Abstract:
Nanoscale graphene structure fabrication techniques are provided. An oxide nanowire useful as a mask is formed on a graphene layer and then ion beam etching is performed. A nanoscale graphene structure is fabricated by removing a remaining oxide nanowire after the ion beam etching.

Description:
TECHNICAL FIELD 
     The described technology relates generally to fabricating a graphene nano-device. 
     BACKGROUND 
     Graphene shows stable characteristics and high electric mobility, and has accumulated considerable interest as a material for use in next generation semiconductor devices. However, in order to show semiconductor characteristics, the graphene is typically required to be formed as a channel having a nanoscale line width, since the graphene basically has a metallic characteristic. 
     For example, it is currently understood that the graphene is required to have a line width of 1-2 nm in order to have a silicon band gap, i.e., about 1.11 eV. However, it is not possible to cut the graphene to such a narrow nanoscale line width (less than 3 nm) by presently available semiconductor processing techniques. Accordingly, graphene semiconductor devices are not yet practically realized although there has been considerable interest in using graphene. 
     SUMMARY 
     Techniques for fabricating a nanoscale graphene structure are provided. In one embodiment, a method for fabricating a nanoscale graphene structure includes forming an oxide nanostructure on a grapheme layer; aligning the oxide nanostructure in a predetermined direction on the grapheme layer, performing anisotropic etching by using the aligned oxide nanostructure as a mask, and removing a remaining oxide nanostructure after the anisotropic etching. 
     In another embodiment, a method for fabricating a nanoscale graphene structure includes forming a metal layer on a grapheme layer, forming a molecule layer pattern having a hydrophobic molecule layer in a first region on the metal layer, aligning an oxide nanostructure in a second region on the metal layer where the hydrophobic molecule layer is not formed, performing anisotropic etching using the aligned oxide nanostructure as a mask, and removing a remaining oxide nanostructure and a remaining metal layer nanostructure after the anisotropic etching. 
     In yet another embodiment, a method for fabricating a nanoscale graphene structure includes forming a sacrificial layer on a grapheme layer, forming a metal layer on the sacrificial layer, forming a molecule layer pattern having a hydrophobic molecule layer in a first region on the metal layer, aligning an oxide nanostructure in a second region on the metal layer where the hydrophobic molecule layer is not formed, performing anisotropic etching using the aligned oxide nanostructure as a mask, and removing a remaining oxide nanostructure, a remaining metal layer nanostructure, and a sacrificial layer nanostructure after the anisotropic etching. 
     The Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. The Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1F  illustrate a process of a method for fabricating a graphene structure according to a first example embodiment. 
         FIG. 2  is a flowchart that shows a method for fabricating a graphene structure according to the first example embodiment. 
         FIGS. 3A-3G  illustrate a process of a method for fabricating a graphene structure according to a second example embodiment. 
         FIG. 4  is a flowchart that shows a method for fabricating a graphene structure according to the second example embodiment. 
         FIGS. 5A-5F  illustrate a process of a method for fabricating a graphene structure according to a third example embodiment. 
         FIG. 6  is a flowchart that shows a method for fabricating a graphene structure according to the third example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain example embodiments will be shown and described, with reference to the Figures, simply by way of illustration. As those skilled in the art will appreciate, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the disclosure. 
     In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
     It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     In the following example embodiments, techniques for fabricating a graphene structure of a nanoscale line width using an oxide nanostructure as a mask are disclosed. In the following description, a nanowire is taken as an example of the nanostructure used as a mask. However, it should be understood that a nanostructure of various other shapes such as a circle, an ellipse, and the like may also be used. 
     An oxide nanowire having a covalent bond shows stronger bonding than a metal having a metallic bond, and shows a far lower etch-rate with respect to ion beam milling than a metal. Therefore, an oxide nanowire may be used as a mask in order to remove peripheral materials when an etching period is appropriately controlled. 
     Hereinafter, a method for fabricating a graphene structure according to a first example embodiment is described in detail with reference to  FIG. 1  and  FIG. 2 . 
     As shown in  FIG. 1(A) , a graphene layer  120  is formed on a substrate  110  (S 110  in  FIG. 2 ). In the present example embodiment, a silicon substrate is used as the substrate  110 , but the substrate  110  is not limited thereto. A solid substrate of any other material appropriate for fabricating a device may be used as the substrate  110 . 
     The graphene layer  120  may be formed on the substrate  110  through various methods, and one example technique is hereinafter described in detail. 
     Expandable graphite is processed in a gas chamber at 1000° C. and a 3% hydrogen (H 2 ) atmosphere, and is dispersed in dichloroethane by ultrasonic waves for about 30 minutes. Thereby a thin graphene is dispersed in a solution. When the substrate  110  is applied with the solution with the dispersed graphene and then rinsed, the graphene layer  120  is formed on the substrate  110 . 
     Subsequently, as shown in  FIG. 1(B) , oxide nanowires  130  are formed on the graphene layer  120  (S 120  in  FIG. 2 ). In the present example embodiment, vanadium oxide (e.g., V 2 O 5 ) nanowire is used as the oxide nanowires  130 . The oxide nanowire  130  may be formed on the graphene layer  120  in various ways, and as one example, the vanadium oxide nanowire is formed on the graphene layer  120  as follows. 
     Induced electric dipoles may be easily formed at graphene on its surface, and such graphene formed with induced electric dipoles shows affinity to vanadium oxide nanowire having a negative charge. When a substrate applied with graphene (hereinafter called a “graphene substrate”) is dipped in a vanadium oxide nanowire solution, nanowires adhere to a surface of a graphene layer. In this case, the affinity between the vanadium oxide nanowire and the substrate may be increased by applying a positive voltage to the graphene substrate. At this point, the oxide nanowires  130  are formed without directivity and are aligned in arbitrary directions. 
     As shown in  FIG. 1(C) , the graphene substrate applied with oxide nanowires  130  aligned in arbitrary directions is dipped in ultrapure water and then pulled out of the ultrapure water along a desired alignment direction (S 130  in  FIG. 2 ). The oxide nanowires  130  are thereby realigned on the graphene layer  120  along the pulling direction by surface tension as shown in  FIG. 1(D) . 
     Subsequently, as shown in  FIG. 1(E) , the graphene substrate with the realigned oxide nanowires  130  is placed in a focused ion beam (FIB) apparatus, and then an ion beam milling process is performed (S 140  in  FIG. 2 ). That is, an ion beam milling etching process, which is a type of anisotropic etching process, is performed using the nanowires  130  aligned on the graphene layer  120  as a mask. 
     An oxide nanowire having a covalent bond shows stronger bonding than graphene having a metallic bond, and shows a far lower etch-rate with respect to ion beam milling than graphene. Therefore, an oxide nanowire may be used as a mask in order to remove graphene at the periphery of the mask when an etching period is appropriately controlled. 
     That is, as shown in  FIG. 1(E) , when the ion beam etching is performed on the graphene pattern on which the oxide nanowires  130  are aligned, the graphene layer  120  under the nanowires  130  remains but the graphene layer  120  of other regions are removed since the nanowires  130  act as a mask. 
     After the ion beam etching, the substrate  110  is rinsed using a buffer solution (e.g., an aqueous solution of 1M NaCl) for about 10 minutes (S 150  in  FIG. 2 ). The result is that the oxide nanowires  130  are fully removed and only a graphene structure of the nanowire scheme remains as shown in  FIG. 1(F) . 
     In the first example embodiment, a vanadium oxide nanowire is taken as an example of the oxide nanowire  130  used as a mask since the vanadium oxide nanowire may be easily formed in a very narrow nanoscale size. 
     Other than the vanadium oxide, any material that has strong resistivity with respect to an ion beam may also be used. As an example, oxide materials such as vanadium pentoxide (V 2 O 5 ) (other vanadium oxides VxOy may also be used), zinc oxide (ZnO 5 ), and silicon dioxide (SiO 2 ) typically show high resistivity with respect to an ion beam. This is partly because the bonding strength is high. Additionally, since the oxides are typically insulators, charges generated when exposed to the ion beam do not flow but are accumulated, and the accumulated charges may redirect the ion beam. 
     Hereinafter, a method for fabricating a nanoscale graphene structure according to a second example embodiment is described with reference to  FIG. 3  and  FIG. 4 . In this second example embodiment, an oxide nanowire is placed at a specific position and direction on a molecule layer pattern, instead of aligning the oxide nanowires using surface tension as was described above with reference to  FIG. 1(C) . 
     Nanowires having an oxide surface are not assembled with a hydrophobic molecule layer but are assembled with a hydrophilic molecule layer or a solid surface that is charged with opposite polarity with respect to the oxides. 
     A method for fabricating a nanoscale graphene structure described hereinafter employs a technique for forming an oxide nanowire at a specific position and direction on a hydrophilic molecule layer utilizing the selective assembling characteristic, which is hereinafter referred to as a selective assembly method. 
     As shown in  FIG. 3(A) , a graphene layer  260  is formed on a substrate  250  (S 210  in  FIG. 4 ). The graphene layer  260  may be formed on the substrate  250  by various methods, as has been mentioned in the description of the first exemplary embodiment. 
     Subsequently, as shown in  FIG. 3(B) , a metal layer  270  is deposited on a surface of the graphene layer  260  by using a thermal evaporator or a sputter (S 220  in  FIG. 4 ). In the present example embodiment, gold is used for the metal layer  270 , but other metals may also be used. 
     Subsequently, as shown in  FIG. 3(C) , a molecule layer pattern  280  including a hydrophobic molecule layer pattern  284  and a hydrophilic molecule layer pattern  282  that are charged with positive charges is formed on the metal layer  270  (S 230  in  FIG. 4 ). 
     In subsequent processes, the hydrophobic molecule layer  284  prevents absorption of oxide nanowires, and the hydrophilic molecule layer  282  helps the absorption of the oxide nanowires by increasing affinity thereto. Although the oxide nanowire may be formed without forming the hydrophilic molecule layer  282 , according to the second example embodiment, by applying a positive voltage to the metal layer  270  after forming the hydrophilic molecule layer  282 , the absorption of the oxide nanowires on the hydrophilic molecule layer  282  is facilitated. 
     The molecule layer pattern  280  may be formed using various techniques such as, by way of example, microcontact printing, photolithography, and dip-pen nanolithography (DPN). Since vanadium oxide nanowires having negative charges are used as oxide nanowires in the second example embodiment, a material such as octadecanethiol (ODT) is patterned as a hydrophobic molecule layer  284  on the metal layer  270 , and a material such as cysteamin is patterned as the hydrophilic molecule layer  282 . 
     Subsequently, as shown in  FIG. 3(D) , the substrate  250  patterned with the molecule layer is dipped in a vanadium oxide nanowire solution, and vanadium oxide nanowires  290  are selectively assembled with the hydrophilic molecule layer  282  that is positively charged (S 240  in  FIG. 4 ). In this case, the vanadium oxide nanowires  290  may be aligned at a resolution of nanometer scale. 
     Subsequently, as shown in  FIG. 3(E) , an ion beam milling is applied on the substrate  250  on which the vanadium oxide nanowires  290  are aligned (S 250  in  FIG. 4 ). As a result of the ion beam milling, the molecule layer pattern  280 , the metal layer  270 , and the graphene layer  260  are removed, but the vanadium oxides  290  remain. That is, the vanadium oxide nanowires  290  act as a mask, and accordingly, the metal layer  270  and graphene layer  260  under the vanadium oxide nanowires  290  remains after the ion beam exposure. 
     Hereinafter, the metal layer structure under the vanadium oxide nanowires  290  that remains after the ion beam etching is referred to as “metal nanowires.” 
     Subsequently, as shown in  FIG. 3(F) , the substrate  250  is rinsed using a buffer solution (e.g., an aqueous solution of 1M NaCl) for about 10 minutes (S 260  in  FIG. 4 ) after the ion beam exposure, so that the vanadium oxide nanowires  290  are fully removed and only metal nanowires and the graphene nanostructure remain. 
     Finally, as shown in  FIG. 3(G) , the metal nanowires are removed from the substrate  250  by a metal etching solution (S 270  in  FIG. 4 ), and only the graphene nanostructure remains on the substrate  250 . In this second example embodiment, a mixed solution of nitric acid and hydrochloric acid may be used as the metal etching solution. The molecule layer above the metal nanowire is also removed when the metal nanowires are removed. 
     In the second example embodiment, the vanadium oxide nanowires  290  used as a mask of the ion beam etching are aligned on the metal layer  270  as shown in  FIG. 3(D) . The metal layer  270  is formed prior to the vanadium oxide nanowires  290  since it may be difficult to align the vanadium oxide nanowires  290  directly on the graphene layer  260  by using a selective assembly process. 
     Hereinafter, a method for fabricating a nanoscale graphene structure according to a third example embodiment is described with reference to  FIG. 5  and  FIG. 6 . Similar to as described above in the second example embodiment, oxide nanowires are formed by using a selective assembly process in the third example embodiment. However, different from the second example embodiment, a process of depositing a sacrificial layer is added prior to depositing a metal layer on a graphene layer. In the description below, processes that are substantially the same as described in the second example embodiment are described in an abbreviated manner for convenience of description and better understanding. 
     As shown in  FIG. 5(A) , a graphene layer  360  is formed on a substrate  350  (S 310  in  FIG. 6 ). Subsequently, as shown in  FIG. 5(B) , a sacrificial layer  300  is deposited on a surface of the graphene layer  360  by using a thermal evaporator or a sputter (S 320  in  FIG. 6 ). In this third example embodiment, aluminum may be used for the sacrificial layer  300 . Aluminum shows a relatively high ionization tendency, and thus it is easily removed since it is easily oxidated and etched. A material other than aluminum may also be used for the sacrificial layer  300  as long as the material is oxidated and etched easier than a metal layer that is subsequently formed. 
     Subsequently, as shown in  FIG. 5(C) , a metal layer  370  is deposited on the sacrificial layer  300  (S 330  in  FIG. 6 ), and then a molecule layer pattern  380  including a hydrophobic molecule layer pattern  384  and a hydrophilic molecule layer pattern  382  charged with positive charges is formed on the metal layer  370  (S 340  in  FIG. 6 ). In this third example embodiment, gold (Au) may be used for the metal layer  370 . 
     Subsequently, as shown in  FIG. 5(C) , vanadium oxide nanowires  390  are aligned on the substrate  350  patterned with the molecule layer, by a selective assembly process (S 350  in  FIG. 6 ). 
     Subsequently, as shown in  FIG. 5(D) , the molecule layer pattern  380 , the metal layer  370 , the sacrificial layer  300 , and the graphene layer  360  are removed in a region that is not covered with the vanadium oxide nanowires  390 , by applying the ion beam to the substrate  350  on which the vanadium nanowires  390  are aligned (S 360  in  FIG. 6 ). 
     Subsequently, the vanadium oxide nanowires  390  are removed from the substrate  350  by using a buffer solution (e.g., an aqueous solution of 1M NaCl) (S 370  in  FIG. 6 ) after the ion beam exposure, and then the sacrificial layer  300  is fully removed as shown in  FIG. 5(F)  (S 380  in  FIG. 6 ). 
     In the third example embodiment, instead of removing the metal layer  370  by a metal etching solution, the sacrificial layer  300  of aluminum is etched by dipping the substrate  350  in a tetramethylammonium hydroxide (TMAH) solution so that the metal layer  370  and the molecule layer pattern  380  formed on the sacrificial layer  300  may be removed by separation. In contrast to the second example embodiment, the sacrificial layer  300  is removed using a TMAH solution that is alkaline in this third exemplary embodiment, instead of removing the metal layer  370  using a metal etching solution that is strongly acidic. Therefore, a graphene nanostructure may be formed without causing damage to the graphene. 
     The present disclosure may be embodied in other specific forms without departing from its basic features or characteristics. Thus, the described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within their scope.