Patent Publication Number: US-9426885-B2

Title: Multi-layer micro-wire structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 14/261,490 filed Apr. 25, 2014 (now U.S. Pat. No. 9,296,013 issued on Mar. 29, 2016), entitled “Making Multi-Layer Micro-Wire Structure” by Spath et al., the disclosure of which is incorporated herein. 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 14/032,213, filed Sep. 20, 2013, entitled “Micro-Wire Touchscreen with Unpatterned Conductive Layer” by Burberry et al. (now U.S. Pat. No. 9,304,636 issued on Apr. 5, 2016), and to commonly-assigned U.S. patent application Ser. No. 13/779,917, filed Feb. 28, 2013, entitled “Multi-Layer Micro-Wire Structure” by Yau et al. (now abandoned), the disclosures of which are incorporated herein. 
     FIELD OF THE INVENTION 
     The present invention relates to micro-wire electrical conductors. 
     BACKGROUND OF THE INVENTION 
     Transparent conductors are widely used in the flat-panel display industry to form electrodes for electrically switching the light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square). 
     Conventional transparent conductors are typically coated on a substrate to form a patterned layer of a transparent, conductive material, such as indium tin oxide or other metal oxide. Such materials are increasingly expensive and relatively costly to deposit and pattern. Moreover, metal oxides have a limited conductivity and transparency, and tend to crack when formed on flexible substrates or when curved. Conductive polymers are also known, for example polyethylene dioxythiophene (PEDOT). However, such conductors have a relatively low conductivity and transparency. 
     More recently, transparent electrodes including very fine patterns of conductive micro-wires have been proposed. For example, capacitive touch-screens with mesh electrodes including very fine patterns of conductive elements, such as metal wires or conductive traces, are taught in U.S. Patent Application Publication No. 2010/0328248 and U.S. Pat. No. 8,179,381, which are hereby incorporated in their entirety by reference. As disclosed in U.S. Pat. No. 8,179,381, fine conductor patterns are made by one of several processes, including laser-cured masking, inkjet printing, gravure printing, micro-replication, and micro-contact printing. The transparent micro-wire electrodes include micro-wires between 0.5μ and 4μ wide and a transparency of between approximately 86% and 96%. 
     Conductive micro-wires are formed in micro-channels embossed in a substrate, for example as taught in CN102063951, which is hereby incorporated by reference in its entirety. As discussed in CN102063951, a pattern of micro-channels is formed in a substrate using an embossing technique. Embossing methods are generally known in the prior art and typically include coating a curable liquid, such as a polymer, onto a rigid substrate. The polymer is partially cured (through heat or exposure to light or ultraviolet radiation) and then a pattern of micro-channels is embossed (impressed) onto the partially cured polymer layer by a master having a reverse pattern of ridges formed on its surface. The polymer is then completely cured. A conductive ink is then coated over the substrate and into the micro-channels, the excess conductive ink between micro-channels is removed, for example by mechanical buffing, patterned chemical electrolysis, or patterned chemical corrosion. Metal nano-particle compositions are known, for example as disclosed in U.S. Patent Application Publication No. 2011/0303885. The conductive ink in the micro-channels is cured, for example by heating. In an alternative method described in CN102063951, a photosensitive layer, chemical plating, or sputtering is used to pattern conductors, for example using patterned radiation exposure or physical masks. Unwanted material (photosensitive resist) is removed, followed by electro-deposition of metallic ions in a bath. 
     It is useful to form many electronic devices on flexible substrates. Flexible substrates are robust in the presence of mechanical shock and enable a wide variety of useful end-product form factors that are not readily achieved with electronic devices formed on rigid substrates. In particular applications, electronic devices are formed on flexible substrates in a flat configuration and then the electronic devices and flexible substrates are bent or otherwise mechanically manipulated to form non-planar shapes, for example a cylindrical shape or portion of a cylindrical shape. Since most electronic fabrication processes rely on flat substrates, the ability to form electronic devices in a flat configuration and then bend or curve the electronic device permits conventional manufacturing equipment designed for conventionally rigid and flat substrates to be used for making devices that are ultimately used in non-flat arrangements. 
     Polymer layers are used to conduct light in channels formed in a substrate, for example as disclosed in U.S. Pat. No. 7,371,452. Resins used for blocking light and formed in channels are discussed in U.S. Pat. No. 8,269,404. However, these disclosures do not provide conductive micro-wires. 
     In useful arrangements, conductive micro-wires are used in electronic devices to form apparently transparent electrodes or to provide conductors in electronic circuits. There is a need, therefore, for robust and manufacturable micro-wire structures that enable improved conductivity in non-flat configurations. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a multi-layer micro-wire structure resistant to cracking, comprises: 
     a substrate having a surface; 
     one or more micro-channels formed in the substrate; 
     an electrically conductive first material composition forming a first layer located in each micro-channel; and 
     an electrically conductive second material composition having a greater tensile ductility than the first material composition forming a second layer located in each micro-channel, the first material composition and the second material composition in electrical contact to form an electrically conductive multi-layer micro-wire in each micro-channel, whereby the multi-layer micro-wire is resistant to cracking. 
     The present invention provides an electrically conductive micro-wire structure resistant to cracking having improved electrical conductivity and robustness that is formed in a flat configuration and used in a curved configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used to designate identical features that are common to the figures, and wherein: 
         FIG. 1  is a cross section of a multi-layer micro-wire structure in an embodiment of the present invention; 
         FIG. 2  is a cross section of another multi-layer micro-wire structure in an embodiment of the present invention; 
         FIG. 3  is a more detailed cross section of the multi-layer micro-wire structure corresponding to the embodiment of  FIG. 1 ; 
         FIG. 4  is a more detailed cross section of the multi-layer micro-wire structure corresponding to the embodiment of  FIG. 3 ; 
         FIG. 5  is a cross section of a multi-layer micro-wire structure with a gap in an alternative embodiment of the present invention; 
         FIG. 6  is a cross section of a multi-layer micro-wire structure with a gap in a curved embodiment of the present invention; 
         FIG. 7  is a cross section of a multi-layer micro-wire structure having a third layer in an embodiment of the present invention; 
         FIGS. 8-11  are cross sections of various multi-layer micro-wire structures having a third layer in alternative embodiments of the present invention; 
         FIG. 12  is a plan view of two arrays of electrodes in separate layers and extending in orthogonal directions useful in understanding the present invention; 
         FIGS. 13-14  are cross section of alternative multi-layer micro-wire structures having an unpatterned conducting layer in an embodiment of the present invention; 
         FIGS. 15-19  are flow charts illustrating various methods of making the present invention; and 
         FIG. 20  is a perspective of a three-dimensional substrate in a curved capacitive touch screen according to an embodiment of the present invention. 
     
    
    
     The Figures are not necessarily to scale, since the range of dimensions in the drawings is too great to permit depiction to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed toward multi-layer micro-wire structures formed in a substrate that are capable of conducting electrical currents. The electrically conductive multi-layer micro-wire structures provide improved transparency, conductivity, and flexibility, especially in a curved configuration. 
     Referring to  FIG. 1  in an embodiment of the present invention, a multi-layer micro-wire structure  5  includes a substrate  40  having a substrate surface  41 . One or more micro-channels  60  are formed in the substrate  40  and extend into the substrate  40  from the substrate surface  41 . The micro-channels  60  have a micro-channel bottom  63 , micro-channel sides  62 , and a micro-channel top  61 . The micro-channel top  61  can be open and correspond to the substrate surface  41 . An electrically conductive first material composition  12  forms a first layer  10  located in each micro-channel  60 . An electrically conductive second material composition  22  having a greater tensile ductility than the first material composition  12  forms a second layer  20  located in each micro-channel  60 . The first material composition  12  and the second material composition  22  form an electrically conductive multi-layer micro-wire  50  in each micro-channel  60 . The micro-channel  60  can be completely or only partially filled with the first and second layers  10 ,  20 . In a useful embodiment, the substrate  40  is a transparent or flexible substrate  40 . 
     As shown in  FIG. 1 , the second material composition  22  is between the first material composition  12  and the substrate surface  41  of the substrate  40  or micro-channel top  61 . Alternatively, as shown in  FIG. 2 , the first material composition  12  is between the second material composition  22  and the substrate surface  41  of the substrate  40  or micro-channel top  61  of the micro-channel  60 . In either configuration, multi-layer micro-wire  50  includes both the first layer  10  and the second layer  20 . In one arrangement of the present invention, the first layer  10  is thicker than the second layer  20  or is more conductive. In another arrangement of the present invention, the second layer  20  is thicker than the first layer  10 . In different embodiments, first layer  10  is more electrically conductive than second layer  20  or second layer  20  is more electrically conductive than first layer  10 . First or second layer  10 ,  20  can have different optical properties. 
     In an embodiment, the first and second layers  10 ,  20  of the multi-layer micro-wire structure  5  form separate layers in the micro-channel  60  that have a clearly defined interfacial boundary separating the first and second layers  10 ,  20 . Referring to  FIGS. 3 and 4 , in another arrangement the first material composition  12  infuses the second material composition  22  or alternatively the second material composition  22  infuses the first material composition  12  so that the first and second layers  10 ,  20  can overlap. By infuse is meant that at least a portion of the second material composition  22  of the second layer  20  intermingles with at least a portion of the first material composition  12  of the first layer  10 .  FIG. 3  corresponds to the layer arrangement of  FIG. 1  and  FIG. 4  corresponds to the layer arrangement of  FIG. 2 . The second material composition  22  in the second layer  20  infuses a portion of the first layer  10 , for example some but not all of the first material composition  12  of the first layer  10  is coated by the second material composition  22 . Elements of the first material composition  12  are on the surface of the first layer  10  and the second layer  20  and also within the first layer  10  and the second layer  20 , thereby forming two at least partially intermingled layers. Infusing the first material composition  12  of the first layer  10  with the second material composition  22  of the second layer  20  improves electrical conductivity between the first and the second layers  10 ,  20  particularly, as discussed further below, when the multi-layer micro-wire  50  is stressed or strained by bending or folding the multi-layer micro-wire  50  and the substrate  40 . 
     Because the second material composition  22  has greater tensile ductility than the first material composition  12 , the second material composition  22  maintains its electrical conductivity when subjected to bending or folding better than the first material composition  12  and the second material composition  22  conducts electricity along the multi-layer micro-wire  50  even if the first material composition  12  is fractured or absent. 
     As shown in the embodiments of  FIGS. 3 and 4 , the first material composition  12  includes conductive particles  92 , for example a cured conductive ink including metal or metallic nano-particles that are sintered together and cured to form an electrical conductor. Conductive inks including metallic nano-particles that are cured, for example by heating, to form micro-wires are known. In another embodiment, the second material composition is, or includes, a conductive polymer, PEDOT, or a polyaniline. In an embodiment, the sintered nano-particles of the first material composition  12  form a porous solid and the second material composition  22  infuses the pores within the first material composition  12  without breaking electrical connections between the conductive particles  92  or otherwise disturbing the solid first material composition  12 . By infusing the first material composition  12  with the second material composition  22 , the electrical conductivity of the first material composition  12  is maintained and effective electrical conductivity between the first material composition  12  and the second material composition  22  is provided. Experiments have demonstrated that sintered conductive inks have poor tensile strength, are more brittle, but have better electrical conductivity than conductive polymers. Conductive polymers are more ductile but have a lower electrical conductivity than cured nano-particle inks. Thus, the multi-layer micro-wire  50  of the present invention provides both improved electrical conductivity and ductility, especially under mechanical strain. 
     Experiments have demonstrated that micro-wires including sintered conductive particles  92  found in conductive inks are susceptible to cracking when physically manipulated, for example by folding or bending the substrate  40  (FIGS.  1  and  2 ). Such cracks reduce the conductivity of cured conductive inks or form an electrical open that does not conduct electricity. In other experiments, it has been demonstrated that the process of locating conductive inks in the micro-channels  60  can in some cases fail to completely fill the micro-channels  60  and the process of curing the conductive ink in the micro-channels  60  can result in cracks that extend partially through the cured conductive ink and reduce conductivity or result in cracks that extend completely through the cured conductive ink resulting in electrical opens. For example, the surface energy of the micro-channel  60  micro-channel side  62  or micro-channel bottom  63  ( FIG. 1 ) can prevent the conductive ink from coating the micro-channel  60  thoroughly and, as the conductive ink cures (for example by drying or heating), the conductive ink can form micro-cracks that reduce or eliminate electrical conductivity in the cured conductor. 
     Thus, referring to  FIG. 5 , in another embodiment of the present invention at least one multi-layer micro-wire  50  in substrate  40  includes at least one crack in the first material composition  12  in the first layer  10  forming a gap  44  in the first material composition  12  where the first material composition  12  is partially or completely absent in the micro-channel  60 . The second material composition  22  in the second layer  20  forms an electrical connection that bridges the gap  44 .  FIG. 5  illustrates a cross section of the multi-layer micro-wire  50  along the length of the multi-layer micro-wire  50  rather than across the width of multi-layer micro-wire  50  as in  FIGS. 1-4 . The second layer  20  and the second material composition  22  can, but need not, extend to the micro-channel bottom  63  of the micro-channel  60  in which the multi-layer micro-wire  50  is formed. The second material composition  22  conducts electricity from one side of the gap  44  to the other, thereby improving the conductivity of the multi-layer micro-wire  50 . 
     Referring further to  FIG. 6  in another lengthwise cross section of multi-layer micro-wire  50 , a corner  42  of an object  46  is shown with the multi-layer micro-wire  50  of the present invention bent or wrapped around the corner  42  by bending or wrapping the substrate  40  into a three-dimensional configuration, for example around another object  46 , such as a display. A micro-crack in the first material composition  12  forms a gap  44  that impedes the flow of electricity through the first material composition  12 . The second material composition  22  bridges the gap  44  to improve the current flow through the multi-layer micro-wire  50 . Note that in both  FIGS. 5 and 6 , it is possible that the gap  44  forms an electrical open in the first material composition  12  or that the conductivity of the first material composition  12  is reduced but not eliminated. Alternatively, in an embodiment the micro-crack forming the gap  44  is a partial crack that does not extend all of the way through the first material composition  12  of the first layer  10 . In an embodiment in which conductive particles  92  form a substantial part of the first layer  10 , a gap  44  can result when the number of conductive particles  92  that are sintered together across the micro-channel  60  in the gap  44  is smaller than the number of sintered conductive particles  92  elsewhere in the multi-layer micro-wire  50 . As illustrated in  FIG. 6 , a single conductive particle  92  conducts electricity across the gap  44  around the corner  42  of the object  46  and on the substrate  40 . Elsewhere in the first layer  10 , two or more conductive particles conduct electricity along the length of the first layer  10 . Thus, according to an embodiment of the present invention, the gap  44  is an area of reduced conductivity in the first material composition  12  of the first layer  10 . The gap  44  is formed in various ways, particularly by coating a conductive ink, curing a conductive ink, or stressing or straining a cured conductive ink. The present invention is not limited by the way in which the gaps  44  are formed, or whether the gaps  44  have reduced or partial conductivity or are completely non-conductive thereby forming an electrical open. 
     As is readily understood by those knowledgeable in the mechanical and electrical arts, stressing or straining a conductor by bending it around a corner or into a three-dimensional configuration can form micro-cracks in the conductor that inhibits the flow of electricity along the conductor. Because the second material composition  22  has greater tensile ductility than the first material composition  12 , the second material composition  22  is more resistant to mechanical strain and less likely to develop cracks. Hence, the present invention provides improved electrical conductivity and resistance to stress or strain in a conductor, especially when formed in a flexible substrate  40  that is flexed, bent, curved, or wrapped around another object. 
     Thus, the multi-layer multi-wire structure  5  of the present invention provides a parallel conductive path along the micro-channels  60  in the substrate  40  parallel to the substrate surface  41  such that the net conductivity of the multi-layer multi-wire  50  is enhanced even if one layer (the more brittle layer, e.g. the first layer  10 ) is more electrically conductive but has defects, faults, or flaws, for example produced by mechanical strain or difficulty in coating or curing. The first and second material compositions  12 ,  22  are in electrical contact where the second material composition  22  is infused with the first material composition  12  and the second material composition  22  provides electrical conductivity where defects, faults, or flaws in the first material composition  12  are present. 
     In a further embodiment of the present invention, the first or second material composition  12 ,  22  is light-absorbing or includes carbon black. Alternatively, as shown in  FIG. 7 , in an embodiment of the present invention, the multi-layer micro-wire  50  in the micro-channel  60  includes a third layer  30  located in the micro-channel  60  on a side of the second layer  20  opposite the first layer  10 . In one embodiment, the third layer  30  includes a third material composition  32  that is different from either the first or the second material compositions  12 ,  22 . The third material composition  32  can be light absorbing and include, for example, carbon black. If the third material composition  32  is electrically conductive, in an embodiment it is a part of the multi-layer micro-wire  50 . 
     Referring to  FIG. 8  in another embodiment, the third material composition  32  in the third layer  30  is the same as or includes the first material composition  12  in the first layer  10  and is a part of the multi-layer micro-wire  50 . In this embodiment, as in  FIG. 7 , the third layer  30  is located in the micro-channel  60  on a side of the second material composition  22  of the second layer  20  opposite the first layer  10  and adjacent to the micro-channel top  61 . 
     Referring to  FIG. 9  in a different embodiment of the multi-layer micro-wire  50 , the third material composition  32  in the third layer  30  is the same as or includes the second material composition  22  in the second layer  20  and is a part of the multi-layer micro-wire  50 . In this embodiment, the third layer  30  is located in the micro-channel  60  on a side of the first material composition  12  of the first layer  10  opposite the second layer  20  and adjacent to the micro-channel top  61 . In either of the embodiments illustrated in  FIGS. 8 and 9 , the greater tensile ductility of the second material composition  22  improves the net conductivity of the first material composition  12  in the presence of cracks or gaps  44  ( FIGS. 5, 6 ) in the first material composition  12 . 
     In various embodiments of the multi-layer micro-wire  50 , the electrically conductive third material composition  32  forms a third layer  30  located in the micro-channel top  61  of the micro-channel  60  ( FIG. 7 ), located in the micro-channel  60  between the first and second layers  10 ,  20  ( FIG. 9 ), or located on the micro-channel bottom  63  of the micro-channel  60  ( FIG. 10 ) to form the multi-layer micro-wire  50  of the present invention. Referring to  FIGS. 10 and 11 , the electrically conductive third material composition  32  in the third layer  30  can enhances the conductivity of the multi-layer micro-wire  50  by providing greater tensile ductility than the first material composition  12  or the second material composition  22  or by providing increased electrical conductivity. As shown in  FIG. 10 , the first layer  10  is between the second layer  20  at the micro-channel top  61  and the third layer  30  at the micro-channel bottom  63 . As shown in  FIG. 11 , the third layer  30  is between the second layer  20  at the micro-channel top  61  and the first layer  10  at the micro-channel bottom  63 . 
     Turning to  FIG. 12 , in an embodiment, the multi-layer micro-wires  50  of the present invention are used to form one or more layers of a plurality of separate, spaced-apart electrodes  52 , each electrode  52  including a plurality of electrically connected multi-layer micro-wires  50 . In  FIG. 12 , for clarity only the multi-layer micro-wires  50  of the top layer and horizontal electrodes  52  are shown. In an embodiment, the electrodes  52  are electrically disconnected. 
     In an embodiment, the arrays of electrodes  52  are used to form a capacitive touch screen. In such an embodiment, the multi-layer micro-wires  50  of the present invention can form the electrical conductors of sense electrodes or drive electrodes. In a further embodiment, the capacitive touch screen is curved or wrapped around a corner or edge of a three-dimensional object, or has a three-dimensional configuration. Thus, the present invention enables a curved capacitive touch screen  99  that has a three-dimensional configuration, is curved, is wrapped around a corner or edge of a three-dimensional object or surface, or is adhered to a curved or three-dimensional surface, such as a cylinder, as shown in  FIG. 20 . By having a three-dimensional configuration is meant that the substrate surface  41  of the substrate  40  is not substantially flat or planar, or is perceptibly curved to an observer or user in at least one dimension. 
     Referring back to  FIG. 2 , the electrically conductive first material composition  12  in the first layer  10  is located only in each micro-channel  60  and not on the substrate surface  41 , as shown. Alternatively, as shown in  FIG. 1 , the electrically conductive second material composition  22  in the second layer  20  is located only in each micro-channel and not on the substrate surface  41 . The first and second layers  10 ,  20  are not necessarily flat or planar and, as noted above, can intermingle or are located together in a variety of configurations. 
     As shown in  FIG. 12 , the multi-layer micro-wires  50  form the separate, spaced apart electrodes  52  located in the micro-channels  60  ( FIG. 1 ). In the embodiments of  FIGS. 13 and 14 , in addition to the multi-layer micro-wires  50  an unpatterned conductive layer  70  is electrically connected to the multi-layer micro-wires  50  and thus electrically connects the multi-layer micro-wires  50  and electrodes  52  ( FIG. 12 ) and the multi-layer micro-wires  50 . The unpatterned conductive layer  70  is electrically connected to the first layer  10 , the second layer  20 , or both. In an embodiment, the unpatterned conductive layer  70  has the second material composition  22  and is formed in a common coating step with the second layer  20 . Essentially, the unpatterned conductive layer  70  is the portion of the second material composition  22  between the micro-channels  60  on the substrate surface  41 . Alternatively, the unpatterned conductive layer  70  includes the second material composition  22  of the multi-layer micro-wire  50  since, in an embodiment they are formed of the same materials in a common step. In an embodiment, the unpatterned conductive layer  70  is an isotropically conductive optically clear adhesive (OCA). Such an electrically conductive optically clear adhesive can adhere other elements in a device, for example another substrate  40 , a dielectric layer, a protective layer, or cover glass to the multi-layer micro-wire structure  5 . An electrically conductive optically clear adhesive forming the unpatterned conductive layer  70  can adhere a drive electrode layer to the substrate of a sense electrode layer in a capacitive touch screen. 
     As discussed with reference to  FIGS. 1 and 2 , either the first material composition  12  is between the second material composition  22  and the substrate surface  41  or the second material composition  22  is between the first material composition  12  and the substrate surface  41 .  FIG. 13  corresponds to the arrangement of  FIG. 1 . In  FIG. 13 , the second material composition  22  is coated over the substrate surface  41  and the first material composition  12  to form the second layer  20  and the unpatterned conductive layer  70 . The first material composition  12  and the first layer  10  are therefore at the micro-channel bottom  63  of the micro-channels  60  and are typically coated before the second material composition  22 . 
     In contrast, referring to  FIG. 14 , the second material composition  22  is deposited first and coats the substrate surface  41 , and the micro-channel sides (walls)  62  and the micro-channel bottom  63  of the micro-channels  60  in the substrate  40 . The first material composition  12  of the first layer  10  is then coated in the micro-channels  60  and over the second material composition  22  of the second layer  20 . The unpatterned conductive layer  70  is the same as that of  FIG. 13 , but the first material composition  12 , as in  FIG. 2 , is over the second material composition  22  and can extend to the substrate surface  41  and the micro-channel top  61  in the micro-channels  60  to form the multi-layer micro-wires  50 . In an embodiment, the unpatterned conductive layer  70  is only a few microns thick, for example less than 20 microns, less than 10 microns, less than 5 microns, or less than 2 microns thick. Alternatively, the unpatterned conductive layer  70  has the same thickness as the second layer  20  or is thinner than the second layer  20 . 
     As noted above, the unpatterned conductive layer  70  can include the second material composition  22 . Alternatively, the unpatterned conductive layer  70  includes the first material composition  12 . 
     Multi-layer micro-wires  50  of the present invention are useful in forming wires to connect electrical components on a substrate, particularly substrates that are flexible or that are flexed. In an embodiment, the multi-layer micro-wires  50  form electrodes that are used in capacitive touch screens. Multi-layer micro-wire structures  5  of the present invention are operated by providing electrical signals to the multi-layer micro-wires  50  or by sensing electrical signals from the multi-layer micro-wires  50 . Integrated circuits and electrical circuits using or connected to electrically conductive wires are commonly used and known in electrical system designs and the present invention is not limited to any particular design, structure, or application. 
     Referring to  FIG. 15  in a method of the present invention, the multi-layer micro-wire structure  5  is made by first providing a substrate  40  in step  100 . Micro-channels  60  are formed in or on the substrate  40  in step  110 . In step  120 , the first layer  10  is formed by locating the first material composition  12  in the micro-channels  60 . In step  130 , the second layer  20  is formed by locating the second material composition  22  in the micro-channels  60 . In an embodiment, the first material composition  12  is provided as a conductive ink or the second material composition  22  is provided as a conductive polymer. In an embodiment, the second material composition  22  is a polymer that is coated as a liquid, for example by spin, hopper, or curtain coating, and then cured. Coating and curing methods for polymers are well known in the art. 
     In a useful method of the present invention, the substrate  40  is arranged in a flat configuration during step  110 . By flat is meant that an imprinting stamp imprints the substrate  40  without objectionable flaws in the imprinted micro-channels  60  due to a misalignment between the orientation of the stamp and the orientation of the substrate  40 . In one embodiment, the substrate  40  is parallel to the stamp or to the portion of the stamp that impresses the substrate  40  (ignoring the relief structure of the stamp). In an embodiment, the method of  FIG. 15  is employed in a roll-to-roll process in which the substrate  40  is provided in a roll configuration, unrolled for processing (e.g. coating, imprinting, or curing), and then rolled again. When unrolled for processing, the substrate  40  need only have a sufficiently large radius of curvature that the micro-channels  60  of the imprinting step have no objectionable flaws. 
     In one embodiment of the present invention, the first material composition  12  and the first layer  10  are formed in step  120  before the second layer  20  is formed by locating the second material composition  22  in the micro-channels  60  in step  130 , forming the multi-layer micro-wire structure  5  illustrated in  FIG. 1 . In another embodiment of the present invention, the second layer  20  is formed by locating the second material composition  22  in the micro-channels  60  are formed in step  130  before the first material composition  12  and the first layer  10  in step  120 , forming the multi-layer micro-wire structure  5  illustrated in  FIG. 2 . In either embodiment, the electrically conductive second material composition  22  has a greater tensile ductility than the first material composition  12 . In optional step  140 , the substrate is bent, wrapped, or curved. In an embodiment, the first and second material compositions  12 ,  22  are cured material compositions. In a useful method of the present invention, the material composition that is deposited first is only partially cured before the material composition that is deposited second. The step of curing the material composition deposited second then also cures the material composition that is deposited first. In this way, the first and second material compositions  12 ,  22  are adhered together and have improved electrical connectivity. 
     Referring to  FIG. 16  in an embodiment, the first material composition  12  is coated over the substrate surface  41  and micro-channels  60  in step  112  and then removed from the substrate surface  41  but not from the micro-channels  60  in step  114 . Alternatively, the second material composition  22  is coated over the substrate surface  41  and micro-channels  60  in step  112  and then removed from the substrate surface  41  but not from the micro-channels  60 . The material in the micro-channels  60  is cured in step  116 . In an embodiment, the first material composition  12  is cured separately from the second material composition  22  in step  116 . In another embodiment, the first material composition  12  is cured at the same time as and together with the second material composition  22  in step  116 . The curing step  116  can reduce the volume of the first or second material compositions  12 ,  22 , for example by evaporating solvents. In a further embodiment, the first or second material composition  12 ,  22  is exposed to an HCl vapor. Exposure to an HCl vapor can enhance the conductivity of silver conductive particles  92  in a layer of a multi-layer micro-wire  50 . 
     In an embodiment, the substrate surface  41  is wiped to remove the first or second material composition  12 ,  22  from the substrate surface  41  between the micro-channels  60 . In another embodiment, the substrate surface  41  is wiped to remove a portion of the first or second material composition  12 ,  22  from the micro-channels  60  so that each micro-channel  60  is only partially filled. 
     As shown in  FIG. 12 , the multi-layer micro-wires  50  of the present invention form a plurality of electrically distinct electrodes  52 . Each electrode  52  includes a plurality of electrically connected multi-layer micro-wires  50 . Referring to  FIG. 17 , in an embodiment the substrate  10  is coated with the unpatterned conductive layer  70  electrically connected to the electrodes in step  105 , as shown in  FIGS. 13 and 14 . Referring to  FIG. 13 , the second material composition  22  is coated after the first material composition  12  so that the unpatterned conductive layer  70  is located on the micro-channel top  61  of each micro-channel  60  and not located on the micro-channel bottom  63  of each micro-channel  60 . Alternatively, referring to  FIG. 14 , the second material composition  22  is coated before the first material composition  12  so that the unpatterned conductive layer  70  is located on the micro-channel sides  62  and micro-channel bottom  63  of each micro-channel  60 . 
     Referring to  FIG. 18 , in another embodiment of the present invention, a third layer  30  is located in the micro-channel  60  on a side of the second layer  20  opposite the first layer in step  135 . The third layer  30  includes the first material composition  12 . In an alternative embodiment of the present invention, the third layer  30  is located in the micro-channel  60  on a side of the first layer  10  opposite the second layer  20 . The third layer  30  includes the second material composition  22 . In yet another embodiment, the third layer  30  includes a third electrically conductive material composition and is located in the micro-channel  60  between the first and second layers  10 ,  20  or located on the micro-channel top  61  of the micro-channel  60  (as shown in  FIGS. 7-11 ). 
     In a useful method of the present invention, referring to  FIG. 19 , an underlying substrate is provided in step  100  and an uncured curable layer provided on the underlying substrate in step  210 . In an embodiment, the underlying substrate and the cured layer form the substrate  40  and the surface of the cured layer opposite the underlying substrate forms the substrate surface  41 . The curable layer is provided, for example by spin, hopper, or curtain coating a cross-linkable resin. Such materials and methods are known in the prior art. 
     A stamp is provided in step  205  and used to imprint micro-channels  60  in the curable layer in step  215 . The curable layer is cured in step  220  to form micro-channels  60  in the cured layer. Conductive ink is provided in step  225 , for example by coating the substrate  40  and micro-channels  60  with the conductive ink and then removing the conductive ink from the substrate surface  41  leaving conductive ink in the micro-channels  60 . In step  230 , the conductive ink is cured to form a conductor. In an embodiment, the cured conductive ink is the first material composition  12 . Imprinting stamps, imprinting methods, and conductive inks are known in the art. 
     In an embodiment, a conductive layer is coated over the curable layer in optional step  212 . When the micro-channels  60  are imprinted, the conductive layer is located on at least a portion of the sides and bottom of the micro-channels  60 . When the conductive ink is cured in the micro-channels  60 , the conductive layer and the conductive ink form the second and first material compositions  22 ,  12 , forming the multi-layer micro-wire  50  in each micro-channel  60  and the unpatterned conductive layer  70  of an embodiment of the present invention, as shown in  FIG. 14 . Alternatively the cured layer and the cured conductive ink are coated with a conductive layer to form a multi-layer micro-wire  50  in each micro-channel  60  and the unpatterned conductive layer  70 , as shown in  FIG. 13 . If the conductive layer is removed from the substrate surface  41  before the conductive material (the second material composition  22 ) is cured, the multi-layer micro-wire structure  5  of  FIG. 1 or 2  results. 
     The unpatterned conductive layer  70  is useful for reducing electromagnetic interference in the multi-layer micro-wire structure  5  of the present invention. In particular, the electrodes  52  on a side of the unpatterned conductive layer  70  opposite a source of electromagnetic interference experience reduced signal noise when used to detect currents in the electrodes  52 . When used to form a capacitive touch screen, the presence of the unpatterned conductive layer  70  also increases capacitance between the driver and sensor electrodes  52  in the first and second layers  10 ,  20 , thereby reducing the voltage needed to sense changes in the capacitive field, for example due to touches, thereby improving efficiency. 
     Methods of the present invention provide advantages over the prior art. Additive techniques are less costly than traditional subtractive methods using photolithographic tools, for example including etching. Disclosed methods are applicable to roll-to-roll manufacturing techniques, increasing manufacturing rates and decreasing manufacturing costs. The costs of substrates, materials, and tooling are reduced. 
     The designation of first or second with respect to material compositions or layers is arbitrary and does not necessarily specify order or structure. Thus, depending on the embodiment of the present invention, first layer  10  is formed on second layer  20  or second layer  20  is formed on first layer  10 . In any specific example or embodiment, the first or second material composition or layer designations can be reversed without changing the nature of the invention. 
     Different materials coated in separate layers over patterned substrates are known. In contrast, the multi-layer micro-wires  50  of the present invention are formed in the micro-channels  60  and not over the substrate surface  41  of the substrate  40  and can have a narrow width and extend into the substrate  40 . Conventional substrate deposition and patterning methods, for example using sputtering to form a layer and then coated photo-resist with masked exposure to pattern a substrate are problematic or expensive, especially for such high-aspect ratio conductive structures, and can involve extensive subtractive processing, for example etching. Although it is known to form conventional micro-wires, the multi-layer micro-wires  50  of the present invention are structured multi-layer micro-wires  50  having at least the first and second layers  10 ,  20 . Such structured multi-layered micro-wires  50  are not known in the prior art and provide advantages as disclosed herein. 
     The first or second material compositions  12 ,  22  can be provided in one state and then processed into another state, for example converted from a liquid state into a solid state, to form a layer. Such conversion can be accomplished in a variety of ways, for example by drying or heating. Furthermore, the first or second material composition  12 ,  22  can include a set of materials when located and be processed to include a subset of the set of materials, for example by removing solvents from the material composition. For example, a material composition including a solvent is deposited and then processed to remove the solvent leaving a material composition without the solvent in place. Thus, according to embodiments of the present invention, a deposited material composition is not necessarily the same material composition as that found in a processed layer (e.g. first or second layer  10 ,  20 ). 
     According to various embodiments of the present invention, the first and second layers  10 ,  20  of the multi-layer micro-wires  50  have different electrical, mechanical, optical, or chemical properties. In useful embodiments, the multi-layer micro-wire  50  includes the first layer  10  located farther from the substrate surface  41  or the micro-channel top  61  than the second layer  20  and the first layer  10  is more electrically conductive than the second layer  20 . 
     The multi-layer micro-wire  50  including the first and second layers  10 ,  20  formed in the substrate  40  can have a width less than a depth or thickness so that the multi-layer micro-wire  50  has an aspect ratio (depth/width) greater than one. The multi-layer micro-wire  50  can be covered with a protective layer to protect it from scratches or other environmental damage, including mechanical or chemical damage. The protective layer can be formed over just the multi-layer micro-wire  50  or over a more extensive portion of the substrate surface  41 . 
     A layer need not continuously cover another layer in the multi-layer micro-wire  50  (not shown). In an embodiment, the first layer  10  completely covers the micro-channel top  61  or the second layer  20 . In another embodiment, the second layer  20  completely covers the micro-channel top  61  or the first layer  10 . Alternatively, the first layer  10  covers only a portion of the micro-channel top  61  or the second layer  20 . In another embodiment, the second layer  20  covers only a portion of the micro-channel top  61  or the first layer  10 . 
     In various embodiments of the present invention, the multi-layer micro-wire  50  has a width less than or equal to 10 microns, 5 microns, 4 microns, 3 microns, 2 microns, or 1 micron. Likewise, the micro-channel  60  has a width less than or equal to 20 microns, 10 microns, 5 microns, 4 microns, 3 microns, 2 microns, or 1 micron. In some embodiments, the multi-layer micro-wire  50  can fill the micro-channel  60 ; in other embodiments the multi-layer micro-wire  50  does not fill the micro-channel  60 . 
     In an embodiment, the first or second layer  10 ,  20  is solid. In another embodiment, the first or second layer  10 ,  20  is porous. The first or second material composition  12 ,  22  can include conductive particles  92  (or light-absorbing particles) in a liquid carrier (for example an aqueous solution). The liquid carrier can be located in the micro-channels  60  and heated or dried to remove the liquid carrier, leaving a porous assemblage of the conductive particles  92  that can be sintered to form a porous electrical conductor in a layer. 
     Electrically conductive multi-layer micro-wire structures  5  and methods of the present invention are useful for making electrical conductors in transparent micro-wire electrodes (e.g. electrodes  52 ) and for electrical conductors in general, for example as used in busses. A variety of micro-wire patterns can be used and the present invention is not limited to any one pattern. The multi-layer micro-wires  50  can be spaced apart, form separate electrical conductors, or intersect to form a mesh electrical conductor in the substrate  40 , as illustrated in  FIG. 12 . The micro-channels  60  can be identical or have different sizes, aspect ratios, or shapes. Similarly, the multi-layer micro-wires  50  can be identical or have different sizes, aspect ratios, or shapes. The multi-layer micro-wires  50  can be straight or curved. 
     Electrically conductive micro-layer micro-wire structures  5  of the present invention are useful, for example in touch screens such as projected-capacitive touch screens that use transparent micro-wire electrodes  52  and in displays. Electrically conductive multi-layer micro-wire structures  5  can be located in areas other than display areas, for example in the perimeter of the display area of a touch screen, where the display area is the area through which a user views a display. 
     When used in display systems, the multi-layer micro-wires  50  of the present invention provide an advantage in that when the substrate  40  is flexed, for example, by adhering the substrate  40  to a curved surface, the multi-layer micro-wires  50  continue to conduct electricity. 
     First layer  10  or second layer  20  can have a color or be reflective. U.S. Patent Application Publication No. 2008/0257211 discloses a variety of metallic colored inks and its contents are hereby incorporated by reference. 
     In various embodiments, the first or second material compositions  12 ,  22  can include conductive particles  92 , for example metal nano-particles such as silver nano-particles. The metal can be silver or a silver alloy or other metals, such as tin, tantalum, titanium, gold, or aluminum, or alloys thereof. The metal nano-particles can be sintered to form a metallic electrical conductor. First or second material compositions  12 ,  22  can include light-absorbing materials or particles such as carbon black, a dye, or a pigment. In one embodiment, the second material composition  22  includes carbon black, a black dye, or a black pigment and the first material composition  12  includes silver nano-particles. 
     Conductive inks including metallic particles are known in the art. In useful embodiments, the conductive inks include nano-particles, for example silver, in a carrier fluid such as an aqueous solution. The carrier fluid can include surfactants that reduce flocculation of the metal particles, humectants, thickeners, adhesives and other active chemicals. Once deposited, the conductive inks are cured, for example by heating. The curing process drives out the solution and sinters the metal particles to form a metallic electrical conductor. Conductive inks are known in the art and are commercially available. 
     In a useful embodiment, a material composition having conductive particles  92  and (optionally) light-absorbing particles in a liquid carrier is located in the micro-channel  60 . The material composition is processed, for example by drying, heating, or treatment with hydrochloric acid to remove the liquid carrier and agglomerate conductive particles  92 . 
     Curing material compositions to form layers (first or second layers  10 ,  20 ) or adhering the layers to each other or to substrate  40  can be done by drying or heating. In particular, if micro-channel  60  is formed in a polymer layer, heating the polymer layer slightly can soften the polymer so that particles, for example black pigment or carbon black particles or conductive particles  92 , in the first or second material compositions  12 ,  22  can adhere to the polymer. Such heating can be done by convective heating (putting substrate  40  into an oven) or by infrared radiation. Heating with infrared radiation has the advantage that light-absorbing particles, for example black particles, differentially absorb the infrared radiation and locally heat up the substrate  40  (that can be transparent), thus providing a more efficient adhesion or drying process for a material composition. Adhesion of the first or second layers  10 ,  20  to the substrate  40  or to each other is advantageous because such adhered layers are more resistant to mechanical abrasion and are thus more environmentally robust. 
     Conductive ink formulations useful for the present invention are commercially available, as are substrates, substrate coating methods, and micro-patterning methods for forming micro-channels. Curable polymer layers are well known as are method for coating, patterning, and curing them. Light-absorbing materials are also known and can be made into coatable material compositions using techniques known in the chemical arts. 
     In any of these cases, conductive inks or other conducting materials are conductive after they are cured and any needed processing completed. Deposited materials are not necessarily electrically conductive before patterning or before curing. As used herein, a conductive ink is a material that is electrically conductive after any final processing is completed and the conductive ink is not necessarily conductive at any other point in the multi-layer micro-wire  50  formation process. 
     According to various embodiments of the present invention, the substrate  40  is any material having the substrate surface  41  in which the micro-channels  60  can be formed. For example, glass and polymer are suitable materials known in the art from which the substrates  40  can be made into sheets of material having substantially parallel opposed sides, one of which is the substrate surface  41 . The substrate  40  can be a rigid or a flexible substrate and can have opposing substantially parallel and extensive surfaces. In a useful embodiment, the substrate  40  is substantially transparent, for example having a transparency of greater than 90%, 80% 70% or 50% in the visible range of electromagnetic radiation. The substrates  40  can include a dielectric material useful for capacitive touch screens and can have a wide variety of thicknesses, for example 10 microns, 50 microns, 100 microns, 1 mm, or more. In various embodiments of the present invention, the substrates  40  are provided as a separate structure or are coated on another underlying substrate, for example by coating a polymer substrate layer on an underlying plastic substrate. Such substrates  40  and their methods of construction are known in the prior art. The substrate  40  can be an element of other devices, for example the cover or substrate of a display or a substrate, cover, or dielectric layer of a touch screen. According to embodiments of the present invention, the multi-layer micro-wires  50  extend across at least a portion of substrate  40  in a direction parallel to the substrate surface  41  of substrate  40 . In an embodiment, the substrate  40  of the present invention is large enough for a user to directly interact therewith, for example with an implement such as a stylus or with a finger or hand. The substrates of integrated circuits are too small for such interaction. 
     The micro-channel  60  is a groove, trench, or channel formed in the substrate  40  or a layer coated on an underlying substrate forming the substrate  40  and extending from the substrate surface  41  into the substrate  40  and, in various embodiments, having a cross-sectional width in a direction parallel to substrate surface  41  less than 20 microns, for example 10 microns, 5 microns, 4 microns, 3 microns, 2 microns, 1 micron, or 0.5 microns, or less. In an embodiment, the cross-sectional depth of the micro-channel  60  is comparable to the width. The micro-channels  60  can have a rectangular cross section, as shown. Other cross-sectional shapes, for example trapezoids, are known and are included in the present invention. The first and second layers  10 ,  20  can have different depths or the same depth. The width or depth of a layer is measured in cross section. 
     A conductive layer of the multi-layer micro-wires  50  can be metal, for example silver, gold, aluminum, nickel, tungsten, titanium, tin, or copper or various metal alloys including, for example silver, gold, aluminum, nickel, tungsten, titanium, tin, or copper. The multi-layer micro-wires  50  can include a thin metal layer composed of highly conductive metals such as gold, silver, copper, or aluminum. Other conductive metals or materials can be used. Alternatively, the multi-layer micro-wires  50  can include cured or sintered metal particles such as nickel, tungsten, silver, gold, titanium, or tin or alloys such as nickel, tungsten, silver, gold, titanium, or tin. Conductive inks can be used to form multi-layer micro-wires  50  with pattern-wise deposition or pattern-wise formation followed by curing steps. Other materials or methods for forming multi-layer micro-wires  50  can be employed and are included in the present invention. 
     In an example and non-limiting embodiment of the present invention, each multi-layer micro-wire  50  is from 5 microns wide to one half micron wide and is separated from neighboring multi-layer micro-wires  50  by a distance of 20 microns or less, for example 10 microns, 5 microns, 2 microns, or one micron. 
     Methods and device for forming and providing substrates, coating substrates, patterning coated substrates, or pattern-wise depositing materials on a substrate are known in the photo-lithographic arts. Likewise, tools for laying out electrodes, conductive traces, and connectors are known in the electronics industry as are methods for manufacturing such electronic system elements. Hardware controllers for controlling touch screens and displays and software for managing display and touch screen systems are well known. These tools and methods are usefully employed to design, implement, construct, and operate the present invention. Methods, tools, and devices for operating capacitive touch screens are used with the present invention. 
     The present invention is useful in a wide variety of electronic devices. Such devices can include, for example, photovoltaic devices, OLED displays and lighting, LCD displays, plasma displays, inorganic LED displays and lighting, electrophoretic displays, electrowetting displays, dimming mirrors, smart windows, transparent radio antennae, transparent heaters and other touch screen devices such as resistive touch screen devices. 
     The invention has been described in detail with particular reference to certain embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
         
           5  multi-layer micro-wire structure 
           10  first layer 
           12  first material composition 
           20  second layer 
           22  second material composition 
           30  third layer 
           32  third material composition 
           40  substrate 
           41  substrate surface 
           42  corner 
           44  gap 
           46  object 
           50  multi-layer micro-wire 
           52  electrodes 
           60  micro-channel 
           61  micro-channel top 
           62  micro-channel side 
           63  micro-channel bottom 
           70  unpatterned conductive layer 
           92  conductive particles 
           99  curved capacitive touch screen 
           100  provide substrate step 
           105  coat substrate with conductive material step 
           110  form micro-channels step 
           112  coat substrate and micro-channels with material step 
           114  remove material from substrate surface step 
           116  cure material in micro-channels step 
           120  locate first material forming first layer step 
           130  locate second material forming second layer step 
           135  locate third material forming third layer step 
           140  bend substrate step 
           205  provide stamp step 
           210  provide curable layer step 
           212  coat conductive layer 
           215  imprint micro-channels in curable layer step 
           220  cure curable layer step 
           225  provide conductive ink in micro-channels step 
           230  cure conductive ink in micro-channels step