Abstract:
In a method of manufacturing a field emitter, a patterned conductive layer is formed on a substrate, an upper surface of the conductive layer is coated with a mixture of a field emission material and metal powder, the mixture is thermally treated to improve adhesion of the mixture to the conductive layer, and a field emission material and a metal deposited on a portion of the substrate other than the conductive layer are removed. Accordingly, the lifespan and field emission characteristic of the field emitter are greatly improved, and a large area field emitter having excellent characteristics that cannot be realized in the conventional art is fabricated.

Description:
CLAIM OF PRIORITY  
       [0001]     This application makes reference to, incorporates the same herein, and claims al benefits accruing under 35 U.S.C. §119 from an application earlier filed in the Korean Intellectual Property Office on 5 Feb. 2004 and there duly assigned Ser. No. 10-2004-0007524.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a method of manufacturing a field emitter, by which method a large field emitter having an excellent adhesion is simply formed on a metal electrode.  
         [0004]     2. Description of the Related Art  
         [0005]     A conventional field emitter has been used in various fields according to a field emission characteristic and a field emission effect thereof. Particularly, since carbon nanotubes were developed in the early 1990&#39;s, much research into the manufacture of field emission displays using a carbon nanotube thin films has been conducted. In a field emitter using carbon nanotubes, carbon-family materials including carbon nanotubes have poor adhesion to a substrate formed of silicon, glass, or the like. Hence, forming a film of carbon-family materials on a substrate is difficult. Even when the film is formed and used as a field emission emitter for a long period of time, the carbon nanotubes may be detached from the substrate due to low adhesion of the film. Hence, manufacturing of a wide field emitter is difficult.  
         [0006]     Conventional methods of manufacturing a field emitter of a field emission display using carbon nanotubes can be roughly divided into two methods. A first method involves growing carbon nanotubes from a substrate, and a second method involves mechanically forming carbon nanotubes on a substrate by pasting the substrate with grown carbon nanotubes. These conventional methods have the following problems. When the first method is used, formation of a wide field emitter is substantially difficult because adhesion to a silicon substrate, a glass substrate, an indium tin oxide (ITO) substrate, etc. is poor. Hence, much research into the manufacturing of a wide field emitter is required.  
         [0007]     When the second method is used, many impurities, such as, a binder, a resin, a filter, or the like, are included in the carbon nanotubes. The impurities adversely affect field emission by the carbon nanotubes, and have a bad influence upon the durability and stability of the field emitter. While mechanical polishing, such as milling, is performed, many defects are detected. As a result, the life span of the field emitter is short.  
         [0008]     When materials other than carbon nanotubes are used as field emission materials, they also may have low adhesion to a lower substrate or an electrode. The low adhesion of the field emission materials directly affects the performance and durability of the field emission display.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a method of manufacturing a large-area carbon nanotube field emitter, by which method adhesion between a field emission material, such as carbon nanotubes, and an electrode is improved, and the durability and field emission of the field emitter are also improved.  
         [0010]     According to an aspect of the present invention, a method of manufacturing a field emitter includes the steps of: forming a patterned conductive layer on a substrate; coating an upper surface of the conductive layer with a mixture of a field emission material and metal powder; thermally treating the mixture to improve an adhesion of the mixture to the conductive layer; and removing a field emission material and a metal deposited on a portion other than the conductive layer.  
         [0011]     The field emission material may be a carbon-family material, a metal or a semiconductor material. The carbon-family material may include at least one of carbon nanotubes and a carbon horn.  
         [0012]     The step of forming the patterned conductive layer includes: forming a photoresist layer so as to expose a predetermined upper area of the substrate; coating the exposed upper area of the substrate and an upper surface of the photoresist layer with a conductive material; and forming a conductive layer by removing the photoresist layer and a conductive material deposited on the photoresist layer.  
         [0013]     The steps of coating the upper surface of the conductive layer with the mixture and thermally treating the mixture include: coating an exposed portion of the substrate and the conductive layer with a mixture of the field emission material, the metal powder and a suitable solvent at a predetermined ratio; and thermally treating the mixture at a temperature of about 600° C. so as to increase adhesion between the mixture and the conductive layer.  
         [0014]     The metal powder may be silver, copper, zinc or nickel.  
         [0015]     The diameter of the metal powder may be about 0.01 to 100 μm.  
         [0016]     The mixture of the field emission material, the metal powder, and the solvent may be coated on the exposed portion of the substrate and the conductive layer using spin coating or printing.  
         [0017]     The substrate may be a silicon substrate, a glass substrate, or an indium tin oxide substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     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:  
         [0019]      FIGS. 1A through 1G  are cross-sectional views illustrating a method of manufacturing a field emitter according to an embodiment of the present invention;  
         [0020]      FIGS. 2A and 2B  are pictures of a field emitter which is fabricated using carbon nanotubes as a field emission material in a method according to an embodiment of the present invention;  
         [0021]      FIG. 3A  is a graph showing emitted fields of conventional field emitters that use carbon nanotubes as field emission materials, and an emitted field of a field emitter according to an embodiment of the present invention that uses carbon nanotubes as a field emission material, versus an applied voltage; and  
         [0022]      FIG. 3B  is a graph showing emitted fields of a conventional field emitter that use carbon nanotubes, and a field emitter according to an embodiment of the present invention that use carbon nanotubes, versus time. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIGS. 1A through 1G  are cross-sectional views illustrating a method of manufacturing a field emitter according to an embodiment of the present invention. Referring to  FIG. 1A , first, a patterned photoresist (PR) layer  12  is formed on a substrate  11  and is exposed and patterned to form grooves  12 ′, each having a predetermined width. The grooves  12 ′ indicate locations where a field emission material is formed. Depending on a desired size, each of the grooves  12 ′ has a predetermined shape. This process is not necessary for formation of a single field emitter but is necessary for formation of a plurality of field emitters on a single substrate so as to manufacture a field emitter array for mass production of field emitters. The type of substrate  11  used is not limited, so that a glass substrate, a light-transmissive indium tin oxide (ITO) substrate or the like may be used as the substrate  11 .  
         [0024]     Thereafter, referring to  FIG. 1B , a conductive material  13  is disposed on the PR layer  12  and the substrate  11  so as to form an electrode and so that a potential can be applied to a field emission material. Generally, a metal capable of being used as an electrode in a semiconductor device is deposited on the PR layer  12  and the substrate  11 . Any deposition method used in a general semiconductor manufacturing process may be used to deposit the conductive material  13 . particularly, when the conductive material  13  is a metal, a vapor deposition method, such as sputtering, ion beam deposition or evaporation, is used. Hence, the conductive material  13  is deposited on the patterned PR layer  12  and the grooves  12 ′ so as to form a stepped surface. Then, as illustrated in  FIG. 1C , a conductive layer  14  remains on the substrate  11 .  
         [0025]     As illustrated in  FIG. 1D , a field emission material and metal nano powder are mixed at a desired ratio to form a mixture, and an exposed portion of the substrate  11  and the conductive layer  14  are coated with the mixture using spin coating or screen printing such that the mixture has a desired thickness. When a carbon-family material, such as, carbon nanotubes, is used as the field emission material, a necessary solvent is also mixed with the mixture. Consequently, a mixture layer  15 , composed of the field emission material and the metal nano powder, is formed on the exposed portion of the substrate  11  and the conductive layer  14 .  
         [0026]     The metal nano power is used to increase an adhesion between the conductive layer  14  and the field emission material, so that a conductive fine powder is used as the metal nano powder. Typically, metal nano powder having a size of a submicron to several microns (about 0.01 to 100 μm) is used, and may include several metal powders, such as Ag, Cu, Zn, Ni and the like, having low melting points. The field emission material may be a carbon-family material, a metal, a semiconductor material, or the like. The carbon-family material may be carbon nanotubes, a carbon horn, or the like. The metal may be tungsten. The semiconductor material may be Si. Palladium oxide (PLO) may also be used as the field emission material. Any material used as a general field emission material may be used as the field emission material according to an embodiment of the present invention.  
         [0027]     As illustrated in  FIG. 1E , the mixture layer  15  formed of the field emission material and the metal nano powder on the conductive layer  14  undergoes thermal treatment. When a solvent is included in the mixture layer  15 , the solvent usually has a boiling point lower than a temperature for thermal treatment. For example, alcohol having a boiling point lower than the temperature for thermal treatment is preferably used as the solvent. When ITO is used to form the substrate  11 , the thermal treatment is preferably performed at no more than 600° C., and a material having a boiling point lower than this temperature is preferably used as the solvent. When carbon nanotubes are used as the field emission material, most of the typically used solvents may be used as the solvent. When the mixture layer  15  undergoes such thermal treatment, the metal and the field emission material excluding the solvent remain on the exposed portion of the substrate  11  and the conductive layer  14 . The remaining metal adheres to the conductive layer  14  due to the thermal treatment, and the remaining field emission material sticks to the metal. Consequently, the remaining field emission material and the remaining metal form a mixture layer  15 ′.  
         [0028]     As illustrated in  FIG. 1F  and  FIG. 1G , the mixture layer  15 ′ formed on the substrate  11  and the conductive layer  14  undergoes surface processing. The surface processing may be performed using a typically used method, and may use ultrasonic waves or an adhesive tape. Even when just an adhesive tape is used, the mixture layer  15 ′ formed on the conductive layer  14  has excellent adhesion to the conductive layer  14 . Thus, during this surface processing, only a portion of the mixture layer  15 ′ that contacts the surface  11  is removed. The metal adheres to the conductive layer  14  due to the thermal treatment, and fixes the field emission material, which is mixed with the metal. Hence, referring to  FIG. 1F  and  FIG. 1G , a plurality of protrusions  17  of the field emission material are formed on the mixture layer  15 ′ remaining after the thermal treatment. Consequently, the field emitter according to an embodiment of the present invention can be completely formed.  
         [0029]      FIGS. 2A and 2B  are pictures of a field emitter which is fabricated using carbon nanotubes as a field emission material in a method according to an embodiment of the present invention. In  FIG. 2A , a conductive layer of a desired size is formed on a substrate  21 , and a mixture layer  22  composed of carbon nanotubes and metal nano powder is deposited on the conductive layer using spin coating or screen printing. The substrate  21  is formed of ITO, and the metal nano powder is formed of Ag. The conductive layer has a 2×2 cm size, and the mixture layer  22  is deposited on the conductive layer to have a 3×3 cm size.  
         [0030]      FIG. 2B  illustrates a specimen obtained by removing a solvent from a specimen of  FIG. 2A  using thermal treatment, and by performing surface processing on the specimen of  FIG. 2A . Referring to  FIG. 2A , when the mixture layer  22  formed on the substrate  21  undergoes surface processing, only a mixture layer  23  of 2×2 cm size adhering to an upper surface of the conductive layer remains on the substrate  21 . In other words, a portion of the mixture layer  22  excluding the mixture layer  23  of 2×2 cm size is removed by thermal treatment. This means that adhesion between the conductive layer and carbon nanotubes is significantly greater than adhesion between the substrate  21  and carbon nanotubes as described above.  
         [0031]      FIGS. 3A and 3B  are graphs showing measured electrical characteristics of a field emitter which uses carbon nanotubes as a field emission material, according to an embodiment of the present invention. Referring to  FIG. 3A , when an identical potential is applied, the field emitter according to an embodiment of the present invention has greater field emission than conventional field emitters, namely, paste ( 1 ) and paste ( 2 ), that use carbon nanotubes as field emission materials. When a potential of 4 V/μm is applied, the field emitter according to the embodiment of the present invention emits a field of about 1200 μA/cm 2 , whereas the conventional field emitters emit a field of no more than 200 μA/cm 2 . In other words, when a field emitter is fabricated using a carbon nanotube paste as in the conventional art, many impurities, such as a binder material, a resin, a filter and the like, are included in the field emitter and adversely affect the field emission of the field emitter.  
         [0032]      FIG. 3B  is a graph showing field emission characteristics of field emitters versus a time during which the field emitters are used. Referring to  FIG. 3B , the size of a conventional field emitter continuously decreases according to the period of time during which the conventional field emitter is used. On the other hand, the field emitter according to an embodiment of the present invention emits a small field that does not greatly vary according to the period of time during which the field emitter is used.  
         [0033]     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 detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.