Patent Publication Number: US-2005136787-A1

Title: Method of forming carbon nanotube emitter and method of manufacturing field emission display using the same

Description:
CLAIM OF PRIORITY  
      This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for METHOD OF FORMING CARBON NANO TUBE EMITTER AND METHOD OF MANUFACTURING FIELD EMISSION DISPLAY USING THE SAME earlier filed in the Korean Intellectual Property Office on 4 Dec. 2003 and there duly assigned Serial No. 2003-87475.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method of forming a carbon nanotube emitter and a method of manufacturing a field emission display using the same. More particularly, the present invention relates to a method of forming a high purity carbon nanotube emitter using interdiffusion between a photoresist and a carbon nanotube and a method of manufacturing a field emission display using the same.  
      2. Description of the Related Art  
      Displays that illuminate varying visual images including text, are one of the main devices for information transmission media, and are conventionally used as PC monitors or television receivers. The displays can be largely classified as either cathode ray tubes (CRTs) using high-speed electrons emitted from a heated cathode and flat panel displays; both types of these displays have recently undergone very rapid improvement. Flat panel displays are divided into liquid crystal displays (LCDs), plasma display panels (PDPs), and field emission displays (FEDs).  
      FEDs are displays that emit light through the collision of phosphors of an anode with electrons emitted from field emitter arrays aligned on a cathode under the influence of a strong electric field created by a gate electrode.  
      Microtip emitters made of a metal such as molybdenum (Mo) were conventionally used as the field emitters for FEDs. More recently, carbon nanotube (CNT) emitters have mainly been used. CNT emitter-based FEDs have many advantages such as their wide viewing angle, high resolution, low power consumption, and temperature stability, and thus are especially suitable for use in various areas such as car navigation equipment and electronic viewfinders. In particular, CNT emitter-based FEDs can be used as alternative displays for personal computers, PDA (personal data assistant) terminals, medical equipment, HDTVs (high definition televisions), and the like. CNT emitters can also be used as field emitters for back lights of LCDs.  
      Such CNT emitters are generally formed by exposing a CNT paste to light in one of two manufacturing techniques. In one technique, known as a front-side method, a cathode, insulating layer that provides an emitter hole, and gate electrode that have been formed on a substrate are covered with a paste of a CNT, the CNT paste is selectively cured by exposing portions of the CNT paste to ultra-violet radiation irradiated toward the front side through a mask, and the unexposed CNT paste is removed. The exposed CNT paste which remains, is fired to form CNT emitters with a predetermined shape.  
      In the second technique, known as a back-side exposure method, an insulating layer providing an emitter hole and a gate electrode are formed upon a cathode, the insulating layer and gate electrode are coated with a layer of photoresist, and the photoresist and the exposed portions of the cathode are coated with a CNT paste which is selectively exposed to ultraviolet light radiated toward a back side of the substrate and the portion of the CNT paste that is radiated, and the photoresist, are removed, and the remaining CNT paste is fired to form CNT emitters with a predetermined shape.  
      These front-side and back-side techniques for making CNT emitters are based on the photosensitivity of a CNT paste, and thus, have limited ability to accommodate an increase in the content of CNTs. Furthermore, since a thick CNT paste film is used, the dose of a high energy (about 1,000 mJ or more) is required to expose the CNT paste. In addition, light scattering generated in a CNT paste during exposure renders the formation and alignment of the desired pattern difficult.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method of forming a high purity carbon nanotube emitter using interdiffusion between a photoresist and a carbon nanotube, and a method of manufacturing a field emission display using the same.  
      According to one aspect of the present invention, there is provided a method for forming a carbon nanotube emitter by coating a photoresist onto a substrate having an electrode thereon, followed by patterning to form a photoresist dot on the electrode. The substrate is coated with a carbon nanotube paste to cover the photoresist dot and a carbon nanotube emitter is formed on the electrode by interdiffusion between the photoresist dot and the carbon nanotube paste through drying. The carbon nanotube paste covering the carbon nanotube emitter is then removed.  
      The carbon nanotube emitter may be made of a mixture of the photoresist and the carbon nanotube paste.  
      The photoresist may be a positive photoresist.  
      The photoresist may use novolak as a base material and the carbon nanotube paste may use Texanol as a viscosity modifier.  
      The carbon nanotube emitter may be formed by interdiffusion between the novolak of the photoresist and the Texanol of the carbon nanotube paste. Here, the carbon nanotube emitter may be formed by diffusing the Texanol toward the carbon nanotube paste in order to dissolve the novolak and diffuse the dissolved novolak toward the carbon nanotube paste.  
      The drying may be performed by heating the carbon nanotube paste at approximately 80° C. for approximately 20 minutes.  
      The carbon nanotube paste covering the carbon nanotube emitter may be removed by development with a developer. The developer may be either acetone or a sodium carbonate (Na 2 CO 3 ) solution.  
      According to another aspect of the present invention, there is provided a method for manufacturing a field emission display by forming sequentially a cathode, an insulating layer, and a gate electrode on a substrate, and forming an emitter hole to expose a portion of the cathode. The substrate is coated with photoresist and the photoresist is patterned to form a photoresist dot on the exposed portion of the cathode within the emitter hole. The substrate is coated with a carbon nanotube paste in order to cover the photoresist dot. A carbon nanotube emitter is formed on the cathode by interdiffusion between the photoresist dot and the carbon nanotube paste through drying. The carbon nanotube paste covering the carbon nanotube emitter is then removed. 
    
    
     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:  
       FIGS. 1A through 1E  are views in a sequence that illustrate one method for forming carbon nanotubes;  
       FIGS. 2A through 2E  are views in a sequence that illustrate another method for forming carbon nanotubes;  
       FIGS. 3A through 3D  are views in a sequence that illustrate a method of forming carbon nanotube emitters according to the principles of the present invention;  
       FIGS. 4A and 4B  are, respectively, photographs of photoresist dots and carbon nanotube emitters formed on a substrate;  
       FIGS. 5A through 5E  are views in a sequence that illustrate a method of manufacturing a field emission display according to the principles of the present invention;  
       FIG. 6  is a photograph of a screen of a field emission display using carbon nanotube emitters according to the principles of the present invention; and  
       FIG. 7  is a graph that illustrates the current-voltage characteristics of a field emission display using carbon nanotube emitters according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Turning now to the drawings,  FIGS. 1A through 1E  are views that illustrate a sequence in a method for forming CNT emitters for FEDs using a front-side exposure method in which a CNT paste is exposed to light.  
      First, referring to  FIG. 1A , cathode  12 , insulating layer  14  having an emitter hole, and gate electrode  16  are sequentially formed on substrate  10 . CNT paste  20  is coated on the substrate  10  by a printing technique so as to cover cathode  12 , insulating layer  14 , and gate electrode  16 . Then, a layer of CNT paste  20  is selectively exposed to ultraviolet (UV) light irradiated toward a front side of substrate  10  by using a mask  30 , as is shown in  FIG. 1B . At this time, an UV-exposed portion of CNT paste  20  is cured. Then, an unexposed portion of the CNT paste  20  is removed by using a developer such as acetone, as is shown in  FIG. 1C . As a result, only an exposed CNT paste  20 ′ remains in the emitter hole. Then, exposed CNT paste  20 ′ shrinks by firing to form CNT emitters  21  with a predetermined shape, as shown in  FIG. 1D . Finally, CNT emitters  21  are surface-treated with an adhesive tape so that pure CNTs  21  a are formed on the tips of CNT emitters  21 , as is shown in  FIG. 1E .  
       FIGS. 2A through 2E  are views that illustrate a sequence in a method for forming CNT emitters for FEDs using a back-side exposure method.  
      First, referring to  FIG. 2A , a cathode  52 , an insulating layer  54  having an emitter hole, and a gate electrode  56  are sequentially formed on a substrate  50 . A sacrificial layer  40  made of photoresist is coated on the substrate  50  so as to cover the cathode  52 , insulating layer  54 , and gate electrode  56  and insulating layer  54  is patterned to expose the portion of the cathode  52  within the emitter hole. Then, a CNT paste  60  is coated on the entire surface of the resultant structure of  FIG. 2A  by a printing method and is then selectively exposed to UV light irradiated toward a back side of substrate  50 , as is shown in  FIG. 2B . At this time, an UV-exposed portion of CNT paste  60  is cured. Then, an unexposed portion of CNT paste  60  is removed using a developer such as acetone and sacrificial layer  40  is removed, as is shown in  FIG. 2C . As a result, only an exposed CNT paste  60 ′ remains in the emitter hole. Then, the exposed CNT paste  60 ′ shrinks by firing to form CNT emitters  61  with a predetermined shape, as is shown in  FIG. 2D . Finally, the CNT emitters  61  are surface-treated with an adhesive tape so that pure CNTs  61  a are formed on the tips of the CNT emitters  61 , as is shown in  FIG. 2E .  
      Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. The same reference numerals indicate the same constitutional elements throughout the drawings.  
       FIGS. 3A through 3D  are views taken in a sequence that illustrate a method for forming carbon nanotube emitters constructed as an embodiment of the present invention.  
      First, a substrate  110  is prepared with an electrode  112  of a predetermined shape formed thereon. Substrate  110  is generally a glass substrate and the electrode  112  may be made of a transparent conductive material, such as ITO (indium thin oxide). Electrode  112  may be formed in a predetermined shape such as a striped shape on the surface of substrate  110 .  
      Next, photoresist dots  140  are formed on the exposed surface of electrode  112 , as is shown in  FIG. 3A . Photoresist dots  140  are formed by coating a layer of photoresist on substrate  110  having electrode  112  thereon followed by patterning. A photograph of photoresist dots  140  that have been formed on substrate  110  is shown in  FIG. 4A . Preferably, the photoresist is a positive photoresist. The positive photoresist includes a sensitizer that is sensitive to light, a base material such as resin, and an organic solvent that dissolves the base material. In this embodiment, novolak is used as the base material. As is better explained in U.S. Pat. No. 5,139,925 for Surface Barrier Silylation Of Novolak File Without Photoactive Additive Patterned With 193 NM Excimer Laser by Mark A. Hartney, issued on the 18 th  of Aug. 1992, as well as in other references, novolak resist is a cresol-formaldehyde copolymer, that in the practice of the instant exemplars, may be compounded with a photoactive component (i.e., a PAC).  
      Next, a carbon nanotube paste  120  is coated onto substrate  110  having electrode  112  and photoresist dots  140  formed thereon to cover photoresist dots  140 , as is shown in  FIG. 3B . Carbon nanotube paste  120  is generally coated by a printing method. In this embodiment, carbon nanotube paste  120  includes Texanol as a viscosity modifier. Texanol is a trademark of Eastman Chemical Co., located in Kingsport, Tenn.  
      Next, the carbon nanotube paste  120  is dried under a predetermined condition. For this, preferably, the carbon nanotube paste  120  is heated at about 80° C. for about 20 minutes. During the drying, interdiffusion between the photoresist dots  140  and the carbon nanotube paste  120  occurs, as is shown in  FIG. 3B . In greater detail, first, the Texanol of carbon nanotube paste  120  diffuses toward photoresist dots  140  and then dissolves the novolak which is a component of the photoresist that forms photoresist dots  140 . The novolak thus dissolved diffuses outwardly from photoresist dots  140  toward carbon nanotube paste  120 .  
      Photoresist dots  140  are transformed to carbon nanotube emitters  150  made of a mixture of photoresist and carbon nanotube paste  120  by this interdiffusion, as is shown in  FIG. 3C . At this time, carbon nanotube emitters  150  may be made to have a high purity by adjustment of the content of carbon nanotube paste  120 .  
      Subsequently, carbon nanotube paste  120  covering carbon nanotube emitters  150  is removed by a developer so that only carbon nanotube emitters  150  remain on electrode  112 , as is shown in  FIG. 3D . The developer may be either acetone or a sodium carbonate (Na 2 CO 3 ) solution. A photograph of carbon nanotube emitters  150  formed on substrate  110  is shown in  FIG. 4B .  
      As described above, according to the present invention, carbon nanotube emitters  150  can be easily formed in a desired shape by using the interdiffusion between photoresist dots  140  and carbon nanotube paste  120 , instead of by using an ultraviolet exposure method.  
       FIGS. 5A through 5E  are views that illustrate a method for manufacturing a field emission display s another embodiment of the present invention.  
      First, cathode  212 , insulating layer  214 , and gate electrode  216  are sequentially formed on substrate  210  and emitter hole  260  intended for exposure of a portion of cathode  212 , is then formed, as is shown in  FIG. 5A . Substrate  210  may be a glass substrate. Cathode  212  may be made of a transparent conductive material such as ITO and gate electrode  216  may be made of a conductive metal such as chromium (Cr).  
      In grater detail, a cathode layer  212  made of ITO is deposited to a predetermined thickness on substrate  210  and then patterned to a predetermined shape, for example a striped shape, to form cathode  212 . Insulating layer  214  is formed to a predetermined thickness on the entire surface of cathode  212  and substrate  210  and then a gate electrode layer is formed on insulating layer  214 . Gate electrode layer  216  is formed by sputtering a conductive metal to a predetermined thickness and conductive metal is patterned to a predetermined shape to form gate electrode  216 . Then, a portion of insulting layer  214  exposed through gate electrode  216  is etched to form emitter hole  260 . At this time, a portion of cathode  212  is exposed through emitter hole  260 .  
      Next, photoresist dot  240  is formed on the portion of cathode  212  exposed through emitter hole  260 , as is shown in  FIG. 5B . In detail, photoresist dot  240  is formed by coating a photoresist on the entire surface of the resultant structure of  FIG. 5A , followed by patterning to provide the structure shown in  FIG. 5B .  
      As mentioned earlier herein, a positive photoresist is preferable. The positive photoresist includes a novolak as a base material.  
      Next, carbon nanotube paste  220  is coated on the entire surface of the resultant structure of  FIG. 5B  to cover photoresist dot  240 , as is shown in  FIG. 5C . Carbon nanotube paste  220  may be coated by a printing method. As described above, carbon nanotube paste  220  includes texanol as a viscosity modifier.  
      Next, carbon nanotube paste  220  is dried under a predetermined condition. For this, preferably, carbon nanotube paste  220  is heated at about 80° C. for about 20 minutes. During the drying, interdiffusion between photoresist dot  240  and carbon nanotube paste  220  occurs, as is shown in  FIG. 5C . Interdiffusion between photoresist dot  240  and carbon nanotube paste  220  is described in the above, and thus, a repetition of the detailed description thereof is omitted here.  
      Photoresist dot  240  is transformed to a carbon nanotube emitter  250  made of a mixture of the photoresist and carbon nanotube paste  220  through the interdiffusion, as is shown in  FIG. 5D . At this time, carbon nanotube emitter  250  can have a high purity by adjustment of the content of carbon nanotube paste  220 .  
      Finally, carbon nanotube paste  220  covering carbon nanotube emitter  250  is removed by a developer so that only carbon nanotube emitter  250  remains on cathode  212 , as is shown in  FIG. 5E . The developer may be either acetone or a sodium carbonate (Na 2 CO 3 ) solution.  
       FIG. 6  is a photograph of a screen of a field emission display using carbon nanotube emitters formed according to the principles of the present invention. Referring to  FIG. 6 , it can be seen that the field emission display manufactured according to the present invention can provide the same image quality as a conventional field emission display.  
       FIG. 7  is a two-coordinate graph that illustrates the current-voltage characteristics of a field emission display manufactured according to the principles of the present invention. Referring to the graph of  FIG. 7 , the field emission display manufactured according to the present invention exhibits enhanced current (I)-voltage (V) characteristics, relative to a conventional field emission display. As is apparent from the above descriptions, the present invention provides the following advantages.  
      First, a carbon nanotube emitter can be formed in a desired shape by interdiffusion between a carbon nanotube paste and a photoresist. This technique for the formation of carbon nanotube emitters can be easily applied to the fabrication of a field emission display or to a back lit display.  
      Second, since the carbon nanotube emitter is formed without using one of the conventional exposure methods, there is no need to consider the light transmittance of the carbon nanotube paste. Therefore, the carbon nanotube emitter can be formed with a high purity by increasing the content of carbon nanotubes in the carbon nanotube paste.  
      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.