Patent Publication Number: US-2021195763-A1

Title: Flexible display apparatus and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/156,355, filed on May 17, 2016, which claims priority to and the benefit of Korean Patent Application No. 10-2015-0100514, filed on Jul. 15, 2015, in the Korean Intellectual Property Office, and entitled: “Flexible Display Apparatus and Manufacturing Method Thereof,” the entire contents of each of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     1. Field 
     One or more exemplary embodiments relate to a flexible display apparatus and a manufacturing method thereof. 
     2. Description of the Related Art 
     Among display apparatuses, an organic light-emitting device may be a next-generation display apparatus, because organic light-emitting devices may have fast response times as well as wide viewing angles and excellent contrast. 
     SUMMARY 
     Embodiments may be realized by providing a flexible display apparatus, including a flexible substrate including a bending area and a non-bending area; a display unit on the flexible substrate; and an encapsulation unit covering the display unit, the encapsulation unit including a first inorganic film, a second inorganic film, and an organic film between the first inorganic film and the second inorganic film, the organic film having a first thickness in the bending area and a second thickness greater than the first thickness in the non-bending area. 
     The non-bending area may include a first non-bending area and a second non-bending area, and the bending area may be between the first non-bending area and the second non-bending area. 
     The bending area may include a first bending area and a second bending area, and the non-bending area may be between the first bending area and the second bending area. 
     The first bending area may have a first curvature, and the second bending area may have a second curvature smaller than the first curvature. 
     The organic film in the first bending area may be thinner than the organic film in the second bending area. 
     The organic film may be formed according to a liquid phase deposition process. 
     Embodiments may be realized by providing a manufacturing method of a flexible display apparatus, the method including providing a flexible substrate including a bending area and a non-bending area; forming a display unit on the flexible substrate; forming a first inorganic film covering the display unit; forming an organic film on the first inorganic film, the organic film having a first thickness in the bending area and a second thickness greater than the first thickness in the non-bending area; and forming a second inorganic film on the organic film. 
     The non-bending area may include a first non-bending area and a second non-bending area, and the bending area may be between the first non-bending area and the second non-bending area. 
     The bending area may include a first bending area and a second bending area, and the non-bending area may be between the first bending area and the second bending area. 
     The first bending area may have a first curvature, and the second bending area may have a second curvature smaller than the first curvature. 
     The organic film in the first bending area may be thinner than the organic film in the second bending area. 
     The organic film may be formed according to a liquid phase deposition process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates a schematic plan view of a flexible display apparatus according to an embodiment; 
         FIG. 2  illustrates a schematic side view of a bent structure of the flexible display apparatus of  FIG. 1 ; 
         FIG. 3  illustrates a schematic cross-sectional view taken along a line of  FIG. 1 ; 
         FIG. 4  illustrates a schematic cross-sectional view of the inside of the flexible display apparatus of  FIG. 1 ; 
         FIG. 5  illustrates a schematic plan view of a flexible display apparatus according to an embodiment; 
         FIG. 6  illustrates a schematic side view of a bent structure of the flexible display apparatus of  FIG. 5 ; and 
         FIG. 7  illustrates a schematic cross-sectional view taken along a line VII-VII′ of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey exemplary implementations to those skilled in the art. 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     It will be understood that although the terms “first”, “second”, etc., may be used herein to describe various components, these components should not be limited by these terms. These components are only used to distinguish one component from another. 
     It will be further understood that the terms “comprises” and/or “comprising” used herein specify the presence of stated features or components, but do not preclude the presence or addition of one or more other features or components. It will be understood that when a layer, region, or component is referred to as being “formed on,” another layer, region, or component, it can be directly or indirectly formed on the other layer, region, or component. That is, for example, intervening layers, regions, or components may be present. Further, it will be understood that when a layer is referred to as being “under” another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. 
     Sizes of elements in the drawings may be exaggerated for convenience of explanation. In other words, since sizes and thicknesses of components in the drawings are arbitrarily illustrated for convenience of explanation, the following embodiments are not limited thereto. 
     In the following examples, the x-axis, the y-axis and the z-axis are not limited to three axes of the rectangular coordinate system, and may be interpreted in a broader sense. For example, the x-axis, the y-axis, and the z-axis may be perpendicular to one another, or may represent different directions that are not perpendicular to one another. 
     When a certain embodiment may be implemented differently, a specific process order may be performed differently from the described order. For example, two consecutively described processes may be performed substantially at the same time or performed in an order opposite to the described order. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list 
       FIG. 1  illustrates a schematic plan view of a flexible display apparatus  1  according to an embodiment,  FIG. 2  illustrates a schematic side view of a bent structure of the flexible display apparatus  1  of  FIG. 1 , and  FIG. 3  illustrates a schematic cross-sectional view taken along a line of  FIG. 1 . 
     Referring to  FIGS. 1 through 3 , the flexible display apparatus  1  according to an embodiment may include a flexible substrate  100  having a bending area BA and a non-bending area NBA, a display unit  200  disposed on the flexible substrate  100 , and an encapsulation unit  300 . The encapsulation unit  300  according to an embodiment may include a first inorganic film  310 , a second inorganic film  330 , and an organic film  320  disposed between the first inorganic film  310  and the second inorganic film  330 . 
     The flexible substrate  100  may have a characteristic of flexibility and may include various materials, for example, a metal or a plastic material of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or polyimide. The flexible substrate  100  may include a display area DA corresponding to the display unit  200  and a peripheral area (PA) surrounding the display area DA. 
     The flexible substrate  100  may have the characteristic of flexibility, and the flexible substrate  100  may have a structure that is bendable and foldable. In the present embodiment, the flexible substrate  100  may have the bending area BA and the non-bending area NBA. As illustrated in  FIG. 1 , the bending area BA may be between a first bending area BA 1  and a second bending area BA 2 . In the flexible display apparatus  1  according to the present embodiment, a first non-bending area NBA 1  and a second non-bending area NBA 2  may be arranged to face each other with respect to an axis in an extension direction of the bending area BA.  FIG. 2  illustrates, for example, the first non-bending area NBA 1  and the second non-bending area NBA 2  that may be parallel in a folded structure. An angle formed by the first non-bending area NBA 1  and the second non-bending area NBA 2  may vary according to a bendable degree of the bending area BA. 
     The display unit  200  may be disposed on the flexible substrate  100 . The display unit  200  may be disposed on an outer side or inner side of the flexible substrate  100 . 
     Referring to  FIG. 3 , the display unit  200  may be disposed on the flexible substrate  100 , and the display area DA may be an area in which the display unit  200  may be disposed. The display unit  200  may include a plurality of pixels and thin film transistors and capacitors (CAPs) to supply signals to the respective pixels. The display unit  200  may be an organic light-emitting display unit or a liquid crystal display unit. The present embodiment is a case in which the display unit  100  is an organic light-emitting display unit. 
     The encapsulation unit  300  may include the first inorganic film  310 , the second inorganic film  330 , and the organic film  320  disposed between the first inorganic film  310  and the second inorganic film  320 . The encapsulation unit  300  may cover the display unit  200 , and seal the display unit  200  from external oxygen and moisture. The first inorganic film  310  and the second inorganic film  330  may be an inorganic compound, for example, silicon oxide or silicon nitride. 
     The organic film  320  disposed between the first inorganic film  310  and the second organic film  330  may have a first thickness t 1  in the bending area BA and a second thickness t 2  in the non-bending area NBA. The first thickness t 1  of the organic film  320  may be less than the second thickness t 2  of the organic film  320 . The second thickness t 2  of the organic film  320  disposed in the first and second non-bending areas NBA 1  and NBA 2  may be thicker than the first thickness t 1  of the organic film  320  disposed in the bending area BA. 
     The organic film  320  may be formed by using a liquid phase deposition process, for example, an inkjet printing process. As described above, the organic film  320  may be formed to have the first and second non-bending areas NBA 1  and NBA 2  thicker than the bending area BA. The organic film  320  may be formed by controlling the amount of an organic compound applied during a process of forming the organic film  320 . 
     The organic film  320  may minimize stress at the bending area BA when the flexible display apparatus  1  according to an embodiment is bent or folded at the bending area BA. When the flexible display apparatus  1  is bent, there may exist a neutral plane without stress since the tensile force may be the same as the compression force inside the flexible display apparatus  1 . The stress inside the flexible display apparatus  1  may increase according to a distance from the neutral plane. When a structure vulnerable to the stress is disposed above an upper side or below a lower side with respect to the neutral plane, the structure may be exposed to continuing stress by receiving the tensile force and the compression force. The stress applied to the structure may be reduced by partially reducing the thickness of the flexible display apparatus  1 , and moving the neutral plane to a position of the structure. 
     In the flexible display apparatus  1  according to an embodiment, a thickness of the organic film  320  of the encapsulation unit  300  formed on the bending area BA may be partially adjusted. A position of the neutral plane may be controlled by forming the organic film  320  of the encapsulation unit  300  to be relatively thinner than in the first and second non-bending areas NBA 1  and NBA 2 . A flexible display apparatus  1  that may have minimized stress may be realized by disposing the structure vulnerable to the stress on the neutral plane. 
       FIG. 4  illustrates a schematic cross-sectional view of the inside of the display unit  200  of the flexible display apparatus  1  of  FIG. 1 . 
     The display unit  200  may include a plurality of pixels and the thin film transistors TFTs and CAPs electrically connected to the pixels to supply power to the pixels. The display unit  200  may be an organic light-emitting unit or a liquid crystal display unit. The present embodiment is a case in which the display unit  200  is an organic light-emitting unit. The thin film transistor TFT may include a semiconductor layer  202  including amorphous silicon, polycrystalline silicon, or an organic semiconductor material, a gate electrode  204 , a source electrode  206   s , and a drain electrode  206   d . A structure of the thin film transistor TFT will be explained in detail below. 
     First, a buffer layer  201  including silicon oxide or silicon nitride may be disposed on the flexible substrate  100  to flatten a surface of the flexible substrate  100  and prevent introduction of impurities into the semiconductor layer  202  of the thin film transistor TFT. The semiconductor layer  202  may be disposed on the buffer layer  201 . 
     The gate electrode  204  may be disposed on the semiconductor layer  202 , and the source electrode  206   s  and the drain electrode  206   d  may be electrically connected to each other according to a signal applied to the gate electrode  204 . The gate electrode  204  may have adhesiveness with an adjacent layer, a surface flatness of a stacked layer, and workability, and may include a single layer or a multilayer including one or more of aluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium (Mg), Gold (Au), Nickel (Ni), Neodymium (Nd), iridium (Ir), Chromium (Cr), Lithium (Li), Calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), or copper (Cu). 
     A gate insulation film  203  including silicon oxide and/or silicon nitride may be disposed between the semiconductor layer  202  and the gate electrode  204  to insulate the semiconductor layer  202  and the gate electrode  204  from each other. 
     An interlayer insulation film  205  may be disposed on the gate electrode  204 . The interlayer insulation film  205  may include a single layer or a multilayer including a material, such as, silicon oxide or silicon nitride. 
     The source electrode  206   s  and the drain electrode  206   d  may be disposed on the interlayer insulation film  205 . The source electrode  206   s  and the drain electrode  206   d  may be electrically connected to the semiconductor layer  202  through a contact hole formed in the interlayer insulation film  205  and the gate insulation layer  203 . The source electrode  206   s  and the drain electrode  206   d  may have electrical conductivity and may include a single layer or a multilayer including one or more of aluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium (Mg), Gold (Au), Nickel (Ni), Neodymium (Nd), iridium (Ir), Chromium (Cr), Lithium (Li), Calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), or copper (Cu). 
     A protection layer may be formed to cover the thin film transistor TFT to protect the thin film transistor having the above-described structure. The protection layer may include an inorganic compound, such as, silicon oxide, silicon nitride or silicon oxynitride. 
     A planarization film  207  may be disposed on the flexible substrate  100 . The planarization film  207  may flatten an upper side of the thin film transistor TFT and protect the thin film transistor and various components when an organic light-emitting device is disposed on the thin film transistor TFT. The planarization film  207  may include an acryl-based organic compound or benzoncylobutene (BCB), for example. As illustrated in  FIG. 4 , the buffer layer  201 , the gate insulation layer  203 , the interlayer insulation layer  205 , and the planarization film  207  may be formed on the entire flexible substrate  100 . 
     A pixel-defining film  208  may be disposed on the thin film transistor TFT. The pixel-defining film  208  may be disposed on the above-described planarization film  207  and may have an opening. The pixel-defining film  208  may define a pixel area on the flexible substrate  100 . 
     The pixel-defining film  208  may include an organic insulation film, for example. The organic insulation film may include acrylic polymer such as poly(methyl methacrylate) (PMMA), polystyrene (PS), a poly derivative having a phenol group, imide polymer, acryl ester-based polymer, amide-based polymer, fluorine-based polymer, p-xylene polymer, vinyl alcohol polymer, or a mixture thereof. 
     An organic light-emitting device  240  may be disposed on the pixel-defining film  208 . The organic light-emitting device  240  may include a pixel electrode  210 , an intermediate layer  220  including an emission layer EML, and an opposite electrode  230 . 
     The pixel electrode  210  may be a (semi-)transparent electrode or a reflective electrode. When the pixel electrode  210  is a (semi-)transparent electrode, the pixel electrode  210  may be, for example, indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium oxide (In 2 O 3 ), indium gallium oxide (IGO), or aluminum zinc oxide (AZO). When the pixel electrode  210  is a reflective electrode, the pixel electrode  210  may include a reflective layer having, for example, silver (Ag), magnesium (Mg), aluminum (Al), platinum (Pt), palladium (Pd), gold (Au), Nickel (Ni), Neodymium (Nd), Iridium (Ir), chromium (Cr), and/or a mixture thereof, and a layer having indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), indium oxide (In 2 O 3 ), indium gallium oxide (IGO), or aluminum zinc oxide (AZO). In an embodiment, various materials may be usable, and a structure thereof may be variously changed to, for example, a single layer or a multilayer. 
     The intermediate layer  220  may be disposed on the pixel area defined by the pixel-defining film  208 . The intermediate layer  220  may include an emission layer EML that may emit light according to an electrical signal. In addition to the emission layer EML, the intermediate layer  220  may be, for example, a single layer or a complex structure including a hole injection layer (HIL) disposed between the emission layer EML and the pixel electrode  210 , a hole transport layer (HTL), and an electron transport layer (ETL) disposed between the emission layer (EML) and the opposite electrode  230 , or an electron injection layer (EIL). In an embodiment, the intermediate layer  220  may have various structures. 
     The opposite electrode  230  covering the intermediate layer  220  including the emission layer EML and disposed opposite to the pixel electrode  210  may be formed on the whole area of the flexible substrate  100 . The opposite electrode  230  may be a (semi-) transparent electrode or a reflective electrode. 
     When the opposite electrode  230  includes a semi-transparent electrode, the opposite electrode  230  may include a metal layer having a low-work function, for example, lithium (Li), calcium (Ca), lithium fluoride calcium (LiF/Ca), lithium fluoride aluminum (LiF/Al), aluminum (Al), silver (Ag), magnesium (Mg), and/or a mixture thereof, and a (semi-) transparent conductive layer of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO), or indium oxide (In 2 O 3 ). When the opposite electrode  230  includes a reflective electrode, the opposite electrode  230  may include a layer including lithium (Li), calcium (Ca), lithium fluoride calcium (LiF/Ca), lithium fluoride aluminum (LiF/Al), aluminum (Al), silver (Ag), magnesium (Mg), and/or a mixture thereof. In an embodiment, a structure and a material of the opposite electrode  230  may be changeable variously. 
     Referring to  FIG. 4 , the encapsulation unit  300  may be disposed on the flexible substrate  100  to cover the display unit  200 . The encapsulation unit  300  may be a multilayer including at least one inorganic film and an organic film  320 . The encapsulation unit  300  may effectively seal the display unit  200  from oxygen and moisture from outside by including the multilayer. In the present embodiment, the encapsulation unit  300  may include the first inorganic film  310 , the second inorganic film  330 , and the organic film  320  disposed between the first inorganic film  310  and the second inorganic film  330 . 
     The first inorganic film  310  and the second inorganic film  330  may include an inorganic compound and may include the same material or different materials. The materials forming the first inorganic film  310  and the second inorganic film  330  may be independent, for example, one or more of silicon nitride, aluminum nitride, zirconium nitride, titanium nitride, hafnium nitride, tantalum nitride, silicon oxide, aluminum oxide, titanium oxide, tin oxide, cerium oxide, or silicon oxynitride (SiON). 
     The organic film  320  may include an organic compound and may include one or more of acryl resin, methacrylic resin, polyisoprene, vinyl resin, epoxy resin, urethane resin, cellulose resin, or perylene resin. The organic film  320  may be formed by using a liquid phase deposition process, for example, an inkjet printing process. 
       FIG. 4  illustrates a cross-sectional view of the display unit  200  including a boundary of the bending area BA and the non-bending area NBA. As stated above, the organic film  320  may have the first thickness t 1  in the bending area BA and the second thickness t 2  in the non-bending area NBA. The first thickness t 1  of the organic film  320  in the bending area BA may be less than the second thickness t 2  of the organic film  320  in the non-bending area NBA. The flexible display apparatus  1  according to an embodiment may reduce, minimize, or eliminate stress applied to the display unit  200  during bending and folding by partially adjusting a thickness of the organic film  320  of the encapsulation unit  300 . The organic film  320  in the bending area BA may be formed to be relatively thinner than the organic film  320  in the non-bending area NBA to control a position of the neutral plane inside the flexible display apparatus  1 . The structure vulnerable to stress may be positioned on the neutral plane to realize a flexible display apparatus  1  that may have minimized stress. 
       FIG. 5  illustrates a schematic plan view of a flexible display apparatus  2  according to an embodiment,  FIG. 6  illustrates a schematic side view of a bent structure of the flexible display apparatus  2  of  FIG. 5 , and  FIG. 7  illustrates a schematic cross-sectional view of the flexible display apparatus  2  along a line VII-VII′ of  FIG. 5 . A display unit  200  of the flexible display apparatus  2  illustrated in  FIGS. 5 to 7  may have the same structure as or a similar structure to the display unit  200  of  FIG. 4 , and descriptions of the display unit  200  of  FIGS. 5 to 7  correspond to the descriptions of the display unit of  FIG. 4 . 
     Referring to  FIGS. 5 to 7 , the flexible display apparatus  2  according to an embodiment may include a flexible substrate  100 , the display unit  200  disposed on the flexible substrate  100  having a bending area BA and a non-bending area NBA, and an encapsulation unit  300 . The encapsulation unit  300  according to an embodiment may include a first inorganic film  310 , a second inorganic film  330 , and an organic film  320  disposed between the first inorganic film  310  and the second inorganic film  330 . 
     The flexible substrate  100  may have a characteristic of flexibility and may include various materials, for example, a metal material or a plastic material such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or polyimide. The flexible substrate  100  may include a display area formed with the display unit  200  and a peripheral area disposed to surround the display area. 
     The flexible substrate  100  may have a bendable or foldable structure due to, for example, the flexible characteristic. In the present embodiment, the flexible substrate  100  may include a bending area BA and a non-bending area NBA, and the non-bending area NBA may be between a first bending area BA 1  and a second bending area BA 2  as illustrated in  FIG. 5 . The flexible display apparatus  2  according to the present embodiment may be bendable such that the first bending area BA 1  and the second bending area BA 2  have predetermined curvatures. As illustrated in  FIG. 6 , the first bending area BA 1  may have a first curvature radius R 1  and the second bending BA 2  may have a second curvature radius R 2  greater than the first curvature radius R 1 . The first bending area BA 1  may have a curvature greater than that of the second bending area BA 2 . In an embodiment, the first bending area BA 1  and the second bending area BA 2  may have a same curvature. 
     The display unit  200  may be disposed on the flexible substrate  100 . The display unit  200  may be disposed inside or outside the flexible substrate  100 . 
     Referring to  FIG. 7 , the display unit  200  may be disposed on the flexible substrate  100  and may be disposed in a display area DA. The display unit  200  may include a plurality of pixels and also include thin film transistors TFTs and CAPs to supply electrical power to the respective pixels. The display unit  200  may be an organic light-emitting display unit or a liquid crystal display unit. The present embodiment is a case in which the display unit  200  is an organic light-emitting display unit. 
     The encapsulation unit  300  may include the first inorganic film  310 , the second inorganic film  330 , and the organic film  320  disposed between the first inorganic film  310  and the second inorganic film  330 . The encapsulation unit  300  may cover the display unit  200  and seal the display unit  200  from external oxygen and moisture. The first inorganic film  310  and the second inorganic film  330  may include an inorganic compound, for example, silicon oxide or silicon nitride. 
     The organic film  320  disposed between the first inorganic film  310  and the second inorganic film  3330  may have a thickness of the non-bending area NBA thicker than those of the bending areas BA 1  and BA 2 . In the present embodiment, the first bending area BA 1  may have a third thickness t 3 , the second bending area BA 2  may have a fourth thickness t 4 , and the non-bending area NBA may have a fifth thickness t 5 . The third and fourth thicknesses t 3  and t 4  of the organic film  320  in the bending areas BA 1  and BA 2  may be relatively less than the fifth thickness t 5  of the organic film  320  in the non-bending area NBA. 
     As describe above, the first bending area BA 1  may have a first curvature radius R 1  and the second bending BA 2  may have a second curvature radius R 2  greater than the first curvature radius R 1 , and the first bending area BA 1  may have a curvature greater than that of the second bending area BA 2 . The organic film  320  disposed in the first bending area BA 1  may have the third thickness t 3  less than the fourth thickness t 4  of the organic film  320  in the second bending area BA 2 . 
     The organic film  320  may be formed by using the liquid phase deposition process, for example, the inkjet printing process. A thickness of the organic film  320  may be adjusted by controlling the amount of an organic compound applied during forming the organic film  320 . 
     The organic film  320  of the flexible display apparatus  2  according to an embodiment may minimize the stress in the bending areas BA 1  and BA 2  when being bent or folded in the bending areas BA 1  and BA 2 . When the flexible display apparatus  2  is bent, there may exist a neutral plane without stress since the tensile force and the compression force may be same inside the flexible display apparatus  2 , and the stress in the flexible display apparatus  2  may increase according to a distance from the neutral plane. When a structure vulnerable to the stress is disposed on an upper side or a lower side with respect to the neutral plane, the structure may be exposed to continuing stress from the tensile force and the compression force. The stress that may be applied to the structure may be reduced when the neutral plane moves to a position of the structure by partially adjusting a thickness of the flexible display apparatus  2 . 
     In the flexible display apparatus  2  according to an embodiment a thickness of the organic film  320  of the encapsulation unit  300  formed on a bending area may be controlled. The organic film  320  in the bending areas BA 1  and BA 2  may be relatively thinner than that of the non-bending area NBA to control a position of the neutral plane. By disposing the structure vulnerable to the stress on the neutral plane, a flexible display apparatus  2  having minimized stress may be realized. 
     Embodiments include the flexible display apparatus, which has been explained. Embodiments also include a manufacturing method of a flexible display apparatus to manufacture such a flexible display apparatus. 
     Referring  FIGS. 1 to 4 , the method may include preparing the flexible substrate  100  having the bending area BA and the non-bending area NBA. The flexible substrate  100  may have a flexible characteristic and may include various materials, for example, a metal material or a plastic material such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or polyimide. In the present embodiment, the flexible substrate  100  may include the bending area BA and the non-bending area NBA, and the bending area BA may be disposed between the first non-bending area NBA 1  and the second non-bending area NBA 2  as illustrated in  FIG. 1 .  FIG. 2  illustrates, for example, a structure in which the first non-bending area NBA 1  and the second non-bending area NBA 2  may be disposed parallel to each other to form a folded structure. In an embodiment, it may be possible to change the structure to various forms according to a bending degree of the bending area BA. 
     The method may include forming the display unit  200  on the flexible substrate  100 . The display unit  200  may be disposed inside or outside of the flexible substrate  100 . 
     Referring to  FIG. 4 , various components of the display unit  200  of the flexible display apparatus  1  may be disposed on the flexible substrate  100 . Common layers, such as, the buffer layer  201 , the gate insulation film  203 , the interlayer insulation film  205 , may be formed on the entire flexible substrate  100 . After the buffer layer  201  is formed, a patterned semiconductor layer  202  may be formed on the buffer layer  201 . The gate electrode  204 , the source electrode  206   s , and the drain electrode  207   d  as components of the thin film transistor TFT may be formed together with the pattern semiconductor layer  202 . 
     The planarization film  207  having a flat upper surface may be formed on the entire flexible substrate  100  to cover the thin film transistor TFT. A patterned pixel electrode  210  may be formed on the planarization film  207 . The pixel electrode  210  may be electrically connected to the thin film transistor TFT through a via hole formed in the planarization film  207 . Thereafter, the pixel-defining film  208  may be formed to expose a center portion of the pixel electrode  210  and to cover a boundary portion of the pixel electrode  210 . The pixel-defining film  208  may define the pixel area. The intermediate layer  220  including a light-emitting layer may be formed in the pixel area defined by the pixel-defining film  208 . The intermediate layer  220  may have one layer as a common layer to partially correspond to the flexible substrate  100  or the other layer as a patterned layer patterned to correspond to the pixel electrode  210 . Thereafter, the opposite electrode  230  may be disposed on the entire flexible substrate  100 . 
     Now referring to  FIGS. 3 and 4 , the encapsulation unit  300  may be formed on an upper side of the opposite electrode  230 . In the drawings, the encapsulation unit  300  is illustrated as, for example, directly formed on the encapsulation unit  300 . In an embodiment, a functional layer, such as, a capping layer (CLP), may be formed on the opposite electrode  230  and then the encapsulation unit  300  may be formed thereon. 
     First, the first inorganic film  310  may be formed on the opposite electrode  230 . Thereafter, the organic film  320  may be formed on the first inorganic film  310 . The organic film  320  may be formed by using the liquid phase deposition process, for example, the inkjet printing process. The second inorganic film  330  may be formed on the organic film  320 . The first inorganic film  310  and the second inorganic film  330  may include an inorganic compound, for example, silicon oxide or silicon nitride. The encapsulation unit  300  may include a multilayer having at least one inorganic film and the organic film  320 . The encapsulation unit  300  having such a structure may seal the display unit  200  from external oxygen and moisture. 
     The organic film  320  disposed between the first inorganic film  310  and the second inorganic film  330  may have the first thickness t 1  in the bending area BA and the second thickness t 2  in the first and second non-bending areas NBA 1  and NBA 2 . The first thickness t 1  of the organic film  320  may be thinner than the second thickness. The second thickness t 2  of the organic film  320  in the first and second non-bending areas NBA 1  and NBA 2  may be thicker that the first thickness t 1  of the organic film  320  in the bending area BA. The organic film  320  may have a thickness in the first and second non-bending areas NBA 1  and NBA 2  relatively thicker than the bending area BA by adjusting the amount of the organic compound when the organic film  320  may be formed. 
     The organic film  320  of the flexible display apparatus  1  according to an embodiment may minimize the stress in the bending area BA when being bent or folded in the bending area BA. When the flexible display apparatus  1  is bent, there may exist a neutral plane without stress since the tensile force and the compression force may be same inside the flexible display apparatus  1 , and the stress in the flexible display apparatus  1  may increase according to a distance from the neutral plane. When a structure vulnerable to the stress is disposed on an upper side or a lower side with respect to the neutral plane, the structure may be exposed to continuing stress from the tensile force and the compression force. The stress that may be applied to the structure may be reduced when the neutral plane moves to a position of the structure by partially adjusting a thickness of the flexible display apparatus  1 . 
     In the flexible display apparatus  1  according to an embodiment a thickness of the organic film  320  of the encapsulation unit  300  formed in the bending area BA may be partially adjusted. A position of the neutral plane may be controlled by forming the organic film  320  in the bending area BA to be relatively thinner than in the first and second non-bending areas NBA 1  and BNA 2 . A flexible display apparatus  1  that may have minimized stress may be realized by displacing the structure vulnerable to the stress on the neutral plane. 
     By way of summation and review, an organic light-emitting device may include thin film transistors and organic light-emitting elements formed on a substrate, and the organic light-emitting elements themselves may emit light. The organic light-emitting device may be used as a display unit of a small product, such as, a mobile phone, or a large product, such as, a television. 
     A flexible display apparatus may include a flexible substrate including a material, such as, a resin composite, rather than a comparative glass substrate to have flexibility. Mechanical damage to a layer disposed in a bending area of a flexible display apparatus may occur, due to, for example, stress applied to the bending area while bending the flexible display apparatus. 
     One or more exemplary embodiments relate to a flexible display apparatus that may have reduced stress inside the flexible display apparatus during bending and a manufacturing method thereof. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.