Patent Publication Number: US-2022223821-A1

Title: Flexible display and manufacturing method thereof

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 16/786,657, filed on Feb. 10, 2020, which is a Continuation of U.S. patent application Ser. No. 16/358,375, filed Mar. 19, 2019, which is a Continuation of U.S. patent application Ser. No. 15/240,820, filed Aug. 18, 2016, which claims priority to Korean Patent Application No. 10-2015-0118837 filed in the Korean Intellectual Property Office on Aug. 24, 2015, the entire contents of which are incorporated herein by reference 
    
    
     BACKGROUND 
     Field 
     The described technology generally relates to a flexible display and a manufacturing method thereof, and more particularly, to a flexible display device with a bending area. 
     Description of the Related Technology 
     Since organic light-emitting diode (OLED) displays are self-luminous, they do not require a separate light source, in contrast to liquid crystal displays (LCDs). This can enable the manufacture of OLED displays that are relatively thin and lightweight compared to LCDs. In addition, OLED displays have other high-quality characteristics, such as low power consumption, high luminance, fast response speeds, etc. 
     OLED displays include a substrate, a thin film transistor formed on the substrate, an OLED which is controlled by the thin film transistor, and a plurality of insulating layers formed between electrodes of the thin film transistor. Recently, flexible OLED displays have been developed that include a flexible substrate in which a bending area is formed. 
     The above information disclosed in this Background section is only intended to facilitate the understanding of the background of the described technology and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One inventive aspect is a flexible display and a manufacturing method thereof that can suppress cracking and the spread of cracking in an insulating layer due to bending of the flexible display along a bending area. 
     Another aspect is a flexible display device including: a flexible substrate including a bending area; an insulating layer disposed on the flexible substrate; a groove in the insulating layer within the bending area; a stress relaxation layer disposed on the groove; and a plurality of wires disposed on the insulating layer and the stress relaxation layer. 
     The flexible substrate can further include a display area, and the bending area can be positioned outside the display area. The flexible display device can further include a display unit located in the display area. The plurality of wires can be electrically connected to the display unit. The stress relaxation layer can contact the flexible substrate. 
     The bending area can be bent based on the bending axis, and the groove of the insulating layer can extend along a direction parallel to the bending axis. The groove of the insulating layer can be formed in a plurality, and a plurality of grooves can be spaced apart from each other along a length direction of the plurality of wires. 
     A top surface of the stress relaxation layer and a top surface of the insulating layer can be positioned at the same height from a surface of the flexible substrate. The insulating layer can include a plurality of inorganic layers stacked on the flexible substrate, and the groove of the insulating layer can be located in at least one inorganic layer including the topmost inorganic layer of the plurality of inorganic layers. 
     The plurality of inorganic layers can include at least two of a barrier layer, a buffer layer, a gate insulating layer, and an interlayer insulating layer. The plurality of wires can be covered by an organic layer. The stress relaxation layer can include an organic insulation material. 
     Another aspect is a flexible display device including: a flexible substrate, an inorganic insulating layer, a groove, an organic insulating layer, and a plurality of wires. The flexible substrate can include a display area in which a display unit is located and a bending area outside the display area. The inorganic insulating layer can be disposed on the flexible substrate. The groove can be located in the inorganic insulating layer within the bending area. The organic insulating layer can be disposed on the groove. The plurality of wires can be electrically connected to the display unit, and can respectively include a first portion disposed on a top surface of the inorganic insulating layer and a second portion disposed on a top surface of the organic insulating layer. 
     The first portion and the second portion can be positioned at the same height from a surface of the flexible substrate. The bending area can be bent based on a bending axis, and the groove of the inorganic insulating layer can extend along a direction parallel to the bending axis. 
     The organic insulating layer can contact the flexible substrate. The inorganic insulating layer can include a barrier layer, a buffer layer, a gate insulating layer, and an interlayer insulating layer that are sequentially stacked on the flexible substrate. The groove of the inorganic insulating layer can be located in at least one of the barrier layer, the buffer layer, the gate insulating layer, and the interlayer insulating layer. 
     Another aspect is a manufacturing method of a flexible display device, including: forming an insulating layer on a display area and a non-display area of a flexible substrate; forming a groove in the insulating layer within the non-display area; forming a stress relaxation layer by filling the groove with an organic insulation material; forming a plurality of wires on the insulating layer and the stress relaxation layer within the non-display area; and forming a bending area by bending at least some of the non-display area. 
     The bending area can be bent based on a bending axis, and the groove of the insulating layer can extend along a direction parallel to the bending axis. 
     A top surface of the stress relaxation layer and a top surface of the insulating layer can be positioned at the same height from a surface of the flexible substrate. The insulating layer can include a plurality of inorganic layers stacked on the flexible substrate, and the groove of the insulating layer can be located in at least one inorganic layer including the topmost inorganic layer of the plurality of inorganic layers. 
     According to at least one embodiment, it is possible to reduce or prevent cracking and the spread of cracking in an insulating layer of a flexible display device by reducing stress on a bending area thereof. Further, it is possible to prevent the occurrence of a step shape being formed in wires by providing a flat surface over which the wires are formed by using a stress relaxation layer, such that short circuit(s) between the wires can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic perspective view of a flexible display according to an exemplary embodiment. 
         FIG. 2  illustrates a schematic cross-sectional view of a flexible display according to an exemplary embodiment. 
         FIG. 3  illustrates a schematic perspective view of the flexible display illustrated in  FIG. 1  in an unfolded configuration. 
         FIG. 4  illustrates a cross-sectional view of an exemplary variation of the flexible display illustrated in  FIG. 2 . 
         FIG. 5A  illustrates an enlarged cross-sectional view of a display unit of the flexible display illustrated in  FIG. 3 . 
         FIGS. 5B, 5C, and 5D  respectively illustrate exemplary variations of  FIG. 5A   
         FIG. 6  illustrates an enlarged cross-sectional view of a bending area of the flexible display illustrated in  FIG. 3 . 
         FIG. 7A  illustrates an enlarged cross-sectional view of a first exemplary variation of the bending area illustrated in  FIG. 6 . 
         FIG. 7B  illustrates an enlarged cross-sectional view of a second exemplary variation of the bending area illustrated in  FIG. 6 . 
         FIG. 7C  illustrates an enlarged cross-sectional view of a third exemplary variation of the bending area illustrated in  FIG. 6 . 
         FIG. 7D  illustrates an enlarged cross-sectional view of a fourth exemplary variation of the bending area illustrated in  FIG. 6 . 
         FIG. 8  illustrates an enlarged perspective view of a bending area of a flexible display corresponding to a comparative example. 
         FIG. 9  illustrates a flowchart of a manufacturing method of a flexible display according to an exemplary embodiment. 
         FIG. 10A  illustrates a schematic cross-sectional view of a flexible display at a first step illustrated in  FIG. 9 . 
         FIG. 10B  illustrates a schematic cross-sectional view of a flexible display at a second step illustrated in  FIG. 9 . 
         FIG. 10C  illustrates a schematic cross-sectional view of a flexible display at a third step illustrated in  FIG. 9 . 
         FIG. 10D  illustrates a schematic cross-sectional view of a flexible display at a fourth step illustrated in  FIG. 9 . 
         FIG. 10E  illustrates a schematic cross-sectional view of a flexible display at a fifth step illustrated in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     The present disclosure will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the disclosure are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present disclosure. 
     Throughout the specification, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it may be directly on the other element or intervening elements may also be present. Further, the word “on” means positioned on or below the object, and does not necessarily mean positioned on the upper side of the object based on the orientation of the object with respect to the direction of gravity. 
     Throughout the specification, unless explicitly described to the contrary, the word “comprise” will be understood to imply the inclusion of stated elements, but not the exclusion of any other elements. In the drawings, the sizes and thicknesses of respective elements may be exaggerated for the sake of clarity, and the present disclosure is not necessarily limited to such illustrated sizes and thicknesses. 
       FIGS. 1 and 2  respectively illustrate a schematic perspective view and a schematic cross-sectional view of a flexible display according to an exemplary embodiment.  FIG. 3  illustrates a schematic perspective view of the flexible display device illustrated in  FIG. 1  in an unfolded configuration. 
     Referring to  FIGS. 1 to 3 , a flexible display  100  includes a flexible substrate  110 , a display unit  120 , an insulating layer  130 , a stress relaxation layer  140 , and a plurality of wires  150  that are disposed on the flexible substrate  110 . Further, the flexible display  100  can further include a chip on film (COF)  160  (or a flexible printed circuit (FPC)) and a printed circuit board (PCB)  170 . 
     The flexible substrate  110  can be formed of an organic material including at least one of polyimide, polycarbonate, polyethylene, polyethylene terephthalate, and polyacrylate. The flexible substrate  110  can be bendable and can be light transmissive. 
     The flexible substrate  110  includes a display area (DA) within the display unit  120 . The display unit  120  includes a plurality of pixels, and can display an image via a combination of light emitted from the pixels PX. Each of the pixels PX includes a pixel circuit and an organic light-emitting diode (OLED). The pixel circuit includes at least two thin film transistors and at least one storage capacitor and can control the emission of light from the OLED. A detailed structure of the display unit  120  will be described below. 
     The flexible substrate  110  includes a bending area (BA). The flexible substrate  110  includes a non-display area (NDA) surrounding the display area (DA), and the bending area (BA) can be included in the non-display area. For example, the bending area (BA) can be a pad region included in the non-display area. 
     A plurality of pad electrodes  180  are disposed on an edge of the flexible substrate  110  in the pad region. A plurality of wires  150  are disposed on the pad region and the wires  150  electrically connect a plurality of signal lines (e.g., scan lines, data lines, driving voltage lines, etc.) that are respectively located in the display unit  120  to a plurality of pad electrodes  180 . 
     The pad electrodes  180  are connected to an output wire portion  161  of the chip on film  160 , and an input wire portion  162  of the chip on film  160  is connected to an output wire portion  171  of the printed circuit board (PCB)  170 . In  FIG. 2 , reference numeral  163  denotes a driving chip mounted on the chip on film  160 . The printed circuit board (PCB)  170  outputs a control signal for controlling the driving chip  163  to the chip on film  160 . The chip on film  160  outputs various signals and power to the pad electrodes  180  in order to display images. 
     When the pad region is arranged to be parallel to the display area (DA) (refer to  FIG. 3 ), the dead space outside of the display unit  120  increases. The pad region of the flexible display device  100  of the present exemplary embodiment can be bent to form the bending area (BA). 
     Then, since the edge of the flexible substrate  110  on which the pad electrodes  180  are located overlaps the display area (DA) at a rear side of the display area (DA), the dead space outside of the display unit  120  can be reduced or minimized. 
     The bending area (BA) is bent with respect to a bending axis (BX)(refer to  FIG. 2 ). A center of curvature of the bending area (BA) is positioned at the bending axis (BX). The bending axis (BX) is parallel to an x-axis with reference to  FIGS. 1 and 2 . 
     The insulating layer (or inorganic insulating layer)  130  is disposed on the entire flexible substrate  110 , and the wires  150  are disposed on the insulating layer  130 . The insulating layer  130  is interposed between electrodes included in the display unit  120  to electrically insulate the electrodes from each other. The insulating layer  130  can include a plurality of layers, such as a barrier layer, a buffer layer, a gate insulating layer, an interlayer insulating layer, etc., and can include an inorganic material such as a silicon oxide (SiO.sub.2) or silicon nitride (SiNx). The wires  150  include metal. 
     The insulating layer  130  can be considerably less flexible than the wires  150  and can be brittle leading to the insulating layer  130  being vulnerable to being broken when applied with external force(s). Accordingly, the insulating layer  130  of the bending area (BA) can be broken by tensile force caused by bending, such that crack(s) can be formed, and an initially occurring crack can be spread to other areas of the insulating layer  130 . A crack in the insulating layer  130  can lead to open circuits in the wires  150 , which leads to display defects in the flexible display device  100 . 
     In the flexible display device  100  of at least one exemplary embodiment, the insulating layer  130  includes a groove (GR) located in the bending area (BA), and the stress relaxation layer (or organic insulating layer)  140  is disposed on the groove (GR) of the insulating layer  130 . 
     The groove (GR) and the stress relaxation layer  140  can extend along a direction (y-axis direction) parallel to the bending axis (BX).  FIG. 2  illustrates the exemplary embodiment in which one groove (GR) and one stress relaxation layer  140  are located in the insulating layer  130 . 
       FIG. 4  illustrates a cross-sectional view of an exemplary variation of the flexible display illustrated in  FIG. 2 . In  FIG. 4 , the insulating layer  130  includes a plurality of grooves (GR) and a plurality of stress relaxation layers  140  that are respectively spaced apart from each other along a length direction of the wire  150 . The grooves (GR) and the stress relaxation layers  140  respectively extend along the direction (y-axis direction) parallel to the bending axis (BX). 
     Although two grooves (GR) and two stress relaxation layers  140  are exemplarily shown in  FIG. 4 , the number and the position of the groove (GR) and the stress relaxation layer  140  are not limited to those shown in  FIG. 4 . 
     Referring to  FIGS. 1 to 4 , the stress relaxation layer  140  is located to fill the groove (GR) of the insulating layer  130 , and the wires  150  are disposed on the insulating layer  130  and the stress relaxation layer  140 . The thickness of the stress relaxation layer  140  is substantially equal to a depth of the groove (GR). Accordingly, a top surface of the stress relaxation layer  140  and a top surface of the insulating layer  130  are positioned at substantially the same height from a surface of the flexible substrate  110 , and the wires  150  do not have a step in height at a boundary between the insulating layer  130  and the stress relaxation layer  140 . 
     For example, the wires  150  are positioned flat from one side contacting the electrode of the display unit  120  to the other side contacting the pad electrode  180  without bending. Here, “bending” means to being bent in a thickness direction (z-axis direction) of the flexible substrate  110 . 
     The stress relaxation layer  140  includes a material that is less brittle and more flexible than the insulating layer  130 . For example, the stress relaxation layer  140  can include an organic insulation material, and can include at least one of polyimide, acrylate, and epoxy. The stress relaxation layer  140  can fill the entire groove (GR), and tightly contacts a lateral wall of the groove (GR) so that no gap(s) are formed between the insulating layer  130  and the groove (GR). 
     The flexible display device  100  of the present exemplary embodiment can reduce or prevent the occurrence of cracking in the insulating layer  130  due to bending stress by the groove (GR) in the insulating layer  130  of the bending area (BA). In addition, it is possible to prevent a step from occurring in the wires  150  traversing the bending area (BA) by the stress relaxation layer  140  in the groove (GR) of the insulating layer  130 . 
     When the wires  150  are disposed after forming the groove (GR) in the insulating layer  130  when there is no stress relaxation layer, the wires  150  are located to have a large step along the thickness direction (z-axis direction) of the flexible substrate  110 . In this situation, when the wires  150  are formed by depositing a metal layer and then patterning the metal layer through a method such as photolithography, the metal layer may remain in an undesired region, thus leading to short circuit(s) between the wires  150 . 
     In the flexible display device  100  of at least one exemplary embodiment, the stress relaxation layer  140  can fill the groove (GR) of the insulating layer  130  to form a substantially flat surface. Accordingly, when the wires  150  are formed by depositing the metal layer and then patterning the metal layer, since it is possible to accurately perform the patterning, the short circuit between the wires  150  can be prevented. For reference, the wires  150  are formed in the flat pad region, and then the pad region is bent for the bending area (BA) to be formed. 
       FIGS. 5A and 6  respectively illustrate an enlarged cross-sectional view of a display unit and a bending area of the flexible display device illustrated in  FIG. 3 . 
     Referring to  FIGS. 5A and 6 , a barrier layer  131  can be disposed on the flexible substrate  110 . The barrier layer  131  serves to block moisture and/or oxygen from permeating through the flexible substrate  110 , and can be formed with a plurality of layers in which silicon oxides (SiO.sub.2) and silicon nitrides (SiNx) are alternatively and repeatedly stacked on each other. A buffer layer  132  can be disposed on the barrier layer  131 . The buffer layer  132  provides a flat surface for forming a pixel circuit, and can include a silicon oxide (SiO.sub.2) or a silicon nitride (SiNx). 
     A semiconductor layer  121  is disposed on the buffer layer  132 . The semiconductor layer  121  can include a polysilicon or oxide semiconductor, and the semiconductor layer  121  including the oxide semiconductor can be covered by a separate passivation layer (not shown). The semiconductor layer  121  includes a channel region, which is not doped with an impurity, and a source region and a drain region, which are positioned at opposite sides of the channel region and are doped with an impurity. 
     A gate insulating layer  133  is disposed on the semiconductor layer  121 . The gate insulating layer  133  can be formed as a single layer of a silicon nitride (SiNx) or a silicon oxide (SiO.sub.2) or a stacked layer of a silicon nitride (SiNx) and a silicon oxide (SiO2). A gate electrode  122  and a first capacitor plate  125   a  are disposed on the gate insulating layer  133 . The gate electrode  122  overlaps a channel region of the semiconductor layer  121 . 
     A first interlayer insulating layer  134  can be disposed on the gate electrode  122  and the first capacitor plate  125   a , and a second capacitor plate  126   a  is disposed on the first interlayer insulating layer  134 . The second capacitor plate  126   a  overlaps the first capacitor plate  125   a , and the first capacitor plate  125   a  and the second capacitor plate  126   a  form a storage capacitor  202   a  using the first interlayer insulating layer  134  as a dielectric material. The gate electrode  122  and the first and second capacitor plates  125   a  and  126   a  can include one or more of: Au, Ag, Cu, Ni, Pt, Pd, Al, Mo, etc. 
     A second interlayer insulating layer  135  can be disposed on the second capacitor plate  126   a , and a source electrode  123  and a drain electrode  124  are disposed on the second interlayer insulating layer  135 . The first and second interlayer insulating layers  134  and  135  can be formed as a single layer of a silicon oxide (SiO.sub.2) or a silicon nitride (SiNx) or a stacked layer of a silicon oxide (SiO.sub.2) or a silicon nitride (SiNx). 
     The source electrode  123  and the drain electrode  124  are respectively connected to the source region and the drain region of the semiconductor layer  121  through a via hole formed in the first and second interlayer insulating layers  134  and  135  and the gate insulating layer  133 . The source electrode  123  and the drain electrode  124  can be formed as a metal multilayer such as Mo/Al/Mo or Ti/AV/Ti. On the other hand, the second capacitor plate  126   a  can include the same material as the source and drain electrodes  123  and  124 , and in this case, one interlayer insulating layer is provided. 
     The pixel circuit includes a switching thin film transistor, a driving thin film transistor  201 , and a storage capacitor  202   a , but the switching thin film transistor is omitted in  FIG. 5A  to facilitate the understanding of the pixel circuit and for convenience. Further, the structures of the driving thin film transistor  201  and the storage capacitor  202   a  are not limited to those of  FIG. 5A . 
       FIGS. 5B, 5C, and 5D  respectively illustrate other examples of the storage capacitor. 
     Referring to  FIG. 5B , a first capacitor plate  125   b  can include the same material as the semiconductor layer  121  on the buffer layer  132 , and a second capacitor plate  126   b  can include the same material as the gate electrode  122  on the gate insulating layer  133 . In this embodiment, the first capacitor plate  125   b  and the second capacitor plate  126   b  form a storage capacitor  202   b  using the gate insulating layer  133  as a dielectric material. 
     Referring to  FIG. 5C , a first capacitor plate  125   c  can include the same material as the semiconductor layer  121  on the buffer layer  132 , and the second capacitor plate  126   c  can include the same material as the source electrode  123  and the drain electrode  124  on the second interlayer insulating layer  135 . In this embodiment, the first capacitor plate  125   c  and the second capacitor plate  126   c  form a storage capacitor  202   c  using the gate insulating layer  133 , the first interlayer insulating layer  134 , and the second interlayer insulating layer  135  as a dielectric material. 
     Referring to  FIG. 5D , the storage capacitor  202   d  can include a first capacitor plate  125   d , a second capacitor plate  126   d , and a third capacitor plate  126   e . The first capacitor plate  125   d  can include the same material as the semiconductor layer  121  on the buffer layer  132 , and the second capacitor plate  126   d  can include the same material as the gate electrode  122  on the gate insulating layer  133 . The third capacitor plate  126   e  can include the same material as the source electrode  123  and the drain electrode  124  on the second interlayer insulating layer  135 . 
     The structures of the storage capacitors ( 202   a ,  202   b ,  202   c , and  202   d ) are not limited to those shown in  FIGS. 5A to 5D , and they can be variously modified. 
     Referring back to  FIGS. 5A and 6 , the driving thin film transistor  201  is covered by a planarization layer  112 , and connected to an OLED  203  to drive the OLED  203 . The planarization layer  112  can include an organic insulation material or an inorganic insulation material, or can be formed of a combination of the organic insulation material and the inorganic insulation material. The OLED  203  includes a pixel electrode  127  and an emission layer  128 , and a common electrode  129 . 
     The pixel electrode  127  is separately formed in each pixel on the planarization layer  112 , and is connected to the drain electrode  124  of the driving thin film transistor  201  through a via hole formed on the planarization layer  112 . A pixel-defining layer (or partition wall)  113  is disposed on the planarization layer  112  and on an edge of the pixel electrode  127 . The emission layer  128  is disposed on the pixel electrode  127 , and the common electrode  129  is disposed on the entire display area (DA) regardless of the pixels. 
     One of the pixel electrode  127  and the common electrode  129  injects holes into the emission layer  128 , and the other injects electrons into the emission layer  128 . The electrons and holes are recombined with each other in the emission layer  128  to generate an exciton, and light is emitted by energy generated when the exciton falls from an excited state to a ground state. 
     The pixel electrode  127  can include a reflective layer, and the common electrode  129  can include a transparent layer or a semi-transmissive layer. Light emitted from the emission layer  128  is reflected by the pixel electrode  127 , and passes through the common electrode  129  to be emitted to the environment. When the common electrode  129  includes the semi-transmissive layer, some of light reflected by the pixel electrode  127  is re-reflected by the common electrode  129 , thus the pixel electrode  127  and the common electrode  129  form a resonant structure, such that light-extracting efficiency can be improved. 
     The OLED  203  is covered by an encapsulator or encapsulation layer  190 . The encapsulator  190  seals the OLED  203 , such that deterioration of the OLED  203  caused by moisture and/or oxygen included in the environment can be reduced or prevent. The encapsulator  190  can include a structure in which inorganic layers and organic layers are stacked, for example, the encapsulator  190  can include a first inorganic layer  191 , an organic layer  192 , and a second inorganic layer  193 . 
     In the bending area (BA), the wires  150  ( FIG. 6  illustrates one wire) can be located in the same layer as the source and drain electrodes  123  and  124  and can include with the same material as the source and drain electrodes  123  and  124 . The wire  150  can be covered by the organic layer, for example, the planarization layer  112 . The insulating layer  130  below the wire  150 , which includes multiple layers, can include the barrier layer  131 , the buffer layer  132 , the gate insulating layer  133 , and the first and second interlayer insulating layers  134  and  135 . 
     The groove (GR) of the insulating layer  130  can be located in the entire insulating layer  130  along the thickness direction (z-axis direction) of the flexible substrate  110 . In this case, the stress relaxation layer  140  contacts the flexible substrate  110 . Further, the groove (GR) of the insulating layer  130  can be located in a subset of the insulating layer  130 , and can be located in at least one inorganic layer including the topmost inorganic layer  135  of the inorganic layers ( 131 ,  132 ,  133 ,  134 , and  135 ) forming the insulating layer  130 . 
       FIG. 7A  illustrates an enlarged cross-sectional view of a first exemplary variation of the bending area illustrated in  FIG. 6 . Referring to  FIG. 7A , a groove GR 1  of the insulating layer  130  can be located in the second interlayer insulating layer  135 .  FIG. 7B  illustrates an enlarged cross-sectional view of a second exemplary variation of the bending area illustrated in  FIG. 6 . 
     Referring to  FIG. 7B , a groove GR 2  of the insulating layer  130  can be located in the first interlayer insulating layer  134  and the second interlayer insulating layer  135 . 
       FIG. 7C  illustrates an enlarged cross-sectional view of a third exemplary variation of the bending area illustrated in  FIG. 6 . Referring to  FIG. 7C , a groove GR 3  of the insulating layer  130  can be located in the gate insulating layer  133  and the first and second interlayer insulating layers  134  and  135 .  FIG. 7D  illustrates an enlarged cross-sectional view of a fourth exemplary variation of the bending area illustrated in  FIG. 6 . Referring to  FIG. 7D , a groove GR 4  of the insulating layer  130  can be located in the buffer layer  132 , the gate insulating layer  133 , and the first and second interlayer insulating layers  134  and  135 . 
     In all the exemplary embodiments shown in  FIG. 6  and  FIGS. 7A to 7D , the grooves (GR, GR 1 , GR 2 , GR 3 , and GR 4 ) of the insulating layer  130  are filled with the stress relaxation layer  140  to provide a substantially flat surface for locating the wires  150 . 
       FIG. 8  illustrates an enlarged perspective view of a bending area of a flexible display corresponding to a comparative example. 
     Referring to  FIG. 8 , a flexible display of a comparative example does not include a stress relaxation layer. In this situation, the wires  151  are located along a top surface of the insulating layer  130 , a lateral surface of the groove (GR), a bottom surface of the groove (GR) (e.g., a top surface of the flexible substrate  110 ), a lateral surface of the groove (GR), and the top surface of the insulating layer  130 , and have a large step formed along the thickness direction (z-axis direction) of the flexible substrate  110 . 
     Typically, the wires  151  are formed by depositing a metal layer over an entire surface and patterning the metal layer through a method such as photolithography. However, when a metal layer is formed on a non-flat surface with a step, the metal layer is not appropriately patterned in a region in which the step is formed, and thus the metal layer may remain in an undesired portion or region. The remaining metal layer can contact an adjacent wire  151  such that a short circuit is formed between the wires  151 . Reference numeral  152  denotes an undesired remaining metal layer in  FIG. 8 . 
     However, in the flexible display  100  of at least one exemplary embodiment, since the stress relaxation layer  140  fills the groove (GR) of the insulating layer  130 , the metal layer for the wires  150  is disposed on the flat surface. Accordingly, the wires  150  can be accurately patterned without the metal layer remaining, and thus the short circuit between the wires  150  can be prevented. 
       FIG. 9  illustrates a flowchart of a manufacturing method of a flexible display according to an exemplary embodiment. 
     Referring to  FIG. 9 , a manufacturing method of the flexible display includes: a first step S 10  for forming an insulating layer on a display area and a non-display area of a flexible substrate; a second step S 20  for forming a groove in the insulating layer of the non-display area; a third step S 30  for forming a stress relaxation layer by filling an organic insulation material into the groove; a fourth step S 40  for forming a plurality of wires on the insulating layer and the stress relaxation layer of the non-display area; and a fifth step S 50  for forming a bending area by bending at least a portion of the non-display area. 
       FIG. 10A  illustrates a schematic cross-sectional view of a flexible display at a first step illustrated in  FIG. 9 . 
     Referring to  FIG. 10A , a flexible substrate  110  can include an organic material including at least one of polyimide, polycarbonate, polyethylene, polyethylene terephthalate, and polyacrylate in first step S 10 . The flexible substrate  110  can be bendable and can be light transmissive. 
     The flexible substrate  110  can be divided into a display area (DA) in which a display unit is located and a non-display area (NDA) outside of the display area (DA). The insulating layer  130  is disposed on both the display area (DA) and the non-display area (NDA) of the flexible substrate  110 . Although not illustrated, electrodes for forming thin film transistors and a storage capacitor are located in the display area (DA) while forming the insulating layer  130 . 
       FIG. 10B  illustrates a schematic cross-sectional view of a flexible display at a second step illustrated in  FIG. 9 . 
     Referring to  FIG. 10B , a groove (GR) is formed in the insulating layer  130  of the non-display area (NDA) in the second step S 20 . As shown in  FIG. 6  and  FIGS. 7A to 7D , the insulating layer  130  can include a plurality of inorganic layers ( 131 ,  132 ,  133 ,  134 , and  135 ) stacked on the flexible substrate  110 , and the groove (GR) of the insulating layer  130  can be located in at least one inorganic layer including the topmost inorganic layer  135  of the inorganic layers ( 131 ,  132 ,  133 ,  134 , and  135 ). 
       FIG. 10C  illustrates a schematic cross-sectional view of a flexible display at a third step illustrated in  FIG. 9 . 
     Referring to  FIG. 10C , a stress relaxation layer  140  fills the groove (GR) of the insulating layer  130  formed in the third step S 30 . The stress relaxation layer  140  can include an organic insulation material, and the thickness of the stress relaxation layer  140  is substantially equal to a depth of the groove (GR). Accordingly, a top surface of the stress relaxation layer  140  and a top surface of the insulating layer  130  are positioned at substantially the same height from a surface of the flexible substrate  110 . 
       FIG. 10D  illustrates a schematic cross-sectional view of a flexible display at a fourth step illustrated in  FIG. 9 . 
     Referring to  FIG. 10D , a plurality of wires  150  ( FIG. 10D  illustrates one wire) are formed on the insulating layer  130  of the non-display area (NDA) and the stress relaxation layer  140  in the fourth step S 40 . The wires  150  can be formed by depositing a metal layer on the insulating layer  130  and patterning the metal layer through a well-known photolithography process. 
     Although not illustrated, the remaining electrodes for forming thin film transistors in the display area (DA) can be formed while forming the wires  150 . After the wires  150  are formed, a display unit  120 , including a plurality of OLED and an encapsulator  190  sealing the display unit  120 , can be formed in the display area (DA). 
     Further, a plurality of pad electrodes (not shown) can be formed at an edge of the flexible substrate  110  within the non-display area (NDA). The wires  150  can connect a plurality of signal lines formed in the display unit  120  to the pad electrodes. 
     Since the stress relaxation layer  140  fills the groove (GR) of the insulating layer  130 , when a metal layer is deposited for forming the wires  150 , a step is not formed in the metal layer. That is, there is no bending portion along the thickness direction of the flexible substrate  110  in the metal layer, and thus no remaining layer occurs when the metal layer is patterned, thereby preventing a short circuit between wires  150 . 
       FIG. 10E  illustrates a schematic cross-sectional view of a flexible display at a fifth step illustrated in  FIG. 9 . 
     Referring to  FIG. 10E , at least a portion of the non-display area (NDA) in which the wires  150  are formed is bent such that the bending area (BA) is formed in the fifth step S 50 . The bending area (BA) is bent based on a bending axis (BX), and the groove (GR) of the insulating layer  130  can extend along a direction parallel to the bending axis (BX) (refer to  FIG. 1 ). By forming the groove (GR) and the stress relaxation layer  140  in bending area (BA), stress applied to the insulating layer  130  within the bending area (BA) decreases, thereby reducing or preventing cracking in the insulating layer  130 . 
     While this disclosure has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.