Abstract:
A field emission display (FED) is provided. The FED has an emitter structure where the emitter, a conductor and a cathode electrode are so arranged to produce a certain electric field about the emitter. The electric field about the emitter causes the electron beam emitted from the emitter to have improved focus and have less dispersion. This causes the electron beam to hit the intended pixel without exciting phosphor layers in neighboring pixels, thus improving image quality.

Description:
BACKGROUND OF THE INVENTION 
   This application claims the priority of Korean Patent Application Nos. 2003-84963 and 2004-35534, filed on Nov. 27, 2003 and May 19, 2004, respectively, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
   1. Field of the Invention 
   The present invention relates to a field emission display and, more particularly, to a field emission display having an emitter structure that improves focusing characteristics of electron beams, thus improving image quality. 
   2. Description of the Related Art 
   Display devices, which account for one of the most important parts of conventional data transmitting media, have been used in personal computers and television receivers. The display devices include cathode ray tubes (CRTs), which use high-speed heat electron emission, and flat panel displays, such as a liquid crystal display (LCD), a plasma display panel (PDP), and a field emission display (FED), which have been rapidly developing in recent years. 
   Of those flat panel displays, an FED is a display device that enables an emitter arranged at regular intervals on a cathode electrode to emit electrons by applying a strong electric field to the emitter to radiate light by colliding the electrons with a fluorescent material coated on the surface of an anode electrode. Since the FED forms and displays images thereon by using the emitter as an electron source, the quality of the images may vary considerably depending on the material and structure of the emitters. 
   Early FEDs use a spindt-type metallic tip (or a micro tip) formed of molybdenum (Mo) as an emitter. In order to arrange such metallic tip-type emitter in an FED, however, an ultramicroscopic hole should be formed, and molybdenum should be evenly deposited on the entire surface of a screen, which requires the use of difficult techniques and expensive equipment and thus results in an increase in manufacturing costs. Therefore, there is a clear limit in manufacturing a wide screen FED. 
   In the industry of FEDs, research on methods of forming a flat emitter of an FED, which can emit sufficient amounts of electrons even at a low driving voltage and, eventually, can simplify processes of manufacturing the FED, is under way. Current trends in the FED industry show that carbon-based materials, for example, graphite, diamond, diamond-like carbon (DLC), fulleren (C60), or carbon nano-tubes (CNTs), are suitable for the manufacture of a flat emitter and the CNTs, in particular, are considered most desirable because they can successfully emit electrons even at a low driving voltage. 
   In order to have an FED display images of good quality, the electron beam emanating from the emitter must be focused and must not disperse too much so that only the phosphor layer in the intended pixel and not phosphor in neighboring pixels are impacted by the electron beam. Therefore, what is needed is an FED with superior image quality brought on by an improved design of the emitter so that the electron beam emanating from the emitter is focused and does not disperse too much so that the electron beam hits phosphor in the desired pixel and not phosphor in neighboring, unintended pixels. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide an improved FED. 
   It is also an object of the present invention to provide a design for an FED that improves image quality by better controlling the amount of dispersion of electron beams emanating from an emitter. 
   It is also an object of the present invention to provide an FED and an emitter design that improves the focusing characteristics of electron beams emanating from the emitter. 
   It is still an object of the present invention to provide an improved design for an emitter in an FED that results in an improved image quality. 
   These and other objects can be achieved by an improved field emission display (FED) design. The FED includes a first substrate, a cathode electrode formed on the first substrate, a conductive layer formed on the cathode electrode to have a first aperture, through which the cathode electrode is partially exposed, an insulation layer formed on the conductive layer to have a second aperture, which is connected to the first aperture, a gate electrode formed on the insulation layer to have a third aperture, which is connected to the second aperture, emitters formed on the cathode electrode exposed through the first aperture, the emitters being disposed a predetermined distance apart from each other at either side of the first aperture, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, having an anode electrode and a fluorescent layer formed thereon. 
   A cavity may be formed in the cathode electrode between the emitters so that the first substrate can be exposed therethrough. The first, second, and third apertures and the cavity may be rectangles extending in a longitudinal direction of the cathode electrode. The widths of the third and second apertures may be larger than the width of the first aperture, and the width of the cavity is smaller than the width of the first aperture. The predetermined distance between the emitters may be smaller than the width of the first aperture, and the width of the cavity may be smaller than the distance between the emitters. The width of the third aperture may be the same as the width of the second aperture. The width of the third aperture may be larger than the width of the second aperture. 
   Conductive layers may be formed at both sides of the cathode electrode and may extend in the longitudinal direction of the cathode electrode, and the first aperture may be formed between the conductive layers. Conductive layers may be formed at both sides of the cathode electrode to have a predetermined length, and the first aperture may be formed between the conductive layers. The conductive layer may be formed on the cathode electrode to surround the first aperture. The conductive layer may include an insulation material layer formed to cover a top surface and side surfaces of the cathode electrode and a metal layer formed on the insulation material layer. A plurality of first apertures, a plurality of second apertures, and a plurality of third apertures may be formed for each pixel, and the emitters may be formed in each of the plurality of first apertures. The emitters may be formed of a carbon-based material. The emitters may be formed of carbon nano-tubes. 
   According to another aspect of the present invention, there is provided a field emission display (FED). The FED includes a first substrate, a cathode electrode formed on the first substrate, a conductive layer formed on the cathode electrode to have a first circular aperture, through which the cathode electrode is partially exposed, an insulation layer formed on the conductive layer to have a second circular aperture, which is connected to the first circular aperture, a gate electrode formed on the insulation layer to have a third circular aperture, which is connected to the second circular aperture, an emitter formed as a ring on the cathode electrode exposed through the first circular aperture, the emitter being disposed along an inner circumference of the first circular aperture, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, on which an anode electrode and a fluorescent layer having a predetermined pattern are formed. 
   A cavity may be formed in the cathode electrode in the emitter to be circular so that the first substrate can be exposed therethrough. A plurality of first circular apertures, a plurality of second circular apertures, and a plurality of third circular apertures may be formed for each pixel, and the emitter may be formed in each of the plurality of first circular apertures. 
   According to another aspect of the present invention, there is provided a field emission display (FED). The FED includes a first substrate, a cathode electrode formed on the first substrate, an insulation material layer formed on the cathode electrode, a conductive layer formed on the insulation material layer, a first aperture formed through the insulation material layer and the conductive layer so that the cathode electrode can be partially exposed therethrough, an insulation layer formed on the conductive layer to have a second aperture, which is connected to the first aperture, a gate electrode formed on the insulation layer to have a third aperture, which is connected to the second aperture, emitters formed on the cathode electrode exposed through the first aperture, the emitters being disposed at both sides of the first aperture so that they can be a predetermined distance apart from each other, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, on which an anode electrode and a fluorescent layer having a predetermined pattern are formed. The conductive layer may be insulated from the cathode electrode by the insulation material layer. 
   According to another aspect of the present invention, there is provided a field emission display (FED). The FED includes a first substrate, a cathode electrode formed on the first substrate, an insulation material layer formed on the cathode electrode, a conductive layer formed on the insulation material layer, a first circular aperture formed through the insulation material layer and the conductive layer so that the cathode electrode can be partially exposed therethrough, an insulation layer formed on the conductive layer to have a second circular aperture, which is connected to the first circular aperture, a gate electrode formed on the insulation layer to have a third circular aperture, which is connected to the second circular aperture, an emitter formed as a ring on the cathode electrode exposed through the first circular aperture, the emitter being disposed along an inner circumference of the first circular aperture, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, on which an anode electrode and a fluorescent layer having a predetermined pattern are formed. The conductive layer may be insulated from the cathode electrode by the insulation material layer. 

   
     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 and 1B  are a cross-sectional view and a plan view, respectively, of a field emission display (FED); 
       FIGS. 2A and 2B  are cross-sectional views of other FEDs; 
       FIG. 3  is a cross-sectional view of an FED according to a first embodiment of the present invention; 
       FIG. 4  is a plan view of the FED of  FIG. 3 ; 
       FIGS. 5A ,  5 B, and  5 C are perspective views of three examples of a conductive layer formed on each cathode electrode of the FED of  FIG. 3 ; 
       FIGS. 6 ,  7 , and  8  are cross-sectional views of variations of the FED of  FIG. 3 ; 
       FIG. 9  is a plan view of an FED according to a second embodiment of the present invention; 
       FIGS. 10A and 10B  are a plan views of an FED according to a third embodiment of the present invention; 
       FIGS. 11A ,  11 B, and  11 C are diagrams illustrating electron beam emission simulation results of the FED of  FIG. 1 ; 
       FIGS. 12A ,  12 B, and  12 C are diagrams illustrating electron beam emission simulation results of the FED of  FIG. 3  in a case where no cavity is formed in each cathode electrode of the corresponding FED; 
       FIGS. 13A ,  13 B, and  13 C are diagrams illustrating electron beam emission simulation results of the FED of  FIG. 3  in a case where a cavity is formed in each cathode electrode of the corresponding FED; 
       FIGS. 14A ,  14 B, and  14 C are diagrams illustrating electron beam emission simulation results of the FED of  FIG. 3  in a case where the width of the cavity formed in each cathode electrode of the corresponding FED has been changed; 
       FIGS. 15A ,  15 B, and  15 C are diagrams illustrating electron beam emission simulation results of the FED of  FIG. 7 ; and 
       FIGS. 16A and 16B  are diagrams illustrating electron beam emission simulation results of the FED of  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Turning now to the figures,  FIGS. 1A and 1B  are a cross-sectional view and a plan view, respectively, of an FED  90 . Referring to  FIGS. 1A and 1B , the FED  90  has a triode structure made of a cathode electrode  12 , an anode electrode  22 , and a gate electrode  14 . The cathode electrode  12  and the gate electrode  14  are formed on a rear substrate  11 , and the anode electrode  22  is formed at the bottom of a front substrate  21 . A fluorescent layer  23  is formed of R, G, and B fluorescent materials, and a black matrix  24  is formed on the bottom surface of the anode electrode  22  so as to improve contrast. The rear substrate  11  and the front substrate  21  are a predetermined distance apart from each other. The predetermined distance between the rear substrate  11  and the front substrate  21  is maintained by a spacer  31  disposed between the rear substrate  11  and the front substrate  21 . When manufacturing the FED  90 , the cathode electrode  12  is formed on the rear substrate  11 , an insulation layer  13  and the gate electrode  14 , both perforated by minute apertures  15 , are deposited on the rear substrate  11 , and an emitter  16  is formed in each of the apertures  15  on top of the cathode electrode  12 . 
   The FED  90  of  FIGS. 1A and 1B , however, may lack good color purity and general picture quality for the following reasons. Most of the electrons emitted from the emitter  16  come from edges of the emitter  16 . The electrons are converted into an electron beam, and the electron beam proceeds to the fluorescent layer  23 . However, when proceeding to the fluorescent layer  23 , the electron beam may disperse due to a voltage of several to dozens of volts applied to the gate electrode  14 , in which case, the electron beam illuminates not only a fluorescent material of a desired pixel but also fluorescent materials of other pixels adjacent to the desired pixel. 
   In order to minimize the tendency of the electron beam emitted from the emitter to disperse toward the fluorescent layer  23 , a plurality of emitters, each having a smaller area than the emitter  16  corresponding to one pixel, can be disposed on the cathode electrode  12  in each of the apertures  15 . In this case, however, there is a clear limit as to the number of emitters that can be satisfactorily formed for each pixel having a predetermined size, the entire area of the emitter  16  for illuminating a fluorescent material of one pixel decreases, and an electron beam is not focused sufficiently. 
   In order to prevent an electron beam from dispersing when proceeding to a fluorescent layer, another FEDs respectively having structures, which are illustrated in  FIGS. 2A and 2B , can be considered. The FEDs  92  and  93  of  FIGS. 2A and 2B  respectively each include an additional electrode disposed near a gate electrode to enhance the focusing characteristics of electron beams. 
   More specifically, in the FED  92  of  FIG. 2A , a focusing electrode  54 , which is ring-shaped, is disposed around a gate electrode  53 . In the FED  93  of  FIG. 2B , a double gate structure having a lower gate electrode  63  and an upper gate electrode  64  is provided to focus electron beams. However, the FEDs of  FIGS. 2A and 2B  have a relatively complicated structure. In addition, the structure of the FEDs  92  and  93  of  FIG. 2A  or  2 B, in which an emitter  52  or  62 , which is a metallic micro-tip, is formed on a cathode electrode  51  or  61 , has not yet been proven satisfactorily fruitful when it comes to its application to an FED having a flat emitter. 
   In the meantime, U.S. Pat. No. 5,552,659 Macaulay et al. discloses an electron emitter that reduces electron emission divergence by imposing restrictions on a ratio between the thickness of a non-insulation layer formed on a substrate where the electron emitter is formed and the thickness of a dielectric layer and a ratio between the diameter of a hole formed through the non-insulation layer, the dielectric layer, and a gate layer formed on the dielectric layer and the thickness of the non-insulation layer. However, it is very difficult to manufacture the electron emitter because the electron emitter has a very complicated structure in which a plurality of holes are formed to correspond to each pixel, and a plurality of electron emitters are formed in each of the holes. In addition, there are spatial restrictions in manufacturing the electron emitter. Therefore, there is a limit in maximizing the number and area of emitters corresponding to each pixel, and the lifetime of the emitters may be shortened when driving the emitters for a long time. 
   Turning now to  FIGS. 3 and 4 ,  FIGS. 3 and 4  are a cross-sectional view and a plan view, respectively, of a field emission display (FED)  100  according to a first embodiment of the present invention. Referring to  FIGS. 3 and 4 , the FED  100  includes two substrates, i.e., a first substrate  110 , which is also referred to as a rear substrate, and a second substrate  120 , which is also referred to as a front substrate. The rear substrate  110  and the front substrate  120  are formed so that they can be separated from each other by a predetermined distance. A spacer  130  is disposed between the rear substrate  110  and the front substrate  120  so that the predetermined distance therebetween can be maintained. The rear and front substrates  110  and  120  are typically formed of glass substrates. 
   A structure that can emit electrons is formed on the rear substrate  110 , and a structure that can realize images using the emitted electrons is formed on the front substrate  120 . More specifically, a plurality of cathode electrodes  111  are arranged on the rear substrate  110  at regular intervals in a predetermined pattern, for example, as stripes. The cathode electrodes  111  are formed by depositing a conductive metallic material or a transparent conductive material, such as indium tin oxide (ITO), on the rear substrate  110  to a thickness of, for example, several hundreds to several thousands of Å and patterning the deposited conductive metallic material or transparent conductive material as stripes. The material of the cathode electrodes  111  may be determined depending on how emitters  115  are formed, which will be described in greater detail later. 
   Cavities  111   a , having a width Wc are preferably formed in the cathode electrodes  111  and perforate cathode electrodes  111  so that the rear substrate  110  can be exposed therethrough. Each of the cavities  111   a  is disposed between emitters  115 . It is within the scope of the invention not to have any cavities formed perforating the cathode electrode  111 . Also, it is within the scope of the invention to have more than one cavity per pixel, as will be discussed in  FIGS. 9 and 10 . For the FED  100  of  FIG. 3 , there will be a one-to-one correspondence between the cavities  111   a  perforating the cathode electrode  111  and the pixels  125 . In addition, the cavities  111   a  may be formed, in consideration of the shape of their respective pixels  125 , as rectangles extending longer in the longitudinal (or +/−y) direction of the cathode electrodes  111 , i.e., rather than in the latitudinal (+/−x) direction. 
   A conductive layer  112  is formed on each of the cathode electrodes  111  so as to be electrically connected to each of the cathode electrodes  111 . The conductive layer  112  may be formed to a thickness of about 2-5 μm by coating a conductive paste on each of the cathode electrodes  111  using a screen printing method and plasticizing the conductive paste at a predetermined temperature. First apertures  112   a  having width W 1 , through which the cathode electrodes  111  are partially exposed, are formed in and perforate the conductive layer  112 . The first apertures  112   a  may be formed as rectangles that extend longer in the longitudinal direction of the cathode electrodes  111  (i.e., the Y direction) than in the latitudinal direction of the cathode electrodes  111  (i.e., the X direction) so that first aperture  112   a  can correspond to one of the pixels  125 . In a case where the cavities  111   a  are formed in the cathode electrodes  111 , as described above, the first apertures  112   a  are formed to have a width W 1 , which is larger than a width W c  of the cavities  111   a , so that they can be connected to their respective cavities  111   a.    
   An insulation layer  113  is formed on the conductive layer  112 . The insulation layer  113  is formed on the entire surface of the rear substrate  110  so that not only the top surface of the conductive layer  112  but also the rear substrate  110  exposed between the cathode electrodes  111  can be covered with the insulation layer  113 , as shown in  FIG. 3 . The insulation layer  113  may be formed to a thickness of about 10-20 μm by coating a paste-type insulating material on the rear substrate  110  using a screen printing method and plasticizing the insulating material at a predetermined temperature. Second apertures  113   a  having width W 2  are formed in the insulating layer  113  to perforate the insulating layer  113  so that they can be connected to their respective first apertures  112   a . The second apertures  113   a  may be formed as rectangles that extend longer in the longitudinal direction of the cathode electrodes  111  (i.e., the Y direction) rather than in the latitudinal direction (i.e., the X direction) so that the second apertures  113   a  can form a one-to-one correspondence with the pixels  125 . In addition, the second apertures  113   a  are formed to have a width W 2 , which is larger than the width W 1  of the first apertures  112   a . Accordingly, the conductive layer  112  is partially exposed through the second apertures  113   a.    
   A plurality of gate electrodes  114  are formed on the insulation layer  113  at regular intervals in a predetermined pattern, for example, as stripes. The gate electrodes  114  extend in a direction perpendicular to the longitudinal direction of the cathode electrodes  111  (the Y direction), i.e., in the X direction. The gate electrodes  114  may be formed by depositing a conductive metal, e.g., chrome (Cr), on the insulation layer  113  using a sputtering method and patterning the conductive metal into stripes. Third apertures  114   a  having width W 3 , which are connected to their respective second apertures  113   a , are each formed in and perforate the gate electrodes  114 . The third apertures  114   a  have the same shape as the second apertures  113   a . The third apertures  114   a  may have a width W 3 , which is the same as the width W 2  of the second apertures  113   a  as in  FIG. 3  or a width greater than W 2  as in  FIG. 6 . 
   The emitters  115  are formed on each of the exposed portions of the cathode electrodes  111  exposed through the first apertures  112   a . The emitters  115  are formed to have a smaller thickness than the conductive layer  112  and are formed to be flat on the cathode electrodes  111 . The emitters  115  emit electrons when affected by an electric field generated by voltage applied between the cathode electrodes  111  and the gate electrodes  114 . In the present invention, the emitters  115  are formed of a carbon-based material, for example, graphite, diamond, diamond-like carbon (DLC), fulleren (C 60 ), or carbon nano-tubes (CNTs). Preferably, the emitters  115  are formed of CNTs, in particular, so that they can smoothly emit electrons even at a low driving voltage. 
   In the present embodiment of  FIGS. 3 and 4 , the emitters  115  are disposed at either side of each of the first apertures  112   a  so that they are a predetermined distance apart from each other. For example, two emitters  115  may be disposed in a first aperture  112   a  in contact with side surfaces of exposed portions of the conductive layer  112 . The emitters  115  may be formed as parallel bars extending in the longitudinal direction of the first apertures  112   a  (i.e., the Y direction). Accordingly, the emitters  115  have a larger area than the emitters of  FIGS. 1A ,  1 B,  2 A,  2 B and Macaulay &#39;659, and thus can guarantee a longer lifetime than those of  FIGS. 1A ,  1 B,  2 A,  2 B and Macaulay &#39;659 when driven for a long time. In addition, in a case where the cavity  111   a  is formed between the emitters  115 , as described above, a distance between the emitters  115  is smaller than the width W 1  of each of the first apertures  112   a  but larger than the width W c  of each of the cavities  111   a.    
   The emitters  115  may be formed in various manners. For example, in a first method, the emitters  115  may be formed by coating a photosensitive CNT paste on the top surface of the rear substrate  110 , applying ultraviolet (UV) rays to the bottom surface of the rear substrate  110  to selectively expose the photosensitive CNT paste, and developing the photosensitive CNT paste. In this case, the cathode electrodes  111  should be formed of a transparent conductive material, i.e., ITO, and the conductive layer  112  and the insulation layer  113  should be formed of an opaque material. 
   Alternatively, in a second method, the emitters  115  may be formed in the following manner. A catalyst metal layer of Ni or Fe is formed on the top surface of each of the cathode electrodes  111  exposed through the first aperture  112   a , and CNTs are vertically grown from the surface of the catalyst metal layer by supplying a carbon-based gas, such as CH 4 , C 2 H 2 , or CO 2 , to the catalyst metal layer. Still alternatively, in a third method, the emitters  115  may be formed by depositing photoresist in the first aperture  112   a , patterning the photoresist so that the photoresist can remain only on predetermined portions of the top surfaces of the cathode electrodes  111  where the emitters  115  are to be formed, coating a CNT paste on the remaining photoresist, and heating the rear substrate  110  to a predetermined temperature to enable the CNT paste to thermally react to the remaining photoresist. The second and third methods of forming the emitters  115  are free from the restriction of the first method of forming the emitters  115  as to the materials of the cathode electrodes  111 , the conductive layer  112  and the insulation layer  113 . 
   Turning now to  FIGS. 5A ,  5 B and  5 C,  FIGS. 5A ,  5 B, and  5 C illustrate three examples of the conductive layer  112  formed on one of the cathode electrodes  111 . Referring to  FIG. 5A , conductive layers  112  are respectively formed at both sides of a cathode electrode  111  to extend in the longitudinal (+/−y) direction of the cathode electrode  111 , in which case, a first aperture  112   a  is formed between the conductive layers  112 . Emitters  115  are formed between the conductive layers  112  to have a predetermined length in the longitudinal (+/−y) direction of the conductive layers  112  and contact side surfaces of the conductive layers  112 . A cavity  111   a  is formed in the cathode electrode  111  between the emitters  115  to have the same length as the emitters  115 . 
   Referring to  FIG. 5B , conductive layers  112  are formed at either side of a cathode electrode  111  to have a predetermined length, and a first aperture  112   a  is formed therebetween. In the case of  FIG. 5B , the conductive layers  112  are illustrated as having the same length as emitters  115 . 
   Referring to  FIG. 5C , a conductive layer  112  is formed in the form of a closed polygon on a cathode electrode  111  so as to completely surround a first aperture  112   a . All of the four sidewalls of a first aperture  112   a  are defined by the conductive layer  112 . Accordingly, emitters  115  are completely surrounded by the conductive layer  112 . 
   Referring now to  FIGS. 3 and 4 , the structure formed on the front or second substrate  120  will now be discussed. An anode electrode  121  is formed on the bottom surface of the front substrate  120 , which faces the top surface of the rear substrate  110 , and fluorescent layers  122  are formed of R, G, and B fluorescent materials on the anode electrode  121 . The anode electrode  121  is formed of a transparent conductive material, such as ITO, so that visible rays emitted from the fluorescent layers  122  can pass therethrough. The fluorescent layers  122  are formed to extend in the longitudinal direction parallel to the cathode electrodes  111 , i.e., in the Y direction. 
   Black matrices  123  may be formed among the fluorescent layers  122  on the bottom surface of the front substrate  120  so as to improve contrast. A metallic thin layer  124  may be formed on the fluorescent layers  122  and on the black matrices  123 . The metallic thin layer  124  is formed of aluminium to have such a small thickness (e.g., several hundreds of Å) so that electrons emitted from the emitters  115  can easily pass therethrough. The R, G, and B fluorescent materials of the fluorescent layers  122  emit visible rays when excited by electron beams emitted from the emitters  115 , and the visible rays emitted from the R, G, and B materials of the fluorescent layers  122  are reflected by the metallic thin layer  124 . Thus, the amount of visible light radiated from the entire FED increases, and eventually, the brightness of the entire FED increases as well. In a case where the metallic thin layer  124  is formed on the front substrate  120 , the anode electrode  121  may not necessarily be formed because the metallic thin layer  124  can serve as a conductive layer, i.e., an anode electrode, when voltage is applied thereto. 
   The rear substrate  110  and the front substrate  120  are located a predetermined distance apart from each other so that the emitters  115  can face the fluorescent layers  122 . The rear substrate  110  and the front substrate  120  are bonded to each other by applying a sealing material (not shown) around them. As described above, the spacer  130  is disposed between the rear substrate  110  and the front substrate  120  so as to maintain the predetermined distance between the rear substrate  110  and the front substrate  120 . 
   The operation of the FED according to the preferred embodiment of the present invention will now be described. When predetermined voltages are applied to the cathode electrodes  111 , the gate electrodes  114 , and the anode electrode  121 , an electric field is formed among them so that electrons are emitted from the emitters  115 . At this time, a voltage of zero to minus dozens of volts, a voltage of several to dozens of volts, and a voltage of hundreds to thousands of volts are applied to the cathode electrodes  111 , the gate electrodes  114 , and the anode electrodes  121 , respectively. The conductive layer  112  is in contact with the top surface of the cathode electrodes  111 , and thus the same voltage applied to the cathode electrodes  111  is applied to the conductive layer  112 . The emitted electrons are converted into electron beams, and the electron beams are led to the fluorescent layers  122  so that they can eventually collide with the fluorescent layers  122 . As a result, the R, G, and B fluorescent materials of the fluorescent layers  122  are excited and emit visible rays. 
   As described above, since the emitters  115  are disposed at either side of each of the first apertures  112   a , electron beams, which are formed of electrons emitted from the emitters  115 , are focused rather than to be widely dispersed. In addition, since the conductive layer  112  is disposed at either side of the emitters  115 , the electron beams can be efficiently focused due to an electric field formed by the conductive layer  112 . 
   Moreover, the cavity  111   a  may be formed in each of the cathode electrodes  111  so that the emitters  115  can be surrounded by equipotential lines of an electric field formed around the emitters  115 . Due to the electric field, current density increases, and a peak in the current density is precisely located in each of the pixels  125  of the fluorescent layers  122 . It is possible to more efficiently focus electron beams by adjusting the width W c  of the cavity  111   a.    
   As described above, color purity of an image can be enhanced by improving the focusing of electron beams emitted from the emitters  115 , and the brightness of the image can be enhanced by precisely placing a peak in current density in each of the pixels  125 . Therefore, it is possible to realize an image with high picture quality. Advantages of the FED according to the preferred embodiment of the present invention will be described in greater detail later with reference to  FIGS. 11A through 13C . 
   Turning now to  FIG. 6 ,  FIG. 6  is a cross-sectional view of one variation of an FED according to the first embodiment of the present invention. Referring to  FIG. 6 , FED  106  is similar to FED  100  in  FIG. 3  except that the width W 3  of third aperture  114   a  is larger and thus not equal to the width W 2  of second aperture  113   a . By forming the third apertures  114   a  to have a larger width W 3  than the width W 2  of the second apertures  113   a , a distance between the cathode electrodes  111  and their respective gate electrodes  114  can be lengthened, and thus, the voltage withstanding characteristics of the FED according to the first embodiment of the present invention can be improved. 
   Turning now to  FIG. 7 ,  FIG. 7  illustrates yet another FED  107  according to the present invention, FED  107  being another variant of FED  100  of  FIG. 3 . Referring to  FIG. 7 , the FED  107  includes a conductive layer  112 ′ that may include an insulation material layer  1121  formed on each of the cathode electrodes  111  and a metal layer  1122  formed to cover the top surface and side surfaces of the insulation material layer  1121 , so that the metal layer  1122  is electrically connected to the cathode electrodes  111  so as to serve basic functions of the conductive layer  112 ′. More specifically, the conductive layer  112 ′ may be formed by forming the insulation material layer  1121  on each of the cathode electrodes  111  and forming the metal layer  1122  on the insulation material layer  1121  through a deposition, sputtering, or plating method. The metal layer  1122  can serve as a passivation layer that protects the conductive layer  112 ′ from an etchant when forming the second apertures  113   a  in the insulation layer  113  using the etchant. Therefore, it is possible to prevent damage to the conductive layer  112 ′ caused by the etchant that is used to make the second apertures  113   a . More specifically, the conductive layer  112  of  FIG. 6  may be damaged by the etchant because it is formed of a conductive paste. However, the conductive layer  112 ′ of  FIG. 7  is not aversely affected by the etchant because its surface is formed of the metal layer  1122 . 
   Turning now to  FIG. 8 ,  FIG. 8  illustrates yet another variant to FED  100  of  FIG. 3 . Referring to FED  108  in  FIG. 8 , an insulation material layer  1123  is formed on the cathode electrodes  111 , and a conductive layer  112 ″ is formed on the top surface of the insulation material layer  1123  so that the conductive layer  112 ″ can be disposed as much apart from the cathode electrodes  111  as the thickness of the insulation material layer  1123  and can be electrically isolated from the cathode electrodes  111  by the insulation material layer  1123 . Unlike FED  107 , conductive layer  112 ″ in FED  108  does not include the insulation material  1123 . Therefore, unlike FED  107  of  FIG. 7 , conductive layer  112 ″ is not electrically connected to the cathode electrode  111 . In this case, the conductive layer  112 ″ may be connected to a different power source from a power source connected to the cathode electrodes  111 , and thus a different voltage from a voltage applied to the cathode electrodes  111  can be applied to the conductive layer  112 ″. Therefore, it is possible to maximize the electron beam-focusing effect of the conductive layer  112 ″ by controlling the voltage applied to the conductive layer  112 ″ independently of the voltage applied to the cathode electrodes  111 . Accordingly, the conductive layer  112 ″ can serve as an independent electrode, i.e., a focusing electrode. 
   The conductive layer  112 ″ may be formed by forming the insulation material layer  1123  on the cathode electrodes  111  and depositing a conductive metallic material on the top surface of the insulation material layer  1123  through a sputtering or plating method. Since the conductive layer  112 ″ is formed of a metallic material rather than to be formed of a conductive paste, the conductive layer  112 ″ can be prevented from being damaged by an etchant used in an etching process for forming the second apertures  113   a  in the insulation layer  113 . 
   The rest of the elements of the FED  108  of  FIG. 8  are the same as their respective counterparts of the FED  100  of  FIG. 3  except that the first apertures  112   a  are formed in the insulation material layer  1123  and in the conductive layer  112 ″ at regular intervals and the emitters  115  disposed in each of the first apertures  112   a  are formed in contact with side surfaces of the insulation material layer  1123  exposed through each of the first apertures  112   a . In the FED  108  of  FIG. 8 , a longitudinal end of the conductive layer  112 ″ may be electrically connected to each of the cathode electrodes  111 , in which case, the same voltage can be applied to the conductive layer  112 ″ and the cathode electrodes  111 . 
     FIG. 9  is a plan view of an FED  200  according to a second embodiment of the present invention. The FED according to the second embodiment of the present invention has the same cross-sectional structure as the FED according to the first embodiment of the present invention, and thus a cross-sectional view of the FED according to the second embodiment of the present invention will not be presented. 
   Referring to  FIG. 9 , in each pixel  225 , a plurality of first apertures  212   a , for example, two first apertures  212   a  are formed in a conductive layer  212 , two second aperture  213   a  are formed in an insulation layer  213 , and two third apertures  214   a , are formed in a gate electrode  214 . Emitters  215  are formed in each of the first apertures  212   a . Unlike FED  100  of  FIG. 3 , there is now more than one set of apertures for each pixel in FED  200 . The emitters  215 , like the emitters  115  in the first embodiment of the present invention, are formed on a cathode electrode  211  and exposed through the first aperture  212   a . In addition, the emitters  215  are disposed at either side of each of the first apertures  212   a  so that they are at a predetermined distance apart from each other. A plurality of cavities  211   a , for example, two cavities  211   a , may be formed in the cathode electrode  211  corresponding to each pixel  225 . 
   Other elements of the FED  200  according to the second embodiment of the present invention are the same as their respective counterparts of the FED  100  according to the first embodiment of the present invention, and thus their descriptions will be omitted. The variations of the FED according to the first embodiment of the present invention, shown in  FIGS. 6 ,  7 , and  8 , may also be applied to the FED  200  according to the second embodiment of the present invention. 
     FIGS. 10A and 10B  are a plan views of an FED  300  according to a third embodiment of the present invention.  FIG. 10A  focusses on a single emitter and  FIG. 10B  shows how may circular emitter structures correspond to a single pixel  325 . The FED  300  according to the third embodiment of the present invention has the same cross-sectional structure as the FED  100  according to the first embodiment of the present invention, and thus a cross-sectional view of the FED  300  according to the third embodiment of the present invention will not be presented. 
   Referring to  FIG. 10A , a first aperture  312   a  formed in a conductive layer  312 , a second aperture  313   a  formed in an insulation layer  313 , and a third aperture  314   a  formed in a gate electrode  314  are all circular in shape instead of rectangular as in the first embodiment. An inner diameter D 3  of the third aperture  314   a  and an inner diameter D 2  of the second aperture  313   a  are larger than an inner diameter D, of the first aperture  312   a . In addition, the inner diameter D 3  of the third aperture  314  may be the same as the inner diameter D 2  of the second aperture  313   a.    
   An emitter  315 , which is ring-shaped, is formed on a cathode electrode  311  exposed through the first aperture  312   a  along an inner circumference of the first aperture  312   a . An inner diameter D E  of the emitter  315  is smaller than the inner diameter D 1  of the first aperture  312   a . The emitter  315 , like the emitters  115  in the first embodiment of the present invention, may be formed of a carbon-based material, e.g., CNTs. 
   In the third embodiment of the present invention, like in the first embodiment of the present invention, a cavity  311   a , which is circular, may be formed to perforate the cathode electrode  311 . The cavity  311   a  is disposed inside the emitter  315 . Therefore, an inner diameter DC of the cavity  311   a  is smaller than the inner diameter D, of the first aperture  312   a  and the inner diameter DE of the emitter  315 . 
   In the third embodiment of the present invention as illustrated in  FIG. 10B , a plurality of first apertures  312   a , a plurality of second apertures  313   a , and a plurality of third apertures may be provided for each pixel  325 , in which case, the emitter  315  is formed in each of the plurality of first apertures  312   a . The rest of the elements of the FED  300  according to the third embodiment of the present invention are the same as their respective counterparts of the FED  100  according to the first embodiment of the present invention, and thus their descriptions will be omitted. 
   The variations of the FED according to the first embodiment of the present invention, shown in  FIGS. 6 ,  7 , and  8 , may also be applied to the FED according to the third embodiment of the present invention. In other words, the inner diameter D 3  of the third aperture  314   a  formed in a gate electrode  314  may be larger than the inner diameter D 2  of the second aperture  313   a  formed in the insulation layer  313 , and the conductive layer  312  may include an insulation material layer formed on the cathode electrode  311  and a metal layer formed on the insulation material layer. In addition, the conductive layer  312  may be formed on the top surface of the insulation material layer, which is formed on the cathode electrode  311 . 
   It is to be appreciated that features from various embodiments and from various variations of embodiments may be mixed and matched to form an FED within the scope of the present invention. The aperture sizes may be rectangular, circular, have a one-to-one correspondence with the pixels or have a many-to-one correspondence with the pixels, the relative sizes of the apertures may vary and the presence or absence of a cavity are all within the scope of the present invention. 
   Empirical simulation results of an FED according to a preferred embodiment of the present invention and the FEDs of  FIGS. 1A and 1B  will now be described in the following paragraphs. In electron beam emission simulations, the FED  90  of  FIGS. 1A and 1B  and the FED  100  according to the first embodiment of the present invention, shown in  FIG. 3 , were respectively selected for an empirical comparison. More specifically, the FEDs according to the first through third embodiments of the present invention have almost the same cross-sectional structure and thus have almost the same electron beam emission characteristics, and thus, the FEDs of  FIGS. 3 ,  6 ,  7 , and  8  were selected as exemplary embodiments of the present invention for the electron beam emission simulations. Therefore, the FEDs according to the first embodiment and their variations were empirically tested and test results for the FEDs  200  and  300  according to the second and third embodiments are not shown as they are essentially the same as that of the first embodiment. 
   Before the simulations, design dimensions of the FED&#39;s tested were fixed. For example, screens of the FED  90  of  FIGS. 1A and 1B  and the FEDs according to the first embodiment of the present invention were each set to have an RGB trio pitch of about 0.69 mm in a case where they were designed to have an aspect ratio of 16:9, a diagonal line length of 38 inches, and a horizontal resolution of 1280 lines so as to realize high definition (HD)-level picture quality. In this case, in the FED according to the first embodiment of the present invention, an insulation layer  113  is preferably set to have a height of 10-20 μm, a conductive layer  112  is preferably set to have a height of 2-5 μm, first apertures  112   a  formed in the conductive layer  112  are preferably set to have a width W 1  of 60-80 μm, second apertures  113   a  formed in the insulation layer  113  are preferably set to have a width W 2  of 70-90 μm, third apertures  114   a  formed in gate electrodes  114  are preferably set to have a width W 3  of 70-95 μm, and cavities formed in cathode electrodes  111  are preferably set to have a width W c  of 10-30 μm. However, the above-mentioned elements of the FED according to the first embodiment of the present invention may have different measurements from those set forth herein, depending on the size, aspect ratio, and resolution of the screen of the FED according to the first embodiment of the present invention. 
     FIGS. 11A through 11C  illustrate electron beam emission simulation results of the FED  90  of  FIGS. 1A and 1B . Referring to  FIG. 11A , an electron beam emitted from an emitter  16  of the FED  90  disperses widely toward fluorescent layers  23  of the FED  90 . The vertical axis in  FIG. 11B  represents current density. Referring to  FIG. 11B , peaks in the current density are located near the edges of a pixel, rather than the center of the pixel, because most electrons are emitted from the edges of the emitters  16 , as described above. If a central portion of the pixel has a low current density, fluorescent materials of the pixel cannot be sufficiently excited, thereby decreasing the brightness of an image displayed on the screen of the FED  90 . Particularly, in a case where emitters are not exactly arranged where they are supposed to be arranged, or in a case where front  21  and rear  11  substrates of the FED  90  are not precisely aligned with each other when bonding them together, peaks in current density are likely to be located near the edges of each pixel of the FED  90 , which results in a considerable decrease in color purity. Referring to  FIG. 11C , the spot of an electron beam arriving at a fluorescent layer of the FED undesirably encroaches upon another pixel. In short, the FED  90  of  FIGS. 1A and 1B  may end up in low color purity and low picture quality. 
     FIGS. 12A through 12C  illustrate electron beam emission simulation empirical results of the FED  100  according to the first embodiment of the present invention as shown in  FIG. 3 , modified for the case where there is no cavity  111   a  perforating cathode electrode  111  (hereinafter referred to as modified FED  100 ). Referring to  FIG. 12A , electron beam emitted from emitters  115  that are respectively arranged at both sides of a first aperture  112   a  of this modified FED  100  according to the first embodiment of the present invention are more focused and less dispersed than the electron beams of FED  90  of  FIGS. 1A and 1B . This improvement in the electron beam of the modified FED  100  is caused by the electric field formed by the conductive layer  112 . Referring to  FIG. 12B , peaks in current density are generally located in a central portion of a pixel, unlike the empirical results of FED  90  illustrated in  FIG. 11B . 
   Accordingly, as shown in  FIG. 12C , the size of the spot of an electron beam arriving at a fluorescent layer is much smaller in this modified FED  100  than in FED  90 , and thus it is possible to solve the problem of the FEDs of  FIGS. 1A ,  1 B,  2 A,  2 B and Macauley &#39;659 that an electron beam aimed at one pixel encroaches upon another pixel as well. Even though current density is generally lower in the electron beam of modified FED  100  than in FED  90 , color purity of an image is higher for modified FED  100  than for FED  90  because the focusing characteristics of electron beams emitted from the emitters  115  of the modified FED  100  according to the first embodiment of the present invention are considerably improved, compared to FED  90  of  FIGS. 1A and 1B . In addition, since peaks in the current density are located in a central portion of each pixel for modified FED  100 , the brightness of an image displayed on the screen of the modified FED  100  according to the first embodiment of the present invention can be compensated for. 
   Turning to  FIGS. 13A ,  13 B and  13 C,  FIGS. 13A through 13C  illustrate electron beam emission simulation empirical results of the FED  100  according to the first embodiment of the present invention, shown in  FIG. 3 , in a case where there is a one-to-one correspondence between cavities  111   a  perforating cathode electrode  111  and pixels  125 . 
   Referring to  FIG. 13A , due to the cavity  11   a  formed in each cathode electrode  111  of the FED  100  of  FIG. 3 , an electric field is formed around the emitters  115  so that the emitters  115  can be surrounded by equipotential lines of the electric field. Due to the electric field, electron beams emitted from the emitters  115  that are respectively disposed at both sides of a first aperture  112   a  can be efficiently focused proceeding toward fluorescent layers  122 . 
   Referring to  FIG. 13B , a peak in current density is precisely located in a central portion of a pixel. Accordingly, as shown in  FIG. 13C , the size of the spot of an electron beam arriving at a fluorescent layer  122  is much smaller in a case where a cavity  111   a  is formed in each cathode electrode  111  of the FED  100  according to the first embodiment of the present invention than in a case where no cavity  111   a  is formed in each cathode electrode  111  of the corresponding modified FED  100 . In addition, current density is higher in a case where a cavity  111   a  is formed in each cathode electrode  111  of the FED  100  according to the first embodiment of the present invention than in a case where no cavity  111   a  is formed in each cathode electrode  111  of the corresponding modified FED  100  as well as the FEDs of  FIGS. 1A ,  1 B,  2 A and  2 B. Therefore, by forming a cavity  111   a  in each cathode electrode  111  of an FED, it is possible to enhance the focusing characteristics of electron beams, increase current density, place a peak in the current density in a central portion of each pixel of the FED, and eventually improve the color purity and brightness of the FED. 
   Turning now to  FIGS. 14A ,  14 B and  14 C,  FIGS. 14A through 14C  illustrate electron beam emission simulation empirical results of the FED  100  according to the first embodiment of the present invention, shown in  FIG. 3 , in a case where the width Wc of the cavity  111   a  formed in each cathode electrode  111  of the corresponding FED has been changed so that it is larger than the FEDs whose results are shown in  FIGS. 13A ,  13 B and  13 C. 
   Referring to  FIG. 14A , an electric field is formed around the emitters  115  so that the emitters  115  can be better surrounded by equipotential lines of the electric field than in FIG.  12 A. Referring to  FIG. 14B , a peak in current density is precisely located in a central portion of a pixel. Accordingly, as shown in  FIG. 14C , the size of the spot of an electron beam arriving at a fluorescent layer  122  is much smaller than in  FIG. 13C . In addition, the current density is also much higher in  FIG. 14C  than in  FIG. 13C . Therefore, by adjusting the width Wc of a cavity  111   a  formed in each cathode electrode  111  of FED  100 , it is possible to considerably increase current density, efficiently focus electron beams, and eventually realize high quality images. 
     FIGS. 15A ,  15 B, and  15 C are diagrams illustrating empirical results of electron beam emission simulation results of the FED  107  of  FIG. 7 . Referring to  FIG. 15A , due to a conductive layer  112 ′, which is formed of an insulation material layer  1121  and a metal layer  1122 , and a cavity  111   a , which is formed in a cathode electrode  111 , an electric field is formed around emitters  115  so that the emitters  115  can be surrounded by equipotential lines of the electric field. Accordingly, electron beams emitted from the emitters  115  can be efficiently focused. Therefore, as shown in  FIG. 15B , peaks in current density are precisely located in their respective pixels. In addition, as shown in  FIG. 15C , the size of a spot of an electron beam on a fluorescent layer  122  is very small. As described above, the FED  107  of  FIG. 7  can have the same effects as the FED  100  of FIG. 
     FIGS. 16A and 16B  are diagrams illustrating electron beam emission simulation results of the FED  108  of  FIG. 8 . Referring to  FIGS. 16A and 16B , the FED  108  of  FIG. 8 , in which a conductive layer  112 ″ is formed on the top surface of an insulation material layer  1123  so that it can be insulated from a cathode electrode  111 , has the same effects as the FEDs  100  and  107  of  FIGS. 3 and 7 . The FED  108  of  FIG. 8  can focus electron beams more efficiently than the FEDs  100  and  107  of  FIGS. 3 and 7  by adjusting a voltage applied to the conductive layer. 
   As described above, the FEDs according to the present invention can improve the focusing characteristics of electron beams emitted from emitters resulting in increased color purity of images and thus realize high quality images. In addition, the FED according to the present invention can improve the brightness of images by precisely placing a peak in current density in each pixel. 
   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.