Abstract:
Embodiments of the disclosed technology include patterning a graphene sheet for biosensor and electronic applications using lithographic patterning techniques. More specifically, the present disclosure is directed towards the method of patterning a graphene sheet with a hard mask metal layer. The hard mask metal layer may include an inert metal, which may protect the graphene sheet from being contaminated or damaged during the patterning process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority to U.S. Provisional Patent Application with Ser. No. 62/350,629 filed on Jun. 15, 2016, the contents of which are incorporated herein by its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosed technology relates generally to depositing a hard mask coating onto a surface of a graphene sheet. More specifically, the present invention relates to depositing a hard mask coating onto a surface of a graphene sheet to provide a protective layer. 
       BACKGROUND 
       [0003]    Graphene is composed of a single thin layer of carbon atoms that are bonded together in a repeating pattern of hexagons. Graphene has many extraordinary properties, which includes high mechanical strength, high electron mobility, and superior thermal conductivity. Because graphene is a great thermal and electrical conductor, graphene material is often used to construct graphene based biosensors, transistors, integrated circuited, and other electronic applications. 
         [0004]    While there has been much academic interest in the application and utilization of graphene, attempts to commercialize graphene based electronic devices and sensors have largely failed. As such, much of the currently known techniques for handling and preparing graphene are limited to techniques that are only suitable for research and academic purposes, and thus fail to take into consideration manufacturing costs, product assembly requirements, and the need for long-term durability. 
         [0005]    More specifically, current methods for preparing and handling graphene often leave the graphene sheet exposed to the environment conditions when constructing the graphene based devices. Because the graphene sheets are left exposed without a protective layer, the graphene is often contaminated and even damaged, especially when exposed to the air for periods of time. Additionally, the exposed graphene surface may further be susceptible to damage during the processing, constructing, and packaging of the graphene based electronic devices and sensors. 
         [0006]    While some limited methods for providing a protective layer on the surface of graphene sheets are currently available, such methods may still damage the graphene. For example, one method may include depositing the surface of the graphene sheet directly with a photoresist or polymethylmethacrylate (hereinafter “PMMC”) layer. However, such direct contact with the graphene sheet often leaves a residue on the graphene that cannot be completely removed, even with the application of acetone or other solvents. As such, any remaining photoresist or PMMC residue on the surface of the graphene significantly lowers the quality of the graphene and is further likely to degrade the overall performance of the graphene based electronic devices and sensors. 
         [0007]    Other methods may also include providing a protective copper layer on the surface of the graphene sheet. However, copper is also known to leave a contaminating residue on the graphene sheet that is difficult to completely remove, which also lowers the quality of the graphene and further likely to degrade the performance of the graphene based electronic devices and sensors. As such, there currently is a need to provide a way for providing a temporary protective layer on a graphene sheet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The technology disclosed herein, in accordance with one or more various embodiments, is described in detail with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict typical or example embodiments of the disclosed technology. These drawings are provided to facilitate the reader&#39;s understanding of the disclosed technology and shall not be considered limiting of the breadth, scope, or applicability thereof. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale. 
           [0009]      FIG. 1  illustrates the different progressions of depositing a temporary protective layer onto a surface of a graphene sheet according to one embodiment. 
           [0010]      FIG. 1A  illustrates a graphene sheet transferred onto a substrate according to one embodiment. 
           [0011]      FIG. 1B  illustrates a graphene sheet with a temporary metal layer according to one embodiment. 
           [0012]      FIG. 1C  illustrates a graphene sheet with a temporary metal layer removed from a surface of the graphene sheet according to one embodiment. 
           [0013]      FIG. 2  illustrates the different progressions of patterning a graphene sheet with a hard mask layer according to one embodiment. 
           [0014]      FIG. 2A  illustrates a graphene sheet transferred onto a substrate according to one embodiment. 
           [0015]      FIG. 2B  illustrates a graphene sheet with a metal layer according to one embodiment. 
           [0016]      FIG. 2C  illustrates a graphene sheet with a metal layer and a photoresist layer for lithographic patterning according to one embodiment. 
           [0017]      FIG. 2D  illustrates a graphene sheet with a metal layer and a photoresist layer exposed to a radiation source to create a pattern template according to one embodiment. 
           [0018]      FIG. 2E  illustrates a graphene sheet with a metal layer and a photoresist layer exposed to a metal etchant solution to pattern the metal layer according to one embodiment. 
           [0019]      FIG. 2F  illustrates a graphene sheet with a metal layer and a photoresist layer exposed to a plasma etchant solution to pattern the graphene sheet according to one embodiment. 
           [0020]      FIG. 2G  illustrates a patterned graphene sheet with a metal layer according to one embodiment. 
           [0021]      FIG. 2H  illustrates a patterned graphene sheet according to one embodiment 
       
    
    
       [0022]    The figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention can be practiced with modification and alteration, and that the disclosed technology be limited only by the claims and the equivalents thereof. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    The following description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the disclosed embodiments. The present embodiments address the problems described in the background while also addressing other additional problems as will be seen from the following detailed description. Numerous specific details are set forth to provide a full understanding of various aspects of the subject disclosure. It will be apparent, however, to one ordinarily skilled in the art that various aspects of the subject disclosure may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid unnecessarily obscuring the subject disclosure. 
         [0024]    Some embodiments disclosed herein include depositing a temporary protective layer onto a surface of a graphene sheet. It should be noted that depositing layers onto the graphene sheet may include a wide range of techniques as appreciated by one of ordinary skill in the art, such as coating techniques, focused ion beam, filament evaporation, sputter deposition, and electrolysis by way of example only. 
         [0025]    This temporary protective layer may protect the graphene from the environment and prevent the contamination of the graphene sheet. By way of example only, the temporary protective layer may include a thin metal layer to protect the graphene from contamination or harm during the packaging and assembly of the graphene based device. The thin metal used to coat the graphene sheet may include gold, by way of example only. Because gold is an inert metal that has the characteristic property of being resistant to corrosion and oxidation, depositing the graphene sheet with a gold layer may protect the graphene. Additionally, due to gold&#39;s characteristically inert qualities, the temporary gold layer on the surface of the graphene sheet may further provide thermal protection and prevent oxidation, especially when the graphene is exposed to high temperature treatments during epoxy curing, oven baking, and burn testing. Furthermore, the temporary gold coating may also protect the graphene from potentially being contaminated during wire bonding, encapsulation, wafer dicing, and cleaning. Thus, this allows the graphene to be handled in a factory setting for large manufacturing production. 
         [0026]    However, other inert metals may also be used to temporarily coat the graphene sheet, which may include, but are not limited to, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and the like. 
         [0027]      FIG. 1A  illustrates a graphene sheet  115  transferred onto a substrate, such as a wafer  105 , according to one embodiment. The graphene sheet  115  may be grown using chemical vapor deposition, especially when producing large sheets of graphene. However, other methods of growing the graphene layer may also include, but are not limited to, reducing oxidized graphite on a selected substrate or via mechanical exfoliation techniques. 
         [0028]    The graphene sheet  115  may then be transferred onto another surface, such as the surface of an electronic chip or sensor. More specifically, the surface of the electronic chip or sensor may include the surface of a thin semi-conductor material, such as a wafer  105 . The wafer  105  may serve as a foundation upon which the proper electronic integrated circuits can be applied. By way of example, the wafer  105  may be a silicon substrate or a silicon dioxide substrate. Additionally, the wafer  105  may be coated with platinum  110 , whereby the platinum  110  acts as the bottom electrode to create the proper electrical connections. 
         [0029]    Next, the graphene sheet  115  may be treated with a temporary thin metal layer, such as a metal layer  120 , as illustrated in  FIG. 1B . The metal layer  120  may act as a mask or protective barrier configured to protect the graphene from being contaminated or degraded during the preparation of the graphene. To properly coat the graphene sheet  115  with the metal layer  120 , the wafer  105  with the graphene sheet  115  may be placed in an electron beam evaporation chamber. Electron beam evaporation is a physical vapor disposition technique whereby an intense electron beam is generated from a filament and steered via electric and magnetic fields to strike source material, such as gold pellets, and to vaporize it within a vacuum environment. As such, by using the electron beam evaporation technique, a thin metal layer  120  may be slowly deposited onto the graphene sheet  115 . 
         [0030]    By way of example only, the metal layer  120  may include a gold metal layer. However, it should be noted that the metal layer  120  may include any inert metal that does not negatively react with graphene. Additionally, the metal layer  120  may be any inert metal that has the characteristic property of being resistant to corrosion and oxidation. By way of example only, such inert metals may include, but are not limited to ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and the like. 
         [0031]    The temporary gold metal layer  120  may range in a thickness from 10 nanometers to 1 micrometer. By way of another example, the gold metal layer  120  may be applied onto the graphene sheet  115  by dipping the graphene sheet  115  into a gold plating solution. Additionally, other methods of depositing a temporary metal layer may be provided. For example, other methods may also include utilizing a focused ion beam, filament evaporation, sputter deposition, electrolysis, and the like to provide a temporary gold metal layer  120  on the surface of a graphene sheet  115 . 
         [0032]    Once the metal layer  120  is deposited on top of the graphene sheet  115 , metal leads or connections may be established. Furthermore, additional processes or constructions may be made on top of the gold metal layer  120  that are now layered on the surface of the graphene sheet  115 . Again, due to inert nature of the metal layer  120 , the metal layer  120  may help protect the graphene from being contaminated or degraded, even as the graphene is being prepped and assembled for use within select graphene based devices. 
         [0033]    Once the preparation of the graphene completed, the metal layer  120  may now be removed, as illustrated in  FIG. 1C . For example, the metal layer  120  may be washed with potassium iodide solution for a duration ranging from 30 seconds to 5 minutes, or for a time period where all trace of the metal layer  120  is removed from the surface of the graphene sheet  115 . The now fully exposed graphene sheet  115  may then be washed with deionized water to remove any remaining metal layer particles or other materials left on the surface of the graphene sheet  115 . The now exposed graphene sheet  115  may further be available for any additional processes required to further manufacture and assemble a proper and functional graphene based device. 
         [0034]    In other embodiments, a metal layer placed on top of a graphene sheet may include additional coatings or layers that are deposited on top of the metal layer, as illustrated in  FIG. 2 . More specifically,  FIG. 2A  illustrates a graphene sheet  215  transferred onto a substrate according to one embodiment. Next, the graphene sheet  215  may be treated with a thin metal layer, such as a gold metal layer  220 , as illustrated in  FIG. 2B . The gold metal layer  220  may act as a mask or barrier configured to protect the graphene from being contaminated or degraded. As discussed above, the thin metal layer may be any inert metal inert metal that has the characteristic property of being resistant to corrosion and oxidation, such that depositing the graphene sheet with a gold layer protects the graphene from being contaminated or degraded. As such, the thin metal layer may include any of the following inert metals or combination thereof: ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and the like. 
         [0035]    The graphene sheet  215  with the gold metal layer  220  may additionally be coated with a photoresist layer  225 , as further illustrated in  FIG. 2C . The photoresist layer  225  may include photosensitive material that experiences a change in its physical properties when exposed to a radiation source. By selectively exposing the photoresist layer  225  with radiation, such exposed areas of the photoresist layer  225  may be etched away, thus exposing portions of the metal gold layer  220  directly underneath the photoresist layer  225 . In other words, the photoresist layer  225  may act as a template for accurately patterning the graphene sheet  215  underneath the photoresist layer  225 . Thus the pattern etched onto the photoresist layer  225  via the radiation source may be used as a template to etch the same pattern onto the graphene sheet  215  directly below the photoresist layer  225 . 
         [0036]    As further illustrated in  FIG. 2D , once the photoresist layer  225  is exposed with a radiation source with a particular pattern etched onto the photoresist layer  225 , the etched away portions of the photoresist layer  225  expose portions of gold metal layer  220  directly below. 
         [0037]    To pattern the graphene sheet  215  underneath the gold metal layer  220 , the gold metal layer  220  must also be etched away in accordance to the pattern etched onto the photoresist layer  225 , as further illustrated in  FIG. 2E . To do so, for example, the wafer  205  may be submerged in an etchant solution that etches away only the exposed gold metal layer  220  not covered with the photoresist layer  225 . For example, the etchant solution may be a potassium iodide solution. However, it should be noted that the etchant solution need not be limited to a potassium iodide solution, but instead, may also include a cyanide based chemical or an aqua regia with a mixture of nitric and hydrochloric acids. Because the etchant solution only reacts with the gold and not the photoresist layer  225 , only the exposed gold metal layer  220  is removed and thus forms a negative space pattern in accordance to the pattern etched onto the photoresist layer  225 . 
         [0038]    With the photoresist layer  225  and the gold metal layer  220  now etched away in accordance to the etched template from the photoresist layer  225 , the graphene sheet  215  is now exposed, as further illustrated in  FIG. 2E . As such, the graphene sheet  215  may now be patterned accordingly to the specific pattern that etched away specific portions of the photoresist layer  225 . For example, the wafer  205  with the graphene sheet  215  may undergo plasma etching, which only removes the exposed graphene sheet  215  in areas where the gold metal layer  125  was removed from the etchant solution, as illustrated in  FIG. 2F . As such, the graphene sheet  215  is now patterned accordingly. 
         [0039]    Upon proper patterning of the graphene sheet  215  via plasma etching, the photoresist layer  225  may now be removed, as further illustrated in  FIG. 2G . For example, the wafer  205  with the photoresist layer  225  may be rinsed with acetone for 2 to 10 minutes followed by isopropanol alcohol for another 1 to 5 minutes, which will effectively and have completely dissolved the photoresist layer  225 . This now completely exposes the remaining gold metal layer  220  on the wafer  205 . However, it should be noted that other solvents may also be used, such as acetone, methyl isopropyl ketone and the like. 
         [0040]    Next, the gold metal layer may now be removed, as illustrated in  FIG. 2H . For example, the gold metal layer  220  may be washed with potassium iodide solution for 30 seconds to 5 minutes such that the thin metal gold layer  220  is no longer coated on the surface of the graphene sheet  215 . The exposed graphene sheet  215  may then be washed with deionized water to remove any remaining gold particles or other materials on the surface of the graphene sheet  215 . The now exposed patterned graphene sheet  215  is available for any further additional processing or preparation. 
         [0041]    While various embodiments of the disclosed technology have been described above, it should be understood that they have been presented by way of example only, and not of limitation. Likewise, the various diagrams may depict an example architectural or other configuration for the disclosed technology, which is done to aid in understanding the features and functionality that can be included in the disclosed technology. The disclosed technology is not restricted to the illustrated example architectures or configurations, but the desired features can be implemented using a variety of alternative architectures and configurations. Indeed, it will be apparent to one of skill in the art how alternative functional, logical or physical partitioning and configurations can be implemented to implement the desired features of the technology disclosed herein. Also, a multitude of different constituent module names other than those depicted herein can be applied to the various partitions. Additionally, with regard to flow diagrams, operational descriptions and method claims, the order in which the steps are presented herein shall not mandate that various embodiments be implemented to perform the recited functionality in the same order unless the context dictates otherwise. 
         [0042]    Although the disclosed technology is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the disclosed technology, whether or not such embodiments are described and whether or not such features are presented as being a part of a described embodiment. Thus, the breadth and scope of the technology disclosed herein should not be limited by any of the above-described exemplary embodiments. 
         [0043]    Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term “including” should be read as meaning “including, without limitation” or the like; the term “example” is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; the terms “a” or “an” should be read as meaning “at least one,” “one or more” or the like; and adjectives such as “conventional,” “traditional,” “normal,” “standard,” “known” and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, where this document refers to technologies that would be apparent or known to one of ordinary skill in the art, such technologies encompass those apparent or known to the skilled artisan now or at any time in the future. 
         [0044]    The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. The use of the term “module” does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations. 
         [0045]    Additionally, the various embodiments set forth herein are described in terms of exemplary block diagrams, flow charts and other illustrations. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives can be implemented without confinement to the illustrated examples. For example, block diagrams and their accompanying description should not be construed as mandating a particular architecture or configuration.