Patent Publication Number: US-2022223807-A1

Title: Flexible display apparatus and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Division of U.S. patent application Ser. No. 16/578,408, filed Sept. 23, 2019, which claims priority to and the benefit of Korean Patent Application No. 10-2018-0137566, filed Nov. 9, 2018, each of which is hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND 
     Field 
     Exemplary embodiments/implementations of the invention relate generally to a flexible display apparatus and a manufacturing method thereof, and more specifically, to a transparent flexible display apparatus and a manufacturing method thereof. 
     Discussion of the Background 
     A display apparatus is an apparatus for displaying an image. In recent years, an organic light-emitting display apparatus has been spotlighted. The organic light-emitting display apparatus has such features as low power consumption, high luminance, and high responding speed. 
     The organic light emitting apparatus may have a flexible property by adopting a flexible organic light emitting display panel and a transparent property by adopting a transparent organic light emitting display panel. In order to manufacture the organic light emitting display panel, a method of bonding a flexible substrate on a substrate, then performing a following process, and separating the flexible substrate from the substrate is introduced. 
     However, in a process of using laser to separate the flexible substrate from the substrate, they may not separated uniformly from each other, or the flexible display apparatus is thermally deteriorated to degrade its transmittance. 
     The above information disclosed in this Background section is only for understanding of the background of the inventive concepts, and, therefore, it may contain information that does not constitute prior art. 
     SUMMARY 
     The present disclosure provides a transparent flexible display apparatus that has a high transmittance by easily separating a substrate and a flexible substrate without being irradiated with laser. 
     The present disclosure also provides a method of manufacturing a transparent flexible display apparatus in which a substrate and a flexible substrate is not easily separated in a process of forming a display unit due to direct contact between the substrate and the flexible substrate. 
     Additional features of the inventive concepts will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the inventive concepts. 
     An embodiment of the inventive concept provides a method of manufacturing a flexible display apparatus, the method including: preparing a substrate; forming a first charge adhesive layer having a first charge on the substrate; forming a second charge adhesive layer having a second charge, which is opposite to the first charge, on the first charge adhesive layer; forming a first charge adhesive pattern and a second charge adhesive pattern by removing an edge of each of the first charge adhesive layer and the second charge adhesive layer; forming a flexible substrate on the substrate on which the first charge adhesive pattern and the second charge adhesive pattern are formed; forming a display unit on the flexible substrate; cutting the substrate, the first charge adhesive pattern, the second charge adhesive pattern, the flexible ubstrate, and the display unit along a cutting line; and separating the substrate and the flexible substrate. 
     In an embodiment, the first charge adhesive layer may include graphene oxide. 
     In an embodiment, the second charge adhesive layer may include montmorillonite, molybdenum disulfide, or hexagonal boron nitride (hBN). 
     In an embodiment, the forming of the first charge adhesive pattern and the second charge adhesive pattern may remove a portion of each of the first charge adhesive layer and the second charge adhesive layer by spraying a gas in a plasma state in which nitrogen and air are mixed. 
     In an embodiment, a portion of a top surface of the substrate may be exposed by the forming of the first charge adhesive pattern and the second charge adhesive pattern. 
     In an embodiment, the flexible substrate may contact the portion of the substrate, which is exposed by the first charge adhesive pattern and the second charge adhesive pattern, by the forming of the flexible substrate. 
     In an embodiment, the forming of the flexible substrate may form the flexible substrate by applying a polymer material on an area greater than that occupied by the first charge adhesive pattern and the second charge adhesive pattern. 
     In an embodiment, the cutting line may be defined to overlap the first charge adhesive pattern and the second charge adhesive pattern on a plane. 
     In an embodiment, the cutting line may have a closed loop shape on a plane, and be defined along an edge of each of the first charge adhesive pattern and the second charge adhesive pattern. 
     In an embodiment, the second charge adhesive layer may be remained on the flexible substrate by the separating of the substrate and the flexible substrate. 
     In an embodiment of the inventive concept, a method of manufacturing a flexible display apparatus includes: preparing a substrate; forming a lower first charge adhesive layer having a first charge on the substrate; forming a lower second charge adhesive layer having a second charge, which is opposite to the first charge, on the lower first charge adhesive layer; forming an upper first charge adhesive layer having the first charge on the lower second charge adhesive substrate; forming an upper second charge adhesive layer having the second charge on the upper first charge adhesive layer; forming a lower first charge adhesive pattern, a lower second charge adhesive pattern, an upper first charge adhesive pattern, and an upper second charge adhesive pattern by removing an edge of each of the lower first charge adhesive layer, the lower second charge adhesive layer, the upper first charge adhesive layer, and the upper second charge adhesive layer; forming a flexible substrate on the substrate on which the lower first charge adhesive pattern, the lower second charge adhesive pattern, the upper first charge adhesive pattern, and the upper second charge adhesive pattern; forming a display unit on the flexible substrate; cutting the substrate, the lower first charge adhesive pattern, the upper second charge adhesive pattern, the upper first charge adhesive pattern, the upper second charge adhesive pattern, the flexible substrate, and the display unit along a cutting line; and separating the substrate and the flexible substrate. 
     In an embodiment, the lower first charge adhesive layer may include graphene oxide, and the upper first charge adhesive layer may include polyethyleneimine (PEI). 
     In an embodiment, each of the lower second charge adhesive layer and the upper second charge adhesive layer may include montmorillonite, molybdenum disulfide, or hexagonal boron nitride (hBN). 
     In an embodiment, the upper first charge adhesive pattern and the upper second charge adhesive pattern may be remained on the flexible substrate by the separating of the substrate and the flexible substrate. 
     In an embodiment, the lower second charge adhesive pattern, the upper first charge adhesive pattern, and the upper second charge adhesive pattern may be remained on the flexible substrate by the separating of the substrate and the flexible substrate. 
     In an embodiment of the inventive concept, a flexible display apparatus includes: a flexible substrate; a circuit element layer disposed on the flexible substrate; a display element layer disposed on the circuit element layer and including an organic light emitting diode; an encapsulation layer disposed on the display element layer to seal the display element layer; and a charge adhesive layer that contacts a lower portion of the flexible substrate, has a charge, and includes montmorillonite, molybdenum disulfide, or hexagonal boron nitride (hBN). 
     In an embodiment, the charge adhesive layer may have a bottom surface having an concave-convex shape. 
     In an embodiment, the concave-convex of the bottom surface of the charge adhesive layer may be randomly distributed. 
     In an embodiment, the concave-convex of the bottom surface of the charge adhesive layer may have a height of about 1 nm or less. 
     In an embodiment, the flexible display apparatus may further include a lower charge adhesive layer that contacts a lower portion of the charge adhesive layer, and the lower charge adhesive layer may include polyethyleneimine (PEI). 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention, and together with the description serve to explain the inventive concepts. 
         FIG. 1  is a flowchart representing a method of manufacturing a flexible display apparatus according to an embodiment of the inventive concept; 
         FIGS. 2A, 3A, 4A, and 5A  are plan views illustrating a manufacturing process of forming a flexible display apparatus according to an embodiment of the inventive concept, and 
         FIGS. 2B, 3B, 4B, 5B, 6, and 7  are cross-sectional views illustrating the manufacturing process of forming a flexible display apparatus according to an embodiment of the inventive concept; 
         FIG. 8  is a graph showing a test result of adhesion forces of a first charge adhesive pattern and a second charge adhesive pattern; 
         FIG. 9  is a photograph showing a bottom surface of a transparent flexible display module according to an embodiment of the inventive concept in  FIG. 7 ; 
         FIG. 10A  is a plan view illustrating a transparent flexible display module DPA in  FIG. 7 , and  FIG. 10B  is a cross-sectional view taken along line II-II′ of  FIG. 10A ; and 
         FIGS. 11, 12, and 13  are cross-sectional views illustrating a manufacturing process for forming a flexible display apparatus according to another embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of various exemplary embodiments or implementations of the invention. As used herein “embodiments” and “implementations” are interchangeable words that are non-limiting examples of devices or methods employing one or more of the inventive concepts disclosed herein. It is apparent, however, that various exemplary embodiments may be practiced without these specific details or with one or more equivalent arrangements. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring various exemplary embodiments. Further, various exemplary embodiments may be different, but do not have to be exclusive. For example, specific shapes, configurations, and characteristics of an exemplary embodiment may be used or implemented in another exemplary embodiment without departing from the inventive concepts. 
     Unless otherwise specified, the illustrated exemplary embodiments are to be understood as providing exemplary features of varying detail of some ways in which the inventive concepts may be implemented in practice. Therefore, unless otherwise specified, the features, components, modules, layers, films, panels, regions, and/or aspects, etc. (hereinafter individually or collectively referred to as “elements”), of the various embodiments may be otherwise combined, separated, interchanged, and/or rearranged without departing from the inventive concepts. 
     The use of cross-hatching and/or shading in the accompanying drawings is generally provided to clarify boundaries between adjacent elements. As such, neither the presence nor the absence of cross-hatching or shading conveys or indicates any preference or requirement for particular materials, material properties, dimensions, proportions, commonalities between illustrated elements, and/or any other characteristic, attribute, property, etc., of the elements, unless specified. Further, in the accompanying drawings, the size and relative sizes of elements may be exaggerated for clarity and/or descriptive purposes. When an exemplary 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. Also, like reference numerals denote like elements. 
     When an element, such as a layer, is referred to as being “on,” “connected to,” or “coupled to” another element or layer, it may be directly on, connected to, or coupled to the other element or layer or intervening elements or layers may be present. When, however, an element or layer is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. To this end, the term “connected” may refer to physical, electrical, and/or fluid connection, with or without intervening elements. Further, the D1-axis, the D2-axis, and the D3-axis are not limited to three axes of a rectangular coordinate system, such as the x, y, and z-axes, and may be interpreted in a broader sense. For example, the D1-axis, the D2-axis, and the D3-axis may be perpendicular to one another, or may represent different directions that are not perpendicular to one another. For the purposes of this disclosure, “at least one of X, Y, and Z” and “at least one selected from the group consisting of X, Y, and Z” may be construed as X only, Y only, Z only, or any combination of two or more of X, Y, and Z, such as, for instance, XYZ, XYY, YZ, and ZZ. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms “first,” “second,” etc. may be used herein to describe various types of elements, these elements should not be limited by these terms. These terms are used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the teachings of the disclosure. 
     Spatially relative terms, such as “beneath,” “below,” “under,” “lower,” “above,” “upper,” “over,” “higher,” “side” (e.g., as in “sidewall”), and the like, may be used herein for descriptive purposes, and, thereby, to describe one elements relationship to another element(s) as illustrated in the drawings. Spatially relative terms are intended to encompass different orientations of an apparatus in use, operation, and/or manufacture in addition to the orientation depicted in the drawings. For example, if the apparatus in the drawings is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. Furthermore, the apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations), and, as such, the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. As used herein, the singular forms, “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It is also noted that, as used herein, the terms “substantially,” “about,” and other similar terms, are used as terms of approximation and not as terms of degree, and, as such, are utilized to account for inherent deviations in measured, calculated, and/or provided values that would be recognized by one of ordinary skill in the art. 
     Various exemplary embodiments are described herein with reference to sectional and/or exploded illustrations that are schematic illustrations of idealized exemplary embodiments and/or intermediate structures. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments disclosed herein should not necessarily be construed as limited to the particular illustrated shapes of regions, but are to include deviations in shapes that result from, for instance, manufacturing. In this manner, regions illustrated in the drawings may be schematic in nature and the shapes of these regions may not reflect actual shapes of regions of a device and, as such, are not necessarily intended to be limiting. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure is a part. Terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and should not be interpreted in an idealized or overly formal sense, unless expressly so defined herein. 
     Hereinafter, a display apparatus according to an embodiment of the inventive concept will be described with reference to the accompanying drawings. 
       FIG. 1  is a flowchart representing a method of manufacturing a flexible display apparatus according to an embodiment of the inventive concept,  FIGS. 2A, 3A, 4A, and 5A  are plan views illustrating a process of manufacturing the flexible display apparatus according to an embodiment of the inventive concept, and  FIGS. 2B, 3B, 4B, 5B, 6, and 7  are cross-sectional views illustrating a process of manufacturing the flexible display apparatus according to an embodiment of the inventive concept.  FIGS. 2B, 3B, 4B, and 5B  are cross-sectional views taken along line I-I′ of  FIGS. 2A, 3A, 4A, and 5A , respectively. 
     Referring to  FIG. 1 , first, silicon powder is prepared in operation S 10 . 
     A substrate  100  of  FIG. 2B  may be a glass substrate, a polymer film, or a silicon wafer that support a flexible substrate  300  (refer to  FIGS. 4A and 4B ) in order to form a flexible display apparatus. However, the embodiment of the inventive concept is not limited thereto. For example, the substrate  100  may be made of any material having a charge. 
     Thereafter, a process of treating a surface of the substrate  100  so that the surface of the substrate  100  (refer to  FIGS. 2A and 2B ) has a second charge that is opposite to a first charge may be added. Here, the substrate  100  may be plasma-treated to have the second charge. In an embodiment of the inventive concept, the second charge may be a negative charge. 
     Referring to  FIGS. 1, 2A, and 2B , a first charge adhesive layer having the first charge is formed on the substrate  100  in operation S 10 . 
     A solution containing a graphene oxide having the first charge may be applied on the substrate  100 . Besides dip coating, a coating method may be one selected from the group consisting of spray coating, spin coating, screen coating, offset printing, inkjet printing, pad printing, knife coating, kiss coating, gravure coating, brushing, ultrasonic micro-spraying coating, and spray-mist spraying coating. However, the embodiment of the inventive concept is not limited thereto and any coating method may be used as needed. 
     Thereafter, the first charge adhesive layer  210   a  having the first charge may be formed by performing a process of removing foreign substances in the graphene oxide solution applied on the substrate  100  and a process of drying. In an embodiment of the inventive concept, the first charge may be a positive charge. 
     Thereafter, a second charge adhesive layer  220   a  having the second charge is formed on the first charge adhesive layer  210   a  in operation S 20 . 
     The second charge adhesive layer  220   a  may include one of transparent montmorillonite, molybdenum disulfide (MoS2), and hexagonal boron nitride (hBN). A solution containing montmorillonite, molybdenum disulfide (MoS2), or hexagonal boron nitride (hBN) having the second charge may be applied on the first charge adhesive layer  210   a . Besides dip coating, a coating method may be one selected from the group consisting of spray coating, spin coating, screen coating, offset printing, inkjet printing, pad printing, knife coating, kiss coating, gravure coating, brushing, ultrasonic micro-spraying coating, and spray-mist spraying coating. However, the embodiment of the inventive concept is not limited thereto and any other coating method may be included. 
     Thereafter, the second charge adhesive layer  220   a  having the second charge may be formed by performing a process of removing foreign substances in the solution containing montmorillonite, molybdenum disulfide (MoS2), or hexagonal boron nitride (hBN) and a process of drying. In an embodiment of the inventive concept, the second charge may be a negative charge. 
     Each of the montmorillonite and the molybdenum disulfide may be substantially a transparent material. Accordingly, as described later, although the solution may partially remain on a rear surface of a flexible substrate while manufacturing the transparent flexible display, high transmittance may be secured. 
     In an embodiment of the inventive concept described with reference to  FIGS. 2A and 2B , although a pair of the first charge adhesive layer  210   a  and the second charge adhesive layer  220   a  is exemplarily described, the process described with reference to  FIGS. 2A and 2B  may be repeated to form two or more pairs thereof. 
     Thereafter, referring to  FIGS. 1, 3A, and 3B , a first charge adhesive pattern  210  and a second charge adhesive pattern  220  are formed by removing a portion of an edge of each of the first charge adhesive layer  210   a  and the second charge adhesive layer  220   a  in operation S 30 . 
     A first area AR 1  and a second area AR 2  may be defined on the laminated first and second charge adhesive layers  210   a  and  220   a  on a plane. The first area AR 1  may have a rectangular shape. The second area AR 2  may surround the first area AR 1  as an edge area of the first area AR 1 . However, the embodiment of the inventive concept is not limited thereto. For example, the second area AR 2  may surround three sides of the first area AR 1  instead of completely surrounding the first area AR 1 . 
     Thereafter, a portion of each of the first charge adhesive layer  210   a  and the second charge adhesive layer  220   a , which corresponds to the second area AR 2 , may be removed by spraying a gas in a plasma state, in which nitrogen and air are mixed. 
     The operation S 30  is performed, and then a portion of a top surface of the substrate  100  may be exposed. 
     Referring to  FIGS. 1, 4A, and 4B , thereafter, a flexible substrate  300  is formed on the substrate  100  on which the second charge pattern  220  is formed in operation S 40 . 
     The flexible substrate  300  is a substantially transparent or transflective plastic substrate having flexibility. The flexible substrate  300  may be made of the material selected from the group consisting of polyester, polyvinyl, polycarbonate, polyethylene, polyacetate, polyimide, polyethersulphone (PES), polyacrylate (PAR), polyethylenenaphthalate (PEN), and polyethyleneterephehalate (PET). 
     The flexible substrate  300  may be formed by applying and curing a polymer material on the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . Here, the polymer material may be applied on an area greater than that occupied by the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . The applied polymer material may entirely cover the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . Besides dip coating, a coating method may be one selected from the group consisting of spray coating, spin coating, screen coating, offset printing, inkjet printing, pad printing, knife coating, kiss coating, gravure coating, brushing, ultrasonic micro-spraying coating, and spray-mist spraying coating. However, the embodiment of the inventive concept is not limited thereto. For example, the coating method may include another coating method. 
     Through the operation S 40 , the flexible substrate  300  may contact a portion of the top surface of the substrate, which is exposed in the operation S 30 . 
     Thereafter, referring to  FIGS. 1, 5A, and 5B , a display unit  400  is formed on the flexible substrate  300  in operation S 50 . 
     The display unit  400  may be formed by sequentially forming a circuit element layer, a display element layer, and an encapsulation layer. A structure of the display unit  400  will be described later, and description on a detailed method of forming the display unit  400  will be omitted. 
     Referring to  FIGS. 4A, 4B, 5A, and 5B , a central area CTA and a peripheral area RPA may be defined on the flexible substrate  300 . The central area CTA overlaps the first charge adhesive pattern  210  and the second charge adhesive pattern  220 , and the peripheral area PRA does not overlap the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . 
     The first charge adhesive pattern  210  and the second charge adhesive pattern  220 , which have polarities opposite to each other, between the substrate  100  and the flexible substrate  300  on the central area CTA. The Van der Waals force, which is a kind of molecular force, acts between the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . Accordingly, an adhesion force between the substrate  100  and the flexible substrate  300  on the central area CTA is relatively low. 
     The substrate  100  and the flexible substrate  300  directly contact each other on the peripheral area PRA. The adhesion force between the substrate  100  and the flexible substrate  300  on the peripheral area PRA is relatively greater than that between the substrate  100  and the flexible substrate  300  on the central area CTA. The adhesion force between the substrate  100  and the flexible substrate  300  may increase at the peripheral area PRA in which the substrate  100  and the flexible substrate  300  directly contact each other through a process of forming the display unit  400  on the flexible substrate  300 , particularly, a process of forming a barrier layer on the flexible substrate  300  for preventing moisture or oxygen from entering. 
     In forming the display unit  400 , a module on which the substrate  100 , the first charge adhesive pattern  210 , the second charge adhesive pattern  220 , and the flexible substrate  300  are formed may be moved or turned-over. In peripheral are PRA, the flexible substrate  300  directly laid on the substrate  100  with stronger adhesion than center area CTA that has different charge areas therebetween. Without the peripheral area PRA where the flexible substrate  300  directly contacts the substrate  100 , the substrate  100  and the flexible substrate  300  may be easily separated, and subsequent processes may not be performed, because the substrate  100  and the flexible substrate  300  in the center area CTA have a low adhesion force due to the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . 
     In the method of manufacturing the flexible display apparatus according to an embodiment of the inventive concept, since the substrate  100  and the flexible substrate  300  directly contact each other in the peripheral area PRA to have a relatively great adhesion force therebetween, the substrate  100  and the flexible substrate  300  may not be easily separated in the process of forming the display unit  400 . 
     The first charge adhesive pattern  210  and the second charge adhesive pattern  220  disposed between the substrate  100  and the flexible substrate  300  have a relatively weak adhesion force than that of an area in which the substrate  100  and the flexible substrate  300  directly contact each other. However, the first charge adhesive pattern  210  and the second charge adhesive pattern  220  are necessary to have an adhesion force not less than a reference value. When the first charge adhesive pattern  210  and the second charge adhesive pattern  220  have an adhesion force less than the reference value, and the module on which the substrate  100 , the first charge adhesive pattern  210 , the second charge adhesive pattern  220 , and the flexible substrate  300  are formed is turned over, although the substrate  100  and the flexible substrate  300  are attached to each other in the peripheral area PRA, as the substrate  100  and the flexible substrate  300  are separated from each other in the central area CTA, the flexible substrate  300  may be deflected due to an own weight. 
     Thus, the first charge adhesive pattern  210  and the second charge adhesive pattern  220  may have an adhesion force of a range of about 1 gf/in to about 10 gf/in. 
       FIG. 8  is a graph showing a test result of adhesion forces of the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . In  FIG. 8 , adhesion forces when two to four pairs of first charge adhesive pattern  210  and the second charge adhesive pattern  220  are provided. 
     When the two to four pairs of first charge adhesive pattern  210  and the second charge adhesive pattern  220  are provided, the adhesion force therebetween satisfies the range of about 1 gf/in to about 10 gf/in, particularly a range of 3 gf/in to about 8 gf/in. 
     Referring to  FIGS. 5A to 5B , the substrate  100 , the first charge adhesive pattern  210 , the second charge adhesive pattern  220 , the flexible substrate  300 , and the display unit  400  are cut along a cutting line CTL in operation S 60 . 
     The cutting line CTL may overlap the first charge adhesive pattern  210  and the second charge adhesive pattern  220  on a plane. The cutting line CTL may have a closed loop shape on the plane and be defined along an edge of each of the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . 
     Accordingly, the peripheral area PRA of the flexible substrate PRA is removed, and only the central area CTA remains. That is, the substrate  100  and the flexible substrate  300  are attached to each other by only the first charge adhesive pattern  210  and the second charge adhesive pattern  220 . 
     Referring to  FIG. 6 , as the substrate  100 , the first charge adhesive pattern  210 , the second charge adhesive pattern  220 , the flexible substrate  300 , and the display unit  400  are cut, a display module DM is generated. 
     Thereafter, referring to  FIGS. 1 and 7 , the substrate  100  and the flexible substrate  300  are separated from the display module DM in operation S 70 . 
     The substrate  100  and the flexible substrate  300  may be separated by using a vacuum suction device. The substrate  100  and the flexible substrate  300  may be separated by fixing the display unit  400  on a vacuum suction state, and then suctioning the substrate  100  to apply a force to the substrate  100  in an upward direction. However, the embodiment of the inventive concept is not limited thereto. For example, the substrate  100  and the flexible substrate  300  may be separated through various other methods. 
     As a result of separating the substrate  100  and the flexible substrate  300 , a flexible display module DPA is generated. 
     The second charge adhesive pattern  220  may remain at the bottom of the separated flexible substrate  300 , and the first charge adhesive pattern  210  may remain at a top surface of the substrate  100 . 
     Since the second charge adhesive pattern  220  is substantially transparent, the second charge adhesive pattern  220  may not substantially affect a light transmittance of the flexible display module DPA. 
     Thereafter, the flexible display module DPA is cut to form a plurality of display panels DP. Thereafter, a transparent flexible display apparatus may be manufactured by using the display panel DP. 
       FIG. 9  is a photograph showing a bottom surface of the transparent flexible display module DPA according to an embodiment of the inventive concept in  FIG. 7 . 
     Referring to  FIGS. 7 and 9 , when the second charge adhesive pattern  220  and the first charge adhesive pattern  210  are separated, a portion of a surface of the second charge adhesive pattern  220  may be also removed. Accordingly, the bottom surface of the second charge adhesive pattern  220  may have concave-convex shape. The concave-convex shape of the second charge adhesive pattern  220  may be randomly distributed on the plane. Also, the concave-convex of the second charge adhesive pattern  220  may have a height T 1  of about 1 nm or less. 
     Through the method of manufacturing the flexible display apparatus according to an embodiment of the inventive concept, since the adhesion force between the substrate  100  and the flexible substrate  300  have a relatively lower than that between the first charge adhesive pattern  210  and the second charge adhesive pattern  220 , the substrate  100  and the flexible substrate  300  may be easily separated without performing an additional process. 
     When a process of weakening the adhesion force between the substrate  100  and the flexible substrate  300  by irradiating laser from therebelow is additionally performed, besides the additional process, the bottom surface of the flexible substrate  300  is burned or becomes to have a yellow color (yellowing, yellow index increase) due to the laser irradiation. Thus, the additional process is not suitable for the transparent flexible display apparatus. 
     The method of manufacturing the flexible display apparatus according to an embodiment of the inventive concept may prevent the bottom surface of the flexible substrate  300  from burning, and being yellow by easily separating the substrate  100  and the flexible substrate  300  without laser radiation, and thus enabling to manufacture the transparent flexible display apparatus with a higher transmittance. 
       FIG. 10A  is a plan view illustrating the transparent flexible display module DPA in  FIG. 7 , and  FIG. 10B  is a cross-sectional view taken along line II-II′ of  FIG. 10A .  FIG. 10B  is a cross-sectional view illustrating a portion of the display panel DP, particularly, a portion corresponding to one pixel. 
     Referring to  FIGS. 10A and 10B , the transparent flexible display module DPA may include a plurality of display panels DP. The display panels DP may be spaced apart from each other. The display panels DP may be individually cut and adopted to the display apparatus. 
     Referring to  FIG. 10B , the display unit  400  may include a circuit element layer CL, a display element layer DPL, and an encapsulation layer TFE. Although not shown, the display unit  400  may further include at least one of a touch sensor, a reflection preventing layer, and a window member. 
     The circuit element layer CL may include signal lines, a scan line GL, a data line DL, and a power line PL. Also, the circuit element layer CL may include a plurality of transistors and a capacitor. In  FIG. 10B , a first transistor T 1  is exemplarily described. 
     The circuit element layer CL may include a barrier layer BR, an active layer ACT, a gate insulation layer GI, a gate electrode GE, an interlayer insulation layer ILD, input and output electrodes SE and DE, and an intermediate insulation layer VLD. 
     The barrier layer BR is disposed on the flexible substrate  300  and prevents foreign substances from being introduced beyond the barrier layer BR. 
     Although not shown, the display unit  400  may further include a buffer layer (not shown) disposed on the barrier layer BR. The buffer layer (not shown) improves a coupling force between the flexible substrate  300  and layers disposed thereon. The buffer layer BR and the barrier layer (not shown) may be selectively provided or omitted. 
     The active layer ACT is disposed on the barrier layer BR. The active layer ACT may serve as a channel region of the first transistor T 1 . The active layer ACT may be selected from amorphous silicon, polysilicon, and a metal oxide semiconductor. 
     The gate insulation layer GI may be disposed on the active layer ACT. The gate insulation layer GI may insulate the gate electrode GE from the active layer ACT. 
     The gate electrode GE may be disposed on the gate insulation layer GI. The gate electrode GE may overlap the active layer ACT. 
     A first conductive layer (not shown) providing signal lines may be disposed on the same layer as the gate electrode GE. 
     The interlayer insulation layer ILD is disposed on the gate electrode GE. The interlayer insulation layer ILD electrically insulates the gate electrode GE from the input and output electrodes SE and DE. The interlayer insulation layer ILD may contain an inorganic material. The inorganic material may include silicon nitride, silicon oxynitride, silicon oxide, and the like. 
     The input and output electrodes SE and DE are disposed on the interlayer insulation layer ILD. The input and output electrodes SE and DE may be electrically connected to the active layer ACT through first and second contact holes CH 1  and CH 2  defined in the interlayer insulation layer ILD and the gate insulation layer GI, respectively. 
     A second conductive layer (not shown) providing signal lines may be disposed on the same layer as the input and output electrodes SE and DE. 
     Although the display panel DP has a top-gate structure in which the gate electrode GE is disposed on the active layer ACT in an embodiment of the inventive concept, the display panel DP may have a bottom-gate structure in which the gate electrode GE is disposed below the active layer ACT in another embodiment. 
     The intermediate insulation layer VLD is disposed on the input and output electrodes SE and DE. The intermediate insulation layer VLD may provide a planarized surface. The intermediate insulation layer VLD may include an organic material. The organic material may include at least one of an acrylic-based resin, a methacrylic-based resin, a polyisoprene-based resin, a vinyl-based resin, an epoxy-based resin, a urethane-based resin, a cellulose-based resin, a siloxane-based resin, a polyimide-based resin, a polyamide-based resin, or a perylene-based resin. 
     The display element layer DPL is disposed on the intermediate insulation layer VLD. The display element layer DPL may include a pixel defining layer PDL and a display element. In an embodiment of the inventive concept, the display element may be an organic light emitting diode OLED. The organic light emitting diode OLED includes a first electrode AE, a hole control layer HCL, a light emitting layer EML, an electron control layer ECL, and a second electrode CE. 
     The pixel defining layer PDL may contain an organic material. The first electrode AE is disposed on the intermediate insulation layer VLD. The first electrode AE is connected to the output electrode DE through a third contact hole CH 3  passing through the intermediate insulation layer VLD. A first opening OP 1  is defined in the pixel defining layer PDL. The first opening OP 1  of the pixel defining layer PDL exposes at least a portion of the first electrode AE. 
     A light emitting area PXA and a non-light emitting area NPXA disposed adjacent to the light emitting area PXA may be defined on the transparent flexible display panel DP. The non-light emitting area NPXA may surround the light emitting area PXA. In the embodiment, the light emitting area PXA may be defined in correspondence to a portion of the first electrode AE, which is exposed by the first opening OP 1 . 
     A hole control layer HCL may be disposed in common on the light emitting area PXA and the non-light emitting area NPXA. Although not separately shown, a common layer such as the hole control layer HCL may be disposed in common on a plurality of organic light emitting diodes OLED. 
     The light emitting layer EML is disposed on the hole control layer HCL. The light emitting layer EML may be disposed on an area corresponding to the first opening OP 1 . That is, the light emitting layer EML may be divided and provided on each of the plurality of organic light emitting diodes OLED. The light emitting layer EML may contain an organic material and/or an inorganic material. Although the patterned light emitting layer EML is exemplarily illustrated in the embodiment, the light emitting layer EML may be disposed in common on the plurality of organic light emitting diodes OLED. Here, the light emitting layer EML may generate white light. Also, the light emitting layer EML may have a multilayer structure. 
     The electron control layer ECL is disposed on the light emitting layer EML. Although not separately shown, the electron control layer ECL may be disposed in common on the plurality of organic light emitting diodes OLED. 
     The second electrode CE is disposed on the electronic control layer ECL. The second electrode CE is disposed in common on the plurality of organic light emitting diodes OLED. 
     The encapsulation layer TFE seals the display element layer DPL. The encapsulation layer TFE is disposed on the second electrode CE. The encapsulation layer TFE is disposed in common on the plurality of organic light emitting diodes OLED. In the embodiment, the encapsulation layer TFE directly covers the second electrode CE. In another embodiment of the inventive concept, a capping layer covering the second electrode CE may be further disposed between the encapsulation layer TFE and the second electrode CE. Here, the encapsulation layer TFE may directly cover the capping layer. 
     The encapsulation layer TFE includes at least one inorganic layer (hereinafter, referred to as an encapsulation inorganic layer). The encapsulation layer TFE may further include at least one organic layer (hereinafter, referred to as an encapsulation organic layer). The encapsulation inorganic layer protects the display element layer DPL against moisture/oxygen, and the encapsulation organic layer protects the display element layer DPL against foreign substances such as dust particles. The encapsulation inorganic layer may include a silicon nitride layer, a silicon oxynitride layer, a silicon oxide layer, a titanium oxide layer, or an aluminum oxide layer. The encapsulation organic layer may include an acrylic-based organic layer. However, the embodiment of the inventive concept is not limited thereto. 
       FIGS. 11 and 12  are cross-sectional views illustrating a manufacturing process for forming a flexible display apparatus according to another embodiment of the inventive concept. 
       FIG. 11  is a cross-sectional view illustrating processes corresponding to  FIG. 4B  and the operation S 40  in  FIG. 1  in another embodiment of the inventive concept. 
     Referring to  FIG. 11 , the processes corresponding to the operations S 10  to S 40  in  FIG. 1  will be described, and processes, which are not described, will be followed by descriptions related to  FIGS. 1 to 4B . 
     Referring to  FIG. 11 , the first charge adhesive pattern  230  and the second charge adhesive pattern  240  are formed on the substrate  100 . The first charge adhesive pattern  230  has a first charge, and the second charge adhesive pattern  240  has a second charge. 
     The first charge adhesive pattern  230  has a lower first charge adhesive pattern  231  and an upper first charge adhesive pattern  233 . The second charge adhesive pattern  240  has a lower second charge adhesive pattern  241  and an upper second charge adhesive pattern  243 . 
     The lower first charge adhesive pattern  231 , the lower second charge adhesive pattern  241 , the upper first charge adhesive pattern  233 , and the upper second charge adhesive pattern  243  may be sequentially laminated on the substrate  100 . The lower first charge adhesive pattern  231 , the lower second charge adhesive pattern  241 , the upper first charge adhesive pattern  233 , and the upper second charge adhesive pattern  243  may be formed such that coating, cleaning, and drying processes of each of the lower first charge adhesive layer, the lower second charge adhesive layer, the upper first charge adhesive layer, and the upper second charge adhesive layer are performed on the substrate  100 , and then an edge of each of the four layers is removed. 
     Although the first charge adhesive pattern  230  and the second charge adhesive pattern  240  each having two layers are exemplarily illustrated in  FIG. 11 , each of the first charge adhesive pattern  230  and the second charge adhesive pattern  240  may have three or more layers. 
     In an embodiment of the inventive concept, the lower first charge adhesive pattern  231  may include graphene oxide, the lower second charge adhesive pattern  241  may include montmorillonite, molybdenum disulfide, or hexagonal boron nitride (hBN), the upper first charge adhesive pattern  233  may include polyethyleneimine (PEI), and the upper second charge adhesive pattern  243  may include montmorillonite, molybdenum disulfide, or hexagonal boron nitride (hBN). 
       FIGS. 12 and 13  are cross-sectional views illustrating a state, in which processes corresponding to the operations S 50  to S 70  in  FIG. 1  are performed, in the embodiment of the inventive concept. 
     When a process corresponding to the operation S 70  in  FIG. 1  is performed, the substrate  100  and the flexible substrate  300  may be separated so that at least the lower first charge adhesive pattern  231  remained on the substrate  100 , and at least one of the lower second charge adhesive pattern  241 , the upper first charge adhesive pattern  233 , and the upper second charge adhesive pattern  243  remained on the flexible substrate  300 . In  FIG. 12 , a state in which, the upper first charge adhesive pattern  233  and the upper second charge adhesive pattern  243  remained on the flexible substrate  300 , is exemplarily illustrated. However, the embodiment of the inventive concept is not limited thereto. For example, the upper first charge adhesive pattern  233 , the upper second charge adhesive pattern  243 , and the lower second charge pattern  241  may remain on the flexible substrate  300  as seen in  FIG. 13 . 
     The polyethyleneimine (PEI) has a transmittance greater than that of the graphene oxide. 
     In the embodiment of the inventive concept described with reference to  FIGS. 11 and 12 , one layer of the plurality of first charge adhesive pattern  230 , which is disposed adjacent to the flexible substrate  300 , contains polyethyleneimine (PEI), and the other layer thereof, which is disposed adjacent to the substrate  100 , includes graphene oxide. Thereafter, when the substrate  100  and the flexible substrate  300  are separated, a layer containing the polyethyleneimine (PEI) of the plurality of first charge adhesive patterns  230  remains on the flexible substrate  300 , and a layer containing the graphene oxide is removed together with the substrate  100 . Accordingly, when the plurality of first and second charge adhesive patterns  230  and  240  are used, as the graphene oxide is used, adhesive reliability may improve during the manufacturing process, and, at the same time, manufacturing the transparent flexible display apparatus having a higher transmittance. 
     The method of manufacturing the flexible display apparatus according to the embodiment of the inventive concept, may prevent the bottom surface of the flexible substrate from burning, and from becoming yellow by easily separating the substrate and the flexible substrate without laser irradiation, and thus manufacturing the transparent flexible display apparatus having a higher transmittance. 
     Through the method of manufacturing the flexible display apparatus according to the embodiment of the inventive concept, since the substrate and the flexible substrate directly contact each other in the peripheral area to have a high adhesion force, the substrate and the flexible substrate may not be easily separated in the process of forming the display unit. 
     Although the exemplary embodiments of the present invention have been described, it is understood that the present invention should not be limited to these exemplary embodiments but various changes and modifications can be made by one of ordinary skill in the art within the spirit and scope of the present invention as hereinafter claimed. 
     Hence, the real protective scope of the inventive concept shall be determined by the technical scope of the accompanying claims.