Abstract:
An electron emission device with conductive layers for preventing accumulation of static charges on an insulating layer of the device is shown that does not require an independent driving circuit. The device includes cathode electrodes formed on a substrate and separated from gate electrodes by an insulating layer formed over the cathode electrodes, all inside a partial vacuum chamber. Crossings of cathode and gate electrodes form the display areas while in the non-display areas of the insulating layer, that are susceptible to accumulation of electrostatic charge, conductive layers are formed parallel to the cathode or gate electrodes, for the most part separated from these electrodes by the insulating layer. Outside the device chamber, the conductive layers are electrically coupled to their corresponding electrodes. Conductive layers thus formed and coupled discharge accumulated static charge over the insulating layers inside the device to the outside circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0038989 filed on May 31, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electron emission device, and in particular, to an electron emission device which has an electrode structure for preventing the electrostatic charges from being accumulated on the insulating layer. 
   2. Description of Related Art 
   Generally, electron emission devices are classified into a first type where a hot cathode is used as an electron emission source, and a second type where a cold cathode is used as the electron emission source. The cold cathode electron emission devices, in turn, include field emitter array (FEA) devices, surface conduction emitter (SCE) devices, metal-insulator-metal (MIM) devices, metal-insulator-semiconductor (MIS) devices, and ballistic electron surface emitter (BSE) devices. 
   Electron emission devices may have different structures depending on their specific type. However, most types include two substrates separated by some form of a spacer and forming a vacuum chamber in the space between the two substrates. An electron emission structure with driving electrodes is formed at one of the substrates to emit electrons. Phosphor layers and an electron accelerating electrode are formed on the other substrate to emit light and display the desired images. The driving electrodes are usually formed with two electrodes placed perpendicular to each other. 
   The rate of electron emission is controlled through operating the driving electrodes by the well-known matrix address technique. An insulating layer is formed between the first and the second electrodes to electrically insulate the two from each other. The substrate with the electron emission structure, and the substrate with the phosphor layers are usually parallel to each other with a distance in between. A sealing material, such as a frit, is used to seal the substrates to each other to form the vacuum chamber. The vacuum chamber, thus formed, is partitioned into a display area and a non-display area. 
   In electron emission devices with the above conventional structures, the insulating layer in the display area is usually covered with one or two electrodes. On the other hand, the insulating layer in the non-display area around the frit-coated sealing line is not covered by electrodes while being exposed to the vacuum inside the chamber. As a result of this structure, static charges are accumulated on the insulating layer of conventional electron emission devices in the non display areas and cause device failures such as abnormal operation, arcing, and flashover. 
   In order to prevent these problems, U.S. Pat. No. 5,929,560 discloses a field emission display device where an ion shield layer is formed on the insulating layer in the non-display area to prevent the accumulation of static charges on the insulating layer. The ion shield layer is electrode layer supplied with a voltage independently from the electrodes placed at the display area, and prevents static charges from accumulating on the insulating layer in the non-display area. 
   In conventional techniques, including the ion shield technique explained above, because the ion shield layer receives its driving voltage from an IC separate from the IC used for driving the emission electrode, the number of structural components and therefore the cost of production, are increased. 
   SUMMARY OF THE INVENTION 
   In one exemplary embodiment of the present invention, there is provided an electron emission device which prevents the static charges from being accumulated on the insulating layer without introducing a separate driving IC. 
   In an exemplary embodiment of the present invention, an electron emission device includes first electrodes formed on a substrate with a predetermined pattern, and an insulating layer formed on the substrate while covering the first electrodes. Second electrodes are formed on the insulating layer with a predetermined pattern. At least two conductive layers are formed at the periphery of the insulating layer parallel to the first electrodes while partially covering the insulating layer. The conductive layers are electrically coupled to the first electrodes. 
   The conductive layers are in one to one correspondence with the first electrodes. The respective conductive layers are electrically connected to the corresponding first electrodes. 
   The first electrode has an end portion exposed to the outside of the insulating layer, and the conductive layer contacts the lateral side of the insulating layer as well as the top surface of the first electrode. 
   The electron emission device further includes electron emission regions electrically connected to one of the first and the second electrodes. 
   The second electrode and the insulating layer have opening portions partially exposing the first electrode, and the electron emission regions are formed on the first electrode within the opening portions. The electron emission regions contact the second electrodes. 
   In another exemplary embodiment of the present invention, an electron emission device includes first and second substrates facing each other, and first electrodes formed on the first substrate with a predetermined pattern. An insulating layer is formed on the first substrate while covering the first electrodes. Second electrodes are formed on the insulating layer with a predetermined pattern. At least two conductive layers are formed on the periphery of the insulating layer parallel to the first electrodes while partially covering the insulating layer. The conductive layers are electrically connected to the first electrodes. At least a third electrode is formed on the second substrate. Phosphor layers are formed on a surface of the third electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified diagram showing, in perspective, a portion of one embodiment of an electron emission device constructed in accordance with the invention. 
       FIG. 2  is a simplified diagram of a partial cross-sectional view of one embodiment of an electron emission device constructed in accordance with the invention. 
       FIG. 3  is a simplified diagram showing, in perspective, a portion of a second embodiment of an electron emission device constructed in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   As seen in  FIG. 1 , in one embodiment the electron emission device  100  includes first substrate  2  and second substrate  4  parallel to each other. The substrates  2 ,  4  are assembled by attaching them to each other via a sealing member  20  leaving a distance in between the substrates  2 ,  4 . The inner space between the substrates  2 ,  4  is exhausted to be in a partial vacuum state hence creating a vacuum chamber between the substrates. 
   As a set of first electrodes, a number of cathode electrodes  6  are formed, in a stripe pattern, on the first substrate  2 . Stripes of cathode electrodes  6  are spaced apart from one another and are formed, for example, along the y-axis of the drawing in  FIG. 1 . An insulating layer  8  is formed on the surface of the first substrate  2  covering the cathode electrodes  6 . A number of gate electrodes  10  are formed on the insulating layer  8 , in another stripe pattern, as a set of second electrodes. Stripes of gate electrodes  10  are spaced apart from one another and run along a direction perpendicular to the direction of cathode electrodes  6  stripes. For example, if the cathode electrodes  6  run along the y-axis in the drawing of  FIG. 1 , then the gate electrodes  10  run along the x-axis of the same drawing. The regions where the cathode electrodes  6  and the gate electrodes  10  cross paths are called pixel regions. The area of the substrate  2  where the pixel regions are located, and where, thereby, electron emissions are substantially realized, is called the display area. Non-display area may not correspond to the display area. In some embodiments, the non-display area may correspond to the regions near the margins and perimeter of the vacuum chamber where the two substrates are attached together. 
   Conductive layers  22  cover portions of the insulating layer  8  and are electrically coupled to the cathode electrode  6  outside the vacuum chamber. In one embodiment a number of conductive layers  22  may be formed on the portions of the insulating layer  8  in the non-display areas. For example, the conductive layers  22  may be formed in stripes over the insulating layer  8  proceeding in a direction perpendicular to the gate electrodes  10 . In some example embodiments, the stripes of conductive layers  22  stop near the inner perimeter of the vacuum chamber and do not reach the gate electrodes  10 . In this embodiment, the conductive layers  22  may be parallel to the cathode electrodes  6  running along and over the cathode electrodes  6  where the cathode electrodes run under the insulating layer  8  and the conductive layers  22  run over the insulating layer  8 . There may be a one to one correspondence between the conductive layers  22  and the cathode electrodes  6 . 
   The areas of highest concern for accumulation of static charges are the non-display areas. Some of the non-display areas may be located near the perimeter of the vacuum chamber where the insulating layer  8  may be exposed and may accumulate charge without an opportunity to discharge the charge through metal or other conductive material. As a result, in some embodiments, the conductive layers  22  may not extend along the entire length of the cathode electrodes  6 . The conductive layers  22  shown in  FIG. 1  extend only partially into the vacuum chamber and stay generally near the inner perimeter of the chamber. 
   Red, green and blue phosphor layers  14  are arranged on a surface of the second substrate  4  facing the first substrate  2  with a distance in between. Black layers  16  are located between the phosphor layers  14  to enhance screen contrast. As a third set of electrodes, an anode electrode  18  is formed by depositing a conductive layer, for example a metallic layer based on aluminum, over the phosphor layers  14  and the black layers  16 . The anode electrode  18  is coupled to a high voltage required for accelerating electron beams and heightens screen brightness generated by the phosphor layer  14  through creating a metal back effect. 
     FIG. 2  is a cross-sectional view of the electron emission device  100  of  FIG. 1  in the yz plane of these drawings, cutting along cathode electrodes  6  and across gate electrodes  8 . As seen in  FIG. 2 , in each pixel region, one or more holes or wells, referred to as gate wells  8   a ,  10   a  are formed. The gate wells start in the gate electrodes  10  and end in the insulating layer  8  and are hence referred to as  10   a  corresponding to the portion of the well in the gate electrode  10 , or  8   a  corresponding to the portion in the insulating layer  8 . Gate wells  8   a ,  10   a  are capable of partially exposing the cathode electrode  6 . 
   Electron emission regions  12  may be formed on the cathode electrode  6  within the gate wells  8   a ,  10   a . In one embodiment, the electron emission regions  12  may be comprised of a material capable of emitting electrons under the application of an electric field. For example, the electron emission regions  12  may be formed with carbon nanotube, graphite, graphite nanofiber, diamond, diamond-like carbon, C60, silicon nanowire, composites of these material, or like material. The formation of the electron emission regions  12  may be made by direct growing, screen printing, chemical vapor deposition, sputtering, or similar processes. As also seen in  FIG. 2 , the end portions of the conductive layers  22  are extended to the outside of the sealing member  20  while spreading over the lateral side of the insulating layer  8  and the top surface of the cathode electrodes  6 , where the conductive layers  22  contact the cathode electrodes  6 . 
   When driving voltages are applied to the cathode electrodes  6  and gate electrodes  10 , an electric field is formed around the electron emission regions  12  due to the voltage difference between the cathode electrodes  6  and gate electrodes  10 . Electrons are emitted from the electron emission regions  12  under the influence of the electric field thus created. The anode electrode  18  may be coupled to a high positive voltage required for accelerating electron beams generated in the emission regions  12 . Both the acceleration of the electrons and the metal back effect created by the anode increase screen brightness. 
   In another embodiment, the anode electrode  18  may be formed with a transparent conductive material such as indium tin oxide (ITO) instead of a metallic material. In this embodiment, first an anode electrode (not shown) is formed on the second substrate  4  with a transparent conductive material, then phosphor layers  14  and black layers  16  are formed on the anode electrode. If required, in some embodiments, a metallic layer may be formed on the phosphor layers  14  and the black layers  16  to increase the screen brightness. The anode electrode  18  may be formed on the entire surface of the second substrate  4 . In other embodiments, the anode electrode  18  may be formed only on parts of the second substrate  4  according to a predetermined pattern. 
   Conductive layers  22 , in the electron emission device  100 , may be used to prevent static charges from accumulating on the portions of the insulating layer  8  in the non-display areas. The conductive layers  22  cover the portions of the insulating layer  8  in the non-display area inside of the sealing member  20 , near the internal perimeter of the vacuum chamber, to prevent the static charges generated during the driving of the electron emission device from being accumulated on the insulating layer  8 . Because the conductive layers  22  are electrically coupled to the cathode electrodes  6 , the conductive layers  22  are driven and controlled by the driving IC for the cathode electrodes  6 . Accordingly, in this embodiment of the electron emission device  100 , the cathode electrodes  6  and the conductive layers  22  can be driven together with the basic electrode driving IC. 
   In one embodiment, the conductive layers  22  may be formed together with the gate electrodes  10  by depositing a conductive layer onto the insulating layer  8 , and patterning it. 
     FIG. 3  is a partial perspective view of another embodiment  200  of the electron emission device of the present invention. 
   As seen in  FIG. 3 , a number of gate electrodes  24  are arranged on a first substrate  2  with a distance in between the gate electrodes  24 , that are deposited or formed in parallel stripes. An insulating layer  8  is formed on the entire surface of the first substrate  2  over the gate electrodes  24 . The insulating layer  8  covers the gate electrodes  24 . A number of cathode electrodes  26  are formed on the insulating layer  8  spaced apart from one another. The cathode electrodes  26  are deposited or formed in parallel stripes that are perpendicular to the gate electrode  24  stripes. Electron emission regions  28  are formed on one side or edge of the cathode electrodes  26 . Electron emission regions  28  are formed within wells, depressions, indentations, notches, pits, or hollowed portions  26   a  formed on one edge of the cathode electrodes  26 . 
   In the embodiment of the electron emission device  200  shown in  FIG. 3 , conductive layers  30  are formed or placed over portions of the insulating layer  8  in the non-display area. The conductive layers  30  may cover the insulating layer  8  in the non-display area. The conductive layers  30  help prevent the accumulation of static charges on the insulating layer  8 . The conductive layers  30  extend on one side to the inside wall of the sealing member  20 , through the sealing member  20 , and to the outside of the vacuum chamber on the other side of the sealing member  20 , where the conductive layers  30  are electrically coupled to the gate electrodes  24  that were formed or placed under the insulating layer  8 . Accordingly, the conductive layers  30  may be driven by the driving IC for the gate electrodes  24 . In some embodiments, a separate driving IC may be used for the gate electrodes  24 . 
   As explained above, the connection between the conductive layers  30  and the gate electrodes  24  prevents static charges from accumulating on the insulating layer  8 . This, in turn, may help prevent problems related to the accumulation of the static charges, such as device abnormality, arcing, and flashover. 
   The electron emission device  100  and the method of preventing the accumulation of static charges may be used with any of the electron emission devices including, for example, FEA devices, SCE devices, MIM devices, MIS devices, BSE devices, or the like. 
   Although, the foregoing describes exemplary embodiments of the present invention, it should be understood that many variations or modifications of the basic inventive concept, taught here, will fall within the spirit and scope of the present invention as defined in the appended claims.