Abstract:
A novel protection structure for protecting field emission elements in a field emission display device from burnout damage due to electrical current surges induced to the device cathode by ionized gases in the device. The protection structure includes one or multiple reduction plates or electrodes which are typically provided on the cathode. The reduction plate or plates are negatively-charged and attract positively charged gas ions. Consequently, induction of electrical current surges to the cathode is avoided, thereby preventing burnout damage to the field emission elements.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to field emission display panels or devices, and more particularly, relates to a field emission display device having at least one reduction plate or electrode which deflects ionic emission gas away from the field emission components of the device to prevent damage to the field emission components.  
         [0003]     2. Background of the Invention  
         [0004]     In recent years, flat panel display devices have been developed and used in electronic applications such as personal computers. One of the popularly-used flat panel display devices is an active matrix liquid crystal display which provides improved resolution. However, liquid crystal display devices have many inherent limitations that render them unsuitable for a number of applications. For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield. Moreover, the liquid crystal display devices require a fluorescent back light which draws high power while most of the light generated is wasted. A liquid crystal display image may be difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications.  
         [0005]     Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels. One of such devices is a field emission display device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to conventional thin film transistor (TFT) liquid crystal display panels.  
         [0006]     A most drastic difference between an FED and an LCD is that, unlike the LCD, the FED utilizes colored phosphors to produce its own light. The FEDs do not require complicated, power-consuming back lights and filters and, as a result, almost all the light generated by an FED is visible to the user. Moreover, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.  
         [0007]     In an FED, electrons are emitted from a cathode and impinge on phosphors coated on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. In contrast to a conventional CRT device, each pixel or emission unit in an FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference exists between a cathode and a gate electrode which extracts electrons from the cathode and accelerates them toward the phosphor coating. The emission current, and thus, the display brightness, is strongly dependent on the work function of the emitting material. To achieve the necessary efficiency of an FED, the cleanliness and uniformity of the emitter source material are very important.  
         [0008]     In order for electrons to travel in an FED, most FEDs are evacuated to a low pressure such as 10 −7  torr in order to provide a log mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus grid to collimate electrons drawn from the microtips.  
         [0009]     In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized. In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide. The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well. A sacrificial material layer such as nickel is deposited to prevent deposition of nickel into the emitter wall. Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes there above. An emitter cone is left when the sacrificial layer of nickel is removed.  
         [0010]     In an alternative design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon, followed by patterning the oxide and selectively etching to form silicon tips. Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer. In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel displays. The microtips can be formed of conducting materials such as metals or doped semi-conducting materials. In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microtips is highly desirable. A proper resistivity of the interlayer enables the device to operate in a stable condition. In fabricating such FED devices, it is therefore desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n +  doped amorphous silicon. The conductivity of the n +  doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film.  
         [0011]     Generally, in the fabrication of an FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded. For instance, a low pressure of 10 −7  torr is normally required. In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels. For instance, in conventional FED devices, glass spheres or glass crosses have been used for maintaining such spacings in FED devices. Elongated spacers have also been used for such purposes.  
         [0012]      FIG. 1A  shows an enlarged cross-sectional view of a conventional field emission display device  10 . The FED device  10  is formed by depositing a resistive layer  12  of typically an amorphous silicon base film on a glass substrate  14 . An insulating layer  16  of a dielectric material and a metallic gate layer  18  are then deposited and formed together to provide metallic microtips  20  and a cathode structure  22  is covered by the resistive layer  12  and thus, a resistive but somewhat conductive amorphous silicon layer  12  underlies a highly insulating layer  16  which is formed of a dielectric material such as SiO 2 . It is important to be able to control the resistivity of the amorphous silicon layer  12  such that it is not overly resistive but yet, it will act as a limiting resistor to prevent excessive current flow if one of the microtips  20  shorts to the metal layer  18 .  
         [0013]     A completed FED structure  30 , including an anode  28  mounted on top of the structure  30 , is shown in  FIG. 1B . It is to be noted, for simplicity, that the cathode layer  22  and the resistive layer  12  are shown as a single layer  22  for the cathode. The microtips  20  are formed to emit electrons  26  from the tips of the microtips  20 . The gate electrodes  18  are provided with a positive charge, while the anode  28  is provided with a higher positive charge. The anode  28  is formed by a glass plate  36  which is coated with phosphorous particles  32 . An intermittent conductive indium-tin-oxide (ITO) layer  34  may also be utilized to further improve the brightness of the phosphorous layer when bombarded by the electrons  26 . This is shown in a partial, enlarged cross-sectional view of  FIG. 1C . The total thickness of the FED device is only about 2 mm, with vacuum pulled in-between the lower glass plate  14  and the upper glass plate  36  sealed by sidewall panels  38  (shown in  FIG. 1B ).  
         [0014]     The conventional FED devices formed with microtips shown in  FIGS. 1A-1C  produce a flat panel display device of improved quality when compared to liquid crystal display devices. However, a major disadvantage of the microtip FED device is the complicated processing steps that must be used to fabricate the device. For instance, the formation of the various layers in the device, and specifically, the formation of the microtips, requires a thin film deposition technique that utilizes a photolithographic method. As a result, numerous photomasking steps must be performed in order to define and fabricate the various structural features in the FED. The CVD deposition processes and the photolithographic processes involved greatly increase the manufacturing cost of an FED device.  
         [0015]     In a co-pending application Ser. No. 09/377,315, filed Aug. 19, 1999, assigned to the common assignee of the present invention, a field emission display device and a method for fabricating such device of a triode structure using nanotube emitters as the electron emission sources were disclosed. In the triode structure FED device, the device is constructed by a first electrically insulating plate, a cathode formed on the first electrically insulating plate by a material that includes metal, a layer formed on the cathode of a high electrical resistivity material, a layer of nanotube emitters formed on the resistivity layer of a material of carbon, diamond or diamond-like carbon wherein the cathode, the resistivity layer and the nanotube emitter layer form an emitter stack insulated by an insulating rib section from adjacent emitter stacks, a dielectric material layer perpendicularly overlying a multiplicity of the emitter stacks, a gate electrode on top of the dielectric material layer, and an anode formed on a second electrically insulating plate overlying the gate electrode. The FED device proposed can be fabricated advantageously by a thick film printing technique at substantially lower fabrication cost and higher fabrication efficiency than the FEDs utilizing microtips.  
         [0016]     In another co-pending application Ser. No. 09/396,536, filed Sep. 15, 1999, assigned to the common assignee of the present invention, a field effect emission display device and a method for fabricating the diode structure device using nanotube emitters as the electron emission sources were disclosed. In the diode structure FED device, the device is constructed by a first glass plate that has a plurality of emitter stacks formed on a top surface. Each of the emitter stacks is formed parallel to a transverse direction of the glass plate and includes a layer of electrically conductive material such as silver paste and a layer of nanotube emitter on top. The first glass plate has a plurality of rib sections formed of an insulating material in-between the plurality of emitter stacks to provide electrical insulation. A second glass plate is positioned over and spaced-apart from the first glass plate with an inside surface coated with a layer of an electrically conductive material such as indium-tin-oxide. A multiplicity of fluorescent powder coating strips is then formed on the ITO layer each for emitting a red, green or blue light when activated by electrons emitted from the plurality of emitter stacks. The field emission display panel is assembled together by a number of side panels that join the peripheries of the first and second glass plate together to form a vacuum-tight cavity therein.  
         [0017]      FIGS. 2A and 2B  show a schematic view of a conventional FED device  40 . The FED device  40  includes a cathode  42  which is spaced from an anode  46 . Multiple field emission elements  44  are provided in electrical contact with the cathode  42  for emitting electrons  52  toward the anode  46 . A voltage source  48  is provided to apply a voltage potential which establishes an electric field  50  between the cathode  42  and the anode  46 .  
         [0018]     During operation of the FED device  40 , oxygen and nitrogen are typically present at low pressures between the cathode  42  and the anode  46 . When the FED device  40  is energized, a voltage potential is applied by the voltage source  48 , between the cathode  42  and the anode  46 , to establish the electric field  50 . High-energy electrons  52  are emitted from the field emission elements  44 , toward the anode  46 . These high-energy electrons  52  strike the nitrogen and oxygen gas and form positive nitrogen and oxygen ions, as shown in  FIG. 2B . The nitrogen and oxygen ions discharge to the cathode  42 , causing a surge of the electrical current passing to the cathode  42  and field emission elements  44 . This magnified electrical current tends to burn and damage the field emission elements  44 . Accordingly, a protection structure is needed for deflecting a discharge path of ionized gases away from a cathode in an FED device to prevent electrical surging and burn-out damage to field emission elements in the device.  
       BRIEF SUMMARY OF THE INVENTION  
       [0019]     An object of the present invention is to provide a novel protection structure for preventing burn-out damage to field emission elements in a field emission display device.  
         [0020]     Another object of the present invention is to provide a novel field emission display device provided with a protection structure having at least one reduction plate or electrode for altering the discharge path of ionized gases and preventing the gases from inducing an electrical surge which may otherwise cause burnout damage to field emission elements in the device.  
         [0021]     Still another object of the present invention is to provide a novel field emission display device having a protection structure which may include multiple reduction plates or electrodes interspersed among field emission elements on a cathode in the device to alter the discharge path of ionized gases in the device and prevent current-induced burnout damage to the field emission elements.  
         [0022]     A still further object of the present invention is to provide a novel field emission display device which includes a protection structure that substantially prolongs the lifetime of field emission elements in the device.  
         [0023]     Another object of the present invention is to provide a novel field emission display device having a protection structure which may include multiple, elongated reduction plates or electrodes that run parallel to and between rows of field emission elements in the device.  
         [0024]     Yet another object of the present invention is to provide a novel field emission display device having a protection structure that is arranged in a meshwork- or net-shaped configuration among field emission elements in the device.  
         [0025]     In accordance with these and other objects and advantages, the present invention is generally directed to a novel protection structure for protecting field emission elements in a field emission display device from burnout damage due to electrical current surges induced to the device cathode by ionized gases in the device.  
         [0026]     The structure includes one or multiple reduction plates or electrodes which are typically provided on the cathode. A voltage source is electrically connected to the reduction plate or plates to alter the discharge path of the ionized gases from the device cathode to the reduction plate or plates. Consequently, induction of electrical current surges to the cathode is avoided, thereby preventing burnout damage to the field emission elements.  
         [0027]     In a typical embodiment of the invention, multiple reduction plates or electrodes are interspersed among the field emission elements in the device. In one embodiment, the multiple reduction plates or electrodes are elongated and run parallel and adjacent to rows of field emission elements in the device. In another embodiment, the multiple reduction plates or electrodes are arranged in a meshwork- or net-shaped configuration among the field emission elements in the device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1A  is an enlarged, cross-sectional view of a cathode and field emission element structure of a conventional field emission display device;  
         [0029]      FIG. 1B  is a cross-sectional view of a complete conventional field emission display device structure;  
         [0030]      FIG. 1C  is a cross-sectional view of a conventional field emission display device, illustrating electron bombardment of a conductive layer on the anode of the device;  
         [0031]      FIG. 2A  is a schematic of a conventional field emission display device, illustrating ionization of oxygen and nitrogen gas in the device by high-energy electrons emitted by the field emission elements;  
         [0032]      FIG. 2B  is a schematic of the conventional field emission display device, as shown in  FIG. 2A , illustrating discharge of oxygen and nitrogen ions to the cathode of the device;  
         [0033]      FIG. 3  is a schematic of a field emission display device of the present invention, illustrating discharge of positive oxygen and nitrogen ions to a negatively-charged reduction plate or electrode;  
         [0034]      FIG. 4  is a perspective, partially schematic, view of one embodiment of the field emission display device of the present invention, illustrating elongated reduction plates or electrodes arranged parallel and adjacent to rows of field emission elements of the device; and  
         [0035]      FIG. 5  is a perspective, partially schematic, view of another embodiment of the field emission display device of the present invention, illustrating reduction plates or electrodes arranged in a meshwork- or net-shaped pattern among the field emission elements of the device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The present invention is directed to a field emission display device which includes a structure for deflecting the discharge path of gas ions away from a cathode. This prevents surges in electrical current from being drawn to the cathode and inducing burnout damage to field emission elements provided in electrical communication with the cathode. Consequently, the lifetime of the device is substantially prolonged.  
         [0037]     Referring initially to  FIG. 3 , wherein a schematic of a field emission device  54  according to the present invention is shown. The field emission device  54  includes a cathode  56  provided in electrical communication with multiple field emission elements  58 . Each of the field emission elements  58  may be any type of dischargeable tip suitable for emitting high-energy electrons, such as carbon nanotubes, for example. An anode  60  is disposed in spaced-apart relationship to the cathode  56  and the field emission elements  58 . The cathode  56  and the anode  60  may be any electrically-conducting metal. An operating voltage source  62  is electrically connected to the cathode  56  and the anode  60  to establish an electric field  64  there between.  
         [0038]     In accordance with the present invention, a protection structure  68  includes at least one reduction plate or electrode  70  which is provided in the field emission device  54 , typically on the cathode  56 . The reduction plate  70  is preferably any electrically-conductive metal. An insulation layer  72 , which is an electrically-insulating material, typically separates the reduction plate  70  from the cathode  56 . A bias voltage source  74  is electrically connected to the reduction plate  70  for applying a negative voltage thereto, as hereinafter further described.  
         [0039]     In operation of the FED device  54 , the operating voltage source  62  applies an operating voltage potential of typically about 1000V between the cathode  56  and the anode  60 , to establish the electric field  64 . Simultaneously, the bias voltage source  74  applies a negative bias voltage of typically about −1 to −30 V to the reduction plate  70 . High-energy electrons  66  are emitted from the field emission elements  58  and strike a phosphors target (not shown) provided on the anode  60 , to emit light from the target. These high-energy electrons  66 , in transit from the field emission elements  58  to the target, strike molecular nitrogen and oxygen in the device  54 , thereby ejecting electrons from the nitrogen and oxygen and forming N +  and O +  ions.  
         [0040]     Due to the negative charge of the reduction plate  70 , applied by the bias voltage source  74 , the N +  and O +  ions are deflected away from the cathode  56 , along a gas discharge path  76 , to the reduction plate  70 . Accordingly, the N +  and O +  ions are prevented from contacting the cathode  56 , thereby preventing ion-induced surges in electrical current to the cathode  56  which would otherwise tend to damage the field emission elements  58 . At the reduction plate  70 , the N +  and O +  ions are reduced back to molecular nitrogen and oxygen as follows: 
 
N 2   + +e − →N 2  
 
O 2   + +e−→O 2  
 
         [0041]     A first exemplary structure of FED device according to the present invention is illustrated in  FIG. 4 . As shown in  FIG. 4 , a FED device  80  includes a cathode plate  81  having a plurality of elongated, parallel cathode strips  82  thereon, anodes  84  spaced-apart from the cathode plate  81 , and an operating voltage source  85  electrically connected to the cathode strips  82  and anodes  84 . Multiple, spaced-apart field emission elements  83  are provided on each of the cathode strips  82 . Each of the field emission elements  83  may be any type of dischargeable tip suitable for emitting high-energy electrons, such as carbon nanotubes, for example.  
         [0042]     A protection structure  87  of the FED device  80  includes multiple, elongated reduction plates or electrodes  89  that are provided on the cathode plate  81 . The reduction plates  89  extend parallel and adjacent to the cathode strips  82  on which the field emission elements  83  are provided. A bias voltage source  90  is electrically connected to each reduction plate  89  of the protection structure  87  for applying a negative bias voltage to the reduction plate  89 . Accordingly, the negative bias voltage applied by the bias voltage source  90  imparts a negative charge to the reduction plates  89  which attracts positive nitrogen and oxygen ions thereto and prevents current-induced damage to the field emission elements  83 , as heretofore described with respect to the protection structure  68  of  FIG. 3 .  
         [0043]     The reduction plates  89  may be fabricated on the cathode plate  81  at the same as the cathode strips  82 . In manufacture, a metal material, i.e., the metal cathode plate  81 , is first deposited on a substrate (not shown), using conventional deposition techniques. Photolithography techniques are then used to form a first mask (not shown) which defines the location and geometry of the cathode strips  82  and the reduction plates  89  on the cathode plate  81 . The cathode plate  81  is then etched to form the cathode strips  82  and the reduction plates  89  according to the pattern defined by the first mask. A wet etching method may be used to precisely control the geometry and size of the cathode strips  82 . Next, a second mask (not shown) is formed on the cathode strips  82  and the reduction plates  89  to define the geometry and location of the field emission elements  83  on the cathode strips  82 , followed by etching and fabrication of the field emission elements  83 . In this structure, the reduction plates  89  and the cathode strips are formed on a same plane and are parallel and alternately spaced-apart. Each of the reduction plate  80  provides protection for its adjacent field emission elements  83 .  
         [0044]     In addition to the elongated and parallel structure described above, the reduction plates  89  can also be formed in a meshwork-shape or a net-shape according to another exemplary embodiment of the present invention, which is shown in  FIG. 5 . As shown in  FIG. 5 , an FED device  92  includes a cathode plate  93 ; multiple, elongated, parallel cathode strips  94  fabricated on the cathode plate  93 ; anodes  96  disposed in spaced-apart relationship to the cathode plate  93 ; and an operating voltage source  97  electrically connected to the cathode strip  94  and anodes  96 . Multiple field emission elements  95  are provided on each of the cathode strips  94  for emitting high-energy electrons toward the anode  96 .  
         [0045]     A meshwork-shaped or net-shaped protection structure  99  including reduction plates  101  is provided on the cathode plate  93  of the FED device. The reduction plates  101  are formed on the top of the cathode plate  93  and is separated from the cathode strips  94  by an insulation layer  100 . Accordingly, the reduction plates  101  along with the underlying insulation layer  100  impart a meshwork- or net-shaped configuration to the protection structure  99 .  
         [0046]     A bias voltage source  103  is electrically connected to the reduction plates  101  of the protection structure  99 . The bias voltage source  103  applies a negative voltage to the protection structure  99  to attract positive nitrogen and oxygen ions formed by the high-energy electrons emitted by the field emission elements  95 . This prevents the ions from contacting the cathode strips  94  and inducing surging of an excessive electrical current to the cathode strips  94  and field emission elements  95 , as heretofore described with respect to the FED device  54  of  FIG. 3 .  
         [0047]     The manufacturing of the FED device  92  is described below. Initially, a first metal layer is deposited on a substrate (not shown) to form the cathode plate  93 . A first mask (not shown) is then patterned on the cathode plate  93  to etch the cathode strips  94  therein. After the first mask is removed from the cathode plate  93 , the insulator layer  100  is deposited over the cathode plate  93  and cathode strips  94 . Next, a second metal layer for the reduction plates  101  is deposited on the insulator layer  100 , followed by formation of a second mask (not shown) using a negative photoresist to define the geometry and location of the light emission elements  95 . The second metal layer is then etched away the region for forming the field emission elements  95 , leaving the reduction plates  101 . Afterward, keeping the second mask unremoved, the regions where the second metal layer is removed are then deposited with materials for the light emission elements  95 . After the light emission elements  95  are formed, the structure of the FED device  92  as shown in  FIG. 5  is completed.  
         [0048]     The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.  
         [0049]     Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.