Abstract:
A method of forming carbon fibers at a low temperature below 450° C. using an organic-metal evaporation method is provided. The method includes: heating a substrate and maintaining the substrate at a temperature of 200 to 450° C. after loading the substrate into a reaction chamber; preparing an organic-metal compound containing Ni; forming an organic-metal compound vapor by vaporizing the organic-metal compound; and forming carbon fibers on the substrate by facilitating a chemical reaction between the organic-metal compound vapor and a reaction gas containing ozone in the reaction chamber.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2005-0131879, filed on Dec. 28, 2005, and all the benefits accruing therefrom under 35 U.S.C. §119(a), the contents of which are herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming carbon fibers using a metal-organic chemical vapor deposition method, and more particularly, to a method of forming carbon fibers at a low temperature below 450° C. using a metal-organic chemical vapor deposition method. 
     2. Description of the Related Art 
     A great deal of research has gone into the application of carbon nanotubes or carbon fibers to field emission devices (“FEDs”), fuel cells, semiconductor devices, and the like, since the discovery of the superior structural and electrical characteristics of carbon nanotubes and carbon fibers. Particularly, carbon fibers provide many advantages when used as emitters of FEDs, such as low driving voltage, high brightness, and competitive prices. Conventional methods of forming carbon fibers include arc discharge, laser ablation, chemical vapor deposition (“CVD”), and plasma enhanced chemical vapor deposition (“PECVD”). However, when forming carbon fibers using these methods, a high temperature of greater than 800° C. is required, which can adversely affect the potential fabrication of devices that may be envisioned using carbon nanotube technology. A catalyst material or a plasma process can be used to form carbon fibers at a relatively low temperature. However, these processes still require a relatively high temperature greater than 600° C. Therefore, there is a limit to the reduction of the process temperature for forming carbon fibers when using these processes. Accordingly, there is a need for a method of forming carbon fibers at a low temperature. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method of forming carbon fibers at a low temperature below 450° C. using a metal-organic chemical vapor deposition method. 
     According to an embodiment of the present invention, a method of forming carbon fibers comprises: heating a substrate and maintaining the substrate at a temperature of 200 to 450° C. after loading the substrate in a reaction chamber; preparing an organic-metal compound containing Ni; forming an organic-metal compound vapor by vaporizing the organic-metal compound; and forming carbon fibers on the substrate by facilitating a chemical reaction between the organic-metal compound vapor and a reaction gas containing ozone in the reaction chamber. 
     According to another embodiment of the present invention, a method of manufacturing a field emission device comprises: providing a substrate; sequentially forming a stacked structure comprising a cathode disposed on a surface of the substrate, an insulating layer disposed on a surface of the cathode opposite the substrate, and a gate electrode disposed on a surface of the insulating layer opposite the cathode; forming at least one emitter hole that exposes the cathode by patterning the insulating layer and the gate electrode; coating a photoresist on the entire exposed surface of a stacked structure that comprises the cathode, the insulating layer, and the gate electrode; removing the portion of the photoresist coated on the portion of the cathode in the emitter hole; maintaining the temperature of the substrate at 200 to 450° C.; preparing an organic-metal compound that includes Ni; forming an organic-metal compound vapor by vaporizing the organic-metal compound; forming carbon fibers in the emitter hole on the cathode by facilitating a chemical reaction between the organic-metal compound vapor and a reaction gas containing ozone in the emitter hole; and removing the photoresist from the stacked structure. 
     The organic-metal compound may be one material selected from the group consisting of Ni(C 5 H 5 ) 2 , Ni(CH 3 C 5 H 4 ), Ni(C 5 H 7 O 2 ) 2 , Ni(C 11 H 19 O 2 ) 2 , Ni(C 7 H 16 NO), and Ni(C 7 H 17 NO) 2 . The organic-metal compound may also be provided as a solution comprising n-heptane as a solvent. The concentration of the organic-metal compound in the n-heptane is 0.05 to 0.5M. The vaporization temperature of the organic-metal compound may be maintained 140 and 200° C. Ozone may be supplied at a flow rate of 150 g/m 3  or greater. The carbon fibers may be grown vertically. The substrate may be a glass substrate, a sapphire substrate, a plastic substrate, or a silicon substrate. 
     In another embodiment, carbon fibers may be grown in a low temperature process, that is, at a temperature of 200 to 450° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic drawing illustrating a process of forming carbon fibers according to an embodiment of the present invention; 
         FIGS. 2A and 2B  are respectively a scanning electron microscope (SEM) image and a cross-sectional view of carbon fibers grown on a substrate using the process of forming carbon fibers of  FIG. 1 ; 
         FIG. 3  is a Raman spectrum of the carbon fibers shown in  FIG. 2 ; 
         FIGS. 4A through 4I  are cross-sectional views illustrating a method of forming carbon fibers according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “disposed on” another element, the elements are understood to be in at least partial contact with each other, unless otherwise specified. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is a schematic drawing illustrating a process of forming carbon fibers according to an embodiment of the present invention.  FIGS. 2A and 2B  are respectively a scanning electron microscope (“SEM”) image and a cross-sectional view of carbon fibers grown on a substrate using the process of forming carbon fibers of  FIG. 1 .  FIG. 3  is a Raman spectrum of the carbon fibers shown in  FIG. 2 . 
     Referring to  FIG. 1 , after loading a substrate  10  in a reaction chamber  50 , the substrate  10  is heated. The reaction chamber  30  can be evacuated by pumping gases out of an exhaust port  51 , and reactant gases can similarly be introduced to the reaction chamber by one or more inlet ports (e.g., inlet ports  52  and  53 ). The substrate  10  is maintained at a temperature of 200 to 450° C., preferably, at 350° C. The material constituting the substrate  10  is not specifically limited. The substrate  10  can be, for example, a glass substrate, a sapphire substrate, a plastic substrate, or a silicon substrate. 
     An organic-metal compound that includes Ni is prepared. Then, an organic-metal compound vapor is formed by vaporizing the organic-metal compound. In an exemplary embodiment, the organic-metal compound can be Nickel 1-dimethlamino-2-methyl-2-butanolate (also abbreviated as both Ni(dmamb) 2  and as Ni(C 7 H 17 NO) 2 ), where the ligand is an alkoxide ligand derived from 1-dimethylamino-2-methyl-2-butanol. In an embodiment, the organic-metal compound can be selected from the group consisting of Ni(C 5 H 5 ) 2 , Ni(CH 3 C 5 H 4 ), Ni(C 5 H 7 O 2 ) 2 , Ni(C 11 H 19 O 2 ) 2 , Ni(C 7 H 16 NO), and Ni(C 7 H 17 NO) 2 . In an embodiment of the present invention, the organic-metal compound can be provided as a solution comprising n-heptane as a solvent. In this case, the concentration of the organic-metal compound in n-heptane is maintained at 0.05 to 0.5M. The vaporization temperature of the organic-metal compound is maintained at 140 to 200° C., specifically at about 180° C. 
     Next, a chemical reaction is facilitated by supplying the organic-metal compound vapor by an inlet port (e.g., inlet port  53 , shown in the figure as providing Ni(C 7 H 17 NO) 2  vapor) and a reaction gas containing ozone (O 3 ) (e.g., inlet ports  52 , shown in the figure as providing the ozone) to grow carbon fibers  20  on the substrate  10 . Ozone may be supplied at a flow rate of 150 g/m 3  or more to ensure a sufficient chemical reaction with the organic-metal compound vapor. 
     As depicted in  FIGS. 1 ,  2 A, and  2 B, carbon fibers  20  can be vertically grown on the substrate  10  using the process described above. By analyzing the Raman spectrum of  FIG. 3 , a G-band (correlating to the helical chirality of the arrangement of 6 membered rings in the nanotube) and a D-band (correlating to the diameter of the nanotube) can be confirmed in the carbon fibers  20  formed according to the process described above. 
     According to an embodiment of the present invention, the carbon fibers can be grown at a low temperature of 200 to 450° C. The organic-metal compound used for a source material in manufacturing the carbon fibers according to an embodiment of the present invention readily dissolves at a relatively low temperature, i.e., below 450° C., thereby enabling a reduction of the carbon fiber growing temperature. In particular, since the organic-metal compound contains Ni, which is a catalyst material suitable for growing carbon fibers at a low temperature, the Ni can serve as a catalyst when the organic-metal compound dissolves, and a ligand material, which is combined with the Ni metal, can be used as a carbon source. As a result, in the method of forming carbon fibers according to an embodiment, the carbon source and the Ni catalyst are provided simultaneously by supplying only the organic-metal compound. Accordingly, in embodiments, a catalyst material deposition process can be omitted, that is, the method of forming carbon fibers is simpler than a conventional 2-step process that includes a catalyst material deposition process and a carbon source supplying process. 
     Embodiment-Forming Carbon Fibers 
     A 0.1 to 0.2 M Ni(C 7 H 17 NO) 2  solution was prepared as a source material (organic-metal compound) by dissolving Ni(C 7 H 17 NO) 2  in n-heptane. Then, carbon fibers having a diameter of a few tens of nm were grown by reacting ozone with the source material using a CVD method. 
     The temperature of the substrate was adjusted to 250 to 350° C., and the organic-metal source material was supplied to a reactor after evaporation using an evaporator. The evaporator was maintained at a temperature of about 180° C., and the organic-metal source material was supplied onto the substrate using a shower-head method. After the organic-metal source material was supplied onto the substrate, ozone was supplied to the reactor as a reaction gas. Through the processes described above, while the organic-metal source material (organic-metal compound) was dissolving, carbon fibers were synthesized using Ni as a catalyst and a ligand as a carbon source. The growth of the carbon fibers was closely related to the amount of ozone supplied. That is, when the ozone flow rate was greater than 150 g/m 3 , the carbon fibers grew well. In the present experiment, the time required to grow the carbon fibers was approximately 10 minutes, and a Si substrate was used. 
     Since the carbon fibers formed in this way are emitters, i.e., carbon fibers that emit electrons according to a voltage applied thereto, the process of forming the carbon fibers can be used for manufacturing a field emission device. 
     Hereinafter, a method of manufacturing a field emission device will now be described with reference to the accompanying drawings. 
       FIGS. 4A through 4I  are cross-sectional views illustrating a method of forming carbon fibers according to an embodiment of the present invention. In the method of forming carbon fibers, parts that have described above will not be repeated. 
     Referring to  FIGS. 4A through 4C , after providing a substrate  101 , a stacked structure  100  is formed, in which a cathode  102  is disposed on a surface of the substrate  101 , an insulating layer  104  is disposed on a surface of the cathode  102  opposite the substrate  101 , and a gate electrode  106  is disposed on a surface of the cathode  102  opposite the insulating layer  104 , and in which the above layers are formed sequentially. Afterward, at least one emitter hole  110  that exposes the cathode  102  is formed by etching/patterning a predetermined region of the insulating layer  104  and the gate electrode  106  ( FIG. 4B ) to form an etched, stacked structure  200  ( FIG. 4C ). 
     The material constituting the substrate  101  is not specifically limited. The substrate  101  can be, for example, a glass substrate, a sapphire substrate, a plastic substrate, or a silicon substrate. The cathode  102  and the gate electrode  106  can be formed of a conductive material, for example, a metal such as Al, Ag, Cu, etc., or a conductive oxide such as indium tin oxide (ITO). The insulating layer  104  can be formed of an insulating material such as SiO 2 . Also, each of the cathode  102 , the insulating layer  104 , and the gate electrode  106  can be formed using a thin film deposition method known to those skilled in the art, for example, physical vapor deposition (PVD) such as sputtering, thermal evaporation, or chemical vapor deposition (CVD), and thus, a description thereof will be omitted. 
     Referring to  FIG. 4D , a photoresist  108  is coated on the entire exposed surface of the etched stacked structure  200  (from  FIG. 4C ) to provide a coated, etched stacked structure  300  including the cathode  102 , the insulating layer  104 , and the gate electrode  106 . Then, referring to  FIGS. 4E and 4F , the portion of the cathode  102  in the emitter hole  110  is exposed by selectively exposing/developing the portion of the photoresist  108  coated on the cathode  102  ( FIG. 4E ). In embodiment, to selectively remove a portion of the photoresist  108 , an exposure process, a developing process, and an etching process can be sequentially performed. After the selective removal of the photoresist, an imaged, etched stacked structure  400  is obtained ( FIG. 4F ). 
     Referring to  FIG. 4G , the substrate  101  is heated and maintained at a temperature of 200 to 450° C., and then an organic-metal compound vapor and a reaction gas containing ozone are supplied to the emitter hole  110  to facilitate a chemical reaction therebetween. Thus, carbon fibers  120  are grown on the cathode  102  in the emitter hole  110  to provide carbon fiber intermediate structure  500 . The carbon fibers  120  can further be formed on the photoresist  108 . The carbon fibers  120  grown on the photoresist  108  can be removed together with the photoresist  108  in a subsequent “lift-off” process ( FIG. 4H ). The method of growing the carbon fibers  120  has described in detail above, and said description will not be repeated. 
     Referring to  FIG. 4I , when the photoresist  108  remaining on the stacking structure is etched, a field emission display (FED)  600  as depicted in  FIG. 4I  can be realized. The above carbon fiber growing process is a one-step process, and the carbon fibers  120  are readily formed in the emitter hole  110 . Therefore, the FED can be formed through a simple and easy process, thereby reducing manufacturing costs. 
     According to an embodiment, carbon fibers can be grown in a low temperature process using an organic-metal chemical vapor deposition method in which the temperature is maintained at 200 to 450° C. In the method of forming carbon fibers according to the present invention, an organic-metal compound used for a source material decomposes at a relatively low temperature, i.e., 450° C. or less, which is advantageous for reducing the carbon fiber growing temperature. Particularly, since the organic-metal compound contains Ni, which is a catalyst required for growing the carbon fibers, when the organic-metal compound decomposes, Ni acts as a catalyst and the ligand material that has combined with Ni is used as a carbon source material. As a result, in the method of forming carbon fibers, the carbon source and the Ni catalyst are supplied simultaneously by supplying only the organic-metal compound. Accordingly, in the present invention, a catalyst material deposition process can be omitted, that is, the method of forming carbon fibers is simpler than a conventional 2-step process that includes a catalyst material deposition process and a subsequent carbon source supplying process. 
     Also, since the carbon fibers formed in this way are emitters, i.e., are carbon fibers that emit electrons according to a voltage applied thereto, the process of forming the carbon fibers can be used for manufacturing a FED. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.