Abstract:
A thermal electron emission backlight device comprises: a first substrate and a second substrate disposed in parallel and separated by a predetermined distance from each other; a first anode electrode and a second anode electrode facing the first anode electrode, the first and second anode electrodes being formed on inner surfaces of the first substrate and the second substrate, respectively; cathode electrodes disposed at predetermined intervals and in parallel with each other between the first substrate and the second substrate; a phosphor layer formed on the second anode electrode; and a plurality of spacers disposed between the first substrate and the second substrate so as to maintain the predetermined distance therebetween. When a predetermined voltage is applied to the cathode electrodes, thermal electrons are emitted from the cathode electrodes.

Description:
CLAIM OF PRIORITY  
       [0001]     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 BACKLIGHT DEVICE EMITTING THERMAL ELECTRON earlier filed in the Korean Intellectual Property Office on the  23   rd  of May 2005 and there duly assigned Ser. No. 10-2005-0043158.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a thermal electron emission backlight device and, more particularly, to a back light device that emits white light by exciting a phosphor layer using thermal electrons.  
         [0004]     2. Related Art  
         [0005]     A backlight device which emits white light is installed on a rear surface of a liquid crystal display (LCD). A plasma type cold cathode electrode tube has been mainly used as the backlight. However, the cold cathode cold cathode electrode tube is not environmentally friendly since the cold cathode electrode tube uses mercury. In addition, it is becoming expensive since the structure is more complicated as it is bigger by using a light guide plate. In this regard, a mercury-free and flat panel type backlight is required. One exemplary backlight is a backlight which employs carbon nanotubes (CNTs).  
         [0006]     A triode structure field emission device, as disclosed in U.S. Pat. No. 5,760,858, can solve the above problem, but the manufacturing process of the backlight is as complicated as the manufacturing process of a field emission display (FED). That is, the backlight is manufactured by a semiconductor manufacturing process, including deposition of a plurality of thin films and photolithography, thereby having a high manufacturing cost and a low yield when compared to the manufacturing of an LCD panel having a simple structure.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention relates to a thermal electron emission backlight device having high brightness by using a thermal electron emitting unit in a space between a rear substrate and a front substrate.  
         [0008]     According to an aspect of the present invention, a thermal electron emission backlight device comprises: a first substrate and a second substrate disposed in parallel and separated by a predetermined distance from each other; a first anode electrode and a second anode electrode facing the first anode electrode, the first and second anode electrodes being formed on inner surfaces of the first substrate and the second substrate, respectively; a plurality of cathode electrodes disposed at predetermined intervals and parallel to each other between the first substrate and the second substrate; a phosphor layer formed on the second anode electrode; and a plurality of spacers disposed between the first substrate and the second substrate so as to maintain the predetermined distance therebetween; wherein, when a predetermined voltage is applied to the cathode electrodes, thermal electrons are emitted from the cathode electrodes.  
         [0009]     The second anode electrode may be a high reflective electrode.  
         [0010]     The thermal electron emission backlight device may further comprise a reflection film between the second anode electrode and the second substrate.  
         [0011]     The thermal electron emission backlight device may further comprise a reflection film on a lower surface of the second substrate.  
         [0012]     The thermal electron emission backlight device may further comprise a phosphor layer on an outer circumference of the spacers.  
         [0013]     The spacers may be conductive spacers which electrically connect the first anode electrode to the second anode electrode.  
         [0014]     The spacers may comprise a cylinder formed of a non-metal and a reflection film formed between an outer circumference of the cylinder and the phosphor layer.  
         [0015]     The cathode electrode may be formed of tungsten.  
         [0016]     The thermal electron emission backlight device may further comprise an electron emitting source material disposed on an outer circumference of the cathode electrode.  
         [0017]     The thermal electron emission backlight device may further comprise a carbon group material disposed on a surface of the electron emitting source  
         [0018]     The thermal electron emission backlight device may further comprise a phosphor layer disposed on the first anode electrode.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     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:  
         [0020]      FIG. 1  is a cross-section view of a backlight device for a liquid crystal display (LCD);  
         [0021]      FIGS. 2A and 2B  are cross-section views of a thermal electron emission backlight device according to an embodiment of the present invention;  
         [0022]      FIG. 3  illustrates a simulation result of the flow of thermal electrons in a thermal electron emission backlight device according to an embodiment of the present invention;  
         [0023]      FIG. 4  is a photographed image of light emission from a backlight according to an embodiment of the present invention; and  
         [0024]      FIGS. 5A and 5B  are cross-section views of a thermal electron emission backlight device according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     the thermal electron emission backlight device according to the present invention will now be described more fully with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. In the drawings, the thicknesses of layers and regions are exaggerated for clarity.  
         [0026]      FIG. 1  is a cross-section view of a backlight device for a liquid crystal display (LCD).  
         [0027]     Referring to  FIG. 1 , a spacer (not shown) is disposed between a front substrate  1  and a rear substrate  4 . Walls (not shown) between the front substrate  1  and the rear substrate  4  are sealed. A cathode electrode  5  having a plate type shape or a stripe shape is formed on the rear substrate  4 . A field emission source  6 , such as carbon nanotubes (CNTs), is formed on the cathode electrode  5 . An anode electrode  2 , which is a transparent electrode, is formed on the front substrate  1 . A phosphor layer  3  is coated on the anode electrode  2 .  
         [0028]     When a predetermined voltage is applied to the cathode electrode  5  and the anode electrode  2 , electrons are emitted from the field emission source  6 , and collide and excite the phosphor layer  3 . Light emitted from the phosphor layer  3  enters into a liquid crystal display (LCD) panel through the anode electrode  2  and the front substrate  1 .  
         [0029]     A flat backlight device has non-uniform brightness since the electron emission is concentrated on an edge of the cathode electrode  5 . Also, in the diode structure as described above, a desired amount of anode current cannot be readily obtained since the control of electron emission is difficult. For example, in a CNT backlight having a diagonal length of 5 inches, to obtain a brightness of 10,000 Cd/m 2 , the backlight must be operated at an anode voltage of approximately 10 kV, which is a high voltage, and at an anode current of 0.5 to 0.7 mA. Meanwhile, in a diode structure, when a distance between the anode electrode  2  and the cathode electrode  5  is 5 mm, the anode current amount of more than a few mA is generated at anode voltage of 5 kV. That is, in the diode method, the backlight is operated at a low voltage and high current. Thus, realizing a high efficiency backlight is difficult.  
         [0030]      FIGS. 2A and 2B  are cross-section views of a thermal electron emission reflective type backlight device according to an embodiment of the present invention.  
         [0031]     Referring to  FIGS. 2A and 2B , a first substrate  110  and a second substrate  120  are spaced apart from each other by a predetermined distance, such as 5-50 mm, by spacers  140 . The first substrate  110  can be formed of a transparent material, such as glass. Light emitted from a phosphor layer  124  passes through the first substrate  110  and a rear surface of an LCD. A first anode electrode  112 , which can be an indium tin oxide (ITO) transparent electrode, is disposed on an inner surface of the first substrate  110  in a flat panel shape. A second anode electrode  122  is disposed in a flat panel shape on an inner surface of the second substrate  120 . A plurality of cathode electrodes  130  arranged parallel to each other are disposed between the first anode electrode  112  and the second anode electrode  122 . The cathode electrodes  130  may respectively have a cylindrical shape. A thermal electron emitting material layer  132 , such as (Ba,Sr,Ca)CO 3 , having a thickness of 5-20 μm is formed on an outer surface of the cathode electrodes  130 .  
         [0032]     An electron emitting source  134 , for example, a carbon group material such as CNT or graphite powder, can be coated on a surface of the thermal electron emitting material layer  132 .  
         [0033]     The cathode electrodes  130  may be formed of tungsten W with a diameter of 10-250 μm. The thermal electron emitting material layer  132  can be formed to a thickness of 10 μm. The cathode electrodes  130  can be disposed at a distance of 0.3-20 mm from the first and second anode electrodes  112  and  122 , respectively.  
         [0034]     A direct current (DC) voltage Va of 3-30 kV can be applied to the first anode electrode  112  and the second anode electrode  122 , respectively. As depicted in  FIG. 2 , when the spacer  140  is formed of a conductive material, the same voltage is applied to the first anode electrode  112  and the second anode electrode  122 . A DC or alternate current (AC) voltage of a few to a few tens of volts can be applied to both ends of the cathode electrodes  130 , depending on the material and length of the cathode electrodes  130 .  
         [0035]     The first anode electrode  112  can be a transparent electrode formed of, for example, ITO. Although it is not shown in  FIG. 2 , a phosphor layer can be formed to a predetermined thickness, for example, 0.2 to 6 μm, on an inner surface of the first anode electrode  112 . The phosphor layer  124  is excited by thermal electrons emitted from the thermal electron emitting material layer  132  and electrons emitted from the electron emitting source  134 , and emits visible light.  
         [0036]     The second anode electrode  122  can be formed of a high reflectance material, such as Al. A phosphor layer  124  is formed to a predetermined thickness, for example, 3 to 15 μm on the second anode electrode  122 . The phosphor layer  124  is excited by thermal electrons emitted from the thermal electron emitting material layer  132  and electrons emitted from the electron emitting source  134 , and emits visible light.  
         [0037]     In  FIGS. 2A and 2B , if the phosphor layer  124  is not coated on the first anode electrode  112 , the backlight device is a reflection type, and if the phosphor layer  124  is coated on the first anode electrode  112 , the backlight device is a combination of the reflection type and a transmission type.  
         [0038]     Wall frames  160  are formed on an outer rim between the first substrate  110  and the second substrate  120 . The wall frames  160  are bonded with melted frit glass so as to seal an inner side of the backlight device. The cathode electrodes  130  can be formed through the wall frames  160 , and at least an end of each of the cathode electrodes  130  is tensioned toward the outside. The tension structure of the cathode electrodes  130  can be formed by a conventional technique for tensioning a filament in a vacuum fluorescent display (VFD). Therefore, the detailed descriptions thereof will be omitted.  
         [0039]     The spacers  140  are formed of a ceramic material, such as glass or aluminum, and can be formed in a cylindrical column having a thickness of approximately 50-500 μm.  
         [0040]     A metal layer  142  can be coated on outer circumferences of the spacers  140  to a thickness of 0.02 to 1 μm. When the metal layer  142  is coated on the circumferences of the spacers  140 , the first anode electrode  112  is electrically connected to the second anode electrode  122 . Accordingly, a voltage Va applied to the first anode electrode  112  and the second anode electrode  122  is the same. When the metal layer  142  is not formed, different voltages Va are respectively applied to the first anode electrode  112  and the second anode electrode  122 .  
         [0041]     Also, a phosphor layer  144  having a thickness of 3 to 10 μm may be coated on an outer surface of the metal layer  142 . The phosphor layer  144  is excited by accelerated electrons, and emits visible light. In this case, the metal layer  142  may be formed of a high reflectance material, such as Al.  
         [0042]     The operation of the field emission type thermal electron emission backlight device according to an embodiment of the present invention will now be described with reference to  FIGS. 2A and 2B .  
         [0043]     A DC voltage of 10 kV is applied to the first anode electrode  112  and the second anode electrode  122 , and a DC voltage of 8 V is applied to both ends of the cathode electrodes  130 . Thus, thermal electrons are emitted from the thermal electron emitting material layer  132 , and the electrons excite the phosphor layers  124  and  144 . Then, the phosphor layers  124  and  144  emit white visible light, and the white visible light is supplied to an LCD panel through the first anode electrode  112  and the first substrate  110 . In this case, white light headed toward the second substrate  120  heads toward the first substrate  110  by reflecting at the second anode electrode  122  which is a reflection film. Although it is not shown in  FIG. 1 , if a phosphor layer is formed on the first anode electrode  112 , the phosphor layer excited by thermal electrons emits white light. The white light heads the first substrate  110  passing through the phosphor layer.  
         [0044]     In  FIGS. 2A and 2B , if an electron emitting source  134 , i.e., CNTs, is coated on a surface of the thermal electron emitting material layer  132 , the CNTs also emit cold electrons by field emission effect. The cold electrons can also excite the phosphor layers  124  and  144  to generate white visible light from the phosphor layers  124  and  144 .  
         [0045]      FIG. 3  illustrates a simulation result of the flow of thermal electrons in a thermal electron emission backlight device according to an embodiment of the present invention. Voltage of 5 kV and 4.5 kV are applied to the first anode electrode (upper anode) and the second anode electrode (lower anode), respectively. This simulation is similar in that the same voltage is applied to the first and second anode electrodes.  
         [0046]     Referring to  FIG. 3 , thermal electrons emitted from the thermal electron emitting material layer  132  in response to a voltage applied to the cathode electrodes  130  are uniformly distributed with overlapping in the backlight device. Accordingly, the thermal electron emission backlight device according to the present invention has uniform brightness, and can be used for large size LCDs. Although it is not depicted in  FIG. 3 , in the backlight device according to an embodiment of the present invention, the degree of dispersion of thermal electrons can be controlled by varying the voltage applied to the first anode electrode  112  and the second anode electrode  122 . For example, when a voltage which is higher than a voltage applied to the first anode electrode  112  is applied to the second anode electrode  122 , relatively more thermal electrons from cathode electrode  130  can reach the second anode electrode  122  relative to the first anode electrode  112 , and thus a reflection-type backlight can be realized.  
         [0047]      FIG. 4  is a photographed image of light emission from the backlight device according to an embodiment of the present invention. The cathode electrodes  130  used in  FIG. 4  are formed of tungsten W to a thickness of 10 μm. A thermal electron emitting material layer  132  formed of(Ba,Sr,Ca)CO 3  is coated on an outer circumferential surface ofthe cathode electrodes  130  to a thickness of 10 μm. The diagonal length of the cathode electrodes  130  is 5 inches, and a voltage of 6V is applied thereto. A voltage of 10 kV is commonly applied to both the first anode electrode  112  and the second anode electrode  122 . The first substrate  110  is disposed 15 mm apart from the second substrate  120 . As seen in  FIG. 4 , even if only one cathode electrode (the cathode electrode in  FIG. 4  is indicated by a black line) is used, the brightness of the backlight unit is uniform on the 5-inch substrate, and the value is as high as 12,000 Cd/m 2.    
         [0048]     In  FIG. 2 , the anode electrode  122  is formed of aluminum Al, which is a high reflective material, but the present invention is not limited thereto. The purpose of the second anode electrode  122  is to reflect electrons. Therefore, an additional reflection layer, such as an Al layer, can be disposed between the second anode electrode  122  and the second substrate  120  or on a lower surface of the second substrate  120 , and a transparent electrode, such as an ITO electrode, can be used as the second anode electrode  122 .  
         [0049]      FIGS. 5A and 5B  are cross-section views of a thermal electron emission backlight device according to another embodiment ofthe present invention. The same reference numerals are used to indicate elements identical with those depicted in  FIG. 2 , and thus a detailed description thereof will be omitted.  
         [0050]     Referring to  FIGS. 5A and 5B , a first substrate  210  and a second substrate  220  are spaced apart by a predetermined distance by spacers  240 . The first substrate  210  can be formed of a transparent material, such as glass. The first substrate  210  is a member through which light emitted from a phosphor layer  214  passes, and is disposed on a rear surface of an LCD. A first anode electrode  212 , which can be an ITO transparent electrode, is disposed in a flat panel shape on an inner surface of the first substrate  210 . A second anode electrode  222  is disposed in a flat panel shape on an inner surface of the second substrate  220 . A plurality of cathode electrodes  230 , parallel to each other, are disposed between the first anode electrode  212  and the second anode electrode  222 . The cathode electrodes  230  may each have a cylindrical shape. A thermal electron emitting material layer  232 , such as (Ba,Sr,Ca)CO 3 , may be formed on an outer surface of the cathode electrodes  230 .  
         [0051]     An electron emitting source material  234  (for example, a carbon group material such as CNTs or graphite powder) may be coated on a surface of the thermal electron emitting material layer  232 .  
         [0052]     The cathode electrodes  230  may be formed of tungsten W with a diameter of 10-250 μ. The cathode electrodes  230  may be disposed at a distance of 0.3-20 mm from the first and second anode electrodes  212  and  222 , respectively.  
         [0053]     A direct current (DC) voltage Va of 3-20 kV can be applied to the first anode electrode  212  and the second anode electrode  222 , and a DC or AC voltage Vc of a few to a few tens of volts can be applied to the cathode electrodes  230 , depending on the material and length of the cathode electrodes  230 .  
         [0054]     A phosphor layer  214  with a predetermined thickness, such as 0.2-6 μm, is coated on the second anode electrode  212 . A phosphor layer  224  with a predetermined thickness, such as 3-15 μm, is coated on an inner surface of the second anode electrode  222 . The phosphor layers  214  and  224  emit visible light when they are excited by thermal electrons emitted from the thermal electron emitting material layer  232  and electrons emitted from the electron emitting source material  234 .  
         [0055]     The second anode electrode  222  may be formed of a material having high reflectance, such as aluminum (Al).  
         [0056]     The spacers  240  are formed of a ceramic material, such as glass or aluminum, and can be formed in a cylindrical column having a thickness of approximately 50-500 μm. A phosphor layer  244  may further be formed on outer circumferential surfaces of the spacers  240  with a thickness of approximately 3-10 μm.  
         [0057]     A highly reflective material film, such as an Al reflection film  242 , can further be formed between the spacers  240  and the phosphor layer  244 .  
         [0058]     Wall frames  260  are formed on an outer rim between the first substrate  210  and the second substrate  220 . The wall frames  260  are bonded with melted frit glass so as to seal an inner side of the backlight device. The cathode electrodes  230  can be formed so as to pass through the wall frames  260 , and at least one end of each of the cathode electrodes  230  is tensioned toward the outside.  
         [0059]     The operation of the field emission type thermal electron emission backlight device will now be described with reference to  FIG. 5 .  
         [0060]     A DC voltage of 10 kV is applied to the first anode electrode  212  and the second anode electrode  222 , and a DC voltage of 8 V is applied to both ends of the cathode electrodes  230 . Then, thermal electrons are emitted from the thermal electron emitting material layer  232 , and the electrons excite the phosphor layers  214 ,  224  and  244 . As a result, the phosphor layers  214 ,  224  and  244  emit white visible light which is supplied to an LCD panel through the first anode electrode  212  and the first substrate  210 .  
         [0061]     In  FIG. 5 , if an electron emitting source  234  (i.e., CNTs) is coated on a surface of the thermal electron emitting material layer  232 , cold electrons can be emitted from the CNTs. The phosphor layers  214 ,  224  and  244  can be excited by the cold electrons, and thus can generate white visible light.  
         [0062]     The thermal electron emission backlight device according to the present invention uses a first anode electrode together with a second anode electrode. Therefore, thermal electrons emitted from cathode electrodes are uniformly distributed between the first and second anode electrodes, thereby improving brightness. Accordingly, no diffuser is required, thereby reducing manufacturing cost.  
         [0063]     Also, the backlight device can be readily manufactured using only anode electrodes, glass substrates on which phosphor layers are formed, and wires for forming cathode electrodes.  
         [0064]     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 detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.