METHODS FOR FABRICATING OPTICALLY TRANSPARENT

CONDUCTIVE MESH

TECHNICAL FIELD

[0001] The present disclosure relates to lithography and optically transparent conductive meshes. More specifically, the present disclosure relates to methods for the fabrication of optically transparent conductive meshes using electrospun polymer nanofiber masks.

BACKGROUND

[0002] Transparent conductive films are of interest in many applications ranging from touch screen displays to electromagnetic interference shielding, from photovoltaic cells to deicing applications. Indium tin oxide (ITO) has been the material of choice for many of these applications due to its high optical transparency, good electrical conductance, and high chemical and environmental stability. However, ITO has poor mechanical stability, high raw material cost, and significant absorption loss of the ultraviolet (UV) and infrared (IR) portion of the spectrum. An extensive search has been devoted to alternative materials, such as carbon-based materials, conductive polymers, metallic wire, and graphene, to realize flexible transparent conductive films with good light transmission, and high conductivity. Although some of these materials provide mechanical, each system has one or more drawbacks such as, but not limited to, surface roughness, low optical transmissivity, and low electrical conductivity.

[0003] Transparent conductive meshes or nanomeshes have a wide variety of applications from electromagnetic beam steering to electromagnetic interference (EMI) shielding, and heating. Rolling mask lithography has been employed to define deterministic nanowire meshes on transparent substrates. Random nanowire meshes have been also used for making transparent conductors, for example by electrospinning a solution of silver (Ag) nanowires. However, such processes that start from finite length nanowires to form a mesh or nanomesh may suffer from discontinuous conduction paths and high conductivity meshes have not been achieved.

[0004] Metallic meshes can provide optical transmissivity and electrical conductance along with a wide transmission spectrum without bandgap limitations. Such metallic wire networks have been utilized in some of the aforementioned applications, such as electromagnetic interference shielding, window deicers, heat reflectors, and transparent electrodes. However, the fabrication of continuous metallic meshes or metallic wires with submicron linewidths on meter scale substrates is very expensive. Therefore, metallic meshes have not been used for a wide range of applications.

[0005] To achieve optical transparency and electrical conductivity, transparent conductive oxides (TCOs), such as indium tin oxide (ITO), zinc oxide (ZnO), and aluminum-doped zinc oxide (AZO), are traditionally used as coatings on dielectric substrates. Alternatively, random Ag nanowire meshes or carbon nanotube composite films may be used for RF antennas that are transparent in the visible. The electrical conductivity can be measured by sheet resistance. The sheet resistance that can be achieved using such random nanowire meshes or TCOs may vary from about 20 Ohm/sq to about 100 Ohm/sq and for radio frequencies (RFs) of interest (about 5 GHz to about 30 GHz or 5G bands), resonant patches or non-resonant patch antennas have strong absorbing characteristics when the sheet resistance is in the range of about 20 Ohm/sq to about 100 Ohm/sq.

[0006] Conventional methods for conductive mesh or nanomesh production are either by direct templating or directly depositing conductive nanofibers. Direct templating uses a heat treatment and restricts the materials to be used to a narrow range of polymers if flexible substrates are desired. However, direct conductive nanofiber spinning cannot produce high conductivity meshes.

[0007] Thus, there remains a need for an improved method for fabricating optically transparent, continuous, metallic conductive mesh, that is flexible in feature sizes of meshes or nanomeshes, is compatible with a wider range of polymer substrates, and is environmentally friendly.

SUMMARY OF THE DISCLOSURE

[0008] It has been discovered that nanofiber mask methods for producing conductive meshes or nanomeshes can utilize lithography, as in Rolling Mask Lithography (RML), in place of a heat treatment step. The disclosed nanofiber mask methods provide process flexibility and material quality. In addition, the backward compatibility of the disclosed nanofiber mask methods with RML allows rapid production of nanowire meshes with a large volume and a high throughput. These discoveries have been exploited to develop the present disclosure, which, in part, is directed to methods of fabrication using polymer nano masks. [0009] In one aspect, a method includes depositing a metal layer over an optically transparent substrate to form a metalized substrate, applying a positive photoresist onto the metalized substrate, and depositing a plurality of opaque or dye-doped polymer nanofibers onto a positive photoresist to form a polymer nanofiber mask. The method also includes exposing the positive photoresist to UV light to form an exposed portion and an unexposed remaining portion of the positive photoresist under the polymer nanofiber mask and contacting the exposed portion of the positive photoresist to a developing solution to dissolve and remove the exposed portion. The method also includes contacting the metalized substrate with a wet etching solution to remove a portion of the metal layer that is not covered by the polymer nanofiber mask and removing the unexposed remaining portion of the positive photoresist by contact with a solvent that does not react with the substrate, thereby forming the metallic nanowire mesh.

[0010] In some examples, the optically transparent substrate includes a polymer or glass. The polymer substrate may be flexible. The glass substrate may be rigid.

[0011] In some examples, which may be combined with each of the examples described above, the polymer includes transparent polyethylene terephthalate (PET), polyimides, polymethyl methacrylate (PMMA), fluoropolymers, polyesters, biaxially-oriented polyethylene terephthalate, polycarbonate, polyvinylidene fluoride (PVDF), polystyrene, and/or polyamides, among others.

[0012] In some examples, which may be combined with each of the examples described above, the optically transparent substrate includes silica, quartz, silicon, germanium, ZnSe, ZnS, CaF2, and/or MgF2.

[0013] In some examples, which may be combined with each of the examples described above, the metal layer includes one or a combination of aluminum, copper, nickel, coppernickel alloy, silver, or gold having a thickness of about 5 nm to about 5 pm. The metal layer may be adhered to the substrate with am adhesion layer. For example, the adhesion layer may include chromium or titanium and may be up to about 20 nm thick chromium or titanium adhesion layer.

[0014] In some examples, which may be combined with each of the examples described above, the method further includes forming a dye-doped water-soluble polymer solution by mixing a dye-doping with a water-soluble polymer prior to depositing a plurality of opaque or dye-doped polymer nanofibers.

[0015] In some examples, which may be combined with each of the examples described above, the depositing step includes electrospinning the opaque polymer nanofibers from the dye-doped water-soluble polymer solution. [0016] In some examples, which may be combined with each of the examples described above, the dye-doping includes one or more of Sudan IV, Sudan Black, Rhodamine family of dyes, osmium tetroxide or Congo Red, or nanoparticles including TiCh, ZnO, C60 fluorenes, and/or carbon nanotubes, wherein the dye absorbs visible light and UV light. The dye-doping may be added to make the polymer opaque and absorb visible light. More specifically, the dye-doping is added to absorb the light having wavelength of about 365 nm to about 450 nm near UV portion of light. The mentioned dyes have relatively high absorption coefficient within this spectral range.

[0017] In some examples, which may be combined with each of the examples described above, the polymer nanofibers include PS, PVA, chitosan, PVDF, cylodextrins, and/or PAN, among others.

[0018] In some examples, which may be combined with each of the examples described above, the depositing a metal layer includes sputter coating or thermal or electron beam deposition, or electroless wet deposition, or a combination of vacuum deposition and subsequent electrochemical deposition.

[0019] In some examples, which may be combined with each of the examples described above, the method also includes removing the plurality of opaque polymer nanofibers by exposing the plurality of opaque polymeric nanofibers to water after exposing the positive photoresist to UV light or after removal of a portion of the metal.

[0020] In another aspect, a method is provided for producing nanowire meshes using a negative photoresist. The method for producing a nanowire mesh includes applying a negative photoresist over an optically transparent substrate and depositing a plurality of opaque or dye-doped polymer nanofibers onto the negative photoresist to form a polymer nanofiber mask. The method also includes exposing the negative photoresist to UV light to form an exposed portion and an unexposed remaining portion under the polymer nanofiber mask. The method also includes contacting the unexposed remaining portion of the negative photoresist to a developing solution to dissolve and remove the unexposed portion of the negative photoresist, thereby exposing a portion of the substrate. The method also includes drying the substrate and depositing a metal layer over the exposed portion of the substrate and the exposed portion of the negative photoresist. The method further includes removing the exposed portion of the negative photoresist by contact with a solvent that does not react with the substrate, thereby forming the nanowire mesh. [0021] In some examples, which may be combined with each of the examples described above, the optically transparent substrate includes a polymer or a glass.

[0022] In some examples, which may be combined with each of the examples described above, the polymer includes PET, polystyrene, polyimide, PVDF, biaxially-oriented polyethylene terephthalate, polycarbonate (PC), fluoropolymers, and/or siloxene elastomer. [0023] In some examples, which may be combined with each of the examples described above, the metal layer includes aluminum or other metals as recited above and has a thickness ranging from about 5 nm to about 5 pm.

[0024] In some examples, which may be combined with each of the examples described above, the method further includes forming a dye-doped water-soluble polymer solution by mixing a dye-doping with a water-soluble polymer.

[0025] In some examples, which may be combined with each of the examples described above, the depositing step includes electrospinning the opaque polymer nanofibers from the dye-doped water-soluble polymer solution.

[0026] In some examples, which may be combined with each of the examples described above, the dye-doping includes one or more of osmium tetroxide or Congo Red, or nanoparticles including TiCh, ZnO, C60 fluorenes, and/or carbon nanotubes, wherein the dye absorbs visible light and UV light.

[0027] In some examples, which may be combined with each of the examples described above, the polymer nanofibers include PS, PVA, PVDF, chitosan, cylodextrins, and/or PAN, among others.

[0028] In some examples, which may be combined with each of the examples described above, depositing a metal layer includes electron beam or thermal evaporation.

[0029] In some examples, which may be combined with each of the examples described above, the method also includes removing the plurality of opaque polymer nanofibers by exposing the plurality of opaque polymeric nanofibers to water after exposing the positive photoresist to UV light or after removal of a portion of the metal.

[0030] In another aspect, a method is provided for producing nanowire meshes without using a photoresist. The method for producing a nanowire mesh includes depositing a metal layer over an optically transparent substrate to form a metalized substrate and depositing a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate. The method also includes reflowing the plurality of polymer nanofibers to form a polymer nanofiber mask and etching the substrate using a wet etching solution to remove the metal that is not protected by the polymer nanofiber mask. The method further includes contacting the polymer nanofiber mask with a solvent to dissolve and remove the polymeric nanofiber mask from the etched metallized substrate, the substrate not reacting with the solvent, thereby producing the nanowire mesh.

[0031] In some examples, which may be combined with each of the examples described above, reflowing the nanofibers may include passing the nanofiber coated substrate through a chamber including the saturated solvent atmosphere.

[0032] In some examples, which may be combined with each of the examples described above, the solvent may include acetone, chloroform, toluene, hexane, and/or tetrahydrofuran (THF).

[0033] In additional examples, which may be combined with each of the examples described above, reflowing the nanofibers may include spraying a layer of solvent over the nanofibers to cause the nanofibers to reflow due to solvent vapor exposure to stick to the metalized substrate and removing the solvent to form the polymer nanofiber mask on the metalized substrate.

[0034] In some examples, which may be combined with each of the examples described above, the depositing a plurality of polymer nanofibers includes electrospinning a polymer solution including the plurality of polymer nanofibers soluble in the solvent onto the metalized substrate to form the nanofiber coated substrate.

[0035] In additional examples, which may be combined with each of the examples described above, the polymer nanofibers include PS, PVDF, acrylonitrile butadiene styrene (ABS), polyamide, PAN, chitosan, and/or cylodextrins.

[0036] In some examples, which may be combined with each of the examples described above, the optically transparent substrate includes a polymer or a glass.

[0037] In additional examples, which may be combined with each of the examples described above, the polymer includes PET, biaxially-oriented polyethylene terephthalate, polycarbonate, polyimide, and/or fluoropolymers.

[0038] In some examples, which may be combined with each of the examples described above, the metalized layer includes aluminum, silver, gold, copper, and/or copper-nickel alloy and has a thickness ranging from about 5 nm to about 5 pm.

[0039] In another aspect, another method is provided for producing nanowire meshes without using a photoresist. The method includes depositing a metal layer over an optically transparent substrate to form a metalized substrate. The method may also include depositing a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate. In addition, the method may also include reflowing the plurality of polymer nanofibers to form a polymer nanofiber mask, wherein the reflowing the nanofibers including spraying a layer of solvent over the nanofibers to cause the nanofibers to reflow due to solvent vapor exposure to stick to the metalized substrate; and removing the solvent to form the polymer nanofiber mask on the metalized substrate. Additionally, the method may include etching the substrate using a wet etching solution to remove the metal that is not protected by the polymer nanofiber mask. The method may also include dissolving the polymer nanofiber mask by using a solvent without reacting with the substrate.

[0040] In a further aspect, another method is provided for producing nanowire meshes without using a photoresist. The method includes depositing a metal layer over an optically transparent substrate to form a metalized substrate. The method may also include depositing a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate. The method may additionally include reflowing the plurality of polymer nanofibers to form a polymer nanofiber mask, the reflowing the nanofibers including spraying a layer of solvent over the nanofibers to cause the nanofibers to reflow due to solvent vapor exposure to stick to the metalized substrate; and removing the solvent to form the polymer nanofiber mask on the metalized substrate. The method may also include etching the substrate using a wet etching solution to remove the metal that is not protected by the polymer nanofiber mask. In addition, the method may include dissolving the polymer nanofiber mask by using a solvent without reacting with the substrate, thereby producing the nanowire mesh.

[0041] In another aspect, a method is provided for producing nanowire meshes without using a photoresist. The method includes depositing a metal layer over an optically transparent substrate to form a metalized substrate. The method may also include depositing a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate. In addition, the method may include reflowing the plurality of polymer nanofibers to form a polymer nanofiber mask, the reflowing the nanofibers including selectively heating the polymer nanofibers using a carbon dioxide (CO2) laser source to melt the polymer nanofibers. The method may also include etching the substrate using a wet etching solution to remove the metal that is not protected by the polymer nanofiber mask. In addition, the method may include dissolving the polymer nanofiber mask by using a solvent without reacting with the substrate, thereby producing the nanowire mesh. [0042] In some examples, which may be combined with each of the examples described above, the method may further include using a plurality of needles for electrospinning the polymer nanofibers and embedding nanofiber spinning into a roll-to-roll production line.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The foregoing and other objects of the present disclosure, the various features thereof, as well as the disclosure itself, may be more fully understood from the following description when read together with the accompanying drawings which:

[0044] FIG. 1 A is a schematic representation showing an exemplary method using electrospinning needle(s) to generate opaque dye-doped water-soluble nanofibers over a positive tone photoresist disposed over a metalized substrate according to an embodiment of the disclosure;

[0045] FIG. IB is a schematic representation showing an exemplary method exposing the positive tone photoresist of FIG. 1 A to UV light to produce a nanowire mesh according to an embodiment of the disclosure;

[0046] FIG. 2 is a schematic representation showing another exemplary method using electrospinning needle(s) to generate opaque dye-doped water-soluble nanofibers over a negative tone photoresist disposed over a blank substrate to produce a nanowire mesh according to an embodiment of the disclosure;

[0047] FIG. 3 is a schematic representation showing a chamber saturated with solvent vapor for reflowing electrospun nanofibers according to an embodiment of the disclosure; [0048] FIG. 4 is a schematic representation showing a carbon dioxide (CO2) laser source for selectively heating and melting electrospun nanofibers to reflow the electrospun nanofibers according to an embodiment of the disclosure;

[0049] FIG. 5 is a flow chart illustrating the steps of producing nanowire meshes using a positive photoresist according to an embodiment of the disclosure;

[0050] FIG. 6 is a flow chart illustrating the steps of producing nanowire meshes using a negative photoresist according to an embodiment of the disclosure;

[0051] FIG. 7 is a flow chart illustrating the steps of producing nanowire meshes by reflowing polymer nanofibers by solvent vapor exposure without using any photoresist according to an embodiment of the disclosure; and [0052] FIG. 8 is a flow chart illustrating the steps of producing nanowire meshes by reflowing polymer nanofibers by using a CO2 laser to selectively heat and melt the nanofibers without using any photoresist according to an embodiment of the disclosure.

DETAILED DESCRIPTION

[0053] The disclosures of these patents, patent applications, and publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed herein. The instant disclosure will govern in the instance that there is any inconsistency between the patents, patent applications, and publications and this disclosure. [0054] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The initial definition provided for a group or term herein applies to that group or term throughout the present specification individually or as part of another group unless otherwise indicated.

[0055] To explain the invention well-known features of lithography known to those skilled in the art of plasmon-enhanced fluorescence technology and cryptographic security have been omitted or simplified in order not to obscure the basic principles of the invention. Parts of the following description will be presented using terminology commonly employed by those skilled in the art of plasmon-enhanced fluorescence. It should also be noted that in the following description of the invention repeated usage of the phrase “in one embodiment” does not refer to the same embodiment.

[0056] As used herein, the articles “a” and “an” refer to one or more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element. Furthermore, the use of the term “including” as well as other forms, such as “include,” “includes,” and “included,” is not limiting.

As used herein, the term “about” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which it is used. As used herein when referring to a measurable value such as an amount, a temporal duration, and the like, the term “about” is meant to encompass variations of ±20% or ±10%, including ±5%, ±1%, and ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.

[0057] “Nanomesh” or “nanowire mesh” as used here encompasses an inorganic, nanostructured, two-dimensional material. [0058] “Electrospinning” as used herein refers to a fiber production method that uses electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in hundred nanometers.

[0059] “Photoresist” as used herein encompasses a light-sensitive material used in photolithography to form a patterned coating on a surface. The photoresist either breaks down or hardens where it is exposed to light, such as UV light.

[0060] “Positive photoresist” as used herein refers to a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to a photoresist developer. The unexposed portion of the photoresist remains insoluble to the photoresist developer. [0061] “Negative photoresist” as used herein encompasses a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble in a photoresist developer, while the unexposed portion of the photoresist is dissolved by the photoresist developer.

[0062] “Photolithography process” as used herein encompasses a process including coating a substrate with a light-sensitive organic material. A patterned mask is then applied to the surface to block light so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is then applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer dissolves away the regions that have been exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened (either polymerized or cross-linked) by light, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.

[0063] A “Rolling Mask Lithography (RML) process” refers to for a process by which large-area fabrication of two-dimensional metallic meshes of wires with sub-micron line widths, which is also referred to as nanowire mesh, which is optically transparent. The wire mesh or metallic mesh can have conductivity and transparency over alternative technologies and can be fabricated for large-area products and flexible devices of roll-to-roll fashion. The RML process allows for patterning the wire mesh with the mesh design for the target application transparency, haze, and/or electrical specifications. The wire mesh is also referred to as wire metallic mesh in the disclosure. [0064] Reference will now be made to specific examples illustrating the disclosure. It is to be understood that the examples are provided to illustrate exemplary embodiments and that no limitation to the scope of the disclosure is intended thereby.

[0065] The present disclosure provides methods for removing heat treatment and replacing the heat treatment with photolithography, such that process flexibility and good material quality can be achieved. The present disclosure provides methods of producing meshes or nanomeshes using electrospun polymer nanofibers as masks.

[0066] In some aspects, the disclosed methods may use photolithography and photoresist, and also use water soluble dye-doped electrospun polymer nanofibers to form a polymer nanofiber mask over the photoresist. The methods may also include a wet etching process to remove an unprotected portion of a metal layer over an optically transparent substrate under the polymer nanofiber mask. The photolithography may use a positive photoresist or a negative photoresist. The polymer nanofibers are washed away by water exposure.

[0067] In other aspects, the disclosed methods may also use reflowing of electrospun polymer nanofibers to form a polymer nanofiber mask without using any photoresist, where the polymer nanofibers are solvent-soluble. The methods also include a wet etching process to remove an unprotected portion of a metal layer over an optically transparent substrate under the polymer nanofiber mask. The methods also include removing the polymer nanofiber mask by using a solvent that does not react with the substrate. In some variations, the reflowing of electrospun polymer nanofibers may use solvent vapor exposure. In some variations, the reflowing of electrospun polymer nanofibers may also use a CO2 laser to selectively heat and melt the polymer nanofibers.

[0068] The disclosed method is different from a conventional method for fabricating two- dimensional aluminum (Al) nanowire networks offering transparent conductors can use wet etching of Al metalized polymer film using polystyrene (PS) nanofiber masks. Nanowire networks can be prepared by electrospinning polystyrene nanofibers with 283 nm in diameter on a metalized film, bonding the nanofibers to the Al metalized film by annealing at 200°C, etching unmasked Al, and then dissolving the PS nanofiber mask. The nanowire width and the area fraction of the Al nanowire network can be controlled readily by the nanofiber diameter and the deposition time of the PS nanofiber mask, respectively. The nanowire networks thus prepared are flexible and exhibit 80% optical transmittance and sheet resistance of 45 Q/sq, indicating that they can be an indium-tin-oxide (ITO) replacement. However, the conventional method uses a heat treatment, e.g., annealing, and also restricts the materials to be used to a narrow range of polymers, if flexible substrates are used.

[0069] The conventional method also uses electrospinning for forming conductive meshes or metallic meshes. For example, about 50 nm Al nanowire network was formed by electrospinning. However, direct conductive nanofiber electrospinning cannot produce high conductivity meshes.

[0070] A schematic representation showing an exemplary method using electrospinning needle(s) to generate opaque dye-doped water-soluble nanofibers over a positive tone photoresist disposed over a metalized substrate according to an embodiment of the disclosure is depicted in FIG. 1 A. A schematic representation showing an exemplary method exposing the positive tone photoresist of FIG. 1 A to UV light to produce a nanowire mesh according to an embodiment of the disclosure is depicted in FIG. IB.

[0071] A positive photoresist setup and method 100 using a positive tone resist or positive photoresist over a metalized substrate are shown in FIGs.lA and IB. The positive photoresist setup and method 100 uses dye-doped opaque polymer nanofibers in lithography on the metalized substrate. The positive photoresist method 100 is backward compatible with the RML process, without requiring an elastomeric phase mask.

[0072] As illustrated in FIG. 1 A, a positive photoresist setup and method 100 uses a positive photoresist 106 disposed over a metal layer 104 that is disposed over an optically transparent substrate 102, which can be a polymer, such as PET, or a glass. As such, the positive photoresist 106 is coated onto the metalized substrate. Electrospun dye-doped opaque nanofibers 110 can be deposited over the positive photoresist coated metalized substrate by electrospinning using electrospinning needle(s) 108.

[0073] As illustrated in FIG. IB, the positive photoresist coated metalized substrate 102 with the electrospun dye-doped opaque nanofibers 110 is subsequently exposed to UV light from a UV source 114. As such, the exposed portion of the positive photoresist becomes soluble in a developing solution, while the unexposed portion of the positive photoresist becomes insoluble in the developing solution. The nanofibers 110 can be then washed away by water exposure. The substrate 102 is then developed in a photoresist developer or a developing solution, such as a solvent, such that the exposed portion of the positive photoresist 106 is dissolved in the developing solution and thus removed. After the development of the positive photoresist 106, the positive photoresist-coated metalized substrate 102 is contacted with (e.g., immersed in) a wet etching solution, which can remove the exposed metal and open the regions that are not protected by the photoresist. Then, the positive photoresist is stripped off by using a solvent such as, but not limited to, acetone, chloroform, tetrahydrofuran (THF), hexane, or toluene. As such, method 100 forms the metallic nanowire mesh 116. Exemplary positive photoresists include AZ1518, or other AZ series resists, which are provided by MicroChemicals GmbH, Nicolaus-Otto-Str. 39, D-89079 Ulm, Germany. Tetramethylammonium hydroxide (TMAH or TMAOH) or hydroxy based developers such as PPD-450, which are provided by Transene Company, Inc., Danvers Industrial Park, 10 Electronics Avenue, Danvers, MA 01923 USA, can be used to develop the films.

[0074] A schematic representation showing another exemplary method using electrospinning needle(s) to generate opaque dye-doped water-soluble nanofibers over a negative tone photoresist disposed over a blank substrate to produce a nanowire mesh according to an embodiment of the disclosure is depicted in FIG. 2. The negative photoresist method or process 200 is different from the positive photoresist process 100 as illustrated in FIGs. 1 A and IB. The negative photoresist method 200 starts with a blank substrate 202 coated with a negative tone photoresist 204 without a metal layer in between. The negative photoresist method 200 still uses dye-doped opaque polymer nanofibers to form a polymer nanofiber mask used in photolithography on the substrate 202. The negative photoresist method 200 is backward compatible with the RML process, without requiring an elastomeric phase mask.

[0075] As illustrated in FIG. 2, substrate 202, which may be a polymer substrate or a glass substrate, is coated with the negative tone photoresist or the negative photoresist 204. For example, the polymer substrate may be an optically transparent polymer, such as polyethylene terephthalate (PET). Then, opaque nanofibers or dye-doped nanofibers 210 are electrospun over the negative photoresist 204 by electrospinning needle(s) 208. A film stack including the nanofibers 210, the negative photoresist 204, and the substrate 202 moves along a direction 212 as pointed by an arrow. Then, the photoresist 204 with the nanofibers 210 is exposed to UV light. As such, the exposed portion of the negative photoresist becomes insoluble or hardened in a developing solution, while the unexposed portion of the negative photoresist becomes soluble in the developing solution. After the UV exposure, the nanofibers are washed away by water exposure. Then, the film stack including photoresist 204 and substrate 202 moves into a developer tank 213 containing a developer solution or a solvent, such that the unexposed portion of the negative photoresist 204, under the nanofiber mask, dissolves in a developing solution and is thus removed. Exemplary negative photoresists include Shipley UVN-2, which is provided by Micro Resist Technology GmbH, Kbpenicker Str. 325, 12555 Berlin, Germany.

[0076] Next, a metal layer 214 is deposited over the substrate that is not protected by the negative photoresist 204 and also deposited over the remaining insoluble or harden portion of the negative photoresist 204. After metalizing the substrate, a lift-off procedure 215 is performed. The lift-off procedure 215 may include using a solvent to remove the remaining insoluble or harden portion of the negative photoresist 204. Solvents such as, but not limited to, acetone, chloroform, THE, or MIB. As such, a metallic mesh or nanomesh 216 is produced.

[0077] In some variations, the electrospinning parameters can be selected such that the mesh or nanomesh can be completely random or aligned to a direction to varying degrees. For example, typical source-target separations are about 1 cm to about 20 cm. A single syringe flow rates may be 0.2 mL/h. Deposition voltages may range from about 5 kV to about 60 kV, and substrate speeds may range from about 0.01 m/min to about 100 m/min. Concentrations of polymer solutions are about 5% to about 30% by weight, with salt additives to control conductivity.

[0078] In addition, the size of electrospun needles 208 can be selected to vary the diameters of the nanofibers 210. For example, the nanofibers 210 may have diameters ranging from about 0.2 pm to about 20 pm.

[0079] In some variations, multiple needles 108 or 208 can be used to cover large areas, and the nanofiber spinning can be embedded into a roll-to-roll production line.

[0080] In some variations, multiple needles 108 or 208 can be used to vary the size, density and uniformity of nanofibers to realize non-uniform distribution of sheet resistance across the printed area.

[0081] The methods 100 and 200 use dye-doped polymer nanofibers, which act as photolithography masks. The dye-doped polymer nanofibers may be obtained by adding a dye to a polymer solution containing water-soluble polymer nanofibers, which can be used in an electrospinning process. The dye or dye-doping, such as Sudan IV, Sudan Black, rhodamines, Congo Red, and osmium tetroxide, may have absorption in wavelengths of blue color and UV lights. The dye-doping may also include nanoparticle doping, such as TiCh, ZnO, C60 fluorenes, or carbon nanotubes. The dye-doping can be used to dye the water- soluble polymer nanofibers to make the nanofibers opaque. The dye-doping may have absorption in wavelengths of visible light and UV lights. It is desirable to have high optical absorption between about 365 nm and about 450 nm.

[0082] In some variations, chemical modifications may be used to make the dyes uniformly dispersed in the polymer solution.

[0083] In some variations, polymers can be soluble in mild solvents, which may not interfere with the photoresist exposure and the process of dissolving the photoresist in a photoresist developer. The mild solvents may also not react with the substrate.

[0084] The photolithography methods using either positive photoresist or negative photoresist have several advantages over conventional methods, including environmental friendliness, flexibility in feature sizes of meshes or nanomeshes, and compatibility with a wider range of polymer substrates without using a heat treatment. In some variations, the feature size may be about 1 pm to about 2 pm width. In some variations, the wire width may be about 2 to about 3 pm. In some variations, the metal layer may be up to 5 pm thick. [0085] A schematic representation showing a chamber saturated with solvent vapor for reflowing electrospun nanofibers according to an embodiment of the disclosure is depicted in FIG. 3. A reflow setup 300 uses solvent solubility of nanofibers to form a polymer nanofiber mask. As illustrated in FIG. 3, a metalized substrate includes a metal layer or metal film 304 over an optically transparent substrate 302. The metalized substrate may be coated with electrospun polymer nanofibers 306, which are soluble in a solvent, such as acetone. The metalized substrate coated with the nanofibers 306 is then placed in chamber 308 with an atmosphere that is saturated by solvent vapor 310, such as acetone. The substrate 302 may be selected to be resistant to the solvent, such as acetone. The polymer nanofibers 306 may reflow due to solvent vapor exposure and then stick to the metalized substrate 302 in a way that the polymer nanofibers 306 can act as a polymer nanofiber mask during a wet etching process. The substrate 302 is dried and then immersed in an etching solution to etch or remove the exposed metal layer that is not protected by the polymer nanofiber mask. Then, the polymer nanofiber mask is removed by dissolving the polymer nanofibers 306 in a solvent (e.g., acetone) that does not react with the substrate 302, thereby resulting in a metallic mesh or nanomesh or a conductive transparent conductor.

[0086] Alternatively, the solvent can be applied to the film using spraying a thin layer of solvent, such as acetone. The amount of the solvent may be adjusted so that the nanofibers do not completely dissolve but only reflow to form a polymer nanofiber mask. [0087] In some variations, the reflowing step may use localized fiber melting starting with a metalized substrate. A schematic representation showing a CO2 laser source for selectively heating and melting electrospun nanofibers to reflow the electrospun nanofibers according to an embodiment of the disclosure is depicted in FIG. 4. As illustrated in FIG. 4, a metal layer is deposited over an optically transparent substrate 402. Polymer nanofibers are coated over the metal layer by using electrospinning. The polymer nanofibers are soluble in a solvent, such as acetone. Then, the substrate is subjected to a controlled CO2 laser exposure from a CO2 laser source. Because the metal layer reflects most of the CO2 laser, the areas without any nanofibers are not heated. The CO2 beam can be expanded and scanned over the metalized substrate coated with electrospun nanofibers to achieve uniform reflow. The polymer nanofibers, such as polystyrene, can absorb the CO2 laser to be heated and melted and thus can reflow to stick to the metalized substrate 402 and act as a polymer nanofiber mask. A film stack including the substrate 402, the polymer nanofiber mask is then immersed in a wet etchant or wet etching solution to remove an exposed portion of the metal layer that is not protected by the polymer nanofiber mask. The polymer nanofiber mask is then removed by dissolving in a solvent (e.g., but not limited to, acetone, chloroform, THE, or MIB.) that does not react with the substrate 402, thereby resulting in a metal mesh or nanomesh or a conductive transparent conductor.

[0088] An exemplary method 500 for producing nanowire meshes using a positive photoresist is depicted in FIG. 5. Although the example method 500 depicts a particular sequence of operations, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the function of the method 500. In other examples, different components of an example device or system that implements the method 500 may perform functions at substantially the same time or in a specific sequence.

[0089] According to some examples, method 500 may include depositing a metal layer over an optically transparent substrate to form a metalized substrate at block 505. Method 500 may also include applying or coating a positive photoresist onto the metalized substrate at block 510. Method 500 may also include depositing a plurality of opaque or dye-doped polymer nanofibers onto the positive photoresist to form a polymer nanofiber mask at block 515. For example, the electrospinning needles 108 illustrated in FIG. 1A may deposit a plurality of opaque or dye-doped polymer nanofibers onto the positive photoresist to form a polymer nanofiber mask. Method 500 may also include exposing the positive photoresist to UV light to form an exposed portion of the positive photoresist and an unexposed remaining portion of the positive photoresist under the polymer nanofiber mask at block 520.

[0090] Method 500 may also include washing away the plurality of opaque polymer nanofibers by water exposure at block 525.

[0091] Method 500 may also include contacting the exposed portion of the positive photoresist with a developing solution to remove the exposed portion at block 530. For example, the exposed portion of the positive photoresist in a developing solution is dissolved in the developing solution to remove the exposed portion.

[0092] Method 500 may also include contacting the metalized substrate with a wet etching solution to remove a portion of the metal layer that is not protected by the polymer nanofiber mask at block 535. For example, the metallized substrate is immersed in a wet etching solution.

[0093] Method 500 may further include removing the unexposed remaining portion of the positive photoresist using a solvent without reacting with the substrate to form the metallic nanowire mesh at block 540.

[0094] An example method 600 for producing nanowire meshes using a negative photoresist is depicted in FIG. 6. Although the example method 600 depicts a particular sequence of operations, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the function of the method 600. In other examples, different components of an example device or system that implements the method 600 may perform functions at substantially the same time or in a specific sequence.

[0095] According to some examples, method 600 may include applying or coating a negative photoresist over an optically transparent substrate at block 605.

[0096] Method 600 may also include depositing a plurality of opaque or dye-doped polymer nanofibers onto the negative photoresist to form a polymer nanofiber mask at block 610. For example, the electrospinning needles 208 as illustrated in FIG. 2 may deposit a plurality of opaque or dye-doped polymer nanofibers onto the negative photoresist to form a polymer nanofiber mask. [0097] Method 600 may also include exposing the negative photoresist to UV light to form an exposed portion of the negative photoresist and an unexposed remaining portion of the negative photoresist under the polymer nanofiber mask at block 615.

[0098] Method 600 may also include washing away the plurality of opaque polymer nanofibers by water exposure at block 620.

[0099] Method 600 may also include contacting the unexposed remaining portion of the negative photoresist with a developing solution or dissolving the unexposed remaining portion of the negative photoresist in a developing solution to remove the unexposed portion of the negative photoresist to expose a portion of the substrate at block 625.

[00100] Method 600 may also include depositing a metal layer over the exposed portion of the substrate and the exposed portion of the negative photoresist at block 630.

[00101] Method 600 may further include removing the exposed portion of the negative photoresist using a solvent without reacting with the substrate at block 635.

[00102] An exemplary method 700 for producing nanowire meshes by reflowing polymer nanofibers by solvent vapor exposure without using any photoresist is depicted in FIG. 7. Although the method 700 depicts a particular sequence of operations, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the function of the method 700. In other examples, different components of an example device or system that implements the method 700 may perform functions at substantially the same time or in a specific sequence.

[00103] According to some examples, method 700 may include depositing a metal layer over an optically transparent substrate to form a metalized substrate at block 710.

[00104] Method 700 may also include depositing a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate at block 720. For example, the electrospinning needles as illustrated in FIGs. 1 A-1B and FIG. 2 may deposit a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate.

[00105] Method 700 may also include reflowing the plurality of polymer nanofibers to form a polymer nanofiber mask by solvent vapor exposure or solvent exposure at block 730. For example, the chamber 308 including saturated solvent vapor as illustrated in FIG. 3 may reflow the plurality of polymer nanofibers to form a polymer nanofiber mask. [00106] In some variations, reflowing the plurality of polymer nanofibers may include passing the nanofiber coated substrate through a chamber including the saturated solvent atmosphere.

[00107] In some variations, reflowing the nanofibers may include spraying a layer of solvent over the nanofibers to cause the nanofibers to reflow due to solvent vapor exposure to stick to the metalized substrate and removing the solvent to form the polymer nanofiber mask on the metalized substrate.

[00108] Method 700 may also include etching the substrate using a wet etching solution to remove the metal that is not protected by the polymer nanofiber mask at block 740.

[00109] Method 700 may further include dissolving the polymer nanofiber mask by using a solvent without reacting with the substrate at block 750. For example, the solvent does not react with the substrate.

[00110] An example method 800 for producing nanowire meshes by reflowing polymer nanofibers by using a CO2 laser to selectively heat and melt the nanofibers without using any photoresist is depicted in FIG. 8. Although the example method 800 depicts a particular sequence of operations, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the function of the method 800. In other examples, different components of an example device or system that implements the method 800 may perform functions at substantially the same time or in a specific sequence.

[00111] According to some examples, method 800 may include depositing a metal layer over an optically transparent substrate to form a metalized substrate at block 810.

[00112] Method 800 may also include depositing a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate at block 820. For example, the electrospinning needles as illustrated in FIGs. 1 A and IB and FIG. 2 may deposit a plurality of polymer nanofibers onto the metalized substrate to form a nanofiber-coated substrate.

[00113] Method 800 may also include reflowing the plurality of polymer nanofibers to form a polymer nanofiber mask at block 830. For example, the CO2 laser source 402 as illustrated in FIG. 4 may reflow the plurality of polymer nanofibers to form a polymer nanofiber mask. The reflowing the nanofibers may include selectively heating the polymer nanofibers using the carbon dioxide (CO2) laser source to melt the polymer nanofibers. [00114] Method 800 may also include etching the substrate using a wet etching solution to remove the metal that is not protected by the polymer nanofiber mask at block 840.

[00115] Method 800 may further include dissolving the polymer nanofiber mask by using a solvent without reacting with the substrate at block 850.

[00116] Nanowire mesh has a filling ratio defined by the ratio of the area of the mesh lines to the area of the substrate under the wire mesh. The filling ratio affects the optical transmission and electrical conductivity of the wire mesh. For example, a filling ratio of 1 :50 corresponds to 95% transmission. The optical transmission is also referred to as transmittance in the disclosure. The wire materials and wire parameters, such as thickness and width may be selected to provide higher optical transmission or higher conductivity. For instance, wider wires of 500 nm to 2 pm wide may provide about 10 times higher conductivity than narrower wires of 50 nm to 200 nm wide for the same filling ratio.

[00117] The dimensions of mesh lines of the wire mesh may affect properties, including haze, optical transmission, and electrical conductivity (or sheet resistance).

[00118] In some variations, the metallic mesh may include linear or curved segments or lines, which are connected.

[00119] In some variations, the conductive film is a planar metallic film.

[00120] In some variations, the wire mesh may enable radio frequency (RF) high pass or low pass and/or bandpass characteristics.

[00121] Nanowire mesh includes metallic meshes and provides high broadband light transmission, strong EMI shielding effectiveness, and good electrical conductivity. The effectiveness of each application, e.g., EMI shielding effectiveness, transparent antenna, heater, etc., is determined by the properties of the nanowire mesh pattern, such as metallic mesh stripe width, periodicity, thickness, among others. In some variations, nanowire mesh can have multiple uses for one or more applications. The nanowire mesh can be designed to meet the specifications of the applications.

[00122] In some variations, a method of fabrication using the Rolling Mask Lithography (RML) process is provided. The method may use a mask having connected and disconnected line patterns in a selected direction. The selected direction and the distance between successive connected line patterns are configured to fabricate a metallic nanowire network with anisotropic electrical conductivity.

[00123] In some variations, a method of fabrication using RML process is provided. The method may use a mask having mask pattern features that create contact pads. The mask pattern features smaller than 3 pm separation creating openings in the photoresist and thus creating contact pads with feature sizes larger than 3 pm.

[00124] In some variations, the method may use a secondary mask having a mask pattern that create connected and disconnected line patterns. The mask pattern features can vary between 50 pm and 100 mm. The pattern features can be configured to control the interaction of incident radio waves with the metallic nanowire network.

[00125] The present methods can be used in many applications, including, but not limited to, transparent EMI shielding, radio frequency (RF) beam forming and redirection, transparent Radar absorbers, transparent heaters, and de-icing or heating elements, de-fogging elements, transparent antennas, touch sensors, among others.

[00126] The present disclosure refers to nanomeshes and to meshes more generally. Therefore, the present disclosure should be read to include an additional disclosure of the disclosed aspects, examples and subject matter in general, in which the qualifier “nano” is omitted from “nanowire”, “nanotube”, “nanoparticle”, “nanostructure”, “nanofiber”, etc, to read “wire”, “tube”, “particle”, “structure”, “fiber”, etc, unless the context dictates differently.

EQUIVALENTS

[00127] Those skilled in the art will recognize or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.