Patent Publication Number: US-2006008584-A1

Title: Method of forming carbon nanotube

Description:
CLAIM OF PRIORITY  
      This application claims all benefits accruing under 35 U.S.C. §119 from Korean Patent Application entitled “METHOD FOR FORMING CARBON NANOTUBE,” assigned serial No. 2003-84726 filed on Nov. 26, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method of forming a carbon nanotube, and more particularly, to a method of forming a carbon nanotube having a fine diameter using plasma.  
      2. Description of the Related Art  
      The main applications of display devices in information transfer media are monitors of personal computers and television screens. The display devices can be divided into cathode ray tubes (CRT), which use high speed thermionic emission, and flat panel displays. The flat panel displays include liquid crystal display (LCD) devices, plasma display panel (PDP) devices, and field emission display (FED) devices.  
      An FED device is a display device in which light is emitted from a fluorescent material of anodes due to the collision of electrons. The electrons are emitted from field emitters of cathodes to which a strong electric field is applied by a gate electrode.  
      A micro-tip composed of a metal such as molybdenum (Mo) is commonly used as the field emitter. Recently, carbon nanotube (CNT) emitters have been widely used. Since an FED device using a CNT emitter has advantages of a wide viewing angle, high resolution, low power consumption, and temperature stability, it is highly applicable to car navigation devices or view finders for electronic image displaying devices. Also, the FED device using the CNT emitter can be used as a monitor for a personal computer, a personal data assistant (PDA), a medical apparatus, or a high definition television. A CNT emitter can also be used as a field emitter for a backlight used in a liquid crystal device.  
      When forming a carbon nanotube, a chemical vapor deposition (CVD) method is generally used. More specifically, a catalyst metal layer can be formed to a predetermined thickness with a catalytic metal by magnetron sputtering or electron beam deposition on a surface of an electrode formed on a substrate. The carbon nanotube is grown vertically on a surface of the catalytic metal layer by injecting a H 2 , N 2 , or Ar gas together with a carbon containing gas such as CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , CO, or CO 2  into a reaction chamber at a temperature of 500˜900° C.  FIGS. 1A and 1B  are scanning electron microscope (SEM) images of the surface of a catalytic metal layer and a carbon nanotube grown on the surface of the catalytic metal layer after heat treatment, respectively. Referring to  FIG. 1A , particles with sizes of a few tens of nm are formed on the catalytic metal layer, and referring to  FIG. 1B , the carbon nanotubes having the diameters corresponding to the sizes of the particles in  FIG. 1A  are formed.  
      The carbon nanotube can also be formed using a plasma enhanced chemical vapor deposition (PECVD). In this case, a SEM image of a surface of a catalytic metal layer before a carbon nanotube is grown is shown in  FIG. 2 . Referring to  FIG. 2 , the particles having almost the same sizes depicted in  FIG. 1A  are formed on the catalytic metal layer. And, the carbon nanotubes having the diameters corresponding to the sizes of each of particles are formed.  
      When forming the carbon nanotube by a conventional CVD method, the carbon nanotube has a relatively large diameter. If the diameter of the carbon nanotube is large, the operating voltage of a device including the carbon nanotube is large.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to solve the above and/or other problems.  
      It is also an object of the present invention to provide an improved display device.  
      It is another object of the present invention to provide a method of forming a carbon nanotube having a fine diameter.  
      It is a further object of the present invention to provide a method of controlling a diameter of a carbon nanotube.  
      In order to achieve the above and other objectives, the first preferred embodiment of the method of forming a carbon nanotube on an electrode comprises forming a polyimide layer on the electrode, etching the polyimide layer and the electrode to form a plurality of protrusions on the electrode, forming a catalyst layer on the surface of the electrode between the protrusions, and forming the carbon nanotube on the catalyst layer.  
      It is preferred that the electrode is formed of molybdenum (Mo), chrome (Cr), and/or tungsten (W), and deposited to a thickness of 1,000˜10,000 Å using an electron beam evaporation method or a sputtering method.  
      The polyimide layer is preferably formed by coating polyimide on the electrode, soft-baking the polyimide, and curing the soft-baked polyimide. Preferably, the polyimide is coated on the electrode using a spin coating method or a method using surface tension. The polyimide is preferably soft-baked at a temperature of 95° C. and cured at a temperature of 350° C., and a thickness of the polyimide layer is preferably about 1 to about 10 μm, and more preferably a few μm.  
      A plurality of protrusions are formed on the surface of the polyimide layer formed on the electrode. The protrusions of the electrode are formed to a shape corresponding to the protrusions of the polyimide layer by etching the polyimide layer and the surface of the electrode. A gap between the protrusions of the electrode can be about 1 to about 50 nm, and more preferably a few nm.  
      It is preferred that the polyimide layer and the surface of the electrode are etched using a reactive ion etching (RIE) method. In the RIE method, plasma generated from a reaction gas that includes SF 6 , O 2 , or CHF 3  can be used.  
      The method may further comprise removing the polyimide remaining on the surface of the electrode before forming the catalyst layer.  
      The catalyst layer is preferably composed of at least one selected from the group consisting of W, Ni, Fe, Co, Y, Pd, Pt, and Au. The catalyst layer may be formed using a sputtering method or an electron beam evaporation method, and a thickness of the catalyst layer is 0.5˜2 nm.  
      The carbon nanotube is preferably formed using a thermal CVD method or a plasma enhanced CVD method. The carbon nanotube is grown on a surface of the catalyst layer using a gas containing carbon, and the carbon containing gas can be at least one selected from the group consisting of CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , CO and CO 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
       FIG. 1A  is an SEM image of a heat treated surface of a catalytic metal layer for forming the carbon nanotube by a conventional CVD method;  
       FIG. 1B  is an SEM image of the carbon nanotube grown by a conventional CVD method on the surface of the catalytic metal layer shown in  FIG. 1A ;  
       FIG. 2  is an SEM image of a surface of a catalytic metal layer before forming the carbon nanotube by a plasma enhanced CVD method;  
       FIGS. 3A through 3F  are cross-sectional views for describing a method of forming the carbon nanotube according to an exemplary embodiment of the present invention;  
       FIG. 4  is an SEM image of a cross-sectional view of a polyimide layer formed on an electrode; and  
       FIG. 5  is an SEM image of a plurality of protrusions formed on an electrode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. Like reference numerals refer to like elements throughout the drawings.  
       FIGS. 3A through 3F  are cross-sectional views for describing a method of forming the carbon nanotube according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 3A , an electrode  102  is formed on a substrate  100  which is preferably composed of glass. The electrode  102  can be composed of molybdenum (Mo), chrome (Cr), or tungsten (W). The electrode can be deposited to a thickness of 1,000˜10,000 Å by electron beam evaporation or sputtering.  
      Referring to  FIG. 3B , a polyimide layer  104  is formed on the electrode  102 . Preferably, polyimide (Pl) is coated to a predetermined thickness on the electrode  102 . Then, the polyimide coat is soft-baked and cured to form the polyimide layer  104 . The polyimide is coated on the electrode  102  preferably to a thickness of approximately 1 to 10 μm, and more preferably a few μm by a spin coating method or a method using surface tension. The coated polyimide on the electrode  102  is soft-baked at a temperature of approximately 95° C., and the curing can be performed at a temperature of approximately 350° C. An organic material contained in the polyimide is removed in these processes.  
       FIG. 4  is an SEM image of a cross-sectional view of the polyimide layer  104  formed on the electrode  102  and the substrate  100 . Referring to  FIG. 4 , a plurality of tiny protrusions are observed on a surface of the polyimide layer  104 .  
      Referring to  FIG. 3C , by etching the polyimide layer  104 , a plurality of protrusions  104   a  are formed on the surface of the polyimide layer  104 . The polyimide layer  104  can be etched by a reactive ion etching (RIE) method. Preferably, the surface of the polyimide layer  104  is etched using plasma generated by a reaction gas injected into a reaction chamber. A reaction gas such as sulfur hexafluride (SF 6 ), oxygen, or trifluoromethane (CHF 3 ) with a flow rate of 7.5, 92.5, or 7.5 sccm (standard cubic centimeter per minute), respectively, is injected into the reaction chamber with a pressure of approximately 67.5 mtorr. The supplying power can be approximately 235 W.  
      The etching is continued until the upper surface of the electrode  102  is etched through the polyimide layer  104 . Referring to  FIG. 3D , a plurality of protrusions  102   a  corresponding to the protrusions  104   a  formed on the polyimide layer  104  are formed on the electrode  102 . At this time, a gap between adjacent protrusions  102   a  on the surface of the electrode  102  is approximately 1 to 50 nm, and more preferably a few nm.  
       FIG. 5  is an SEM image of a plurality of protrusions  102   a  formed on the electrode  102 . Referring to  FIG. 5 , a plurality of protrusions  102   a  corresponding to the protrusions  104   a  of the polyimide layer  104  are formed on the electrode  102 .  
      Then, the surface of the electrode  102  is cleaned by removing the polyimide remaining between the protrusions  102   a.    
      Next, referring to  FIG. 3E , a catalyst layer  106  is formed between the protrusions  102   a  of the electrode  102 . Preferably, the catalyst layer  106 , on which carbon nanotubes can grow, is formed by depositing a catalyst on the surface of the electrode  102  using a sputtering method or an electron beam evaporation method. The catalyst layer  106  is formed only between the protrusions  102   a  of the electrode  102  because the catalyst layer  106  is formed relatively thin with a thickness of approximately 0.5˜2 nm. The catalyst can be at least one selected from the group consisting of W, Ni, Fe, Co, Y, Pd, Pt, and Au.  
      Referring to  FIG. 3F , the carbon nanotube  108  is formed on a surface of the catalyst layer  106  using a thermal CVD method or a plasma enhanced CVD method. Preferably, while injecting a gas containing carbon into a reaction chamber, the gas of the chamber is maintained at approximately 500˜900° C., and the carbon nanotube is grown vertically from the surface of the catalyst layer  106 . The carbon containing gas can be at least one selected from the group consisting of CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , CO and CO 2 . The carbon nanotube  108  grown in this way has a diameter of approximately 1 to 50 nm, and more preferably a few nm.  
      As described above, in the method of forming the carbon nanotube according to embodiments of the present invention, the carbon nanotube having a fine diameter can be produced by growing the carbon nanotube between protrusions formed on an electrode using plasma. Therefore, the use of the carbon nanotube in a device can reduce the operating voltage and can improve a field emission characteristic of the device.  
      While this invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.