Abstract:
Provided is a field emission device using carbon nanotubes. The field emission device includes a substrate, a cathode, a gate insulating layer, an electron emitter, and a gate electrode. The cathode is formed on the substrate. The gate insulating layer is formed on the cathode and has a well exposing a portion of the cathode. The electron emitter is formed on the exposed portion of the cathode. The gate electrode is formed on the gate insulating layer and has a gate hole corresponding to the well. The gate electrode further includes a cylindrical electrode part that forms a focusing electric field from the gate hole toward a proceeding path of an electron beam. Accordingly, a focusing electric field can be formed around an electron beam emitted from the electron emitter so as to converge and focus the electron beam passing through the focusing electric field. As a result, color purity, brightness, and durability can be improved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the priority of Korean Patent Application No. 2002-64345, filed on Oct. 21, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a field emission device, and more particularly, to a field emission device having improved electron emission efficiency, brightness, color purity, and durability due to a focusing electric field.  
           [0004]    2. Description of the Related Art  
           [0005]    As shown in FIG. 1, in a field emission device (FED) using carbon nanotubes (CNTs), a cathode  2  is formed on a substrate  1 , and a gate insulating layer  3  is formed on the cathode  2 . The gate insulating layer  3  has a well  3   a  that exposes a portion of the cathode  2 . An electron emitter  4  is formed of carbon nanotubes on the exposed portion of the cathode  2 . A gate electrode  5  with a gate hole  5   a  corresponding to the well  3   a  is formed on the gate insulating layer  3 .  
           [0006]    A process of manufacturing a conventional FED having the above-described structure will be described in brief with reference to FIGS. 2A through 2J.  
           [0007]    As shown in FIG. 2A, a cathode  2  is formed of a transparent conductive material such as ITO on a substrate  1  made of glass. Actually, a plurality of cathodes  2  are formed in parallel strips. To form the cathode  2 , a process of depositing ITO on the entire surface of the substrate  1  and patterning the ITO is performed.  
           [0008]    Referring to FIG. 2B, a first insulator  3 ′ is coated on the cathode  2 , and then heated. Thereafter, a second insulator  3 ″ having a lower etching rate to an etchant than the first insulator  3 ′ is coated on the first insulator  3 ′, and then heated. As a result, a gate insulating layer  3  having a thickness of about 10 microns is completed.  
           [0009]    As shown in FIG. 2C, chromium (Cr) is deposited on the gate insulating layer  3  to form a gate electrode  5 .  
           [0010]    As shown in FIG. 2D, a photoresist layer  6  is coated on the gate electrode  5 . Thereafter, as shown in FIG. 2E, the photoresist layer  6  is patterned to form a window  6   a  corresponding to a gate hole  5   a  and a well  3   a  in the photoresist layer  6 . Next, a portion of the gate electrode  5  exposed by the window  6   a  is dry etched.  
           [0011]    Referring to FIG. 2F, an etchant is supplied through the window  6   a  to etch the gate insulating layer  3 . Here, since the first insulator  3 ′ of the gate insulating layer  3  has a higher etching rate than the second insulator  3 ″ of the gate insulating layer  3 , the well  3   a  shown in FIG. 2F is formed.  
           [0012]    As shown in FIG. 2G, the gate electrode  5  is patterned to broaden the gate hole  5   a . Due to patterning of the gate electrode  5 , the gate electrode  5  on the gate insulating layer  3  is divided into a plurality of gate electrodes which are arranged in parallel strips.  
           [0013]    Referring to FIG. 2H, a photoresist  7  is properly coated on the gate electrode  5 , and then patterned so that a portion of the cathode  2  in the center of the floor of the well  3   a  is exposed.  
           [0014]    As shown in FIG. 2I, a CNT paste  4   a  containing photoresist is coated on the photoresist  7 . Here, the CNT paste  4   a  fills the well  3   a.    
           [0015]    Referring to FIG. 2J, an exposure and development process is performed using a pattern mask (not shown) so that the CNT paste  4   a  remains in the center of the floor of the well  3   a . As a result, an electron emitter  4  is formed on the cathode  2 .  
           [0016]    Through the above-described process, a lower substrate including an electron gun structure having a cathode, a gate electrode, and so forth is completed. Next, the lower substrate is heated, and then sealed to a front substrate, which is coated with R, G, and B fluorescent materials, with a predetermined gap between the two substrates.  
           [0017]    In the electron gun structure shown in FIG. 1 or FIG. 2J, an electron beam emitted from the electron emitter  4  diverges due to mutual repulsion of electrons in the electron beam and a strong electric field is formed by applying a positive voltage to the gate electrode  5 . As a result, the electron beam is defocused, which increases the size of a spot formed on the fluorescent material. Then, the electron beam lands on other regions adjacent to its intended target fluorescent material. Thus, color purity is degraded, which results in poor image quality. Also, as the positive voltage of the gate electrode  5  increases, the electron beam further diverges after exiting the gate hole  5   a . Thus, most of the intensity of the electron beam radiated onto a fluorescent material is at the periphery of a corresponding pixel. If divergence of the electron beam is not minimized, load on the electron emitter  4  increases during high current driving or a long driving period, thereby reducing the life span of the electron gun structure.  
           [0018]    These problems occur in a spint type FED using metal micro tips as an electron emitter. Thus, for the spint type FED, a method of forming a double gate was introduced in order to reduce divergence of the electron beam. However, as is known, the spint type FED has a complicated structure, is not suitable as a wide-screen FED, and is expensive.  
         SUMMARY OF THE INVENTION  
         [0019]    Accordingly, the present invention provides an FED capable of effectively focusing an electron beam emitted from an electron emitter.  
           [0020]    The present invention provides a FED having improved color purity and clearness due to effective focusing of an electron beam.  
           [0021]    According to an aspect of the present invention, there is provided a field emission device including a substrate, a cathode, a gate insulating layer, an electron emitter, and a gate electrode. The cathode is formed on the substrate. The gate insulating layer is formed on the cathode and has a well exposing a portion of the cathode. The electron emitter is formed on the exposed portion of the cathode. The gate electrode is formed on the gate insulating layer and has a gate hole corresponding to the well. The gate electrode further includes a cylindrical electrode part that forms a focusing electric field from the gate hole toward a proceeding path of an electron beam.  
           [0022]    It is preferable that the cylindrical electrode part is a Bellmouse type electrode part that broadens in the direction of propagation of the electron beam.  
           [0023]    The electron emitter is micro tips or carbon nanotubes.  
           [0024]    According to another aspect of the present invention, there is also provided a method of manufacturing a field emission device. A first insulating layer and a second insulating layer having a higher etching rate than the first insulating layer are sequentially coated on a substrate on which a cathode is formed, to form a gate insulating layer. A first mask having a window with a predetermined diameter is formed on the gate insulating layer to form a well in the gate insulating layer. An etchant is supplied through the window to form a well that broadens upward in the gate insulating layer beneath the window of the first mask and exposes a portion of the cathode. A gate electrode is deposited on the gate insulating layer. A portion of the gate electrode on the bottom and lower inner wall of the well is removed to form a gate hole corresponding to the cathode in the gate electrode. An electron emitter is formed on the exposed portion of the cathode.  
           [0025]    It is preferable that after forming the well, the first mask is removed. Preferably, when removing the portion of the gate electrode, a second mask having a window corresponding to the gate hole is formed, etched, and removed so as to form the gate hole.  
           [0026]    It is preferable that a third mask is formed on the gate electrode to expose only a portion of the cathode and cover the remaining area of the cathode so as to form the electron emitter.  
           [0027]    It is preferable that after forming the third mask, a carbon nanotube paste containing photoresist is coated, and then patterned by photolithography so as to form the electron emitter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    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:  
         [0029]    [0029]FIG. 1 is a schematic cross-sectional view of a conventional FED;  
         [0030]    [0030]FIGS. 2A through 2J are cross-sectional views for explaining a process of manufacturing the conventional FED shown in FIG. 1; and  
         [0031]    [0031]FIG. 3A is a schematic cross-sectional view of a single gate type FED according to an embodiment of the present invention;  
         [0032]    [0032]FIG. 3B is a schematic cross-sectional view of a single gate type FED according to another embodiment of the present invention;  
         [0033]    [0033]FIG. 4A is a schematic cross-sectional view of a double gate type FED according to an embodiment of the present invention;  
         [0034]    [0034]FIG. 4B is a schematic cross-sectional view of a double gate type FED according to another embodiment of the present invention;  
         [0035]    [0035]FIG. 5 is a view for explaining the principle of focusing an electron beam by a Bellmouse type electrode part of a gate electrode according to the present invention;  
         [0036]    [0036]FIGS. 6A through 6L are cross-sectional views for explaining a process of manufacturing the single gate type FED shown in FIG. 3;  
         [0037]    [0037]FIGS. 7A through 7D are views illustrating results of a simulation carried out for the conventional FED; and  
         [0038]    [0038]FIGS. 8A through 8D are views illustrating results of a simulation carried out for an FED according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    Hereinafter, an FED and a method of manufacturing the FED according to the present invention will be described in detail with reference to the attached drawings. The FED of the present invention will be described as having a single gate structure. However, the FED may have a double gate structure without departing from the scope of the present invention.  
         [0040]    [0040]FIG. 3A is a schematic cross-sectional view of a single gate type FED using micro tips as an electron emitter according to an embodiment of the present invention. Referring to FIG. 3A, a cathode  21  is formed on a substrate  20 . A gate insulating layer  22  is formed on the cathode  21 . The gate insulating layer  22  has a well  22   a  which exposes a portion of the cathode  21 . An electron emitter  23  is formed of micro tips on the exposed portion of the cathode  21 . A gate electrode  24 , which has a gate hole  24   a  corresponding to the well  22   a , is formed on the gate insulating layer  22 .  
         [0041]    In the above structure, a characteristic part of the present invention is a Bellmouse type cylindrical electrode part  24   b  that forms a focusing electric field around an electron beam passed through the gate hole  24   a  of the gate electrode part  24 . As shown in FIG. 3A, the cylindrical electrode part  24   b  preferably has a Bellmouse shape which gradually broadens in the direction of electron beam propagation. The cylindrical electrode part  24   b  forms an electric field that converges or focuses the electron beam emitted from the electron emitter, i.e., the micro tips. FIG. 5 is a view for explaining the principle of forming an electric field using the cylindrical electrode part  24   b  and focusing an electron beam by the electric field. As shown in FIG. 5, a positive electric lens L, much like an optical convex lens, is formed by the cylindrical electrode part  24   b  (electric lens forming part). The positive electric lens L serves as a focusing lens that focuses a passing electron beam toward the central beam axis using an electric field. The theory behind the positive electric lens L is general electrodynamics, and thus will not be further described.  
         [0042]    [0042]FIG. 3B is a schematic cross-sectional view of a single gate type FED using CNTs as an electron emitter. Referring to FIG. 3B, a cathode  21  is formed on a substrate  20 . A gate insulating layer  22  is formed on the cathode  21 . The gate insulating layer  22  has a well  22   a  which exposes a portion of the cathode  21 . An electron emitter  23   a  is formed of CNTs on the exposed portion of the cathode  21 . A gate electrode  24 , which has a gate hole  24   a  corresponding to the well  22   a , is formed on the gate insulating layer  22 .  
         [0043]    [0043]FIG. 4A is a schematic cross-sectional view of a double gate type FED using micro tips as an electron emitter according to an embodiment of the present invention. As shown in FIG. 4A, a cathode  31  is formed on a substrate  30 . Micro tips, i.e., an electron emitter  35 , is formed on the cathode  31 . A first gate insulating layer  32  and a second gate insulating layer  33 , which form a well  36  enclosing the electron emitter  35 , are sequentially stacked on the cathode  31 . A first gate electrode  32   a  is interposed between the first and second gate insulating layers  32  and  33 . A second gate electrode  34  having a gate hole  34   a  corresponding to the well  36  is formed on the second gate insulating layer  33 . As described previously, a cylindrical electrode part characterizing the present invention, preferably a Bellmouse type electrode part  34   b , is formed at the second gate electrode  34 .  
         [0044]    A double gate type FED shown in FIG. 4B has an electron emitter  35   a  formed of CNTs instead of the electron emitter  35  formed of the micro tips shown FIG. 4A. The remaining elements of the double gate type FED shown FIG. 4B are the same as those of the double gate type FED shown in FIG. 4A.  
         [0045]    As described above, an FED according to the present invention is characterized in that a cylindrical electrode part for forming a focusing electric field, preferably a Bellmouse type electrode part, is formed at a gate electrode. The Bellmouse type electrode part is most effective in a single gate type FED using CNTs as an electron emitter as shown in FIG. 3B. A double gate type FED can effectively focus an electron beam without a cylindrical or Bellmouse type electrode part. However, also in the double gate type FED, a cylindrical or Bell mouse type electrode part characterizing the present invention can be formed at a second gate electrode. Thus, an electron beam can be further effectively focused.  
         [0046]    Hereinafter, a method of manufacturing the single gate type FED shown in FIG. 3B will be described. Methods of manufacturing FEDs according to other embodiments of the present invention can be easily understood through this description.  
         [0047]    As shown in FIG. 6A, ITO is deposited on a substrate  20 , and then patterned, thereby forming a cathode  21 .  
         [0048]    As shown in FIG. 6B, a gate insulating layer  22  is formed on the cathode  21 . Here, the gate insulating layer  22  includes first and second gate insulating layers  22 ′ and  22 ″ having different etching rates. The second gate insulating layer  22 ″ has a higher etching rate to an etchant than the first gate insulating layer  22 ′. Each of the first and second gate insulating layers  22 ′ and  22 ″ undergoes coating and heating processes. For example, the first gate insulating layer  22 ′ is formed of 7870K of Noritake Co. to a thickness of about 5 microns, and the second gate insulating layer  22 ″ is formed of 7972C of Noritake Co. to a thickness of about 10 microns.  
         [0049]    As shown in FIG. 6C, a photoresist mask  41  having a window  41   a  necessary for forming a well of a gate is coated on the gate insulating layer  22 .  
         [0050]    As shown in FIG. 6D, a Bellmouse-shaped well  26  is formed by supplying an etchant through the window  41  a of the photoresist mask  41 . The Bellmouse-shaped well  26  broadens upward due to a difference between the etching rates of the first and second gate insulating layers  22 ′ and  22 ″.  
         [0051]    As shown in FIG. 6E, the photoresist mask  41  is stripped by ashing. Next, as shown in FIG. 6F, a gate electrode  24  is formed on the gate insulating layer  22  using a sputtering method.  
         [0052]    As shown in FIG. 6G, a photoresist mask  42  is formed on the gate electrode  24 , and then patterned, thereby forming a window  42   a  that is opened to expose the floor and lower inner wall of the Bellmouse-shaped well  26 . Here, the photoresist mask  42  has a pattern necessary for forming the window  42   a  and the gate electrode  24 . In the present embodiment, the first gate insulating layer  22 ′ and the window  42  are further formed.  
         [0053]    As shown in FIG. 6H, the gate electrode  24  is patterned by wet or dry etching using the photoresist mask  42  to form a gate hole  24   a  corresponding to the window  42   a  in the gate electrode  24 . During this patterning process, the gate electrode  24  is divided into a plurality of patterns as in a general patterning process.  
         [0054]    As shown in FIG. 6I, the photoresist mask  42  is stripped by ashing. Thereafter, as shown in FIG. 6J, a photoresist mask  43  is formed. The photoresist mask  43  is spin coated and patterned to form a well-shaped window  43   a  exposing the floor of the Bellmouse-shaped well  26 .  
         [0055]    As shown in FIG. 6K, a CNT paste  23  containing photoresist is coated on the photoresist mask  43  using a printing method. As a result, the well-shaped window  43   a  is filled with the CNT paste  23 .  
         [0056]    As shown in FIG. 6L, the CNT paste  23  is patterned by exposure and development processes to remove a portion of the CNT paste  23  at the edge of the well-shaped window  43   a , thereby forming an electron emitter  23   a  in the center of the inside of the well-shape window  43   a . A portion of the CNT paste  23  remaining on the photoresist mask  43  is removed by lifting up the photoresist mask  43 .  
         [0057]    An FED having a desired structure can be manufactured through the above-described processes.  
         [0058]    In order to observe effects of an FED having the above-described structure according to the present invention, simulations were carried out for a conventional FED shown in FIG. 7A and an FED of the present invention shown in  8 A.  
         [0059]    [0059]FIGS. 7B and 8B each illustrate enlarged portions around gate electrodes of the FEDs shown in FIG. 7A and 8A, and FIGS. 7C and 8C each illustrate trajectories of divergent electron beams around the gate electrodes of the FEDs shown in FIG. 7A and 8A. As can be seen in FIGS. 7C and 8C, in the FED of the present invention shown in FIG. 8C, the electron beam is focused at a narrower angle due to the gate electrode than in the conventional FED shown in FIG. 7C. When the electron beam diverges in the conventional FED shown in FIG. 7C, part of the electron beam is intercepted by the edge of the gate hole, which causes a leakage current from the gate electrode.  
         [0060]    [0060]FIGS. 7D and 8D each illustrate trajectories of electron beams emitted from the FEDs shown in FIGS. 7A and 8A. As can be seen in FIGS. 7D and 8D, a radius of an electron beam emitted from the FED of the present invention shown in FIG. 8A is narrower than that of an electron beam emitted from the conventional FED shown FIG. 7A. According to calculation, the simulations showed that in the present invention, electron beams reaching a front substrate on which an anode and a fluorescent material are formed are focused with an approximately 10% smaller width than in the conventional FED. Also, in the conventional FED, the width of a well of a gate insulating layer was limited to 30 microns due to the height of the gate insulating layer. However, in the FED according to the present invention, the width of a well of the first gate insulating layer  22 ′ of a gate insulating layer can be adjusted by adjusting an area of the gate insulating layer to be etched. Thus, the well can be minutely formed to a width of 30 microns or less.  
         [0061]    As described above, according to the present invention, since electron beams can be effectively focused, an FED having high color purity and brightness can be manufactured. Since the FED according to the present invention can form electron beams having a desired width using a single gate electrode, the FED of the present invention does not need a complicated double gate electrode. However, if the FED is desired to have higher color purity, brightness, and performance than existing double gate electrode type FEDs, a cylindrical electrode part, preferably a Bellmouse type electrode part, can be formed at a final gate electrode, i.e., a second gate electrode.  
         [0062]    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.