the substrate  202  may have a hydrophilic function group capable of improving adhesiveness through hydrogen bonding. 
     In this instance, the first line  210  and the second line  212  may also be made of the same substance as the substrate  202 , thereby improving adhesiveness with respect to the substrate  202 . This is because, when made of the same polymer substance, additional bonding or hydrogen bonding between function groups enables strong bonding. 
     Besides this, the substrate  202  of the OLED  200  and the polymer substance  182  need to be made of a substance having a low level of hardness to cope with folding or bending. 
     Meanwhile, when the OLED  200  adopts a bottom emission scheme, the second electrode  236  may be made of the above-mentioned conductor  180 . As used herein, the bottom emission scheme refers to a scheme in which light emitted from the organic layer  234  is reflected by the second electrode  236  and discharged outwards in the direction of the first electrode  230 . 
       FIG. 4  is a sectional view taken along B-B′ of the flexible OLED of  FIG. 2 . 
     Referring to  FIG. 4 , the flexible OLED  200  may include a first transistor T 21  formed on a substrate  202 , a first insulation film  229  formed on the first transistor T 21 , and a first electrode  230 , a bank  232 , a second electrode  236 , and a protection layer  238 , which are successively formed on the first insulation film  229 . 
     The first transistor T 21  may include a first semiconductor layer  211 , a gate insulation film  213 , a first gate electrode  214 ′, a first source electrode  210 ′, a first drain electrode  222 , and a second insulation film  207 . 
     In this regard, the first transistor T 21  may be an oxide transistor having a first semiconductor layer  211  made of an oxide semiconductor, such as IGZO(Indium Gallium Zinc Oxide), but embodiments of the invention are not limited thereto, and the first transistor T 21  may be a transistor made of a LTPS (Low Temperature PolySilicon) semiconductor or a transistor made of an amorphous silicon semiconductor. 
     In addition, although the first transistor T 21  has been illustrated in a top gate scheme, i.e. the first gate electrode  214 ′ is positioned on the first semiconductor layer  211 , embodiments of the invention are not limited thereto, and a bottom gate scheme may also be adopted. 
     Meanwhile, the gate insulation film  213  and the second insulation film  207  may be organic films or inorganic films such as silicon oxide (SiOx) or silicon nitride (SiNx). 
     In this instance, at least one of the first gate electrode  214 ′, the first source electrode  210 ′, and the first drain electrode  222 , which are formed on the insulation films  213 ,  207  positioned on the substrate  202 , may be made of a conductor  180 ; the polymer substance  182  of the first gate electrode  214 ′, the first source electrode  210 ′, and the first drain electrode  222  may be made of the same substance as that of the insulation films  213 ,  207  and, for example, may be made of a polyimide-based substance. This can advantageously improve adhesiveness through additional bonding or hydrogen bonding between function groups, as described above. 
     This advantageously increases adhesiveness between the conductor  180  and the insulation films  213 ,  207 . In addition, the fact that the metal nanowire structure  184  is inserted into and firmly coupled to the polymer substance  182  advantageously improves electric conductivity, surface uniformity, reliability regarding fatigue fracture resulting from bending and folding, durability, and the like. 
     Having described the structure of the conductor  180  and the display device  200 , to which the conductor  180  is applied, a method of manufacturing a flexible display device  200  including the conductor  180  will now be described. 
       FIG. 5A  to  FIG. 5E  schematically illustrate a method of manufacturing a conductor formed on a flexible display device according to another embodiment of the invention. The drawings are solely for convenience of description and do not limit embodiments of the invention, and various methods or processes may be used to manufacture the conductor. 
     Referring to  FIG. 5A  to  FIG. 5E , a method of manufacturing a flexible display device  200  may include the operation of: forming a polymer substance  582  having adhesiveness on a substrate or on an insulation film  570  positioned on the substrate; applying multiple metal nanowires  583  onto the polymer substance  582 ; and forming a conductor  580 , which has a metal nanowire structure  584  inserted into the polymer substance  582 , by cross-connecting the multiple metal nanowires  583  to form a metal nanowire structure  584  and insert it into the polymer substance  582 . The metal nanowire structure  584  may be, for example, a silver nanowire structure. 
     As illustrated in  FIG. 5A , a step of forming a polymer substance  582  and multiple metal nanowires  583  successively on a substrate or an insulation film  570  (hereinafter, referred to as a base layer) is performed. 
     The base layer  570  may be a lower substrate of the flexible display device  200 , and the insulation film  570  may be an insulation film formed on the substrate or a gate insulation film formed on a gate of a transistor. 
     The polymer substance  582  having adhesiveness may be a substance capable of improving adhesiveness with respect to the base layer  570  or adhesiveness with respect to the metal nanowire structure  584 . For example, the substance may include polyimide-based polymer. In addition, such polyimide-based polymer may have a hydrophilic function group that improves adhesiveness through hydrogen bonding. In this instance, the polymer substance  582  and the base layer  570  may be made of the same substance. 
     Meanwhile, the polymer substance  582  may include a photosensitive substance, which may be a photo acryl-based substance, for example, but is not limited thereto. This will be described later in more detail. 
     The metal nanowires  583  may be pre-processed with an anti-corrosive agent before/after coating or deposition onto the base layer  570 . For example, the metal nanowires  583  may be pre-coated with barrier-forming anti-corrosive agents, such as BTA and dithiothiadiazole. Furthermore, the metal nanowires  583  may be processed with an anti-tarnish solution. 
     The viscosity, corrosiveness, adhesiveness, and dispersiveness of the metal nanowires  583  may be adjusted by additives and binders. Suitable additives and binders may include, for example, carboxy methyl cellulose (CMC), 2-hydroxy ethyl cellulose (HEC), hydroxy propyl methyl cellulose (HPMC), methyl cellulose (MC), poly vinyl alcohol (PVA), tripropylene glycol (TPG), xanthan gum (XG), ethoxylates, alkoxylate, ethylene oxide, propylene oxide, and copolymers thereof. 
     Schemes of applying the metal nanowires  583  may include spin coating, nozzle coating, slit coating, and printing, and deposition schemes may include chemical vapor deposition and physical vapor deposition, but the schemes are not limited thereto. 
     Meanwhile, a step of forming a conductor  580 , which has a metal nanowire structure  584  inserted into a polymer substance  582 , by subjecting metal nanowires  583  to plasma treatment is illustrated in  FIG. 5B  and  FIG. 5C . 
     The step of forming a conductor  580  may be performed by one of pressurization, plasma treatment, light sintering-type welding treatment, and heating-type welding treatment, and is not limited thereto. 
     The plasma treatment scheme may be one of thermal plasma, cold plasma, which is a type of glow discharge occurring under various gases with removed pressure, and hybrid plasma, and may be, for example, cold plasma treatment conducted under an oxygen gas condition. 
     The light sintering scheme can instantly deliver a high level of light energy at about 1500° C. within a very short period of time of 20 ms. In addition, use of transparent light of visible rays does not damage the transparent substrate at all, thereby avoiding any influence on the upper layers of the metal nanowires  583  and the underlying base layer  570 . 
     On the other hand, the heating scheme, which relies on electricity, is as follows: when a predetermined voltage is applied, the resulting Joule heating generates heat, which welds the metal nanowires  583 ; the amount of generated heat is in proportion to the square of current and is in proportion to the size of resistance. 
     A display device, such as a flexible display or a folding display, may be repeatedly bent or folded. As a result, the metal nanowire structure  584  may be broken, but can be reconnected by the heating scheme using electricity. In other words, the heating-type welding is advantageous in that broken parts of the metal nanowire structure  584  can be joined again by heat generated when a voltage is applied to the display device and drives it. 
     On the other hand, the metal nanowires  583  can be welded by the above-mentioned plasma treatment or heating treatment, for example, and simultaneously inserted into the polymer substance  582 . In other words, a metal nanowire structure  584  is formed and simultaneously inserted, thereby forming a conductor  580 . 
     Thereafter, a step of patterning the formed conductor  580  is illustrated in  FIG. 5D  and  FIG. 5E . 
     The method of manufacturing a flexible display device  200  may further include a step of patterning the conductor  580  by using a mask  590  to conduct exposure and development treatments, after the operation of forming a conductor  580 , when, in the operation of forming a polymer substance  582 , the polymer substance  582  is formed by mixing a photosensitive substance  585  with it. 
     Specifically, the polymer substance  582  may be mixed with a photosensitive substance  585  and formed on the base layer  570 . As used herein, the photosensitive substance  585  refers to a substance, the solubility of which varies inside a developer when a specific wavelength of light is received, so that exposed parts or the other parts can be selectively removed during the following development process. For example, the photosensitive substance may be a photo acryl-based substance, but is not limited thereto. 
     Although the exposure process illustrated in  FIG. 5D  adopts a negative type, in which unexposed parts are removed by the developer, embodiments of the invention are not limited thereto and may also adopt the positive type, in which the exposed parts are removed. 
     Referring to  FIG. 5D , a step of radiating light using an exposure mask  590 , which has a transmission portion  590   a  and a shielding part  590   b , is performed. Although not illustrated, a PEB (post exposure bake) process may be performed after the exposure step. 
     Thereafter, a development process may be performed using a developer composed of a water-soluble alkali solution, e.g. potassium hydroxide (KOH), and a TMAH (TetraMethyl-Ammonium-Hydroxide) aqueous solution to pattern a conductor  580 , such as the first to fifth lines  210 ,  212 ,  214 ,  216 ,  218 , the first to third gate electrodes  214 ′,  216 ,  228 , the first to third source electrodes  210 ′,  226 ,  212 ′, and the first to third drain electrodes  222 ,  225  illustrated in  FIG. 2 . 
     According to such a patterning scheme, a photosensitive substance  585  is mixed with a polymer substance  582  and applied onto the base layer  570 , so that the operation of applying a photoresist layer and the process of stripping the photoresist layer that remains after the development process can be omitted, thereby making processes simpler, improving the yield ratio, and reducing the manufacturing cost. 
       FIG. 6  illustrates schematic top views of a process for forming a metal nanowire structure of a flexible display device according to another embodiment of the invention. 
     The metal nanowire structure  584  can be formed by welding multiple separate metal nanowires  583  to each other. In embodiments of the invention, the metal nanowire structure  584  can be formed by cross-connecting or cross-linking the multiple separate metal nanowires  583  to each other. As described above, one of pressurization, plasma treatment, light sintering-type welding treatment, and heating-type welding treatment may be adopted to enable the cross-connecting or cross-linking. In embodiments of the invention, application of heat to the multiple separate metal nanowires  583  and/or the polymer substance  582 . Thus, the metal nanowire structure  584  are disposed in the polymer substance  582  by being enveloped therein. Also, In addition, the metal nanowire structure  584  may be a silver nanowire structure or may contain silver, but is not limited thereto. Other metals or material may be used for the nanowire structure. 
     Furthermore, the metal nanowire structure  584  has multiple metal nanowires  583  coupled randomly. A metal nanowire  583  may be connected, at at least one part thereof, to another nanowire  583 ; or one nanowire  583  may be connected to a number of other nanowires  583 . In embodiments of the invention, the metal nanowires  583  may be formed into a mesh-sheet or a mesh prior to being formed into the metal nanowire structure  584 . If the metal nanowires  583  are the mesh, one or more sheets of the mesh may be used. Strands of the metal nonowire  583  may intersect, may be parallel, or both. Such intersection and/or parallel arrangement may be regular or irregular. Additionally, the metal nanowires  583  may be formed into a tube or other geometric shapes when in the mesh or sheet form. Also, each strand of the metal nanowires  583  need not be in the form of a wire having a circular cross section, but may have other shapes, or may have irregular shapes along their lengths. 
     Therefore, in connection with the flexible display device  200 , the metal nanowire structure  584  may be used as wiring or electrodes, based on electric conductivity of the metal. Furthermore, the random coupling type can secure strong coupling force in instance of repeated bending or folding. In addition, the electric conductivity can be advantageously maintained even when a part of the metal nanowire structure  584  is broken. 
       FIG. 7A  is a graph illustrating a relationship between resistance and bending cycle in connection with a flexible display device according to another embodiment of the invention, and  FIG. 7B  is a table corresponding to the graph of  FIG. 7A . 
       FIG. 7A  and  FIG. 7B  illustrate results of a bending test when the radius of curvature (R) is 4.5, and the ratio of deformation caused by external force, i.e. strain, is 1.5%. In this regard, the resistance refers to line resistance; the initial resistance value is 4.0Ω; the resistance value after bending at 50,000 cycles is 4.1Ω; in the instance of 100,000 cycles, 4.1Ω; and, in the instance of 200,000 cycles, 4.2Ω. The ratio of increase of the resistance value is only 5% over 200,000 times of bending. 
     The above-mentioned strain ε (unit: %) is defined by equation (1) below. The radius of curvature (R) refers to the radius of curvature during bending, d 1  refers to the thickness of the substrate  202  of the flexible display device  200 , and d 2  refers to the thickness of the conductor  180 ,  580  formed on the substrate  202 . 
     
       
         
           
             
               
                 
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     It is clear from the test results that the flexible display device according to embodiments of the invention have reliability against environments of repeated bending and folding. 
     In other words, the metal nanowire structure  184 ,  584 , which is formed by multiple metal nanowires  583  intersecting with each other, has multiple random welding points so that the metal nanowire structure  184 ,  584  can secure strong coupling force and stability. 
     Therefore, even when a part of the metal nanowire structure  184 ,  584  is broken or has a defect, the multiple connections still improve the reliability of the flexible display device  200 . Furthermore, when a voltage is applied to the flexible display device  200 , broken parts of the metal nanowire structure  184 ,  584  can be advantageously welded again by Joule heating. 
     In addition, since the metal nanowire structure  184 ,  584  is inserted into a polymer substance  182 ,  582  and thereby forms a conductor  180 ,  580 , strong coupling between the metal nanowire structure  184 ,  584  and the polymer substance  182 ,  582  can be maintained. This improves reliability of the flexible display device  200  against fatigue fracture. 
     In summary, the metal nanowire structure  184 ,  584  itself has reliability or stability against fatigue fracture, and the conductor  180 ,  580  is integrally formed by inserting the metal nanowire structure  184 ,  584  into a polymer substance  182 ,  582 , so that the ratio of increase of resistance is only 5% through 200,000 times of bending tests. 
     Advantageous effects of a flexible display device  200 , to which the above-described embodiments of the invention are applied, are as follows: first, adhesiveness between the metal nanowire structure  184 ,  584  and the polymer substance  182 ,  582  is improved, and adhesiveness between the polymer substance  182 ,  582  and the lower layer (i.e. substrate) or the insulation film  570  can also be improved. 
     Furthermore, the insertion of the metal nanowire structure  184 ,  584  into the polymer substance  182 ,  582  not only gives the conductor  180 ,  580  strong coupling force, but also reduces the sheet resistance. In other words, signal lines or electrodes using silver nanowires generally have a problem of high sheet resistance, but the conductor  180 ,  580  according to embodiments of the invention can have a low sheet resistance value of 1Ω/□ or less, for example, 0.4Ω/□ to 0.6Ω/□. 
     Furthermore, the structure of the conductor  180 ,  580  can also reduce the thickness, thereby reducing the panel thickness of the flexible display device  200 . 
     On the other hand, the flexible display device  200 , which includes a conductor  180 ,  580 , can secure a high level of electric conductivity due to the high level of electric conductivity of the metal nanowire structure  184 ,  584 , e.g. silver nanowire structure. 
     In general, in the instance of a signal line or an electrode using silver nanowires, the uneven surface may pose a problem. In other words, since the nanowires make the surface uneven, electric characteristics of the flexible display device  200  may change, depending on the surface condition, or a defect may occur. 
     However, the conductor  180 ,  580  according to embodiments of the invention has a metal nanowire structure  184 ,  584 , which is a conductive substance, inserted into a polymer substance  182 ,  582 , so that the surface uniformity can be improved, thereby making it unnecessary to form a separate layer for flattening the surface. 
     Finally, the conductor  180 ,  580  may include a photosensitive substance  585 , so that it can be patterned solely by an exposure process using a mask and a development process using a developer. Therefore, the process of applying a photoresist, the etching process after development, and the process of stripping the photoresist can be omitted, thereby advantageously reducing the number of processes, decreasing the manufacturing costs, and improving the yield ratio. 
     Although various embodiments of the invention have been described up to now with reference to the accompanying drawings, the embodiment of the invention are not limited to them. 
     Further, the terms “includes”, “constitutes”, or “has” mentioned above mean that a corresponding structural element is included unless context dictates otherwise. Accordingly, it should be interpreted that the terms may not exclude but further include other structural elements. All the terms that are technical, scientific or otherwise agree with the meanings as understood by a person skilled in the art unless defined to the contrary. Common terms as found in dictionaries should be interpreted in the context of the related technical writings not too ideally or impractically unless the disclosure expressly defines them so. 
     Although the embodiments of the invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention. Accordingly, the embodiments disclosed in the invention are merely to not limit but describe the technical spirit of the embodiments of the invention. Further, the scope of the technical spirit of the invention is limited by the embodiments of the invention. The scope of the embodiments of the invention shall be construed on the basis of the accompanying claims in such a manner that all of the technical ideas included within the scope equivalent to the claims belong to the embodiments of the invention.