Abstract:
Embodiments of the present disclosure include a metal mesh structure and a method of fabrication thereof. The metal mesh structure includes a metal mesh formed on a substrate. The metal mesh is a 2D or 3D pattern of lines. The lines in the first and second set are characterized by a linewidth that is less than 2 microns. Such metal mesh structures are fabricated through rolling mask lithography. This abstract is provided to comply with rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure is related to lithography methods. More specifically, aspects of the present disclosure are related to nanometer-scale metal mesh devices and methods of fabrication thereof. 
     BACKGROUND 
     Photolithography fabrication methods have use in a wide variety of technological applications, including micro-scale and nano-scale fabrication of solar cells, LEDs, integrated circuits, MEMs devices, architectural glass, information displays, and more. 
     Roll-to-roll and roll-to-plate lithography methods typically use cylindrically shaped masks (e.g. molds, stamps, photomasks, etc.) to transfer desired patterns onto rigid or flexible substrates. A desired pattern can be transferred onto a substrate using, for example, imprinting methods (e.g. nanoimprint lithography), the selective transfer of materials (e.g. micro- or nano-contact printing, decal transfer lithography, etc.), or exposure methods (e.g. optical contact lithography, near field lithography, etc.). Some advanced types of such cylindrical masks use soft polymers as patterned layers laminated on a cylinder&#39;s outer surface. Unfortunately, lamination of a layer on a cylindrical surface creates a seam line where the edges of the lamination layer meet. This can create an undesirable image feature at the seam when the pattern is repeatably transferred to a substrate by using the cylindrical mask. 
     In addition to fabricating a mask having a seamless polymer layer, it would be desirable to fabricate polymer layers with smooth surfaces that are thick and uniform for use in subsequent rolling lithography fabrication methods. 
     It is within this context that the present invention arises. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a cross-sectional view of an example of a rolling mask lithograph (RML) apparatus that can be used to pattern of large areas of substrate material. 
         FIG. 1B  shows a three-dimensional view of the apparatus and substrate depicted in  FIG. 1A . 
         FIG. 2A  depicts a cross-sectional view of alternative apparatus useful for patterning of large areas of substrate material. 
         FIG. 2B  depicts a three-dimensional view of a metal mesh structure having a staggered pattern. 
         FIG. 2C  depicts a three-dimensional view of a metal mesh structure having a continuous non-staggered pattern. 
         FIGS. 3A-3E  illustrate an example of fabrication of metal mesh structures using a metal etch technique. 
         FIGS. 4A-4D  illustrate an example of fabrication of metal mesh structures using a “lift-off” technique. 
         FIGS. 5A-5B  are scanning electron micrographs of metal mesh structures fabricated on a glass substrate using a metal etch technique. 
         FIGS. 6A-6B  are scanning electron micrographs of metal mesh structures fabricated on a glass substrate using a “lift-off” technique. 
         FIGS. 7A-7B  are scanning electron micrographs of metal mesh structures fabricated on a polymer substrate using a “lift-off” technique. 
         FIGS. 8A-8B  are a sequence of schematic diagrams illustrating printing a pattern using rolling mask nanolithography in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In certain aspects of the present disclosure rolling mask lithography is used to produce metal mesh structures characterized by linewidths less than 2 microns. Such aspects utilize radiation transmitted through a rotating mask to generate a nanopattern in a layer of radiation-sensitive material on a substrate. Although the rotating mask used to generate the nanopattern within the layer of radiation-sensitive material may be of any configuration which is beneficial, and a number of these are described below, a hollow cylinder is particularly advantageous in terms of imaged substrate manufacturability at minimal maintenance costs. 
       FIG. 1A  and  FIG. 1B  depict an example of a rolling mask lithograph (RML) apparatus  100  that can be used to pattern of large areas of substrate material. In the apparatus  100 , a radiation transparent cylinder  106  has a hollow interior  104  in which a radiation source  102  resides. An exterior surface  111  of the cylinder  106  is patterned with a specific surface relief  112 . The cylinder  106  rolls over a radiation sensitive material  108  (e.g., a photoresist) which overlies a substrate  110 . 
     The radiation sensitive material  108  is imaged by radiation passing through surface relief  112 . The cylinder  106  rotates in the direction shown by arrow  118 , and radiation from a radiation source  102  passes through the nanopattern  112  present on the exterior surface  103  of rotating cylinder  106  to image the radiation-sensitive layer  108  on substrate  110 , providing an imaged pattern  109  within the radiation-sensitive layer  108 . The radiation-sensitive layer  108  is subsequently developed to provide a nanostructure on the surface of substrate  110 . In  FIG. 1B , the rotatable cylinder  106  and the substrate  110  are shown to be independently driven relative to each other as shown by arrow  120 . In another embodiment, the substrate  110  may be kept in dynamic contact with a rotatable cylinder  106  and moved in a direction toward or away from a contact surface of the rotatable cylinder  106  to provide motion to an otherwise static rotatable cylinder  106 . In yet another embodiment, the rotatable cylinder  106  may be rotated on a substrate  110  while the substrate is static. 
     The specific surface relief  112  may be etched into the exterior surface of the transparent rotating cylinder  106 . In the alternative, the specific surface relief  112  may be present on a film of polymeric material which is adhered to the exterior surface of rotating cylinder  106 . The film of polymeric material may be produced by deposition of a polymeric material onto a mold (master). The master, created on a silicon substrate, for example, is typically generated using an e-beam direct writing of a pattern into a photoresist present on the silicon substrate. Subsequently the pattern is etched into the silicon substrate. The pattern on the silicon master mold is then replicated into the polymeric material deposited on the surface of the mold. The polymeric material is preferably a conformal material, which exhibits sufficient rigidity to wear well when used as a contact mask against a substrate, but which also can make excellent contact with the radiation-sensitive material on the substrate surface. One example of the conformal materials generally used as a transfer masking material is polydimethylsiloxane (PDMS), which can be cast upon the master mold surface, cured with UV radiation, and peeled from the mold to produce excellent replication of the mold surface. 
     To produce a metal mesh, the specific surface relief  112  would form a pattern that corresponds to the desired mesh pattern, which may be characterized in terms of a desired pitch and linewidth. In aspects of the present disclosure, it is desirable for the mesh pattern to be characterized by first and second sets of intersecting parallel lines. The lines in the first set are parallel to each other and not parallel to the lines in the second set. Likewise, the lines in the second set are parallel to each other and not parallel to the lines in the first set. The lines in the first and second sets extend continuously across the pattern in a non-staggered fashion. The lines in the two sets are characterized by a linewidth that is less than 2 microns, preferably less than 1 micron, potentially less than 0.5 micron, possibly less than 0.1 micron. The lines in the first set may be regular spaced apart from each other by a first characteristic pitch. Likewise the lines in the second set may be regularly spaced apart from each other by a second characteristic pitch. The first and second pitches may be the same or different. Each characteristic pitch is greater than the linewidth and is up to 10 millimeters. The metal lines in the mesh are also characterized by a thickness, which can be from 50 nm to 10 microns. Although examples are described herein in which the metal mesh has a pattern in the form of regularly intersecting parallel lines, the pattern of the metal mesh may be a 2D or 3D pattern of lines, not necessarily parallel. 
       FIG. 2A  shows a cross-sectional view of another example of an apparatus  200  useful in patterning of large areas of substrate material. In  FIG. 2A , the substrate is a film  210  upon which a pattern is imaged by radiation which passes through surface relief  212  on a first (transparent) cylinder  206  while film  210  travels from roll  211  to roll  213 . A second cylinder  215  is provided on the backside  209  of film  210  to control the contact between the film  210  and the first cylinder  206 . The radiation source  202  which is present in the hollow space  204  within transparent cylinder  206  may be a mercury vapor lamp or another radiation source which provides a radiation wavelength of 365 nm or less. The surface relief  212  may be a phase-shift mask, for example, where the mask includes a diffracting surface having a plurality of indentations and protrusions, as discussed above in the Background Art. The protrusions are brought into contact with a surface of a positive photoresist (a radiation-sensitive material), and the surface is exposed to electromagnetic radiation through the phase mask. The phase shift due to radiation passing through indentations as opposed to the protrusions is essentially complete. Minima in intensity of electromagnetic radiation are thereby produced at boundaries between the indentations and protrusions. An elastomeric phase mask conforms well to the surface of the photoresist, and following development of the photoresist, features smaller than 100 nm can be obtained. 
     The surface relief  212  would form a desired mesh pattern with a desired pitch and linewidth. As mentioned above, the mesh pattern can be characterized by first and second sets of intersecting parallel lines. The lines in the first set are parallel to each other and not parallel to the lines in the second set. Likewise, the lines in the second set are parallel to each other and not parallel to the lines in the first set. The lines in the first and second sets extend continuously across the pattern in a non-staggered fashion. In comparison,  FIG. 2B  depicts a three-dimensional view of a metal mesh structure having a staggered pattern. Such patterns are common when using edge lithography to form a mesh pattern. As shown in the enlarged view of  FIG. 2B , the lines along the x direction at the intersection do not line up with each other.  FIG. 2C , on the other hand, depicts a three-dimensional view of a metal mesh structure having a continuous non-staggered pattern. As shown in the enlarged view of  FIG. 2C , the lines along the x direction at the intersection are lined up with each other comparing to  FIG. 2B . Such a metal mesh structure can be fabricated using phase lithography. 
     A key advantage of using RML to fabricate mesh structures is that it allows for fabrication of mesh structures on substrates of unlimited length. The width of continuous mesh structures that can be formed using RML is constrained mainly by the length of the cylinder mask  106 . Substrates up to 1 to 2 meters wide and of practically unlimited length, e.g., up to several hundred meters in length or more may be patterned with metal mesh using RML as described in aspects of the present disclosure. 
     There are several ways of forming metal mesh structures using RML. For example, as shown in  FIGS. 3A-3E , a metal etch technique may be used. In this technique, a metal layer  301  is formed on a substrate  302 , as shown in  FIG. 3A . The substrate  302  can be, e.g., glass or a polymer material. Deposition of metal materials can be implemented using physical vapor deposition (PVD), sputtering, chemical vapor deposition (CVD), atomic layer deposition (ALD), MVD and other vacuum-based techniques. Non-vacuum methods can also be used, like sol-gel, electroplating, electroless plating, and the like. 
     A layer of photo-sensitive material  304  is deposited over the metal layer  301 , as shown in  FIG. 3B . The photosensitive material  304  could be a photoresist. The photoresist could be a positive resist or a negative resist. A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The portion of the photoresist that is unexposed remains insoluble to the photoresist developer. A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer. 
     The photosensitive layer  304  can be patterned using RML, e.g., as discussed above, and then developed. The developed photosensitive layer includes a pattern of openings  305  that expose underlying portions of the metal layer  301 , as shown in  FIG. 3C . The patterned photosensitive material  304  and metal layer  301  can be subjected to an etch process that removes portions of the metal layer exposed by the openings  305  in the resist layer, as shown in  FIG. 3D . The etch process can be an anisotropic process, such as a plasma etch process or ion milling. Remaining portions of the photosensitive material  304  can then be removed leaving behind the patterned metal  301  as shown in  FIG. 3E . 
     In an alternative implementation, metal mesh structures may be formed by deposition of materials through a template can be followed by lift-off of template materials (photoresists, etc.), e.g., as shown in  FIGS. 4A-4D . In this technique, a layer of photosensitive material  404  (e.g., a positive or negative resist) is formed on a substrate  402 , as shown in  FIG. 4A . The substrate  402  can be, e.g., glass or a polymer material. The photosensitive layer  404  can be patterned using RML, e.g., as discussed above, and then developed. The developed photosensitive layer includes a pattern of openings  405  that expose underlying portions of the substrate  402 , as shown in  FIG. 4B . 
     A layer of metal  401  is deposited over the patterned photosensitive material  404  as shown in  FIG. 4C . Deposition of the metal layer  401  can be implemented using physical vapor deposition (PVD), sputtering, chemical vapor deposition (CVD), atomic layer deposition (ALD), MVD and other vacuum-based techniques. Non-vacuum methods can also be used, like sol-gel, electroplating, electroless plating, and the like. One preferred metal deposition technique that is useful for forming metal mesh structures is to deposit metal-containing materials from a liquid phase (e.g. as a metal ink) onto the substrate through the patterned photosensitive layer, e.g., using a roller. The metal material may also be sprayed onto the template and substrate. Also, other coating methods for liquid film deposition could be used such as, but not limited to, slot die and gravure coating. An example of such a technique is described, e.g., in U.S. Pat. No. 8,334,217, which is incorporated herein by reference. Metal-containing materials can be chosen to attach only to template materials or only to substrate material exposed through the template. The width and pitch of the metal mesh structures is determined by the corresponding pitch and width in the patterned rolling mask that is used to pattern the photosensitive layer. The thickness of the metal structures can be controlled by optimization of process transfer speed, viscosity of precursor, number of contact cycles with the roller, and other processing parameters. 
     The patterned photosensitive material  404  is then removed in a lift-off process taking with it overlying portions of the metal layer  401 . Portions of the metal layer that are in direct contact with the substrate remain behind following the liftoff process, leaving behind a pattern metal layer as shown in  FIG. 4D . Some implementations that use a metal-containing ink to form the metal layer  401  include a sintering step to solidify the patterned metal layer. The sintering could take place before lift-off or afterwards. 
     Using lift-off in conjunction with RML does not require etching the metal layer, e.g., with plasma etch. Plasma etch is a vacuum process that is not compatible with processing of large area flexible substrates. Lift-off also allows for recycling of the metal portions that have been removed in the lift-off process. Lift-off may be advantageously implemented in conjunction with deposition of metal by evaporation, as opposed to sputtering. The evaporation is more anisotropic than sputtering and produces metal lines with smoother sides. Smoother metal lines are particularly advantageous in OLED applications. An OLED (organic light-emitting diode) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current. This layer of organic semiconductor is situated between two electrodes. Generally, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as mobile phones, handheld games consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications. A common problem in manufacture of OLED devices that use metal mesh for one of the electrodes is that the rough metal mesh tends to short through an overlying layer of OLED material to the other electrode. Normally, this is addressed by covering the rough metal lines with a smoothing layer and then depositing the OLED material over the smoothing layer. Using lift-off in conjunction with metal evaporation produce smooth-sided metal mesh structures and the need for a smoothing layer can be avoided. 
     Lift-off in conjunction with metal ink deposition is highly desirable because it removes any vacuum operation from the manufacturing process. Thus metal mesh fabrication can be implemented in a roll-to-roll process, as opposed to a batch process. In addition to being a vacuum process, etching can result in a roughened substrate and/or roughened metal line edges. The difference between etching and lift-off can be clearly seen by comparing  FIGS. 5A-5B  with  FIGS. 6A-6B  and  7 A- 7 B. In  FIGS. 5A-5B , aluminum mesh structures are formed on a glass substrate using RML and metal etching. Such mesh structures have exhibited 96% transparency and sheet resistance of 3.5Ω/□. In  FIGS. 6A-6B , silver mesh structures are formed on a glass substrate using a combination of RML, metal evaporation, and liftoff. Such mesh structures have exhibited 95% transparency and sheet resistance of 3Ω/□. Aspects of the present disclosure allow for manufacture of metal mesh structures on polymer substrates. For example,  FIGS. 7A-7B  depict silver mesh structures formed on a polymer substrate (polyethylene terephthalate (PET)) using a combination of RML, metal evaporation, and liftoff. Such mesh structures have exhibited 96% transparency and sheet resistance of 5 Ω/□. 
     In certain embodiments according to the present disclosure, the photosensitive material is exposed by passing the substrate past the cylinder two or more times. For sufficiently small values of the pitch and linewidth, the linear pattern of exposure resulting from one pass is unlikely to line up with each other. As a result, lines from one pass are likely to end up between lines of a previous pass. By careful choice of the pitch, linewidth, and number of passes it is possible to end up with a pattern of lines in the photosensitive material that has a pitch smaller than the pitch of the lines in the pattern on the cylinder. In addition, each consecutive exposure is done with some misalignment to avoid “Moire” effect”. 
       FIGS. 8A-8B  are a sequence of schematic diagrams illustrating printing a pattern using rolling mask nanolithography. The substrate  110  translates in a first direction (e.g., to the right in this example) and the cylindrical mask rotates in a corresponding first sense (e.g., counterclockwise in this example) and in a second pass the substrate  110  translates in a second direction (e.g., to the left in this example) opposite the first direction and the cylindrical mask rotates in a second sense opposite the first sense (e.g., clockwise in this example). In the first pass, the lines (or spots) in the pattern  112  are transferred to the photosensitive material as a result of exposure of the photosensitive material to radiation from the source  102  through the pattern  112 . In the second pass, previously unexposed portions of the photosensitive material  108  located between neighboring exposed lines or spots are exposed. The linewidth (or spot size) remains more or less the same for both passes, but the resulting pattern in the photosensitive material has a smaller pitch due to the exposure of previously unexposed portions between neighboring exposed portions. 
     It is noted that the foregoing is only one example and embodiments of the present invention are not limited to the implementation depicted in  FIGS. 8A-8B . Alternatively, the substrate  110  may pass the rotating cylindrical mask two times in the same direction (e.g., two times to the right) with the cylinder  106  rotating the same way (e.g., counterclockwise) for each pass. 
     Other variations are possible. For example, two passes may be accomplished using two rotating cylindrical masks with corresponding light sources, elastomeric films, and patterns. The two rotating cylindrical masks may be arranged in tandem and passing the substrate, e.g., on a conveyor belt such that the substrate passes past one mask and then the other. The two patterns on the masks may have parallel lines characterized by the same pitch and linewidth of the lines in the two patterns or the two patterns may have slightly different pitch and/or linewidth. In another example, a single rotating cylindrical mask may have two different patterns on different portions. The patterns may be characterized by different pitch, linewidth (or spot size), or different shaped spots with the same pitch or different pitches. In a first pass, the photosensitive material may be exposed to the first pattern and in a second pass, the photosensitive material may be exposed to the second pattern. In certain embodiments, the two or more passes may be done at slightly different angles. Specifically, the two or more passes include a first pass of the substrate at an angle relative to an axis of the cylindrical mask and a second pass at a different angle relative to the axis of the cylindrical mask. Alternatively, the second pass may be at a different angle relative to an axis of a different rolling transparent mask. Details of possible variations are described in commonly owned, co-pending PCT Application No. PCT/2012/059388, to Boris Kobrin et al., filed Oct. 9, 2012, and entitled “LITHOGRAPHY WITH REDUCED FEATURE PITCH USING ROTATING MASK TECHNIQUE”, the entire contents of which are herein incorporated by reference. 
     Although the foregoing example describes an embodiment in which periodic patterns of regularly spaced lines are used, embodiments of the invention are not limited to such implementations. Alternatively, non-periodic patterns may be used. Furthermore, embodiments of the present invention are flexible with respect to the feature size in the patterns. For example, it may be desirable to print patterns with different linewidths in different passes. Specifically, wide lines may be printed in a first “roll”/exposure and narrow lines may be printed in a second exposure. Density of the mesh metal pattern/structure can be controlled by number of exposures, misalignment in orthogonal directions, and misalignment by angle. 
     Aspects of the present disclosure are useful for technological applications, such as touch screen displays, organic light emitting diode (OLED) lighting systems, optical antennae, electromagnetic interference (EMI) shielding, transparent heaters, and electrochromic windows. 
     Aspects of the present disclosure enable submicron printing, resulting in a two-dimensional mesh made up of 300-800 nm width regular metal lines that are invisible to human eye. This is advantageous since sub-micron line metal mesh structures do not exhibit visible Moiré patterns that would be undesirable, e.g., in display structures. 
     Metal mesh electrodes produced in accordance with aspects of the disclosure can exhibit sheet resistance R of less than 4Ω/□ and optical transmission T greater than 96% with low haze factor. 
     Aspects of the present disclosure allows for formation of metal mesh on large rigid and flexible substrates, up to 2.2 meters in width and of unlimited length. 
     Aspects of the present disclosure also allow for low cost production technology (e.g., down to $5 per square meter) production technology 
     Unlike silver nanowire (AgNW) technology, aspects of the present disclosure do not compromise haze and transparency with sheet resistance &lt;30Ω/□. Also, unlike AgNW technology, aspects of the present disclosure can produce transparent metal grid and traces in the same process step/layer 
     Aspects of the present disclosure are believed to provide the only known technology capable of producing sub-micron metal mesh on glass or film on industrial scale. 
     Those of ordinary skill in the art will readily appreciate that various aspects of the present disclosure may be combined with various other aspects without departing from the scope of the present disclosure. 
     More generally it is important to note that while the above is a complete description of the preferred embodiments of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature described herein, whether preferred or not, may be combined with any other feature described herein, whether preferred or not. In the claims that follow, the indefinite article “a”, or “an” when used in claims containing an open-ended transitional phrase, such as “comprising,” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. Furthermore, the later use of the word “said” or “the” to refer back to the same claim term does not change this meaning, but simply re-invokes that non-singular meaning. The appended claims are not to be interpreted as including means-plus-function limitations or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for” or “step for.”