Abstract:
A method for fabricating patterned graphene structures, which adopts a photolithographic etching process to fabricate patterned graphene structures, comprises steps: providing a substrate; forming a catalytic layer on the substrate; forming a carbon layer on the catalytic layer; heating the carbon layer to a synthesis temperature to form a graphene layer. A photolithographic etching process is performed on the catalytic layer before formation of the carbon layer. Alternatively, a photolithographic etching process is performed on the carbon layer before heating. Alternatively, a photolithographic etching process is performed on the graphene layer after heating. Compared with the laser etching process, the photolithographic etching process is suitable to fabricate large-area patterned graphene structures and has advantages of high productivity and low cost.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a graphene fabrication method, particularly to a patterned graphene fabrication method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Graphene is an allotrope of carbon, which is a material formed of 2-dimensional 6-carbon hexagonal cells. Graphene features transparency, high electric conductivity, high thermal conductivity, high strength-to-weight ratio, and fine ductility. Therefore, the academia and industry have invested a lot of resources in introducing graphene into the existing electronic element processes and anticipate that graphene can promote the overall performance thereof. At present, graphene is mainly applied to transistors, electrodes of lithium batteries, photosensors, and transparent electrodes of touchscreens, LED, solar cells, etc. 
         [0003]    A U.S. Pat. Pub. No. 2010/0237296 disclosed a graphene fabrication method, which reduces a single-layer graphite oxide into graphite in a high boiling point solvent. Firstly, disperse a single-layer graphite oxide in water to form a dispersion liquid. Next, add a solvent to the dispersion liquid to form a solution. The solvent is selected from a group consisting of N-methlypyrrolidone, ethylene glycol, glycerin, dimethlypyrrolidone, acetone, tetrahydrofuran, acetonitrile, dimethylformamide, amine, and alcohol. Next, heat the solution to a temperature of about 200° C. Then, obtain single-layer graphite with a purification process. A U.S. Pat. Pub. No. 2010/0323113 disclosed a graphene synthesis method, which maintains a hydrocarbon compound at a temperature of 40-1,000° C. to implant carbon atoms into a substrate made of a metal or an alloy. With decrease of temperature, carbon deposits and diffuses out of the substrate to form graphene layers. 
         [0004]    A U.S. Pat. Pub. No. 2011/0102068 disclosed a graphene-based device and a method for using the same. The graphene-based device comprises a laminate structure, a first electrode, a second electrode, and a dopant island. The laminate structure includes a conductive layer, an insulating layer and a graphene layer. The conductive layer is electrically coupled to the graphene layer via the insulating layer. The first and second electrodes are respectively electrically coupled to the graphene layer. The dopant island is electrically coupled to an exposed surface of the graphene layer, and the exposed surface is disposed between the first and second electrodes. The graphene layer is fabricated with an ex-foliation process or a chemical vapor deposition process. 
         [0005]    For some applications, such as touchscreens or LED, the transparent electrodes need specified patterns or structures. Conventionally, the patterns or structures are fabricated with laser etching after the graphene layer has been done. However, laser etching is time-consuming, especially for high-definition patterns. Further, the laser etching apparatuses are expensive. Therefore, patterning graphene layers with laser etching has disadvantages of low efficiency and high cost. 
       SUMMARY OF THE INVENTION 
       [0006]    The primary objective of the present invention is to overcome the conventional problem that laser etching of graphene layers has disadvantages of low efficiency and high cost. 
         [0007]    To achieve the abovementioned objective, the present invention proposes a patterned graphene fabrication method, which comprises steps: providing a substrate, forming a catalytic layer on the substrate, coating a carbon layer on the catalytic layer, photolithographically etching the carbon layer to form a patterned carbon layer, and heating the patterned carbon layer to a synthesis temperature to obtain a patterned graphene layer. 
         [0008]    To achieve the abovementioned objective, the present invention proposes a patterned graphene fabrication method, which comprises steps: providing a substrate, forming a catalytic layer on the substrate, photolithographically etching the catalytic layer to form a patterned catalytic layer, forming on the patterned catalytic layer a carbon layer including a patterned area covering the patterned catalytic layer and a non-patterned area covering the substrate, heating the carbon layer to a synthesis temperature to make the patterned area of the carbon layer form a patterned graphene layer. 
         [0009]    To achieve the abovementioned objective, the present invention proposes a patterned graphene fabrication method, which comprises steps: providing a substrate, forming a catalytic layer on the substrate, forming a carbon layer on the catalytic layer, heating the carbon layer to a synthesis temperature to form a graphene layer, photolithographically etching the graphene layer to form a patterned graphene layer. 
         [0010]    Compared with the conventional technologies, the patterned graphene fabrication method of the present invention has the following advantages:
   1. The present invention patterns the carbon layer of graphene layer with a photolithographic etching process, which is much more efficient than the laser etching process. Therefore, the method of the present invention has high productivity and is suitable to fabricate large-size patterned graphene layers.   2. The apparatuses of photolithographic etching are easy to acquire with a lower cost than that of the laser etching apparatuses. Therefore, the method of the present invention can fabricate patterned graphene layers with a lower cost.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1A-1F  are sectional views schematically showing the process of a patterned graphene fabrication method according a first embodiment of the present invention; 
           [0014]      FIG. 2  is a top view of the patterned graphene layer according to the first embodiment of the present invention; 
           [0015]      FIGS. 3A-3G  are sectional views schematically showing the process of a patterned graphene fabrication method according a second embodiment of the present invention; 
           [0016]      FIG. 4  is a top view of the patterned graphene layer according to the second embodiment of the present invention; 
           [0017]      FIGS. 5A-5G  are sectional views schematically showing the process of a patterned graphene fabrication method according a third embodiment of the present invention; and 
           [0018]      FIGS. 6A-5F  are sectional views schematically showing the process of a patterned graphene fabrication method according a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    The present invention pertains to a patterned graphene fabrication method. Refer to  FIGS. 1A-1F  sectional views schematically showing a patterned graphene fabrication method according a first embodiment of the present invention. Firstly, provide a substrate  10   a . In this embodiment, the substrate  10   a  is made of a material immiscible with carbon. The substrate  10   a  may be made of a metallic material or a ceramic material, such as copper, aluminum, silicon dioxide, aluminum oxide, or silicon carbide. The present invention does not constrain that the substrate  10   a  must be made of the abovementioned materials. In the present invention, the substrate  10   a  can be made of any material, which does not form a solid solution with carbon, i.e. does not form a homogeneous phase with carbon. Next, as shown in  FIG. 1B , form a catalytic layer  20   a  on the substrate  10   a  with an evaporation deposition process or a PVD (Physical Vapor Deposition) process. The catalytic layer  20   a  is made of iron, cobalt, nickel, manganese, or an alloy of the abovementioned metals. Next, as shown in  FIG. 1C , form a carbon layer  30   a  on the catalytic layer  20   a  with a deposition process. The deposition process may be a spin-coating process, a sputtering process, or an evaporation deposition process. The carbon layer  30   a  is made of graphite or a carbon-containing polymer. The carbon-containing polymer is selected from a group of consisting of acrylic resins, phenol formaldehyde resins, epoxy resins, and polymers containing long chains or hexagonal benzene rings. 
         [0020]    After the carbon layer  30   a  has been formed on the catalytic layer  20   a , photolithographically etch the carbon layer  30   a . As shown in  FIG. 1D , form a photoresist layer  40   a  on the carbon layer  30   a  firstly. Next, sequentially perform an exposure process and a development process on the photoresist layer  40   a . As shown in  FIG. 1E , place a photomask  50   a  over the photoresist layer  40   a . In this embodiment, the photoresist layer  40   a  is made of a negative photoresist material; the photomask  50   a  is a perforated structure containing a light permeable area  52   a  and a light impermeable area  51   a . The light impermeable area  51   a  defines at least one sacrifice area  41   a  in the photoresist layer  40   a . The sacrifice area  41   a  is below the light impermeable area  51   a  and bordered by dashed lines in  FIG. 1E . Light is projected on the photoresist layer  40   a  to enable the chemical reaction and cross link of the portion of photoresist layer  40   a , which is below the light permeable area  52   a . A development agent is used to dissolve and remove the portion of the photoresist layer  40   a , which is below the light impermeable area  51   a  and not illuminated by light, i.e. remove the sacrifice areas  41   a . Thus, a portion of the carbon layer  30   a  is revealed. The selections of the negative photoresist material, the development agent, and the wavelength and intensity of the light are mature conventional technologies and will not repeat herein. 
         [0021]    Next, perform an etching process on the carbon layer  30   a  to remove a portion of the carbon layer  30   a  corresponding to the sacrifice areas  41   a . The etching process may be a chemical etching process or a reactive ion etching (RIE) process. Next, remove the photomask  50   a , and use an appropriate solvent to dissolve the negative photoresist material. Thus is obtained a patterned carbon layer  31   a , as shown in  FIG. 1F . Then, heat the patterned carbon layer  31   a  to a synthesis temperature for a given interval of time to obtain a patterned graphene layer  70   a . The synthesis temperature is preferably between 700 and 1,200° C. The patterned carbon layer  31   a  may be heated in vacuum or in an atmosphere of ammonia gas, argon, nitrogen, or a mixture of argon and hydrogen, a mixture of nitrogen and hydrogen. For the above mixtures, the volume concentration of hydrogen is preferably between 0 and 50%. In this embodiment, the given interval of time is preferably between 1 and 300 minutes. Refer to  FIG. 2  a top view of the patterned graphene layer according to the first embodiment of the present invention. Preferably, each graphene structure of the patterned graphene layer  70   a  has a width of less than 7 μm. In this embodiment, the etching process simultaneously etches the carbon layer  30   a  and the catalytic layer  20   a . However, the etching process may only etch the carbon layer  30   a  in practical fabrication processes. 
         [0022]    Refer to  FIGS. 3A-3G  sectional views schematically showing a patterned graphene fabrication method according a second embodiment of the present invention. Firstly, provide a substrate  10   b . In this embodiment, the substrate  10   b  is made of a material miscible with carbon, such as iron, cobalt or nickel. Next, as shown in  FIG. 3B , form an isolation layer  60  on the substrate  10   b . The isolation layer  60  must be made of a material immiscible with carbon. In the present invention, the isolation layer  60  is preferably made of silicon dioxide, aluminum oxide or silicon carbide. Next, as shown in  FIG. 3C , form a catalytic layer  20   b  on the substrate  10   b . Similar to the first embodiment, the catalytic layer  20   b  is formed on the substrate  10   b  with an evaporation disposition process or a PVD process; the catalytic layer  20   b  is made of iron, cobalt, nickel, manganese, or an alloy of the abovementioned metals. Next, as shown in  FIG. 3D , deposit a carbon layer  30   b  on the catalytic layer  20   b  with a deposition process. The deposition process may be realized with a spin-coating process, a sputtering process, or an evaporation disposition process. The carbon layer  30   b  is made of graphite or a carbon-containing polymer. The carbon-containing polymer is selected from a group consisting of acrylic resins, phenol formaldehyde resins, epoxy resins, and polymers containing long chains or hexagonal benzene rings. 
         [0023]    After the carbon layer  30   b  has been formed on the catalytic layer  20   b , photolithographically etch the carbon layer  30   b . As shown in  FIG. 3E , form a photoresist layer  40   b  on the carbon layer  30   b  firstly. Next, sequentially perform an exposure process and a development process on the photoresist layer  40   b . As shown in  FIG. 3F , place a photomask  50   b  over the photoresist layer  40   b . In this embodiment, the photoresist layer  40   b  is made of a negative photoresist material; the photomask  50   b  is a perforated structure containing a light permeable area  52   b  and a light impermeable area  51   b . The light impermeable area  51   b  defines at least one sacrifice area  41   b  in the photoresist layer  40   b . The sacrifice area  41   b  is below the light impermeable area  51   b  and bordered by dashed lines in  FIG. 3F . Light is projected on the photoresist layer  40   b  to enable the chemical reaction and cross link of the portion of photoresist layer  40   b , which is below the light permeable area  52   b . A development agent is used to dissolve and remove the portion of the photoresist layer  40   b , which is below the light impermeable area  51   b  and not illuminated by light, i.e. remove the sacrifice areas  41   b . Thus, a portion of the carbon layer  30   a  is revealed. Next, perform an etching process on the carbon layer  30   b  to remove a portion of the carbon layer  30   b  corresponding to the sacrifice areas  41   b . The etching process may be a chemical etching process or a reactive ion etching (RIE) process. Next, remove the photomask  50   b  to obtain a patterned carbon layer  31   b , as shown in  FIG. 3G . 
         [0024]    Then, heat the patterned carbon layer  31   b  to a synthesis temperature for a given interval of time to obtain a patterned graphene layer  70   b . The synthesis temperature is preferably between 700 and 1,200° C. The patterned carbon layer  31   b  may be heated in vacuum or in an atmosphere of ammonia gas, argon, nitrogen, or a mixture of argon and hydrogen, a mixture of nitrogen and hydrogen. For the above mixtures, the volume concentration of hydrogen is preferably between 0 and 50%. In this embodiment, the given interval of time is preferably between 1 and 300 minutes. Refer to  FIG. 4  a top view of the patterned graphene layer according to the second embodiment of the present invention. Preferably, each graphene structure of the patterned graphene layer  70   b  has a width of less than 7 μm. In this embodiment, the etching process simultaneously etches the carbon layer  30   b , the catalytic layer  20   b  and the isolation layer  60 . However, the etching process may only etch the carbon layer  30   b  or the carbon layer  30   b  plus the catalytic layer  20   b  in practical fabrication processes. 
         [0025]    Refer to  FIGS. 5A-5G  sectional views schematically showing a patterned graphene fabrication method according a third embodiment of the present invention. Firstly, provide a substrate  10   c . Next, as shown in  FIG. 5B , form a catalytic layer  20   c  on the substrate  10   c . Next, photo lithographically etch the catalytic layer  20   c . As shown in  FIG. 5C , form a photoresist layer  40   c  on the catalytic layer  20   c  firstly. Next, sequentially perform an exposure process and a development process on the photoresist layer  40   c . As shown in  FIG. 5D , place a photomask  50   c  over the photoresist layer  40   c . In this embodiment, the photoresist layer  40   c  is made of a negative photoresist material; the photomask  50   c  is a perforated structure containing a light permeable area  52   c  and a light impermeable area  51   c . The light impermeable area  51   c  defines at least one sacrifice area  41   c  in the photoresist layer  40   c . The sacrifice area  41   c  is below the light impermeable area  51   c  and bordered by dashed lines in  FIG. 5D . 
         [0026]    Light is projected on the photoresist layer  40   c  to enable the chemical reaction and cross link of the portion of photoresist layer  40   c , which is below the light permeable area  52   c . A development agent is used to dissolve and remove the portion of the photoresist layer  40   c , which is below the light impermeable area  51   c  and not illuminated by light, i.e. remove the sacrifice areas  41   c . Thus, a portion of the catalytic layer  20   c  is revealed. Next, perform an etching process on the catalytic layer  20   c  to remove a portion of the catalytic layer  20   c  corresponding to the sacrifice areas  41   c . The etching process may be a chemical etching process, a reactive ion etching (RIE) process, or another equivalent etching process having the same effect. Next, remove the photomask  50   c  to obtain a patterned catalytic layer  21 , as shown in  FIG. 5E . 
         [0027]    Refer to  FIG. 5F . After the photolithographic etching process is completed, form a carbon layer  30   c  on the catalytic layer  20   c . The carbon layer  30   c  includes a patterned area  32  covering the patterned catalytic layer  21  and a non-patterned area  33  covering the substrate  10   c . In this embodiment, the carbon layer  30   c  is made of graphite or a carbon-containing polymer. Then, heat the carbon layer  30   c  to a synthesis temperature for a given interval of time, whereby the patterned area  32  of the carbon layer  30   c  becomes a patterned graphene layer  70   c , as shown in  FIG. 5G . The synthesis temperature is preferably between 700 and 1,200° C. The carbon layer  30   c  may be heated in vacuum or in an atmosphere of ammonia gas, argon, nitrogen, or a mixture of argon and hydrogen, a mixture of nitrogen and hydrogen. For the above mixtures, the volume concentration of hydrogen is preferably between 0 and 50%. In this embodiment, the given interval of time is preferably between 1 and 300 minutes. According to requirement of practical fabrication, the non-patterned area  33  of the carbon layer  30   c  may be removed before or after heating. In this embodiment, the non-patterned area  33  is removed before the patterned area  32  becomes the patterned graphene layer  70   c.    
         [0028]    Refer to  FIGS. 6A-6G  sectional views schematically showing a patterned graphene fabrication method according a fourth embodiment of the present invention. Firstly, provide a substrate  10   d . Next, as shown in  FIG. 6B , form a catalytic layer  20   d  on the substrate  10   d . Next, as shown in  FIG. 6C , form a carbon layer  30   d  on the catalytic layer  20   d . The carbon layer  30   d  is made of graphite or a carbon-containing polymer. The carbon-containing polymer is selected from a group of consisting of acrylic resins, phenol formaldehyde resins, epoxy resins, and polymers containing long chains or hexagonal benzene rings. After the carbon layer  30   d  has been formed on the catalytic layer  20   d , heat the carbon layer  30   d  to a synthesis temperature for a given interval of time to obtain a graphene layer  71 . The synthesis temperature is preferably between 700 and 1,200° C. The carbon layer  30   d  may be heated in vacuum or in an atmosphere of ammonia gas, argon, nitrogen, or a mixture of argon and hydrogen, a mixture of nitrogen and hydrogen. For the above mixtures, the volume concentration of hydrogen is preferably between 0 and 50%. 
         [0029]    Next, photolithographically etch the graphene layer  71 . As shown in  FIG. 6D , form a photoresist layer  40   d  on the graphene layer  71  firstly. Next, sequentially perform an exposure process and a development process on the photoresist layer  40   d . As shown in  FIG. 6E , place a photomask  50   d  over the photoresist layer  40   d . In this embodiment, the photoresist layer  40   d  is made of a negative photoresist material; the photomask  50   d  is a perforated structure containing a light permeable area  52   d  and a light impermeable area  51   d . The light impermeable area  51   d  defines at least one sacrifice area  41   d  in the photoresist layer  40   d . The sacrifice area  41   d  is below the light impermeable area  51   d  and bordered by dashed lines in  FIG. 6E . Light is projected on the photoresist layer  40   d  to enable the chemical reaction and cross link of the portion of photoresist layer  40   d , which is below the light permeable area  52   d . A development agent is used to dissolve and remove the portion of the photoresist layer  40   d , which is below the light impermeable area  51   d  and not illuminated by light, i.e. remove the sacrifice areas  41   d . Thus, a portion of the graphene layer  71  is revealed. Next, perform an etching process on the graphene layer  71  to remove a portion of the graphene layer  71  corresponding to the sacrifice areas  41   d . The etching process may be a chemical etching process or a reactive ion etching (RIE) process. Next, remove the photomask  50   d , and use an appropriate solvent to dissolve the negative photoresist material. Thus is obtained a patterned graphene layer  72 , as shown in  FIG. 6F . 
         [0030]    In the third and fourth embodiments, the substrates  10   c  and  10   d  are made of a material immiscible with carbon; the substrates  10   c  and  10   d  may be made of a metal or a ceramic material, such as copper, aluminum, silicon dioxide, aluminum oxide, or silicon carbide. In the third and fourth embodiments, the catalytic layers  20   c  and  20   d  are formed with an evaporation disposition process or a PVD process; the catalytic layers  20   c  and  20   d  are made of iron, cobalt, nickel, manganese, or an alloy of the above-mentioned metals. In the third and fourth embodiments, the carbon layers  30   c  and  30   d  are formed on the catalytic layers  20   c  and  20   d  with a deposition process; the deposition process may be a spin-coating process, a sputtering process, or an evaporation deposition process. In the third and fourth embodiments, the substrates  10   c  and  10   d  may be alternatively made of a material miscible with carbon, such as iron, cobalt or nickel, and an isolation layer made of a material immiscible with carbon is formed on the substrates  10   c  and  10   d  before formation of the catalytic layers  20   c  and  20   d.    
         [0031]    In the abovementioned embodiments, the patterned graphene strictures are in form of a plurality of parallel strip-like structures. However, the present invention does not constrain that the patterned graphene structures must be in form of parallel strips. In the present invention, the graphene structure may be in form of an arbitrary geometrical shape, such as a triangle, a rectangle, etc. In the abovementioned embodiments, the photoresist layers  40   a ,  40   b ,  40   c  and  40   d  are made of a negative photoresist material. However, the photoresist layers  40   a ,  40   b ,  40   c  and  40   d  may be alternatively made of a positive photoresist material if it is required in practical fabrication. 
         [0032]    In conclusion, the present invention patterns the carbon layer or the graphene layer with a photolithographic etching technology. If the carbon layer is photolithographically etched into a patterned carbon layer before graphene synthesis, the patterned carbon layer is converted into patterned graphene structures. The photolithographic etching technology is far more efficient than the laser etch technology. Therefore, the present invention has productivity much higher than that of the laser etch-based conventional technology. Further, the present invention is suitable to fabricate large-size patterned graphene structures. Besides, the apparatuses of the photolithographic etching process are easy to acquire with a lower cost in comparison with the apparatuses of the laser etching process. Therefore, the present invention also has advantages of simple fabrication processes and high cost efficiency. Hence, the present invention possesses utility, novelty and non-obviousness and meets the condition for a patent. Thus, the Inventors file the application for a patent. It is appreciated if the patent is approved fast. 
         [0033]    The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention, which is based on the claims stated below.