Patent Publication Number: US-2007096630-A1

Title: Field emission backlight unit and its method of operation

Description:
CLAIM OF PRIORITY  
      This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for FIELD EMISSION TYPE BACKLIGHT UNIT AND METHOD OF OPERATING THE SAME earlier filed in the Korean Intellectual Property Office on the 2 nd  of Nov. 2005 and there duly assigned Serial No. 10-2005-0104360.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a field emission backlight unit, and more particularly, to a field emission backlight unit with increased luminous efficiency and its method of operation.  
      2. Description of the Related Art  
      Flat panel displays can be generally divided into emissive displays and passive displays. Emissive displays include Cathode Ray Tubes (CRTs), Plasma Display Panels (PDPs), and Field Emission Displays (FEDs), and passive displays include Liquid Crystal Displays (LCDs). Of these displays, LCDs have the advantages of being light weight and having a low power consumption. However, they do not generate light. That is, the LCDs display an image using light from an external device. Therefore, the image cannot be seen in a dark place. To solve this disadvantage, backlight units are installed behind the LCDs.  
      Conventional backlight units mainly use Cold Cathode Fluorescent Lamps (CCFLs) as a line luminescence source and Light Emitting Diodes (LEDs) as a point luminescence source. However, conventional backlight units have high manufacturing costs due to their structural complexity, and have a high power consumption due to light reflection and transmittance being required since the light sources are located on one side of the backlight unit. In particular, as the size of an LCD increases, the achievement of uniform brightness is more difficult.  
      Recently, to solve the above drawbacks, flat field emission backlight units have been developed. The flat field emission backlight units have low power consumption compared to the backlight units that uses the conventional CCFLs, and have an advantage of having relatively uniform brightness on a wide light emitting region. The field emission backlight unit can be used for illumination.  
      In a field emission backlight unit, an upper substrate and a lower substrate are spaced apart and face each other. An anode electrode is formed on a lower surface of the upper substrate, and a phosphor layer is formed on a lower surface of the anode electrode. A cathode electrode is formed on an upper surface of the lower substrate. The cathode electrode can have a flat shape.  
      An insulating layer is formed on the cathode electrode, and a plurality of parallel strip shaped gate electrodes are arranged on the insulating layer. The gate electrodes and the insulating layer respectively include gate apertures and cavities. A plurality of emitters formed of an electron emission material, for example, Carbon Nanotubes, are disposed on the cathode electrode exposed through the gate apertures. A plurality of spacers for uniformly maintaining a gap between the upper substrate and the lower substrate are disposed therebetween.  
      In the structure described above, electrons are emitted from the emitters disposed on the cathode electrode when a voltage is supplied between the cathode electrode and the gate electrodes. The electrons are accelerated by a voltage supplied to the anode electrode to excite the phosphor layer, thereby emitting visible light.  
      However, some electrons emitted from the cathode electrode accumulate at the insulating layer between the gate electrodes, and generate an arc discharge due to the high voltage supplied to the anode electrode. The arc discharge damages the backlight unit.  
     SUMMARY OF THE INVENTION  
      The present invention provides a field emission backlight unit that prevents an insulating layer, on which electrons accumulate, from generating an arc discharge by forming the gate electrode so that the insulating layer does not face an anode electrode.  
      The present invention also provides a method of operating the field emission backlight unit.  
      According to one aspect of the present invention, a field emission backlight unit is provided including: an upper substrate and a lower substrate spaced apart from each other and facing each other; an anode electrode arranged on a lower surface of the upper substrate; a phosphor layer arranged on a lower surface of the anode electrode; cathode electrodes arranged on an upper surface of the lower substrate; an insulating layer having cavities adapted to expose the cathode electrode; a flat panel shaped gate electrode arranged on the insulating layer and having gate apertures respectively connected to the cavities; and an emitter arranged on the cathode electrode; the gate electrode is adapted to receive a ground voltage and the cathode electrode is adapted to receive a negative voltage.  
      The cathode electrode preferably includes a plurality of strip shaped electrodes spaced apart from each other.  
      A pulsed DC voltage is preferably supplied to the cathode electrode.  
      The cathode electrode preferably includes a conductive material adapted to transmit ultraviolet rays and the gate electrode preferably includes a conductive material adapted to prevent ultraviolet rays from passing therethrough.  
      The emitter preferably includes Carbon Nanotubes (CNTs).  
      A plurality of spacers are preferably adapted to maintain a uniform gap between the upper substrate and the lower substrate.  
      According to another aspect of the present invention, a method of operating a field emission backlight unit including: an upper substrate and a lower substrate spaced apart from each other and facing each other; an anode electrode arranged on a lower surface of the upper substrate; a phosphor layer arranged on a lower surface of the anode electrode; cathode electrodes arranged on an upper surface of the lower substrate; an insulating layer having cavities adapted to expose the cathode electrode; a flat panel shaped gate electrode arranged on the insulating layer and having gate apertures respectively connected to the cavities; and emitters arranged on the cathode electrodes is provided, the method including: supplying a ground voltage to the gate electrodes; and supplying a negative voltage to the cathode electrodes to emit electrons from the emitter.  
      Supplying a negative voltage to the cathode electrodes preferably includes supplying a pulsed DC voltage to the cathode electrodes to sequentially emit electrons from the emitters on the cathode electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the present invention and many of the attendant advantages thereof, will be readily apparent as the present invention 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:  
       FIG. 1  is a cross-sectional view of a field emission backlight unit;  
       FIG. 2  is a cross-sectional view of a field emission backlight unit according to an embodiment of the present invention;  
       FIG. 3  is a graph of variations in a light emission current with an increase in a negative voltage supplied to a cathode electrode, according to an embodiment of the present invention;  
       FIGS. 4A through 4E  are cross-sectional views of a method of manufacturing the field emission backlight unit of  FIG. 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a cross-sectional view of a field emission backlight unit.  
      Referring to FIG. I, an upper substrate  20  and a lower substrate  10  are spaced apart and face each other. An anode electrode  22  is formed on a lower surface of the upper substrate  20 , and a phosphor layer  24  is formed on a lower surface of the anode electrode  22 . A cathode electrode  12  is formed on an upper surface of the lower substrate  10 . The cathode electrode  12  can have a flat shape.  
      An insulating layer  14  is formed on the cathode electrode  12 , and a plurality of parallel strip shaped gate electrodes  16  are arranged in to each other on the insulating layer  14 . The gate electrodes  16  and the insulating layer  14  respectively include gate apertures  16   a  and cavities  14   a . A plurality of emitters  18  formed of an electron emission material, for example, Carbon Nanotubes (CNTs), are disposed on the cathode electrode  12  exposed through the gate apertures  16   a . Although not shown, a plurality of spacers for uniformly maintaining a gap between the upper substrate  20  and the lower substrate  10  are disposed therebetween.  
      In the structure described above, electrons are emitted from the emitters  18  disposed on the cathode electrode  12  when a voltage is supplied between the cathode electrode  12  and the gate electrodes  16 . The electrons are accelerated by a voltage supplied to the anode electrode  22  to excite the phosphor layer  24 , thereby emitting visible light.  
      However, some electrons emitted from the cathode electrode  12  accumulate at the insulating layer  14  between the gate electrodes  16 , and generate an arc discharge due to the high voltage supplied to the anode electrode  22 . The arc discharge damages the backlight unit.  
      The present invention is described more fully below with reference to the accompanying drawings in which exemplary embodiments of the present invention are shown. In the drawings, like reference numerals refer to like elements.  
       FIG. 2  is a cross-sectional view of a field emission backlight unit according to an embodiment of the present invention.  
      Referring to  FIG. 2 , an upper substrate  120  and a lower substrate  110  are spaced apart and face each other. The upper substrate  120  and the lower substrate  110  are generally formed of glass. An anode electrode  122  is formed on a lower surface of the upper substrate  120 , and a phosphor layer  124  is formed on a lower surface of the anode electrode  122 . The anode electrode  122  can be formed of a transparent conductive material, for example, Indium Tin Oxide (ITO), so that visible light emitted from the phosphor layer  124  can pass therethrough.  
      The anode electrode  122  can be formed as a thin film on the entire lower surface of the upper substrate  120 . The phosphor layer  124  can be formed by respectively coating red R, green G, and blue B phosphor materials in a predetermined pattern on the lower surface of the anode electrode  122 , or can be formed by coating a mixture of the red R, green G, and blue B phosphor materials on the entire lower surface of the upper substrate  120 .  
      The strip shaped cathode electrode  112  is formed to a thickness of 1000 to 3000 Å on the surface of the lower substrate  110 . The cathode electrode  112  is formed of a conductive material that can transmit ultraviolet rays, such as ITO.  
      An insulating layer  114  that exposes the cathode electrode  112 , such as an SiO 2  layer, is formed on the lower substrate  110 . The insulating layer  114  can be formed to a thickness of approximately a few to a few tens of μm, and includes cavities  114   a  that expose the cathode electrode  112 . A gate electrode  116  having gate apertures  116   a  connected to the cavities  114   a  is formed on the insulating layer  114 . The gate electrode  116  is formed as a thin film having a thickness of approximately 1000 to 3000 Å. The gate electrode  116  can be formed of a conductive material that does not transmit ultraviolet rays, such as Cr or Ag.  
      The gate electrode  116  can be formed in a flat shape. The gate electrode  116  prevents an arc discharge caused by collision of electrons accumulated on the insulating layer  114  with the anode electrode  122 .  
      A plurality of emitters  118  that emit electrons in response to a voltage supplied to the cathode electrode  112  and the gate electrode  116  are formed on the cathode electrode  112  exposed through the gate apertures  116   a . The emitters  118  are formed of, for example, Carbon Nanotubes (CNTs). When the emitters  118  are formed of CNTs, electrons are emitted at a relatively low driving voltage. Although not shown in  FIG. 2 , a plurality of spacers for uniformly maintaining a gap between the upper substrate  120  and the lower substrate  110  are disposed therebetween.  
      A method of operating the field emission backlight unit according to an embodiment of the present invention is as follows. To drive the field emission backlight unit having the above structure, a ground voltage Vg is supplied to the gate electrode  116  and a negative cathode voltage Vc, for example, a −60V DC pulse voltage with a period of 60 μs, is supplied to the cathode electrode  112 . Thus, the current in the field emission backlight unit can be held constant and the electrons are sequentially emitted from the emitter  118  by supplying a pulse voltage to the cathode electrode  112 , thereby obtaining uniform brightness from the backlight unit.  
       FIG. 3  is a graph of variations in a light emission current with an increase in a negative voltage supplied to a cathode electrode, according to an embodiment of the present invention. Referring to  FIG. 3 , a light emission current increases with the increase in the anode voltage at a constant cathode voltage, and also increases with the increase in the negative voltage of the cathode voltage. In the backlight unit of  FIG. 1 , a gate voltage of approximately 80V is necessary to obtain a light emission current of 2 mA when a voltage of 4 kV is supplied to the anode electrode. However, in the present embodiment, when a ground voltage is supplied to the gate electrode, a cathode voltage of approximately −27V is necessary. This shows that the field emission backlight unit according to the present invention needs a lower voltage than other backlight units to emit light with the same brightness, that is, the luminous efficiency of field emission backlight unit according to the present invention is improved. An arc discharge is not observed when an anode voltage of 10 to 15 kV is supplied to the field emission backlight unit according to the present invention.  
      In the field emission backlight unit according to the present invention, a high brightness can be realized by increasing an anode voltage since no arc discharge is observed at increased anode voltages.  
       FIGS. 4A through 4E  are cross-sectional views of a method of manufacturing the field emission backlight unit of  FIG. 2 . The same reference numerals are used for elements substantially identical with those depicted in  FIG. 2 , and accordingly, detailed descriptions thereof have been omitted.  
      Referring to  FIG. 4A , after sputtering an ITO layer to a thickness of 0.25 μm on the lower substrate  110  formed of glass, a strip shaped cathode electrode  112  is formed by patterning the ITO layer. Next, an insulating layer  114 , for example, an SiO 2  layer, covering the cathode electrode  112 , is deposited to a thickness of a few tens of μm on the lower substrate  110 . Next, a gate electrode  116  is formed on the insulating layer  114  by sputtering a Cr layer to a thickness of 0.25 μm. The purpose of forming the cathode electrode  112  using a material that transmits ultraviolet rays and the purpose of forming the gate electrode  116  using a material that does not transmit the ultraviolet rays is to perform a back exposure, which will be described later.  
      Referring to  FIG. 4B , after coating a photosensitive film P on the gate electrode  116 , a region Pa corresponding to the cathode electrode  112  is exposed.  
      Next, the exposed region Pa is removed through a developing process. The gate electrode  116  is exposed through the removed exposed region Pa. Gate apertures  116   a  are formed by wet etching the exposed portion of the gate electrode  116  using the photosensitive film P as an etch mask. Next, cavities  114   a  that expose the cathode electrode  112  are formed in the insulating layer  114  by etching the insulating layer  114  using the photosensitive film P as an etch mask.  
       FIG. 4C  shows a resultant product after the photosensitive film P is removed.  
      Referring to  FIG. 4D , after a CNT paste  117  that contains a negative photosensitive material is coated to cover the resultant product including the exposed cathode electrode  112 , the CNT paste  117  on the cathode electrode  112  which is exposed in the gate hole  116   a  is back-exposed using ultraviolet rays through the lower substrate  110 . Next, as depicted in  FIG. 4E , CNT emitters  118  are formed on the cathode electrode  112  through developing and baking processes.  
      The next process, such as bonding the upper substrate and the lower substrate after forming the anode electrode and the phosphor layer on the upper substrate, is well known in the art, and accordingly, detailed descriptions thereof have been omitted.  
      As described above, a field emission backlight unit according to the present invention prevents an insulating layer from being exposed to an anode electrode by forming a flat shaped gate electrode, thereby preventing the formation of an arc discharge. Therefore, the field emission backlight unit according to the present invention can have a high brightness by supplying a high anode voltage.  
      Also, according to a method of operating the field emission backlight unit according to the present invention, a driving voltage can be reduced by supplying a DC pulse negative voltage to the strip shaped cathode electrode.  
      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 modifications in form and detail can be made therein without departing from the spirit and scope of the present invention as defined by the following claims.