Patent Publication Number: US-7905756-B2

Title: Method of manufacturing field emission backlight unit

Description:
CLAIM OF PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATION 
     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 UNIT, METHOD OF DRIVING THE BACKLIGHT UNIT, AND METHOD OF MANUFACTURING LOWER PANEL earlier filed in the Korean Intellectual Property Office on the Jan. 8, 2004 and there duly assigned Serial No. 2004-1102. Furthermore, this application is a divisional of Applicants&#39; Ser. No. 10/980,793 filed in the U.S. Patent &amp; Trademark Office on 4 Nov. 2004 now U.S. Pat. No. 7,288,884, and assigned to the assignee of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a backlight unit for a liquid crystal display and, more particularly, to a field emission backlight unit. 
     2. Related Art 
     In general, flat panel displays are largely classified into light emitting displays and light receiving displays. Light emitting flat panel displays include cathode ray tubes (CRTs), plasma display panels (PDPs), and field emission displays (FEDs), and light receiving flat panel displays include liquid crystal displays (LCDs). Among these flat panel displays, LCDs have the advantages of light weight and low power consumption, but have a disadvantage in that, since they form an image not by emitting light by itself but by receiving light from an outside source, the image cannot be viewed in a dark place. To solve this problem, a backlight unit for emitting light is installed at a rear surface of the LCD so that the LCD can form an image in a dark place. 
     A conventional backlight unit uses a linear or a point light source. Typically, a cold cathode fluorescent lamp (CCFL) is used as the linear light source, and a light emitting diode (LED) is used as the point light source. However, the conventional backlight unit is disadvantageous in that, since its structure is complex, manufacturing costs are high, and since the light source is disposed at the side of the backlight unit, power consumption is high when light is reflected and transmitted. In particular, as the LCD becomes larger, it becomes more difficult to achieve uniform brightness with the conventional backlight unit. 
     Accordingly, in recent years, a field emission backlight unit having a planar light emitting structure has been suggested. The field emission backlight unit has lower power consumption and more uniform brightness over a larger area than the backlight unit using the typical CCFL. 
     Korean Patent Publication No. 2002-33948 discloses a conventional field emission backlight unit. An indium tin oxide (ITO) electrode layer and a fluorescent layer are sequentially stacked on a bottom surface of an upper substrate. A thin metal layer and a carbon nanotube layer are sequentially stacked on a lower substrate. The upper substrate and the lower substrate are bonded to each other with a spacer therebetween. A glass tube for vacuum ventilation is installed in the lower substrate. 
     In the backlight unit constructed as above, if a voltage is applied between the ITO electrode layer and the thin metal layer, electrons are emitted from the carbon nanotube layer and collide against the fluorescent layer. As a result, fluorescent materials in the fluorescent layer become excited and emit visible light. 
     However, the conventional field emission backlight unit has a diode-type field emission structure in which the ITO electrode layer disposed on the upper substrate is used as an anode and the thin metal layer disposed on the lower substrate is used as a cathode. Since a high voltage used for emitting electrons is directly applied between the anode and the cathode, this diode-type structure is vulnerable to local arcing. If such local arcing occurs, brightness cannot be kept uniform over the entire surface of the backlight unit, and the ITO electrode layer, the fluorescent layer, and the carbon nanotube layer gradually become damaged, thereby reducing the lifespan of the backlight unit. 
     SUMMARY OF THE INVENTION 
     The present invention provides a field emission backlight unit having a triode-type field emission structure, which can ensure uniform brightness and prolong lifespan. 
     The present invention further provides a method of driving a field emission backlight unit so as to ensure uniform brightness and prolonging lifespan. 
     The present invention further provides a method of manufacturing a lower panel of the field emission backlight unit. 
     According to an aspect of the present invention, there is provided a field emission backlight unit comprising: a lower substrate; first electrodes and second electrodes alternately formed in parallel lines on the lower substrate; emitters disposed on at least the first electrodes of the first and second electrodes; an upper substrate spaced apart from the lower substrate by a predetermined distance such that the upper and lower substrates face each other; a third electrode formed on a bottom surface of the upper substrate; and a fluorescent layer formed on the third electrode. 
     The emitters may be made of carbon nanotubes. The first electrodes and second electrodes may include indium tin oxide electrode layers formed on the lower electrode and thin metal layers formed on the indium tin oxide electrode layers. 
     The emitters may be disposed on only the first electrodes such that the first electrodes serve as cathodes, the second electrodes serve as gate electrodes, and the third electrode serves as an anode. 
     In this case, the plurality of emitters may be disposed along both edges of the first electrodes at predetermined intervals. A plurality of emitter grooves may be formed along both edges of the first electrodes, and the emitters may be formed in the plurality of emitter grooves. 
     Also, the emitters may be disposed on both the first electrodes and the second electrodes such that the first electrodes and the second electrodes serve as cathodes and gate electrodes alternately, and the third electrode serves as an anode. 
     In this case, the plurality of emitters may be disposed along both edges of both the first electrodes and the second electrodes at predetermined intervals. The emitters disposed on the first electrodes and the emitters disposed on the second electrodes may be arranged by turns. A plurality of emitter grooves may be formed along both edges of both the first electrodes and the second electrodes, and the emitters may be formed in the plurality of emitter grooves. 
     According to another aspect of the present invention, there is provided a method of driving a triode-type field emission backlight unit including a lower panel on which first electrodes, second electrodes, and emitters disposed on both the first electrodes and the second electrodes are formed, and an upper panel on which a third electrode is formed, the method comprising the steps of: applying a cathode voltage to the first electrodes, a gate voltage to the second electrodes, and an anode voltage to the third electrode so as to emit electrons from the emitters disposed on the first electrodes; applying a gate voltage to the first electrodes, a cathode voltage to the second electrodes, and an anode voltage to the third electrode so as to emit electrons from the emitters disposed on the second electrodes; and repeating the above steps. 
     According to still another aspect of the present invention, there is provided a method of manufacturing a lower panel of a field emission backlight unit, the method comprising the steps of: forming a conductive material layer on a transparent substrate; patterning the conductive material layer in parallel lines to form alternating first electrodes and second electrodes, and forming a plurality of emitter grooves at predetermined intervals along both edges of at least the first electrodes; coating a photoresist material layer on the substrate on which the first electrodes and the second electrodes are formed; patterning the photoresist material layer to expose the emitter grooves; coating a carbon nanotube paste on the photoresist material layer and in the emitter grooves; selectively exposing the carbon nanotube paste to form carbon nanotube emitters in the emitter grooves; and stripping the photoresist material layer and removing unexposed portions of the carbon nanotube paste. 
     The conductive layer forming step may comprise: forming an indium tin oxide electrode layer on the substrate; and forming a thin metal layer on the indium tin oxide electrode layer. 
     The emitter groove forming step may comprise forming the emitter grooves along both edges of both the first electrodes and the second electrodes. 
     The first and second electrode forming step may comprise: coating a photoresist material layer on the conductive material layer; patterning the photoresist material layer using a photolithography process; etching the conductive material layer using the patterned photoresist material layer as an etching mask; and stripping the photoresist material layer. 
     The carbon nanotube paste coating step may comprise coating the carbon nanotube paste using a screen printing method. 
     The carbon nanotube emitter forming step may comprise exposing the carbon nanotube paste from a rear surface of the substrate. 
    
    
     
       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: 
         FIG. 1  is a cross-sectional view of a field emission backlight unit; 
         FIG. 2  is a partial sectional view of a field emission backlight unit according to a first preferred embodiment of the present invention; 
         FIG. 3  is a partial perspective view of a lower panel of the backlight unit of  FIG. 2 ; 
         FIG. 4  is a partial perspective view of a modified example of the lower panel of the backlight unit of  FIG. 2 ; 
         FIG. 5  is a diagram illustrating simulation results of electron beams emitted from the backlight unit of  FIG. 2 ; 
         FIG. 6  is a photograph illustrating light-emission test results of the backlight unit of  FIG. 2 ; 
         FIG. 7  is a partial sectional view of a field emission backlight unit according to a second preferred embodiment of the present invention; 
         FIG. 8  is a partial perspective view of a lower panel of the backlight unit of  FIG. 7 ; 
         FIG. 9  is a schematic plan view of the lower panel of the backlight unit of  FIG. 7  for explaining a method of driving the backlight unit; and 
         FIGS. 10A  thru  10 I are schematic perspective views for explaining steps of manufacturing the lower panel of the backlight unit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. In the drawings, whenever the same element reappears in a subsequent drawing, it is denoted by the same reference numeral. 
       FIG. 1  is a cross-sectional view of a field emission backlight unit. Referring to  FIG. 1 , an indium tin oxide (ITO) electrode layer  2  and a fluorescent layer  3  are sequentially stacked on a bottom surface of an upper substrate  1 . A thin metal layer  6  and a carbon nanotube layer  4  are sequentially stacked on a lower substrate  7 . The upper substrate  1  and the lower substrate  7  are bonded to each other with a spacer  5  therebetween. A glass tube  8  for vacuum ventilation is installed in the lower substrate  7 . 
     In the backlight unit constructed as above, if a voltage is applied between the ITO electrode layer  2  and the thin metal layer  6 , electrons are emitted from the carbon nanotube layer  4  and collide against the fluorescent layer  3 . As a result, fluorescent materials in the fluorescent layer  3  become excited and emit visible light. 
     The field emission backlight unit has a diode-type field emission structure in which the ITO electrode layer  2  disposed on the upper substrate  1  is used as an anode and the thin metal layer  6  disposed on the lower substrate  7  is used as a cathode. Since a high voltage used for emitting electrons is directly applied between the anode and the cathode, this diode-type structure is vulnerable to local arcing. If such local arcing occurs, brightness cannot be kept uniform over the entire surface of the backlight unit, and the ITO electrode layer  2 , the fluorescent layer  3 , and the carbon nanotube layer  4  gradually become damaged, thereby reducing the lifespan of the backlight unit. 
       FIG. 2  is a partial sectional view of a field emission backlight unit according to a first preferred embodiment of the present invention, and  FIG. 3  is a partial perspective view of a lower panel of the backlight unit of  FIG. 2 . 
     Referring to  FIGS. 2 and 3 , the field emission backlight unit includes a lower panel  110  and an upper panel  120 , which are spaced apart by a predetermined distance and face each other. The lower panel  110  and the upper panel  120  are constructed so as to be suitable for triode-type field emission. 
     Specifically, the lower panel  110  includes a transparent lower substrate  111  which may be made of glass, first electrodes  112  and second electrodes  114  which are formed on the lower substrate  111  and which act as cathodes and gate electrodes, respectively, and carbon nanotube emitters  116  which are disposed on the first electrodes  112 . 
     The upper panel  120  includes a transparent upper substrate  121  which may be made of glass, a third electrode  122  which is formed on a bottom surface of the upper substrate  121  and which acts as an anode, and a fluorescent layer  123  which is formed on the third electrode  122 . 
     The lower panel  110  and the upper panel  120  are spaced apart and face each other, and are bonded to each other with a sealing material (not shown) coated on perimeters thereof. As seen in  FIG. 2 , a spacer  130  is installed between the lower panel  110  and the upper panel  120  to maintain the predetermined distance between the lower panel  110  and upper panel  120 . 
     To be more specific, the first electrodes  112  are arranged in parallel lines on the lower substrate  111  of the lower panel  110  to serve as cathodes, and the second electrodes  114  are arranged in parallel lines on the lower substrate  111  of the lower panel  110  to serve as gate electrodes. The plurality of first electrodes  112  and the plurality of second electrodes  114  are alternately arranged in the same plane. The first electrodes  112  and the second electrodes  114  may include transparent conductive indium tin oxide (ITO) electrode layers  112   a  and  114   a , respectively, formed on the lower substrate  111 , and conductive thin metal layers  112   b  and  114   b , respectively, formed on the ITO electrode layers  112   a  and  114   a , respectively, and made of chrome. 
     However, the first electrodes  112  and the second electrodes  114  may include only the ITO electrode layers  112   a  and  114   a . The ITO electrode layers  112   a  and  114   a  disadvantageously have a high line resistance. Accordingly, it is preferable in manufacturing a large backlight unit that the thin metal layers  112   b  and  114   b , acting as bus electrodes for reducing the line resistance of the ITO electrode layers  112   a  and  114   a , respectively, are formed on the ITO electrode layers  112   a  and  114   a , respectively. 
     As previously mentioned, the plurality of first electrodes  112  and the plurality of second electrodes  114  are made of the same materials and are formed in the same plane. Therefore, as will be described when addressing the manufacturing method, the first electrodes  112  and the second electrodes  114  can be simultaneously manufactured, thereby simplifying manufacturing processes and reducing manufacturing costs. 
     The emitters  116  are formed on the first electrodes  112  that serve as the cathodes. The emitters  116  emit electrons when an electric field is formed by a voltage applied between the first electrodes  112  and the second electrodes  114 . The emitters  116  are made of carbon nanotubes (CNTs). The CNTs can smoothly emit electrons at a relatively low driving voltage. Further, as will be described when addressing the manufacturing method, if a CNT paste is used, the CNT emitters  116  can be easily formed on a larger substrate, and accordingly, a larger backlight unit can be manufactured. 
     According to the first preferred embodiment of the present invention, the plurality of CNT emitters  116  are disposed at predetermined intervals along both longitudinal edges of the first electrodes  112 . To be more specific, a plurality of emitter grooves  115  are formed at predetermined intervals along both longitudinal edges of the first electrodes  112 , and the CNT emitters  116  are formed in the emitter grooves  115 . Since bottom surfaces of the CNT emitters  116  are in contact with a top surface of the transparent lower substrate  111 , as will be described when addressing the manufacturing method, the CNT emitters  116  can be formed by exposing the CNT paste from a rear surface of the lower substrate  111 . 
       FIG. 4  illustrates a modified example of the lower panel of the backlight unit of  FIG. 3 . Referring to  FIG. 4 , CNT emitters  116 ′ are formed on a top surface of the first electrodes  112  along both longitudinal edges of the first electrodes  112 . Accordingly, the emitter grooves  115  shown in  FIG. 3  are not required, thereby further simplifying the structure of the first electrodes  112 . However, it is impossible to form the CNT emitters  116 ′ by the aforesaid backside exposure. Thus, the CNT emitters  116 ′ should be formed by a frontal exposure using an exposure mask. 
     The CNT emitters  116  and  116 ′ can be formed by various other well-known methods instead of backside and frontal exposure using CNT paste. For example, the CNT emitters  116  and  116 ′ may be formed by chemical vapor deposition. The chemical vapor deposition is performed by forming catalytic metal layers made of nickel or iron on portions on which the emitters are to be formed, and supplying gas containing carbon, such as CH 4 , C 2 H 2 , or CO 2 , to vertically grow carbon nanotubes from surfaces of the catalytic metal layers. 
     Referring to  FIGS. 2 and 3  again, the third electrode  122  formed on the bottom surface of the upper substrate  121  serves as an anode, and is made of transparent conductive ITO through which visible light emitted from the fluorescent layer  123  can pass. The third electrode  122  may be formed as a thin film on the entire bottom surface of the upper substrate  121 , or may be formed in a predetermined pattern, for example, a stripe pattern, on the bottom surface of the upper substrate  121 . 
     The fluorescent layer  123  is formed on a bottom surface of the third electrode  122 , and is made of red (R), green (G), and blue (B) fluorescent materials. The R, G, and B fluorescent materials may be individually coated on the bottom surface of the third electrode  122  in a predetermined pattern, or may be mixed and then coated on the entire bottom surface of the third electrode  122 . 
     A method of driving the field emission backlight unit according to the first preferred embodiment of the present invention will now be explained. 
     In the field emission backlight unit according to the first preferred embodiment, if predetermined voltages are applied to the first electrodes  112 , the second electrodes  114  and the third electrode  122 , respectively, an electric field is formed between the electrodes  112 ,  114  and  122 , and electrons are emitted from the CNT emitters  116 . A cathode voltage ranging from zero to negative tens of volts is applied to the first electrodes  112 , a gate voltage ranging from a few to hundreds of volts is applied to the second electrodes  114 , and an anode voltage ranging from hundreds to thousands of volts is applied to the third electrode  122 . The electrons emitted from the emitters  116  bombard the fluorescent layer  123 . Accordingly, the R, G and B fluorescent materials of the fluorescent layer  123  are excited to emit white visible light. 
     As described above, since the field emission backlight unit has the triode-type field emission structure, it can perform more stable field emission than a conventional backlight unit having a diode-type field emission structure. 
       FIG. 5  is a diagram illustrating simulation results of electron beams emitted from the backlight unit of  FIG. 2 , and  FIG. 6  is a photograph illustrating light-emission test results of the backlight unit of  FIG. 2 . In this case, the first electrodes  112  are grounded, a gate voltage of 100 volts is applied to the second electrodes  114 , and an anode voltage of 2000 volts is applied to the third electrode  122 . 
     First, referring to  FIG. 5 , since the first electrodes  112  functioning as the cathodes and the second electrodes  114  functioning as the gate electrodes are formed in the same plane, electrons emitted from the CNT emitters  116  are spread while traveling to the third electrode  122  that functions as the anode. If the electrons are spread in this manner, the entire surface of the fluorescent layer  123  formed on the third electrode  122  can be uniformly excited. 
     As a result, as shown in  FIG. 6 , uniform brightness is obtained all over the light emitting surface of the upper panel  120 . Here, the brightness is approximately 7000 cd/m 2 . 
       FIG. 7  is a partial sectional view of a field emission backlight unit according to a second preferred embodiment of the present invention, and  FIG. 8  is a partial perspective view of a lower panel of the backlight unit of  FIG. 7 . 
     Referring to  FIGS. 7 and 8 , a backlight unit includes a lower panel  210  and an upper panel  220  which are spaced apart from each other by a spacer  230 . The lower panel  210  includes a lower substrate  211 , first electrodes  212  and second electrodes  214  formed on the lower substrate  211 , and CNT emitters  216  and  218  disposed on the first electrodes  212  and the second electrodes  214 , respectively. 
     The first electrodes  212  and the second electrodes  214  in the second preferred embodiment are arranged in the same form as in the first preferred embodiment, and may include ITO electrode layers  212   a  and  214   a  formed on the lower substrate  211  and thin metal layers  212   b  and  214   b  formed on the ITO electrode layers  212   a  and  214   a  as in the first preferred embodiment. 
     However, the first electrodes  212  and the second electrodes  214  serve as cathodes and gate electrodes alternately. To this end, the CNT emitters  216  and  218  are formed on the first electrodes  212  and the second electrodes  214 , respectively. That is, the plurality of CNT emitters  216  are disposed at predetermined intervals along both longitudinal edges of the first electrodes  212 , and the plurality of CNT emitters  218  are disposed at predetermined intervals along both longitudinal edges of the second electrodes  214 . To easily form the CNT emitters  216  and  218  using a backside exposure method, a plurality of emitter grooves  215  and  217  are formed along both edges of the first electrodes  212  and the second electrodes  214 , respectively. In particular, it is preferable that the CNT emitters  216  and  218  are arranged by turns, such that the CNT emitters  216  formed on the first electrodes  212  face the second electrodes  214 , and the CNT emitters  218  formed on the second electrodes  214  face the first electrodes  212 . Consequently, electrons can be more smoothly emitted from the CNT emitters  216  and  218 . 
     On the other side, the modified example of the lower panel of the backlight unit of  FIG. 4  can be applied to the second preferred embodiment of the present invention. 
     The upper panel  220  includes an upper substrate  221 , a third electrode  222  formed on a bottom surface of the upper substrate  221  and serving as an anode, and a fluorescent layer  223  formed on the third electrode  222 . The detailed construction of the upper panel  220  is the same as that of the upper panel  120  in the first preferred embodiment. 
     A method of driving the backlight unit according to the second preferred embodiment of the present invention will now be explained with reference to  FIG. 9 . 
       FIG. 9  is a schematic plan view of the lower panel of the backlight unit of  FIG. 7 . 
     Referring to  FIG. 9 , the plurality of first electrodes  212  formed on the lower substrate  210  are connected to a first wire  241  for application of a voltage, and the plurality of second electrodes  214  alternating with the first electrodes  212  are connected to a second wire  242  for application of a voltage. The first electrodes  212  and the second electrodes  214  function as cathodes and gate electrodes alternately, as described above. 
     In further detail, if at the same time that an anode voltage of hundreds to thousands of volts is applied to the third electrode  222  formed on the upper substrate  221  shown in  FIG. 7 , a cathode voltage of zero to several tens of volts is applied to the first electrodes  212  through the first wire  241 , and a gate voltage of a few to hundreds of volts is applied to the second electrodes  214  through the second wire  242 , the first electrodes  212  function as cathodes such that electrons are emitted from the CNT emitters  216  formed on the first electrodes  212 . Next, if a gate voltage is applied to the first electrodes  212  through the first wire  241 , and a cathode voltage is applied to the second electrodes  214  through the second wire  242 , the second electrodes  214  function as cathodes such that electrons are emitted from the CNT emitters  218  formed on the second electrodes  214 . If the above steps are repeated, electrons are alternately emitted from the CNT emitters  216  formed on the first electrodes  212  and the CNT emitters  218  formed on the second electrodes  214 . The emitted electrons are formed into a beam and radiated onto the fluorescent layer  223  formed on the upper substrate  221  shown in  FIG. 7 . Accordingly, fluorescent materials of the fluorescent layer  223  are excited and emit white visible light. 
     In the method of driving the backlight unit according to the second preferred embodiment of the present invention, alternating emission of electrons from the CNT emitters  216  formed on the first electrodes  212  and the CNT emitters  218  formed on the second electrodes  214  prolongs the life of the CNT emitters  216  and  218  more than in the first preferred embodiment. That is, if a time interval between the application of gate voltage to the first electrodes  212  and the application of gate voltage to the second electrodes  214  is made two times longer than in the first preferred embodiment, the load applied to the CNT emitters  216  and  218  is reduced, and thus the lifespan is prolonged, while the same brightness as in the first preferred embodiment can be obtained. On the other hand, if the time interval between the application of gate voltage to the first electrodes  212  and the application of gate voltage to the second electrodes  214  is maintained the same as in the first preferred embodiment, the lifespan of the CNT emitters  216  and  218  is the same as in the first preferred embodiment, but the number of electrons emitted within the same time is increased, and thus brightness is further improved. 
     The method of driving the backlight unit according to the second preferred embodiment has an advantage in that it can control the time interval between application of the gate voltages to the first electrodes  212  and to the second electrodes  214 , thus appropriately adjusting the lifespan and brightness of the CNT emitters  216  and  218 . 
     Steps of manufacturing the lower panel of the backlight unit according to the present invention will now be explained with reference to  FIGS. 10A  thru  10 I. 
       FIGS. 10A  thru  10 I are schematic perspective views of the lower panel of the backlight unit according to the present invention. 
     As described above, the lower panels of the first and second preferred embodiments have similar structures, except that the CNT emitters of the first preferred embodiment are formed only on the first electrodes, while the CNT emitters of the second preferred embodiment are formed on both the first electrodes and the second electrodes. Accordingly, the manufacturing method will be explained based on the lower panel of the backlight unit according to the first preferred embodiment shown in  FIG. 3  and, for the lower panel of the backlight unit according to the second preferred embodiment shown in  FIG. 8 , only the difference will be explained. 
     Referring to  FIG. 10A , the transparent lower substrate  111 , for example, a glass substrate, having a predetermined thickness is prepared. Subsequently, the ITO electrode layers  112   a  and  114   a  are formed on the prepared lower substrate  111 . The ITO electrode layers  112   a  and  114   a  may be formed by depositing transparent conductive ITO materials on the entire surface of the lower substrate  111  to a predetermined thickness, for example, hundreds to thousands of Å. 
     Next, as shown in  FIG. 10B , the thin metal layers  112   b  and  114   b  are formed on the ITO electrode layers  112   a  and  114   a , respectively. The thin metal layers  112   b  and  114   b  may be formed by sputtering conductive metal materials, e.g., chrome, on the entire surface of the ITO electrode layers  112   a  and  114   a , respectively, to a predetermined thickness. 
     Next, as shown in  FIG. 10C , a photoresist (PR) material layer is coated on the entire surface of the thin metal layers  112   b  and  114   b.    
     Next, as shown in  FIG. 10D , the PR material layer is patterned in parallel lines by a photolithography process including exposure and development. In this case, a plurality of grooves  115 ′ corresponding to the emitter grooves  115  shown in  FIG. 3  are formed at predetermined intervals along both edges of odd or even lines of the PR material layer. 
     Meanwhile, when the lower panel of the backlight unit according to the second preferred embodiment of the present invention shown in  FIG. 8  is manufactured, the grooves  115 ′ are formed along both edges of all the lines of the PR material layer. Here, it is preferable that the grooves  115 ′ formed in two adjacent lines of the PR material layer are arranged by turns. 
     Next, the thin metal layers  112   b  and  114   b  and the ITO electrode layers  112   a  and  114   a  are etched using the patterned PR material layer as an etching mask, and then, the PR material layer is stripped off. Then, as shown in  FIG. 10E , the first electrodes  112  and the second electrodes  114 , including the ITO electrodes  112   a  and  114   a  and the thin metal layers  112   a  and  114   b , are formed in parallel lines on the lower substrate  111 . The plurality of emitter grooves  115  are formed along both edges of the first electrodes  112 . 
     In the meantime, in the step described with reference to  FIG. 10D , when the grooves  115 ′ are formed along both edges of all the lines of the PR material layer to manufacture the lower panel of the backlight unit according to the second preferred embodiment of the present invention shown in  FIG. 8 , the emitter grooves  115  are formed along both edges of both the first electrodes  112  and the second electrodes  114 . 
     Next, as shown in  FIG. 10F , a PR material layer is coated on the entire surface of the resultant structure of  FIG. 10E  once again. 
     Next, as shown in  FIG. 10G , the PR material layer is patterned using a photolithography process, including exposure and development, to expose the emitter grooves  115 . 
     Next, as shown in  FIG. 10H , a photosensitive CNT paste  119  is coated to a predetermined thickness on a surface of the resultant structure of  FIG. 10G  using a screen-printing method. Thereafter, light, (e.g., ultraviolet rays) is applied from a rear surface of the lower substrate  110  to selectively expose the CNT paste  119 . In this case, only the CNT paste  119  within the emitter grooves  115  is exposed to the ultraviolet rays so as to be cured. 
     In the meantime, the CNT paste  119  can be exposed from a front surface of the lower substrate  110 , but this case requires an exposure mask, which is inconvenient. If backside exposure is used, a separate exposure mask is not needed. 
     Next, if the PR material layer is removed using a developer, such as acetone, unexposed portions of the CNT paste  119  are also lifted off along with the removed PR material layer. Accordingly, as shown in  FIG. 10I , only the exposed CNT paste within the emitter grooves  115  is left to form the CNT emitters  116 . Through these steps, the lower panel  110  of the backlight unit according to the first preferred embodiment of the present invention is completed as shown in  FIG. 10I . 
     As described above, since the backlight unit according to the present invention has the triode-type field emission structure, more stable field emission can be ensured. 
     Since the first electrodes and the second electrodes serving as the cathodes and the gate electrodes are formed in the same plane and electrons emitted from the CNT emitters are spread out while being directed toward the third electrode, uniform brightness can be obtained over the entire light emitting surface of the upper panel. 
     Further, since the first electrodes and the second electrodes are made of the same materials and are formed in the same plane, and thus, can be manufactured simultaneously, manufacturing processes can be simplified and manufacturing costs are reduced. 
     Furthermore, since CNT emitters are used, electrons can be smoothly emitted, even at a relatively low driving voltage. 
     Moreover, since the method of driving the backlight unit of the present invention can control the time interval between applications of the gate voltages to the first electrodes and to the second electrodes, the lifespan of the CNT emitters can be prolonged, and brightness can be improved. 
     In addition, since the manufacturing method of the present invention employs CNT paste, the CNT emitters can be more easily formed on a larger substrate, and since the method uses backside exposure, an additional exposure mask is not required. 
     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.