Patent Publication Number: US-2016234930-A1

Title: Stretchable transparent electrode and method of fabricating same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2015-0018038, filed on Feb. 5, 2015, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure herein relates to a stretchable transparent electrode and a method of fabricating the same. 
     A stretchable electronic device may still maintain an electrical function even when a substrate is expanded by a stress applied from an outside. The stretchable electronic device has a potential application in a variety of fields such as a robot sensor skin, a wearable communication device, a built/attached-in body type bio device, a next-generation display beyond a limitation of a simple bendable and/or flexible device. 
     The stretchable electronic device may include interconnections of a stretchable material having conductivity instead of a metal material. A conductive stretchable material mainly includes a conductive material such as a conductive polymer, carbon nanotube, and graphene. However, the conductive stretchable material may have a drawback of high electrical resistance compared to a metal material while having a high expansion ability. 
     SUMMARY 
     The present disclosure provides a stretchable transparent electrode having more enhanced electrical conductivity. 
     The present disclosure also provides a method of fabricating a stretchable transparent electrode having more enhanced electrical conductivity. 
     The object of the present disclosure is not limited to the aforesaid, but other objects not described herein will be clearly understood by those skilled in the art from descriptions below. 
     An embodiment of the inventive concept provides a stretchable transparent electrode including a first substrate having an uneven surface, a first conductive film conformally covering the uneven surface of the first substrate to have an uneven top surface, a second conductive film conformally covering the first conductive film to have an uneven top surface, and a second substrate covering the second conductive film, wherein one of the first and second conductive films is a metal film and the other is a graphene film. 
     In an embodiment, the first conductive film may be a graphene film and the second conductive film may be a metal film. 
     In an embodiment, the first conductive film may be a metal film and the second conductive film may be a graphene film. 
     In an embodiment, a light may pass through the metal film. 
     In an embodiment, the metal film may include a crack. 
     In an embodiment, the stretchable transparent electrode may further include a third conductive film conformally covering the second conductive film, and the third conductive film may be a graphene film. 
     In an embodiment, the uneven surface of the first substrate may include convex portions and concave portions, and the convex portions and the concave portions may be alternately and repeatedly arranged in a first direction and may extend in a second direction crossing the first direction. 
     In an embodiment, the uneven surface of the first substrate may include convex portions and concave portions, and the convex portions and the concave portions may be alternately and repeatedly arranged in a first direction and in a second direction crossing the first direction. 
     In an embodiment, the first and second substrates may include polydimethylsiloxane PDMS or polyurethane. 
     In other embodiments of the inventive concept, a method of fabricating a stretchable transparent electrode, the method including forming a mold structure having an uneven surface, forming a polymer film on the uneven surface of the mold structure, separating the polymer film from the mold structure to form a first substrate having an uneven surface, conformally forming a graphene film on the uneven surface of the first substrate, and conformally forming a metal film on the graphene film. 
     In an embodiment, the method may further include forming a second substrate on the metal film, and the first and second substrates may include polydimethylsiloxane PDMS or polyurethane. 
     In an embodiment, the method may further include conformally forming a second graphene film on the metal film. 
     In an embodiment, the forming the mold structure may include preparing a mother substrate, patterning the mother substrate to form trenches, and forming a photoresist film filling the trenches on the mother substrate, and concave portions of the photoresist film may be formed on the trenches and convex portions of the photoresist film may be formed on projecting surfaces of the mother substrate due to a step difference between bottom surfaces of the trenches and projecting surfaces of the mother substrate disposed between the trenches. 
     In an embodiment, the forming the mold structure may include preparing a mother substrate, forming a photoresist film on the mother substrate, patterning the photoresist film to form a pattern including angled convex portions and angled concave portions, and performing a reflow process on the photoresist film to change the angled convex portions into rounded convex portions and the angled concave portions into rounded concave portions so as to form a photoresist film having an uneven surface. 
     In an embodiment, the forming the mold structure may include preparing a mother substrate, forming a photoresist film on the mother substrate, and forming the photoresist film curved in a round shape by using a gray scale photomask. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying drawings are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the inventive concept and, together with the description, serve to explain principles of the inventive concept. In the drawings: 
         FIG. 1  illustrates a cross-sectional view of a transparent electrode according to a first embodiment of the present inventive concept; 
         FIG. 2  illustrates a cross-sectional view of a transparent electrode according to a second embodiment of the present inventive concept; 
         FIG. 3  illustrates a cross-sectional view of a transparent electrode according to a third embodiment of the present inventive concept; 
         FIG. 4  illustrates a cross-sectional view of a transparent electrode according to a fourth embodiment of the present inventive concept; 
         FIGS. 5A to 5D  illustrate cross-sectional views of a method of fabricating a transparent electrode according to the first embodiment of the present inventive concept; 
         FIGS. 6A to 6C  illustrate cross-sectional views of one example of a method of fabricating a mold structure; 
         FIGS. 7A to 7C  illustrate cross-sectional views of another example of a method of fabricating a mold structure; 
         FIGS. 8A to 8C  illustrate cross-sectional views of still another example of a method of fabricating a mold structure; and 
         FIGS. 9A and 9B  illustrate perspective views of a curved surface according to an embodiment of the present inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Advantages and features of the present invention, and implementation methods thereof will be clarified through following embodiments described with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Further, the present invention is only defined by scopes of claims. Like reference numerals refer to like elements throughout. 
     In the following description, the technical terms are used only for explaining a specific exemplary embodiment while not limiting the present invention. The terms of a singular form may include plural forms unless referred to the contrary. The meaning of “include,” “comprise,” “including,” or “comprising,” specifies mentioned components, steps, operations and/or elements but does not exclude other components, steps, operations and/or elements. 
     Additionally, the embodiment in the detailed description will be described with sectional views and/or plan views as ideal exemplary views of the present invention. In the figures, the dimensions of layers and regions are exaggerated for an effective description of technical content. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Therefore, the embodiments of the present invention are not limited to the specific shape illustrated in the exemplary views, but may include other shapes that may be created according to manufacturing processes. For example, an etched region illustrated as a right angle may have rounded or curved features. Areas exemplified in the drawings have general properties, and are used to illustrate a specific shape of a device region. Thus, this should not be construed as limited to the scope of the present invention. 
       FIG. 1  illustrates a cross-sectional view of a transparent electrode according to a first embodiment of the present inventive concept. 
     Referring to  FIG. 1 , the transparent electrode may include a first flexible substrate  100 , a graphene film  200 , a metal film  300 , and a second flexible substrate  400 . The first flexible substrate  100  may be a flexible substrate. The flexible substrate may include, for example, polydimethylsiloxane (PDMS) or polyurethane. A top surface of the first flexible substrate  100  may be uneven in a round shape. For example, the top surface of the first flexible substrate  100  may include convex portions  111  and concave portions  113 . 
     Specifically, referring to  FIG. 9A , the convex portions  111  and the concave portions  113  of the first flexible substrate  100  may be alternately and repeatedly arranged in a first direction X. Also, the convex portions  111  and the concave portions  113  may extend in a second direction Y crossing the first direction X. 
     Referring to  FIG. 9B , the convex portions  111  and the concave portions  113  of the first flexible substrate  100  may be alternately and repeatedly arranged in the first and second directions X and Y. 
     Referring back to  FIG. 1 , the graphene film  200  may be applied to the first flexible substrate  100 . The graphene film  200  is conformally formed on the top surface of the first flexible substrate  100 , so that a top surface of the graphene film  200  may have a profile same as the top surface of the first flexible substrate  100 . For example, the top surface of the graphene film  200  may be uneven in a round shape. 
     The graphene film  200  may be coated with a metal film  300 . The metal film  300  is conformally formed on the top surface of the graphene film  200 , so that a top surface of the metal film  300  may have a profile same as the top surface of the graphene film  200 . For example, the top surface of the metal film  300  may be uneven in a round shape. A light may pass through the metal film  300 . The metal film  300  may have the thickness of several nanometers. The metal film  300  may include a metal material such as tungsten (W), copper (Cu), aluminum (Al), chromium (Cr), molybdenum (Mo), silver (Ag), or gold (Au). 
     According to an embodiment of the inventive concept, the metal film  300  is in contact with the graphene film  200  to be able to have a function to complement electrical conductivity of the graphene film  200  with high electrical resistance. Thus, the transparent electrode may have a work function of a desired value. 
     The metal film  300  may be coated with a second flexible substrate  400 . The second flexible substrate  400  may be a flexible substrate. The flexible substrate may include, for example, polydimethylsiloxane (PDMS) or polyurethane. The second flexible substrate  400  may completely cover the top surface of the metal film  300 . The second flexible substrate  400  may have a flat top surface. The second flexible substrate  400  may play a role of protecting the top surface of the metal film  300 . In addition, the second flexible substrate  400  is disposed opposite the first flexible substrate  100 , so that the graphene film  200  and the metal film  300  may be disposed between the first and second flexible substrates  100  and  400 . Accordingly, when the first and second flexible substrates  100  and  400  are folded and/or stretched, stress applied to the graphene film  200  and the metal film  300  may be minimized 
       FIG. 2  illustrates a cross-sectional view of a transparent electrode according to a second embodiment of the present inventive concept. For the sake of brevity, the elements and features of this example that are similar to the first embodiment previously shown and described will not be described in much further detail. 
     Referring to  FIG. 2 , the metal film  300  may include a crack  310 . When the first flexible substrate  100  and the second flexible substrate  400  are folded and/or stretched, the crack  310  may be formed as stress is applied to the metal film  300 . As the crack  310  is formed, a current may fail to be transferred through the metal film  300 . However, the metal film  300  is in contact with the graphene film  200 , thus being able to have a function locally lowering an electrical resistance of the graphene film  200 . Accordingly, the metal film  300  may enhance the electrical conductance of the graphene film  200 . 
       FIG. 3  illustrates a cross-sectional view of a transparent electrode according to a third embodiment of the present inventive concept. For the sake of brevity, the elements and features of this example that are similar to the first and second embodiments previously shown and described will not be described in much further detail. 
     Unlike the first embodiment, the order in which the graphene film  200  and the metal film  300  are formed may be changed. Referring to  FIG. 3 , the metal film  300 , the graphene film  200 , and the second flexible substrate  400  may be sequentially disposed on the first flexible substrate  100 . 
       FIG. 4  illustrates a cross-sectional view of a transparent electrode according to a fourth embodiment of the present inventive concept. For the sake of brevity, the elements and features of this example that are similar to the first and third embodiments previously shown and described will not be described in much further detail. 
     Referring to  FIG. 4 , the graphene film  200 , the metal film  300 , and a second graphene film  500  may be sequentially disposed on the first flexible substrate  100 . For preventing oxidation of the metal film  300 , the second graphene film  500  may be formed on the metal film  300 . The second graphene film  500  is conformally formed on the metal film  300 , so that a top surface of the second graphene film  500  may have a profile same as the top surface of the metal film  300 . For example, the top surface of the second graphene film  500  may be uneven in a round shape. The second flexible substrate  400  may be disposed on the second graphene film  500 . 
       FIGS. 5A to 5D  illustrate cross-sectional views of a method of fabricating a transparent electrode according to the first embodiment of the present inventive concept. 
     Referring to  FIG. 5A , a mold structure  10  is formed. A top surface of the mold structure  10  may be uneven in a round shape. Referring to  FIG. 9A , the top surface of the mold structure  10  may include concave portions  13  and concave portions  13  may be alternately and repeatedly arranged in a first direction X. The convex portions  11  and the concave portions  13  may extend in a second direction Y crossing the first direction X. Referring to  FIG. 9B , the convex portions  11  and the concave portions  13  of the mold structure  10  may be alternately and repeatedly arranged in the first and second directions X and Y. The mold structure  10  may be any one of a silicon substrate, a glass substrate, an insulating substrate, a polymer substrate, and a plastic substrate. 
     A method of forming the mold structure  10  will be described in detail with reference to  FIGS. 6A to 6C, 7A to 7C, and 8A to 8C . 
     Referring to  FIGS. 5B and 5C , a polymer film  50  is formed on the top surface of the mold structure  10 . The polymer film  50  may be formed by coating and curing a polymer material on the top surface of the mold structure  10 . The polymer film  50  may include, for example, polydimethylsiloxane (PDMS) or polyurethane. 
     The mold structure  10  is overturned to separate the polymer film  50 , so that a first flexible substrate  100  with an uneven surface is formed. The uneven surface of the first flexible substrate  100 , formed by contacting the top surface of the mold structure  10  may have a profile same as the top surface of the mold structure  10 . The uneven surface of the first flexible substrate  100  may be the top surface of the first flexible substrate  100 . For example, the top surface of the first flexible substrate  100  may include convex portions  111  and concave portions  113 . Referring to  FIG. 9A , the convex portions  111  and the concave portions  113  are alternately and repeatedly arranged in the first direction X, and may extend in the second direction Y crossing the first direction X. Referring to  FIG. 9B , the convex portions  111  and the concave portions  113  of the first flexible substrate  100  may be alternately and repeatedly arranged in the first and second directions X and Y. 
     Referring to  FIG. 5D , graphene film  200  may be conformally formed on the top surface of the first flexible substrate  100 . The graphene film  200  may be formed on the top surface of the first flexible substrate  100  by a physical method, a chemical method, and a chemical vapor deposition method (CVD). Alternatively, a graphene is grown on a seed film (not shown) and is separated from the seed film, and then is transferred on the top surface of the first flexible substrate  100  to form the graphene film  200 . A top surface of the graphene film  200  may have a profile same as the top surface of the first flexible substrate  100 . 
     Metal film  300  may be conformally formed on the top surface of the graphene film  200 . The metal film  300  may be deposited, for example, by a physical vapor deposition (e.g., sputtering). The top surface of the metal film  300  may be formed to have a profile same as the top surface of the graphene film  200 . The metal film  300  may be formed thin so that a light may pass therethrough. For example, the metal film  300  may be formed to have a thickness of several nanometers. The metal film  300  may include a metal material such as tungsten (W), copper (Cu), aluminum (Al), chromium (Cr), molybdenum (Mo), silver (Ag), or gold (Au). 
     Referring back to  FIG. 1 , a second flexible substrate  400  may be formed on the metal film  300 . A polymer material is applied to the metal film  300  and is cured to form the second flexible substrate  400 . Top surface of the second flexible substrate  400  may have a flat surface. 
       FIGS. 6A to 6C  illustrate cross-sectional views of one example of a method of fabricating a mold structure. 
     Referring to  FIGS. 6A and 6B , a mother substrate  20  is prepared. The mother substrate  20  may be any one of a silicon substrate, a glass substrate, an insulating substrate, a polymer substrate, and a plastic substrate. Trenches  21   a  may be formed by patterning the mother substrate  20 . Projecting surfaces  21   b  of the mother substrate  20  may be disposed between the trenches  21   a.  The mother substrate  20  may be patterned by performing any one process of a wet etching and a dry etching. 
     Referring to  FIG. 6C , a photoresist film  22  may be applied to a surface of the mother substrate  20  having trenches  21   a  formed thereon. The photoresist film  22  may fill the trenches  21   a.  The photoresist film  22  may be applied to the mother substrate  20  due to a low step coverage property caused by a step difference between a bottom surface of the trenches  21   a  and the projecting surfaces  2  lb of the mother substrate  20 . Accordingly, the top surface of the photoresist film  22  may include convex portions  11  and concave portions  13 . The convex portions  11  may be portions of the top surface of the photoresist film  22  applied to the projecting surfaces  21   b  of the mother substrate  20 , and the concave portions  13  may be portions of the top surface of the photoresist film  22  applied to the trenches  21   a.  The mold structure  10  may include the mother substrate  20  and the photoresist film  22 . 
       FIGS. 7A to 7C  illustrate cross-sectional views of another example of a method of fabricating a mold structure. 
     Referring to  FIGS. 7A and 7B , a photoresist film  22  may be applied to a mother substrate  20 . Patterns  23  may be formed by patterning the photoresist film  22 . The patterns  23  may be configured by angled convex portions  23   a  and angled concave portions  23   b  disposed between the convex portions  23   a.  The photoresist film  22  may be patterned by performing any one process of a wet etching, a dry etching, and a photolithography process. 
     Referring to  FIG. 7C , a reflow process may be performed on the photoresist film  22 . The reflow process may apply a temperature (T&gt;Tg) higher than the glass transition temperature Tg of the photoresist film  22  to the photoresist film  22 . By the reflow process, the angled convex portions  23   a  may be changed into convex portions  11  of a round shape and the angled concave portions  23   b  may be changed into concave portions  13  of a round shape. 
       FIGS. 8A to 8C  illustrate cross-sectional views of still another example of a method of fabricating a mold structure. 
     Referring to  FIGS. 8A and 8B , a photoresist film  22  may be applied to a mother substrate  20 . A grayscale photomask  30  may be disposed on the photoresist film  22 . The grayscale photomask  30  may pass different amount of light therethrough by regions. That is, an exposure amount of light may be differentiated by regions by using the grayscale photomask  30 . 
     Referring to  FIG. 8C , a development process may be performed on the photoresist film  22 . Since the exposure amount of light exposed to the photoresist film  22  is differentiated by using the grayscale photomask  30 , an amount of the photoresist film  22  removed when developed may be changed according to a region of the photoresist film  22 . For example, the amount of the photoresist film  22  removed in regions having a more exposure amount may be larger than that in regions having a less exposure amount. Accordingly, the top surface of the photoresist film  22  may be formed to have convex portions  11  and concave portions  13 . The convex portions  11  of the photoresist film  22  may be regions exposed to a smaller amount of light, and the concave portions  13  of the photoresist film  22  may be regions having exposed to a larger amount of light. 
     The stretchable transparent electrode according to embodiments of inventive concept may include a graphene film and a metal film disposed on a flexible substrate. The metal film is in contact with the graphene film, thus being able to enhance the electrical conductance of the graphene film with high electrical resistance. Accordingly, a transparent electrode with enhanced electrical conductivity may be provided. 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings to fully explain the present invention in such a manner that it may easily be carried out by a person with ordinary skill in the art(hereinafter, ‘those skilled in the art’) to which the present invention pertains. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.