Patent Publication Number: US-2022236616-A1

Title: Display substrate, method for manufacturing display substrate, and display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 U.S.C 119 to Chinese Patent Application No. 202110089823.6, filed on Jan. 22, 2021, in the China National Intellectual Property Administration. The entire disclosure of the above application is incorporated herein by reference. 
     FIELD 
     The present disclosure relates to the technical field of display, in particular to a display substrate, a method for manufacturing the display substrate, and a display device. 
     BACKGROUND 
     At present, people have higher and higher requirements for full display of display products, and full display and narrow borders have become the trend of display products. 
     A display panel typically includes a display region and a fanout region for wiring located at the periphery of the display region. The width of the fanout region directly determines the width of a bezel of the display panel. In order to reduce the width of the fanout region, the industry has developed a double-layer metal wiring method, each metal layer is provided with a plurality of signal wirings. 
     SUMMARY 
     Some embodiments of the present disclosure provide a display substrate, including a substrate including with a fanout region; 
     the display substrate includes a first metal layer, a flat layer, and a second metal layer which are sequentially formed on the substrate; an orthographic projection of the flat layer on the substrate at least covers an orthographic projection of the fanout region on the substrate; in the fanout region, a surface, furthest from the substrate, of the flat layer includes a highest point having a maximum distance to a horizontal plane where the substrate is located and a lowest point having a minimum distance to the horizontal plane where the substrate is located, and a difference between the maximum distance and the minimum distance is less than or equal to 1000 angstroms; and 
     the first metal layer includes a plurality of first metal wirings located in the fanout region, the second metal layer includes a plurality of second metal wirings located in the fanout region, and an orthographic projection of the first metal wirings on the substrate at least partially overlaps with an orthographic projection of the second metal wirings on the substrate. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, the maximum distance is equal to the minimum distance. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, the display substrate further includes a first insulating layer located between the first metal layer and the flat layer, wherein an orthographic projection of the first insulating layer on the substrate covers the substrate. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, the display substrate further includes a second insulating layer located between the flat layer and the second metal layer, wherein an orthographic projection of the second insulating layer on the substrate at least covers the orthographic projection of the fanout region on the substrate. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, a thickness of the flat layer is greater than a thickness of the first metal layer. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, a width of each first metal wiring is identical to a width of each second metal wiring, and a gap width between adjacent first metal wirings is identical to a gap width between adjacent second metal wirings. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, an orthographic projection of the first metal wirings on the substrate completely overlaps with an orthographic projection of the second metal wirings on the substrate. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, the substrate further includes a display region located at one side of the fanout region, the first metal layer includes a gate electrode located in the display region, and the second metal layer includes a source electrode and a drain electrode which are located in the display region; 
     an orthographic projection of the flat layer on the substrate also covers an orthographic projection of the display region on the substrate, and the orthographic projection of the second insulating layer on the substrate also covers the orthographic projection of the display region on the substrate; 
     the display substrate further includes a first transparent conductive layer located between the second insulating layer and the second metal layer, a passivation layer located on one side, deviating from the substrate, of the second metal layer, and a second transparent conductive layer located on one side, deviating from the substrate, of the passivation layer; and 
     the first transparent conductive layer is electrically connected with the drain electrode, the second transparent conductive layer includes a plurality of transparent signal lines located in the display region, and an orthographic projection of the first transparent conductive layer on the substrate overlaps with an orthographic projection of the plurality of the transparent signal lines on the substrate. 
     Optionally, in the above display substrate provided by embodiments of the present disclosure, the substrate further includes a via hole region located between the fanout region and the display region, the first metal layer includes a first transfer electrode located in the via hole region, and the second metal layer includes a second transfer electrode located in the via hole region; 
     the orthographic projection of the flat layer on the substrate also covers an orthographic projection of a part of the via hole region on the substrate, and the orthographic projection of the second insulating layer on the substrate also covers the orthographic projection of a part of the via hole region on the substrate; 
     the second transparent conductive layer includes a plurality of transparent wirings located in the via hole region; 
     a part of the transparent wirings are electrically connected with the first transfer electrode by a via hole sequentially running through the passivation layer, the second insulating layer, the flat layer, and the first insulating layer; and 
     a part of the transparent wirings are electrically connected with the second transfer electrode by a via hole running through the passivation layer. 
     Accordingly, some embodiments of the present disclosure also provide a method for manufacturing the display substrate, including: 
     providing a substrate; wherein the substrate is provided with a fanout region; 
     forming a first metal layer on the substrate; wherein the first metal layer includes a plurality of first metal wirings located in the fanout region; 
     forming a flat layer on one side, deviating from the substrate, of the first metal layer; wherein an orthographic projection of the flat layer on the substrate at least covers an orthographic projection of the fanout region on the substrate; in the fanout region, a surface, furthest from the substrate, of the flat layer includes a highest point having a maximum distance to a horizontal plane where the substrate is located and a lowest point having a minimum distance to the horizontal plane where the substrate is located, and a difference between the maximum distance and the minimum distance is less than or equal to 1000 angstroms; and 
     forming a second metal layer on one side, deviating from the substrate, of the flat layer; wherein the second metal layer includes a plurality of second metal wirings located in the fanout region, and an orthographic projection of the first metal wirings on the substrate at least partially overlaps with an orthographic projection of the second metal wirings on the substrate. 
     Optionally, in the above method provided by embodiments of the present disclosure, the method further includes, before forming the flat layer on one side, deviating from the substrate, of the first metal layer: 
     forming a first insulating layer located between the first metal layer and the flat layer; wherein an orthographic projection of the first insulating layer on the substrate covers the substrate. 
     Optionally, in the above method provided by embodiments of the present disclosure, the forming the flat layer on one side, deviating from the substrate, of the first metal layer specifically includes: 
     forming the flat layer having a thickness greater than a thickness of the first metal layer on one side, deviating from the substrate, of the first metal layer. 
     Optionally, in the above method provided by embodiments of the present disclosure, the method further includes, after forming the flat layer, and before forming the second metal layer: 
     forming a second insulating layer located between the flat layer and the second metal layer; wherein an orthographic projection of the second insulating layer on the substrate at least covers the orthographic projection of the fanout region on the substrate. 
     Optionally, in the above method provided by embodiments of the present disclosure, forming the plurality of the first metal wirings specifically includes: 
     coating one side, deviating from the substrate, of the first metal layer with a first photoresist layer; and 
     exposing and developing the first photoresist layer to form the plurality of the first metal wirings; 
     forming the plurality of the second metal wirings specifically includes: 
     coating one side, deviating from the substrate, of the second metal layer with a second photoresist layer; and 
     exposing and developing the second photoresist layer to form the plurality of the second metal wirings corresponding to the plurality of the first metal wirings. 
     Accordingly, some embodiments of the present disclosure also provide a display device, including the above display substrate provided by embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural schematic diagram of a display substrate in the related art; 
         FIG. 2  is a schematic top view of a display substrate provided by some embodiments of the present disclosure; 
         FIG. 3  is a schematic cross-sectional view along the CC═ direction in  FIG. 2 ; 
         FIG. 4  is another schematic cross-sectional view along the CC′ direction in  FIG. 2 ; 
         FIG. 5  is another schematic cross-sectional view along the CC′ direction in  FIG. 2 ; 
         FIG. 6  is a flow diagram of a method for manufacturing a display substrate provided by some embodiments of the present disclosure; 
         FIG. 7  is a flow diagram of another method for manufacturing a display substrate provided by some embodiments of the present disclosure; 
         FIG. 8  is a flow diagram of another method for manufacturing a display substrate provided by some embodiments of the present disclosure; 
         FIG. 9  is a flow diagram of another method for manufacturing a display substrate provided by some embodiments of the present disclosure; 
         FIG. 10  is a flow diagram of another method for manufacturing a display substrate provided by some embodiments of the present disclosure; 
         FIG. 11A  is a structural schematic diagram of the display substrate corresponding to  FIG. 5  after fabrication step  1  is implemented; 
         FIG. 11B  is a structural schematic diagram of the display substrate corresponding to  FIG. 5  after fabrication step  2  is implemented; 
         FIG. 11C  is a structural schematic diagram of the display substrate corresponding to  FIG. 5  after fabrication step  3  is implemented; 
         FIG. 11D  is a structural schematic diagram of the display substrate corresponding to  FIG. 5  after fabrication step  4  is implemented; 
         FIG. 11E  is a structural schematic diagram of the display substrate corresponding to  FIG. 5  after fabrication step  5  is implemented; 
         FIG. 11F  is a structural schematic diagram of the display substrate corresponding to  FIG. 5  after fabrication step  6  is implemented; and 
         FIG. 12  is a structural schematic diagram of a display device provided by some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In order to make the objects, technical solutions and advantages of embodiments of the present disclosure more clear, the technical solutions of embodiments of the present disclosure will be clearly and completely described below in conjunction with the accompanying drawings of embodiments of the present disclosure. Obviously, the described embodiments are part of embodiments of the present disclosure, rather than all of embodiments. Moreover, embodiments of the present disclosure and the features in embodiments may be combined with each other without conflict. Based on the described embodiments of the present disclosure, all other embodiments obtained by those of ordinary skill in the art without inventive effort fall within the scope of protection of the present disclosure. 
     Unless otherwise defined, the technical or scientific terms used in the present disclosure should have the general meanings understood by those of ordinary skill in the art to which the present disclosure belongs. Similar terms such as “including” or “comprising” used in the present disclosure mean that elements or objects appearing before the term encompass elements or objects listed after the term and equivalents thereof, without excluding other elements or objects. Similar terms such as “connection” or “connected” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. “Inner”, “outer”, “upper”, “lower” and the like are only used to indicate relative positional relationships, which may change accordingly when the absolute positions of the described objects change. 
     It should be noted that the size and shape of each figure in the drawings do not reflect the true scale, and the purpose is only to schematically illustrate the contents of the present disclosure. And the same or similar reference signs throughout represent the same or similar elements or elements having the same or similar function. 
     With the development of a display technology, display products need to be designed with narrow bezels to achieve perfect visual effects. The metal wirings of a lower bezel are a peripheral circuit connecting the effective display region of the display panel and a driver chip, which are called fanout signal lines. Therefore, it is necessary to reduce the line width and line spacing of the fanout region to reduce the occupied area of the fanout, thus realizing a narrow bezel. Wherein, the sum of the line width and the line spacing is called a fanout pitch. 
     An existing wiring design in the fanout region use two layers of metal to be overlapped to make the fanout region to reduce the signal line layout space in the fanout region, as shown in  FIG. 1 , a substrate  1 ′, a first metal layer  2 ′ and a second metal layer  3 ′ are included, wherein the first metal layer  2 ′ and the second metal layer  3 ′ are stacked on the substrate  1 ′, an insulating layer  4 ′ is formed between the first metal layer  2 ′ and the second metal layer  3 ′, and the first metal layer  2 ′ is exposed, developed and etched by a photoresist process to form a plurality of first metal wirings  21 ′. However, when manufacturing metal wirings on the second metal layer  3 ′, firstly, the second metal layer  3 ′ is coated with a photoresist layer  5 ′, and the second metal layer  3 ′ is also not flat due to the unflatness of the insulating layer  4 ′, so the thickness of the photoresist layer  5 ′ coating the second metal layer  3 ′ is not uniform, resulting in poor exposure uniformity and affecting the fabrication accuracy of the second metal wirings on the second metal layer  3 ′. For example, in actual production, under the condition of ensuring that the metal wirings do not have open and short defects, the first metal wirings  21 ′ of the first metal layer  2 ′ can be made at a line width of 2.5 μm, a line spacing of 2.5 μm and a fanout pitch of 5.0 μm, but due to the influence of the process, the second metal layer  3 ′ can be made only at a line spacing of 3.0 μm and a fanout pitch of 5.5 μm. If one wants to make the fanout pitch of the second metal layer  3 ′ be the same as the fanout pitch of the first metal layer  2 ′ to further achieve a narrow bezel design, the second metal layer  3 ′ needs to be made at a line width of 2.0 μm, and the line width open risk is large, so the second metal layer  3 ′ needs to change the design pitch to 5.5 μm. Thus, the fanout pitch size is limited by the resolution of the second metal layer  3 ′, which can only be made to be 5.5 μm, thereby affecting the narrow bezel design of the display panel. 
     In view of this, some embodiments of the present disclosure provide a display substrate, as shown in  FIG. 2  and  FIG. 3 ,  FIG. 2  is a schematic top view of the display substrate,  FIG. 3  is a schematic cross-sectional view along the CC′ direction in  FIG. 2 , and the display substrate includes: a substrate  1  provided with a fanout region B 1 . 
     The display substrate includes a first metal layer  2 , a flat layer  6 , and a second metal layer  3  which are sequentially formed on the substrate  1 ; an orthographic projection of the flat layer  6  on the substrate  1  at least covers an orthographic projection of the fanout region B 1  on the substrate  1 ; in the fanout region B 1 , a surface, furthest from the substrate  1 , of the flat layer  6  has a highest point having a maximum distance to a horizontal plane where the substrate  1  is located and a lowest point having a minimum distance to the horizontal plane where the substrate  1  is located, and the difference between the distance from the highest point to the horizontal plane where the substrate  1  is located and the distance from the lowest point to the horizontal plane where the substrate  1  is located is less than or equal to 1000 angstroms to guarantee flatness of the surface away from the substrate  1  in the flat layer  6 . 
     The first metal layer  2  includes a plurality of first metal wirings  21  located in the fanout region B 1 , the second metal layer  3  includes a plurality of second metal wirings  31  located in the fanout region B 1 , and an orthographic projection of the first metal wirings  21  on the substrate  1  at least partially overlaps with an orthographic projection of the second metal wirings  31  on the substrate  1 ; optionally, only the plurality of the second metal wirings  31  located in the fanout region B 1  are illustrated in  FIG. 2 , and the plurality of the first metal wirings  21  located in the fanout region B 1  are located below the second metal wirings  31 . 
     According to the above display substrate provided by embodiments of the present disclosure, the flat layer  6  is arranged between the first metal layer  2  and the second metal layer  3 , and the orthographic projection of the flat layer  6  on the substrate  1  at least covers the orthographic projection of the fanout region B 1  on the substrate  1 , in the fanout region B 1 , the surface, furthest from the substrate  1 , of the flat layer  6  has the highest point having the maximum distance to the horizontal plane where the substrate  1  is located and the lowest point having the minimum distance to the horizontal plane where the substrate  1  is located, and the difference between the distance from the highest point to the horizontal plane where the substrate  1  is located and the distance from the lowest point to the horizontal plane where the substrate  1  is located is less than or equal to 1000 angstroms; in this way, the surface, facing the second metal layer  3 , of the flat layer  6  is relatively flat, thus the second metal layer  3  deposited on the flat layer  6  is also relatively flat, when the second metal layer  3  is subsequently patterned by using a photoresist process to form the plurality of the second metal wirings  31 , uniform photoresist thickness of the fanout region B 1  can be guaranteed, and the fabrication accuracy of the second metal wirings  31  of the second metal layer  3  can be improved, so that the second metal wirings  31  with the same width and spacing as that of the first metal wirings  21  can be formed. Thus, the etching resolutions of the metal wirings in the fanout region B 1  corresponding to the first metal layer  2  and the second metal layer  3  in the solutions of the present disclosure can be the same, thereby reducing the wiring space of the fanout region B 1 , and further realizing the narrow bezels. 
     In some embodiments, the distance from the highest point in the surface, furthest from the substrate, of the flat layer to the horizontal plane where the substrate is located is equal to the distance from the lowest point in the surface, furthest from the substrate, of the flat layer to the horizontal plane where the substrate is located. Equality means that the difference between the distance from the highest point to the horizontal plane where the substrate is located and the distance from the lowest point to the horizontal plane where the substrate is located is within an error range, for example, the difference between the distance from the highest point to the horizontal plane where the substrate is located and the distance from the lowest point to the horizontal plane where the substrate is located is less than or equal to 100 angstroms. 
     In some embodiments, the material of the flat layer may be an organic resin material, such as polyimide, but is not limited thereto. 
     In some embodiments, since the material of the flat layer is generally an organic material, the adhesion between the organic material and the metal layer is poor and a subsequently fabricated film layer is easily detached from the substrate, in the above display substrate provided by embodiments of the present disclosure, as shown in  FIG. 4 , the display substrate further includes a first insulating layer  4  located between the first metal layer  2  and the flat layer  6 , wherein an orthographic projection of the first insulating layer  4  on the substrate  1  covers the substrate  1 . Optionally, the first insulating layer  4  may employ silicon nitride SiNx, silicon oxide SiOx, or a composite layer of SiNx/SiOx, and the first insulating layer  4  may improve adhesion between the subsequently fabricated film layer and the substrate  1 . 
     In some embodiments, on one hand, due to poor adhesion between the flat layer and the metal layer, and on the other hand, due to the presence of particles in the flat layer of the organic material, which affects the device performance, in the above display substrate provided by the embodiments of the present disclosure, as shown in  FIG. 5 , the display substrate further includes a second insulating layer  7  located between the flat layer  6  and the second metal layer  3 , wherein an orthographic projection of the second insulating layer  7  on the substrate  1  at least covers the orthographic projection of the fanout region B 1  on the substrate  1 . Optionally, the second insulating layer  7  may employ silicon nitride SiNx, silicon oxide SiOx or a composite layer of SiNx/SiOx. 
     In some embodiments, in order to further guarantee the flatness of the surface of the flat layer, in the above display substrate provided by some embodiments of the present disclosure, as shown in  FIGS. 3-5 , the thickness of the flat layer  6  needs to be greater than the thickness of the first metal layer  2 , and the specific thickness of the flat layer  6  is selected according to the actual needs, which is not limited. 
     In some embodiments, since the flat layer with better flatness is arranged between the first metal layer and the second metal layer, the second metal wirings with the same width and spacing as that of the first metal wirings can be formed when the second metal wirings are fabricated in the second metal layer by a photoresist exposure, development and etching process, so in the above display substrate provided by some embodiments of the present disclosure, as shown in  FIGS. 3-5 , a width d 1  of each first metal wiring  21  is the same as a width d 2  of each second metal wiring  31 , which may be 2.5 μm; and a gap width w 1  between adjacent first metal wirings  21  is the same as a gap width w 2  between adjacent second metal wirings  31 , which may be 2.5 μm. Thus, the etching resolutions of the metal wirings in the fanout region B 1  corresponding to the first metal layer  2  and the second metal layer  3  can be the same, thereby reducing the wiring space of the fanout region B 1 , and further achieving the narrow bezels. 
     In some embodiments, as shown in  FIGS. 3-5 , an orthographic projection of the first metal wirings  21  on the substrate  1  completely overlaps with an orthographic projection of the second metal wirings  31  on the substrate  1 . That is, the center lines of both the first metal wirings  21  and second metal wirings  31  coincide in the thickness direction of the display substrate. Of course, it is a preferred solution that the orthographic projection of the first metal wirings  21  on the substrate  1  completely overlaps with the orthographic projection of the second metal wirings  31  on the substrate  1 , and in other embodiments, the effect of reducing the border can be achieved as long as the two are partially overlapped, but complete overlapping of the orthographic projections can make the effect of shortening the narrow border optimal. 
     In some embodiments, the display substrate can be a display substrate for an OLED display screen or a display substrate for an LCD display screen, and the embodiments of the present disclosure are exemplified by the display substrate being the display substrate for the LCD display screen. Specifically, by taking the structure shown in  FIG. 5  as an example, the film layer structures of other regions in the display substrate will be described in detail, and the same film layer structures as those in  FIG. 5  in  FIGS. 3 and 4  refer to the schematic diagram of  FIG. 5 . 
     In some embodiments, as shown in  FIGS. 2 and 5 , the substrate  1  is further provided with a display region AA located at one side of the fanout region B 1 , the first metal layer  2  includes a gate electrode  22  located in the display region AA, and the second metal layer  3  includes a source electrode  32  and a drain electrode  33  which are located in the display region AA; optionally, the display substrate further includes an active layer  11  located between the second insulating layer  7 , and the source electrode  32  and the drain electrode  33 , and the gate electrode  22 , the active layer  11 , the source electrode  32  and drain electrode  33  constitute a driving transistor in the display region AA. 
     In order to improve the performance of devices fabricated in the display region AA, an orthographic projection of the flat layer  6  provided by embodiments of the present disclosure on the substrate  1  also covers an orthographic projection of the display region AA on the substrate  1  to improve the flatness of the display region AA, and the condition that the surface flatness of the flat layer  6  in the display region AA satisfies can refer to the description of the fanout region B 1 ; and the orthographic projection of the second insulating layer  7  on the substrate  1  also covers the orthographic projection of the display region AA on the substrate  1  to solve the problem of particles appearing in the display region AA. 
     The display substrate further includes a first transparent conductive layer  8  located between the second insulating layer  7  and the second metal layer  3 , a passivation layer  9  located on one side, deviating from the substrate  1 , of the second metal layer  3 , and a second transparent conductive layer  10  located on one side, deviating from the substrate  1 , of the passivation layer  9 ; optionally, due to taking the display substrate being the display substrate for the LCD display screen as an example, the first transparent conductive layer  8  may be a pixel electrode layer and the second transparent conductive layer  10  is a common electrode layer; the material of the first transparent conductive layer  8  and the material of the second transparent conductive layer  10  may be a transparent conductive material such as ITO. 
     The first transparent conductive layer  8  is electrically connected with the drain electrode  33 , the second transparent conductive layer  10  includes a plurality of transparent signal lines  101  located in the display region AA, and an orthographic projection of the first transparent conductive layer  8  on the substrate  1  overlaps with an orthographic projection of the plurality of the transparent signal lines  101  on the substrate  1 ; optionally, when the LCD display screen performs display, a common electrode signal is loaded to a common electrode through the transparent signal lines  101 , a driving signal is loaded to a pixel electrode through the driving transistor, and the liquid crystal of the LCD display screen is driven to flip by an electric field formed between the common electrode and the pixel electrode, and light passes through the liquid crystal layer to realize display. 
     In some embodiments, the plurality of the transparent signal lines of the second transparent conductive layer need to be led out to the fanout region corresponding to the first metal layer and the second metal layer through a via hole to realize electrical connection with a peripheral driver chip (IC), since the second transparent conductive layer is located in a different layer from the first metal layer and the second metal layer, it needs to be transferred to the fanout region corresponding to the first metal layer and the second metal layer through the via hole, thus in the above display substrate provided by the embodiments of the present disclosure, as shown in  FIGS. 2 and 5 , the substrate  1  is further provided with a via hole region B 2  located between the fanout region B 1  and the display region AA, the first metal layer  2  includes a first transfer electrode  23  located in the via hole region B 2 , and the second metal layer  3  includes a second transfer electrode  34  located in the via hole region B 2 . 
     In order to further improve the fabrication flatness of a film layer in the via hole region B 2 , the orthographic projection of the flat layer  6  provided by embodiments of the present disclosure on the substrate  1  also covers an orthographic projection of a part of the via hole region B 2  on the substrate  1 , the condition that the surface flatness of the flat layer  6  in the via hole region B 2  satisfies can refer to the description of the fanout region B 1 , and the orthographic projection of the second insulating layer  7  on the substrate  1  also covers the orthographic projection of a part of the via hole region B 2  on the substrate  1 . 
     The second transparent conductive layer  10  includes a plurality of transparent wirings  102  located in the via hole region B 2 . 
     A part of the transparent wirings  102  are electrically connected with the first transfer electrode  23  by a via hole V 1  sequentially through the passivation layer  9 , the second insulating layer  7 , the flat layer  6  and the first insulating layer  4 , and a part of the transparent wirings  102  are electrically connected with the second transfer electrode  34  by a via hole V 2  through the passivation layer  9 , thereby leading the transparent wirings  102  of the second transparent conductive layer  10  to the fanout region B 1  corresponding to the first metal layer  2  and the second metal layer  3 , respectively, achieving a design in which the fanout region B 1  adopts double-layer metal wirings, and facilitating a narrow bezel design. 
     It should be noted that the display region AA in  FIG. 2  shows only part of the transparent signal lines  101 , and of course, the display region AA also includes gate lines, data lines, power lines, etc., which are all electrically connected with the metal wirings (the first metal layer  2  or the second metal layer  3 ) of the fanout region B 1  to be electrically connected to a driver chip (IC) to realize the signal transmission function. 
     Based on the same inventive concept, some embodiments of the present disclosure also provide a method for manufacturing the display substrate, as shown in  FIG. 6 , including: 
     S 601  providing a substrate; wherein the substrate is provided with a fanout region; 
     S 602  forming a first metal layer on the substrate; wherein the first metal layer includes a plurality of first metal wirings located in the fanout region; 
     S 603  forming a flat layer on one side, deviating from the substrate, of the first metal layer; wherein an orthographic projection of the flat layer on the substrate at least covers an orthographic projection of the fanout region on the substrate; in the fanout region, a surface, furthest from the substrate, of the flat layer has a highest point having a maximum distance to a horizontal plane where the substrate is located and a lowest point having a minimum distance to the horizontal plane where the substrate is located, and the difference between the distance from the highest point to the horizontal plane where the substrate is located and the distance from the lowest point to the horizontal plane where the substrate is located is less than or equal to 1000 angstroms; and 
     S 604  forming a second metal layer on one side, deviating from the substrate, of the flat layer; wherein the second metal layer includes a plurality of second metal wirings located in the fanout region, and an orthographic projection of the first metal wirings on the substrate at least partially overlaps with an orthographic projection of the second metal wirings on the substrate. 
     According to the method for manufacturing the display substrate provided by embodiments of the present disclosure, the flat layer is fabricated between the first metal layer and the second metal layer, and the orthographic projection of the flat layer on the substrate at least covers the orthographic projection of the fanout region on the substrate, the surface, away from the substrate, of the flat layer includes the highest point and the lowest point, and the difference between the distance from the highest point to the horizontal plane where the substrate is located and the distance from the lowest point to the horizontal plane where the substrate is located is less than or equal to  1000  angstroms; in this way, the surface, facing the second metal layer, of the flat layer is relatively flat, thus the second metal layer deposited on the flat layer is also relatively flat, when the second metal layer is subsequently patterned by using a photoresist process to form the plurality of the second metal wirings, uniform photoresist thickness of the fanout region can be guaranteed, and the fabrication accuracy of the second metal wirings of the second metal layer can be improved, so that the second metal wirings with the same width and spacing as that of the first metal wirings can be formed. Thus, the etching resolutions of the metal wirings in the fanout region corresponding to the first metal layer and the second metal layer in the solutions of the present disclosure can be the same, thereby reducing the wiring space of the fanout region, and further realizing the narrow bezels. 
     In some embodiments, since the material of the flat layer is generally an organic material, the adhesion between the organic material and the metal layer is poor, and the subsequently fabricated film layer is easily detached from the substrate, and in the above fabrication method provided by the embodiment of the present disclosure, as shown in  FIG. 7 , before the flat layer is formed on one side, deviating from the substrate, of the first metal layer, the method further includes: 
     S 602 ′ forming a first insulating layer located between the first metal layer and the flat layer; wherein an orthographic projection of the first insulating layer on the substrate covers the substrate. Optionally, the first insulating layer may employ silicon nitride SiNx, silicon oxide SiOx, or a composite layer of SiNx/SiOx, and the first insulating layer may improve adhesion between the subsequently fabricated film layer and the substrate. 
     In some embodiments, in order to further guarantee the flatness of the surface of the flat layer, in the above fabrication method provided by the embodiments of the present disclosure, forming the flat layer on one side, deviating from the substrate, of the first metal layer specifically includes: 
     forming the flat layer having a thickness greater than a thickness of the first metal layer on one side, deviating from the substrate, of the first metal layer. In particular, the specific thickness of the flat layer is selected according to the actual needs, which is not limited. 
     In some embodiments, on one hand, due to poor adhesion between the flat layer and the metal layer, and on the other hand, due to the presence of particles in the flat layer of the organic material, which affects the device performance, in the above method provided by embodiments of the present disclosure, as shown in  FIG. 8 , after forming the flat layer, and before forming the second metal layer, the method further includes: 
     S 603 ′ forming a second insulating layer located between the flat layer and the second metal layer; wherein an orthographic projection of the second insulating layer on the substrate at least covers the orthographic projection of the fanout region on the substrate. In particular, the second insulating layer may employ silicon nitride SiNx, silicon oxide SiOx, or a composite layer of SiNx/SiOx, the second insulating layer may improve adhesion between the subsequently fabricated film layer and the substrate  1 , as well as solve the problem that the presence of particles in the flat layer affects the device performance. 
     In some embodiments, in the above method provided by embodiments of the present disclosure, as shown in  FIG. 9 , forming the plurality of the first metal wirings may include: 
     S 901  coating one side, deviating from the substrate, of the first metal layer with a first photoresist layer; and 
     S 902  exposing and developing the first photoresist layer to form the plurality of the first metal wirings. 
     As shown in  FIG. 10 , forming the plurality of the second metal wirings may specifically include: 
     S 1001  coating one side, deviating from the substrate, of the second metal layer with a second photoresist layer; and 
     S 1002  exposing and developing the second photoresist layer to form the plurality of the second metal wirings corresponding to the plurality of the first metal wirings. 
     By taking  FIG. 5  as an example, the method for manufacturing the display substrate provided by embodiments of the present disclosure is described in detail below. 
     (1) The first metal layer  2  is formed on the substrate  1  by a conventional film formation process, one side, deviating from the substrate  1 , of the first metal layer  2  is coated with the first photoresist layer (not shown), the first photoresist layer is exposed and developed, the first metal layer  2  is subjected to an etching process to form the gate electrode  22  located in the display region AA, the first transfer electrode  23  located in the via hole region B 2 , and the first metal wirings  21  located in the fanout region, the material of the first metal layer  2  can be Al/Mo and the thickness of the first metal layer  2  can be about 3400 angstroms; next, the first insulating layer  4  is deposited on one side, deviating from the substrate  1 , of the first metal layer  2  by using a PECVD chemical vapor deposition process, the material of the first insulating layer  4  may adopt silicon nitride SiNx, silicon oxide SiOx or a composite layer of SiNx/SiOx, and the first insulating layer  4  has a thickness of about 2000 angstroms; then, the flat layer  6  is fabricated on one side, deviating from the substrate, of the first insulating layer  4  by a gluing baking process, the material of the flat layer  6  may be an organic material, and the flat layer  6  has a thickness of about 5000 angstroms, as shown in  FIG. 11A . 
     (2) The second insulating layer  7  is deposited on one side, deviating from the substrate  1 , of the flat layer  6  by using the PECVD chemical vapor deposition process, the material of the second insulating layer  7  may adopt silicon nitride SiNx, silicon oxide SiOx or a composite layer of SiNx/SiOx, and the second insulating layer  7  has a thickness of about 2000 angstroms; the PECVD chemical vapor deposition process is then used to deposit a a-Si material on one side, deviating from the substrate  1 , of the second insulating layer  7 , the thickness of the a-Si material may be about 1800 angstroms, and then, the silicon island pattern of the active layer  11  is formed by exposure, dry etching and stripping processes, as shown in  FIG. 11B . 
     (3) The first transparent conductive layer  8  is formed on the second insulating layer  7  by conventional sputtering film-forming, photoetching, and etching processes, the material of the first transparent conductive layer  8  may be a transparent conductive material such as ITO, and the thickness of the first transparent conductive layer  8  may be about 400 angstroms, as shown in  FIG. 11C . 
     (4) The second metal layer  3  is evaporated based on the step (3) by using a sputtering process, the material of the second metal layer  3  may be a Mo/Al/Mo three-layer metal layer, the corresponding thickness of the three-layer metal layer is about 150/2000/800 angstroms, at this time, the surface of the second metal layer  3  corresponding to the fanout region B 1  is a flat surface, one side, deviating from the substrate  1 , of the second metal layer  3  is coated with the second photoresist layer  12 , and the surface of the second photoresist layer  12  is a flat surface, as shown in  FIG. 11D . 
     (5) The second photoresist layer  12  is exposed and developed, an etching process is performed on the second metal layer  3  to form the source electrode  32  and the drain electrode  33  which are located in the display region AA, the second transfer electrode  34  located in the via hole region B 2 , and the second metal wirings  31  located in the fanout region, the material of the second metal layer  2  can be Al/Mo and the thickness of the second metal layer  2  can be about 3400 angstroms, since the surface of the second metal layer  3  is flat, at this time, the exposure resolution of the fanout region B 1  is equivalent to that of the first metal layer  2 , and the line width and line spacing of the first metal wirings  21  of the first metal layer  2  are consistent with those of the second metal wirings  31  of the second metal layer  3 , as shown in  FIG. 11E . 
     (6) A film is formed on one side, deviating from the substrate  1 , of the second metal layer  3  by using the PECVD chemical vapor deposition process, the passivation layer  9  with the via holes V 1  and V 2  is fabricated in the via hole region B 2  by an exposure and etching process, the material of the passivation layer  9  may be SiNx, and the thickness of the passivation layer  9  may be about 6000 angstroms, as shown in  FIG. 11F . 
     (7) The second transparent conductive layer  10  is formed on one side, deviating from the substrate  1 , of the passivation layer  9  by using a sputtering film-forming process, the transparent signal lines  101  located in the display region AA and the transparent wirings  102  located in the via hole region B 2  are formed by the exposure and etching process, the material of the second transparent conductive layer  10  may be a transparent conductive material such as ITO and the thickness of the second transparent conductive layer  10  may be about 700 angstroms, as shown in  FIG. 5 . 
     By the steps (1)-(7), the display substrate shown in  FIG. 5  can be formed. 
     In summary, the line width and line spacing of the first metal wirings  21  of the first metal layer  2  finally formed by the above solution of the present disclosure may be 2.5 μm/2.5 μm, and the fanout pitch is 5.0 μm; the line width and line spacing of the second metal wirings  31  of the second metal layer  3  may be 2.5 μm/2.5 μm, and the fanout pitch is 5.0 μm; however, in the prior art, due to the limitation of the exposure resolution of the second metal layer, the line width and line spacing can only be 2.5 μm/3.0 μm, and the fanout pitch is 5.5 μm. Therefore, the solutions of the present disclosure can reduce the wiring space in the fanout region, thereby further realizing the narrow bezel design. 
     In addition, some embodiments of the present disclosure only need to form the flat layer between the second metal layers, and the adopted process flow does not need to add a mask, has high compatibility with the prior art and saves cost. 
     Based on the same inventive concept, some embodiments of the present disclosure also provide a display device, including the above display substrate provided by embodiments of the present disclosure. The principle of solving the problem of the display device is similar to that of the aforementioned display substrate, so the implementation of the display device can refer to the implementation of the aforementioned display substrate, and the repetition will not be repeated here. 
     In some embodiments, the above display device provided by embodiments of the present disclosure may be an organic light-emitting display device or a liquid crystal display device, which is not limited here. 
     In some embodiments, the above display device may be a full-screen display device, a flexible display device, or the like, which is not limited here. 
     In some embodiments, the above display device may be a full-screen cell phone as shown in  FIG. 12 . Of course, the above display device may also be any product or component with a display function, such as a tablet computer, a television, a display, a notebook computer, a digital photo frame, a navigator, and the like. Other essential components of the display device should be understood by those of ordinary skill in the art and will not be described herein and should not be taken as a limitation of the present disclosure. 
     According to the display substrate, the method for manufacturing the display substrate, and the display device provided by embodiments of the present disclosure, the flat layer is arranged between the first metal layer and the second metal layer, and the orthographic projection of the flat layer on the substrate at least covers the orthographic projection of the fanout region on the substrate, in the fanout region, the surface, furthest from the substrate, of the flat layer has the highest point having the maximum distance to the horizontal plane where the substrate is located and the lowest point having the minimum distance to the horizontal plane where the substrate is located, and the difference between the distance from the highest point to the horizontal plane where the substrate is located and the distance from the lowest point to the horizontal plane where the substrate is located is less than or equal to 1000 angstroms; in this way, the surface, facing the second metal layer, of the flat layer is relatively flat, thus the second metal layer deposited on the flat layer is also relatively flat, when the second metal layer is subsequently patterned by using a photoresist process to form the plurality of the second metal wirings, uniform photoresist thickness of the fanout region can be guaranteed, and the fabrication accuracy of the second metal wirings of the second metal layer can be improved, so that the second metal wirings with the same width and spacing as that of the first metal wirings can be formed. Thus, the etching resolutions of the metal wirings in the fanout region corresponding to the first metal layer and the second metal layer in the solutions of the present disclosure can be the same, thereby reducing the wiring space of the fanout region, and further realizing the narrow bezels. 
     While the preferred embodiments of the present disclosure have been described, those skilled in the art may make additional changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including preferred embodiments and all changes and modifications falling within the scope of the present disclosure. 
     Obviously, those skilled in the art can make various changes and modifications to the embodiments of the present disclosure without departing from the spirit and scope of the embodiments of the present disclosure. Thus, if these changes and modifications of the embodiments of the present disclosure fall within the scope of the claims of the present disclosure and its equivalent technology, the present disclosure is also intended to include these changes and modifications.